The development of carbon capture and storage (CCS) in India: A critical review

Carbon capture and sequestration (CCS) is a three-tier process- carbon capture, transport and storage. The cap- ture consists of pre-combustion, oxy-combustion and post-combustion capture. Transport of CO 2 is most viable through pipelines. The biotic CO 2 storage occurs through terrestrial or oceanic pathways and can be simulated naturally or artiﬁcially. The abiotic/geologic storage is achieved through sequestering CO 2 in depleting/depleted hydrocarbon reserves, in deep saline aquifers or through mineral carbonation. At the district level, 64 out of 641 districts (2013 government reports) accounted for ∼ 60% of the total CO 2 emissions . Controlling CO 2 emissions comes with the challenge of sustainable socio-economic growth of the country- a demanding task for the econ- omy. Indian organizations have made international collaborations. India holds a substantial geological sequestration potential in its basaltic rocks, coal seams, depleted oil reserves, soils, deep saline aquifers and sedimentary basins. At this point, no carbon capture and storage / clean development mechanism projects are operational in the country. The next 10-15 years would be very crucial for India to attain technological advancement to deploy large-scale CCS projects. “In ceremony- a to the death of its ﬁrst glacier, killed due to climate ”

after the snowball phase by trapping solar energy that presumably favoured the first multi-cellular life on the Earth ( Hyde et al. 2000 ). An appropriate concentration of CO 2 in the atmosphere is essential for life on Earth. CO 2 concentration plays an important part in photosynthesis, which in turn drives the food chain on the Earth. But excess of anything turns hostile-and that has been happening with the atmospheric CO 2 whose concentration has well exceeded the maximum limit of ∼ 350 ppm ( Hansen et al. 2008 ).
Till the industrial revolution, anthropogenic carbon production mainly from wood-burning and other sources were at equilibrium with natural carbon uptake processes e.g., photosynthesis and oceanatmosphere carbon flux. However, the situation changed after the 1780s with the industrial revolution. It is estimated that since then ∼ 15-40% of additional anthropogenic carbon emissions have happened. These excess emissions would continue to be in the atmosphere for at least a millennium and would take thousands of years to be removed by natural processes alone ( Harde 2017 ).
The present-day concentration of CO 2 in the atmosphere is responsible for 26% of global warming ( Bhui 2021 ). On the other hand, the global energy demand is predicted to double by 2030 with the majority of it being met with fossil fuel sources, because of their low-cost and the existing infrastructures. This would cause the global mean temperature to rise by 2 °C by 2065 ( Lau et al. 2021 ) against the Intergovernmental Panel on Climate Change (IPCC) target of limiting global temperature rise to 2 °C by 2100 (Paris Agreement, 2015;Bui et al. 2018 ). To achieve the Paris agreement goal, by 2050 the emissions would need to be cut by 50-80% from the 1990 levels. The Agreement was a successor to other such assents, viz., Geneva Convention (1979), Montreal Protocol (1987), Kyoto Protocol (1997 and the Doha amendment (2012) ( Yoro and Daramola 2020 ; Table S1). The international energy bodies have targeted to make half the CO 2 emission due to energy production by 2050 ( Haszeldine 2009 ;Lu et al. 2020 ).

General points
Increasing temperatures have made the Earth vulnerable to several issues such as the sea-level rise and associated issues of coastal erosion, flooding, saline water intrusion and infrastructural damages ( Nazarnia et al. 2020 ). Extreme weather conditions such as increased frequency of cyclones ( Knutson et al. 2010 ), flood, drought ( Prospero and Lamb, 2003 ;Gleick 2014 ) and forest fire ( Flannigan et al. 2006 ) have also been noted in recent years.

Atmospheric CO 2 levels
Atmospheric CO 2 level over the last 300 years has changed rapidly. The timeframe ranging from 1780s till the present has seen the most incredible and life-changing developments. The levels of CO 2 changed drastically to over 100 ppm within ∼ 250 years. Whereas the pre-1750 level lingered at ∼ 280 ppm ( Wigley, 1983 ), the current level exceeded 400 ppm, much above the accepted limit of 350 ppm ( Azar and Rodhe, 1997 ). In the last decade, the atmospheric CO 2 concentration has increased by > 2 ppm y − 1 ( Yoro and Daramola 2020 ). The current CO 2 level stands at 413.08 ppm (14-Sept-2021, Mauna Loa Observatory, Hawaii).
In the geologic past, however, CO 2 levels much exceeded this threshold and reached 4400 ppm by natural means during the Triassic-Jurassic mass extinction ( Schaller et al. 2011 ) characterized by extreme volcanism forming the Central Atlantic Magmatic Province ( Blackburn et al. 2013 ). Such values were mainly attributed to volcanic activity.
Currently, China accounts for maximum emission, ∼ 26%of the global emissions. For the USA, India and Russia, the emission percentages with respect to global values are 13.7, 7.0 and 4.8, respectively ( Yoro and Daramola 2020 ). Annually about 51 billion tons of greenhouse gases are added up in the atmosphere. The aim has been to make that number zero ( Gates 2021 ).

Consequences of the increasing atmospheric CO 2 level
Climate change has become a reality. The steady increase in sea-level and average global temperature is the outcome. Since 1880, the average temperature of the Earth has increased by 0.8 0 C, which in turn rose the global mean sea at ∼ 1.7 mm y − 1 , being 3.0 mm y − 1 since 1993 ( Church and White, 2011 ;Dieng et al. 2017 ). The sudden spike since 1993 has been attributed to the rapid loss of the Greenland ice sheets ( Chen et al. 2017 ). Rising sea levels and mean global temperature are the major repercussions of the rising atmospheric reservoir of CO 2 . A perpetual increase from the current levels will only result in severe consequences in future. Rising sea levels will submerge the highly populated coastal areas of the world (such as Bangladesh), leading to acute pressure on the existing land to host the ever-growing population that is expected to touch 9.6 billion by 2050 ( Tripathi et al. 2019 ). Concentrated efforts need to be taken now if such hazardous projections must be nullified.

Possible solutions
As the world population grows, there will be a complementary increase in energy demand. The production of this energy will further add to the ever-increasing CO 2 level of the atmosphere. Places such as Hong Kong, China and Singapore have already started transitioning to lowcarbon transport options viz., electric rails and metro and discouraged the use of personal cars ( Senthilkumar 2021 ).
This situation needs to be addressed fast in three possible ways ( Figueroa et al., 2008 ): (i) to opt for alternate sources of clean energy; (ii) reduce the intensity of CO 2 production by focusing on cleaner forms of combustion or to choose cleaner fuel (anthracite or bituminous) with high carbon-content; and (iii) focus on the development of efficient carbon-capture and sequestration technologies.
India holds a sizeable share in the global growing energy demand, 69% of which are met through fossil fuels, out of which 44% is coalbased ( IEA 2020a ). This seems viable since India has the third-largest coal reserve in the world. As on 01-April-2019, the official figure stands at 326.05 BT, as measured up to a depth of 1200 m ( Government of India, GOI 2020a ). Projections indicate that India's emission could stand at around 5.6 BT in the business-as-usual scenario when as per India's Intended Nationally Determined Contributions (INDC), non-fossil fuels would contribute to 40% of its total electricity installed capacity (Section 3.3) . India will probably account for ∼ 25% of the increased energy demand from 2017 and 2040. Coal-based energy would meet ∼ 42% of the incremental demands ( Ray 2021 ). These trends indicate that fossil fuel will be continued to be used as a power source well into the 2040s, with projected emissions reaching their peak in 2043 ( Frank 2015 ). An average 500 MW thermal power plant can emit 2-3 MT CO 2 annually ( Yoro and Daramola 2020 ), and coal-based energy generation is slated to be in the range of 330 -441 GW in 2040, increasing from 175 GW in 2017 ( Goel et al. 2021b ).
In such a global and Indian scenario, the only method, besides using clean energy sources and policy interventions, is carbon capture and sequestration (CCS; Yoro and Daramola 2020 ) Fig. 1 . portrays the carbon cycle with and without sequestration Leung et al. (2014) . has summarized CO 2 reduction strategies. Meeting the net-zero emission target within this century is impossible without CCS ( Global CCS Institute, GCCSI 2020 ). An estimate by the IEA state that to meet the Paris agreement goals, an additional investment of USD 9.7 trillion, by 2050 would be required in Carbon capture, utilisation and storage (CCUS) deployment ( IEA 2019 ). Wei et al. (2021) also estimated a similar number at USD 8.2 trillion as per the "global cost effective CCUS layout strategy ". Moving ahead with IEA's estimate in absolute terms, this is almost 3.4 times the size of India's GDP. However, the cost of not implementing it (climate change-related disasters) far eclipses this value. Between 2000 and 2019, the global economy incurred a loss of USD 4.78 trillion. The last decade (2010-2019) saw the highest loss at USD 2.98 trillion. In the prior decade (2000 -2009), the number was USD 1.8 trillion. There was an increase of USD 1.1 trillion in one decade ( AON 2019 ). If we project from here, assuming no increase in decadal losses, the number  Bhui et al. 2021). stands at USD 13.72 trillion by 2050. If an increase of USD 1.1 trillion is factored in per decade, the losses stand at USD 17.02 trillion by 2050. In economic terms, an investment of USD 9.7 trillion to tackle a loss of USD 17.02 trillion seems feasible in the current context. Such investments also entail certain risks, especially the geological storage of CO 2 (Section 2.4.3c). However, risk assessment studies involving the selection of sites, reservoir characteristics and monitoring of CO 2 movement, in early stages as well as in simulation, have significantly decreased the possibility of such mishaps (global case) ( Hardisty et al. 2011 ).
There are already commercial sites operating in Sleipner (Norway) and Gorgon (Australia) (Section 2.4.3b6, Fig. 2 ). CCS would also enable a "just transition ", which would create new jobs in the net-zero industry, allowing re-use or continued use of available infrastructure and deferring their shut-down costs (GCCSI 2021). Bergstorm and Ty (2017) analysed the total cost and benefit of CCS technologies and concluded that the private, public and social benefits of the technology in mitigating global warming outweigh its cost. Research on CCS in the previous decade present CCS as a critical option in tackling climate change ( Bonto et al. 2021 ). A review by Seigo et al. (2014) presents an allaround view of how the public perceives the CCS technology based on 13 variables. These variables are a part of Energy Technology Acceptance Framework (ETAF) developed by Huijts et al. (2012) . The public opinion is weighed on the basis of acceptance, Knowledge, Experience, Trust, Fairness, Technologies Affect, Perceived Costs, Perceived Risks, Perceived Benefits, Outcome Efficacy and Problem Perception. However, there has not been a clear conclusion on what the majority seems to agree with due to lack of enough knowledge dissemination among the public. Such efforts, however, can be undertaken. Smit et al. (2014) preferred recycling of CO 2 over its sub-surface storage. Methanol production from captured CO 2 holds a great prospect as a transportation fuel and in the petrochemical industry ( Bhui 2021 ). However, at present, recycled products of CO 2 utilized for the chemical industry cannot solve the problem of elevated CO 2 concentration in the atmosphere. Infrastructural dearth, capital and safety issues trouble carbon utilisation into fuels ( Kanjilal et al. 2020 ). Section 4.1 presents some promising prospects. Arehart et al. (2021) review concluded that inte-gration of carbon-based materials in buildings and construction could prove to be a safe sequestration option above ground.
CCS applied to an existing emitter will only act as a transition technology for its decarbonisation ( Lau et al. 2021 ). CCS can decrease the carbon footprint of fuels by ∼ 90% ( Senthilkumar 2021 ). This will allow the use of fossil fuels until they are replaced by relatively cleaner energy sources (Table S2: advantages and disadvantages of other methods). It will be beneficial, particularly for India being a coal-dependent country ( Global CCS Institute, GCCSI 2020 ). During 2003-04, efforts were made to explore the potential of supercritical and ultra-supercritical thermal power plants that have efficiency values of ∼ 44% and ∼ 50%, respectively. This was to avoid old thermal power plants running at < 35% efficiency. Investments, however were not made to install carbon capture technologies due to economic reasons ( Donev et al. 2018 ;Verma 2021 ).
Currently, the global reality is different, and this becomes particularly important for 'hard-to-abate' industries ( Fig. S1: emission sources of hard-to-abate industries). These industries account for 20-30% of the global emissions and mainly consist of cement, petrochemical and steel industries. These industries heavily depend on fossil fuels as reducing agents (coal in iron and steel industries) and as a feedstock in their production (e.g., petroleum in petrochemical industries, calcium carbonate in cement industry; Leeson et al. 2017 ;IEA 2020b ). Paltsev et el. (2021) concluded that with Industrial CCS technology deployment, the global cost for reaching the Paris target would be 12% less by 2075 and 71% less by 2100, as opposed to without CCS options.
All these industries would primarily depend on post-combustion capture technology to reduce their emissions. Post-combustion capture allows retrofitting the existing infrastructure at the end of the cycle for carbon capture (Section 2.2.2) . This technique also stands true for thermal power plants that are major emission sources in the power sector. Post-combustion capture can also increase the industrial production 3.7 to 7 times the 2010 levels by 2100 as opposed to meagre 1.6 times without such a capture ( Paltsev et al. 2021 ).
In the cement industry,60 -70% of CO 2 is generated as process emissions during clinker production [calcium carbonate (CaCO 3 ) breaks down into calcium oxide (clinker) and CO 2 ], which account for ∼ 33 % of emissions ( IEA 2020b ). For such industries, CCS seems to be the only viable option, besides technology upgration e.g., clinker substituted by blast furnace slag and fly ash in the cement industry ( Xavier and Oliveira 2021 ). Chemical looping combustion can reduce emission in the cement industry ( Gu et al. 2015 ; Section 2.2.4 ) Hargis et al. (2021) . developed a new CaCO 3 cement using CO 2 -rich industrial flue gas, calcium and alkali. All the feedstocks essentially are industrial waste products, e.g., those of thermal power and acetylene production plants. Flue gas is produced in thermal power plants and acetylene production produces carbide lime sludge as a waste product. This cement, which is completely manufactured using waste products of different industries (primarily CO 2 ), has advantages over traditional cement such as lighter weight, shorter production cycles and similar compressive strength ( > 40 MPa) to traditional types of cement.
The iron and steel industries account for ∼ 31% of the industrial emissions. Out of this, the blast furnace (where iron ore is smelted) emits 65-70% of the emissions, followed by a coking coal plant (where coal converts to coke in the blast furnace, Ashour 2018 ) at 27% and sinter plant (where iron ore dust is agglomerated and sent to the blast furnace) at 6% ( Pérez-Fortes et al. 2014 ;Leeson et al. 2017 ). The primary emission here comes from coal (in the form of coke) to reduce iron to a relatively pure form, which is further processed for making other products. Carbon capture technology can be retrofitted to these sources in a steel plant to capture CO 2 , reducing the emission of the plants. This can minimize emission causing reducing agents such as hydrogen, polymer/coke blends and lignin. However, these alternatives are in their initial stages of research. One type of hydrogen production itself depends on hydrogen, and polymer/coke blend is not a complete replacement as it still uses coke. Lignin is most promising, but its current production is too low to meet the demand ( Sahajwalla et al. 2019 ).
The petrochemical industry accounts for ∼ 10% of the industrial emissions. Within them, the boilers and furnaces account for ∼ 65% of the emissions, followed by gasifiers at ∼ 10% ( Leeson et al. 2017 ). Petroleum alongside coal is a major driver of the industrial wheels. Replacing petroleum can only be done if there is an established alternative fuel source. There are few renewable options that are slowly gaining attention.
Worldwide petrochemical plants are high-value assets with certain economies completely dependent on them (e.g., Middle East, Russia; Snyder et al. 2020 ). The only option left then is CCS technologies that reduce the emission intensity of the petrochemical industry.
Post-combustion capture also stands true for the thermal power plants that are major emission sources in the power generation sector. Apart from post-combustion capture, pre-combustion capture technologies viz., IGCC, IRCC (Section 2.2.1) , and oxy-combustion capture (Section 2.2.3) have considerable potential in decreasing the emission intensity of thermal power plants and at the same time allowing for a smooth transition to cleaner energy sources and more energy-efficient plants. CCS retrofitting has the potential of bringing down the emissions of thermal power plants to nearly zero by 2047 ( Vishal et al. 2021a ).
As of 2020, there are 65 commercial CCUS facilities worldwide, out of which 26 are operating, three are under construction, 13 are in advanced development, 21 are in early development and 2 have been suspended ( Fig. 2 ; Global CCS Institute, GCCSI 2020 ). Altogether, they sequester 40 MT of CO 2 annually. To put this into perspective, the global emissions in 2019 stood at 52 BT. If the total conversion rate remains intact, the existing plants will take 130 years to sequester, provided there are no further emissions. This is an impossible scenario and hence more such plants need to develop. A special report by Intergovernmental Panel on Climate Change, IPCC (2018) reviewed 90 scenarios to restrict global warming to 1.5 °C. Together, they need to meet the permanent sequestration of 10 BT by 2050 to attain the 1.5 °C target. The current sequestration potential is thus rendered extremely insufficient. Around 2000 CCS plants are required to meet the IPCC targets decided in the Paris Summit ( Senthilkumar 2021 ).
In this context of the CO 2 dilemma, this article provides an overview of CCS with the primary focus on reviewing the potentials, technologies, and current scenarios in the Indian CCS arena Goel (2008) . presented CCS studies from the Indian perspective. However, in the last 13 years, numerous advancements have taken place in the field, therefore an updated review article was needed. We also discuss a few case studies to provide promising technologies and feasibility studies that hold huge potential in bringing India onto the global CCS map. We describe the prospects of CCS in India. We understand that each sub-section in this article has the potential to develop as standalone contributions.

Contents of this review article
In this article, Section 1 briefly introduces the current energy situation in India and the world, the trend of CO 2 increase, its consequences, and the importance of CCS-and especially the present Indian scenario Section 2 . elucidates CCS-its various components, techniques, and technologies. Section 3 is an overview of India's carbon footprint, its trend over the years and emission hotspots of the country. The section concludes with India's emission projections in the coming decade and the effect of Covid-19 in the temporary thwarting of emissions. Section 4 elaborates the need for carbon sequestration in India along with major research and developments in this century. This is followed by the sequestration potential of India in biotic and abiotic means. Section 5 presents a few Indian case studies that hold significant potential for carbon sequestration if scaled-up Section 6 . describes the prospects in terms of CCS-CDM and ACT proposals. Section 7 concludes the review by mentioning a probabilistic timeline for the deployment of CCS in India.

General points
The history of carbon sequestration goes back to the 1920s when CO 2 was separated from the natural gas in the gas wells. Soon it was realised that the captured gas could augment the process of oil extraction. The process came to be known as the Enhanced Oil Recovery (EOR), which gained momentum in the 1970s and the 1980s ( Gupta and Paul 2019 ). This process also locked down captured CO 2 into those geologic formations from where oil was extracted, not allowing it to add to its global atmospheric reservoir ( IEAGHG 2013 ). CO 2 -EOR reduces the viscosity of oil, thereby enhancing its extraction by ∼ 43% , while in some studies the extraction enhancement was ∼ 10-22% ( Karmakar 2016 ). However, the claim that it enhances the extraction rate is irrefutable. Currently, there are 18 CO 2 -EOR projects worldwide ( Fig. 2 ), out of which six operate on CO 2 obtained from power plants ( Elmabrouk et al. 2017 ).
Carbon sequestration is a three-tier process ( Shahhosseini HR et al., 2017 ): (i) carbon capture from emission plants or directly from the air; (ii) conversion into suitable forms to be transported and deposited to sequestration sites; and (iii) the final sequestration of the carbon.

Carbon capture
The most important procedure is to capture carbon from flue gases (other methods in Fig. S2).There are three pathways: (i) pre-combustion capture, (ii) oxy-combustion capture, and (iii) post-combustion capture ( Padurean et al. 2012 ;Jain et al. 2016 ).

Pre-combustion capture
As the name suggests, C is captured, in the form of CO 2 , before the fuel combusts. Two leading technologies that make this happen are the Integrated Gasification Combined Cycle (IGCC) and Integrated Reformed Combined Cycle (IRCC; Lorenzo et al. 2013). In the latter, a syngas production process called auto-thermal reforming ( Feron and Hendriks, 2005 ) is combined with a cycle power generation plant. In the former case, a gasification process is combined with the combined cycle power generation plant ( Di Lorenzo et al. 2013 ).
In a typical IRCC ( Fig. 3 a), there is an Auto-thermal reformer (ATR), two Water-Gas shift (WGS) reactors, one pre-reformer, and a single CO 2 capture section. The heavy hydrocarbons, e.g., ethane, butane and propane ( Mokhatab et al. 2012 ) in the natural gas are converted to CO 2 and H 2 . Following this, the natural gas is converted into syngas (mainly containing CO, CO 2 and H 2 ) in the ATR. The syngas undergoes the WGS reaction in the WGS reactor, by converting CO into CO 2 . The heat produced during the process is added to the Heat-Recovery Stream Generator (HRSG), to remove the heat and generate steam. CO 2 is liquified and stored, resulting in a carbon-free fuel ( Nord et al. 2009 ;Di Lorenzo et al. 2013 ).
IGCC is a coal-driven power generation technology that is more efficient and environment friendly than a typical coal-powered plant ( Emun et al. 2010 ). It leads to a substantial decrease in the release of pollutants and can be a useful retrofitting option for existing thermal power plants ( Yoro and Daramola 2020 ). In IGCC, coal is decarbonised before combustion. Just as the natural gas is used as an input in IRCC, in IGCC, inputted coal or other carbon-based feeds such as coke etc. under pressure reacts with O 2 and steam to generate the syngas ( Fig 3 b). The O 2 is supplied by the Air separation unit (ASU). Steam is added to the syngas in the WGS reactor after the syngas passed through the syngas cooling section. Heat generated from the cooling section is added to the HRSG (like IRCC). The acid gases removal (AGR) section removes the acid gases from the syngas. The resultant CO 2 is captured, and H 2 -rich fuel is obtained. This H 2 is called "blue hydrogen " as opposed to "greyhydrogen " when no carbon capture takes place and "green hydrogen " when water is electrolysed to obtain hydrogen using renewable energy ( Kanniche and Bouallou 2007 ;Descamps et al. 2008 ;Gibbins and Chalmers 2008 ;Di Lorenzo et al. 2013 ;Wood 2020 ).

Post-combustion capture
Post-combustion capture involves the capturing of carbon from the flue gas generated at plants concerned with the burning of fossil fuels. It is the only process that is industrially employed, which can be seen in TMC Mongstad (0.3 million tonnes y − 1 ) and BD3 SaskPower (1 million tonne y − 1 ) in Norway and Canada, respectively ( Liang, 2015 ). It is also the most widely researched and adaptable method, with various technologies being used for it e.g., solvent-based absorption, membraneseparation, mineralization, adsorption-driven, cryogenic capture and microalgae-based carbon capture ( Mokhtar et al. 2012 ;Kanjilal et al. 2020 ). Amongst solvent-based PCC, amine-based solvents are widely used. Monoethanolamine (MEA) is most widely used for its high-CO 2 reactivity and high capture efficiency ( ∼ 90%) ( Kanjilal et al. 2020 ). Membranes are semi-permeable structures that separate CO 2 from the gaseous mixtures emitted out after fuel combustion ( Carrera et al. 2017 ). Membrane-based PCC is significantly advantageous in high surface areas, considerably reducing the equipment size and increasing efficiency. Polytetrafluorethylene (PTFE) is the most widely used PCC membrane in many pilot studies. However, it has still not been used in industry ( Zhao et al. 2016 ) presumably because of its high cost and further research is underway ( Merkel et al. 2010 ). The process of mineralization is mineral carbonation where CO 2 is converted into stable carbonates for their storage, especially in areas that lack suitable geological formations ( Zevenhoven and Fagerlund 2009 ). It is also a process of storage as opposed to the other three that deal with only carbon capture and is more environment-friendly than geologic sequestration ( Helwani et al. 2016 ). Hence Section 2.4.3c details mineral carbonation. Adsorption is a process of CO 2 capture by making it interact with a solid (molecular sieves or activated carbon) or chemical adsorbent such as modifying low-cost carbons using polyethyleneimine ( Drage et al. 2009 ). In the last few decades, ionic solvents have been found to be better adsorbent than the other media ( Das et al. 2021 ). Ionic liquids provide several advantages over other methods due to easy regeneration, low solvent loss and low environmental impact ( Farsi and Soroush 2020 ; Fig. S3-detailed classification of ionic liquids). Nanomaterials such as nanomembranes, nanoparticles and nanosheets are gaining worldwide acceptance as adsorbents ( Pant et al. 2021 ). Other materials such as activated carbon, zeolites, amine-functionalized silica, porous organic frameworks and metal organic frameworks (MOFs) (subcategories of MOFs include MOF-glass membranes, MOF-Covalent organic frameworks membranes, MOF based mixed matrix membranes) ( Demir et al. 2022 ) are also expanding the range of adsorbent materials for post-combustion capture ( Siegelman et al. 2021 ). Biochar based adsorbents also have potential due to their wide availability, low cost, renewable nature and highly porous structure ( Qiao and Wu 2022 ). (MOFs Cryogenic capture involves lowering the temperature of flue gas and separating the solidified CO 2 from the flue gas (Ahuja 2021; Table S3). A review by Buckingham et al. (2022) shows that process intensification can be used by integrating the CO 2 adsorption process into the chain of reactions that generate CO 2 . This can have advantages in terms of increased energy efficiency. Section 2.4.4 . discusses capture (and sequestration/utilization) through microalgae. There are several other methods that are under research (Fig. S4).

Oxy-combustion capture
Here there is no real 'capture' of CO 2. The fossil fuel itself is burnt in an oxygen-rich environment, thus allowing for a cleaner and fuller combustion of the fossil fuels, greatly reducing the CO and the SO 2 contents ( Jain et al. 2016 ). The process does not involve any membranes or absorbents and is thus significantly cost-effective for new plants, but the cost increases if old plants are retrofitted. A review by Yadav and Mondal (2022) concludes that the overall cost of carbon capture is less in Oxy-combustion capture. Another advantage is that almost pure (90%) CO 2 can be directly compressed and stored without the need for further purification as in PCC ( Gopan et al. 2014 ;Jain et al. 2016 ). Maximum energy penalty for CO 2 capture was found to be prevalent if membrane-based capture systems are adopted in comparison to amineand ammonia-based systems .
The challenges of this technology are primarily related to retrofitting an old plant which is caused by high temperatures during combustion and air that leaks into the system. These factors negatively affect performance ( Yadav and Mondal, 2022 ). . involve a high energy penalty ( ∼ 15%), which decreases the efficiency of the system. In the above cases, a significant amount of energy is spent to separate and obtain a pure stream of CO 2 that is further stored or processed. Chemical Looping Combustion (CLC) is a cost-effective alternative to other methods ( Kumar and Parwani 2021 ).
CLC uses a metal oxide as an oxygen source ( Jain et al. 2016 ). The looping, as the method is named, is between two chambers: oxidation and reduction. In the oxidation chamber, also called the air reactor, a metal is oxidised in air to obtain its oxide. This metal oxide acts as an oxygen carrier that reacts with fuel (any hydrocarbon C x H y ) in the reduction chamber, also called the fuel reactor ( Lisbona et al. 2020 ). The metal oxide (Me x O y ) is reduced to its metal form (Me), CO 2 and H 2 O. The reduced metal is again looped into the oxidation chamber, where the process resumes. The pure CO 2 stream from the reduction chamber can be compressed for storage, transportation or utilisation. This significantly reduces the cost of obtaining CO 2 from flue gas. The primary roadblock in this method is the metal that is used for continuous cycling without much physical and chemical degradation. Another concern is the energy required for cycling solid metal between the chambers. As of now, there are no operating facilities employing this technology anywhere in the world ( Verma et al. 2015 ;Jain et al. 2016 ;Kumar and Parwani 2021 ) Table 1 . summarizes the technologies discussed above.

Transportation
The carbon capture is followed by its transportation from the point of capture to the point of its final sequestration. The captured carbon is compressed to liquefy itself, to smoothen its transportation process. After liquefaction, the most convenient way to transport it is through pipelines. However, small shipments of a few tonnes are also transported through trucks over short inland distances-from large point sources to ports for further transport via ships. Ships are already transporting ∼ 1000 tonnes of food-quality CO 2 , in Europe ( Global CCS Institute, GCCSI 2012 ). However, shipments of small quantities would not be viable if the large-scale prospect of carbon sequestration is considered. Pipelines are the most suitable means to transport CO 2 in the scale of the current requirement and have been conveniently in business since 1970s. CO 2 is moved in a supercritical phase under high pressure. This dense phase CO 2 is prone to gradient changes and contamination. This requires specific and continuous inspection and adds up to the transportation cost ( Kumar et al. 2020 ).
As of 2015, the total length of such pipelines stood at 8000 km globally Holloway et al. (2008) . suggested developing a central system of interconnected pipes to collect CO 2 emitted from different artificial sources/industrial plants that can perform sequestration at some specific location. However, such a step has not so far been taken in India, possibly due to the economic feasibility issues.

General point
The final sequestration of carbon is the last stage in the three-tier process. It allows the final storage of the captured and transported carbon into carbon sinks. There are various types of carbon sequestration options depending on the technological advancement of the economy involved, the nature of the carbon to be sequestered and the nature of the sink itself. Eccles et al.'s (2012) engineering modeling revealed that a sequestration site could range from < 100 to > 1000 km 2 , and that thick low permeability reservoirs should be preferred. "Engineering-economic model has demonstrated that CO 2 transport is as important as to consider the sequestration sites ( Chandel et al. 2010 ). " Based on the above parameters, carbon sequestration has been broadly categorised ( Lal, 2008 ;Kambale and Tripathi, 2010 ) as: (i) biotic sequestration and (ii) abiotic sequestration. See Chandel et al. (2015) and Boindala and Anmala (2019) for review of different models of geological CO 2 sequestration.

Biotic sequestration
This refers to the biotic media through which carbon is sequestered. It requires a close symbiosis between plants and animals. The process is highly efficient due to the involvement of living beings in the storage process due to the input of less energy from outside the natural cycle. The primary process that aids in the biotic sequestration of carbon is photosynthesis. Carbon capture through photosynthesis is called phytoremediation and plants can act as potential CO 2 sequestration options. Different species have different potentials and certain plants work better in certain environments such as urban or industrial areas ( Govindaraju et al. 2021 ).

Oceanic sequestration.
Oceans are vast harbours of plant life. The oceanic photosynthesis of these zones, especially the phytoplankton photosynthetic process itself, sequesters ∼ 45 Pg C yr − 1 ( Lal 2008 ). These phytoplanktons then deposit on the ocean floor, thus sequestering carbon. Oceans have consumed 25% of anthropogenic CO 2 and act as a buffer towards extreme climatic catastrophes ( Fig. S5: oceanic carbon flux). However, this has acidified the ocean water ( Turley et al. 2010 ). Another method of stimulating phytoplankton growth has been hypothesized ( Kambale and Tripathi 2010 ). The ocean, if fertilized with iron, can stimulate the growth of phytoplankton ( Street and Paytan, 2005 ), which in turn would increase the rate of photosynthesis and increase the overall biomass. The former would consume more CO 2 , while the latter would sequester it when the phytoplanktons die.

Terrestrial sequestration.
It is the most common type of carbon sequestration globally though with different intensity depending on the latitudinal extent being considered. Its intensity is highest near the tropical zone (0 0 -10 0 N/S) and decreases towards the poles. The sequestration is directly proportional to the availability of biomass, which is itself guided by the latitudinal position. The sequestration occurs through the process of photosynthesis, where plants capture atmospheric CO 2 to produce their food by utilising sunlight and water, and release oxygen ( Lal 2008 ).
Terrestrial carbon sequestration has been occurring since plants appeared on the Earth, much before animals in the Late Precambrian (Knauth and Kennedy 2009). This natural sequestration, a part of the global carbon cycle, has played a vital role in maintaining the homeostasis of the planet. However, since the CO 2 levels exceeded the natural limit of 300 ppm, the natural sequestration process is not proving to be enough leading to a continuous rise in average global temperature (Section 1.1) . There are three interdependent components of terrestrial sequestration viz., forests, wetlands and soils ( Lal 2008( Lal , 2010. • Forests (both afforested and reforested) store a major portion of the global carbon ( Tong et al. 2020 ) and store it as lignin, an important constituent of the cellulose of the cell wall of a plant cell. This cellulose accounts for 50% of all cellular carbon in the biosphere ( Zeikus 1981 ). The average rate of carbon sequestration through the forest is ∼ 1.58 BT C yr − 1 ( Lorenz and Lal 2010 ), with a potential of storing up to 87 BT by 2050 ( Sohngen and Mendelsohn 2003 ). Urban forests can play a dual role by providing green areas in dense settlements and performing sequestration in their biomass (e.g., roots, branches, leaves). Studies mention that urban forests can sequester 18 kg C yr − 1 per tree, with roadside trees sequestering more C than those found in isolated areas ( Govindaraju et al. 2021 ). • The soil ecosystem includes all the types of soil on the Earth. Soil contains twice the carbon contained in the atmosphere and three times that of trees. It is a major carbon reservoir ( Yadava and Thokchom 2021 ). Soil carbon sequestration differs from geological sequestration, although both methods follow similar processes. While geological sequestration is abiotic and requires carbon storage beyond 1000 m depth, soil carbon sequestration is biotic and requires storage up to 1 m depth ( Lal 2008 ). • Wetland ecosystems include bogs, peats, marshes and other forms of histosols. They sequester C as Soil Organic Matter (SOM). They hold ∼ 20-30% of the world soil carbon while occupying a mere 5-8% of land area ( Nahlik and Fennessy, 2016 ). Since the last ice age, around 13,500 years ago, wetlands have sequestered carbon at a steady rate of 0.1 BT C yr − 1 ( Lal 2008 ).
Mauthausen and Dooly (2019) ran a Monte Carlo simulation across tropical, subtropical, temperate and boreal forests on their sequestration potentials if those were reforested as per various sequestration scenarios. The data on sequestration parameters were taken from various literatures to model the sequestration of the above forest types. The median value obtained was 151.9 GT of C till 2150 ( Fig. 4 ). This shows that 151.9 GT of C will be sequestered till 2150.

Abiotic sequestration
Abiotic or engineered sequestration occurs without the action of the biotic components. Its rate, intensity and frequency can be altered. It is a more viable option than biotic sequestration as the technologies can be developed and refined to increase the efficiency. Global studies estimated a total geological sequestration capacity between 10,000 -30,000 BT of CO 2 . Such estimates are highly prospective and thus highly uncertain ( Budinis et al. 2018 ). Wei et al. (2021) estimated the capacity to be 2082 BT, using a method developed by United States Department of Energy. Brown et al. (2013) estimated the sequestration potential of the location Florida Panhandle through numerical modelling and arrived at a value of 4.55 BT van der Meer and Yavuz (2009) . calculated the sequestration capacity of Rotliegend Formation (The Netherlands). An empirical equation based on storage efficiency, aquifer volume, porosity and CO 2 density at depths was used to arrive at a total value of 104.12 MT of sequestration potential. Thibeau et al. (2014) through 3D flow modelling ascertained the sequestration capacity of four sandstone formations from four different countries. These are Mt. Simon (USA), Basal (USA & Canada), Bunter (UK) and Rotliegend (The Netherlands). The calculated sequestration values were 13.3, 16.2, 2.23 and 0.33 BT, respectively. Mt. Simon, Basal and Rotliegend are the onshore areas. The values for Rotliegend Formation differ significantly between those estimated by Van der Meer and Yavuz (2009) and by Thibeau et al. (2014). In any case, these are empirical estimates. Vangkilde-Pedersen (2009) calculated a tentative geological sequestration capacity of Europe under the "GeoCapacity " Project. Empirical method was used considering various parameters such as: • aquifer area, volume, thickness, CO 2 density at depths, storage efficiency for aquifers • Storage capacity, recovery factor, CO 2 at reservoir conditions, gas and oil formation volume factor, original gas and oil in place, the volume of injected and produced water for hydrocarbon reservoirs • Storage capacity, producible gas, CO 2 density and CO 2 to CH 4 exchange ratio for coal fields The total capacity of potential European sites stood at 325 BTin aquifers, 30 BTin hydrocarbon fields and 1.5 BT in coal fields. ( Faiz et al. 2007 ) calculated the sequestration capacity of coal seams in the Southern Sydney Basin, Australia. Calculations were made using fieldcollected data and previously published work. The calculations yielded values of 350 MT (with 100% CBMR) and 175 MT (with 50% CBMR).
Besides the EU GeoCapacity project, other regional initiatives to estimate geologic storage potentials are North American Carbon Storage Atlas and CO 2 atlas for the Norwegian Continental Shelf ( Ringrose 2020 ).

Oceanic injection.
Oceanic injection envisages injecting liquified CO 2 in the ocean water at ⩾1000 m depth for carbon storage. The total capacity of oceanic sequestration far exceeds the amount of carbon (5000 -10000 GT) that the world fossil fuels can produce ( Herzog et al. 2001 ).
Overturning of the ocean is also a major phenomenon and driver of oceanic life. Therefore, inevitably the injected CO 2 would leak into the atmosphere. However, overturning time ranges from 300-1000 years and various data suggest that ∼ 20% of the injected CO 2 would eventually leak. The leakage periods would vary inversely with the depths. Hence, 1000 m is the minimum depth for carbon storage ( Herzog et al. 2001 ). At 1000 m depth, CO 2 would remain as a droplet plume ( Herzog et al. 2002 ), whereas at 3000 m, liquefied CO 2 can exist in the form of a lake in a depression ( Benson and Orr, 2008 ;Fig. S6). This method can actively sequester 2 BT of CO 2 annually. Those are valued across the entire geological media, i.e., the total capacity ( Bose and Satyanarayana 2021 ).

Geological sequestration.
General points. Geologic carbon sequestration occurs in depleted hydrocarbon reservoirs and deep saline aquifers. CO 2 is also used in the EOR, Enhances Gas Recovery (EGR) and Enhanced Coal Bed Methane Recovery (ECBMR) techniques as a method of passive sequestration in depleted/depleting hydrocarbon reservoirs. The primary purpose is to accentuate the recovery of oil, gas or methane from sources that have reached their output limit of oil, gas or methane ( Riley 2010 ) Smit et al. (2014) . pointed out that in any geological modelling of CCS, one must consider the effect of (i) weathered rocks since that has a much lower permeability and (ii) organic and biological matters in the sequestrated CO 2 .
Depleted hydrocarbon reserves. Depleted oil and gas reservoirs are currently the most viable option for sequestration. Economically, these reservoirs have already been thoroughly researched, developed, scientifically explored and have the necessary infrastructure required to extract the hydrocarbon. The same approach can be used, with minor modification if required, to inject CO 2 into them. CO 2 injection in a tight hydrocarbon reservoir has two purposes (i) sequestration of CO 2 , and (ii) enhanced oil recovery from the reservoir ( Jia et al. 2020 ). However, such an injection can lead to induced seismicity ( Vasco et al. 2020 ). In particular, a basin with fractures and faults can induce seismicity if CO 2 is injected, or it can elevate pore pressure in the basin (review in Zakharova et al. 2020 ; also see Dasgupta and Mukherjee 2020). To avoid these, reservoirs are to be monitored geodetically ( Vasco et al. 2020 ). Monitoring is also required due to pressure difference during and after injection. Such issues manifest as land surface deformations. The all-weather monitoring capabilities of Inferometric Synthetic Aperture Radar (In-SAR) can find applications in the long term CO 2 migration monitoring (by monitoring land surface deformation) ( Zhang et al. 2022 ). Other remote sensing techniques such as Active microwave remote sensing and LiDAR also hold significant potential in terms of monitoring capabilities . A review by Noble et al. (2012) concludes that biological monitoring using DNA fingerprinting and bacterial counts can also be used for efficient monitoring. Given that technology is advancing quickly, biological monitoring demands exploration. Leakages also can manifest due to geochemical interaction and temperature/pressure differentials between the CO 2 and the host rock in which it is injected ( Gholami et al. 2021 ). Biological monitoring has more potential in detecting leaks and other environmental changes that happen due to those leakages ( Noble et al., 2012 ). The Barmer basin (Rajasthan, western India) is a tight reservoir, but carbon sequestration has not been started there.
Geologically too, tight reservoirs are well developed to hold off the CO 2 because of the already present geological structures that contained the oil and gas for millions of years. The prerequisite is to store the CO 2 at > 800 m depth ( Riley 2010, Ringrose 2020 to ensure the necessary density to keep the CO 2 in a super-critical or liquid phase (Fig. S7 for CO 2 -depth density relation). However, the density also depends on local temperature and geothermal gradients ( Ringrose 2020 ). The EOR, EGR and ECBMR are applied to both sequester carbon and enhance the production of oil, gas and methane from their respective reservoirs.
Enhanced Oil Recovery (EOR). In EOR, CO 2 is injected into an oil well to reduce its viscosity, enabling an increase oil production ( Section 2.1 ). This process has been in pract i ce since its first execution in Oklahoma ( Karmakar 2016 ). Since then, 136 active EOR projects have been in practice worldwide ( Ettehadtavakkol 2016 ). This method is deployed after the primary and secondary phases, which involve using CO 2 and water to extract the oil, respectively. The remaining oil is then extracted by guiding alternating streams of water and supercritical CO 2 , in the tertiary phase, through the parts of the field where recoverable oil remains. This phase can enhance the life cycle of the oil field for decades ( Riley 2010 ;Mariyamma P et al., 2015 ).
Enhanced Gas Recovery (EGR). CO 2 can be used to increase the productivity of depleting gas fields by using CO 2 to displace the natural gas since CO 2 is heavier. Both EGR and EOR are considered suitable, and relatively safe techniques of sequestration as both possess natural sealing mechanisms that held the oil and gas for millions of years before they were extracted ( Riley 2010 ;Mariyamma P et al., 2015 ).
Enhanced coal bed methane recovery (ECBMR). Carbon sequestration in coal seams can enhance the coal bed methane recovery. Methane con-stitutes ∼ 95% of the total number of gases present in the coal seams. This simple process involves the replacement of pre-existing CH 4 by injecting CO 2. The cost of either one of the processes can be used for dual purposes, with an affinity of coal for CO 2 providing an additional advantage ( Vishal et al. 2012 ;Singh and Mohanty 2014 ;Sen, 2017 ). Methane produced is used as an energy source rather than releasing it into the atmosphere, which can defeat the purpose of sequestering CO 2 as methane is 23 times more potent GHG than CO 2 ( Riley 2010 ). A study by Tambaria et al. (2022) concluded that moisture, maceral and pore size and characteristics of injected gas are the three most important factors that determine the efficiency of the ECBMR process Table 6 . of Leung et al. (2014) lists the ECBMR plants.
These three options, but more specifically the EOR, is favoured in industries because the cost of capture is balanced or even exceeded by the cost of additional oil that is recovered. However, Sekera and Lichtenberger (2020) have concluded in their review that carbon capture is a feasible technology only when the captured carbon is stored and not commercially utilised in any manner. Usage of captured carbon for oil recovery simply shifts the emission source.
Deep saline aquifers. Saline aquifers contain brackish water and hence are of no major use due to the presence of excess salt Dayal et al. (2008) . proposed measuring the baseline concentration for atmospheric CO 2 and soil carbon content (organic and inorganic) in shallow saline aquifers with sequestration potential. Borehole data, if any, can help to monitor and model seepage. Reservoir simulation models point out that a mere 0.01-0.1 % of the volume of saline aquifers can be used for storage considering 50 years of injection ( Budinis et al. 2018 ). Notwithstanding, the volumetric carbon sequestration capacity of deep saline aquifers (100 -10,000 BT; Bradshaw et al. 2007 ) remains greater than any other geological sequestration sites ( Herzog 2009 ;Global CCS Institute, GCCSI 2020 ). If the models consider aquifers with traps, the number comes down to 320 BT ( Koide et al. 1992 ) and 200 BT ( Hendriks and Blok, 1995 ) Van der Meer and Yavuz (2009) . considered Bradshaw et al. (2007) 's estimate to be highly unreliable. Recently, Wei et al. (2021) have estimated the mean capacity to be 1914 BT and the range between 888 -5126 BT. Active aquifer sequestration sites e.g., in Sleipner (Norway), has been storing 1 MT CO 2 annually in the Utsira Formation of the North Sea since 1996 ( Global CCS Institute, GCCSI 2020 ). The Shenhua CCS project also sequesters 0.1 MT CO 2 in the saline formations of the Ordos Basin, China ( Diao et al. 2014 ; global sequestration map of deep saline aquifers in Fig. S8).
Ribeiro e Sousa (2012) referred that the injected CO 2 in rocks can do one of these: (i) displace existing fluid, (ii) dissolve in the existing fluid, (iii) reach with minerals, (iv) do a combination of the stated three possibilities. The CO 2 can be sequestered in the following four ways ( Radha and Navrotsky 2013 ;Mariyamma P et al., 2015 ;Potdar and Vishal 2016 ;Tsang and Niemi 2017 ): i) Structural trapping-CO 2 can be trapped as plumes at the aquifer top, with its upward movement stopped by structural caprocks. Such plumes lead to mineral precipitation, with a direct correlation between the plume size and the rate of precipitation ( Maalim et al. 2021 ; Fig. S8). ii) Capillary trapping-CO 2 injecting into the pore of the aquifer rock and trapped as bubbles is also a possible sequestration mechanism (Fig. S10). iii) Dissolution -CO 2 can be dissolved in aquifer water. This CO 2 saturated water becomes denser and migrates to the bottom of the aquifer as finger-like projections. iv) Mineralization-CO 2 can also transform and deposit as mineral carbonates after reaction between the aquifer minerals, CO 2 and water.
Deep saline aquifers represent a key sequestration option for CO 2. H owever, the four processes described above require a significant amount of time, up to 100s years. A review by Rathnaweera and Ranjith (2020) shows that adding nanoparticles in the injected CO 2 can signifi-cantly reduce the mixing time as well as prevent back migration of CO 2, thus preventing leakages.
Metal oxide + CO2 → Metal carbonates + Heat (1) Mineral carbonation is presumably a safer alternative to the other geological sequestration techniques since carbon is immobilized into stable carbonate as rocks ( Sanna et al., 2014 ;del Real and Vishal, 2016 ). This process can safely sequester ∼36,000 BT of CO 2 , which far exceeds the sequestration potential of geological reservoirs. The only downside is that this process is extremely time-taking, and emission rates far exceed it. Research is underway to understand the optimum way to increase the rate of natural carbonation ( Yuen et al. 2016 ).
This process can be either ex-situ or in-situ. In in-situ carbonation, the capture and storage of carbon occur underground and is also one of the associated processes in sequestration in deep saline aquifers (Section 2.4.3b.6). In ex-situ, the capture occurs on the surface and the storage takes place in suitable repositories, specifically in the large continental flood basalts across the world such as the Columbia River flood basalts and the Deccan trap basalts ( McGrail et al. 2006 ;Jain et al. 2016 ;Mukherjee et al. 2017Mukherjee et al. , 2020. Basalts terrains with (i) impermeable layers within the interflow zones, and (ii) sills and dykes are suitable for CO 2 sequestration. Here the impermeable layers, sills and dykes act as seals or caprocks ( Zakharova et al. 2020 ). In the Deccan trap (India), the dominant rock type is basalt (Misra et al. 2014) and it consists of bole beds as the inter-flow layers (Ghosh et al. 2006), and dykes ( Misra and Mukherjee, 2015 ) in many places. Those specific locations in the Deccan trap are to be investigated for sequestration operation, also keeping in mind that basaltic terrain alone is not the only criterion to succeed in sequestration. Krawczyk et al. (2015) emphasized the need to estimate in-situ stress magnitudes and vulnerability to deformation at the planned CCS location in rocks. The degree to which CO 2 moves laterally and vertically, when basalt flow that lies above the target sequestration locations is critical in the context of CCS in basaltic traps ( McGrail et al. 2008 ).
Nevertheless, abiotic sequestration options such as depleted oil and gas fields and saline aquifers have been the primary sequestration options for the last ∼ 70 years. These methods have some inherent drawbacks such as less efficiency, high energy penalty to capture carbon from large point sources, infrastructural costs, and feasibility issues for permanent storage (due to leakage risks) in the geological reservoirs ( Celia and Bachu 2003 ;Xie and Economides 2009 ;De Silva G et al., 2015 ;Onishi et al. 2019 ).

Sequestration through microalgae
Microalgae sequestration has been extensively studied in recent years. Biotic sequestration options have been adopted but not widely due to land constraints and ecological factors. This is because biotic sequestration depends on either afforestation or reforestation ( Smith and Torn 2013 ). Microalgae sequestration is considered a sustainable alternative to the above methods. This technique can be readily applied to point sources and can be utilized in the transportation sector not only for capturing carbon but also for the production of biofuels (using the microalgae), leading to a circular system ( Onyeaka et al. 2021 ). Microalgae can be symbiotically used with bacteria in wastewater treatment plants. The CO 2 generated by the bacteria, during the process of decomposing waste, is used by the microalgae to sequester it, and in turn, the oxygen produced by the microalgae helps to sustain the bacteria. This way, the waste gets decomposed and algal biomass is also produced. This system needs further research and investigation to overcome its challenges such as inefficient CO 2 mixing in the water and inefficient algal growth in industrial effluents. Employing this natural symbiosis can lead to cost effective and highly efficient wastewater treatment cum sequestration plants ( Viswanaathan et al., 2022 ).
Till now, > 25000 species have been identified that can naturally fix CO 2 by photosynthesis. Each species has unique adaptations such as halotolerance ( Chlorella ), SO x tolerance ( Scenedesmus obliquus ) and thermotolerance ( Picochlorum ). Under favourable conditions, they tend to generate exponentially with capture efficiency 10-50 times more than that of terrestrial plants. One kg of microalgal biomass can fix 1.38 kg of CO 2. Subsequently, microalgae can be grown using CO 2 from industrial sources or flue gases expelled by the large point sources. A review by Thomas et al. (2016) concludes that the strategy of using flue gases directly for microalgal growth can be a fruitful method. Certain compounds need to be eliminated from flue gases such SOx and NOx, for optimum growth of microalgae. The cost of artificial microalgae culture posed some drawbacks, but this cost can be offset by using microalgae for biofuels, biofertilizers, wastewater treatment etc. ( Osman et al. 2020 ). Table S4 presents by-products of different microalgae. However, uniform CO 2 diffusion in the artificial culture medium is a current challenge in this field ( Vale et al. 2020 ).

India's carbon footprint
India consists of 28 states and 8 union territories. The country's population is > 1.3 billion ( United Nations (UN) 2019 ). Meeting the energy demands of such a huge population is a challenge, especially when it is a key contributor to economic growth (Paul and Bhattacharya, 2004). Narain (2007) long back pointed out not so good prospect of CCS in India. However, situation has changed significantly as the GOI has started taking CCS seriously. Shahbaz et al. (2017) conclude that relation between economic growth and energy consumption can follow growth (energy → economic growth), conservative (economic growth → energy), feedback (energy ↔ economic growth) or neutrality (energy ≠economic growth) hypotheses depending on the country. After reviewing 17 Indian studies, Shahbaz et al. (2017) found that 14 of them indicate some relation (growth, conservation and feedback) between economic growth and energy consumption, and the remainder indicate a neutral relation. India's economic growth and energy consumption can therefore be correlated.
Between 1980 to 1991, the Indian GDP had an average growth rate of 3%. However, the Indian economy experienced a revolutionary turn in 1991 when the finance minister of the country introduced liberalisation, privatization, and globalisation (LPG) policy. The policy allowed the country to recover from a severe economic crisis and accelerated economic growth ( Ravan 2014 ). The aim was to boost economic growth along with reducing poverty and unemployment. The aim was certainly achieved. From 1991 to 2019, the average annual GDP growth rate stood at 6.375% ( Tiwari 2011 ;World Bank 2020a. However, this growth came at a cost. In the industrial sector, the country flourished manifold, and subsequently, their energy consumption increased. This energy, > 80% of which was derived from the combustion of fossil fuels, almost quadrupled the CO 2 emissions of India. While the 1994 emissions stood at 779 million tonnes, the emission values in 2019 stood at 3.6 BT ( Garg et al. 2017 ; UNEP 2020) ( Fig. 5 ). This relationship, however, has been non-linear. An empirical analysis by Javid and Khan (2020) concludes that the average growth rates of emissions (5.5%) and GDP (6.3%) were almost identical between 1990 -2009, while between 2010 -2016, the average growth rate of emission (5.13%) has been less than the average GDP growth rate (7.34%) As of 2019, India's CO 2 emission per capita stood at 2.68 metric tonnes, which shows a constant increase ( Table 2 ).
These energy demands are met through fossil fuel combustion (Section 1) at the cost of voluminous CO 2 released. Added to this is the industrial emission ( Table 4 ). The power sector (dominated by coal) contributes ∼ 50% of the total CO 2 emissions, followed by manufacturing and construction ( ∼22%), transport ( ∼ 11%), industries ( ∼ 5%)   Table 3 ). In the transport sector, road transport accounts for 94.5% of the total CO 2 emissions ( Senthilkumar 2021 ). Out of 20-30% from industrial sources, the cement industry, oil refineries and iron and steel account for ∼ 7, 6 and 5% emissions, respectively ( Goel et al. 2021b ).

Major emission zones
Although the above data covers the entire country, there are certain zones or "hotspots " with the highest rates ofemission owing mainly due to the presence of large emission sources. Industries and thermal power plants are the two chief agents. See Mishra et al. (2019) for ONGC's initiatives in planning India's First CO2-EOR Project in relation to the thermal power plants. Table 4 presents the hotspot states, districts and sources.
25 out of 641 districts of India ( Table 5 ) are amongst the top emitters of CO 2 , which are distributed among 14 states of the country. There is a skewed distribution of these districts among the states.
These 25 districts accounted for ∼ 44% of the total CO 2 emission of the country in 2013, and the rest 56%, were distributed among the remaining 616 districts ( Garg et al. 2017 ). The share of these 25 districts increased from 35% in 2005 to reach 44% in 2013. This highlighted that the energy production and consumption increase in the states as well as the increase in inequality of energy consumption and expenditure.
In 2013, the CO 2 emissions of these 25 districts were > 15 MT and district-wise average CO 2 emission stood at 3.07 MT. A total of 64 districts accounted for 60% of the total CO 2 emissions and the remainder, 40% was distributed among the remaining 577 districts. This shows a clear disparity in the volume of emissions. Since the emission patterns are directly proportional to the energy consumption patterns, it also indicates the difference in the energy consumption patterns of the districts.
After analysing several government reports and independent works, Mohan et al. (2019) presented the leading states in emissions from various sectors ( Table 6 ). Uttar Pradesh and Maharashtra are amongst the top three emitter states. Even the agricultural sector emission is led by Uttar Pradesh, with Punjab and Haryana following next.

India's CO 2 emission projections
One year after submitting its Intended Nationally Determined Contribution (INDC), India officially ratified the Paris Accord in 2016. India has committed to meet 40% of its electricity demands from nonfossil fuel-based energy by 2030 ( Fig. 6 ). By 2030 it planned to reduce its emission intensity by 33-35% of the 2005 levels. The INDC also includes provisions for an additional carbon sink for 2.5 -3 BT of carbon ( International Energy Agency (IEA) 2021 ; Ray 2021 ). This does sound like an ambitious goal given that India must tackle its core issue of grassroot development along with transitioning its energy sources.
India's emission intensity in 2005 stood at 0.47 Mt CO 2 per $1000 of GDP. Considering the reduction goal of 33 -35% of 2005 levels, the emission intensity in 2030 should ideally stand between 0.3149 to 0.3055Mt CO 2 per $1000 of GDP. The emission intensity of India in 2019 stood at 1.27 Mt CO 2 per $1000 of GDP. Note a constant decrease from 1.45 Mt CO 2 per $1000 of GDP in 2016 followed by 1.29, 1.33 in 2017 and 2018 (estimations made from data in Niharika and Basu, 2021 ). Interestingly, China's emission intensity in 2019 stood at 0.97     Mt CO 2 per $1000 of GDP. In thermal power generation with ∼ 35% efficiency, India's emission intensity stands at 0.9 Kg CO 2 /kWh ( Shell Global and TERI 2021 ). Adjusting the values for a 33-35% decrease, the intensity should be 0.603-0.585 Kg CO 2 /kWh. Assuming an ideal scenario, if India achieves its target for the emission intensity by 2030, the country's CO 2 emission would stand at 5.6 BT in 2030 ( Frank 2015 ) du Can et al. (2019) ., however predicted the value to be 4.0 BT in 2030 and 7.4 BT in 2050. It also shows that India's emission intensity in 2030 would decrease by 34% from 2005 level, thus achieving it INDC target. This would happen even if India stuck to Business-As-Usual (BAU) scenario, without taking a load of new initiatives ( Frank 2015, du Can et al. 2019. The CO 2 emission projections differ in both studies. It concludes that the absolute emission of India is not decreasing soon. If the emission intensity decreases to 0.31 Mt CO 2 per $1000 of GDP, it means that energy production is more efficient; i.e., there is less emission per $1000 of GDP. India has not announced any peak year of emission. Prakash Javadekar, the Minister of Environment, Forest, and Climate Change stated in 2015: "The world is not expecting... India to announce its peaking year " and "Countries know where India stands and what its requirements [development needs] are and therefore nobody has asked us for [the] peaking year " ( Khadka 2015 ). Robiou du Pont et al. (2017) concluded that in order to stay in line with the Paris agreement goals, India would have to peak its emission by 2033. Shell Global and TERI (2021) , in their Net Zero Emissions (NZE) by 2050 scenario, have chalked out a probable future pathway for India's sectoral emissions ( Fig. 7 ). According to their NZE scenarios: • Electricity generation sector -By the second half of the 2020s, ∼ 75% of thermal power plants would be decommissioned and by 2050 solar and wind energy would constitute 90% of total electricity generation. The entire electrical system would grow four times, yet emissions would be half of the current values. • Transport sector -The demand for electric vehicles will increase in the 2020s. Subsequently, there would be either electric vehicles or fuel cell vehicles. Fuel cell vehicles would be powered by green hydrogen obtained from the electrolysis of water. • Residential, commercial and agriculture -The residential and commercial sectors' complete transition to renewable energy-based electricity by 2050 while the agricultural sector will continue to depend on solar energy and biodiesel. • Industry -Industrial emissions would reach a plateau after 2025.
There would not be a significant decrease due to lack of readily available technology. However, there would be a continued transition. This sector would need CCS technologies to decrease their emission during the transition proce ss (Section 1) . The NZE scenario estimated 1.3 BT of residual CO 2 from the industrial sector. 0.9 BT of this residual CO 2 can be sequestered biotically (forests, wetlands and mangroves), while the remaining 0.4 BT would need some form of geologic sequestration. Thus, this NZE scenario considers CCUS as an essential part of the policy, if 2050 is taken as a Net-Zero year. This would require the Indian government to frame CCS focussed policies and aim technological deployment in the near term, probably within this decade.

Carbon emissions during COVID-19
The Covid-19 pandemic hit the entire world in 2020, forcing a global shutdown of business, transport, industry, and overall normal functions. Because of this shutdown, the planet experienced a major global fall of atmospheric CO 2 level, since the second world war ( Debata et al., 2018 ). The CO 2 in April 2020 were 17% less than the mean levels of 2019 ( Le Quéré et al., 2020 ). In India, the decline was that of 205 Mt, a 15.4% decline in the first half of 2020, as compared to the same period in 2019 . However, such decline came at the cost of economic slowdown. After the pandemic (hopefully) gets over, the CO 2 concentration in the atmosphere is bound to increase.

Carbon sequestration in India
If the increase in CO 2 emission remains unchecked, then the per capita emission values would double in a decade, posing another challenge for the Indian Government apart from the already lingering issues of sustained socio-economic growth ( Saleth et al. 2020 ;Bagchi 2020 ), population boom ( Kurinjimalar and Prasanna, 2017 ), poverty ( Liu, 2020 ) and inequality gap ( Sankar 2020 ). The current challenge is to address these issues while continuously increasing the production and sustainable consumption of (cleaner) energy ( Maji 2019 ). Even though the world is pushing towards cleaner forms of energy, low per capita income, and high coal reserves (326.05 billion tonnes as of 01-April-2019 ( Government of India, GOI 2020a ) still render coal-generated power to be the cheapest in India ( Bhattacharyya, 2018 ). Even though alternatives like solar and wind are gradually coming up, coal would continue to be the main source of power for India in 2030, at around 60% ( Shearer et al. 201 7 ; Section 1 ). Thus, carbon capture and storage are necessary technology that should be developed in the country soon. India is yet to do substantial developments in the field of CCS and implement them (Section 4.1) .

Research & Developments in India
In 2003, India became the founder member of the Carbon Sequestration Leadership Forum (CSLF), launched by the US Department of Energy (Goel 2021b), and since the G8 meeting in Gleneagles (2005) Goel (2008) presented the following three thrust areas for CCS in the Indian context: (i) pre-combustion carbon capture research, (ii) modelling studies for geological sequestration, and (iii) monitoring of sequestration locations. Since then, active research has been carried out in the field of CCS to compete in the international arena. With NPCS, India became one of the few developing economies that began R&D investments in CCS ( TERI 2013 ; Mukherjee et al. 2014 ;Goel et al., 2021a ). This included a call for proposals for research in the field of CCS ( Chakroborty 2008 ).
DST has identified four thrust areas for active research under the aegis of NPCS: (i) CCS process development, (ii) policy and development studies, (iii) network terrestrial agro-forestry sequestration modelling, and (iv) bio-sequestration through micro algae bio-fixation ( TERI 2013 ; Mukherjee et al. 2014 ). Under the ambit of NPCS, India hosted an international workshop on R&D challenges in CCS technology in 2007. Experts and young scientists attended it from 19 countries. DST also presided over the formation of the "Indian CO 2 Sequestration and Applied Research Network " to coordinate sequestration R&D among various institutions and stakeholders ( Goel et al. 2021b ). Under the National Action Plan on Climate Change (NAPCC) 2008, Government of India (GOI) proposed the 9 th mission for Clean Coal Technology (CCT). Bharat Heavy Electrical Limited (BHEL) collaborated with Tiruchi Regional Engineering College-Science and Technology Entrepreneurs Park (TREC-STEP) and organized four skill leverage and training programs on CCT-CCS in 2011-2012. The programs were funded by the European Union and implemented the initiatives of the 9 th mission ( Goel et al. 2021b ).
Besides, DST and the Research Council of Norway (RCN) commenced a joint programme to research climate change, including an innovative domain of metal-organic framework (MOFs) in Indian Institute of Petroleum, Dehradun (Uttarakhand). MOFs are used in adsorption technologies during post-combustion capture ( TERI 2013 ; Goel et al. 2021b ). Climate change research is also one of the thrust areas of research under the Norwegian Program for Research Cooperation in India (IND-NOR) ( RCN 2018 ). In 2008, a joint meeting was held amongst DST, Department of Environment, Food and Rural Affairs (DEFRA, UK) and the Integrated Research for Action and Development (an Indian NGO). An Indo-US CCS research programme has also been initiated and is the most active collaboration working in the field. DST established an independent network to study CCS in India named ICOSAR ( Shackley and Verma, 2008 ). Maulana Azad National Institute of Technology (MANIT), Bhopal became the first Indian Institute to start a dedicated course on carbon sequestration ( Goel et al. 2021b ).
Under MI, DST and DBT have launched Carbon Capture Innovation Challenge, whose scope is to enable emission intensive industries and sources to move towards near-zero CO 2 emissions. A call was launched in 2018 for joint R&D with other MI member countries to identify breakthrough technologies in CCUS. Under this, 17 proposals were recommended from DST and 3 from DBT ( Ministry of Science and Technology, Government of India 2018 ).
CCS initiatives got more R&D support from the Government when it became a part of Mission Innovation along with European Union and 21 other countries in 2015 ( Goel et al. 2021a ). The most promising research and technological developments in India in the last two decades have been elucidated below ( Fig. 8 ).
• Several Indian Public Sector Units also came forward. For example, the Oil and Natural Gas Commission (ONGC) expressed interest to establish an EOR project to increase the crude oil extraction from the Ankleshwar oil field, Gujarat ( Kumar et al. 2018 ;Kumar et al. 2020 ).
The plan was to transport CO 2 from the Hazira processing plant (Gujarat) to the depleted onshore reserve of Ankleshwar to enhance the oil recovery ( Chakroborty 2008 as referred in TERI, 2013 ). • A pilot reactor was set up at Hazira processing plant to use the captured CO 2 for microalgal growth, which were later used for the production of biogas. Two different setups were studied. In one setup, only Chlorella sp . was used, while the other had a consortium of Chlorococcumhumicola , Scenedesmus quadricauda and Chlorellavulgaris. The average CO 2 capture and yield rate of these microalgae was 30 g CO 2 m − 2 day − 1 and 18 g m − 2 day − 1 , respectively. The mean CH 4 yield stood at 386 litres CH 4 kg − 1 and 228 CH 4 kg − 1 for the Chlorella sp. and the consortium, respectively .
• In 2012, the Department of Atomic Energy (DAE) led a consortium with BHEL, National Thermal Power Corporation (NTPC) and Indira Gandhi Centre for Atomic Research (IGCAR) to develop ultra-supercritical technology for thermal power plants ( Goel et al. 2021b ). • Coal India Limited (CIL) and the Geological Survey of India (GSI) collaborated to study the feasibility of Indian un-mineable coal seams for sequestration through ECBMR ( Goel et al. 2021b ( Goel et al. 2021b ). • National Aluminium Corporation (NALCO) is aiming for biosequestration with its coal-powered plant in Orissa. It has involved a high-end bio-technology company, Indo-Can Technology Solutions (ICTS) to achieve its objectives. The bio-sequestration will be executed by algae, which will be cultivated within a pond, enclosed in a 0.18-acre area. The flue gas, after being cooled, will be fed in the pond, which would accelerate the growth of algae. The microalgae will later be retrieved for producing biofuel. In this pilot project, the theoretical maximum production of biomass stood at 291.5 t h − 1 yr − 1 ( Pradhan R R et al., 2017 ). A demonstration plant at the Indian Oil Corporation Limited (IOCL) is under construction near the Panipat refinery (Haryana). It would capture industrial CO 2 and covert into ethanol (C 2 H 5 OH) and 2,3-Butanediol (C 4 H 10 O 2 ; Ray 2021 ). Viswanaathan and Sudhakar (2019) reviewed the potential of microalgae in CCUS and concluded that microalgae have huge potential in biofuel production, wastewater treatment and bioremediation. Such biotic fuel manufacture options will be especially helpful in meeting India's fuel demand and play a major role in helping India achieve its INDC targets ( Ray 2021 ). • Seshadri and Sashirekha (2018) conducted a seven-year preliminary study in the Murugappa Chettiar Research Centre (MCRC) on the bio-sequestration potential of Scenedesmus, an algal species. Two different waste streams were used. Liquid waste from a sugar mill and CO 2 -rich gaseous waste from a distillery leads to consumption of 6000 m 3 and production of (300-500) gram/m 3 /day of algal biomass. If the process is scaled up, this could lead to the sequestration of 1500 t Cha − 1 yr − 1 and production of 100 tonnes of biomass per hectare annually. This biomass can be used in several ways, as mentioned in the previous paragraph. • The Department of Chemical Engineering, IIT Bombay has been researching cyano-bacteria to develop them as cell factories that can convert atmospheric CO 2 , into useful products such as biofuel ( TERI, 2013 ). However, such work is in a nascent stage and more research is needed ( Mondal et al. 2017 ( Goel et al. 2021b ). Bharat Heavy Electrical Limited (BHEL) has also established a 6.5 MW IGCC power plant to study and implement the prospects of pre-combustion capture technologies in India. However, a minimum capacity of 100 MW is to be experimented with to understand the nuances such as system optimization and hot gas clean-up ( Sethi, 2017 ). • The company CarbonClean (India) has come up with an innovative technology that converts the emitted CO 2 into baking soda. The firm, which comprises two chemists from IIT Kharagpur, who are also the inventors of the technology. The technology is currently installed in Tuticorin Alkali Chemicals, without subsidy, and can utilise 60,000 tonnes of CO 2 per year. The latest addition to its steam boilers has already led to zero CO 2 emission from the plant-a crucial breakthrough ( Harrabin 2017 ). • In IIT Delhi, Subbarao et al. (2018) have patented a water scrubbingbased technology. It can produce bio-methane from biogas with > 95% methane concentration. This can act as a substitute fuel for vehicles running on natural gas, such as the CNG autorickshaws. They can also be injected into the natural gas grid. The process of water scrubbing also produces Bio-CO 2 as a by-product. This BioCO 2 can be used for algae cultivation, production of CaCO 3 and dry ice. Besides, it can be a natural alternative to synthetic CO 2. • Tripathi (2018) conducted an experimental study to understand the effect of increased atmospheric CO 2 level of 585 ppm, which is being projected to reach by 2060, under the business-as-usual scenario ( Smith and Myers, 2018 ). The plant chosen was rice. The experiment was conducted in a free-air CO 2 enrichment ring under two scenarios-at the ambient value of 400 ppm and the predicted value of 585 ppm. It was observed that the photosynthesis rate, leaf area per plant and leaf area index increased in the latter conditions. However, the effects can be seen in terms of nutritional deficiency in the plant. For example, Smith and Myers (2018) Kumar et al. (2020) reviewed the status of CCS studies from the Deccan traps as follows. CCS, by natural means, has taken place in the Deccan trap. This is indicated by the specific association of rock types (limestone and inter-calcareous facies below and between lava flows). Tholeiitic basalt lava flows of the Mandla area are a potential site for long-term CO 2 storage. Deccan basalt water-CO 2 saturated experiments prove partial and complete carbonation reactions. • Kumar et al. (2008) pointed out that even though the Deccan trap basalts and the Columbia River Basalts resemble chemically and mineralogically, the scientific deductions on the latter regarding the CCS has not worked out for the Decca trap-rocks. This means that the insitu rock testing and determination of properties will be the key step ahead. Similarly, McGrail et al. (2008) stated that carbonation rates in basalt samples vary significantly in India, USA and African basalts despite resembling mineralogy and chemistry.
• NTPC under its NTPC Energy Technology Research Alliance (NE-TRA) scheme has initiated research in carbon sequestration. One such programme includes the setting up of a 10MW CO 2 capture plant. The pre-feasibility study of this plant was conducted by IIT-Bombay and Carbon Clean Solutions Limited. The gas would be captured from the flue gas and will be used in producing soda ash, methanol, and urea ( IEA 2020a ; Goel et al. 2021b ). NETRA also signed an MOU with ONGC to set up a carbon capture plant at the Jhanor Gandhar thermal power plant (Gujarat). The captured CO 2 will be used for EOR in ONGC's Jhanor oil field ( IEA 2020a ). Other such projects include capturing CO 2 from Cuddalore power plant under IL&FS Tamil Nadu Power Company Limited (ITPCL) and using it for EOR in oilfields in Kamalapuram and other oil fields in Cauvery basin ( Ranjan 2018 ). • An EOR demonstration project is under evaluation by ONGC and IOCL to capture CO 2 from the Koyali refinery (Gujarat). The captured CO 2 will be used for EOR in the Gandhar Oil Field (Gujarat). The feasibility study is being carried out by the Institute of Reservoir Studies, ONGC ( Goel et al. 2021b ). IIT Bombay along with Upstream for Carbon capture , a taskforce under Ministry of Petroleum and Natural Gas, are the knowledge partners in the project (V. Vishal, per. comm.). Apart from this individual study, ONGC is also hiring consultants for conducting overarching carbon capture and transportation. After this, the point sources will be ranked according to their economic feasibility in capturing and transporting CO 2 for EOR ( Ranjan 2018 ). • A collaboration between Central Electricity Authority (CEA), Bureau for Energy Efficient (BEE) and Germany aimed at assessing emissions from Thermal Power Plants and consequent remedial measures ( Goel et al. 2021b ). • Since 2009, Climate Change Research Institute (CCRI), an NGO, has been organizing capacity development courses, training, and work-shops in the field of CCS. They have been supported by the GOI and private sector. • A Memorandum of Understanding (MoU) was signed between Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR, Bangalore) and the Breath Applied Sciences, a JNCASR-raised company, was signed on 05-June-2020. The MoU aims technology transfer for converting CO 2 to methanol. The pilot mode can convert up to 300 kg day − 1 , but its capacity can be increased to 500 T day − 1 at an industrial scale. Tata Steel wishes to adopt the technology into their plants ( DST 2020a ). • In the transport sector, the GOI is working to increase renewable energy capacity, especially in the railway due to their high consumption of electricity. To reduce emissions, the GOI decided to skip the Bharat V and directly implement Bharat VI emission standards in 2020 ( Senthilkumar 2021  All the R&D and technologies mentioned are at various levels of readiness and hence would take different amounts of times to scale up Vishal et al. (2021b) . ascertained the Technology Readiness Levels (TRLs) of various CCUS technologies and compared their status with the global state of the art equivalent (Fig. S12).

The probable cost of CCS implementation in India
The primary cost in CCS is in the domain of carbon capture, accounting for 60-80% of the total CCS system costs (IEA 2008). However, in India such a study has not been undertaken to check the feasibility of CCS cost as a whole. However, the cost of retrofitting of existing coal power plants with capture technology has been simulated. Based on technologies available in 2010, CO 2 capture in a power-plant would have increased the cost of electricity by 25-50% ( Nanoni and Goswami 2010 ). Rao and Kumar (2014), using the Integrated Environmental Control Model (IECM), did a more detailed analysis. The study was conducted on four thermal power plants viz., in Trombay (Maharashtra), Rama-gundam (Telengana), Dahanu (Maharashtra) and Badarpur (Delhi). The plants were chosen for their proximity to a potential carbon sink. The model was simulated for a post-combustion capture scenario using a monoethanolamine-based system, as it was the cheapest. The simulation study pointed out an increased expenditure of electricity production by INR 2.2-2.6 per kWh. This rate of increase is in the range of 63-76% of the current production rate. The present average rate of production is INR 3.38-3.45 per kWh. This increase in expenditure would mean that electricity production would have to be increased to cover the extra cost.
An indirect approach by Anandarajah and Gambhir (2014) provides a different perspective in this regard. In the study, two low carbon scenarios were analyzed (L1, L2) using a cost-optimization model called TIAM-UCL. L1 included CCS, while L2 did not. It was found that in case of L2, emission level would decrease with greater contribution from renewables to compensate for the lack of CCS technologies. This, however, triples the marginal emission abatement cost till 2050 in L2 than L1. Thus, the cost of including CCS in India's climate policies (L1) will be beneficial in the long run, whereas solely depending on renewables (L2) would incur more expenditure.

India's sedimentary basins
India has a total of 26 sedimentary basins. These basins, covering ∼ 3.14 million km 2 area, are of two distinct types based on whether they are up to or beyond the 200 m isobaths. The basin on the land up to 200 m isobaths line cover ∼ 1.79 million km 2 while basin area beyond the 200 m isobaths covers ∼ 1.35 million km 2 . These basins have an estimated potential ranging from 500 GT to 1000 GT (Kalbende2015). The 26 basins have been classed into five categories depending on their hydrocarbon prospects ( Table 7 ; Kalbende 2015 ; review in Singh and Singh 2016 ). Storage potential for specific sites are to be estimated before carbon capture projects are to be initiated ( Goldar and Dasgupta 2022 ). Holloway et al. (2008) stated that the Indian Purana/Proterozoic sedimentary basins, e.g., Cuddapah, Chhattisgarh and Vindhyan have quite limited porosity and permeability data and therefore, for the time being, will not be suitable sites for CO 2 sequestration Bhandari et al. (2008) . considered the Ganges, the Vindhyan and the Rajasthan basins to be suitable sites for CO 2 sequestration. These authors also stated that at two locations, Palwal (Haryana) and Tumsar (Maharashtra), deep aquifers have been studied and that more studies are needed to confirm whether these can be good locations for sequestration.

Storage potential of geological formations
The country has an estimated storage potential of 500-1000 BT. The deep saline aquifer formations have the highest potential of 300-400 BT followed by the Deccan trap basaltic rocks (200-400 BT), un-mineable coal seams ( ∼5 BT) and exhausted hydrocarbon reservoirs (5-10 BT) (Kalpende 2015). These estimates were made using empirical equations in which the areas geographical, geochemical, and geological characteristics were taken into consideration ( Singh et al. 2006 ) Fig. 9 . presents the sequestration potential and suitable zones.
The 5,00,000 km 2 region in the north-western Deccan trap ( Mukherjee et al. 2017 ;Mukherjee et al. 2020 ) seems to be a promising prospect with storage capacities up to 400 GT Holloway et al. (2008) . however pointed out that the Deccan trap and the Rajmahal trap basalts are unsuitable because of the present-day technological issues. Deccan trap consists of thick lava flows, mostly > 3000 m thick at the western flank. The Saurashtra region (southern Gujarat) largely meets the prerequisites for carbon sequestration . Research and developments are still underway. Geochemical, geophysical, and fluid-rock behaviour modelling of the area needs to be conducted to test the viability of the region as a sequestration zone. A study by Punnam et al. (2021) concludes that at an optimal injection rate, CO 2 can be   sequestered efficiently through residual and structural trapping mechanisms if optimal injection points are chosen. Kumar et al. (2008) chalked out methodologies for a pilot study for CCS in the Deccan trap. These include: • Feasibility studies in areas with a thickness of ∼ 800 m, including adsorbed soil gas surveys (ASGSs), drill location studies and magnetotelluric analysis • Modelling and simulation for borehole drilling and subsequent CO 2 injection • Actual drilling and injection of CO 2 at a rate of ∼ 100 tonnes per day at 2000 psi. This should be done for 10 days • Monitoring and analysis of results including verification for mineralization using observation boreholes Singh (2008) identified the potential coal-bearing basins and calculated their sequestration potential using empirical equations. The author suggests that proximate analysis and vitrinite reflectance percentage are suitable parameters to estimate the sequestration potential of the coal beds Table 8 . summarizes the coal-bearing basins along with their potential capacity for sequestration.
Deep saline aquifers also hold considerable potential around Gujarat and Rajasthan coastal area, especially in the Ankleshwar oil field (Section 5.1) . The Ganga foreland basin is considered a potential site for sequestration. The fluvial sandstones of the Siwalik formations hold good sequestration potentials (Section 5.3) . The region is capped by siltstone having low permeability, thus not allowing any large-scale movement through it. This eliminates the chance of CO 2 escaping to the atmosphere, which is one of the hindrances in choosing a site. Further, the foreland is near a few large-scale point sources. Thus, sequestration in the nearby region can reduce the transport cost ( Holloway et al. 2009;Kalbende 2015 ). The storage potential described, however, seems to be underestimated (for coal) and overestimated (for saline aquifers)  ). The reason given by Singh et al. (2021) states that while the estimation methods for saline aquifers was largely borrowed from the methods used by United States Department of Energy (US DOE), the estimated methods for coal formations assumed higher coal consumption and hence less area for sequestration. Thus, it was suggested that estimations be made in a fresh manner.
The latest initiative in calculating the CO 2 storage capacity of India's geological formations was undertaken by Vishal et al. (2021b) . Based on the global assessment methods, a systematic theoretical assessment was made. The estimations were made for four storage pathways viz. deep saline aquifers (291 GT), basaltic rocks (97-316 GT), ECBMR (3.7 GT) and EOR (3.4 GT).

Terrestrial carbon sequestration in India
Terrestrial carbon sequestration (Section 2.4.2b) is a form of biotic sequestration that has immense potential in India. Soil stores both organic and inorganic carbon in the form of biomass and pedogenic CaCO 3 , respectively Lal (2004) . presented a detailed analysis of the sequestration potential of the Indian soils. The organic carbon pool of the soil was estimated at 2.1 BT up to 30 cm depth and 6.3 BT up to 150 cm depth. The inorganic carbon pool was estimated at 19.6 BT up to 100 cm depth. However, in another work ( Pal et al. 2015 ), different significant magnitudes, 2.997 BT and 3.403 BT, have been estimated up to 150 cm depth. Although the numbers differ widely, the sequestration potential of Indian soil and trees in general are significant. Gupta (2015) studied how climate change can alter soil carbon sequestration. GOI launched Green India Mission (GIM) under NAPCC to harness this potential in urban and peri-urban areas. The NIM aims to enhance the green cover across 2000 km 2 of urban and peri-urban areas ( Govindaraju et al. 2021 ). The mangroves particularly are considered to have 50 times more sequestration potential than the terrestrial trees because they allocate more carbon below ground than the latter ( Bhatt and Kathiresan 2012 ;Alongi 2014).
Using stratified random sampling, Pandey and Pandey (2013) estimated the carbon sequestered by the mangroves in Gujarat (India) using 316 plots of 10m * 100 m. This constitutes ∼ 0.03% of the total mangrove area. The total value came out to be 8.116 MT of carbon. The study by Sahu et al. (2016) in the Mahanadi mangrove delta, using Pandey and Pandey's (2013) methodology yielded a magnitude of 0.977 MT. A more comprehensive study was conducted by Rani et al. (2021) using radiocarbon dating of three stations in the Cochin estuary (Kerala). These cores were then analysed for the C-13 and N-15 isotopes, along with organic carbon, nitrogen content and bulk density. The calculated organic and inorganic carbon sequestration rate in the Cochin mangroves was estimated to be 2.95 t C ha − 1 yr − 1 .
Like natural vegetation, agricultural techniques such as agroforestry were recognised under the Kyoto Protocol (1997) for their sequestration potential ( Nair et al. 2009 ;Abbas et al. 2017 ). It is even a more lucrative option in a country like India, due to the scarcity of land resources. Agroforestry techniques combine forest and agricultural produce on the same land, thus significantly enhancing farmers' income and promoting biodiversity ( Pandey 2007 ). Their sequestration potential is also significant. In a tree-crop combination, biomass per unit of water significantly increases, leading to more carbon sequestration. A study conducted in 51 districts across 16 Indian states estimates that the sequestration potential of the agroforestry systems of these states is to be 7.23 MT ( Dhyani et al. 2020 's review). Yadava (2010) assessed the carbon stock of soils in Manipur. Vegetation type and environmental factors govern the pool, emissions, and sequestration of the soil. The output showed that the pine forests had the largest carbon stock (295.00 t C ha − 1 ), followed by oak (65.11t C ha − 1 to 127.52 t C ha − 1 ), and Dipterocarpus (3.21 t C ha − 1 to 3.77 t C ha − 1 ). Soil has its natural carbon content. Its degradation releases this locked-up carbon into the atmosphere. Soil degradation is already a widespread issue in India Yadava and Thokchom (2021) . conducted a study to ascertain the CO 2 loss due to soil degradation. Three different Dipterocarpus forest sites with dissimilar conditions were chosen in the Chandel district (Manipur). Site 1 was the control site, site 2 was a recently logged site (underwent logging a month before sampling) and site 3 was slashed and burned (underwent burning a year before sample collection). 12 samples were collected from each site between September 2017 and August 2018. The depth of collection ranged 0 to 10 cm below the ground level. The CO 2 loss rate was estimated using the al-kali absorption method. The rate varied between 263.5 to 609.9 mg CO 2 m − 2 hr − 1 at site 1, 233.4 to594.8 mg CO 2 m − 2 hr − 1 at site 2 and 308.7 to 700.2 mg CO 2 m − 2 hr − 1 at site 3. The mean values at these sites stood at 330.74 ± 2.16, 308.36 ± 2.06 and 388.97 ± 1.96, respectively. The highest value of CO 2 loss was observed in the burnt area due to rapid degradation caused by initial burning, which caused more microbial activities hence augmented decomposition and enhanced CO 2 flux. Ragula and Chandra (2020) calculated the C stock of roadside trees in Bilaspur (Chattisgarh state). Five sample plots (100 m x 10 m) were selected in six zones within the city. The C stock was calculated empirically, and the total value stood at 148.18 t C. ( Raha et al. 2020 ) assessed the C stock of three distinct forest types in Sagar district in Madhya Pradesh. The chosen forest types were Boswellia Forest, Dry deciduous teak forest and Dry deciduous mixed forest. Random sampling was carried out by placing 42 quadrats (60 m x 20m); 14 in each forest. The C stock was calculated to be 75.3 ± 6.1 t C ha − 1 in Dry deciduous teak forest, 81.3 ± 5.6 t C ha − 1 in Dry deciduous mixed forest and 104.7 ± 5.4 t C ha − 1 in Boswellia Forest. ( Moharana et al. 2021 ) conducted a similar study to ascertain the soil C stock in the Suratgarh block of Rajasthan. Before 1960s, the study area was desert land; however, due to canal intervention in the last 1960s, the area became suitable for agriculture Moharana et al. (2021) . collected 150 soil samples from 4 LULC types (single crop, double crop, plantation crop and sand dunes). The soil samples were collected from the surface up to 90 cm in depth, and geostatistical analysis was conducted to calculate the soil C stock. The soil C stock stood at 92.25 t C ha − 1 . This shows that restoring desert lands can also help in sequestering carbon Mir et al. (2021) . calculated the C stock of community managed forests in Khasi hills of Meghalaya. Fifteen such forests were analyzed. Sampling was done using a belt transect (250 m x 20 m) in each forest. The C stock was calculated empirically. The total carbon stock was calculated at 107.53 ± 9.7 t C ha − 1 .  took a composite approach to quantify the SOC in a part of Lahaul Valley, Himachal Pradesh. This treacherous Himalaya valley has a cold arid climate. Hence, the soil samples were only collected from the arable lands, from the surface up to a depth of 30 cm. These samples were then tested using the rapid titration method to ascertain their SOC content. Ordinary kriging was undertaken to estimate the SOC content from unsampled locations. The average SOC content was estimated to be 14.41 g kg − 1 . Several other studies have been conducted in Western Himalayas (Tables S5, S6). Govindaraju et al. (2021) conducted three studies in Neyveli Lignite Corporation Campus (NLCC) & Reserve Forests of Panchamalai in Tamil Nadu and at the Delhi Ridge (Delhi). The study conducted in NLCC was to identify the most suitable sequestration species in and around the NLCC campus. The study concluded that Mangifera Indica (Mango tree) and Azadirachta Indica (Neem tree) are the most resilient trees that can grow around industrial areas along with having a significant sequestration potential. The study in Panchamalai aimed to find the sequestration potential of trees based on their altitude of growth. The study concluded that the reserve forests had stored 3081.41 tonnes of CO 2 . The most potential trees were found to grow at a 580-830 m altitude above the MSL. A case study estimated the total sequestration potential of green cover in Delhi to be 3.1 MT y − 1 . It also concludes that ∼ 90 tonnes of CO 2 per hectare could be released annually if the Delhi Ridge Forests face deforestation. Sharma et al. (2020) calculated the sequestration potential of the trees in the Amity University Campus, Noida. A total of 1997 trees were enumerated, and their sequestration potentials were estimated empirically from the tree characteristics. The total annual sequestration was calculated to be 139.9 tonnes Henry et al. (2013) . and Cifuentes et al. (2015) have presented three allometric equations and guidelines to use them. Table S7 summarizes other studies of estimating sequestration potential.
Soil degradation is a key hindrance in sequestration. The five most affected states of such degradation are Mizoram, Himachal Pradesh, Kerala, Nagaland and Tripura, with values of 89.2, 75, 67.1, 60 and 59.9% of the degraded area with respect to their total geographical areas, re-spectively ( Bhattacharyya et al., 2015 ). However, soil can be restored by putting soil conservation and erosion control methods into practice. These conservation techniques if materialised could lead to a soil carbon sequestration potential of 30 BT ( Pal et al. 2015 ).

General points
CO 2 -EOR has been the first abiotic sequestration method that came into operation in the oil industry. Although the reasons were/are economical, it still acted as a sequestration method since the 1970s. The Kelly-snider oil field in Texas, USA was the first to use CO 2 -dependent EOR, using naturally occurring CO 2 transported from New Mexico and Texas (USCOC 2012).
After the primary (unaided) recovery from the field, water is injected to maintain reservoir pressure. This water is brine, which is recovered from the reservoir during oil production and used for secondary recovery. CO 2 is used in the tertiary recovery to further increase reservoir oil output. Secondary and primary recoveries hold prime importance for economic sustenance during oil extraction from reservoirs. During primary recovery, ∼20% of the total oil is recovered. Using secondary and tertiary recovery mechanisms can further increase production by 15 and 20%, respectively, thus increasing the lifetime of an oil field. In the process, although CO 2 is produced in the producer well, it is re-injected, and the total injected CO 2 can stand ∼ 60%. ( Gozalpour et al. 2005 ; US Chamber of Commerce (USCOC) 2012 ).

Sequestration potential at the Ankleshwar oil field (Gujarat)
Unlike coal, India does not boast a dependable oil inventory. Seven Indian basins are presently under commercial operation ( "category 1 " row in Table 8 ). The total recoverable oil reserves of India as on 01-April-2020 stood at 603.6 MT and oil production in 2019-20 stood at 32.17 MT. The import value stood at 226.95 MT. Oil and related products amounted to 27.1 % of the total Indian imports in 2019-20 ( Government of India (GOI) 2020b ). This has been a major cause of India's trade deficit. This amount of oil import adds up to an incremental 300 MT of carbon that India imports ( Ray 2021 ).
The Ankleshwar oilfield is a Cenozoic anticline having a deltaic origin and is located in the Cambay basin ( Ganguli et al. 2016a ;Ganguli et al. 2016;Surabhi et al. submitted). The Cambay basin contains a thick Paleogene sedimentary column deposited after Palaeocene over the Deccan traps ( Srivastava et al. 2015 ;Ganguli et al. 2016a ). The field has been under active production for about the last 56 years and has neared its maturity. As in April 2011, the total production from the oil field stood at 65.35 MT, which is ∼ 49% of the total reserves in place (Ganguli et al. 2016), along with a water cut value of ∼ 88% ( Ganguli et al. 2016b ). Ganguli (2017) developed a model to assess the carbon sequestration potential through the EOR pathway of this oil field. The initial datasets were provided by the ONGC. A similar EOR study was conducted earlier by Vedanti and Sen (2009) , using in-situ combustion in the heavy oil field of Balol, also located in the Cambay basin ( Ganguli et al. 2016a ). The process of in-situ combustion means a certain part of heavy, highly viscous oil undergoes combustion. The heat generated reduces viscosity, and production increases. However, this study focussed on using seismic data to study fluid movement during different combustion phases.
Fracture pressure and pore pressure of the Ankleshwar Formation were calculated ( Ganguli 2017 ) by using the equation of Mathews and Kelly (1967) and Eaton (1975 ;recent review by Dasgupta and Mukherjee 2020): Here PP: pore pressure FP: fracture pressure S V : vertical stress S h : minimum horizontal stress P hyd : hydrostatic pressure DT n : sonic travel time in shale DT: observed sonic travel time during well-logging The result concluded that the Ankleshwar is a great potential field for carbon sequestration. The simulation shows that CO 2 injection increases the oil recovery percentage from 56.8 to 71.6%. It has a potential for safe sequestration of 15.04 MT along with a 10.4% increment in the oil production of the original reserves, i.e., ∼ 134 MT ( Ganguli et al. 2016a ;Ganguli 2017 ).

General points
India has the third-largest coal reserves at 326.05 BT (Section 4) ( Government of India (GOI), 2020a ). The total production stood at 714.88 MT, with surface mines accounting for 650.58 MT and underground mines accounting for 64.3 MT (DGMS 2017) as referred in Singh and Hajra (2018) . India's coal demand is expected to increase by 4.6% each year, which would put an equal strain on its production ( Finkelman et al. 2021 ). This strain manifested itself in October 2021 when the coal stock of India's several thermal power plants hit critically low levels. This led some states to impose partial load-shedding in order to compensate for it ( Perumal 2021 ).
Coal can store a substantial amount of CH 4 , a greenhouse gas(Section 2.4.3b5). This has caused several related disasters in the past, the most recent one being in Pakistan where 23 mine workers died in the Marwar coalfield, Baluchistan province ( Radio free Europe radio liberty (RFERL) 2018 ). Such disasters can be averted if the CH 4 entrapped within the coal seams can be captured. The chemical properties of both the gases are such that while CO 2 is adsorbed into the coal structure, it readily replaces the CH 4 present there, thus also enhancing the production of CH 4 ( Vishal et al. 2012 ).
In India, out of 326.05 BT of coal reserves, ∼ 99% are the Gondwana coal, formed during the Permian Period, and merely 1% are the Tertiary coal, formed during the Eocene Period. 88% of Gondwana coal is utilised in power generation. All kinds of coal contain CH 4 . In CBMR, coals are classified based on their gassiness, i.e., methane emitted per tonne of mined coal. ( DGMS 1967 ) categorized underground coals (as surface coals lose their methane because of their exposure) into three degrees based on their gassiness; degree I ( < 1 m 3 ), degree II (1-10 m 3 ) and degree III ( > 10 m 3 ). Out of 342 working underground coal mines all over India ( DGMS2014 ), 242 are degree I, 90 are degree II and 13 are degree III.
The Jharia coalfield ( Mukhopadhyay 2019 ) has been a major candidate in CBMR, with possibilities being considered for the last 15 years. Till now, the DGH has allotted 33 coal fields, covering an area of 26000 km 2 , for virgin coal bed methane recovery. Out of this, four have been operating since 2007; two in the Raniganj coalfield ( Mukhopadhyay 2019 ) (operated by GEECL and EOL each), one in the Jharia coalfield (operated by ONGC) and one in the Sohagpur coal field (operated by RIL) ( Vishal et al. 2012 ;Chatterjee and Paul 2016 ;Singh and Hajra 2018 ).

Coal structure and behaviour
Coal is a sedimentary rock formed under elevated pressure and temperature for geologically long periods. This results in coal displaying a dual-porosity structure, consisting of both macropores (size > 50nm) and micropores (size < 2.0 nm) ( Zdravkov et al. 2007 ). A difference in pore size also affects the flow of materials (such as CO 2 ), resulting in the application of different laws while studying such differential flows through the same medium. Gases follow Fick's law while diffusing through the micropores, and they follow Darcy's law while flowing through the coal cleats ( Vishal et al. 2012( Vishal et al. , 2013a( Vishal et al. , 2018. CO 2 injected into a coal body can stay in the cleats and micropores and lead to recovery of coal bed methane ( Ribeiro e Sousa 2012 ).
CO 2 sequestration in coal mines remained mostly a concept till ∼ 2012 with only a few actual efforts made. Abandoned deep ( > 500 m) or ultra-deep ( > 800 m) mines that may be difficult to explore coal ( Ribeiro e Sousa 2012 ) can be good targets for CCS. However, the leakage issue needs study in such cases as well ( Piessens 2012 )   concluded that the permeability of Indian coal at low confinement decreases with injection pressure. However, permeability increases with higher confinements and higher injection pressures. Other studies by Vishal et al. (2017a) and Vishal et al. (2017b) concluded that coal permeability is lower for CO 2 in a supercritical phase than in its liquid phase. The reason can be attributed to the high affinity of coal for the supercritical variety. To overcome this, injection pressure induced fracturing can be used to counteract the swelling. However, it should be done in a controlled way such that the fracture does not propagate throughout the entire coal seam. Vishal et al. (2012Vishal et al. ( , 2013bVishal et al. ( , 2018 simulated the sequestration potential through the ECBMR pathway. A simulator, COMET3, was employed to understand the behaviour of coal while sequestration, its capacity of sequestration and the extraction of CH 4 .
The numerical parameters of the simulated coal blocks and the model parameters are summarised in Tables 9 and 10 , respectively.

Simulations
Simulations were made for 4000 days ( Vishal et al. 2013 b;Vishal et al. 2018 ) and 7300 days ( Vishal et al. 2012 ) to study the sequestration potential for ECBMR. In the work by Vishal et al. (2012) ( Fig. 10 ), it was observed that for the initial period of ∼ 3000 days, a higher rate of CO 2 injection is followed by a lower rate of injection of CO 2 ( Table  11 ), which keeps on decreasing further and remains constant till the end of the studied time period. The pattern was repeated in Vishal et al. (2018) ( Fig. 11 ). The total CO 2 injection in Vishal et al. (2018) stood at 220Mm 3 at ∼ 4000 days ( Fig. 11 c). Both the studies ( Vishal et al. 2012 ; give similar results, except that the simulation period in the former is almost double than the latter. However, the value of CO 2 adsorption in Vishal et al. (2012) at 3650 days is ∼ 135 Mm 3 , which is more than that for 4000 days in Vishal et al. (2018) .
The adsorption of CO 2 leads to the release of the coal-bed methane ( Figs. 10 ,b,d), which was another conjecture of the simulations. The onset of CH 4 production is marked by the initial release of water contained within the coal block. The simulated CH 4 production stood at 141, 74.22 and 56.63 Mm 3 in Vishal et al. (2012) , Vishal et al. (2013 b) and Vishal et al. (2018) , respectively.

Carbon sequestration in deep saline aquifers: A case study from the Ganga basin
Indian deep saline aquifers hold the highest potential for carbon sequestration owing to their geology that has stored and restricted the flow of brackish water for a geologically long period. Major states containing saline aquifers occur both within and outside the Ganga basin, e.g., in Uttar Pradesh, Gujarat, Rajasthan, Punjab, Andhra Pradesh, Tamil Nadu and Karnataka ( Bhandari 2014 ;Central Ground Water Board (CGWB) 2020 ). Major CO 2 emitting sources are also located in nearby areas (Section 4.2.2) . On the other hand, the water being brackish holds little importance for social or economic uses ( Kumar, 2015 ). Thus deepsaline aquifers can be of great potential for carbon sequestration in India ( Chadha 2016 ). This can also counterbalance the carbon footprint of groundwater irrigation in India, that stands between 45.3 MT and 62.3 MT ( Rajan et al. 2020 ).

Table 10
Model parameters of Vishal et al. (2012Vishal et al. ( , 2013bVishal et al. ( , 2018  The saline aquifers of Uttar Pradesh ( Fig. 12 ) hold the most potential for storage. This is because the depths of these aquifers are 1100 mbgl. This satisfies a preliminary criterion in CO 2 storage, that it should take place at depths exceeding 800 m. (detail in Section 2.4.3b2). The entire Ganga basin is overlain by fertile silt that is at places are > 5000 m deep. This causes this region to be one of the most agriculturally productive regions of the country. Since the silt depositions run to a significant depth, deep vertical electrical sounding (VES) surveys were carried out at 12 identified locations reaching > 750 m depth. Below such a depth, CO 2 exists in a super-critical phase ( Fig. 13 ) ( Chadha 2016 ). Chadha (2016) considered that to sequester CO 2 in the saline aquifers, displacement of water to accommodate the CO 2 is considered Eqn 4 . was used to approximate the amount of displaced water.
Here F f : fluid faction, VSA: volume of the saline aquifer. Based on the above calculation, it was found that an aquifer area of ∼ 15 km 2 could be saturated with CO 2 . The CO 2 sequestration potential of the studied area is 48.3 MT with a storage efficiency of 2%. This value is close to Thibeau and Mucha (2011) efficiency value of 1.4% from the Utsira Formation (Sleipner). However, the magnitude can vary from less than 1% to even more than 10% ( Bachu, 2015 ) on a globalscale depending on the geological characteristics of the aquifer; injection rate, duration and strategy of CO 2 and the characteristics (permeability and capillary entry pressure) of the confining aquitards.

Sequestration potential of greenstone belts of Dharwar: Kolar & Chitradurga (Karnataka state)
The greenstone belts of the Dharwar craton contain abundant alkaline silicate, which reacts with CO 2 to form their respective carbonates, thus safely sequestering the carbon Mani et al. (2008) . studied the sequestration potential of two greenstone belts in Chitradurga and Kolar ( Fig. 14 ). The mean length and thickness of the Chitradurga belt are 450 km and 10 km, while the mean length and width of the Kolar belt are 80 and 6 km, respectively. An empirical approach was adopted. The weight percentage of MgO in 1 tonne of serpentine is 35-49 % ( Goff and Lackner 1998 ), which sequesters ∼ 1.5 tonnes of CO 2 . Thus, ∼ 1 tonne of MgO would sequester 1 tonne of CO 2 . In order to ascertain, an approximate value Eqn 5 was used: Here T: amount of CO 2 sequestered, p: % of MgO in the ultramafic, a: area (effective area is assumed as 20% up to 1 km depth).
Only ultramafic komatiites have been included in the analysis for their high MgO content, t = thickness (taken as 1 km), d = mean density of the ultramafic, and = mean porosity (2%). The calculation estimated a sequestration potential of 2.94 MT for the Kolar belt and 4.7 MT in the Chitradurga belt.

Carbon sequestration using Eastern Cottonwood tree (Poplar): A case study from the Nainital district (Uttarakhand)
Biotic sequestration (oceanic and terrestrial) holds significant potential (Section 2.4.2b). Within terrestrial biotic sequestration, the niche of agroforestry sequestration has been recognised by the Kyoto Protocol as a method with multiple benefits apart from carbon sequestration. These include supplemental income through wood selling, cropland protection, supporting pollination etc. ( Abbas et al. 2017 ;Gupta et al. 2017 ). Afforestation is also included under the CDM by Kyoto Protocol ( Gera 2012 ). For this purpose, a case study of biotic sequestration has been included. Gera (2012) conducted detailed modelling to ascertain the sequestration potential of Poplar trees, using PROject based COmprehensive Mitigation Assessment Process (PRO-COMAP), a spreadsheet-based model that has been designed for these types of studies. Similar studies using this model have also been conducted ( Rootzén et al. 2010 ;Wani et al. 2012 ;Malhotra 2017 ). Gera (2012) carried out the study in three villages Kyaribandobasti, Kanchanpur Choi, and Nandpur (Nainital district, Uttarakhand). The trees are planted in two ways. In block plantation, 500 trees are planted with five trees planted every 4 m and in row and bund plantation, each tree is planted 2 m apart. The field data collected from plantations were Above Ground Biomass (AGB), Soil Organic Carbon (SOC), Below Ground Biomass (BGB) and Woody litter. The AGB and BGB for PRO-COMAP were recorded quadrat-wise, on three different poplar age groups. A quadrat is a quadrilateral structure used in biotic samplings, such as species of animals or plants ( Gleason 1920 ) . In block plantation, a quadrat of size 25 m * 20 m was considered for the data collection. In bund and row plantations, the area was divided into strips; 10 m * 125 m in block plantations and three random quadrats/strip (signifying three age groups) were considered for the data collection. The mean annual increment (increase in average tree height) was calculated using the volume formula detailed in Dhanda and Verma (2001) . The biomass and wood density were calculated using the biomass expansion factor (1.1) and the wood density factor (0.35). The biomass and wood density was used to calculate the AGB. The AGB was multiplied by 0.27 to obtain the Below Ground Biomass (BGB) ( Intergovernmental Panel on Climate Change (IPCC) 2003 ). SOC was measured at previously se- lected locations, at depths of 15, 30 and 45 cm. Woody litter data is not mentioned in the study. However, SOC and woody litter have negligible impacts on the model.
The model was rendered with the collected data for 30 years. Two scenarios were modelled: with the wood product and without the wood product. In the former, trees would be cut every 6 years (average maturing period of the tree) and in the latter, the tree would be left for 30 years. In block plantation, the sequestration values were 1.33 t C h − 1 yr − 1 and 2.41 t C h − 1 yr − 1 for 'without wood' and 'with wood' simulations. In bund/row plantations, the sequestration values were 1.05 t C h − 1 yr − 1 and 1.80 t C h − 1 yr − 1 for without wood and with wood, respectively.

Prospects
India's emission projection stands at 5.3 BT by 2030 (Section 3.3) . Such figures cannot be thwarted without mainstream CCUS infrastructure retrofitted in the hard-to-abate industries such as cement, iron and steel and thermal power. The infrastructure of these industries serves as the backbone of the Indian economy and cannot be replaced easily. This is highly pertinent to thermal power plants. They drive the energy production in India and given that energy access and security is still a big issue at hand, CCS can make the thermal energy 'green', while efforts are pursued to transition to cleaner energy sources ( Sharma, 2018 ).
India is taking substantial leaps in the field of renewable energy. Ministry of Power, Government of India (2021) reported the total installed capacity of electricity at 382.73 GW as of 30 th April 2021. Out of this 95, GW ( ∼ 20%) is renewable. The Government aims to increase this to 175 GW by 2022 ( International Energy Agency(IEA) 2021 ). This would severely cut the dependence on thermal power plants and hence curb emissions. The Indian private company Reliance Industries has included CCS in its net-zero commitments.
Curbing new emissions and sequestering CO 2 requires huge expenditure. There has not been a detailed analysis of CCS cost in India except a study concerning thermal power plants (Section 4.2) . This becomes more pressing in India, given its multidimensional developmental needs. A nodal organization comprising of representatives from the Maharatna and Navratna companies can be set up which can supervise the CCUS activities in India. Besides Government funds, the major energy conglomerates such as NTPC, ONGC and CIL etc., should work towards allocating funds for CCUS projects ( Ranjan 2018 ).
CCS infrastructure is highly cost-intensive, with the only United States of America somewhat succeeding in creating a CCS infrastructure (SG and TERI 2021). There are also some examples in Europe, the most well-known being Sleipner (Section 2.4.3b6).

What is a Clean Development Mechanism?
The CDM is an important clause that allows a symbiotic relationship between the developed and the developing countries. The mechanism allows the developed countries to set up emission reduction projects and undertakings in the developing countries. This allows the developed countries to procure Certified Emission Reduction (CER) credits to their name. These credits allow them to reach their emission reduction targets. This mechanism satisfies the twin aim of both the developed and the developing countries involving an interchange of skill, knowledge and technology ( Zomer et al. 2008 ;Lema and Lema 2013 ).

CCS-CDM
The CDM consists of a project cycle that is to be satisfied before its execution. The cycle consists of the steps outlined below. These have been mainly compiled from Kalbende (2015) with cross-verifications from Thorne and La Rovere (1999) , ( UNEP 2005 )  The mechanism commences with a specific project blueprint that would set the base of the project. This design should be both ambitious and conceivable. Since the present work deals with CCS, the CDM project design in this scenario should set forth baseline emission data according to which the designated project would proceed. This baseline emission date will be extracted depending on the actual emission of the source, technological advancement and human resource potential.
In the Indian context, the project design and development include several other factors. Carbon capture and storage procedures require understanding and preparation for all possible outcomes. Since the storage of CO 2 in geological repositories at > 800 m depths ( Riley 2010 ) is of primary concern here, accidental leakages needs to be accounted for. A subsidiary plan accounting for the accidental or unfortunate leakages needs to be worked on. The financial fluctuation between the buyers and sellers are also to be looked after in the plan itself. This is because the prices of the CER units are subject to volatility, and thus the buyer and the seller needs to negotiate and agree on all the common acceptable terms to both parties.

National approval
After agreement on the negotiated terms by both parties, the CDM project design requires the approval of the national authority. All the countries involved in the project must designate an authoritative body to preside over the viability of the project, its impact and outcomes across all levels. This authoritative body becomes the point of contact between the countries involved. The body must ensure that the project design is congruent with the international laws and conventions. These are: • The UNCLOS agreement (1982) • The London convention (1972) on the prevention of marine pollution due to dumping of wastes and other matter • The OSPAR convention (1992) for the Protection of Marine Environment of The North Atlantic. • The Kyoto protocol (1992)

Validation & registration
The national approval of the project is followed by the preparation of an official project document that contains detailed information on the following points .
• General description • Baseline analysis • Project timeline and credit period • Monitoring plan • Greenhouse gas emission values by sources • Environmental impact assessment (EIA) reports • Comment of the involved stakeholders After the submission of the detailed document, the project is validated by the designated authority. Then the project is passed on to the executive committee that comprises 10 members as per the UNO's regulations for the project registration.

Project financing
The next step is to secure adequate funding for the CDM project. However, in any circumstances, public funding is meant for the development expenditure, and in no way the CDM project finances should impede any ongoing development process.

Implementation and monitoring
After the financing is procured, the project is set forth for implementation. A CCS-CDM project takes a comparatively longer implementation period due to the scale of the projects.

Verification and issue of CER units
The designated body would then verify the results of CDM projects. The body needs to ensure that the project has been executed without deviating from the issue's guidelines and regulations. After the designated body is thoroughly convinced of the results of CDM projects, they will proceed with the issuance of CER units.
The CER certification would recognise the success of the CDM project. This CER unit can be used by developed countries to lower their CO 2 reduction targets.

CCS-CDM in India
Currently, there are no CCS-CDM projects operational in India. India has the status of a developing economy. Meeting the power needs and alleviating the widespread poverty is the major concern for the Government. Tackling the energy crisis economically seems to be the top priority of the Government at this moment and deploying the CCS project would only thwart the economic aspect of tackling the energy crisis. In 2006, two CCS-CDM proposals were submitted to the Government in India. However, none of them materialised. The GOI recognises CCS as a technology of the future in the Indian context. The year 2030 can be envisaged as the year when the CCS projects might begin taking effect. Until then, R&D is the only thing that the Government is presumably concerned. However, the Government still is lenient upon CCS-CDM projects if the interested international party is willing to cover all costs ( Shackley and Verma 2008 ;Viebahn et al. 2014 ).

Other methods and existing schemes
Besides global methods such as CCS-CDM, an explicit method such as carbon pricing or carbon tax, where a tax is imposed on the amount of emissions, can prove beneficial. Currently, according to IMF, this tax should be close to USD 75 per tonne by 2030 to achieve the Paris goals. However, no such explicit mechanism exists in India. Certain implicit methods seek to serve the same purpose ( Chandra 2021 ).
• Coal cess -This was implemented in 2010. It introduced excise duty on coal. Its pricing reached INR 400 per tonne in 2016. The excise collected from it was allotted to the National Clean Energy Fund (NCEF) that finances clean energy research and initiatives. This scheme was, however, not implemented properly. Between 2010-11 and 2017-18, only 34% of the collected duty was allotted to NCEF, further out of which only 50% was utilised. In 2017, however, this scheme was abolished ( Shakti and EY 2018 ;Chandra 2021 ). • Perform, achieve or Trade Scheme (PAT) -The first cycle of PAT scheme was launched in 2012-17, coordinated by Indian Energy Exchange (IEE). Its sets energy reduction targets (ERTs) for high emission industrial sources. The ERTs achievement is acknowledged with Energy Saving Certificates (ESCs). The ESC is equal to 1 tonne of oil. Inability to achieve the ERTs requires the companies to buy ESCs. The combined reduction from the first and second cycles  was 92 tonnes of CO 2 . The fourth PAT cycle commenced in 2020 ( Bhandari and Shrimali, 2018 ;Chandra 2021 ). • Renewable Purchase Obligation (RPOs): Similar to PAT, RPOs set a target for Indian states to produce a certain amount of energy from renewable sources to meet their requirements. The states that do achieve the target are awarded Renewable energy certificates (RECs). The RPOs are tradable at energy exchanges. In 2019-20, only four states met their RPOs ( Mishra 2020 ;Chandra 2021 ). • Internal Carbon Pricing (ICP): ICP is a voluntary scheme where private organisations set a price on their carbon emissions and use the fund to transition to low emissions technologies. This is done in three ways: i) Internal or private carbon fee where the revenue generated is funnelled towards low emission alternatives ii) Implicit price where company measures the amount to meet government emission reduction targets. This enables them to track the revenue, invest in their low emission targets and minimize their carbon footprint iii) The Shadow price is where a company sets a theoretical carbon price and use that as an index to fund their low emission incentives.
As of 2019, 22 companies have implemented ICP, out of which four companies have to implement implicit price, three have implemented private or internal price, nine have implemented shadow price, 1 has a carbon offset (where emission made from one source is offset by reducing from another source) price, three have a combination of implicit and shadow and one has a combination of implicit and private or internal. This initiative has been gaining traction in the private sector ( Chandra 2021 ; Centre for Climate and Energy solutions(CCES) 2021 ).
Bhat and Mishra (2020) 's analyses show that such taxes have been quite effective in increasing the R&D in the clean energy sector. This, however, is still very ineffective in increasing the share of cleaner energy sources in the Indian energy mix, which is still dominated by fossil fuels (primarily coal). A pronounced shift in the energy production source is needed to move towards a cleaner energy mix. An increase in publicprivate partnership investment in renewable energy also has the ability to decrease carbon emissions in India ( Kirikkaleli and Adebayo, 2021 ).

Latest initiatives: Accelerating CCUS technologies (ACT) ( DST 2020b ; Global CCS institute (GCCSI) 2020 )
The most recent initiative of the Indian Government has been the ACT. ACT is an international consortium to fund and accelerate the development of CCUS technology. India has recently become a member of the ACT and joined hands with Netherlands, Denmark, Alberta province in Canada, the Nordic Region, France, Greece, Germany, Italy, Norway, Romania, Turkey, Spain, Switzerland, the UK and the USA. Under this initiative, India has pledged a million Euros ( ∼88 million INR) to support the Indian projects.
Under the ACT, DST has invited proposals ranging from small-scale research projects to pilot sites. This proposal should be in collaboration with at least three ACT countries. Any such project should be industrially scalable.
The ACT proposal call is a two-step process: • Call for pre-proposal (stage 1) -This remained open up to 10-Nov-2020. • Call for full-proposal (Stage 2) -Participants selected in stage 1 move to stage 2. This was open till 15-March-2021.
The selected projects would start from September 2021.

Conclusions
The article extensively reviewed the current scenario and prospects of CCS in India. Such a review was much needed as we enter a new decade where climate-related action will be at the forefront. CCS will play a major role in it. Given the vastness of the topic, this article does not delve deeper into individual technologies and other subtopics.
The article provides an overarching view of CCS in the Indian context and presents a coherent picture of the current situation. Each section can be further studied for a more detailed analysis of its full potential and limitations. Several initiatives are in the R&D stage (Section 4.1) . India also possesses substantial geological repositories (Section 4.3.2) and biotic sequestration capabilities (Section 4.4) . Some simulation and feasibility studies have already been carried out (Section 5) , and some are underway (Section 4.1) . All the case studies discussed (Section 5) are highly promising and scalable. Both the biotic and abiotic options need to be at the forefront for a holistic approach to CCS. The sequestration options in the saline aquifers and depleted hydrocarbon reserves hold more potential given that there are several operational sites in the world including India. All the projects discussed under R&D (Section 4.1) hold significant promise, although some are still in the experimentation stage. These projects can complement each other to tackle the issue of CCS in India.
The public-private partnership framework has the ability to play a major role in this regard. These options need to be complemented with suitable, adaptable, and scalable policy measures targeted at carbon sequestration in India. All of these need to work towards decreasing the per unit cost of carbon capture as that still accounts for the major expenditure in sequestration. Continued R&D and focused policies are the key things to achieve this goal.
The next 10-15-year period is crucial for the development of CCS technologies in India. India would look to cover the technological gap in its power production and distribution sector, thus also enhancing the chances of successful deployment of CCS technologies to the power plants post-2030. This step would ensure India's position on global energy as well as in the carbon reduction map.

Conflict of interest
Authors declare that they do not have any conflict of interest with anyone regarding this article.

Acknowledgements
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. This work is an outcome of research work in the National Centre of Excellence in Carbon Capture and Utilisation (NCOE-CCU) funded by the Department of Science and Technology (DST, New Delhi) under the Union Ministry of Science and Technology (India). Mohit Kumar Puniya (Survey of India, Dehradun, India) computerized several figures. RS: preparing and finalizing the ms. SM: supervision and finalizing the ms. We thank Prof. Vikram Vishal (IIT Bombay) for making thorough internal review of this article in two rounds and updating us on CCS issues. Mr. Meenakshi Sundaram (retired from ONGC) is thanked for updating us about the CCS scenario in India during the initial phase of this review work. This article is dedicated to Rhythm Shah, Firdush Hussain, Arpita Chakraborty, Sonal Janagal and Sushmita Mastud (Department of Earth Sciences, IIT Bombay) for participating and winning the final round of CarboNIL (EAGE Minus CO 2 Challenge), organized by the European Association of Geoscientists and Engineers (EAGE): https://eage.org/students/minus-co2challenge/ < https://nam11.safelinks.protection.outlook.com/?url = https %3A%2F%2Feage.org%2Fstudents%2Fminus-co2-challenge%2F& data = 04%7C01%7Cz.french%40elsevier.com%7Ce10ee5948a264fe9f 80608da003c48be%7C9274ee3f94254109a27f9fb15c10675d%7C0% 7C0%7C637822556407811978%7CUnknown%7CTWFpbGZsb3d8eyJWI joiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0 %3D%7C3000&sdata = qnOmRV3y1ypc%2FBdl%2BWIFUvZJyInKjp9 NMNHFXPkkk%2BE%3D&reserved = 0 > . Dr. Chunfei Wu (Chief Editor) and anonymous reviewers are thanked for providing detail review comments in two rounds. We express gratitude to the proofreading team, Zoe French (Journal Manager) and Rahul Nayal (Data Administrator) from Elsevier for support.

Supplementary materials
Supplementary material associated with this article can be found, in the online version, at doi: 10.1016/j.ccst.2022.100036 .