David Dempsey, Karan Titus, Rebecca Peer

Geothermal electricity generation has low carbon emissions compared to hydrocarbon alternatives. Nevertheless, recent attention on emissions of magmatic CO2 and other non-condensable gases (NCG) has prompted interest in their capture and reinjection. The geothermal industry is uniquely placed to effect CO2 sequestration due to existing reinjection infrastructure (e.g., wells, pipes, pumps) and detailed characterization of the subsurface resource.
Dissolving CO2 into reinjected fluid is one way to dispatch emissions without creating a buoyant free phase in the reservoir. The CO2 can be sourced as NCG capture from the produced geothermal fluid or from a hybrid fuel energy scheme like bioenergy carbon capture and sequestration (geothermal-BECCS). The latter approach allows for CO2 concentration in the reservoir to be increased beyond the natural state, turning the geothermal system into a carbon sink. Depending on the local context, spillover benefits could include increased electricity generation, derisking of marginal wells, and additional revenue from CO2 emission offset schemes.
In this study, we develop first-order estimates for the global CO2 sequestration potential of installed geothermal power plants retrofitted for geothermal-BECCS. We estimate the potential carbon dioxide removal (CDR) rate (MtCO2/yr) on a plant-by-plant basis using a mass and energy balance model with fluid throughput rates inferred from plant capacity, technology, and resource temperature. We also calculate potential revenues from CO2 removal and boosted electricity.
We estimate that existing geothermal systems have the potential for 60 million tonnes of CO2 removal annually (MtCO2/yr) at a value of $6 billion. On average, hybridized cycles generate about 30% more electricity at a value of $3 billion annually. However, biomass fuel requirements to realize this scale CDR are 37 Mt/yr and may represent a key bottleneck for many countries. The United States had the highest potential CDR rate at 21.6 MtCO2/yr, followed by Indonesia, the Philippines, and Turkey with 6-7 MtCO2/yr, and other countries (New Zealand, Italy, Kenya, Mexico, Iceland, Japan) in the range of 2-3.5 MtCO2/yr. Herein, we discuss the caveats, limitations and risks that geothermal-BECCS would need to overcome to realize these benefits.