Carbon dioxide removal capacity of converting harmful macroalgal blooms in South Korea to biochar: a Monte Carlo assessment

SeoYeon Kim , BongKwan Song 

Blooms of the drifting brown alga Sargassum horneri (golden tides) and of green-tide Ulva spp. strand on the coasts of South Korea every year and are collected by local governments. Pyrolysing this biomass to biochar has been proposed as a way to store carbon, but its carbon dioxide removal (CDR) capacity has not been quantified. We built a Monte Carlo model (100,000 draws) of the technical 100-year net CDR of slow pyrolysis at about 600 °C, defined as the carbon stored in biochar minus life-cycle emissions from haulage, four drying routes, process energy, kiln methane and soil application, for officially collected biomass (Tier 1) and all biomass stranded on the coast (Tier 2). With open-air pre-drying, net CDR per dry tonne was 0.156 t CO₂e (90% interval 0.061 to 0.253) for S. horneri and 0.056 t CO₂e (−0.088 to 0.159) for Ulva. Every route without open-air pre-drying was a net emitter (medians −0.168 to −1.471 t CO₂e per dry tonne). National net CDR under open-air drying was 155 t CO₂e yr⁻¹ (49 to 280) for Tier 1 and 176 t CO₂e yr⁻¹ (−20 to 393) for Tier 2. With renewable electricity and process heat, Tier 2 rose to 364 t CO₂e yr⁻¹, and the gross storage of all stranded biomass, before any emissions, was 624 t CO₂ yr⁻¹. Sand carried with beach-cast Ulva diluted the product to a median carbon content of 6%, far below the 40% minimum of the Korean biochar fertiliser standard. The most influential inputs, biochar carbon content, sand content and plant electricity, lack Korean measurements. The CDR capacity of Korean bloom biomass is marginal; its value lies in waste management rather than climate mitigation.