Net CO2 removal of rice husk biochar as soil amendment depending on energy reuse in the production stage

Rice husk is a highly useful biomass for biochar production in Korea, because of convenience of relatively homogeneous feedstock supply. From this, greenhouse gas emissions (GHG) in biomass sourcing stage and use stage of local-used biochar could be negligible, and thereby the net CO2 removal of rice husk biochar with soil application could be considered as the value of biochar organic carbon sequestered over 100-year minus GHG by energy consumption of the production stage. GHG by renewable energy consumption is regarded as zero in international biochar C credit certification methodologies, so energy reuse could increase the net CO2 removal of rice husk biochar. This study assessed the influences of the energy reuse percentage, including renewable energy and thermal energy reuse, in biochar production on the net CO2 removal of rice husk biochar locally used as soil amendment. The total amount of rice husk production in Korea in 2023 was estimated at 800 thousand tons. The economic pyrolysis temperature for rice biochar production as a soil amendment could be 500°C because H/C molar ratio was less than 0.7, biochar yield was relatively high, and the soil improvement effect was large. If 10% of the total amount of rice husk in Korea is converted to biochar at a pyrolysis temperature of 500°C, the amount of organic carbon sequestered over 100 years by soil-amendment is estimated as 58 thousand tons CO2-eq, which corresponds to the 2030 biochar utilization CO2-eq sequestration goal in Korea. This amount corresponds to the net carbon sequestration when biochar is produced with 100% renewable energy. As the proportion of renewable energy or reused thermal energy decreases, the net CO2 removal decreases, showing maximum 70% cut. Therefore, this study suggests that it is necessary to maximize the net amount of sequestered carbon per ton biochar by securing energy-efficient production facilities.Estimated net carbon sequestration of soil-amended rice husk biochar at 500°C pyrolysis using 10% of total rice husk produced in Korean city/county with different production energy use.