Biowaste to Biochar: A Techno-Economic and Life Cycle Assessment of Biochar Production from Food-Waste Digestate and its Agricultural Field Application

Abstract

Biochar has high potential for long-term atmospheric carbon storage in terrestrial environments, contributing to meet UK and global greenhouse gas emission reduction targets. This study investigates greenhouse gas emissions and techno-economics associated with biochar produced from anaerobic digestate using hydrothermal carbonisation with post carbonisation at higher temperatures. While digestates are challenging to valorise, owing to high moisture contents, these low-value feedstocks have steady availability with minimal competition for other applications. The study scope includes food-waste digestate supply, stable biochar production, agricultural field application including co-benefits, and transportation activities. Minimising digestate transport through co-location of biochar production with anaerobic digestion demonstrated greenhouse gas mitigation costs of <£100 tCO2eq-1 (125USD tCO2eq-1). Stable biochar effectively removes atmospheric greenhouse gases, achieving net emissions of 1.53-1.63 tCO2eq t-1biochar, a result dominated by the storage of carbon in biochar (1.94 tCO2eq t-1biochar). Using 50% of UK’s projected available food-waste digestate by 2030 offers 85.4 ktCO2eq p.a. sequester potential, requiring 28 individual 20 kt p.a. biochar facilities. Sensitivity analysis demonstrates heavy influence on the gate fee charged to digestate processing, emphasising its importance for economic success of biochar production. Potential technology enhancements to reduce fossil-fuel use and investigating the co-benefits of biochar’s agricultural applications require further study.

Keywords: biochar, anaerobic digestate, food-waste, life cycle assessment, techno-economic assessment, greenhouse gas removal, carbon price