Biochar from agricultural wastes: Environmental sustainability, economic viability and the potential as a negative emissions technology in Malaysia

Biochar used for soil amendment is considered a viable negative emissions technology as it can be produced easily from a wide range of biomass feedstocks, while offering numerous potential agricultural benefits. This research is the first to present a comprehensive sustainability assessment of large-scale biochar production and application in Malaysia. The five feedstocks considered comprise the country's most abundant agricultural wastes from palm oil (empty fruit bunches, fibres, palm fronds and shells) and rice (straw) plantations. Combined with process simulation, life cycle assessment and life cycle costing are used to assess the sustainability of biochar production via slow pyrolysis at different temperatures (300-600 °C), considering two functional units: i) production and application of 1 t of biochar; and ii) removal of 1 t of CO2from the atmosphere. The cradle-to-grave system boundary comprises all life cycle stages from biomass acquisition to biochar use for soil amendment. The positive impacts of the latter, such as carbon sequestration, fertiliser avoidance and reduction in soil N2O emissions, are also included. The global warming potential (GWP) is net-negative in all scenarios, ranging from -436 to -2,085 kg CO2 eq./t biochar and -660 to -933 kg CO2 eq./t CO2 removed. Per t of biochar, the systems with shells have the lowest GWP and those with straw the highest, all showing better performance if produced at higher pyrolysis temperatures. However, the temperature trend is opposite for all other 17 impacts considered, with fibres being the best option and fronds the worst for most categories. Per t CO2 removed, fronds have the highest impact in eight categories, including GWP, and shells the lowest in most categories. All impacts are lower for biochar production at higher temperatures. The main hotspot is the pyrolysis process, influencing the majority of impact categories and contributing 66-75 % to the life cycle costs. The costs range from US$116-197/t biochar and US$60-204/t CO2 removed. The least expensive systems per t biochar are those with straws and per t CO2 removed those with shells, while fronds are the worst option economically for both functional units. Utilising all available feedstocks could remove 6-12.4 Mt of CO2 annually, reducing the national emissions from the agricultural sector by up to 54 % and saving US$36.05 M annually on fertilisers imports. These results will be of interest to policy makers in Malaysia and other regions with abundant agricultural wastes.