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.
Forest carbon sequestration is a promising negative emissions technology as it is relatively simple and inexpensive. Its potential for climate mitigation could be particularly high in tropical rainforests as they can store more carbon at a faster rate. However, the understanding of its sustainability is limited due to the scarcity of studies. Focusing on reforestation, this work is the first to present an extensive environmental and economic assessment of forest CO2 sequestration in a tropical rainforest country such as Malaysia. Life cycle assessment (LCA) and life cycle costing (LCC) are combined to evaluate the sustainability of reforestation with four tropical tree species: keruing (Dipterocarpus spp.), meranti (Shorea leprosula), rubber (Hevea brasiliensis) and kapok (Ceiba Pentandra). Considering the horizon of 60 years, the system boundaries comprise site infrastructure, land clearing, tree planting and growing, and forest management. The dynamics of greenhouse gas (GHG) and other air emissions are also considered over the period, including carbon sequestration and land use change, nitrous oxide and ethylene emissions. All tree species lead to a net-negative GHG emissions, ranging from -558 to -808 kg CO2 eq./t CO2 removed, with kapok having the highest and rubber the lowest sequestration potential. The latter has the highest values in 13 other impact categories considered, while keruing is the best option for 12 impacts. The main environmental hotspot for all species is the forest management stage. The LCC range from -US$17 to US$12/t CO2 removed. Reforestation on lands available in Malaysia could remove 105.9-473.3 Mt CO2 over 60 years, or 1.8-7.9 Mt CO2/yr, equivalent to 7.4-33 % of the agricultural emissions. Rubber and kapok could generate US$640 M and US$8.06 bn in profits over 60 years from latex and fibre, respectively. Therefore, reforestation has a significant potential to help tropical countries, such as Malaysia, achieve net-negative emissions, while at the same time boosting the economy.