Characterization of Biochar Produced from Cacao Pod Husk with Smallholder Farmers in Farm-Scale Reactors

Purpose: Cacao pod husks, comprising 70% of the crop weight, are usually discarded back on the plot representing a waste stream, even though it could be used to produce biochar, a valuable soil amendment. Biochar is usually produced in large-scale reactors; however, producing it in farm-scale reactors could be more affordable and socially equitable for the farmers, and more sustainable. Here, we investigate whether cacao husk biochar can be produced using farm-scale reactors and has properties suitable for use as soil amendment. Method: With Malaysian cacao farmers, we fabricated two reactor setups operating under pyrolysis and gasification. We characterized its properties according to International Biochar Initiative standards and compared the two production processes. Results: Both reactor setups reliably converted cacao husks into biochar. The biochars passed all toxicology tests. Gasifier biochar largely contained more nutrients (total P, K, Ca, Mg, and S) and volatile matter than retort biochar, likely because gasification required quenching before complete thermochemical conversion to preserve yield. As quenching induces thermal shock, gasifier biochar had higher pore volumes (0.02 vs. 0.002 cm3/g), pore sizes (20.08 vs. 9.61 nm), and surface areas (48.58 vs. 8.34 m2/g) relative to retort biochar. The gasifier reactor also required less setup time (30 vs. 120 mins) and capital cost, but had longer post-processing times, lower yields (13% vs. 33% feedstock weight), and lower pyrolysis temperatures. Conclusion: The production of cacao husk biochar with small-scale reactors proved successful and cost-efficient, and could be used to produce biochar locally at the waste source. Highlights: · Two types of biochar reactors designed to fit ~10 kg of dried cacao husks. · Both gasifier and retort biochars passed all international toxicological standards. · Gasifier biochar had higher nutrients and greater surface area than retort. · Gasifier had lower costs but lower yield (13%) compared to retort (33%). · Gasifier required less setup time but longer post-processing.