Quality of Biochars Made from Eucalyptus Tree Bark and Corncob Using a Pilot-Scale Retort Kiln

Abstract

In many developing countries, traditional earth-mound kilns are still the principal technology for biochar production. The present study focused on the production and characterization of two biochars from common agricultural and forestry residues in Cameroon: corncob and eucalyptus tree bark. A pilot-scale retort kiln, made from local material, was constructed for biochar production. Its production efficiency varied between 33 and 68%, compared to the 10–22% obtained with traditional biochar production systems. Both biochars exhibited good agronomic properties and fulfilled key quality criteria for soil carbon sequestration, as described by the European Biochar Certificate (EBC) and the International Biochar Initiative (IBI). Organic carbon (Corg) > 50%, H/Corg < 0.7, and potential toxic elements far below environmental threshold values. Both biochars passed seed germination and worm inhibition toxicity tests. They were characterized by similar physical properties, apart from greater water absorption observed for corncob biochar (CCB). Chemical properties differed; CCB had higher buffer capacity, and was richer in soluble potassium (5473 ppm) and in graphitic like carbon (37.7%) compared to eucalyptus bark biochar (EB) (897 ppm and 24.9%). The latter was less alkaline (pH 8.1 vs. 9.3 for CCB), had more than twice its electrical conductivity (0.68 vs. 0.25 dS m−1 for CCB) and ash content (10.1 vs. 5.3%); EB had higher sum of cations. Our results demonstrated that the constructed pyrolyser could be a current and feasible alternative to be used by farmers in Cameroon, to produce a quality biochar product from common residues.