Influence of Torrefaction on Lignocellulosic Woody Biomass of Nigerian Origin

Abstract

Torrefaction process is a thermal treatment that can improve quality of lignocellulosic biomass into a carbon-rich and hydrophobic feedstock which is applicable as fuel and metallurgical reductant. Biomass (Melina and Teak wood) of Nigerian origin was subjected to mild (240o C) and severe (300o C) torrefaction treatment at different residence times (30 and 60 min) and particle sizes (+0.5 - 2 mm and +4 - 6.35 mm). Raw biomass and biochar from torrefaction were subjected to proximate, ultimate, higher heating value and SEM analyses. The mass yield obtained for mild treatment conditions for both biomass was in the range of 72 - 84 (wt. %) compared to 40 - 54 (wt. %) under severe treatment conditions. However, 33 - 56 % increment in higher hating value was observed for severe treatment conditions as against 11 - 17 % of mild treatment condition. This ultimately led to a 60 - 72 (wt. %) energy yield for severe treatment conditions and 73 - 94 (wt. %). The fixed carbon content increased from the range of 8 - 11 (wt. %) to 20 - 61 (wt. %) after torrefaction. The volatile matter content under mild condition was reduced by 7 - 10 % for both biomass as against 41 - 47 % under severe treatment condition. The fuel ratio increased from 0.11 and 0.15 for Melina and Teak woods respectively to a range of 0.22 - 0.25 for mild treatment conditions and 0.97 - 1.75 for severe treatment condition. The H/C and O/C atomic ratios of biochar were lowered towards that of sub-bituminous coal and peat. A honey-comb-like structure with cylindrical holes were observed for biochar compared to the fibrous and spongy nature of the raw biomass. Biomass of Nigerian origin were improved under torrefaction and thus can be suitable as feedstock in thermal or metallurgical applications.