Effect of torrefaction severity on the energy recovery from amazonian wood residues for decentralized energy conversion systems

Abstract

Brazil features over 196 isolated energy systems, mainly in the Amazonia, relying on diesel-fired conversion for about 96 % of their energy supply. Given diesel's cost and pollution, there's a significant potential for waste wood from sustainable forest management. This study originally assessed torrefaction (225–275 °C, 60 min) to enhance the energy density of a blend (AB) consisting of six (16.66 %) waste wood: Peltogyne lecointei, Erisma uncinatum, Martiodendron elatum, Handroanthus incanus, Dipteryx odorata, and Allantoma decandra. Torrefaction was evaluated through severity indexes, morphological modification, analytical (proximate, ultimate, and calorific) characterizations and kinetic modeling. TGA assessed the torrefied blend's combustion behavior, and related emissions were determined numerically. Torrefaction modifies the raw material by significantly reducing H/C (from 1.87 to 1.05) and O/C (from 0.70 to 0.47) ratios. Considering AB₂₇₅, fixed carbon sees a 159 % increase, and volatile matter (VM) decreases by 68.3 %. The low ash (0.63 %) in the final product indicates the potential for direct burning and blending for low-ash biofuel. The higher heating value improved from 20.22 to 21.64 MJ kg⁻¹ (1.07 energy densification). Morphological analysis indicated increasing particulate matter and enhanced porosity. The two-step kinetic modeling precisely predicted the solid yield, with R² values of 0.9979, 0.9951, and 0.9996 for AB₂₂₅, AB₂₅₀, and AB₂₇₅. Torrefaction improved thermal stability, impacting ignition dynamics due to lower O/C and VM. Emission factors from the combustion of torrefied products reported lower emissions than diesel, coal and other biomasses: CO₂, NOx, and SO₂ at 1281.67–1487.48, 1.12–1.72, and 0.16–0.25 kg ton⁻¹, respectively.