Pyrolysis kinetics and combustion characteristics of local firewood species and charcoal produced by slow pyrolysis

Firewood and charcoal are relatively abundant in sub-Saharan Africa and have low cost of production, hence their continued dominance as an energy source. However, very few studies have documented any information on pyrolysis kinetics and combustion performance characteristics for firewood and charcoal in sub-Saharan Africa. This information is needed to develop environmentally friendly combustion reactors suitable for these firewood species and charcoal as well as guide policy decisions on fuels appropriation for different purposes. Therefore, this study seeks to identify the thermo-chemical conversion characteristics of five local commonly used firewood species, namely Dichrostachys cinerea, Morus lactea, Piliostigma thonningii, Combretum molle, and Albizia grandibracteata and their charcoals produced after slow pyrolysis. Firewood was characterized by a fibrous network, while charcoal showed a dentritic network structure with micropores. The pyrolysis process resulted in a significant increase in lignification in the process of charcoal formation with highest lignin values recorded for Albizia grandibracteata. The lowest rate of lignification was observed for Morus lactea firewood species. Peak temperatures for firewood species and charcoal ranged between 515.4-621.7 degrees C and 741.6-785.9 degrees C. Activation energy of Morus lactea was 16.3 kJ/mol, which was the lowest among firewood species. Activation energy of Piliostigma thonningii was the highest among firewood species at 29.5 kJ/mol. For charcoal, activation energy was highest for both Morus lactea and Dichrostachys cinerea (35.9 kJ/mol). Piliostigma thonningii charcoal had the lowest combustion performance results with maximum burning rate at 0.68%/min, flammability index at 0.18 x 10(-5)%/min degrees C, and combustion characteristic index at 0.02 x 10(-8)%/min(2)degrees C-3.