Wood fuel carbonisation and charcoal characterisation of six agroforestry tree species of Ethiopia

A study was done on wood fuel carbonisation and charcoal characterisation of six agroforestry tree species of Ethiopia, namely, Acacia abyssinica, Acacia seyal, Albizia gumifera, Croton macrostachys, Grevillea robusta and Millettia ferruginea. Wood charcoal was produced using partable, cylindrical, "mini" steel kiln. Physico-chemical and thermal properties of wood fuel and charcoal and carbonisation productivity were analysed. The important analytical values were proximate analysis, calorific value, specific gravity, volumetric density and moisture content on wet basis. Carbonisation productivity (both energy and weight basis) was also determined. Significant differences in observed values were tested isomg analysis of variance and mean values were compared by Duncan's Multiple Range Test. Linear regression models for correlated properties were also examined. The differences obtained in proximate analysis, thermal and physical properties of wood fuel and charoal between species were significant It was also observed that the conversion of wood into charcoal resulted in considerable chage in volatile matter and fixed carbon percentage, decreasing and increasing respectively. The volatile matter, ash and fixed carbon contents of wood were 74.62 plus or minus 0.56 percent, 1.86 plus or minus 0.33 percent and 21.22 plus or minus 0.16 percent respectively. In contrast charcoal has 21.15 plus or minus 0.33 percent volatile matter, 3.25 plus or minus 0.44 percent ash and 70.95 plus or minus 0.77 percent fixed carbon. The potential net heating value of wood was on average 2592 plus or minus 100.36kcal/kg. The average gross calorific value was 4423 plus or minus 44.92kcal/kg for wood and 7158 plus or minus 93.83kcal/kg for charcoal. Actual wood substance specific gravity was 1.23 plus or minus 0.03g/cm cubic. There were no significant differences in volumetric density of charcoal between species. The average value was 0.34 plus or minus 0.01g/cm cubic. Differences in mass yield, balanced mass yield, gross energy yield and energy efficacy of carbonisation were significant. Both values were very low for Croton macrostachys and comparatively high for Grevillea robusta and Acacia seyal. The average charcoal recovery from steel kiln method was 34.87 plus or minus 5.1 percent on anhydrous wood mass basis, 31.2 plus or minus 4.7 percent on weighted mass yield and 24.6 plus or minus 2.7 percent on gross energy yield basis. The energy value of wood was upgraded to 2693 plus or minus 354.8kj/kg of wet wood using steel kiln. Volumetric estimates of wood fuel carbonised and charcoal produced were found unreliable basis for comparison of carbonisation productivity of different wood fuel species. The relationship between wood fuel net calorific value(y) and its moisture content on wet basis(x) was the prominent one with linear regression model y equal to 43713 minus 49.84x.