|dc.description.abstract||Today approximately one-and-half billion people around the world, especially in the developing world, use biomass based source of energy for cooking and heating. Conversion efficiency of the biomass in many cases is low resulting in unsustainable use of the biomass resource and negative environmental impacts. In the earth charcoal-making kiln widely used in the developing nations, the conversion efficiency is on average as low as 6 - 10 percent on dry basis. The low conversion efficiency is a source of greenhouse gases and causing deforestation around most African cities. However, the earth charcoal-making kiln has been reported to have potential for improvement of the kiln conversion efficiency.
This research work was about improving the conversion efficiency of the earth charcoal-making kiln by applying a scientific approach using a numerical method. To achieve this, a 3-Dimensional transient numerical model of the earth charcoal-making kiln was developed. The CFD software code of PHOENICS was used to mathematically model and simulate the major factors influencing carbonisation processes in the kiln for their effect on the kiln conversion efficiency.
The results showed that several factors have some influence on the carbonisation process and the kiln conversion efficiency. Low moisture content wood gave relative conversion efficiency of 57.7 percent and fresh wood had 27.2 percent on a dry basis. The smaller diameter logs gave 92.8 percent relative conversion efficiency and large diameter logs 60 percent. The wood weight distribution of smaller diameter logs (5.0 - 20.0 cm) gave a relative conversion efficiency of 92.8 percent while distribution of large diameter logs (35.0 - 50.0 cm) resulted in 16.2 percent relative conversion efficiency. The crosswise type of wood Arrangement gave a relative conversion efficiency of 49.1 percent as opposed to 41.2 percent for longitudinal loading. A kiln of width 1.5 m resulted in a relative conversion efficiency of 3.6 percent while kilns of width between 2.0-2.5 m had a relative conversion efficiency of up to 92.8 percent. The kilns of length 3.5 m had relative conversion efficiency of 85.2 percent. The thinner insulation thickness of 10.0 cm gave relative conversion efficiency of 48.2 percent. A thicker insulation layer of 40.0 cm had relative conversion efficiency of 28.4 percent. A crosswise loaded kiln carbonizing against the prevailing wind direction had a relative conversion efficiency of 64.2 as opposed to 36.9 percent for one carbonising along the prevailing wind direction. The relative conversion efficiency for the longitudinal loaded kiln carbonising along the prevailing wind direction was 63.2 percent and 52.0 percent for similar kiln carbonising agaist the wind direction.
From the numerical model results, the optimised overall conversion efficiency and charcoal yield for the crosswise loaded kiln were calculated to be 12.36 percent and 14.05 percent respectively, while for the longitudinally loaded kiln these figures were 18.50 percent and 21.02 percent respectively. In both cases, this is an improvement on the reported unimproved earth charcoal making kilns average conversion efficiencies of 6 – 10 percent. The figures also agree well with other researchers findings in the field on related works.||en_US