Production of Biochar for Soil Application: A Comparative Study of Three Kiln Models

Biochar has potentials for soil fertility improvement, climate change mitigation and environmental reclamation, and charred biomass can be deliberately incorporated into soil for long-term carbon stabilization and soil amendment. Many different methods have been used for biochar production ranging from laboratory to industrial scales. However, in countryside of developing countries, biomass is generally used for cooking but not charred. Biochar production techniques at farmer scale have remained poorly developed. We developed and tested biochar production kilns for farmers with a dimension of 50.8 cm × 38.1 cm (height × diameter), using three different setups for optimizing oxygen (O₂) limitation and syngas circulation: airtight with no syngas circulation (Model I), semi-airtight with external syngas circulation (Model II) and semi-airtight with internal syngas circulation (Model III). A comparative assessment of these biochar production kiln models was made considering biochar pyrolysis time, fuel to biomass ratio, biochar to feedstock ratio and thermogravimetric index (TGI). Among the models, the best quality biochar (TGI = 0.15) was obtained from Model I kiln taking the longest time for pyrolysis (12.5 h) and the highest amount of fuel wood (1.22 kg kg^-1 biomass). Model III kiln produced comparatively good quality biochar (TGI = 0.11), but with less fuel wood requirement (0.33 kg kg^-1 biomass) and shorter pyrolysis time (8.5 h). We also tested Model III kiln in a three times larger size under two situations (steel kiln and pit kiln). The biochar to feedstock ratio (0.38) and quality (TGI = 0.14) increased slightly for the larger kilns. Quality of biochar was found to be mainly related to pyrolysis time. The costs for the biochar stove and pit kiln were US$ 65-77, while it was US$ 154 for the large size steel kiln. Model III kiln can potentially be used for both cooking and biochar production at farmer scale.