Emerging trends in appropriate kiln designs for small-scale biochar production in low to middle income countries

This paper reviews information about the design and performance of existing biochar production kilns to identify areas for further research and development. The kiln designs are categorized into flame curtain kilns, drum kilns, pyrolytic top-lit-updraft cookstoves, retort kilns, masonry kilns and others. Depending on the design, reviewed kilns can attain yield efficiency of 10-46 % and produce biochar containing 26-87 % fixed carbon with a calorific value of 14-40 MJ kg -1. Comparison of available kiln designs is challenging as some of the data required are either unavailable or derived from experiments that use a single feedstock. However, a qualitative schematic comparison indicates that drum retort kilns are best suited for most applications in low-income settings. These designs would likely be improved to approach the theoretical maximum efficiency whilst improving the quality of biochar and reducing emission levels, by combining appropriate design features from other kilns.