Biochar production and characterisation — A field study

Biochar is a solid residue produced from pyrolysis, which is the heating of biomass in the absence (or near absence) of oxygen. The use of biochar can potentially aid in mitigating climate change through carbon sequestration and improve soil quality by the addition of nutrients, increased water holding capacity and providing a liming effect in soils. The physical and chemical properties of biochars can vary depending on pyrolysis conditions and feedstocks used. The majority of research to date focuses on biochar production in laboratory conditions or from commercial reactors and there is less research on biochar production using appropriate technology for resource-constrained communities. The current work details preliminary results of the characterisation of biochars produced using an earth pit and a prototype top-lit up-draft (TLUD) stove. Analyses were conducted for yield, elemental composition, pH and water uptake of the biochar. Five types of feedstocks were used — pea straw, single-source woodchips, multi-source woodchips, sticks and branches, and leaves and twigs. The results indicate that as expected feedstock impacts both quality and quantity of biochar produced, and different production methods may be more suitable for different feedstock. The earth pit produces a larger yield than the TLUD stove for pea straw and fallen biomass (12 v/v% for pea straw, 6 v/v% for leaves and twigs, and 25 v/v% for sticks and branches compared with 2 v/v% for each, respectively). However, the TLUD stove produces higher quantities of biochar for single-source wood chips (34 v/v% from the TLUD stove vs 10 v/v% from the earth pit). The difference in elemental composition for biochar from different feedstock material is significant, however, the production method has little impact, indicating that the degree of devolatilisation from each method is similar for each feedstock. The pH of all biochars is high and in the range of 9.3–10, which may be attributed to their high ash content. In addition, pea straw biochar has a greater water uptake compared with biochar from the other wood-based feedstocks, as expected. In a humanitarian context, the results indicate that both production methods are suitable for producing a highly alkaline biochar with a similar water uptake capability. The high alkalinity of all biochars shows that acidic soils could potentially be improved by the biochar produced from either system. Drought-susceptible soils can potentially be improved to a slightly greater degree with addition of pea straw biochar.