Operating regimes for intra-carbonisation of sawdust with low external fuel requirements

• Intra-carbonisation of loose biomass waste such as sawdust for downstream charcoal briquettes production is an understudied concept. • An open mound reactor configuration was explored as a low-cost option that has the least intensive technical design, operations and maintenance requirements. • The study found that the best conditions for carbonisation using this reactor configuration were: biomass samples with a low amount of fines; smaller heap sizes for samples with fines; lower external fuel (3–4 kg of charcoal) for slower heating rates; and moderate residence times of 15 to 26 h for a mass range of 46 to 56 kg of sawdust. • The carbonised sawdust properties, especially GCV, were comparable to similar research and resulting charcoal briquettes had a maximum GCV of 23.9 MJ/kg. • The yields of carbonised biomass were between 35 % and 58 %. Investigations of an understudied concept of intra-carbonisation of loose sawdust waste were carried out with a view to promote their valorisation by low-income communities for better socio-economic inclusivity. Very few studies have investigated the intra-carbonisation of such loose waste biomass with respect to reactor configuration, biomass size, external fuel use and residence time and their effects on the yield and quality of carbonised biomass produced. As a starting point, the open mound reactor configuration was explored as a low-cost option that has the least intensive technical design, operations and maintenance requirements. To allow multiple heat transfer modes, including convection, the study found that the best conditions for carbonisation using this reactor configuration were: biomass samples with a low amount of fines; smaller heap sizes for samples with fines; lower external fuel (3–4 kg of charcoal) for slower heating rates; and moderate residence times of 15 to 26 h for a mass of 46 to 56 kg of sawdust. The carbonised sawdust properties, especially GCV, were comparable to similar research. The best briquettes were obtained from the large particle size material carbonised with 3–4 kg of fuel with an air supply to the fuel chamber, while the configuration without an air supply and the longest residence time produced the worst briquettes in terms of fuel value. The yields of carbonised biomass were between 35 % and 58 %, where the latter reflected low conversion levels since unconverted biomass has a higher density than charred sawdust due to high moisture and volatile contents.