Small-scale autothermal thermochemical conversion of multiple solid biomass feedstock

Abstract

The thermochemical conversion of four types of biomass in a batch-fed reverse downdraft process for heat generation in cookstoves is investigated. Fuel switching is widely considered inefficient because many combustion devices do not respond well to changes in fuel. Here, the use of agricultural by-products, represented by wheat straw, sheep manure, cow manure, and wood pellets is addressed. Two air supply rates within the oxygen-limited regime, where the fuel consumption is linearly dependent on the air supply, are investigated. At higher air supply rates, in the reaction-limited regime, low carbon yields lead to the exposure of the ash fraction to high temperatures, such that the resultant ash melting has detrimental effects on the process. Generally, no detrimental impact of the ash content on the conversion process within the oxygen-limited regime could be identified. The release of gaseous products, evaluated through cold gas efficiency, increases linearly from 24 to 54% with higher air flow, corresponding to increasing process temperatures from 690 to 980 °C, and is largely fuel type independent. The char produced from all feedstocks fall within the highest classification for biochars, based on its elemental composition and determined by international protocols. This emphasises the potential of the investigated process for a combined production of producer gas and biochar from a variety of low-value biomass feedstocks.