Feedstock dependence of emissions from a reverse-downdraft gasifier cookstove

Abstract

The present study investigates the combustion process of the producer gas from a gasifier cookstove, for four solid biomass fuels: wood pellets (WP), wheat straw (WS), sheep manure (SM) and cow manure (CM). It was found that more primary air and/or a deeper fuel bed reduces tars in the producer gas and increases the combustion efficiency, especially from low-ash-containing WP. At higher air supply rates, indications of a strong influence from the fuel ash content on the emissions were found. Although more combustible gases and fewer tars are produced in the conversion process, a substantial increase in particulate matter (PM) emissions is noted. At low air supply rates, the emissions of particulates with an aerodynamic diameter ≤2.5 μm (PM2.5) released from the combustion process are in the range of 6–30 mg⋅MJreleased⁻¹ (WP<WS<SM<CM), low when compared with similar devices. However, when the air supply is increased by a factor of three, the PM₂.₅ emissions almost double for WP and increase more than ten fold for CM. At lower air supply rates, low emissions of both PM and CO are achieved. This is likely due to lower peak temperatures (reducing ash devolatilisation) and larger char yields (to retain ash particulates) from the thermochemical solid biomass conversion process. This shows that low air supply rates and the combined production of heat, for cooking, and char, for subsequent applications, may achieve substantial benefits for the emissions of pollutants from gasifer cookstoves.