Investigation of the utilization of forest woodchips in a commercial small-scale open-top gasifier

Abstract

The present work investigates the gasification of forest woodchips, which also contain needles, branches and cones, in an 85-kW open-top gasifier with 60 %–100 % load. This lower quality fuel has higher contents of ash, bark and fines compared to typical requirements of commercial small-scale gasifiers. The woodchips were utilized in unsieved and sieved (>10 mm) fraction. By using gas analysis and temperature measurements at a high local and temporal resolution, three main novel results were found: 1) The results of this work confirm that preferential paths in the fuel bed, caused by the fines content, led to a worse gas quality. The positive effects of a load reduction (i.e., increasing hydrogen and decreasing tars contents) were not found with the unsieved fuel. 2) Opposite trends for the methane concentration with the load were found with unsieved and sieved fuel. The preferential paths probably prevented the complete decomposition of longer-chain hydrocarbons and increased the intermediate product methane, while the non-methane hydrocarbon content reduced at a similar rate with both fuel fractions. 3) When using unsieved fuel, reducing the load massively decreased the fluctuations of the air mass flow and the total hydrocarbon content. With sieved fuel, however, a satisfying process stability was already achieved in all loads. Therefore, an appropriate load management was identified to enable the utilization of fuels with higher fines contents. With these new findings, forest chips in unsieved and sieved form can be successfully used in small-scale open-top gasification.