Influence of primary and secondary air supply on gaseous emissions from a small-scale staged solid biomass fuel combustor

Abstract

The emissions from traditional biomass combustion systems for cooking and heating are, globally, the main cause for premature mortality as a result of air pollution. A staged combustion process that separates the thermochemical conversion of the solid fuel and the combustion of the released products offers potential to reduce harmful emissions for solid fuel combustion and could, therefore, help mitigate the issue. In the present study, the fundamental combustion behavior of a small-scale staged combustor was investigated, with a focus on an independent systematic analysis of relevant parameters. Natural and forced draft conditions as well as a combination of both were tested. The relative location of primary to secondary air was also varied. When lighting the fuel, higher air flows lead to faster ignition and lower emissions. A steady-state combustion phase is achieved when gasification products are burned with secondary air, which occurs mainly while the solid fuel is being pyrolyzed. After the steady-state phase, char remains as the solid pyrolysis product. Gasification of the remaining char was found to release great amounts of CO, which are emitted from the combustor, in the case of natural draft secondary air (SA). With higher air flows of forced SA, an exceptionally high nominal combustion efficiency [NCE = XCO2/(XCO + XCO2)] can be achieved in the steady-state phase. Forced SA flows cause a longer duration of the steady-state phase from the combustion of raw biomass gasification products into the combustion of char gasification products. This extension leads to a significant reduction of emissions of incomplete combustion. Additionally, smaller distances between the SA inlet and the fuel stack caused lower emissions of incomplete combustion. The combination of forced draft primary air and natural draft SA presented worse combustor performance than under natural draft conditions.