Gradual combustion of wood logs by the use of preheated air

Abstract

The tendency to automate operation and minimize human involvement in small-scale biomass boilers has led to an increase in pellet usage. Standardized biomass-derived solid fuels are getting a bigger share in the market. In this battle, traditional firewood is not yet ready to give in. Compared with the standardized fuels, the processing of firewood is cheaper, emits smaller amounts of parasite emissions whereas complete combustion and automation require further development. To achieve these requirements, wood log gasification is a promising intermediate step. The paper aims to design gasification and combustion chambers for a 25-kW wood-log-fired water boiler. For both processes, preheated air is used. Its appropriate distribution and preheating are the main tasks that are realized by the use of a CFD model. In the gasification chamber, the oxidation should take place at least 20 cm before the introduction of the secondary (combustion) air, which is introduced by the use of many openings. Their numbers and positions are envisaged to achieve as complete as possible combustion in a larger area and to divert the flame. A gradual introduction of air in a larger area should reduce carbon monoxide (CO), nitrogen oxides (NOx), and the emissions of volatile organic compounds (VOC), whereas the diversion of the flame should reduce the emissions of particulate matter.