Parameters influencing wet biofuel drying during combustion in grate furnaces

Abstract

Mainly grate-fired furnaces are used for heat production, as these systems are designed for high thermal efficiency with low emission levels of gaseous pollutants when firing good quality biomass with 30–45% moisture content. However, high demand for biomass during the winter makes it necessary to necessitates use wet biofuel of lower quality with varying moisture content that can reach up to 60% wt. causing the burning instability and incomplete combustion. These negative effects could be avoided by optimizing the furnace control to intensify biomass drying on the grates, but the parameters affecting wet biofuel drying must be known. In order to expand the knowledge of the wet biofuel drying process and establish the parameters that influence the drying process most significantly, a special rig based on characteristics of a 6 MW grate furnace was produced and drying experiments simulating the primary air flow in the furnace, the recirculation of the flue gases and the primary air flow together with exposure to thermal radiation were performed. The results revealed that the radiation from surfaces has the biggest impact on biomass drying when preheated air of the temperature of 200 °C is supplied and the moisture content of the biomass is reduced to 0.66 of its initial value. It was also determined that the drying intensity depends on the mixing period and the drying intensity can be increased by up to 15% by additional mixing. Influence of the drying agent temperature depending on the particle size was established as well.