Kaolin as additive in wood pellet combustion with several mixtures of spruce and short-rotation-coppice willow and its influence on emissions and ashes

In this study ten mixtures of spruce (Picea abies), short-rotation-coppice (SRC) willow (Salix spec.; Clone Tordis) and kaolin were pelletised and burned in a residential biomass boiler (12 kW). Focus is on usability of SRC willow in exchange of forestal biomass (spruce), as well as on kaolin as fuel additive for particulate emission reduction in small-scale biomass combustion. Moreover, fuel blends were intended to match quality criteria for the European market according EN 14961-2. Initially, ten raw material mixtures were pelletized and the produced blend pellets were physically and chemically characterized. Combustion experiments with these fuel blends show differences in gaseous emissions, chemical composition of emitted particles and boiler ash. Kaolin addition reduced emitted particle mass even in lowest kaolin concentrations (0.2 wt.%) in every combination of spruce and SRC-willow. Chemical analysis shows decreased K-contents in the collected particles for fuels with kaolin addition, but an increased share of Zn.