Influence of biomass properties on technical and environmental performance of a multi-fuel boiler during on-farm combustion of energy crops

More knowledge on combustion of agricultural crops is needed because of increased interest in using farm-grown biomass for energy production purposes. Presently, uncertainty regarding fuel quality and combustion-related emissions hinders the sustainable development of the agricultural biomass industry. The aim of this study was to evaluate the influence of physicochemical properties of biomass on gas, namely carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), nitrogen monoxide (NO), nitrogen dioxide (NO2), ammonia (NH3), sulfur dioxide (SO2) and hydrogen chloride (HCl), and particulate emissions during on-farm combustion of wood and four dedicated energy crops: short-rotation willow, miscanthus, switchgrass and reed canary grass. Different shapes (pellets, chips and chopped grasses) and harvest seasons (fall and spring) were investigated. The thermal efficiency of the 29-kW boiler (63-75%) varied depending on energy load, excess air and fuel ash content. Wood emitted less CO (57-94%), NOx (64-74%), SO2 (0-93%) and PM (49-82%) during combustion than the four other solid biofuels. Higher emissions from the latter are due to their higher nutrient content. Pelletized products usually generated less CO (27-86%), CH4 (15-98%) and PM (28-50%) than uncompressed materials. Reaching a constant combustion process with chopped biomass fuels turned out very difficult because of their heterogeneous particles and low density. The present work also revealed that delaying harvest from fall to spring reduced NOx (0-11%), SO2 (11-54%) and PM (0-37%) emissions. The 20-60% decrease of several chemical elements in the biomass due to leaching is likely responsible. Wood, willow, fall- and spring-harvested switchgrass and miscanthus pellets and willow chips seemed suitable to be burned in appropriate small-scale combustion units according to fuel indexes evaluation.