Particulate concentrations during on-farm combustion of energy crops of different shapes and harvest seasons

The increasing energy costs and environmental concerns of farms have motivated the growing interest of agricultural producers in using farm-grown biomass as a substitute to fossil fuels for heat production. However, the use of non-woody biomass is facing challenges due to variability regarding chemical composition and fuel properties that may induce problems during combustion such as particulate matter (PM). The aim of this work was to measure and compare total PM concentrations during on-farm combustion of wood and four agricultural crops: short-rotation willow, switchgrass, miscanthus and reed canary grass. In order to study the influence of physicochemical properties, different shapes (pellets, chips and chopped grasses) and harvest seasons (fall and spring) were also evaluated. In this context, a representative small-scale (29 kW), multi-fuel boiler for light commercial use was utilized. The boiler was also non-catalytic so that the burning took place in a single combustion chamber. Overall, twelve different biomass fuels were tested and each product was burned three times. Mean PM concentration of wood (416 mg Nm−3 at 7 vol% O2) was lower than that of the four dedicated energy crops (505-1417 mg Nm−3 at 7 vol% O2). However, because of the high variability between the experiments, no statistical significance was observed at P > 0.1 level except in one case. The PM amounts were high compared to literature data and Quebec's environmental regulation mainly because of the boiler system used. Except for willow, pelletized products decreased PM levels by 22-52% compared to chopped materials. Bulky biomass of low density was unable to reach steady-state conditions and produced compounds associated with incomplete combustion including PM. Spring-harvested biomass fuels showed a PM reduction up to 48% compared to fall-harvested crops. This was likely due to a 20-60% decrease of several chemical elements in the biomass, namely S, Cl, K and P which are the main constituents of fly ash.