Torrefied and pyrolyzed pellets and their environmental impact in household heating

The transition from original feedstock to torrefied and pyrolyzed biofuels was investigated on a small-scale automatic pellet boiler. Thermal treatment significantly improves fuel quality; however, several technological difficulties were encountered. The entrainment of unburnt fuel particles led to severe CO, TOC and PM emissions as well as a drop in power output. All these issues were exacerbated when firing herbaceous biomass. It was found that most of the emitted particles are smaller than 1 μm in aerodynamic diameter, which necessitates additional flue gas filtering. Furthermore, 16 selected PAHs were individually monitored to evaluate the effect of thermal treatment on PAHs emissions and composition. On average, the most abundant PAH was naphthalene (4135 μg·kgfuel−1), followed by phenanthrene (246 μg·kgfuel−1), acenaphthylene (145 μg·kgfuel−1), fluoranthene (97 μg·kgfuel−1) and pyrene (95 μg·kgfuel−1). Benzo(a)pyrene contributed minimally to the total PAHs emissions (on average 16.6 μg·kgfuel−1). The 16 selected PAHs constitute only about 0.1–1.1 % of the total organic carbon emitted from solid biofuel combustion.