Small-scale Pellet Boiler Emissions – Characterization and Comparison to Other Combustion Units

Residential combustion has been identified as one of the main sources of particulate matter (PM); fine particles are important because of their adverse effects on human health and the environment. In this thesis, fine particle emissions originating from residential pellet combustion were characterized in different operational situations and using different raw materials as pellet fuel. Moreover, pellet combustion emissions were compared with those from other combustion technologies used for heat production, including batch-wise operated wood stoves, residentialscale oil boilers and medium-scale plants utilizing wood chips and heavy fuel oil. Particle emissions from pellet combustion in modern pellet boilers are low compared to those of other wood-fueled residential appliances; these particles primarily consist of fine inorganic fly ash. Decreasing air-to-fuel ratio of primary combustion while increasing air-to-fuel ratio of secondary combustion affected the release of fine fly ash without deteriorating the combustion conditions, resulting in significant reductions in fine particle emissions. In low-load conditions, the amount of fine fly ash particles remained constant, but fine particle emissions increased due to soot formation. The pellet raw material largely affected fuel originated fine fly ash emissions, whereas emissions of incomplete combustion remained low. The addition of peat to biomass fuels was found to decrease fine particle emissions due to the increased binding of alkali metals in the bottom ash. The use of bark or straw pellets resulted in elevated fine fly ash particle emissions because of the larger alkali metal contents in the fuel. Thus, only stem-wood-based pellets should be used in current residential-scale combustion technology to prevent additional particle emissions. Combustion technology had a substantial effect on particle and gaseous emissions in the studied batch-wise operated appliances, although the emissions are generally higher than from pellet combustion. In batch combustion, fine particles are mainly formed of elemental carbon and organic matter, although fine fly ash is also present in these particles. Based on the analyzed particle components, polycyclic aromatic hydrocarbons (PAHs) were found to be the most sensitive compound for combustion quality, indicating that the harmfulness of PM from different sources may vary more than what is estimated for particle mass emissions alone. Although particle emissions from light fuel oil combustion are low, this combustion produces more PAHs than pellet combustion. Heavy fuel oil exhibited similar PM emissions as pellet boilers; however, significantly more PAHs and heavy metals were also produced. There are numerous options for reducing emissions using combustion technology and fluegas cleaning. Overall, combustion technology provides the possibility of decreasing emissions from residential combustion; however, firmer emission regulations are needed to increase the utilization of low-emission technologies.