Fine Particle and Gaseous Emissions from a Small-Scale Boiler Fueled by Pellets of Various Raw Materials

The market of pellet fuels has undergone drastic changes in Europe in recent years. Traditional raw materials have become inaccessible due to unavailability or dramatic increases in prices. It is possible to use other feasible raw materials for pellet production. However, novel raw materials can have disadvantages with respect to usability and emissions, and these issues need to be considered before such raw materials are taken into wide-scale production. Such factors are especially important in small-scale pellet combustion. Fine particle and gaseous emissions from 21 pellet fuels of various raw materials were studied in a residential pellet boiler. These fuels represented a wide range of stem wood, bark, and forest residues from different tree species, as well as peat, agricultural biomass, and different mixtures of these. Fuel characteristics, fine particle mass (PM₁) emissions, number emissions, number size distribution, PM₁ chemical composition, and the main gaseous emissions were measured. The smallest PM₁ emissions were measured from peat pellets, followed by wood-peat pellets and woody biomass pellets containing only small amounts of bark. Peat also reduced the PM₁ and CO emissions when blended with woody biomass pellets. Pellets containing bark produced clearly higher PM₁ emissions due to the higher content of ash-forming elements, such as potassium, chlorine, and sodium, in the fuels. The highest PM₁ emissions were seen with straw pellets. Linear correlations were found between emissions and fuel characteristics: fuel K content, fuel K+Na+Cl content, and fuel (Na+K)*2Cl/S molar ratio being the most important parameters. The results show that biomass raw materials significantly affect particle emissions in combustion. The addition of peat to biomass can result in lower particle emissions but higher emissions of SO₂ and HCl.