Influence of wood species and additives on emission behavior of wood pellets in a residential pellet stove and a boiler

Wood pellets on the German pellet market mainly consist of spruce wood, but other wood species are increasingly used during pellet production. Moreover, additives such as starch or kaolin can be added to improve fuel quality and combustion behavior. Consequently, wood pellets vary in their physical and chemical properties and this variation might affect combustion behavior in stoves and boilers. In this study, eight different wood species were used for pellet production at the Technology and Support Centre (TFZ). Three types of starch and one type of kaolin were added to spruce wood without bark, using two additivation levels (starch: 0.5 w-% and 1.8 w-%, kaolin: 0.17 and 0.34 w-%). All pellet assortments were analyzed according to international standards for solid biofuels and additional methods. Fuels were combusted in a 6-kW pellet stove and a 15-kW pellet boiler. Selection of wood species had a significant impact on CO emissions for the pellet stove ranging between 85 and 4189 mg/m(3) and on TPM emissions ranging between 23 and 263 mg/m(3). Similar combustion behavior was observed for the pellet boiler but on a lower emission level. Kaolin reduced CO and TPM emissions as well as slag formation when the additivation level was set according to stoichiometric calculations. When kaolin was overdosed, it had a negative impact on TPM emissions mainly in the pellet boiler, probably due to entrainment. Regarding particles from incomplete combustion, a negative impact of overdosed kaolin was observed. Overall, at the current state of the art, the pellet stove showed larger sensitivity towards variable pellet qualities compared to the automatic pellet boiler. This calls for further improvements in fuel design and stove technology.