Influence of pellet length, content of fines, and moisture content on emission behavior of wood pellets in a residential pellet stove and pellet boiler

Abstract

In Germany, wood pellets must fulfill high quality standards to be utilized in small-scale furnaces. According to current legislation, i.e., the 1st BImSchV, only ENplus A1–certified pellets may be used. Despite these strict requirements, physical fuel parameters can vary within the permissible range and variation usually increases due to fuel transport and storage. Fluctuations in physical fuel parameters may have a strong influence on combustion, especially in pellet stoves with a low degree of automation regarding fuel control and air supply. During this study, pellet length, moisture content, and the content of fines of spruce wood pellets were varied artificially at three levels for each parameter. All pellets were analyzed according to international standards for solid biofuels. Fuels were combusted in a 6 kW pellet stove and selected assortments in a 15 kW pellet boiler. For the pellet stove, pellet length had a significant influence on gaseous but not on total particulate matter (TPM) emissions. Both, rather short (10.3 mm) and long (17 mm) pellets caused an increase in emissions. In contrast, for the boiler, no significant effect of pellet length on gaseous and TPM emissions was observed. A low moisture content (3 w-%) led to increased CO, organic gaseous carbon (OGC), and TPM emissions in the pellet stove. This could be especially relevant for furnaces with very simple or no heat output control as it is common in most pellet stoves. High content of fines (3 to 10 w-%) led to a significant increase in gaseous emissions (CO up to fivefold, OGC up to tenfold) and TPM emissions (up to 1.4-fold) in the pellet stove most likely due to elevated particle entrainment. Overall, the state-of-the-art pellet stove showed larger sensitivity towards variable pellet qualities compared to the automatically stoked pellet boiler; thus, improvements in fuel design and stove technology are recommended. Overall, this study has provided important insights into the influence of pellet length, fines content, and moisture content on emissions from pellet stoves and pellet boilers, which can help drive future technical development in the areas of fuel flexibility and emission reduction.