Particulate emissions from a 350 kW wood pellet heater

The particulate mass and size distribution was investigated for a biomass wood-pellet air heater with a direct comparison with an equivalent oil-fired burner using the same cross-flow air heater system. Five wood pellet fuels were investigated from different sources and the influence on particle mass and size distribution was determined. The influence of burner excess air on gaseous and particulate emissions was determined. The optimum excess air for minimum emissions and maximum thermal efficiency for the pellet burner was higher at 42% than for the oil burner at 23%. The thermal efficiency of the pellet heater was determined to be slightly less than that of the oil heater. The main reason for this was operation of the heater at higher excess air levels with pellets. The hydrocarbon and particulate carbon fraction emissions were lower for the pellet burner but the CO and NOx emissions were higher. Composition differences between different pellet manufacturers, due to the use of different wood sources, were significant and this produced significant variation in the stoichiometric A/F, which without oxygen feedback control, resulted in different excess air levels for the same pellet feed rate. This resulted in a significant influence of pellet composition on emissions due to excess air variations. Particulate mass and number emissions were low for the biomass pellet burner and similar to the oil burner, provided both burners were at their optimum excess air operational condition. Particulate emissions increased dramatically if the excess air was reduced to 23%.