Effects of Air Staging and Load on Fine-Particle and Gaseous Emissions from a Small-Scale Pellet Boiler

Fine-particle and gaseous emissions from a modem small-scale pellet boiler were studied with different air-staging settings and under different load operations. Commercial wood pellets were used in the boiler, which worked with a top-feed fuel input. Partial load operation experiments included half-load 12.5 kW and low-load 7 kW. In air-staging experiments, the amounts of primary air were decreased by 71 and 82% while simultaneously keeping constant total air/fuel ratios. This was found to result in considerably lower emissions than during normal full-load operation with factory settings. The reduction in fine-particle emissions was based on both a decrease in alkali metal emissions and emissions of unburnt carbonaceous particles. However, when the amounts of secondary air were decreased by 17 and 33%, there were higher emissions of both fine particles and gases during full-load operation, especially of emission components originating from incomplete combustion. The primary air/secondary air ratio correlated with CO, PM1, K, EC, and SO4 emissions. Furthermore, correlations were found between the primary air/fuel ratio and particle geometric mean diameter (GMD) and between the secondary air supply and GMD. These correlations were seen with all measured points, which indicates that they apply to all of the operational situations that were studied. The results show that there is significant potential for decreasing particle emissions from automated pellet combustion systems by optimizing combustion air staging in the furnace.