Effect of Thermal Storage on the Emissions and Efficiency Performance of a Wood-Pellet-Fired Residential Boiler

Wood pellet boilers for residential heating applications offer the promise of low emissions, high efficiency, and automatic operation. However, when operated in the field, these units operate often at very low loads causing them to cycle. In this study, the performance of a 25 kW modern pellet boiler under emulated field conditions and fixed nominal loads of 15 and 100% has been studied in a lab-with and without a buffer tank. A dilution tunnel approach was used for the measurement of particulate emissions, in accordance with US certification testing requirements under EPA Methods 28 WHH and 28 WHH PTS. Results show that increasing the amount of thermal storage used decreases cycling rates leading to decreased emissions and increased efficiency. Without thermal storage, integrated efficiency over a 15% load test period was 57%, compared to 74% when thermal storage was used. Particulate emissions were 180 and 64 mg/MJ for the 15% load case without and with thermal storage, respectively.