Comparison of Emission Characteristics of Small-Scale Heating Systems Using Biomass Instead of Coal

Biomass combustion is mainly used for heat production in a wide range of units. Few research studies have been carried out to investigate the overall performance of small-scale residential heating systems when replacing coal with biomass in these units. Sheffield University (SUWIC) has carried out an extensive series of experimental tests at a small-scale residential heating plant. Various tests were conducted on two units: a wood pellet fired boiler and a coal fired boiler. In addition, mathematical modeling work using FLIC and FLUENT codes was carried out in order to simulate the overall performance of the small-scale biomass and coal-fired heating systems. The main objective of this study was to investigate the changes to the overall performance of the plant (e.g., combustion characteristics and emissions) when replacing coal with wood pellets. The concentrations of CO, NOx, particulate matter, and SO2 in the flue gases were measured. Emission factors for both boilers were also compared. FLIC code integrated with FLUENT was employed to model the combustion process of wood pellets and coal in both boilers. The measured emission data were used in the development and validation of the modeling work. The modeling work helped to demonstrate the relationship between the fuel combustion behavior and pollutants formation in the residential boilers. Results showed that pollutant emissions from both boilers were within relative emission limits whereas pollutant emission factors for the wood pellet-fired boilers were lower than for the coal-fired boiler. The modeling analysis showed that the high CO emissions from the coal-fired boiler were attributed to the less mixing intensity and low residential time of flue gas in the furnace. For small-scale residential boilers, biomass with a high volatile fraction could be an ideal solid fuel to get higher combustion efficiency and low pollutant emissions. This study has provided valuable information that can be used to help improve future furnace design and retrofit existing small-scale heating systems to reduce overall emissions.