Performance of Low-NOx and Conventional Storage Water Heaters Operated on Biogas and Natural Gas

Abstract

Abstract The U.S. has high renewable methane potential. Biogas can be obtained from many organic sources like landfills, wastewater, animal manure, industrial, institutional, and commercial organic waste. Biogas can also be produced from lignocellulosic biomass; also, there is attention in biofuel and biogas production from various kinds of algae. In the U.S., 5% of the natural gas utilized in the electric power sector and 56% of the natural gas in transportation can be directly replaced with biogas. In a typical Californian household, around 50% of the 354 therms Natural Gas energy supplied goes to water heating. Hence, a large amount of emission from residential spaces is due to water heating units. Introducing renewable natural gas (RNG) or biomethane to natural gas pipelines has shown great potential for greenhouse gas mitigation. However, the upgrade process is energy and cost intensive. Assuming primary biogas composition as methane (CH4) and carbon dioxide (CO2), an understanding of the tolerance for residential appliances (significant pipeline delivery point use) to biogas addition could save cleaning/upgrading costs. Focusing on the combustion performance of two representative models of storage water heaters (low-NOx and conventional) in California, this research addresses how much CO2 in natural gas can be tolerated without losing critical performance parameters for reliable operation. Characteristics like blow-off, ignition time, flame structure, efficiency, and emissions (NOx, NO, N2O, CO, CO2, UHC, CH4, and NH3) at different concentrations of CO2 in natural gas are investigated. The pilot operation becomes unstable for the low-NOx water heater beyond 10% CO2, and the probability of blow-off increases. At both 5% and 10% CO2 addition, a stable though the increasingly flat flame is observed, and pilot operation is stable both during idling and while the main burner relights. For the conventional gas storage water heater, a stable flame is established up to 25% CO2 addition, with the flame becoming increasingly shorter beyond 30% CO2 addition. On the other hand, the stable pilot operation could not be established even at 5% CO2 addition, which proved to be the limiting factor for the operation of this device. A similar trend of NOx/NO decrease and CO/UHC increase with increasing CO2 percentage was observed for both water heaters. Further, methane emissions from water heaters during active and idle operations and pilot light’s role were quantified. The present study show 10% CO2 can be added to natural gas without any significant loss of efficiency for the low-NOx storage water heater. We found that both the water heaters emit CH4 during active and idle operation, of which more CH4 is emitted during the idle process. This study can inform policymakers on allowing higher composition variability for the pipeline gas and research into modifying water heater burners for increased tolerance to biogas with reliable performance and a simultaneous decrease in pollutant emissions.