The US has high renewable methane potential. Biogas can be sourced from many organic sources like landfill, wastewater, and animal manure, industrial, institutional and commercial organic waste.&nbsp; Biogas can be produced from lignocellulosic biomass, also, there is attention in biofuel and biogas production from various kinds of algae. In the US, 5% of the natural gas utilized in the electric power sector and 56% of the natural gas is transportation can be directly replaced with biogas.&nbsp; 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.&nbsp; 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 (NO, N2O, NO, CO, CO2, UHC, CH4, and NH3) at different concentrations of CO2 in natural gas are investigated. For the Low NOx water heater beyond 10% CO2 addition, the pilot operation becomes unstable and the probability of blow off increases. At both 5% and 10% CO2 addition a stable though increasingly flat flame is observed, and pilot operation is stable both during idling and relight of the main burner.&nbsp; For the conventional gas storage water heater, a stable flame is established up to 25% CO2 addition, with the flame becoming increasingly more compact beyond 30% CO2 addition. Stable pilot operation, on the other hand, could not be established even at 5% CO2 addition which proved to be the limiting factor for operation of this device. A similar trend of NOx and NO decrease and CO and UHC increase with increasing CO2 percentage was observed for both water heaters.&nbsp; 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 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.

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

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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