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Natural Gas Initiative is a cross-campus effort of the Precourt Institute for Energy.

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Methane and NOx Emissions from Residential Stoves and Water Heaters

Event Details:

Thursday, February 10, 2022

With Eric Lebel, Colin Finnegan, and Zutao Ouyang

Methane emissions from natural gas appliances remain the least characterized portion of the fossil-fuel supply chain. Here we examine water heaters and stoves from 35 and 53 California homes, respectively, to quantify methane emissions (and NOx emissions for stoves) from natural gas leaks and incomplete combustion while off, turning on or off, and in steady-state operation. We found that individual tankless water heaters emitted 2390 [95% CI: 2250, 2540] g CH4 yr−1 on average, 0.93% [0.87%, 0.99%] of their natural gas consumed, primarily from on/off pulses. Storage water heaters emitted 1400 [1240, 1560] g CH4 yr−1 on average, 0.39% [0.34%, 0.43%] of their natural gas consumption. Scaling our measured emissions by the number of storage and tankless water heaters in the United States (56.8 and 1.2 million, respectively), water heaters overall emitted an estimated 82.3 [73.2, 91.5] Gg CH4 yr−1, 0.40% [0.35%, 0.44%] of all natural gas consumed by these appliances. Meanwhile, we estimated that natural gas stoves emit 0.8−1.3% of the gas they use as unburned methane and that total U.S. stove emissions are 28.1 [95% confidence interval: 18.5, 41.2] Gg CH4 year−1. More than three-quarters of methane emissions we measured originated during steady-state-off. In addition to methane emissions, co-emitted health-damaging air pollutants such as nitrogen oxides (NOx) are released into home air and can trigger respiratory diseases. In 32 homes, we measured NOx (NO and NO2) emissions and found them to be linearly related to the amount of natural gas burned (r2 = 0.76; p ≪ 0.01). Emissions averaged 21.7 [20.5, 22.9] ng NOx J−1, comprised of 7.8 [7.1, 8.4] ng NO2 J−1 and 14.0 [12.8, 15.1] ng NO J−1. With these rates of NOx emissions, it is possible that cooking processes can exceed the EPA’s 1-hr outdoor standard of NO2 in a small sized kitchen with little to no ventilation.

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