Unburned Methane Emissions from Residential Natural Gas Appliances

Methane, the primary component of natural gas (NG), is a potent greenhouse gas. NG is a common fuel for residential appliances because of low cost, high energy density, and relatively clean combustion. NG exhaust contains some unburned methane due to inevitable incomplete combustion. A field campaign measuring methane concentrations in exhaust from residential NG appliances was conducted in Boston and Indianapolis to determine their contribution to overall emissions. NG space heating, water heating, and cooking appliances were measured in 100 homes. Appliance exhaust typically exhibits a brief methane concentration spike during ignition and extinguishment and relatively low concentrations during steady-state operation. Exceptions to this pattern include ovens, suboptimal stove burners, and tankless water heaters, which either have a different operating pattern or nontrivial steady-state concentrations. Findings were combined with appliance usage and prevalence assumptions to estimate total emissions. Annually, ∼30 [97.5% CI: 19-160] Gg of methane emissions can be attributed to U.S. residential NG appliances, corresponding to ∼830 [530-4500] Gg carbon dioxide equivalent (CO₂e₁₀₀). This accounts for ∼0.1% [0.08-0.7%] of U.S. anthropogenic methane emissions (which account for ∼10% of total U.S. greenhouse gas emissions) and corresponds to an emission factor of 0.38 g/kg of NG consumed (0.038% [0.024%-0.21%]).