Observations of Methane Emissions from Natural Gas-Fired Power Plants

Current research efforts on the atmospheric impacts of natural gas (NG) have focused heavily on the production, storage/transmission, and processing sectors, with less attention paid to the distribution and end use sectors. This work discusses 23 flights at 14 natural gas-fired power plants (NGPPs) using an aircraft-based mass balance technique and methane/carbon dioxide enhancement ratios (ΔCH₄/ΔCO₂) measured from stack plumes to quantify the unburned fuel. By comparing the ΔCH₄/ΔCO₂ ratio measured in stack plumes to that measured downwind, we determined that, within uncertainty of the measurement, all observed CH₄ emissions were stack-based, that is, uncombusted NG from the stack rather than fugitive sources. Measured CH₄ emission rates (ER) ranged from 8 (±5) to 135 (±27) kg CH₄/h (±1σ), with the fractional CH₄ throughput lost (loss rate) ranging from -0.039% (±0.076%) to 0.204% (±0.054%). We attribute negative values to partial combustion of ambient CH₄ in the power plant. The average calculated emission factor (EF) of 5.4 (+10/-5.4) g CH₄/million British thermal units (MMBTU) is within uncertainty of the Environmental Protection Agency (EPA) EFs. However, one facility measured during startup exhibited substantially larger stack emissions with an EF of 440 (+660/-440) g CH₄/MMBTU and a loss rate of 2.5% (+3.8/-2.5%).