Methane Exhaust Measurements at Gathering Compressor Stations in the United States

Unburned methane
entrained in exhaust from natural gas-fired compressor
engines (“combustion slip”) can account for a substantial
portion of station-level methane emissions. A novel in-stack, tracer
gas method was coupled with Fourier transform infrared (FTIR) species
measurements to quantify combustion slip from natural gas compressor
engines at 67 gathering and boosting stations owned or managed by
nine “study partner” operators in 11 U.S. states. The
mean methane emission rate from 63 four-stroke, lean-burn (4SLB) compressor
engines was 5.62 kg/h (95% CI = 5.15–6.17 kg/h) and ranged
from 0.3 to 12.6 kg/h. The mean methane emission rate from 39 four-stroke,
rich-burn (4SRB) compressor engines was 0.40 kg/h (95% CI = 0.37–0.42
kg/h) and ranged from 0.01 to 4.5 kg/h. Study results for 4SLB engines
were lower than both the U.S. EPA compilation of air pollutant emission
factors (AP-42) and Inventory of U.S. Greenhouse Gas Emissions and
Sinks (GHGI) by 8 and 9%, respectively. Study results for 4SRB engines
were 43% of the AP-42 emission factor and 8% of the GHGI emission
factor, the latter of which does not distinguish between engine types.
Total annual combustion slip from the U.S. natural gas gathering and
boosting sector was modeled using measured emission rates and compressor
unit counts from the U.S. EPA Greenhouse Gas Reporting Program. Modeled
results [328 Gg/y (95% CI = 235–436 Gg/y) of unburned methane]
would account for 24% (95% CI = 17–31%) of the 1391 Gg of methane
emissions for “Gathering and Boosting Stations”, or
6% of the net emissions for “Natural Gas Systems” (5598
Gg) as reported in the 2020 U.S. EPA GHGI. Gathering and boosting
combustion slip emissions reported in the 2020 GHGI (374 Gg) fall
within the uncertainty of this model.