Methane Exhaust Measurements at Gathering Compressor Stations in the United States

Area of Science:

• Environmental Science
• Atmospheric Chemistry
• Energy Engineering

Background:

• Unburned methane from natural gas compressor engines, known as combustion slip, contributes significantly to station-level methane emissions.
• Accurate quantification of combustion slip is crucial for understanding and mitigating greenhouse gas emissions from the natural gas sector.

Purpose of the Study:

• To quantify methane emission rates from natural gas compressor engines using a novel in-stack tracer gas method.
• To compare measured emission rates with existing emission factors from the U.S. EPA.

Main Methods:

• Employed an in-stack tracer gas method combined with Fourier transform infrared (FTIR) spectroscopy.
• Measured combustion slip from 67 gathering and boosting stations across 11 U.S. states.
• Modeled total annual combustion slip for the U.S. natural gas gathering and boosting sector.

Main Results:

• Mean methane emission rate for 63 four-stroke, lean-burn (4SLB) engines was 5.62 kg/h, 8-9% lower than EPA AP-42 and GHGI estimates.
• Mean methane emission rate for 39 four-stroke, rich-burn (4SRB) engines was 0.40 kg/h, significantly lower than EPA estimates (43% of AP-42, 8% of GHGI).
• Modeled annual combustion slip was 328 Gg/y, accounting for 24% of gathering and boosting station methane emissions.

Conclusions:

• Combustion slip from natural gas compressor engines represents a substantial source of methane emissions.
• Measured emission rates, particularly for 4SLB engines, suggest existing emission factors may overestimate contributions.
• The study provides critical data for refining greenhouse gas inventories and informing emission reduction strategies.