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Compressor engine methane emissions
Exhaust methane from gathering compressor drivers, estimated from direct measurement of 133 engines rather than from a catalogued factor. Pick your engine type and see what the measured factor and the three published inventory factors each say about the same fleet.
Factors last reviewed
The numbers, in one place
| Quantity | Value | Source |
|---|---|---|
| 4SLB measured emission factor | 5.62 kg CH4/h/unit | Vaughn et al. 2021, Table 2 |
| 4SLB fuel-input basis | 1.15 lb CH4/MMBtu | Vaughn et al. 2021, Table 1 |
| 4SRB measured emission factor | 0.4 kg CH4/h/unit | Vaughn et al. 2021, Table 2 |
| GHGRP factor, same 4SLB engine | 0.01 kg CH4/h/unit | GHGRP Subpart C, as applied in Table 2 |
| Gap between them | 562x | Measured against reported |
Your fleet
Costed per unit from the study's measured hourly rates.
What each emission factor says
The same fleet, costed five ways. The amber rows are what was measured in the field. The others are the published factors an inventory would otherwise apply.
kg CH4 per hour on a logarithmic axis. Each gridline is a tenfold step, which is the only way to show factors that differ by more than two orders of magnitude on one chart.
| Emission factor | kg CH4/h | t CH4/yr | t CO2e/yr | Against measured |
|---|---|---|---|---|
| Study measured | 5.77 | 50.5 | 1,415 | - |
| Study emission factor | 5.62 | 49.2 | 1,378 | - |
| AP-42 | 6.13 | 53.7 | 1,504 | 1.1x higher |
| GHGI | 7.2 | 63.1 | 1,766 | 1.3x higher |
| GHGRP | 0.01 | 0.088 | 2.45 | 562x lower |
Read this before you use the number
- Crankcase venting is not included
- None of these estimates cover unburned methane from crankcase ventilation systems. On 4SLB engines the authors note this may add another 3 to 20 percent.
- Every 4SRB tested had after-treatment
- All 4SRB engines in the study were equipped with non-selective catalytic reduction. The AP-42 factor is based on tests of units that did not have exhaust after-treatment, so an uncontrolled 4SRB is not represented by the study factor.
- Two 4SLB subtypes behave very differently
- The study found markedly different emissions from two groups of engines that both fall under the single AP-42 4SLB classification. Pre-chamber ultra-lean combustion appears to be the distinguishing characteristic. A fleet weighted towards one subtype will not match the class average.
- Individual engines vary enormously
- Across the 63 lean-burn units measured, emission rates ran from 0.3 to 12.6 kg of methane an hour, a fortyfold spread. The rich-burn units ran from 0.01 to 4.5. A class average describes a fleet, and it will be wrong for most single engines in it. If a specific unit matters, measure it.
- These are rates while running
- A factor in kg per hour applies to an operating engine. Annual totals here multiply by the operating hours you enter, so entering 8,760 assumes the unit never stops.
Why one engine has four emission factors
Combustion slip is fuel that leaves the exhaust unburned. It is invisible to a leak survey, because nothing is leaking, and it is not what a stack test for criteria pollutants looks for. So it has largely been carried in inventories by factor rather than by measurement, and the factors were built at different times for different purposes.
This study put instruments on 133 compressor drivers at 67 gathering stations, across 11 states and 9 operators. It measured exhaust flow with an in-stack tracer and methane concentration by FTIR, then cross-checked the flows against EPA Method 2 and Method 19. The result is the only one of these four factors that comes from recent, direct, facility-scale measurement.
the GHGRP combustion slip emission factor is inadequate for internal combustion engines and should not be used in the GHGI
Vaughn et al. 2021, Results and Discussion
The gap is not uniform. AP-42 lands close to measurement for lean-burn units. For rich-burn units the measured factor is 43 percent of the AP-42 value, which the authors attribute to every rich-burn engine in the study carrying catalytic after-treatment while the AP-42 factor describes uncontrolled engines. The GHGI factor applies a single value to all reciprocating engines. Only the GHGRP factor is wrong by orders of magnitude, in the same direction, for every engine class.
Methane combustion slip by engine type
Per unit, in kilograms of methane per operating hour, with 95 percent confidence intervals. The study measured four-stroke lean-burn and four-stroke rich-burn engines only, so those are the only two columns that exist.
| Basis | 4SLB (kg CH4/h) | 4SRB (kg CH4/h) |
|---|---|---|
| Study measured | 5.77 (5.03 to 6.5) | 0.4 (0.24 to 0.68) |
| Study emission factor | 5.62 (5.15 to 6.17) | 0.4 (0.37 to 0.42) |
| AP-42 | 6.13 (5.63 to 6.73) | 0.94 (0.87 to 1) |
| GHGI | 7.2 (6.55 to 7.97) | 5.09 (4.72 to 5.41) |
| GHGRP | 0.01 (0.01 to 0.01) | 0.01 (0.01 to 0.01) |
Vaughn et al. 2021, Table 2. Averages across the tested units.
On a fuel-input basis
The same comparison in pounds of methane per MMBtu of fuel burned. This is the only basis that covers two-stroke lean-burn engines and gas turbines, neither of which this study measured.
| Basis | 4SLB | 4SRB | 2SLB | IGT |
|---|---|---|---|---|
| Study | 1.15 | 0.1 | - | - |
| AP-42 | 1.25 | 0.23 | 1.45 | 0.0086 |
| GHGI | 1.27 | 1.27 | 1.27 | 0.0044 |
| GHGRP | 0.002 | 0.002 | 0.002 | 0.002 |
Vaughn et al. 2021, Table 1, in lb CH4/MMBtu on a fuel-input basis. A dash means that basis publishes no factor for that engine class.
What the difference is worth nationally
Scaled to US gathering compressor units, the choice of factor moves the national total by more than two orders of magnitude. The study estimate of 328 Gg a year accounts for 24 percent of the 1,391 Gg the 2020 US inventory attributes to gathering and boosting stations, and 6 percent of net emissions for natural gas systems. Applying the reporting-program factor the same way gives 1 Gg. Combustion slip reported in the 2020 GHGI for gathering and boosting, 374 Gg, falls within the uncertainty of the study model.
| Basis | kg CH4/h | Gg CH4/yr | 95% CI |
|---|---|---|---|
| Study | 37,437 | 328 | 235 to 436 |
| AP-42 | 49,367 | 432 | 306 to 576 |
| GHGI | 72,779 | 638 | 447 to 850 |
| GHGRP Subpart C | 125 | 1 | 1 to 1 |
Vaughn et al. 2021, Table 3. Gathering compressor units only.
Questions
What is methane combustion slip?
It is fuel gas that passes through an engine without burning and leaves in the exhaust. It is not a leak and not a vent, so leak surveys do not find it, and it is separate from the carbon dioxide produced by the fuel that did burn.
What is the methane emission factor for a 4SLB compressor engine?
The factor developed from direct measurement of 63 four-stroke lean-burn units is 5.62 kg of methane per hour per unit, with a 95 percent confidence interval of 5.15 to 6.17. On a fuel-input basis that is 1.15 lb of methane per MMBtu.
Why does the GHGRP give such a different answer?
The Greenhouse Gas Reporting Program applies one combustion slip factor to every internal combustion driver, which works out to 0.01 kg per hour for a 4SLB against a measured 5.62. That is roughly 562 times lower. The study authors concluded it "the GHGRP combustion slip emission factor is inadequate for internal combustion engines and should not be used in the GHGI".
Does this include crankcase emissions or seal vents?
No. These figures are exhaust only. Crankcase ventilation was not measured and the authors note it may add another 3 to 20 percent on lean-burn engines. Rod packing and seal vents are separate sources entirely and are not covered here.
Which factor should I use for reporting?
Use the one the programme you report under specifies, which for US federal greenhouse gas reporting is the GHGRP factor whether or not it matches measurement. Use the measured factor when you are estimating what a site actually emits, for example for OGMP 2.0 Level 4 or 5 reconciliation, mitigation planning, or checking an inventory against a measurement campaign.
Source
- Methane Exhaust Measurements at Gathering Compressor Stations in the United States
- Vaughn, T. L.; Luck, B.; Williams, L.; Marchese, A. J.; Zimmerle, D. Environmental Science & Technology 2021, 55(2),
1190-1196. Open access under CC BY-NC-ND. doi:10.1021/acs.est.0c05492
A novel in-stack tracer gas method based on EPA Method ALT-012, coupled with FTIR species measurement, on engines operating as found under normal daily circumstances.
Machine-readable copy of every factor on this page, with provenance: /data/engine-emission-factors.json
Modelling a whole site, not one engine
Combustion slip is one source among dozens at a compressor station. MAES models a site mechanistically, equipment by equipment, which is what OGMP 2.0 Level 4 and 5 reporting asks for.
See how MAES models a site