The EPA believes methane emissions from compressors are under reported. Considering this, the EPA implemented several changes and updates to calculation methodologies to account for the potentially overlooked volumes.
New Approach to Calculations
For existing methodologies, the EPA will only accept calculations on a per compressor basis versus previous acceptance of reporting a total engine count for an operating basin. Operators can now elect to directly measure emissions from compressors or use the default emission factors. By expanding the methodologies to individual compressors, runtime hours and natural gas composition can and must be considered.
Added Categories
In addition to updated methodologies, two new categories specific to compressors have been added in the form of crankcase venting and methane slip of the natural gas used to operate the engine.
Crankcase venting will be calculated per compressor with a standard emission factor of 0.83 kilogram of methane per hour per engine. Similar to the above-mentioned hours, operating hours are represented in the calculation.
Methane, or combustion slip, is a term to reference how efficient natural gas compressor engines are at combusting fuel to run. EPA claimed through general studies that different engine types had a corresponding kg of methane per million British thermal unit in the table below.
Engine type (stroke & rich burn/lean burn) | EF (kg CH4/MMBTU) |
2 LB | 0.658 |
4 RB | 0.045 |
4 LB | 0.522 |
2025 emissions from combustion of compressor engines can now be calculated one of two ways:
Using the emission factors above based on engine type
Utilize test data from unit stack testing (i.e. Method 320), generally for JJJJ compliance.
If an engine has been tested in the reporting year, the report MUST be used to calculate emissions from combustion. If no test was done, default emission factors based on the engine type will be used in the calculation below (BSCF, HHV of gas analysis, HP of engine, time run).
With the emission factors provided above, several example calculations have been completed to demonstrate the significant increase in emissions utilizing the methane slip emission factors:
Engine Model | HP | bscf (BTU /hp-hr) * | FHV (btu/scf)* | hours run per year | CH4 emissions (tpy) |
3516-TALE (4LB) | 1,340 | 9,498 | 1,552 | 8760 | 58.2 |
G3508 ULB (4LB) | 690 | 7,700 | 1,552 | 8760 | 24.3 |
G3516B (4LB) | 1,380 | 8,500 | 1,552 | 8760 | 53.6 |
CG137-8 (4RB) | 400 | 8243 | 1552 | 8760 | 1.3 |
G3306TA (4RB) | 211 | 9006 | 1552 | 8760 | 0.75 |
*Generic numbers
By comparison, if those same four stroke lean burn engines above were tested and met the general requirement of 0.7 grams/hp-hr of VOC for JJJJ the emissions would be considerably less. Using the total hydrocarbons (THC) breakout emission factor to encompass the methane, the emission factor would range from 0.5 to 2 g/hp-hr. The estimated emission reduction from utilizing the tests could reduce emissions by engine by 45% to 88%.
Engine Model | THC emissions (tpy) [based on 0.5 g/hp-hr] | THC emissions (tpy) [based on 2 g/hp-hr] | Reduction Range (%) |
3516-TALE (4LB) | 6.47 | 25.87 | 55 to 88 |
G3508 ULB (4LB) | 3.33 | 13.32 | 45 to 86 |
G3516B (4LB) | 6.66 | 26.65 | 50 to 87 |
Recommendations:
Get all four stroke lean burn engines stack tested every year following Methods 320, 18, 25A, or ASTM D6348-12.
Ensure that your stack tester contractors include methane and/or the THC in the summary portion of the report for accessibility
Consider stack testing two stroke lean burn engines if applicable
Push owner companies of natural gas driven engines to adhere to catalyst checks and replacement schedules
Keep records of engine run times to get an accurate hourly run time for the year
