Air Starters

Natural gas is typically used to start internal combustion engines for compressors, generators, and pumps. Replacing natural gas with compressed air can eliminate emissions of methane from engine starts.

In the natural gas industry, internal combustion engines for compressors, generators, and pumps are often started using small gas expansion turbine starter motors. Natural gas is typically used to start these engines. Replacing natural gas with compressed air eliminates emissions of methane from engine starts. This practice simply fills the startup volume tank with compressed air to support the frequency of engine startups. No facility changes are necessary except for a high-pressure air fill connection.  

When using air as the startup gas, either the high-pressure startup gas system must be very tight (i.e., no leakage) or the air supply must be made just prior to startups to ensure an adequate volume of high-pressure air is available. Resupply of compressed air must be arranged on a schedule coinciding with the required frequency of engine startups. 

While use of compressed air does eliminate methane emissions from the starter mechanism process, it must be recognized that unnecessary methane emissions can still result from operational inefficiencies due to failed starts, shutdowns, and restarts. A failure during loading the compressor in an initial start may require unloading or venting the compressor casing, resulting in additional potential emissions dependent on the outlet of that vent.   

This practice is applicable to all compressors with natural gas expansion turbine engine starters. A stationary or mobile air compressor is required for this practice.  

Where available from a gas processing plant nitrogen rejection unit, nitrogen can also be substituted for compressed air.

Methane emission reductions can be determined by taking the difference in emissions from the source before and after the specific mitigation action was applied. For replacing natural gas with compressed air for engine startups, this means calculating emissions from starts using natural gas and subtracting zero (because air starters do not emit methane). While using actual measurements may provide a more accurate representation of emissions/reductions from individual equipment at a given time, emissions from starts can be reasonably calculated using an emission factor as follows. 

ER = C × EF 

Where: 

ER = Emissions reduction estimate (kg CH₄/year) 

C = Number of compressors (i.e., count of compressors that start using natural gas) 

EF = Emission factor (kg CH₄/yr/compressor) 

Assumptions: 

• Use the most current “compressor starts” emission factor. Emission factors are generally developed to be representative of long-term averages for all applicable emission sources. EPA updates the emission factors from the Natural Gas Systems section of the Inventory of U.S. Greenhouse Gas Emissions and Sinks (“Greenhouse Gas Inventory”, or “GHGI”) every year so specific emission factors may change. To find the current emission factor, navigate to the GHGI website for Natural Gas and Petroleum Systems and click on the page for the most recent inventory. On that page, you will find links for Annex 3.5 (Methodology for Estimating CH₄, CO₂, and N₂O Emissions for Petroleum Systems) and Annex 3.6 (Methodology for Estimating CH₄, CO₂, and N₂O Emissions for Natural Gas Systems). Methane emission factors can be found in Table 3.5-3 (Petroleum Systems) and Table 3.6-2 (Natural Gas Systems).   
• The GHGI emission factor for compressor starts includes assumptions on the number of compressor starts per year. This emission factor does not include emissions from unloading the compressor (blow down) before each start, or fugitive emissions from leaking volume tank connections and valves. 

The calculation methodology in this emissions reduction section is based upon current information and regulations (as of August 1, 2023). EPA will periodically review and update the methodology as needed.

In addition to reducing emissions of methane, using air starters in lieu of natural gas starters may:  

• Reduce air pollution: The combustion of natural gas produces air pollutants, such as nitrogen oxides, sulfur oxides, carbon monoxide, volatile organic compounds, and hazardous air pollutants.   
• Reduce gas starter corrosion and maintenance costs: If sour gas containing significant amounts of hydrogen sulfide is used to start the engine, this can cause equipment to rust and deteriorate. 

Almasi, A. (2018, December 17). Considerations for reciprocating compressors for processing units. Processing Magazine. https://www.processingmagazine.com/pumps-motors-drives/article/15587660/considerations-for-reciprocating-compressors-for-processing-units

Brun, K. & Kurz, R. (2019). Compression machinery for oil and gas. Gulf Profession Publishing. https://doi.org/10.1016/C2017-0-02064-5

Chemical Processing. (2019, August 28). Treat your reciprocating compressor right. https://www.chemicalprocessing.com/processing-equipment/motors-drives/article/11304977/treat-your-reciprocating-compressor-right