Electric Compressor Motors

Internal combustion engines are used in the natural gas industry in many applications and are often powered using a portion of the produced gas stream. Using internal combustion engines can result in methane emissions from leaks in the fuel gas supply lines to the engine, compressor blowdown for re-start, gas starters for the engine, and incomplete combustion. Installing an electric motor in place of a gas-fired engine can reduce methane emissions, increase operational efficiency, and reduce maintenance costs.

Internal combustion engines are used in the natural gas industry primarily in remote locations where there is no reliable electrical power. Applications include gas compressors, liquid pumps, and electrical generators at wellheads, on drilling rigs, in gathering/boosting stations, processing plants, and transmission compressor stations. Electric motors require less maintenance than internal combustion engines and can result in improved efficiency and reliability. The capital costs and the energy costs (i.e., electricity versus natural gas fuel) are higher for an electric motor compared to those for a gas-driven engine. The economics may vary significantly for transmission companies because of contractual agreements.

The major technical consideration for conversion from gas-fired engines to electric motors to drive compressors and pumps is the availability of an uninterrupted electrical power supply. While major facilities often have an existing power supply or their own power generation system, many smaller and remote facilities do not. For these remote facilities with a single compressor or pump (e.g., wellheads), the installation of power transmission lines in addition to an electric motor may not be economically feasible or reliable. However, remote facilities with an available and reliable electrical power source and spare equipment (e.g., one electric motor driver with a back-up gas engine), such as gas gathering, processing and transmission stations, may be good candidates for this technology.

Methane emissions 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-powered engines with electric motors, this means calculating the unburned hydrocarbon (methane) emissions from the natural gas engines, plus gas starter vents and fugitive emissions, and subtracting zero (because electric motors do not emit methane). Compressors are still depressurized (blown down) before restarts with electric motors, but restarts are more reliable than using gas starters. While using actual measurements may provide a more accurate representation of emissions/reductions from individual equipment at a given time, emissions from natural gas-powered engines can be reasonably calculated using emission factors as follows.  

ER = (ES × T × EF_C) + (C × EF_S)  

Where: 

ER = Emissions Reductions estimate (kg CH₄/year) 

ES = Engine size (hp) 

T = Annual operating time (hours/year) 

EF_C = Emission factor – gas engine combustion (kg CH₄/hp-hr) 

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

EF_S = Emission factor – gas engine starters (kg CH₄/year-compressor) 

Assumptions: 

• Use the most current “gas engine combustion” and “compressor starts” emission factors. 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 Excel 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. 
• Annual operating hours – use actual hours when available. 

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 electric motors instead of gas driven engines may: 

• Reduce noise output at the location: Electric motors are generally quieter than gas-powered engines. 
• Provide operational flexibility: If fuel-quality gas is not required at remote sites (i.e., to power an internal combustion engine), there may be less need to treat produced gas with a glycol dehydrator, opting instead for injecting methanol to control hydrates in gas gathering lines.  

IPIECA. (2022, November). Compressors. https://www.ipieca.org/resources/energy-efficiency-solutions/efficient-use-of-power/compressors-2022/

Quincy Compressors. (2020, November 25). Electric vs. natural gas air compressors: Your guide to knowing what’s the best choice for you. https://www.quincycompressor.com/electric-vs-natural-gas-air-compressors/