Greenhouse gas (GHG) regulations are likely to have a significant impact on the natural gas
industry. Limits on equivalent carbon dioxide emissions (CO2e) incorporate both CO2 and
methane emissions. Methane emissions from engines, though significantly lower than CO2, can
have an impact on CO2e because CH4 is multiplied by 25 to account for a larger warming
potential. This project explores options to reduce CO2e emitted from integral compressors, with a focus on reducing exhaust methane emissions.
The current GHG reporting rule for stationary reciprocating natural gas engines is outlined in
subpart C of the EPA’s Greenhouse Gas Reporting Program (GHGRP). The results from this
study found the methane emissions factor in the current reporting rule was not an attainable level for reciprocating natural gas engines. The level was three orders of magnitude lower than the best performing engine configurations. As a result, the replacement of this emission factor with the AP-42 emission factor is suggested to accurately represent the potential emissions from reciprocating natural gas engines.
The large bore engine test bed (LBET), a Cooper Bessemer GMV-4, at the Engines and Energy
Conversion Laboratory (EECL) was the primary data source for this study. The LBET has been
used extensively to evaluate different industry technologies at various operating conditions,
generating results analogous to actual field engines. The study found trapped equivalence ratio
had the greatest effect on methane emissions. As the in-cylinder mixture became leaner, there
was an increased propensity for the engine to encounter partial combustion or misfire, increasing the amount of unburned hydrocarbons (mostly CH4) in the exhaust.
High pressure fuel injection (HPFI) and pre-combustion chambers (PCCs) increase the incylindermix ing and ignition energy, respectively, allowing for the extension of the operational
lean limit. The extension of the lean limit is significant to operators because it allows the engine
to operate at lower NOx permit levels. For the same NOx level, HPFI decreases CH4 and CO2e
levels compared to cam driven low pressure fuel injection. Achieving moderate NOx levels (~5
g/bhp-hr) can be achieved with either HPFI or PCCs, but HPFI is preferred because CH4 and
CO2e emissions can be simultaneously reduced compared to uncontrolled operation.
The only uncontrolled parameter that significantly affected CO2e emissions was fuel quality.
New natural gas drilling methods have enabled access to shale gas formations. Natural gas from
these formations often has high ethane levels. The elevated ethane content alters the in-cylinder
chemical kinetics to produce more CO2 while maintaining relatively constant methane
emissions. This results in an increase in CO2e emissions.
Replacement of integral compressor engines with gas turbines or electric motors is often
considered by operating companies for various reasons. HP replacement of integral compressors could significantly increase GHG emissions and replacement with gas turbines will likely increase GHG emissions. The impact on GHG emissions resulting from the replacement of integral compressors with electric motors could increase or decrease GHG emissions depending on the source of the electricity used to power the motors.