Abstract
Compressor stations utilizing large-bore natural gas engines transport natural gas through pipelines worldwide. One emission class regulated by the Environmental Protection Agency (EPA) is volatile organic compounds (VOCs), which are nonmethane, nonethane, nonaldehyde hydrocarbons. The combination of a gas chromatograph (GC) and a flame ionization detector (FID) can measure VOCs, following EPA Method 18/25A. The Fourier transform infrared spectrometer (FTIR) also measures VOCs, following EPA Method 320. Multiple VOC calculation techniques are utilized, some combining measurements from separate analyzers. Two basic methods of extracting exhaust gas are direct extraction and Tedlar bag sampling. In this study, various VOC quantification methods are evaluated. Exhaust gas was sampled from a Cooper-Bessemer GMV lean-burn engine and a Caterpillar G3304 rich-burn engine. The GMV was tested in three configurations: open chamber spark ignition, precombustion chamber (PCC) ignition, and PCC ignition with high-pressure fuel injection. Ignition timing sweeps were performed on both engines, and a fuel variability test was performed on the GMV. Results showed that the Gasmet and MKS FTIRs’ (Method 320) VOC measurements deviate significantly from the HP GC when measuring low molar concentrations, albeit below regulatory limits. A common VOC quantification approach is subtracting the sum of methane and ethane FTIR measurements from a total hydrocarbon measurement utilizing a FID. This method produces uncertainties of 190% and overestimates VOC concentration by an average of 100%. The Tedlar bag sampling method produced VOC measurements within −2% of the direct extraction method.
