Experimental evaluation of stack testing methods for accurate VOC measurement
Date
2019
Authors
King, Brenna Allison, author
Olsen, Daniel, advisor
Quinn, Jason, committee member
Carter, Ellison, committee member
Journal Title
Journal ISSN
Volume Title
Abstract
There are more than 1,400 natural gas compressor stations that utilize large-bore, two-stroke natural gas engines in the United States to transport natural gas through pipelines across the country. Because of the long operating lives associated with these engines, it is important for emissions to be monitored and technology to be improved to ensure the engines are meeting current emissions standards. One emission class that is currently regulated by the Environmental Protection Agency (EPA) is volatile organic compounds (VOCs). VOCs are defined as non-methane, non-ethane hydrocarbons and have negative environmental effects, especially in the formation of ozone and fine particulates that create smog. The combination of a Gas Chromatograph (GC) and a Flame Ionization Detector (FID) can be used to measure methane, ethane, and VOCs. The use of a GC/FID to quantify hydrocarbon concentration is in compliance with EPA Method 18/25A. In some cases, this approach is mandated by regulatory bodies. The Fourier Transform Infrared Spectrometer (FTIR) can also be used to measure VOCs in engine exhaust gas, following EPA Method 320. However, there is concern that Method 320 is not as accurate as Method 18/25A. The main objective of this research is to provide data and analysis with both measurement methods from different engine types, conditions, and fuel quality to determine whether Method 320 is acceptable for VOC quantification. iii Exhaust gas was sampled from engines of different types and configurations: the GMV-4 lean burn testing with open chamber spark ignition, pre-combustion chamber ignition, and high-pressure fuel injection with electronic fuel valves and the Caterpillar 3304 rich burn testing with a three-way catalyst. For the GMV-4 configurations, an ignition timing sweep was performed, including retarding and advancing ignition timing from the nominal 18°aTDC. In addition, fuel ethane and fuel higher hydrocarbons were added to the natural gas fuel supply separately to determine the effects fuel variability has on emissions and engine performance. For the Caterpillar 3304 configuration, only an ignition timing sweep was performed. It was concluded that the HP 5890 Series II GC utilizing EPA Method 18/25a is the most accurate method for VOC quantification. Both the Gasmet and MKS FTIRs (EPA Method 320) overestimate total VOC concentration compared to the HP GC by approximately 18 percent and 12 percent, respectively. However, in most cases the differences were within uncertainty bounds. A common process currently used for VOC quantification, which subtracts the methane and ethane measurements from the MKS FTIR (utilizing EPA Method 320) from the THC measurement from the Siemens 5-Gas analyzer, is not an accurate method as it creates large uncertainty up to 193 percent and overestimates total VOC concentration by nearly 100 percent relative to the HP GC.