An Experimental and Theoretical Study of Fuel Composition on Noxious Emissions in a Natural Gas Engine Operating on Wellhead Gas

The use of natural gas in spark-ignited internal combustion engines optimized for minimum emissions has repeatedly shown a significant reduction in exhaust emissions over that of gasoline. Pronounced variations in unprocessed natural gas composition can however present an emissions problem for engines used in natural gas recovery where unrefined wellhead gas is used as the fuel. This study is twofold involving both experimental analysis and theoretical development with a computer model that simulates wellhead gas combustion. On the experimental side, Fourier transform infrared spectroscopy (FTIR) was used to analyze emissions of a 2.4L four-cylinder spark-ignited natural gas engine operating on fuels of varying composition. A comparative assessment is made between CO, NO, THC, CH4, and CH2O emissions of the engine operating on refined pipeline natural gas and those of the engine operating on the same gas with added CO2, N2, and C3H8 diluents. Diluents were added to the fuel individually to isolate the effect of each and to approximate wellhead gas. Additionally, a burn rate analysis was conducted which shows changes in the rate of fuel energy liberation with changes in diluent concentration. On the theoretical side, a two zone computer model of engine operation was developed that would simulate operation of the engine under varying fuel composition as found in various natural gas recovery wells throughout the United States. Results show that exhaust concentrations of NO and THC were strongly affected by addition of both inert and reactive diluent due to their strong dependence on in-cylinder temperature. Emissions of CO, CH4, and CH2O were also found to depend on diluent concentration; however, to a much lesser extent with emissions of CO being seemingly unaffected by addition of N2 for the compositions tested. Burn rate analysis shows that the introduction of inert constituents to the fuel decreases the fuel burn rate while addition of propane increases the burn rate. The whole of the analysis indicates a strong dependence of emissions on fuel composition and that significant potential exists for emissions reduction of engines operating on unprocessed natural gas.