NOx and hydrocarbon emissions from a diesel engine fueled on digester gas

A small, single cylinder diesel engine was set up to investigate the impact of engine operating conditions on emissions of hydrocarbons and NOx. The engine was operated on a synthetic digester gas (CO2/CH4 mixture) that was typical of the gas produced at waste water treatment plants. The test engine was instrumented to measure torque, rpm, temperatures and pressures, flow rates of gases and exhaust gas composition. The impact of digester gas and exhaust gas recirculation on NOx emissions was measured with a chemiluminescent analyzer. For some runs the non-methane organic gases (NMOG), CO2, O-2, and methane concentrations were also measured with a gas chromatograph to provide a more complete emissions picture. Higher engine loads generally resulted in higher NOx emissions, caused by increased combustion temperatures. An increased concentration of digester gas lowered the NOx emissions up to a certain point due to a decreased thermal NO-formation rate. Increasing the rate of digester gas beyond this point resulted in higher NOx emissions, possibly as a result of enhanced rates of prompt NO-formation. Hence, there appeared to be an optimum level of digester gas input to the engine to minimize NO emissions. In contrast to the behavior of the total NOx emissions, NO2 fell steadily with rising rates of digester gas concentration. The NOx emissions were reduced from 795 ppm on pure diesel operation at full load to 242 ppm at the optimum digester gas concentration. Increasing the amount of digester gas resulted in a decreased fuel conversion efficiency.