Predicting the performance and exhaust NOX emissions of a spark-ignition engine generator fueled with methane based biogases containing various amounts of CO2

The effect of methane-CO2 blends with varying CO2 concentrations (corresponding to various biogas compositions) on combustion and emission characteristics were investigated numerically for a gas engine generator. CO2 variation in a methane-based biogas fuel produced effects similar to the CO2 effects of exhaust gas recirculation (EGR), decreasing the cylinder temperature, pressure, heat release rate, and flame speed. By design of experiment (DOE) analysis, the maximum brake torque (MBT) timing was calculated for various values of the spark timing and excess air ratio (EAR). As the EAR increased from 1.2 to 1.8 and the CO2 content decreased, the MBT timing was far advanced from top dead center (TDC). At 50% CO2 content, incomplete combustion phenomena were detected in the leaner region at EAR values above 1.6. In spite of the performance degradation of the engine itself, the addition of CO2 up to 50% in volumetric content improved the electrical generating efficiency by an average of 6.4%, with lower NOX formation.