The pollutant emissions of a four-stroke spark ignition gas engine generator operating on biogas–LPG blends of varying excess air ratios and LPG concentrations were investigated in this paper. Experiments were carried out at a constant engine speed of 1400 rpm and a constant electric power output of 3.5 kW. The experimental results showed that NOx emissions were elevated at an excess air ratio of around 1.2 as the LPG concentration was increased. CO2 emissions decreased as the excess air ratio and LPG concentration increased, due to lean-burn conditions and LPG combustion. These results demonstrated that the addition of LPG to biogas enabled the effective generation of electricity using a gas engine generator through lean-burn combustion. The adding of LPG of 0%, 5%, and 10% in volume is interchangeable. Mixing inert gas such as N2 and CO2 in the end user side to increase the gas MN, thereby increasing its antiknock tendency, furthermore, the measures such as the installation of closed-loop control engine with oxygen sensor in the exhaust and adjust air-fuel ratio accurately and adjusting engine ignition timing, to adapt the changes in gas composition to ensure that the engine working conditions are not significantly affected, to meet the requirements of natural gas interchangeability.