Combustion and Emission Characteristics of a Spark Ignition Engine Fuelled with Biogas from Two-Phase Anaerobic Digestion (T-PAD)

The performance of a small-scale spark ignition gas engine fuelled with model biogases from two-phase anaerobic digestion (T-PAD) was experimentally studied. Combustion characteristics, in terms of engine power output and thermal efficiency, and engine exhaust emissions of unburned hydrocarbon, CO and NO_x were measured. The biogases from T-PAD, rich in hydrogen, were modelled with mixtures ofCH₄/H₂/CO₂ with a varyingCO₂volumetric fraction from 25% - 30% and H₂/CH₄ volumetric ratio from 0 to 3/7. Increasing CO₂ volumetric fraction in the biogas decreased the engine power output, while increasing H₂/CH₄ ratio increased the engine power output especially at high excess air ratios (the ratio of the actual air to the stoichiometric air > 1.4). For a given CO₂volumetric fraction, increasing H₂/CH₄ ratio in the biogas from 0 to 3/7 significantly increased the thermal efficiency, reduced the unburned hydrocarbon and CO emissions and slightly increased NO_x emission under the conditions with the excess air ratios greater than 1.4. When the engine was operating under its best condition (excess air ratio from 1.2 to 1.4), increasing H₂/CH₄ ratio did not remarkably change the engine power output, thermal efficiency, the unburned hydrocarbon and CO emissions, but significantly increased the NO_x emission. This study proved the feasibility of using the hydrogen-rich T-PAD biogas as a renewable fuel for power generation in small-scale spark ignition engines.