Combustion and emission characteristics of simulated biogas from Two-Phase Anaerobic Digestion (T-PAD) in a spark ignition engine

The performance of hydrogen-rich biogas from two-phase anaerobic digestion (T-PAD) in a 3.2 kW spark ignition gas engine was studied experimentally and by numerical modelling. The biogases from T-PAD were modelled with H₂/CH₄/CO₂ mixtures of varying CO₂ fraction from 25 to 30 vol% and H₂/CH₄ volumetric ratio from 0 to 3/7. The engine performance in terms of power output, thermal efficiency (power/chemical energy input rate), and exhaust emissions of unburned hydrocarbon, CO and NO_x were measured. The combustion performance and engine emission characteristics were also modelled using the CHEMKIN Pro Spark Ignition Engine Module. Increasing CO₂ 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 actual air/the stoichiometric air) > 1.4. For a given CO₂ fraction, increasing the H₂/CH₄ ratio from 0 to 3/7 significantly increased the thermal efficiency, reduced the unburned hydrocarbon and CO emissions but slightly increased NO_x emission at the excess air ratios greater than 1.4. When the engine was operating under high loads at 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. The modelling results suggest that the amelioration of the overall engine performance by increasing H₂/CH₄ ratio under ultra-lean (excess air ratio > 1.4) conditions was due largely to the reduction in the combustion duration and thereby the improvement of combustion efficiency. At high loads and excess air ratio from 1.2 to 1.4, increasing H₂/CH₄ ratio led to a higher cylinder temperature and greater heat loss, resulting in a significant increase in NO_x emission and a slight decrease in the overall thermal efficiency. The numerical results agreed with the experimental observation, and provided an insightful understanding of the experimental phenomena.