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

Abstract

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 NOx 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 NOx 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 NOx 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 NOx 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.