Performance and sensitivity analysis of raw biogas combustion under homogenous charge compression ignition conditions

This study aims to investigate the feasibility of using raw biogas as the main fuel in homogeneous charge compression ignition (HCCI) engines and to identify the effects of different trace elements found in biogas on engine performance and emissions. This study was conducted using many models that simulate the operation of HCCI process. These models include the chemical kinetics, combustion model, and the heat transfer models. Raw biogas composition was selected based on the most available traces. The traces that were studied are Hydrogen Sulfide (H₂S), Ammonia (NH₃), Carbon monoxide (CO), Nitrogen (N₂), Oxygen (O₂), and Water vapor (H₂O). Other parameters like the equivalence ratio, intake temperature, and pressure were also studied. It was found that raw biogas mixtures could produce higher power outputs in the presence of H₂S and NH₃. However, H₂S and NH₃ were found to increase the SO_x and NO_x percentages owing to their higher in-cylinder temperatures. Other traces such as CO and H₂O can reduce the power output, but at the same time, they can reduce the engine NO_x emissions but not the CO emissions. Engine performance was positively affected by increasing the intake pressure and equivalence ratio within the studied ranges. However, higher intake temperatures and reduce the engine efficiency and increase the engine emissions. On the other hand increasing the compression ratio (CR) increase the system efficiency but up to a certain limit. Considering the exhaust gas residual (EGR), this parameter reduce system performance but at the same time lower the NO_x emissions significantly. A sensitivity analysis for the mentioned parameters indicates that EGR is the most effective parameter while CR are the least significant one.