CFD Analysis of Different Biogas Upgrading Levels for Dual-Fuel Operation in Diesel Engines

Abstract

<div class="section abstract"><div class="htmlview paragraph">As the transportation sector continues to increase its energy demand and present stricter environmental regulations, the use of biofuels has been gaining more attention. Among them, one of the most promising options is biomethane - a methane-rich fuel produced from biogas upgrading. Despite presenting excellent combustion properties and composition comparable to natural gas, this green fuel requires a proper biogas processing technology that may lead to a high final cost. On the other hand, the direct use of unprocessed biogas may cause operational issues in the engine, since it may present corrosive contaminants and a high CO₂ concentration that affects the combustion and decrease storage efficiency. Therefore, a balance between upgrading level and good engine operation could lead a reliable engine performance without the need of high processing costs. In this sense, the present study aims to discuss the effect of different biogas upgrading levels over engine performance and pollutant formation, focusing on fuels with distinct methane concentrations and their use as a partial energy source in dual-fuel diesel engines. For this analysis, a CFD simulation was carried out using a multi-component, 178-species reaction mechanism, which was experimentally validated for biogas and biomethane at various energy substitution fraction (ESF). Compared to diesel operation, all tested fuels led to delayed combustion, more homogeneous temperature profiles, and lower NO_x emissions. Biogas upgrading level did not visually affect temperature and NO_x distribution inside the cylinder, but final NO_x emissions were up to 24.2% lower for biogas with lower methane concentrations. Finally, slightly delaying the diesel injection led to lower NO_x, lower UHC, and increased power, but the optimal delay was lower for dual-fuel operation due to the delayed combustion of both biogas and biomethane.</div></div>