Using Petroleum and Biomass-Derived Fuels in Duel-fuel Diesel Engines

Abstract

There is worldwide interest in developing renewable energy sources in a sustainable manner. Syngas and biogas offer significant potential in this context, as these fuels offer great flexibility with regard to their production and utilization. This chapter provides an overview of research dealing with the combustion and emission characteristics of these fuels, both as stand-alone fuels or by blending with petroleum fuels. Conversion methods for producing these fuels from different biomass sources are also briefly reviewed. While the syngas composition can vary widely, it generally has lower heating value, lower density, higher mass diffusivity, higher flame speeds, and wider flammability limits compared to hydrocarbon fuels. Moreover, its combustion leads to almost zero soot emission, although NO x emission may be a concern depending upon its composition and operating temperatures. Similarly, biogas has lower heating value compared to hydrocarbon fuels, and its ignition and combustion characteristics can vary noticeably depending upon its composition. While there have been few studies focusing directly on biogas combustion, there is extensive literature on methane combustion, including ignition, extinction, flammability limits, flame speeds, cellular instabilities, and emissions. Fundamental combustion aspects requiring further research include cellular instabilities, flame stabilization and blowout behavior, turbulent flames, and emission characteristics. Such efforts would lead to the development of optimized systems for producing these fuels and provide guidelines for optimizing their composition for a given set of operating conditions. The use of syngas and biogas in dual-fuel diesel engines has been a subject of numerous experimental and computational studies. A general observation is that the engine performance and emission characteristics are significantly modified by the presence of gaseous fuel. While the heat release in a diesel engine generally occurs through a hybrid combustion mode, involving rich premixed combustion and diffusion combustion, that in a dual-fuel engine also involves a lean combustion mode with a propagating flame. The dual-fuel operation at high-load conditions can provide significant reduction in soot and CO soot emissions, while maintaining the engine efficiency, provided the injection characteristics including the amount of pilot fuel can be optimized. However, the NO x emission may increase, requiring a suitable strategy for lowering the temperatures. The dual-fuel strategy may be less effective at low load, resulting in lower thermal efficiency and higher UHC and CO emissions. Future work should be directed at optimizing the various parameters, such as injection timing, amount of pilot fuel injected, EGR, and multiple injections.