Hydrogen (H2) as a carbon-free fuel has attracted several waves of global interest in the past half a century. H2 can be directly burnt to provide heat and power or converted in fuel cells to generate electricity with high efficiency. However, three fundamental challenges for H_2 fuel are its low volumetric energy density, low ratio of low-heating-value to high-heating-value (LHV/HHV = 0.85), and that it does not occur naturally. These challenges make it especially difficult for H_2 to be used as a carbon-free transport fuel. Clearly, H_2 needs a carrier that is easy to bulk-manufacture, handle, transport, store and use. Among very few H-rich and C-free substances, ammonia (NH3) stands out as an excellent H2 carrier. Indeed, NH3 is combustible and could potentially become a clean transport fuel for direct use in internal combustion engines (ICEs). This technical review examines the current state of knowledge of NH3 as a fuel in ICEs on its own or in mixtures with other fuels. A particular case of interest is to partially dissociate NH3 in-situ to produce an NH3/H2 mixture before injection into the engine cylinders. A key element of the present innovation, the presence of H_2 is expected to allow easy control and enhanced performance of NH3 combustion. Against this backdrop, the key physical and thermochemical properties of NH3 are collected and compared to those of conventional and alternative fuels. The basic combustion characteristics and properties of NH3 and its mixtures with H2 are summarised, providing a theoretical basis for evaluating NH_3 combustion in ICEs. The combustion chemistry and kinetics of NH3 combustion and mechanisms of NOx formation and destruction are also discussed. The potential applications of NH3 in conventional ICEs and advanced homogenous charge compression ignition (HCCI) engines are analysed.
展开▼
