Review on the production and utilization of green ammonia as an alternate fuel in dual-fuel compression ignition engines

Expectations on hydrogen, being a carbon free fuel, are faced with challenges related to its storage and distribution, accompanied by limitations in the development of required infrastructure. Ammonia has been identified as potential enabler for carbon-free economy with well-established infrastructure and can also serve as a hydrogen energy carrier owing to its higher gravimetric hydrogen density. Liquid ammonia has higher volumetric energy density (15.6 MJ/L) as compared to compressed hydrogen (5.6 MJ/L at 70 MPa) and can be stored at relatively low pressures. Compression ignition engines have higher installed capacity than spark ignition engines since it finds applications in automotive vehicles, marine industry and power generation sector. However, toxic emissions released by diesel engines have affected its sustainability, and utilization of green alternate fuels like ammonia offers the best solution for decarbonization of these engines to meet greenhouse gas emission targets with reduced global warming potential. A detailed literature study has been performed in this article highlighting the challenges and strategies for utilization of ammonia as a fuel for compression ignition engines in dual fuel combustion mode with secondary fuels like diesel, dimethyl ether, kerosene, hydrogen and other alternate fuels. Higher auto-ignition temperature of ammonia has a detrimental effect on combustion and performance characteristics, which, however, can be enhanced by incorporating advanced injection strategies. Ammonia has carbon free combustion but low laminar burning velocity and long quenching distance accompanied with fuel bound nitrogen result in unburned ammonia and nitrogen based emissions, hence there must be an emission after-treatment device downstream to restrict these emissions. Techniques for synthesis of green ammonia are also discussed with a note on technology readiness levels, techno-economic feasibility, ammonia fuel properties and reaction pathways for combustion.