Mayank Mittal

Rising concerns about the dependence of modern energy systems on fossil fuels have raised the requirement for green alternate fuels to pave the roadmap for a sustainable energy future with a carbon-free economy. Massive expectations of hydrogen as an enabler for decarbonization of the energy sector are limited by the lack of required infrastructure, whose implementation is affected by the issues related to the storage and distribution of hydrogen energy. Ammonia is an effective hydrogen energy carrier with a well-established and mature infrastructure for long-distance transportation and distribution. The possibility for green ammonia production from renewable energy sources has made it a suitable green alternate fuel for the decarbonization of the automotive and power generation sectors. In this work, engine characteristics for ammonia combustion in spark ignition engines have been reported with a detailed note on engines fuelled with pure ammonia as well as blends of ammonia with gasoline, hydrogen, and methane. Higher auto-ignition temperature, low flammability, and lower flame speed of ammonia have a detrimental effect on engine characteristics, and it could be addressed either by incorporating engine modifications or by enhancing the fuel quality. Literature shows that the increase in compression ratio from 9.4:1 to 11.5:1 improved the maximum power by 59% and the addition of 10% hydrogen in supercharged conditions improved the indicated efficiency by 37%. Challenges and strategies for the utilization of ammonia as combustible fuel in engines are discussed by considering the need for technical advancements as well as social acceptance. Energy efficiency for green ammonia production is also discussed with a due note on techniques for direct synthesis of ammonia from air and water.

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.

Non-carbon nature and higher hydrogen energy density of ammonia have gained a lot of interest as an energy asset and green fuel. In this work, the effects of ammonia fuel share and engine operating load on the combustion stability and engine characteristics (combustion and performance) of gaseous (NH3/CH4) fuelled SI engine have been reported. The lower heating value and flame propagation speed of fuel mixture reduce with the increase in ammonia share (0–60% at 8 Nm engine load) and thus results in detrimental engine performance and higher cycle-to-cycle combustion variations (1.36%–14.9% COV of IMEP). A rise in operating load from 8 Nm to 16 Nm increases the flame propagation speed of the fuel mixture (60% ammonia share) and improves engine performance and combustion stability (14.9%–4.3% COV of IMEP). The results of this study indicate that the substitution of methane with ammonia could be maximized at higher engine operating loads to get the benefit of clean fuel utilization.