Mayank Mittal

Biogas is a renewable gaseous fuel and has the potential to replace fossil fuels for sparkignition engines; however, a higher volumetric proportion of CO2in biogas degrades the engine characteristics significantly. Biogas upgradation techniques are limited by higher fuel costs, and strenuous modifications would be required for improving engine physical parameters. In this study, experimental investigations were performed with hydrogenenriched biogas to enhance low operating load limit and engine characteristics, and to the best of authors' knowledge, studies related to operating range and low load enhancement by hydrogen addition in biogas fueled engines are not reported in literature. Gaseous-fuels blending setup was developed to fabricate the gaseous fuel mixtures in desired proportions and moderate amounts of hydrogen (5, 10, 20, and 30% by vol.) were blended with biogas. The experiments were conducted on a single-cylinder SI engine operated at the compression ratio of 10:1 and 1500 rpm for stationary applications. It was found that the coefficient of variation (COV) of indicated mean effective pressure decreased from 10% in case of biogas to 8.69, 6, 3.05, and 1.66%, respectively, for 5, 10, 20, and 30% hydrogen cases at 6 N·m loading condition. Low operating load limit enhanced from 6 N·m in case of biogas to 5.3, 2.2, 1.5, and 0.8N·m, respectively, for 5, 10, 20, and 30% of hydrogen share in the fuel mixture and brake thermal efficiency also improved with hydrogen enrichment. Carbon-based emissions decreased with hydrogen addition, whereas oxides of nitrogen increased but it was well below the baseline case with pure methane. Overall results indicated that hydrogen enrichment enhances the low load limit and engine characteristics of biogas-fueled SI engines for stationary power generation applications in rural areas.

Internal combustion engines play a major role in biogas-based stationary power generation applications in rural areas, and serious progress on effective utilization of bio-resources by considering engine stability is not achieved yet. In the present study, combustion characteristics and cycle-to-cycle variations (CCVs) of a spark-ignition (SI) engine fueled with gasoline, biogas, and surrogate of bio-methane are analyzed. A single-cylinder, four-stroke SI engine (with a flexible gaseous fuel system) was operated at a couple of load points (8 Nm and 11.5 Nm) with a rotational speed of 1500 rpm. CCVs are analyzed using a statistical approach considering 1000 consecutive engine cycles for each operating condition. Results at 8 Nm showed relatively higher CCVs of indicated mean effective pressure (IMEP), peak in-cylinder pressure (Pmax), and flame initiation duration (FID) for biogas compared to methane. It is also found that a linear relationship existed between Pmax and its corresponding location (θPmax) for methane, whereas the hook-back phenomenon was observed for biogas, which is because of the successive slow and fast burning cycles as noted in the return map of the IMEP.

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.

Limitations on the upgradation of biogas to biomethane in terms of cost effectiveness and technology maturity levels for stationary power generation purpose in rural applications have redirected the research focus towards possibilities for enhancement of biogas fuel quality by blending with superior quality fuels. In this work, the effect of hydrogen enrichment on performance, combustion and emission characteristics of a single-cylinder, four-stroke, water-cooled, biogas fuelled spark-ignition engine operated at the compression ratio of 10:1 and 1500 rpm has been evaluated using experimental and computational (CFD) studies. The percentage share of hydrogen in the inducted biogas fuel mixture was increased from 0 to 30%, and engine characteristics with pure methane fuel was considered as a baseline for comparative analysis. The CFD model is developed in Converge CFD software for a better understanding on combustion phenomenon and is validated with experimental data. In addition, the percentage share of hydrogen enrichment which would serve as a compromise between biogas upgradation cost and engine characteristics is also identified. The results of study indicated an enhancement in combustion characteristics (peak in-cylinder pressure increased; COVIMEP reduced from 9.87% to 1.66%; flame initiation and combustion durations reduced) and emission characteristics (hydrocarbon emissions reduced, and NOx emissions increased but still lower than pure methane) with increase in hydrogen share from 0 to 30% in biogas fuelled SI engine. Flame propagation speed increased and combustion duration reduced with hydrogen supplementation and the same was evident from the results of the CFD model. Performance of the engine increased with increase in hydrogen share up to 20% and further increment in hydrogen share degraded the performance, owing to heat losses and the enhancement in combustion characteristics were relatively small. Overall, it was found that 20% blending of hydrogen in the inducted biogas fuel mixture will be effective in enhancing the engine characteristics of biogas fuelled engines for stationary power generation applications and it holds a good compromise between biogas upgradation cost and engine performance.

Increasing demand for energy accompanied by environmental concerns has raised the requirement for limiting the use of fossil fuels in energy generation and transportation applications. Among the green and renewable energy-based solutions, biogas is quite promising since it could be implemented for power generation applications (engines driving generators and pump sets) in rural areas, at domestic and industrial scales with lower capital investment and production cost by using the agricultural crop residues and other domestic biomass sources as raw materials. However, the composition of biogas varies depending on the raw materials, and higher concentration of carbon dioxide in biogas results in combustion variations affecting engine durability. This review focuses on the role of biogas in achieving sustainable development goals with an emphasis on its utilization in gaseous fuelled spark-ignited engines. Recent progress in biogas production and upgradation techniques are also detailed. Challenges related to the stability and characteristics of biogas fuelled spark-ignited engines could be addressed by either modifying the physical parameters of the engine or by enhancing the fuel quality (upgradation to biomethane or blending with hydrogen). A comprehensive review on the effects of these approaches on the performance, combustion, and emission characteristics of biogas-fuelled engines is discussed in detail with a note on engine operating parameters.

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.

Rising pollution levels and stringent emission regulations have raised the requirement for the utilization of alternative green fuels as a sustainable energy source for engine applications. The economic and environmental benefits of using biomethane or renewable natural gas (>95% methane) in diesel engines have made it a promising solution. Sound levels caused by combustion noise, however, is an area of concern in the early stages of engine development process, and to the best of our knowledge studies related to the impact of gaseous fuel energy share on combustion noise in dual fuel diesel engines have not been reported in the literature. In the present work, experimental research on the combustion noise of a biomethane augmented dual fuel common-rail direct injection (CRDI) diesel engine has been carried out with a correlational design of its performance and combustion characteristics at various engine operating conditions. Comparative results on combustion and performance characteristics of different fuel compositions at various engine loads revealed that there is a better utilization of gaseous fuel at full load conditions resulting in improved power output and reduced heat transfer losses. Cylinder pressure spectra derived from recorded in-cylinder pressure were used for carrying out the quantitative analysis of sound levels. The results suggested that an increment in methane share is effective in controlling the sound pressure levels to 95 dB (at 1.5 Newton-meter [Nm] and 13.5 Nm loading conditions) and it is lower than the diesel-only operational levels at all loading conditions.

Higher levels of air pollutants and greenhouse gas emissions from fossil fuel-based internal combustion engines have raised environmental concerns and it has resulted in stringent emission regulations. Recent emission norms have made it a mandatory requirement to adopt after-treatment emission control systems for controlling and reducing emission levels to meet the required emission standards. This chapter will discuss the recent research efforts in modelling engine emissions, and after-treatment systems like oxidation catalyst filters, particulate filters, and selective catalytic reduction systems. Detailed discussions on the mechanisms for soot emission modelling and NOx formation are covered in this chapter. Literature studies on one-dimensional, two-dimensional, and three-dimensional modelling of filters and catalysts coupled with CFD studies for application in three-way catalytic filters, NOx reduction catalysts, electrically heated catalysts, and particulate filters (soot accumulation and regeneration) are also discussed. Future directions for modelling advanced after-treatment systems to reduce regulated and unregulated emissions are also explored.

Humans contemporarily depend on fossil fuels for most of their energy needs which however is depleting at an alarming rate, forcing researchers to search for alternate and sustainable ways. The potential of ammonia and hydrogen as carbon-free fuels in energy systems is very promising. Hydrogen is the cleanest fuel presently available. The use of hydrogen in internal combustion engines, however, is constrained due to its low density, shorter flame-quenching distance, and complex storage and infrastructure. Ammonia is a hydrogen energy carrier (17.65% hydrogen by weight) with high hydrogen energy density, and it has a well-established storage/transportation infrastructure, and thus has the potential to mitigate the challenges faced due to hydrogen storage, distribution, and infrastructure drawback. Green ammonia produced from renewable sources can also contribute to carbon-neutrality targets. Using ammonia as a single fuel in an internal combustion engine faces several challenges due to its high auto-ignition temperature (~930 K), low flame velocity, slow chemical kinetics, and high unburnt ammonia emissions. Ammonia utilization in IC engines could be improved by enhancing the fuel quality, incorporating physical modifications in the engines (compression ratio, fuel injection strategies, etc.). This chapter discusses the key aspects of conventional and green ammonia production, highlighting the world energy outlook and a detailed literature study on the engine characteristics and challenges for ammonia-fueled engines with a due note on strategies for improving ammonia utilization and the possible enormous impact on various energy sector segments.