Ammonia and Hydrogen for Green Energy Transition

In the pursuit of a sustainable energy future, this comprehensive overview navigates the complexities of alternative fuel combustion, focusing on hydrogen and ammonia, two carbon-free fuels crucial for a low-carbon economy. Hydrogen and ammonia, derived from renewable sources, stand as promising alternatives, their efficient production methods aligning with the world's escalating energy demands. However, their adoption is challenged by nitrogen oxide (NO_x) emissions, prompting an in-depth analysis of NO_x formation pathways. Detailed examinations of equivalence ratios, fuel mixtures, and combustion temperatures provide crucial insights, steering the development of effective mitigation strategies. In the realm of hydrogen-powered engines, where the promise of clean transportation meets the challenge of NO_x emissions, innovative control methods take the spotlight. Techniques such as Variable Valve Timing (VVT), injection timing, and Exhaust Gas Recirculation (EGR) are dissected. Notably, direct hydrogen injection emerges as a pivotal strategy, offering precise control over combustion parameters and NO_x production. These methods, in synergy, present a holistic approach to NO_x reduction and efficient engine operation, shaping the future of sustainable transportation. This holistic exploration not only sheds light on the advancements in hydrogen and ammonia synthesis technologies but also emphasizes the importance of responsible combustion practices. By understanding NO_x formation intricacies and implementing innovative mitigation strategies, researchers, engineers, and policymakers are empowered to pave the way for a greener, more sustainable energy landscape. With a focus on harmonizing environmental conservation and burgeoning energy needs, this discussion provides a comprehensive roadmap, steering the world toward a cleaner, more sustainable energy future for generations to come.

Continuous fossil fuel extraction and its use, environmental pollution, and future energy security are key alarming concerns for the modern civilization. Most of the countries at the beginning of this century witnessed huge diesel-powered vehicular registration. Diesel engine and its noxious nitrogen oxides (NO_x) and soot emissions caused a shock to the diesel engine market and its existence in dispute. That is why transportation industries are in strict focus on carbon neutrality by developing alternative fuels. Despite NO_x and soot emissions reduction, decarburization of automotive and power generation industries is mandatory to tackle the greenhouse gas (GHG) emissions target. The absence of carbon-containing compounds in ammonia (NH₃) makes it a promissing carbon-free fuel to achieve the decarburization of power generation industries. Ammonia possesses major advantages like, easy liquefaction, storage at low pressure and ambient temperature or low temperature and ambient pressure, less expensive storage systems, and high energy density than natural gas (NG) or hydrogen (H₂). Ammonia can efficiently be combusted with diesel or other low auto-ignition fuels for the significant reduction of carbon-based emissions. This paper summarizes the challenges, present scope, and future potentials of green ammonia as a carbon-free fuel. In this paper, the production process of NH₃, its characteristics as a fuel, potential role as a H₂ carrier, combustion behavior, and overall technological advancement have been discussed.

Considering the extent of combustion in energy conversion devices, every pathway to sustainable energy generation foresees a big role for carbon-free fuels like ammonia and hydrogen. However, while hydrogen is a high-performance fuel with improved environmental benefits, its uses are mostly limited to fuel cells. This is due to the complexities involved in handling, storing, and transporting hydrogen. Accordingly, ammonia, an efficient hydrogen carrier, promises to be an excellent alternative owing to its relative ease in production, storage, transportation, and high hydrogen density. However, low burning velocity and high NOx production in ammonia combustion propose challenges to developing dedicated energy conversion devices solely based on ammonia. This chapter will discuss, in detail, the recent research on ammonia combustion and its qualitative and quantitative comparison against hydrogen and conventional fuels. Moreover, a discussion on how ammonia and hydrogen can fit into the current technology space will be presented. The reader should note that it is out of scope to discuss all the combustion-oriented devices such as engines, turbines, and others; therefore, the article will mostly focus on the combustion process relevant to gas turbines. Overall, the lessons we will learn from assessing the combustion of ammonia and hydrogen are the foundational building blocks of how energy space will shape in the future.

The use of ammonia as a fuel has been recognized as a promising way for various energy and power devices toward carbon neutrality. However, significantly different combustion characteristics of ammonia from conventional fossil fuels pose great challenges to its applications. Among them, the extremely low combustion reactivity of ammonia can lead to poor flame stability and low combustion efficiency, which consequently hinders its direct utilization in existing combustion devices. Therefore, this chapter focuses on the challenge and reviews the emerging attempts in which enhancement strategies have been proposed and tested in laboratory burners or real-scale combustors. Details of the combustion enhancement strategies, such as reactive fuel-cofiring, fuel-cracking, pre-heating, oxygen-enrichment, and combustion auxiliary technologies have been introduced from theoretical, diagnostic, numerical, and technical aspects. These state-of-the-art progresses greatly enable a deeper mechanistic understanding of ammonia combustion enhancement strategies and further development of clean and efficient ammonia combustion technologies, thus facilitating applications of ammonia in practical combustion devices, such as internal combustion engines, aero-engines, land-based gas turbines, furnaces, and boilers.

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.

According to the International Energy Agency (IEA), “green” hydrogen will be a critical component of the world's transition to a sustainable energy future since it is one of the most promising and clean energy carriers for both transportation and power generation. Faced with the challenge of lowering greenhouse gas emissions and shifting to greater use of renewable energies, ammonia (NH₃) is becoming more widely recognised as a viable “green carrier” transportation fuel. However, its combustion properties (high minimum ignition energy and auto-ignition temperature, poor combustion speed compared to conventional hydrocarbon fuels) have limited its employment to this point. The application of a combustion promotor can enhance the combustibility of NH₃. Combustion promoters are more reactive fuels that are used in conjunction with ammonia to increase ignitability and flame speed. Common ICE fuels like diesel and gasoline, as well as alternative fuels like hydrogen, methane, ethanol, and natural gas, might be used as combustion boosters for NH₃. The objective of this chapter is to discuss the potential of ammonia as a fuel for dual-fuel spark-ignition engines and possibilities for future transportation. This chapter reviews the possible use of NH₃ as fuel with combustion buster emphasis on Gasoline, Hydrogen, and Methane. We also focused on advanced technology such as jet turbulence ignition as a possible option for NH₃ combustion research.

Decarbonizing the transportation sector is an eminent research frontier to counter the global challenge of climate change and local pollution. The hydrogen economy is pitched as a viable short and long-term solution to mitigate these challenges. However, the low volumetric energy density of hydrogen as an energy vector has been detrimental to its wide-scale adoption. Ammonia, with multiple production routes and 1.4 times higher volumetric energy density than liquid hydrogen, presents an attractive alternative for carbon-free IC engine fuel. The challenges of low flame speed, poor ignitibility, and ammonia toxicity require considerable changes to the existing engine design for ammonia combustion. Although limited in research, the use of pre-chamber for ammonia-fuelled engines is promising due to its strong turbulence generation capability and well-distributed spatial ignition characteristics. This work reviews pre-chamber-assisted IC engines operated in single and dual fuel modes using ammonia.

Ammonia is recognized as one of the most effective hydrogen carriers and as an excellent candidate for supporting the decarbonization of the energy sector. Nevertheless, its combustion characteristics make it hard to be used in standard combustion processes. For example, its low reactivity, that leads to a very low laminar flame speed, does not allow to easily stabilize processes based neither on deflagrative nor diffusive flame structure. At the same time, it yields to the formation of a relevant amount of nitrogen oxides (NO_x), not acceptable for their environmental impact. On this basis, it is striking to break the mold and consider processes that go beyond the stabilization mechanisms of standard combustion processes, thus avoiding the main issues related to ammonia combustion. From this point of view, one of the most promising conversion technologies of ammonia is Moderate or Intense Level of Dilution (MILD) combustion, already proven to be really fuel flexible. In this chapter, the main characteristic of ammonia combustion in MILD condition will be explored based on the knowledge available in the literature. Firstly, kinetics of ammonia will be discussed in relation to this combustion regime, highlighting reaction subsets active in the temperature range characteristic of MILD combustion. In this context, it will be also discussed the crucial role of ammonia in third-body efficiency reactions. Moreover, possible burner configurations for MILD combustion of ammonia and its impact on stability range and NO_x emissions will be discussed, showing the fundamental role of such a combustion regime in the energy transition.

Hydrogen is considered as one of the promising green energy alternatives to non-renewable fuels to accommodate the ever-increasing demand for energy resources. The growing application of hydrogen at its ‘technical grade, i.e., 99.999% pure’ in the fuel cell and as blending with natural gas in the gas grid has enormously increased the demand to produce extremely pure hydrogen for all practical purposes. Ammonia, being easy to liquefy, and store, and with high hydrogen content (17.6% w/w) chemicals, is considered as one the important carbon-free sources of hydrogen. Among different existing technologies, membrane reactors have been extensively explored mainly because of the feasibility of simultaneously decomposing ammonia using a catalyst and in-situ separate hydrogen using a membrane. Conventionally, packed bed membrane reactors (PBMR) are extensively explored for the process of ammonia decomposition. However, catalytic membrane reactors (CMR) are considered as advanced membrane reactors mainly because of the constraint of diffusional mass transfer resistance and temperature non-uniformity in PBMR. The application of materials such as Ruthenium (Ru) based catalysts and Palladium (Pd) based inorganic membranes are extensively explored owing to their high performance. However, their industrial application is constrained due to the high price, limited availability, and the brittle nature of the Pd-membrane. In this chapter, the recent developments in the technologies for catalytic membrane reactor-based hydrogen production from ammonia are discussed. A comprehensive insight on the material selection for catalyst and membrane preparation, reactor design, and its coupling with sustainable processes of solar energy will be elaborated.

Hydrogen is a simple, lighter, and common element found in the composition of many materials, especially fuels of interest. With growing interest toward hydrogen being a non-carbon fuel and its potential toward decarbonizing the aviation and power industry. It is necessary to understand the fundamentals of hydrogen chemical kinetics. The chapter reviews and discusses the critical elements of hydrogen chemical kinetics by introducing the basis of chemical kinetics and exploring possible reaction pathways for hydrogen oxidation with oxygen and air. The critical reactions of major intermediate species and NO_x emissions are emphasized. The effect of considering appropriate intermediate species like HO₂ and H₂O₂ are discussed and provided a brief on the explosion limits of H₂–O₂ along with ignition delay and laminar speeds.

The major concerns around conventional fuels used in the aviation industry include pollution due to excessive emissions and an increase in carbon footprint apart from the rapid depletion of fossil deposits. Fossil fuels and their derivatives are the major sources of harmful emissions from aircraft. The exponential growth in the air transportation sector calls for a much cleaner, safer form of fuel, preferably renewable, like hydrogen. This article is a review on the use of hydrogen as a fuel for aircrafts. Hydrogen is a readily and abundantly available career of energy, with almost zero emissions. The emissions of hydrogen fuel only include small traces of water vapor and nitrogen oxide, which results in a considerable reduction in the carbon footprint. It has a higher efficiency than most of the common fuels currently in use. Design constraints and overall cost are some cons that need to be addressed. This paper is a holistic review of the recent research done on hydrogen energy, and intends to identify the trends in the development of hydrogen energy, and the obstacles in popularizing it. Identifying the gaps in the literature and further prospects for hydrogen fuel for aircrafts are also objectives of this study.

Similar to hydrogen, ammonia is a zero-carbon fuel that can be synthesized from renewable energy sources such as solar and wind. Due to its better feasibility for production, preservation, and distribution, ammonia has been considered sustainable to meet the requirements of the future energy fields that are developing toward a low-carbon economy. However, the broad deployment of ammonia as fuel is limited by \({\text{NO}}_{\text{x}}\) emissions. This chapter presents the pathways of ammonia mixture reactions and the production routes of \({\text{NO}}_{\text{x}}\) emissions with different equivalence ratios. Some critical intermediate radicals are revealed for \({\text{NO}}_{\text{x}}\) formation. It is found that many factors affect the chemical reaction pathways of ammonia-based fuels, such as equivalence ratio, fuel mixture, pressure and temperature, and so forth. Ammonia combustion and \({\text{NO}}_{\text{x}}\) emissions have been investigated under different conditions on both laboratory and industrial scales. It was found that the \({\text{NO}}_{\text{x}}\) productions peaked at Φ = 0.8–0.9 for various ammonia/hydrogen blends. The NO productions from ammonia-based flames were effectively decreased with rich blends because of more generated \({\text{NH}}_{\text{i}}\) (i = 0, 1, 2) radicals. An overall equivalence ratio of 1.20 was suggested for two-stage combustion to improve combustion efficiency and emission performance. Furthermore, some practical controlling techniques, e.g., thermal \({\text{DeNO}}_{\text{x}}\), two-stage combustion, humidification, and plasma-assisted combustion, are introduced for \({\text{NO}}_{\text{x}}\) mitigation.

Ammonia is gaining attention for decarburizing the energy sectors with high volumetric energy density and stable industry as favorable features compared to hydrogen. The low reactivity of ammonia is successfully addressed to approve ammonia as fuel. However, higher fuel NOx emissions are a major concern that limits ammonia utilization in practical combustion systems as NOx is a highly reactive and poisonous atmospheric pollutant. This suggests the need for NOx reduction techniques for commercializing ammonia as fuel. This review work is aimed at presenting different NOx reduction strategies for ammonia combustion with favorable results. Initially, the chemical kinetics of ammonia oxidation is understood to emphasize the crucial NOx and De-NOx pathways. Later, the majorly explored NOx reduction strategies for ammonia combustion such as different fuel injection techniques, raising combustor pressure, humidification, air staging, fuel staging, and mild combustion are discussed in detail at various operating conditions. This work systematically elaborates the underlying De-NOx chemistry for each NOx reduction strategy. The mitigation of NOx emissions during ammonia combustion is of utmost importance in order to preserve the environment, ensure the well-being of the public, adhere to regulatory requirements, contribute to global sustainability objectives, and exhibit corporate accountability. The implementation of this initiative yields extensive advantages for both the natural environment and the broader societal framework.

Hydrogen as an alternative fuel in spark ignition engines presents a promising avenue for achieving sustainable transportation. However, the combustion characteristics of hydrogen often result in elevated nitrogen oxide (NOx) emissions. This book chapter delves into a comprehensive exploration of diverse control methods aimed at mitigating NOx emissions in hydrogen-fueled spark ignition engines. The chapter embarks on an analysis of the combustion intricacies of hydrogen, elucidating its propensity for high-temperature combustion and the consequent NOx formation through thermal and prompt mechanisms. It subsequently delves into a detailed investigation of multiple control techniques, including Variable Valve Timing (VVT), Injection timing, Ignition timing (IT), Exhaust Gas Recirculation (EGR), and Engine design optimization. These methods are scrutinized for their potential to modulate combustion parameters, thereby managing temperatures and curbing NOx. A focal point of the chapter is the role of direct injection of hydrogen, which allows for precise control over mixture distribution and combustion phasing. This technique proves instrumental in mitigating NOx by facilitating leaner combustion and controlling heat release. Each strategy's advantages, challenges, and integration feasibility are evaluated. The synergy between these control methods and their potential for simultaneous NOx reduction and efficient engine operation is thoroughly discussed. In summation, this chapter synthesizes a spectrum of control strategies encompassing VVT, Injection timing, Ignition timing (IT), EGR, Engine design, and Direct-injection (DI) hydrogen, all directed at abating NOx emissions in hydrogen-fueled spark ignition engines. It serves as a pivotal reference for researchers, engineers, and policymakers invested in advancing clean and sustainable hydrogen technologies.