Ammonia as a clean energy for internal combustion engines: A review and preliminary CFD investigation

Abstract

The emissions from internal combustion engines (ICEs) contribute to climate change, air pollution, and pose risk to human health. This review is the first part of a research study that aims to employ computational fluid dynamics (CFD) to develop a clean and efficient combustion strategy focussing on ammonia as an alternative fuel. This article discusses background related to ammonia economy, safety, and its impact on the environment and human health. Combustion characteristics of ammonia are analysed in this article. The review explores various techniques such as blended and dual fuels that are employed to optimize the ignition, including developments in recent ammonia commercial engines. It discusses challenges for minimizing nitrogen-based emissions and also delves into investigations of hydrogen generation through ammonia decomposition, with a focus on microwave heating as an efficient method. Literatures on CFD models tailored to ICEs are also examined. The methodologies of CFD model and techno-economic approach of ammonia decomposition by microwave heating are described. Key findings referred that nearly 4% of ammonia production currently utilizes sustainable sources. Hydrogen with selective catalytic reduction reduces the temperature of reaction to below 200 °C and produces water vapour as a byproduct. Ammonia decomposition by microwave heating occurs at temperatures 100–200 °C lower than conventional methods. Moreover, the localized ammonia decomposition by microwave heating for hydrogen production proves to be 4.5% more efficient than hydrogen storage for transportation. The preliminary CFD investigation from this study suggests the feasibility of using ammonia as a carbon-free fuel for spark ignition ICEs. Compared to methane (CH₄) combustion, ammonia shows a 2.5% increase in thermal efficiency and approximately 40% reduction in NOx emissions. This study lays the groundwork for future exploration of ammonia and hydrogen as potential cornerstone for cleaner and more sustainable ICEs.

Keywords

CFD combustion emissions engines ammonia sustainable fuel

Introduction

There is a crucial international demand for reducing carbon emissions, as well as replacing fossil fuel to meet Net Zero carbon emissions target by 2050.¹ The emissions are gases discharged from fossil fuel combustion. The sources of emissions are classified in USA into about 28% transportation, 25% electric power generation, 23% industrial plants, 13% commerce and residence, and 10% agriculture.² These emissions are called greenhouse gases (GHGs) and nitrogen oxides. GHGs impact on the environment, which contributed to rise the global average temperature by 1.2 °C between approximately 1900 and 2020 as documented in the sex assessment report by Intergovernmental Panel on Climate Change (IPCC).³ The effects of emissions are global warming, ice melting, desertification, acid rains, and ozone layer depletion.⁴ The prolonged exposure to polluted air causes heart attack, respiratory illnesses, asthma, and lung cancer.⁵ The implications of these conditions can be increase in number of hospital admissions, emergency rooms, and death. Additionally, the demand for energy is expected to be two to three times higher by 2050 due to the growing world population and economic development, while fossil fuels are finite resources which will be depleted in next generations.⁶ Therefore, it is vital to achieve decarbonization or Net Zero carbon emissions by switching from fossil fuel to green-renewable energy,³ and cut off carbon emissions from transportation,⁷ power,⁷ industry⁸ and commercial sectors.⁹ A critical step in this process is identifying a replacement to fossil fuels used for internal combustion engines (ICEs). The substitute would be an energy having no carbon, sustainable, efficient, and cost-effective. Hence, research efforts in the last years have been concentrated to reduce emission of exhaust gases,¹⁰ and particulate matter¹¹ using various techniques centred in traditional fuel replacement, energy cell employment, hybrid system use, combustion process development, and in-situ emissions capture. The fossil fuels can be altered or mixed with biofuels. There are several research studies that used this technique such as biodiesel,¹² biodiesel-diesel-ethanol,¹³ biodiesel-alcohol,¹⁴ and biodiesel-hydrogen¹⁵ in diesel engine, and biogas as a clean energy for power generation and heating.¹⁶ Another technique related to fuels is using water injection in gasoline injection engine,¹⁷ and water emulsion fuel.¹⁸ Furthermore, techniques of hybrid systems can be seen in research, such as power plant combined with solid oxide fuel cells,¹⁹ and power generator with waste of thermal energy and R245fa Rankine cycle,²⁰ solar organic Rankine systems,²¹ and hybrid batteries – diesel generator.²² Recently, a technique that has been reported to reach a high-level of efficiency with low-level of emissions, which is a homogeneous charge compression ignition (HCCI) engine fuelled with hydrogen and nitric oxide.²³ On the other hand, air pollution control devices,²⁴ and metal-aided microwave irradiation for in-situ fast soot oxidation, which regenerates the diesel particulate filter while minimizing energy cost²⁵ are techniques used to develop emission trappers in ICEs. Overall, many techniques are used to reduce the emissions, however, there is another promising method for fuelling vehicles by clean energy that requires further study, which is ammonia.

The objective of this article is to carry out a literature review as a foundation for a broader study aims to developing a CFD model for ammonia combustion in ICEs to investigate critical aspects, such as effect of concentrations of fuel components (ammonia and hydrogen), engine design parameters, and operating conditions on ICE performance and emissions. This review focuses on the state-of-the-art ammonia combustion technology, combustion characterization of ammonia, economy, safety, environmental and health impacts, techniques for reduction of NOx emissions from ICEs, and computational modelling of ICEs, supported with research methodology and preliminary results of CFD investigation.

Economy and safety

Ammonia is produced in the form of fertilizers for agricultural purposes. It is produced by using a reaction between nitrogen and hydrogen under a catalytic system. Nitrogen is secured from air, which contains 78% nitrogen. Hydrogen can be obtained from water breaking. The annual manufacture of ammonia is about 180 million tons, which contributes to about 2% of global energy production.²⁶ The Haber–Bosch method is primarily employed to manufacture ammonia. Productions can be categorized into two according to their sources. First production is called brown ammonia, which is synthesized by extracting N₂ via liquefaction from air and reforming of natural gas or coal producing H_2.²⁷ This process produces a byproduct of about 290 Mt of CO₂ as an annual global total. The production of ammonia, mainly, via fossil fuel consumes about 1.8%–3.0% of all global energy. Also, it is possible to produce ammonia from electrolysed hydrogen, where the estimated production is around 0.5% of global ammonia.²⁸ The second type is named green ammonia as it is environmentally friendly.²⁷ In this type, ammonia is synthesized by utilizing a sustainable source, such as wind, solar, ocean tidal, thermal energy, nuclear, and biomass fuel as a power for water electrolysis producing H₂.²⁹ The amount of ammonia produced utilizing renewable sources is roughly 4%.³⁰ Power consumption to produce 1 kg of green NH₃ is about 14.248 kW/kg,³¹ while for brown NH₃ it is estimated by 17.113 kW/kg.³² Currently, Envision has officially commissioned the largest and advanced green hydrogen and ammonia production plant in the world located in Chifeng, China. The plant is completely supplied by power produced from wind and solar-based systems and delivers 320,000 ton of green ammonia yearly.³³ Ammonia can compete hydrogen economically as shown in Table 1.

Table 1.

Comparison between ammonia and hydrogen related to economy.

Economic criteria	Ammonia	Hydrogen	Ref.
Production efficiency by energy	51.4% green source ³¹	52% by electrolysis	³⁴
	61% methane source ³²	79% by plasmolysis
	74% power source ³²	80% by methane
Storage efficiency by energy	93%	77% at 300 K	³⁵
Storage efficiency by energy	93%	20% at 20 K
Transport efficiency by energy	93%	87% at 1 MPa	³⁵
Transport efficiency by energy	93%	31% at 70 MPa
Availability	Cylinders, refrigerants and fertilizers	Cylinders	³⁶
Safety	Ammonia is safer than other fuels such as hydrogen, propane and gasoline		³⁶

Notes: The efficiency by energy means the division of produced or stored or carried content energy on consumed chemical and electrical energies. Transport efficiency calculated over a distance of 1000 mile.

Significantly, the use of H₂ vehicles needs a great and costly infrastructure. Construction of stations for hydrogen fuelling is beneficial when there is an increase in the amount of H₂ fuel vehicles production, nevertheless, vehicle production of H₂ fuel may be halted if the infrastructure of hydrogen is improperly grown.³⁷ However, another way to provide hydrogen is ammonia decomposition. The speculated cost of unit H₂ produced from ammonia decomposition including capital and operating costs was about 6.27 $/kg for 30 m³/h while the cost of hydrogen production from gasoline was 10.65 $/kg. The capital cost of ammonia decomposition was calculated as 10.8% and the operating cost was 89.2% of the total cost considering the use of Ru-based catalysts (Ru/La(x)-Al₂O₃).³⁸ In addition to that ammonia is possible to be stored as a gas or liquid in tanks. For example, insulated tanks can be used to store about 50,000 ton of ammonia at temperature of −33 °C and pressure of 1 bar. Liquid NH₃ may be stored in 60 km³ tank, which contains energy near to 211 GWh, equals to an annual generation of around 30 wind turbines.³⁹ The profit is that NH₃ production can be sustainable from variable sources, cost effective, world-wide storage, carried by vehicles with large amount and low cost, and safe.³⁶

Environmental and health impacts

Ammonia is considered by National Fire Protection Association, USA as a toxic substance without colour, having a strong-penetrating odour.²⁶ The effect of ammonia on human health depends on concentration and time of exposure. Table 2 shows different legislations about limitations of ammonia exposure and their effects on human health. NH₃ is an alkaline, which has the ability to react with other materials such as acid, halogen, and oxygenizing agent.²⁶ Ammonia mixing with water (pH 11.6) is a corrosive liquid for metals, such as copper, brass, and zinc creating a greenish/blue colour.²⁶ Interestingly, hydrogen easily diffuses through metals making metal parts more brittle and capable of breakage.⁴⁰ Anhydrous ammonia is non-flammable while vapour of ammonia in air is flammable, and it is possible to explode if it is exposed to ignition, however, ammonia is less explosive than other fuels.

Table 2.

A view on legislations/guidelines relating to ammonia exposure limitations.

Organization	Exposure concentration	Exposure time	Effect	Ref.
UK Health and Safety Executive (HSE)	25 ppm	TWA	Max. safe limit	⁴²
UK Health and Safety Executive (HSE)	35 ppm	STEL	Max. safe limit	⁴²
Occupational Safety and Health Administration (OSHA) Europe	20 ppm	TWA	Max. safe limit	⁴³
Occupational Safety and Health Administration (OSHA) Europe	50 ppm	STEL	Max. safe limit	⁴³
National Fire Protection Association (NFPA), USA	20–100		No impact on health	⁴¹
	400–3000	TWA	Irritation of eyes, ears, nose and throat
	2000–3000	STEL	May be fatal
	5000–10,000		Rapidly fatal
Environmental Protection Agency (EPA), USA	30		AEGL 1: No impact on health	⁴⁴
	110	TWA	AEGL 2: Impaired health
	220	STEL	AEGL 2: Impaired health
	390	TWA	AEGL 3: Life-threatening or death
	2700	STEL	AEGL 3: Life-threatening or death

TWA: time-weighted average, 8 h; STEL: short-term exposure limit, 10 min; AEGL: acute exposure guideline levels.

The risk assessment of ammonia leakage on health and environment comparable with other fuels is illustrated in Table 3.⁴¹

Table 3.

Ammonia impact on health and environment when compared to other fuels.

Substance	Health	Flammability	Risk assessment
Ammonia (gas/liquid)	3	1/0	0 = zero hazard, 4 = severe hazard
Hydrogen	0	4
Gasoline	1	3
LPG	1	4
Natural gas	1	4
Methanol	1	3

The risk management of ammonia use is dependent on the industrial environments, whether static plants such as electricity production and power generation or transportation, especially vehicles, which is classified in Table 4.⁴¹

Table 4.

Ammonia risk management in power transportation and power generation.

Classification	Power transportation	Power generation
Safety	Very critical	Not critical
Cracking	Expensive	Easily done
Storage tank weight	Critical	Not a matter
Storage tank robustness	Require indestructible tanks	Available tanks can be used
Distribution	Complicated	Relatively simple
Start up	Problematic	Not many start ups
Operation	Pumps operated by non-professionals	Delivered or handled by professionals

On the other hand, ammonia depletes ozone indirectly via the production of NOx in the atmosphere.⁴⁵ Nitrous compounds are catalytically destroying ozone according to the following equations:

N O x = N O + N O_{2}

(1)

N O + O_{3} \to N O_{2} + O_{2}

(2)

N O_{2} + O \to N O + O_{2}

(3)

n e t : O + O_{3} \to 2 O_{2}

(4)

An investigation on the negative health impact of environment pollution on Pearl River Delta, which is a region of a significant economy in China, concluded that the mortality through a short term in 2013 reached between 13,217 and 22,800, and the loss in total economy was 1.4%–2.3% of the local gross domestic product due to NO₂ and PM₁₀ emissions, and O₃ depletion.⁴⁶

Ammonia characterizations for combustion

This section discusses five fields of research concerning NH₃ as a fuel, which are (1) ammonia as an alternative clean fuel; (2) combustion features; (3) characterization of blended fuel; (4) characterization of dual fuel; (5) commercial ammonia ICEs.

Ammonia as a clean alternative fuel

Ammonia can be a clean fuel and alternative to fossil fuels because it contributes significantly to reduction of CO₂ emissions compared with carbon-based fuel.⁴⁷ Also, a review⁴⁸ showed that ammonia is the best replacement for hydrogen to be used as a fuel for engines for multiple justifications. Firstly, ammonia is possibly supplied as a liquid in tanks at atmospheric temperature and low pressure. Secondly, guides of safety and management for distribution and processing of NH₃ are internationally published. Thirdly, the products of combustion are water and nitrogen. Moreover, it is possible to add ammonia to fossil fuels to reduce carbon emissions, where many vehicles could be fueled by 90% gasoline mixed with 10% liquid ammonia or perhaps it will be more in the future.⁴⁹ Hydrogen-assisted NH₃ is also an alternative clean fuel enhancing NH₃ combustion characteristics.⁴⁰

Combustion features

The concentrations of ammonia combustion emissions were experimentally measured by using premixed laminar burner fuelled by ammonia mixed with methane at various equivalence ratios (0.8, 0.9 and 1).⁵⁰ The measurements included discharges of NOx, CO and unburned NH₃ in different conditions. 300 W was the input thermal energy. The molar fraction of ammonia with respect to a mixture of NH₃ and CH₄ was variable from 0 to 0.7. Results showed that the ammonia fuel had a low flammability carbon-free energy. Amount of NOx emissions grew with an increase in ammonia fraction until it reaches 0.5 of the mixture. Emissions of NOx dropped with equivalence ratio reduction towards lean conditions. Emissions of CO and unburned NH₃ were properly low articulating that the combustions of CH₄ and NH₃ were complete processes. In the same trend, other experiments used alumina–zirconia burner of porous media that was supplied with two types of premixed fuels, which were NH₃/CH₄/air and NH₃/H₂/air to measure emission concentrations of NOx, CO, and unburned NH₃.⁵¹ The experiment consequences of burning NH₃/CH₄ and NH₃/H₂ stated that the ammonia autoignition was conducted at high temperature, the flame speed was low, and vaporization heat was high. The maximum emissions of NOx from NH₃/CH₄ mixture combustion were at approximately 0.5 of NH₃ molar fraction whereas a great emission from NH₃/H₂ burning was at around 0.5 and 0.8 molar fraction of NH₃. The amount of CO emissions referred that the CH₄ combustion was complete. However, NH₃ existence in exhaust gases meant its combustion was incomplete.

A study that was conducted to predict the ability of fuels containing nitrogen to be employed for each of compression ignition (CI) and spark ignition (SI) engines showed ammonia having the ability to be used for two types of ICEs as a carbon-free fuel.⁵² However, there were disadvantages, such as temperature of autoignition and heat of vaporization were very high, speed of flame was low, and flammability was limited by 16–25% (volume in air). It was mentioned that a pure ammonia fuel for CI engines demanded a big compression ratio like (35:1), and fuels of high cetane number can be used with NH₃ to enhance the features of ignition. Pure ammonia fuel for SI engines required multi spark plugs of high-power igniter and long duration spark for the purpose of igniting the NH₃ at multiple locations to break the resistance of ammonia to combustion. Therefore, the suggestion was using natural gas and synthetic gasoline directly in the vehicle instead of producing ammonia from natural gas and using it as a fuel due to the energy content (LHV) of NH₃ is 18.8 MJ/kg while it is 50 MJ/kg for natural gas. Nevertheless, using biomass could be the best sustainable source to produce ammonia.

On the other hand, it was documented that NH₃ is a hydrogen carrier, which includes 17.8% hydrogen by mass, and it is available and capable of being stored and transported. It is possible for ammonia to be used for fueling different power usages despite the process of ammonia combustion being of high ignition energy and NOx emissions with low burning velocities. Low flammable speed and NOx emission can be controlled by developing ammonia synthesis catalysts or blended fuels.⁵³ Therefore, the proposed fuels were hydrogen, dimethyl ether (DME), gasoline, diesel, and biodiesel to be mixed with NH₃ to overcome the downsides of ammonia fuel.⁵⁴

Results of an experimental work about blending gases of NH₃ and H₂ with air in SI engine⁵⁵ showed that a mixture of up to 20% hydrogen (by volume) frustrated misfires, and gave a better work and efficiency close to stoichiometric conditions. H₂ of higher fractions caused reduction in efficiency due to the increase in thermal losses through walls and increased NOx and NH₃ slip. The duration of combustion was low due to the low velocity of burning mixtures. A less than 10% volume of H₂ to NH₃ enhanced the combustion and engine stability.^56,57 Best value of hydrogen to ammonia ratio for stability and approximately zero NH₃ emissions during the period of cold engine start until warmed up was 2:1 by mole under lean conditions.⁵⁸ Hydrogen accelerated the initial combustion and the overall combustion.^59,60

The findings of mixing liquid ammonia with dimethyl ether for fuelling high-pressure CI engine⁶¹ revealed that the use of NH₃ with dimethyl ether led to delay in ignition and limitation in conditions of engine load because the temperature of ignition was elevated, and combustion rate or flame speed was weak. Pressure and temperature of combustion decreased due to existing of NH₃, producing a high emission of CO and HC. It also raised emissions of NOx and NH₄.

Ammonia can be mixed with gasoline efficiently through a feature named solubility, which means it can be kept in a liquid phase by using emulsifier, such as ethanol or methanol. Pure gasoline can emulsify NH₃ of 23 g/L at temperature and pressure of 286.65 K and 345 kPa, respectively, while gasoline mixed with each of 10% ethanol and 10% methanol keeps about 51.4 g/L and 61.8 g/L, respectively of NH₃ in a liquid phase at similar conditions of temperature and pressure.⁶² Gasoline content of 30% as an igniter necessitated a 10:1 compression ratio for ideal operation.

A comparison between two dual fuels of NH₃/diesel and H₂/diesel for a high-pressure heavy-duty engine was run out experimentally and numerically.⁶³ The major fuels were H₂ and NH₃ that were directly injected at maximum pressure of 500 bar. H₂ and NH₃ were ignited by a slight amount of diesel as a pilot fuel. Diesel was directly injected at a pressure up to 1800 bar. The comparison showed that the ignition of ammonia was delayed without any further actions because ammonia has lower flammability, higher evaporation energy and higher ignition temperature than hydrogen. This meant NH₃ requiring a high value of activation energy for combustion, which delayed the starting of heat release rate (HRR). However, utilizing high ammonia injection pressure and temperature improved the vaporization speed and shifted the heat release rate early. Additionally, the preheating of NH₃ enhanced the combustion. Furthermore, double pilot fuels led to ignite NH₃ early. The heating of NH₃ to 450 °C with a very high injection pressure showed that the trend of the HRR of ammonia approximately close to the HRR trend of hydrogen. The hydrogen burned almost near to the injector, and a long-term operation might be damaging the injection due to the elevated flame temperature close to the injector.

The characteristics of NH₃ compared to CH₄ and H₂ are recorded in Table 5. One of the significant features is that the high-octane number of ammonia (130) resists and reduces knocking in reciprocating SI engines.^35,55 An interesting observation from this research is that two different values have been reported for the flash point of liquid ammonia 132 °C^64,65 and 11 °C^66–69 as shown in Table 5. It can be deduced that more accurate research is required to remove this uncertainty.

Table 5.

Ammonia characteristics compared to other gases at sea level conditions.

Properties	Unit	Ammonia (NH₄)	Methane (CH₄)	Hydrogen (H₂)	Ref.
Boiling point	°C	−33.2	−161.6	−252.72	⁷⁰
Freezing point	°C	−77.5	−182	−259.01	⁷⁰
Flash point (liquid)	°C	11, 132	−188	N/A (gas)	⁷¹
Density	kg/m³	0.73	0.66	0.08	²⁶
Dynamic viscosity	X10⁻⁵ Ps	9.9	11	8.8
Octane	number	130	120	100
Low heating value (energy content)	MJ/kg	18.80	50.05	120
Laminar flame velocity	m/s	0.07	0.38	3.51
Autoignition temperature	°C	651	580	536
Ignition energy	mJ	8	1.3	0.018	⁷²
Flammability limit in air	vol%	15–28	4.4–16.4	4.7–75
Stoichiometric air/fuel ratio	mass	6.04	9.53	34.3
Specific heat capacity	kJ/kg K	2.2	2.2	14
Storage pressure	MPa	0.8 (liquid)		35–70 (compressed)	³⁵
Storage capacity	kg/L	0.38 at (1 MPa)		0.024 at (70 MPa)
Energy density	MJ/L	7.1 at (1 MPa)	55.5 at (1 MPa)	2.9 at (70 MPa)
Octan number		130	120	>100	⁷³

Characterizations of blended fuel

Ammonia can be blended with other types of fuels to enhance the features of combustion for the blended fuel. The mixing of ammonia with gasoline prevents knock operation in the SI engine.⁷⁴ The amount of fuel mixing was limited to 30% gasoline and 70% ammonia on an energy basis for normal operation in spark ignition engines. Higher ratios of ammonia can be used for engines with superchargers. Subsequently, an experiment compared 100% gasoline with 80% ammonia to 20% gasoline.⁷⁵ The results stated that the exhaust temperature increased from 369.9 °C to 480.1 °C because ammonia was ignited under a higher temperature. The power output decreased from 3689.2 W to 3572.8 W due to the energy content of gasoline was higher than that of NH₃. Efficiencies of energy and exergy decreased by about 7% and 8.5%, respectively, due to the reduction in output power. The reduction in emissions of CO₂ was 0.0434 g/s. Next, it was explored⁷⁶ that the combustion of premixed NH₃ with H₂ in air inside spark ignition engines produced low NO comparable with hydrocarbon fuels. Best conditions for mixture of NH₃ with H₂ were equivalence ratios of range 1.0–1.05, and hydrogen fractions of 40–60%. Compression ratios of high values were appropriate for the mixture due to the reduction in knock. A recent experimental study on the combustion process of ammonia/hydrogen in air showed that there was an ability to improve the combustion characteristics of ammonia/hydrogen under high fraction of ammonia by employing a turbulent jet ignition.⁷⁷ However, in order to tackle the challenges of engine operating at cold start and unstable combustion in a SI engine fuelled with pure ammonia, a numerical investigation was done to crack ammonia by heating it with recycled exhaust gases using reformer producing hydrogen under a temperature of approximately 773 K.⁷⁸ The results of mixing ammonia with hydrogen demonstrated that the increase in H₂ ratio from 2.5% to 12.5% shortened the duration of combustion by 34.9 CA degrees. Moreover, the pressure inside the cylinder was increased to a maximum value of 7.12 MPa at λ = 0.9 due to the excess in exothermic heat release while the minimum value of the pressure and the heat release rate inside the cylinder were at λ = 1.2 due to the decrease in velocity of burned mixture. Mixing 10% H₂ of the reformate at stoichiometric conditions achieved 96.3% combustion efficiency and 43.6% thermal efficiency. NOx emissions were under 1.05 g/kWh at λ less than 1.0 and all values of hydrogen fractions, while the highest value of NOx was 24.4 g/kWh at λ = 1.2 and hydrogen fraction 12.5%. Unburned NH₃ increased with a decrease in λ and hydrogen fraction due to the drop in temperatures in crevices preventing the complete combustion of ammonia. N₂O emissions reduced with an increase in hydrogen from 2.5% to 12.5% but they increased with λ increase.⁷⁸ Table 6 shows a summary about blended ammonia.

Table 6.

Summary about spark ignition engines fuelled by bended ammonia.

Fuel	Mix base	Investigation	Findings			Ref.
80% ammonia20% gasoline	Volume	Experimental	7% thermal efficiency ( $η_{t h}$ ) reduced compared to gasoline			⁷⁵
1:2 NH₃ / H₂	Mole	Experimental	Stable cold engine start			⁵⁸
40–60% NH₃ to H₂	Volume	Numerical	Lowest NOx at λ = 1			⁷⁶
80% NH₃ 20% H₂	Volume	Experimental	Case	λ	H₂	⁵⁵
			Max. $η_{t h}$	=1	<20%
			Down $η_{t h}$ Up NOx, NH₃		>20%
90% ammonia 10% hydrogen	Volume	Experimental	Stable operation, 40% NOx reduced compared to 100% ammonia			⁵⁷
87.5–97.5% ammonia to hydrogen	Volume	Numerical	Optimum	λ	H₂	⁷⁸
			Max. $η_{t h}$	=1	10%
			Min. NOx	<1	2.5%
			Min. N₂O	<1	12.5%
			Min. NH₃	>1	12.5%

Crucially, the research findings in literatures manifest no standard for fuel fraction can combine performance and emissions related to combustion of pure ammonia and ammonia with hydrogen blends. This highlights there is a need for establishing more comprehensive research to innovate the optimum value and condition for ammonia use in ICEs that can be harnessed by Net Zero industry.

Characterizations of dual fuel

The combustion features of ICEs are possibly developed by using ammonia as an additional or secondary fuel. Experimentally, the combustion and emission of a dual fuel of (diesel and ammonia) in CI engine were investigated.⁷⁹ The investigation referred that replacing 40% (by energy) or less of diesel with ammonia fuel lowered emissions of NOx compared to 100% diesel fuel. However, emissions of CO and HC were greater due to the reduction in temperature of combustion. Emissions of soot were not significantly affected by low fraction of ammonia. While, replacing 40–60% (by energy) diesel with ammonia caused ignition delay, raising the quantity of unburned NH₃ due to ammonia is a big reluctant for autoignition. The peak in-cylinder pressure and combustion efficiency dropped due to the reduction in temperature of combustion. The engine brake specific fuel consumption was high. Moreover, the increase in ammonia fuel above 60% (by energy) led to higher NO emissions and specific fuel consumption. Emissions of soot decreased as a result of the reduction in the amount of soot producer, which was diesel. Emission of NH₃ were more than the safe limit. Furthermore, a review about ammonia as a fuel for a CI engine presented that ammonia was a carbon-free fuel that could be employed for diesel engines.⁸⁰ However, ammonia fuel as a low autoignition temperature required diesel for combustion. Dual fuel combustion containing ammonia produced huge emissions of unburned NH₃ and NOx because ammonia was nitrogen-based fuel. The advanced injection strategies could take part to improve performance of engines and reduce pollutant emissions. Hence, an experiment was conducted using two direct injections in a spark-ignition engine to enhance the engine performance.⁸¹ First injection was gasoline, and the second injection was gaseous ammonia. In the experiment the time taken for ammonia injection was figured out. The results showed the increasing ammonia reduced the engine power. The optimal injection for ammonia was 320 BTDC and 370 BTDC at low load and high load, respectively. The peak in-cylinder pressures of the dual fuel of gasoline and ammonia was a little bit less than gasoline alone due to the depressed value of flame speed and temperature of ammonia fuel lowered the temperature of overall combustion. The efficiency by using ammonia-gasoline was close to efficiency of gasoline alone. The growth in the amount of injected ammonia fuel caused a growth in the emissions of unburned NH₃ and NOx, and decreased CO emission. The combustion and emission characterizations of dual fuel (diesel and ammonia) spray engine were studied numerically at low- and high- pressure injections.⁸² Diesel was used as a pilot igniter, then diesel and liquid ammonia were injected as a main fuel. The consequences showed that the best ratio was about 80% by energy for ammonia at low pressure dual fuel (LPDF) injection. The raise in ratio of ammonia increased the possibility of misfire happening. The optimum ratio of ammonia at high pressure dual fuel (HPDF) injection was achieved at 97%. The comparison between HPDF injection and pure diesel injection stated that the indicated thermal efficiencies were similar, and HPDF injection decreased the greenhouse gas emissions and NOx with a slight increase in emissions of N₂O and NH₃. The comparison between LPDF injection and pure diesel injection indicated that LPDF injection has the possibility to rise the indicated thermal efficiency, however, the contribution of LPDF to reduce emission of NOx, N₂O and NH₃ was limited due to the difficulty in burning ammonia. Recently, it was demonstrated numerically that the utilization of ammonia energy fraction between 30% and 70% in dual fuel (diesel and ammonia) reactivity-controlled compression ignition (RCCI) engine with early injection of diesel indicated mean effective pressure and lower the emissions of HC, CO, N₂O, and unburned NH₃ compared to diesel RCCI engine.⁸³ Also, there is another recent research study that employed dual fuel of diesel and ammonia to operate CI engine experimentally using ammonia share from zero to 55% by energy.⁸⁴ Research results concluded that the increasing replacement of diesel by ammonia pumped in-cylinder pressure slightly, climbed peak of heat release rate by just above 20 J/deg, and enhanced each of the combustion efficiency and engine efficiency by a range 3%–4% while increased specific fuel consumption by about 100 g/kWh. The increase in ammonia energy fraction from 23% to 55% increased the delay of ignition time, and the duration of combustion. The increase in ammonia shares up to 55% slumped soot emissions from 0.47 g/kWh to 0.012 g/kWh and plunged CO by roughly 25% whereas the trend of CO₂ increased by 30 g/kWh at 11% ammonia and then collapsed by 200 g/kWh. The increase in ammonia fraction until 44% by energy increased emissions of NOx by 30% in comparison with using diesel alone. Recently, an experimental work was carried out⁸⁵ to predict impacts of ammonia and biodiesel mixture on the efficiency and emission of IC engine. The findings showed that the rise in ammonia fraction to 70% by energy caused a decrease in brake thermal efficiency from 31.8% (pure biodiesel) to 29.4% (highest ammonia fraction) because the combustion of the mixture is unsuccessful at full load. Ammonia boosts the rate of pressure rise. There was a delay in the start of combustion when the ammonia contribution was increased. The emissions of CO₂ and CO were reduced by 510 g/kWh and 30.1 g/kWh, respectively when ammonia contribution was maximized to 69.4%. Another work that was carried out to investigate experimentally and numerically the leverage of ammonia/diesel ratio on performance and emissions of engines concluded that the indicated thermal efficiency elevated by growing ammonia to 84.2% as input energy lowered the GHGs compared to diesel, increased NOx and NH₃ emissions, produced N₂O offset the reduction of CO₂.⁸⁶ The detailed results showed that in-cylinder peak pressure rose with ammonia increasing to 61.62% by energy. Combustion starting and duration dipped by 32 CA and 6.8 CA, respectively, due to increasing the ammonia energy share. Ammonia at 84.2% by energy dropped drastically the emissions of CO, CO₂, and particulate matter, while NO₁, NO₂, N₂O, and unburned ammonia emissions were increased. Ammonia combustion produces N₂O of 90 ppm. Additionally, diesel should be replaced with ammonia at 35.9% as a minimum to lower the GHGs emission.

On the other hand, the dual fuel of H₂ and NH₃ for fuelling an engine of heavy-duty truck was simulated using a special design.⁸⁷ The design replaced the spark plug and diesel injector with a fuel injector and added a small volume pre-chamber connected with the main chamber via orifice. The pre-chamber supplied with another spark plug and fuel injector to control the ignition starting. The simulation ran the engine in different modes, and the results revealed that the fuel efficiency was improved due to a reduction in the amount of fuel heat that was used to rise the temperature of the cold NH₃ to ignition temperature.

Commercial ammonia internal combustion engines

The major worldwide commercial ammonia-powered ICEs that have undergone full-scale bed tested produced near zero carbon emissions are listed in Table 7, according to the Ammonia Energy Association on March 11, 2025,⁸⁸ and NYKline on 14 May 2025.⁸⁹

Table 7.

Full-scale tested commercial ammonia fuelled ICEs.

Make	Brand	Specific	Fuel	Performance findings	Emission findings	Commerce
WinGD, Swiss	X-DF-A	LNG-designed1-cylinder2-stroke52 cm-boreRate 80 rpmpropulsion CI	Dual-fuel ammonia,5% pilotmethanol	Full-load same thermal efficiency of diesel fuel	NH₃ 10 ppmN₂O 3 ppm, NOx below diesel	Available for order
MAN Energy Solutions (ES), RCC	ME-LGIA	LNG-designed1-cylinder2-stroke50 cm-borefull 167 rpmpropulsion CI	Dual-fuel ammonia,5% pilotmethanol	Full-load stable combustion	N₂O virtually zero,NOx 40% less oil fuel	Ready 2026
NYK, IHI, and ClassNK, Japan	tugboat Sakigake	LNG-designed4-strokepropulsion CI	Dual-fuel ammonia,5% pilot fuel	Successful	N₂O virtually zero,	To deliver in November 2026.

RCC: Research Centre Copenhagen.

However, several companies and institutions are playing a role in development of ammonia powered engines either blended fuel spark ignition or dual fuel compression ignition as listed in Table 8.⁷⁰

Table 8.

Key players involved in development of ammonia fuelled engines.

Fuel	Spark ignition engines	Compression ignition engines
Ammonia-hydrocarbon	Bigas International (Italy) fitted NH₃ engine into Toyota GT86-R Marangoni sports car	Hydrofuel Inc., (Canada)
	South Korean Institute for Energy Research (KIER)	Sturman Industries (USA)
	Research group at the Xiamen University (China)	Caterpillar Inc.
	Caterpillar Inc.	MAN Energy Solutions
		University of Minnesota
		Iowa State University
		MITSUI E&S commenced a test operation of large-bore, low-speed, two-stroke commercial MITSUI-MAN ES engine⁸⁸
Ammonia-hydrogen	Hydrofuel Inc., (Canada)	No feasible⁹⁰
	KIER
	Iowa State University
	Toyota Central R&D Labs Inc.
	Nevada Corporation, Hydrogen Energy Centre (USA)

Reduction of NOx emissions from ICEs

The techniques for reduction of NOx emissions from ICEs are classified into (1) general techniques to reduce NOx; (2) techniques for reduction of NOx emissions by ammonia decomposition; and (3) reduction of NOx emissions by ammonia decomposition via microwave.

General techniques to reduce NOx emissions from ICEs

The selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) are the most effective techniques used in exhaust gas aftertreatment of cars and ships for NOx reduction. The SCR is activated by injecting the exhaust gas flow with chemicals, such as ammonia, urea, hydrogen, hydrocarbons, and carbon monoxide that react with NOx and unburnt NH₃ converting them into N₂ and H₂O with catalyst assistance. SNCR is an injection of nitrogen content selective reductant, such as NH₃ and urea into a post-combustion flue gas to react with NOx, producing N₂. Ammonia is an efficient factor for reduction of NOx emissions. Urea is a safe and low-toxicity alternative to ammonia. Hydrocarbon-based SCR systems are lowering operating temperatures and improving efficiency and it is environmentally friendly, particularly in low-temperature operations.⁹¹ Other common techniques for NOx emission reduction are EGR, TWC, LNT, HAM, and fuel emulsions. EGR is exhaust gas recirculation, which recirculates exhaust gas into intake air to lower combustion temperature and NOx. TWC is a three-way catalytic converter, works under stoichiometric conditions, to convert NOx, CO, and HC to N₂, CO₂, and H₂O. LNT is lean NOx trap that stores NOx during lean operation and reduces it during rich operation. Humid air motor (HAM) adds water vapor to intake air to lower NOx. Fuel emulsions (water in fuel) reduce NOx through the decrease in combustion temperature. Table 9 shows the difference between cars and ships related to NOx techniques and regulations.^92,93 Also, there are works that were conducted trying to mitigate NOx emissions from ICEs, especially the ammonia combustion engines are demonstrated in Table 10.

Table 9.

Nox emission reductions and regulations used in cars and ships.

Category	Passenger car engine	Ship engine
Typical engine type	Small gasoline/diesel ICE	Large 2-stroke/4-stroke marine
Fuel type	Gasoline, Diesel	Heavy Fuel Oil (HFO), Marine Diesel Oil (MDO), LNG
Selective catalytic reduction (SCR)	Used in diesel vehicles; small urea tank	Large reactors; requires heating, dosing systems, and urea tanks
Exhaust gas recirculation (EGR)	Common; compact design	Complex; needs water scrubbers due to high sulphur
3-Way catalytic converter	Standard for gasoline cars	Not used
Lean NOx trap (LNT)	Used in lean-burn or old diesel	Not used
Water-based	Rare	Emulsions
Fuel switching	Rare (except hybrids)	Used with LNG-fuelled ships
Regulatory body	Euro 6/7 (EU), EPA Tier 3/4 (US)	IMO MARPOL Annex VI (Tier I, II, III), Emission control areas (ECAs)

Table 10.

Works aimed to reduce NOx emissions from ICEs.

Technique	Investigation	Topic	Findings	Suggestion	Reference
Adding H₂ to NH₃	Experiment	Effect of varying amount of air to different ratios of ammonia with hydrogen on combustion processes and its emissions	Mixing ammonia with 10% by volume hydrogen improved engine efficiency and work for a SI engine while higher level of H₂ caused a higher loss in the heat of combustion.NOx and unburned ammonia were still the considered factors in the emissions of the ammonia/hydrogen combustion.	Use selective catalytic reduction (SCR) technology in an exhaust after treatment to address the emissions of NOx and unburned NH₃	⁹⁴
Adding H₂ to NH₃	Experiment and numerical	Ammonia combustion	Adding hydrogen in conditions of lean combustion increased the production of NOx	Supply the exhaust gas after-treatment with (SCR) device.	⁹⁵
Adding hydrogen and nitrogen	Numerical	Effect of adding hydrogen, syngas, and nitrogen on combustion and emissions of a heavy-duty natural gas/diesel RCCI engine	Adding H₂ and syngas at operations of low and medium loads optimized the flame speed, combustion efficiency, power and thermal efficiency compared to increased diesel fraction, as well as CO and HC emissions were plummeted. NOx emissions were greatly increased.Adding N₂ worked as a dilution gas, which reduced the value of peak pressure at high load operation.Adding H₂ and N₂ enhanced the ignition timing compared to gasoline, leading to high loads operating.	Employing catalytic fuel to reduce the emissions of RCCI engine.	⁹⁶
In-cylinder inoculation of ammonia	Numerical and experiment	Analysed NOx emissions in a commercial engine of a diesel-hydrogen dual fuel	In comparison with diesel engine, 10% hydrogen ratio increased the peak pressure and maximum temperature by 5.3% and 5.7%, respectively. Emissions of CO₂, CO and HC decreased but emissions of NOx increased up to 18.3%.NOx emission reduced to 70% by inoculating 3% (NH₃ into the cylinder), when the temperature was about 1200 K. An excessive quantity of ammonia caused increasing in unburned NH₃ in the exhaust.		⁹⁷
After combustion ammonia injection	Experiment	Determining NOx amounts in two flames, which were NH₃ in air and NH₃/H₂ in air using a shock tube under variable operating conditions	The flame of pure NH₃ had low speed and high ignition delay time. The flame of NH₃ and H₂ mixture raised flame speed and NOx emissions.	Reaction of NH₃ and O₂ produces NO. NH₃ increase in after ignition burns NO.	⁵¹
Selective non-catalytic reduction	Simulation	One and two-zone sprays of urea as a NOx reducing agent in a catalytic exhaust gas after-treatment.	One-zone spray produced incorrect speculation for performance of the SCR	Must use two-zone spray rather than one-zone	⁹⁸
Premixing fumigated NH₃ with a pilot fuel	Simulation	Employed ammonia with each of diesel, kerosene-diesel, and kerosene as a pilot fuel	Ammonia growing to 80% by energy fraction increased the peak pressure in the cylinder and the indicated thermal efficiency. The fumigated ammonia reduced the emission of CO₁ and CO₂.The emission of NOx decreased due to the use of fumigated ammonia while it increased when the fraction of ammonia fumigation accessed 60% by energy even the combustion temperature was low		⁹⁹
Water injection	Numerical and experiment	NOx reduction by comparing direct in-cylinder ammonia injection with in-cylinder water injection in a marine diesel engine	NOx reduction by ammonia injection at top dead centre (TDC) was smaller than that by water injection because water reduced excessively the fuel temperature inside the cylinder, where NOx is very sensitive to the temperature.The injection of water achieved a highest value (57.1%) for NOx depletion at 2.1 CA ATDCAmmonia injection at 58.4 CA ATDC achieved NOx fall by about 78.1%.	Injection of NH₃ near TDC works as a fuel instead of NOx inhibitor.	¹⁰⁰
Multiple injection of fuel	Review	Fueling CI engines with ammonia	The successful operation of ammonia was 40%–80% fraction within compression ratio from 35:1 to 100:1 as published in the literature.NOx emissions were produced from a high combustion temperature whilst unburned NH₃ resulted from a low combustion temperature.	Emissions of N₂O, NOx, and NH₃ from an ammonia dual-fuel engine can be reduced by controlling quantity and injection number.	⁸⁰
Injection conditions	Numerical and experiment	Using KIVA3V code with reaction mechanism of C₇H₁₆. Measuring in-cylinder pressure, NOx, and CO concentrations at the exhaust were at five engine loads considering same injected fuel mass.	Firstly, advanced injection timing increased average in-cylinder peak pressure and NOx, and lowered CO. Secondly, advanced injection duration dropped average in-cylinder peak pressure and NOx, and raised CO.		¹⁰¹
Add H₂ to SCR	Review	Innovative catalysts for the selective catalytic reduction of NOx with H₂	H₂ enhances SCR at low temperature. The reaction temperature reduces to below 200 °C. Water vapour was the byproduct. The absence of O₂ in the reaction forms N₂O or NH₃ byproducts.	Considering safety tests to mitigate the risk of H₂ storage.	¹⁰²

Techniques for reduction of NOx emissions by ammonia decomposition

To overcome the challenges associated with ammonia combustion and its emissions, several studies suggested alternative approach such as decomposing NH₃ to H₂ and N₂ using hydrogen as a fuel¹⁰³ or ignition promoter.¹⁰⁴ Hydrogen reduces nitrogen-based emissions. However, ammonia decomposition inside an engine's cylinder is difficult to be controlled and the endothermal reaction reduces the temperature and pressure inside the cylinder. Hence, ammonia decomposition should be before the engine intake. The common method that is employed for rising ammonia temperature uses heat recovery of exhaust gases incorporating a reformer.^72,78 The required temperature for catalytic decomposition of ammonia is approximately 500 °C, however, the temperature of exhaust gases is less than 400 °C,¹⁰⁵ especially at cold start. Hence, other methods were considered to raise the temperature of ammonia such as, chemical interaction¹⁰⁶ and plasma.¹⁰⁷ Also, catalytic furnaces or electric heaters were used together with exhaust gases to increase NH₃ temperature as demonstrated in Table 11.

Table 11.

Experiments on decomposition of ammonia to hydrogen by using catalytic reformer and energy recovery of exhaust gas.

			Inlet to reformer			Exhaust from reformer				Finding
No	Process	Catalyst	Gases	Mix %	Flow rate	Temp. (°C)	Gases	Flow rate	Engine	Decomposition temp. (°C)	Reference
1	Combustion - decomposition (catalyst + tube furnace 400 °C + exhaust components 150–400 °C)	Ru-Al₂O₃ (16 g) vertical stainless-steel reactor 15 mm diameter in a furnace	NH₃N₂O₂/NH₃Exhaust(Co, Co₂HC, NONO₂, N₂O)	50–10050–00.06–0.175	2–4 L/min2–0 L/mintotal 4 L/minGHSV 36,000/h	400–600	H₂N₂NH₃	2.6–3.0 L/min-1.05–0.28 L/min	Diesel of constant load 1500 rpm	400–800	¹⁰⁵
2	Decomposition (catalyst + electric heater 2 kW, 600 °C + exhaust heat)cold start	Ru bed 3 mm thick (0.2 L)	NH₃	100	5000 L/h.kgcat(0.9 m³/h)	450	H₂N₂	1380 L/h-	NH₃/H₂ used in gasoline 4-stroke 2-cylinder SI 5000 rpm. H₂ cooler before engine.	450	¹⁰⁸
3	Decomposition (+2.6 W)catalyst + electric furnace (−76 W) + exhaust heat (450–550 °C)(−9 W)	Rhodium-Platinum22 mm diameter77 mm length650 °C	NH₃N₂	199	Total GHSV 16,000 mL/h.gcat(3.4 g/h)	400	H₂N₂	--	Gasoline/H₂ direct injection.Reduce fuels consumption and CO2 by 30.5%	550	¹⁰⁹
4	Combustion - decomposition(+2.6 W)catalyst + electric furnace (−76 W) + exhaust heat (450–550 °C)(−9 W) + exhaust components (−17.4 W)	Rhodium-Platinum (Rh-Pt)0.5 L22 mm diameter77 mm length650 °C	NH₃N₂O₂Exhaust(H₂, H₂O, CO, CO₂, HC, NO, NO₂)	186.5–60.90.712.5–38.1	Total GHSV 16,000mL/h.gcat(3.358 g/h)	400	H₂N₂NH₃H₂OCOCO₂HCNONO₂		Gasoline/H2 direct injection.Reduce fuels consumption and CO₂ by32%	550

Reduction of NOx emissions by ammonia decomposition via microwave

Several experiments used microwave heating with catalyst to assist the exhaust gases or replace them at cold engine start in order to facilitate ammonia heating and decomposition producing hydrogen fuel as shown in Table 12.

Table 12.

Experimental works using microwave heating with catalyst for ammonia decomposition to hydrogen at atmospheric pressure.

	Specifications				Findings (decomposition)
No.	Frequency (GHz)	Power (W)	Catalyst	FlowratemL/h.gcat(mL/min)	Temperature (°C)	Conversion (%)	Remarks	Reference
1	2.3–2.7 Single mode	0–100	5%Co/Al₂O₃, 100 mg, particle size 250–500 µm	(40)	600	Start after15 s100% in 60 s.	Catalyst with addition of 10 mg SiC powder improved microwave heating efficiency	¹¹⁰
2	2.45	60	12 wt% Molybdenum incorporated mesoporous carbon	36,000(60)	400	100	Highest conversion of ammonia was 49% with conventional heating with catalytic reaction at 600 °C	¹¹¹
3	2.45	20–40	7.7 wt% Iron incorporated mesoporous carbon catalysts	36,000	450	100	Conversion in conventional heating with catalytic reactor was at 600 °C	¹¹²
4	2.45	2000	Carbon supported cobalt (mesoporous carbon and activated carbon)	36,000 catalyst 0.1 g	350–400	100	Conversion in conventional heating with catalytic reactor was at 550 °C	¹¹³
5	2.45	-	Co-Fe@AluminaCo@AluminaFe@AluminaMix 0.1 g catalyst with 0.1 g mesoporous carbon	36,000(60)	400	100	Conversion in conventional heating with catalytic reactor was at 550 °C	¹¹⁴

CFD studies related to ICEs

The review of literatures that studied CFD models for analysing ammonia combustion, or performance and emissions of ICEs is shown in Table 13.

Table 13.

CFD models related to combustion process in ICEs.

No.	Case study	Engine and fuel type	Simulation code	Turbulence model	Chemical reaction mechanism model	Flame propagation modelSpray model	Boundary conditions, mesh elements, time step	Code validation	Ref.
1	Analysis of a selective catalytic reduction (SCR) system with NH₃ as an agent for reducing NOx emissions	Aftertreatment of a single cylinder diesel engine	AVL FIRE, ESE aftertreatment module	k-ε	GmBH FIRE	-	BC: No slip at wall.Element size: 2 mm	Close agreement NOx conversion to N₂ + H₂O	¹¹⁵
2	Developed software package entitled reaction mechanism generator (RMG)	Applied to different fuels	Python version of RMG contain CanThem	-	Automatic generation of chemical kinetic mechanism	-	-	Succeed for molecules containing C, H, O, S, and N	¹¹⁶
3	Reduction of NOx emissions by injection of NH₃ into the combustion chamber	Diesel-hydrogen engine, 10-cylinder, 4-stroke, 128 mm bore, 142 mm stroke, 1-intake valve, 1-exhaust valve, 1-diesel direct injector	OpenFOAM based on the finite volume method, C++ programming language	-	-	-	Element no.: 315000Time step: 0.1 CA	Error 3.8–6.7% in pressure	⁹⁷
4	Effect of swirl flame, secondary air injection, and inlet pressure on emissions of NOx, unburned NH₃, and H₂	NH₃ with air premixed combustor of a gas turbine like	OpenFOAM	k-Large eddy simulation (LES)	Partially stirred reactor mode (PaSR), CHEMKIN-PRO	Swirl flame calculated	WALE model, Element volume: 0.25 mm³	No validation	¹¹⁷
5	Investigation on the combustion and emission of multiple pilot fuels injection of Kerosene-diesel and NH₃, and injection timing variation	Diesel CI engine, 4-cylinder, 2.5 L displacement	KIVA4-CHEMKIN	(RNG) k-ε	CHEMKIN	Spray breakup model: Kelvin–Helmholtz Rayleigh–Taylor	-	Well agree of pressure and apparent heat release rate	⁹⁹
6	Developed 2-D CFD kinetic model to predict the turbulent reacting flow for NH₃/H₂ premixed	Combustion under gas turbine conditions	Developed code	Large-eddy-simulation (LES)	Reduced chemical-kinetics mechanism model for NOx emission	-	Time step: 0.2 s	Acceptable prediction relative to the full mechanism	¹¹⁸
7	Developed a 1-D kinetic model to assess the best reaction mechanisms describing NH₃/CH₄ blends	Combustion in a gas turbine	Developed code	Chemical Reactor Network (CRN) for swirl burner	CHEMKIN-PRO	Perfect stirred reactor (PSR) for premixing and primary flame.Plug flow reactor (PFR) for post flame	-	Best agreement with Tian and Teresa mechanism	¹¹⁹
8	Effects of reformed exhaust gas recirculation on the HC and CO emission	SI engine, fuel LNG, 4-stroke, 6-cylinder, 123 mm bore, 145 mm stroke, 2-intake valve, 2-exhaust valve, 1-spark plug	Converge CFD	(RNG) k–ε	SAGE	GRI-Mech 3	BC: Intake air temperature 330 K, intake pressure 1.72 bar	Error 6% peak pressure, 5% emissions	¹²⁰
9	Injecting a pilot diesel fuel to ignite a mixture of NH₃ and air	Dual-fuel CI engine, 4-cylinder, 4-stroke, 105 mm bore, 127 mm stroke, 1-intake valve, 1-exhaust valve	AVL BOOST	-	-	-	-	Error 2% pressure	¹²¹
10	Analysis of NOx Emission reduction via direct in-cylinder NH₃ injection	Commercial Marine diesel engine, 6-cylinder, 4-stroke, 2-intake valve, 2-exhaust valve, direct injector	OpenFOAM	C++ for RANS and k–ε	Kinetic scheme: Ra and Reitz.NOx formation model: Yang.NOx reduction model: Miller and Glarborg	Spray breakup Model: Kelvin–Helmholtz Rayleigh–Taylor (KHRT)Evaporation model: Dukowicz	BC: Intake Temperature 514 K, intake pressure 2.72 barElement no.: 501769-1264873	Satisfactory Agree of pressure and heat release rate	¹⁰⁰
11	Use high-pressure dual-fuel NH₃ and H₂ for combustion in heavy-duty engines	Diesel engine, 170 mm bore, 210 mm stroke, 2-intake valve, 2-exhaust valve, dual fuel injector	CONVERGECFD	RANS and k–ε	Redlich–Kwong equation of state	Spray breakup model: Kelvin–Helmholtz Rayleigh–Taylor (KHRT)Particles model for liquid spray: LangragianDroplet collision: No time counter (NTC) method	BC: Wall, inlet pressure 6.5 bar, exhaust pressure 5,2 bar.Element size: 1 mm	Satisfying results of pressure and heat release rate	⁶³
12	Comparison between low- and high-pressure injection	CI engine, dual-fuel of diesel-pilot-ignition ammonia, 95 mm bore, 102 mm stroke	CONVERGE code package	(a) LES(b) RNG k−ε	SAGE	GRI3.0Spray breakup model: KHRTDroplet collision: NTCEvaporation: Frossling model	BC: Intake temperature 322 K, intake pressure 1.3 bar.Element size: 1–1.8 mm, adaptive mesh refinement 0.25–0.45 mm	Good agree for ignition delay under different start of pilot diesel injection timing	⁸²
13	Combustion and emission characteristics	SI engine of NH₃ added with H₂-rich gas from exhaust-fuel reforming, 4- stroke, 6-cylinder, 123 mm bore, 145 mm stroke, 2-intake valve, 2-exhaust valve, 1-spark plug	CONVERGE v2.4 CFD tool	RNG k- ε	SAGE	Okafor mechanism	Max. grid size 8 mm, adaptive mesh refinement 0.5–2 mm	Validation published in (Long et al., 2018)	⁷⁸

Methodology

CFD model for internal combustion engines

The methodology for assessing the thermal engine efficiency, work, and power of ICEs involves a comprehensive analysis of various parameters over time and space. These parameters include pressure, temperature, density, chemical heat release, and thermal losses within the engine cylinder. To achieve this, finite volume method with SIMPLE algorithm is employed for solving sets of unsteady three-dimensional governing equations.^122–124 These equations encompass mass, momentum, energy, chemical reaction mechanisms, and state equations. Additionally, the methodology incorporates the re-normalized group k–ε model, which is pivotal in accurately modelling turbulent flow. This model is essential for understanding and predicting the complex behaviour of fluids within the engine. Furthermore, flame propagation models are integrated into the analysis providing a more detailed and precise representation for the process of combustion. The flowchart in Figure 1 shows the methodology of CFD model for spark-ignition engines that is designed by authors of this article. This holistic approach ensures a thorough understanding of the internal dynamics of ICEs, leading to more efficient and environmentally friendly engine designs. The CFD investigations were applied on two different premixed spark ignition ICEs as shown in Table 14, by employing ANSYS Forte. The computational geometry was built with moving piston and valves. The computational mesh of the engine geometry was generated automatically on-the-fly during the simulation by Ansys Forte using 2- and 2.5-mm global mesh size and refined by utilizing additional meshes as shown in Table 15. The CFD model setup are stated in Table 16.

Figure 1.

Flowchart of the methodology of CFD model for spark-ignition ICE.

Table 14.

Specifications of two spark ignition engines used for CFD investigation.

Parameter	Gasoline premixed SIE	LNG premixed SIE
Cylinders	1	6
Strokes	4	4
Bore	89 mm	123 mm
Stroke	79.5 mm	145 mm
Displacement	0.5 L	1.723 L / cylinder
Compression ratio	12.5	11.5
Connecting rod length	138.1 mm	290 mm
Intake valves per cylinder	2	2
Exhaust valves per cylinder	2	2
Engine speed	1500 rpm	1362 rpm
Piston offset	0	0
Fuel	IC₈H₁₈ + H₂O + CO₂	CH₄ + H₂

Table 15.

Additional mesh types.

Mesh type	Fraction of global mesh size
Solution adaptive mesh of temperature (SAM-T)	0.5
Solution adaptive mesh of velocity (SAM-V)	0.5
Solution adaptive mesh of G-equation (SAM-G)	0.125
Surface refinement wall boundaries, such as head, liner, piston, valves, and manifolds	0.5, except 0.25 for valves ends that seat to the port
Continuative flow at open boundaries of inlet and outlet	0.5
Squish region near TDC at the end of compression stroke	0.25
Spark region	0.125

Table 16.

CFD model setup of Figure 1.

Parameter	Description
Boundary wall temperature (Gasoline SIE)	385 K, piston 420 K, exhaust valves 497 K
Initial temperature (Gasoline SIE)	330 K @ 350 crank angle (CA) degree
Initial pressure Gasoline SIE)	0.8 bar @ 350 CA degree
Boundary wall temperature (LNG SIE)	550 K, liner 400 K
Initial temperature (LNG SIE)	550 K @ 620 CA degree
Initial pressure (LNG SIE)	1.825 bar @ 620 CA degree
Compression computation	Equation of state
Transport model	Eulerian two-phase flow
Turbulent model	Renormalization group (RNG) k-ε
Spark start (Gasoline SIE)	688 (CA) degrees
Spark start (LNG SIE)	692 (CA) degrees
Duration of the spark	10 CA degrees
Energy release rate for spark	30 J/sec
Efficiency of energy discharge spark	0.5
Chemical reaction computation	Ansys Chemkin Reaction Workbench
Autoignition	During 620–850 CA degrees.
Laminar flame speed	Power law
Solution convergence	1 × 10⁻¹²

Techno-economic approach on hydrogen re-production by ammonia decomposition using microwave heating

The objective of this section is to pay attention on storage and heat energy investment by microwave heating to support the thought of ammonia hydrogen fuelling.

The key concept of energy fraction for ammonia decomposition by chemical equilibrium states that the energy content of H₂ produced from NH₃ decomposition is more than the reactant NH₃ as shown in equations (5)–(7).

Equilibrium : N H_{3} \to 1.5 H_{2} + 0.5 N_{2} Δ H = + 46.19 \frac{kJ}{mol}

(5)

Mass fraction : (1 g) {NH}_{3} \to (0.177 g) H_{2} + (0.823 g) N_{2}

(6)

Mixture fraction by energy : (18.8 kJ) {NH}_{3} \to (21.24 kJ) H_{2} + (0) N_{2}

(7)

The microwave heating, aforementioned, outweighs conventional heating in aspects that are articulated in Table 17. Technically, microwave heating reduces the temperature of ammonia decomposition by about 100 °C–200 °C in comparison to conventional heating as revealed in Table 12. It should be noted that ammonia is a limited absorber for electromagnetic radiation, since the dielectric constant of ammonia is 16.61 at 20 °C.¹²⁵ Therefore, a good absorber of electromagnetic radiation, for example, carbon¹²⁶ is required to promote the absorption and dissipation of electromagnetic energy as a thermal energy, and then conduct heat to ammonia. The proposed technique to prevent the spread of electromagnetic waves is using meshes of spaces’ length shorter than electromagnetic wavelength.

Table 17.

Comparison between conventional heating and microwave heating.

Feature	Conventional heating	Microwave heating
Method	Convert electrical energy to thermal energy or transfer heat	Convert electrical energy to electromagnetic energy by magnetron and then to thermal energy by dielectric heating
Heating control	Low	High
Operating cost	Heat loss to surrounding and through walls	Less temperature, time and energy consumption. No heat loss
Check up	Periodically	Periodically
Maintenance	Required for long-term sustainability	Not required.Disposal after a specified term
Emissions	The source or grid emissions for microwave heating are less.The emissions of materials inside the microwave cavity are lower because heating created from inside to outside.
Risk	Fire or electric shock due to failure in moving parts or electrical connection.Damage due to thermal stress	Skin burns due to electromagnetic leakage.Magnetron damage due to empty cavity
Overall risk assessment	Lower for microwave
Regulations	Risk management for safety	Risk management for safety and control
Life-cycle cost	Longer life span with less efficiency	Less cost considering above points

The economic benefit for re-production of H₂ from NH₃ using microwave heating system can be analysed from a comparison between (a) re-production efficiency and (b) storage efficiency of H₂.

NH₃ production from H₂ → Storage of NH₃ → H₂ production from NH₃

Efficiency of NH₃ production from H₂ by Haber–Bosch process:

η_{1} = 61 %

³²:

Efficiency of H₂ production from NH₃ by microwave heating:

η_{2} = \frac{e n e r g y c o n t e n t_{H 2}}{\frac{p o w e r c o n s u m e d}{{\dot{m}}_{N H 3}} + e n e r g y c o n t e n t_{N H 3}} \times 100 %

(8)

Energy content for H₂ and NH₃ are available in Table 5.

The consumed microwave power is 100 W as shown in Table 12.

$\overset{\dot{}}{m_{N H 3}}$ is the mass flow rate of ammonia, and it can be calculated from density and volume flow rate. ρ_NH3 = 0.73 kg/m³ (Table 5); v_NH3 = 60 mL/min (Table 12) = 60 × 10⁻⁶ m³/min

m_{\dot{N} H 3} = ρ . v = 0.73 \times 10^{- 6} \frac{kg}{s}

(9)

η_{2} = \frac{120 \times 10^{6}}{\frac{100}{0.73 \times 10^{- 6}} + 18.8 \times 10^{6}} \times 100 %

(10)

η_{2} = 77 %

Efficiency of ammonia storage:

Energy density for NH₃ and H₂ are 7.1 MJ/L and 2.9 MJ/L, respectively from Table 5.

η_{3} = \frac{e n e r g y d e n s i t y_{N H 3}}{e n e r g y d e n s i t y q_{H 2} + e n e r g y d e n s i t y_{N H 3}} \times 100 %

(11)

Efficiency of ammonia storage, $η_{3} = 71 %$ re-production efficiency of H₂:

η_{p} = η_{1} \times η_{2} \times η_{3}

(12)

η_{p} = 0.61 \times 0.77 \times 0.71 = 0.3335 = 33.35 %

(13)

(b) Storage of H₂:

η_{s} = \frac{e n e r g y d e n s i t y_{H 2}}{e n e r g y d e n s i t y q_{H 2} + e n e r g y d e n s i t y_{N H 3}} \times 100 %

(14)

η_{s} = \frac{2.9}{2.9 + 7.1} \times 100 % = 29 %

(15)

Therefore, the system efficiency of re-production of H₂ (η_p) is around 4.5% more than the efficiency of H₂ storage (η_s).

Preliminary results of current CFD research

The CFD software was verified by comparing the simulation results of gasoline engine with Ansys-Forte tutorials¹²⁴ and the results showed a good agreement as shown in Figure 2. The CFD model was validated by comparing the investigation results of LNG engine with experimental results.¹²⁰ The results stated that the operation time of 2 mm global mesh size lasted 4 times longer than that of 2.5 mm. Overall uncertainty of peak pressure, peak pressure location, and chemical heat release rate by using 2.5 mm global mesh size was 2.4%.

Figure 2.

CFD investigation results show pressure, heat release rate, NO₁, NOx and CO₁ versus crank angles of gasoline spark ignition engine.

The current CFD investigation is on replacing methane (LNG) with ammonia using same LNG engine and conditions to compare the effect of each of ammonia and methane combustion on spark ignition engine performance and emissions. The comparison results of 0.2668 mole fraction ammonia that equals (in energy) to 0.1 mole fraction methane are shown in Figure 3 for NOx vs equivalence ratio and Figure 4 for performance and emission parameters at equivalence ratio 1. The results state that NH₃ in the conditions of current investigation can be combusted in a spark ignition ICEs producing in comparison to CH₄ a mitigation in NOx emissions within the range of 0.8–1.2 equivalence ratio achieving around 40% reduction at equivalence ratio 1 due to the decrease in air-nitrogen. The thermal efficiency was increased by about 2.5% due to the increase in in-cylinder temperature and pressure, in addition to zero carbon emissions.

Figure 3.

Preliminary results of the current CFD investigation show the effect of replacing CH₄ by NH₃ on NOx emissions of spark ignition ICE.

Figure 4.

Preliminary results of the current CFD investigation show the effect of replacing CH₄ by NH₃ on performance and emissions of spark ignition ICE.

Conclusions

The conclusions that are analysed from the article are:

Ammonia as a sustainable fuel: Approximately 4% of ammonia production is derived from sustainable sources.

Ammonia as a clean fuel: Ammonia is a free carbon energy. The undesired byproducts, such as unburned NH₃ and NOx produce from the ammonia combustion can be treated by a selective catalyst reduction system.

Economy of storage and transport: Ammonia is a hydrogen energy carrier. The energy of ammonia stored at 1 MPa is higher by about 2.5 times in comparison with that of hydrogen stored at 70 MPa.

Safety of storage: Ammonia can be stored in insulated tanks at 0.8 MPa while hydrogen should be stored under high pressure of about 70 MPa. The boiling point of ammonia is −33 °C whereas it is −161 °C for methane and −252 °C for hydrogen.

Environment and health risks: Ammonia is a highly toxic, corrosive, low flammable, non-active material without a reaction agent. It is preferred to be used for power generation than power transport for less risk. Productions of ammonia combustion are harmful gases for health and environment.

Significant combustion characteristics of ammonia: The temperature and energy required for attaining ammonia combustion are high, which are 651 °C and 8 mJ, respectively causing great NOx emissions. Speed of flame is low. Flammability is limited by 16–25% (volume). Energy content (18.8 MJ/kg) is low comparable with other fuels.

Impact of ammonia blends: Blending ammonia with 10% hydrogen by volume tackles ammonia autoignition problem and improves engine efficiency and work output in spark ignition engines. However, the concentrations of hydrogen and oxygen affect the amount of NOx production.

Impact of ammonia dual fuel: Replacing up to 40% by energy of diesel with ammonia fuel lowers emissions of NOx compared to 100% diesel fuel. However, emissions of CO and HC are greater due to the reduction in temperature of combustion.

Reduction of NOx emissions using innovative techniques: The in-cylinder ammonia injection at 58.4 CA ATDC achieves up to a 78.1% reduction in NOx emissions. The selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) promise further NOx emission reduction. Premixed hydrogen injection enhances further the overall combustion and engine efficiency and catalyst-aftertreatment hydrogen injection reduces NOx emissions.

Efficiency of microwave heating for ammonia decomposition: Ammonia decomposition using microwave heating occurs at temperatures 100–200 °C lower than conventional methods. This process results in an ammonia-to-hydrogen conversion efficiency of 100% at about 400 °C. This approach makes an approximately 4.5% increase in economic efficiency comparable to traditional hydrogen storage and transport methods. Additionally, the integration of microwave-heated ammonia decomposition into ICEs provides potentially a continuous supply of hydrogen with a risk less than the hydrogen storage.

Advancements in computational fluid dynamics (CFD) modelling: The preliminary CFD results in comparison to CH₄ state that it is potential to use ammonia as a clean fuel for spark ignition engines with 2.5% rise in thermal efficiency, zero carbon emissions and close to 40% lowering in NOx emissions.

Currently, a CFD model is under development to explore the optimum concentration of mixing H₂ to NH₃ and to optimize boundary conditions with operating conditions to get a best performance and less emissions for ICEs.

Footnotes

Acknowledgements

The authors would like to acknowledge Teesside University for the financial and technical support to conduct this research work. We would also like to thank Mr Oluwagbemiga A. Fabunmi from Teesside University for his valuable time for proofreading of this article and providing feedback.

ORCID iDs

Haitham B Al-Wakeel

Tariq G Ahmed

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Haitham B Al-Wakeel is a Researcher in Engineering and Energy Technology at Teesside University. His area of research is computational fluid dynamics (CFD), energy decarbonization and investment, internal combustion engines, carbon capture and combustion, hydrogen, ammonia combustion, turbomachinery, microwave heating and heat recovery.

Tariq G Ahmed is a Chartered Engineer, Researcher and Senior Lecturer in Engineering (Net Zero) at Teesside University. His area of research is CFD modelling, Life cycle assessment, hydrogen production and application, applications of geopolymer in carbon capture, hydrogen and ammonia clean fuels and carbon neutral cement production.

Hafiz MF Amin is a Senior Lecturer in Mechanical and Aerospace Engineering at Teesside University. His research focuses on clean and efficient energy systems, emission reduction and decarbonisation.

Nashwan Dawood is a Research Director of NetZero Industrial Innovation Centre at Teesside University. His area of research is construction management, digital construction, smart energy, hydrogen and decarbonisation.

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