LNG in some respects is considered a cleaner fossil fuel as it produces less SOx, NOx, and PM in comparison to heavy fuel oil and distillate fuels, however, methane (CH₄) is a primary component of natural gas and is a powerful GHG and has a radiative force that is 30 times stronger than CO₂ over a period of 100 years [12]. This means that the potential benefits of natural gas are being questioned due to the leakage of methane (methane slip) during production, transportation, and combustion, especially at low speed and low load operations of natural gas compression engines [12]. It is crucial to gain a better understanding of the GHG emissions from LNG related activities to achieve national and international climate goals and global climate objectives.

At normal temperature and pressure, LPG is in gaseous form, however, when it is under pressure or cooled, it changes to a liquid form which is the handling principle onboard LPG carriers. LPG is a suitable and environmentally friendly fuel option and due to its unique features and cost-effectiveness, it can be considered as a good choice for certain vessel types [13].

While LPG has a higher energy content than some other alternative fuels such as alcohols, which makes it suitable for use in modern dual-fuel engines, however, it has not been widely adopted as compared to LNG due to its lower ability to reduce emissions and the different safety concerns it poses [14].

Methanol (CH₃OH or MeOH), also known as wood alcohol, is a clear, colorless liquid that is widely used as a solvent, fuel, and chemical intermediate [15]. As a marine fuel, methanol has gained attention for its potential to reduce emissions, increase energy efficiency and decrease the dependence on fossil fuels.

Methanol offers several advantages as a marine fuel and does not have the same storage and handling requirements as LNG or heavy fuel oil (HFO) as it is a liquid at normal temperatures, does not need to be stored at low temperatures or require specialized materials for tanks and pipes which on the other hand require more space onboard than LNG or HFO for the same voyage distance [16]. When methanol is burnt, it produces carbon dioxide and water, and the emissions from burning fossil-derived methanol in ship engines are much lower than those of heavy fuel oil.

Hydrogen is the most prevalent element in the universe, and it has the greatest energy storage capacity per unit weight of any energy carrier [17]. One of the mostly discussed alternative marine fuel is Hydrogen which is gaining attention as a potential solution to the shipping industry's need to reduce emissions and transition to cleaner energy sources.

One way to use hydrogen as a fuel is by keeping it in a liquid state, which can then be utilized in modified gas turbines or internal combustion engines (ICEs) while another technology that utilizes hydrogen is fuel cells (FC) as a promising alternative to traditional marine fuels, which use catalytic reactions on the ship to convert traditional hydrocarbon fuels into hydrogen to generate electricity where the primary byproduct of FC when utilizing hydrogen is water only, and making it a highly environmentally friendly option [18].

The storage of hydrogen for maritime applications poses unique challenges compared to stationary or automotive applications, hence the storage of hydrogen for ships is more difficult than in these other scenarios [19].

The use of biofuels in marine applications can potentially decrease the output of pollutants such as SOx, NOx, and Particular Matter (PM), resulting in a better air quality worldwide and specifically in harbors in comparison to the usage of marine diesel or heavy fuel oil [20]. Biofuels have been shown to decrease overall carbon emissions because of the absorption of carbon from the atmosphere during the growth of the biomass and may become more important in the future if produced from more renewables [21]. Another advantage that makes biofuels an attractive option for marine use is that they can be utilized as a ready-to-use drop-in fuel with minimal modifications required for existing ship engines and infrastructure such as bunkering vessels [22]. There are various types of biofuels that can be used for marine applications, and some of those are straight vegetable oils (SVO), biodiesel (FAME), hydrotreated vegetable oil (HVO), biomethane, and biogas.

Ammonia (NH₃), a chemical compound made up of one nitrogen atom and three hydrogen atoms, is used as an inhibitor in certain enzymatic reactions and is toxic to the nervous system which can be both artificially created in industrial settings and naturally produced through bacterial actions and the decay of organic material [23]. When it is combusted, it produces only water and nitrogen, making it a near-zero-emission fuel [24].

The use of ammonia as a marine fuel has the potential to provide significant economic and environmental benefits such as having the potential to reduce ship fuel costs and improve energy efficiency along with its huge potential to significantly reduce GHG emissions that shipping industry has been fighting for.

There are three commonly used methods for producing ammonia: using natural gas which is called steam methane reforming (SMR), coal gasification, or water electrolysis combined with the Haber-Bosch (H-B) process and most of the ammonia produced currently is from fossil fuels and primarily made through the combination of SMR and the H-B process [25].

While it is possible to produce green ammonia using electrolysis and renewable energy, this method is more energy-intensive and less efficient, resulting in higher costs and additionally, the equipment used in electrolysis is costly, which further increases the expense of green ammonia production [25].

More research and development are required to reduce the cost of green ammonia production, but the advantages of utilizing ammonia as an alternative fuel make it a potential alternative for the future. This study examined the benefits and drawbacks of using ammonia as an alternative fuel, and it is obvious that more study is needed to make it a viable choice.

Fuels with high energy density (kJ/kg) and specific energy (kJ/m³) are preferred ones since they require a smaller storage tank and have a lighter weight. Overall system efficiency and energy consumption or losses are normally incorporated in determining the fuel mass and volume for a more precise assessment of the mass and volume for fuels onboard the ship. The gravimetric energy density and volumetric energy density of alternative fuels studied are shown in Figure 1 and HFO is used as base fuel for comparison and according to data indicated that only the mass and volume requirements are shown and if the storage tank capacities are also considered, then there will be a significant change for certain fuels like those require cryogenic/refrigerated or pressurized fuel containment systems such as Hydrogen (H₂) which has gravimetric densities lower than 10 MJ/kg for LH₂ including containment systems, compared to 119.93 MJ/kg for the fuel only while for the LNG gravimetric and volumetric densities of ~22.2 MJ/kg and ~13 MJ/L including storage systems, respectively, compared to ~53.6 MJ/kg and ~21 MJ/L for the fuel only [26].

Figure 1: 
Comparison of volumetric and gravimetric energy densities of alternative fuels [26][27]

Another major concern is safety which is needed to be assessed, particularly due to the toxicity and flammability, and for that reason the use of alternative fuels in maritime is strictly regulated and more under development such as for Ammonia. To have a better understanding on this, an overview of critical properties connected with alternative fuels is assessed and shown in Table 1.

The development of rules for the marine application of alternative fuels is crucial as it ensures equivalent safety standards and reduces the risk levels involved. By implementing equivalent safety principles in the design and operation of vessels, the use of alternative fuels can be made more reliable and sustainable. The potential benefits of NH₃ as alternative fuel, such as reducing emissions and improving efficiency, can only be fully realized if the necessary precautions are taken to mitigate the increased risk levels for the toxicity that can only come with new technologies. Therefore, the development of comprehensive rules must be a priority to ensure a safer and greener future for the maritime industry.

Numerous research and development projects are underway, working to establish standards and protocols for the safe and efficient use of NH₃ as a fuel. As these standards are developed and refined, the conditions and applications for the use of NH₃ in the maritime industry will become clearer.

Engine designers are currently researching both combustion concepts, but due to the fuel slip issues inherent in the Otto cycle process, the difficulty in initiating NH₃ combustion, and its slow burn characteristics, the Diesel cycle is thought to be the most suitable combustion concept: in fact, it is being selected by MAN-ES for their ME-LGIA engine concept [29]. While a significant progress has done, the main concern remains to be the fuel combustion as the fuel injectors have received extensive development and testing, with a focus on the materials employed to support not just efficient combustion but also lifecycle maintenance [30].

Wartsila, another major engine manufacturer is researching both combustion concepts, including the use of NH₃ and methane in Otto (or Diesel) cycles to minimize CO₂ emissions [29]. The company’s 25 medium-speed 4-stroke engine which can already run on diesel, LNG, or liquid carbon-neutral biofuels, was recently introduced and it can be simply upgraded to run on future carbon-free fuels as they become available such as NH₃ [31].

WinGD, one of the key engine manufacturers for low pressure LNG Dual Fuel (DF) engines run on Otto cycle, and Hyundai Heavy Industries are partnering to create the first WinGD two-stroke engine capable of using NH₃ as fuel by 2025 [32].

In addition to developments happen progressively on technology side, there are also significant progress has been made for regulatory sides for adoption of NH₃. The Maritime Safety Committee’s (MSC) 105^th session, which convened from April 20 to 29, agreed that the IMO should include "Development of nonmandatory standards for the safety of ships utilizing ammonia as fuel" to its work schedule which will be carried out by the Sub-Committee on Cargo and Container Carriage (CCC), with a completion date of 2023 [33].

Despite its potential benefits, the use of ammonia as a marine fuel still faces a few problems and barriers that must be overcome before it can be extensively utilized and adopted in a wide range.

Presently the main issue with NH₃ is production from renewables as green NH₃ due to WtW GHG emission issues, however current production methods make it more expensive than the conventional NH₃. The limited availability of infrastructure for bunkering with ammonia makes it difficult to implement this fuel on a large scale. This is especially true in remote and coastal areas, where it is difficult to build the infrastructure required to handle this fuel.

As NH₃ is a hazardous material, and there are significant risks associated with its handling and storage those include fires, explosions, and toxic gas releases, which can pose a threat to the safety of ship crews and the surrounding environment especially if this will be associated with also lack of experience and appropriate training as all will create additional safety risks, as it is essential to have trained personnel who understand the unique properties and hazards associated with this fuel.

The use of NH₃ as a marine fuel is not yet regulated in the same way that traditional fuels, however dissections at IMO level are ongoing as NH₃ is considered a toxic product and is currently not permitted for use under IGC code, which will require amendment in the long term.

Because of the properties of NH₃, its use presents some challenges as it has a substantially lower heating value compared to other hydrocarbons. NH₃ also has a low flammability due to its narrow equivalency ratio (0.63 to 1.4) and high auto ignition temperature. The adiabatic flame temperature of NH₃ is 1800 ⁰C, which is lower than that of H₂ (2110 ⁰C), CH₄ (1950 ⁰C), and C₃H₈ (2000 ⁰C) and as a result, radiation heat transmission is reduced, which is critical during combustion and heat transfer [34].

Fuel Cell and combustion-based systems are the two major approaches which can be used to utilize NH₃. In a simple expression, FC systems employ a chemical process to transform NH₃ energy into electrical power, whereas combustion-based systems use the heat created by burning NH₃ to power a mechanical device such as an engine or turbine.

There are structured and systematic approaches used for risk assessment and decision-making and in this case Structured What-If Technique (SWIFT) method is used to assess and evaluate the potential risks and uncertainties. SWIFT is a systematic and structured approach that involves identifying hypothetical scenarios by asking "what-if" questions. Here is the outline of steps involved in SWIFT method applied:

• • 1^st step – Definition of scope and objective
• • 2^nd step – Scenarios identification
• • 3^rd step – Analyzing scenarios and consequences
• • 4^th step – Assess the risks
• • 5^th step – Identification of contingencies
• • 6^th step – Review, revise and document the results

The NH₃ fuel supply to the engine is supplied via the fuel supply system (FSS) during dual-fuel operation as shown in Figure 3 and Table 3 along with all system components studied and evaluated.

Figure 3: 
NH₃ fuel supply system schematic

In the typical arrangement for gas carriers where the cargo is used as fuel, there are two approaches for machinery space concepts, namely “Gas Safe” machinery space or “Non-hazardous” machinery space and ESD Machinery space.

For the spaces where the likelihood of fuel leakage may be higher, such as fuel-preparation rooms, additional safety measures and operational safeguards will be required. The fire and explosion hazards for NH₃ fuel must also be evaluated, but the toxicity risk will require suitable type of gas detector and subsequent alert levels which must be identified during the detailed HAZID study. For this study Gas Safe machinery space concept is used and a simplified HAZID result of which is mentioned under 5.3 is prepared based on this approach.

The purpose of the risk assessment and HAZID study is to identify the potential major hazards relative to the operational configuration of a proposed NH₃-fueled vessel at an early stage of concept development. This analysis helps in reviewing the effectiveness of selected safety measures and, where required, expand the safety measures to achieve tolerable levels of residual risk. The study seeks to:

• • Identify hazards arising from NH₃ as a marine fuel.
• • Evaluate the potential consequences of these hazards and assess their probability of occurrence.
• • Develop practical safeguards and risk mitigation measures
• • Provide clear and actionable recommendations for mitigating hazards.
• • Ensure that the use of NH₃ as a marine fuel meets the safety standards stipulated in the IMO IGC code.
• • Establish a robust framework for future safety-assessment studies.

To evaluate the importance of each hazard based on its frequency and severity, a risk matrix as shown in Table 4 is prepared. The risk levels are assigned based on the intersection of the severity level and frequency level. For example, a hazard with a Severity Level of 5 (Catastrophic) and a Frequency Level of 5 (Almost Certain) would be considered an "Extremely High" hazard with a residual risk (RR) factor of 25. Conversely, a hazard with a Severity Level of 1 (Minimal) and a Frequency Level of 1 (Rare) would be considered a "Low" hazard.

The project is only a conceptual design, but approach is based on present dual fuel systems used for LPG and Methanol carriers and their similarities along with the additional safety concerns identified under HAZID worksheet which is conducted separately and not included in this study due to its length.

During the HAZID worksheet development, 4 'extremely high' and 11 'high' risk scenarios were identified, which will require mitigation during the design phase and the summary of risk ranking along with the HAZID nodes those are developed and selected based on Table 3 are shown in the Table 5.