Hydrogen production may be categorised into three main groups: green (based on renewable energy), blue (based on coal gasification and natural gas, along with CCS hydrogen generating systems) and red (based on fossil fuels alone) (based on conventional fossil fuels). Green hydrogen may also be created utilising renewable energy sources, despite the fact that the majority of it is currently produced via the CO₂-intensive steam methane reforming process. Electrolysis is a typical method that uses an electrical current to separate water into oxygen and hydrogen and creates green hydrogen without any direct emissions of carbon dioxide. Renewable energy sources may be used to produce the necessary electricity. The expense of producing hydrogen, particularly for green hydrogen, is a significant hurdle. The cost of manufacturing hydrogen using steam reforming is around three times greater than the cost of producing one unit of energy using natural gas. Hydrogen will cost almost twice as much to produce using electrolysis with 5 cents/kWh of energy compared to hydrogen produced using natural gas. Lower hydrogen concentrations may be transported via the existing natural gas pipeline infrastructure, which will also assist in reducing CO₂ emissions from the existing natural gas reforming plants.

There are a number of factors drawing attention to hydrogen fuel, but the following are the most important ones:

• Different energy sources can be used to produce hydrogen.
• Since hydrogen is the least polluting, using hydrogen in fuel cells or combustion processes results in the production of water.
• All energy needs may be met by hydrogen, which is also utilised in residential applications, hydrogen fuel cell automobiles, energy carriers, and integrated heating and power generation systems. Hydrogen also covers all requirements for energy

By using off-grid offshore wind energy to generate clean fuel (hydrogen/ammonia), hydrogen may significantly contribute to the decarbonization of the marine sector. Energy may be transported and potentially stored by hydrogen, which is the best substance for both. Some important hydrogen generation techniques include: landfill gas dry reformation; coal gasification; H₂S methane reformation; naphtha reformation; methane/natural gas pyrolysis; steam-iron process; steam reforming of waste oil; partial oxidation of heavy oil and coal; grid electrolysis of water; high-temperature water electrolysis; chloralkali electrolysis; solar and PV water electrolysis; photolysis of water; and biomass gasification.

According to the literature and study reports [34,35], there are three main forms of hydrogen generation based on the use of various technologies and proposed sources. The idea of colour has been developed as a result of the use of important sources in the generation of hydrogen. The production of hydrogen from fossil fuels results in the release of CO₂ and other greenhouse gases. Grey hydrogen is the term used to describe this method of producing hydrogen and its used source [36]. Blue hydrogen came into being as a result of the use of grey hydrogen with carbon capture technology to lower the quantity of greenhouse gas emissions [37,38]. In order to produce hydrogen for sustainable transportation, fossil fuels including industrial gas, by-product gas and natural gas typically release pollutants and greenhouse gases into the atmosphere [39,40,41]. A variety of technical advancements for hydrogen generation have been documented in the literature as a means of reducing the pollutants released into the environment. Therefore, recent literature [42,43,44] has reported on the use of renewable energy resources for hydrogen production. Green hydrogen is another by-product of electrolysers made from renewable energy sources. It has also been noted that bioenergy sources such as the burning of biomass and biomethane can also create green hydrogen. Researchers and businesses are paying increasing attention to the improvement of green hydrogen generation, since green hydrogen produced using diverse approaches has net zero gas emissions [45,46].

Current conventional hydrogen production facilities integrate carbon capture and storage (CCS) or carbon capture and utilisation (CCU) systems to collect the discharged CO₂ emissions. Alternately, to fulfil the worldwide need for hydrogen, which is currently being satisfied by renewable energy sources, conventional hydrogen production pathways are employed. These approaches provide a sizable part of hydrogen. The majority of conventional hydrogen is produced from fossil fuels, mainly by partially oxidizing methane and reforming natural gas. Unprocessed natural gas contains gaseous hydrogen sulphide (H₂S), which is removed from the gas during the first stage of processing by desulfurization. The Claus procedure is used to extract sulphur from gaseous H₂S. In Claus plants, the gaseous hydrogen sulphide generated interacts with oxygen gas to extract sulphur. Another common technique for producing hydrogen is coal gasification. Air separation is the initial phase, which extracts oxygen from the air and delivers the same to the gasifier. One of the three frequently used methods for air separation: membrane separation, cryogenic air separation, or pressure swing adsorption, is employed. Coal pyrolysis follows the air separation process. In order to gasify coal, the pyrolysis stage needs two inputs: steam and oxygen. Coal is broken down into volatile and char in this process. The process of quenching, which applies fast cooling, comes after the gasification stage [47]. The next step is a cooling unit for the syngas, where heat is collected for syngas and may be utilised for a variety of things including electricity generation, hot water production, and space heating. The watergas shift reaction comes after the syngas cooling unit, which converts CO into CO₂, the component that prior to the water gas shift process separates hydrogen sulphide (H₂S) and carbon dioxide (CO₂). After the hydrogen purification unit, there is an acid gas removal step where hydrogen is frequently separated from other gases using the pressure swing adsorption technique. The blue hydrogen production method is depicted in Figure 3.

Purple hydrogen is produced using nuclear energy, and other forms of hydrogen may be produced using thermochemical processes thanks to the high temperatures of the nuclear reactor. The fission of uranium atoms produces nuclear energy. Heat from nuclear energy is used to create steam, which is then used in turbines to create electricity [48,49].

Nuclear power plants do not directly burn fuel; thus, they do not emit any greenhouse gases in that way. Because it can be controlled in the reactors used in nuclear power facilities, fission reaction is utilised. The fission process in the nuclear reactor serves as the heat source for the thermal energy in a nuclear power plant [50,51,52,53,54]. Nuclear reactor heat is used to make steam, which is subsequently utilised in a turbine to produce electricity using a generator similar to conventional stations that produce thermal energy. Nuclear energy is created by the nuclear reactor’s atoms splitting process, which transforms water into steam and drives a turbine to generate electricity. The most popular way to use this thermal energy is to turn a turbine to generate electricity, which may then be used to power fuel cells to create hydrogen [55,56]. Other ways to use this thermal energy include steam reforming techniques, water electrolysis for the creation of hydrogen, and thermochemical water splitting cycles.

Hydrocarbons split to generate turquoise hydrogen. This may be accomplished using a variety of methods. The approach that has advanced the most in research and is most likely to be commercialised is the plasma process for producing carbon black and hydrogen. Other techniques include cold plasma, methane catalytic conversion and molten metal pyrolysis via thermal splitting [57,58]. Hydrocarbons such as natural gas are utilised as feedstock and process energy for all of these operations, and electricity is the source of both. Methane splitting potentially requires 38 kJ/mol H₂, while water electrolysis requires 285 kJ/mol H₂ and steam-methane reforming requires 252 kJ/mol H₂. Methane splitting requires high temperatures and heat losses [59,60].

Grey hydrogen is the result of the steam reformation of coal or natural gas without the addition, usage, or storage of carbon. A by-product of other chemical processes produces more than 40% of grey hydrogen [61].

White hydrogen has also been unofficially coined by the North American Council for Freight Efficiency [62]. Grey hydrogen is mostly used in the petrochemical industry and in the production of ammonia [63]. The main disadvantage of grey hydrogen is due to the significant CO₂ emissions that are generated during hydrogen synthesis, which are estimated to be around 830 Mt of CO₂ yearly [64]. However, a tried-and-true technique that generates hydrogen at a fair price is natural gas steam reforming (SMR) without CCUS. During the procedure, the water is heated and natural gas is pre-treated. The methane is subsequently broken down into syngas in the reformer with steam.

Another important method of producing grey hydrogen is coal gasification, which is sometimes referred to as brown hydrogen in the literature. Since coal has the largest reserves of any fossil fuel in the world, this production method is also frequently used. China, in particular, generates a substantial quantity of hydrogen by coal gasification due to the high cost of natural gas and the abundance of coal reserves [59]. Four main types of coal, including lignite (low rank), sub-bituminous coal (low rank), bituminous coals (middle rank), and anthracites, are widely used as gasification feedstock (high rank) [61,62,63]. Figure 4 shows grey hydrogen production.