Environmental Assessment of Hydrogen Utilization in Various Applications and Alternative Renewable Sources for Hydrogen Production: A Review

Abstract

Rapid industrialization is consuming too much energy, and non-renewable energy resources are currently supplying the world’s majority of energy requirements. As a result, the global energy mix is being pushed towards renewable and sustainable energy sources by the world’s future energy plan and climate change. Thus, hydrogen has been suggested as a potential energy source for sustainable development. Currently, the production of hydrogen from fossil fuels is dominant in the world and its utilization is increasing daily. As discussed in the paper, a large amount of hydrogen is used in rocket engines, oil refining, ammonia production, and many other processes. This paper also analyzes the environmental impacts of hydrogen utilization in various applications such as iron and steel production, rocket engines, ammonia production, and hydrogenation. It is predicted that all of our fossil fuels will run out soon if we continue to consume them at our current pace of consumption. Hydrogen is only ecologically friendly when it is produced from renewable energy. Therefore, a transition towards hydrogen production from renewable energy resources such as solar, geothermal, and wind is necessary. However, many things need to be achieved before we can transition from a fossil-fuel-driven economy to one based on renewable energy.

1. Introduction

According to existing knowledge on the subject, published articles, and internet sources, no one knows the exact depletion of fossil fuels [1]. However, other experts predict that if we continue to burn fossil fuels at our current rate, we will deplete all of our fossil fuel supplies by 2060 [2]. Since the turn of the century, global hydrogen consumption has increased by 50%, and it now stands at 90 million tons, with more on the way. At the moment, the principal consumers of hydrogen are chemical firms and refineries [3]. As new technologies are embraced and the climate worsens, nations are moving toward a green hydrogen economy [4,5].

Ambitions have resulted in the developing of carbon-free energy resources such as hydrogen-based renewable energy technology. As a result, a large quantity of H₂ is needed. This amount of hydrogen utilization is responsible for 27% of ammonia and 11% of methanol production. Urea and ammonia nitrate are by-products of ammonia production. Conversely, methanol is used to generate formaldehyde, methyl methacrylate, and other chemicals. To meet world steel demand, the blast furnace-basic oxygen furnace (BF-BOF) uses hydrogen to transform iron ore into steel, accounting for around 3% of overall H₂ use [6,7,8]. Hydrogen can also be used in many different ways because they have many different uses, such as hydro-treating biofuels and improving oil sands. Furthermore, hydrogen can generate a high temperature that can be used for melting, gasification, drying, and other applications [9]. In addition, the welding process, hydrogenation, superconductivity, and rocket engines are all frequent uses of H₂ [10,11,12], as will be discussed in greater detail in the next sections. Figure 1 represents the hydrogen utilization in different processes.

The by-products and the production process determine the environmental impact of hydrogen production [13]. Hydrogen in the market place currently is primarily produced by natural gas steam reforming process. CO₂ is the primary byproduct of this process, and its release contributes to climate change and global warming. As a result, hydrogen derived from fossil fuels contributes to climate change in the same way as direct fossil fuel combustion. Conversely, hydrogen production from renewables such as wind, solar and geothermal are environmentally friendly during the combustion and production cycles [4]. As a result, the adoption of a hydrogen economy must be complemented by the development of environmentally friendly hydrogen.

Producing hydrogen is vital for a cleaner energy sector. Nevertheless, it is essential to evaluate the suitability of different H₂ production and utilization options from a life-cycle aspect. To gain a more comprehensive understanding of a product’s impact on sustainability, the Life Cycle Sustainability Assessment (LCSA) methodology can be utilized. By considering economic, environmental, and social factors, LCSA allows for a holistic evaluation of a product system’s performance. This helps to identify potential improvement areas and promotes sustainable practices across all stages of a product’s life cycle [14]. The adoption of renewable-based hydrogen as an energy source can significantly reduce the negative environmental impacts of current fossil fuel usage [15]. Shifting away from non-renewable sources towards cleaner, sustainable energy can mitigate many harmful effects on human activities and ecosystems [16,17]. Renewable hydrogen can result in significantly lower environmental implications. Hydrogen can be produced using renewable energy sources such as wind, geothermal, solar energy, and others. To our best knowledge, no review article has been published that discusses the source of hydrogen utilization in various applications and its environmental impact. A brief section on hydrogen production through renewable energy sources is also discussed. Day by day, the consumption of hydrogen has been rising; therefore, to sort out the future demand and supply of hydrogen, renewable technologies could provide the initial support for the hydrogen industry.

This study provides a detailed analysis of the potential benefits and challenges associated with the use of hydrogen as a fuel source, and it sheds light on the environmental implications of hydrogen utilization in various applications. The findings of this research have important implications for the development of sustainable energy policies and the transition to a low-carbon economy.

2. Industrial Utilization of Hydrogen

The utilization of hydrogen in a range of applications has the potential to play a crucial role in decarbonizing the energy system and mitigating climate change. However, the production, storage, and utilization of hydrogen present significant challenges that must be addressed to fully realize the benefits of this technology. The cost, efficiency, and environmental impact of hydrogen production methods vary considerably, and careful consideration is needed to identify the most viable options for different applications. The safe handling and storage of hydrogen, which is highly flammable and explosive, also requires specialized infrastructure and equipment. In addition, the production of high-purity hydrogen required for certain applications, such as rocket engines and superconductivity, is challenging and requires significant quality control measures. Despite these challenges, the potential benefits of hydrogen utilization, including improved energy security, and reduced greenhouse gas emissions.

2.1. Utilization of Hydrogen in Ammonia Production

In 1909, Fritz Haber invented the ammonia process, which involves the conversion of nitrogen and hydrogen into a useful product. The Haber-Bosch process reacts atmospheric nitrogen with hydrogen to produce ammonia (NH₃), which is 17.8% hydrogen by weight. Equation (1) described the ammonia production process as follows:

$$3{\mathrm{H}}_2+{\mathrm{N}}_2\leftrightarrow 2\mathrm{N}{\mathrm{H}}_3$$

(1)

Figure 2 illustrates the block flow diagram of the Haber process. Nitrogen is extracted from the air, while hydrogen is extracted from natural gas or coal. The expense of ammonia is primarily attributable to hydrogen, as nitrogen separation is not expensive.

The ammonia process’s capital and operating costs are decreased by employing an active catalyst and lowering the temperature and pressure. Due to decreased energy consumption and operation at lower temperatures and pressures, ammonia produced via electrochemical is more environmentally friendly than ammonia produced using the commercialization process [18]. The state-of-the-art technology is utilized for the sustainable production of ammonia by the unit process. Ammonia is a renewable source of hydrogen storage, medicines, fertilizer, and various other applications [19].

The use of Protonic ceramic electrochemical cells (PCECs) for ammonia synthesis offers a promising alternative to traditional methods that require high pressures and temperatures. This has the potential to reduce energy consumption and carbon dioxide emissions associated with ammonia synthesis. PCECs could offer several benefits, including the ability to operate at lower pressures and temperatures and reduce reliance on fossil fuels [20].

2.2. Utilization of Hydrogen in Methanol Production

Syngas, a mixture of hydrogen, carbon monoxide, and carbon dioxide, is used to make methanol. For direct CO₂ hydrogenation, metal-based catalysts (ZnO/CuO/Al₂O₃) are utilized. This exothermic reaction bypasses the cost-intense syngas generation steps. Furthermore, methane reformation at temperatures above 700 °C is preferred. Equations (2) and (3) describe the methanol reactions as follows:

$$\mathrm{C}\mathrm{O}+2{\mathrm{H}}_2\leftrightarrow \mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{H}$$

(2)

$${\mathrm{C}\mathrm{O}}_2+3{\mathrm{H}}_2\leftrightarrow \mathrm{C}{\mathrm{H}}_3\mathrm{O}\mathrm{H}+{\mathrm{H}}_2\mathrm{O}$$

(3)

In contrast, a reverse water gas shift process captures carbon dioxide [21,22]. Hydrogen derived from renewable sources is more versatile and preferred. Mitigating greenhouse emissions and carbon dioxide capture is critical during methanol synthesis [23]. Methanol is a hydrogen carrier that can be easily converted to hydrogen using a catalytic process in a fuel reformer at temperatures between 200 and 300 °C. However, it is a much more expensive process than hydrogen produced from methanol [24]. The process flow diagram of the methanol process is shown in Figure 3 [25]. The utilization of H₂ in methanol is depicted in Figure 4.

2.3. Utilization of Hydrogen in the Reduction in Iron Ore Process

The eco-friendly hydrogen-reduction process for manufacturing iron has the advantage of employing H₂ as a reducing agent. In a two-step reaction, the first ion-exchange iron oxide forms on the surface of FeS particles when the reaction occurs between FeS and CaO. Secondly, iron oxide is reduced in the presence of hydrogen to form a layer of metallic iron. Hydrogen-reduction in FeS-CaO kinetics has been affected by the temperature, which is dominant for the heating rate-controlling mechanism [26]. The dry reforming reaction of methane is described by the following reaction Equation (4).

$$\mathrm{C}{\mathrm{H}}_4+{\mathrm{C}\mathrm{O}}_2\leftrightarrow 2{\mathrm{H}}_2+2\mathrm{C}\mathrm{O}$$

(4)

Due to the dehydration process, the FeOOH in the limonite ore is transformed into Fe₂O₃, increasing the surface area of the limonite ore, because nano-pore particles are destroyed at higher temperatures due to sintering [27]. The reaction temperature of the reduced iron ore increased as the concentration of the reducing gas (H₂) was raised. At lower temperatures, water vapors are eliminated from the product generated during the reaction. Hydrogen is more efficient and cleaner than carbon monoxide and can be recycled and reused in the furnace [28]. When the reduction in low-grade limonite ore with high moisture content is carried out in a rotary drum reactor in the presence of hydrogen-nitrogen as a reducing agent, the isothermal reduction in hematite ore is followed by the first-order reaction model below 850 °C. Above 900 °C, due to the formation of the dense iron layer, the reaction follows the model of the diffusion control model [29]. Direct reduction in iron ore pellets with and without biomass is isothermally reduced at a high temperature (1123–1323 K). Equations (5)–(7) describe the reaction of the use of pure hydrogen as a reducing agent:

$$3\mathrm{F}{\mathrm{e}}_2{\mathrm{O}}_3+{\mathrm{H}}_2\to 2{\mathrm{F}\mathrm{e}}_3{\mathrm{O}}_4+{\mathrm{H}}_2\mathrm{O}$$

(5)

$$\mathrm{F}{\mathrm{e}}_3{\mathrm{O}}_4+{\mathrm{H}}_2\to 3\mathrm{F}\mathrm{e}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}$$

(6)

$$\mathrm{FeO} ({{\mathrm{Fe}}_2}^{+}+{\mathrm{O}}_2)+{\mathrm{H}}_2/2\mathrm{H}/2(\mathrm{H}+{\mathrm{e}}^{-}) \to \mathrm{Fe}+{\mathrm{H}}_2{\mathrm{O}}_{ \left(\mathrm{g}\right)}$$

(7)

The reduction in the iron ore process is illustrated in Figure 4 [30]. Smelting reduction in iron ore is completed by thermal plasma or non-thermal plasma. During the plasma process, the structure of molten iron ore is usually made up of anions and cations, as well as complexes of anions and cations.

The steel industry is the backbone of the global economy. Steel plays a vital role in the foundations of developed countries. High levels of income per capita are essential for the welfare of people and a happy region, but to reach such a number, it is required to enhance steel consumption per capita substantially. No country has ever achieved higher growth without this factor. Despite the influence of the pandemic, through its different regional impacts, the global steel industry was fortunate to end 2020 with only a minor contraction in steel demand. 51 kg of hydrogen is needed per tonne of steel output [30].

2.4. Utilization of Hydrogen in the Hydrogenation Process

Electrochemical hydrogenation of soybean oils is carried out with platinum nanoparticles [31]. Hydrogen is electrolyzed at the anode side to form H⁺ when the gas pressure increases (3 MPa) throughout this process. As a result, the electrolyzer conductivity and hydrogen solubility in electrolytes both improve. Catalysts made of Ni, Pd, or Pt are used to overcome unsaturated fatty acids in fats and oils. Hydrogenation enhanced the oil’s saturated fat content, oxidation resistance, and melting point. A fuel cell structure-based polymer electrolyte membrane electrochemical reactor (PEMER) is used for electrochemical reduction. On the catalyst’s surface, hydrogen atoms are produced and added to the fatty acids’ unsaturated bonds. The overall hydrogenation reaction is described by Equations (8)–(11).

$${\mathrm{H}}_2\mathrm{O}\leftrightarrow \frac{1}{2}{\mathrm{O}}_2+2\mathrm{H}+2{\mathrm{e}}^{-}$$

(8)

$$2\mathrm{H}+2{\mathrm{e}}^{-}\leftrightarrow 2{\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}$$

(9)

$${\mathrm{H}}_{2 \left(\mathrm{g}\mathrm{a}\mathrm{s}\right)}\to 2\mathrm{H}+2{\mathrm{e}}^{-}$$

(10)

$$2{\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}+\mathrm{R}-\mathrm{C}\mathrm{H}=\mathrm{C}\mathrm{H}-\mathrm{R}\leftrightarrow \mathrm{R}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2-\mathrm{R}$$

(11)

A side reaction is responsible for the formation of hydrogen by the combination of adsorbed hydrogen atoms, which does not affect the product yield. The reactions Equations (12) and (13) are [32]:

$$2{\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}\to {\mathrm{H}}_{2 \left(\mathrm{g}\mathrm{a}\mathrm{s}\right)}$$

(12)

$${\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}+{\mathrm{H}}^{+}+{\mathrm{e}}^{-}\to {\mathrm{H}}_{2 \left(\mathrm{g}\mathrm{a}\mathrm{s}\right)}$$

(13)

The use of hydrogen to hydrogenate CO₂ is another promising way to reduce greenhouse gas emissions. This method utilized negative electrodes for protonic ceramic electrochemical cells that can selectively convert CO₂ to CO with high efficiency and selectivity as follows in Equation (14) [33].

$${\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\leftrightarrow \mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}$$

(14)

Biodiesel’s oxidation stability and cold flow characteristics are improved by partial hydrogenation [34]. Direct hydrogenation and catalytic transfer hydrogenation are two methods for partial hydrogenation. Hydrogen is fed directly into the reactor in direct hydrogenation, while hydrogen is provided from the hydrogen donor in catalytic transfer hydrogenation. The catalytic transfer process is inferior to direct hydrogenation because it synthesizes trans-isomers at higher temperatures, which is bad for the fuel [35]. Figure 5 illustrates the hydrogenation process. The essential prerequisite for the hydrogenation process is hydrogen. Hydrogen can potentially remove criteria pollutants such as sulfur and nitrogen by converting them into hydrogen sulfide and ammonia [36]. Nitrogen is sent to an amine treatment unit while the carcinogenic substance is scrubbed down to less than 200 parts per million. The quality of naphtha, kerosine, and diesel is improved by hydro-treating in the presence of Ni, Co, and Mo catalysts. The Pt catalyst is used to increase the RON to 95–96. Industries use a blend of light straight run and heavy straight run to improve pure petroleum [37]. Producing 100 kg of synthetic crude oil through direct hydrogenation requires 7.4 kg of hydrogen.

2.5. Utilization of Hydrogen in Rocket Engines

Metastable hydrogen can be used to create highly compressible, lightweight, and strong rocket propellants. A suitable propellant has a defined impulse and range (liquid hydrogen-oxygen-based modern rockets have 460 s). If metallic hydrogen is used as metastable in rocket propellants, hydrogen is considered a strong chemical. Metallic hydrogen, similar to cryogenic fuel, does not require cooling due to its high density. Liquid fuel and oxidizer are injected into the chamber, where the reaction releases the hot gases at high pressure through the nozzle.

The recombination of hydrogen molecules releases 216 MJ/kg of energy, more than the amount required in space shuttle engines [38]. Hybrid fuel has the advantage of reducing the de-bonding and crack sensitivity, thus increasing the specific impulse of the rocket engine. As well, hybrid fuel-based rockets are capable of throttling, thrust termination, or restart whenever required [39]. Liquid oxygen (LOx)–hydrogen and LOx–kerosene are employed as propellants in liquid rocket engines. When combined with liquid oxygen, liquid hydrogen has a high specific impulse and superior combustion properties. Combustion is fueled by propellant injection, which results in a rise in temperature and pressure [40]. Hydrogen is a necessary fuel used in a rocket engine. Figure 6 represents the schematic diagram of a rocket.

2.6. Utilization of Hydrogen in Superconductivity

Liquid hydrogen is a coolant because of its latent and specific heat, low viscosity, and boiling point. When liquid hydrogen is used as a superconductor in a refrigerator, the heat transfer characteristics of the film boiling region are of great importance. A higher heat transfer coefficient in the film boiling region is primarily due to the flow rate of liquid hydrogen. Acceleration of the gas-liquid phase improves the heat transfer coefficient [41]. The hydrogen sulfide and the lanthanum hydride have the potential to become superconductors due to their hydrogen content and the fact that they are superconductors due to the alteration of the chemical bond to become a superconductor under pressure. At higher pressures, the lanthanum hydride stabilizes and forms the compound LaH₁₀ which is rich in hydrogen content [42]. A critical high temperature, usually 39 K, MgB₂ wire-based superconductor was developed for practical use of superconducting. The wire-based superconductors are used more often due to the lower material and production costs. During the heating of liquid hydrogen with a small MgB₂ coil immersed in an external magnetic field, the temperature of the liquid hydrogen rises from 21 to 30 K. The magnetic field allows good heat conduction between the coil layers [43].

2.7. Utilization of Hydrogen in Welding Process

Gases, including hydrogen, are used as welding, brazing, cutting, and shielding gases because they are colorless, odorless, non-toxic, tasteless, and combustible. It is also used in the flame process. As well as affecting the temperature and energy distribution, shielding gas is used in arc welding to influence the shape of the material. Gas mixtures containing hydrogen and argon are used in TIG and MIG welding and cutting processes. Hydrogen makes a weld root shield combined with argon and is explosive when mixed with oxygen. Water vapor is created when hydrogen and oxygen combine. The release of hydrogen in arc welding occurs when two tungsten electrodes are introduced into the arc, which causes a rapid increase in the plasma temperature in the core, which results in the dissociation of the gas with an endothermic reaction followed by the following reactions Equations (15) and (16):

$${\mathrm{H}}_2+0.5{\mathrm{O}}_2\to {\mathrm{H}}_2\mathrm{O}+\mathrm{e}\mathrm{n}\mathrm{e}\mathrm{r}\mathrm{g}\mathrm{y}\mathrm{e}\mathrm{m}\mathrm{i}\mathrm{t}\mathrm{t}\mathrm{e}\mathrm{d}$$

(15)

$${\mathrm{H}}_2\leftrightarrow \mathrm{H}+\mathrm{H}-422\mathrm{K}\mathrm{J}$$

(16)

It can be welded if this steam of atomic hydrogen gas is used on the metal surface. They will return the energy gained from the arc, and the flame temperature will reach 3700 °C [44]. Ionization potential-based gases such as helium and a few percentages of pure hydrogen provide a stronger arc and more efficient welding. By TIG or MIG welding, a variety of martensitic, duplex, high alloy ferritic, and super duplex steels can be combined to make a high alloy stainless steel. When hydrogen is heated at the range of 3000 to 4500 (K), its thermal conductivity is ten times greater than that of argon, which increases the energy concentration. Hydrogen produces gas bubbles and crakes in the steel because hydrogen solubility in molten metals is very high. To melt a large amount of material, hydrogen is added to the shielding gas in arc welding, increasing the arc power and affecting the arc’s static characteristics. The higher the hydrogen content in the argon gas, the greater the melting efficiency of MIG by up to 30% to 50% due to the stronger voltage drop [45]. Figure 7 signifies the use of hydrogen in the welding process.

2.8. Utilization of Hydrogen in Hydrogen Chloride Production

When chlorine gas (Cl₂) and hydrogen gas (H₂) mix directly, hydrogen chloride is generated; this reaction Equation (17) occurs almost instantly above 250 degrees Celsius (482 degrees Fahrenheit).

$${\mathrm{H}}_2+\mathrm{C}{\mathrm{l}}_2\leftrightarrow 2\mathrm{H}\mathrm{C}\mathrm{l}$$

(17)

As can be seen, the reaction generates heat and appears to be accelerated by moisture. Figure 8 represents one of the most common techniques for producing hydrogen chloride, both in the laboratory and on a large industrial scale, which is the reaction of a chloride, primarily sodium chloride (NaCl), with sulfuric acid (H₂SO₄). Hydrogen chloride is used in various processes, including cleaning, pickling, electroplating metals, tanning leather, and refining and manufacturing different products. It produces hydrochloric acid when it comes into contact with water [46,47].

2.9. Other Applications of Hydrogen Utilization

H₂ is a promising energy carrier with many potential applications in sustainable energy systems. One notable application is as a clean fuel for fuel cells, which convert hydrogen into electricity through electrochemical reactions without generating harmful emissions. Fuel cells have already found practical applications in transportation and stationary power generation [48]. In addition, water/steam electrolysis driven by renewable electricity offers an opportunity for green hydrogen production. This process involves the splitting of water molecules into hydrogen and oxygen, where the required electricity is sourced from renewable sources, such as solar or wind power. By producing hydrogen from renewable sources, this technology provides a pathway for storing renewable energy and facilitating the integration of intermittent renewable sources into the grid [49]. Therefore, hydrogen has the potential to play a vital role in the transition to a more sustainable energy system.

3. Environmental Assessment of Hydrogen Utilization in Various Applications

A key tool for supporting decision-making for industrial sustainability is life cycle assessment (LCA) [50,51,52]. Indirectly, ammonia is considered an H₂ storage compound since it has a hydrogen content of 17.6 wt% [53]. Several comparative LCA studies have been performed on producing sustainable ammonia pathways. These include Bicer et al., (2016) [54] on various methods for ammonia production by using a comparative LCA approach and Arora et al. (2018) [55] on the energy evaluation system approach of multiple sources of renewable energy into ammonia integrated production plants. Frattini et al. (2016) [56] and Tallaksen and Reese (2013) [57] carried out the same LCA methodology by selecting 1 kg of ammonia as a Functional Unit for its production utilizing renewable sources and fossil fuels. LCA results of Arora et al. showed that the value of the global warming potential of natural gas SMR was 2.81 kg_{CO2eq/kg NH3}. In comparison, the GWP values for biomass and coal gasification were 1.2 kg_{CO2eq/kg NH3} and 4.22 kg_{CO2eq/kg NH3} respectively. The LCA results presented by Makhlouf et al. (2015) [58] showed a high GWP value (1.44 t_{CO2eq/t of NH3}) due to high emissions of GHG.

Table 1. GHG emissions and key consumption for conventional and renewable technologies for ammonia production (1-tonne production of ammonia).

Technology	Kg of Emitted CO2 per Tonne of NH3	Water Consumption (kg H2O per Tonne of NH3	Efficiency %	Energy (Heat and Electricity Consumption (kWh per Tonne of NH3	Capital Cost (per Day/Tonne NH3	References
Electrolysis of water with Haber ammonia process driven by wind/solar	Negligible	ca. 1.5880	∼54%	ca. 12,000	750,000	[59,60,61]
SMR with Haber ammonia process	ca. 1.8	ca. 0.6561	∼60–66%	ca. 9500	500,000	[59,60,62,63]

shows the GHG emissions and key consumption for conventional and renewable technologies for ammonia production.

Hydrogen may serve as a sustainable intermediary element in producing methanol based on renewable hydrogen. Fernández-González et al. (2022) [64] performed LCA of various methanol production pathways. The LCA results showed that conventional methanol production has a GWP of 0.584 kg _CO2e/kg of Methanol (Figure 9). The main contributor to GWP was CH_4, which is utilized as a feedstock and energy used during the process, which is mainly come from non-renewable sources.

If this energy is replaced with renewable once (such as wind), the conventional methanol production process’s GWP values will decrease to 0.295 kg _CO2e/FU. On the other hand, the direct GWP of the vulcanol process was 0.178 kg _CO2e/FU (FU stands for functional unit), with electricity accounting for roughly 80% of H₂ generation. If the energy used during the process is substituted which renewable energy, the Vulcanol process must be attributed −1.450 kg_CO2e/FU negative emissions, resulting in a net −1.272 kg_CO2e/FU GWP value. These results showed that by replacing the conventional methanol production process (fMeOH= fossil-based methanol) with the hydrogen-based Vulcanol process (hMeOH), more than 1.5 kg_CO2e/FUwould avoid by the Vulcanol process.

Seven percent of the world’s GHG emissions are caused by iron and steel production. The decarbonization objectives would not be achieved with incremental adjustments to the primary steel production methods currently in use. It is possible to significantly reduce emissions from iron and steel production by substituting hydrogen generated from water electrolysis with coke, which is used as a reducing agent in blast furnaces. Direct iron ore reduction using hydrogen with an electric arc furnace is a promising technology for producing carbon-free steel. Although, the GWP of this process is also 1101 kg_CO2/tls. Compared to other steel production processes (such as blast furnace basic oxygen furnace which has an emission factor of 1688 kg_CO2/tls), this process has low emissions. A decrease of 170.26 Mt_CO2 per year would be achieved if non-renewable energy is replaced with renewable energy sources [65].

In the chemical production of vegetable oils, hydrogenation is a widely used and important process. Hydrogenation of vegetable oil generally produces solid fat with appropriate texture and consistency. Catalytic transfer hydrogenation (CTH) is an alternative oil hydrogenation method that utilizes hydrogen donors as organic molecules at ambient pressure. De Souza Schneider et al. (2013) [66] performed a laboratory-scale environmental assessment of the CTH process using Leopold Matrix for environmental impact identification. The following criteria were used to determine each activity’s environmental impact: value (negative or positive), space (strategic, local, or regional), order (indirect or direct), time (long, short, or medium), plasticity (irreversible or reversible) and dynamics (permanent, temporary or cyclical). Environmental assessment results showed that the CHT process is cleaner technology having environmental impacts negative (67.48%), reversible (95.32%), direct (80.12%), temporary (95.33%), and local (78.95%).

The environmental impacts of rocket emissions vary significantly depending on the propellant type. For example, Solid rocket motors (SRMs), which utilize solid fuel made of aluminum and an oxidizer such as ammonium perchlorate (NH₄ClO₄), and Liquid rocket engines (LRE), which use primarily oxidizer and liquid hydrogen. Because of its high impulsive force, liquid hydrogen propellant (LOx/LH₂) is usually employed for higher stages of rockets, and its combustion exhaust is usually emitted at high altitudes. Engine exhaust from LOx/LH₂ propellant burning engines is primarily comprised of water vapor; hence it has very few environmental concerns. The only environmental concern share by the LOx/LH₂ propellants is the production of nitrous oxide, produced through atmospheric nitrogen combustion at high exhaust plume temperature.

Table 2. Primary products emissions of most commonly utilized space rocket propellants.

Propellants	Primary Products Emissions	Advantages	Disadvantages
LOx/LH2	NOx, OH, H2, H2O	High impulsive power The ecological impact is low because of water vapor exhaust	Arduous to handle due to the explosive risk and temperature requirements. Low density. Cryogenic storage is required due to the considerably low LH2 boiling point (−252.87 °C).
LOx/RP-1 (Kerosene)	NOx, OH, COx, H2O, CO2, and soot	Handling is easy Propellant density is high More affordable compared to liquid H2 propellants	Emissions of black soot and CO2 contribute to climate change
Solid Al/NH4ClO4 ± HTPB	AL2O3, NOx, CO2, H2O, HCl and soot	High thrust Easy storage Simple engine design High density	No shutdown or throttling High environmental impact Specific impulse is relatively low
Hypergolic N2O4/UDMH ± N2H4	NOx, CO2, N2. H2O	Simple engine design Long storage periods	Handling is difficult due to safety concerns High toxicity

shows the primary product emissions of the most commonly utilized space rocket propellants.

The utilization of H₂ in HCl production is generally a safer process and has minimal environmental impacts. Although, HCl has some major environmental concerns. Because it is highly corrosive, HCl poses risks to monuments made of limestone, metal, and buildings [67]. Aquatic organisms may affect by the high concentrations of dissolved HCl gas in a body of water caused by accidental spills. As a result of HCl air emissions, the high HCl gas solubility leads to acid rain concerns, which can induce aquatic habitats and soil acidity above critical threshold levels. A promising treatment method for significant HCl amount is cyclic H₂ and Cl economy. A novel Cu-Cl cycle that utilizes HCl waste as a raw material and produces H₂ and Cl. The Cu-Cl cycle CO₂ emissions are less (8.7 kg_{CO2/kg H2}) compared to other processes, such as SMR, as the process can utilize a significant HCl quantity, which is also considered a greenhouse gas [68]. Figure 10 shows the GHG emissions of the Cu-Cl cycle and other hydrogen production technologies.

4. Future Utilization

Renewable energy can provide a greater contribution to the environment by utilizing hydrogen. With the extensive utilization of clean hydrogen in other fields such as fertilizers, steel, and petrochemical industries, it is obvious to anticipate the ever-increasing demand for hydrogen in the future. Hydrogen is undergoing an extraordinary economic boom, with more and more policies and projects being implemented worldwide. It concludes that it is time to scale up the applications and bring prices down to make hydrogen readily available. The slope of hydrogen utilization will rise in the upcoming years. The forecast for ammonia production from 2026 to 2030 will increase to 289.8 million metric tons per year, which means it will require 51.58 million metric tons of hydrogen in the future [69]. Most hydrogen is consumed in refineries, by the metals and fertilizer industries, and by the food and beverage industries. Petroleum refineries use hydrogen to lower the sulfur content of fuels. Domestic and international demand for diesel fuel has increased refinery demand for hydrogen as sulfur content regulations have stiffened. By 2050, oil demand will rise by 103.2 million tons per year, which adequately requires 663.68 million tons per year of hydrogen [70].

The struggle for decarbonization is getting at a faster pace. To decrease global emissions by 50% by 2030 and to limit global warming temperature to two degrees: Hydrogen is the key component to achieve the said goals. There is a surge in H₂ demand across various industries. Hydrogen is a cleaner fuel being adopted briskly by some sectors and evolving in other fields. Industrial chemicals have hydrogen as a key element of their chemical configuration, and their hydrogen dependency is partial in some and full in others, i.e., ammonia and methanol. Hydrogen demand for these chemicals is expected to be 57 Mt/year by 2030 that was 44 Mt/year in 2018. Moreover, methanol demand is predicted to grow at 3.6 percent per year by 2030. Likewise, for steel production currently, the major share is carried by the Blast furnace steel production method, which is expected to be surpassed by the direct route. Hence, increase in H₂ demand. Furthermore, reduction in H₂ cost by electrolytic method and carbon reduction may occupy steel production. Similarly, other industries are expected an elevation of H₂ use in the near future [71].

In addition, Russia has undertaken a concentrated drive to gain 15% of the world’s hydrogen market by the end of the decade. To achieve the export goals of 0.2 million tons and 2 million tons by 2024 and 2030, respectively, 3.9 billion dollars will be required every year [72]. Similarly, Europe intends to achieve carbon neutrality by the mid-century and meet its climate goal within the next eight years. H2 can be employed as an energy carrier (BEV) in electric vehicles. These eco-friendly and zero-carbon-emission cars, similar to fuel cell electric vehicles (FCEVs), are powered by hydrogen and emit only water. Many countries are attempting to increase the number of hydrogen refueling stations, but Japan is leading the charge, with plans to increase the number to 325 by 2025 [73]. The declining trend in refinery H₂ utilization is incompatible with climate goals. As a replacement, Europe is also expected to import synthetic liquid fuels [74]. On the other hand, the chemical and steel sectors would see higher demand. Similarly, India intends to reach net-zero emissions by the year 2060. The primary driving factors are the decrease in the cost of renewable energy technologies and improvements in H₂ production efficiency. A TERI estimate anticipates a two-fold increase in H₂ use in the next 30 years [75].

China is also the world’s largest H₂ generator. Previously, coal was used to provide China’s energy needs, but this has caused major environmental problems. Carbon neutrality is advancing rapidly to attain neutrality by 2060. It is planned to increase H₂ base power projects, filling stations, and fuel cell systems to make it 10% of the principal energy source, up from about 4% now [76]. As hydrogen utilization is increasing day by day, researchers and industries need to emphasize renewable sources for hydrogen production.

5. Renewable Sources for Hydrogen Production

Global energy demand has increased due to industrialization and economic sustainability, particularly in emerging countries. Global energy demand is predicted to be 600–1000 EJ by 2050 [77]. Natural gas, crude oil, and coal account for over 80% of global energy consumption [78]. If biomass or waste materials are used as energy sources, the cost of creating hydrogen will likely be lower. Petroleum-based fuels are only found in a few places across the world. At the current rate of fossil fuel consumption, the reserves are predicted to be depleted in less than 50 years. Growing concerns about CO₂ emissions into the air point to the need for a new type of energy from carbon-neutral renewable sources with the least negative environmental impact [79]. The hydrogen economy’s future is largely contingent on the availability of a low-cost, environmentally friendly hydrogen supply [5]. Governments worldwide are investing in alternative fuels, and multiple research initiatives on hydrogen storage, production, and use have facilitated hydrogen as the future fuel [80]. According to reports, the transportation industry consumes more than 30% of energy in the European Union (EU) due to growing rates of people and goods mobility [81].

Depending on current improvements in different hydrogen generation processes, the most energy is required for hydrogen synthesis by water electrolysis [82]. Several studies have been conducted in various nations to investigate the viability of shifting to a hydrogen economy. Models such as the Italian-Markal model, the scenario-based model of Germany [83], the Danish energy system, and the Balmorel model [84,85]. Taiwan’s general equilibrium model-energy for hydrogen [86], the dynamic Austria framework [87], the UK THESIS model [88], and the MARKAL model of Swiss are a few examples of studies regarding the viability of using hydrogen as an energy carrier in various countries energy mix. Despite countries worldwide’ attempts to reduce their dependency on fossil fuels, global energy consumption is still rising. Global energy demand increased by 2.3% in 2018, according to the 2018 Worldwide Energy and CO₂ Status Report, the largest increase in a decade. Hydrogen fuel cells are a promising solution for future global energy supply. According to the European Commission Roadmap, renewable energy resources are expected to provide 36% of the world’s energy demand by 2025 and 69% by 2050. With hydrogen accounting for 11% in 2025 and 34% in 2050. By 2030, coal and crude oil use will be cut by 36.5% and 40.7%, respectively, if hydrogen-generating technology obtains a lot of support [89]. The two commercial markets that require a substantial amount of hydrogen are the direct reduction in iron ore and the manufacturing of ammonia. Nearly half of the H₂ generated today is utilized in manufacturing ammonia, whereas H₂ is used in the chemical reaction that converts iron ores to iron in roughly a third of the world’s steel production. The transportation industry contributes one-fifth of worldwide CO₂ emissions, accounting for nearly 60% of global oil usage. As a result, we need to start using alternative transportation fuels such as ethanol, hydrogen, and biodiesel instead of traditional fuels such as gasoline and diesel in the world’s future energy plan.

Renewable energy sources provide a viable alternative to fossil fuels. Due to its relatively high availability and low cost, geothermal energy for hydrogen production could be a good alternative. Geothermal energy has been used for power generation, cooling, and heating since it is derived from the natural heat of the earth’s core and has environmentally favorable properties [90]. Heat or electricity is produced from geothermal energy. Electrolysis or hybrid cycles can be used to produce hydrogen using this heat and electricity. A cleaning step is required to collect hydrogen directly from geothermal steam [91]. Geothermal energy produces high-temperature steam, which can be used to generate electricity or power heat pumps. Geothermal resources have the highest temperatures in volcanic regions. The national power company of Iceland carried out recent research in Iceland on deep drilling, which demonstrated that steam extraction at 500–600 °C at 4–5 km of depth is feasible. Over 100 different thermochemical cycles have been proposed for hydrogen production, but only 25 have been demonstrated to be feasible [92]. Researchers proposed the copper–chloride (Cu–Cl) as the most promising lower-temperature cycle, and the sulphur–iodine (S–I) cycle [93] and the Br–Ca–Fe cycle as the best high-temperature cycles. Heat flow optimization is essential for high energy conversion efficiency in thermochemical hydrogen production [94]. The process of producing hydrogen using geothermal energy is depicted in Figure 11 [95,96].

Another promising alternative for hydrogen production is solar energy [97]. Solar energy is a clean, renewable, and sustainable energy (RSE) source, with an average irradiation of 120,000 TW at the earth’s surface [98]. Solar energy is the planet’s largest energy source but only supplies 0.06% of the world’s electrical demand. About 50 years ago, the first solar power instrument was introduced, and since then, the technology has advanced dramatically [99]. Solar to hydrogen system (SHS) and splitting water using solar energy have been proposed to end the world’s addiction to fossil fuel resources and, eventually, to tackle the global warming phenomenon [100]. The photovoltaic energy in SHS breaks down H₂O molecules into H₂ and O₂, which are then used in hydrogen fuel cells to generate electricity when the sun is not shining.

In 1995, the first SHS infrastructure was built in El Segundo, California. About 50–70 m³ of hydrogen per day is produced by the combination of sophisticated PV cells and electrolyzes. Since then, several SHSs have been established to investigate the sustainability of solar-based hydrogen generation as well as strategies to reduce costs. Rodriguez et al. [101] stipulated that a PV-Electrolyzer strategy is commercially compatible with other alternative hydrogen production techniques, such as electrolysis and steam methane reforming, using polluting energy sources. Clarke et al. [102] suggested a stand-alone hydrogen production system that uses a PV solar array to energize the PEM electrolysis device directly. Tributsch et al. [103] estimated that the SHS efficiency for commercial silicon-based solar cells is nearly 8–14% if the water electrolysis system operates at 70–75% energy efficiency. Kelly et al. [104] suggested certain ways of orienting photovoltaic cells for optimal solar energy capture on cloudy days, which could increase by over 40% solar energy capture. Furthermore, since the solar facilities’ efficiency decreases due to the PV cell’s higher temperature, PV system heat should be transmitted to electrolysis, where higher temperatures improve system efficiency [105].SHS achieves its overall energy conversion efficiency by multiplying PV efficiency with electrolyzer efficiency. As a result, solar thermal systems (STS) are more efficient than PV systems.

Table 3. Routes for solar energy and hydrogen production processes.

Solar Energy Routes for Hydrogen Production	Thermal Energy	Solar only	Water Thermolysis (STH = 1–2%)
			Thermo chemical cycles (STH = 20%)
			H2SSplitting
		Hybrid with fossil fuels	Thermo catalytic H2S cracking
			Reforming (Require CO2 sequestration)
			Gasification (Require CO2 sequestration)
			Cracking (Require C sequestration)
		Electrical Energy	PV-Electrolysis (STH = 40%)
		Photonic Energy	Photo catalysis (STH = 0.2%)
			Photo electrochemical (STH = 12%)
	Hybrid Options	Electrical with thermal energy	Steam/High Temp. Electrolysis (STH = 35–40%)
			Hybrid thermo chemical water- Splitting cycles
			Solar thermal power generation and Electrolysis
			Steam/High Temp. Electrolysis (STH = 35–40%)
		Electrical with photonic energy	Photo electrolysis
		Biochemical with thermal energy	Thermophilic digestion
		Biochemical with photonic energy	Artificial photosynthesis
			Photo fermentation

shows the different methods’ solar-to-hydrogen (STH) conversion efficiencies [106].

Hydrogen production from wind energy is another clean RSE source. Compared to other energy sources, it has the least negative environmental impact [107]. Wind energy fluctuates due to weather factors [108], which increases the demand for load frequency control in balancing power. A water electrolysis-based wind-to-hydrogen (WTH) technique could solve such challenges. The excess electricity is stored as hydrogen, which can be converted back to electricity when system congestion is relieved, or wind potential is low [109]. For both isolated [110] and connected grid systems [111], electrolytic hydrogen was investigated as a potential storage medium in networking balancing for wind power. Furthermore, the wind-to-hydrogen system has much potential for use as a clean fuel in light-duty cars to help reduce GHG emissions. Figure 12 shows the hydrogen and electricity generation from wind power [112]. The cost of hydrogen produced from wind power is higher than hydrogen produced from non-renewable fuels. This was investigated by Gregor et al. [113] by performing a life cycle assessment (LCA) study of electricity and hydrogen production from solar and wind technologies as renewable sources and hydrogen production from non-renewable fuels such as gasoline and natural gas. There are two factors which are attributed to the high cost of hydrogen production from wind energy: (1) energy losses as a result of inefficiencies in the processes of energy conversion (2) and the new equipment installation cost. Furthermore, when the efficiency of a hydrogen fuel cell vehicle is two times that of an internal combustion vehicle, wind to hydrogen for hydrogen fuel cell vehicles rather than gasoline may result an economically viable GHG emissions mitigation [114].

Table 4. Efficiency and cost of hydrogen production from renewable processes.

Renewable Hydrogen Production	Efficiency % (HHV)	Cost $/Kg
Electrolysis (Wind)	75–80	6–7
Electrolysis (Solar)	75–80	7–8
Biomass gasification	65–70	3–5
Photo electrochemical	10–12	10–20
Wind-to-hydrogen	60–75	4–6
Geothermal electrolysis	70–80	4–7
Biomass pyrolysis	55–65	2–4

shows the efficiency and cost of hydrogen production from various renewable processes [115,116,117,118].

6. Potential Challenges in Hydrogen Utilization

Hydrogen has the same challenges regardless of the application being considered. Hydrogen is difficult to operate, store, handle, and keep safe at workstations. Concerned with the special physical properties of hydrogen, there is a need for high-level instrumentation for its handling. It produces colorless flames, which are hard to detect, posing a critical safety issue. Specially designed material is essential for storing hydrogen as it penetrates into the material, which causes hydrogen embrittlement [119,120]. When hydrogen is used in large-scale applications or for commercial purposes, safety is paramount and must be taken seriously. Aside from its ability to defrost, liquid hydrogen is easier to handle and considered less hazardous. Despite that, when it leaks or is subjected to temperatures above the boiling point (20.4 K [121]), it evaporates, posing several safety concerns that must be considered during storage container design, valves and valves and joint support systems, and instrumentation and control systems. When hydrogen gas leaks, it can be more hazardous than hydrocarbon fuels.

The possible contamination of a hydrogen stream with oxygen or air is a serious issue when handling liquid hydrogen. When liquid hydrogen leaks and evaporates, it disperses rapidly due to its small molecular size. This reduces the hydrogen localized concentration in the air and the duration of a potential hazard. However, it increases the combustible cloud size. Whereas H₂ has a considerably lower energy barrier (0.017 mJ) and the largest flammability range (4 to 74%) [122] for air combustion, it burns very readily with only a small ignition/sparking source, making it more difficult to extinguish the combustion flame.

7. Hydrogen Energy, Current Development, Challenges and Future Scope

Hydrogen is considered a promising energy carrier due to its high energy content, clean combustion, and the ability to produce electricity via fuel cells. With the growing global demand for clean energy, the development of H₂ technologies has received considerable attention over the past few decades. In this section, we will discuss the current achievements, challenges, proposed strategies, and future perspectives regarding H₂ technologies.

7.1. Advancements in H₂ Technologies

The advancements in H₂ technologies can be seen in various aspects, such as H₂ production, storage, transportation, and utilization. In terms of H₂ production, the development of renewable energy sources has provided opportunities for green H₂ production via water/steam electrolysis [123]. Moreover, the development of advanced catalysts and membrane materials has improved the efficiency and selectivity of H₂ production via reforming and other chemical processes [124]. In terms of H₂ storage and transportation, the development of new materials, such as metal-organic frameworks (MOFs) and carbon nanotubes (CNTs), has improved the H₂ storage capacity and safety. In terms of H₂ utilization, the application of H₂ in fuel cells has been greatly improved, with the development of advanced fuel cell materials and systems [125].

7.2. Challenges in H₂ Technologies

Despite the advancements in H₂ technologies, there are still many challenges that need to be addressed. One of the main challenges is the high cost of H₂ production, which is mainly due to the high cost of renewable energy sources and advanced materials. Another challenge is the low energy density and safety issues associated with H₂ storage and transportation. Furthermore, the performance and durability of fuel cells are still not sufficient for commercial applications. In addition, the lack of infrastructure for H₂ production, storage, and transportation is also a major challenge that needs to be addressed [126].

7.3. Strategies for Addressing Challenges in H₂ Technologies

To address the challenges in H₂ technologies, various strategies have been proposed. One strategy is to reduce the cost of H₂ production by increasing the efficiency of H₂ production processes and reducing the cost of renewable energy sources and advanced materials. Another strategy is to improve the energy density and safety of H₂ storage and transportation by developing new materials and technologies. In addition, the development of advanced fuel cell materials and systems is essential for improving the performance and durability of fuel cells. Moreover, the establishment of a comprehensive H₂ infrastructure is crucial for the widespread application of H₂ technologies [127,128].

7.4. Future Perspectives for H₂ Technologies

The future perspectives for H₂ technologies are promising. The increasing global demand for clean energy, coupled with the development of renewable energy sources, will provide more opportunities for green H₂ production. The development of advanced materials and technologies will continue to improve the efficiency, selectivity, energy density, and safety of H₂ production, storage, and transportation. The performance and durability of fuel cells will also be greatly improved, with the development of advanced fuel cell materials and systems. Moreover, the establishment of a comprehensive H₂ infrastructure will enable the widespread application of H₂ technologies in various sectors, such as transportation, power generation, and industrial processes [129,130,131].

8. Conclusions

This review article indicates the overall scenario of hydrogen utilization by a sustainable and eco-friendly process in various sectors. Hydrogen is considered one of the key chemicals for refineries and chemical firms. The world’s energy resources are running out, but the demand for hydrogen is increasing. This review is focused on hydrogen utilization in various applications such as ammonia, methanol, hydrogenation processes, rocket engines, and iron ore reduction processes. As new technologies are embraced, and the climate continues to worsen, nations are moving towards a green hydrogen economy. Only if hydrogen is created using renewable energy sources will its usage as fuel be environmentally friendly. The hydrogen fuel on the market currently available is mainly produced from natural gas steam reforming, which is a carbon-intensive process with greenhouse gas emissions similar to direct fossil fuel combustion. Hydrogen derived from renewable energy sources, on the other hand, is ecologically beneficial as its combustion and production do not result in carbon emissions. Furthermore, the sources of hydrogen production from renewable sources such as solar, wind, and geothermal are also discussed in this review. Governments worldwide are investing in alternative fuels, and a number of research programs on hydrogen storage, production, consumption, and application have paved the way for hydrogen to become the future fuel.