Routes for Hydrogen Introduction in the Industrial Hard-to-Abate Sectors for Promoting Energy Transition

: This paper offers a set of comprehensive guidelines aimed at facilitating the widespread adoption of hydrogen in the industrial hard-to-abate sectors. The authors begin by conducting a detailed analysis of these sectors, providing an overview of their unique characteristics and challenges. This paper delves into speciﬁc elements related to hydrogen technologies, shedding light on their potential applications, and discussing feasible implementation strategies. By exploring the strengths and limitations of each technology, this paper offers valuable insights into its suitability for speciﬁc applications. Finally, through a speciﬁc analysis focused on the steel sector, the authors provide in-depth information on the potential beneﬁts and challenges associated with hydrogen adoption in this context. By emphasizing the steel sector as a focal point, the authors contribute to a more nuanced understanding of hydrogen’s role in decarbonizing industrial processes and inspire further exploration of its applications in other challenging sectors.


Introduction
The "hard to abate" sectors are those industries or sectors that face significant challenges in reducing their greenhouse gas emissions and transitioning to more sustainable practices. These sectors typically involve processes or activities that rely heavily on fossil fuels or emit substantial amounts of carbon dioxide (CO 2 ), making it difficult to implement traditional decarbonization measures [1].
The term hard-to-abate highlights the complexities and barriers faced by these sectors in achieving emission reduction targets. Some examples of hard-to-abate sectors include steel production, cement, glass and paper manufacturing, chemicals, and heavy transportation [2,3].
The challenges in decarbonizing these sectors stem from the intrinsic nature of their operations, which often require high-temperature heat sources or involve energy-intensive processes. Finding viable alternatives to fossil fuels and developing technologies capable of reducing emissions without compromising productivity and competitiveness are key challenges [4].
Innovative solutions, such as the use of hydrogen, carbon capture, utilization, and storage (CCUS), electrification, and circular economy approaches, are being explored to tackle these challenges [5].
The role of hydrogen in decarbonization should not be underestimated, as it offers versatility and compatibility with existing industrial processes [6,7]. However, it is crucial to recognize that hydrogen serves as a carrier of energy and must be produced sustainably. Therefore, it is essential to establish a clear understanding of the energy balance and energy demand within each process, and carefully consider this aspect in all new projects. Failing to do so may result in missed opportunities for cost-effectiveness and

Hard-to-Abate Sectors: Energy Analysis and the Possible Role of Hydrogen
Hard-to-abate industrial processes encompass a range of activities within sectors such as cement, metallurgy, glass, paper, and chemicals (these sectors include both traditional applications in chemical production and refineries, as well as innovative uses in green chemistry). While electrification serves as an effective pathway for decarbonization in many sectors, it may not be a straightforward solution for all industries.
Certain sectors, referred to as hard-to-abate sectors, face unique challenges in achieving their sustainability objectives. For these industries, hydrogen emerges as a promising solution in enabling energy transition. The adoption of hydrogen technologies offers opportunities for these sectors to diversify their energy sources, optimize their operations, and contribute to overall decarbonization efforts. It presents a pathway that complements electrification and enables the decarbonization of processes. The International Energy Agency's report highlights the significant role of industrial hard-to-abate sectors as major contributors to global CO 2 emissions, as depicted in Figure 1 [10]. Industry stands out as a substantial emitter, accounting for approximately 9 gigatonnes of CO 2 emissions. Within the industrial sector, energy-intensive industries play a prominent role, with the iron and steel sector alone contributing to almost 30% of the global industrial CO 2 emissions. offering a comprehensive overview of the role and potential impact of green hydrogen in driving decarbonization efforts.

Hard-to-Abate Sectors: Energy Analysis and the Possible Role of Hydrogen
Hard-to-abate industrial processes encompass a range of activities within sectors such as cement, metallurgy, glass, paper, and chemicals (these sectors include both traditional applications in chemical production and refineries, as well as innovative uses in green chemistry). While electrification serves as an effective pathway for decarbonization in many sectors, it may not be a straightforward solution for all industries.
Certain sectors, referred to as hard-to-abate sectors, face unique challenges in achieving their sustainability objectives. For these industries, hydrogen emerges as a promising solution in enabling energy transition. The adoption of hydrogen technologies offers opportunities for these sectors to diversify their energy sources, optimize their operations, and contribute to overall decarbonization efforts. It presents a pathway that complements electrification and enables the decarbonization of processes. The International Energy Agency's report highlights the significant role of industrial hard-to-abate sectors as major contributors to global CO2 emissions, as depicted in Figure 1 [10]. Industry stands out as a substantial emitter, accounting for approximately 9 gigatonnes of CO2 emissions. Within the industrial sector, energy-intensive industries play a prominent role, with the iron and steel sector alone contributing to almost 30% of the global industrial CO2 emissions. However, it is important to note that other industries also make substantial contributions to CO2 emissions, including cement manufacturing and various other sectors, such as glass and paper. These industries are widespread worldwide and play a significant role in the overall emissions profile.
Hydrogen has the potential to play a significant role in the decarbonization of the "hard to abate" sectors. Hydrogen offers the advantage of being a versatile energy carrier, capable of being produced from renewable sources and used in various applications, and with different roles. The understanding that hydrogen serves as an intermediate carrier necessitates a comprehensive and thoughtful analysis. While hydrogen is often discussed in the literature with a focus on specific details, there appears to be a lack of holistic and However, it is important to note that other industries also make substantial contributions to CO 2 emissions, including cement manufacturing and various other sectors, such as glass and paper. These industries are widespread worldwide and play a significant role in the overall emissions profile.
Hydrogen has the potential to play a significant role in the decarbonization of the "hard to abate" sectors. Hydrogen offers the advantage of being a versatile energy carrier, capable of being produced from renewable sources and used in various applications, and with different roles. The understanding that hydrogen serves as an intermediate carrier necessitates a comprehensive and thoughtful analysis. While hydrogen is often discussed in the literature with a focus on specific details, there appears to be a lack of holistic and general analysis. It is important to recognize that hydrogen is not readily available; it must be produced through various methods. Merely examining the intricacies of hydrogen without considering the broader context can result in an incomplete understanding of its role in the energy transition. A comprehensive analysis should encompass the entire value chain of hydrogen, including at least its production, storage, transportation, and utilization. Transportation and storage are crucial components for the effective deployment of hydrogen. While this paper may not extensively cover these aspects, it is important to acknowledge that the specific considerations for transportation and storage can vary significantly depending on the context and location. By examining the interdependencies and complexities associated with hydrogen, a more robust understanding of its potential and challenges can be achieved, enabling informed decision making and the development of effective strategies for its integration into energy systems.

Hydrogen Production through Electrolysis
Many methods are discussed for producing hydrogen, including chemical, electrochemical, and thermochemical, as well as biological.
Nowadays, hydrogen production is based on fossil fuel technologies [11]; in 2021, about 62% of global hydrogen generation was carried out using natural gas without carbon capture, utilization and storage (CCUS), 19% using coal, 18% as a byproduct, and 0.7% using oil; only 0.7% of global H 2 production used fossil fuels with CCUS, and 0.04% electricity.
The main fossil-fuels-based technologies for hydrogen generation are steam methane reforming (SMR), coal gasification, liquid hydrocarbons partial oxidation, and auto thermal reforming. Steam methane reforming is the most common method for producing hydrogen on an industrial scale. SMR involves reacting natural gas (methane) with steam at high temperatures (800-1000 • C); this process produces a mixture of hydrogen and carbon monoxide (CO), known as syngas, which is then processed to separate and purify hydrogen. In coal gasification, the fossil fuel is processed by oxygen and steam, in fixed/moving/fluid/entrained beds, to produce syngas; steam serves both as a thermal moderator and as a reactant, increasing the fraction of hydrogen in the outgoing synthesis gas. In partial oxidation, mainly liquid hydrocarbons from oil processing react with oxygen (or more rarely air) to produce H 2 and CO. Auto thermal reforming, unlike SMR, autonomously supplies the heat requirement, burning part of the natural gas instead of reforming it.
Biomass gasification is also employable for hydrogen generation; organic materials like biomass or waste are converted into syngas, which contains hydrogen, carbon monoxide, and other gases. Hydrogen can then be separated from the syngas.
All these processes require CCUS to produce low-emissions hydrogen. Alternative methods of generating molecular hydrogen are electrolysis (electrochemical water splitting and co-electrolysis), thermolysis and thermochemical processes, and biological processes (such as fermentation and microbial electrolysis).
Thermochemical water splitting and thermal decomposition processes mainly use high temperatures to obtain hydrogen and oxygen molecules from water, while electrochemical technologies employ primarily electricity.
Additional processes are photobiological and photoelectrochemical processes, which use photosynthetic microorganisms or materials that directly convert sunlight into hydrogen by splitting water molecules, and fermentative hydrogen production, in which certain microorganisms can produce hydrogen as a byproduct of their metabolic processes.
Each method has its advantages and disadvantages in terms of efficiency, energy source, environmental impact, and applicability to different scales and situations, and the choice of method depends on factors like the availability of feedstocks, energy sources, environmental considerations, and intended application.
Water electrolysis can employ electricity produced by renewable energy sources from a decarbonization perspective. In the International Energy Agency's outlook to 2030 [11], lowemissions hydrogen appears fundamental for energy transition, with electrolysis powered by green electricity being the main route of production.
Water electrolysis is a modular technology; thus, the size can be scaled up to adapt it to final uses, although this requires paying attention to real system performance.
Energies 2023, 16, 6098 5 of 23 Furthermore, the technology readiness level (TRL) of electrolysis technologies is the highest among low-emissions hydrogen production processes [11]; water electrolysis technologies have a TRL from 6 (large prototype) to 9 (early adoption), while, e.g., thermal decomposition has a TRL equal to 4 (small prototype), and thermochemical water splitting only 3 (concept), and fossil fuel processes with CCUS have TRL values 5-6. In conclusion, only low-temperature electrolysis is near maturity (TRL 11), although a considerable amount of research and experience with green hydrogen is still necessary to help achieve the energy transition.
Water electrolysis is a process that uses an electrical current to split water molecules into hydrogen and oxygen. It requires the use of an electrolyzer, which mainly consists of two electrodes separated by an electrolyte. When sufficient electrical current is provided, hydrogen gas is produced at the cathode, and oxygen gas is produced at the anode.
In the absence of external energy sources, the natural tendency of water is to remain in its stable molecular form. The decomposition of water into hydrogen and oxygen requires energy input to overcome the energetic barrier and drive the reaction in the forward direction. In the process of hydrogen production through electrolysis, energy is indeed required. The energy consumption for electrolysis can vary depending on several factors, including the electrolysis technology used and the operating conditions.
The production of electrolytic hydrogen from water stands out as one of the most practical and promising technologies for the large-scale production of "green" hydrogen, allowing the utilization of renewable energy sources, such as solar or wind power, to generate electrical power for hydrogen production without carbon dioxide emissions.
The hydrogen produced can be used as feedstock, alternative fuel, or to generate electricity as needed once it is stored.
Fuel cells operate by electrochemically reacting hydrogen with oxygen from the air to produce electricity, with heat and water as byproducts. This electrochemical process offers a clean and efficient way to convert the chemical energy stored in hydrogen into usable electrical power, with significantly higher efficiencies compared to traditional combustionbased power generation.
Extensive research efforts are being conducted to develop functional components and systems for electrochemical technologies in hydrogen production.
Each of these electrochemical technologies offers unique advantages in terms of operational temperature, efficiency, scalability, and suitability for specific applications. Research activities focus on improving the performance, durability, and cost-effectiveness of these technologies to enable their widespread adoption in hydrogen production and utilization systems.

Theoretical Elements of Electrolysis
Low-temperature electrolysis is described from the splitting reaction of the liquid water molecule: whose enthalpy change in standard conditions (1 atm and 25 • C), ∆H • , equals 285.8 kJ per mol of reaction [12].
The ∆H • of the reverse reaction, H 2 combustion and steam condensation, is a positive value corresponding to the higher calorific value of hydrogen: 39.4 kWh/kg (about 2.016 g H2 /mol reac ).
Electrolysis performed at high temperatures uses steam according to the reaction in which water is in gaseous form: Energies 2023, 16, 6098 6 of 23 whose standard enthalpy change is 241.8 kJ/mol reac [12]. The ∆H • of the reverse reaction, H 2 combustion, corresponds to the lower calorific value (LHV) of hydrogen, about 33.3 kWh/kg.
Water electrolysis is a nonspontaneous process, and the reaction free energy change is a large positive quantity. The change in free energy of a system for a constant-temperature process is the difference between the enthalpy change and the product of the absolute temperature and the entropy change, Equation (3): ∆G corresponds with the electrical energy demand of the electrolytic process, T∆S the thermal energy demand, and ∆H the total energy demand required. Figure 2 shows the mass and energy flows of the electrolytic process.
The ΔH° of the reverse reaction, H2 combustion and steam condensation, is a positive value corresponding to the higher calorific value of hydrogen: 39.4 kWh/kg (about 2.016 gH2/molreac).
Electrolysis performed at high temperatures uses steam according to the reaction in which water is in gaseous form: whose standard enthalpy change is 241.8 kJ/molreac [12]. The ΔH° of the reverse reaction, H2 combustion, corresponds to the lower calorific value (LHV) of hydrogen, about 33.3 kWh/kg.
Water electrolysis is a nonspontaneous process, and the reaction free energy change is a large positive quantity. The change in free energy of a system for a constant-temperature process is the difference between the enthalpy change and the product of the absolute temperature and the entropy change, Equation (3): ΔG corresponds with the electrical energy demand of the electrolytic process, TΔS the thermal energy demand, and ΔH the total energy demand required. Figure 2 shows the mass and energy flows of the electrolytic process. The electrical energy demand in standard conditions, ΔG°, of liquid water electrolysis equals 232.7 kWh per mol of reaction [12] (=32.7 kWh per kg of hydrogen), while for steam electrolysis, 228.6 kJ/molreac [12] (=31.5 kWh/kgH2) is necessary.
Bi et al. [13] represented graphically how the electrical demand decreases considerably with increasing the temperature at which electrolysis is carried out, especially when the process happens with steam instead of liquid water. For temperatures above 100 °C, the total energy demand increases slightly as the temperature increases, because the increase in the heat demand is greater than the decrease in the required electricity.
The change in free energy represents the amount of electrical work that can be obtained or must be provided in a reaction [12], as in Equation (4): where n = 2 is the number of moles of electrons (e − ) transferred per mole of hydrogen in the overall redox equation of the electrochemical process, F = 96,485.3 C/mole− is the Faraday constant, and Ecell is the cell voltage, i.e., the difference between the electrodes' The electrical energy demand in standard conditions, ∆G • , of liquid water electrolysis equals 232.7 kWh per mol of reaction [12] (=32.7 kWh per kg of hydrogen), while for steam electrolysis, 228.6 kJ/mol reac [12] (=31.5 kWh/kg H2 ) is necessary.
Bi et al. [13] represented graphically how the electrical demand decreases considerably with increasing the temperature at which electrolysis is carried out, especially when the process happens with steam instead of liquid water. For temperatures above 100 • C, the total energy demand increases slightly as the temperature increases, because the increase in the heat demand is greater than the decrease in the required electricity.
The change in free energy represents the amount of electrical work that can be obtained or must be provided in a reaction [12], as in Equation (4): where n = 2 is the number of moles of electrons (e − ) transferred per mole of hydrogen in the overall redox equation of the electrochemical process, F = 96,485.3 C/mol e− is the Faraday constant, and E cell is the cell voltage, i.e., the difference between the electrodes' reduction potentials. By the respective standard free-energy change value, E • cell is calculated to be 1.23 V for liquid water electrolysis, and 1.18 V for steam electrolysis.
Assuming the reaction is reversible, and carrying out the electrolytic process at constant temperature and pressure, E • cell corresponds with the reversible cell voltage, U rev , which is defined as the minimum voltage to be applied between the electrodes for electrolysis to take place. In not ideal conditions, the cell voltage is always higher than U rev due to the irreversibilities of the real electrolytic process. These efficiency losses lead to an increase in the voltage (overpotentials, η v ) required for water electrolysis compared to the theoretical one, as shown in Equation (5) to calculate the operational cell voltage: where η v act is the activation overpotential, η v ohm is the ohmic overpotential, and η v conc is the concentration or diffusion overpotential. The activation overpotential is related to the reactions' activation energy, and the catalyst and operational temperature increase can reduce this efficiency loss. The ohmic overpotential is due to ionic, electrical, and contact resistances in the electrolytic cell; the current density, cell materials and design, and temperature affect this overpotential [14]. η v conc is related to mass transport, more difficult at high current densities; if H 2 and O 2 are not removed as fast as they are produced, their concentration increases, decreasing the reaction kinetics [14].

Technical Data on Electrolysis
According to the data available from the literature and market, summarized in Table 1, it is evident that the energy consumption related to hydrogen production in commercially available devices is notably higher than the theoretical values discussed in Section 3.1. Nowadays, the average specific energy consumption (ASEC) falls within the range of 55 to 60 kWh per kilogram of hydrogen produced for low-temperature electrolyzers, and 40-42 kWh/kg for high-temperature electrolyzers, although commercial equipment with such performance levels during the useful life is not currently available. These figures highlight the fact that there are efficiency losses and energy requirements beyond the idealized values in practical hydrogen production systems.
Furthermore, it should be noted that the energy consumption data for hydrogen production are partially dependent on the size of the electrolyzer. It is unrealistic to expect the same energy consumption value to be applicable across a wide range of systems, spanning from a few kilowatts to several megawatts. Larger-scale electrolyzers may exhibit different energy efficiency levels and operational characteristics compared to smaller units. Factors such as system design, materials, and operating conditions can significantly influence the energy consumption of H 2 production. Therefore, when considering the implementation of H 2 production systems, it is essential to take into account the specific characteristics and scale of the electrolyzer to accurately assess its energy requirements.
Technological advancements in low-temperature and high-temperature electrolysis and process optimizations can lead to improvements in energy efficiency, potentially reducing the energy consumption per unit of hydrogen produced.
Electrolyzers' First Law efficiency can be evaluated as in Equation (6): which results in around 58% for low-temperature electrolyzers, and about 81% for hightemperature electrolyzers, using the current ASECs identified in Table 1. For a real evaluation of electrolyzers' First Law efficiency, the input thermal energy should also be considered, as shown in Figure 2.
Thus, by identifying the declared energy consumption of electrolyzers as electricity consumption, it is possible to define an electrical efficiency (η el ) as the ratio of the electricity demand of the electrolytic process and the average specific energy consumption of the electrolyzer, Equation (7): evaluating the electricity demand in standard conditions, ∆G • , to have a fixed term of comparison depending only on operating the electrolysis at low or high temperatures. Through the current ASECs identified above (Table 1), the electrical efficiency is around 0.57 for low-temperature electrolyzers and about 0.77 for high-temperature electrolyzers. The electrical energy demand decreases steadily as the temperature increases, and it decreases significantly above 100 • C; if the ∆G were evaluated at the operating temperature instead of in standard conditions, the electrical efficiency would surely assume lower values, especially for high-temperature electrolyzers. Table 2 summarizes the main data and results of Sections 3.1 and 3.2. In conventional low-temperature electrolysis, such as alkaline (ALK), proton exchange membrane (PEM), and anion exchange membrane (AEM) electrolyzers, the water-splitting reaction occurs at relatively low temperatures, typically around 60 to 80 • C.
Considering low-temperature real applications, their energy consumption is of the order of 55-60 kWh per kg of hydrogen produced, and this means that in the first step about 40% of the energy is lost.
Increasing temperature in electrolysis processes can lead to significant reductions in electrical consumption.
High-temperature electrolysis, such as that using solid oxide (SO) cells, is a promising approach that utilizes elevated temperatures, typically above 600 • C and often in the range of 800 to 1000 • C, to drive the electrochemical reactions more efficiently, resulting in improved energy efficiency and reduced electricity requirements.
At elevated temperatures, the electrolysis reactions become more favorable thermodynamically. The higher operating temperature allows for faster reaction kinetics, enabling higher production rates of hydrogen. All this leads to lower overpotentials required; thus, less electrical energy is needed to drive the reactions, resulting in reduced electricity consumption. However, it is important to note that high-temperature electrolysis comes with challenges, such as material compatibility, thermal management, and system integration. High temperatures require appropriate materials for cell components and sealing, and efficient heat transfer mechanisms are necessary to maintain the desired operating conditions. These technical considerations need to be addressed to realize the full potential of high-temperature electrolysis.
Overall, high-temperature electrolysis holds promise for reducing electrical energy consumption, making it an area of active research and development. By leveraging the benefits of elevated temperatures, high-temperature electrolyzers can contribute to the advancement of efficient and sustainable hydrogen production systems.
Continuous research on H 2 production by water electrolysis is very important to increase the performance of low-temperature and high-temperature electrolyzers, in various operating conditions, and during their useful life, especially to produce green H 2 Energies 2023, 16, 6098 9 of 23 from renewable energy. Electrolysis is a complex process that is still being studied as regards its optimization. However, to carry out energy assessments on industrial processes, it is sufficient to refer to the current literature and market data, despite the remaining uncertainties.

Pathways to H 2 Introduction in the "Hard to Abate" Sectors
The decarbonization of the hard-to-abate sectors poses significant challenges, but one potential solution lies in the electrification of industrial processes using green energy sources. While this approach holds promise, its implementation is not without difficulties, particularly in sectors like steel, cement, and glass that rely heavily on high-temperature processes and large-scale equipment. Substituting thermal energy with electricity in these processes is often impractical due to technical limitations and the sheer size of the components involved. Many industrial processes operate at extremely high temperatures that can exceed the capabilities of electrical technologies. Traditional high-temperature processes often use the heat generated as a byproduct for various purposes within the industrial facility. Transitioning to electrified systems may result in the loss of this useful waste heat. Electrification of high-temperature processes requires robust electrical infrastructure capable of delivering the required power levels consistently and reliably. Depending solely on electricity can introduce issues related to power grid stability. In general, we can affirm that transitioning from well-established conventional processes to electrified alternatives may require significant modifications to existing equipment and facilities.
Green hydrogen has emerged as a viable pathway towards decarbonization in these sectors. Its unique ability to store energy, for long periods too, and generated from renewable sources, presents a valuable opportunity. Green hydrogen can serve as an intermediary energy carrier, enabling the efficient utilization of renewable energy across different applications. By producing hydrogen through electrolysis powered by green electricity, excess renewable energy can be stored and converted into hydrogen for subsequent use.
The integration of green hydrogen offers several advantages. Firstly, it addresses the intermittency of renewable energy sources by providing a means of energy storage that can be utilized as needed. This flexibility enhances the stability and reliability of energy supply, particularly in industries that require continuous and high-energy operations. Secondly, green hydrogen can be effectively utilized in processes that cannot be easily electrified, such as those involving high temperatures or specialized equipment. By leveraging hydrogen as a clean and versatile fuel or as a chemical-reducing agent, the hard-to-abate sectors can significantly reduce their carbon emissions.
To unlock the full potential of green hydrogen, concerted efforts are needed in advancing electrolysis technologies, expanding renewable energy infrastructure, and establishing supportive policies and regulations. Collaboration between industry stakeholders, research institutions, and policymakers is crucial to drive innovation and overcome existing challenges. By embracing green hydrogen as a valuable tool for decarbonization, the hardto-abate sectors can move closer to achieving their sustainability goals while benefitting from a reliable and clean energy source. This paper aims to address several key aspects related to the use of hydrogen in hard-to-abate industrial sectors, such as steel, glass, paper, and others, for decarbonization purposes. The following objectives are outlined:

(a) Identification of Industrial Processes
The first objective is to identify the main industrial processes within each hard-to-abate sector that have the potential to incorporate hydrogen for decarbonization. This involves a comprehensive analysis of the different steps and processes involved in these sectors, assessing where hydrogen can be effectively utilized as a clean energy source.

(b) Analysis of Possible Hydrogen Routes in the Process
This study will analyze and evaluate the various routes available for hydrogen use in the hard-to-abate sectors. This includes examining different technologies and assessing the state-of-the-art solutions in terms of their feasibility, efficiency, and environmental impact. The goal is to identify the most suitable hydrogen-based solutions for each sector, considering factors such as process requirements, energy efficiency, and emissions reduction potential.
(c) Use of Produced Hydrogen as a Fuel Another aspect to be explored is the broader role of hydrogen in the hard-to-abate sectors. The first opportunity is to use hydrogen as a fuel, in particular, the use of hydrogen as fuel in blended combustion; blended combustion refers to the practice of combining hydrogen with another fuel, such as natural gas, for thermal power production in industrial processes. Another benefit is the versatility of blended combustion. The mixture of hydrogen and natural gas, with H 2 percentages up to 20-30%, can be utilized in existing combustion systems without significant modifications to existing equipment. This enables a smoother transition towards decarbonization, as industries can leverage their existing infrastructure and equipment while gradually incorporating higher proportions of hydrogen into the fuel blend.
When hydrogen is used as an alternative fuel in burners, carbon dioxide emissions are reduced, but nitrogen oxide (NOx) levels may increase. Therefore, to enable the reduction of NOx production in hydrogen combustion, it is necessary to adopt NOx abatement methods, such as flameless combustion which reduces temperature peaks in combustion to limit NOx formation, and/or hydrogen oxy-combustion.
Natural gas has a lower heating value typically in the range of 45-48 MJ/kg, considering a rather high methane content; assimilating natural gas with methane, a lower heating value of 50 MJ/kg ≈ 36 MJ/Nm 3 can be assumed, while hydrogen has a lower heating value of 120 MJ/kg ≈ 11 MJ/Nm 3 .
Thus, the lower heating value of the blend can be defined as: which, using a reference value of 1 MW for the thermal power transferred to the treated material, allows evaluating the blend flow rate required, for example, in weight: . m blend = 1 MW η burner · 3600 s h LHV blend (9) and, thus, the flow rates of H 2 and methane by Equations (10) and (11): . m H2 = % H2 100 · . m blend (10) .
Tables 3 and 4 show the lower heating value of the combustible blend, and the flow rates of the blend, hydrogen, and methane, varying the percentage of H 2 in the combustible blend, respectively, in volume and weight. H 2 volumetric flow rates appear much greater than those for mass due to the very low density of hydrogen (about 0.09 kg/Nm 3 ). Assuming stoichiometric combustion, the reduction of carbon dioxide emissions is also estimated as the percentage of H 2 varies (Tables 3 and 4).
The burner efficiency also depends on the percentage of hydrogen used; an average efficiency is assumed, for all hydrogen percentages evaluated, for a burner with air preheating: η burner = 0.85-0.86, according to [15].
To supply by hydrogen 1 MW of heat requirement for material treatment, around 35 kg/h of H 2 must be employed (Table 4), which nowadays is producible by about 2 MW of low-temperature electrolyzers or around 1.4 MW of high-temperature ones (Table 1, neglecting degradation and system energy losses).
Blended combustion also offers flexibility in terms of varying hydrogen concentrations. The proportion of hydrogen in the blend can be adjusted based on specific requirements and process conditions. This adaptability allows industries to optimize combustion performance, energy efficiency, and emissions reduction according to their unique needs.
Moreover, blending hydrogen with natural gas can enhance the combustion process itself. Hydrogen has a high flame speed and wide flammability range, which can improve the stability and efficiency of combustion. The addition of hydrogen to the fuel mixture can lead to faster and more complete combustion, resulting in higher combustion efficiency and reduced pollutant emissions.
In analogy to Franco et al. [16], energy indicators of H 2 industrial use can be enucleated. The heat requirement for material treatment in the industrial process is assumed equal to 1 MW, as a reference value.
In the planning stage, common target examples for the percentage of heat requirement coverage (HRC) by hydrogen can be 10%, 15%, and 20%, as shown in Table 5. The HRC by hydrogen equals the non-renewable primary energy saving (NRPES), by replacement of fossil energy, as in Equations (12) and (13): . m CH4,ante · LHV CH4 · 100 = % H2 · LHV H2 LHV blend (13) With 1 MW of thermal power requirement for material treatment as the reference value, in Table 5 it can be seen that to cover 20% of the heat requirement with hydrogen (thus, to reduce CO 2 emissions by 20%, assuming stoichiometric combustion, Equations (12) and (13)), about 7 kg/h of H 2 are necessary in the combustible blend. This hydrogen flow rate could be produced as green H 2 by using around 400 kW low-temperature electrolyzers or 300 kW high-temperature ones, with present average nominal efficiencies (Table 1) and neglecting system energy losses.
Other relevant aspects include assessing the potential use of hydrogen as a chemical element within the principal processes of hard-to-abate industries, such as in refining, manufacturing, and other key operations. Additionally, this study will investigate the application of hydrogen as an energy storage system, enabling the utilization of renewable energy sources by storing excess energy in the form of hydrogen for later use.
By addressing these objectives, it is possible to provide valuable insights into the integration of hydrogen in hard-to-abate industrial sectors. It will contribute to the understanding of how hydrogen can be effectively employed to decarbonize these sectors, as a fuel, a reducing agent, and an energy storage solution, ultimately supporting the transition to more sustainable and low-carbon industrial processes.

Steel Sector
The steel sector is characterized by its high energy consumption and it is the industrial sector with the highest CO 2 emissions ( Figure 1); thus, it presents unique challenges and opportunities for decarbonization. According to common statistical data, steel production requires a fossil energy demand of approximately 3.3 MWh (equivalent to about 12 GJ) per tonne of crude steel [2]. In the following Section 5, we will have the opportunity to better investigate the data with a detailed analysis of the various processes.
The steel industry is known for its energy-intensive processes, primarily in the form of the high-temperature heat required for iron ore reduction and steelmaking. This poses challenges for electrification due to the difficulty of replacing thermal energy with electricity. To achieve deep decarbonization, the sector requires transformative measures to reduce its reliance on fossil fuels. Hydrogen produced by renewable energy sources offers a carbon-neutral alternative to traditional fossil fuels used in steelmaking, such as coal and natural gas, and can play a crucial role by serving as a clean and high-temperature energy carrier. One possible application of hydrogen is surely hydrogen use as a reducing agent in direct reduced iron (DRI) technology. In this process, hydrogen reacts with iron oxide to produce direct reduced iron, which can be further processed into steel. By replacing carbon-based reducing agents with hydrogen, the carbon emissions associated with iron and steel production can be significantly reduced [17].
Another way hydrogen can contribute to decarbonization in the steel sector is through blended combustion. By blending hydrogen with natural gas, the carbon intensity of the combustion process can be lowered. This approach allows for a gradual transition by leveraging existing infrastructure while reducing carbon emissions [18][19][20].
In scenarios where power plants are situated in proximity to industrial facilities like steel plants, green hydrogen can play a dual role, both as an energy storage medium and as a means of integrating renewable energy sources. During times of low electricity demand or when renewable power generation exceeds immediate requirements, excess renewable electricity can be utilized for the electrolysis of water, producing green hydrogen. The stored hydrogen can then be utilized during peak demand or when the renewable energy supply is limited, providing a reliable and flexible energy source for steel production.

Cement Sector
Cement production is the industrial sector with the second-highest carbon dioxide emissions, as shown in Figure 1. Cement manufacturing requires about 0.5 MWh (approximately 1.8 GJ) of fossil energy per tonne of clinker [2].
Cement production involves high-temperature processes, such as limestone calcination and clinker production, which require substantial amounts of thermal energy. These processes typically rely on fossil fuels like coal and natural gas, leading to significant CO 2 emissions. Hydrogen can play a crucial role in decarbonizing the cement sector by serving as a clean and sustainable alternative to fossil fuels [21].
Hydrogen can be utilized as an alternative fuel in cement kilns, replacing or supplementing traditional fossil fuels.
Clinker, a key ingredient in cement production, is produced through the heating of limestone and other materials in a kiln. This process generates substantial CO 2 emissions due to the calcination of limestone. Hydrogen, when used as a reducing agent, can potentially replace the traditional carbon-intensive calcination process.
The cement sector can also benefit from the use of hydrogen as an energy storage solution and for integrating renewable energy sources in the production process.

Glass Sector
The glass sector encompasses various processes involved in the manufacturing of glass products. Manufacturing requires about 2.5 MWh of fossil energy per tonne of flat glass, and around 1.6 per tonne of container glass [2]. These processes typically involve high-temperature operations, such as glass melting, refining, and forming.
One possibility is the use of hydrogen as a direct fuel in glass furnaces, replacing fossil fuels in the form of blended combustion. Hydrogen oxy-combustion, in experimentation, can produce only water vapor as a byproduct, even if steam plays a critical role in the Earth's greenhouse effect due to its ability to trap heat in the atmosphere, and its interactions with other greenhouse gases. In any case, this substitution can significantly reduce the carbon footprint of glass production [22].
Another potential application of hydrogen is as a reducing agent in glass manufacturing. Hydrogen can act as an alternative to carbon-based reducing agents, such as coke or coal, in the production of specific types of glass. By using hydrogen, the industry can avoid carbon emissions associated with traditional reduction processes. Moreover, hydrogen can play a significant role in energy storage and the effective management of power generated from renewable sources.

Pulp and Paper Sector
The pulp and paper sector is the industrial sector with the fifth-highest CO 2 emissions (Figure 1). Its fossil energy demand varies from about 1.4 MWh per tonne of board and packaging paper, to around 2 for tissue and graphic paper, to about 3.4 MWh per tonne of chemical pulp [2]. The paper sector encompasses various processes involved in the production of paper and related products. These processes typically include wood pulping, papermaking, and paper coating. The industry relies heavily on thermal energy, primarily obtained from fossil fuels, for tasks such as drying, heating, and steam generation. As in the other sectors, one possible application is the use of hydrogen for thermal processes in place of fossil fuels. Hydrogen can also be utilized in the pulping process as a chemical agent for delignification, a crucial step in paper production.

Case Study: Implementing Hydrogen in the Steel Sector
Steel has been a central component of industrial society since its inception, symbolizing the advent of the industrial era. The economic significance of steel is exemplified by its substantial production growth over the years. In 1950, at the onset of post-war recovery, global steel production stood at almost 190 million tonnes; this figure skyrocketed to 850 Mtonnes in 2000 and reached the staggering maximum of 1962 Mtonnes in 2021 [23].
Crude steel production for the 64 countries reporting to the World Steel Association was 1885 Mtonnes in 2022 [23,24], as shown in Figure 3 and Table 6.
Another potential application of hydrogen is as a reducing agent in glass manufacturing. Hydrogen can act as an alternative to carbon-based reducing agents, such as coke or coal, in the production of specific types of glass. By using hydrogen, the industry can avoid carbon emissions associated with traditional reduction processes. Moreover, hydrogen can play a significant role in energy storage and the effective management of power generated from renewable sources.

Pulp and Paper Sector
The pulp and paper sector is the industrial sector with the fifth-highest CO2 emissions (Figure 1). Its fossil energy demand varies from about 1.4 MWh per tonne of board and packaging paper, to around 2 for tissue and graphic paper, to about 3.4 MWh per tonne of chemical pulp [2]. The paper sector encompasses various processes involved in the production of paper and related products. These processes typically include wood pulping, papermaking, and paper coating. The industry relies heavily on thermal energy, primarily obtained from fossil fuels, for tasks such as drying, heating, and steam generation. As in the other sectors, one possible application is the use of hydrogen for thermal processes in place of fossil fuels. Hydrogen can also be utilized in the pulping process as a chemical agent for delignification, a crucial step in paper production.

Case Study: Implementing Hydrogen in the Steel Sector
Steel has been a central component of industrial society since its inception, symbolizing the advent of the industrial era. The economic significance of steel is exemplified by its substantial production growth over the years. In 1950, at the onset of post-war recovery, global steel production stood at almost 190 million tonnes; this figure skyrocketed to 850 Mtonnes in 2000 and reached the staggering maximum of 1962 Mtonnes in 2021 [23].
Crude steel production for the 64 countries reporting to the World Steel Association was 1885 Mtonnes in 2022 [23,24], as shown in Figure 3 and Table 6.  Table 6. Top ten steel producing countries in 2022 and respective million tonnes produced between January and May 2023. Adapted from Ref. [24].  Asia emerges as the dominant region, with major producers including China, India, and Japan, closely followed by Europe and North America, Figure 3. In Table 6, it can be noticed that the top ten steel-producing countries in 2022 together account for about 85% of the global production. These statistics highlight the vital role that steel plays in various sectors and its contribution to economic development and infrastructure worldwide.
Steel production is a continuous process that demands significant energy input and results in high emissions. The steel production cycle can be divided into two primary phases: the upstream phase and the downstream phase. The upstream phase involves the transformation of raw materials into semi-finished products. This phase includes processes such as iron ore mining, coke production, and iron and steelmaking in blast furnaces or electric arc furnaces. During this phase, the raw materials are converted into steel billets, slabs, or other semi-finished forms. The downstream phase involves the further processing of these semi-finished products to obtain the final steel products. This phase encompasses various operations, such as the hot or cold working of sheets (plates), sheets and rolls (plates and coils), as well as additional processing of semi-finished products like slabs and billets. The downstream phase also includes several treatments and applications to enhance the quality and characteristics of the steel. These treatments may involve chemical processes like pickling to remove surface oxide, mechanical processing such as cold rolling to refine the shape and thickness, heat treatment to adjust mechanical properties, and surface treatments like galvanizing to prevent oxidation and impart specific properties to the steel. Figure 4 provides an overview of the steel process with its main production routes: raw materials such as coal, iron ore, and scrap are processed by, respectively, coke production, agglomeration, and collecting and sorting; then, steel production involves ironmaking (by blast furnace or direct reduction), steelmaking (by basic oxygen or electric arc furnace), casting, semi-finishing, and finishing.
Currently, most of the primary steel production (70%) utilizes the conventional blast furnace (BF) and converter system with oxygen injection (BOF), as shown in Figure 5 [25]; the remaining portion is mainly derived from recycled scrap in electric arc furnaces and induction furnaces (EF), and about 7% of the crude steel production employs DRI (direct reduced iron) and electric arc furnaces (EAF). This distribution highlights the prevailing dominance of traditional production methods and the growing significance of recycling processes in mitigating environmental impacts.   The costs of steel production are divided into various components, with raw materials and energy inputs typically accounting for 60-80% [26], and the remaining portion allocated to labor, depreciation, and maintenance. Given its energy-intensive nature, the steel industry relies on both thermal and electrical energy, with a final energy requirement of around 17 gigajoules per tonne of steel produced by natural-gas-based DRI-EAF (ngDRI-EAF), and about 21 GJ/tonne for the BF-BOF process [26]. Moreover, the  The costs of steel production are divided into various components, with raw materials and energy inputs typically accounting for 60-80% [26], and the remaining portion allocated to labor, depreciation, and maintenance. Given its energy-intensive nature, the steel industry relies on both thermal and electrical energy, with a final energy requirement of around 17 gigajoules per tonne of steel produced by natural-gas-based DRI-EAF (ngDRI-EAF), and about 21 GJ/tonne for the BF-BOF process [26]. Moreover, the The costs of steel production are divided into various components, with raw materials and energy inputs typically accounting for 60-80% [26], and the remaining portion allocated to labor, depreciation, and maintenance. Given its energy-intensive nature, the steel industry relies on both thermal and electrical energy, with a final energy requirement of around 17 gigajoules per tonne of steel produced by natural-gas-based DRI-EAF (ngDRI-EAF), and about 21 GJ/tonne for the BF-BOF process [26]. Moreover, the environmental impact of steel production is substantial, primarily attributed to CO 2 emissions during coke processing and blast furnace reduction, resulting in an approximate production of 1-2 tonnes of CO 2 per tonne of steel produced by the BF-BOF or ngDRI-EAF processes [26].
To fully grasp the potential role of hydrogen, it is essential to understand the energy and mass balance of the steel production process. The authors developed a representation, depicted in Figures 6-8, to shed light on this aspect. It is important to note that the data presented were derived from a model and may not reflect precise real-world values. However, they offer a representative insight into the technology and its characteristics. environmental impact of steel production is substantial, primarily attributed to CO2 emissions during coke processing and blast furnace reduction, resulting in an approximate production of 1-2 tonnes of CO2 per tonne of steel produced by the BF-BOF or ngDRI-EAF processes [26].
To fully grasp the potential role of hydrogen, it is essential to understand the energy and mass balance of the steel production process. The authors developed a representation, depicted in Figures 6-8, to shed light on this aspect. It is important to note that the data presented were derived from a model and may not reflect precise real-world values. However, they offer a representative insight into the technology and its characteristics. A comprehensive analysis of the BF-BOF route, Figure 6, reveals that the greatest energy consumption is attributable to the operation of the blast furnace. The total energy requirement amounts to approximately 19 GJ in terms of final energy per metric ton of steel. This value aligns closely with the findings reported in the existing literature, which indicate energy consumption ranging from 18 to 22 GJ per tonne of steel, as shown in Table 7.
Obviously, considering the conventional BF-BOF process, another promising avenue lies in the application of hydrogen for blended combustion, synergistically combining it with natural gas. This approach optimizes combustion efficiency and reduces emissions, contributing to the overall sustainability of the steel sector.
An alternative technology for steel production is the DRI-EAF process, which currently satisfies 7% of global steel demand. DRI plants typically employ a shaft furnace fueled by methane instead of coal. The chemical process involves reforming, either internal or external to the furnace, to generate syngas consisting of CO and H2. This syngas enables the direct reduction of iron at temperatures around 800 °C, resulting in the formation of solid iron known as DRI (direct reduced iron).
Hydrogen serves as an effective reducing agent in the DRI process, facilitating high productivity and iron metallization exceeding 90%, ensuring complete oxidation. However, it is important to note that hydrogen reduction reactions are endothermic, leading to related energy requirements in these plants. In contrast, CO sustains the process through exothermic reactions. Numerical examples of the DRI-EAF process demonstrate the lower demand for iron ore, which is introduced in a porous form to enhance the reduction rate.
The energy consumption in the ngDRI-EAF process ranges from 14 to 18 GJ per tonne of steel, as reported in Table 7. Figure 7 provides an approximate energy and mass balance of the process. The consistency between the observed energy consumption values and those documented in the literature reinforces the validity and reliability of the data. It underscores the significance of the furnace operation in determining the overall energy consumption in steel production. Understanding this energy-intensive aspect is crucial for identifying opportunities to optimize energy efficiency and explore potential pathways for decarbonization. To reduce CO2 emissions in the steel industry, a transformation of the production process is underway. This transformation involves the adoption of various techniques such as direct reduced iron (DRI) with different reduction degrees, hot briquetted iron (HBI), hydrogen plasma smelting reduction (HPSR), and electric smelters for low-grade ores. To uphold its commitment to zero waste, the industry must gain a comprehensive understanding of byproducts like slag.
The contribution of hydrogen in the steel sector encompasses various dimensions that offer transformative benefits. Primarily, hydrogen serves as a crucial facilitator for the integration of green energy sources into steel production. By functioning as a storage medium, hydrogen enables the effective utilization of renewable energy, addressing the challenges associated with its direct application. Moreover, hydrogen demonstrates its potential as a versatile chemical-reducing agent within the steel process, particularly in technol- When considering the feedstock of hydrogen, there are two options: either producing hydrogen on-site through electrolysis or sourcing it from an external supply chain. In both cases, if hydrogen is generated by renewable energy, CO2 emissions can be significantly reduced, up to 80% compared to the BF-BOF process [27]. This reduction in emissions highlights the environmental benefits of utilizing hydrogen in the DRI process.
Integrating renewable energy sources for electricity production and implementing hydrogen as a reducing agent in the H2DRI process present opportunities for reducing carbon emissions and achieving greater sustainability in steel production. The potential for emissions reduction and the use of renewable energy sources make H2DRI a promising pathway for decarbonizing the steel industry and transitioning towards more environmentally friendly steelmaking processes. Assessing all energy intensities in terms of final energy in Table 7, the H2DRI-EAF energy requirement appears smaller than that for BF-BOF, and comparable and potentially smaller than that for ngDRI-EAF, with a much larger share of electricity. A comprehensive analysis of the BF-BOF route, Figure 6, reveals that the greatest energy consumption is attributable to the operation of the blast furnace. The total energy requirement amounts to approximately 19 GJ in terms of final energy per metric ton of steel. This value aligns closely with the findings reported in the existing literature, which indicate energy consumption ranging from 18 to 22 GJ per tonne of steel, as shown in Table 7. Obviously, considering the conventional BF-BOF process, another promising avenue lies in the application of hydrogen for blended combustion, synergistically combining it with natural gas. This approach optimizes combustion efficiency and reduces emissions, contributing to the overall sustainability of the steel sector.
An alternative technology for steel production is the DRI-EAF process, which currently satisfies 7% of global steel demand. DRI plants typically employ a shaft furnace fueled by methane instead of coal. The chemical process involves reforming, either internal or external to the furnace, to generate syngas consisting of CO and H 2 . This syngas enables the direct reduction of iron at temperatures around 800 • C, resulting in the formation of solid iron known as DRI (direct reduced iron).
Hydrogen serves as an effective reducing agent in the DRI process, facilitating high productivity and iron metallization exceeding 90%, ensuring complete oxidation. However, it is important to note that hydrogen reduction reactions are endothermic, leading to related energy requirements in these plants. In contrast, CO sustains the process through exothermic reactions. Numerical examples of the DRI-EAF process demonstrate the lower demand for iron ore, which is introduced in a porous form to enhance the reduction rate.
The energy consumption in the ngDRI-EAF process ranges from 14 to 18 GJ per tonne of steel, as reported in Table 7. Figure 7 provides an approximate energy and mass balance of the process. The consistency between the observed energy consumption values and those documented in the literature reinforces the validity and reliability of the data. It underscores the significance of the furnace operation in determining the overall energy consumption in steel production. Understanding this energy-intensive aspect is crucial for identifying opportunities to optimize energy efficiency and explore potential pathways for decarbonization.
To reduce CO 2 emissions in the steel industry, a transformation of the production process is underway. This transformation involves the adoption of various techniques such as direct reduced iron (DRI) with different reduction degrees, hot briquetted iron (HBI), hydrogen plasma smelting reduction (HPSR), and electric smelters for low-grade ores. To uphold its commitment to zero waste, the industry must gain a comprehensive understanding of byproducts like slag.
The contribution of hydrogen in the steel sector encompasses various dimensions that offer transformative benefits. Primarily, hydrogen serves as a crucial facilitator for the integration of green energy sources into steel production. By functioning as a storage medium, hydrogen enables the effective utilization of renewable energy, addressing the challenges associated with its direct application. Moreover, hydrogen demonstrates its potential as a versatile chemical-reducing agent within the steel process, particularly in technologies like direct reduced iron (DRI).
This direct utilization in hydrogen-based DRI (H 2 DRI) enhances process efficiency and promotes decarbonization. Figure 8 provides a possible mass and energy balance of the process H 2 DRI-EAF, with hydrogen generated by low-temperature electrolysis.
In the case of H 2 DRI, the energy requirement experiences an average increase, estimated to be in the range of 15-18 GJ per tonne of steel, as shown in Table 7. However, this increase is primarily attributed to a larger share of electricity in the process. If the electricity used is generated from renewable sources, the overall energy consumption would be lower compared to the conventional BF-BOF process. Additionally, the electric share in the shaft furnace can be further reduced through heat recovery from waste fumes.
When considering the feedstock of hydrogen, there are two options: either producing hydrogen on-site through electrolysis or sourcing it from an external supply chain. In both cases, if hydrogen is generated by renewable energy, CO 2 emissions can be significantly reduced, up to 80% compared to the BF-BOF process [27]. This reduction in emissions highlights the environmental benefits of utilizing hydrogen in the DRI process.
Integrating renewable energy sources for electricity production and implementing hydrogen as a reducing agent in the H 2 DRI process present opportunities for reducing carbon emissions and achieving greater sustainability in steel production. The potential for emissions reduction and the use of renewable energy sources make H 2 DRI a promising pathway for decarbonizing the steel industry and transitioning towards more environmentally friendly steelmaking processes. Assessing all energy intensities in terms of final energy in Table 7, the H 2 DRI-EAF energy requirement appears smaller than that for BF-BOF, and comparable and potentially smaller than that for ngDRI-EAF, with a much larger share of electricity.
The potential of hydrogen to revolutionize green energy integration and process optimization in the steel industry is undeniably significant. However, accurately assessing the scale of this transformation is paramount, given the quite high power requirements involved. The high energy demands inherent to steelmaking processes present challenges in evaluating feasible pathways for hydrogen implementation. Careful consideration of the scale and magnitude of the problem is essential to ensure realistic and effective solutions. To provide a rough estimate, 1 kW of power from a photovoltaic (PV) plant under average climatic conditions corresponds to approximately 1000 kWh of energy produced (or 3.6 GJ) over a year of operation. Thus, the production of one tonne of steel aligns with the annual output of a PV power plant ranging from 0.5 to 5 kW, with reference to the EAF technologies' share of electricity evaluated in Table 7. When taking into account the production capacity of a steel factory, it becomes evident that a substantial power installation is essential to adequately address the significant energy demands of a small-to medium-sized steel plant, which might have an annual production capacity ranging from around 500,000 to 2 million tonnes of steel [26]. This assessment delves into the intricate task of evaluating the real-world implementation of hydrogen in the steel industry, considering the substantial power levels required. It emphasizes the need for a comprehensive understanding of the energy landscape, process requirements, and technological capabilities to accurately gauge the viability and scalability of hydrogen-based solutions.
The complex interplay between hydrogen, green energy integration, and process optimization in the steel industry requires careful analysis in the coming years.

Conclusions
This paper explored hydrogen's potential applications in industrial hard-to-abate sectors, emphasizing its role in decarbonization. It reviewed hydrogen production methods, stressing sustainable production. Current efficiencies are approximately 60% for lowtemperature and 80% for high-temperature electrolyzers. The electrical efficiency assumes lower values for the high-temperature technology, evaluating the electrical energy demand at the operating temperatures, instead of in standard conditions. Research and experience are essential to enhance performance across various operating conditions, aiming to produce green hydrogen from renewable energy sources. The subsequent analysis delved into the diverse possibilities for hydrogen utilization in the different hard-to-abate sectors. In particular, this paper provided insights into the multiple ways hydrogen can contribute to decarbonization efforts in the hard-to-abate industries.
One prominent strategy is the use of hydrogen as an alternative fuel in blended combustion, where it can be combined with fuels like natural gas to reduce carbon emissions. This approach offers a practical and incremental pathway towards decarbonization, leveraging the existing infrastructure while achieving significant emission reductions.
According to our analysis of blended combustion, about 40 Nm 3 /h of H 2 is necessary in the combustible blend for hydrogen to cover the 10% of 1 MW (reference value) of heat requirement for material treatment. This hydrogen flow rate could be produced as green H 2 by low-temperature electrolyzers around 200 kW, or by 150 kW high-temperature ones, with present average nominal efficiencies and neglecting system energy losses.
The second relevant use referred was the use of hydrogen as a chemical element within the industrial processes themselves. In sectors such as steel, glass, and paper, hydrogen can act as a valuable reducing agent, enabling cleaner and more sustainable production methods. By replacing carbon-intensive inputs with hydrogen, these industries can significantly reduce their environmental impact. Finally, hydrogen demonstrates potential as an energy storage system, allowing the integration of renewable energy sources in the hard-to-abate sectors. By storing excess renewable energy in the form of hydrogen, it can be utilized during periods of high demand or when renewable energy generation is low, ensuring a more reliable and sustainable energy supply.
Finally, this paper analyzed hydrogen introduction into the steel sector, focusing on the BF-BOF and DRI-EAF processes. Implementing hydrogen in DRI process is very promising. Leveraging renewable energy and utilizing hydrogen as a reducing agent in the H 2 DRI process offers a chance to cut carbon emissions and enhance steel production sustainability. The potential emissions reduction, combined with renewable energy use, positions the H 2 DRI route as a promising avenue for decarbonizing steel production. Comparing energy intensity, H 2 DRI-EAF's energy demand seems lower than that for BF-BOF, with a greater use of electricity. This strategy aligns with greener steelmaking, paving the way for a more environmentally conscious approach in the industry. The mass and energy balance analysis of different steel processes reveals a significant energy demand, ranging from 14 to 22 GJ per tonne of steel produced. These high energy requirements pose a challenge for substantial renewable energy integration in the sector, even with the inclusion of hydrogen.
While the issue is clearly defined, it is crucial to conduct additional research to tackle the constraints linked to hydrogen's application in industrial contexts, as in hard-to-abate sectors, where the use of hydrogen for decarbonization is at the first step and requires continuous research, innovation, and implementation to achieve its full potential.
Author Contributions: Conceptualization, A.F. and C.G.; formal analysis, A.F. and C.G.; methodology, A.F. and C.G.; data curation, C.G.; supervision, A.F.; writing-original draft preparation, A.F.; writing-review and editing, A.F. and C.G. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.

Acknowledgments:
The authors would like to express their sincere appreciation for the support and guidance provided by Isabella Biasci throughout the development of the analysis presented in Section 5. Biasci's insights have significantly contributed to the comprehensive understanding of the role of hydrogen in decarbonizing the steel industry. Acknoledgements are also due to Lino Iallorenzi, Leandro Nannipieri and Lorenzo Becherini of Liberty Magona, for the profitable exchange of ideas on the topic of decarbonization of the steel industry.

Conflicts of Interest:
The authors declare no conflict of interest.