Renewable Electricity and Green Hydrogen Integration for Decarbonization of “Hard-to-Abate” Industrial Sectors

Abstract

This paper investigates hydrogen’s potential to accelerate the energy transition in hard-to-abate sectors, such as steel, petrochemicals, glass, cement, and paper. The goal is to assess how hydrogen, produced from renewable sources, can foster both industrial decarbonization and the expansion of renewable energy installations, especially solar and wind. Hydrogen’s dual role as a fuel and a chemical agent for process innovation is explored, with a focus on its ability to enhance energy efficiency and reduce CO₂ emissions. Integrating hydrogen with continuous industrial processes minimizes the need for energy storage, making it a more efficient solution. Advances in electrolysis, achieving efficiencies up to 60%, and storage methods, consuming about 10% of stored energy for compression, are discussed. Specifically, in the steel sector, hydrogen can replace carbon as a reductant in the direct reduced iron (DRI) process, which accounts for around 7% of global steel production. A next-generation DRI plant producing one million tons of steel annually would require approximately 3200 MW of photovoltaic capacity to integrate hydrogen effectively. This study also discusses hydrogen’s role as a co-fuel in steel furnaces. Quantitative analyses show that to support typical industrial plants, hydrogen facilities of several hundred to a few thousand MW are necessary. “Virtual” power plants integrating with both the electrical grid and energy-intensive systems are proposed highlighting hydrogen’s critical role in industrial decarbonization and renewable energy growth.

1. Introduction

The urgent need to decarbonize industrial sectors has intensified the exploration of the role of hydrogen produced with renewable energies, particularly within hard-to-abate sectors, such as steel, petrochemical, glass, cement, and paper and cardboard. Transitioning hydrogen into a practical energy vector demands a dual focus: a shift towards renewable energy sources and a meticulous examination of all conversion phases [1].

The use of hydrogen produced from renewable sources in hard-to-abate sectors could be highly beneficial and strategic for further increasing the penetration of intermittent renewable sources like wind and solar in global energy systems, which already exceeds 1000 GW [2]. In 2022, the global installed capacity of solar photovoltaic energy reached 1200 GW. This growth in the solar photovoltaic market reflects a global shift towards renewable and sustainable energy technologies. China and the United States lead the global PV market, with 307 GW and 122 GW of installed solar PV capacity, respectively, and some countries, like Chile and Honduras, had the highest share of photovoltaic energy mix in the total energy produced in 2022 [3].

Among renewable sources, wind and photovoltaic have the highest growth trends due to their simple and easily replicable technology. However, because of the significant variability and intermittency of these resources, many countries have been experiencing saturation in their grid’s absorption capacity for several years. That makes it difficult to create significant increases in penetration without systems that can synchronize production and energy use. Examples include smart grids [4], microgrids [5], and other relevant concepts, like energy communities [6], which are generally small-scale systems. However, this problem led to increasing the use of renewables in sectors where the end uses are primarily thermal. Associating renewable energies with highly energy-intensive sectors allows for substantial further developments, and in these sectors, hydrogen can be of great assistance.

Direct electrification will play a growing role in industry, with important contributions in multiple applications. Some of these solutions are already mature or close to technological maturity and include the use of electric furnaces to replace natural gas boilers or the use of heat pumps for all low- and medium-temperature heating operations. However, the use of electrical boilers is not always practical. Furthermore, in many sectors it is not easy to replace existing ovens with electric ovens, both for technical reasons, as the production processes have been developed over decades, and for economic reasons, because the ovens still have a useful life of at least 10–15 years. For this reason, it is very useful to utilize a combination of renewable energy and hydrogen so that hydrogen can indirectly serve as an intermediary to increase the penetration of renewable energy sources.

The role of hydrogen in these industries is multifaceted, serving as a versatile resource not only in terms of alternative fuel for burners or a system for energy storage, but as a raw material of foundational chemical components and as a catalyst for process innovation. Hydrogen emerges as a relevant element, not only for mitigating environmental impact by curbing emissions but also because it plays a central role in the definition of innovative processes [7,8]. However, the multifaceted role of hydrogen and dissecting its involvement in all the sectors traditionally resistant to clean energy transition need to be investigated in detail, because an overall energy-saving strategy must be determined, and this is not always easy to achieve. From electrolysis to hydrogen storage and end-use applications, each phase of the “hydrogen process” demands meticulous evaluation of energy aspects. Achieving viable margins requires a nuanced understanding of the energy efficiency levels throughout the entire process [9].

This work sheds light on the critical dimensions necessary for successful coupling, emphasizing the need for a minimum scale to facilitate seamless integration of hydrogen in the hard-to-abate industrial sector. Currently, individual phases of the process are quite far from reaching optimal efficiency levels, posing a challenge to the realization of a fully functional and sustainable hydrogen economy [10].

Considering that hydrogen from renewable energies will become a complementary technology, in addition to electricity, as part of an overall optimization of the energy system, the focus of this paper is to identify and delineate the minimum objectives that must be achieved for harmonized and efficient hydrogen integration with a specific focus in the “hard-to-abate” industrial sector. While the hydrogen supply chain encompasses various elements, current research focuses specifically on advancements in electrolysis [11,12], storage techniques [13,14], and on the practice of blended combustion, where hydrogen is combined with natural gas, offering a strategic approach to sectoral decarbonization [15,16,17]. Regarding hydrogen storage, the ongoing research emphasizes advancements aimed at enhancing the efficiency of compression technologies, including the development of advanced compressor designs and materials. This focus is particularly pertinent to gaseous hydrogen, given its simpler development with respect to its liquid and chemical forms, with the overarching goal of establishing a trade-off between energy consumption and augmenting storage capacity. In terms of blending hydrogen with natural gas within existing gas networks and leveraging ready combustion technologies to facilitate a gradual transition to a low-carbon energy system, the aim is to illustrate a viable strategy for increasing the hydrogen percentage. Another element of interest is the integration of hydrogen as a chemical agent in the industrial process, a topic that is well-developed in some specific sectors, like the steel industrial sector [18,19,20]. The various elements concerning hydrogen must be considered in a general analysis. Considering the intricacies of the process and delineating key milestones, this paper aspires to contribute to the practical realization of an effective roadmap for the future of hydrogen integration in the main hard-to-abate sectors with specific reference to some specific sectors. The authors’ original contribution lies in the evaluation based on typical mass and energy balances available in the literature, which are not always easily applicable to specific contexts, to quantify the possible contribution of hydrogen, taking into consideration the final uses of the energy, maintaining the processes currently available, and evaluating possible changes to the processes. Although some considerations can be considered of a general nature, to better quantify some aspects, a specific context is taken as reference, such as that of the steel industry.

After this introductive chapter, the article is structured into four main sections, followed by a concluding section. The first section provides an overview of industries facing challenges in reducing emissions and explores the potential role of green hydrogen, derived from renewable sources of hydrogen, as the only relevant solution for decarbonization efforts. Section 3 analyzes and discusses methods and technologies related to green hydrogen production and storage. Section 4 examines the utilization of hydrogen as a fuel in burner systems with specific reference to specific industrial sectors. Section 5 focuses on the application of hydrogen as a chemical agent in industrial processes, with a specific emphasis on its use in the steel sector. In particular, the authors attempt to highlight how hydrogen can also be interesting as an element to support relevant innovation in the production process, while paying attention to the energy balances of the process.

2. Hard-to-Abate Industry and the Prospect of Decarbonization: The Possible Role of Electricity and Green Hydrogen

The current topic of energy transition is a challenge that affects all sectors, from transport to construction and from energy to industry. Global energy-related CO₂ reached the level of 37 Gtons in 2022 [21]. With reference to the industrial field, it can be noted from the IEA report that approximately 9 Gtons of CO₂ were emitted by industry in general, mainly by energy-intensive industries. In particular, the steel sector contributed to 7.4% of global CO₂ emissions. The production of main large-volume materials, including metals, cement, glass, basic chemicals, timber, plastic, rubber, and paper, can be divided in two groups of processes, identified as upstream and downstream.

Downstream manufacturing sub-sectors, i.e., those closer to the final consumer, tend to account for larger shares of total value added than those upstream, which produce the main input materials for the downstream industries and use more relevant energy amounts. Among heavy industrial sectors worldwide, the steel industry ranks first in carbon dioxide (CO₂) emissions (Figure 1).

The significant level of emissions is largely connected to the need to use significant quantities of fossil fuels, which can be used directly in the production process (think of the coal used to produce steel) and for the fueling of ovens that require often high temperatures. Looking at data obtainable from IEA, most of the process heat above 500 °C (around 96%) is used in the three subsectors of steel, basic chemistry, and non-metallic materials (cement and pulp and paper); the steel industry alone contributes 48% [22]. Proposals linked to possible future prospects with a view to decarbonization have been drawn up in all sectors. Among these, the increase in the share of electricity, possibly produced through renewable sources, and the systematic transition to electric ovens are certainly relevant. However, it is not always easy to replace ovens powered by fossil fuels with electric ovens without distorting the production process to some extent.

In this regard, the examples of the steel and glass sectors are relevant. An increasingly significant introduction of hydrogen could play an important role. Green hydrogen produced by water electrolysis using renewable sources can be used in several sectors as shown in Figure 2. Indeed hydrogen can be used either as a chemical agent to be introduced into industrial processes, modifying them appropriately, or as a co-fuel or even as a pure fuel in ovens and burners or in energy storage. Not all “hard-to-abate” industries will see hydrogen playing the same role. While hydrogen can significantly contribute to decarbonizing some high-energy-intensive sectors like steel or ammonia production, its relevance may not be as pronounced in others. In other industrial sectors, hydrogen might primarily serve as an alternative energy source or energy carrier to reduce carbon emissions associated with melting and production processes. However, its application might not be as direct as in other sectors. Therefore, it is essential to recognize that the role of hydrogen can vary significantly from one industry to another, and its effectiveness depends on its adaptation to the specific needs and operational conditions of each sector. For all the sectors, hydrogen as a carrier could therefore play a fundamental role in industrial processes, but its inclusion must be evaluated carefully, and analyses of the energy costs linked to its inclusion should be conducted. The industrial sectors considered “hard-to-abate” are those in which reducing greenhouse gas emissions to very low levels or eliminating them altogether is particularly challenging due to the intrinsic characteristics of the involved industrial processes. These sectors represent a significant challenge in the transition to a low-CO₂ economy. The main reasons why these sectors are considered “hard to abate” include the following:

-

High levels of direct emissions: Industrial sectors, such as steel, cement, and heavy chemical production, emit large amounts of CO₂ during production processes due to the direct use of carbon.

-

Complex chemical processes: Some sectors, like the chemical industry, involve complex chemical processes that require the use of specific chemicals or reactions that are challenging to substitute with low-carbon alternatives.

-

High energy demand: Sectors like heavy industry, ferrous material production, and glass manufacturing require significant amounts of energy of the thermal type to heat furnaces or power production. This high energy demand results from the combustion of high-temperature fossil fuels and is difficult to replace with different (cleaner) energy sources.

-

Long plant lifecycles: Industrial plants often have long lifecycles and require significant investments to be replaced or upgraded. Consequently, even if cleaner technologies exist, it may be costly and complex to replace existing plants with updated versions.

-

Continuous demand for products: Sectors such as the automotive industry are closely tied to the continuous demand for products, which also makes it difficult to renew production lines.

For “hard-to-abate” sectors, an integrated approach is needed that involves the adoption of cleaner technologies, energy efficiency, fossil fuel replacement with renewable energy sources, and the development of innovative solutions for increasing the penetration of renewable energy sources. For this reason, it is appropriate to consider the possible implications of innovation processes, especially from a mass and energy balance point of view.

3. Hydrogen Production and Storage

Electrification is, in general, a way to implement the decarbonization of many sectors. However, not all sectors can be easily decarbonized in such a way; so-called hard-to-abate sectors can reach their sustainable goals by means of integration of hydrogen and renewable energy. In these sectors, real energy transition based on hydrogen will serve to allow the penetration of renewable sources. It must be considered that hydrogen is a secondary resource; therefore, it must be included within a supply chain from production to transport and storage up to final uses, and the energy costs of the entire production chain must be considered. To date, hydrogen is mainly produced using fossil resources. One of the most widespread methods is steam reforming of methane, which covers more than 60% of the demand, with a low production yield and high energy consumption.

When using hydrogen to support the increase in the penetration of renewable energy, it is necessary to keep in mind the entire hydrogen supply chain, from production to end uses, also considering the need to accumulate hydrogen. While many routes are possible for the generation of green hydrogen (Figure 3 provides only one possible route), not all are convenient from a technical point of view.

Table 1. Methods for hydrogen production: basic reactions and typical efficiencies.

Process	Basic Reaction	Typical Maximum Efficiency
Steam methane reforming	$\mathrm{C}{\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{O}+3{\mathrm{H}}_2$	0.48
Carbon gasification	$\mathrm{C}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{O}+{\mathrm{H}}_2$	0.17
Water electrolysis	${\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_2+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2$	0.60

presents an example calculation of the energy consumption of the various production processes, from which the steam reforming of methane and the electrolysis of water can be seen as the two processes with the highest energy yield to focus on for a possible increase in hydrogen demand in view of its use in “hard-to-abate” industrial sectors. Considering these two processes, they allow the production of zero-emission hydrogen: the first with the addition of a carbon capture system, while the second through the electrolysis of water powered by renewable energy sources.

3.1. Hydrogen Production with Water Electrolysis

Focusing on the topic of water electrolysis, the most developed technologies are alkaline (ALK), proton exchange membrane (PEM), and solid oxide (SOEC), which differ in operating conditions (T and P) and the electrolytes from which they take their name. The commercialized and most developed technologies are ALK and PEM, although they have the strong disadvantage of being made of high-quality materials, which increase the cost and reduce the useful life, while solid SOEC could perhaps be relevant in the future industrial field thanks to lower electricity consumption due to the use of energy derived from heat recovery, strongly present in “hard-to-abate” industrial sectors. This type of production is of interest for energy transition, which is why research is very active in this specific sector. Research on electrolysis is still ongoing. It can be said that the topic of electrolysis is quite mature from a technological standpoint; however, when considering its integration into a production process, it is important to highlight that a significant percentage of energy is lost in the process (over 30%). The electrolytic process can be summarized according to the actual state of the art, analyzed by the authors of the present paper in [12], that evidences a value of 0.6 for the efficiency of the electrolysis process by the following synthetic equation, in which the water and energy requirements to obtain 1 kg of hydrogen are outlined:

$$8.8 \mathrm{k}\mathrm{g} {\mathrm{H}}_{2}O\stackrel{55 \mathrm{k}\mathrm{W}\mathrm{h}}{\to }1 \mathrm{k}\mathrm{g} {\mathrm{H}}_2 + 7.8 \mathrm{k}\mathrm{g} {\mathrm{O}}_2$$

(1)

Considering the energy value of hydrogen, 120 MJ/kg (33.3 kWh), and the lower hearing value (LHV) of 144 MJ/kg (40 kWh), at least 15 kWh are lost in the process. Furthermore, another significant issue is the durability of electrolyzers, which is not so high, along with the stability of their performance.

Table 2. Technologies for hydrogen production and reference data.

Parameters	ALK	PEM	SOEC
Operating temperature [°C]	60–90	50–80	800–1000
Operating pressure [bar]	1–30	1–35	1–5
Electrolyte	NaOH (20–25% wt)/KOH (30–35% wt)	Solid polymer	Mixed oxide based on zirconium oxide stabilized with yttrium oxide, YSZ
Electrodes	Ni/Co/Ru/Mo	Titanium	Ni/YSZ, perovskite (ABO3)-like conducting oxides
Area of a cell [cm2]	>4	<0.03	<0.01
Charge factor with respect to nominal power [%]	20–100	5–100	5–100
Efficiency (referred to HHV) [%]	50–70	50–78	65–97
Stack size [MW]	1	1	0.005

summarizes the main relevant data that can be associated with the different technologies. With respect to alkaline (ALK) electrolysis and proton exchange membrane (PEM) electrolysis, high-temperature electrolysis technologies and solid oxide electrolysis (SOEC) promise rather high efficiencies, but if we look closely, they shift the demand a lot on the thermal front and the high-efficiency values and therefore refer to cases in which waste heat can be available at high temperatures. In any case, these technologies are not currently commercially available.

3.2. Hydrogen Storage and Its Role in “Load Leveling” between Renewable Power and Final Uses

Hydrogen plays a significant role as an energy accumulator, especially for excess energy generated from renewable sources. Hydrogen storage, an important topic analyzed by the authors of this work, offers a crucial solution for energy transition in “hard-to-abate” industries. It serves as an alternative to electrochemical storage, aligning production with utilization by storing substantial amounts of energy in gaseous or liquid form. Produced via water electrolysis using excess renewable energy, hydrogen can be converted back into energy during peak demand periods, mitigating renewable energy variability and supporting a sustainable, resilient energy system. Different storage methodologies are also proposed to support hydrogen integration in hard-to-abate industrial sectors. The main techniques to store hydrogen in pure form are compressed gaseous hydrogen and cryogenic liquid hydrogen, in which the H₂ storage density is only influenced by the storage pressure and temperature because there is no appreciable chemical or physical interaction between H₂ and the storage device. In metal hydrides, hydrogen is stored in atomic form as a part of the hydride molecule through a reversible chemical reaction.

Compression in gaseous form is certainly the most convenient and practicable form. At an industrial level, the accumulation pressure generally varies between 200 and 300 bar. However, attention must be paid to the energy that is subtracted to carry out the compression, which will obviously be subtracted from the process chain. The work required for compression, w_ms, expressed in kWh/kg_H2, can be estimated as shown in Figure 4 with multi-stage compression (in this case, three stages), considering the value of the specific enthalpy for hydrogen for each point, and the isentropic efficiency for the compressor for each stage (${\eta }_{is})$.

$$w_{ms}=h_2-h_1+h_4-h_3+h_6-h_5={\displaystyle \frac{h_{2is}-h_1}{{\eta }_{is,Istage}}}+{\displaystyle \frac{h_{4is}-h_3}{{\eta }_{is,IIstage}}}+{\displaystyle \frac{h_{6is}-h_5}{{\eta }_{is,IIIstage}}}$$

(2)

Figure 4, in which red and blue lines represent the isobaric and isenthalpic curves, respectively, represents the case.

Table 3. Minimum required values for specific work of compression considering different cases.

Storage Pressure	From 1 Bar, 5 Stages		From 30 Bar, 3 Stages
[bar]	Tmax [°C]	wms [kWh/kg]	Tmax [°C]	wms [kWh/kg]
200	150	2.8	95	1
300	160	3.5	110	1.25

, which reports some of the calculations produced in [23] to identify the minimum levels linked to existing technologies, shows the energy subtracted in the compression process in gaseous form, although in theoretical conditions, it can also be of the order 10% of the calorific value of the fuel.

This value can even double if commercially available compressors are considered. Fortunately, the value can be reduced considering that the output pressure from the electrolyzers is often already quite high (for example, in the order of 30 bar).

In the following sections, we will closely examine the role of hydrogen across various industrial sectors. It is evident that, given some renewable energy is dissipated during hydrogen production, minimizing the need for compression is crucial. Therefore, hydrogen should be integrated primarily with continuously operating industrial processes to minimize the necessity for storage.

4. Green Hydrogen as an Alternative Fuel in Industrial Burners

After examining the possible systems to produce hydrogen, the role of hydrogen as a co-fuel mixed with methane is analyzed. Compared to methane, hydrogen has different chemical–physical properties, which, when mixed, bring about changes to both the combustion and the characteristics of the flame. Hydrogen combustion determines high flame speed, approximately 6 times that of methane, which leads to short, compact, and turbulent flames and problems with backfire and flashbacks, which is why premixed flames are not recommended. Moreover, combustion with hydrogen causes a different adiabatic flame temperature, which changes the design temperature of the furnace. Careful control of the mixing is therefore necessary to maintain product specifications. A wider flammability range will therefore produce more combustion management problems but with a lower minimum ignition energy.

Although CO₂ emissions decrease, high temperatures lead to high NO_X emissions, particularly thermal ones; therefore, dry low NO_X or flameless configurations are recommended for future burner configurations. Finally, the small size of hydrogen leads to losses from possible welds, and the chemical reactivity with steel alloys causes the material to become embrittled, which is more a problem of piping and storage than of the burner itself given the high operating speeds. A further consideration is the useful heat released.

Table 4. Comparison of hydrogen and methane from the perspective of combustion.

Properties	Hydrogen	Methane
Density at environmental conditions (ρ) [kg/m3]	0.089	0.657
Lower heating value (LHV) [MJ/kg]	120	50
Lower heating value (LHV) [MJ/m3]	10	33
Higher heating value (HHV) [MJ/kg]	144	55
Higher heating value (HHV) [MJ/m3]	12	38
Autoignition temperature [°C]	585	540
Adiabatic flame temperature (air) [K]	2400	2100
Adiabatic flame temperature (oxygen) [K]	3100	2300
Flammability limits [%volume]	4–75	5.3–15
Minimum ignition energy [MJ]	0.02	0.29
Flame speed in air [m/s]	2.65	0.4

provides a comparison of the two fuels. Apart from other general elements of comparison, hydrogen is more flammable and easier to ignite than methane, with a wider explosive range and higher flame speed. Hydrogen combustion produces only water vapor, while methane combustion produces both water vapor and CO₂. Hydrogen releases more energy per unit mass than methane, resulting in higher combustion temperatures. Due to its high flammability and low ignition energy, hydrogen poses more significant safety risks, requiring stringent measures to prevent accidental ignition and explosion.

When blending or replacing one fuel with another, the power range and stability of combustion are influenced, as well. Combining hydrogen and natural gas at different densities influences the flow rate that passes through the same nozzle and the amount of heat exchanged.

Therefore, it is necessary to evaluate the rate of release of useful heat using some specific indicators, like the Wobbe index or other indicators. Carrying out the calculations according to the formula presented, which depends on the calorific value and density of the hydro-methane mixture in relation to the density of the air, it is noted that when the percentage varies, the parameter does not have a linear trend but presents a valley around 80% of H₂ and is therefore the most unfavorable percentage for combustion. Instead, it is noted that a 20% H₂ mixture has the same heat exchanged as a 99% H₂ mixture, which is why burner manufacturers recommend methane hydrogen blending on methane burners operating up to 20% by volume for combustions in which the process requires T > 750 °C. For higher percentages, a change in burner technology and in the design of the oven and the flue gas lines is necessary, even if it is not advantageous on a thermal level. As regards emissions, it is noted that as the hydrogen content increases, the %CO₂ saved increases, although not linearly given the calculation in relation to the different volume and density. The chemical–physical properties of H₂ when it is mixed with CH₄ determine changes in the characteristics of the flame compared to that of methane alone, which are more evident as the hydrogen content in the mixture increases. To take advantage of the blending technique in burners operating in the production lines, it is necessary to keep in mind that the heat exchange mode could be different, based on the %H₂, to the point of having to foresee significant changes in the oven or in the fume recovery path. Hydrogen brings changes during combustion, which are summarized in the list below:

-

Hydrogen contributes to combustion a quantity of heat 2.4 times greater than the 1 kg of CH₄, but 3 times less than the m³ of CH₄ due to the low density of the H₂. This translates into large dimensions or high pressures of storage, a jet speed approximately 3 times higher, and greater volumetric flow rates of the mixture.

-

The flames have different shapes, which are increasingly shorter and compact but more turbulent given the flame speed of H₂, which is approximately 6 times greater than that of CH₄.

-

The high flame temperature of H₂ can be harmful for some thermal specifications (for example, in the polymerization oven in the painting line, with the risk of burning the tape).

-

The addition of hydrogen involves an expansion of the flammability field, increasing its reactivity, diffusivity, and reaction speed with problems in the management and safety of the flame and high risks of flashbacks; hence, premixed combustion is not recommended. Burner manufacturers [24,25] used experimental tests to declare the feasibility of blending up to 20% H₂ by volume without making significant changes to the burner technology and the conditions of use for temperatures above 750 °C, given how the properties of combustion change. Going beyond these percentages does not seem convenient in terms of the useful heat produced, as demonstrated by the trend of the Wobbe index, which, for a specific gas, is defined as follows:

$$W_I={\displaystyle \frac{HHV}{\sqrt{\frac{{\rho }_{gas}}{{\rho }_{air}}}}}=HHV/\sqrt{RD}$$

(3)

where RD is the ratio between the density of natural gas and air. The Wobbe index has dimensionally the same unit of the calorific value. G is a specific gravity factor, which is assumed as the ratio between the density of the gas and the density of air, at the same temperature and pressure [26]. The Wobbe index helps ensure that different gas compositions can be safely and efficiently used in gas-fired appliances without causing operational issues, such as flame instability or incomplete combustion. Gas appliances are typically designed to operate within a certain range of Wobbe index values to maintain consistent performance. The concept is that gases with a similar Wobbe index but different compositions can be replaced with each other, as they release the same amount of energy in a furnace through the same nozzle with similar feed pressure. If the same concept is applied to a mixture of hydrogen and natural gas (NG), with x% of hydrogen in the mixture, the Wobbe index can be defined as follows:

$$W_{I_{Mix}}={\displaystyle \frac{HH{V}_{Mix}}{\sqrt{\frac{\left(1-x\%\right){·\rho }_{NG}+x\%·{\rho }_{H_2}}{{\rho }_{air}}}}}$$

(4)

$$HH{V}_{Mix}=\left(1-x\%\right)·HH{V}_{NG}+x\%·HH{V}_{H_2}$$

(5)

In fact, by providing for a change of fuel, the power range and stability of combustion are different, and the release rate of different fuels at the same flow rate can be very different. This is verified using the Wobbe index, a parameter used as an indicator of the interchangeability between gaseous fuels. Figure 5 provides the different values of the Wobbe index for a mixture of natural gas (approximated to methane) and hydrogen, with different volumetric percentages of hydrogen. Although in general, the combustion analysis should take into consideration some aspects related to the flame and the combustion speed, a general energy analysis is still useful. The advantage of using a mixed mixture can be better appreciated by considering the aspect linked to CO₂ emissions. It can be seen that by maintaining a percentage of hydrogen below 20% by volume, there is an interesting decrease in CO₂ emissions, resulting precisely from the presence of hydrogen in the chemical reaction of hydrogen combustion. Table 4 shows the potential of blended combustion from the point of view of the potential reduction of CO₂ emissions; the evaluation is made in this case starting from a simple mass balance. As can be seen, there is a potential for reducing emissions related to combustion processes by up to approximately 21%, which can be achieved with a hydrogen fraction equal to 20%. The values in Table 4 refer to a maximum blending of 20% in line with the values currently considered admissible based on the technology. The data reported in

Table 5. Estimated CO₂ reduction in blended combustion of mixtures natural gas (NG)/hydrogen (H₂).

Mixture	HHV [MJ/kg]	LHV [MJ/kg]	Volume CO2/Volume Reactant	%CO2 Reduction (Potential)
NG	38	33	1.12
NG + 5% H2	35.8	32.4	1.04	7%
NG + 10% H2	34.7	31.3	0.99	12%
NG + 15% H2	33.5	30.1	0.94	16%
NG + 20% H2	32.3	28.9	0.88	21%
H2	12	10	0	100%

are directly calculated by the authors of the present paper, developing a mass and energy balance of reaction. The balance is carried out based on the thermal power introduced into the combustion chamber, considering the different calorific powers of methane and hydrogen. From Table 5, we can see that as the percentage of hydrogen increases in the mixture, the calorific value per unit of volume decreases due to the low density of hydrogen compared to natural gas; on the contrary, the calorific value per unit of mass increases. For comparison, the results theoretically obtainable with total hydrogen combustion are reported in the final row of the table.

5. Hydrogen Produced with Green Electricity as a Chemical Agent in the Process: The Relevant Case of the Steel Sector

In the previous section, we focused our attention on the possible role of green hydrogen as an alternative fuel to be used instead of blend-type mixtures with natural gas to decarbonize the various sectors. However, this is certainly not the only possibility to investigate because hydrogen is already used in many sectors, such as the chemical and petrochemical sector, in which it could therefore be generated through green electricity. It also shows some further interesting possibilities in other energy-intensive sectors, such as glass and steel. As is known, hydrogen is used in many industrial sectors as a reducing agent. It is well known that most of the H₂ production in the world comes from natural gas (via steam methane reforming) or coal (via coal gasification). The direct production of hydrogen by means of water electrolysis using renewable electricity also enables a significant penetration of green energy in the aforementioned sectors. However, green H₂ produced through renewable-powered water electrolysis can be used as a carbon-free feedstock and reductant to significantly reduce carbon emissions.

In industries such as steelmaking, ammonia, and chemical manufacturing in general, hydrogen can serve as a raw material or as a reducing agent for various processes. For example, hydrogen can replace coal in the direct reduction of iron ore (DRI) to produce steel, contributing to the reduction of carbon dioxide emissions significantly. However, hydrogen is also a critical raw material for ammonia production, as it serves as the key ingredient in the Haber–Bosch process, which is used to synthesize ammonia (NH₃) from nitrogen (N₂) and hydrogen (H₂). In the Haber–Bosch process, hydrogen acts as a reducing agent, providing the necessary electrons to convert nitrogen gas into ammonia. This process requires a large amount of hydrogen to ensure high conversion rates and ammonia yields. In any case, there are many possible uses of hydrogen within industrial processes;

Table 6. Hydrogen usage in different industries.

Industry	Hydrogen Use
Refinery	Hydrogen is used for the desulphurization of products such as diesel and petrol.
Fertilizer	Hydrogen is used for manufacturing ammonia, which in turn is used as a feedstock for ammonia-derived fertilizers, such as urea and di-ammonium phosphate.
Methanol	Hydrogen is used as a main feedstock to produce methanol, which is currently produced from natural gas.
Hydrogen peroxide	Hydrogen is used in the first step, i.e., hydrogenation of a working solution of a four-step hydrogen peroxide manufacturing process.
Steel	Hydrogen can be used as a reducing agent in steel manufacturing by integrating it instead of natural gas into a direct reduced iron (DRI) process.
Float glass	Hydrogen is used as a getter gas to prevent oxidation over the tin baths used in float glass manufacturing process; the glass formed on the baths is made without defects.
Optic fiber	Hydrogen is used for refractive index increment: optical fiber is immersed in a hydrogen environment to absorb hydrogen up to several mol% in advance of UV irradiation to reduce defect formation in cables.
Pharma	Hydrogen is used to produce hydrogen peroxide and active pharmaceutical ingredient, which is used in various medicine manufacturing.

summarizes the main ones.

Very interesting is the case of the steel sector. Steel is an indispensable material that is widely used in infrastructure, buildings, transport vehicles, household appliances, medical equipment, and many other industries, such as energy (including electric vehicles, wind turbines, and solar photovoltaic (PV) structures) [27]. The demand for steel closely follows the development of an economy, particularly in the early stages of industrialization. Steel production has increased steadily over time. In absolute terms, steel production has risen from 190 million tons in 1950 to almost 1.85 billion tons in 2023 [27,28]. In 2023, the total crude steel production was 1878.5 Mt, with China, India, and Japan leading as producing countries, followed by the United States, Russia, South Korea, Germany, Turkey, Brazil, and Iran [27]. Currently, about 75% of global steel is produced in emerging markets and developing economies, as represented in Figure 6 [29].

The technology that has established itself over the last hundred years both for maximum productivity and efficiency and for excellent economics is the blast furnace (blast furnace) powered by carbon coke. The process is known by the acronym BF-BOF (blast furnace–boiling oxygen furnace). It is characterized by high investment costs and production capacity in the order of millions of tons per year and relevant energy use, based on coal (as primary matter) and natural gas as fuel in the burners.

As it is clear from the previous considerations, the steel industry is extremely carbon intensive. The main reason for this is the industry’s extensive use of metallurgical coal in blast furnaces to produce iron, the primary component of steel. Around 90% of the emissions from steel production arise from this process [30]. New steelmaking processes that do not rely on metallurgical coal, such as DRI, are emerging as viable alternatives. DRI is a form of iron produced through direct reduction, bypassing the carbon-intensive blast furnace process. DRI is produced at temperatures below iron’s melting point. A basic oxygen furnace (BOFs) is replaced by an electric arc furnace (EAF), and this can be partially supported with renewable energy. There are two types of direct reduction plant DRP that have been established over the years by two companies: Midrex and Tenova HYL (Energiron Direct Reduction Technology) [31,32].

In secondary steel production, steel scrap is reclaimed and re-melted in an EAF without the need for a new iron ore reduction process. Figure 7 provides the main routes for steel production considering the primary process, which is followed by downstream processing, where finished steel products are obtained.

By making some evaluations on standard processes based on the methodology defined in [33], it is possible to carry out mass and energy balances typical of the process, from which, based on the evaluations made using a simple one-dimensional model, the results that can be estimated are shown in

Table 7. Typical values for mass and energy balance of BF-BOF process (for 1 ton of steel).

Process	Pulverized Coal [kg]	Coke [kg]	Iron Ore [kg]	Oxygen [kg]	Energy Use [GJ]	Thermal Fraction of Energy Used [%]
BF-BOF	200	300	1600	80	20	95

. Energy uses related to the production process can be estimated in the order of magnitude of 18–25 GJ/ton of steel produced. Most of this energy used in the production process is thermal; therefore, natural gas is often the primary energy source for the sector.

As can be seen from the data in Table 7, most of the energy demand (about 20 GJ/ton of steel produced or about 5.55 MWh/ton or 5.55 kWh/kg of steel, considering the equivalence of 1 kWh and 3.6 MJ) is thermal energy, and only a reduced amount results in electricity (5%). The possible role of hydrogen as a fuel in blended combustion with natural gas has already been analyzed in the previous section. However, the role of hydrogen in this sector can also contribute to modifying the production process and progressively decarbonizing the sector.

Using hydrogen to produce steel is not a new concept. In recent years, direct reduction iron technology (DRI) has been developed, which today satisfies 7% of global steel production. Alternative reduction technologies include hydrogen-based direct reduction processes and electrolytic reduction methods. Most are not well-developed and require huge amounts of green energy, but they hold the promise of carbon-neutral steelmaking [34]. Most of these plants consist of a furnace (shaft furnace) powered by methane rather than coal. Through reforming, which can be inside or outside the oven depending on the system, CO + H₂ syngas is generated, which allows the direct reduction of iron. The novelty of this technology is therefore precisely the chemical process.

Furthermore, the final product is iron in the solid phase since the reductions take place at 800 °C and not at the melting temperatures, directly forming the DRI. Figure 8 provides a scheme of the process with evidence of all the energy fluxes. Hydrogen, used as a reductant, has excellent kinematic properties, favoring the productivity and metallization of iron Fe > 90%, i.e., complete oxidation. From a thermal point of view, however, there are some disadvantages, since the reduction reactions with H₂ are endothermic. An innovative version of DRI technology is represented by the scheme of Figure 9, in which hydrogen is obtained using an electrolyzer. Therefore, in these plants, the total energy requirement increases due to the efficiency of the electrolytic process. Furthermore, the increase in energy expenditure also corresponds to a significant increase in the use of electricity, especially in the electrolysis process. Considering the production of 1 ton of H₂, approximately 70 kg of hydrogen are needed, according to [19]; therefore, considering a need of approximately 55 kWh/kg, consequently, approximately 3850 kWh are necessary for each ton of steel produced. From the point of view of CO₂ emissions produced, there is still an overall reduction if electricity generated with renewables is used.

Table 8. Specific energy requirements and emissions in steel production processes based on authors’ evaluation (for 1 ton of steel).

Process	Iron ore Pellets [kg]	Lump Iron [kg]	Water [kg]	Oxygen [kg]	Thermal Energy [GJ]	Electricity [GJ]
DRI–NG	1200	500		90	13	2
DRI-H2	1100	470	1000	40	2.5	14.5

provides a comparative analysis between the two DRI processes, with specific attention to the mass flow rate involved in the processes, while

Table 9. Specific energy requirements and emissions level in typical steel production processes.

Process	Specific Energy	Specific CO2 Emissions
BOF	18–25 GJ/steel	1.8–2 tCO2/steel
DRI–NG	13–17 GJ/steel	0.7–1.2 tCO2/steel
DRI–H2	15–20 GJ/steel	Dependent on the energy used for H2 prod.

provides a comparison of energy consumption estimates per unit of finished product considering the traditional blast furnace process (BOF) and the two DRI processes, including the evolution based on the total use of hydrogen. The table also includes an estimate of the expected level of CO₂ emissions per ton of product, elaborated according to the model developed in [35] and based on some basic information extracted from [36,37]. In Table 8, the estimation of CO₂ emissions associated with the last process is omitted because in this case, it heavily depends on the type of energy resource used to generate the electricity introduced into the process (mainly in the electrolyzer). As can be seen based on the evaluations made in this article, the DRI process leads to a possible overall reduction in energy use in steel production processes, even if this is obviously conditioned in the hydrogen production processes by the efficiency of the electrolyzers. Today, around 500 DRI plants are available globally, with a total capacity of 119 Mton, which represents approximately 7% of global steel production. DRI technology is used primarily in the Middle East/North Africa (44%), followed by Asia (25%) and Latin America (17%) [38]. Considering the value of global production and the 500 available plants, the average productivity per single DRI plant of approximately 250,000 tons of steel per year can be estimated.

As in the steel sector, achieving significant reductions in CO₂ emissions through hydrogen may be quite challenging. This applies similarly to different “hard-to-abate” sectors, where identifying contributions from hydrogen other than those as a co-fuel may be even more difficult.

6. Integration of DRI–H₂ Process with PV Plants

To reduce potential emissions, steel production processes should be combined with renewable energy use that allows the production of electricity and reduces emissions. Great relevance can be attributed, for example, to the possible integration of steel production processes with a photovoltaic (PV) system.

Considering the production capacity of a steel plant, which, for a small to medium size, could be 1 million tons of steel annually, a large PV plant is required. The sizing of the system, P_PV, determined by the standard test conditions (STC), is quite simple. It was carried out assuming that the productivity for each kW of peak power can be estimated by determining the solar irradiation in the specific site, considering an appropriate balance of system efficiency for the PV plant, η_BOS (it usually assumes a value between 0.75 and 0.85), and considering the ideal specific energy produced by a PV plant, $H_{SN}$, expressed in kWh/kW of the peak power for the period considered (a day, a month, a year), that is, a function of the energy irradiated in the specific location:

$$E_{PV}=({\eta }_{BOS}·H_{SN})·P_{PV}$$

(6)

Obviously, the real power (P_PV,real) of the PV plant under real operation is different and can be estimated considering the real operating conditions of the place in which PV plants are installed, according to one of the models available in the literature, as in [39].

Considering some typical values for specific electricity consumption for a typical electrolyzer, as discussed in [12], $E_{el}$ (e.g., 55 kWh for 1 kg of hydrogen) and the typical mass of hydrogen required for producing a ton of steel in a typical benchmark DRI-H₂ process, $M_{H2-S}$ (e.g., 70 kg for each ton of steel produced according to some evaluation developed in [10,35]) the size of the PV plant necessary to support the industrial process, producing a well-defined annual quantity of steel, M_S, can be evaluated based on a basis of annual energy production by means of the following equation:

$$P_{PV}={\displaystyle \frac{M_S·M_{H2-S}{·E}_{el}}{({\eta }_{BOS} ·H_{SN})}}$$

(7)

Table 10. Power of the PV plant required for typical steel plants (based on typical annual production).

Annual Steel Production of the Plant [tons]	Annual Green Hydrogen Required [tons]	Annual Energy Produced [MWh]	Daily Hydrogen Prod. (Max) [tons]	Daily Hydrogen Prod. (Min) [tons]	Power of the PV Plants [MW]
1,000,000	69,818.2	3,840,000	335.1	55.6	3200
250,000	17,454.2	960,000	83.8	13.96	800
100,000	6981.8	384,000	33.5	5.59	320
15,000	1047.3	57,600	5.8	0.84	48

shows the typical sizes of photovoltaic plants that would be required to power existing steelmaking systems worldwide with green energy. The data refer to an annual basis, taking as reference a case in which there is a level of sunlight estimated at 1500 kWh/m² per year. Similarly, the minimum and maximum values are estimated starting from typical values of the incident daily radiation, evaluated in the range between 1.2 kWh/m² day, the typical winter value, and 8 kWh/m² day, a typical summer value in central-south Italy. The annual trend of incident solar radiation is shown in Figure 10.

While this analysis could be complemented by a statistical evaluation based on actual insolation values for different types of days, such an addition would not significantly alter the overall conclusions of this work. A more detailed analysis of this specific case would require a more complex model that accounts not only for the statistical analysis of solar radiation but also for specific environmental conditions, as illustrated in more structured models, such as those in [39], which is beyond the scope of this work.

The analyses presented in Table 10 summarize some general results obtainable by linking typical photovoltaic production under certain climatic conditions with the production capacity requirements of variously sized plants.

As can be seen, these results tend to be very large-scale installations, which would undoubtedly require substantial investments. Moreover, it is not feasible to construct plants directly tied to the production system. Nonetheless, these solutions can still be considered.

Hydrogen-based DRI production has been proven at the precommercial scale, and the first commercial scale plants are at the advanced project stage: the conditions for energetic transition seems to be favorable, but considering this and the economic situation, a large fraction of the projected growth in activity from hard-to-abate sectors is expected to come from developing economies, mainly in Asia, while action within the most developed countries appears to be critical [40].

The conditions for decarbonizing the steel sector seem favorable because many plants in Europe have reached the end of their useful life and could be rebuilt on new foundations, explicitly implementing innovative technologies. Consider, for example, the cases of the steel plants in Piombino and Taranto in Italy, which could be reconstructed based on DRI–H₂ technology and green electricity.

The proposed analysis provides a broad-spectrum energy analysis of the issue of integrating renewable energy, specifically, photovoltaic generation, within an industrial sector. The aim of this analysis is to define the order of magnitude of various variables, which results in a general model. The focus is on the overall energy perspective, but some second-level variables, which could be very significant, are inevitably overlooked. These include the dynamic aspects of the system, such as the time-dependent nature of photovoltaic energy production and load variations. Nevertheless, the primary goal was to analyze the problem on a dimensional scale.

The integration of renewable energy through hydrogen in the steel sector can achieve an additional outcome: facilitating the further expansion of renewable electricity (mainly photovoltaic and wind) for non-electric energy uses, which are currently by far the most significant, accounting for about 85% of the total on a world scale. This is clear from an analysis of major international reports, such as [41].

7. Conclusions

The decarbonization of the various hard-to-abate industrial sectors will necessitate a multifaceted approach tailored to the unique characteristics of each industry. Fundamental shifts, rather than incremental steps, are essential for a successful transition in these sectors. Direct electrification will become increasingly prominent, while hydrogen is poised to play a substantial supplementary role. This article has demonstrated the potential of hydrogen, produced from renewable sources (“green hydrogen”), as a viable strategy for decarbonizing sectors classified as “hard-to-abate”.

Hydrogen has the potential to facilitate partial or complete powering of various industrial processes through renewable energy. Hydrogen’s role in these sectors is significant, both as a process material and as an alternative fuel. Despite the technological limitations associated with electrolysis (with current efficiencies around 60%) and the energy requirements for hydrogen storage (approximately 10% of the stored energy), the potential integration of hydrogen remains a promising avenue warranting thorough consideration. In particular, the possible applications of hydrogen in specific sectors have been focused, with the steel industry emerging as particularly promising. In the steel and iron industrial sector, hydrogen shows significant potential both as a reductant in the direct reduction iron (DRI) process—which currently accounts for about 7% of global steel production—and as a co-fuel that can be blended with natural gas in existing furnaces. Mixtures of up to 20% hydrogen with natural gas are already being used, resulting in measurable reductions in CO₂ emissions. While it is challenging to precisely quantify the overall decarbonization impact of hydrogen in these sectors, it undeniably represents an intriguing and valuable prospect.

The connection between green electricity and hydrogen is crucial for the decarbonization of hard-to-abate sectors. However, it is important to recognize that powering a typical industrial production plant demands substantial power capacities. As discussed in Section 6, a next-generation DRI plant based on the use of green hydrogen with an annual output of approximately 1 million tons of steel would require a photovoltaic capacity of about 3200 MW to be supported.

Thus, directly coupling photovoltaic plants to industrial facilities appears to be infeasible. Nevertheless, integrating renewable energy generation within a complex energy system alongside large industrial plants can be a pivotal element in enhancing the penetration of renewable power. Achieving this transition will undoubtedly require a systemic and strategic vision.

The steelmaking sector may be one of the most promising for achieving concrete decarbonization of the production processes in the future. Beyond the conventional strategies of blended combustion, which can be applied across various contexts, and the electrification of furnaces, the development of DRI technology offers a broader spectrum of possibilities. Moreover, there are significant investments in this new technology, particularly in Asia.

Certainly, the use of hydrogen must be carefully considered because it is essential to ensure that the energy balance is as favorable as possible. Hydrogen is an energy carrier, and each phase of its utilization (production through electrolysis, storage, and final use) is characterized by its own efficiency.