Hydrogen Oxyfuel Combustion for Energy-Intensive Industries

*Esin Iplik, Martin Adendorff and David Muren*

## **Abstract**

Hydrogen has been seen as a decarbonization enabler for the last few decades, and in the last couple of years, there have been many investments in its production through renewables and use in different industrial applications. It is often researched for energy storage, and combustion is an excellent alternative to recover the energy stored in hydrogen. It might be the most viable alternative, especially when it comes to energy-intensive metal and glass production processes. The utilization of hydrogen as a fuel in these processes would reduce greenhouse gas emissions significantly, considering their share in total emissions. Since these industries already benefit from oxyfuel combustion with traditional fuels for fuel savings, part of the infrastructure already exists for hydrogen oxyfuel combustion. Fuel change is expected to require some minor adjustments other than simply changing the oxidizer; however, each industry has specific points to consider. This chapter investigates metal and glass production processes based on their needs and challenges in using hydrogen oxyfuel combustion for heating. Additionally, possible exhaust gas stream improvements are suggested to recover energy and reduce emissions. Finally, safety aspects of hydrogen and oxygen use are discussed together with the community acceptance of hydrogen use.

**Keywords:** hydrogen, oxyfuel combustion, hydrogen combustion, decarbonization, energy-intensive

### **1. Introduction**

Increasing renewable energy penetration in the electricity grid brought the discussion of power-to-x. Energy storage concepts are necessary for efficient energy utilization and also for grid stability, especially for uncontrollable and intermittent solar and wind energy [1]. Most of the power-to-x methods use electrolyzers as a core energy converter and suggest hydrogen usage within Haber–Bosch, methanation, Fischer–Tropsch, or other processes [2]. These strategies are classified as power-tochemicals, power-to-liquid, power-to-heat, and power-to-power, based on their end use. At first, power-to-gas was a general term to describe the energy stored as a gas, excluding the end use. However, due to the different routes suggested, the use of power-to-x was expanded to include them all. Web of Science (www.webofknowledge.com) search for "power-to-x" gives 279 results, and over 50% of them are from

Germany. A search for "power-to-gas" results in 1736 scientific publications, and again Germany is the major contributor with 377 of the listed results. Finally, the "power-to-heat" search gives 471 results, and the yearly distribution of the articles with the given search terms is given in **Figure 1**. There is an increasing interest in heat demand and using stored renewable energy for heat. While part of the research is about district heating or residential heating, industrial heat demand is also investigated.

As of 2022, the European Clean Hydrogen Alliance has over 750 projects in its pipeline, 172 of these projects have an end use of hydrogen in industry. The main themes are the use of hydrogen for industrial heating, steel manufacturing, feedstock for ammonia, and synthetic fuels. Since these are application projects rather than research and are usually driven by industry, they show commitment, a level of maturity of relevant technologies, and investment possibilities (www.ech2a.eu).

Power-to-heat is an integral part of the discussion for industry because heat demand is roughly 70% of the total industrial energy demand in Europe [3]. While the wood industry needs 60% of the process heat around 80–100°C, ferrous and nonferrous metal processes require almost the same amount of their process heat over 1000°C [4]. Industrial heating with hydrogen can be achieved *via* fuel cells followed by electric heating or hydrogen combustion. Direct electrical heating should always be considered when it is technologically and economically feasible. However, since there are some technical challenges with high temperatures, heat generation by combustion is relevant for several industrial applications. While electrification and decarbonization of electricity production would decrease emissions significantly, hydrogen combustion can fill the gap in high-temperature heat demand. In 2019, around 38% of the heat demand in Europe was supplied by natural gas and other gasses in Europe, followed by 30% renewables and over 20% by solid fossil fuels [5].

The core of power-to-x and green hydrogen production relies on electrolyzers using renewable energy. There are three major types of water electrolyzers. The installed capacity and number of publications of each method are given in **Table 1**. The alkaline electrolyzer is the most mature and established technology. The installed capacity was 176 MW in 2020, and it still finds research ground. There were 576 articles published in 2020 about this technology; over 50% of the articles were from China. Proton exchange membrane (PEM) electrolyzers have an 89 MW installed capacity, and with its short

**Figure 1.** *Number of articles found on Web of Science for power-to-gas, power-to-x, and power-to-heat concepts.*

*Hydrogen Oxyfuel Combustion for Energy-Intensive Industries DOI: http://dx.doi.org/10.5772/intechopen.106152*


#### **Table 1.**

*Major water electrolysis routes, their installed capacity, and the number of publications in 2020.*

response time, PEM electrolysis is well suited to renewable energy storage. China and Germany were at the top of the list of countries contributing to the 268 articles on this technology. Solid-oxide electrolyzers are high-temperature systems and are considered to be advantageous for integration into high-temperature industries. With 142 publications, still high investment costs, short life cycles, and only around 1 MW of installed capacity, this technology is not considered to be commercially viable yet. Further details and technological advances can be found in recent review papers about alkaline [8], PEM [9], and solid-oxide [10] electrolyzers.

Oxygen is also produced with water electrolysis; however, the focus has been on the hydrogen side up to now, and oxygen is not captured. Industrial oxygen is produced from the air *via* air separation technologies that consume electricity and then used in wastewater treatment, combustion, and other applications. Oxygen-enriched combustion and oxyfuel combustion are not new concepts for some industries. Oxyfuel combustion is preferred for fuel saving or capacity increasing. Fuel saving up to 50% is possible depending on the flue gas temperature. Eliminating the nitrogen from the oxidizing mixture decreases the flue gas volume significantly, decreasing the flue gas's heat loss. While hydrogen is not a cheap fuel, its oxyfuel combustion is an excellent way to reduce costs.

An illustration for airfuel combustion and oxyfuel combustion for hydrogen is shown in **Figure 2**. While airfuel can melt roughly 3 ton/h aluminum burning 1200 Nm3 /h hydrogen with air, oxyfuel can achieve this production rate with 780 *Nm*<sup>3</sup> *=h* hydrogen. It can also be used to increase capacity, and in that case, the fuel flow rate is kept constant at 1200 Nm3 /h to melt aluminum at 6 ton/h rate. Of course, converting from airfuel to oxyfuel requires investment, and there is also the continuous cost of oxygen, but usually, a new furnace installation costs more to increase the capacity.

This chapter explains hydrogen oxyfuel combustion, its chemistry, possible applications, and safe handling. For the applications, steel, aluminum, and glass industries are selected. The reason is the high-temperature requirements of the processes and their emissions. The traditional fuel choices of these processes vary between natural gas, different crude oil-based fuels, and LPG depending on the country and the facility. The steel industry has one of the highest greenhouse gas emissions, which accounts for around 7% of the global emissions [11]. Aluminum production accounts for roughly 1% of the global emissions, and aluminum has a higher energy consumption per kg product than steel [12]. Compared with metals, glass has a lower emission contribution [13]; however, considering the even higher process temperatures, a limited possibility for complete electrification (based on currently available technology), and to protect the optical properties of the product, a limited possibility to use solid biomass [14], hydrogen oxyfuel combustion could make a difference for this industry.

**Figure 2.**

*Comparison of airfuel and oxyfuel combustion for aluminum melting. Note: These values are rough estimates and can vary between furnaces. No air preheating is considered. (a) Hydrogen airfuel combustion. (λ = 1.1), (b) Hydrogen oxyfuel combustion with lower fuel needs. (λ = 1.03), (c) Hydrogen oxyfuel combustion with higher capacity. (λ = 1.03).*

### **2. Hydrogen oxyfuel combustion chemistry**

Combustion of hydrogen with pure oxygen is a single reaction, given in Eq. (1), and the product is water.

$$\text{H}\_2 + \frac{1}{2}\text{O}\_2 \rightarrow \text{H}\_2\text{O} \tag{1}$$

Hydrogen oxyfuel combustion has other reactions if the furnace is not air tight. While in a laboratory, tightness is highly likely to be achieved, under practically all industrial conditions, air in-leakage into a furnace is the norm, and only the amount and location of the in-leakage can be controlled, based on the furnace pressure and process parameters. Energy efficiency and combustion efficiency are of course directly affected by these leaks.

Hydrogen combustion has the adiabatic flame temperature of 2377 K with air, and 3072 K with oxygen under stoichiometric conditions and atmospheric pressure [15]. The enthalpy of combustion is �241.8 kJ/mol H2 [16]. If the furnace is air tight, without nitrogen or carbon present in the gas mixture, NOx and CO2 emissions are of course zero. When the furnace leaks, undesired side reactions generate NOx, which is produced only *via* the thermal route (Zeldovich mechanism, given in Eq. (2), (3), and (4)) since the fuel does not contain nitrogen compounds. These undesired reactions can be significantly reduced by lowering the peak flame temperature, as a high temperature is required for oxygen and nitrogen dissociation reactions. Additionally, lower nitrogen partial pressure (less air drawn into the furnace) and lower oxygen partial pressure (no excess oxygen) limit these reactions. Further information on Zeldovich reactions can be found in [17].

*Hydrogen Oxyfuel Combustion for Energy-Intensive Industries DOI: http://dx.doi.org/10.5772/intechopen.106152*

$$\text{N}\_2 + \text{O} \rightarrow \text{NO} + \text{N} \tag{2}$$

$$\text{N} + \text{O}\_2 \rightarrow \text{NO} + \text{O} \tag{3}$$

$$\text{N} + \text{OH} \rightarrow \text{NO} + \text{H} \tag{4}$$

Industrial furnaces are typically operated with a slight positive pressure (in the range of a few mm of water column) to minimize air ingress. In this case, the amount of nitrogen in the furnace can be reduced to a great extent, and NOx emissions can be minimized. Specific furnace designs as well as frequent charging operations on some types of furnaces and the associated air ingress, however, can make NOx control very challenging. The problems associated with these leaks are not limited to the emissions, as the combustion efficiency and furnace temperature also suffer, requiring increased energy input to maintain operating conditions, while the product quality may suffer as well.

Along with oxyfuel combustion, O2-enhanced combustion (sometimes referred to as oxygen enrichment) is sometimes discussed in the literature [18]. Besides the potential for increased NOx generation, it brings with it certain operational drawbacks, especially if the enrichment level and power load changes are required during operation. These processes must be sized for airfuel combustion, which has the highest flue gas flow rate. As the O2 enrichment level increases, the flue gas flow rate decreases significantly, and this fact limits the lowest possible power level, while still maintaining a suitable furnace pressure. Complete oxyfuel combustion offers a simpler control of the process conditions if the flue gas system is appropriately redesigned.

Maintaining the furnace pressure is vital to prevent leakage, and it is directly dependent on the flue gas flow rate. The difference between natural gas oxyfuel and hydrogen oxyfuel in terms of flue gas volumes is insignificant when compared with the difference between airfuel and oxyfuel. If the flue gas duct is sized based upon natural gas oxyfuel combustion, maintaining the furnace pressure of hydrogen oxyfuel operation would be no concern. Of course, there are some process-specific requirements, and the following two sections discuss two metal processes and glass production for hydrogen oxyfuel combustion.

### **3. Metal production processes**

#### **3.1 Steel production**

Steel production is based on ore (primary) or scrap (secondary) processing. The primary production starts with a blast furnace producing pig iron, followed by a basic oxygen furnace that produces steel by oxidizing the excess carbon and other undesired elements. A blast furnace uses coke for the reduction of iron oxides. There are also direct reduction applications with natural gas and hydrogen, but the blast furnace route dominates the primary steel production. Secondary production is carried out in electric arc furnaces (EAFs) that are charged with scrap material. Both of the routes have high-temperature requirements.

The iron and steel industry has higher greenhouse gas emissions than other industrial processes. China has the highest steel production from ore, which is roughly 66% of the world's total. EU-28 countries, Japan, and South Korea follow China in primary production [19]. Secondary production is a quarter of total steel production [20], and the

leading region is North America, followed by EU-28 countries and India. Both routes can utilize hydrogen oxyfuel combustion in auxiliary and downstream processes, that is, for ladle preheating and reheating furnaces. In fact, secondary production already uses natural gas oxyfuel combustion. However, the world's total hydrogen production capacity is not enough to decarbonize the entire industry. In total, only 4% of the global hydrogen production relies on water electrolysis [21] and the rest is produced through fossil fuel-based routes, such as steam methane reforming. All leading steel-producing countries have green hydrogen policies and investment plans; however, usually, these plans include the transport and mobility sector instead of the industrial applications. Noteworthy green hydrogen integration in steel production projects takes place in Sweden and Germany, namely, Hybrit [22], H2 Green Steel [23], and Salcos [24].

For both primary and secondary productions, oxyfuel combustion to supply hightemperature heat demand is discussed in the literature. Although the secondary production is based on EAFs, oxyfuel burners are frequently included to shorten the cycle time and to reduce electricity consumption [25]. For primary steel production, there is blast furnace retrofitting suggestions in the literature with hydrogen injection [26] and coke oven gas recirculation (contains over 50% H2 and 30% methane) [27], next to a more fundamental direct reduced iron route [28]. While the first one suggests using lower amounts of carbon compounds and instead includes hydrogen in the mix, the latter eliminates these substances and suggests using only the hydrogen itself for reduction. The reduction reactions of CO and H2 are given below, and the H2 reactions are faster.

$$\text{2Fe}\_2\text{O}\_3 + \text{H}\_2 \to \text{2Fe}\_3\text{O}\_4 + \text{H}\_2\text{O} \tag{5}$$

$$\text{Fe}\_3\text{O}\_4 + \text{H}\_2 \rightarrow \text{3FeO} + \text{H}\_2\text{O} \tag{6}$$

$$\text{FeO} + \text{H}\_2 \rightarrow \text{Fe} + \text{H}\_2\text{O} \tag{7}$$

$$\text{3Fe}\_2\text{O}\_3 + \text{CO} \rightarrow \text{2Fe}\_3\text{O}\_4 + \text{CO}\_2 \tag{8}$$

$$\text{Fe}\_3\text{O}\_4 + \text{CO} \rightarrow \text{3FeO} + \text{CO}\_2 \tag{9}$$

$$\text{FeO} + \text{CO} \rightarrow \text{Fe} + \text{CO}\_2 \tag{10}$$

The investment impact would be higher in the primary production plants, considering their significantly higher capacity and much higher emissions. The alternative blast furnace design is expected to decrease the emissions as lower amounts of carbon compounds would be required, and the DRI route eliminates the emissions.

An important point to consider is product quality as changing the process gases, and the resultant different flue gas mixture might cause some undesirable side reactions. For steel production, hydrogen embrittlement is a nonnegligible concern when atomic hydrogen is present. If, however, the burner design and the furnace control are well performed, unburned hydrogen would not exist as a precursor for this problem. Another concern is the scale layer and its morphology. In hot rolling mills, a scale layer is formed on the steel, and then, it is removed by a descaler. If the morphology of the scale changes due to a change in the production, the descaler might cause defects on the surface. Tests at different temperatures and atmospheres show a slight difference in the scale thickness between natural gas and hydrogen combustion [29]. The same work shows no decarburization in hydrogen oxyfuel combustion conditions, which is promising. This, on the other hand, needs further investigation for high carbon steel as the information does not specify the samples' content. A full-scale trial run with hydrogen oxyfuel combustion for steel reheating showed no change in the scale formation next to other quality indicators [30].

#### **3.2 Aluminum production**

Primary aluminum production starts with bauxite, and alumina is extracted by the Bayer process and purified by the Hall–Heroult process afterward. The very high energy requirement of the primary production route further increases the importance of secondary production, which requires around 94% less energy per kg of aluminum [31]. In 2020, worldwide, aluminum had an over 30% recycle rate, which increases every year [32]. However, high-quality aluminum production requires more energy if produced *via* the secondary route [33]. Both routes have high-temperature requirements. China is the leading country of primary aluminum production with over 50% [34]. India, Russia, and North America are other important producers [35]. While China and Russia mainly rely on primary production, North America has a higher secondary production capacity [36]. In Europe, Germany, the United Kingdom, and Italy are the important secondary producers [37].

Oxyfuel combustion is used in aluminum melting and remelting processes and can reduce dross formation in primary and secondary production processes. There are very specific concerns related to hydrogen oxyfuel combustion due to the change in the furnace atmosphere. H2 is very soluble in molten aluminum, and it might cause pinholes, passages, and blisters if remains dissolved when the molten aluminum solidifies [31]. This, of course, can be avoided by burner design and proper process operation. It should also be considered that the only source of H2 dissolution is not the fuel. A water dissociation reaction occurs at the liquid aluminum surface and generates atomic hydrogen [38] and alumina, according to Eq. (11).

$$\text{2Al} + \text{3H}\_2\text{O} \rightarrow \text{Al}\_2\text{O}\_3 + \text{6H} + \tag{11}$$

Water is of course present at a very high partial pressure in the combustion gases of hydrogen oxyfuel combustion; therefore, further research and trial runs are essential to determine the effect and the importance of this reaction. However, dissolved H2 is efficiently removed by the degassing process before casting [39]. Another concern is increased dross formation due to the previously given water dissociation and higher oxygen partial pressure in the furnace atmosphere, causing the reaction given in Eq. (12).

$$\text{4Al} + \text{3O}\_2 \rightarrow \text{2Al}\_2\text{O}\_3 \tag{12}$$

CO2 is known to decrease dross losses, and combined trials of hydrogen oxyfuel combustion and inert gas injection can be used to compare against a reference natural gas combustion scenario. Hydrogen as a reduction agent was also suggested to recover iron and aluminum from the by-product of the Bayer process, bauxite residue, and it was found successful in lab trials [40]. It is important to note that for emission-free aluminum production, the use of hydrogen for heating and decarbonization of the electricity used for the electrolysis should be complemented by carbon-free anodes instead of graphite for Hall–Heroult process since this electrolytic method contributes to the emissions significantly.

### **4. Glass production process**

Glass production is energy-intensive in the melting and fining steps, with furnace temperatures of up to 1600°C. The melting process is primarily carried out in

regenerative furnaces with a very small number using recuperative furnaces; both rely on heat recovery since there is significant waste heat in the hot flue gases. In some geographies, these furnaces have been replaced by full oxyfuel furnaces, usually either due to very strict environmental legislation or very high fuel prices. For specialty glasses, oxyfuel furnaces are not uncommon, and for small capacities, electric furnaces can be used; an electric furnace, however, has a shorter lifetime [13]. The glass production is mainly local since product transportation poses a high risk of loss. Recycling rates are high, especially for container glass in developed countries (78% in EU28 and 32% world). Unlike the metal industry, recycling and primary production are carried out in the same furnace. Emissions are different based on the production location, and currently, in Europe and North America natural gas is the fuel of choice. Over 60% of glass production emissions are due to combustion [41]. The rest of the CO2 is formed in calcination reactions of soda ash and limestone given in Eq. (13) and (14).

$$\text{Na}\_2\text{CO}\_3 \rightarrow \text{Na}\_2\text{O} + \text{CO}\_2 \tag{13}$$

$$\text{CaCO}\_3 \rightarrow \text{CaO} + \text{CO}\_2 \tag{14}$$

A recent review on the decarbonization of the glass industry focuses on higher recycling rates and changing the fuel [42]. Due to the above reactions, glass production would still emit CO2 with hydrogen oxyfuel combustion. However, the flue gas is expected to be more suitable for carbon capture methods with fewer soot particles and higher CO2 concentrations after condensation of the water content.

There are particular product and furnace design concerns resulting from hydrogen oxyfuel combustion in glass melting. Foaming and, therefore, limited heat transfer is a primary concern. This also happens in existing operations and is solved by decreasing the air ratio to form CO that breaks down the foam. The high water vapor concentration of the furnace atmosphere is believed to exacerbate the issue. Another concern is a reduced furnace lifetime due to refractory damage caused or accelerated by the higher water vapor content.

There are a number of projects focusing on the use of hydrogen oxyfuel combustion for the glass industry investigating if these issues will occur and how they can be mitigated. There are projects in Germany and the U.K. testing hydrogen and enriched natural gas for glass melting that are HyGlass [43], Kopernikus [44], and HyNet [45]. By the end of 2021, HyGlass was finalized, and the trials had demonstrated the success of hydrogen airfuel combustion. The heat distribution was found homogenous with hydrogen combustion [46]. From the economic point of view, hydrogen is an expensive fuel for airfuel combustion. On an industrial scale, hydrogen oxyfuel combustion is expected to play a significant role, most likely in combination with electrical heating in the so-called hybrid furnaces, and the glass industry requires dedicated projects to ensure compatibility.

### **5. Economizer and latent heat utilization**

Heat recovery is a crucial part of the aforementioned industries. Due to the high temperatures required for the process chemistry, the flue gases also have high temperatures. Current practices usually show partial waste heat recovery for basic oxygen, electric arc, and glass furnaces. Annual energy loss to flue gas in hightemperature applications is relatively high. Blast furnaces and electric arc furnaces

have over 10% heat loss with the flue gas; this value is over 25% for glass melting furnaces and finally over 65% for aluminum melting furnaces [47]. Decreasing the amount of flue gas with hydrogen oxyfuel combustion will partly solve this problem, and the rest of the energy can be recovered using different systems. Waste heat recovery systems have certain restrictions due to materials, capital costs, and maintenance issues.

Heat pipes can be used for over 1000°C flue gas temperatures [48]. These systems utilize phase transition and are often used in solar collectors to keep the temperature difference low [49]. Tests in a highly corrosive environment show that with a suitable material selection, a long lifetime can be expected, and maintenance needs can be kept minimal [50].

Below 500°C, condensing economizers can recover the heat. As the name suggests, latent heat of the flue gas stream is also utilized in a condensing economizer. Usually installed by the district heat producers to increase the total efficiency [51, 52], condensing economizers are also discussed for the flue gas heat recovery. However, the heat recovered would be useful only for low-temperature heat demand. The risk of corrosion and deposition has research focused on coatings [53, 54]. One advantage of hydrogen oxyfuel combustion here is the complete elimination of sulfur gases, which reduces the risk of corrosion since sulfidation is one of the main corrosion mechanisms.

Additionally, the high water vapor content of the flue gas would provide a higher latent heat recovery. The lower volume and the higher water vapor ratio of the flue gas raise questions about condensation in the flue gas duct, but this can be used to cut energy losses with careful planning. A cleaner flue gas stream makes it easier to plan heat recovery systems and their maintenance.

## **6. Safety aspects and community engagement**

Like all combustible gases, hydrogen has several important safety requirements. Due to a few prominent accidents in the past (Hindenburg and Challenger), it has a reputation for being very dangerous. Years after these accidents, thorough investigations have determined that hydrogen was not the reason behind them. When compared with many other conventional fuels, hydrogen has its benefits. Unlike gasoline or diesel, hydrogen is nontoxic and neither does it contaminate the environment. A leak in a confined space would decrease the O2 concentration, resulting in an asphyxiation hazard. As it is lighter than air an outdoor leak is highly unlikely to create an explosive mixture; instead, it will disperse upward and escape in the atmosphere or, if ignited, it would burn until the hydrogen source has been removed. A hydrogen flame has low radiation due to its lack of carbon-containing combustion products, for example, soot. This fact decreases the risk of secondary fires but also makes it harder to detect a hydrogen flame as it is almost invisible when burning in the air in bright outdoor conditions [55], which necessitates fast detection of a leak in a confined space. The difference between a natural gas and hydrogen oxyfuel flame can be seen in **Figure 3**.

This broad flammability range makes a hydrogen flame very stable, and it is difficult to provoke a hydrogen flame to lift off of a burner. Its very low ignition energy also poses a risk, while at the same time simplifying the ignition of a hydrogenfired burner. Anti-static clothes and tools should always be used when working with hydrogen or hydrogen-containing piping or equipment. In addition, all equipment

**Figure 3.** *Natural gas (upper) and hydrogen (lower) oxyfuel flames with the same burner power.*

must be correctly grounded, and all vents and chimneys protected by a lightning conductor. As mentioned previously, hydrogen is a very light gas; therefore, adequate ventilation would decrease the risks. If adequate ventilation is always present, then small-to-moderate hydrogen leaks cannot accumulate. Additionally, hydrogen requires a high oxygen concentration to create a detonative mixture (18–59% [56]); therefore, even in a confined space, it is more likely to burn than to detonate.

Due to its very small molecular size, it can diffuse rapidly through some porous materials or systems that are considered gas tight with other gases. As a result, threaded connections and fittings should be avoided, or kept to an absolute minimum, while welded joints should be prioritized. Valves and flanges are susceptible to leaks and therefore require specialized sealing elements, for example, tongue and groove flanges. Ensure that all gaskets, seals, and lubricants are hydrogen compatible as hydrocarbons are cracked over time and purged out of the material. Some materials, such as cast iron, cannot be used for hydrogen piping elements since it loses ductility over time with diffusing hydrogen [57]. Copper and copper /tin- /zinc-based alloys should not be used for hydrogen service.

Although hydrogen is being discussed more often than ever due to environmental concerns, industrial applications have used hydrogen for many decades. Ammonia production is an excellent example of high-volume hydrogen usage showing that it is possible to use this gas without any problems if the appropriate safety requirements are complied with.

The safe handling of oxygen is frequently overlooked since it occurs naturally, and it is essential for our survival. High purity oxygen has very strict safety requirements, and accidents can result in very serious injuries and deaths. While oxygen is not flammable, many compounds that are practically inert in air, burn readily in oxygen. Even stainless steel can burn at high oxygen concentrations and pressures. The flammability range of hydrogen in oxygen is from 4–94% [58]. Breathing pure oxygen can cause dizziness, vision loss, and loss of consciousness. Oxygen saturation of clothing is a severe fire hazard since almost all clothing materials ignite readily and burn vigorously when saturated. All piping, components, and instruments in an oxygen system

#### *Hydrogen Oxyfuel Combustion for Energy-Intensive Industries DOI: http://dx.doi.org/10.5772/intechopen.106152*

must be free of all hydrocarbons (e.g., oil and grease, to name some), organic contamination, dirt, moisture, and particles. Valves and rotating instruments have special requirements to eliminate potential ignition sources. Valves should always be opened very slowly to prevent ignition due to particle impingement or adiabatic compression. Magnesium and titanium alloys should be avoided for the oxygen-containing components and piping. Oxygen is slightly heavier than air and can therefore accumulate in pits, cellars, and underground rooms.

Since both hydrogen and oxygen are stored under pressure, the appropriate personal protective equipment (PPE) should always be used when working with them. Flame retardant and anti-static clothing, safety glasses, safety shoes, and portable gas analyzers are typically the minimum protective equipment used in industrial applications where hydrogen and/or oxygen are present. Atmospheric monitoring equipment should be positioned according to gas specifications and possible leak locations and should be calibrated and maintained regularly. If cylinders are used for the gas supply, these should be clearly marked, well secured, and correctly stored.

It is worth noting that using these gases together does not pose a significantly higher risk than other oxidizer fuel combinations. Hydrogen and oxygen do have a much broader combustion range and a mixture will ignite very easily, hydrogen has other features that make it "less dangerous" than other fuels; for example, LPG being significantly heavier than air will readily accumulate in underground rooms, cellars, sewers, and pits. Each gas has its own specific requirements for storage, transportation, and use and these must be fully complied with at all times. These requirements might differ, which does not mean that one is more dangerous than the other. Although most of it is not green, hydrogen is produced on an industrial scale, transported by trailers, rail, or pipelines, and has been safely used in many different industrial applications for decades. Oxygen is produced by air separation units and used in industry, hospitals, and homes for life support. Knowing and understanding the risks is the first step toward safety, and this should not cause anxiety but instead should bring confidence.

### **7. Conclusions**

In the last decades, research efforts focused on hydrogen as an environmentally friendly energy carrier, which brought with it the discussion of its conversion back to energy. While hydrogen is used in fuel cells to generate electricity, its combustion provides thermal energy. Since green hydrogen is not a cheap fuel, hydrogen oxyfuel combustion can be used to reduce fuel consumption and the energy loss to the flue gases. The potential of three high-temperature industries is discussed for hydrogen oxyfuel combustion applications. The change in the furnace atmosphere and the possible product interactions are presented based on the literature. In this aspect, only a limited number of lab-scale experiments are documented for the aluminum and glass industries. Pilot-scale trials have the potential to show the full effect on products, energy gain, and environmental advancements. The iron and steel industry is demonstrating a dedicated effort toward carbon-free production methods. Certainly, their experience in handling hydrogen on a large scale will have an effect on the other industries. Thanks to the change in flue gas stream, possible gains are suggested to decrease emissions and increase heat recovery. These, however, require conceptual design and feasibility analysis. Finally, safety precautions for hydrogen and oxygen use are stated to run hydrogen oxyfuel combustion with the lowest risk possible.
