© D. Astesiano et al., 2023

1 The challenge for the decarbonization of steel industry

To meet the goal of transforming the steel industry into a climate-neutral sector by 2050, EU is mainly focused on the decarbonization of the upstream processes, meaning the set of operation to obtain liquid steel [1,2]. This goal can be achieved by means of two different pathways. In the so-called “primary route”, iron ore is fed into blast furnaces together with fossil raw materials, which act as reducing agents. This solution covers roughly 60% of EU production and 70% of world one, creating about 1800 kg per ton of crude steel produced of Scope 1 CO₂ emissions (i.e., the CO₂ emissions directly generated at the stack) [3,4]. On the other hand, the “secondary route”, in which liquid steel is produced from scrap by means of Electric Arc Furnace (EAF), is less CO₂-intensive (∼330–470 kg CO₂ per ton): about 80–120 kg CO₂ per ton as direct emissions (Scope 1), while indirect emissions (Scope 2, i.e., the CO₂ emissions coming from the purchase of electricity that was generated using fossil sources) are in the range of 250–350 kg CO₂ per ton [5] (Fig. 1).

Nevertheless, in the “secondary route” the contribution of downstream processes (reheating, heat treatment, etc.) to the total CO₂ emissions of the steel production cycle is already comparable with the upstream. In this scenario, the downstream CO₂ emissions will also be relevant with respect to the final CO₂ reduction scenario and need to be addressed as soon as possible. To reach the steel industry’s long-term vision and reach the target of greenhouse gas (GHG) emissions reduction set by the European commission (−55% by 2030 and then −85% by 2050 [7]), a holistic approach including both upstream and downstream processes will be of fundamental importance for both the “primary” and “secondary” routes as envisioned in the Strategic Research and Innovation Agenda (SRIA) of the Clean Steel Partnership (CSP) [5].

Aside this decarbonization pathway, the steel sector suffers from a strong inertia due to its characteristics of being very capital intensive, operating in a highly competitive global market and being characterized by an investment cycle between 20 and 30 years. To guide asset investments in the steelmaking industry, sustainable scenarios for an energy transition strategy must be identified. The transition must be managed, taking the opportunities of plant modernization by exploiting new technologies, and, at the same time, securing the capital investment.

This paper continues the presentation of Tenova solutions for the decarbonization of downstream processes described by Della Rocca et al. [8], in which the development of the “Hydrogen Ready” and “SmartBurner” technologies applied to a side-mounted burner (Tenova TSX) are described. The present paper extends these technologies to three additional burner families and shows how they improves a wider range of downstream processes in terms of NO_x emissions and energy efficiency. Furthermore, the paper presents the first industrial installation of a “Hydrogen Ready” burner, together with the setup of the first H₂-ready heat treatment furnace.

Fig. 1 Carbon emission intensity divided into direct and indirect (electricity) emissions for the different liquid steel production routes (BF/BOF: blast furnace/basic oxygen furnace; EAF: electric arc furnace; NG-DRI: natural gas based direct reduction) and hot rolling mill process [6]. Intensité des émissions de carbone répartie entre les émissions directes et indirectes (électricité) pour les différentes méthodes de production d’acier liquide (BF/BOF : blast furnace (haut fourneau)/basic oxygen furnace (convertisseur basique à oxygène) ; EAF : electric arc furnace (four à arc électrique) ; NG-DRI : réduction directe basée sur l’utilisation de gaz naturel) et process de laminoir à chaud [6].

2 Heating technologies in downstream processes

The evolution of combustion technologies for heating furnaces (reheating and heat treatment processes) is motivated by the reduction of fuel consumption while maintaining, or reducing, nitrous oxides (NO_x) emissions. The first solution in this direction was developed before the 1990: by means of a central recuperator installed directly on the furnace, the heat of the flue gases is used to pre-heat the combustion air up to 400–600 °C. After the 1990s the evolution of such technology led to the development of regenerative systems, in which a ceramic bed regenerator installed directly on the burners increase air temperature up to 1100 °C (an example is the Tenova TRG and TRGD burner families).

Although increasing the combustion air temperature leads to higher system efficiencies and less fuel consumption, this approach has a detrimental effect on NO_x emissions (Fig. 2). An effective solution to this problem was identified in the early 2000s with the Moderate or Intense Low-oxygen Dilution (MILD) combustion (a.k.a. flameless combustion) that exploits a combustion regime characterized by a flame invisible to human eyes. This is obtained by means of a strong dilution of the reactive mixture (fuel and air/oxygen) with the flue gases inside the furnace. The result is a sort of “diffuse combustion” (Fig. 3) that allows to strongly reduce the temperature gradients inside the chamber and avoids the hotspots generated by traditional flame combustion regimes (which are responsible for high quantities of “thermal NO_x”¹) [11]. The application of the flameless technology in the steel sector (see Tenova TSX and TRX burner families) allowed to reach very fine control of the heating procedure in downstream processes where the final product quality represents a crucial aspect (i.e., heat treatment furnaces).

Today, combustion systems for downstream processes that combine both regenerative and flameless technologies (like the Tenova TRGX burner families) are commercially available, providing higher furnace efficiency and lower fuel consumption with respect to central recuperator solutions. This evolution means a lower environmental impact in terms of NO_x and CO₂ emissions. Further information about the Tenova burner families can be found on the company website [12].

Current efforts in R&D&I are focused on a further reduction of the carbon footprint of reheating and heat treatment furnaces. To this end, hydrogen combustion represents the latest frontier of industrial combustion technologies. Nevertheless, hydrogen firing is generally associated to a net increase of NO_x emissions at the stack [13]: the higher gas reactivity leads to more intense flames compared to natural gas combustion, producing high quantities of thermal NO_x.

Hydrogen is not a new component in fuel mixture used in the steelmaking sector: Blast Furnace Gas (BFG), Basic Oxygen Furnace Gas (BOFG) and Coke-Oven Gas (COG) contain variable amounts of H₂, even up to 60% in volume. Their application in high-efficiency burners with pre-heated air usually produces very intense flame and leads to a significant increase in NO_x emissions, mainly by “thermal” effect [14] (Fig. 2). To avoid these phenomena, proper control strategies are required.

Fig. 2 NOx emissions for the same furnace equipped with natural gas and COG combustion systems [9]. Émissions de NOx pour un même four selon qu’il est équipé d’un système de combustion au gaz naturel ou au COG [9].

Fig. 3 Example of “flame” (left) and MILD/“flameless” (right) combustion in an industrial furnace [10]. The burner is placed on the left side of each picture. Under “flame” mode (left panel), it is possible to observe the development of a region in the furnace atmosphere emitting a strong light, identifying the zone where the high-temperature visible flame develops. Under MILD/“flameless” conditions (right panel), no high-temperature visible-light-emitting regions are observed in the furnace atmosphere. Exemple de combustion « avec flamme » (à gauche) et MILD/« sans flamme » (à droite) dans un four industriel [10]. Le brûleur se trouve sur la gauche de chaque image. En mode « avec flamme » (image de gauche), on peut observer, dans l’atmosphère du four, le développement d’une région qui émet une puissante lumière, il s’agit de la zone dans laquelle se forme une flamme visible à haute température. Dans des conditions MILD/« sans flamme » (image de droite), on n’observe aucune zone à haute température émettant une lumière visible dans l’atmosphère du four.

3 Results

The previous work by Della Rocca et al. [8] showed how the “Hydrogen Ready” concept coupled with the flameless technology represent an effective solution to enable the use of hydrogen in the steel sector while securing the investments. In the present work, the concept of a combustion system able to accommodate any hydrogen enrichment percentage in traditional fuels, ranging from 0% up to 100% hydrogen feeding, without any equipment modification and without any alteration in process performance (heating uniformity, NO_x emissions, etc.) is extended to three more burner families. The goal is to demonstrate how this “Hydrogen Ready” technology represents an effective solution for several different applications in the downstream processes of the steel sector (Fig. 4). All the burners presented here feature also the “SmartBurner” technology. More information about “Hydrogen Ready” and “SmartBurner” technologies can be found in [8].

The development process for a “Hydrogen Ready” burner exploits the know-how acquired in several years of NG flameless combustion, exploiting a design process where engineering, virtual prototyping, laboratory experiments and industrial tests are combined in a synergistic approach (Fig. 5). In particular, the virtual prototyping, based on Computational Fluid Dynamics (CFD) simulations coupled with detailed chemistry, allows to optimize the burner geometry in terms of thermal distribution and emissions.

Fig. 4 Tenova “Hydrogen Ready” combustion system roadmap, showing the burner families already equipped with the “Hydrogen Ready” technology and the future developments. Feuille de route pour le système de combustion « Hydrogen Ready » de Tenova qui présente les gammes de brûleurs déjà dotées de la technologie « Hydrogen Ready », ainsi que leur évolution à venir.

Fig. 5 Tenova approach for burner development/industrialization cycle. Approche de Tenova concernant le cycle de développement/d’industrialisation des brûleurs.

3.1 Tenova TRKSX H₂ flameless burner

The Tenova TRKSX H₂/NG burner is a new flame/flameless self-recuperative burner for treatment furnaces. The burner is equipped with a metallic counter-current heat exchanger that uses the exhaust gases sucked from the furnace to pre-heat the combustion air. This solution allows to reach a burner efficiency η_b² as high as 75%. Figure 6 shows the performance of the TRKSX 2.5 (210 kW thermal power) in terms of NO_x emissions for the complete set of H₂/NG mixture, both in flame and flameless regimes.

It can be observed that NO_x emissions are always below the critical threshold of 100 ppm (at 3% excess oxygen) in any operating conditions, also in flame mode. This aspect is quite important for the industrial application, because even when the burner is operated in flame mode (i.e., whenever the furnace temperature is below the fuel self-ignition temperature, like during start-up) the respect of the emissions regulations must be guaranteed. Moreover, emissions for the flameless combustion are always below the lowest value of the limits defined in the Best Available Technologies (BAT) for the ferrous metals processing industry (channeled NO_x emissions to air in hot/cold rolling – new plants), and set to 80 mg/Nm³ at 3% O₂ on dry fume base (about 44 ppm at 3% O₂) [15].

Figure 6 shows that the TRKSX burner in flame mode produces higher NO_x emissions with increasing H₂ concentration in the fuel mixture. For the pure NG case (furnace temperature 1050 °C), 78 ppm of NO_x at 3% oxygen are measured. Then, a 10% raise is observed when hydrogen addition reaches up to 75% V/V, followed by a steep increment up to 90 ppm NO_x at 3% oxygen when the burner is fueled with 100% H₂ (meaning a variation of ∼ 20% with respect to the NG case). These trends confirm previous observation by other authors [8,14]. The furnace temperature also plays a significant role in the NO_x emissions: moving from 860 °C to 1050 °C means increasing more than 30% the NO_x emitted at the stack. This effect is related not only to the higher temperature where combustion takes place, but also at the effect produced by the hotter exhaust gas on the combustion air (which tends to be preheated at higher temperatures).

The measurements reported in Figure 6 confirm the efficiency of the flameless technology in drastically reducing NO_x emissions. This result is true not only with NG, but also with NG/H₂ mixtures and full H₂ combustion, with a significant decrease in NO_x emissions between 72% (22 ppm NO_x at 3% oxygen, 100% NG, furnace temperature 1050 °C) and 82% (16 ppm NO_x at 3% oxygen, 100% H₂, furnace temperature 1050 °C). It is relevant to note how the shape of the emission profile changes with the H₂ enrichment: differently from the flame mode, the flameless combustion presents a maximum of NO_x emissions when the fuel mixture contains between 25% and 75% of H₂. This behavior highlights a sort of increased reactivity due to the interaction between NG and H₂. Moreover, this peak is sensible to the system temperature: in fact, it appears at lower hydrogen enrichment when the system is hotter. A similar behavior is reported also by other authors [16].

From the data reported, it is quite clear the capability of flameless combustion in limiting the NO_x formation by thermal route, thanks to the strong dilution of the reacting mixture with the exhaust gases. This is true not only for NG, but also for the highly reactive H₂. Preliminary analyses suggest that the high reactivity of hydrogen may also be responsible for the peak observed in the NO_x emission profiles with flameless combustion: when the H₂ concentration reaches a critical value in the mixture, it seems to promote the NG combustion and the general system reactivity, leading to higher NO_x production. These results are in line with the dynamics observed in a previous study done on a side mounting flameless burner for reheating applications (Tenova TSX) [8].

Fig. 6 Tenova TRKSX burner. Left panel: installation setup used in the test and picture (visible spectrum) of the combustion chamber with 100 H2 under flame (top) and flameless (bottom) conditions. Right panel: measured NOx emissions at different hydrogen enrichment level for flame and flameless combustion mode. Brûleur TRKSX de Tenova. Illustration de gauche : configuration d’installation utilisée lors du test et illustration (spectre visible) de la chambre de combustion dans des conditions avec (haut) et sans (bas) flamme à 100 % de H2. Illustration de droite : émissions de NOx mesurées à différents niveaux d’enrichissement en hydrogène pour les modes de combustion avec et sans flamme.

3.2 Tenova TRGX H₂ flameless burner

The Tenova TRGX burner combines flameless combustion with regenerative technology [17], and provides the state-of-the-art performance in terms of both combustion efficiency and NO_x emissions reduction. The combustion system consists in a burner equipped with a ceramic (honeycomb) regenerator placed on the back, and works in two successive phases: at first, the burner recovers heat from flue gases by passing them inside the regenerator (“recovery mode”). Then, the burner is switched to the firing phase and the air passes through the regenerator, to be preheated before passing through the burner head. Usually, two burners are placed one in front of the other, working alternatively in recovery and firing mode (Fig. 7).

Figure 8 shows the performances in terms of NO_x emissions for the 1 MW (gas based) hydrogen-ready version of TRGX burner fueled with mixture from 0% up to 100% H₂. Similarly to the Tenova TSX and Tenova TRKSX, the TRGX shows an increase in NO_x emissions with the H₂ quantity in the fuel mixture in flame mode: for pure NG 95 ppm of NO_x at 3% oxygen are measured and, after a slightly increase of ∼ 5% in NO_x with hydrogen enrichment up to 50%, higher H₂ quantities lead to a steep increase in the NO_x production, reaching a +19% with respect to NG case for the pure hydrogen (113 ppm at 3% oxygen). The use of flameless combustion is again an effective solution to abate nitrous oxides production also in the case of strong air preheating: with a furnace temperature of 1250 °C and a combustion air preheated up to 1090 °C, the NO_x emission curve is basically constant around 33 ppm at 3% oxygen for any NG/H₂ mixture. This means a reduction ranging from 65% for pure NG combustion, up to 71% for pure H₂. Moreover, these results highlight the independence of the NO_x emissions from the H₂ enrichment level for the Tenova TRGX burner.

Due to the high thermal capacity and exchange area of the ceramic honeycomb regenerators, the burner efficiency η_b is up to 93% with furnace at 1250 °C (Fig. 9). As shown in the graph, by increasing the quantity of hydrogen in the fuel mixture, the fume outlet temperature increases: when the percentage of fumes aspirated by the burner is kept constant (about 80%), the ratio between the quantity of sucked waste gases and the amount of air passing through the regenerator increases with the higher H₂ content (from 0.88 for pure NG up to 0.97 for hydrogen). Therefore, the burner efficiency η_b decreases when hydrogen content increases, while the global efficiency η_g³ remains almost constant and equal to 85%.

Fig. 7 Regenerative burner concept. Top: burner #1 works in recovery mode while burner #2 works in firing mode. Bottom: the two burners switched the working mode. Concept de brûleur régénératif. En haut : le brûleur n° 1 fonctionne en mode récupération tandis que le brûleur n° 2 est en mode combustion. En bas : le mode de fonctionnement des deux brûleurs a été inversé.

Fig. 8 Tenova TRGX burner. Left panel: installation setup used in the tests. Right panel: measured NOx emissions at different hydrogen enrichment level for flame and flameless combustion mode. Brûleur TRGX de Tenova. Illustration de gauche : configuration d’installation utilisée lors des tests. Illustration de droite : émissions de NOx mesurées à différents niveaux d’enrichissement en hydrogène pour les modes de combustion avec et sans flamme.

Fig. 9 Regenerator efficiency and waste gas temperature vs. H2 content in the fuel mixture for the Tenova TRGX burner. Efficacité du régénérateur et température des gaz de fumées par rapport à la teneur en H2 du mélange de carburant pour le brûleur TRGX de Tenova.

3.3 Tenova THSQ H₂ flameless burner: ongoing developments

The “Hydrogen Ready” technology is under development also for the Tenova THSQ burners, a family of combustion systems suitable for batch annealing furnaces, heat treatment furnaces and other special heat treatment applications. This burner features different combustion techniques to reduce pollutant formation, such as air and fuel staging, as well as flameless combustion. Figure 10 shows the burner performances in terms of normalized NO_x emissions at different H₂-enrichment levels for both flame and flameless combustion. As can be seen, the preliminary tests confirm the trends already described in the previous paragraphs. Under flame condition, the burner shows a strong increase in the NO_x emissions for H₂ enrichment levels higher than 75%: with pure NG, the Tenova THSQ burner produces 90 ppm of NO_x at 3% oxygen, which becomes 110 ppm at 50% H₂ and almost the double at 100% H₂ (170 ppm at 3% O₂). Even if the development of the Hydrogen Ready technology for the Tenova THSQ flameless burner family is not yet completed, preliminary tests show that the flameless combustion is again an effective solution to reach very low emissions level of NO_x also with this type of burner when fueled with NG/H₂ mixtures, reaching a potential reduction ratio of almost 80%.

Fig. 10 Tenova THSQ burner. Left panel: installation setup used in the test. Top panel: picture (visible spectrum) of the combustion chamber under flame conditions for different hydrogen enrichment levels and different angles (top row: front; bottom row: lateral). Bottom panel: measured NOx emissions at different hydrogen enrichment level for flame and flameless combustion mode. Brûleur THSQ de Tenova. Illustration de gauche : configuration d’installation utilisée lors du test. En haut : illustration (spectre visible) de la chambre de combustion dans des conditions avec flamme pour différents niveaux d’enrichissement en hydrogène et différents angles (ligne du haut : de face ; ligne du bas : de côté). En bas : émissions de NOx mesurées à différents niveaux d’enrichissement en hydrogène pour les modes de combustion avec et sans flamme.

4 Industrial application of hydrogen combustion in downstream processes

The adoption of hydrogen in downstream processes is related to several different aspects, in which the availability of a compatible combustion system is only a detail of a wider picture. Figure 11 tries to summarize the main enablers that make this transformation occur, highlighting the ones, which represent an obstacle for the decarbonization process.

The possibility to use hydrogen in a furnace is directly dependent on the availability of a combustion system able to burn it and to guarantee the process performances without overcoming the strict emission regulations (Fig. 11, top left). As extensively discussed in this paper, today these aspects do not represent a limiting factor thanks to state-of-the-art burners available on the market, which are able to use any NG/H₂ mixture and feature the flameless technology (Tenova TSX, TRKSX, TRGX and THSQ).

Another important aspect is related to the process since the combustion system fueled with hydrogen must avoid altering the final product surface quality, like scale formation, scale adhesion and decarburization (Fig. 11, top right). In this direction, some data are available for COG gases with high H₂ content [9,18]. Moreover, laboratory tests show that for some specific steel grades the scale formed with H₂-rich atmospheres increases up to 8–10% for the line pipe steel, and up to 16% for the casing steel (Fig. 12) [19]. Nevertheless, a more comprehensive work is required with the aim of including as many steel grades as possible.

A critical aspect when talking about hydrogen use in the steel sector is represented by the system integration, meaning how to ensure a stable and continuous feed of hydrogen in steel mills (Fig. 11, bottom left). In general, it is possible to foresee two scenarios: a local production scenario, where H₂ is produced directly in the steel mill, versus a remote one, where hydrogen is delivered in continuous (through a pipe) or batch way (like gas cylinders). In both cases, different equipment (like pipes, valves, storage devices, power generation and distribution systems, etc.) is required and needs to be integrated in the steel mill with the furnaces, ensuring the respect of all the local laws and safety rules. In this direction, a series of projects started by Tenova allow to build a knowledge base of industrial guidelines with the goal of reducing the investment risk for hydrogen use in hot rolling furnaces. The first example is the “TenarisDalmine Zero Emissions” project, in which a single H₂ Ready burner is tested in an industrial furnace and fed with hydrogen produced on-site by means of an electrolyzer. The goal of the project is to test the complete H₂ value chain at a representative industrial scale inside the TenarisDalmine steel mill. Figure 13 shows an aerial view of the site, together with the layout of the hydrogen production unit, the hydrogen storage and the hydrogen pipeline for the burner feeding.

The burner selected for this project is the Tenova TRKSX, which is already installed on the selected furnace of the TenarisDalmine site (Fig. 14). The installation of the electrolyzer unit, supplied by SNAM [20], is expected to happen in 2023 and will provide a stable source of hydrogen on site, allowing to feed the burner with NG/H₂ mixtures up to 100% H₂.

The natural extension of the TenarisDalmine Zero Emission project is the supply of the first heat treatment furnace completely equipped with “Hydrogen Ready” burners (Fig. 15). This movable hood furnace is designed to treat special steel bars up to 12 m long, and it is equipped with thirty-four TRKSX self-recuperative flameless burners ready to work with any NG/H₂ mixture, up to 100% H₂.

The last enabler is represented by the availability of low-CO₂ (i.e., green) hydrogen (Fig. 11, bottom right), which represents a key element in a decarbonization scenario. It summarizes the problem of the availability of some relevant resources (i.e., zero-carbon electricity) that are essentials to produce significant quantities of green hydrogen. This aspect represents the real bottleneck for a decarbonization roadmap of the steel industry, even if it is an external and cross-industry factor [21–23]. Figure 16 reports an example of the need of renewable energy and green hydrogen for the conversion of a medium-size reheating furnace. As it can be observed, a typical industrial unit with a productivity of 800 000 tons/year consumes about 25 million of normal cubic meters of NG. Switching to a full green hydrogen scenario, the same process conditions will be guaranteed with a consumption of around 81 million of normal cubic meters of H₂ (based on the Low Heating Value, LHV). This means that the corresponding electric power to generate this amount of green hydrogen is around 60 MW (for a single furnace).

Fig. 11 Enablers for hydrogen combustion in downstream processes. The icon on the top-left angle of each panel clarifies if the factor represents an obstacle (red stop sign) or not (green thumb-up sign) to the usage of H2 in downstream processes. The man-at-work sign identifies the factors for which specific activities are ongoing to move from the “obstacle” state to the “not-an-obstacle” one. The figures summarize the problems related to each enabler, while the text identifies specific solutions/actions allowing the status change from “obstacle” to “not-an-obstacle”. Facilitateurs pour la combustion d’hydrogène dans les process en aval. L’icône figurant dans le coin supérieur gauche de chaque case indique si le facteur constitue un obstacle (panneau d’arrêt rouge) ou non (symbole de pouce levé vert) à l’utilisation de H2 dans les process en aval. Le panneau indiquant des travaux signale les facteurs pour lesquels des activités spécifiques sont en cours pour passer de l’état « obstacle » à l’état « pas un obstacle ». Les illustrations récapitulent les problèmes liés à chaque facilitateur tandis que le texte fournit des solutions/actions spécifiques permettant de passer de l’état « obstacle » à l’état « pas un obstacle ».

Fig. 12 Scale formation on different steel grades due to the interaction of the product with the combustion atmosphere generated from a variety of NG/H2 mixtures [19]. Formation de calamine sur différentes qualités d’acier en raison de l’interaction du produit avec l’atmosphère de combustion générée à partir de divers mélanges de GN/H2 [19].

Fig. 13 Layout of the hydrogen value chain installation at the TenarisDalmine site. Disposition de l’installation de la chaîne de valeur de l’hydrogène sur le site de TenarisDalmine.

Fig. 14 Tenova TRKSX H2/NG self-recuperative burner as installed at TenarisDalmine. Brûleur TRKSX H2/NG de Tenova tel qu’il est installé à TenarisDalmine.

Fig. 15 Photos of the first full “Hydrogen Ready” furnace delivered to TenarisDalmine by Tenova. Photos du premier four entièrement « Hydrogen Ready » livré à TenarisDalmine par Tenova.

Fig. 16 Example of the renewable energy and green hydrogen demands for a medium-size reheating furnace. Exemple d’exigences en matière d’énergies renouvelables et d’hydrogène vert pour un four de réchauffage de taille moyenne.

5 Conclusions

The global energy transition scenario pushes for the decarbonization of the steel industry, even if the sector suffers from a strong inertia due to its characteristics of being very capital intensive, operating in a highly competitive global market and being characterized by an investment cycle between 20 and 30 years. Nevertheless, the envisioned availability of high quantities of green hydrogen in the near future represents an opportunity to integrate all the steel production steps into a more general hydrogen-based production route, eventually in combination with electrified solutions in hybrid configurations. This approach potentially constitutes a big advantage, both from the economic, environmental, and social acceptability points of view.

A key challenge right now is the so-called “valley of death” between the research activity and the deployment of innovative technologies: since the energy transition of the steel industry entails high technological and economic risks, it is very important for the companies to identify the most suitable solutions that guarantee a reduction of CO₂ emissions and, at the same time, ensure the required capital investments for plant modernization in a long-time vision.

The concept of “Hydrogen Ready” burner fits perfectly with this strategy, ensuring the investments for several years: it gives steel producers a great flexibility in terms of fuel mixture, allowing the use of any H₂ enrichment level that will be provided in the near future. Furthermore, the adoption of the flameless technology ensures to not overcome the strictly regulations in terms of emissions.

Despite all the above-mentioned topics, some aspects still represent an open point. As a matter of facts, the use of mixture containing a high concentration of H₂ for some downstream process may impact on the product quality, due to the different atmospheres produced during combustion. For this reason, it is necessary to investigate the effects of combustion atmospheres on scale formation, scale adhesion and decarburization. Moreover, the system integration, meaning how to ensure a stable and continuous feed of hydrogen to the production units in terms of supply and storage, represents a critical aspect for the steel industry. Industrial demonstration projects are currently ongoing and can help to clarify all these aspects, allowing to generate best practices useful to be exploited in the near future when a higher quantity of low-CO₂ (i.e., green) hydrogen will be available.

References

All Figures

Fig. 1 Carbon emission intensity divided into direct and indirect (electricity) emissions for the different liquid steel production routes (BF/BOF: blast furnace/basic oxygen furnace; EAF: electric arc furnace; NG-DRI: natural gas based direct reduction) and hot rolling mill process [6]. Intensité des émissions de carbone répartie entre les émissions directes et indirectes (électricité) pour les différentes méthodes de production d’acier liquide (BF/BOF : blast furnace (haut fourneau)/basic oxygen furnace (convertisseur basique à oxygène) ; EAF : electric arc furnace (four à arc électrique) ; NG-DRI : réduction directe basée sur l’utilisation de gaz naturel) et process de laminoir à chaud [6].

In the text

	Fig. 2 NOx emissions for the same furnace equipped with natural gas and COG combustion systems [9]. Émissions de NOx pour un même four selon qu’il est équipé d’un système de combustion au gaz naturel ou au COG [9].
In the text

Fig. 3 Example of “flame” (left) and MILD/“flameless” (right) combustion in an industrial furnace [10]. The burner is placed on the left side of each picture. Under “flame” mode (left panel), it is possible to observe the development of a region in the furnace atmosphere emitting a strong light, identifying the zone where the high-temperature visible flame develops. Under MILD/“flameless” conditions (right panel), no high-temperature visible-light-emitting regions are observed in the furnace atmosphere. Exemple de combustion « avec flamme » (à gauche) et MILD/« sans flamme » (à droite) dans un four industriel [10]. Le brûleur se trouve sur la gauche de chaque image. En mode « avec flamme » (image de gauche), on peut observer, dans l’atmosphère du four, le développement d’une région qui émet une puissante lumière, il s’agit de la zone dans laquelle se forme une flamme visible à haute température. Dans des conditions MILD/« sans flamme » (image de droite), on n’observe aucune zone à haute température émettant une lumière visible dans l’atmosphère du four.

In the text

	Fig. 4 Tenova “Hydrogen Ready” combustion system roadmap, showing the burner families already equipped with the “Hydrogen Ready” technology and the future developments. Feuille de route pour le système de combustion « Hydrogen Ready » de Tenova qui présente les gammes de brûleurs déjà dotées de la technologie « Hydrogen Ready », ainsi que leur évolution à venir.
In the text

	Fig. 5 Tenova approach for burner development/industrialization cycle. Approche de Tenova concernant le cycle de développement/d’industrialisation des brûleurs.
In the text

Fig. 6 Tenova TRKSX burner. Left panel: installation setup used in the test and picture (visible spectrum) of the combustion chamber with 100 H2 under flame (top) and flameless (bottom) conditions. Right panel: measured NOx emissions at different hydrogen enrichment level for flame and flameless combustion mode. Brûleur TRKSX de Tenova. Illustration de gauche : configuration d’installation utilisée lors du test et illustration (spectre visible) de la chambre de combustion dans des conditions avec (haut) et sans (bas) flamme à 100 % de H2. Illustration de droite : émissions de NOx mesurées à différents niveaux d’enrichissement en hydrogène pour les modes de combustion avec et sans flamme.

In the text

	Fig. 7 Regenerative burner concept. Top: burner #1 works in recovery mode while burner #2 works in firing mode. Bottom: the two burners switched the working mode. Concept de brûleur régénératif. En haut : le brûleur n° 1 fonctionne en mode récupération tandis que le brûleur n° 2 est en mode combustion. En bas : le mode de fonctionnement des deux brûleurs a été inversé.
In the text

Fig. 8 Tenova TRGX burner. Left panel: installation setup used in the tests. Right panel: measured NOx emissions at different hydrogen enrichment level for flame and flameless combustion mode. Brûleur TRGX de Tenova. Illustration de gauche : configuration d’installation utilisée lors des tests. Illustration de droite : émissions de NOx mesurées à différents niveaux d’enrichissement en hydrogène pour les modes de combustion avec et sans flamme.

In the text

	Fig. 9 Regenerator efficiency and waste gas temperature vs. H2 content in the fuel mixture for the Tenova TRGX burner. Efficacité du régénérateur et température des gaz de fumées par rapport à la teneur en H2 du mélange de carburant pour le brûleur TRGX de Tenova.
In the text

Fig. 10 Tenova THSQ burner. Left panel: installation setup used in the test. Top panel: picture (visible spectrum) of the combustion chamber under flame conditions for different hydrogen enrichment levels and different angles (top row: front; bottom row: lateral). Bottom panel: measured NOx emissions at different hydrogen enrichment level for flame and flameless combustion mode. Brûleur THSQ de Tenova. Illustration de gauche : configuration d’installation utilisée lors du test. En haut : illustration (spectre visible) de la chambre de combustion dans des conditions avec flamme pour différents niveaux d’enrichissement en hydrogène et différents angles (ligne du haut : de face ; ligne du bas : de côté). En bas : émissions de NOx mesurées à différents niveaux d’enrichissement en hydrogène pour les modes de combustion avec et sans flamme.

In the text

Fig. 11 Enablers for hydrogen combustion in downstream processes. The icon on the top-left angle of each panel clarifies if the factor represents an obstacle (red stop sign) or not (green thumb-up sign) to the usage of H2 in downstream processes. The man-at-work sign identifies the factors for which specific activities are ongoing to move from the “obstacle” state to the “not-an-obstacle” one. The figures summarize the problems related to each enabler, while the text identifies specific solutions/actions allowing the status change from “obstacle” to “not-an-obstacle”. Facilitateurs pour la combustion d’hydrogène dans les process en aval. L’icône figurant dans le coin supérieur gauche de chaque case indique si le facteur constitue un obstacle (panneau d’arrêt rouge) ou non (symbole de pouce levé vert) à l’utilisation de H2 dans les process en aval. Le panneau indiquant des travaux signale les facteurs pour lesquels des activités spécifiques sont en cours pour passer de l’état « obstacle » à l’état « pas un obstacle ». Les illustrations récapitulent les problèmes liés à chaque facilitateur tandis que le texte fournit des solutions/actions spécifiques permettant de passer de l’état « obstacle » à l’état « pas un obstacle ».

In the text

	Fig. 12 Scale formation on different steel grades due to the interaction of the product with the combustion atmosphere generated from a variety of NG/H2 mixtures [19]. Formation de calamine sur différentes qualités d’acier en raison de l’interaction du produit avec l’atmosphère de combustion générée à partir de divers mélanges de GN/H2 [19].
In the text

	Fig. 13 Layout of the hydrogen value chain installation at the TenarisDalmine site. Disposition de l’installation de la chaîne de valeur de l’hydrogène sur le site de TenarisDalmine.
In the text

	Fig. 14 Tenova TRKSX H2/NG self-recuperative burner as installed at TenarisDalmine. Brûleur TRKSX H2/NG de Tenova tel qu’il est installé à TenarisDalmine.
In the text

	Fig. 15 Photos of the first full “Hydrogen Ready” furnace delivered to TenarisDalmine by Tenova. Photos du premier four entièrement « Hydrogen Ready » livré à TenarisDalmine par Tenova.
In the text

	Fig. 16 Example of the renewable energy and green hydrogen demands for a medium-size reheating furnace. Exemple d’exigences en matière d’énergies renouvelables et d’hydrogène vert pour un four de réchauffage de taille moyenne.
In the text