Decarbonisation and the Impact of Hydrogen on Refractory Linings in the Iron and Steel Industry

In recent years, the world has seen diverse initiatives promoting emission and energy consumption reductions. Many countries and companies worldwide are striving to achieve shared energy and climate goals within the next decade, with the main industry sectors considering various alternatives, such as continuous improvement, process enhancements, and carbon capture, among other possibilities. The use of hydrogen as a fuel and a process gas is one of the most important trends for reducing CO₂ emissions in a relatively short time.

Hydrogen can be produced in various ways, resulting in a colour-based classification system, typically divided into main colours and others (Table I). The main colours of hydrogen production are: (1) Green, (2) Grey/brown/black, and (3) Blue. The first group highlights hydrogen sources such as biomass, biomethane, and all alternative routes of water electrolysis using renewable energy (e.g., solar, wind, and hydroelectric) that avoid using or generating any greenhouse gases. The second group includes all processes based on hydrocarbons, which generate CO₂ that is afterwards released into the atmosphere. The third group encompasses the same processes as the second but includes carbon capture and storage at the end of the production chain. From these main groups, additional colours such as pink, turquoise, and brown are derived, representing other possible routes to produce hydrogen. Each of these methods impacts emissions and energy consumption in different ways [3]. 

Recent global statistics show that hydrogen production has been increasing in the last years. For example, in 2022, total hydrogen production was about 95 million tonnes, 3% higher than the previous year. Currently, most production is performed using fossil fuels, as the majority of low-emission hydrogen production processes are still projects under development or at the feasibility study stage. In total, these low-emission plants accounted for only 0.7% of overall worldwide production in 2022. Nevertheless, a significant increase of up to 30% is already planned by 2030 [4]. In most industries, once hydrogen has been produced, it can be used either as fuel or a process gas. Different impacts on the refractory lining are expected, as there is a main difference between both cases, which relates to the direct contact of refractory materials with hydrogen gas.

One of the main planned developments in the iron and steelmaking industry is the increased use of hydrogen in DRI units, upstream of the EAF or electric smelting furnace/ BOF routes (Figure 1). Although current DRI technologies already use a gas mixture based on H₂ and CO, efforts are underway to develop an even cleaner process where the use of carbon can be reduced and substituted by up to 100% hydrogen. Wear mechanisms in the refractory linings may change in these DRI units, as well as in the subsequent EAF, where part of the scrap may be substituted by DRI.

For over 70 years, various DRI technologies have been developed around the globe as an alternative to the BF route. With these technologies, the reduction of oxidised iron ore occurs in a gaseous environment using a mixture of reducing gases, usually based on H₂ and/or CO. These processes achieve a very high level of metallisation of the iron ore, which is then used in steelmaking shops.

Global DRI production was 127.36 million tonnes in 2022 [6]. This represents about 7% of the steel production during that year, which totalled 1885 million tonnes worldwide [7]. Although the impact of DRI on the industry is currently still low, it is expected that the installed capacity will increase significantly in the coming years due to recent net-zero emission initiatives.

The state-of-the-art DRI process typically uses a H₂/CO gas ratio of about 1.6 on average, operating at temperatures around 900 °C. The partial pressures can vary depending on the technology, and in some cases this ratio may even rise to nearly 3.9 [8]. This is due to the use of natural gas as the main feedstock for the reformation stage, to prepare the gas mixture used as a reductant. Although current DRI technologies still produce some CO₂ in their off-gases, these processes potentially reduce CO₂ emissions by 38–61% compared to the standard BF route [9].

Decarbonising the steel industry may require several different actions from most of the important players in this market segment. For example, in the ironmaking stage carbon capture, BF enhancements, and/or the implementation of DRI units are feasible solutions to reduce emissions effectively.

The main DRI original equipment manufacturers (OEMs) are also developing new processes that are targeting 100% hydrogen-based units. These emerging technologies may require updated refractory concepts, as changes to the process parameters, which in most cases affect the gas composition, could lead to more aggressive wear mechanisms in the refractory lining, primarily caused by the enriched hydrogen atmosphere.

Only a few investigations have been conducted to analyse the behaviour of different refractory materials in pure hydrogen atmospheres [10] and the different methods and types of furnaces used led to various results that in some cases were contradictory. However, overall it has been confirmed that one of the oxides contained in refractory materials, namely SiO₂, reacts very easily with H₂. The resulting silica loss is a phenomenon where hydrogen reacts with the refractory material, reducing the brick or castable, causing a detrimental effect on the properties of the refractory lining. This reaction is primarily driven by temperature and pressure, occurring more rapidly at higher temperatures. In contrast, the opposite behaviour is observed for pressure, where higher pressures somehow slow down the same reaction [10,11].

Thermodynamic modelling can be used to calculate the weight loss of SiO₂ and SiO₂-containing refractory raw materials (e.g., mullite) due to the reduction of SiO₂ by pure hydrogen at increasing temperatures (Table II). This reaction accelerates in cases where either the SiO₂ content is higher, the temperature is higher, or the partial pressure is lower. These observations also align with studies of SiO₂-containing refractory brick samples exposed to the same atmospheres and temperature conditions [12].

One of the main challenges that refractory producers and OEMs are now facing is understanding the possible impacts of changes in operating conditions that increase the hydrogen content. However, recent laboratory studies provided an insight into the reduction mechanism of silica containing refractories [11]: First, the glassy phase, prone to react with hydrogen, forms SiO gas; second, SiO₂ is reduced in the mullite phase; and third, secondary alumina forms over time when an Al₂O₃/SiO₂ ratio higher than 2.6 is reached (Figure 2).

The conclusions from these studies were substantiated by postmortem analyses performed on refractory bricks taken from state-of-the-art DRI units after years in operation. A representative microstructure of a sample recovered from the reduction zone showed that the glassy phase had been attacked by the reducing gas mix on the outer most part of the hot face (Figure 3 delineated by red dots), while the mullite network remained intact and was not affected by reduction (Figure 3 highlighted by red arrows).

Once the high-grade DRI has been produced, it is delivered to the steel shop in either pellet or briquette form, cold or hot, and charged into the EAF through its roof. In general, use of DRI in the EAF offers advantages, such as controlling residual elements like Cu or Mo, and tends to reduce nitrogen levels in crude steel. This enables the production of high-quality steel grades, which are currently produced mainly through the BF-BOF route. However, its use presents many challenges for the furnace operation, including higher electrical energy consumption, higher slag volumes, and longer tap-to-tap times compared to a standard EAF charged with 100% scrap. The main reason for these changes is related to the DRI chemical and physical properties. Two of the most important parameters affecting the furnace operation are the degree of metallisation, defined as the level of metallic iron in the pellet, and the gangue content, which is mainly formed from acidic oxides such as SiO₂ and Al₂O₃ (Table III).

The expected variances in EAF process conditions may directly affect the refractory consumption in the steel shop. For example, an increased SiO₂ content in the DRI would require more lime to reach the optimal basicity, while also necessitating more electrical energy to melt the additional slag formers. Additional studies have shown that if the EAF charging mix contains more than 20% DRI, there may be a negative impact on productivity, refractory lifetime, power-on time, consumables, and energy consumption [14]. Furthermore, the need for high-performance refractory lining grades and maintenance mixes may also increase [15].

Usually, for cases of increased DRI charge ratios in the EAF, some basic requirements must be fulfilled:

• Focus on health and safety in the work environment for refractory personnel.
• Stable, balanced lining wear behaviour during the campaign, targeting an optimum refractory consumption.
• Minimum downtimes due to maintenance.
• Sustainable refractory lining concepts, with a reduced product carbon footprint.
• Circular economy concepts that avoid landfilling.

To support customers in these and other areas, RHI Magnesita offers a range of refractory solutions for the EAF (Table IV).