The average annual temperature of the earth is rising since the industrial revolution. This is mainly due to the burning of the fossil fuels which increase the emissions of carbon di-oxide (CO2) in the atmosphere. Prior to the industrial revolution, 280 ppm (0.028 %) of the atmospheric air consisted of CO2, and this has increased to around 413 ppm (0.0413 %) in the year early 2019. Fig 1 shows global rise in the annual temperature and the concentration of CO2 on the earth during last 800,000 years. The data of atmospheric CO2 is provided by the U.S. National Oceanic Atmospheric Administration (NOAA). Since no direct measurements exist, the respective information has been derived from ice cores through the European Project for Ice Coring in Antarctica (EPICA).
Fig 1 Occurrence of global warming
Global warming, in fact, is the result of ‘too much of a good thing’. Without the atmosphere, the surface of the earth would be all but frozen. As sunlight enters the atmosphere, it is absorbed by oceans and continents, which warm up. Much of the heat is then radiated back toward space in the form of energy-rich infra-red light. This is where ‘greenhouse gases’ come into play. These gases consisting mainly of water vapour, CO2, and methane interact with the infra-red light and keep it from leaving the atmosphere as it heads for space. As a consequence, the ‘good thing’ happens and the atmosphere retains the heat. It is just that too much of the warming effect has negative effects of making the atmosphere too warm.
Fig 1 shows that the amount of CO2 in the atmosphere has risen from 280 ppm to 413 ppm since the industrial revolution. Carbon dating shows that this increase is connected to the burning of fossil fuels (coal, oil, and natural gas). During the same period, average global temperatures have been reported to have risen by 1 deg C. While 1 deg C does not seem to be high, it is believed that any further increase can have serious consequences such as disappearing of the sea ice, receding of the glaciers resulting into a rise in sea levels, which is presently measured at 3.3 millimetres per year on an average. For the avoidance of the ill effects of the climate chance, the global warming is required to be kept below the 2 deg C.
Iron and steel industry is the single largest sector in terms of total global fossil and industrial emissions, making up around 7 % to 9 % of greenhouse gas (GHG) emissions. It is the largest industrial emitter and at present responsible for around 8 % of global final energy demand. Hence, it is a prime focus for the governments. On the other hand, steel is vital to modern economies and so the global demand for steel is expected to grow to meet rising social and economic welfare needs. It is also a critical input for the clean energy transition. The generation and use of electricity depend in part on the ferro-magnetic properties of steel and its alloys. Steel is a key input material for wind turbines, transmission and distribution infrastructure, hydro-power and nuclear power plants, among other critical energy sector assets.
While being a facilitator of the clean energy transition, steel is also a large contributor to the present challenge the world face in meeting the climate goals. Direct CO2 emissions from the sector are around 2.6 giga tons of carbon dioxide (Gt CO2) per year, or around a quarter of industrial CO2 emissions, owing to its large dependence on coal and coke as fuels and reducing agents. A further 1.1 Gt CO2 of emissions is attributable to the use of its off-gases, along with other fuels, to generate the electricity and imported heat it consumes.
The high reliance on coal in the present primary steel production, long-lived capital assets, and the exposure of the sector to international trade and competitiveness make this transition towards near-zero emissions of CO2 challenging. It is for these reasons that the sector is sometimes referred to as among those which are ‘hard to abate’.
Meeting of the demand of the iron and steel products presents challenges for the iron and steel sector as it seeks to plot a more sustainable pathway while remaining competitive. Hence, iron and steel producers have a major responsibility to reduce energy consumption and greenhouse gas emissions, develop more sustainable products and enhance their competitiveness through innovation, low-carbon technology deployment, and resource efficiency.
Recent studies estimate that the global steel industry can find around 14 % of steel organizations’ potential value is at risk if they are unable to decrease their environmental impact. Hence, de-carbonization is to be a top priority for remaining economically competitive and retaining the industry’s permit to operate. Moreover, long investment cycles of 10 years to 15 years, multi-billion financing needs, and limited supplier capacities make this issue even more relevant and lock in significant lead times for addressing the de-carbonization challenge.
The iron and steel industry has recognized that long term solutions are needed to tackle the CO2 emissions produced during the production of steel. As a result, the steel industry has been highly proactive in improving energy consumption and reducing the CO2 emissions. Improvements in energy efficiency have led to reductions of around 50 % in energy needed to produce a ton of crude steel since 1975 in most of the top steel producing countries. Further improvements in energy efficiency are being done by making the maximum possible use of the state-of-the-art technologies.
Green steelmaking consists of the use of those processes which result into reduction in CO2 emissions. Development work for the green steelmaking processes is being done in European Union, USA, Canada, Brazil, Japan, South Korea, Australia, and China. For the development of the technologies for the green steelmaking, five key directions are being explored. These directions are (i) technologies involving coal usage, (ii) technologies involving use of hydrogen, (iii) technologies involving electrons, (iv) technologies involving use of biomass, and (v) technologies involving carbon capture, use, and / or storage (CCUS). The pathways to the break-through technologies for cutting the CO2 emissions from the ore-based steel production routes are shown in Fig 2.
Fig 2 Pathways to breakthrough technologies for green steelmaking
In European Union the breakthrough technologies are being developed under the ULCOS (Ultra-Low CO2 Steelmaking) programme. Under this programme, development work is being carried out for (i) ‘top gas recycling blast furnace’(TGR-BF) with CO2 capture, use, and / or storage (CCUS), (ii) HIsarna process with CCUS involving smelting reduction, (iii) ULCORED with CCUS which involves a new direct reduction (DR) concept, and (iv) electrolysis. Besides these ULCOS is also working on the use of carbon from sustainable biomass as well as hydrogen based steelmaking.
In USA, the development work is being carried out with ‘public private partnership’ between American Iron and Steel Institute (AISI) and the US Department of Energy (DOE), and Office of Industrial Technology. Two projects represent significant steps. These projects are (i) suspension hydrogen reduction of iron oxide concentrate, and (ii) molten oxide electrolysis (MOE). In the nearer term, development work is being done by the AISI members for ‘the paired straight hearth furnace’, a coal based DRI and molten metal process for long range replacement of blast furnaces and coke ovens.
In Japan the development work is carried out under COURSE50 programme involving six steel and engineering organizations, the Japan Iron and Steel Federation, and New Energy and Industrial Technology Development Organization. The research and development goals of the programme are (i) reduction of CO2 emissions from the blast furnace iron ore reduction with other reducing agents (hydrogen), (ii) reforming coke oven gas aiming at enhancing the hydrogen content by utilizing waste heat, (iii) high-strength and high reactivity coke for reduction with hydrogen. The development work is also being carried out towards capture of CO2 from blast furnace gas involving (i) chemical and physical absorption to capture, separate and recover CO2, and (ii) reduction of energy requirement for capture, separation, and recovery using waste heat from the steel plant.
In South Korea the development work is carried out involving POSCO, RIST, POSLAB, and POSTECH. Three promising routes of CO2 break-through solutions have been identified. These are (i) carbon lean steelmaking consisting of carbon lean FINEX process, and pre-reduction and heat recovery of hot sinter, (ii) carbon capture and storage of steelmaking by CO2 absorption using ammonia solution, and CO2 sequestration in ocean gas field, and (iii) hydrogen steelmaking by iron ore reduction in FINEX process using hydrogen-enriched syngas, and hydrogen enriched blast furnace process.
Emerging technologies for the reduction or elimination of the carbon emissions from the steelmaking process can be divided into two distinct categories namely (i) carbon capture, use, and / or storage (CCUS), and (ii) alternative reduction of iron ore. CCUS employs different methods to capture CO2 emissions. It either stores them (for example, in geological formations such as exhausted undersea gas reservoirs) or processes the emissions for onward utilization. Alone, CCUS cannot achieve carbon neutrality. But it can result into a negative CO2 balance if fossil fuels used in the steelmaking process are replaced by the biomass.
The second type of potential technologies involves replacement of coke or natural gas with alternative reducing agents for the iron ore. These include hydrogen and direct electric current. The advantage of these technologies is that theoretically they can make steel production fully green. However, most of them likely need even more time and funds for setting up as compared to CCUS.
The most promising of the new CCUS and alternative reduction technologies as well as the technology of hydrogen based direct reduction are discussed below.
Technologies with CCUS
In these technologies, CO2 which is emitted during process of operation is separated from other gases and is captured. The captured CO2 is then either transported through a pipeline or shipped to an onshore or offshore storage location or used. Processes for CCUS include post / pre-combustion capture, compression, transport, and store / use. Fig 3 shows CCUS scheme for the simplified blast furnace – basic oxygen furnace (BF-BOF) steelmaking route.
Fig 3 CCUS scheme for simplified BF-BOF route
The main advantage is that CCUS systems can be quite easily integrated into existing conventional brown field plants. And as the technology is not specific to steelmaking, other industries can also share development and infrastructure costs. Further, future operating costs are largely predictable.
The main disadvantage is that CCUS is not fully carbon neutral, as the carbon capture process alone captures only around 90 % of CO2. Also, there are some other challenges. Public acceptance of carbon storage is not certain which puts the first movers into a disadvantageous position. Further presently, excepting minor onshore storage locations, the sea offers the only suitable large storage location, and this necessitates considerable transportation efforts. In addition, utilization of emissions is also to ensure that there is no carbon release at a later stage for the process to be carbon neutral. Also, CCUS equipment increases maintenance burdens and shutdown times with a significant impact on the operating costs.
There are some pilot projects which have been taken up for the processing of emissions such as CO2 to make synthetic fuel. But this is at present not carbon neutral as CO2 is emitted at a later stage.
Biomass based ironmaking with CCUS
The basic idea behind these technologies is that carbon-neutral biomass partially replaces fossil fuels in pre-processing or as an iron ore reducing agent. The examples are carbon-rich ‘chars’ made from raw biomass (raw algae, grass, wood etc.) are used to produce a substitute coke, or biogas is injected into a shaft furnace instead of natural gas. Processes based on these technologies include pyrolysis and hydrothermal carbonization. CCUS systems take care of any remaining carbon emissions.
Biomass alone can cut upto 40 % to 60 % of CO2 emissions and in combination with CCUS can achieve carbon-neutral steelmaking. In the shorter term, biomass is an instant partial replacement for fossil fuels, allowing quick-win emission reductions at existing plants. CO2 from emissions can also be recycled using CCUS to produce fresh biomass.
However, cultivation of biomass is difficult. Environmentally, it can lead to deforestation, pollution, and reduced biodiversity, and socially it affects the food prices and agricultural land use. Hence, political and social acceptance has a high risk. In addition, biomass has a lower calorific value than fossil fuels, limiting its use in large blast furnaces or resulting into lowering efficiencies. Further, due to its high water content, it can also be too heavy for use in large blast furnaces.
A study on the use of by the Swedish research group SWEREA at an SSAB steel plant in Lulea has identified potential for a 28 % reduction in CO2 emissions with the biomass based ironmaking.
Hydrogen based shaft furnace for direct reduced iron
In the process, instead of a carbon reducing agent such as reformed natural gas, hydrogen is used for reducing iron ore pellets to ‘direct reduced iron’ (DRI or sponge iron). The reaction takes place in a shaft furnace. The produced DRI is then fed into an electric arc furnace and by adding carbon; it is turned into steel by further processing. DRI can also be fed into a blast furnace in the form of ‘hot briquetted iron’ (HBI). This significantly increases the blast furnace efficiency and reduces the coke consumption. The most common similar process technologies are the Midrex and Energiron processes.
In hydrogen based reduction, the iron ore is reduced through a gas-solid reaction, similar to the DRI route of production. The only differentiating factor is that the reducing agent is pure hydrogen instead of carbon mono oxide gas, syngas, or coke. The reduction of iron ore by hydrogen occurs in two or three stages. For temperatures higher than 570 deg C, hematite (Fe2O3) ore is first transformed into magnetite (Fe3O4), then into wustite (FexO), and finally into metallic iron whereas at temperatures below 570 deg C, magnetite is directly transformed into iron since wustite is not thermo-dynamically stable.
The reduction reactions involved in the reduction of iron ore by hydrogen are represented by the equations (i) 3 Fe2O3 + H2 = 2 Fe3O4 + H2O, (ii) x Fe3O4 + (4x-3) H2 = 3 FexO + (4x-3) H2O, and (iii) FexO + H2 = x Fe + H2O where x is equal to 0.95. As indicated by these reactions, iron ore reduction with hydrogen releases harmless water vapours (H2O) instead of the greenhouse gas CO2. The overall reaction for the reduction of hematite ore with H2 is Fe2O3 + 3H2 = 2Fe + 3H2O which is endothermic reaction with a heat of reaction, delta H at 298 deg C = 95.8 kJ/mol, which is negative for the energy balance of the process and demands an addition of energy with the injected reduction gas / gas mixture. The focus in developing the production line is optimization based on the reduction temperature, kinetics of the reaction, pellet composition, and technology for the preheating of the reduction gas.
The stoichiometric consumption of H2 for reducing hematite ore (Fe2O3) is 54 kg per ton of iron. Hence, a 1 million ton per year steel plant needs a hydrogen plant which has a capacity as much as 70,000 cum / hour of hydrogen at standard temperature and pressure (STP). With hydrogen as reduction gas, it is important to anticipate the change in the behaviour of the reactor as compared to the reactor with hydrogen-carbon mono oxide mixtures as the reduction gas. Several factors can interact in different ways, such as kinetics, thermodynamics, heat transfer, and gas flow.
The process makes the whole primary steelmaking route carbon neutral and fossil fuel-free in case green electricity is completely used for the process. Other advantage for the process is the high production flexibility. The process is easy to start and stop, and the ability of the technology to use smaller units enables greater scalability. In addition, the ability to feed DRI as HBI into a blast furnace – basic oxygen furnace steelmaking system means existing conventional brown field plants can be used while shaft furnace / EAF production is ramped up.
The process still needs iron ore pellets, and producing them can cause significant emissions depending on the heat source of the pellet plant. Supplying the necessary amount of hydrogen is also a problem and efficient large-scale electrolyzers need to be developed. In addition, as the process relies on vast amounts of cheap green energy, steel producing countries are to import hydrogen or pre-processed iron, hurting their value chains, if they fail to significantly ramp up their own green energy production. There is also uncertainty around future operating costs which are related to the prices of hydrogen and electricity. Fig 4 shows hydrogen based shaft furnace for direct reduced iron.
Fig 4 Hydrogen based shaft furnace for direct reduced iron
HYBRIT process uses hydrogen based shaft furnace for the DRI production. HYBRIT is short for ‘HYdrogen BReakthrough Ironmaking Technology’. On 4 April 2016, the three Swedish companies—SSAB, LKAB, and Vattenfall AB launched a project aimed at investigating the feasibility of a H2 based DRI production process, with CO2 emission-free electricity as the primary energy source. A joint venture company was formed, HYBRIT Development AB, with the three companies being owners. This has given full access to top competence in the entire value chain from energy production, mining, ore beneficiation and pellet production, direct reduction, melting, and the production of crude steel. A pre-feasibility study on H2 based direct reduction was carried out in 2017. The study concluded that the proposed process route is technically feasible and, in view of future trends on costs for CO2 emissions and electricity, it is also economically attractive for conditions in northern Sweden / Finland.
HYBRIT process replaces coal with hydrogen for the direct reduction of iron, combined with an electric arc furnace. The process is almost completely fossil-fuel free, and result into substantial reduction in its greenhouse gas emissions. The process is among several initiatives which use a hydrogen-direct reduction / electric arc furnace setup, combining the direct reduction of iron ore by use of hydrogen with an electric arc furnace for further processing into steel. The product from the hydrogen-direct reduction process is DRI or sponge iron, which is fed into an electric arc furnace, blended with suitable shares of scrap, and further processed into steel.
The principle flowsheet of the HYBRIT production process is shown in Fig 5. The main characteristics of the process are (i) non fossil fuels are used in pellet production, (ii) hydrogen is produced with electrolysis using fossil-free electricity, (iii) storage of hydrogen in a specially designed unit is used as a buffer to the grid, (iv) a shaft furnace is used for iron ore reduction, (v) tailor-made pellets are used as iron ore feed, (vi) the reduction gas / gas mixture is preheated before injection into the shaft, (vii) the product can either be DRI or HBI free of carbon or carburized, and (viii) the DRI / HBI is melted together with recycled scrap in an electric arc furnace.
Fig 5 Principle flowsheet of the HYBRIT production process
The use of hydrogen produced by water electrolysis using fossil-free electricity to reduce iron ore pellets in a shaft furnace is the main alternative chosen for the HYBRIT initiative. Under this initiative, a conversion to a fossil-free value chain from the mine to the finished steel includes many issues to be developed where also local market and geographical conditions are taken into consideration. Sweden has a unique situation with overcapacity in electrical power in the northern part of the country, vicinity to iron ore mines, good access of biomass and steelworks, and a strong network between industry, research institutes, and universities.
The HYBRIT process falls within a category of technological concepts which are substantially closer to the commercial deployment. It is based on the use of hydrogen as a reducing agent, with the hydrogen being produced through electrolysis based on renewable electricity. From an environmental standpoint, the most important advantage of this is that the exhaust from this process is water (H2O) instead of CO2, with a consequent reduction in GHG emissions. As with conventional DRI steelmaking, the iron produced using hydrogen-based DRI route can be further processed into steel using commercially available electric arc furnace technology. The hydrogen production and electric arc furnace steelmaking steps can be made carbon-free if the electric power and hydrogen are produced using renewable sources such as PV (photovoltaic) solar / wind / hydro-powered electrolysis, photo-chemical hydrogen production, or solar-thermal water splitting.
Hydrogen based fluidized bed process for direct reduced iron
As with the shaft furnace version, this technology uses hydrogen to reduce iron ore and produce DRI to feed into an electric arc furnace. The differences are that reduction occurs in a fluidized bed rather than a furnace, and finely processed iron ore fines / concentrates are used instead of pellets. Fluidized beds are reactor chambers which can continuously mix solid feed stocks with a gas to produce a solid. The similar processes are FINEX and Circored.
The use of fines over iron pellets has the advantage of removing the need to pelletize and thus cutting of costs and the high CO2 emissions involved in the process. In addition, fluidized bed reactors have fewer internal sticking problems than shaft furnaces, achieving higher metallization (around 90 % to 95 %).
The process shares the same issues regarding the hydrogen supply, electrolyzer and operating cost as the shaft furnace method. The electricity supply is also to be 100 % green to achieve carbon neutrality. In addition, the use of fluidized bed reactors in steelmaking is less developed than shaft furnaces, and hence needs higher investment. Fig 6 shows the hydrogen based fluidized bed process for direct reduced iron.
Fig 6 Hydrogen based fluidized bed process for direct reduced iron
Hydrogen based Fine-Ore Reduction (or HYFOR for short) is the world’s first direct-reduction process for iron-ore concentrates from ore beneficiation which does not need any pre-processing of the material like sintering or pelletizing. This reduces CAPEX and OPEX costs. The process is capable of processing a wide variety of ores, e.g. hematite and magnetite.
HYFOR process has been developed by Primetals Technologies. The new technology can be applied to all types of beneficiated ore. It works with particle sizes of less than 0.15 mm for 100 % of the feedstock, while allowing a maximum grain size of 0.5 mm. Because of the large particle surface, the process achieves high reduction rates at low temperatures and pressures.
As a primary reducing agent, the new process uses hydrogen. Hydrogen can be from renewable energy or alternatively hydrogen rich gases from other gas sources like natural gas pyrolysis or conventional steam reformers. As yet another alternative, HYFOR can run on hydrogen rich waste gases. Depending on the source of the hydrogen, this leads to a low or even zero CO2 emission for the resulting DRI.
A pilot plant for testing purposes has been commissioned in April 2021 voestalpine Stahl Donawitz, Austria. The plant features a modular design with a rated capacity of 250,000 tons per module per year, making it suitable for all sizes of steel plants. The purpose of the pilot plant is to provide practical evidence for this breakthrough process and to serve as a testing facility, collecting enough data to set up an industrial-scale plant at a later stage.
First tests have been successfully executed in April 2021 and May 2021. The scale of one test run is in the range of processing of 800 kg iron ore. The HYFOR pilot plant is going to be operated for at least 2 years in multiple campaigns to test various ore types and to evaluate the optimal process parameters for the next scale up step. Smooth operation assumed, a hot briquetting unit is going to be added to verify the hot briquetting step as well as the HBI quality to be expected from the HYFOR technology.
HYFOR process drastically reduces CO2 emissions as well as helps producers to effectively deal with the challenge of reduced iron-ore quality, which has become more acute as of late, resulting in an increased need to beneficiate the ores. Rising demand for iron-ore pellets for blast furnaces and direct-reduction plants has led to higher prices for iron ore, especially pellet premium. With HYFOR process, it is possible to use pellet-feed fine ore directly and benefit from the rising global supply of ultra fines.
The HYFOR pilot plant at voestalpine Donawitz consists of three parts namely (i) a preheating-oxidation unit, (ii) a gas-treatment plant, and (iii) the core which is the new and unique reduction unit. In the preheating-oxidation unit, fine-ore concentrate is heated to around 900 deg C and fed to the reduction unit. The reduction gas is 100 % hydrogen which is supplied from a gas supplier located outside the plant boundary. A waste heat recovery system which harnesses heat from the off-gas ensures optimal energy use and a dry de-dusting system takes care of dust emissions from the processes. The hot direct reduced iron (HDRI) leaves the reduction unit at a temperature of around 600 deg C before it is cooled down and discharged from the HYFOR pilot plant.
The hot direct reduced iron which leaves the reduction unit at a temperature of around 600 deg C can be subsequently direct transported and fed into an electric arc furnace or used to produce hot briquetted iron. Hot briquetted Iron is for supply to the market. The next step is going to be the addition of a hot briquetting testing facility to test the hot briquetted iron characteristics.
The aim of the HYFOR pilot plant is to verify this break-through process and to serve as a testing facility to provide the data basis for upscaling the plant size to an industrial-scale prototype plant as the next development step.
Suspension based ironmaking
Suspension based ironmaking is also known as ‘flash ironmaking technology’. This process begins with the ultrafine grinding of low grade iron ore to produce iron ore concentrate. The iron ore is to be ground to particles of less than 100 micrometers in diameter. The ultrafines are then reduced using hydrogen in a high-temperature ‘flash’ reactor for just a few seconds, directly producing iron once carbon is added. The iron ore concentrate can also be pre-reduced at a lower temperature in a separate reactor before being added to the flash reactor. Fig 7 shows principle of suspension based ironmaking.
Fig 7 Suspension based ironmaking
Flash ironmaking transformational technology is being developed by a consortium of organizations and institutes in USA under the financial support of American Iron and Steel Institute. This technology is based on the direct gaseous reduction of iron oxide concentrate in a flash reduction process. The technology has the potential to reduce energy consumption by 32 % to 57 % and lower CO2 emissions by 61 % to 96 % compared with the average present BF based operation. This technology is suitable for an industrial operation which converts iron ore concentrate (less than 100 microns) to steel without further treatment.
This technology produces iron while bypassing pelletization or sintering as well as coke making steps. Further, the process is intensive due to the fact that the fine particles of the concentrate are reduced at a fast rate at 1,150 deg C to 1,350 deg C. Thus, the needed residence times in this process is of the order of seconds rather than the minutes and hours required for pellets and even iron ore fines. 90 % to 99 % reductions take place in 2 seconds to 7 seconds at 1,200 deg C to 1,500 deg C. The energy requirement of the process with H2 as reducing gas is 5.7 GJ (1,360 Mcal) /ton of liquid iron.
The direct reduction of iron ore to steel in one reactor eliminates the need for ironmaking and sintering or pelletization. It has considerable cost and emission benefits. It also produces ‘cleaner’ steel as the high temperatures and fast reaction times ensure fewer impurities.
With H2 as reducing gas, CO2 emissions are 0.04 tons of CO2 per ton of liquid iron. These emissions are 2.5 % of the emissions of the BF route of ironmaking. The flash iron process is performed at a high enough temperature so that individual particles have enough energy to close down the pores created from oxygen removal. Hence, the individual particles are far less susceptible to catching fire from rapid oxidation. The University of Utah performed research on small samples of powder and determined they are not pyrophoric.
The process is to be applied to the production of iron as a feed to the steelmaking process or a part of a continuous direct steelmaking process. Justified by experimental data obtained during the previous phase of the project, scale-up development work is presently ongoing at the University of Utah. Testing in a laboratory flash furnace has resulted in the establishment of a kinetics database over wide ranges of operating conditions and a complete design of a more advanced bench reactor. With an objective to develop an industrially viable flash iron technology, a comprehensive bench scale testing campaign is planned. The deliverables from this phase of the project are expected to be the determination of the scalability of the process, substantive process simulation results, and fundamental engineering data leading to the design and construction of an industrial pilot plant. The flowsheet of the flash ironmaking technology is given at Fig 8.
Fig 8 Flowsheet of flash ironmaking Technology
Plasma direct steel production
In the plasma direct steel production process iron ore, raw or in the form of fines or pellets, is reduced using hydrogen plasma in a plasma steelmaking reactor. At the same time, carbon is added to the reactor to produce steel. Hydrogen plasma is hydrogen gas which has been heated or electrically charged to separate, or ionize, it into its constituent particles. The process can use either thermal plasma (produced by directly heating hydrogen) or non-thermal plasma (produced by passing a direct current or microwaves through the hydrogen).
The process removes the need for pre-processing of iron ore and allows for lower reactor temperatures. It is also highly integrated, with some methods (for example, hydrogen plasma smelting reduction) needing only a single step. This makes it commercially attractive. The technology has the potential to reduce costs considerably. It also offers higher product quality and better production flexibility.
The technology is at a very early stage of development, with an optimal process and full reactor design yet to be developed. Its commercial feasibility is also still to be proven. As part of its Sustainable Steel (SuSteel) project, the Austrian steelmaker voestalpine has built a small pilot hydrogen plasma reduction reactor at its Donawitz site. Plasma direct steel production process is shown in Fig 9.
Fig 9 Plasma direct steel production process
There are two types of electrolytic processes. These are (i) electrolysis, and (ii) electro-winning. These two process variants are called as ULCOWIN and ULCOLYSIS under the ULCOS programme. ULCOWIN process operates slightly above 100 deg C in a water alkaline solution populated by small grains of ore. In this process iron ore is ground into an ultrafine concentrate, leached and then reduced in an electrolyzer at around 110 deg C. The resultant iron plates are fed into an electric arc furnace, which turns it into steel. ULCOLYSIS operates at steelmaking temperature (around 1,550 deg C) with a molten salt electrolyte made of a slag (pyro-electrolysis). This process transforms iron ore at into liquid steel using electricity as a reducing agent. Fig 10 shows electrolytic processes for steel production.
Fig 10 Electrolytic processes for steel production
The electrolysis process has been developed from scratch within the ULCOS programme and, hence, is still operating at laboratory scale. Although it holds the promise of zero emissions, if it has access to green electricity, time is needed to scale it up to a commercial size (10 to 20 years). ULCOWIN process consists of alkaline electrolysis of iron ore. Electrolysis is normally used to produce metals other than steel and needs large amounts of electricity. The process is to depend on a CO2-lean electricity source such as renewable power, hydro power, or nuclear power. ULCOLYSIS is the molten oxide electrolysis. Molten oxide electrolysis works by passing an electric current through molten slag fed with iron oxide. The iron oxide breaks down into liquid iron and oxygen gas. No CO2 is produced. Process emissions are further reduced with a CO2-lean electricity source.
Since the electrolytic processes skip the upstream stages needed in other production routes, such as producing coke or H2 as reducing agents, these processes have the potential to become the most energy-efficient steelmaking technologies, especially electrolysis. They also promise to significantly lower CAPEX as, in the case of electrolysis, only very few equipments are needed. The process is also relatively inflexible compared to the hydrogen direct reduction process since it cannot be stopped easily.