Iron and steel production 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. At the same time, steel is the primary material component for modern industrial societies. Further increase in the consumption of steel is going to take place because of the need to expand and improve infrastructure and to increase global standards of living at a pace sufficient to reach a satisfactory level. Hence, there is a challenge lies to find a process of making steel which allow for societal use of steel as a material, while at the same time avoiding the negative environmental impacts of its production. In the context of reducing the CO2 (carbon dioxide) emissions of steelmaking, the hydrogen (H2) based steelmaking route is presently receiving much attention since it provides an attractive choice.
Reduction of iron ore with H2 is well known since the reduction reactions with H2 also take place both in the production of hot metal in a blast furnace (BF) as well as in the production of direct reduced iron (DRI) / hot briquetted iron (HBI) in a shaft furnace. In the BF ironmaking sources of H2 are humidified hot air blast and injected pulverized coal. In case of the steelmaking route based on DR-EAF (direct reduction – electric arc furnace), it has always been historically characterized by the use of H2, which is normally generated from natural gas (NG) through catalytic reformers. Since the hydrocarbon source is NG, the H2 produced can be of variable concentration, and mixed with carbon monoxide (CO), depending on the oxidant ratio being used.
H2 reduction of iron ore has been studied for many decades. There were pioneering investigations in Sweden by Wiberg and Edstrom already in the 1950s, which have been followed by a number of investigations describing the kinetics of the process as well as the reactivity of the produced product. The first commercial scale H2 direct reduction of iron ore (H2-DRI) plant based on fluidized bed technology, Circored, was built by Cliffs and Associates Ltd. at Point Lisas Industrial Complex in Trinidad in 1998. The plant had a designed of 500,000 tons per annum of HBI. The plant did not succeed commercially and was closed down in 2016.
In the present day global scenario, several activities are going on to find a process which utilizes reduction of iron ore with H2 so as to reduce emissions of CO2 greatly, if not totally eliminate it. There are numerous issues which are to be overcome before the success is achieved that can provide stiff competition to the present processes being used for ironmaking and steelmaking. Some of the main issues which are to be overcome are described in subsequent paragraphs.
One of the important issues is to find cost effective method for the production of bulk H2 which uses power produced from processes with lean CO2 emissions. The sources of renewable energy are solar energy, wind energy, water energy, marine energy, and geothermal energy. In the nuclear enegy also fossil free fuel is used. Out of all the renewable energies, the two most dependable sources of renewable energy are photovoltaic (PV) solar energy, and wind turbines. However, both of these technologies are characterized by fluctuating electrical power provision due to the volatile nature of solar radiation and the wind, so that there are times when supply of electricity is scare and times when it is abundant. To integrate a high proportion of wind and solar power into the energy system, a large-scale storage solution is needed to compensate for the temporal imbalances between production and demand.
The second issue is the storing of the bulk H2 (high-pressure gaseous storage or cryogenic liquefied storage). H2 storage gives benefits from an electricity sourcing / pricing perspective, but it is not as such critical for the process concept. Today, the most cost efficient alternative for the H2 storage is underground pressurized storage, where there is storing of H2 in the underground salt formations. In the present day scenario, this is the only technology for H2 storage which has been tested on an industrial scale. Other solutions which are attracting a lot of interest nowadays include utilizing natural gas pipelines and conversion to ammonia or hydrocarbons as intermediate H2 storage. Initial evaluation of the Lined Rock Cavern (LRC) technology used presently in Sweden for natural gas is considered to be promising. Other alternative methods for H2 storage under development are storage in metal hydrides and in porous materials.
The third issue is that the iron produced through reduction by H2 contains no carbon (C) unlike hot metal and DRI / HBI which contain C in varying percentage. C is needed in steelmaking for the C boil. Also steel derives its properties because of its C content and hence varying percent of C is present in different grades of steels. However, real technical difficulties in this regards are not anticipated since some additional C can be added in the EAF.
The fourth issue is CO2 emissions occurring during mining, processing, and beneficiation of the iron ore and during the production of pellets as well as the transportations of these materials. The processes utilizing H2 reduction preferably needs pellets as the feed material. In the respect, there is possibility available to eliminate some of the CO2 emissions by the use of renewable energy sources.
The first process is the large-scale production of H2, which is presently being achieved by steam reforming of methane. This option can be retained and even optimized for H2-based ironmaking, e.g., by targeting a 97 % to 98 % purity of H2 instead of the normal 99.9 % plus purity. However, since based on a fossil resources, the performance in terms of CO2 mitigation remains overall average, unless a CO2 capture unit is added, which represents a strategy different from the one pursued presently. The other preferable option is to produce H2 by water electrolysis. H2 production needs to be fossil-free, and thus, the appropriate production method is water electrolysis with CO2-lean electricity, i.e., renewable or nuclear electric power. The challenge is to achieve massive production of H2 in acceptable economic conditions. Although water electrolysis is a well-known technology, some developments are needed to reach the target of massive amounts of H2 which is CO2-lean and, above all, which is affordable for ironmaking. The size of the plant can be achieved by multiplying the electrolytic cells. New, improved technologies have also been identified, such as proton exchange membranes and high pressure or high temperature electrolysis.
The second process is producing iron by the direct reduction of iron ore in a shaft furnace which is operated with H2 only. It is the heart of the H2 ironmaking process. The shaft furnace is fed with pellets or lump ore at the top, which descend by gravity and encounter a rising flow of H2, fed laterally at mid-height of the reactor and exiting at the top. The reduction reactions take place in the upper section between the reducing gas outlet and inlet. The conversion to iron is completed at the level of the gas inlet. Below, a conical section can be used to cool the DRI, but preferably using H2 instead of methane (CH4). The rest of the gas circuit is much simpler than that in the conventional DRI process with reformed NG, with the top gas consisting of H2-H2O is cooled to condense water. The separated H2 is recycled, mixed with fresh H2 from the electrolysis plant, and reheated to the desired temperature (around 800 deg C to 900 deg C).
From the mathematical modeling of the reduction zone of a shaft furnace operated with 100 %. H2, it has been found that, due to the fast reduction kinetics with H2, complete metallization can be theoretically achieved faster than that with H2-CO mixture, resulting into reactors which are smaller than the present DR shafts.
The third process is the melting of the C-free DRI in an EAF to produce steel. A H2 based reduction process results in a zero carbon iron product, indicating that a fossil-free C source is needed to produce steel product of required chemistry in the steelmaking step.
In the best H2 based steelmaking route studied in ULCOS (ultra low CO2 steelmaking) project (Fig1), H2 is considered to be produced by water electrolysis using hydraulic or nuclear electricity. Iron ore is considered to be reduced to DRI by H2 in a shaft furnace, and C-free DRI is considered to be treated in an EAF to produce steel. This route shows promising performance regarding CO2 emissions which is less than 300 kg CO2 / ton of steel, including the CO2-cost of electricity with the emissions from the DR (direct reduction) furnace itself being almost zero. This represents 85 % cut in CO2 emissions as compared to the present around 1,850 kg CO2 / ton of steel of the BF-BOF (basic oxygen furnace) route. This new route thus is a more sustainable way for making steel. However, its future development is largely dependent on the emergence of a so-called H2 economy, when this gas becomes available in large quantities, at competitive cost, and with low CO2 emissions for its production.
Fig 1 Hydrogen based steelmaking route
In H2-based reduction, the iron ore is reduced through a gas-solid reaction, similar to the DRI routes. The only differentiating factor is that the reducing agent is pure H2 instead of CO gas, syngas, or coke. The reduction of iron ore by H2 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 thermodynamically stable.
The reduction reactions involved in the reduction of iron ore by H2 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 H2 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 requires a H2 plant which has a capacity as much as 70,000 cum / hour of H2 at standard temperature and pressure (STP). With H2 as reduction gas, it is important to anticipate the change in the behaviour of the reactor as compared to the reactor with H2-CO mixtures as the reduction gas (Fig 2). Several factors can interact in different ways, such as kinetics, thermodynamics, heat transfer, and gas flow. Regarding kinetics, laboratory studies on the reduction of iron oxides with CO, H2, and CO–H2 mixtures have clearly shown that, all else being equal, the kinetics with H2 are faster (up to 10 times) than those with CO. However most of the reaction features are very similar to that of the reduction by CO and many mechanisms are common to both of them. However, there are also significant differences.
Fig 2 Comparison of the reduction kinetics of hematite pellets with H2, CO, and H2-CO mixtures
The first major factor is the thermodynamics, which favour CO at low temperatures, as evidenced by the Chaudron diagram (Fig 3). The vertical blue arrows represent the driving force for the wustite-to-iron reduction, which increases with temperature with H2 and decreases with temperature with CO.
Fig 3 Chaudron phase diagram of the iron phase domain as a function of temperature and oxidizing power of the gas
The second factor is the heat of the reduction reactions as shown in the Tab 1. The hematite-to-magnetite reaction is less exothermic with H2 than with CO, the magnetite-to-wustite reaction is more endothermic, and chiefly, the wustite-to-iron reaction is endothermic with H2 and exothermic with CO. Overall, the balance is an endothermic reduction with H2 and an exothermic reduction with CO. As a result, the temperature and compositions in the shaft greatly change with the inlet gas composition. When leaving the gas injection zone, the temperature decreases due to methane cracking, but with a higher CO content, the bed is maintained at a higher temperature as a result of the exothermic heat of the reduction reactions, whereas the temperature is lower with more H2.
|Tab1 Heat values of reduction reactions|
|Reaction||Delta H, 800 deg C|
|3Fe2O3 +H2 + 2 Fe3O4 = H2O||– 6,020|
|3Fe2O3 + CO = 2 Fe3O4 + CO2||– 40,040|
|Fe3O4 + H2 = 3 FeO + H2O||46,640|
|Fe3O4 + CO = 3 FeO + CO2||18,000|
|FeO + H2 = Fe + H2O||16,410|
|FeO + CO = Fe + CO2||– 17,610|
|Note: A minus sign indicates an exothermic reaction|
Even if in all cases, more H2 than CO is overall utilized for the three reductions (a result of the kinetics), the latter effect, i.e., the reduction by CO in the central zone in case of CO gas, is decisive regarding the final metallization degree. Also, when using only H2 (both at the reducing gas inlet and at the bottom inlet) the colder central zone does not exist, the temperatures are more uniform radially, and the reduction, due to the efficient kinetics, goes to completion (100 % metallization).
As seen above, the reduction with H2 is endothermic, whereas it is exothermic with CO. On the other hand, thermodynamics are more favourable with H2 than with CO above 800 deg C. This makes the industrial operation different. With H2, the hot gas fed has to bring enough calories to heat and maintain the solid at a temperature sufficiently high for the reaction to occur. Operating with a gas flow rate higher than stoichiometry is hence necessary. Kinetics is also reported to be faster with H2. This in turn can modify the morphology of the final product (iron), which depends on a competition between diffusion and chemical reaction. In particular, the formation of whiskers seems a specific feature of the reduction by H2. Whiskers are iron grains protruding from the wustite phase and growing as fingers toward the exterior of the particles. The whiskers make the iron-iron contacts more frequent and can thus explain the phenomenon of sticking of the solid particles, sometimes experienced in industrial reactors operated with high H2 content. Another awkward phenomenon observed with the H2 reduction is the occurrence, at some temperatures, of a slowing down at the end of the reaction to reach the last percent of conversion degree.
In a shaft furnace with pure H2, there is no C source. Because of endothermic nature of the reactions, a large amount of heat is absorbed, and the inner temperature at the bulk material layer decreases rapidly. As a result, the reduction reactions which need to consume a high amount of heat causes deterioration of the gas utilization rate. The quantity of H2 as a heat carrier is to be increased to maintain the preferred productivity. As an example, when the pressure at the top is 0.4 MPa, the quantity of H2 with a temperature of 900 deg C is to be at least 2,600 cum per ton of DRI to meet the heat demand of shaft furnace reduction. If the H2 addition remains unchanged, the DRI output is one-third less than that of the present, leading to a great increase in production cost of the DRI.
The specific gravity of H2 is low, and the density of H2 is only 1/20 times that of CO. As a result, the entering H2 gas molecule rapidly escapes upward. In comparison with the path and direction of a mixed reducing gas, those of H2 in a furnace change so quickly that H2 cannot stay in the high-temperature zone at the bottom part of the shaft furnace to complete the task of reducing iron ore pellets. Theoretically, DRI products can also reach the designed index by maintaining the entering H2 with a pressure above 1 MPa and a temperature above 1,000 deg C.
Further, H2 is an extremely flammable and explosive substance, and the shaft furnace needs highly efficient and long-term stable production. If the shaft furnace system is allowed to work for a long time under the ultimate conditions of high temperature and high pressure, safety becomes an issue. In short, direct reduction rate and production efficiency are affected by several factors, such as H2 proportion, temperature, pressure, gas utilization rate, residence time of iron ore, heat transfer, mass transfer, and shaft furnace design.
The HYBRIT initiative
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 gives 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 H2 for the direct reduction (DR) of iron, combined with an EAF. The process is almost completely fossil-free, and result into substantial reduction in its greenhouse gas emissions. The process is among several initiatives which use an H2-DR / EAF setup, combining the direct reduction of iron ore by use of H2 with an EAF for further processing into steel. The product from the H2-DR process is DRI or sponge iron, which is fed into an EAF, blended with suitable shares of scrap, and further processed into steel.
The principle flowsheet of the HYBRIT production process is shown in Fig 4. The main characteristics of the process are (i) non fossil fuels are used in pellet production, (ii) H2 is produced with electrolysis using fossil-free electricity, (iii) storage of H2 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 C or carburized, and (viii) the DRI / HBI is melted together with recycled scrap in an EAF.
Fig 4 Principle flowsheet of the HYBRIT production process
The use of H2 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 H2 as a reduction agent, with the H2 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 H2-based DRI route can be further processed into steel using commercially available EAF technology. The H2 production and EAF steelmaking steps can be made C-free if the electric power and H2 are produced using renewable sources such as PV (photovoltaic) solar / wind / hydro-powered electrolysis, photo-chemical H2 production, or solar-thermal water splitting.
Flash ironmaking technology using hydrogen
A new transformational technology for alternate ironmaking is being developed by a consortium of organizations and institutes under the financial support of American Iron and Steel Institute in USA. This technology is based on the direct gaseous reduction of iron oxide concentrate in a flash reduction process. The novel ‘flash ironmaking technology (FIT) 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 reduces iron ore concentrate in a flash reactor with a suitable reductant gas such as H2 or natural gas, and possibly bio / coal gas or a combination thereof. It is the first flash ironmaking process. This technology is suitable for an industrial operation which converts iron ore concentrate (less than 100 microns) to metal without further treatment.
This transformative 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 required 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 % -99 % reductions take place in 2 seconds to 7 seconds at 1200 deg C to 1500 deg C. The residence time is a combination of speed of reaction due to temperature, size of the feed material and amount of excess gas / distance from equilibrium line. The energy requirement of the process with H2 as reduction gas is 5.7 GJ (1,360 Mcal) /ton of liquid iron. The Fe/FeO equilibrium diagram is shown in Fig 5.
Fig 5 Fe/FeO equilibrium diagram
The heating portion of the reactor is where the induction heating coil heats up the graphite susceptor. The susceptor heats up refractory wall by radiation. Both susceptor and refractory heat up the gas and particle by convection and radiation. After being heat up to the temperature, gas and particles enter the reaction zone, where good insulation is assumed so wall condition is set to be adiabatic. After the reaction zone, there is a cooling zone with cooling panel to cool gas and particles. Fig 6 shows the flowsheet of flash ironmaking technology.
Fig 6 Flowsheet of flash ironmaking Technology
In case of FIT with H2 as reducing gas, CO2 emissions are 0.04 tons of CO2 per ton of liquid iron. These emissions are 2.5 % f 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 uses gaseous reducing agents such as NG, H2, syngas or a combination thereof. It 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 currently 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 FIT, 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.