Carbon Neutral Steelmaking
Carbon Neutral Steelmaking
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 consumption of steel is going to take place because of the need to expand and improve infrastructure and increase global standards of living at a pace sufficient to reach a satisfactory level. Hence, there is a challenge lies in finding pathways which allow for societal use of steel as a material, while at the same time avoiding the negative environmental impacts of its production.
Traditional iron and steel production processes are associated with very high environmental GHG emissions. The present emissions from iron and steel plants are at the level of around 1.8 tons of CO2 (carbon dioxide) per ton of crude steel (tCO2/tCS). Although there have been significant ongoing efforts to reduce the GHG emissions from steelmaking over the last several decades, major technological breakthroughs are indeed still needed if the iron and steel sector is to keep up with the across-the-board emissions reductions needed under the Paris Agreement, which aims to limit the global temperature rise at the turn of this century to well below 2 deg C above pre-industrial levels, and states that efforts are to be directed towards a more ambitious target of only 1.5 deg C of temperature rise.
CO2 emissions from the iron and steel industry are amongst the most difficult to decrease, since carbon is used as a stoichiometric reducing agent in the production of iron and steel in most of the steel plants. This carbon ends up as a CO / CO2 mixture in the steel plant gases, which are combusted to generate heat, electricity, and more CO2. Strategies for carbon, capture and storage (CCS), for carbon, capture, and utilize (CCU), or for avoiding CO2 exist in iron and steel production, but are highly dependent on the availability of renewable electric power for the production of hydrogen (H2).
The present estimates of future steel demand vary widely with projected annual growth rate fluctuating between 1.4 % and 3.3 %. With this rate of growth, the projected steel demand by 2025 is as high as 2.4 billion tons. Partial decarbonization of this growing iron and steel industry can only be achieved through efficiency improvements and the integration of renewable electric power in conventional steelmaking routes, whereas, complete decarbonization needs new zero-carbon and / or negative emissions technologies. On the other hand, attempts to decarbonize the iron and steel production processes have not reached to a level of a large-scale industrial adoption, in spite of substantial on-going research and development (R&D) efforts which are going on presently. The feasibility and applicability of CCS in the context of steelmaking remain highly doubtful in the present scenario.
In the conventional blast furnace / basic oxygen furnace (BF / BOF) route, carbon (in the form of coke and coal) is used to drive the endothermic reduction reactions as well as for providing the high temperatures which are needed. A typical BF-BOF process produces 1.6 tCO2/tCS to 2.2 tCO2/tCS. Considerable regional differences in steel-related emission exists, with some steel producing countries having much higher CO2-emissions footprint compared to the OECD (Organization for Economic Co-operation and Development) countries. There are also significant differences between different steelmaking routes such as BF-BOF, DRI-EAF (direct reduced iron- electric arc furnace) and scrap-EAF. Through technological improvements, steel plants have steadily reduced their fuel consumption rate over the last five decades to the point that the BF-BOF route can now be considered to be largely optimized. The most efficient BFs in the world are now operating within around 5 % above the theoretical minimum in terms of their CO2-emissions.
The transition of heavy industry in general and iron and steel industry in particular, towards decarbonization has, up until very recently, not received much attention from the energy and climate research point of view. However, with industrial emissions almost as high as those from power generation, mitigation measures for deep decarbonization are to be pursued with vigour in the iron and steel industry. The deep decarbonization strategies for iron and steel industry can follow largely two distinct routes as given below.
- Changing of the existing processes which are to be carried out to remove inherent dependencies on fossil fuels. This route tends to rely heavily on the use of electric power, either directly or through the H2 produced by electrolysis.
- Keeping of the existing processes as such and addressing emissions through a combination of CCS and CCU application and a shift to renewable sources for process energy.
The categorization of these two routes as applied to iron and steel industry has several characteristics as well as challenges and opportunities pertaining to their implementation as described below. Fig 1 gives expected reduction in the CO2 emissions for various alternatives.
Fig 1 Expected reduction in the CO2 emissions
Changing existing processes
With the growing availability of low-cost renewable electricity, different forms of electrification are increasingly seen as attractive options for decarbonization of the iron and steel industry. A number of different technological solutions have been suggested to this end. Among the more promising technologies is the electrolysis or ‘electro-winning’ of the iron ore. This entails the use of electric power as the reducing agent, similar to the production of aluminum (Al) from aluminum oxide (Al2O3). This process is still in an early stage of development and has so far only been tried at a laboratory scale, but it represents a high-efficiency steelmaking option with the promise of large emission reductions in the long term.
Electro-winning is one of the oldest electrolytic techniques used for extraction of metals from their ores by using electricity. This technology is normally used for electro-winning of metals like lead, copper and rare-earth elements. There are some commercially-available ore-specific electro-winning technologies for iron namely (i) the Boucher process, electro-refining in FeSO4-FeCl2 solution, (ii) the Eustis process, electro-winning in FeCl2 solution using iron sulphide ore, and (iii) the Pyror process, electro-winning in FeSO4 solution using iron sulphide ore. However, the more generally applicable electro-winning of Fe from iron-ore has only been established at a laboratory scale. Depending on the carbon footprint of the electricity mix used for electrolysis, this route can be potentially carbon free. In a futuristic scenario, in which the global primary energy supply is dominated by renewable sources, this technology offers significant carbon reduction potential. The present European SIDERWIN initiative, which is a project under the Horizon 2020 framework with a target of CO2 emissions and energy consumption reduction of 87 % and 31 % (compared to BF-BOF route), respectively aims to validate this technology at the pilot scale, and demonstrate a technology readiness level (TRL) by 2022.
Another electrolytic route which has recently received interest for steelmaking is the molten oxide electrolysis of iron ore (also known as pyro-electrolysis). The process is similar to the standard method for the reduction of Al from Al2O3 through the Hall -Heroult process, in which Al2O3 is dissolved in a 800 deg C bath of molten cryolite (aluminum sodium fluoride) and then electrolyzed between anodes of graphite (above) and a cathode of molten aluminum (below). The operation of a similar process for iron ore reduction at very high temperatures is expected to yield a potential decrease in energy consumption compared to the low-temperature electrolysis routes. Proofs-of-concept have been demonstrated, but the technical feasibility with acceptable efficiencies is still elusive. Challenges include the corrosivity of molten electrolytes, lack of suitable anode materials, and limited mechanistic understanding of very high temperature electrolytic processes. Although steel production by molten oxide electrolysis offers potential economic and environmental advantages over classic extractive metallurgy, its feasibility is far from being convincingly demonstrated as an immediate zero-carbon alternative.
A study has compared the energy requirement and CO2-emissions from four different steel production routes, namely, BF-BOF reference case, BF-BOF with carbon capture (BF-CCS), H2- DRI and electro-winning process. The electro-winning pathway had an energy requirement 50 % lower than the reference BF-BOF case, followed by H2-DRI and BF-CCS at 28 % and 13 % lower energy requirements respectively. Importantly, the analysis also concludes that more than 50 % reduction in CO2- emissions is not possible through the BF-CCS route, whereas, both H2-reduction and electro-winning routes can eventually lead to complete decarbonization of the iron and steel sector. The market entries for H2-DRI and electro-winning are not expected until 2035 and 2040 respectively.
The stoichiometric consumption of H2 for the reduction of hematite is 54 kg per ton of iron. A 1 Mtpa (million tons per annum) iron and steel plant needs a H2 plant capacity of as much as 70,000 cum at STP/hour. Large-scale of H2 production is presently 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 overall remains 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, provided that the electric power needed is fossil-free. 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.
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 (carbon monoxide) gas, syngas, or coke. HYBRIT, short for ‘HYdrogen BReakthrough Ironmaking Technology’, is a joint venture between three Swedish companies namely SSAB, LKAB and Vattenfall. It aims to completely eliminate carbon from steelmaking, using H2-reduction. The HYBRIT process falls within a category of technological concepts which are substantially closer to the commercial deployment.
HYBRIT process 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 carbon-free if the electric power and H2 are produced using renewable sources such as PV (photovoltaic) / wind / hydro-powered electrolysis, photo-chemical H2 production, or solar-thermal water splitting.
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. Typical flow sheet of H2-DR/EAF process is shown in Fig 2.
Fig 2 Typical flowsheet of hydrogen direct reduction process
Although this specific combination of processes has not been implemented at the commercial scale, several of the individual components are already widely used in the global iron and steel industry. EAF-based steel makes up around 30 % of annual global production. The DR process is also widely employed, being the basis for around 7 % of total global iron production and is normally integrated with the EAF. While pure H2 has been used commercially as the reducing agent in direct reduction, existing DRI production capacity relies on natural gas which is steam-reformed to get the reduction agent, a mixture of CO and H2. Recently, zero reform (ZR) process has been developed which has substantially reduced the consumption of natural gas.
As the cost of natural gas is a key factor for the economic viability of this setup, most DRI production is located in regions which are rich in low-cost natural gas (CH4). Despite the fact that several components of the H2-DR/EAF setup have been tested and deployed in industrial settings, key challenges still remain for the process. These are related to process integration, product qualities, scale-up of H2 infrastructure (production and storage) and the integration of an H2-DR/EAF iron and steel plant into an energy system based on renewable sources of electricity. One of the main challenges is how to get carbon into the iron in order to make it into steel.
Keeping existing processes
CCS has been an important topic in the study of the control of the GHG emissions. Interest grew in the 1990s with more in-depth analysis of the technology. The concept entered more widely into climate policy discussions in the early 2000s as a potential technology by which global use of fossil fuels can continue without contributing to the GHG levels in the atmosphere. Carbon capture technology itself is rather mature, following commercialization in the mid-20th century in the food and chemicals industries. Storage has also been successfully tried in natural gas reservoirs. In the early phase of CCS studies for the purpose of climate change mitigation, the focus was predominantly on applications to the electric power generation sector, especially coal-based electric power production. However, despite a long list of pilot plants and trial projects, commercial CCS has failed to materialize. This is partly due to the cost overruns, partly due to the public opposition for the underground CO2 storage and partly due to the dropping costs of other less polluting means of power generation such as renewables and natural gas.
Although expectations for the role of CCS in the power sector have decreased, however, recent developments in carbon capture from natural gas power generation, by use of the so-called Allam cycle, appear promising. It is still considered a key option for reducing GHG emissions from heavy industry including iron and steel industry without major changes to the existing processes. CCS in industry has certain distinguishing characteristics when it comes to conditions for implementation. An advantage is that CO2 streams tend to be quite pure in industry compared to the electric power production, which can make the separation and capture stages less complicated. Furthermore, public opposition is expected to be less severe, as there are few renewables-based alternatives for several industries, such as cement industry.
However, iron and steel industry also has some features which can make CCS applications difficult. First, unlike the electric power generation industry, iron and steel industry competes globally, which makes it even more vulnerable to cost increases and more problematic to carry through cost increases to the customers. Another disadvantage for the CCS option is that an industrial site hosts a number of CO2 sources of varying concentration and volumes. Most CCS assessments focus only on the major source of CO2, whereas capturing all CO2 from a plant can prove much more difficult and need major rebuilding. Capturing and storing 50 % to 60 % of the CO2 emissions at an industrial site can cost around USD 70 to USD 80 per ton CO2, according to several assessments. However, for capturing higher shares of emissions, the cost structure is more uncertain. Notably, no reliable cost estimates exist for capturing over 90 % of emissions.
The ‘Ultra-Low Carbon Dioxide Steelmaking’ (ULCOS) project has identified a number of technologies which can support the implementation of CCS in the iron and steel industry. One of these, the TGR-BF (Top Gas Recycling – Blast Furnace) process has been tested successfully in pilot plants, resulting in a 24 % reduction potential in CO2 emissions. However, actual capture and storage of the CO2 was not part of this pilot setup.
In short, presently CCS seems like a more promising solution in industry than in power generation, but there are still inherent problems. First, the potential GHG emission reductions from CCS are limited to around 50 %, due to small and diffuse emission sources, lack of space for installations, and other problems. Secondly, storage-related issues, such as oversight and long-term integrity of storage reservoirs, are still remaining unresolved. Thirdly, CCS comes with very few co-benefits, and the presence of co-benefits has been identified as a key facilitator when it comes to accelerating transition processes. This is a factor which can hinder widespread uptake. If carbon capture is combined with some form of CO2 utilization (CCU), there can be greater opportunities, but there is still a lot of process development is needed to be done. Also, even if the CO2 is utilized as raw material such as in some form of a specialty chemical or fuel, it however eventually ends up in the atmosphere.
The role of biomass
Throughout most of the history of iron and steel processing, biomass was a key resource. Wood-based charcoal acted both as the reduction agent necessary to free iron ore of its oxygen components, as well as the source of energy needed to reach the high temperatures necessary. It was not until around 1875 that coke, produced from coal, took over, although it is important to note that charcoal continued to be used up until the mid-1900s. Charcoal produced from fast-growing eucalypts are still used as the main reduction agent in smaller steel plants in Brazil, but this is likely not feasible in larger iron and steel plants due to the limits imposed by the lower compressive strength of charcoal compared to coke. Also, the large quantities which are needed and significant challenges in maintaining quality make a full shift from coke to bio-coke highly unlikely.
But biomass can still have important role to play in the decarbonization of the iron and steel sector, and several different options have been suggested. At the incremental side of the scale is the possibility of blending 5 % to 10 % charcoal with coking coal in the production of metallurgical coke for use in existing BFs. Another option is to use biomass in the processing of raw iron ore, either as a fuel for the process itself or to produce a composite bio-carbon-iron ore pellet which can then be used in a DR process. There have also been trials aimed at using gasified biomass in DR processes. This approach is feasible but in need of further trials and studies. Höganäs AB, a Swedish firm which uses a coal-based DR process to produce iron powders, has initiated real-world trials of a process using wood gasification to produce both bio-coke, to be used as reduction agent, as well as syngas, to be used for process energy (and possibly also for reduction.
Bio-methane is also going to be an important low-carbon option for heating in the secondary metallurgy process if the coke oven gas from the coke oven and by-product plant is no longer available. In an integrated iron and steel plant, the coke oven gas from the coke oven and by-product plant is used for heating in the secondary metallurgy processes. With a H2-DR concept, this energy has to be replaced. In the long term, different options for electric heating exist, but bio-methane can replace the presently used natural gas and coke-oven gas directly, with minimal changes to the process.
However, the systemic challenges for biomass tend to be substantial. In contrast to coal, biomass resources are not concentrated in a specific place (like a mine), which leads to high procurement costs as biomass from geographically dispersed area are to be collected, processed and transported to the iron and steel plant. Secondly, a growing demand for wood can lead to competition with existing users (such as the forest industry), as well as other sectors aiming to utilize biomass to achieve mitigation ambitions. This can in turn lead to higher prices, unless focus is shifted to forest residues which are less in demand. Finally, in order to ensure GHG emission reductions, it is crucial that biomass is sourced from sustainably managed forests.
In summary, biomass can come to play an important role in both the renewable as well as the CCS route. Both need large amounts of heat in the iron ore processing, secondary metallurgy and hot rolling processes. This can very well be provided through the combustion of bio-methane. In the CCS route, charcoal can at least partially substitute coke, as long as the mechanical stability of the BF charge is maintained. In the HYBRIT concept, biomass is foreseen also to serve as a carbon source for iron and steel processes and potentially also in the downstream metalworking process.