**Lauri Holappa**

Department of Chemical and Metallurgical Engineering, Aalto University School of Chemical Engineering, 02150 Espoo, Finland; lauri.holappa@aalto.fi; Tel.: +358-50-560-83-77

Received: 16 July 2020; Accepted: 16 August 2020; Published: 19 August 2020

**Abstract:** The 2018 IPCC (The Intergovernmental Panel on Climate Change's) report defined the goal to limit global warming to 1.5 ◦C by 2050. This will require "rapid and far-reaching transitions in land, energy, industry, buildings, transport, and cities". The challenge falls on all sectors, especially energy production and industry. In this regard, the recent progress and future challenges of greenhouse gas emissions and energy supply are first briefly introduced. Then, the current situation of the steel industry is presented. Steel production is predicted to grow by 25–30% by 2050. The dominant iron-making route, blast furnace (BF), especially, is an energy-intensive process based on fossil fuel consumption; the steel sector is thus responsible for about 7% of all anthropogenic CO<sup>2</sup> emissions. In order to take up the 2050 challenge, emissions should see significant cuts. Correspondingly, specific emissions (t CO2/t steel) should be radically decreased. Several large research programs in big steelmaking countries and the EU have been carried out over the last 10–15 years or are ongoing. All plausible measures to decrease CO<sup>2</sup> emissions were explored here based on the published literature. The essential results are discussed and concluded. The specific emissions of "world steel" are currently at 1.8 t CO2/t steel. Improved energy efficiency by modernizing plants and adopting best available technologies in all process stages could decrease the emissions by 15–20%. Further reductions towards 1.0 t CO2/t steel level are achievable via novel technologies like top gas recycling in BF, oxygen BF, and maximal replacement of coke by biomass. These processes are, however, waiting for substantive industrialization. Generally, substituting hydrogen for carbon in reductants and fuels like natural gas and coke gas can decrease CO<sup>2</sup> emissions remarkably. The same holds for direct reduction processes (DR), which have spread recently, exceeding 100 Mt annual capacity. More radical cut is possible via CO<sup>2</sup> capture and storage (CCS). The technology is well-known in the oil industry; and potential applications in other sectors, including the steel industry, are being explored. While this might be a real solution in propitious circumstances, it is hardly universally applicable in the long run. More auspicious is the concept that aims at utilizing captured carbon in the production of chemicals, food, or fuels e.g., methanol (CCU, CCUS). The basic idea is smart, but in the early phase of its application, the high energy-consumption and costs are disincentives. The potential of hydrogen as a fuel and reductant is well-known, but it has a supporting role in iron metallurgy. In the current fight against climate warming, H<sup>2</sup> has come into the "limelight" as a reductant, fuel, and energy storage. The hydrogen economy concept contains both production, storage, distribution, and uses. In ironmaking, several research programs have been launched for hydrogen production and reduction of iron oxides. Another global trend is the transfer from fossil fuel to electricity. "Green" electricity generation and hydrogen will be firmly linked together. The electrification of steel production is emphasized upon in this paper as the recycled scrap is estimated to grow from the 30% level to 50% by 2050. Finally, in this review, all means to reduce specific CO<sup>2</sup> emissions have been summarized. By thorough modernization of production facilities and energy systems and by adopting new pioneering methods, "world steel" could reach the level of 0.4–0.5 t CO2/t steel and thus reduce two-thirds of current annual emissions.

**Keywords:** climate warming; carbon footprint; energy saving; emissions mitigation; electricity generation; hydrogen in steelmaking; steel vision **Keywords:** climate warming; carbon footprint; energy saving; emissions mitigation; electricity generation; hydrogen in steelmaking; steel vision

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#### **1. Global Challenge of Climate Warming and Its Rationale 1. Global Challenge of Climate Warming and its Rationale**  Climate change is indisputable. The Intergovernmental Panel on Climate Change's (IPCC) Fifth

Climate change is indisputable. The Intergovernmental Panel on Climate Change's (IPCC) Fifth Assessment Report concluded that, "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century" [1]. The main culprit is anthropogenic greenhouse gas (GHG) emissions, which have doubled since 1970 due to the rapid population growth, expanded industrialization, and increase in standard of living. The observed growth can be seen in Figure 1 [2]. The scale in the figure is in Gt CO<sup>2</sup> equivalent per year. Carbon dioxide is the most important GHG; its emissions are currently at 37 Gt/year. CO<sup>2</sup> content in the atmosphere increased from 300 ppm in 1950 to the current 410 ppm [3]. Additionally, there are other greenhouse gases that are more potent, albeit in lesser amounts. Methane CH<sup>4</sup> is the most significant, followed by NOx, and F-bearing gases. The total equivalent GHG emissions are estimated at about 52 Gt/year and 56 Gt/year when land-use, land-use change, and forestry (LULUCF) are taken into consideration. Assessment Report concluded that, "It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century" [1]. The main culprit is anthropogenic greenhouse gas (GHG) emissions, which have doubled since 1970 due to the rapid population growth, expanded industrialization, and increase in standard of living. The observed growth can be seen in Figure 1 [2]. The scale in the figure is in Gt CO2 equivalent per year. Carbon dioxide is the most important GHG; its emissions are currently at 37 Gt/year. CO2 content in the atmosphere increased from 300 ppm in 1950 to the current 410 ppm [3]. Additionally, there are other greenhouse gases that are more potent, albeit in lesser amounts. Methane CH4 is the most significant, followed by NOx, and F-bearing gases. The total equivalent GHG emissions are estimated at about 52 Gt/year and 56 Gt/year when land-use, land-use change, and forestry (LULUCF) are taken into consideration.

**Figure 1.** Global greenhouse gas emissions, 1970–2018. Modified from [2]. **Figure 1.** Global greenhouse gas emissions, 1970–2018. Modified from [2].

The recent course of GHG emissions indicate rapid climate warming, 3–5 °C by 2100 (Figure 2). The United Nations' Intergovernmental Panel on Climate Change (IPCC) has stated that CO2 concentration must be stabilized at 450 ppm to have a fair chance at avoiding global warming above 2 °C, which was set as a limit at the COP 21/CMP 11 Conference in Paris, December 2015 [1]. Later, this target was brought down to 1.5 °C at the COP 24 meeting in 2018 [4]. In Figure 2, feasible future scenarios are shown, together with the last 50 years' history of equivalent CO2 emissions. Current policies or nationally determined contributions (NDC) are not effective, and more radical actions are needed. The 2 °C pathway means a roughly 50% cut in emissions by 2050, and the 1.5 °C target indicates an 80% cut, respectively. The recent course of GHG emissions indicate rapid climate warming, 3–5 ◦C by 2100 (Figure 2). The United Nations' Intergovernmental Panel on Climate Change (IPCC) has stated that CO<sup>2</sup> concentration must be stabilized at 450 ppm to have a fair chance at avoiding global warming above 2 ◦C, which was set as a limit at the COP 21/CMP 11 Conference in Paris, December 2015 [1]. Later, this target was brought down to 1.5 ◦C at the COP 24 meeting in 2018 [4]. In Figure 2, feasible future scenarios are shown, together with the last 50 years' history of equivalent CO<sup>2</sup> emissions. Current policies or nationally determined contributions (NDC) are not effective, and more radical actions are needed. The 2 ◦C pathway means a roughly 50% cut in emissions by 2050, and the 1.5 ◦C target indicates an 80% cut, respectively.

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**Figure 2.** History of global greenhouse gas (GHG) emissions and different scenarios till 2050 [3]. **Figure 2.** History of global greenhouse gas (GHG) emissions and different scenarios till 2050 [3].

**Figure 2.** History of global greenhouse gas (GHG) emissions and different scenarios till 2050 [3]. The key question is: how can we stop the growth of CO2 emissions and drastically lower the curve in the 2020s? The primary fault lies in fossil fuel being a major source of energy. Although renewable energy has gained publicity since the 1990s, its role is still minor—about 14% of all energy supply, whereas fossil energy represents 81% (Figure 3). The remaining 5% is nuclear power. For electricity generation, the corresponding percentage contribution is 26/63/11 for renewables, fossil, The key question is: how can we stop the growth of CO<sup>2</sup> emissions and drastically lower the curve in the 2020s? The primary fault lies in fossil fuel being a major source of energy. Although renewable energy has gained publicity since the 1990s, its role is still minor—about 14% of all energy supply, whereas fossil energy represents 81% (Figure 3). The remaining 5% is nuclear power. For electricity generation, the corresponding percentage contribution is 26/63/11 for renewables, fossil, and nuclear energy, respectively [4,5]. The key question is: how can we stop the growth of CO2 emissions and drastically lower the curve in the 2020s? The primary fault lies in fossil fuel being a major source of energy. Although renewable energy has gained publicity since the 1990s, its role is still minor—about 14% of all energy supply, whereas fossil energy represents 81% (Figure 3). The remaining 5% is nuclear power. For electricity generation, the corresponding percentage contribution is 26/63/11 for renewables, fossil, and nuclear energy, respectively [4,5].

**Figure 3.** Total primary energy supply by source, globally, 1990–2017 [5]. **Figure 3.** Total primary energy supply by source, globally, 1990–2017 [5].

**Figure 3.** Total primary energy supply by source, globally, 1990–2017 [5]. The global energy production exceeded 14 Gtoe (gigatons oil equivalent) in 2018 [5]. The share The global energy production exceeded 14 Gtoe (gigatons oil equivalent) in 2018 [5]. The share of fossil energy was 11.3 Gtoe, which is in accordance with anthropogenic carbon emissions, including fuel combustion and cement production, 37 Gt CO2/year [2]. The global energy production exceeded 14 Gtoe (gigatons oil equivalent) in 2018 [5]. The share of fossil energy was 11.3 Gtoe, which is in accordance with anthropogenic carbon emissions, including fuel combustion and cement production, 37 Gt CO2/year [2].

#### of fossil energy was 11.3 Gtoe, which is in accordance with anthropogenic carbon emissions, including fuel combustion and cement production, 37 Gt CO2/year [2]. **2. Progress of the Steel Industry and its Role in Energy Consumption and CO<sup>2</sup> Emissions**

The overall progress of world steel production over the last 150 years is shown in Figure 4a [6]. In the early 19th century, the world annual steel production was only a few million tons. After the

breakthrough of new technologies, converter processes, and open hearths, production increased and exceeded 30 Mt in 1900. In 1927, steel production reached 100 Mt and 200 Mt in 1951. The next 30 years after the II World War was a period of "new industrial revolution" with innovative novel processes. Extensive investments were made in the steel industry, with Japan, Soviet Union, United States, and South Korea in the vanguard. The annual steel production reached 700 Mt in the 1970s (record 749 Mt in 1979). The growth then stagnated due to economic crises and political changes until the turn of the millennium, when it reached 850 Mt in 2000. This was the overture to the "boom" with China in the forefront. Since then, the world production has doubled and the record so far is 1,869 Mt, attained in 2019 [6]. China´s share is over 50%. Today, China´s domestic steel demand has reached an "established level" and eventual growth is directed towards export. Meanwhile, India has strongly increased steel production and has risen to second place with 111 Mt/2019. It is plausible that in the near future, the consumption in developing countries will grow. Earlier scenarios predicted continuous growth up to 3000 Mt/year in 2050. Owing to the recession period, the stabilization in China, and the newest goals of "stop climate change", the current scenarios are more conservative and an estimate of 2500 Mt in 2050 can be considered realistic [7,8]. Future scenarios until 2050 are sketched accordingly in Figure 4b. exceeded 30 Mt in 1900. In 1927, steel production reached 100 Mt and 200 Mt in 1951. The next 30 years after the II World War was a period of "new industrial revolution" with innovative novel processes. Extensive investments were made in the steel industry, with Japan, Soviet Union, United States, and South Korea in the vanguard. The annual steel production reached 700 Mt in the 1970s (record 749 Mt in 1979). The growth then stagnated due to economic crises and political changes until the turn of the millennium, when it reached 850 Mt in 2000. This was the overture to the "boom" with China in the forefront. Since then, the world production has doubled and the record so far is 1,869 Mt, attained in 2019 [6]. China´s share is over 50%. Today, China´s domestic steel demand has reached an "established level" and eventual growth is directed towards export. Meanwhile, India has strongly increased steel production and has risen to second place with 111 Mt/2019. It is plausible that in the near future, the consumption in developing countries will grow. Earlier scenarios predicted continuous growth up to 3000 Mt/year in 2050. Owing to the recession period, the stabilization in China, and the newest goals of "stop climate change", the current scenarios are more conservative and an estimate of 2500 Mt in 2050 can be considered realistic [7,8]. Future scenarios until 2050 are sketched accordingly in Figure 4b.

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breakthrough of new technologies, converter processes, and open hearths, production increased and

The overall progress of world steel production over the last 150 years is shown in Figure 4a [6].

**2. Progress of the Steel Industry and its Role in Energy Consumption and CO2 Emissions** 

**Figure 4.** (**a**) World production of steel, BF hot metal, and DRI from 1860 to 2018, including recent steel production in China and estimated recycled steel. (**b**) History of steel production and future **Figure 4.** (**a**) World production of steel, BF hot metal, and DRI from 1860 to 2018, including recent steel production in China and estimated recycled steel. (**b**) History of steel production and future scenarios.

scenarios. Figure 4a also shows the amounts of main raw material of steel: blast furnace hot metal (BFHM), recycled steel (RS), and direct reduced iron (DRI + HBI). Of these, BFHM is mainly charged into converters to make steel, whereas RS and DRI go into electric furnaces. In 2018, 72% of crude steel

came from converter processes based on BF hot metal. Electric furnaces produced 27.6%, utilizing recycled steel scrap as the main iron source, with a minor share of direct reduced iron. The balance 0.4% was produced in open hearths, which is currently a declining technique [6].

Ironmaking is an extremely energy-intensive process, utilizing coal as the main primary energy source. The steel production was responsible for 7–9% of direct emissions from the global use of fossil fuel [9]. The specific emission was estimated at 1.85 t CO2/t steel, corresponding to 3.3 Gt CO<sup>2</sup> in a year at the production rate of 1.8 Gt/a. The predicted growth of steel demand/production by the year 2050 was discussed afore as ending in 2.5 Gt/a (Figure 4b). The "current policy" would lead to annual emission of 4.5–5 Gt CO2, which would be a disaster, as it is insufficient to stop the growth; therefore, we must radically cut emissions. By tracking the "2 ◦C pathway", the emissions could be halved and the "1.5 ◦C pathway" could mean reduction by 80% [3]. In proportion to the steel industry, total emissions should be reduced to 1.5–0.75 Gt CO2/a, corresponding to specific CO<sup>2</sup> emissions of 0.6 − 0.3 t CO2/t steel. The fall from 1.85 t CO2/t steel is, thus, dramatic. A pertinent question follows: By what means can we attain this level by 2050?

The author of this paper examined this problem in 2011 by analyzing production practices in different countries; comparing them with BAT (Best Available Technology) values; estimating emissions from different energy sources (including electricity); and studying the potential of energy-saving actions, and low-carbon and carbon-free innovative technologies [10]. The present contribution, although based on previous studies [10,11], is an updated, generalized version, taking into account the extensive recent developments and numerous works of literature.

#### **3. Review of Means to Cut CO<sup>2</sup> Emissions from the Steel Industry**

We have identified several key factors that make it possible to reduce CO<sup>2</sup> emissions from steel production. Some of them are incremental improvements in current processes, whereas the others can be regarded as radical breakthrough technologies for iron/steel making or energy supply/usage. Both incremental and radical improvements are useful and necessary to be implemented due to the huge scale and inertia for change as well as the urgent schedule of having only few decades to achieve the set goal.

#### *3.1. Improving Energy E*ffi*ciency*

The first key factor is to improve the energy efficiency of current processes—the fastest way to stop the growth of emissions at a moderate cost. A previous comprehensive study by IEA/OECD analyzed steel industries in different countries and showed an energy-saving potential of 4.1 GJ/t steel (corresponding to 20% reduction from the current world average) [12]. The saving potential varied from 1.4 to 8.7 GJ/t in different countries, with the largest savings slated for China, Ukraine, Russia, India, and Brazil. China has made big efforts and decreased its specific energy consumption by 15% from 2006 to 2017 [13]. The total energy consumption or intensity in BF-BOF steel production varied in different countries from 19 to 26 GJ/t crude steel in a recent, rather inclusive benchmarking [14]. Despite great advancements by the world steel industry, energy consumption can still be reduced by 10–15% on average to meet the BAT values by applying best available technologies [15,16]. Even bigger deduction of CO<sup>2</sup> emissions is possible by transfer to low-carbon energy sources.

Any comparison of energy efficiency between different countries or even steel plants is not fair if the boundary conditions (raw materials, energy sources, processes, products etc.) are different. For such comparisons and evaluation of any kind of process changes, it has proved illustrative to set the different process routes (ore-based and scrap-based steel production) at the ends of the *x*-axis and to examine the specific energy consumption (GJ/t steel) against the recycling ratio, defined as percentage of scrap from total Fe input [10,11]. Correspondingly, the specific CO<sup>2</sup> emissions (t CO2/t steel) can be presented as in Figure 5. The % REC means percentage of recycled steel i.e., scrap. The case % REC = 0 corresponds to 100% ore-based BF–BOF (or in the case of smelting reduction SRF–BOF) route steel production. On the other hand, % REC = 100 means 100% recycled steel (scrap) based production

*Current Technology* 

i.e., the EAF route. The direct reduction process (DRI production) cannot be put in a diagram on its own, but the melting stage in EAF, normally with some scrap, must be included. Then, the position on the *x*-axis depends on the scrap/DRI ratio used in the EAF. This kind of representation is apt, e.g., for comparison of the state of the steel industry in different countries as well as evaluation of new development steps. In Figure 5, the present level is outlined. The full BAT line was drawn based on rather conservative data by Worrell et al. [16]. The BAT Line Range was outlined based on different CO<sup>2</sup> emissions from electricity generation, the low line referring to low emission electricity (hydro/nuclear power) and the high line permitting coal/oil/gas as primary energy. In this scale, the current position of "world steel" W is at 1.8 t CO2/t steel vs. 35% REC [9]. The % REC value takes account of the usage of scrap in BOFs. Hence % REC is notably higher than the percentage of EAF production (see e.g., China). Further, Japan, the European Union, Germany, France, Canada, the United States, and Italy were evaluated in Figure [13,14,17–20]. The drop in overall world average from 1.8 t CO2/t steel to the BAT level would mean a reduction by 15–20%. This could be achieved by modernizing plants, adopting best available technologies, and closing old-fashioned units in China and other countries—i.e., a certain "low-carbon retrofit". production. On the other hand, % REC = 100 means 100% recycled steel (scrap) based production i.e.**,** the EAF route. The direct reduction process (DRI production) cannot be put in a diagram on its own, but the melting stage in EAF, normally with some scrap, must be included. Then, the position on the *x*-axis depends on the scrap/DRI ratio used in the EAF. This kind of representation is apt, e.g., for comparison of the state of the steel industry in different countries as well as evaluation of new development steps. In Figure 5, the present level is outlined. The full BAT line was drawn based on rather conservative data by Worrell et al**.** [16]. The BAT Line Range was outlined based on different CO2 emissions from electricity generation, the low line referring to low emission electricity (hydro/nuclear power) and the high line permitting coal/oil/gas as primary energy. In this scale, the current position of "world steel" W is at 1.8 t CO2**/**t steel vs. 35% REC [9]. The % REC value takes account of the usage of scrap in BOFs. Hence % REC is notably higher than the percentage of EAF production (see e.g., China). Further, Japan, the European Union, Germany, France, Canada, the United States, and Italy were evaluated in Figure [13,14,17–20]. The drop in overall world average from 1.8 t CO2/t steel to the BAT level would mean a reduction by 15–20%. This could be achieved by modernizing plants, adopting best available technologies, and closing old-fashioned units in China and other countries—i.e., a certain "low-carbon retrofit".

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0 corresponds to 100% ore-based BF–BOF (or in the case of smelting reduction SRF–BOF) route steel

**Figure 5.** Specific CO2 emissions from fossil fuels and electricity in iron and steel production as a function of recycled steel ratio (% REC). BAT line and range were approximated based on published data [13,14,16–20]. The country codes in the figure are revealed and values commented in the text. **Figure 5.** Specific CO<sup>2</sup> emissions from fossil fuels and electricity in iron and steel production as a function of recycled steel ratio (% REC). BAT line and range were approximated based on published data [13,14,16–20]. The country codes in the figure are revealed and values commented in the text.

The positions of world, EU, and different countries in Figure 5 are only an approximate, as both the published CO2 emissions and estimated % REC values were not necessarily based on equivalent premises. As remarked before, the % REC also rules in the scrap used in converters, whereas DRI is counted as "ore-based" iron raw material. A position in relation to the BAT line relates to the technology level but can also incorporate other factors. For instance, Canada's outstanding position is partly owing to the substantial share of natural gas as the primary energy source. In contrast, Germany´s relatively high value is a result of the big role of coal in ironmaking as well as in electricity generation. Generally, in cases with high EAF share, electricity generation emissions have a strong influence, illustrated by the expanding BAT range. The positions of world, EU, and different countries in Figure 5 are only an approximate, as both the published CO<sup>2</sup> emissions and estimated % REC values were not necessarily based on equivalent premises. As remarked before, the % REC also rules in the scrap used in converters, whereas DRI is counted as "ore-based" iron raw material. A position in relation to the BAT line relates to the technology level but can also incorporate other factors. For instance, Canada's outstanding position is partly owing to the substantial share of natural gas as the primary energy source. In contrast, Germany´s relatively high value is a result of the big role of coal in ironmaking as well as in electricity generation. Generally, in cases with high EAF share, electricity generation emissions have a strong influence, illustrated by the expanding BAT range.

*3.2. Potential Means to Mitigate CO2 Emissions in Ore-Based Production by Improving and Modifying* 
