Decarbonising the iron and steel sector for a 2 °C target using inherent waste streams

The decarbonisation of the iron and steel industry, contributing approximately 8% of current global anthropogenic CO2 emissions, is challenged by the persistently growing global steel demand and limitations of techno-economically feasible options for low-carbon steelmaking. Here we explore the inherent potential of recovering energy and re-using materials from waste streams, high-temperature slag, and re-investing the revenues for carbon capture and storage. In a pathway based on energy recovery and resource recycling of glassy blast furnace slag and crystalline steel slag, we show that a reduction of 28.5 ± 5.7% CO2 emissions to the sectoral 2 °C target requirements in the iron and steel industry could be realized in 2050 under strong decarbonization policy consistent with low warming targets. The technological schemes applied to engineer this high-potential pathway could generate a revenue of US$35 ± 16 and US$40 ± 18 billion globally in 2035 and 2050, respectively. If this revenue is used for carbon capture and storage implementation, equivalent CO2 emission to the 2 °C sectoral target requirements is expected to be reduced before 2050, without any external investments.


Supplementary note 3-Various technological schemes to engineer Pathways 5 and 6
In Pathways 5 and 6, the thermal heat in both BFS and SS is recovered. After energy recovery, BFS is in a glassy state, while SS is in a crystalline state, considering its strong crystallization ability. Regarding waste energy recovery, two types of methods can be applied, namely physical and chemical ones. Based on these methods, five technological schemes can be proposed, named Schemes 1-5.
For the physical method, because of the low thermal conductivities of high-temperature slag [9][10][11]17 , the liquid slag should first be granulated into small droplets to increase the surface between the agent (air) and the slag, named Scheme 1. Therefore, the thermal heat in the slag can be quickly transferred to the agent (air). As a result, a high-temperature agent (air) can be obtained for further utilization, such as power generation, and at the same time, the slag is quickly cooled to obtain a glassy state to be used for cement manufacturing. Currently, a series of dry granulation methods are being developed including air blast and centrifugal methods such as RCAs, SDAs and RCLAs [18][19][20][21][22][23][24][25][26][27][28][29][30][31] .
After granulation, the residual thermal heat transfer in the slag can be further transferred to the cool air, and then the thermal heat in the hot air can finally be transferred to steam for heat utilization. Thus, the physical method consists of three steps, namely air-slag granulation, airslag heat transfer and air-steam heat transfer. Herein, to simplify the analysis, several assumptions are made. First, the granulation step takes place at 1550-1200 °C and 1600-1200 °C for BFS and SS, respectively, and an air flow with a temperature of 600 °C is obtained. The heat transfer efficiency between the air and liquid slag in this temperature range is assumed to be 70% to confirm the rapid cooling of the slag. In the second step, the full heat transfer between the slag and air occurs at 1200-100 °C and an air flow of 600 °C is obtained. The heat transfer efficiency between the air and solid slag in this step is assumed to be 80% since the glassy state has already been obtained, which can fully exchange heat with the air. Finally, the thermal heat in the air is transferred to the steam, and a steam product with a final temperature of 200 °C is obtained.
For the chemical methods, the thermal heat in high-temperature slag acts as the heat source for endothermic reactions such as limestone decomposition, methane reforming and gasification and pyrolysis reactions [47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62] . The chemical methods, especially gasification and pyrolysis reactions, exhibit the advantages of high efficiency of heat transfer; production of valuable syngas such as CO, H2 and CH4; and integration of multiple sectors [9][10][11]17 . For energy recovery using chemical methods, the high-temperature slag still needs to first be granulated into small droplets or particles, and then for gasification or pyrolysis, the slag is mixed with fuel to provide thermal heat to support these reactions. Herein, the gasification method is analyzed due to these special advantages.
Regarding the gasification method, two key issues should be pointed out. The first is the temperature range for granulation. To confirm that a large part of the thermal heat is transferred to the gasification reaction to achieve a high energy efficiency, the granulation of BFS and SS is assumed to take place in a small temperature range, namely 1550-1350 °C and 1600-1400 °C, respectively. This will be realized based on the advancements of granulation technologies. The second issue is the granulation agent that is used. For the physical method, air is generally used, while for the chemical methods, in addition to air, CO2 can be used considering further gasification reactions. For gasification, three agents can be employed, namely pure CO2, pure H2O (steam) and a mixing agent of CO2 and H2O. For CO2 gasification or mixing agent gasification, a CO2 agent is preferred for physical granulation because it is much easier for the heat in CO2 to be used. Hot CO2 can be directly mixed with room temperature CO2 or H2O to act as both a gasification agent and heat source for gasification. However, for air/slag granulation, a loss of energy efficiency will be caused since the thermal heat in air needs to be transferred to the gasification agent such as CO2 or H2O for further gasification reactions.
According to the granulation agent and gasification agent, heat utilization via chemical gasification can be divided into four schemes: air granulation + CO2 gasification, air granulation + H2O gasification, CO2 granulation + CO2 gasification and CO2 granulation + CO2/H2O gasification. The scheme of air granulation+ CO2/H2O gasification is ignored since it does not present apparent advantages over the other schemes and a more complicated process is required.
Here, it is assumed that for BFS and SS, similar energy recovery methods are used since if we use different energy recovery methods for the BFS and SS treatments, the situations will become so complex that the comparison and discussion will lose their meanings.

Scheme 1
The detailed process of Scheme 1 is sketched in Supplementary Fig. 21. Overall, it is divided into three steps: a first step of liquid slag granulation using cool air flow, a second step of full heat transfer between the solid slag and cool air flow, and a third step of heat transfer from the hot air to produce steam. As a result, steam and the solid slag are the main products of Scheme 1. Currently, most granulation methods have been developed for BFS [18][19][20][21][22][23][24][25][26][27][28][29][30][31] , while those for SS are quite limited 37 . Therefore, in this section, the discussion focuses on BFS. For SS, it is assumed that similar technologies will be used considering future technological advancements.
In the first step, the high-temperature BFS is granulated into small droplets and quickly cooled to a glassy state by transferring the thermal heat to cool air with a high flow rate. The temperature of the BFS decreases from 1550 °C to 1200 °C, and the air temperature increases from 25 °C to 600 °C. Due to the requirement of rapid cooling of the liquid slag, we assume that the heat transfer efficiency between the slag and air is 70%. The mass and energy balances of step 1 are expressed by Eq. (1): * , ( ,2 − ,1 ) = ,1 * , * ( ,2 − ,1 ) where and ,1 represent the masses of the BFS and air, respectively; , and , represent the heat capacities of the BFS and air, respectively; and ,1 , ,2 and ,1 , ,2 represent the temperature points of the BFS and steam before and after heat transfer, respectively.
In the second step, the heat transfer between the hot glassy slag and cool air will fully take place, where a fluidized bed can be used with a heat transfer efficiency of 80%. The temperature of the glassy slag will further decrease from 1200 °C to 100 °C, while the air temperature will increase from 25 °C to 600 °C. The mass and energy balances of step 2 are expressed by Eq.
In the third step, full heat transfer from the obtained hot air at 600 °C to room temperature water (25 °C) will occur to produce steam, with a heat transfer efficiency of 80%. In this step, the temperature of the air will decrease from 600 °C to 100 °C, while that of the steam will increase from 25 °C to 200 °C. The mass and energy balances of step 3 are expressed by Eq. (3): where ,1 + ,2 and represent the masses of hot air and steam, respectively; , , 1, and 2, represent the heat capacities of hot air, water at 25-100 °C and steam at 100-200 °C, respectively; represents the latent heat of steam at 100 °C; and ,2 , ,3 and ,1 , ,2 , ,3 represent the temperature points of air and steam, respectively.
Regarding the SS treatment, two other important issues different from those in the BFS treatment should be highlighted. First, the temperature range of the first step for SS is different from that from BFS, i.e., it takes place at 1600-1200 °C. Second, due to the stronger crystallization ability, SS is finally obtained in a crystalline state instead of a glassy state, and 15% more of the thermal energy will be recovered from crystal precipitation.

Scheme 2
Scheme 2 represents a gasification method using air as the granulation agent and CO2 as the gasification agent, as sketched in Supplementary Fig. 22. The process of Scheme 2 will be expressed in detail here, because this is the first scheme using a gasification reaction to treat the high-temperature slag; Schemes 3-5 follow similar processes to Scheme 2, where only the granulation or gasification agent is adjusted. The treatment process of BFS is also detailed here, while for SS, a similar process is employed.
In the first step of Scheme 2, the high-temperature liquid BFS is granulated into small droplets and then glassy particles in a granulator, and additionally, the thermal heat in the BFS is transferred to the cool air. To ensure that a large part of the thermal heat is used for the gasification reaction, the slag temperature in this step decreases from 1550 °C to 1350 °C while the air temperature increases from 25 °C to 600 °C. Because of the severe requirement of slag cooling, it is assumed that the heat transfer efficiency in this step is 70%. In the second step, the gasification reaction takes place in the temperature range of 1350-800 °C. In this step, the gasification fuel will react with CO2 to produce syngas composed of CO, H2, CH4, CO2 and H2O using the thermal heat in the slag. After gasification, the temperature of the BFS will decrease from 1200 °C to 800 °C and the hot syngas at 800 °C is produced.
The third step is related to the deep utilization of the residual heat, including the 600 °C hot air from the first step and the 800 °C BFS and syngas from the second step. Three types of heat utilization methods can be selected. First, the heat can be used to heat the gasification fuel to a gasification temperature of 800 °C. Second, the heat can be used to heat the CO2 gasification agent to 800 °C. Third, the heat can also be used for steam production at a temperature of 200 °C.
In the present schemes, the excess energy of the BFS and hot syngas is used for both heating the gasification agent and steam production. It is assumed that the final temperature of the BFS and syngas is 100 °C. It should be pointed out that the heat transfer efficiency in all these methods of heat utilization is 80%, considering the full heat transfer between different agents.
The energy balance of the whole process is expressed by Eq. (4) as follows: , + , + , + , + = + , + , + where , and , represent the sensible and latent heat of the fuel, respectively; , and , represent the sensible and latent heat of the gasification agent, respectively; represents the thermal heat in the BFS; represents the heat loss in various steps; , and , represent the sensible and latent heat of the syngas, respectively; and represents the thermal energy in the steam product.
For the gasification agent, For the gasification fuel, where (MJ/kg) is the higher heating value of the gasification fuel, which can be estimated based on the chemical compositions, and is expressed as follows 63 Combining Eqs. (10) and (11), an equation expressing the total energy balance in this system can be derived as follows: Regarding SS, a similar process is performed, where the temperature of the first step changes to 1600-1400 °C, the temperature of the second step changes to 1400-800 °C, and SS is obtained in a crystalline state instead of a glassy state.
Another important issue should be highlighted here, i.e., how to utilize the latent heat of the steam in the syngas. With regard to this part of energy, two methods can be used. The first method is that the latent heat of the steam is transferred to the cool fuel or gasification agent with a transfer efficiency of 80%. Another method is that this part of heat is totally lost since the discharge temperature of the syngas is assumed to be 100 °C, which is the condensation temperature of steam under 1 atm. In this study, the first method is discussed to maximize the energy efficiency of the different schemes.

Scheme 3
Scheme 3 represents a gasification method using air as the granulation agent and H2O as the gasification agent, as sketched in Supplementary Fig. 23. The differences between Scheme 3 and Scheme 2 are that in the second step, a steam flow with a temperature of 200 °C is used as the gasification agent, while in the third step, the sensible heat of the 800 °C syngas and BFS is mainly used to heat room temperature water to produce 200 °C steam and heat the gasification fuel; the steam produced will be further used as the gasification agent. It is assumed that the heat transfer efficiencies in each step are the same as those in Scheme 2.
For the energy and mass balances of Scheme 3, Eq. (12) and Eq. (13)

Scheme 4
Scheme 4 represents a gasification method using CO2 as the granulation agent and CO2 as the gasification agent, as sketched in Supplementary Fig. 24. Regarding the detailed process, the differences between Scheme 4 and Scheme 2 are that slag granulation is realized using a high flow rate of CO2 gas and that part of the hot CO2 produced from the first step will be directly used as a gasification agent in the second step. Thus, the energy loss can be decreased because the heat transfer step between the granulation agent and the gasification agent will be avoided.
Therefore, in Eq. (12) and Eq. (13), the energy loss for the first step, 1,1550−1350 , and the energy for steam production, 1 * 2 * 1550−1350 , will vary. As a result, these two types of energy should be modified based on the amount of gasification fuel, with a mole ratio of fuel/CO2 of 2:1; however, for other types of energy in the whole process, the same calculation equations are used.

Scheme 5
Scheme 5 represents a gasification method using CO2 as the granulation agent and a mixing agent of CO2/H2O as the gasification agent, as sketched in Supplementary Fig. 25. Regarding the detailed process, the differences are that, the slag granulation is realized using a high flow rate of CO2 gas and that part of the hot CO2 is directly mixed with steam to work as the gasification agent; therefore, less heat loss is realized. Similar to Scheme 4, the energy loss in the first step, 1,1550−1350 , and the energy for steam production, 1 * 2 * 1550−1350 , will vary. These two values will be adjusted based on the amount of fuel a fuel/CO2/H2O mole ratio of 1:1:1.

Supplementary note 5-Cost-benefit analysis per tonne of BFS and SS
Based on the process analysis, the costs and benefits of the different schemes can be obtained.
There are three types of costs, namely capital costs, operating costs and CO2 price caused by energy consumption. Overall, the benefits are composed of two types, namely product benefits and revenues by CO2 avoided due to energy recovery and emission reduction. The operating costs are further divided into maintenance costs, labour costs, resource costs and energy costs, while the products obtained in these schemes include cooled BFS and SS, steam and syngas.
Regarding CO2 price, both costs and benefits will be considered due to energy consumption and energy recovery. Fourth, the revenue in the cement industry due to the utilization of BFS and SS in cement manufacturing is not included here since the economic analysis focuses on the iron and steel sector. However, for the potential analysis of various pathways, this part is included since we discuss the emission reduction ratio to the 2 °C target requirements on a larger sectoral scale composed of various sectors.
As mentioned above, in the present study, five schemes are considered for the energy recovery and resource recycling of BFS and SS. Here, the cost-benefit analysis of BFS is first discussed.
In Scheme 1, physical granulation is used for energy recovery, which includes three steps, namely slag granulation, heat transfer between air and slag and heat transfer between hot air and steam. In Schemes 2-5, a chemical gasification method is used to recover the waste heat of BFS, which is mainly composed of a quick granulation process and a gasification reaction. It is assumed that the capital cost does not change greatly since for the current level, a reactor composed of a liquid slag granulator and chemical gasification reactor is expected to be developed 47,74 .
Regarding the operating costs, it is assumed that the labour and maintenance costs do not change compared with those in Scheme 1, while the energy costs are assumed to increase by 20% since a new step of chemical gasification is used. However, compared to Scheme 1, a new kind of cost is necessary, namely material costs due to the utilization of coal, biomass and sludge. If coal is utilized, two kinds of costs will be incurred, i.e., fuel costs based on the market price and CO2 costs due to the utilization of coal, a traditional fossil fuel. However, if biomass or sludge is applied, resource costs can be avoided since they are common solid wastes, and additionally, CO2 costs can also be avoided since they are carbon-neutral materials. In fact, the avoidance of resource costs and CO2 emission costs accounts for the important advantages of Schemes 2-5.
Regarding the benefits of Schemes 2-5, in addition to glassy slag, steam and CO2 avoided, a new type of benefit will be produced, namely valuable syngas composed of CO, H2, CH4 and CO2. Regarding the estimation of syngas prices, two methods can be used. On the one hand, the sensible heat and latent heat of syngas are directly transformed into the corresponding energy of natural gas and thus the benefits. On the other hand, syngas is one kind of product from the energy recovery of BFS, similar to the steam product. In the present study, we estimate the steam price by assuming that it is produced by the combustion of natural gas with an energy efficiency of 80%. Thus, here, it can be assumed that syngas can be produced using natural gas with the same efficiency of 80% since in this study we compare the benefits of these various schemes. Therefore, in this study we use the second method to estimate the value of syngas.
Another important issue is the gasification fuel used, coal, biomass or sludge, since if biomass or sludge is used, both the material costs and CO2 penalty can be avoided.
The results of the cost-benefit analysis for the BFS energy recovery using used. However, if solid waste, such as biomass or sludge, is used, the net revenue increases to ~ US$11.1 per tonne of SS mainly because a large amount of the fuel costs and the CO2 emission penalty will be avoided.

Supplementary note 6-Methodology to obtain the heat capacities of BFS and SS
There are two methods to obtain the heat capacities of slag. The first way is to calculate them based on the individual heat capacities of the oxides in the slag following several steps. First, these values can be directly collected based on FactSage software, which is commonly used for slag thermodynamic calculations 75 . Second, the heat capacities of these oxides are summed linearly based on their weight ratios, and thus, the heat capacities of BFS and SS can be derived.  [79][80][81] , which also proves the reasonability of the present methodology.

Supplementary note 7-Control of gasification conditions for gasification equilibrium calculation
For the waste energy recovery from BFS and SS using a gasification method, there are three types of agents that can be used, namely, CO2, H2O (steam) and a mixture of CO2 and H2O. For the gasification methods, the important factors determining the results are the gasification fuel used, gasification temperature and fuel/agent ratio.
Regarding the gasification fuel, two types are considered here, namely, coal and solid wastes, including biomass and sludge. Generally, the content of fixed carbon in coal is much higher than that in biomass and sludge [47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][82][83][84] , and thus, more syngas can be produced. However, the utilization of biomass and sludge has the advantages of emission reduction and waste treatment since they are carbon-neutral solid wastes. Regarding the reaction temperature, in fact, the energy recovery from the slag occurs in a wide temperature range, and therefore, gasification will take place in this temperature range. To simplify the calculations and analysis, the lowest temperature of the slag is taken as the operational gasification temperature. Herein, 800 °C is used as the gasification temperature to ensure that a large part of the thermal heat is recovered for the gasification reaction.

Supplementary note 8-Selection of gasification conditions based on the results of gasification calculations
The CO2 gasification results with varying fuel/agent ratios from 0.5 to 10 are detailed in Supplementary Table 14. As can be observed, at 800 °C with an increasing amount of CO2, the yield of CO continuously increases, while those of H2 and CH4 continuously decrease. As a result, the carbon efficiency continuously increases, while the hydrogen efficiency decreases. It can be noted that the carbon efficiency can reach values higher than 1.0 because the carbon in CO2 is transformed into CO when the CO2/fuel ratio is higher than 1.0. Due to the increasing residual CO2 in the syngas with an increasing amount of CO2, the H2 content in the syngas continuously decreases. However, with an increasing amount of CO2, the CO content first remarkably increases due to the enhanced reaction between CO2 and fixed carbon and then continuously decreases.
A discussion of the yields of these gases can be conducted on the basis of several fundamental reactions 60,85,86 . These fundamental reactions include organic pyrolysis (Eq. 21), char transformation (Eq. 22), the Boudouard reaction (BD, Eq. 23), primary water gas (PWG, Eq. 24), water-gas shift (WGS, Eq. 25), methane formation (MF, Eq. 26), methane steam reforming (MSR, Eq. 27) and methane CO2 reforming (MCR, Eq. 28). We can also observe that, using CO2 as the gasification agent, the yield of CO is greatly larger than those of CO and CH4 due to the dominant reactions of Eq. (23) and Eq. (28). Most importantly, we can see that as the amount of CO2 increases to more than 1.5 moles, the residual carbon disappears due to the enhanced reaction of Eq. (23). Here, to confirm the full gasification of fuel and to ensure a low cost of syngas separation after gasification, a CO2/fuel ratio of 2:1 is employed. CH 1.6 0.7 0.008 → CO+ 2 + CH 4 + CO 2 + 2 O+ NH 3 + NO+ℎNO 2 +Char+......
Char transformation: Char → C+ CO+ 2 + CH 4 + CO 2 + 2 ...... In addition to CO2 gasification and H2O gasification, another choice is to use a mixing agent of CO2 and H2O, which can be used to modify the components of syngas and the reaction kinetics in a target direction. To ensure gasification at a similar level and to simplify the discussion of different strategies, the total molar amount of CO2 and H2O is assumed to be 2.0, and the mole ratio of CO2 to H2O varies from 0.1:1.9 to 1.9:0.1. The results are detailed in Supplementary   Table 16. As can be observed, when the mixing agent is used, there is no fixed carbon remaining.
With an increasing CO2/H2O ratio, the yield of CO gas continuously increases, while that of H2 gas continuously decreases. The yields of CO and H2 are comparable, while the yield of CH4 is much lower, and slightly decreases with an increasing CO2/H2O ratio. As a result, the carbon efficiency gets continuously increased and the hydrogen efficiency continuously decreases with an increasing CO2/H2O ratio. When the ratio is larger than 0.6/1.4, both efficiencies are larger than 1.0 due to the carbon transformation from CO2 and the hydrogen transformation from H2O.
Therefore, in this study, a CO2/H2O mole ratio of 1:1 is employed, and as a result, the final mole ratio of fuel/CO2/H2O is selected to be 1:1:1.
In summary, for the gasification method using different agents including CO2, H2O and a mixture of CO2/H2O, the agent/fuel mole ratios are all 2:1, and for the CO2/H2O mixture, a CO2/H2O mole ratio of 1:1 is employed.

Supplementary Tables
Supplementary Table 1