Analysis of the synergistic benefits of typical technologies for pollution reduction and carbon reduction in the iron and steel industry in the Beijing–Tianjin–Hebei region

With its high energy consumption and pollutant emissions, the iron and steel industry is a significant source of air pollution and carbon emissions in the Beijing–Tianjin–Hebei (BTH) region. To improve air quality and reduce greenhouse gas emissions, a series of policies involving ultra-low emission, synergistic reduction of pollution, and carbon application have been implemented in the region. This study has assessed air pollutant and CO2 emission patterns in the iron and steel industry of the region by employing co-control effects coordinate system, marginal abatement cost curve, and numerical modeling, along with the synergistic benefits of typical technologies. The results have demonstrated that: (1) the intensive production activities pertinent to iron and steel enterprises has contributed greatly to the emission in Tangshan and Handan, where the sintering process is the main source of SO2, NOx, PM2.5, and CO, accounting for 64.86%, 55.15%, 29.98%, and 46.43% of the total emissions, respectively. (2) Among the typical pollution control and reduction measures, industrial restructuring and adjustment of the energy-resource structure have led to the greatest effects on emission reduction. Technologies exhibiting great potential in emission reduction and high-cost efficiency such as Blast Furnace Top Gas Recovery Turbine Unit (TRT) need to be promoted. (3) In Tangshan city with the highest level of steel production, the iron and steel production activities contributed to the concentration of 30.51% of PM2.5, 50.67% of SO2, and 42.54% of NO2 during the non-heating period. During the heating period, pollutants pertinent to the combustion of fossil energy for heating have increased, while iron and steel induced emissions have decreased to 23.7%, 34.32%, and 29.13%, respectively. By 2030, it is speculated that the contribution of the iron and steel industry to air quality will be significantly decreased as result of successful implementation of ultra-low emission policies and typical synergistic reduction technologies.

www.nature.com/scientificreports/near-surface temperature, wind speed, boundary layer height, etc. Accumulation of atmosphere pollutants over a long period can lead to changes in regional climate factors [6][7][8] .Research on the synergistic benefits of air pollution and climate has garnered significant attention from scholars 9 .The concept of "synergies" has also been introduced as early as the IPCC's third report 10 , which refers to the reduction of localized pollutant emissions in conjunction with the reduction of greenhouse gas (GHG) emissions, or the GHG abatement benefits resulting from the reduction of pollutants 11 .Therefore, analyzing the synergistic benefit of typical technologies in the iron and steel industry in the BTH region and identifying the corresponding emission reduction pathways can offer crucial support for pollution prevention and control.
The 14th Five-Year Plan for Ecological Environmental Protection has highlighted the importance of promoting synergies between pollution reduction and carbon emission reduction, which serves as the key to realizing a comprehensive green transformation of the economy and society.Policies such as the "Implementation Plan for the Reduction of Pollutants and Carbon Synergy" has explicitly called for actions to decrease air pollutants, promote energy conservations, and reduce carbon emissions in the iron and steel industries.Conducting a synergistic assessment of typical technologies in the iron and steel industry is crucial for achieving the goal of reducing pollution and carbon emissions 12,13 .Various methodologies for evaluating synergistic effects have also been studied.Mao et al. 14 has proposed three evaluation methods for co-control effects: the coordinate system, co-control cross-elasticity analysis, and unit pollutant abatement cost, all of which have been also applied to the iron and steel industry and other sectors for synergistic control assessment.Gao et al. 15 has proposed the synergy effect assessment index method to determine the synergy effects.Wang 16 has evaluated the synergistic effect of greenhouse gas and local air pollutant emission reduction in Tianjin by constructing a synergistic effect evaluation model.Deng 17 has developed a partial equilibrium model to assess the synergistic emission reduction effect of CO 2 , SO 2 , and NOx.Further analysis of the emission reduction potential and cost assessment of synergistic control technologies can assist in selecting the optimal approach.Yang et al. 18 has evaluated 28 energy-saving technologies in the iron and steel industry in the Yangtze River Delta region using energy-saving supply curves and scenario analysis, establishing a comprehensive bottom-up dynamic optimization model to simulate 48 development pathways for energy-saving and emission reduction technology.The iron and steel industry is a significant source of industrial pollution in the BTH region and the country as a whole.Although methods for assessing synergistic effects have been widely employed, there is still a lack of research on evaluating the synergistic control effect for emission reduction technologies of air pollution and CO 2 in the iron and steel industry, especially in the BTH region.
Identifying emission characteristics, establishing an inventory of pollution sources, and analyzing emission contributions along with numerical models are essential for synergistic control of pollution reduction and carbon reduction.China has initiated the compilation of air pollution emission inventories in the 1980s, with related studies also conducted in the iron and steel field 19 .There are three methods for establishing emission inventories: the online monitoring method, pollution source investigation method, and emission factor method 20 .Zhou et al. 21has compiled a multi-sectoral pollution emission inventory, including the iron and steel industry in Jiangsu Province, relying on the emission factor method.With the use of continuous online monitoring data of pollutants and environmental statistics, Bo et al. 22 has established a nationwide pollution emission inventory based on the main production processes in the iron and steel industry.Tang et al. 23 has developed an emission inventory of the iron and steel industry in China of year 2018 by using a bottom-up method, and assessed the impacts on atmospheric condition.Li et al. 24 has employed a combination of the emission factor method and GIS technology to establish an emission inventory of air pollution sources in the iron and steel industry in Tangshan.When integrated with numerical models, emission inventories can simulate the generation and dissipation of air pollution, offering technical support for air pollution prevention and control 18 .Duan et al. 2 has utilized a bottom-up approach to establish a detailed multi-pollutant emission inventory of the iron and steel industry in the BTH region for the year 2015, using WRF-CAMx model to simulate the emission impact on the regional PM 2.5 concentration.A detailed emission inventory contributes to an accurate understanding of the characteristics of pollution emissions from the iron and steel industry, forming the basis for the development of emission reduction policies [25][26][27] .However, there is still a lack of effort to develop a multi-pollutant emission inventory at process level in the BTH region under the new policy of synergistically reducing pollution and carbon.
To evaluate the synergistic control effect for pollution and carbon emission, along with its impact on air quality under the new policy, this paper is aimed to develop a pollutant emission inventory of the iron and steel industry in the BTH region using the emission factor method to analyze emission characteristics.The study has selected 10 typical technologies from the optimization of industrial structure, adjustment of energy structure, and energy-saving and emission reduction for assessing the synergistic effect.On this basis, the study has constructed scenarios for pollution and carbon reduction.Simultaneously, numerical models have been applied to analyze the impact of iron and steel pollutant emissions on air quality under these scenarios.The relevant findings can provide theoretical support for the future promotion of energy-saving technologies in the iron and steel industry in the BTH region.The study results and relevant technology also provide basis for future scientific research on source apportionment of air pollution and the synergistic emission reduction effect of climate change.

Establishment of emission inventories
This study utilized investigation and research to obtain the pollution production factors, average control efficiency, and activity levels of steel enterprises in the BTH region in 2015.Determining the emission factors required production factors and control efficiencies.The production factors were mainly obtained from Technical Manual for the Preparation of Urban Air Pollutant Emission Inventories and related literature 28,29 .The emission control efficiency of pollutants were determined based on information in Steelmaking Industry Factor Manual in the Manual of Methods and Factors for Accounting for Industrial Source Production and Emissions.In result, emission factors were collected and shown in Table 1.We calculated the 2015 pollutant emissions in the BTH region through top-down emission factor methodology.The formula was shown in Eqs.(1) and (2).
where E p,t,i represented emissions of pollutant i from production process p in enterprise e ; A i,j represented activity level of production process p in enterprise e , which was obtained through research; EF e,i represented emission factors for pollutant i in enterprise e ; PF e,i and PC e,i represented the production factors of the process, and the efficiency of pollution control, respectively.

Emission reduction potential
The emission reductions for a typical technology were established by the emission reduction factor and the level of activity.The formula was shown in Eq. ( 3): where E j,i represented emission reductions from technology j for pollutant i .B represented crude steel produc- tion.Q j ,i represented the emission reduction of per unit production for pollutant i after the implementation of technology j , which factor referred to Zhao 30 .
Emission reduction potential referred to the future emission reductions that can be achieved with the development and diffusion of technologies j .The total emission reduction potential of energy-saving and emission reduction technologies was calculated as shown in Eq. ( 4): where RP j,i represented the total abatement potential for pollutant i after the implementation of technology j ; r j represented technology promotion rate.
This study utilized the calculation of the Integrated Contaminant Emission Reduction potential (ICER) to evaluate comprehensive emission reduction effects.The values selected in this paper were shown in Table 2, which were updated based on the research of Gao et al. 31 .The ICER was calculated as: where ER LAP represented air pollutant emission reductions; ER GHG represented greenhouse gas emission reduc- tions; ER j represented the emission reduction of pollutant j ; α j represented the coefficient by which emission reduction of air pollutant j was converted to ER LAP ; β j represented the coefficient that converted the greenhouse gas emission reduction to ER GHG , for which only CO 2 was considered in this study ( β 1 = 1); W LAP and W GHG represented the coefficients of ER LAP and ER GHG converted to ICER, respectively.

Cost of emission reduction technology
The Marginal Abatement Cost Curve (MACC) served as an important method for assessing climate change policies and determining optimal paths [32][33][34] .This method integrated the abatement potential and the abatement costs, and it was defined as the ratio of the incremental cost of the abatement technology to the abatement potential.The cost of abatement technologies included investment and operating costs, energy saving benefits, and abatement benefits.The formula 30 was provided as following: where C j represented the technology cost of applying technologies j ; I A,j represented the investment cost of the technology j ; I AO,j represented the annual operating investment cost of the technology; E j represented the energy efficiency of the technology j ; AP j represented the emission reduction benefits of the technology j ; I j represented the cost of construction; n represented the repayment period, and relevant data were obtained from the results of the study by Ren 35 ; d represented the discount rate, taken as 20%; S represented energy savings per unit of technology; P represented energy prices per unit, where e and pe stand for electricity and primary energy, respectively, and primary energy prices, electricity prices were weighted average prices of coal and coke, and large industrial electricity prices ,which was 35 yuan/GJ and 0.84 yuan/kW h respectively; AP j represented emission reduction benefits of the technology j ; ∂, β, γ, δ were weighting factors for SO 2 , NOx, PM 2.5 and CO 2 respectively, which was 5730 yuan/t, 6390 yuan/t, 48,980 yuan/t 36 ,and 42.85 yuan/t 18 .
In MACC, the X-axis represents the abatement potential of technology for pollutant i , and the Y-axis repre- sents the unit pollutant abatement cost of the technology.In this paper, 1.5 yuan per kilogram was used as the high and low abatement cost cut-off point.0 yuan per kilogram was used as the positive and negative abatement cost cut-off point 32 .The measures with negative abatement cost were characterized by benefits outweighing costs.This type of technology can achieve pollutant emission reductions while being less costly compared to the other conventional technologies, making them worth for extensive promotion.The measures with low abatement costs resulted no benefit, but the investment cost for this type of technology was low.With the support of government subsidies and the enterprises' own investment, these technologies can progressively undergo upgrade 32 .Measures with high abatement cost required further investment in scientific research and more pilot scale trial after cost reduction is achieved 37 .

Assessment of synergy and effectiveness of pollution and carbon reductions
In this study, Weather Research and Forecasting model coupled to Chemistry (WRF-chem) was used to assess the impact of pollutant emissions from the steel industry in BTH on air quality.The settings used in the modeling were referred to our previous studies 38 .This study applied meteorological data of year 2019 as the initial and boundary condition field, provided by the National Centers for Environmental Prediction (NCEP).July and December were selected for the simulation of non-heating and heating period, respectively.The anthropogenic emission input consisted of power, industry, residential, transportation, and agriculture, which obtained from emission inventory of year 2017 in MEIC (the Multi-resolution Emission Inventory for China) (http:// meicm odel.org.cn).In this study, the grid emission data of the iron and steel industry in the BTH region were updated ) E j = S e,j × P e + S pe,j × P pe (10)   by the method introduced in "Establishment of emission inventories" section.Figure 1 showed the spatial distribution of iron and steel enterprises in the BTH region.On this basis, emissions have been updated for the 2030 scenario, taking into account of the successful implementation of ultra-low emission policy and the widespread adoption of representative low carbon technologies in 2030.Table 3 showed the specific program settings.We defined the impact of emission reduction on air quality as: where V 1 represented the impact of pollutant emissions from the iron and steel industry on air quality in BTH under the baseline scenario (scenario 2); V 2 represented the impact of pollutant emissions from the iron and steel industry on air quality in BTH under the pollution reduction and carbon reduction scenario (scenario 3);   www.nature.com/scientificreports/V * represented impact of implementation of ultra-low emission policy and adoption of typical technologies on air quality improvement compared with the baseline scenario.

Pollution characterization of air pollutant and CO 2 emissions
The emission pertinent to the iron and steel industry in the BTH region were calculated by the method introduced in "Establishment of emission inventories" section.Figure 2 showed that the spatial distribution of major air pollutants from the iron and steel industry in BTH.209.024Mt of crude steel was produced in BTH, with the highest level of production in Hebei province, reaching 188.32 Mt.Among cities of Hebei, Tangshan was the largest crude steel producer, with 82.697 Mt accounting for 39.6% of the total production in BTH.The second largest producer, Handan, contributed 43.582 Mt, accounting for 20.9% of the total production.The total emissions of SO 2 , NOx, PM 2.5 , VOC, CO, and CO 2 were 307.3 Kt, 296.0 Kt, 406.2 Kt, 235.4 Kt, 10,229.2Kt, and 340,459.9Kt, respectively.Due to the large number of iron and steel enterprises and their large scale production activities in Tangshan and Handan, the emission in those two cities were particularly high.The emissions in Tangshan accounts for 42.59%, 37.87%, 35.82% and 44.29% of total emission of PM 2.5 , SO 2 , NOx and CO 2 in BTH, respectively, while emission in Handan made up of 17.74%, 15.25%, 15.65% and 18.01% of total emission of those parameters, respectively.However, the emission inventory established in this study have uncertainty, which was mainly ascribed to random errors from obtaining the activity levels data and selecting the emission factors, which may affect the accuracy of the inventory results.Therefore, it is necessary to compare the established emission inventory results with existing studies, including literature 2 , databased from MEIC and the National Annual Report of Ecological and Environmental Statistics (NARES) (https:// www.mee.gov.cn/).According to the comparison shown in Fig. 3, similar distribution pattern of pollutant emissions was observed.The discrepancy could be mainly explained by difference in boundary condition and methods employed.For example, the calculation performed this study was primarily based on activity levels, while MEIC mainly utilized datasets from Continuous Emission Monitoring System (CEMS) and electric load.NARES obtained the emission level based on direct measurement in the enterprise operation, with more focus on ferroalloy related operations.www.nature.com/scientificreports/ In general, the inventory established in this study was generally consistent with the previously attained results, demonstrating great level of reliability of this inventory.Figure 4 showed the emission of pollutants from different production processes.SO 2 and NOx were mainly emitted from sintering process, accounting for 64.86% and 55.15% of total emissions of these two parameters.Sintering, ironmaking and steelmaking processes accounted for 29.98%, 28.57% and 26.20% of total PM 2.5 emission, respectively.VOC was mainly from sintering and coking, which made up of 31.52% and 59.11% of VOC emission, respectively.The relative contribution of sintering process to CO emission was 46.43%, while CO 2 was mainly from ironmaking, accounting for 60.05% of total emissions.
On this basis, we calculated the pollution emissions from the iron and steel industry in the BTH region in 2020.Figure 5 showed the comparison of air pollutant emissions between 2015 and 2020.In 2020, new policies (e.g."Opinions on Promoting the Implementation of Ultra Low Emissions in the Steel Industry" and "Iron and steel industry ultra-low emission transformation implementation plan") was published to accelerate the implementation of ultra-low emissions in the steel industry 39,40 .To implement the new policy, iron and steel enterprises in the BTH region phased out the outdated production capacity, improved the performance efficiency of pollution treatment facilities, and achieved the peak production level.According to NARES, the emission of SO 2 , NOx and PM were 65.59 Kt, 200.34 Kt and 65.18 Kt in 2020, respectively.According to MEIC the emission of SO 2 , NO x , VOC, PM 2.5 , CO and CO 2 were 65.4 Kt, 117.7 Kt, 115.9 Kt, 96.6 Kt, 9981.9Kt and 350,079.4Kt.Comparing with emissions in 2015, the results showed that the promotion of ultra-low emission retrofit enhanced the emission reduction of pollutants in the production chain.According to National Eco statistical Annual Report, SO 2 , NOx and PM were reduced by 80%, 25% and 92% from 2015 to 2020, respectively, as indicated in Fig. 5a. Figure 5b showed a results comparison based on MEIC inventories.According to MEIC, the reductions of SO 2 , NOx, PM 2.5 , and CO emission were 19.9%, 22.3%, 17.6%, and 34.6%, respectively.An increase in CO 2 emission with a growth rate of 22.8% was observed.The robust level of crude steel production had led to a substantial amount of CO 2 emissions.However, the emission control policy primarily targeted on other conventional pollutants during the 13th Five Year Plan period.According to China Statistical Yearbook 2021, crude steel production in 2020 was 271.49Mt, which was 29.88% higher than level obtained in 2015.The iron and steel industry underwent enormous pressure to reduce CO 2 emissions in the future.While addressing air pollutants, more attention needed to be given in BTH for synergistic CO 2 emission reduction.

Selection of typical technologies and evaluation of synergistic effects
An integrated assessment of various policies (e.g."Peak Carbon Implementation Program in Industry" and "Carbon Neutral Vision and Low Carbon Technology Roadmap for the Steel Sector"), "National Catalogue of Key Low Carbon Technologies for Promotion", and related literature 24,30,33,[41][42][43] was performed to evaluate three major aspects of pollution and carbon reduction policies and technologies: industrial structure optimization, energy structure adjustment, and application of energy-saving technologies.The energy-saving technologies were studied with primary focus on coking, sintering, ironmaking, steelmaking, and rolling.The background information of the selected emission control measures were provided in Table 4. Figure 6 displayed the emission reductions induced by different typical technologies, calculated by Eq. ( 3).Among the 10 listed emission abatement measures, industrial restructuring (i.e.T1) and adjustment of energyresource structure (i.e.T2) exhibited the highest emission reductions level for all types of pollutants.Among the energy saving and emission reduction technologies, T8 and T9 showed the best emission reduction performance.T8 led to the largest PM 2.5 emission reduction, accounting for 29.6% of the total technologically induced emission reduction.T9 resulted in the highest reduction of NOx, accounting for 33.2% of the total technologically induced reduction.Both T8 and T9 exhibited decent performance of emission reduction for SO 2 and CO 2 , accounting for 21.1% and 20.8% of the total SO 2 emission reduction, and 21.1% and 20.7% of the CO 2 emission reduction, respectively.
This study utilized coordinate system analysis to evaluate the effectiveness of various technologies for synergistic control of air pollutants and greenhouse gases.This approach involved plotting the emission reductions performance of air pollutants (X-axis) and CO 2 (Y-axis) achieved by the technology T1-T10 in a two-dimensional coordinate system.If a point fell within the first quadrant, it signified a synergistic reduction effect on both air pollutants and CO 2 .The farther away the points are from the origin, the better the synergy.An angle greater than 45 degrees between the line connecting the data points of the technology performance and the origin and the x-axis indicated that the technology resulted in greater reduction of air pollutants than CO 2 .Conversely, a smaller angle signified a stronger effect on CO 2 reduction.Points in the third quadrant suggested that the technology increased emissions of both pollutants.Points in the second and fourth quadrant indicated a lack of synergistical effect 37 .
In Fig. 7, emission reductions performance induced by T1-10 were plotted into the coordinate system to display the degree of synergistic control effects.The results revealed that all measures are situated in the first quadrant of the coordinates system, indicating synergistic control effects of those typical abatement technologies on SO 2 , NOx, PM 2.5 and CO 2 .Among all points, T1 and T2 were the farthest from the origin, indicating a pronounced synergistic effect on conventional pollutants (SO 2 , NOx, PM 2.5 ) and CO 2 .Lines connecting T4, T7 and T9 to the origin formed less than 45 degrees angles with X-axis, implying greater efficiency of reduction for SO 2 and NOx compared to CO 2 emission reduction.In the coordinate system of synergistic control of PM 2.5 and CO 2 , the angles formed by the line connecting T4, T7 and T9 to the origin and the X-axis were greater than 45 degrees.This contrasting trend suggested that the emission reduction of PM 2.5 was relatively lower than that of emission reduction of CO 2 .Therefore, emission reduction of SO 2 and NO x were also greater than that of PM 2.5 when removing the same amount of CO 2 .

Cost analysis of synergistic abatement of typical technologies
The investment cost, cost of construction and energy savings per unit were determined by referring to relevant literatures 10,[44][45][46] .Based on Eqs. ( 4)- (10), the abatement potential and abatement cost for each technology were calculated.Figure 8 provided the MACC of various abatement technologies.As introduced in "Cost of emission reduction technology" section, 1.5 yuan per kilogram was used as the high and low abatement cost cut-off point in this study.0 yuan per kilogram was used as the positive and negative abatement cost cut-off point 32 .www.nature.com/scientificreports/6 technologies with negative abatement cost technologies (i.e.T5, T6, T7, T8, T9, T10) were identified.These technologies showed effective emission reduction performance for SO 2 , NOx, PM 2.5 , and CO 2 , they also achieved cost reduction.Among these technologies, T5 was the least costly technology for the reduction of SO 2 , NOx, PM 2.5 and CO 2 , with marginal abatement costs of − 2523.16yuan/Kg, − 1971.56 yuan/Kg, − 2885.86 yuan/Kg, and − 5394.14 yuan/Kg, respectively.However, the abatement potential of T5 was low.In the MACC for SO 2 , PM 2.5 and CO 2 , T7 ranked with the second least cost only to T5, however, it exhibited greater abatement potential than     www.nature.com/scientificreports/T5, with emission reductions of 16.1 Kt, 12.2 Kt and 7556.8Kt, respectively.In the MACC of NOx, the cost of implementing T6, T7, T8 and T10 were also relatively low, ranging from − 465.26 to − 303.53 yuan/kg.Among them, T7 and T8 exhibited the highest emission reduction potentials of 6.54 Kt and 6.78 Kt, respectively.Among the technologies with positive marginal cost, T1 and T2 demonstrated higher abatement potential for all types of pollutants, accounting for more than 36% of the total abatement.In order to assess the cost-effectiveness of synergistic reductions in air pollution and greenhouse gases, Eq. ( 5) was used to calculate the reduction potential of integrated pollutants.The MACC of various measures for the combined pollutants were displayed in Fig. 9.According to calculation results, T5, T6, T7, T8, T9 and T10 were among abatement processes with negative abatement cost for dealing with integrated pollutants.T5 exhibited the lowest abatement cost of − 1971.56 yuan/Kg, while T7 and T9 showed the highest abatement potential, with reduction potential of 325.54 Kt and 421.00 Kt, respectively.In comparison, T1 and T2 incurred higher abatement cost, but it showed promising emission reduction result with 75.9% of pollutant reduction achieved.All six technologies with negative abatement cost involved comprehensive utilization of waste heat and waste energy, with the two abatement technologies exhibiting the highest abatement potential classfied under industrial structure and energy ratio adjustment.Therefore, in align with the demand for low-carbon and green development of the iron and steel industry, it would be crucial for BTH region to actively promote the comprehensive utilization of waste heat and energy technology.At the same time, continuously adjustment of industrial structure, optimization of the energy utilization, and the cost reduction were essential to achieve greater benefits.

Analysis of iron and steel emission impact on air quality
In this section, WRF-chem model was used to assess the impact of iron and steel emission on air quality.As described in "Assessment of synergy and effectiveness of pollution and carbon reductions" section, three scenarios were established.Scenario 2 evaluated the impact of emissions from the iron and steel industry on air quality in the BTH region under base year (Eq.11).Scenario 3 studied the effects of pollution and carbon synergy reduction (Eq.12).In scenario 2, all ultra-low emission retrofits were completed and typical technologies were applied till 2030, which would have meet the requirements specified by "Opinions on Promoting the Implementation of Ultra-low Emission in the Iron and Steel Industry".Typical technologies for synergistic reduction of pollution and carbon were evaluated mainly based on results of the cost curve in "Evaluation of typical technologies synergistic effects and cost effectiveness" section.The study selected three best synergistic technologies: Scrap-EAF, hydrogen steelmaking, and TRT.Hydrogen steelmaking was classified into hydrogen-based direct reduction and hydrogen-rich blast furnace smelting.Among them, hydrogen-rich blast furnace smelting was widely used 47 , and can be promoted as the main technology for steel emission reduction.
With the progressive development of technology, the effects of pollution and carbon reduction of each abatement technology had been gradually improved.Considering the data comparability and accessibility, the abatement factors reported by Zhao 30 were utilized.Therefore, the effects of emission reductions were updated in this section.We also obtained the promotion of the technologies in the target year according relevant literatures 18,33 .The emission reductions for the resulted scenarios were calculated according to Eq. ( 4), with results presented in Table 5.
Emissions from iron and steel for the baseline scenario were described in "Pollution characterization of air pollutant and CO 2 emissions" section.Figure 10 showed the comparison of pollutant emissions between the baseline and the synergy reduction scenario pertinent to pollution and carbon emission.To accomplish the synergistical effects of pollution and carbon reduction, enterprises completed the ultra-low emission retrofit and implemented the above three emission reduction technologies.The results showed promising emission reduction effects on SO 2 , NOx and PM 2.5 .At the same time, pollution and carbon reduction measures exhibited a significant effect on CO 2 reduction.Compared with the baseline scenario, emissions of SO 2 , NOx, PM 2.5 and CO 2 from the iron and steel industry in BTH were reduced by 0.2968Mt, 0.248Mt, 0.3765Mt and 128.61Mt, corresponding to a reduction efficiencies of 96.58%, 83.78%, 92.69% and 37.78% respectively.www.nature.com/scientificreports/On the basis of inventory calculation, the WRF-chem model was used to simulate the emission impact on air quality.Figure 11 illustrated the spatial distribution of the impacts on regional air quality (PM 2.5 , SO 2 , NO 2 ) under base year (Eq.11). Figure 11a showed the impact on air quality during non-heating period, and Fig. 11b showed the impact during heating period.Under the baseline scenario, the contribution of pollutant emissions from the iron and steel industry to PM 2.5 , SO 2 , and NO 2 in the BTH region ranges from 0-24 μg/m 3 , 0-28 μg/m 3 , and 0-44 μg/m 3 in the non-heating period.During the heating period, the contribution ranged from 0-46 μg/ m 3 , 0-42 μg/m 3 , and 0-42 μg/m 3 , respectively.The major emission contribution mainly occurred around the cities with intensive industrial activities related to iron and steel enterprises, such as Tangshan, Shijiazhuang and Handan.This simulation was consistent with the spatial distribution of Fig. 2. The high iron and steel emission contributions were also due to inadequacy in implementation of ultra-low emission retrofit project in 2015, which resulted in a relatively low pollution control efficiency.The city such as Beijing had been less affected by iron and steel emission thanks to fewer iron and steel production operation in the city.The monthly average concentrations of PM 2.5 , SO 2 and NO 2 were 2.53 μg/m 3 , 2.34 μg/m 3 and 3.36 μg/m 3 , respectively.However, the rise in PM 2.5 in the neighboring areas due to elevated level of iron and steel industry operation would lead to obvious impact on Beijing's air quality.Among cities with iron and steel production, the relative emission contribution to total PM 2.5 , SO 2 , and NO 2 of BTH region were 30.51%,50.67%, and 42.54% during non-heating period in Tangshan, respectively.In Shijiazhuang, the pollution output of iron and steel industry accounted for 30.74%,43.43%, 47.47% of the total regional pollution concentration of PM 2.5 , SO 2 , and NO 2 , respectively.During heating period, the relative contribution of pollution emissions to total PM 2.5 , SO 2 and NO 2 were 25.31%, 37.03% and 41.24% in Shijiazhuang, respectively, while 23.7%, 34.32% and 29.13% were attributed to iron and steel production in Tangshan.The pollutants emitted from the combustion of fossil energy for heating increased due to the heating season demand.The production activities of iron and steel industries were restricted, which resulted in reduced emission.Table 6 showed the concentration contribution to air quality from iron and steel enterprises in key cities.
Figure 12 showed the spatial distribution of the impacts on regional air quality (PM 2.5 , SO 2 , NO 2 ) under policy of synergistic reduction of pollution and carbon (Eq.12).Compared to the baseline scenario, the contribution of the iron and steel industry to air quality significantly decreased under the pollution reduction and carbon reduction scenario.In the non-heating season, the pollution abatement resulted in the reduction of 9.2 μg/m 3 , 11.75 μg/m 3 , and 15.5 μg/m 3 in the average PM 2.5 , SO 2 , and NO 2 concentration across the modelled region, respectively, while 20 μg/m 3 PM 2.5 , 19 μg/m 3 SO 2 , and 17 μg/m 3 NO 2 were reduced during the heating season.Although intensive iron and steel operations in Tangshan, Shijiazhuang and Handan, their contributions of to total iron and steel emission were limited.As Table 6 shown, in the non-heating season, the contribution of the iron and steel industry in Beijing, Shijiazhuang, and Tangshan to the local PM 2.5 concentration was reduced by 2.08 μg/m 3 , 13.1 μg/m 3 , and 18.65 μg/m 3 , respectively, corresponding to a 51.61%, 67.48%, and 64.55% decrease  Table 6.Impacts on air quality in representative cities in the modeled region under the baseline scenario, the pollution reduction and carbon reduction scenario, and pollution abatement.where V 1 represented the impact of pollutant emissions from the iron and steel industry on air quality in BTH under the baseline scenario (scenario 2); V 2 represented the impact of pollutant emissions from the iron and steel industry on air quality in BTH under the pollution reduction and carbon reduction scenario (scenario 3); V * represented the impact of completion of ultra-low emission policy and diffusion of typical technologies on air quality improvement compared with the baseline scenario.

Pollutant factor
Non-heating period Heating period

Conclusions
In this study, the emissions of iron and steel industry in the BTH region were calculated by emission factor method.In 2015, the emission of SO 2 , NOx, PM 2.5 , VOC, CO, CO 2 were 307.3 Kt, 296.0 Kt, 406.2 Kt, 235.4 Kt, 10,229.2Kt, and 340,459.9Kt.Among them, sintering, pelletizing, coking and ironmaking were the main emission sources.Tangshan and Handan were the largest emission contributors with the highest air pollutant emission associated with iron and steel industry in the BTH region.After implementation of ultra-low emissions policy, the emission of pollutants in the production chain significantly decreased.On this basis, the study evaluated the effectiveness of synergistic control and economic cost of 10 typical emission abatement technologies.The technologies were selected from three aspects: optimization of industrial structure, adjustment of energy ratios, and implementation of energy-saving and emission reduction measures.The results showed that the technologies exhibited synergistic emission reduction effects on SO 2 , NOx, PM 2.5 and CO 2 emission.Flue gas waste heat recovery, dehumidification blast technology for blast furnace, TRT, CCPP, dry recovery technology for converter gas and process control for hot rolling mills were highlighted with negative marginal cost, indicating greater economic efficiency.Scrap iron-electric arc furnace steelmaking and hydrogen steelmaking were more costly but demonstrated higher potential for emission reduction, which should also be widely promoted.
Moreover, WRF-chem model was used to explore the impact on air quality of iron steel emission under different scenarios.According to the simulation, emission mainly occurred around the cities with a high amount of iron and steel production, such as Tangshan, Shijiazhuang and Handan.Under the baseline scenario, the pollutant emissions from the iron and steel industry for PM 2.5 , SO 2 , and NO 2 in the BTH region ranges from 0-24 μg/ m 3 , 0-28 μg/m 3 , and 0-44 μg/m 3 , respectively, during the non-heating period.During the heating period, the concentration ranged from 0-46 μg/m 3 , 0-42 μg/m 3 , and 0-42 μg/m 3 , respectively.By 2030, after implementation of the ultra-low emission policy and typical synergistic reduction technologies, it would be likely that the contribution of the iron and steel industry to air quality significant decreased.For future efforts, it would be crucial to promote abatement measures with high abatement cost and perform pilot program after achieve cost reductions.
of iron and steel sources from industrial sources in the original MEIC inventory Scenario 2 Deduction of iron and steel sources from industrial sources in the original MEIC inventory and addition of the iron and steel sector emissions inventory for 2015 Scenario 3 Deduction of iron and steel sources from industrial sources in the original MEIC inventory and addition of the iron and steel sector emissions inventory for 2030 Vol:.(1234567890)Scientific Reports | (2024) 14:12413 | https://doi.org/10.1038/s41598-024-63338-8

Figure 2 .
Figure 2. Distribution of enterprises and spatial distribution of major air pollutants pertinent to the iron and steel industry in BTH.Note: This figure was generated by the authors by using Arc Geographic Information System (Arcgis) Vision10.8(https:// www.esri.com/ zh-cn/ arcgis/ produ cts/ arcgis-deskt op/ overv iew).

Figure 3 .
Figure 3.Comparison of the inventory results with existing studies.

Figure 4 .
Figure 4. Proportion of pollutant emissions from different processes of iron and steel in BTH region.

Figure 5 .
Figure 5.Comparison of pollutant emissions in 2015 and 2020.
This technology involved cooling the high temperature flue gas and passing it through an electrostatic precipitator.The filtrated flue gas was recovered and the unfiltrated flue gas was ignited and dischargedT10Process control technology for hot rolling mills Rolling This technology combined big data analysis with mathematical modeling to adjust the hot rolling process while monitoring all key parameters of the process

Figure 6 .
Figure 6.Emission reduction of air pollutants and CO 2 by various pollution abatement measures.

Figure 7 .
Figure 7. Coordinate system for synergistic control of air pollutants and CO 2 .

Figure 8 .
Figure 8. MACC of Air Pollutants and CO 2 for iron and steel industry in BTH region.

Figure 9 .
Figure 9. MACC of integrated pollutants for iron and steel sector in BTH Region.

Figure 11 .
Figure 11.Air quality impacts of iron and steel industry emissions under baseline scenarios in the BTH region.Note: This figure was generated by the authors by using NCAR Command Language (Vision 6.4) (URL: 10.5065/ D6WD3XH5).

Figure 12 .
Figure 12.Air quality impacts of iron and steel industry emissions under synergistic emission reduction scenario in BTH region.Note: This figure was generated by the authors by using NCAR Command Language (Vision 6.4) (URL: 10.5065/D6WD3XH5).

Table 1 .
Pollutant emission factors for the iron and steel industry (g•Kg −1 ). a Represents the pollution gas discharged through the exhaust pipe.b Represents the pollution gas discharged irregularly without exhaust pipe.

Table 2 .
Selection of parameters for ICER calculations.aData from appendix 1 of the Environmental Protection Tax Act.bData from the Carbon Trading Platform of China (http:// www.tanji aoyi.com/).cData from appendix 2 of the Environmental Protection Tax Act.LAPeq According to the Environmental Protection Tax Law, the tax for air pollutant ranges from 1.2 to 12 yuan/LAPeq a .The study set 6 yuan/ LAPeq W GHG 0.0046 IAPeq/kgCO 2 eqThis study used the average price of carbon emissions trading piloted in 8 provinces and cities from 2017 to 2020 for discounting b , which is 0.02735 yuan/kg.According to the pollutant tax and carbon price, we obtain the value of W GHG .W GHG = 0.02735 6 = 0.0046 IAPeq/kgCO 2 eq SO 2 α 1 1/0.95LAPeq/kg The air pollutant equivalent conversion factor is the reciprocal of the pollution equivalent.According to the Environmental Protection Tax Law, pollution equivalent values for SO 2 , NOx, and PM 2.5 were 0.95 kg, 0.95 kg, and 2.18 kg, respectively c Vol.:(0123456789) Scientific Reports | (2024) 14:12413 | https://doi.org/10.1038/s41598-024-63338-8

Table 4 .
Description of the selected synergistic control measures.

Table 5 .
Emission reductions from the iron and steel sector under the synergistic scenario of pollution reduction and carbon reduction/10 4 t.Ultra-low emission retrofitBased on the annual reports of the key iron and steel enterprises, we obtained the pollutant removal efficiency and updated the pollutant emission factors 100 24.1917.8330.96Scrap-EAFPollutantemissionintensity of SO 2 , NOx and PM 2.5 was 0.05, 0.12 and 0.05 kg/t, respectively 48 30 1.66 2.74 2.55Hydrogen-rich blast furnace smelting When the blowing volume of ammonia reached 100 m 3 /t, the coke usage can be reduced by 16.6 kg49 20During the heating period, the contribution of the iron and steel industry in Beijing, Shijiazhuang, and Tangshan to local PM 2.5 was reduced by 9.93 μg/m 3 , 10.27 μg/m 3 , and 29.79 μg/m 3 , respectively.The contribution to SO 2 concentration was reduced by 10.44 μg/m 3 , 15.04 μg/m 3 , and 21 μg/m 3 , respectively.The contribution to NO 2 concentration decreased by 13.48 μg/m 3 , 31.21 μg/m 3 , and 21.07 μg/m 3 With the implementation of ultra-low emission retrofits and synergistic control measures, pollutant concentration had reduced more substantially.
Figure 10.Comparison of pollutants emissions in BTH's iron and steel industry between 2015 and 2030.