Analysis of Carbon Dioxide Emissions in Bottom-blowing O 2 -CO 2 -CaO Process in Basic Oxygen Furnace Steelmaking

Abstract

The O₂-CO₂-CaO process, which blows the lime powder from the bottom, offers significant advantages in terms of reducing the consumption of raw and auxiliary materials while improving steel quality. This study examines the carbon dioxide (CO₂) emissions associated with four different converter smelting processes, using industrial test production data from nearly 1000 furnaces in a steel plant. This is the first comprehensive analysis of CO₂ emissions for these processes and quantifies the CO₂ emissions reduction potential of the bottom-blowing O₂-CO₂-CaO process. The study shows a significant difference in CO₂ emissions between the duplex and conventional smelting processes, with the former exhibiting significantly higher emissions. The application of bottom-blowing O₂-CO₂-CaO process led to a decrease of 24.6 kgCO₂/t in the duplex converter smelting process and 30.1 kgCO₂/t in the conventional smelting process.

Keywords

conventional converter duplex converter

Introduction

The Fifth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) underscores that human activities, particularly the emissions of greenhouse gases (GHGs), are the primary drivers of climate change, posing significant threats to both human civilisation and natural ecosystems.¹ Among the six GHGs outlined in the Kyoto Protocol, carbon dioxide (CO₂) stands out as the most influential contributor to the greenhouse effect.² Within this context, the steel industry emerges as a notable player due to its substantial consumption of materials and energy, which often results in high carbon emissions. According to data from the International Energy Agency, global CO₂ emissions from energy-related sources reached approximately 33 billion tonnes in 2019, with the steel industry accounting for nearly 14% of this total.³

China, a pivotal force in global urbanisation and industrialisation, plays a particularly significant role in the steel industry, contributing around 57% of the world’s steel production.⁴ In 2018, the global steel industry was responsible for roughly 7% of the planet’s total carbon emissions from energy consumption.⁵ Notably, within China, the steel industry’s energy-related carbon emissions comprised approximately 13% of the nation’s overall carbon emissions.^6,7 These statistics underscore the considerable impact of the steel industry on global carbon emissions and the urgent need for mitigation efforts. The Chinese government has made a commitment to reach its peak CO₂ emissions by 2030 and attain carbon neutrality by 2060.⁸ Among industries, the iron and steel sector stands out for its significant fossil energy consumption and carbon emissions. Therefore, reducing the CO₂ emissions from this industry is crucial for meeting the ‘dual carbon’ target of achieving both carbon peak and carbon neutrality.

Variations in technological processes, product structures, and raw material sources among iron and steel enterprises result in significant differences in CO₂ emissions. Iron and steel production can be broadly categorised into two types: long-flow steelmaking processes, utilising blast furnace-basic oxygen furnace (BF-BOF), and short-flow steelmaking processes, employing electric arc furnace.⁹ Research by the World Steel Association has revealed that the long-flow steelmaking process emits approximately 3.5 times more CO₂ per tonne of steel compared to the short-flow method.¹⁰

China's steel production predominantly relies on the long-flow steelmaking process, which contributes to the notably high average CO₂ emissions per tonne of steel in the country’s steel industry. In 2021, China’s crude steel output reached 1.03 billion tonnes, representing about 54% of global production. Of this, approximately 90% was produced using the long-flow process, with the short-flow process accounting for the remaining 10%.¹¹ Given these statistics, reducing carbon emissions during long-flow processes, particularly in converter steel production, holds immense significance for advancing the Chinese steel industry's development and environmental sustainability.

Significant strides have been made in developing green and low-carbon technology for ironmaking, particularly in the context of long-flow steelmaking involving BF-BOF systems.⁷ However, scant attention has been paid to similar advancements in the BOF domain. Addressing this gap, we have pioneered a lime powder spraying technique at the base of a converter during steelmaking, a method that underwent rigorous industrial testing and found practical application in the dephosphorisation converter of a steel plant's duplex converter.^12,13

This innovative approach involves introducing lime powder into the molten bath of the converter, carried by a blend of O₂ and CO₂ as the carrier gases, from the converter's base. This method ensures continuous and controllable feeding, facilitating more precise regulation of reactions within the molten steel, and enhancing the utilisation efficiency of oxygen and lime. Industrial tests have demonstrated that this bottom-blowing O₂-CO₂-CaO process in the converter provides favourable thermodynamic conditions for dephosphorisation. It substantially increases the phosphorus distribution ratio, surpassing conventional converter methods by 2–4 times, thereby simultaneously reducing the consumption of raw and auxiliary materials for steelmaking while achieving superior dephosphorisation outcomes.

Furthermore, the adoption of the bottom-blowing O₂-CO₂-CaO process leads to substantial reductions in raw material consumption and iron and steel usage, while also enhancing the recovery and calorific value of basic oxygen furnace gas (BOFG). In this study, we conducted a comprehensive analysis of CO₂ emissions associated with the converter smelting processes, based on industrial test findings. For the first time, CO₂ emissions evaluations were performed across four different converter smelting methodologies: the duplex converter smelting process, duplex converter bottom-blowing O₂-CO₂-CaO smelting process, conventional converter smelting process, and conventional converter bottom-blowing O₂-CO₂-CaO smelting process.

Process description

We successfully implemented the bottom-blowing O₂-CO₂-CaO process in a 300-tonne converter at Shougang Jingtang United Iron & Steel Co. Ltd., located in Tangshan, Hebei, People’s Republic of China. At this steel plant, there are two primary methods for operating the converter steelmaking process. The first method involves both the dephosphorisation and decarbonisation of hot metals within the same converter, known as the conventional converter smelting method. Building upon this, the second method has been developed. It utilises two converters operating jointly: one for dephosphorisation and the other for decarburisation. In this approach, low-phosphorus hot metal from a dephosphorisation furnace is used, known as the duplex converter smelting method. We have independently developed an advanced automated steelmaking model that streamlines the entire steelmaking process. This model handles the loading, pressure charging, blowing, and exhausting of the powder spraying system autonomously. Moreover, it adjusts process parameters independently based on raw material conditions and desired outcomes during the blowing process. This innovation achieves ‘one-key steelmaking’, significantly reducing the need for manual intervention. Consequently, it enhances the stability and reliability of the bottom-blowing system.

In Figure 1, we illustrate the schematic of the bottom-blowing O₂-CO₂-CaO process. This method involves spraying most or all of the lime required for smelting in powder from the converter’s bottom. The maximum flow rate of powder spraying rate is designed to be 240 kg/min, corresponding to a spraying intensity of 0.8 kg/(t·min). Each furnace can receive 6–12 kg/t of powder spraying, effectively meeting the lime demand needed for dephosphorisation. The carrier gas for this process is an O₂-CO₂ mixture, flowing at a rate of 2700 Nm³/h. The bottom-blowing intensity can reach up to 0.2 Nm³/(t·min).

Through the implementation of this technology, we witnessed significant enhancements in the metallurgical performance of dephosphorisation converters. Lime consumption saw a notable decrease from 10.1 kg/t to 7.1 kg/t, while the final slag’s T.Fe content decreased substantially from 25.1% to 8.81%. Additionally, steel material usage decreased on average by 2.3 kg/t, with the dephosphorisation rate increasing by an average of 4.0%. These improvements not only resulted in substantial economic gains but also facilitated the establishment of a low-cost, efficient, and environmentally friendly steel production platform.

In this study, we conducted CO₂ emissions calculations for the converter smelting process based on the industrial test results. We analysed carbon emissions across four different converter smelting techniques: the duplex converter smelting process (Route 1), the duplex converter bottom-blowing O₂-CO₂-CaO process (Route 2), the conventional converter smelting process (Route 3), and the conventional converter bottom-blowing O₂-CO₂-CaO process (Route 4). Schematic diagrams illustrating these processes are presented in Figure 2.

It is important to recognise that because the bottom-blowing O₂-CO₂-CaO process primarily serves for dephosphorisation. Through this process in the duplex smelting procedure, the phosphorus content of the semi-steel endpoint in the dephosphorisation converter is typically reduced to less than 0.030% on average. Consequently, the bottom-blowing O₂-CO₂-CaO method is no longer utilised in the decarburisation converter.

Calculation of CO₂ emissions

The process of calculating CO₂ emissions in the converter smelting involves several key steps for clarity and precision¹⁴: (1) establishing a method for calculating CO₂ emissions, (2) defining the boundaries of the system under examination, (3) developing the CO₂ emissions factor; and (4) executing the calculation for CO₂ emissions.

At the heart of this method lies the principle of maintaining a carbon elemental balance.

Main calculation methods for CO₂ emissions

Currently, there is no standardised method for calculating CO₂ emissions in the iron and steel industry. Various countries and organisations have proposed their own methods, typically falling into three categories³: (1) the IPCC calculation method, which focuses on national-level GHG emissions and only accounts for direct emissions from the iron and steel industry to prevent double counting between sectors; (2) the WSA calculation method, which aligns with GHG emissions agreements and it is suitable for calculating CO₂ emissions at the level of individual iron and steel enterprises, dividing emissions into direct emissions, indirect emissions from purchased electricity, and other indirect GHG emissions; and (3) the LCA method, which offers a bottom-up approach, meticulously calculating emissions throughout the production process for a more detailed and accurate assessment.^15–17

In iron and steel production, the CO₂ emissions associated with a particular input material are determined by multiplying the CO₂ emissions factor of the material by the quantity of input. Within an iron and steel enterprise, the CO₂ emissions per unit of product produced in a specific process are calculated by summing the product of the CO₂ emissions factor (EF) and the input quantity for each material used and then subtracting the cumulative sum of the product of the CO₂ emissions factor deduction value and the output of products and by-products exported from the process. This calculation formula is expressed in Equation (1).

P C E = \sum_{i = 1}^{m} E F_{i} \times M_{i} - \sum_{j = 1}^{n} E F_{j} \times D_{j}

(1)

Where

P C E

stands for process CO₂ emissions (tCO₂),

E F

is the emissions factor (tCO₂/t, tCO₂/m³), M and D are the input amounts of materials and products entering the process, and the output of by-products (t, m³), respectively.

In analysing the CO₂ emissions resulting from both top and bottom re-blowing of the converter, Feng¹⁸ highlighted significant challenges. These challenges stem from the complexities involved in accurately quantifying CO₂ emissions during the transportation process of certain raw materials and variations in processes and equipment upstream. As a result, there exists substantial variability in the indirect CO₂ EFs. To address this, simplifications were made in the parameters used for CO₂ emissions calculations.

Specifically, indirect emissions originating from carbon-containing raw materials, fossil fuels, hot metals, molten steel, scrap, and converter gas were omitted from the calculation process. Instead, the focus was narrowed down to direct CO₂ emissions arising from carbon-containing raw materials at the carbon input stage of carbon, along with indirect CO₂ emissions from non-carbon-containing raw materials and energy sources. Additionally, consideration was given to CO₂ emissions credits obtained from products and by-products. The comprehensive formula for calculating total CO₂ emissions is provided in Equation (2):

E_{C O_{2}} = \sum_{d = 1}^{n} M_{d} \times E F_{d} + \sum_{i = 1}^{m} M_{i} \times E F_{i} - \sum_{c = 1}^{q} M_{c} \times E F_{c}

(2)

Where

E_{C O_{2}}

is the total CO₂ emissions of simplified process (tCO₂),

M_{d}

is the quantity of carbon-containing raw materials and energy at carbon input end (t, km³),

E F_{d}

is the direct CO₂ emissions factor of carbonaceous raw material at carbon input end (tCO₂/t, tCO₂/m³),

M_{i}

is the quantity of non-carbonaceous raw materials and energy at carbon input end (t, km³),

E F_{i}

represents the indirect CO₂ emissions factor of non-carbonaceous raw materials at carbon input end (tCO₂/t, tCO₂/m³),

M_{c}

is the quantity of products and by-products (t, km³), and

E F_{c}

demonstrates the CO₂ emissions deduction factors of products and by-products (tCO₂/t, tCO₂/m³).

To facilitate a clearer comparison of CO₂ emissions levels across four processes (i.e. the duplex converter smelting process, duplex converter bottom-blowing O₂-CO₂-CaO process, conventional converter smelting process, and conventional converter bottom-blowing O₂-CO₂-CaO process), a calculation model was primarily utilised in the calculations in this study’s analyses.

System boundary determination

In this section, we examine the boundaries of the converter smelting process, encompassing both the duplex converter smelting method and the conventional converter smelting method. We designate the boundary of the duplex converter smelting process as Boundary 1, while the boundary specific to the bottom-blowing O₂-CO₂-CaO process in the duplex converter is termed Boundary 2. Similarly, we identify the boundary of the conventional converter smelting process as Boundary 3, with its bottom-blowing O₂-CO₂-CaO process boundary designated as Boundary 4.

It is important to note that our calculation of CO₂ emissions in the converter smelting process focuses solely on direct emissions. This excludes any indirect emissions stemming from carbon-containing raw materials, fossil fuels, hot metals, molten steel, scrap, and BOFG. Instead, we deduct direct CO₂ emissions originating from carbon-containing raw materials at the carbon input stage, as well as indirect CO₂ emissions from non-carbon-containing raw materials and energy sources, along with those generated at the output stage and by-products.

At the input stage, we primarily consider raw and auxiliary materials such as hot metal, scrap, lime, dolomite, and sinter, along with the energy medium comprising electricity, O₂, N₂, CO₂, and Ar, as well as the alloy, aluminium. At the output stage, we account for molten metal, while deductions include BOFG and slag.

Routes 1 and 2 represent duplex smelting processes wherein semi-steel, initially produced by the dephosphorisation converter, undergoes further smelting in the decarburisation converter. The system boundary comprises two distinct components: the dephosphorisation converter and the decarburisation converter. Semi-steel serves as both the output end of the dephosphorisation converter and the input for the decarburisation converter. Due to the semi-steel’s carbon content exceeding 3.0% in the dephosphorisation converter, the CO concentration in the BOFG is low, rendering the BOFG from the dephosphorisation converter non-recyclable. Consequently, deductions from the output of the dephosphorisation converter exclude the BOFG. Deoxidation is unnecessary during the tapping process of the dephosphorisation converter, thus eliminating the need for alloy components at its input. In the smelting process of the decarburisation converter, scrap steel is not added, meaning the input for this converter lacks scrap material. These boundaries are illustrated in Figure 3 as Boundary 1 and Boundary 2.

Boundaries 3 and 4 denote conventional smelting methods where hot metals are directly transformed into low-carbon molten steels. Much like the duplex smelting process, the beginning stage of Boundaries 3 and 4 predominantly involves raw and auxiliary materials (such as hot metal, scrap, lime, dolomite, and sinter ore), energy mediums (electricity, O₂, N₂, CO₂, and Ar), and alloy additives (such as aluminium). On the output side, we obtain steel, while by-products include, BOFG and slag. In the conventional bottom-blown O₂-CO₂-CaO smelting process, lime powder is sprayed from the bottom of the converter into the molten bath, utilising an O₂-CO₂ mixture as the carrier gas. This action serves to lower the temperature in the fire-point region of the bottom-blown air outlet and decelerate the erosion rate of the bottom-blown lance.¹⁹ Consequently, CO₂ is also present in the energy medium at the input end of Boundary 4. Boundaries 3 and 4 are visually depicted in Figure 4.

CO₂ emissions factor determination

The CO₂ emissions factor plays a pivotal role in calculating the CO₂ emissions from the converter process. It essentially quantifies the amount of CO₂ released per unit of energy utilised. This factor can be categorised into three main types based on how it is generated: direct CO₂ emissions factor, indirect CO₂ emissions factor, and CO₂ emissions reduction factor.

The direct CO₂ emissions factor pertains to emissions resulting from the consumption of fossil fuels during the converter smelting process. On the other hand, the indirect CO₂ emissions factor accounts for emissions stemming from the consumption of energy carriers, as well as raw and auxiliary materials in the same process. Lastly, the CO₂ emissions deduction factor involves emissions offset by certain by-products during converter smelting.

By understanding and appropriately applying these factors, one can accurately assess the CO₂ emissions associated with the converter process.

The direct CO₂ emissions factor in the converter smelting process primarily comprises emissions stemming from carbonaceous energy, raw and auxiliary materials, and carbonaceous materials. To prevent redundancy in CO₂ calculations, emissions from the upstream production and transportation of carbonaceous materials are excluded. Instead, only the fixed carbon content of these materials is considered, which is then converted into a CO₂ emissions factor using Equation (3).

E F_{C O_{2}} = C C \times \frac{44}{12}

(3)

Where:

E F_{C O_{2}}

and

C C

are the CO₂ emissions factor and the fixed carbon content of carbonaceous materials, respectively.

In this study, we analysed the carbon contents of various materials: hot metal, semi-steel, molten steel, and scrap, revealing concentrations of 4.30%, 3.20%, 0.03%, and 0.08%, respectively. Calculated using Equation (3), the CO₂ emissions factors were determined as 0.1577, 0.1173, 0.0011, and 0.0029 t CO₂/t for each material, respectively. The compositions of BOFG from both duplex and conventional smelting processes were found to be similar, as detailed in Table 1. Considering the CO₂ emissions factor of the carbon fraction in the gas, the CO₂ emissions factor of BOFG was determined to be 1.551 t CO₂/km³.

Table 1.

BOFG composition.

Composition	CO	CO₂	N₂	O₂	H₂
Content/%	56.0 ± 2.0	23.0 ± 3.2	19.5 ± 1.5	0.8 ± 0.2	0.7 ± 0.1

CO₂: carbon dioxide; BOFG: basic oxygen furnace gas.

Key indirect CO₂ emissions within the defined system boundary stem from the consumption of energy resources such as electricity, water, oxygen, nitrogen, argon, and CO₂. Feng¹⁸ emphasised that these energy resources are typically generated through processes involving electricity, which itself does not inherently contain carbon. Consequently, calculating the CO₂ emissions associated with these resources requires conversion using the CO₂ emissions factor of electricity. Thus, the choice of the CO₂ emissions factor for electricity is pivotal. Following GB/T34194-2017 guidelines, the CO₂ emissions factor for electricity was set at 0.9419 t CO₂MW·h. The specific CO₂ emissions factors for each energy resource are detailed in Table 2.

Table 2.

CO₂ emissions factors for various energy media.

Item	Units (unit)	Emission factor (tCO₂/unit)
Electric	MW·h	0.9419
O₂	km³	0.6146
N₂	km³	0.1295
Ar	km³	4.7057
CO₂	km³	1

CO₂: carbon dioxide.

For the auxiliary materials such as lime, light-burnt dolomite, sinter, and aluminium, Feng¹⁷ proposed to take the default values of WSA, which are 0.7832 tCO₂/t, 0.8619 tCO₂/t, 0.3290 tCO₂/t, and 9.4400 t CO₂/t, respectively; the main solid by-product at the output end is slag, which is recommended to be used for cement production using the average CO₂ emissions crediting factor of slag production, 0.3 tCO₂/t.

In summary, the CO₂ emissions factors of the input, output, and deduction items involved in the system boundary of this paper are shown in Table 3.

Table 3.

CO₂ emissions factors.

Item	Units (unit)	Emission factor (tCO₂/unit)
Hot metal	t	0.1577
Semi-steel	t	0.1173
Scrap	t	0.0029
Lime	t	0.7832
Dolomite	t	0.8619
Sinter	t	0.3290
Aluminium	t	9.4400
Electricity	MW·h	0.9419
O₂	km³	0.6146
N₂	km³	0.1295
Ar	km³	4.7057
CO₂	km³	1.0000
BOFG	km³	1.5510
Slag	t	0.3000
Molten steel	t	0.0011

CO₂: carbon dioxide; BOFG: basic oxygen furnace gas.

Regarding auxiliary materials such as lime, dolomite, sinter, and aluminium, Feng¹⁷ suggested adopting the default values provided by WSA: 0.7832, 0.8619, 0.3290, and 9.4400 t CO₂/t, respectively. The primary solid by-product, slag, recommended for cement production, is assigned an average CO₂ emissions crediting factor of 0.3 t CO₂/t. The CO₂ emissions factors for input, output, and deduction items pertinent to the system boundaries of this study are summarised in Table 3.

Analysis of results

This section delves into the CO₂ emissions calculation method, system boundary, and CO₂ emissions factor to analyse the smelting data from both the duplex and conventional converters in a steel plant. It encompasses the production data of almost 1000 furnaces for each of the four processes and conducts CO₂ emissions calculations accordingly.

Analysis of CO₂ emissions by Route 1 and Route 2

By analysing the material consumption data from approximately 1000 furnaces following Route 1 smelting and nearly 800 furnaces following Route 2 smelting at this steel plant, we observed that the duplex smelting process, particularly with the implementation of the bottom-blowing O₂-CO₂-CaO method, significantly reduces the consumption of both raw and auxiliary materials.

To evaluate the carbon emissions, we focused on two primary aspects: carbon emissions from the dephosphorisation converter and those from the decarbonisation converter in both Route 1 and Route 2. Utilising predefined system boundaries and emissions factors for each substance established earlier, along with field production data on material consumption, we calculated the CO₂ emissions specifically from the dephosphorisation converter of Route 1, as detailed in Table 4.

Table 4.

CO₂ emissions from the dephosphorization converter in Route-1.

Category	Project	Units (unit)	Quantity (unit/t)	Emission factor (tCO₂/unit)	CO₂ emissions (tCO₂)
Raw materials and fluxes	Hot metal	t	0.9361 ± 0.0436	0.1577	0.1476
	Steel scrap	t	0.1210 ± 0.0108	0.0029	0.0004
	Lime	t	0.0109 ± 0.0025	0.7832	0.0085
	Dolomite	t	0.0038 ± 0.0009	0.8619	0.0033
	Sinter	t	0.0090 ± 0.0012	0.3290	0.0029
Dynamic medium	Electricity	MW·h	0.0527 ± 0.0103	0.9419	0.0496
	O₂	km³	0.0142 ± 0.0035	0.6146	0.0087
	N₂	km³	0.0035 ± 0.0011	0.1295	0.0005
	Ar	km³	0.0005 ± 0.0002	4.7057	0.0022
	CO₂	km³	0	1.0000	0
Alloy	Aluminium	t	0	9.4400	0
By-product	BOFG	km³	0	1.0540	0
By-product	Slag	t	0.0420 ± 0.0123	0.3000	0.0126
Product	Semi-steel	t	1.0000	0.1173	0.1173
Total input					0.2237
Total output					0.1299
Net CO₂ emissions					0.0938

CO₂: carbon dioxide; BOFG: basic oxygen furnace gas.

In Route 1, the dephosphorisation converter was utilised to produce semi-steel, which underwent further processing by the decarburisation converter. The CO₂ emissions from the decarburisation converter were determined and are presented in Table 5.

Table 5.

CO₂ emissions from the decarburisation converter in Route-1.

Category	Project	Units (unit)	Quantity (unit/t)	Emission factor (tCO₂/unit)	CO₂ emissions (tCO₂)
Raw materials and fluxes	Hot metal	t	1.0000	0.1173	0.1173
	Steel scrap	t	0	0.0029	0
	Lime	t	0.0135 ± 0.0028	0.7832	0.0106
	Dolomite	t	0.0122 ± 0.0024	0.8619	0.0103
	Sinter	t	0.0041 ± 0.0012	0.329	0.0013
Dynamic medium	Electricity	MW·h	0.0640 ± 0.0207	0.9419	0.0603
	O₂	km³	0.0349 ± 0.0101	0.6146	0.0214
	N₂	km³	0.0110 ± 0.0009	0.1295	0.0014
	Ar	km³	0.0008 ± 0.0002	4.7057	0.0039
	CO₂	km³	0	1.0000	0
Alloy	Aluminium	t	0.0023 ± 0.0004	9.4400	0.0217
By-product	BOFG	km³	0.0890 ± 0.0267	1.0171	0.0905
By-product	Slag	t	0.0411 ± 0.0089	0.3000	0.0123
Product	Semi-steel	t	0.9400 ± 0.0159	0.0011	0.0010
Total input					0.2483
Total output					0.1039
Net CO₂ emissions					0.1444

CO₂: carbon dioxide; BOFG: basic oxygen furnace gas.

Upon analysing the calculations provided in Tables 4 and 5, it becomes evident that in Route 1, the CO₂ emissions from the dephosphorisation converter and the decarburisation converter stand at 0.0938 and 0.1444 t CO₂/t, respectively. Consequently, the total CO₂ emission for Route 1 amounts to 0.2382 t CO₂/t.

Similarly, for Route 2, the CO₂ emissions from both the dephosphorisation converter and the decarburisation converter were calculated, and the results are presented in Tables 6 and 7, respectively.

Table 6.

CO₂ emissions from the dephosphorisation converter in Route-2.

Category	Project	Units (unit)	Quantity (unit/t)	Emission factor (tCO₂/unit)	CO₂ emissions (tCO₂)
Raw materials and fluxes	Hot metal	t	0.9081 ± 0.0543	0.1561	0.1417
	Steel scrap	t	0.1351 ± 0.0379	0.0029	0.0003
	Lime	t	0.0079 ± 0.0041	0.7832	0.0065
	Dolomite	t	0.0030 ± 0.0009	0.8619	0.0026
	Sinter	t	0.0069 ± 0.0031	0.329	0.0023
Dynamic medium	Electricity	MW·h	0.0527 ± 0.0162	0.9419	0.0496
	O₂	km³	0.0119 ± 0.0043	0.6146	0.0073
	N₂	km³	0.0047 ± 0.0018	0.1295	0.0006
	Ar	km³	0.0004 ± 0.0001	4.7057	0.0021
	CO₂	km³	0	1.0000	0
Alloy	Aluminum	t	0	9.4400	0
By-product	BOFG	km³	0	1.0543	0
By-product	Slag	t	0.0350 ± 0.0098	0.3000	0.0105
Product	Semi-steel	t	1.0000	0.1173	0.1173
Total input					0.2127
Total output					0.1278
Net CO₂ emissions					0.0849

CO₂: carbon dioxide; BOFG: basic oxygen furnace gas.

Table 7.

CO₂ emissions from the decarburisation converter in Route-2.

Category	Project	Units (unit)	Quantity (unit/t)	Emission factor (tCO₂/unit)	CO₂ emissions (tCO₂)
Raw materials and fluxes	Hot metal	t	1.0000	0.1173	0.1173
	Steel scrap	t	0	0.0029	0
	Lime	t	0.0132 ± 0.0046	0.7832	0.0104
	Dolomite	t	0.0130 ± 0.0042	0.8619	0.0112
	Sinter	t	0.0039 ± 0.0014	0.3290	0.0013
Dynamic medium	Electricity	MW·h	0.0640 ± 0.0234	0.9419	0.0603
	O₂	km³	0.0365 ± 0.0178	0.6146	0.0224
	N₂	km³	0.0120 ± 0.0026	0.1295	0.0016
	Ar	km³	0.0009 ± 0.0003	4.7057	0.0043
	CO₂	km³	0	1.0000	0
Alloy	Aluminium	t	0.0025 ± 0.0009	9.4400	0.0236
By-product	BOFG	km³	0.1085 ± 0.0239	1.0171	0.1104
By-product	Slag	t	0.0350 ± 0.0136	0.3500	0.0123
Product	Semi-steel	t	0.9600 ± 0.1236	0.0011	0.0011
Total input					0.2523
Total output					0.1237
Net CO₂ emissions					0.1286

CO₂: carbon dioxide; BOFG: basic oxygen furnace gas.

According to the calculations in Tables 4 and 5, it becomes evident that within Route 1, the dephosphorisation converter emits 0.0849 t CO₂ per tonne processed, while the decarburisation converter emits 0.1286 t CO₂/t. This leads to a combined CO₂ emission of 0.2135 t CO₂/t for Route 2.

A comparative analysis of CO₂ emissions between Routes 1 and 2 reveals a notable reduction of 24.6 kg CO₂/t in total CO₂ emissions attributable to the implementation of the bottom-blowing O₂-CO₂-CaO process within the duplex smelting procedure. This reduction primarily stems from the substantial decrease in auxiliary material consumption, notably lime, within the dephosphorisation converter of the duplex smelting process.

The increased flow rate facilitated by bottom-blowing significantly intensifies the stirring energy within the molten bath. By introducing O₂ from the base of the converter, the oxidation process of liquid steel and slag at the molten bath’s surface diminishes. Consequently, the T.Fe content of slag within the bottom-blowing O₂-CO₂-CaO process experiences a significant reduction compared to the conventional method, leading to enhanced metal yield.

Furthermore, bottom-blowing O₂ enhances oxygen utilisation and increases residual heat within the molten bath, thereby optimising scrap utilisation. Increasing the proportion of scrap in the process, given its lower CO₂ emissions factor compared to hot metal, effectively mitigates CO₂ emissions stemming from the converter process.

The bottom-blowing O₂-CO₂-CaO process used for dephosphorisation in converters can be categorised into four levels based on the rate of powder spraying: Route 2-a, Route 2-b, Route 2-c, and Route 2-d.

These routes correspond to powder spraying rates of 100 kg/min, 150 kg/min, 200 kg/min, and 250 kg/min, respectively (see Figure 5(a)).

In comparing Route 1 with the four modes of Route 2, significant improvements are evident. Firstly, there is a marked reduction in lime consumption per tonne of steel as the amount of powder sprayed increases. This reduction underscores the effectiveness of higher powder application rates in the dephosphorisation process.

Secondly, when considering slag T.Fe content and metal yield, the benefits of the bottom-blowing O₂-CO₂-CaO process become even more apparent (Figure 5(b)). The T.Fe content in slag is notably decreased, while metal yield is dramatically enhanced compared to the original process. These findings underscore the substantial improvements achievable through the adoption of the bottom-blowing O₂-CO₂-CaO method.

When comparing the CO₂ emissions of the two processes, it is evident that the emissions from the decarburisation converter are similar in both cases. However, in the dephosphorisation converter, Route 2, which utilises the bottom-blowing O₂-CO₂-CaO process, shows lower CO₂ emissions. This is primarily due to reduced lime consumption and a higher scrap ratio. Consequently, Route 2 exhibits lower total CO₂ emissions compared to Route 1.

Analysis of CO₂ emissions by Route 3 and Route 4

By implementing the bottom-blowing O₂-CO₂-CaO process alongside the conventional smelting method, we have achieved a significant reduction in lime consumption, dropping from 35.44 kg per tonne to 27.7 kg per tonne. This reduction stems from the direct reaction of bottom-blowing O₂-CO₂ with the elements present in the hot metal. Consequently, there has been a marked improvement in oxygen utilisation, bringing down oxygen consumption per tonne of steel from 47.8 Nm³ to 38.6 Nm³. Additionally, bottom-blowing CO₂ interacts with key elements in the hot metal, such as carbon, silicon, iron, and manganese, yielding CO. This process elevates the BOFG from 0.089 to 0.111 km³ per tonne of steel. Moreover, the calorific value of the BOFG rises due to the heightened concentration of CO. For a comprehensive understanding, refer to Table 8 for select smelting parameters pertaining to Routes 3 and 4.

Table 8.

Smelting parameters of Route 3 and Route 4.

	Lime consumptionkg/t	R	T.Fe/%	[P]/%	O₂ consumptionNm³/t	BOFGkm³/t
Route-3	35.44 ± 3.23	4.58 ± 0.89	23.80 ± 2.79	0.00768 ± 0.00123	47.8 ± 5.6	0.108 ± 0.025
Route-4	27.70 ± 2.39	3.90 ± 0.76	16.88 ± 2.35	0.00538 ± 0.00098	38.6 ± 4.3	0.121 ± 0.015

The CO₂ emissions from Routes 3 and 4 are calculated and outlined in Tables 9 and 10.

Table 9.

CO₂ emissions from Route-3.

Category	Project	Units (unit)	Quantity (unit/t)	Emission factor (tCO₂/unit)	CO₂ emissions (tCO₂)
Raw materials and fluxes	Hot metal	t	0.9391 ± 0.1035	0.1577	0.1481
	Steel scrap	t	0.1091 ± 0.0352	0.0029	0.0003
	Lime	t	0.0354 ± 0.0086	0.7832	0.0277
	Dolomite	t	0.0268 ± 0.0056	0.8619	0.0231
	Sinter	t	0.0060 ± 0.0017	0.3290	0.0020
Dynamic medium	Electricity	MW·h	0.0797 ± 0.0235	0.9419	0.07514
	O₂	km³	0.0478 ± 0.0139	0.6146	0.0294
	N₂	km³	0.0135 ± 0.0024	0.1295	0.0017
	Ar	km³	0.0008 ± 0.0003	4.7057	0.0010
	CO₂	km³	0	1.0000	0
Alloy	Aluminium	t	0.0020 ± 0.0005	9.4400	0.0189
By-product	BOFG	km³	0.1080 ± 0.0084	1.0171	0.1098
By-product	Slag	t	0.0920 ± 0.0257	0.3000	0.0276
Product	Semi-steel	t	1.0000	0.0013	0.0013
Total input					0.3302
Total output					0.1387
Net CO₂ emissions					0.1915

CO₂: carbon dioxide; BOFG: basic oxygen furnace gas.

Table 10.

CO₂ emissions from Route-4.

Category	Project	Units (unit)	Quantity (unit/t)	Emission factor (tCO₂/unit)	CO₂ emissions (tCO₂)
Raw materials and fluxes	Hot metal	t	0.8910 ± 0.1351	0.1577	0.1405
	Steel scrap	t	0.1311 ± 0.0224	0.0029	0.0004
	Lime	t	0.0277 ± 0.0121	0.7832	0.0217
	Dolomite	t	0.0251 ± 0.0097	0.8619	0.0216
	Sinter	t	0.0037 ± 0.0016	0.3290	0.0012
Dynamic medium	Electricity	MW·h	0.0717 ± 0.0235	0.9419	0.0675
	O₂	km³	0.0386 ± 0.0113	0.6146	0.0237
	N₂	km³	0.0120 ± 0.0028	0.1295	0.0016
	Ar	km³	0.0019 ± 0.0005	4.7057	0.0089
	CO₂	km³	0.0030 ± 0.0011	1.0000	0.0030
Alloy	Aluminium	t	0.0021 ± 0.0008	9.4400	0.0198
By-product	BOFG	km³	0.1210 ± 0.0113	1.0171	0.1231
By-product	Slag	t	0.0809 ± 0.0206	0.3000	0.0243
Product	Semi-steel	t	1.0000	0.0013	0.0013
Total input					0.3100
Total output					0.1486
Net CO₂ emissions					0.1614

CO₂: carbon dioxide; BOFG: basic oxygen furnace gas.

Based on the findings presented in Tables 9 and 10, Route 3 exhibits CO₂ emissions of 0.1915 t CO₂/t, while Route 4 shows 0.1614 t CO₂/t. In the traditional smelting process, employing the bottom-blowing O₂-CO₂-CaO technique reduces CO2 emissions during the converter process by 30.1 kgCO₂/t. Figure 6 illustrates the distribution of CO₂ emissions between the input and output stages of Routes 3 and 4.

Examining Figure 6(a) and (b), the inputs for Route 3 primarily consist of hot metal, electricity, lime, O₂, and dolomite. Among these, hot metal exhibits the highest CO₂ emissions ratio at 44.84%. Conversely, incorporating scrap at 10.4% results in a significantly lower CO₂ emissions proportion of only 0.10%, indicating the potential for reducing emissions through increased scrap utilisation.

Regarding the output stages, BOFG recovery constitutes the most significant deduction, accounting for 79.17% of emissions, while steel slag deduction contributes 19.89%. Enhancements in both the quantity and quality of the converter gas are instrumental in increasing CO₂ emissions credits at the output stage.

Based on the data presented in Table 10, Route 4 yielded CO₂ emissions of 161.4 kgCO₂ per tonne. Upon comparing Figure 6(c) and (d) with Figure 6(a) and (b), it becomes evident that the CO₂ emissions at the initial stage are comparable to those of conventional smelting post the implementation of the bottom-blowing O₂-CO₂-CaO process.

At the input stage of Route 4, the primary contributors to CO₂ emissions are hot metal, electricity, O₂, lime, and dolomite, mirroring the pattern observed in Route 3. Although the proportion of CO₂ emissions from hot metal in Route 4's input stage slightly increased to 45.32%, a mere 0.48% rise compared to Route 3, the total CO₂ emissions from hot metal decreases from 148.1 to 140.5 kgCO₂/t, marking a reduction of 7.6 kgCO₂/t. Notably, the overall CO₂ emissions at the input stage dropped from 330 to 310 kg CO₂ /t in Route 4, signifying a substantial decrease, albeit with a relatively higher proportion attributed to CO₂ emissions from iron.

Furthermore, the direct injection of lime powder into the molten bath significantly enhances the reaction interface between the lime powder and elements in the molten steel, thereby increasing lime utilisation. A comparison between the input ends of Routes 3 and Route 4 reveals a noteworthy reduction in CO2 emissions from lime, declining from 8.40% to 7.00%.

At the output end, the CO₂ emissions reduction from BOFG recovery in Route 4 escalated by 3.63%, whereas the reduction from slag decreased by 3.56%. This shift can be primarily attributed to the increase in O₂ utilisation through bottom-blowing O₂, which boosts BOFG production, and the curtailed usage of raw and auxiliary materials such as lime, leading to reduced slag production. Both these adjustments, increasing BOFG recovery and diminishing slag output, prove effective in mitigating CO₂ emissions from the converter smelting process.

In our thorough analysis, we have identified that the primary CO₂ emissions stem from various inputs in Routes 3 and 4, including hot metal, slag-making auxiliaries, and energy usage. Optimising the converter process by increasing the scrap ratio and reducing hot metal usage can effectively mitigate CO₂ emissions at the input stage. At the output stage, CO₂ emissions predominantly arise from BOFG, with slag following closely behind. By enhancing the proportion of BOFG during the converter smelting process and minimising slag production, we can significantly decrease CO₂ emissions.

Our industrial tests have demonstrated that employing the bottom-blowing O₂-CO₂-CaO process yields superior molten bath dynamics, higher metal yield, and reduced consumption of raw materials and auxiliaries compared to conventional smelting methods. Notably, this innovative approach reduces the total emissions by 30.1 kgCO₂/t compared to traditional smelting techniques.

Comparative analysis of CO₂ emissions from various processes

Based on the calculations illustrated in Figure 7, a comparison of the CO₂ emissions across different routes reveals significant insights. When contrasting Route 1 with Route 3, it becomes evident that despite the duplex smelting process offering benefits such as enhanced steel quality and a shorter smelting cycle, its CO₂ emissions are 46 kgCO₂/t higher compared to the conventional smelting process.

This increase primarily stems from several factors. Firstly, while the consumption of auxiliary materials is slightly reduced with the duplex converter, there is a notable increase in the utilisation of energy media such as electricity, water, O₂, N₂, Ar, and CO₂ due to the employment of two converters. Secondly, the dephosphorisation converter, crucial for the process, fails to efficiently recover gas due to low gas CO concentration. Additionally, there is a delay of approximately 1–3 min in gas recovery post-blowing during dephosphorisation converter smelting, leading to a 15% reduction in total gas recovery compared to conventional methods.

Consequently, the CO₂ emissions associated with the duplex converter smelting process surpass those of the conventional method, primarily due to these operational differences.

Comparing Route 1 and Route 3 with Route 2 and Route 4, it is evident that regardless of whether it is the duplex converter smelting process or the conventional smelting process, there is a significant reduction in CO₂ emissions following the implementation of the bottom-blowing O₂-CO₂-CaO process. Through a thorough calculation process, it becomes apparent that the decrease in lime, O₂, and other material consumption plays a pivotal role in reducing CO₂ emissions the most. Moreover, the bottom-blowing O₂ enhances the surplus heat of the molten bath, improves scrap utilisation compared to conventional methods, and aids in optimising the raw material structure, all contributing to CO₂ emissions reduction.

Simultaneously, the kinetic conditions of the bottom-blowing molten bath, with its high flow rate, are enhanced, leading to reduced slag's T.Fe content and significantly improved metal recovery rates. Additionally, the bottom-blowing CO₂ facilitates better recovery of BOFG in the converter smelting process, further mitigating CO₂ emissions in the converter process.

The dephosphorisation converter operates with a quick smelting time of just 7 to 8 min, utilising a powder spray ranging from 1.5 to 2.5 t. In contrast, the conventional converter takes approximately 15 min to smelt, using approximately 4 tonnes of powder. This results in a notable reduction in CO₂ emissions due to decreased raw and auxiliary material consumptions and increased ingas recycling credits. Implementing the bottom-blowing O₂-CO₂-CaO process in the conventional method yields a more pronounced carbon reduction effect, reducing CO₂ emissions by 30.1 kg per tonne of steel produced.

Conclusions

Utilising real production data from a steel plant, we conducted an industrial test to evaluate the effectiveness of the converter bottom-blowing O₂-CO₂-CaO process. This involved analysing CO₂ emissions across four distinct converter smelting methods for the first time, aiming to quantify the CO₂ emissions reduction potential of the bottom-blowing O₂-CO₂-CaO process. The key findings are as follows.

The total consumption of auxiliary material in the duplex converter smelting process sees a slight reduction. However, the consumption of energy resources (such as electricity, water, oxygen, nitrogen, argon, and CO₂) increases due to the employment of two converters. Compared to the conventional smelting process, the duplex converter smelting process results in a 40–50 kgCO₂/t increase in emissions.

According to our analysis using the carbon emission model and industrial test data, we observed a reduction in CO₂ emissions were reduced by 24.6 kgCO₂/t for the duplex smelting process and 30.1 kgCO₂/t for the conventional smelting process when employing the bottom-blowing O₂-CO₂-CaO process.

This bottom-blowing O₂-CO₂-CaO process demonstrates remarkable efficacy in curbing CO₂ emissions. When applied to steel plants with an annual production capacity of 20 million tonnes of crude steel, it is expected that this technology could lead to an annual reduction of 600,000 tonnes of CO₂ emissions.

Footnotes

Acknowledgements

The authors would like to express their thanks for the support by the National Nature Science Foundation of China (NO. 52304343 & NO. 52322407& NO.52274313), China Baowu Low Carbon Metallurgy Innovation Foundation-BWLCF202108 and China Postdoctoral Science Foundation-2023M730227.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: China Baowu Low Carbon Metallurgy Innovation Foundation, grant number, BWLCF202108, China Postdoctoral Science Foundation, grant number, 2023M730227, National Natural Science Foundation of China, grant numbers, NO. 52304343, NO. 52322407, NO.52274313.

ORCID iD

Chao Feng

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Analysis of Carbon Dioxide Emissions in Bottom-blowing O 2 -CO 2 -CaO Process in Basic Oxygen Furnace Steelmaking

Abstract

Keywords

Introduction

Process description

Calculation of CO2 emissions

Main calculation methods for CO2 emissions

System boundary determination

CO2 emissions factor determination

Analysis of results

Analysis of CO2 emissions by Route 1 and Route 2

Analysis of CO2 emissions by Route 3 and Route 4

Comparative analysis of CO2 emissions from various processes

Conclusions

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

ORCID iD

References

Calculation of CO₂ emissions

Main calculation methods for CO₂ emissions

CO₂ emissions factor determination

Analysis of CO₂ emissions by Route 1 and Route 2

Analysis of CO₂ emissions by Route 3 and Route 4

Comparative analysis of CO₂ emissions from various processes