CO Reduction Process Technology and Development of Iron Ore Sintering Process

Abstract

Iron ore sintering is a high-energy-consuming industry, and its high dependence on fossil fuels and the low concentration of CO in the sintering flue gas conceal the truth of the large total amount of CO emissions, which leads to the continuous emission of CO in the sintering flue gas has been harmful to the atmosphere and human health, and it is facing the great pressure of CO emission reduction. On the basis of commercially applied sintering technologies, the mechanism and characteristics of CO emission from sintering flue gas are discussed, and feasible ways to control CO emission in multiple aspects of source control, process emission reduction and end-of-pipe treatment are summarized. The core of source abatement is to reduce the fuel ratio, process abatement is to improve the combustion conditions of fuels to enhance the conversion rate of CO to CO₂, and end-of-pipe treatment is to separate or oxidize CO to CO₂ by physical or chemical means. Hydrogen sintering technology is the future development direction for source abatement, steam blowing sintering technology is introduced for process control, and catalytic oxidation technology has great prospects for removing CO from flue gas in end-of-pipe treatment. CO has great prospects, but efforts are needed to develop highly active catalysts with anti-poisoning and long-standing stability. Finally, feasible technical routes for sintering flue gas CO reduction and their challenges are analyzed, and a coordinated multifaceted control of source-process-end sintering technologies is proposed to achieve the goal of high-efficiency sintering flue gas CO reduction.

1. Introduction

Steel is a key product for the global economy. As the backbone of the national economy, the steel industry has contributed positively to the rapid growth of China.¹⁾ Since 2002, China’s steel production has increased by 408.4% in the last two decades to reach 868 million tons in 2021, and China’s GDP has grown from 121774 billion yuan in 2002 to 1143670 billion yuan in 2021 in the last two decades, which shows that the rapid growth of China’s GDP is closely related to the growth of crude steel production. The steel industry has played an important role in supporting China’s GDP growth as shown in Figs. 1 and 2. In addition, in terms of the annual growth rate of pig iron production in China, which reached its maximum growth rate in 2005 and began to decline in 2014 against the backdrop of the gradual start of the closure of outdated production facilities in the steel industry in 2014, with a negative annual growth rate of pig iron production in 2015. Although steel manufacturing is considered a pollution-intensive industrial activity, it remains crucial for the global economy and sustainability. Sintering and lumping of iron ore by iron ore sintering technology is a necessary process in order to maintain stable blast furnace operation. It greatly reduces production fluctuations and, more importantly, expands the sources of obtaining various types of iron ore, making it possible to utilize low-grade iron ore.

Fig. 1. Change in pig iron production and growth rate in China in the past two decades. (Online version in color.)

Fig. 2. China’s GDP and growth rate in the last two decades. (Online version in color.)

Iron ore sintering is a typical industrial process with high energy consumption, high material consumption and heavy pollution. It is reported that energy consumption accounts for about 6–10% of the total energy consumption of iron and steel enterprises.^2,3) Sintered ore is one of the main iron charges used in blast furnaces (BF), accounting for about 75% of the blast furnace charge structure. The energy structure of the sintering process in China is shown in Fig. 3. In China, the annual production of sintered ore exceeds 1 billion tons per year. As the iron ore sintering process is highly dependent on fossil fuels, the sintering industry consumes more than 60 million tons of fossil fuels (coal, coke and natural gas, etc.) per year, and solid fuel consumption accounts for about 78.8% of the total energy consumption of the sintering process. However, the combustion process of fossil fuels will inevitably produce CO.^4,5)

Fig. 3. Energy structure of the sintering process in China. (Online version in color.)

In the sintering process, the main source of CO is the incomplete combustion products of the solid fuels dispensed into the sintered material and the carbonaceous combustion products in the ignition gas, and the following reactions mainly occur after the sintering process begins:⁶⁾

  

$$\mathrm{C}+{\mathrm{O}}_2=\mathrm{C}{\mathrm{O}}_2\hspace{1em}\mathrm{\Delta }{\mathrm{G}}^{\theta }=-395\mathrm{\hspace{0.17em} }350-0.54\mathrm{T}$$(1)

  

$$2\mathrm{C}+{\mathrm{O}}_2=2\mathrm{C}\mathrm{O}\hspace{1em}\mathrm{\Delta }{\mathrm{G}}^{\theta }=-228\mathrm{\hspace{0.17em} }800-171.54\mathrm{T}$$(2)

  

$$2\mathrm{C}\mathrm{O}+{\mathrm{O}}_2=2\mathrm{C}{\mathrm{O}}_2\hspace{1em}\mathrm{\Delta }{\mathrm{G}}^{\theta }=-561\mathrm{\hspace{0.17em} }900+170.46\mathrm{T}$$(3)

  

$$\mathrm{C}{\mathrm{O}}_2+\mathrm{C}=2\mathrm{C}\mathrm{O}\hspace{1em}\mathrm{\Delta }{\mathrm{G}}^{\theta }=-166\mathrm{\hspace{0.17em} }550-171.00\mathrm{T}$$(4)

In addition, the degree of coke combustion is not only influenced by the temperature, but also closely related to the atmosphere and airflow conditions in the material layer of the sintering process.⁷⁾

According to the gas-solid reaction model, as shown in Fig. 4, the carbon combustion reaction can be basically divided into five processes:

① O₂ reaches the surface of coke powder by free diffusion from the gas phase.

② O₂ is adsorbed on the surface of coke powder.

③ The coke and O₂ react on the surface of coke powder.

④ Desorption of CO and CO₂ from the surface of C.

⑤ Gaseous reaction products diffuse outward from the interface into the gas phase by free diffusion.

Fig. 4. Schematic diagram of gas-solid reaction mechanism. (Online version in color.)

After testing the composition of the flue gas of each airbox of the sintering machine, it was found that, CO is mainly produced in the gas and solid fuel combustion stages after sintering and ignition and varies closely with the oxygen content in the feed layer, and the fuel combustion reaction to form CO proceeds more easily when there is not enough oxygen in the fuel.⁸⁾ As can be seen in Fig. 5, the change of CO content in the flue gas can be basically divided into three stages: the stage of gradual increase after the start of ignition, the stage of stable sintering with high emissions and the stage of rapid decrease after reaching the end of sintering. The whole sintering process is accompanied by the generation of CO, but the content basically does not exceed 2.5%.

Fig. 5. The emission contents of flue gas components in the wind-boxes. (Online version in color.)

In addition, CO is a toxic gas that can damage human brain tissue and respiratory system. CO produced in industrial production in China accounts for 43.55% of the total annual emissions in the country, with the highest proportion of CO emissions from the sintering process in the steel industry.⁹⁾ According to statistics, China’s steel industry will consume 670 million tons of coal in 2021, and the sintering process alone will generate 50–60 million tons of CO,¹⁰⁾ The calorific value of unburned CO converted into standard coal amounts to 201.6–241.9 million tons. The low volume fraction of CO in sintering flue gas is difficult to be utilized for resource utilization, and is usually directly discharged into the atmosphere, which not only causes environmental pollution, but also leads to energy waste. Therefore, it is important to carry out CO utilization and treatment in the iron and steel industry to continuously improve the air quality and steadily accomplish the goal of “carbon peaking and carbon neutral”.

Coke powder and coal powder are the main sources of heat for the sintering process and are the most widely used combinations of sintered solid fuels for sintering production. In the actual sintering process, the generation of CO may come from the insufficient combustion of solid fuels or from the incomplete combustion of ignition gas, which is a complex process. To explore some effective measures to reduce CO emissions, we can start from three aspects: source reduction, process control and end-of-pipe treatment.

2. Emission Reduction Technology of Sintering Source Control

Source reduction aims to effectively reduce solid fuel ratios through the implementation of low carbon sintering technologies. On the one hand, low carbon sintering technologies are implemented to improve the utilization of heat within the material layer, such as thick-bed sintering, support sintering and double layer sintering technologies, and on the other hand, carbon containing energy is minimized in the sintering process, such as hydrogen rich fuel injection sintering technologies. The implementation of low carbon thick-bed sintering technology to improve the heat utilization of the material layer achieves a goal of reducing CO.

2.1. Thick Bed Sintering

Thick-bed sintering technology began to be developed in the 1970s and has been rapidly growing in China since then. Thick-bed sintering can take advantage of the self-storage effect of the material layers to reduce solid fuel consumption and reduce emissions of gaseous pollutants such as NOx, COx, SOx, etc., while increasing the yield of the sintering process and improving the sintering strength.^11,12) Optimizing iron ore sinter production by increasing the sinter material layer thickness has become a consensus among steel companies all over the world. As a result, the material layer thickness of sintering machines has been increasing, from 200–300 mm at that time to about 900 mm today, as shown in Fig. 6.

Fig. 6. Evolution of average material bed height in China.

However, as the bed thickness increases, a number of problems can arise with sintering. As the thickness of the bed increases, the resistance of the bed increases, leading to a reduction in the exhaust volume and resulting in a corresponding impact on the heat transfer, mass transfer and physicochemical reactions of the bed.¹³⁾ In this case, the sintering speed and productivity decrease, and the sinter quality becomes uneven. In addition, the thickness of the over-wet zone increases with the thickness of the material layer, which further deteriorates the permeability of the material layer. Without corresponding countermeasures, the advantages of thick-bed sintering are not obvious and may have unintended or opposite effects.

Kasama et al.¹⁴⁾ further confirmed the three-dimensional permeability evolution of the sintered ore cake by high-energy x-ray computed tomography. They noted that the temperature of the combustion zone increases as the flame decreases due to the self-regenerative function of the sintering process. Furthermore, the permeability of the burned melt zone in the sintered bed is highly dependent on the sinter cake loading on the burned melt zone during sintering. Liu et al.¹⁵⁾ analyzed the characteristics of ultra-thick-bed sintering and focused on high permeability and low gas leak rate. Then, the “triple synchronization” theory including liquid phase front, heat transfer front and flame front and the concept of “full active lime enhanced sintering” were proposed to support the thick bed sintering, as shown in Fig. 7. On this basis, TISCO developed a series of integrated technologies including raw material control, pelletizing enhancement and air leakage improvement, and then achieved efficient and stable sintering production with a bed height of 1000 mm on its two 260 m² sintering machines. The implementation of these technologies resulted in productivity of 1.89 t/(m²-h) and solid fuel consumption of only 41.85 kg/ton, while producing sinter ores with tumbling, reduction, and low-temperature reduction degradation indices of RDI+3.15 of 78.24%, 87.17%, and 74.2%, respectively. While promoting the development of ultra-thick bed sintering technology, it also brings significant economic and environmental benefits. Chen et al.¹⁶⁾ quantitatively investigated the effects of decreasing the coke powder ratio on the sintering emission indexes such as layer permeability, layer heat distribution, sintered mineral quantity and quality, sintered flue gas emission per tons of ore and sintered pollutant emission per ton of ore under different layer thickness and ultra-thick bed conditions by sintering cup test, and explored the suitable coke powder ratio for ultra-thick-bed sintering. The results show that when the thickness of sintering layer is gradually increased from 800 mm to 950 mm, increasing the thickness of sintering layer under the same coke powder ratio can significantly improve the quantity and quality of sintering minerals and various sintering indexes, thus creating conditions for reducing the coke powder ratio, and when the layer thickness is increased, the sintering solids combustion consumption is significantly reduced by about 6%.

Fig. 7. Logic of the comprehensive technologies.¹⁴⁾ (Online version in color.)

Thick-bed sintering is the basic measure to achieve energy saving and consumption reduction in the sintering process and to improve the yield and quality. Improving the permeability of the thick-bed and reducing the air leakage in the sintering process are the keys to the implementation of the thick-bed technology to achieve the desired goals. In addition, due to the characteristics of the “automatic heat storage effect” of the material layer, the uneven heat distribution between the upper and lower part of the sintered thick material layer “insufficient heat at the top and excess heat at the bottom” phenomenon will also be more obvious. Therefore, the future research direction for thick material layer sintering can mainly start from the above two aspects, specific measures can include: the segregation method of sinter feeding, hydrogen-rich gas blowing, etc. to improve the heat distribution in the upper and lower part of the material layer; sintering machine sealing equipment modification and other air leakage control measures.

2.2. Stand-support Sintering

With the development of thick-bed sintering technology, more and more technicians in the field have noticed that solving the permeability of thick-bed is the key to the successful implementation of thick-bed sintering.

Higuchi et al.¹⁷⁾ developed the bracket support sintering method in 1995, as shown in Fig. 8. The mechanism of bracket support sintering is to reduce the load of the upper material or sintered ore on the lower material during the sintering process by means of a support plate mounted on the sintering grate to solve as much as possible the adverse effects of reduced pore space between materials, airflow obstruction and shrinkage of the material layer caused by the upper weight and extraction pressure. So that the lower layer of material to get a better permeability, as shown in Fig. 9.

Fig. 8. Schematic diagram of Stand-support sintering. (Online version in color.)

Fig. 9. Schematic diagram of pressure change of Stand-support sintering. (Online version in color.)

Higuchi et al.¹⁷⁾ conducted industrial tests into a sintering machine with a material layer thickness of 750 mm and 600 mm by installing a support plate on the grate of the sintering machine. The experimental results showed that bracket-supported sintering could significantly reduce the sintering time and increase the productivity by about 20%, and the technology was applied to Kimitsu’s sintering machine. Zuo et al.^18,19,20) conducted a sinter cup experiment with a material thickness of 600 mm to further investigate the effects of moisture in the sinter mixture, stand height and support area on productivity and drum index. In addition, their industrial tests over two periods showed a 1.32% reduction in fuel consumption, a 6.34% increase in productivity and a 0.07% reduction in the sinter drum index when the supports were installed on the sinter bed. Wang et al.²¹⁾ investigated the effect of support sintering technology on CO reduction, and the results showed that: when the support height increased from 0 to 500 mm, the total porosity of the sintered material layer increased from 35.4% to 42.5%, the permeability of the material layer became better, the oxygen potential within the material layer increased, and the CO generation decreased. In addition, bracket sintering can effectively solve the problems of poor permeability and slow combustion rate caused by coarse coke particle size, and has better adaptability to large particle size fuels >3 mm.

In the actual production process, it is found that the wear of the support plate is relatively large after a period of use, and it is also accompanied by the problem of sintered material adhesion, so the application in China is still relatively small. As the thickness of the sintered production material layer continues to increase, the Stand-support sintering can definitely play its advantages. Therefore, the later workers can carry out research work on the influence of parameters such as support plate material, support plate height and width on the sintering process.

2.3. Double-layer Sintering Process

The Double Layer Sintering Process (DLSP) with two charging stages and corresponding ignition processes in the same sintering machine can also effectively improve bed permeability and sintering productivity, The double sintering process flow chart is shown in Fig. 10.^22,23,24)

Fig. 10. Schematic diagram of double-layer sintering process. Hu Bed height of upper layer; Hl bed height of lower layer; Lp length of presintering interval. (Online version in color.)

Compared with single layer sintering, the structure and nature of double layer sintered material is more complex as shown in Fig. 11, which can be upper layer, middle layer and lower layer. The air containing 21% O₂ flows downward from the surface of the upper material layer, and a series of reactions occur with Fe and C components of the material layer to consume oxygen, produce NOx and water vapor, etc. When the gas flows to the original material layer, the main composition of the gas is 10% O₂+CO₂+CO+H₂O+N₂, etc. The upper material layer is a single-layer sintering process with the same structure and properties as single-layer sintering, which is divided into sintered ore belt, combustion belt, pre-heat belt, over-wet belt and original material belt from top to bottom. The middle material layer is the pre-sintered sintered ore belt, the sintering process of this material layer is the same as the single layer sintering process, which is a single layer sintering process after a charging process of sintering feed and ignition. Therefore, the height of the material layer is mainly determined by the pre-sintering time. The sintering process of the lower material layer is synchronized with the sintering process of the upper material layer, and the structure of the lower material layer is also divided into sintered ore belt, burning belt, pre-heat belt, over-wet belt and original material belt from top to bottom, and the O₂ is further consumed in this material layer, and its volume fraction is reduced from about 10% to about 4%.²⁵⁾

Fig. 11. Comparison between double-layer pre-sintering reaction and ordinary sintering process. (Online version in color.)

Zhou et al.²⁶⁾ studied the reaction behavior of the sintering process in the double layer sintering process and found that the sintering process is prone to oxygen deficiency in the lower part of the material layer, but by appropriately extending the pre-sintering time, the problem of oxygen deficiency in the lower part of the material layer can be alleviated and the sintered ore drum strength can be significantly improved. Furthermore, the industrial trial of double-layer pre-sintering was carried out for nearly 7 months in 360 m² sintering machine and No. 4 and No. 5 blast furnace in the second sintering workshop of Anshan Iron and Steel General Factory, and the sintered ore production increased by 16.11%, the blast furnace was running smoothly, and the blast furnace utilization coefficient, fuel ratio and air volume were basically the same as the base period, which showed that the new process of double-layer pre-sintering was fully feasible in production practice.²⁷⁾ Sato et al.²²⁾ conducted a sinter pot experiment in 1988 to study the bilayer sintering process with a 750 mm bed height and showed that the bilayer sintering process was beneficial in reducing energy consumption while maintaining sufficient oxygen concentration and high negative pressure. Zhong et al.²³⁾ studied the double-layer sintering with a material layer thickness of 1000 mm by establishing theoretical calculation and sintering pot verification test. The experiment found that the sintering time, sintering productivity and material layer permeability of double-layer sintering were all due to the traditional sintering process. Under the conditions of bed height ratio of 350/650 mm and pre-sintering time of 20 min, the yield, drum strength, productivity and solid fuel consumption are 69.96%, 65.87%, 1.71 t m⁻²·h⁻¹ and 56.71 kg/t respectively. Liu et al.²⁴⁾ investigated the double-layer sintering process with supports, and the results showed that compared with the normal double-layer sintering process, double-layer sintering with supports (DLSP-S) increased the sintering yield and productivity from 64.53% and 1.76 t·m⁻²·h⁻¹ to 66.74% and 2.12 t·m⁻²·h⁻¹. In addition, the permeability of the sintered material layer was significantly improved and the O₂ content of the lower layer increased during the DLSP-S process.

A series of DLSP industrial tests, were conducted by An steel’s, China. They noted that the total thickness of the material layer could reach more than 1000 mm under the condition that the percentage of concentrate exceeded 60%, and the output of individual sintering machines was greatly improved, but the yield of sintered products was reduced. However, the new process needs further improvement to achieve optimal sintering yields and reduce sintering solid fuel consumption and NOx and carbon oxide emissions.

2.4. Hydrogen-rich Fuel Injection Sintering

The main gaseous fuels used in sintering are blast furnace gas, coke oven gas and natural gas used as ignition fuel in the sintering ignition process, and their main components are methane, CO and hydrogen, which are regarded as hydrogen-based gaseous fuels. JFE Steel’s Keihin Daiichi Sintering Plant has developed a technology to inject hydrogen-based gas fuel into the sintering machine.^24,28) A certain concentration of gaseous fuel mixed with air enters the inner layer of the material, reaches the high temperature position above the combustion zone and then ignites, reheats the middle and upper layers of the material and releases heat to supply the middle and upper layers of the material for sintering and ore formation. Ye et al.^29,30) found that, in gas fuel sintering experiments, the temperature profile within the feed layer changes accordingly due to the exothermic reaction of the gaseous fuel, as shown in Fig. 12, where the liquid phase formation time and high temperature time (1200 to 1400°C) are prolonged to help generate high quality calcium ferrate and reduce the cooling rate of the sintered ore to ensure the mechanical strength of the silico-ferrite of calcium and aluminum (SFCA).

Fig. 12. Distribution of zones and temperature along the height direction of the bed at a certain sintering time.²⁸⁾ (① Solid fuel heat supply curve, ② Combined gas-solid fuel heat supply curve) (Online version in color.)

Fan et al.³¹⁾ conducted a combination of sintering cup experiments and mathematical modeling under laboratory conditions to elucidate the deeper mechanisms of hydrogen enrichment during sintering and its effect on sintering properties. They found that sintering can increase the area of liquid phase formation when H₂-rich gas is injected, as illustrated in Fig. 13. It was found that the behavior of H₂ rich gas when injected into the sinter bed is that the combustible gases (H₂, CH₂ and CO) flow downward in the sinter bed under the action of the extracted air and when it reaches the area near the combustion zone, the combustion reaction occurs and the combustion zone of the gaseous fuel will overlap with the combustion zone of the solid fuel, which eventually enlarges the combustion zone in the sinter bed. In turn, it can be seen that the main mechanisms for the improvement of sintering performance by hydrogen-rich gas include the improvement of liquid phase formation area and high temperature (≥1200°C) duration, which contributes to the formation of sufficient calcium ferrate in the binder phase; the reduction of the cooling rate of the binder minerals facilitates the formation of silico-ferrite of calcium and aluminum with high mechanical strength. Such changes not only facilitate the formation of high-quality sintered ore but also reduce the use of coke powder and reduce the emission of CO from the sintering source.

Fig. 13. Influence of H2-rich gas injection on temperature of sintering bed.³⁰⁾ (a) Coke breeze rate was 5.60%; (b) Coke breeze rate was 5.30%. (Online version in color.)

Many researchers have conducted experimental studies on partial replacement of coke powder by hydrogen-based gas fuels in iron ore sintering production.³²⁾ This technology changes the heating rate in the iron ore sintering process and enables high quality production of iron ore sinter using gaseous fuels even at low fuel consumption rates, thus contributing to further reduction of CO emissions.

Fan et al.³³⁾ studied the influence law of different kinds of gas spraying sintering on the quality of sintered ore, and the results showed that under the condition of equal heat spraying, the priority order of the effect of spraying hydrogen-based gas to improve the sintering yield and quality index is methane > coke oven gas > hydrogen; and for the study of the location of hydrogen-rich gas spraying in the sintering process. It has been proposed to adjust the sintering process according to the varying supplementary heat demands observed at different heights within a thick material layer. The sintering hydrogen-rich step-blowing technology as shown in Fig. 14 was developed and studied. By examining the effects of gas injection with equal concentration and step spraying on the quantity and quality index of sintered ore in the spraying interval, under the condition of keeping all other spray-cooling process parameters constant, it was found that hydrogen-rich spraying sintering can significantly improve the quality of sintered ore and reduce the carbon allocation of sintering. This is of great significance to the CO reduction in the sintering process.

Fig. 14. Gas step blowing flow rate variation curve. (Online version in color.)

Nobuyuki OYAMA et al.³⁴⁾ conducted a basic study in the laboratory using an X-ray CT scanner and sintering tank tests to elucidate the effects of natural gas (LNG) injection on temperature distribution, pressure drop, and pore structure in the sintered cake. The results of the study showed that the temperature range above 1200°C after natural gas injection was expanding upward and the liquid phase generation was increasing, which promoted the bonding of 1–5 mm pores and improved the sintering strength. Operational tests were also conducted at JFE Steel to verify the principle of the technology. The technology was put into commercial operation at Keihin Daiichi Sintering Plant in January 2009 and has remained in stable operation since then. Energy efficiency in the sintering process has been greatly improved, and CO₂ emissions from the Keihin No. 1 sintering plant have been reduced by up to about 60000 t/a. Cheng et al.^32,35) chose methane and charcoal as alternative fuels to replace coke powder, with gaseous fuel injected from the top into the melting zone and automatically ignited near the solid fuel burning zone, and considered different methane concentrations and solid fuel inputs. The experimental results show that methane injection can significantly extend the upstream melting zone and improve the sinter strength without increasing the energy consumption, and the experiments show a 2.31% increase in sinter strength at 0.5% methane injection. J. A. de Castro et al.^36,37,38) investigated the effect of gaseous fuels (for example H₂-rich coke oven gas, mixture of coke oven gas and blast furnace gas) on the sintering performance in a steel mill by numerical simulations, which showed that gas injection enlarges the sintering zone and enhances the sintering phenomenon, with an increase in the total amount of liquid phase formation and a slight increase in calcium ferrate with gas injection. It also significantly reduces the amount of solid fuel required for sintering, thus helping to make the process more environmentally friendly.

The implementation of hydrogen-rich sintering requires special attention, as the reaction products of the injected hydrogen-rich gas will generate water, and the additional water will increase the moisture content of the material layer to some extent and deteriorate the permeability of the material layer. Therefore, during the sintering operation, the water content of the quasi-particles of the material should be reduced appropriately.

3. Emission Reduction Technology of Sintering Process Control

3.1. Analysis of CO Path of Process Emission Reduction

Sintering process emission reduction path to control CO emissions lies in the sintering process operating system to improve the utilization rate of carbonaceous fuels that have entered the sintering process at the sintering source, improve the oxidation atmosphere around the fuel, and promote complete combustion of the fuel to reduce the generation of CO. The key point to change the oxygen potential of the material layer is the permeability of the material layer, for example, through multi-angle fuel control technology to improve the granulation effect of the mixture, also through oxygen-enriched sintering technology to improve the oxygen content of the gas stream. In addition, the implementation of steam blowing and flue gas circulation sintering technologies in the sintering process will change the sintering gas medium entering the material layer to optimize the solid fuel combustion environment and reduce the generation of CO.

3.2. Integrated Control of Sintering Fuels

3.2.1. Fuel Particle Size Control

Granulation performance is the decisive factor affecting the permeability of the material layer, which is closely related to the flame leading edge speed, combustion efficiency, sintering productivity and power consumption of the extractor fan. During the sintering process, the heat and gaseous reaction products from the combustion of solid fuels provide the necessary heat and atmosphere conditions for the melting, liquid phase generation and other physicochemical reactions of the sintered material.³⁹⁾ According to the gas-solid reaction kinetic model, when C and O₂ react, the solid carbon combustion rate is controlled by both the interfacial chemical reaction rate and diffusion control, depending on two factors: the carbon and oxygen chemical reaction rate and the diffusion rate of reactants O₂ and reaction products CO₂ and CO on the surface of fixed carbon.⁴⁰⁾ Therefore, the composition of the fuel size is very important for the smooth sintering process. When the fuel particle size is too fine, the specific surface area is larger and the combustion reaction is rapid. However, too fast combustion will lead to a thinner combustion layer and insufficient heat generation resulting in a reduced amount of liquid phase and a lower yield, making the utilization of the fuel decrease and the reaction to generate CO rise.

Tobu et al.⁴¹⁾ elucidated the criteria for different chemical routes of coke reactions so as to find out the effective way to maximize the efficiency of coke combustion by controlling the particle structure in order to avoid the incomplete combustion reaction of coke to produce CO. The results of the study pointed out that composite type of particles (fine particles of coke adhered to the core particles) is the best choice. Compared to a single type of coke pellet, the coke consumption of the composite type is saved by 1.2% and the speed of the thermal front should be increased by 2.3%, a change that will realize the combustion of the fuel with less CO, which will contribute to the reduction of CO emissions from the sintering process. Huang et al.⁴²⁾ conducted a comparative study of the proposed pre-wetting granulation method with conventional granulation methods under laboratory conditions, and more particles in the sintered mixture exhibited stratified fine particles during the pre-wetting granulation process. The effect of pre-wetting granulation is characterized by a decrease in the proportion of fine particles and an increase in the proportion of relatively coarse particles. The effect of prewetting moisture (PWD) on particle size distribution and permeability was compared. The results show that increasing PWD from 65% to 80% leads to a large difference in the proportion of 3–7 mm particles compared to JPU with conventional pelletizing, but the optimal bed permeability remains almost constant at both PWD levels. However, it should be noted that pre-wetting pelletizing of iron ores with high moisture absorption capacity will reduce the bed permeability. Umadevi et al.⁴³⁾ aimed to determine the effects of raw material size on sintering productivity indices, including flame front speed and permeability, as well as sintering quality indices, including microstructure, strength, and RDI. They evaluated the influence of coke particle size and iron ore particle size on granulation and quantitatively examined the distribution of coke powder in quasi-particles (P- and C-type), as depicted in Fig. 15.⁴⁴⁾ Regardless of the type of iron ore and the presence of nuclei, the size of the quasi-particles (P-type) in the particles decreases with the presence of coke particles and becomes smaller as the coke particle size increases. Xiao et al.⁴⁵⁾ carried out an evaluation of the effect of adding coarse-grained iron ore on molten phase formation and sintering structural properties. The correlation established between molten phase mobility and sintering conditions was effective in predicting the sintering behavior of various ore mixtures.

Fig. 15. Diagram of different types of quasi-particles.⁴³⁾ (Online version in color.)

The above research scholars target the sintering granulation process to change the different states of fuel presence in the quasi-pellets, the size of fuel particle size, and the optimization of parameters such as the amount of water and additives added to the granulation process to improve the granulation performance of the mixture, increase the full combustion of the fuel, and reduce the CO emissions generated by the sintering process.

3.2.2. Fuel Distribution Control

One of the most important aspects of sintering operation is that the particle size of the sintering mix gradually becomes coarser and the carbon content (fuel content) gradually decreases from top to bottom along the height of the cart.⁴⁶⁾

Poor fuel distribution is one of the main reasons for the unbalanced heat distribution in the sinter bed. The thermal mode of the upper material layer is weak, and the thermal mode of the lower material layer is enhanced by the excessive automatic heat accumulation in the lower material layer and the improvement of the combustion environment, which leads to some energy wastage. In order to meet the requirements of fuel distribution for sintering production, the segregation method of sinter feeding is often used in production to make a reasonable distribution of fuel on the sintering bed. Kang et al.⁴⁷⁾ and Cheng et al.³⁵⁾ identified thermal patterns that are not uniformly distributed in the height direction of the sintering bed and considered their possible causes in conventional sintering systems. The sintered material layer often faces the issue of upper cooling and lower heating. An effective solution to this problem is to control the fuel distribution by increasing the fuel content in the upper bed and reducing it in the lower bed. This can be achieved in practice by adjusting the operating parameters of the sintering mixture charging unit, which allows for the distribution of solid fuels. Zhao et al.⁴⁸⁾ theoretically investigated the minimization of unbalanced heat distribution in iron ore sintering based on segregated solid fuel content, particle size, bed porosity, stack density and other factors. Simulation results show that fuel content segregation and increased bed permeability have a significant effect on key parameters of the sintered bed and determine the optimum level of segregation for the sintered mixture. Machida et al.⁴⁹⁾ investigated the optimal degree of coke separation for different mineral ratios and introduced a magnetic brake feeder to control the chute installation angle and magnetic flux density by the coke separation was achieved by controlling the installation angle of the chute and the magnetic flux density. Figure 16 is a schematic diagram of the charging device used in the experiment. The charging chute angle and flux density of the magnetic brake feeder were determined to achieve the optimum degree of coke separation. A laboratory charging test simulating a sinter charging unit was conducted to obtain the charging conditions for achieving optimum coke separation in the MBF. It was also applied to MBF at the charging unit of JFE Steel Keihin No. 1 sintering plant to optimize carbon segregation in the sinter bed. As a result, sintering indexes such as productivity and drum index were improved.

Fig. 16. Schematic diagram of charging device used in the charging test.⁴⁸⁾

Through the improvement of charging process of sintering feed operation, it can better realize the reasonable distribution of fuel in the upper part of the material layer with more fuel and less fuel in the lower part, and realize the uniform distribution of heat in the material layer through the automatic heat storage effect of the thick material layer. In addition, in order to better achieve uniform charging process of sintering feed effect production enterprises have gradually developed and applied automatic control charging process of sintering feed technology, through the installation of radar, infrared monitoring and other equipment to monitor the changes in the charging process of sintering feed process of the material layer in real time, while adjusting the degree of opening and closing of the discharge door through PLC automatic control system to precisely control the amount of material. Practice has proved that the air permeability of the material layer will be greatly improved after the implementation of automatic charging process of sintering feed technology.

3.2.3. Fuel Division and Addition Control

Sintered fuel secondary dosing technology is to mix part of the fuel with the sintered material and add the other part of the fuel after the primary mixing of the sintered material, Fig. 17 shows the process flow diagram for the secondary addition of fuel and melt together.⁵⁰⁾ The purpose is to make this part of the fuel wrapped around the surface of the mixture particles to keep the fuel with a larger active reaction surface and increase its combustion rate. Fuel fractionation can improve the combustion and heat transfer conditions of the fuel and accelerate the combustion and heat transfer rate.⁵¹⁾ At the same time, the relative concentration of fuel distribution is conducive to the increase of material layer temperature and the prolongation of high-temperature cooling time, and the solidification of bonded phase is more adequate, thus improving the sintering yield, increasing the sintering utilization coefficient,⁵²⁾ improving the strength of sintered ore, and reducing the consumption of sintering fuel.

Fig. 17. Process flow of coke breeze and limestone coating granulation technology at Kurashiki No. 2 Sinter Plant.^52,53)

Arikata et al.⁵¹⁾ prepared coke powder quasi-particle with different morphologies by controlling the proportion of the second addition of coke powder, and investigated how this morphological variation affected the combustion quality, and proposed a method of preparing quasi-particle by delaying the addition of coke powder during the granulation process in order to investigate the possibility of reducing the consumption of coke powder. It was found experimentally that most of the secondary added coke powder adhered to the surface of the quasi-particle or was present alone, increasing the fuel-air contact and leading to results such as faster sintering and lower yields, although in subsequent experiments it was found that the negative effect of lower yields was reduced after increasing the thickness of the material layer. Finally, different coke phase II addition ratios were compared, including 0%, 50% and 100%, where 50% was preferred because it improved permeability and combustion efficiency. Oyama et al.⁵³⁾ developed a similar granulation method and tested it in a commercial sintering plant, while noting that the key to this granulation process is the control of the mixing time of the secondary addition of coke powder and limestone. The advanced granulation process is characterized by the co-addition of the coke powder and the fusing agent (limestone) in a secondary process. Coke powder and limestone are injected at high speed from the end of the drum mixer via a belt conveyor to achieve coating coverage of the quasi-particle. This process controls the excessive melt reaction between the iron ore and limestone as the two substances are separated in the quasi-particle. Basic studies and commercial plant tests have shown that the ideal melt fluidity produced by the aforementioned segregation enhances the permeability in the sintered bed. The excellent reducibility of the sintered product depends on the diffusible structure of the retained micropores in the residual ore. A new granulation process has been introduced in the 13.5 million ton per year capacity commercial sinter plant of JFE Steel’s West Japan Works, as shown in the Fig. 17. Despite the recent prevalence of poor-quality ore, the new process has significantly improved the productivity and reducibility of the sintered product.

The above researchers have analyzed the sintered fuel size, fuel endowment state in quasi-particle and distribution state within the material layer from multiple perspectives. The key point to reduce CO emission in the sintering process is to promote complete combustion of the fuel and reduce CO generation. The maximum utilization of fuel in the sintering process is achieved by controlling the size of raw fuel, controlling the distribution state of fuel during granulation and deflecting fuel in the upper and lower part of the material layer during charging process of sintering feed. In addition to solid fuels, the injection of gaseous fuels into the feed layer is also increasingly studied, benefiting from its ease of control through flow controllers and piping arrangements, and the simplicity and convenience of the injection into the feed layer location, flow rate and timing during the production process. More and more researchers have started to improve the heat imbalance between the upper and lower layers from gas fuel injection, and have achieved certain results.

3.3. Steam Injection Sintering of the Sintered Material Surface

3.3.1. Analysis of Steam Sintering Abatement Mechanism

During the sintering process, the fuel is burned to release heat, and materials such as flux and iron ore soften and melt at high temperatures, forming a combustion zone where coke and combustion products undergo complete or incomplete combustion reactions, gasification reactions, secondary combustion reactions, etc. The temperature reaches 1150°C–1300°C.

Long et al.⁵⁵⁾ performed a mechanistic analysis of the reaction changes that occurred in the material layer after steam injection from a thermodynamic point of view after steam injection at the material surface, and several new reactions occurred in the sintered combustion zone,⁵⁵⁾ as shown in reactions (9)–(12) and Figs. 18(a), 18(b). It can be seen that reactions (6) and (7) occur above 900°C. Reaction (10) occurs more easily at 900–1100°C, while reaction (9) occurs at temperatures higher than 1100°C. Reaction (11) occurs more easily below 1100°C, especially below 900°C than reactions (9) and (10). Considering the low oxygen content during sintering, the CO and H₂O produced by reactions (6)–(8) may undergo the reactions described in reaction (10).

  

$$\mathrm{C}+{\mathrm{O}}_2=\mathrm{C}{\mathrm{O}}_2$$(5)

  

$$2\mathrm{C}+{\mathrm{O}}_2=2\mathrm{C}\mathrm{O}$$(6)

  

$$2\mathrm{C}\mathrm{O}+{\mathrm{O}}_2=2\mathrm{C}{\mathrm{O}}_2$$(7)

  

$$\mathrm{C}+\mathrm{C}{\mathrm{O}}_2=2\mathrm{C}\mathrm{O}$$(8)

  

$$\mathrm{C}+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)=\mathrm{C}\mathrm{O}+2{\mathrm{H}}_2$$(9)

  

$$\mathrm{C}+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)=\mathrm{C}{\mathrm{O}}_2+2{\mathrm{H}}_2$$(10)

  

$$\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)=\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2$$(11)

  

$$2{\mathrm{H}}_2+{\mathrm{O}}_2=2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)$$(12)

  

$$\mathrm{C}\mathrm{O}+1/2{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)=\mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}$$(13)

Fig. 18. Reaction thermodynamic curves of the sintering process. (a) conventional sintering; (b) steam injection during sintering. (Online version in color.)

Li et al.⁵⁶⁾ used the PFR model to simulate the reduction pathways of H₂O and CO to NO, showing that CO is the key to H radical generation and NO reduction when H₂O is less than 1%. Fan et al.⁵⁷⁾ observed from the kinetics of the sintering process that oxygen is sufficient during the sintering process, but because most of the fuel in the sintering process granulation is wrapped around the material as a nucleation core and high temperature melt is generated in the sintering combustion zone, it is difficult for oxygen to reach the surface of the coke powder and there is a local reducing atmosphere around the coke powder. The presence of steam, which diffuses more rapidly in the sinter bed than oxygen (air), opens a reaction “channel” for the other mixed ore-encapsulated fuels. This allows the oxygen-deficient region to be supplemented by an H₂O–H₂ medium to facilitate the water-gas reaction. As a result, the heat transfer conditions for fuel combustion are improved to a certain extent and complete combustion of incomplete reaction fuels is achieved. The steam reacts with C at high temperature to produce H₂, and H₂ and O₂ produce not only (OH) but also (O), both of which are free radicals that oxidize CO and thus reduce CO emissions. The reaction equation is as follows:

  

$$\begin{array}{l}\mathrm{C}+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)=\mathrm{C}\mathrm{O}+2{\mathrm{H}}_2\\ (\mathrm{S}\mathrm{p}\mathrm{o}\mathrm{n}\mathrm{t}\mathrm{a}\mathrm{n}\mathrm{e}\mathrm{o}\mathrm{u}\mathrm{s}\mathrm{r}\mathrm{e}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\mathrm{a}\mathrm{t}\mathrm{h}\mathrm{i}\mathrm{g}\mathrm{h}\mathrm{t}\mathrm{e}\mathrm{m}\mathrm{p}\mathrm{e}\mathrm{r}\mathrm{a}\mathrm{t}\mathrm{u}\mathrm{r}\mathrm{e}\mathrm{s})\end{array}$$(14)

  

$${\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)=2\left(\mathrm{O}\mathrm{H}\right)$$(15)

  

$${\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{O}}_2\left(\mathrm{g}\right)={\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)+\left(\mathrm{O}\right)$$(16)

  

$$\left(\mathrm{O}\right)+\mathrm{C}\mathrm{O}\left(\mathrm{g}\right)=\mathrm{C}{\mathrm{O}}_2\left(\mathrm{g}\right)$$(17)

From the above explanation, the steam injection during the sintering process can effectively suppress the production of CO, and the emission of CO will be mainly influenced by two reaction paths. Figure 19 shows the schematic diagram of the reaction mechanism.

Fig. 19. Schematic diagram of the mechanism of steam emission reduction. (Online version in color.)

1) At high temperatures, CO from incomplete combustion of coke powder reacts with steam to produce CO₂ and H₂ (H₂O(g) + CO(g) = H₂(g) + CO₂(g)); however, this pathway cannot occur spontaneously at temperatures above 800°C by thermodynamic calculations, so it is not the main pathway to reduce CO emissions.

2) When the temperature is higher than 674°C, the steam enters the feed layer and reacts with C to form H₂ (C + 2H₂O(g) = CO + 2H₂), and H₂ reacts with O₂ to form two free radicals (OH) and (O). These two free radicals will rapidly oxidize CO, which is the key to reduce CO emissions.

3.3.2. Steam Injection Location

The choice of steam blowing position on the sintered material surface not only affects the CO emission, but also more obviously affects the quality and yield of the sintered ore. Luo et al.⁵⁸⁾ noticed that the blowing position was too far ahead, when the high temperature zone was not formed or just formed, and most of the blowing steam entering at this time was easily condensed into water, while the sintering process urgently needed to burn the carbon in the fuel to establish or maintain the sintering high temperature zone, and the steam that was not condensed competed with the sintering process, and only a small amount of steam participated in the reaction of H₂O with C and O₂ through competition, and converted a small amount of CO into CO₂; And because the temperature of steam is much lower than that of the combustion layer, physical heat absorption occurs when steam is close to the combustion layer, while chemical heat absorption occurs after the reaction of steam (C + H₂O = CO + H₂, heat absorption reaction),⁵⁹⁾ which adversely affects the sintering of the upper layer. If the steam injection is delayed, the position of the steam into the sinter layer is delayed, the high-temperature zone is formed and stabilized and moved down to the lower position of the material layer. At this time, the residence time and path of the steam inject in the material layer is too long, and only a small part of it touches the high-temperature zone and plays its role; at the same time, due to the long residence time of the steam, part of the steam encounters the upper cold sintered ore and condenses into water, which is difficult to participate in the reaction of the high-temperature zone, resulting in the reduction effect of CO is insignificant.

Zhou et al.⁶⁰⁾ experimented with different blowing intervals of steam in the sintering process with and without steam blowing sintering conditions to compare the changes in flue gas CO concentration; the results showed that the CO rise in interval I (5–8 min) decreased, the CO emissions in interval II (10–13 min) decreased by 1125–1875 mg/m³, and the CO emissions in interval III (15–18 min) at least decreased by about 875–1375 mg/m³, as shown in Fig. 20. Therefore, the most obvious effect of water vapor spraying in the middle position on CO emission reduction, while the effect of CO emission reduction by water vapor spraying in the lower position is reduced, probably because the more downward the water vapor spraying position is, the longer the water vapor travels through, and the less effective water vapor amount reaches the combustion belt. Long et al.⁵⁵⁾ investigated the effect of steam injection on fuel combustion efficiency and CO emission by comparing the changes of thermodynamic parameters of the sintering process before and after steam injection through laboratory experiments. It was found that the sintering gas medium of H₂O–H₂–N₂–O₂ and the blown-in steam improved the heat transfer conditions of fuel combustion and promoted the water-gas reaction. The Steam injection is optimal 15 minutes after ignition, with a steam blowdown volume of 0.02 m³·min⁻¹. The CO emissions were reduced by 10.91% compared to the base case. The combustion efficiency was 88.83%, which was 6.15% higher than conventional sintering, and solid fuel consumption was reduced by1.15 kg·t⁻¹.

Fig. 20. Effect of steam injection position on CO emission.⁵⁹⁾ (a) CO emission without injection; (b) CO emission in different injection intervals. (Online version in color.)

3.3.3. Steam Blowing Quantity

This process occurs when steam enters the sintered material layer before it reaches the combustion layer, and heat transfer occurs within the material layer.⁶¹⁾ In addition, blowing excess steam has a significant impact on the combustion temperature of the fuel. Since the temperature of the steam is much lower than the combustion layer, physical heat absorption occurs when the steam approaches the combustion layer, and the excess steam eventually enters the flue gas carrying a large amount of heat and increasing heat loss. In addition, excessive vapor entering the material layer will also aggravate the over-wetting of the lower part of the material layer and deteriorate the permeability of the material layer, which is detrimental to the sintering compliance.

Zhou et al.⁶⁰⁾ studied the effect of water vapor blowing on CO emissions for different steam blowing volumes in a sinter cup experiment under the blowing interval of 10–15 min. As shown in Fig. 21, the overall level of CO emissions decreased after water vapor was sprayed, and the more the amount of spraying, the more the CO emission level decreased. When no water vapor was sprayed, the average CO emission during the whole sintering process was 11690 mg/m3; when the water vapor spraying amount on the surface of the sintered material was 0.4%, the average CO emission decreased to 9918 mg/m3; as the water vapor spraying amount increased to 0.9%, the average CO emission continued to decrease to 8255 mg/m3, with a maximum decrease of 29.4%. Luo et al.⁵⁸⁾ also found the same results for the experimental study of steam blowing volume, which showed that increasing the steam blowing volume within a suitable range had a better effect on the reduction of CO. However, during the experiments, it was also found that the increase of steam blowing volume has a relatively large impact on the reduction of sinter ore quality, especially the upper finished sinter ore. The reason for this may be that the contact time between steam and sinter ore surface is too long, which leads to the local deterioration of sinter ore surface quality, and then the formation of regrind or powder, resulting in a lower yield and drum index.

Fig. 21. Influence of steam injection amount on average CO emission. (Online version in color.)

3.3.4. Industrial Production Effect

Gan et al.^57,62) have studied the effect of sintering process steam injection volume and time on sintered CO emissions. The results showed that increasing the by-product steam injection concentration (0.32–0.47 vol%) and extending the injection time (5 min) in the appropriate range (10–15 min) could improve the sinter quality. By reducing the coke powder dosage from 5.60 to 5.45% under the recommended parameters, the CO in the sinter exhaust gas is reduced by 15.16%. The potential economic benefits of steam injection technology were calculated on a 360 m2 sintering machine (annual sinter production of 3.2 million tons), excluding equipment modifications and steam injection costs of ＄300000; a profit of ＄737491.2 per year or ＄0.23 per ton of sintered ore could be realized. Therefore, low-carbon, clean iron ore sintering production can be achieved using byproduct steam. Many enterprises in China have also implemented sintered material surface injection steam production and have achieved good results in reducing CO. Capital Iron and Steel of China⁶³⁾ implemented steam blowing on the sintering material surface, the solid fuel consumption was reduced by 1.64 kg/t and the CO mass concentration in the sintering flue gas was reduced by 900–1200 mg/Nm³. Zhongtian Iron and Steel⁶⁴⁾ reported that by adjusting the parameters of particle size distribution in the sintering mix, the negative pressure of extraction air, and implementing steam blowing technology on the material surface, the CO content in sintering flue gas was significantly reduced from 5208 mg/Nm³ to 4426 mg/Nm³, demonstrating a clear practical effect on CO reduction in sintering flue gas.

In the sintering process, CO emissions from incomplete combustion of coke powder can be reduced by injecting steam. However, the high-temperature zone of the material layer during sintering production is a process that changes continuously from top to bottom; only by spraying steam in the right area can the desired reduction effect be achieved. Figure 22 shows a process flow of steam injection in the middle section of sintering on industry as well. In industrial production, after the sintering ignition begins, it is not suitable for injecting low-temperature steam because the high-temperature zone is small, the highest temperature of the material layer and the high-temperature zone is maintained for a short period of time, and the burning of coke is needed to provide sufficient heat. When the combustion layer reaches the middle of the sintering layer, it is better to inject steam because the hot sintering layer has the ability to heat the steam to a higher temperature. When the steam reaches the high temperature zone, it can react with C to form H₂, and to form (OH) and (O), releasing the heat of reaction. Due to the additional heat added, some reduction in sintering energy consumption and pollutant emissions can be achieved. In industrial applications, the steam temperature, the height of the injection unit from the sintering ore surface and the influence of the injection pressure should be taken into account, as well as the water circulation system and steam boiler system owned by the plant for the treatment and recycling of wastewater from the steel industry. In addition, this injection unit can also be used for gas injection after modification, and better sintering results can be achieved by exploring a suitable coupled injection system.

Fig. 22. Flue gas circulation and steam blowing sintering process flow chart. (Online version in color.)

3.4. Oxygen-rich Sintering

Oxygen-enriched sintering⁴⁷⁾ refers to the sintering process in which a certain percentage of excess oxygen is added to the incoming air to promote fuller combustion of the sintered fuel, making the sintering process react more thoroughly, improving the efficiency of fuel utilization and reducing the generation of incomplete combustion products CO.

Kolesanov et al.⁶⁵⁾ pioneered the field of oxygen enrichment in sintering processes. An experimental sintering plant was built for supplying additional oxygen to the sintering bed. The average efficiency of the productivity of sinteringmachine was increased by 8.4% due to oxygen enrichment in the preheated air mixture. In Japan,⁶⁶⁾ the benefits of oxygen enrichment were examined in sintering tank experiments and computational simulations. The sintered ore was comparable in volume and strength, and oxygen enrichment increased the sinter productivity by 3–4%. Kang et al.^47,67) considered oxygen enrichment in the upper region of the sinter bed to improve coke combustion efficiency, hoping to solve the problem of unbalanced heat distribution in the bed height direction conducted sinter tank tests and numerical simulations. The observed and simulated results based on hard glass tank tests showed that the combustion zone was significantly extended in the oxygen-infused region and the flame leading edge velocity increased slightly under oxygen-enriched conditions. When the oxygen concentration increased to 30%, the thickness of the burning area and the thickness of the melting zone both became wider. After the oxygen concentration returned to 21%, both the melting zone thickness and the burning zone thickness decreased to normal levels. Yuji IWAMI et al.⁶⁸⁾ experimented with oxygen-rich and hydrogen-based gaseous fuel injection comparing four oxygen concentrations (21%, 24%, 28%, and 32%) in the inhaled air, keeping the natural gas concentration constant. As the oxygen concentration increased from 21% to 28%, the sinter strength increased while the sintering time decreased. These changes are mainly attributed to the reaction kinetics at different oxygen concentrations. As shown in Fig. 23(a), under oxygen-enriched conditions, the CH₄ combustion position moves to a lower position by shifting the coke combustion position. In addition, the lowering of the CH₄ ignition point in the oxygen-enriched environment leads to the upward shift of the CH₄ combustion position, as shown in Fig. 23(b). As a result, the distance between the two combustion zones becomes larger with increasing oxygen concentration, which leads to a more preferred thermal mode and higher energy efficiency. However, the experiments show that the increase in sinter quality index saturates at an oxygen concentration of 32%.

Fig. 23. Schematic diagram of the effect of oxygen enrichment on the thermal model.⁶⁶⁾

The combustion environment in a sintered bed can be improved by oxygen enrichment. However, the accelerated oxidation rate of solid fuels in an oxygen-enriched environment and the narrowing of the thickness of the combustion zone will limit the application of oxygen enrichment. Future studies should examine the effect of oxygen concentration and oxygen injection time on the change of thermal pattern of the feed layer in the oxidizing atmosphere and the flue gas CO emission during the sintering process. In terms of application prospects, it is similar to the LNG blow-in method, but the price of oxygen is much lower than LNG, and there is an oxygen generator in the plant, so it is easier to use. As long as the oxygen concentration is accurately controlled and the location of the blow-in is well grasped, it can better promote the combustion and exotherm of the fuel, improve the utilization efficiency of the fuel, and reduce the amount of solid fuel used for sintering production and the emission of flue gas pollutants such as CO.

3.5. Flue Gas Recirculation Sintering

Sintering flue gas recycling technology is a flue gas utilization technology that selectively returns part of the sintering flue gas to be recycled in the circulating flue gas hood on the upper part of the sintering machine dolly after the igniter, which has the double effect of energy saving and emission reduction.⁶⁹⁾ When the circulating flue gas passes through the sintering layer again, the flue gas sensible heat and the secondary combustion exotherm of CO, CH and other compounds in the flue gas reduce the solid combustion consumption, which can not only realize the recovery of sensible heat and latent heat in the sintering flue gas, but also reduce the concentration of CO and nitrogen and sulfur oxides in the discharged flue gas, and reduce the pressure of the subsequent flue gas emission reduction treatment system. According to the different locations of sintering machine flue gas extraction, it can be divided into internal circulation process and external circulation process, with the internal circulation process taking air from the branch duct of sintering machine air box and the external circulation process taking air from the flue duct after the main extractor fan.⁷⁰⁾ Figures 24(a) and 24(b) shows the process flow diagrams of internal and external circulation of sintered flue gas, respectively. The study shows that although the external circulation process is relatively simple in production transformation and the engineering volume is small, the flue gas composition of large flue gas has high moisture content, low oxygen concentration, and is not suitable for secondary sintering of the circulating return layer, and the unqualified sintering atmosphere will lead to poor permeability of the sintering bed, sintering minerals, quality decline, inadequate fuel combustion, and more CO emissions from the sintering process; However, the internal circulation process makes use of its characteristics of being able to adjust the air intake box flexibly, and selects the air intake box at different positions of the sintering machine to achieve the circulating flue gas volume, temperature and composition (including oxygen concentration, moisture content and CO concentration) suitable for the sintering process, and finally achieves the triple goals of reducing the amount of emitted flue gas, reducing production energy consumption and reducing flue gas pollution.⁷¹⁾

Fig. 24. Sintering flue gas circulation process. (a) internal circulation of sintering flue gas (b) external circulation of sintering flue gas. (Online version in color.)

In order to explore the suitable circulating flue gas parameters in sintering process, many researchers have made a lot of efforts and obtained effective reference data to guide the sintering production practice.

Fan et al.⁸⁾ investigated the effect of sintering flue gas recirculation on sintering process flue gas composition changes on sintered ore quality and flue gas pollutant emissions as shown in Fig. 25.

Fig. 25. Influence of FGR on sintering process.⁸⁾ (Online version in color.)

Wang et al.⁷²⁾ A comprehensive mathematical model of FRG sintering was developed especially for the study of the thermal model of the feed layer and the variation of bed porosity in the sintered bed. The effects of flue gas temperature, flue gas O₂ content and flue gas velocity on the thermal model in the sintered bed were determined. Yu et al.⁷³⁾ conducted laboratory-scale flue gas recirculation sintering experiments to investigate the effect on NOx reduction mechanism at different flue gas recirculation ratios. The experimental results showed that the NOx reduction rate was higher as the proportion of circulating flue gas increased. However, the deterioration of the sinter quality index at high ratios of recirculating flue gas indicated the need to find a balance between NOx emission and sinter quality. Subsequent researchers found a synergistic reduction between NO and CO within the material layer of the sintering process. Gan and Zhou et al.^44,74,75) experimentally investigated the effect of circulating flue gas components on NOx emission characteristics. The results show that the conversion of N to NOx in the fuel decreases with increasing concentrations of CO and CO₂ in the circulating flue gas, and CO can reduce the conversion of N to NOx in the fuel by direct reduction of NO or indirect reduction of NOx by reaction with C. The reaction of CO₂ with C to form CO has a significant inhibitory effect on the conversion of N to quasi-particulate NOx in the fuel. This study provides a valuable pathway on how to further achieve joint NOx and CO emission reduction by controlling operating parameters. In addition, Gan et al.⁷⁶⁾ investigated the combustion behavior of CO in the circulating flue gas and its effect on the heat transfer front, flame front and heat in the sinter bed by sintering cup experiments to reveal the mechanism of the effect of CO content in the circulating flue gas on the sintering of iron ore. The results show that CO in the circulating flue gas undergoes post-combustion in the sintering zone when passing through the sintering bed, releasing a large amount of heat and reducing the consumption of solid fuel. The fuel ratio can be reduced from 5% to 4.7% at a CO content of 2% in the circulating flue gas. In order to further reduce the use of primary fuels and seek suitable alternative fuels, Gan et al.⁷⁷⁾ used the biochar sintering technology, which reduced the sintering yield and sinter strength at 40% biochar replacement. The FGR technology was introduced to improve the sintering performance by recovering the sensible heat of the flue gas and the heat released from the oxidation of CO, thus increasing the peak temperature, extending the high temperature duration and improving the sintering quality. Finally, when the FGR ratio was 40%, the sintering performance of biochar with a substitution rate of 40% was comparable to that of coke powder, and the CO produced by combustion was slightly reduced. Li and Yu et al.^78,79) studied the combustion rate and combustion efficiency of the fuel, considering that the appropriate O2 concentration in the circulating flue gas is greater than 15%. The post-combustion of CO can release a large amount of heat and increase the temperature of the sintering bed, which is conducive to melt generation and increases the sintering strength. Therefore, increasing CO concentration is beneficial to the sintering process. With the increase of H₂O(g) content, the gas-solid heat transfer rate increases, and the condensation in the over-wet zone intensifies, so the H₂O content in the circulating flue gas should be less than 8%. In addition, the circulating flue gas temperature should be kept in the range of 150°C to 250°C.

Sintering flue gas recycling technology significantly reduces the emissions of sintering flue gas through the reuse of waste gas, and reduces the burden of end-of-pipe treatment of sintering flue gas. The internal circulation process designed and developed by making full use of the difference of flue gas distribution in the length of the sintering machine is flexible and comprehensive, and has less influence on the sintering production, so it is the flue gas circulation mode that can be adopted in common sintering plants. Based on the production operation characteristics of different sintering machines and energy saving and emission reduction targets, detailed flue gas testing and process effect prediction are the prerequisite and guarantee for the realization of flue gas circulation effect.

4. CO Treatment Technology at Sintering Ends

Sintering flue gas is mainly characterized by large flue gas volume, low temperature, high dust content and high humidity, which determines the complexity of sintering flue gas CO purification. The adoption of low gas resistance, high efficiency and stable removal method is the core key to purify CO under the limited area condition.

4.1. Adsorption Method

The adsorption method uses an adsorbent to separate CO from other gas components in the flue gas. The adsorption performance and the adsorption amount of the adsorbent are related to the temperature and pressure. The adsorption of gases under pressure and the desorption of gases under pressure is called variable pressure adsorption, and the adsorption of gases at room temperature and the desorption of gases at elevated temperature is called variable temperature adsorption. Variable temperature adsorption with a large temperature change makes the life of the adsorbent reduced, while the adsorbent of variable pressure adsorption can be reused, so the application is more extensive. Common adsorbents include: zeolite molecular sieve, activated carbon, activated alumina and silica gel, etc. The related process flow⁷⁾ is shown in Fig. 26. When the equipment is running, the flue gas enters the adsorption zone of the molecular sieve rotor through the fan, CO is adsorbed by the molecular sieve, and the purified flue gas is discharged into the atmosphere through the chimney, the molecular sieve in the adsorption zone releases CO through the cooling zone and desorption zone, and the high concentration of CO is finally discharged into the atmosphere after combustion treatment. Gao et al.⁸⁰⁾ prepared CuCl₂-loaded Y-type zeolite molecular sieve by monolayer dispersion method, and the adsorption capacity of CO reached 3.03 mmol/g at 303 K and 100 kPa, and the selectivity factors (the ratio of partition coefficients of the two components, the larger the value, the better the separation effect) of CO/N₂, CO/CH₄ and CO/CO₂ reached 68.00, 24.73 and 2.83. Peng et al.⁸¹⁾ prepared a Cu(I)-loaded MIL-100(Fe) adsorbent, and the adsorption capacity reached 2.78 mmol/g and the selectivity factor reached 169 at 298 K and 100 kPa. The reason for the significant improvement of the absorption capacity and selectivity was mainly due to the selective p-complexation of CO with Cu(I).

Fig. 26. Zeolite molecular sieve rotor concentration and adsorption technology.⁷⁾ (Online version in color.)

In addition to solid CO adsorbent, more liquid adsorbent used in the chemical industry is also more effective in the adsorption and removal of CO. They mainly include copper ammonia solution and TCA aluminum toluene solution. Although the solution absorption of CO removal efficiency is high, but there are some disadvantages, such as the absorption of CO and CO₂ at the same time, resulting in the increase of the amount of absorption fluid; secondly, the absorption solution of Cu + easy to reduce to metal Cu and block the pipeline, and the absorption solution is corrosive and high requirements for equipment. These characteristics make the method have high investment and operating costs.

The problems of large equipment investment, high energy consumption and high corrosiveness of the absorbent solution for sintered flue gas treatment by solution absorption method cause more restrictions in its sintered flue gas CO treatment. Meanwhile, the adsorption capacity, selectivity and input-output efficiency of these adsorbents in the industrial application of sintered flue gas CO treatment still need to be further researched.

4.2. Strong Oxidizer Oxidation Method

Strong oxidants can oxidize CO in flue gas to CO₂, and the main strong oxidants used in industry are ozone and Fenton’s reagent. The chemical formula of ozone oxidation of CO is: O3+CO→CO₂+O₂, and its oxidation products are friendly to the environment. Without catalyst, the oxidation process requires high ozone concentration and temperature, and the preparation cost of ozone is relatively high, which is not suitable for massive application. KONOVA P et al.⁸²⁾ prepared a metal oxide catalyst CoOx/Al₂O₃ by the deposition-oxidation-precipitation method and compared the oxidation effect of O₃ and O₂ oxidants. The results showed that the oxidation temperature of O₃ was 353 K at a CO conversion rate set at 80%, while the oxidation temperature of O₂ needed to reach the reaction temperature and activation energy of ozone-catalyzed CO oxidation are significantly lower, and this technology is a better technology for low mass concentration CO purification in the future. Fenton’s reagent is an acidic strong oxidizer formed by H₂O₂ and Fe²⁺. At pH 3–5, H₂O₂ and Fe²⁺ react to form a large number of hydroxyl reactive groups, which is the reason for the strong oxidizing property of Fenton’s reagent. Cheng et al.⁸³⁾ investigated the effect of Fenton’s reagent to oxidize CO, and they found that the CO abatement rate was 9%–15% at 80–150°C in a gas with a flow rate of 2.52 m/s at a flow rate of 6.8 L/min, and this result indicates that Fenton’s reagent has limited effect on CO abatement. In addition, the acidic Fenton’s reagent is corrosive to the flue gas pipeline. In conclusion, Fenton’s reagent is not suitable for application in the steel industry.

4.3. Catalytic Oxidation Method

The low CO concentration, high gas flow rate and low flue gas temperature in steel production flue gas make the catalytic oxidation abatement technology a good match with the characteristics of steel flue gas. The catalytic mechanism of catalytic oxidation method contains three main types:

1) Mars-van Krevelen (M-vK) mechanism

The M-vK mechanism^84,85) refers to the fact that CO is adsorbed on the catalyst surface and activated to become adsorbed state CO, which reacts with lattice oxygen on the catalyst surface to produce CO₂, and the consumed lattice oxygen creates oxygen vacancies, which are replenished by oxygen in the adsorbed gas phase, forming a cycle of gas phase oxygen-adsorbed state oxygen-lattice oxygen between the catalyst bulk phase and the surface to achieve continuous oxidation of CO. The MvK mechanism holds that the O₂ for the CO oxidation process originates from the lattice oxygen in the catalyst.

2) Langmuir-Hinshelwood (L-H) mechanism

The L-H mechanism^86,87) was originally proposed by Engel and Ertl for the catalytic reaction of CO on Pt. The mechanism refers to the fact that the active component of the catalyst adsorbs CO and O₂ molecules on the surface, then dissociates and activates O₂, and the activated O reacts with the adsorbed CO on the surface to form CO₂, and the CO₂ molecules are desorbed from the catalyst surface to complete the catalysis. The L-H mechanism considers that the O in the CO oxidation process comes from adsorbed oxygen molecules on the surface of the catalyst or from oxygen atoms generated by the decomposition of adsorbed oxygen on the surface.

3) Eley-Rideal (E-R) mechanism

The E-R mechanism^88,89) refers to the fact that a component of the reactant is adsorbed on its surface by the catalyst and the reaction proceeds through the interaction of atoms or molecules in the chemisorbed state with molecules in the gas phase (or physically adsorbed).

In addition, CO catalytic purification belongs to the category of multiphase catalysis, in which the catalyst changes the reaction pathway by adsorbing CO and O₂ on its surface through different active sites, thus reducing the activation energy required for the reaction and accelerating the reaction.⁹⁰⁾ Depending on the active components of the catalysts, CO catalysts can be divided into noble metal catalysts and non-precious metal catalysts.

Since the 1980s, researchers have been studying CO oxidation catalyzed by precious metals, mainly Au, Pt and Pd.^91,92,93)

MURAVEV. V et al.⁹⁴⁾ prepared a Pd-CeO₂ catalyst and investigated the mechanism of the CO-catalyzed reaction of this catalyst in depth. From the reduction process of Ce ions and Pd ions in the catalyst, it was concluded that the Pd ion doping activated the lattice oxygen in the CeO₂ carrier and promoted the catalytic oxidation of CO, and the CO conversion could reach 100% at 150°C. SPEZZATI G et al.⁹⁵⁾ investigated the effect of CeO₂ carriers with different crystal exposure surfaces on the activity of Pd-based catalysts. The exposed surface of rod CeO₂ crystals was (111), while the exposed surface of cubic CeO₂ crystals was (100). The CO oxidation rate of the rod CeO₂ catalyst could reach 100% at 175°C, while the cubic form needed to be at 225°C to achieve 100% CO conversion.

Non-precious metal catalysts include Co-based and Cu-based catalysts. It has been shown that the active site of Co-based catalysts is Co³⁺, and the CO oxidation performance of the catalysts can be effectively improved by increasing the Co³⁺ content in the catalysts. CAI Y et al.⁹⁶⁾ synthesized a two-dimensional planar structure of Co₃O₄ by ethylene glycol self-assembly and rapid roasting, which makes (112) crystal plane with abundant Co³⁺, unsaturated bonds and oxygen vacancies, and these features make the catalysts at room temperature. However, it was found that a higher temperature was required for the complete catalytic oxidation of CO when the gas contained moisture, indicating that the water resistance of the catalyst needs to be improved. SONG X Z et al.⁹⁷⁾ prepared CuO/CeO₂ catalysts from Ce metal organic skeleton and copper nitrate, and when the Cu to CeO₂ mass ratio was 8%, the highest Cu⁺ content was 30.25%, and the CO conversion was 100% at 120°C, which had a good catalytic effect.

Precious metal catalysts have the advantages of high catalytic activity, good stability and long service life, but they also have major disadvantages, such as high loading of precious metals in the catalysts, which greatly increases the cost of use, and the preparation process is cumbersome and non-renewable after deactivation, which limits the large-scale industrial application of precious metal catalysts, so for low-temperature catalytic purification of CO, non-precious metal catalysts with low price and high catalytic activity become the mainstream choice.

5. Technical Analysis Comparison and Coupling Technology Proposed

The previously described abatement pathways, investment costs, abatement efficiencies and technology comparisons and possible future directions of development that need to be tackled by researchers were summarized and the results are shown in Table 1. In particular, the evaluation of the emission reduction potential of sintering technology is expressed by symbols (+++++), which indicate the emission reduction potential of the sintering technology or the emission reduction contribution to CO through different numbers of symbols. The specific categorization is shown as follows:

Table 1. Comparison of sintering abatement processes.

Direction of emission reduction	Typical technologies	Investment	Secondary pollution	CO reduction potential	Looking forward	Reference
Sintering to reduce emissions at sourced	Thick bed Sintering	Technology improvement	non-polluting	++	Increase sintered bed thickness and develop permeability	52,98)
	Stand-support Sintering	Equipment retrofit	non-polluting	++	Optimization of support plate materials	99)
	Double-Layer Sintering	Equipment retrofit	non-polluting	+++	Improvement of lower sintered bed oxygen content and permeability	100)
	H2-rich gas Sintering	Equipment retrofit	non-polluting	+++	Increased hydrogen-rich fuel ratio	101)
Emission reduction from the sintering process	Integrated control of sintering fuels	Technology improvement	non-polluting	++	Strengthen the development of high-precision and intelligent equipment	39,40)
	Steam injection Sintering	Equipment retrofit	non-polluting	+++	Low price source of oxygen at the factory	58,102)
	O2 injection Sintering	Technology improvement	non-polluting	++	Low price source of oxygen at the factory	27)
	Flue gas recirculation Sintering	Technology improvement and Equipment retrofit	non-polluting	+++	Increased flue gas recirculation rate	103,104)
Sintering end-of-pipe controls	Adsorption Method	Additional processes	Solid wastes	++++	Solid waste disposal or recycling	105)
	Strong oxidizer method of oxidizing	Additional processes	Acid waste liquid	+	Waste liquid treatment or recycling	83)
	Catalytic method of oxidizing	Additional processes	non-polluting	++++	Seeking efficient and stable catalysts that are resistant to H2O, S, and have a long useful life	7)

One plus sign (+): Indicates the lowest level of abatement potential, possibly with only the most basic measures.

Two plus signs (++): Indicates a higher level of emission reduction potential, which may include higher solid fuel utilization and more efficient waste heat utilization systems.

Three plus signs (+++): Indicates a stronger potential to reduce emissions, which may involve the use of cleaner energy sources and direct reductions in CO production.

Four plus signs (++++): Indicates very high emission reduction potential, which may include advanced technology applications such as CO capture and conversion technologies.

Five plus signs (+++++): Indicates the strongest emission reduction potential, which may mean that the company has adopted cutting-edge technologies and strategies for reducing emissions, such as the use of clean energy sources exclusively.

Table 1 presents a comparison of the investment costs associated with sintering abatement processes, CO abatement rates, secondary pollution, and other relevant indicators. The analysis reveals a positive correlation between the investment in abatement processes and the CO abatement rate. While simple process optimizations or equipment modifications can contribute to reducing CO emissions, their effectiveness in achieving high abatement rates is limited. Currently, technological approaches tend to rely more on end-of-pipe treatment, necessitating investments in new equipment and facilities for effective management. From the perspective of the sintering process, the future of emission reduction and sintering technology control is a focal point for researchers. They aim to develop innovative technologies or enhance the utilization of existing emission reduction technologies to maximize their effectiveness. This includes strategies such as increasing the thickness of the sintering layer after improving its permeability and employing hydrogen-rich fuels through spraying techniques. Catalytic oxidation technology is the highest rate of CO reduction and relatively low cost, and does not produce secondary pollutants, the future is the need to find a stronger resistance to toxicity and can be a long-term service of efficient catalyst research and development.

As mentioned before, sinter flue gas CO reduction using a single sintering emission reduction technology implementation has certain limitations, to achieve efficient and stable long-term CO emission reduction targets require integrated control of source, process and end-use emission reduction technologies. In view of this, the following section describes several sintering process overall optimization and emission reduction technology coupling schemes to provide reference for sintering practitioners to work together to explore the future development direction.

5.1. Sintering Flue Gas Internal Recirculation + Oxygen-enriched Sintering Coupling Technology

Internal recirculation of sintering flue gases is a sintering method in which a portion of the head and exhaust plenum flue gases from the head and tail of the sintering machine are selected and returned to the sintering surface after de-dusting to re-participate in the sintering process. The internal recycling flue gas is characterized by higher temperatures and higher oxygen concentrations. Sintering by flue gas recovery and recirculation can reduce the flue gas CO emissions and the total amount of flue gas emissions, and at the same time effectively utilize the CO secondary combustion heat and the sintering high-temperature flue gas waste heat, but the oxygen concentration of the recirculating flue gas is one of the main reasons to limit the recirculation rate of the sintering flue gas. Literature reports^78,79) that when the flue gas circulation rate is 30% and 50%, the oxygen concentration in the flue gas is about 16.99% and 11.63%. The advantage of oxygen-enriched sintering technology is that it can increase the oxygen concentration of the gas stream medium entering the sintered material surface and thus promote the full combustion of the fuel to release heat, and the CO concentration in the flue gas is significantly reduced.⁴⁷⁾ Flue gas internal circulation and oxygen-enriched sintering coupling technology integrates the advantages of flue gas internal circulation sintering and oxygen-enriched sintering technology to realize the double effect of enhancing the oxygen concentration of circulating flue gas and regulating the flue gas temperature. The main technical principle is to use the internal circulation temperature of about 200°C in the high-temperature flue gas as the carrier of oxygen-enriched body, the oxygen plant as a source of oxygen additional oxygen pipeline and circulating flue gas through the gas mixing valve to achieve the circulating flue gas 1% to 5% of the oxygen-enriched circulating flue gas to increase the concentration of flue gas O₂ to 18% to 22% after supplying the sintering process. The technology focuses on using oxygen enrichment to make up for the lack of O₂ concentration in the circulating flue gas, improve the combustion efficiency of the fuel in the sintered material layer to reduce CO emission, and realize the efficient recovery of sintering flue gas waste heat, reduction of solid fuel consumption, and sintering minerals, quality enhancement, and other functional requirements. The process flow diagram is shown in Fig. 27.

Fig. 27. Sintering Flue Gas internal Recirculation + Oxygen-enriched sintering coupling technology process flow chart. (Online version in color.)

5.2. Multi-media Atmosphere Sintering Coupling Technology with Hydrogen-rich Gas + Steam Injection + Hot Air

The heat in a traditional sintering process is provided by solid carbon fuels distributed inside the layer, but the automatic heat storage effect becomes more pronounced as the layer thickness increases. It has been shown that the maximum temperature difference between the lowest and surface burner zones when sintering 900 mm thick-bed can be more than 130°C at maximum.²⁹⁾ When the hydrogen-rich gas enters the material layer, it starts to burn and exotherm above the combustion belt, and makes up heat to the upper part of the material layer to prolong the high temperature holding time of 1200°C–1400°C. Steam blowing sintering has two advantages, one is the diffusion capacity of steam is greater than the air medium, the addition of steam is conducive to the diffusion of sintering air medium in the material layer, to accelerate the heat transfer within the material layer. The second is that the steam is in contact with the fuel inside the material layer can promote the complete combustion of solid fuel and reduce the generation of CO. Ring cooler 150–250°C of high temperature flue gas cycle to the material surface for sintering, high temperature hot air into the surface layer of sintered ore heat time to extend, improve the charging process of sintering feed due to segregation caused by temperature distribution above and below the layer of uneven distribution of temperature problems. In addition, the existence of the hood can also reduce the gas escape and heat loss. The multi-media sintering coupling technology of steam, hydrogen-rich gas and hot air proposed in this paper is implemented in the front section of the sintering cart, where hydrogen-rich gas is sprayed to supplement the heat to reduce the temperature difference between the upper and lower layers of the material layer, and high-temperature steam of 150°C–200°C is sprayed in the middle and back sections to promote the complete combustion of fuel to reduce the emission of CO, which can effectively reduce the energy consumption of sintering process, decrease the carbon consumption of the sintering process, reduce the carbon emission and other pollutants, and improve the quality of the sintered ore. quality of sintered ore. The process flow diagram is shown in Fig. 28.

Fig. 28. Process Flow Diagram of Multi-Media Sintering Coupling Technology with Hydrogen-Rich Gas + Steam Injection + Hot Air. (Online version in color.)

5.3. Double-layer Sintering + Hydrogen-enriched Gas Injection + Stepped Oxygen-enriched Sintering Coupling Technology

The layered pre-sintering of double-layer sintering technology is characterized by effective utilization of the preheating effect of the upper sintering flue gas on the lower layer and the secondary combustion effect of the lower combustion zone on the CO in the flue gas produced by the upper layer to save fuel consumption, maintain the advantages of energy saving and emission reduction of the ultra-thick-bed, and realize the sintering production increase and CO emission reduction. However, although the formation of a pre-sintered ore belt with good permeability in the middle of the layer can appropriately avoid the problem of poor permeability brought about by excessive heat storage in the lower part of the thick-bed and the loading of the layer, it also brings about a certain amount of oxygen deficiency in the lower part of the sintered bed. Research²⁴⁾ has shown that when the gas flow media after the upper sintering process O₂ volume fraction is generally 10%–15%, at this time into the lower sintering process will be in a state of oxygen deficiency. Therefore, in order to ensure the quality of sintered minerals and further play a double-layer sintering emission reduction advantages, the aforementioned sintering coupling technology is proposed, and at the same time, in order to make up for the lack of upper sintering flue gas in the front section of the lower layer of material to make up for the lack of the role of the upper sintering flue gas, At the end of the first ignition and before the second charging process of sintering feed, the sintered material bed is injected with hydrogen-enriched gas to make up the heat, and after the second ignition, according to the sintering process, the oxygen enrichment operation is carried out by using the graded oxygen enrichment to the gas medium with about 3%–5.5% of the supplemental oxygen amount in order to satisfy the demand of sintering, and the process flow is shown in Fig. 29.

Fig. 29. Process flow diagram of coupled technology of Double-layer sintering + Hydrogen-enriched Gas injection + Stepped Oxygen-enriched sintering coupling technology. (Online version in color.)

Among the sintering source, process and end emission reduction coupling technologies examples given in this paper, the first and second coupling technologies have already been realized in some Chinese sintering enterprises. The current application of double-layer sintering technology has not been widely used, and the coupling technology of double-layer sintering for emission reduction is more of an outlook of the authors to provide an idea for further exerting the emission reduction advantages of double-layer sintering technology, and more researchers are needed to optimize and innovate in the future.

6. Conclusion and Prospects

This review article briefly describes the sources and emission characteristics of CO in the iron ore sintering process. During the sintering process, CO is continuously emitted in large amounts but at low concentrations, mainly from sinter head ignition gases and incomplete combustion of solid fuels. Depending on the raw materials and operating conditions of different iron ore sintering plants, CO in sintering flue gas is characterized by low concentration, high emissions and high toxicity.

(1) Source emission reduction control is based on low-carbon sintering technology, from the perspective of energy structure change, reduce the use of fossil energy and seek suitable clean energy into sintering production, which is the development direction of green production of steel. The existing low-carbon sintering technologies such as thick-bed sintering and hydrogen-rich sintering have been widely used, but in the future, we still need the majority of scientific and technological workers to achieve breakthroughs in more effective low-carbon sintering technologies.

(2) Process emission reduction control is an effective means to reduce CO emissions from the perspective of regulating fuel combustion, improving the permeability of the material layer, ensuring full combustion of fuel and improving the utilization rate of fuel. Through multi-angle fuel control, steam blowing, flue gas circulation and oxygen-enriched sintering technology and other coordinated control to improve the fuel combustion environment to reduce sintering CO generation and emission reduction. The process control technology is low-input and only requires equipment modification or technology improvement based on the original sintering production, but the emission reduction effect is limited and further optimization is needed to meet the actual demand.

(3) End-of-pipe treatment is the ultimate guarantee of CO emission reduction, of which catalytic oxidation CO emission reduction technology is a good match with the flue gas treatment of the iron and steel industry. The core of catalytic oxidation technology lies in catalyst development. Precious metal and non-precious metal catalysts for CO oxidation performance can meet the needs of steel flue gas, but need further optimization in terms of anti-poisoning, stability and preparation cost, which is the main breakthrough in the direction of CO emission reduction in the future.

(4) Sintering CO reduction is not a simple stacking of abatement technologies, but the actual situation of production, taking into full account the matching and coordination of the abatement process before the effective coupling to achieve maximum abatement effect. In addition to technical considerations, economic issues must also be considered when updating existing production lines. In particular, the cost of equipment replacement during the technological transformation process should be evaluated in order to provide a valuable and effective reference for decision makers regarding iron ore sintering plants.

(5) In the context of the development of “carbon peaking and carbon neutral”, we need to pay more attention to CO emissions from the flue gas of the iron and steel industry. The current steel industry still has a large demand for fossil fuels, and the incomplete combustion of fossil fuels is the main cause of CO emissions, but the current status and characteristics of CO emissions from the steel production process lack more accurate field data, which brings inconvenience for researchers to develop effective CO emission reduction technologies.

Acknowlegenments

This research was founded by the China Minmetals Science and Technology Special Plan Foundation (2020ZXA01) and by the China Baowu Low Carbon Metallurgical Innovation Foundation (BWLCF202104).

References

• 1)   S. H.  Ma,  Z. G.  Wen and  J. N.  Chen: J Ind Ecol., 16 (2012), 506. https://doi.org/10.1111/j.1530-9290.2011.00418.x
• 2)   Z. C.  Guo and  Z. X.  Fu: Energy, 35 (2010), 4356. https://doi.org/10.1016/j.energy.2009.04.008
• 3)   K.  Wang,  C.  Wang,  X. D.  Lu and  J. N.  Chen: Energy Policy., 35 (2007), 2320. https://doi.org/10.1016/j.enpol.2006.08.007
• 4)   J. J.  Li,  J. C.  Wei,  Y. F.  Wang,  Y. F.  Luo,  Z. W.  Yu,  J. M.  Li and  H. M.  Long: J. Cent. South Univ. (Sci. Technol.), 52 (2021), 2062. https://doi.org/10.11817/j.issn.1672-7207.2021.06.036
• 5)   Y. F.  Luo,  Y. F.  Wang,  J. J.  Li,  Z. W.  Yu,  J. C.  Wei and  H. M.  Long: China Environ. Sci., 41 (2021), 4077. https://doi.org/10.19674/j.cnki.issn1000-6923.20210517.002
• 6)   Q. W.  Liu,  Y.  Cheng,  J.  Li,  W. J.  Zhu,  F.  Li and  D. L.  Li: J. Iron Steel Res., 32 (2020), 633. https://doi.org/10.13228/j.boyuan.issn1001-0963.20190283
• 7)   J. Y.  Liao,  H. X.  Zheng,  M.  Min,  H. Z.  Yu,  J. G.  Kang and  X. H.  Fan: Sintering and Pelletizing., 46 (2021), 17. https://doi.org/10.13403/j.sjqt.2021.02.018
• 8)   X. H.  Fan,  G. J.  Wong,  M.  Min,  X. L.  Chen,  Z. Y.  Yu and  Z. Y.  Ji: J. Cleaner Prod., 235 (2019), 1549. https://doi.org/10.1016/j.jclepro.2019.07.003
• 9)   B.  Zheng,  D.  Tong,  M.  Li,  F.  Liu,  C. P.  Hong,  G. N.  Geng,  H. Y.  Li,  X.  Li,  L. Q.  Peng,  J.  Qi,  L.  Yan,  Y. X.  Zhang,  H. Y.  Zhao,  Y. X.  Zheng,  K. B.  He and  Q.  Zhang: Atmos. Chem. Phys., 18 (2018), 14095. https://doi.org/10.5194/acp-18-14095-2018
• 10)   Y.  Cheng,  J.  Li,  J. W.  Zhu,  Y. T.  Cui,  W. F.  Zhao and  J. Y.  Lu: Multipurp. Util. Miner. Resour., 43 (2022), 22. https://doi.org/10.3969/j.issn.1000-6532.2022.02.004
• 11)   Y.  Liu,  J.  Yang,  J.  Wang,  Z. L.  Cheng and  Q. W.  Wang: Energy, 67 (2014), 370. https://doi.org/10.1016/j.energy.2013.11.086
• 12)   J. S.  Feng,  L.  Zhao,  S.  Zhang and  H.  Dong: Appl. Therm. Eng., 175 (2020), 115370. https://doi.org/10.1016/j.applthermaleng.2020.1153
• 13)   T.  Umadevi,  A.  Brahmacharyulu,  R.  Sah and  P. C.  Mahapatra: Ironmaking Steelmaking, 41 (2014), 410. https://doi.org/10.1179/1743281213Y.0000000133
• 14)   K.  Shunji,  T.  Inazumi and  T.  Nakayasu: ISIJ Int., 34 (1994), 562. https://doi.org/10.2355/isijinternational.34.562
• 15)   Z. J.  Liu,  L. L.  Niu,  S. J.  Zhang,  G. Q.  Dong,  Y. Z.  Wang,  G. L.  Wang,  J.  Kang,  L. Z.  Chen and  J. L.  Zhang: ISIJ Int., 60 (2020), 2400. https://doi.org/10.2355/isijinternational.ISIJINT-2020-219
• 16)   S. G.  Chen,  X. C.  Zhang,  J.  Liang,  D. Q.  Wang,  W.  Pan and  Y. P.  Zhang: China Metall., 33 (2023), 94. https://doi.org/10.13228/j.boyuan.issn1006-9356.20220956
• 17)   K.  Higuchi,  T.  Kawaguchi,  M.  Kobayashi,  Y.  Hosotani,  K.  Nakamura,  K.  Iwamoto and  M.  Fujimoto: ISIJ Int., 40 (2000), 1188. https://doi.org/10.2355/isijinternational.40.1188
• 18)   H. B.  Zuo,  L. H.  Cao,  Z. J.  Liu,  X.  Zhang and  T. J.  Yang: Chin. J. Eng. (Beijing, China), 30 (2008), 1101. https://doi.org/10.13374/j.issn1001-053x.2008.10.013
• 19)   H. B.  Zuo,  J. L.  Zhang,  Z. W.  Hu and  T. J.  Yang: Int. J. Miner. Metall. Mater., 20 (2013), 131. https://doi.org/10.1007/s12613-013-0704-9
• 20)   H. B.  Zuo,  J. L.  Zhang,  X.  Zhang,  Z. J.  Liu and  T. J.  Yang: Chin. J. Process Eng., 9 (2009), 147. 1009−606X(2009)S1−0147−05
• 21)   Y. Z.  Wang,  Z. J.  Liu,  J. L.  Zhang,  Y. P.  Zhang,  L. L.  Niu and  Q.  Cheng: J. Cleaner Prod., 252 (2020), 119855. https://doi.org/10.1016/j.jclepro.2019.119855
• 22)   S.  Sato,  T.  Kawaguchi and  M.  Kato: Trans. Iron Steel Inst. Jpn., 28 (1988), 705. https://doi.org/10.2355/isijinternational1966.28.705
• 23)   Q.  Zhong,  H. B.  Liu,  L. P.  Xu,  X.  Zhang,  M. J.  Rao,  Z. W.  Peng,  G. H.  Li and  T.  Jiang: J. Iron Steel Res. Int., 28 (2021), 1366. https://doi.org/10.1007/s42243-021-00576-4
• 24)   J.  Liu,  M. S.  Zhou,  F. D.  Wu,  H.  Zhang,  L. B.  Xu,  L. W.  Zhai,  W.  Gao and  Q.  Zhong: Metals, 12 (2022), 629. https://doi.org/10.3390/met12040629
• 25)   M. S.  Zhou,  Y. D.  Wang,  S. F.  Han,  D. M.  Zhao,  J. W.  Zhu,  Q.  Zhong and  T.  Jiang: Sintering and Pelletizing., 44 (2019), 23. https://doi.org/10.13403/j.sjqt.2019.06.086
• 26)   M. S.  Zhou,  Y. D.  Wang,  D. M.  Zhao,  J. W.  Zhu,  H. B.  Liu,  Q.  Zhong,  G. H.  Li and  T.  Jiang: 11th International Symposium on High-Temperature Metallurgical Processing, (2020), 639. https://doi.org/10.1007/978-3-030-36540-0
• 27)   T.  Miwa and  K.  Kurihara: Steel Res. Int., 82 (2011), 466. https://doi.org/10.1002/srin.201100031
• 28)   L.  Lu and  O.  Ishiyama: Miner. Process. Extr. Metall., 125 (2016), 132. https://doi.org/10.1080/03719553.2016.1165500
• 29)   H. D.  Ye,  H. Y.  Zhou,  Y. F.  Wang,  Q.  Li,  X. F.  Lu and  Q.  Liu: Iron Steel (Beijing, China), 56 (2012), 134. https://doi.org/10.13228/j.boyuan.issn0449-749x.20210349
• 30)   H. D.  Ye,  H. Y.  Zhou,  Z. C.  Wang,  J. C.  Wei and  Y. F.  Wang: Sintering and Pelletizing., 45 (2020), 1. https://doi.org/10.13403/j.sjqt.2020.05.060
• 31)   X. X.  Huang,  X. H.  Fan,  Z. Y.  Ji,  M.  Gan,  X. L.  Chen,  Y. J.  Zhao and  T.  Jiang: Appl. Therm. Eng., 157 (2019), 113709. https://doi.org/10.1016/j.applthermaleng.2019.04.119
• 32)   Z. L.  Cheng,  S. S.  Wei,  Z. G.  Guo,  J.  Yang and  Q. W.  Wang: J. Cleaner Prod., 161 (2017), 1374. https://doi.org/10.1016/j.jclepro.2017.07.017
• 33)   X. H.  Fan,  M.  Gan,  Z. Y.  Ji,  Z. A.  Zhou and  H. Y.  Zhou: Iron Steel (Beijing, China), 55 (2020), 62. https://doi.org/10.13228/j.boyuan.issn0449-749x.20200219
• 34)   N.  Oyama,  Y.  Iwami,  T.  Yamamoto,  S.  Machida,  T.  Higuchi,  H.  Sato,  M.  Sato,  K.  Takeda,  Y.  Watanabe,  M.  Shimizu and  K.  Nishioka: ISIJ Int., 51 (2011), 913. https://doi.org/10.2355/isijinternational.51.913
• 35)   Z. L.  Cheng,  J. Y.  Wang,  S. S.  Wei,  Z. G.  Guo,  J.  Yang and  Q. W.  Wang: Appl. Energy, 207 (2017), 230. https://doi.org/10.1016/j.apenergy.2017.06.024
• 36)   J. A.  de Castro,  N.  Nath,  A. B.  Francal, V. S. Guilherme1 and Y. Sasaki: Ironmaking Steelmaking., 39 (2012), 605. https://doi.org/10.1179/1743281212Y.0000000008
• 37)   J. A.  de Castro: Adv. Mater. Res. (Durnten-Zurich, Switz.), 918 (2014), 136. https://doi.org/10.4028/www.scientific.net/AMR.918.136
• 38)   J. A.  de Castro,  J. L.  Pereira,  V. S.  Guilherme,  E. P.  da Rocha and  A. B.  Franca: J. Mater. Res. Technol., 2 (2013), 323. https://doi.org/10.1016/j.jmrt.2013.06.002
• 39)   S. H.  Liu,  K. K.  Bai,  G. H.  Ni,  J. J.  Zuo and  H. B.  Zuo: Sintering and Pelletizing., 44 (2019), 17. https://doi.org/10.13403/j.sjqt.2019.06.085
• 40)   T.  Liu,  Q. Y.  Zhang,  J. H.  Sheng,  Y. W.  Jiang,  B. W.  Wang and  Y. L.  Song: Sintering and Pelletizing., 45 (2020), 17. https://doi.org/10.13403/j.sjqt.2020.03.034
• 41)   Y.  Tobu,  M.  Nakano,  T.  Nakagawa and  T.  Nagasaka: ISIJ Int., 53 (2013), 1594. https://doi.org/10.2355/isijinternational.53.1594
• 42)   X. B.  Huang,  X. W.  Lv,  C. G.  Bai,  G. B.  Qiu and  L. M.  Lu: ISIJ Int., 54 (2014), 2721. https://doi.org/10.2355/isijinternational.54.2721
• 43)   T.  Umadevi,  A.  Brahmacharyulu,  A. K.  Roy,  P. C.  Mahapatra,  M.  Prabhu and  M.  Ranjan: ISIJ Int., 51 (2011), 922. https://doi.org/10.2355/isijinternational.51.922
• 44)   H.  Zhou,  P. N.  Ma,  M. X.  Zhou,  Z. Y.  Lai and  M.  Cheng: J. Energy Inst., 92 (2019), 1476. https://doi.org/10.1016/j.joei.2018.08.003
• 45)   Z. X.  Xiao,  L. K.  Chen,  Y. D.  Yang,  X. C.  Li and  M.  Barati: ISIJ Int., 57 (2017), 795. https://doi.org/10.2355/isijinternational.ISIJINT-2016-688
• 46)   X. Y.  Qiao: Liaoning University of Science and Technology, (2022), 23. https://doi.org/10.26923/d.cnki.gasgc.2022.000255
• 47)   H.  Kang,  S.  Choi,  W.  Yang and  B.  Cho: ISIJ Int., 51 (2011), 1065. https://doi.org/10.2355/isijinternational.51.1065
• 48)   J. P.  Zhao,  C. E.  Loo and  B. G.  Ellis: ISIJ Int., 56 (2016), 1148. https://doi.org/10.2355/isijinternational.ISIJINT-2015-686
• 49)   S.  Machida,  T.  Higuchi,  N.  Oyama,  H.  Sato,  K.  Takeda,  K.  Yamashita and  K.  Tamura: ISIJ Int., 49 (2009), 667. https://doi.org/10.2355/isijinternational.49.667
• 50)   M. J.  Hu,  T. J.  Chen,  L. T.  Pan,  X. L.  Zhou,  X. Z.  Huang and  J. W.  Liu: Iron Steel (Beijing, China), 57 (2022), 12. https://doi.org/10.13228/j.boyuan.issn0449-749x.20210680
• 51)   Y.  Arikata,  K.  Yamamoto and  Y.  Sassa: ISIJ Int., 53 (2013), 1523. https://doi.org/10.2355/isijinternational.53.1523
• 52)   G. S.  Feng,  S. L.  Wu and  Z. J.  Zhao: Sintering and Pelletizing., 36 (2011), 1. https://doi.org/10.13403/j.sjqt.2011.01.002
• 53)   N.  Oyama,  H.  Sato,  K.  Takeda,  T.  Ariyama,  S.  Masumoto,  T.  Jinno and  N.  Fujii: ISIJ Int., 45 (2005), 817. https://doi.org/10.2355/isijinternational.45.817
• 54)   N.  Oyama,  K.  Takeda and  N.  Fujii: JFE Technical Report., 22 (2008), 32.
• 55)   Y. F.  Wang,  T.  Yang,  H. Y.  Wang,  L.  Ding,  Y. F.  Luo and  H. M.  Long: J. Iron Steel Res. Int., 30 (2023), 31. https://doi.org/10.1007/s42243-022-00793-5
• 56)   S.  Li,  X. L.  Wei and  X. F.  Guo: Energy Fuels, 26 (2012), 4277. https://doi.org/10.1021/ef300580y
• 57)   Y. F.  Wu,  X. H.  Fan,  Z. Y.  Ji,  M.  Gan,  H. Y.  Zhou,  H. R.  Li,  X. L.  Chen,  Y. J.  Zhao,  R. C.  Zhang and  R. S.  Lai: Environ. Sci. Pollut. Res., 29 (2022), 62698. https://doi.org/10.1007/s11356-022-20059-7
• 58)   Y. F.  Luo,  T.  Yang,  J. H.  Zhou,  T. J.  Chun,  Y. D.  Pei and  H. M.  Long: Iron Steel (Beijing, China), 56 (2011), 47. https://doi.org/10.13228/j.boyuan.issn0449-749x.20210128
• 59)   Y. D.  Pei,  J.  Xiong,  S. L.  Wu,  S. H.  Ou,  H. Y.  Ma,  Z. X.  Zhao and  J. S.  Shi: 9th International Symposium on High-Temperature Metallurgical Processing, The Minerals, Metals & Materials Series, (2018), 785. https://doi.org/10.1007/978-3-319-72138-5
• 60)   H. Y.  Zhou,  X. H.  Fan,  Q.  Li and  M.  Gan: J. Northeast. Univ., Nat. Sci., 42 (2021), 260. https://doi.org/10.12068/j.issn.1005-3026.2021.02.016
• 61)   Z. H.  Li,  R. J.  Zou,  D. K.  Hong,  J. C.  Ouyang,  L. K.  Jiang,  H.  Liu,  G. Q.  Luo and  H.  Yao: Energy Fuels, 34 (2020), 7554. https://doi.org/10.1021/acs.energyfuels.0c00845
• 62)   Y. F.  Wu,  X. H.  Fan,  Z. Y.  Ji,  M.  Gan,  Y.  Tu,  G. G.  Zhao,  H. Y.  Zhou,  Z. Q.  Sun,  X. L.  Chen,  X. X.  Huang and  F. Q.  Zou: J. Cleaner Prod., 374 (2022), 133907. https://doi.org/10.1016/j.jclepro.2022.133907
• 63)   F. M.  Zhang: Iron Steel (Beijing, China), 55 (2020), 11. https://doi.org/10.13228/j.boyuan.issn0449-749x.20190481
• 64)   Q. K.  Li,  G. L.  Li,  G. F.  Yin,  X. D.  Zhou and  Y. D.  Pei: Sintering and Pelletizing., 44 (2019), 70. https://doi.org/10.13403/j.sjqt.2019.04.065
• 65)   F. F.  Kolesanov,  A. A.  Tkachenko,  N. S.  Khlaponin,  V. N.  Krivosheev,  N. M.  Mishchenko,  F. F.  Pilipaitis,  V. I.  Chikurov,  B. M.  Rotmistrovskii and  N. P.  Zaporozhets: Metallurgist, 16 (1972), 452.
• 66)   T.  Kenta,  T.  Higuchi and  T.  Yamamoto: ISIJ Int., 64 (2024), 521. https://doi.org/10.2355/isijinternational.isijint-2023-332
• 67)   H. J.  Kang: koasas.kaist.ac.kr., (2011).
• 68)   Y.  Iwami,  T.  Yamamoto,  T.  Higuchi,  K.  Nushiro,  M.  Sato and  N.  Oyama: ISIJ Int., 53 (2013), 1633. https://doi.org/10.2355/isijinternational.53.1633
• 69)   X. D.  Wang,  C. J.  Hou and  J. L.  Tian: Chin. J. Process Eng., 20 (2020), 997. https://doi.org/10.12034/j.issn.1009-606X.219317
• 70)   H.  Yu,  H. F.  Wang and  C. X.  Zhang: Sintering and Pelletizing., 39 (2014), 51. https://doi.org/10.13403/j.sjqt.2014.01.014
• 71)   Y. D.  Pei,  J. J.  Zhang and  C. L.  Qi: Sintering and Pelletizing., 44 (2019), 74. https://doi.org/10.13403/j.sjqt.2019.02.034
• 72)   G.  Wang,  Z.  Wen,  G. F.  Lou,  R. F.  Dou,  X. W.  Li,  X. L.  Liu and  F. Y.  Su: Int. J. Heat Mass Transfer., 97 (2016), 964. https://doi.org/10.1016/j.ijheatmasstransfer.2016.02.087
• 73)   Z. Y.  Yu,  X. H.  Fan,  M.  Gan,  X. L.  Chen and  W.  Lv: Jom., 69 (2017), 1570. https://doi.org/10.1007/s11837-017-2268-z
• 74)   X. H.  Fan,  Z. Y.  Yu,  M.  Gan,  X. L.  Chen,  Q.  Chen,  S.  Liu and  Y. S.  Huang: ISIJ Int., 55 (2015), 2074. https://doi.org/10.2355/isijinternational.ISIJINT-2015-180
• 75)   H.  Zhou,  P. N.  Ma,  M.  Cheng,  M. X.  Zhou and  Y. W.  Li: ISIJ Int., 58 (2018), 1650. https://doi.org/10.2355/isijinternational.ISIJINT-2018-185
• 76)   X. H.  Fan,  Z. Y.  Yu,  M.  Gan,  X. L.  Chen and  Z. Y.  Ji: J. Cent. South Univ., 21 (2014), 239. https://doi.org/10.1007/s11771-014-2192-0
• 77)   M.  Gan,  X. H.  Fan,  T.  Jiang,  X. L.  Chen,  Z. Y.  Yu and  Z. Y.  Ji: J. Cent. South Univ., 21 (2014), 4109. https://doi.org/10.1007/s11771-014-2405-6
• 78)   H. L.  Zhang,  M. J.  Rao,  Z. Y.  Fan,  Y. B.  Zhang,  G. H.  Li and  T.  Jiang: ISIJ Int., 52 (2012), 2139. https://doi.org/10.2355/isijinternational.52.2139
• 79)   X. H.  Fan,  Z. Y.  Yu,  M.  Gan,  X. L.  Chen,  T.  Jiang and  H. L.  Wen: ISIJ Int., 54 (2014), 2541. https://doi.org/10.2355/isijinternational.54.2541
• 80)   F.  Gao,  Y. Q.  Wang and  S. H.  Wang: Chem. Eng. J., 290 (2016), 418. https://doi.org/10.1016/j.cej.2016.01.054
• 81)   J. J.  Peng,  S. K.  Xian,  J.  Xiao,  Y.  Huang,  Q. B.  Xia,  H. H.  Wang and  Z.  Li: Chem. Eng. J., 270 (2015), 282. https://doi.org/10.1016/j.cej.2015.01.126
• 82)   P.  Konova,  M.  Stoyanova,  A.  Naydenov,  St.  Christoskova and  D.  Mehandjiev: Appl. Catal., A., 298 (2006), 109. https://doi.org/10.1016/j.apcata.2005.09.027
• 83)   H.  Zhou,  Y.  Cheng,  M. X.  Zhou and  Y. G.  Ni: Chin. J. Eng. (Beijing, China), 42 (2020), 70. https://doi.org/10.13374/j.issn2095-9389.2019.04.08.005
• 84)   V.  Sadykov and  S. F.  Tikhov: J. Catal., 165 (1997), 279. https://doi.org/10.1006/jcat.1997.1528
• 85)   G. G.  Jernigan and  G. A.  Somorjai: J. Catal., 147 (1994), 567. https://doi.org/10.1006/jcat.1994.1173
• 86)   S.  Royer and  D.  Duprez: ChemCatChem., 3 (2011), 24. https://doi.org/10.1002/cctc.201000378
• 87)   T.  Engel and  G.  Ertl: Adv. Catal., 28 (1979), 1. https://doi.org/10.1016/S0360-0564(08)60133-9
• 88)   C. H. F.  Peden,  D. W.  Goodman,  M. D.  Weisel and  F. M.  Hoffmann: Surf. Sci., 253 (1991), 44. https://doi.org/10.1016/0039-6028(91)90580-L
• 89)   L. Y.  Wang,  X. X.  Cheng,  Z. Q.  Wang,  X. Y.  Zhang and  C. Y.  Ma: Chem. Ind. Eng. Prog., 35 (2016), 2222. https://doi.org/10.16085/j.issn.1000-6613.2016.07.040
• 90)   R. R.  Gao,  X. R.  Lou,  W. J.  Bai,  W. T.  Mu and  Z.  Li: J. Mol. Catal. (China), 29 (2015), 563. https://doi.org/10.16084/j.cnki.issn1001-3555.2015.06.007
• 91)   Y.  Zhou,  Z. Y.  Wang and  C. J.  Liu: Catal. Sci. Technol., 5 (2015), 69. https://doi.org/10.1039/C4CY00983E
• 92)   D.  Vasilchenko,  P.  Topchiyan,  S.  Berdyugin,  E.  Filatov,  S.  Tkachev,  I.  Baidina,  V.  Komarov,  E.  Slavinskaya,  A.  Stadnichenko and  E.  Gerasimov: lnorg.chem., 58 (2019), 6075. https://doi.org/10.1021/acs.inorgchem.9b00370
• 93)   H.  Masatake,  K.  Tetsuhiko,  S.  Hiroshi and  Y.  Nobumasa: Chem. Lett., 16 (1987), 405. https://doi.org/10.1246/cl.1987.405
• 94)   V.  Muravev,  G.  Spezzati,  Y. Q.  Su,  A.  Parastaev,  F. K.  Chiang,  A.  Longo,  C.  Escudero,  N.  Kosinov and  E. J. M.  Hensen: Nat. Catal., 4 (2021), 469. https://doi.org/10.1038/s41929-021-00621-1
• 95)   G.  Spezzati,  A. D.  Benavidez,  A. T.  DeLaRiva,  Y. Q.  Su,  J. P.  Hofmann,  S.  Asahina,  E. J.  Olivier,  J. H.  Neethling,  J. T.  Miller,  A. K.  Datye and  E. J. M.  Hensen: Appl. Catal., B., 234 (2019), 36. https://doi.org/10.1016/j.apcatb.2018.10.015
• 96)   Y. F.  Cai,  J.  Xu,  Y.  Guo and  J. Y.  Liu: ACS Catal., 9 (2019), 2558. https://doi.org/10.1021/acscatal.8b04064
• 97)   X. Z.  Song,  Q. F.  Su,  S. J.  Li,  S. H.  Liu,  N.  Zhang,  Y. L.  Meng,  X.  Chen and  Z. Q.  Tan: New J. Chem., 43 (2019), 16096. https://doi.org/10.1039/C9NJ04244J
• 98)   J. L.  Zhang,  Y. H.  Kan,  S. J.  Zhang,  Z. J.  Liu,  L. L.  Niu and G. L. G: Iron Steel (Beijing, China), 55 (2020), 56. https://doi.org/10.13228/j.boyuan.issn0449-749x.20200126
• 99)   Z. J.  Liu,  Y. Z.  Wang,  J. L.  Zhang,  Y. P.  Zhang and  H. B.  Zuo: Chin. J. Eng. (Beijing, China), 38 (2016), 200. https://doi.org/10.13374/j.issn2095-9389.2016.02.007
• 100)   J.  Liu,  H.  Zhang,  L. B.  Xu,  Q.  Zhong,  M. S.  Zhou and  T.  Jiang: China Metall., 7 (2023), 1. https://doi.org/10.13228/j.boyuan.issn1006-9356.20230319
• 101)   J. L.  Zhang,  Z. J.  Liu,  K. J.  Li and  T. J.  Yang: Ironmaking, 41 (2022), 1.
• 102)   F. C.  Sun,  X. K.  Zhou,  G.  Li,  Y. F.  Wang,  X. R.  Xie,  X. W.  Lv and  J.  Xu: J. Cent. South Univ., 30 (2023), 786. https://doi.org/10.1007/s11771-023-5280-1
• 103)   H. L.  Wu,  Y. F.  Luo,  J. H.  Zhou,  T. J.  Chun,  Z. W.  Yu,  Y. D.  Pei,  J. L.  Yang and  H. M.  Long: J. Cent. South Univ., 53 (2022), 1179. https://doi.org/10.11817/j.issn.1672-7207.2022.04.003
• 104)   Y. J.  Wang,  T. L.  Zhong,  J. F.  Wang,  X. F.  She and  Q. G.  Xue: Sintering and Pelletizing., 47 (2022), 69. https://doi.org/10.13403/j.sjqt.2022.05.073
• 105)   D.  Sun,  Y. G.  Wang,  Y. J.  Dai,  Q. X.  Zhao,  Z. Y.  Liang and  H. S.  Shao: J. Chin. Soc. Power Eng., 43 (2023), 641. https://doi.org/10.19805/j.cnki.jcspe.2023.05.017