Investigations into NOx Formation Characteristics during Pulverized Coal Combustion Catalyzed by Iron Ore in the Sintering Process

Sintering accounts for about 50% of the total NOx emissions of the iron and steel industry. NOx emissions from the sintering process can be simulated using the emissions from coke combustion. However, the generation and emission law for NOx burning in the sintering process of pulverized coal is still not clear. The formation characteristics of NOx during coal combustion catalyzed by iron ore fines and several iron-containing pure minerals were studied in this paper. The results showed that iron ore fines can improve the NOx emission rate and increase the total NOx emissions during coal combustion. The type and composition of the iron ore fines have an important impact on the generation and emission of NOx in the process of coal combustion. The peak concentration and emissions of NOx in coal combustion flue gas with limonite, hematite or specularite added increased significantly. The peak value for the NOx concentration in the coal combustion flue gas with magnetite or siderite added increased, but the emissions decreased. Therefore, the generation of NOx in the sintering process can to a certain extent be controlled by adjusting the type of iron-containing raw materials and the distribution of the iron-containing raw materials and coal.


Introduction
NO x is one of the predominant pollutants in the iron and steel industry [1]. Sintering is a major emitter of pollutants in the steel industry, producing about 50% of the NO x in this industry [2][3][4]. The emission of NO x has caused great harm to human health and the living environment, such as acid rain, photochemical smog, etc. Therefore, there is an urgent need to reduce the emission of nitrogen oxides [5,6]. In China, the permitted hourly NO x emission concentration since 2019 has been below 50 mg of NO x /Nm 3 (16% O 2 ) for the nose of a sintering machine.
Iron ore sintering is a basic agglomeration process providing sinter for blast furnaces. The sintering mixture (iron-containing raw materials, fuel, flux, return ore, etc.) is mixed with an appropriate amount of water and then paved on the sintering machine trolley after granulation. After the surface of the sintering material is ignited, the fuel inside the sintering material burns and releases heat from top to bottom under the forced suction of the lower bellows. The mixture undergoes a series of physical and chemical reactions under the action of high temperature and finally consolidates into sinter [7,8]. The solid fuels used in conventional sintering are mainly anthracite and coke [9]. NO x can be divided into

Raw Materials
The main chemical composition of the iron ore fines obtained from a Chinese steel and iron group is shown in Table 1. The results showed that the iron grade of the iron ore fines was 61.47% and the loss on ignition was 5.14%. Proximate and ultimate analyses of coal (air-dry basis) are shown in Table 2. The nitrogen content in the pulverized coal was 0.645%. The fixed carbon mass fraction was 79.86%, and the ash and volatile matter were 15.06% and 3.63%, respectively. Note: M ad -moisture on air-dry basis; A ad -ash on air-dry basis; V ad -volatile matter on dry, ash-free basis; F Cad -fixed carbon on air-dry basis.
XRD phase analysis of the iron ore fines and the several major iron-bearing pure minerals used in the experiment are shown in Figures 1 and 2. The results showed that the main phases of the iron ore fines were hematite, magnetite, goethite and quartz. It can be seen from Figure 2 that the purity of the hematite, magnetite, siderite, specularite and limonite was relatively high, allowing these materials to meet the requirements of this experiment.  Note: Mad-moisture on air-dry basis; Aad-ash on air-dry basis; Vad-volatile matter on dry, ash-free basis; FCad-fixed carbon on air-dry basis.
XRD phase analysis of the iron ore fines and the several major iron-bearing pure minerals used in the experiment are shown in Figures 1 and 2. The results showed that the main phases of the iron ore fines were hematite, magnetite, goethite and quartz. It can be seen from Figure 2 that the purity of the hematite, magnetite, siderite, specularite and limonite was relatively high, allowing these materials to meet the requirements of this experiment.

Experimental Apparatus and Method
Micro-sintering experiment were carried out using the experimental system of a tubular electric furnace with SiC heaters (Figure 3) (Changsha Kehui Furnace Technology Co., LTD., Changsha, China) [39]. Two kinds of gas were used in this experiment, O2   10  20  30  40  50  60  70 80 90

Experimental Apparatus and Method
Micro-sintering experiment were carried out using the experimental system of a tubular electric furnace with SiC heaters (Figure 3) (Changsha Kehui Furnace Technology Co., LTD., Changsha, China) [39]. Two kinds of gas were used in this experiment, O 2 (purity: 99.9 vol%) and N 2 (purity: 99.9 vol%); they were provided by Wuhan Minghui Gas Technology Co., LTD (Wuhan, China), and respectively configured using high-pressure gas cylinders, controlled with a glass rotor flowmeter and mixed with a mixer. They were then passed into the corundum tube (outer diameter: 80 mm, inner diameter: 70 mm, height: 1000 mm) in the tubular electric furnace and allowed to flow through the test sample in the crucible (outer diameter: 60 mm, inner diameter: 50 mm, height: 80 mm). A flue gas analyzer (MRU OPTIMA 7, Neckarsulm, Baden-Wurttemberg, Germany) was used to measure the gas composition at the entrance and exit of the reaction system. The measurement accuracy for O 2 and CO 2 was ±0.2%, and that for other gases was ±5 ppm.

Characterization Methods
The chemical compositions of the raw ore were determined with a wavelength dispersive X-ray fluorescence spectrometer (XRF, Rigaku/ZSXPrimus IV) and ICP-AES (Optima 2000DV). The crystalline phase compositions of the materials were detected with an X-ray diffractometer (XRD, D/Max-2500, Rigaku Co., Tokyo, Japan). Proximate analysis of coal was conducted according to the Chinese standard GB/T212-2008. SEM was conducted using a JEOL JSM-6610 scanning electron microscope (JEOL, Japan). The settings for the microscopy were EHT = 20 kV and I probe = 200 pA.
During the micro-sintering experiment, the compositions of the flue gas were detected online, and the peak concentration of NOx, the relative influence factors for the peak concentration of NOx and the relative emission rate of NOx were analyzed and calculated. The calculation formulas are shown in Equations (1) and (3): where ( ) indicates the relative influence factors for the peak concentration of NOx, A micro-sintering experiment device was used to simulate the process of producing nitrogen oxides in the test samples during sintering. The experiment steps were as follows: a dried sample of a certain quality was weighed and the sample placed into the test crucible (a crucible with holes at the bottom), then the heating program of the electric furnace was set. After the temperature in the corundum tube inside the electric furnace was stabilized to the temperature specified in the experiment, the crucible containing samples was placed in the corundum tube. Then, the corundum tube was sealed, the gas valve was opened and the gas composition controlled in terms of the air atmosphere (21% oxygen and 79% nitrogen) and gas flow (2 L/min). The gas flowed in from above the quartz tube, passed through the sample layer and flowed out from the below outlet. During the experiment, a flue gas analyzer was used to measure the flue gas composition in real time, and the data were recorded every 5 s. The reaction temperature was 1100 • C, the amount of pulverized coal in each group was 3 g and the amount of iron-containing pure mineral was 3 g.

Characterization Methods
The chemical compositions of the raw ore were determined with a wavelength dispersive X-ray fluorescence spectrometer (XRF, Rigaku/ZSXPrimus IV) and ICP-AES (Optima 2000DV). The crystalline phase compositions of the materials were detected with an X-ray diffractometer (XRD, D/Max-2500, Rigaku Co., Tokyo, Japan). Proximate analysis of coal was conducted according to the Chinese standard GB/T212-2008. SEM was conducted using a JEOL JSM-6610 scanning electron microscope (JEOL, Tokyo, Japan). The settings for the microscopy were EHT = 20 kV and I probe = 200 pA.
During the micro-sintering experiment, the compositions of the flue gas were detected online, and the peak concentration of NO x , the relative influence factors for the peak concentration of NO x and the relative emission rate of NO x were analyzed and calculated. The calculation formulas are shown in Equations (1) and (3): where P (NO x ) indicates the relative influence factors for the peak concentration of NO x , X 0 NO x is the peak concentration of NO x in pulverized coal combustion flue gas (10 −6 (ppm)) and X NO x is the peak concentration of NO x in the flue gas for roasted samples.
By integrating the NO x volume fractions corresponding to each moment, the total amount of NO x generated during the coal combustion process can be obtained. The calculation formula is given in Equation (2): where E NO x is the NO x emission in the flue gas for roasted samples (mg), Q is the flow rate of the flue gas (L/min), a and b are the beginning and end moments (s), ϕ NO x is the volume fraction of NO x (10 −6 (ppm)), t is the time (s) and M is the NO x molar mass (g/mol). Equation (3) is as follows: where R NO x is the relative emission rate of NO x , E 0 NO x is the total nitrogen oxide emissions from coal combustion flue gas (mg) and E NO x is the total NO x emissions from the combustion flue gas of coal with iron-containing pure minerals added (mg).

Effect of Iron Ore Fines on NO x Emission Characteristics in Pulverized Coal Combustion
It can be seen from Figure 4a and Table 3 that, at the roasting temperature of 1100 • C, the pulverized coal rapidly burned to form nitrogen oxides, reached a peak value of about 244 ppm at 3.2 min, then decreased rapidly and dropped to zero after a period of time. The peak concentration of nitrogen oxides in the flue gas produced by the high-temperature roasting of the iron ore fines was only 30 ppm, indicating that the iron ore fines contained small amounts of nitrogen-containing compounds. When iron ore fines were added, the peak concentration of NO x produced in the pulverized coal combustion increased from 244 ppm to 515 ppm, indicating that the iron ore fines had a relatively large enhancement effect on the NO x emission rate in the pulverized coal combustion. It can be concluded from the relative emission rate of NO x that mixing iron ore can greatly enhance the conversion rate for the total N-NO x in pulverized coal, which increases the total emissions of NO x in the flue gas. According to the analysis in Figure 4b-d, the addition of mixed iron ore reduced the concentration of oxygen and increased the concentration of carbon dioxide during the combustion of the pulverized coal, indicating that the addition of mixed iron ore could accelerate the combustion rate of the pulverized coal, the consumption rate of oxygen  Table 3. Effects of iron ore fines on flue gas composition characteristics in pulverized coal combustion.

Variation in Flue Gas Components during Oxidative Roasting of Iron-Bearing Pure Minerals
It can be seen from Figure 5 and Table 4 that a single piece of iron-containing pure mineral produced a portion of the nitrogen oxides during the roasting process, and the amount of NOx produced was less than that from a single pulverized piece of coal. The order of the peak concentrations of nitrogen oxides in the roasting flue gas of the pure iron-bearing minerals was limonite > siderite > magnetite > hematite > specularite. The NOx generation from specularite and hematite was very small, and the P(NOx) and R(NOx) values did not exceed 0.04. The roasting flue gas of siderite and magnetite contained certain amounts of NOx. The P(NOx) and R(NOx) values were 0.11, 0.11, 0.12 and 0.05, respectively. However, the amount of NOx produced by limonite was relatively large, reaching more than 50 ppm, and the P(NOx) and R(NOx) values were 0.21 and 0.12. Figure 5b-d show that, during the roasting process of a single piece of iron-bearing pure mineral, siderite was oxidized and decomposed at a high temperature, consuming a  Table 3. Effects of iron ore fines on flue gas composition characteristics in pulverized coal combustion.

Variation in Flue Gas Components during Oxidative Roasting of Iron-Bearing Pure Minerals
It can be seen from Figure 5 and Table 4 that a single piece of iron-containing pure mineral produced a portion of the nitrogen oxides during the roasting process, and the amount of NO x produced was less than that from a single pulverized piece of coal. The order of the peak concentrations of nitrogen oxides in the roasting flue gas of the pure iron-bearing minerals was limonite > siderite > magnetite > hematite > specularite. The NO x generation from specularite and hematite was very small, and the P (NOx) and R (NOx) values did not exceed 0.04. The roasting flue gas of siderite and magnetite contained certain amounts of NO x . The P (NOx) and R (NOx) values were 0.11, 0.11, 0.12 and 0.05, respectively. However, the amount of NO x produced by limonite was relatively large, reaching more than 50 ppm, and the P (NOx) and R (NOx) values were 0.21 and 0.12. certain amount of oxygen and at the same time generating a portion of the carbon dioxide and carbon monoxide, while magnetite was oxidized by oxygen at a high temperature and consumed only a small amount of oxygen. Limonite, hematite and magnetite iron ore had very little effect on the concentrations of oxygen and carbon dioxide during the roasting process.     Figure 5b-d show that, during the roasting process of a single piece of iron-bearing pure mineral, siderite was oxidized and decomposed at a high temperature, consuming a certain amount of oxygen and at the same time generating a portion of the carbon dioxide and carbon monoxide, while magnetite was oxidized by oxygen at a high temperature and consumed only a small amount of oxygen. Limonite, hematite and magnetite iron ore had very little effect on the concentrations of oxygen and carbon dioxide during the roasting process.

Influence of Iron-Bearing Pure Minerals on NO x Emission Characteristics in Pulverized Coal Combustion
A comparison of Figures 5a, 6a and 7 and Tables 4 and 5 shows that the types of iron-containing pure minerals had a great impact on the combustion rate of the pulverized coal and the generation and emission rates of NO x in the combustion process. Through a comparison between the flue gas composition in iron-containing pure mineral roasting and that in catalytic pulverized coal combustion using iron-containing pure minerals at 1100 • C, it can be seen that limonite had the greatest catalytic effect on the NO x generation reaction during the pulverized coal combustion, and its P (NOx) and R (NOx) values reached 2.20 and 1.40, respectively, indicating that limonite can greatly enhance the NO x generation rate and the N-NO x conversion rate in pulverized coal combustion. The addition of hematite and specularite resulted in p (NOx) and R (NOx) reaching 1.63 and 1.37, and 1.26 and 1.02, respectively, indicating that hematite and specularite can improve the NO x generation and conversion rates in pulverized coal combustion at the same time. The catalytic effects of siderite and magnetite on the NO x formation reaction in pulverized coal combustion were weak, with P (NOx) and R (NOx) reaching 1.31 and 1.23, 0.97 and 0.84, respectively, indicating that the addition of siderite and magnetite improved the NO x generation rate in pulverized coal combustion but inhibited N-NO x conversion to a certain extent. The addition of siderite and magnetite reduced the R (NOx) of pulverized coal combustion by 2.77% and 15.69%, respectively.
A comparison of Figures 5a, 6a and 7 and Tables 4 and 5 shows that the types of iron-containing pure minerals had a great impact on the combustion rate of the pulverized coal and the generation and emission rates of NOx in the combustion process. Through a comparison between the flue gas composition in iron-containing pure mineral roasting and that in catalytic pulverized coal combustion using iron-containing pure minerals at 1100 °C, it can be seen that limonite had the greatest catalytic effect on the NOx generation reaction during the pulverized coal combustion, and its P(NOx) and R(NOx) values reached 2.20 and 1.40, respectively, indicating that limonite can greatly enhance the NOx generation rate and the N-NOx conversion rate in pulverized coal combustion. The addition of hematite and specularite resulted in p(NOx) and R(NOx) reaching 1.63 and 1.37, and 1.26 and 1.02, respectively, indicating that hematite and specularite can improve the NOx generation and conversion rates in pulverized coal combustion at the same time. The catalytic effects of siderite and magnetite on the NOx formation reaction in pulverized coal combustion were weak, with P(NOx) and R(NOx) reaching 1.31 and 1.23, 0.97 and 0.84, respectively, indicating that the addition of siderite and magnetite improved the NOx generation rate in pulverized coal combustion but inhibited N-NOx conversion to a certain extent. The addition of siderite and magnetite reduced the R(NOx) of pulverized coal combustion by 2.77% and 15.69%, respectively.       According to the comparison of Figure 5b,c and Figure 6b,c, the addition of pure minerals containing iron reduced the contact between the coal and the air, resulting in an overall reduction in coal combustion speed, an increase in oxygen concentration and a decrease in carbon dioxide concentration. At the same time, the combustion cycle of the coal was shortened to varying degrees. The addition of siderite reduced the oxygen concentration in the combustion flue gas of the coal and increased the carbon dioxide concentration, which may have been due to the decomposition of the siderite, resulting in the simultaneous generation of carbon dioxide and consumption of part of the oxygen. Figure 6d shows that the addition of limonite, specularite and hematite increased the CO concentration and the total amount of CO generated from the combustion of coal, while siderite and magnetite increased the amount of CO in the flue gas of the coal combustion. The CO concentration decreased. Magnetite greatly reduced the CO concentration and the total amount of CO in the coal combustion flue gas, and siderite decreased the CO concentration in the coal combustion flue gas but the total amount and emission period increased. This was largely due to some of the CO being generated from the slow decomposition of siderite at high temperatures. Figure 8 shows the XRD patterns for the different combustion residues of pulverized coal with different kinds of iron-containing pure minerals added. It can be seen that the phase compositions of single pulverized coal combustion residues mainly consisted of Fe 2 (SiO 4 ), CaAl 2 (SiO 4 ) 2 , Al 2 SiO 5 , SiO 2 , Al 2 O 3 , Fe 2 O 3 , etc. The phase composition of the pulverized coal combustion residues with iron-containing pure minerals mainly consisted of iron oxide. The main phase of the pulverized coal combustion residues with pure hematite mineral added was Fe 2 O 3 , but it also contained small amounts of Fe 2 (SiO 4 ), CaAl 2 (SiO 4 ) 2 , Al 2 SiO 5 , etc. The main phases of the pulverized coal combustion residues with pure magnetite mineral added were Fe 2 O 3 and Fe 3 O 4 , and a small amount of Al 2 SiO 5 was also detected. The main phase of the pulverized coal combustion residues with pure specularite mineral added was Fe 2 O 3 , and there was also a small amount of Al 2 SiO 5 , etc. The main phases of the pulverized coal combustion residues with pure siderite mineral added were Fe 2 O 3 and Fe 3 O 4 , and a small amount of Al 2 SiO 5 was also detected. The main phase of the pulverized coal combustion residue with pure limonite mineral added was Fe 2 O 3 . In addition, it also contained small amounts of Al 2 SiO 5 and CaAl 2 (SiO 4 ) 2 .

Discussion
Metals 2022, 5, x FOR PEER REVIEW 11 of 15 Figure 8 shows the XRD patterns for the different combustion residues of pulverized coal with different kinds of iron-containing pure minerals added. It can be seen that the phase compositions of single pulverized coal combustion residues mainly consisted of Fe2(SiO4), CaAl2(SiO4)2, Al2SiO5, SiO2, Al2O3, Fe2O3, etc. The phase composition of the pulverized coal combustion residues with iron-containing pure minerals mainly consisted of iron oxide. The main phase of the pulverized coal combustion residues with pure hematite mineral added was Fe2O3, but it also contained small amounts of Fe2(SiO4), CaAl2(SiO4)2, Al2SiO5, etc. The main phases of the pulverized coal combustion residues with pure magnetite mineral added were Fe2O3 and Fe3O4, and a small amount of Al2SiO5 was also detected. The main phase of the pulverized coal combustion residues with pure specularite mineral added was Fe2O3, and there was also a small amount of Al2SiO5, etc. The main phases of the pulverized coal combustion residues with pure siderite mineral added were Fe2O3 and Fe3O4, and a small amount of Al2SiO5 was also detected. The main phase of the pulverized coal combustion residue with pure limonite mineral added was Fe2O3. In addition, it also contained small amounts of Al2SiO5 and CaAl2(SiO4)2.  Figure 9 shows SEM images of pulverized coal combustion residues with different kinds of iron-containing pure minerals added. It indicates that, with the addition of iron-containing pure minerals, the combustion residues of the mixture increased greatly, most of which were the roasting products of the iron-containing pure minerals, which   Figure 9 shows SEM images of pulverized coal combustion residues with different kinds of iron-containing pure minerals added. It indicates that, with the addition of ironcontaining pure minerals, the combustion residues of the mixture increased greatly, most of which were the roasting products of the iron-containing pure minerals, which adhered to the surface of the pulverized coal during the pulverized coal combustion, so that the pulverized coal did not fully burn.

R PEER REVIEW
12 of 15 adhered to the surface of the pulverized coal during the pulverized coal combustion, so that the pulverized coal did not fully burn. As can be seen from Section 3.3, the peak NOx concentration and the total NOx emissions in the flue gas increased significantly after the pure minerals containing high-valence iron elements (limonite, hematite and specularite) were added to the pulverized coal. At the same time, the concentration of CO2 in the flue gas decreased, the generation cycle of the CO2 was shortened and the concentration and total emissions of CO increased. According to the analysis of Figures 8 and 9, with the addition of pure minerals containing high-valence iron elements, the combustion residues of the mixture increased greatly, and most were the roasting products of the iron-containing pure minerals, which adhered to the surface of the pulverized coal during combustion so that the pulverized coal did not fully burn, resulting in a decrease in CO2 concentration and an increase in CO concentration in the flue gas. However, the peak value for the NOx concentration in the flue gas increased significantly, indicating that the addition of pure minerals containing high-valence iron elements reduced the combustion performance of the pulverized coal and catalyzed the formation reaction of NOx in the pulverized coal. Limonite contains crystal water, which evaporates rapidly and forms water vapor during high-temperature roasting. On the one hand, the existence of the water vapor promoted the combustion of the pulverized coal and the generation of NOx. On the other hand, the evaporation of the crystal water led to the formation of many pores on the mineral surface, increased the reaction contact area between the dehydrated iron-bearing minerals and the pulverized coal and enhanced the catalytic effect of Fe2O3 on the NOx formation reaction during the pulverized coal combustion. The catalytic effects of hematite and specularite on NOx formation during pulverized coal combustion mainly resulted from the effective catalytic component Fe2O3 contained in the pure minerals.
The peak value for the NOx concentration in the flue gas increased to a certain As can be seen from Section 3.3, the peak NO x concentration and the total NO x emissions in the flue gas increased significantly after the pure minerals containing highvalence iron elements (limonite, hematite and specularite) were added to the pulverized coal. At the same time, the concentration of CO 2 in the flue gas decreased, the generation cycle of the CO 2 was shortened and the concentration and total emissions of CO increased. According to the analysis of Figures 8 and 9, with the addition of pure minerals containing high-valence iron elements, the combustion residues of the mixture increased greatly, and most were the roasting products of the iron-containing pure minerals, which adhered to the surface of the pulverized coal during combustion so that the pulverized coal did not fully burn, resulting in a decrease in CO 2 concentration and an increase in CO concentration in the flue gas. However, the peak value for the NO x concentration in the flue gas increased significantly, indicating that the addition of pure minerals containing high-valence iron elements reduced the combustion performance of the pulverized coal and catalyzed the formation reaction of NO x in the pulverized coal. Limonite contains crystal water, which evaporates rapidly and forms water vapor during high-temperature roasting. On the one hand, the existence of the water vapor promoted the combustion of the pulverized coal and the generation of NO x . On the other hand, the evaporation of the crystal water led to the formation of many pores on the mineral surface, increased the reaction contact area between the dehydrated iron-bearing minerals and the pulverized coal and enhanced the catalytic effect of Fe 2 O 3 on the NO x formation reaction during the pulverized coal combustion. The catalytic effects of hematite and specularite on NO x formation during pulverized coal combustion mainly resulted from the effective catalytic component Fe 2 O 3 contained in the pure minerals.
The peak value for the NO x concentration in the flue gas increased to a certain extent after the addition of pure minerals (magnetite and siderite) containing low-valence iron elements into the pulverized coal, while the total emissions decreased. The addition of magnetite reduced the concentration of CO 2 in the pulverized coal combustion flue gas, shortened the generation cycle of the CO 2 and reduced the concentration of CO. This may have been due to the reaction of the CO in the flue gas and the NO x at the combustion interface to produce N 2 and CO 2 . Similarly, magnetite can react with NO to produce N 2 at high temperatures. When siderite was added to the pulverized coal, due to its own thermal decomposition under high temperature conditions to produce CO 2 and Fe 3 O 4 , the CO 2 concentration in the overall flue gas increased and the generation cycle of the CO 2 was greatly shortened, but the CO concentration decreased. This may have been due to the reaction between the CO in the flue gas and the NO x at the combustion interface to produce N 2 and CO 2 . Moreover, the Fe 3 O 4 produced by siderite thermal decomposition could react with NO to produce N 2 under high temperature conditions. As can be seen from Figures 8 and 9, with the addition of pure minerals containing low-valence iron elements, a large number of combustion residues adhered to the surface of the pulverized coal during combustion, most of which were the roasting products of the iron-containing pure minerals, so that the pulverized coal did not fully burn. Moreover, the peak value for the NO x concentration in the flue gas increased, but the total emissions decreased. This was because the addition of pure minerals containing low-valence iron elements reduced the combustion performance of the pulverized coal. Fe 3 O 4 and CO in the system could react with NO x to produce Fe 2 O 3 , N 2 and CO 2 , and then Fe 2 O 3 continued to catalyze the formation reaction of NO x during the pulverized coal combustion [33].
To sum up, the type and composition of iron ore powder have a significant impact on the generation and emission of NO x in the pulverized coal combustion process. In the actual process of iron ore sintering, iron ore fines and pulverized coal are fully mixed and granulated in the process of mixing and granulating. Therefore, controlling and adjusting the type and composition of the iron-containing raw materials in the mixture and the distribution of the iron-containing raw materials and the pulverized coal makes it possible to control the generation of NO x in the pulverized coal combustion process to a certain extent, thereby regulating the emission of NO x across the whole sintering process and achieving the purpose of NO x emission reduction. Moreover, the addition of magnetite and siderite can be conducive to the reduction of nitrogen oxide emissions in iron ore sintering flue gas. Magnetite can also release heat in the sintering process, reduce the sintering carbon content, reduce sintering solid fuel consumption and pollutant emissions and help to improve the quality of sinter.

Conclusions
Iron ore fines can improve the NO x emission rate and increase the total NO x emissions during coal combustion. At the same time, they can accelerate the combustion rate of coal and stimulate its full combustion.
The type and composition of iron ore fines have a significant impact on the generation and emission of NO x in the coal combustion process. With the addition of limonite, hematite or specularite, the peak concentration and total emissions of NO x in the flue gas increased significantly, indicating that the addition of limonite, hematite and specularite reduce the combustion performance of coal and catalyze the formation of NO x . With the addition of magnetite or siderite, the peak value for the NO x concentration in the flue gas increased, but the total emissions decreased.
The generation of NO x in the sintering process can be controlled to a certain extent by adjusting the type of iron-containing raw materials and the distribution of the ironcontaining raw materials and coal. Moreover, the addition of magnetite is conducive to reducing emissions of NO x from coal combustion in the process of iron ore sintering. Moreover, magnetite can release heat during sintering, which can help reduce the consumption of solid fuel, reduce pollutant emissions and improve the quality of sinter.