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

Abstract

Sintering accounts for about 50% of the total NO_x emissions of the iron and steel industry. NO_x emissions from the sintering process can be simulated using the emissions from coke combustion. However, the generation and emission law for NO_x burning in the sintering process of pulverized coal is still not clear. The formation characteristics of NO_x 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 NO_x emission rate and increase the total NO_x emissions during coal combustion. The type and composition of the iron ore fines have an important impact on the generation and emission of NO_x in the process of coal combustion. The peak concentration and emissions of NO_x in coal combustion flue gas with limonite, hematite or specularite added increased significantly. The peak value for the NO_x concentration in the coal combustion flue gas with magnetite or siderite added increased, but the emissions decreased. Therefore, the generation of NO_x 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.

1. 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/Nm³ (16% O₂) 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 three types according to its mechanism of formation: thermal NO_x, prompt NO_x and coal NO_x [10,11]. In the sintering process, NO_x mainly comes from coke breeze and anthracite combustion [12,13]. In addition, NO accounts for 95% of the total NO_x [14]. Therefore, NO_x emission from the sintering process could be simulated by that of coke combustion.

At present, most researchers have focused on source control, process control and end-of-pipe treatment technology for NO_x in sintering flue gas. Resource control mainly refers to the use of low-nitrogen coke and biomass carbon in the iron ore sintering process [15,16], the use of new low-nitrogen burners, etc. Process control mainly refers to control of sintering process parameters, sintering additives [5,17], fuel pretreatment [18], gas injection [19], flue gas circulation [20], etc. End-of-pipe treatment technologies include selective SNCR [21,22,23], SCR [24,25,26], activated carbon adsorption [27] and oxidative absorption [28]. However, the high gas flow rate, moisture content and dust concentration in the sintering flue gas lead to many problems with denitration technology, such as high operating costs and secondary pollution [12].

However, few studies have been published on the mechanisms of NO_x production in sintering so far [29]. Although the NO_x emissions from pulverized coal in boilers and in fluidized bed combustion have been fully investigated, the combustion process of coal in the sintering process is very different from fuels in coal-fired systems. In the sintering process, coal is mainly found in the combustion zone and the preheating zone [30], which together comprise a very complex gas, solid and molten liquid phase system, thus entailing a significant distinction in the reaction conditions and atmosphere of coal combustion compared to an ordinary coal combustion system [4]. Therefore, the reaction of NO_x generated by coal combustion in the sintering process is also different from that of the coal combustion system.

In recent years, several studies have investigated the emission and reduction of NO_x in the iron ore sintering process. Calcium ferrite formed in the high-temperature sintering zone has an impact on NO_x emission, catalyzing NO_x decomposition and reducing its emission level [29,31,32,33,34]. NO_x emissions decreased when a specially prepared coke containing CeO₂ was used during the iron ore sintering process [5]. The addition of Fe₃O₄, FeO and Fe can reduce the generation and emission of NO_x in the sintering process [35,36]. Under certain conditions, the presence of CO seems to reduce NO_x, and this reduction reaction is catalyzed by sintered ore, Fe₂O₃, Fe₃O₄ and MgO [37,38].

Although significant research has been carried out to analyze the generation of and reduction in emissions of NO_x in the sintering process, the generation and emission law for NO_x burning in the sintering process of pulverized coal is still not clear. This study focused on NO_x formation characteristics during pulverized coal combustion catalyzed by iron ore fines and several major iron-containing pure minerals in the iron ore sintering process.

2. Materials and Methods

2.1. Raw Materials

The main chemical composition of the iron ore fines obtained from a Chinese steel and iron group is shown in

Table 1. Chemical composition of raw materials/wt.%.

Raw Materials	TFe	FeO	SiO2	CaO	Al2O3	MgO	MnO2	V2O5	LOI
Iron Ore Fines	61.47	2.88	5.4	0.16	1.52	0.08	0.31	0.008	5.14

. 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. Proximate and ultimate analyses of coal (air-dry basis)/wt.%.

C	H	N	S	Mad	Aad	Vad	Fcad
83.73	0.725	0.645	0.186	1.45	15.06	3.63	79.86

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.

. 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.

XRD phase analysis of the iron ore fines and the several major iron-bearing pure minerals used in the experiment are shown in Figure 1 and Figure 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.

2.2. 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 (purity: 99.9 vol%) and N₂ (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 O₂ and CO₂ was ±0.2%, and that for other gases was ±5 ppm.

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.

2.3. 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):

$$P_{N{O}_x}=\frac{X_{N{O}_x}^{\prime }}{X_{N{O}_x}^0}$$

(1)

where $P_{(N{O}_x)}$ indicates the relative influence factors for the peak concentration of NO_x, $X_{N{O}_x}^0$ is the peak concentration of NO_x in pulverized coal combustion flue gas (10⁻⁶ (ppm)) and $X_{N{O}_x}^{\prime }$ 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):

$$E_{N{O}_x}=\frac{Q}{60}\times {\displaystyle {\int }_a^b{\phi }_{N{O}_{\mathrm{x}}}}\times {10}^{-6}dt\times \frac{1000M}{22.4}$$

(2)

where $E_{N{O}_x}$ 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), ${\phi }_{N{O}_{\mathrm{x}}}$ 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:

$$R_{N{O}_x}=\frac{E_{N{O}_x}^{\prime }}{E_{N{O}_x}^0},$$

(3)

where $R_{N{O}_x}$ is the relative emission rate of NO_x, $E_{N{O}_x}^0$ is the total nitrogen oxide emissions from coal combustion flue gas (mg) and $E_{N{O}_x}^{\prime }$ is the total NO_x emissions from the combustion flue gas of coal with iron-containing pure minerals added (mg).

3. Results

3.1. Effect of Iron Ore Fines on NO_x Emission Characteristics in Pulverized Coal Combustion

It can be seen from Figure 4a and

Table 3. Effects of iron ore fines on flue gas composition characteristics in pulverized coal combustion.

Test Conditions	X(NOx)/ppm	P(NOx)	R(NOx)
Coal	244	1	1.00
Iron ore fines	30	0.12	0.05
Coal and iron ore fines (minus the amount of NOx produced by iron ore fines)	515	2.11	1.89

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 and the generation rate of carbon dioxide. At the same time, the concentration of carbon monoxide in the flue gas decreased with the addition of the mixed iron ore, indicating that the addition of the mixed iron ore encouraged the full combustion of the pulverized coal.

3.2. Variation in Flue Gas Components during Oxidative Roasting of Iron-Bearing Pure Minerals

It can be seen from Figure 5 and

Table 4. List of X_(NOx), P_(NOx) and R_(NOx) values during oxidative roasting of iron-bearing pure minerals.

Test Conditions	X(NOx)/ppm	P(NOx)	R(NOx)
Coal	244	1	1.00
Hematite	9	0.04	0.04
Magnetite	27	0.11	0.05
Specularite	9	0.04	0.03
Siderite	28	0.11	0.12
Limonite	51	0.21	0.12

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.

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.

3.3. Influence of Iron-Bearing Pure Minerals on NO_x Emission Characteristics in Pulverized Coal Combustion

A comparison of Figure 5a, Figure 6a and Figure 7 and Table 4 and

Table 5. Effects of iron-bearing pure minerals on X_(NOx), P_(NOx) and R_(NOx) during pulverized coal combustion (minus the amount of NO_x produced by the iron-bearing pure minerals).

Test Conditions	X(NOx)/ppm	P(NOx)	R(NOx)
Coal	244	1	1.00
Coal + Hematite	408	1.63	1.26
Coal + Magnetite	327	1.23	0.84
Coal + Specularite	343	1.37	1.02
Coal + Siderite	346	1.31	0.97
Coal + Limonite	537	1.99	1.40

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.

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.

4. Discussion

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 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 NO_x concentration and the total NO_x 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 Figure 8 and Figure 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 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 Fe₂O₃ 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 Fe₂O₃ 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 CO₂ in the pulverized coal combustion flue gas, shortened the generation cycle of the CO₂ 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 N₂ and CO₂. Similarly, magnetite can react with NO to produce N₂ at high temperatures. When siderite was added to the pulverized coal, due to its own thermal decomposition under high temperature conditions to produce CO₂ and Fe₃O₄, the CO₂ concentration in the overall flue gas increased and the generation cycle of the CO₂ 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 N₂ and CO₂. Moreover, the Fe₃O₄ produced by siderite thermal decomposition could react with NO to produce N₂ under high temperature conditions. As can be seen from Figure 8 and Figure 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. Fe₃O₄ and CO in the system could react with NO_x to produce Fe₂O₃, N₂ and CO₂, and then Fe₂O₃ 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.

5. 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 iron-containing 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.