J. Cent. South Univ. (2020) 27: 469-477

DOI: https://doi.org/10.1007/s11771-020-4309-y

Process control technology of low NO_x sintering based on coke pretreatment

WANG Zhao-cai(王兆才)^{1, 2}, ZHOU Zhi-an(周志安)^{1, 2}, GAN Min(甘敏)²,FAN Xiao-hui(范晓慧)², HE Guo-qiang(何国强)¹

1. Zhongye Changtian International Engineering Co., Ltd., Changsha 410125, China;

2. School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2020

Abstract:

Process control is an effective approach to reduce the NO_x emission from sintering flue gas. The effects of different materials adhered on coke breeze on NO_x emission characteristics and sintering performance were studied. Results showed that the coke breeze adhered with the mixture of CaO and Fe₂O₃ or calcium ferrite significantly lowers the NO_x emission concentration and conversion ratio of fuel-N to NO_x. Pretreating the coke with the mixture of lime slurry and iron ore fines helped to improve the granulation effect, and optimize the carbon distribution in granules. When the mass ratio of coke breeze, quick lime, water and iron ore fines was 2:1:1:1, the average NO_x emission concentration was decreased from 220 mg/m³ to 166 mg/m³, and the conversion ratio of fuel-N was reduced from 54.2% to 40.9%.

Key words:

iron ore sintering; nitrogen oxide; process control; coke breeze pretreatment；

Cite this article as:

WANG Zhao-cai, ZHOU Zhi-an, GAN Min, FAN Xiao-hui, HE Guo-qiang. Process control technology of low NO_x sintering based on coke pretreatment [J]. Journal of Central South University, 2020, 27(2): 469-477.

1 Introduction

Nitrogen oxides, a typical gas pollutant, which is harmful to the environment and human health, should be reduced. In China, 6% of NO_x emission is generated from iron and steel industry [1]. Meanwhile, the NO_x emitted from sintering process accounted for 48%, which is the main emission source in iron and steel industry [2]. In 2019,the “Emission Standard of Air Pollutants for Sintering and Pelletizing of Iron and Steel Industry” [3] was issued by the government of China, which indicated that the limit of NO_x emission was reduced to 50 mg/m³. Therefore, realizing the reduction of NO_x emission is significant for the sustainable development of Chinese iron and steel industry.

The present work on reducing the NO_x emission mainly focuses on the raw material control, process control and terminal treatment. It is a functional way of choosing coal or coke with low nitrogen content [4]. Biomass energy can also be used to replace the traditional solid fuels for reducing NO_x generation [5, 6]. However, optimizing fuel to reduce NO_x emission can not be widely used due to the limitation of resource region and cost. Currently, only two kinds of de-nitration technologies have been popularized in the sintering industry, which are selective catalytic reduction (SCR) [2] and synergistic purification of multi pollutants with activated carbon [7]. Only relying on terminal treatment to reduce the NO_x emission will increase the investment and operation cost. Therefore, process control should be introduced to help to decrease the load of terminal treatment on reducing NO_x emission. Studies made by YU et al [8], GAN et al [9] and PEI et al [10] all showed that adding Ca-Fe oxides into sintering raw materials can reduce the emission of NO_x. It was also found that adding hydrocarbon such as sawdust and flour in the sintering raw materials can reduce NO_x emission as well [11]. The effects of sinter basicity and grain size of limestone on NO_x generation were also investigated [12-14]. Although the sintering flue gas circulation method is also a function way to help to reduce the emission of pollutants [15-17], there are still some problems when it was carried out, including insufficient recirculation ratio, the deterioration of sintering indexes, etc.

The emission of NO_x generated from solid fuels is closely related to the combustion atmosphere of the fuel [18]. NO_x can be formed from fuel-N under the condition of oxygen atmosphere. It should be noticed that NO_x can also be reduced to N₂ through reduction reaction. Therefore, how to reduce the formation of NO_x through controlling the combustion atmosphere of the solid fuel during the combustion process is a promising method. Then the effects of different adhesion layer materials on the NO_x generated during coke breeze combustion were studied, and the feasible materials inhibiting and degrading NO_x were analyzed. Through the experiment of sintering pot, the effects of fuel pretreatment on granulation, fuel distribution and NO_x generation of sintering flue gas were studied as well. Finally, the method of low NO_x sintering based on coke pretreatment was proposed.

2 Experimental

The chemical compositions of sintering raw materials are shown in Table 1. The flux includes dolomite, limestone and quick lime. The basicity of sinter was 1.95. Quick lime was used to pretreat coke surface, of which the CaO content was higher than 80%, and the activity was higher than 250 mL of 4 mol/L HCl. The proportion of coke breeze was 4.31% with its industrial analysis shown in Table 2. The N content in the coke breeze is 0.55%.

Fe₂O₃, CaO and xCaO·yFe₂O₃ were ground to <0.5 mm. To improve the adhesion of Fe₂O₃ and xCaO·yFe₂O₃, 0.06 wt.% sodium carboxymethyl cellulose (CMC), a typical binder, was added with reasonable amount of water. The coke breeze was screened to 1-2 mm. Then 2.0 g coke breeze was attached with different mass of Fe₂O₃, CaO and xCaO·yFe₂O₃, respectively. And then the pretreated coke breeze was dried in the drying box. As shown in Figure 1, the coke combustion test was carried out in a vertical heating furnace, of which the temperature would be set as 1100 °C. And a quartz tube of f38 mm×70 mm was used as the reactor. 25 g sample (pretreated coke breeze) was loaded into the quartz tube and placed on the gasket with uniform holes, which would allow the gas passed through the pellet layer uniformly. During the combustion process, a flue gas analyzer (Vario Plus enhanced, York Co., Austria) was used to detect the flue gas composition, especially the NO_x.

By using a mixer (JJ-5 type, Shanghai Yanggong Co., China), quick lime with moderate water was mixed firstly. Then the coke breeze was added in to be covered with the lime slurry. The pretreated coke breeze was used for further sintering cup trials, as shown in Figure 2. The sintering pot test includes proportioning, mixing, granulating, feeding, igniting, sintering, cooling and quality testing, etc. The total consumption of water and quick lime in the whole sintering process remained unchanged, and only part of them was transferred for coke pretreatment. An analyzer (HW2000 CS, Shanghai Dongruan Zaiweibo Co., China) was used to analyze the content of carbon in granulated pellets with different grain size levels. During the sintering process, the concentration of NO_x was also measured.

Table 1 Chemical compositions and ratios of sintering raw materials (wt.%)

Table 2 Industrial analysis of coke breeze (dry basis)

Figure 1 Diagram of fuel combustion test device (1-Cylinder; 2-Pressure relief valve; 3-Flowmeter; 4-Mixing cylinder; 5-Thermocouple; 6-Dryer; 7-Gas analyzer; 8-Temperature controller; 9-Quartz tube; 10-Loading cup; 11-Vertical electric heating furnace)

Figure 2 Schematic diagram of main equipment of sintering cup test (Unit: mm): (1-Vacuum chamber; 2-Dryer; 3-Gas analyzer; 4-Temperature display instrument; 5-Thermocouple; 6-Sintering cup)

The generation mechanism of NO_x can be divided into three styles [19], namely thermal NO_x, fuel NO_x, prompt NO_x. In general, thermal NO_x is formed at the temperature more than about 1500 °C in the air. While the highest temperature during iron ore sintering process is lower than 1400 °C. Therefore, thermal NO_x is less likely to be generated. As for prompt NO_x, it is usually generated under the condition of low O₂ content. Hence, the amount of prompt NO_x generated during sintering process is also low. Fuel NO_x is considered to be the main source of NO_x during sintering process and it comes from the fuel including coke, coal, etc., in the sintering raw mixture [19, 20].

During the process of fuel combustion, only part of fuel N can be converted into NO_x. The ratio of NO_x actually generated to the theoretical one is defined as the conversion ratio from fuel N to NO_x. As most of NO_x is NO during the combustion process, the conversion ratio is calculated by the concentration of NO.

                       (1)

               (2)

                         (3)

where a(Fuel_N) is the content of N in the fuel, mg/kg; m is the mass of fuel burnt during the period from t_s to t_e, kg;η_N is the conversion ratio of fuel-N, %; [N]_a is the mass of fuel-N convert to NO released from t_s to t_e, mg; C(NO) is the concentration of NO at some point, mg/m³; Q^t_gas is the inlet flow at standard state, m³/min; 0.47 is the percentage of N to NO; [N]_o is the mass of N in fuel, of which the mass is m.

3 Results and discussion

3.1 Effect of fuel pretreatment on conversion ratio of N in coke breeze combustion

The emission of NO_x and the conversion ratio of fuel-N during the combustion process under the heating temperature of 1100 °C in air atmosphere was studied, with the adhesive content of CaO on coke surface kept at 10%, 20%, and 50%, respectively.

From Figure 3, it can be found that the conversion ratio of fuel-N decreased slightly as the amount of CaO increased, and when adhering 50% CaO, the peak value of NO_x emission decreased significantly.

Figure 3 Effect of CaO adhesion on NO_x emission and fuel-N conversion ratio in coke combustion

The emission of NO_x and the conversion ratio of fuel-N during the fuel combustion process at the roasting temperature of 1100 °C in air atmosphere was analyzed, with the adhesive proportion of Fe₂O₃ on coke breeze surface kept at 10%, 20%, and 50%, respectively.

From Figure 4, it can be found that the peak value of NO_x emission and conversion ratio of N all decreased as the amount of Fe₂O₃ increased. The conversion ratio of fuel-N reduces from 56% to 49.8% when the surface of coke is adhered with 50% Fe₂O₃, and the effect of Fe₂O₃ on the conversion of fuel-N is stronger than that of CaO, but it is difficult for Fe₂O₃ to be adhered to the surface of coke breeze.

CaO and Fe₂O₃ were mixed in molar ratio of 1:2, 1:1 and 2:1, respectively. While the ratio of the mixture xCaO+yFe₂O₃ was kept at 20%. Under the roasting temperature of 1100 °C in air atmosphere, the influence of the relative proportion variation of CaO and Fe₂O₃ on NO_x emission and N conversion in the combustion process of coke breeze is studied, as shown in Figure 5.

Figure 4 Effect of Fe₂O₃ adhered on NO_x emission and fuel N conversion ratio in coke combustion

Figure 5 Effect of CaO to Fe₂O₃ in adhesion layer on NO_x emission and fuel-N conversion ratio in coke combustion

Compared with the single adherence of CaO and Fe₂O₃, the mixture of CaO and Fe₂O₃ adhered on the surface of coke breeze greatly improves the inhibition of fuel-N conversion. With the increasing molar ratio of CaO to Fe₂O₃, the NO_x emission and the conversion ratio of N decreased significantly. When the molar ratio of CaO to Fe₂O₃ was 2:1, the conversion ratio of fuel-N decreased from 56% to 42.6%.

The reaction between CaO and Fe₂O₃ in adhesion layer and fuel ash in combustion process was studied, as shown in Figure 6. When the molar ratio of CaO to Fe₂O₃ was 1:2, calcium-diferrite (CaO·2Fe₂O₃) and a small amount of calcium ferrite (CaO·Fe₂O₃) were produced. When the molar ratio of CaO and Fe₂O₃ was 1:1, it was easy to produce calcium ferrite (CaO·Fe₂O₃). When the molar ratio of CaO to Fe₂O₃ was 2:1, a large amount of dicalcium-ferrite(2CaO·Fe₂O₃) was generated.

Figure 6 XRD patterns of combustion residues

To form a kind of calcium-containing ferrite on the surface of the coke breeze during sintering process is the key to realize the process control of low NO_x emission. Therefore, the way of accelerating the generation of calcium ferrite through the coke surface modification was studied, and how it affected the pelletizing efficiency, carbon distribution and NO_x generation was also researched.

3.2 Effect of fuel pretreatment on granulation effect and carbon distribution in granules

The coke breeze pretreated with lime slurry was used for sintering. Table 3 lists the grain size composition and carbon distribution in granules, of which the mass ratio of coke breeze, quick lime and water was 2:1:1. Compared with the base case, under the condition of fuel pretreatment, the proportion of granules larger than 3 mm increased from 80.08% to 81.50%, and the carbon content of particles larger than 5 mm increased. Coke breeze was easier to participate in the granulating process than that of independent existence. This can be attributed to the function of lime slurry, which has modified the coke breeze’s original hydrophobicity into hydrophilicity and the strong adhesion.

Table 3 Effect of coke breeze pretreatment on grain size composition and carbon distribution of granulated sintering pellets

Not changing the concentration of lime slurry (the mass ratio of quick lime to water is 1:1), the effect of lime slurry adhesion on granulating effect and carbon distribution was studied, as shown in Table 4. As the adhered lime slurry increased, the granulating pellets larger than 5 mm were increased, while the particles less than 1 mm tended to decrease. The carbon content of particles larger than 3 mm increased firstly and then decreased. When the mass ratio of coke breeze, quick lime and water was 2:1:1, the amount of the particles with 3-8 mm, was the largest. Meanwhile, the distribution of carbon content is the most reasonable.

Table 4 Effect of lime slurry adhesion on particle size composition and carbon distribution of granulated sintering pellets

When the mass ratio of coke breeze, quick lime and water was 2:1:1, different amounts of iron ore fines were added in the lime slurry and adhered on the surface of coke breeze. The effect of the amount of iron ore fines on the granulating effect and the carbon distribution was studied. As listed in Table 5, the granulation was not affected obviously when the iron ore fines were added into the lime slurry. The proportion of the particles, whose size was 3-5 mm, decreased slightly. The carbon content of the particles larger than 5 mm tended to decrease as well. Therefore, the amount of the iron ore fines added into the lime slurry should not be excessive due to the deterioration of granulation.

3.3 Effect of fuel pretreatment on NO_x generation of sintering flue gas

The effects of lime slurry adhesion on sintering index and NO_x emission were studied by the sintering pot test. The results are shown in Table 6 and Figure 7. As the adhesion of lime slurry increased, the sintering indexes firstly increased and then decreased. The comprehensive sintering indexes were obtained when the mass ratio of coke breeze, quick lime and water was 2:1:1. As the adhesion increased further, the sintering speed and productivity were negatively affected because the coke breeze was completely wrapped out and the combustion process was affected.

The concentration of NO_x in the flue gas and the fuel-N conversion ratio decreased as the adhesion of lime slurry increased, especially when the mass ratio of coke, quick lime and water was 2:1:1. The average NO_x emission concentration reduced from 220 mg/m³ to 179 mg/m³, and the conversion ratio of fuel-N decreased from 54.2% to 47.6%. By increasing the amount of adhesion further, the effect of inhibiting the formation of NO_x was not obvious. The effect of pretreatment on the surface morphology of coke breeze was studied. The results are shown in Figure 8. When the lime slurry was adhered to the surface of the coke breeze, the pores were filled with CaO and the number of surface pores was reduced, so as to prevent the directly contacting between coke breeze and O₂. The CaO on the surface of the coke breeze can react with the Fe_xO_y in raw mixture to form calcium ferrite, which can promote the reduction reaction of NO_x. In addition, the reduction of O₂ content produced reducing gas, which can also promote the reduction of NO_x. Finally, NO_x was reduced during the combustion process.

Table 5 Effect of amount of iron ore fines in lime slurry on particle size composition and carbon distribution of granulated sintering pellets

Table 6 Effect of thickness of lime milk adhesion layer on sintering index

Figure 7 Effect of amount of lime milk adhesion on NO_x emission and fuel-N conversion ratio in sintering process

When the mass ratio of coke breeze, quick lime and water was 2:1:1, by adding different amounts of iron ore fines in the lime slurry and then adhering them on the coke breeze surface, the effects of iron ore fines addition on sintering index and NO_x emission were studied. The results are shown in Table 7 and Figure 9. Obviously, the sintering indexes were slightly influenced. The comprehensive sintering indexes were the best when the mass ratio of coke breeze, quick lime, water and iron ore fines was 2:1:1:1. The average NO_x emission concentration decreased from 220 mg/m³ to 166 mg/m³. The conversion ratio of N reduced from 54.2% to 40.9%. The NO_x reduction ratio can be 25%.

The reason why the NO_x was inhibited is that the calcium ferrite was formed quickly on the surface and reacted with NO_x during the combustion process. However, the formation of NO_x was not inhibited further as the amount of iron ore fines increased further. According to Section 3.1, it can be found that 2CaO·Fe₂O₃ has the highest NO_x reduction ratio. As the amount of iron ore fines increased, the kind of calcium ferrite was changed from 2CaO·Fe₂O₃ to the mixture of 2CaO·Fe₂O₃ and CaO·Fe₂O₃. Therefore, compared with 100% 2CaO·Fe₂O₃, the mixture has a lower NO_x reduction ratio.

Figure 8 Surface microtopography changes before and after pretreatment of coke breeze:

Table 7 Effect of coke breeze adhered with lime milk containing iron ore fines on sintering index

Figure 9 NO_x emission and fuel-N conversion ratio of coke breeze adhered with lime milk containing iron ore fines (C-Coke; L-Lime; W-Water; I-Iron ore)

4 Conclusions

1) The conversion of fuel-N to NO_x was not inhibited obviously when adhering CaO or Fe₂O₃. While adhering their mixture on the surface of the coke breeze, the conversion ratio and the emission concentration of NO_x can be reduced obviously.

2) The surface modification of coke breeze by lime slurry improved the adhesion of coke breeze surface, and improved the granulation effect and carbon distribution as well. However, the additive amount of the iron ore fines should be controlled in a reasonable range to avoid the negative effects.

3) The pretreatment of coke breeze by lime slurry with iron fines reduced the pores on surface of coke breeze, prevented the directly contacting between coke breeze and O₂, and finally inhibited the oxidation reaction of fuel-N to NO_x. Calcium ferrite formed on the surface of coke breeze promoted the reduction reaction of NO_x. However, the adhesion layer of coke breeze should not be too thick; otherwise, the sintering indexes will be negatively affected. When the mass ratio of coke breeze, quick lime, water and iron ore fines was 2:1:1:1, the average NO_x emission concentration was decreased from 220 mg/m³ to 166 mg/m³, and the conversion ratio of fuel-N was reduced from 54.2% to 40.9%, and the NO_x reduction ratio was 25%.

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(Edited by YANG Hua)

中文导读

基于焦粉预处理的低NO_x烧结过程控制技术

摘要：过程控制是降低烧结烟气NO_x排放的有效途径。研究了焦粉粘附不同物质时对NO_x排放和烧结指标的影响。结果表明，当焦粉粘附CaO和Fe₂O₃的混合物或者铁酸钙时能减少燃料氮向NO_x的转化从而降低NO_x排放浓度。用石灰浆和铁矿粉的混合物对焦粉进行预处理，有助于改善制粒效果，优化制粒小球中的碳分布。当焦粉、生石灰、水、铁矿粉以质量比2：1：1：1进行混合预处理后参与烧结，NO_x平均排放浓度可由220 mg/m³降至166 mg/m³, 燃料氮转化率可由54.2%降至40.9%。

关键词：铁矿烧结；氮氧化物；过程控制；焦粉预处理

Foundation item: Project(2017YFC0210302) supported by the National Key R&D Program of China; Projects(U1660206, U1760107) supported by the National Natural Science Foundation of China

Received date: 2019-06-08; Accepted date: 2019-12-09

Corresponding author: ZHOU Zhi-an, Senior Engineer; E-mail: 35394690@qq.com; ORCID: 0000-0003-0829-0789

Abstract: Process control is an effective approach to reduce the NO_x emission from sintering flue gas. The effects of different materials adhered on coke breeze on NO_x emission characteristics and sintering performance were studied. Results showed that the coke breeze adhered with the mixture of CaO and Fe₂O₃ or calcium ferrite significantly lowers the NO_x emission concentration and conversion ratio of fuel-N to NO_x. Pretreating the coke with the mixture of lime slurry and iron ore fines helped to improve the granulation effect, and optimize the carbon distribution in granules. When the mass ratio of coke breeze, quick lime, water and iron ore fines was 2:1:1:1, the average NO_x emission concentration was decreased from 220 mg/m³ to 166 mg/m³, and the conversion ratio of fuel-N was reduced from 54.2% to 40.9%.