J. Cent. South Univ. (2018) 25: 1920-1927

DOI: https://doi.org/10.1007/s11771-018-3882-9

Influence of blast furnace top gas composition and dust on HCl removal with low temperature Ca-based dechlorination agent

TENG Ai-jun(滕艾均)^{1, 3}, HU Bin-sheng(胡宾生)², GUI Yong-liang(贵永亮)², XUE Xiang-xin(薛向欣)^{1, 3}

1. School of Metallurgy, Northeastern University, Shenyang 110819, China;

2. North China University of Science and Technology, Tangshan 063009, China;

3. Key Laboratory of Liaoning Province for Recycling Science of Metallurgical Resources,Shenyang 110819, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract: Using fixed-bed reaction method and changing the gas composition and dust content, the influence of blast furnace top gas composition and dust on HCl removal with low temperature Ca-based antichlor was studied. It was found that, when the content of CO₂ in blast furnace top gas increased, the dechlorination efficiency was getting worse obviously; when the contents of CO and N₂ increased, the dechlorination efficiency was getting better to a certain extent; when the content of H₂ changed, the dechlorination efficiency got no significant change; as the content of dust increased, the dechlorination efficiency got better obviously when the content was less than 15 g/m³, but it would be got worse quickly when the content was more than 20 g/m³, and the best content was 15–20 g/m³; the suitable site of the process of dechlorination was after gravity dust collector and before bag dust collector.

Key words: blast furnace top gas; HCl gas; dechlorination; breakthrough time; penetration chlorine capacity

Cite this article as: TENG Ai-jun, HU Bin-sheng, GUI Yong-liang, XUE Xiang-xin. Influence of blast furnace top gas composition and dust on HCl removal using low temperature Ca-based dechlorination agent [J]. Journal of Central South University, 2018, 25(8): 1920–1927. DOI: https://doi.org/10.1007/s11771-018-3882-9.

1 Introduction

Dry dust removal technology of blast furnace is widely applied to blast furnace dust removal system of each large steel and iron company, because of its high efficiency of dust collection and purification ability, high utilization rate of the gas sensible heat, less water consumption, etc [1]. But compared with wet dust removal technology, the dry dust removal technology is difficult to solve the problem of chlorine content in the gas, such as the salt corrosion of gas pipeline and TRT operation leaves, and the erosion of hot blast stove regenerator checker brick. All of these problems are caused by the chlorine in the gas and these problems are seriously harmful for the normal production of blast furnace. In addition to the improvement of national emphasis about environmental protection recent years and the quality problems of the low grade iron ores, chlorine brings significant impact on the whole production process and it is receiving more attentions gradually. So, it is necessary to remove the HCl gas before the blast furnace top gas into the gas pipeline.The removal of HCl gas in the gas has been carried out extensive researches in petrochemical industry and coal chemical industry, and the research and development of high temperature gas dechlorination agent are relatively wide and mature. Some typical researches are as follows. KRISHNAN [2] studied the performance of HCl desorption in the coal gas by the baking soda in 525–650 °C using fixed bed reaction device and the result showed that the saturated chlorine capacity can be up to 45%. NURA et al [3] studied the aspects of dynamics of CaO and HCl from the reaction kinetics theory and got activation energy of the gas–solid reaction and the reaction gas diffusion through the product layer respectively. In the air and energy engineering research laboratory of USA, JOZEWICZ et al [4] studied the reaction mechanism of HCl with Ca(OH)₂ and CaO and the product structure (CaCl₂·2H₂O and CaClOH) by using DSC thermogravimetric analyzer and X-ray diffraction. WAN [5] studied the reaction mechanism of removing HCl gas by calcium oxide at 600–800 °C and found that the restriction factor is the gas diffusion in the product layer. It can be seen that these studies about HCl desorption under high temperature conditions only aim at the non-blast furnace gas. However, the gas temperature in the blast furnace gas pipeline is ranged from 100 °C (±20 °C) to 300 °C (±50 °C), and the temperature is 150–200 °C between the gravity dust collector and the gravity dust collector. Therefore, it is necessary for us to study the HCl desorption aiming at the blast furnace gas. Because the study of dechlorination for blast furnace gas is relatively few, this research simulates the blast furnace gas under the condition of laboratory, and learns the effects of gas composition and dust content for the low temperature dechlorination agent from the aspects of basic reaction kinetics.

2 Experimental

2.1 Materials

The dechlorination agent used in this present study was selected the H-L low temperature dechlorination agent from a company in Liaoning Province, China. The sample was chemically analyzed using inductively coupled plasma optical emission spectroscopy (ICP-OES), and the chemical composition of the sample is shown in Table 1. Because the reaction circumstance that we simulated was between the gravity dust collector and the bag dust collector, the dust we used in this experiment was a mixture of the gravity dust and bag dust (the mass ratio of gravity dust to bag dust was 1:1). And the chemical compositions of the mixed dust, gravity dust, and bag dust are listed in Table 2. All the gas such as CO, CO₂, N₂, H₂, HCl were of analytical grade.

2.2 Apparatus and procedure

The main devices of this experiment are three parts including gas supply device, reaction device and gas detection device and the schematic diagram is shown in Figure 1. The gas supplying device consists of CO, CO₂, N₂, H₂ and HCl five kinds of gas supply bottles, powder spraying equipment, mixing bottle and gas heater; the reaction device is a cylindrical fixed bed reactor of which the length is 150 cm and the inner diameter is 20 cm; in the fixed bed the temperature remains constant and the dechlorination agent takes reaction with the HCl gas; the gas detection device is used to detect the concentration of HCl gas real-time monitoring at the export of the fixed bed.

According to the average level of the basic components of the gas in the blast furnace gas-pipeline, the reference gas ratio of this experiment was: CO: CO₂: N₂: H₂=20: 20: 59: 1, and the variation range of each gas was as follows: the content of CO was 18%–24%, the content of CO₂ was 16%–22%, the content of N₂ was 56%–63%, and the content of H₂ was 0.5%–2.5%. According to the fluctuation range of dust content between gravitate dust filter and bag dust filter in blast furnace iron making system, the dust content was kept at 10 g/m³ when studying the effect of gas content variety, and the dust content was kept at 0–55 g/m³ when studying the effect of dust content variety. While, the size of dust used in this study was screened to 44–178 μm. According to the fluctuation range of the gas temperature in the blast furnace gas-pipeline, the test temperature was controlled at 150 °C (±1 °C). The initial concentration of HCl gas in the gas was controlled at 100×10^–6 (±0.5×10^–6) and the gas flow rate was 5 L/min. The dechlorination agent was screened to 3–5 mm, and a certain amount of dechlorination agent was put into the fixed bed reactor. The dechlorination agent was set as a cylindrical in which the high-diameter ratio is 1:1 (the diameter is 20 cm).

Table 1 Chemical composition of H-L low temperature dechlorination agent (wt%)

Table 2 Results of analysis of BF dust composition (wt%)

Figure 1 Schematic diagram of HCl removal from blast furnace top gas test equipment:

This study used the breakthrough time and the penetration chlorine capacity to evaluate the effect of the dechlorination agent. When the content of HCl gas was greater than 1×10^{–6 in the gas detection device at the export, it can be seen that the dechlorination agent has been penetrated. The time from the beginning to eject HCl gas to the dechlorination agent penetrated was defined as the breakthrough time, and the chlorine mass percentage of the dechlorination at this time was called the penetration chlorine capacity. The chlorine capacity was analyzed by X-ray fluorescence spectrometer (XRF); the major phases of the dechlorination agent samples were identified by X-ray diffraction (XRD); the surface morphology of the dechlorination agent samples was examined by scanning electron microscopy (SEM).}

3 Results and discussion

3.1 Effect of CO₂ and CO content

The effects of CO₂ content on the breakthrough time and the penetration chlorine capacity of the dechlorination agent are shown in Figure 2. As the CO₂ content increased gradually, the breakthrough time and the penetration chlorine capacity of dechlorination agent got reduced. When the concentration of CO₂ increased from 16% to 22%, the breakthrough time decreased from 398 min to 345 min, reduced by about 13.32%; meanwhile, the penetration chlorine capacity decreased from 18.33% to 16.16%, reduced by about 11.84%. As shown in Figure 3, with the CO content increased gradually, the breakthrough time and the penetration chlorine capacity of dechlorination agent got increased. When the concentration of CO increased from 18% to 24%, the breakthrough time increased from 345 min to 398 min, increased by about 15.36%; meanwhile, the penetration chlorine capacity increased from 16.16% to 18.33%, increased by about 13.43%.

Figure 2 Effects of CO₂ content on dechlorination efficiency of antichlor

Due to the fact that the dechlorination agent used in this test is Ca-based antichlor which contains CaCO₃, the main chemical reactions might happen in the reaction process as follows:

Figure 3 Effects of CO content on dechlorination efficiency of antichlor

CaCO₃(s)+2HCl(g)=CaCl₂(s)+H₂O(g)+CO₂(g)

CaO(s)+2HCl(g)=CaCl₂(s)+H₂O(g)

CaCO₃(s)=CaO(s)+CO₂(g)

The above three chemical reactions are reversible under certain conditions. So from the point of chemical reaction equilibrium, when the concentration of CO₂ increased, the forward reaction would be restrained and it could reduce the rate of reaction, made the breakthrough time and the penetration chlorine capacity decrease accordingly. Figure 4 shows the XRD patterns at different CO₂ contents. The results show that the CO₂ content can be proved to have a negative effect on the dechlorination agent. The specific performance is that the peak intensity of CaClOH at 16% CO₂ content is higher than that at 22% CO₂ content, while the peak intensities of CaCO₃ and CaOH are relatively low.

Because under the normal condition of blast furnace system, the content of CO+CO₂ in the blast furnace top gas keeps a fixed value, when the content of CO increases, the CO₂ would be reduced accordingly, the time of reaction also would be increased, and the penetration chlorine capacity would be increased accordingly. From the comparison of Figure 2 and Figure 3, it can also be informed that the effects of CO and CO₂ on the dechlorination reaction are just opposite.

Figure 4 XRD analysis of antichlor when changing CO₂ content

3.2 Effect of N₂ content

The effects of N₂ content on the breakthrough time and the penetration chlorine capacity of the dechlorination agent are shown in Figure 5. As the N₂ content increased gradually, the breakthrough time and the penetration chlorine capacity of dechlorination agent got increased gradually. When the concentration of N₂ increased from 53% to 62%, the breakthrough time increased from 343 min to 372 min and it gained about 8.45% increment; meanwhile, the penetration chlorine capacity increased from 15.95% to 17.42%, increased by about 9.22%.

Figure 5 Effects of N₂ content on dechlorination efficiency of antichlor

The reason why the increase of N₂ can enhance the efficiency of dechlorination is that: when the concentration of N₂ increased, the CO₂ content would be decreased and it could improve the rate of reaction, and the breakthrough time and the penetration chlorine capacity increased accordingly. Figure 6 shows the XRD patterns at different N₂ contents. The results show that the N₂ content can be proved to have a positive effect on the dechlorination agent. The specific performance is that the peak intensity of CaClOH at 62% N₂ content is higher than that at 53% N₂ content, and the peak intensities of CaCO₃ and CaOH are relatively low.

Figure 6 XRD analysis of antichlor when changing N₂ content

From Figure 4 and Figure 6, it indicates that CaClOH peaks appeared, which can be attributed to the fact that the CaCO₃ was converted to CaCl₂, then the CaCl₂ absorbed H₂O and transformed into CaClOH. The crystal transformation relationship of CaCO₃, CaCl₂ and CaClOH is shown in Figure 7. The crystalline structure of CaCO₃ (Figure 7(a)), with space group Pmcn(62), unit cell parameters of a=4.9614 , b= 7.9671 , c=5.7404 , α=β=90° and γ=120°, has the crystal structure of aragonite, strontianite, and witherite. In the initial reaction process, C—O bonds of CaCO₃ were broken by HCl, forming the orthorhombic structure of CaCl₂. The crystalline structure of CaCl₂ (Figure 7(b)), has space group Pbcn(60), unit cell parameters of a=6.268 , b= 7.619 , c=6.923 , α=β=γ=90°. With the development of reaction, the formed CaCl₂ got with H₂O, the Ca—Cl bonds of CaCl₂ was broken by H₂O, forming the hexagonal structure of CaClOH. The crystalline structure of CaClOH (Figure 7(c)), has space group P63mc(186), unit cell parameters of a=3.8641 , c=9.9044 , α=β=90°, γ=120°.

3.3 Effect of H₂ content

As shown in Figure 8, it can be seen that when the H₂ content increased, the breakthrough time and the penetration chlorine capacity of the dechlorination agent kept stable basically. There are two reasons for this phenomenon: one is that the proportion of H₂ content in blast furnace top gas is very low and the variety of H₂ content has a little effect of dechlorination in the process; the other is that H₂ and dechlorination agent can not take any chemical reaction and the variety of H₂ content only has a slight influence on the fixed value of CO+CO₂. So when changing the content of H₂ gas, both the breakthrough time and the penetration chlorine capacity had no obvious change, only fluctuated around a fixed value, which is shown as the breakthrough time remaining for about 360 min and the penetration chlorine capacity remaining at about 16.81%.

3.4 Effect of dust content

The effect of dust content is shown in Figure 9. it can be seen that: when the dust content increased from 0 to 15 g/m³, the breakthrough time gained about 20.97% increment and the penetration chlorine capacity gained as much as 68.26% increment; when the dust content arrived about 20 g/m³, the breakthrough time and the penetration chlorine capacity of the dechlorination agent reached the top; when the dust content increased from 20 g/m³ to 55 g/m³, the breakthrough time reduced about 12.20% and the penetration chlorine capacity decreased as much as 16.97%.

The SEM images of the surface of the dechlorination agent are shown in Figure 10. Figures 10(a) and (b) show the surface characteristics of the dechlorination agent before reaction, indicating that there are a lot of cracks and wrinkles on the surface, which can not only make the gas enter in easily but also enlarge the reaction area. These are two positive factors for the reaction. Figure 10(c) shows the surface features of the dechlorination agent in the reaction at the initial stage of adding dust, showing that many tiny particles are adsorbed on the surface. Figure 10(d) shows the surface features of the dechlorination agent after adding lots of dust. It can be seen clearly that lots of dust are adsorbed on the surface round and round.

Figure 7 Transformation relationship of CaCO₃, CaCl₂ and CaClOH in reaction process

Figure 8 Effects of H₂ content on dechlorination efficiency of antichlor

Figure 9 Effects of dust content on dechlorination efficiency of antichlor

The dust used in this test is the mixture of the gravity dust and the bag dust. From Table 2, it can be known that a certain amount of alkaline substances was produced in the process, and the chemical reaction may be as follows:

Fe+2HCl=FeCl₂+H₂

2FeO+6HCl=2FeCl₃+2H₂O+H₂

CaO+2HCl=CaCl₂+H₂O

Al₂O₃+6HCl=2AlCl₃+3H₂O

Na₂O+2HCl=2NaCl+H₂O

MgO+2HCl=MgCl₂+H₂O

K₂O+2HCl=2KCl+H₂O

ZnO+2HCl=ZnCl₂+H₂O

From the above reactions, it can be seen that the dust can consume part of HCl to a certain extent, and the water produced by the reaction can also absorb some HCl, so it causes the chlorine capacity increasing significantly at the initial stage of adding dust. According to JAMES [6] on the catalytic properties of the materials and some researches on the catalytic properties of tiny particles [7–16], some tiny particles can react with HCl in the dust adsorbed on the surface of the dechlorination agent as shown in Figure 10(c). And numerous small levels rapid reaction fields are formed, which promoted the absorption of HCl. So the breakthrough time and the penetration chlorine capacity increased quickly at the initial stage of adding dust. Figure 10(d) shows that lots of tiny particles adsorbed on the surface and filled in the cracks and wrinkles at the later stage of adding dust. Figure 11 shows the gas pipe-path formed on the bed of the dechlorination agent. When the gas contains a lot of dust, parts of large particles dust is easy to settle and block the interlayer pores of the dechlorination agent, and part of tiny particles dust adsorbed on the large particles and dechlorination agent surface tightly. This leads to a phenomenon that some gas pipe-paths form on the bed of the dechlorination agent. These gas pipe-paths can decrease the breakthrough time, and then reduce the whole absorption of the dechlorination agent. This causes the breakthrough time and the penetration chlorine capacity reduced gradually when the dust content was more than 20 g/m³. To sum up, the best dust content of dechlorination is between 15 g/m³ and 20 g/m³, the suitable site of the process of dechlorination is after gravity dust collector and before bag dust collector.

Figure 10 SEM images of antichlor’s surface:

Figure 11 Gas pipe-path formed on bed of dechlorination agent

4 Conclusions

1) As the CO₂ content increased, the breakthrough time and the penetration chlorine capacity of the dechlorination agent reduced obviously and the effect of dechlorination became worse.

2) When the CO and N₂ content increased, the breakthrough time and the penetration chlorine capacity of the dechlorination agent increased obviously and the effect of dechlorination became better.

3) With the H₂ content increased, the breakthrough time and the penetration chlorine capacity of the dechlorination agent had no obvious effect.

4) When the dust content was less than 15 g/m³, the effect of dechlorination became better quickly; when the dust content was more than 20 g/m³, the effect of dechlorination got worse obviously; the best dust content for dechlorination was between 15 g/m³ and 20 g/m³.

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(Edited by YANG Hua)

中文导读

高炉炉顶煤气中气体成分和炉尘对常温钙基氯剂脱除HCl的影响

摘要：通过研究高炉炉顶煤气的气体成分和炉尘对低温钙基脱氯剂脱除HCl的影响发现：在高炉炉顶煤气中，随着CO₂含量的增加，脱氯效果明显变差；随着CO含量和N₂含量的增加，脱氯环境明显改善；H₂含量的变化对脱氯效果的影响不大；当炉尘含量处在较低范围时（<15 g/m³），炉尘含量的增加使脱氯效果得到大幅度改善；而当炉尘含量超过一定量（>20 g/m³）后再提高炉尘的含量时，脱氯效果则明显恶化，在含尘量为15~20 g/m³时脱氯效果最佳，脱氯剂的最佳工作位置为重力除尘器之后与布袋除尘器之前。

关键词：高炉炉顶煤气；HCl；脱氯；穿透时间；穿透氯容量

Foundation item: Project(51274080) supported by the National Natural Science Foundation of China; Project(E2013209051) supported by the Hebei Science Foundation and the Steel and Iron Joint Research Foundation Project, China; Project(U1502273) supported by the Joint Funds of the Natural Science Foundation of China and Yunnan Provincial Government; Projects(N150202001, N150203003) supported by the Fundamental Research Funds for the Central Universities, China

Received date: 2017-04-20; Accepted date: 2018-03-11

Corresponding author: TENG Ai-jun, PhD; E-mail: wdtaj2008@163.com; ORCID: 0000-0002-1472-0661