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Discussion on factors affecting the thermal properties of coke Zheng Mingdong and Wang Xiaoyan (School of Chemistry and Chemical Engineering, Anhui University of Technology, Ma'anshan 243002) With the development of science and technology, people have an increasingly detailed understanding of the blast furnace production process, and they also have a deeper understanding of the change process of coke in the blast furnace. The traditional indicators for evaluating coke quality are cold properties and components, which can no longer meet the needs of blast furnace production. To this end, coke thermal property indicators are proposed, usually using coke reactivity (CRI) and post-reaction strength (CSR). There are many factors that affect the thermal properties of coke, and they are also very complex. There is currently no comprehensive explanation. This article explores the relationship between these factors and CRI and CSR from the perspective of raw coal properties. 1 Test method: Industrial analysis, cohesiveness index and ash composition analysis were conducted on 17 types of coal samples in northern Anhui. A 40kg small coke oven coking test was conducted to measure the corresponding coke property indicators. The analysis of coal's Ad, Vdaf, G, Y, Std, M40, M10, CRI, and CSR are all conducted in accordance with national standard methods. 2 Test results and discussion 2.1 Effect of ash on thermal properties (1) The effect of ash on coke can be divided into two aspects. One is the effect of SiO2, gangue and other granular rocks in the ash. ； On the other hand is the influence of alkali metals in ash, see Figure 1. It can be seen from Figure 1 that as the ash content in raw coal increases, the CO2 reactivity of coke becomes larger and the strength after reaction becomes smaller. This is because the thermal expansion of granular rocks such as SiO2 in ash is different from that of coke at high temperatures, resulting in radioactive cracks centered on them, which enlarges the contact surface between coke and CO2 and accelerates the reaction speed. (2) The minerals in ash refer to the oxides of minerals in coal, which include acidic oxides and alkaline oxides. Minerals degrade coke in the blast furnace through the following two pathways. One is through the catalytic effect on the dissolution loss reaction, which intensifies the coke dissolution loss reaction and reduces the strength after the reaction. ； Another way is that minerals can directly interact with carbon, such as direct reduction reaction in a blast furnace, formation of TiC, formation of interlayer compounds of potassium and sodium, etc. Minerals refer to the oxides of minerals in coal, which have a catalytic effect on the carbon dissolution reaction of coke. They include acidic oxides and salts of alkaline oxides. Corrected acid-base index MBI after taking into account all acid and alkali components * for: MBI * =(Na2O+K2O+CaO+MgO+Fe2O3+TiO2+MnO)/(SiO2+A12O3+P2O5) (1) As can be seen from Figure 2, when the alkalinity index increases, the CO2 reactivity of coke also increases, and the post-reaction strength of coke gradually decreases. It shows that alkali metals mainly play a positive catalytic role in the dissolution reaction of coke, and their presence seriously affects the thermal properties of coke. Effective measures should be taken to control the content of alkali metals in production to improve the thermal properties of coke. 2.2 The influence of coalification degree index on the thermal properties of coke. It can be seen from Figure 3 that the volatile matter of coal is closely related to the reactivity and post-reaction strength of coke. As the volatile content of a single coal increases, the CO2 reactivity of coke becomes greater, and the strength after reaction gradually decreases. In particular, coal with a volatile content between 20% and 24% has better coke reactivity and post-reaction strength. This is because during the coking process, volatile matter gradually precipitates as the temperature increases. The higher the volatile matter content, the more or larger the pores of the coke, which causes the pore wall of the coke to become thinner and the specific surface to increase, and the contact surface with CO2 to increase, accelerating its reaction speed. The vitrinite reflectance of coal is an important indicator of the degree of coalification and an important factor affecting the strength of coke. It can be seen from Figure 4 that as the vitrinite reflectance of a single type of coal increases, the CO2 reactivity of the coke decreases, and the intensity of the coke after reaction gradually increases. When R0max (referring to the average maximum reflectance of coke, the same below) is between 1.3 and 1.8, the reactivity and post-reaction strength of coke are better. 2. 3 The influence of caking index on the thermal properties of coke Figure 5 shows that there is a certain relationship between a single coal and caking index and the reactivity and post-reaction strength of coke. The basic rule is that as the caking index of a single coal increases, the reactivity of coke decreases, but the strength after reaction increases. 2.4 The influence of sulfur content on the thermal properties of coke. It can be seen from Figure 6 that there is a certain relationship between the sulfur content in the raw coal and the thermal properties of the coke. As the sulfur content in the coal increases, the thermal properties of the coke become better, which is reflected in the decrease in reactivity and the increase in strength after reaction. The reason is that the sulfur in coke exists in the form of organic sulfur, forming CSC bonds, lengthening the molecular chain, and forming a network structure between molecules, strengthening the structure of the coke. Therefore, the thermal properties of coke are improved. Li Yunyong and others also confirmed in their article that the presence of sulfide has an inhibitory effect on the reactivity of coke. In HCWikison's research, it is also believed that the sulfur in coal can also increase the reactivity of coke. Since sulfur will cause inconvenience to subsequent blast furnace production in actual production, the sulfur content should also be controlled. 3 Establishing a mathematical model 3.1 Coke CO2 reactivity prediction model There are many factors that affect the thermal properties of coke, mainly including the coalification degree of raw coal, caking index, alkalinity catalytic index, etc. This paper selects different combinations of indicators such as ash content, sulfur content, coalification degree, bonding index, and alkalinity catalytic index to establish a prediction model for coke CO2 reactivity index, which is expressed as follows: CRI＝15.72251－0.4124Ad+0.433282Vdaf－0.21519G+144.0259MBI * -7.74784Std (2) tAd=-0.47375, tVdaf=1.597362, tStd=1.50653, tG=1.92717, tMBI * =4.187902 a = 0.2, ta =1.363, ︱tAd ︱＜ta Therefore, it is considered that tAd has little impact on CRI and can be eliminated from equation (2), so there is equation: CRI＝11.27679+0.480049Vdaf－6.05904Std－0.22999G+140.8076MBI * (3) R=0.94518, F=25.13384, s=2.435224 When n=17, a=0.05, check the correlation coefficient R table and F distribution table, and get R=0.641, F=3.01, so the regression equation is 95% sure to be reliable within a certain range, and it is significant at the level of a=0.05. CRI＝7.313158+0.62257Vdaf－0.28439G+147.0583MBI * (4) R=0.93166, F=28.49165, s=2.603278 Use R0max to replace Vdaf to establish the CO2 reactivity prediction model of coke, and obtain the equation: CRI=33.07695-0.33653G+165.6813MBI * -7.25776R0max (5) R=0.91772, F=23.12774, s=2.846191 The test method is the same as equation (3). The test results show that both equations (4) and (5) are 95% sure to be reliable within a certain range, and are significant at the level of a=0.05. 3.2 The post-reaction strength prediction model of coke is the same as the CO2 reactivity prediction model of coke. Different combinations of raw coal property indicators are selected to establish the post-reaction strength prediction model of coke. The model is expressed as follows: CSR=56.95597-0.0084Std-0.31707 Vdaf+0.770489G-273.816MBI * (6) R=0.91624, F=15.69125, s=5.216949 CSR=56.95048-0.31687Vdaf+0.770414G-273.807MBI * (7) R=0.91624, F=22.66513, s=5.012283 CSR=44.34907+3.255481 R0max +0.800981 G－284.375MBI * (8) R=0.91463, F=22.17883, s=5.058043 The test equations (6), (7), and (8) show that these three equations are correlated and are significant at the level of a=0.05. The test data of the small coke oven of Huozhou Coking Co., Ltd. is used to test the correctness and practicality of equations (3), (4), (5), (6), (7), and (8). The results are shown in Figures 7 and 8. It can be seen from Figures 7 and 8 that the prediction effects of the established models are very stable. Since these models are established for Wanbei coal, they show certain systematic errors when used in the production of Nanjing Iron and Steel Co., Ltd. This error can be corrected and eliminated by correcting the constant term. Comparing the standard errors of each model, models (3) and (7) are smaller than the other models and are recommended to be used. 4 Conclusion (1) The higher the ash content in coal, the worse the thermal property index of coke. The alkaline oxides in ash play a positive catalytic role in the dissolution reaction of coke. The greater its content, the greater the reactivity of coke and the lower the strength after reaction. (2) Coal with a volatile content between 20% and 24% and a vitrinite reflectance between 1.3 and 1.8 has better coke reactivity and post-reaction strength. As the caking index of a single coal increases, the reactivity of coke decreases, and the strength after reaction increases. The presence of sulfur can inhibit the CO2 reactivity of coke. As the content increases, the post-reaction strength of coke increases. (3) The established model has certain reliability and stability.