New approaches to coal quality assessment

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New approaches to the evaluation of coking coal quality: Due to shortages and difficulties in obtaining raw coal, and in order to maintain production and ensure the quality of metallurgical coke, efforts have been made to expand the sources of coal used for coking. However, as the properties of different types of coal vary greatly, it is necessary to classify them in order to make efficient use of them. To scientifically classify coal, it is necessary to select appropriate classification criteria. At present, our department primarily uses three monitoring methods for classifying coal: 1. Determining the degree of coal transformation by conducting industrial analysis of the coal ； 2. Determine the caking property of coal by measuring the vitrinite thickness and caking index of the coal ； 3. Determine whether coal blending has occurred through coal and rock analysis. Although there are the aforementioned monitoring methods, there is no indicator that directly characterizes the coking property of coal, whereas the coking property of coal has a significant impact on the quality of coke. In production, we can obtain the vitrinite thickness and caking index, which are indicators of coal’s caking property. However, coal with good caking properties does not necessarily have good bindability, whereas coal with good bindability definitely has good caking properties. To address the existing confusions, we proposed the small iron box experiment, which achieved good results. 1. Determination of the testing method: A small iron box of a specified volume was fabricated. Coal samples obtained from the plant were placed in this box and then fed into a coke oven for carbonization. After a certain coking time, the sample was taken out, quenched to stabilize its moisture content, and the quality of the coke cake was examined (including factors such as thickness, porosity, fusibility, and microstructure). The drum strength was also measured according to specific requirements. 2. Practical applications and effects 1) Determining the coking property of a single coal type: (1) Judgment based on macroscopic characteristics. By conducting small-iron-box tests on bituminous coal, lean coal, coking coal, and fat coal, and observing parameters such as the particle size of the coke, porosity, fusibility, cracks, and the inlaid structure, the visual characteristics of coke formation from each type of coal can be identified: bituminous coal yields coke with high porosity and small coke particles ； Fatty coal coke has many transverse cracks; it contains honeycomb structure at its root, has good fusibility, and the coke pieces are small in size ； Coking coal has large lumps, with uneven distribution of pores, and contains granular coal with weak cohesion embedded within it ； Primary coking coal yields coke with large particle size, good fluidity, low porosity, few cracks, and excellent coking properties ； Lean coal has poor fusibility and poor caking property. (2) Judgment based on drum data. By testing the drum strength of small iron box cokes, the data in the table below were obtained. It can be seen that among different types of coal, hard coking coal has better resistance to crushing compared to bituminous coal and fat coal, as well as better coking properties. On the other hand, fat coal has better wear resistance than hard coking coal, coking coal, and bituminous coal, along with better bonding properties. Project: Crushing strength %, Wear resistance %. Bituminous coal: 60, 32; Rich coal: 77.5, 5.0; Coking coal (Xinlong): 80, 20; Primary coking coal: 83, 10; Lean coal: 0, 0. As can be seen from the above, whether based on macroscopic properties or drum test data, the characteristics of the coke produced in the small iron box experiments reflect the coking ability of each individual type of coal. 2) Determining the coking property of blended coal: At present, there are a wide variety of coking coals in our department, and blending of these coals is quite extensive. Tests using coal petrography analyzers show that such blended coal is generally formed by mixing bituminous coal, fatty coal, coking coal, and lean coal. Due to the low prices of bituminous coal and lean coal, as well as their high gelification index and caking index, merchants add large amounts of these coals when washing coal. After mixing, the gelification index and caking index reach the levels required for coking coal, with values that are fairly close to each other. It is therefore difficult to determine the quality of coal based solely on its gelification index and caking index. The values of volatile matter, gel layer thickness (Y value), and caking index (G value) for the coking coal supplied to the coking department recently are shown in Table 1 below: Table 1: Comparison table of key parameters for various coal types. Coal type | Volatile matter % | Y value (mm) | G value (%) | Taihang | 22.17 | 16.3 | 74.3 | Ruiyuan | 24.59 | 17.54 | 73.7 | Xinxing | 22.23 | 16.8 | 73.7 | Jushachengta | 22.43 | 18.7 | 68.4 | Guifeng | 17.23 | 9.0 | 66.6 | Huacheng | 27.38 | 13.0 | 35.7 The Y value for Taihang coal is 16.3 mm, with a G value of 74.3%; the Y value for Jushachengta coal is 18.7 mm, with a G value of 68.4% ； The values of these two indicators are relatively similar, but through iron box tests, the cokes produced can be seen in the figures below: Figure 1 shows the coke produced from Taihang coal, while Figure 2 shows that of Jushachengta coal. As can be seen from these figures, the coke produced from Taihang coal is more dense, with a lower porosity; its layer thickness is greater, and its coking quality is better ； On the other hand, sand-aggregated coal has a high gas porosity, poor density, and poor fusibility, with granular coal of weak adhesion embedded within it. It can be inferred that when mixing Pusachengta coal, a relatively large amount of bituminous coal and lean coal is incorporated, resulting in coking properties that are inferior to those of Taihang coal. Therefore, in coal yard management, Taihang coal should be stored in the main coking coal storage areas, while Jushachengta coal should be stored in the coking coal storage areas as well. 3) Conduct blending tests to maintain quality while reducing costs. This is achieved by selecting appropriate mixtures for coal blending, carrying out iron drum tests, and assessing the feasibility of the chosen mixtures by observing the condition of the coke cakes and measuring the strength of the rotary drums. This method is relatively direct and objective, takes little time, and avoids subjectivity as well as quality issues. 3. Existing problems 1) The particle size of the test coal is not very uniform ； 2) When testing the rotating drum, the amount of coke is low and the drum size is large, which does not meet the standards; the data obtained are merely comparative figures under identical conditions. To address the above issues, the Ministry of Industry is considering purchasing a small crusher and constructing a small drum to make this experiment more reasonable and scientific. This is some of my humble experience; please take a look and point out any mistakes. There are two images that, for some reason, are not showing up. This post was last edited by Keke on 2009-3-27 at 16:45.]