Prediction of the formation of coking products, yields, and coke quality

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1 Product formation 1.1 The coking process of coal: Coking coal is heated in the absence of air, and its organic components undergo a series of changes as the temperature rises, resulting in gaseous (coal gas), liquid (coal tar), and solid (semi-coke or coke) products. The coking process of coal can be divided into three stages: The first stage (from room temperature to 300°C) is the drying and degassing stage of coal, during which water is released and CH4, CO, and N2 are precipitated. In the second stage (300°C–600°C), depolymerization and decomposition reactions dominate, with coal bonding to form semi-coke. Generally, bituminous coal begins to soften at 300°C, accompanied by the release of gas and coal tar ； Bituminous coals with medium degree of coalification produce phases of gas, liquid, and solid coexisting in a certain temperature range during this period. The third stage (600℃–1000℃) is the stage in which semi-coke transforms into coke; during this stage, polycondensation reactions dominate, producing large amounts of gas (primarily H2), and the semi-coke shrinks to form cracked coke. 1.2 Coking process in the carbonization chamber? The heat required for coking of the coal loaded into the chamber is provided by the furnace walls on both sides; this heat moves from those walls toward the center of the carbonization chamber. Therefore, the coking process is a layered coking process that begins at the walls of the carbonization chambers on both sides and gradually moves toward the center of the carbonization chambers. The coke cake matures when its temperature at the center of the carbonization chamber reaches 950°C to 1050°C. The coking rate reflects the average temperature rise rate in the carbonization chamber. An excessively fast coking rate will lead to an increase in cracks in the coke and a reduction in its size. When a plastic layer forms in the coal material near the walls of the carbonization chamber, the coal material at the top and bottom of the chamber also heats up and develops plastic layers. Meanwhile, gaseous products resulting from the decomposition of the coal are continuously generated. Since the plastic layers on all sides do not allow gases to pass through, the pressure inside the carbonization chamber rises steadily; the plastic layers expand, and this expanding pressure is transmitted to the walls of the chamber through the layers of semi-coke and coke. When the plastic layers converge at the central plane of the carbonization chamber, the expansion pressure of the carbonization chamber reaches its maximum value. The expansion pressure commonly referred to is this maximum value. The expansion pressure depends on the properties of the coal used in the furnace, the bulk density of the charge, and the coking rate. Increasing the expansion pressure helps to bond and melt the coal particles, thereby improving the physical properties of the coke; however, if the expansion pressure is too high, it can cause damage to the walls of the carbonization chamber. 1.3 Dynamics of gas evolution in the carbonization chamber? The gaseous products generated during the coking process of the coal loaded in the carbonization chamber (including steam containing liquid products) flow, in part, through the coal layer between the plastic layers on both sides toward the upper space of the carbonization chamber; this is referred to as \"inward gas flow,\" and it accounts for approximately 10% to 25% of all gaseous products. Additionally, about 75% to 90% of the gaseous products flow toward the upper space of the carbonization chamber through the semi-coke layer and the coke layer, as well as the gaps between the coke and the walls of the carbonization chamber; this is referred to as \"external gas flow\". As they flow in the space above the coke oven, they undergo secondary pyrolysis reactions due to the high temperature. Since the temperature of the carbonization chamber walls is higher than that of the furnace roof area, the secondary thermal decomposition of the gas on the outside occurs much more intensely than that of the gas on the inside. The outer gas phase contains large amounts of hydrogen, benzene, and toluene, whereas the inner gas phase contains large amounts of methane, ethane, low-molecular-weight olefins, and oxygen- and nitrogen-containing organic compounds. The gases generated after the secondary thermal decomposition reach the space at the top of the carbonization chamber and are then discharged through the rising pipe; all of these gas products constitute crude gas. The coking process in the carbonization chamber is periodic, so the composition of the raw gas escaping from the carbonization chamber changes over time as coking progresses. However, since a coke oven consists of multiple carbonization chambers, and each chamber experiences a different coking time, the composition of the crude gas from all chambers remains essentially stable after it is collected in the gas collection pipe and mixed together. By condensing, cooling, washing, and absorbing crude gas, products such as clean gas, coal tar, and crude benzene can be obtained. 2 Product yield 2.1 Coking ratio? The coking ratio (i.e., coal-to-coke ratio) is the percentage of the charged coal (dry) that is converted into coke (dry) through high-temperature dry distillation. The coking rate mainly depends on the coal quality, and is also affected by coking conditions and furnace type. The main methods for calculating the coking rate are as follows: 2.1.1 Determine the coking rate by using the relationship between the ash content of coal and coke. ? Kd·j = Ad·m / Ad·j × 100% (1) ? In this formula, Kd·j represents the coking rate of dry coke relative to dry coal ； Ad·m and Ad·j represent the ash content on a dry basis (%) for coal and coke, respectively. The focal length calculated using equation (1) is often lower than the measured value. Apart from reasons such as sampling and analysis errors, this is mainly due to changes in the ash composition during the high-temperature carbonization process, as well as loss of mass in the coke during its stay in the carbonization chamber and during quenching, which leads to an increase in ash content. 2.1.2 Calculating the coking rate using the volatiles of the coal and coke used in furnace charging: ?Kd·j=100-Vd·m/100-Vd·j×100+Q (2) ?Where Vd·m and Vd·j represent the dry-base volatiles (%) of coal and coke, respectively ； The correction factor Q refers to the carbon increase resulting from secondary cracking after the volatiles in coal have escaped; it is related to factors such as the volatiles in the coal fed into the furnace, the structure of the coke oven, and the operating procedures of the coke oven. Typically, Q is set to 1. 2.1.3 How can the coking degree be determined using the relationship between the volatiles in coal and coke? The formula is Kd·j = 99 – 5/6 Vd·m; here, Vd·m represents the dry-base volatiles of the coal, while the dry-base volatiles of coke are assumed to be Vd·j = 1.2%. 2.1.4 How to determine the coking rate from coal quality and coking operation conditions? Kd·j=103.17-0.75Vd·m-0.0067tJ. In the formula, Vd·m represents the dry-base volatiles of the coal loaded ； tJ is the center temperature of the coke cake (°C, measured 15 minutes before coke pushing). This formula was proposed by Japan; the Kryvorizhia coking plant in the former Soviet Union found it to be suitable for its operational conditions after calibration, and China also recommends its use. 2.2 Net gas yield 2.2.1 Formula for calculating the net gas yield from dry-base coal feed in the former Soviet Union: ? Kd·g=QVd·m (4) Where Q is a coefficient; for bituminous coal, Q=3, and for coking coal, Q=3.3 ； Vd·m represents the volatile matter on a dry basis of the coal loaded. 2.2.2 Formula for calculating the yield of net gas from thermal power plants in China relative to the dry-basis coal fed in: Kd·g = (1–β)AVd·m (5) Where β = 2%–4% (wall leakage rate) ； A=64～72 is a correction factor related to the type of coal ； Vd·m is the dry-base volatiles of the coal loaded. ? The Ministry of Metallurgy recommends this formula as a basis for coking enterprises in China to calculate the quality of gas. 2.3 Yield of Chemical Products 2.3.1 Recovery Rates of Coal Tar and Crude Benzene? The Donbas coal mines in the former Soviet Union proposed that when Vdaf·m = 18%～30%, the recovery rate for coal tar is given by Kd·g = [–18.35 + 1.53Vdaf·m – 0.026(Vdaf·m)²] × (100–Ad/100) (6), while the recovery rate for crude benzene is given by Kd·b = [–1.61 + 0.144Vdaf·m – 0.0016(Vdaf·m)²] × (100–Ad/100) (7).? Anshan Iron and Steel Company’s Chemical Plant proposed that when Vdaf·m = 27.96%～30.37%, the recovery rate for coal tar is Kd·g = [–1.4 + 0.184Vdaf·m] × (100–Aa/100) (8), and the recovery rate for crude benzene is Kd·b = [–0.64 + 0.065Vdaf·m] × (100–Ad/100) (9). In these formulas, Kd·g and Kd·b represent the recovery rates of coal tar and crude benzene respectively, based on the dry basis of the coal used as feedstock; Vdaf·m refers to the volatile matter content on a combustible basis of the coal fed into the furnace. 2.3.2 Ammonia recovery rate ?Kd·A=(17/14)·bNd·m (10) ?Where Kd·A represents the recovery rate of ammonia from the coal based on its dry weight ； 17 is the molecular weight of ammonia ； 14 is the atomic weight of nitrogen ； Nd·m is the nitrogen content on a dry basis of the coal loaded into the furnace (%) ； b is the conversion factor for the total nitrogen content in coal, generally taken as 0.12 to 0.16. The remaining nitrogen is transferred to coke and other nitrogen-containing compounds. 2.4 Yield of the compound product: W = K·Od·m·(18/16)(100–Ad–St·d/100) (11) Where K = 0.437 ； W is the amount of bound water generated per unit mass of coal on a dry basis ； Od·m is the oxygen content on a dry basis of the coal loaded ； 18 is the molecular weight of water ； 16 is the atomic weight of oxygen. ?Generally, the amount of water formed through combination is taken as 2% to 3% of the amount of coal loaded. ?The amount of bound water generated can also be calculated using the following formula: ?W=〔4.64-0.354Vd+0.0118Vd2〕 (12) Where Vd represents the dry-base volatiles of the coal loaded. 3 Coke quality prediction? 3.1 Prediction of chemical composition 3.1.1 Coke ash content? Ad·m=Kd·j·Ad·j (13) Here, Ad·m and Ad·j represent the ash contents on a dry basis for the coal used in coke production and the coke itself, respectively; Kd·j is the coking efficiency (as mentioned earlier). 3.1.2 Sulfur content in coke? The formula for calculating sulfur content in French coke: St·d·j=0.084+0.759St·d·m (14) Where St·d·j and St·d·m represent the total sulfur content on a dry basis (%) for coke and the coal used in production, respectively ； 0.084 and 0.759 are the coefficients. ?Formula for calculating sulfur content in Polish coke: ?St·d·j=0.63St·d·m+0.2 (15) ?In this formula, St·d·j and St·d·m represent the total sulfur content on a dry basis (%) for coke and the coal used in feeding, respectively; 0.63 and 0.2 are coefficients. ?The formula for calculating sulfur content in coke in our country: ?St·d·m=(Kd·j/ΔS)St·d·j (16) ?In this formula, St·d·m and St·d·j represent the total sulfur content on a dry basis (%) for coal and coke respectively ； Kd·j is the total focal length (see above) ； ΔS is the mass ratio of sulfur in the converted coke during the coking process. The Kd·j/ΔS value is generally 1.0 to 1.2. The author believes that Kd·j/ΔS=0.9～1.1 is more in line with the actual conditions of coking production in Shanxi. 3.2 Coke strength prediction? Use coke strength prediction methods to select coal blending schemes in order to meet the requirements for coke strength. The factors affecting coke strength include three aspects: the properties of the raw coal, the preparation of coking coal, and the coefficients of the coking process. Since the latter two are relatively fixed, they have only a minor impact. The main factor is the caking property of coal, which determines two factors: the degree of coalification and the cohesion of the coal. The indicators reflecting coalification degree are usually the volatile matter of coal or the vitrinite reflectance. There are many indicators of coal cohesion, such as the caking index, the maximum fluidity of coal, the expansion degree of coal, the maximum thickness of the gum layer, and the caking index. There are many models used in various countries to predict coke strength. Here, only the method used in China will be introduced: this approach relies on parameters such as the dry ash-free basis volatile matter (Vdaf) of coal, as well as the caking index (G) or the maximum thickness of the gum layer (y), to establish regression equations for predicting coke strength – namely the VM-G(y) method. ?M40=126.147-2.104Vdaf+0.144G, M10=12.974+0.452Vdaf-0.0243G ； ? or M40=126.881-1.947Vdaf+0.227y, M10=9.085+0.201Vdaf-0.363y. ? The optimal range for the coal used in charging the furnace is: Vdaf=28%～32%, G=58%～72% or y=14mm～18mm. The volatiles of the coal used for firing in our province are generally low; the lower limit of the ash-free volatiles on a dry basis for the coal fed into the furnaces can reach 23%–25%. Reference: http://library.dlrtvu.edu.cn:85/~kjqk/kjqbkfyjj/kjqb99/kjqb9906/990621.htm