Coking coal blending

Let me first share my understanding of coal blending: First, coal blending requires that the parameters for coal blending be determined based on the requirements of the coke industry – for example, what the acceptable ranges are for ash content and sulfur content. The ash content in the blended coal must then be kept within these theoretical limits. Additionally, it is necessary to control the moisture content of the coal, the proportion of different particle sizes, and the density of the coal briquettes; these factors affect the density of the coal briquettes and the heating rate of the coke oven, which in turn influences the quality and yield of coke. Finally, and most importantly, it is essential to manage the bonding properties of the coal, its gel layer characteristics, its Giesler flow index, and its Oya expansion degree. These values determine, on a macro level, the bonding properties of the gel layer, the coking behavior, and the fluidity of the gel layer – all of which directly affect the filling of gaps in the coke, thereby influencing the overall appearance and strength of the coke.

Based on the experimental data, the following empirical formulas were derived: CSR = 1.15(G – Vdaf) when Vdaf > 37.5; CSR = 121G – Vdaf when Vdaf < 37.5. M40 = 171.4 – 2.65 Vdaf when 33.5 < Vdaf < 37.5; and M40 = 383.2 – 8.33 Vdaf when G ≥ 80 and Vdaf < 33.5. The derivation process is omitted

I’m not aware of the specifics regarding coal blending for coking; I’ll discuss it in the context of coal blending for gasification. Generally, we pay attention to indicators such as 1) ash content, 2) the ash fusion point of the coal (although what’s really important are its viscosity-temperature properties), 3) sulfur content, 4) fixed carbon, volatile matter, calorific value, total moisture, and internal moisture. The Coal Research Institute once published a book on coal blending for power plants – you might find it useful; it was written by Researcher Jiang Ying

B1. Significance and principles of coal blending: Coke used in blast furnaces is required to have low ash content, low sulfur content, and high strength. However, based on the coking properties of the individual coals mentioned above, it can be seen that most of these coals do not readily yield high-quality metallurgical coke with high mechanical strength in a coke oven. Some coking coals contain excessive amounts of harmful impurities such as ash and sulfur, and therefore do not meet the requirements for blast furnace smelting. Furthermore, blast furnaces, foundries, and other industries have varying requirements regarding coke quality, which cannot be met by coking with a single type of coal. Therefore, it is necessary to combine coal of different grades in order to leverage their respective advantages and disadvantages, and produce coke of various quality specifications to meet the needs of different sectors. Given the specific conditions of China’s coal resources, using blended coal for coking not only enables rational utilization of the coking coal resources in various regions of the country but is also one of the key measures to increase the supply of coal for coking. To ensure coke quality and facilitate production operations, the following principles should be considered when determining the coal blending plan: (1) The properties of the blended coals must be compatible with the plant’s coal pretreatment processes and coking conditions, so as to ensure that the coke quality meets the specified technical standards and satisfies customer requirements. (2) It conforms to the principles of regional coal blending, facilitating the expansion of coking coal resources and the full utilization of weakly bonded coal. (3) It helps increase the yield of coke oven gas and chemical products, controls the expansion pressure generated by heating of the coal charge, and prevents difficulties in coke pushing. (4) Shorten the average distance for coal delivery, facilitating vehicle allocation and preventing the phenomenon of “reverse flow”. (5) The amount of coal supplied is balanced, and the quality is uniform. (6) Reduces production costs, offering economic benefits. To determine the coal blending ratio for coking, in addition to complying with the above basic principles, coal blending tests must be conducted first. Before the test, it is necessary to determine the industrial analysis of each individual coal type, as well as relevant parameters such as the thickness of the gel layer and the G value. Then, the moisture, ash content, sulfur content, volatile matter, y value, and G value in the blended coal are calculated based on certain mixing ratios. If any issues are detected with these parameters, the mixing ratio of the blended coal is adjusted accordingly. Ensure that the coal blending ratio meets the requirements of the coal blending process specifications. Tests are conducted using coking coal proportionally blended in small coke ovens (or iron boxes). The produced coke meets the technical quality specifications, which means that this coal blending ratio is adopted as the one for coke oven production. In coke production, when it is necessary to change the coal blending ratio, it is generally done by appropriately increasing or decreasing the amounts of certain types of coal based on practical experience, or by adjusting the blending ratio according to the quantities of various coals stored in the coal yard.

2. Quality indicators of coal: Blast furnace production requires coke with low ash and sulfur content, high strength, and stable thermal properties. To ensure the quality of coke, the quality of the coal used as a blending material must meet the following standards. (1) Moisture: The amount and stability of moisture in the coal have a significant impact on coke production, quality, and the lifespan of coke ovens. The bulk density of coal is closely related to its moisture content; it is highest for dry coal. As the moisture content increases, the bulk density decreases, reaching its lowest value at 7–8% moisture content, which is around 85% of the bulk density of dry coal. Further increase in moisture content results in a slight increase in bulk density. Additionally, due to the high heat of vaporization of water and the poor thermal conductivity of coal, heat cannot be rapidly transferred to the inner layers of the coal. It is estimated that the amount of heat required to heat 1 kg of coal to 200°C is equivalent to the amount of heat needed to vaporize and heat the 10% water contained in this coal to 200°C. Therefore, before sufficient heat is supplied to completely vaporize the water, the temperature of the coal can only approach the boiling point of water. As a result, within 7–8 hours after coal is charged into the center of the carbonization chamber, its temperature remains at around 110°C. Additionally, when the moisture content in coal is high, the heating rate of the coal slows down, which reduces its caking properties. For the above reasons, for every 1% increase in the moisture content of the coal used as a blending material, the temperature in the vertical flame channel needs to be increased by 5–7°C; the heat required for coking increases by about 30 kJ/kg, and the coking time is prolonged by 10–15 minutes. Thus, when the moisture content of the charging coal fluctuates significantly during production, it becomes difficult to regulate the furnace temperature, resulting in either excessively high or low coke cake temperatures. Therefore, the operating procedures stipulate that the variation in the moisture content of the coal used in coordination between adjacent shifts should not exceed 1%. At the beginning of coal loading, due to the large temperature difference between the walls of the carbonization chamber and the coal, heat is transferred rapidly from the chamber walls to the coal, causing a sharp drop in their temperature. The higher the moisture content of the coal, the greater the drop in the temperature of the chamber walls. When the temperature of these walls drops below 600°C, it causes significant damage to the silica bricks, thereby affecting the service life of the furnace. However, if the moisture content of the coal is too low, it deteriorates the operating conditions; smoking and ignition occur more frequently during coal loading, the amount of tar residues increases, and graphite deposition on the walls of the carbonization chamber accelerates. Therefore, it is advisable to keep the moisture content of the coal fed into the furnace at around 10%. (2) Ash: The ash content of coke comes from the coal blend, therefore strict control over the ash content of the coal blend is necessary. The ash content of the blended coal is entirely transferred to the coke; generally, the overall coke yield in coking is 70–80%. Therefore, the ash content in coke is approximately 1.3 to 1.4 times that of the blended coal. Coal with high ash content results in coke of low strength. In blast furnace processing, coke with high ash content experiences further crack propagation under thermal effects, leading to its pulverization and thereby affecting the permeability of the blast furnace ； On the other hand, at high temperatures, the structural strength of coke decreases and its thermal strength is poor, leading to further degradation of the coke in the blast furnace. The higher the ash content of coke, the more coke and limestone are required in iron production, resulting in a reduced production capacity of the blast furnace. Generally, for every 1% increase in coke ash content, the amount of coke used in blast furnaces increases by 2–2.5%, the amount of limestone used rises by 4%, and pig iron production decreases by 2.2–3.0%. Meanwhile, the large particles in the ash serve as crack initiation points in the coke, thereby reducing its crush strength and worsening its abrasion resistance. Therefore, the ash content of the blending coal should be kept within a certain range. The ash content of the blended coal is calculated as a weighted average based on the ash content of each individual coal type. If one or two types of high-ash refined coal are used in the blending coal, the remaining components must be single types of coal with lower ash content in order to ensure that the ash content of the coke meets the required standards. The ash content of coaling in our country is generally high, at around 11%, with values exceeding 14% in some cases; as a result, the ash content in coke is also high, typically above 13%. The reason for the high ash content in the coal used in our country is the high ash content in the washed coking coal; in particular, coking coal and fat coal have high ash levels and are difficult to wash. High-volatility, low-cohesion coal not only has large reserves, but also has a low ash content and is easy to wash. Therefore, use more coal with high volatile matter and low ash content. Using an appropriate amount of low-ash coal, along with preheated coal for coking or the use of blended coal and rammed coking, is an effective way to reduce the ash content in the blended coal. To stabilize coke quality, adjacent teams should cooperate to keep coal ash content fluctuations within 0.5%. (3) Sulfur content: Sulfur in coal exists in three forms: pyrite, sulfates, and organic sulfur compounds. Sulfur is also a harmful impurity; the sulfur in coke is transferred to iron during refining, reducing the quality of the iron. About 60–70% of the sulfur in coal is transferred to coke. Therefore, the sulfur content in coke is about 80–90% of that in the blended coal. Based on this, the upper limit for the sulfur content in the blending coal can be calculated. The sulfur content of blended coal can be calculated based on the additive properties of individual coals. The coal in the northeastern and northern regions of our country has a low sulfur content. The coal in central-southern and southwestern regions has a higher sulfur content. During the coal blending process, the sulfur content of the blended coal is adjusted by controlling the blending ratio. The sulfur content in coke is generally required to be less than 0.6–1.0%. A high sulfur content in coke will increase the sulfur content in pig iron, thereby reducing its quality ； Additionally, increase slag basicity ； In particular, large fluctuations in the sulfur content of coke have a significant impact on the operating parameters of blast furnaces. For every 0.1% increase in coke sulfur content, the coke ratio for iron production rises by 1.2–2.0%, and pig iron production decreases by more than 2%. When coke is used as a fuel, sulfur burns to produce sulfide gases, which corrode metals. When coke is used for gasification, the resulting syngas must have its sulfides removed before it can be used. Therefore, the sulfur content in the blending coal should be strictly controlled. It is generally required that the sulfur content in the coal blend should not exceed 1%. To reduce the sulfur content in blended coal, desulfurization technologies such as washing and extraction can be employed. However, no economically viable industrial methods have been developed. Based on the current practical situation, using coal varieties with lower sulfur content in the blend is one of the common approaches to reduce the sulfur content of the blended coal. (4) Volatiles: The level of volatiles in the blended coal determines the yield of coal gas and chemical products; it also affects the strength of coke. Production practices at Anshan Iron and Steel Works show that: for every 1% increase in the volatile matter Vdaf of the blended coal, the coke crush strength index M40 decreases by 2%, while the coke abrasion resistance index increases by 0.2%. This is due to the high volatile matter content, which results in a large shrinkage coefficient for the coking coal feedstock, a small average particle size of the coke, and low crush strength. For coking coal used in large blast furnaces, it is appropriate to keep the volatility index of the conventional coking blend at 26–28%. On the premise of ensuring coke quality, as much high-volatile, weakly caking coal as possible should be blended. Taking into account regional resources and customer requirements, the volatile matter content in the blended coal can be controlled within the range of 28–32%. The volatile matter content of the coal blend used for medium and small blast furnaces can be higher. In addition, it is necessary to consider comprehensively in conjunction with the appropriate range of adhesion indicators. The volatile matter content of blended coal can be approximately calculated using additivity, or it can be determined directly. (5) Cohesiveness Cohesiveness is the property of coal that enables it to form a plastic mass during coking. The amount of the liquid phase formed in the plastic mass upon coal heating determines the quality of coal cohesion. To obtain coke with good melting properties and wear resistance, the coal blend must have sufficient cohesion. The cohesiveness of coal is additive. The cohesion index of coal, in the international coal classification, refers to the Roga index and the method of observing the shape of the coke in a standard crucible. Our country adopts the method based on the maximum thickness value y of the gel layer and the bonding index G value. Expansive pressure is a characteristic of coking coal with adhesiveness; non-coking coal does not have expansive pressure. Expansive pressure can enable colloids to be evenly distributed among coal particles, thereby aiding in the bonding of the coal material. Excessive expansion pressure can cause damage to the furnace wall. The safe expansion pressure should be less than 10–15 kPa. Expansion pressure and the y-value can be used as quality indicators for colloids; coal with good caking properties has an expansion pressure of 8–15 kPa and a y-value of 16–18 mm. The Y value for coal blending in production is generally 16–20 mm, while the caking index G value ranges from 58 to 72. Both the y-value and G-value of blended coal can be approximately calculated using additivity, or they can be determined directly. The expansion pressure of blended coal cannot be calculated by adding the expansion pressures of individual coals; therefore, it can only be determined through experiments. When determining the coal blending plan, two points can be used as references: first, within the range of conventional coking coal blending, when the degree of coal metamorphism is high, the swelling pressure is also high ； Second, increasing the bulk density raises the expansion pressure. (6) Fineness: Fineness refers to the percentage by mass of coal particles smaller than 3 mm in the total mass of coal after it has been crushed. At present, in most coking plants in China, the fineness of coking coal is generally controlled at around 80% for top-loading coke ovens, and at around 90% for ramming coke ovens. The fineness variation between adjacent batches should not exceed 1%. If the fineness is too low, the coal particles will not mix evenly, which inevitably leads to an uneven internal structure of the coke and reduces its strength. However, too high a fineness not only increases the power consumption of the crusher and reduces its production capacity, but also makes loading operations more difficult. The increase in coal dust accelerates the clogging of the rising pipes, and the amount of tar residue increases, resulting in substandard quality of the tar ； More importantly, the extremely high fineness causes the cohesive coal to experience a loss of cohesion, thereby reducing the quality of coke. When the fineness of the coal is too high, it reduces the bulk density of the coal loaded into the furnace, which in turn affects the quality of the coke. As the fineness increases from 80% to 90%, the bulk density shows a downward trend. The quality of coke is influenced not only by the fineness of the coal feed but also by its particle size distribution. However, coal is a very complex organic mineral, and its particle size distribution is related to its bulk density. Therefore, the optimal grain size composition is necessary to ensure the uniformity of the coke structure.

1B3. Coal preparation process: Under normal circumstances, the individual types of coal brought into the plant do not meet the quality standards required for coking coal in modern coke ovens. Therefore, it is necessary to combine these different types of coal based on their respective properties, grind them, and mix them together to produce coking coal that meets the required standards. The task of the coal preparation plant is to process the various types of coal brought into the plant into coal materials that meet the quality requirements for coking. A coal preparation workshop generally consists of a raw coal receiving unit, a coal storage yard, a coal blending room, a crusher room, the top level of the coal tower, a distribution device, belt conveyor corridors, and transfer stations. In northern regions, thawing rooms and crusher chambers are sometimes also required. In addition, there are workshop auxiliary facilities such as coal sampling rooms. The purpose of selecting a coal blending process is to expand the sources of coking coal and improve the quality of coke. The difference between various processes lies mainly in the method of crushing the coal material. Coal preparation processes can be classified into the following types: (1) The process of blending coal first and then crushing it. In this process, the individual coals that make up the coking coal are first blended in specified proportions, and then crushed. This process features simplicity, fewer pieces of equipment, and ease of operation; it is suitable for situations where the coal has good caking properties and relatively uniform quality. Most coking plants in China adopt this process. This process cannot control different grinding particle sizes for various coal types, and it is not suitable when the coal quality is poor or the lithological composition is uneven. As shown in Figure 2-3. (2) Process flow of crushing first and then blending coals: In this process, the individual coals that make up the coking coal mixture are first crushed to different fineness levels depending on their properties, then blended in specified proportions, and finally mixed together; hence this process is also known as the separate crushing process. This process has a long duration and is complex; it requires multiple crushers, as well as a mixing device after coal blending, which results in high investment costs and complicated operation. Due to the varying caking and crushability of each type of coal, this process allows for the control of the degree of crushing according to the properties of each coal type, ensuring an optimal particle size range for the coal material and thus helping to improve the quality of the coke. To simplify this process, it is possible to adopt a method in which only a portion of each type of coal is crushed separately and then mixed with other coals for further crushing. Coal types that are generally subjected to pre-crushing have poor crushability. Such as gas coal, so pre-crushing is usually only performed on gas coal. This can improve the particle size distribution of the coal feed; selecting an appropriate pre-crushing fineness and blend coal fineness for different coal blending ratios helps to enhance the quality of coke. Experiments have shown that after pre-grinding bituminous coal for coking, the crush resistance index M40 of the coke increases by more than 2%, while the wear resistance index M10 decreases by more than 0.5%. Figure 2-4 shows a schematic diagram of this process. (3) Select the crushing process. Crushing and screening are combined based on the differences in hardness resulting from the various coal types and lithological compositions involved in coal blending for coking, as well as the desired particle size after crushing. The coal material passes through a screening device, with the larger particles remaining on the screen and being sent to a crusher for further crushing. This eliminates large particles while also preventing the over-fining of coals with strong agglutinative properties, thereby improving the coking process. This process also includes taking a single component, such as anthracite or coking coal dust, crushing it first, then adding it to the coal mixture, and subsequently mixing and crushing everything together. Figure 2-5 shows the production process for adding coke powder to coal material, while Figure 2-6 illustrates the production process for manufacturing cast coke by adding anthracite powder to coal material.

4. Receipt of raw coal: The main raw material for coking plants is coal. Raw coal is generally transported to the plant via rail, road, or water, and is unloaded using various coal-unloading equipment before being conveyed by belt conveyors to the coal storage area or blending room. If the coal washing plant and the coking plant are located close to each other, the washed clean coal from the coal washing plant can be transported directly to the coking plant using a belt conveyor. The main receiving devices for coal delivered by train are the car dumpers and the train coal receiving pits ； The main receiving device for coal delivered by road is the truck coal receiving pit ； Coal delivered by water is unloaded using a ship unloader; after being unloaded, the coal is sent to either a coal storage yard or a coal blending trough using three types of receiving devices. In addition, bridge cranes are also commonly used as loading and unloading equipment; they are generally installed in coal storage areas, serving both as unloading equipment and as storage and transportation facilities. Large coking plants generally use tipplers as the main equipment, supplemented by screw unloaders and coal receiving pits as the coal reception systems, while small and medium-sized coking plants use screw unloaders in combination with coal receiving pits as their coal reception systems. To ensure normal operation of the coke oven and maintain coke quality, it is necessary to keep the quality of the coal used for coking stable. However, due to the complex structural composition and physicochemical properties of coal itself, even coal with the same degree of metamorphism, or even coal from the same mine, does not have identical properties, and its quality can vary. Therefore, it is a very important task to deliver the large quantities of washed coking coal sent from various locations to designated storage sites in accordance with quality and quantity requirements. To this end, strict acceptance procedures must be followed for the coal received: (1) Coking plants must have a rigorous acceptance system for the coal supplied. Therefore, coking plants need to establish a strict acceptance system for the coal in order to ensure stability in its quality. The specific approach is as follows: First, strengthen the management of coal yards, with dedicated personnel in charge of them. The responsibility of the management staff is to verify the type of coal arriving, and to unload, transport, and store the raw coal or refined coal that enters the plant according to their respective types. The coal storage areas should be designated and numbered, and a “coal arrival register” should be established to specifically record the grade of the coal, its origin, the date of arrival, the quantity, the quality after washing and sorting, as well as the location where the coal is stored. If necessary, place wooden signs on the coal piles to avoid confusion. Furthermore, due to the constant changes in the coal yard, it is necessary to establish a proper shift-handover system. The situation regarding the incoming coal should be clarified in person during the handover. Coal arrival managers should directly contact the foremen, weighers, and lab technicians to keep track of the quantity and quality of the coal. (2) The coking plant should take samples of the incoming coal for analysis. In order to check whether the incoming coal meets the technical standards, samples from each batch of coal should be taken and analyzed in accordance with the relevant procedures. The analysis data of the incoming coal in the coking plant laboratory should be compared with the analysis reports from the sending party; if the difference exceeds the allowable limit, comments should be submitted. Coal that does not meet technical standards should be stored separately, and oxidized coal should not be accepted or used. (3) The unloading and storage areas for various types of coal must be kept clean; if the location is changed, that area must be thoroughly cleaned to prevent mixing of coals of different grades. By following the above points and accepting coal from other regions in an orderly manner, it is possible to maintain relative stability in the properties of various types of coal; this is the first step toward ensuring stable coal blending quality.

5. Storage of raw coal (1) Coal storage yard: To ensure continuous and balanced production in the coke ovens as well as stable coke quality, the coal preparation workshop needs to have a coal storage yard with sufficient capacity (in some large steel complexes, the coking plant does not have its own separate coal storage yard; instead, the raw coal is stored in a general material yard). After being unloaded, washed coking coal is transported to coal storage areas, which come in two types: indoor coal yards and outdoor coal yards. There are various types of indoor coal yards; one type consists of several rows of circular or rectangular storage tanks, with discharge openings at the bottom of the tanks and railways above them. The incoming coal is directly unloaded from trains into a trough. The trough is covered with iron grates, which allow large pieces of coal to remain on top of them; they fall down only after being crushed, thus preventing blockages at the discharge outlet. There is a belt conveyor beneath the trough; coal is discharged onto the belt conveyor by a coal unloading machine located below the discharge opening. Such coal yards have a compact layout, which protects the coal from wind and rain, but they require large capital investment, have limited capacity, and do not allow for homogenization of the coal; as a result, they are used less frequently. Another type is a mechanized indoor coal yard, which consists of adding a dry coal shed on top of an outdoor coal yard. Coal can also be protected from wind and rain; its moisture content is easy to control, and homogenization processes can be carried out to reduce losses of coal. This is beneficial for coal blending, coke oven heating, and the quality of coke. Although the initial investment is high, there are certain operating costs involved, and it also has an impact on the capacity of coal storage areas. However, due to its outstanding advantages, this method is preferred in regions with heavy rainfall in the south or where strict requirements are placed on coal moisture content (such as in compacting coking), especially when sufficient funds are available. Currently, some large enterprises still use a coal storage method that involves giant coal storage tanks (with each tank having a capacity of 5,000 to 10,000 tons), allowing for coal to be stored at the top and distributed at the bottom. It requires little space and can prevent coal dust pollution as well as coal loss. However, the investment cost is relatively high, so it has not been widely adopted yet. The coal storage yards in ordinary factories are mechanized open-air coal yards. After the raw coal is unloaded in the coal storage yard, the quality of coal yard management has a direct impact on the quality of coke. Therefore, the following requirements apply to coal yard management: 1) The coal storage yard must have sufficient capacity to ensure an adequate reserve, enabling continuous and steady production in the coke ovens and maintaining consistent coke quality. The capacity of a coal storage yard is influenced by various factors, such as the distance from the coal source, the production status of coal mines, and transportation conditions. When the coal source is close, coal mining operations are stable, and transportation conditions are good, it is possible to keep a slightly smaller stock. Generally, large and medium-sized coking plants in China should maintain a reserve of 10 to 15 days, while smaller coking plants should have an even larger reserve. When the actual reserves approach the allowable minimum level, emergency measures must be taken to avoid affecting production. The capacity determined according to the above standards is referred to as the coal storage yard operating capacity, that is, the amount of coal that the storage yard can hold under normal operating conditions. Taking into account the needs such as the segmentation of various coal piles, the operation and maintenance of yard machinery, as well as the cleaning and leveling of the coal yard, the operational capacity of the coal yard is generally 60–70% of its total capacity, meaning the operating coefficient is 0.6–0.7. The length of the coal storage yard should allow for various possibilities regarding stacking, retrieval, and storage; that is, there should be at least three stacks for each type of coal. When circumstances restrict this, there must still be at least two stacks so that storage and retrieval can be separated. To provide space for the maintenance of coal yard machinery, the total length of the coal yard is about 10 meters longer than the effective length of the coal pile. During storage in the coal yard, further dehydration of the coal helps to reduce the moisture content of the blended coal. Therefore, if the coal yard is located close to the coal washing plant, it should have sufficient storage capacity to help lower the moisture content of the blended coal. 2) Ensure that different coal types are stored separately. For the same type of coal, in order to eliminate and reduce the quality variations associated with coal sourced from different mines or coal seams, quality homogenization should be carried out during storage in the coal yard. Typically, the “layering and spreading” method is used; that is, when storing coal, it is spread out in layers across the entire area designated for that type of coal ； When taking coal, extract it directly from top to bottom along one side of the coal pile. 3) Dust prevention and drainage. When there is strong wind in the area where the coking plant is located, to prevent excessive dust from polluting the atmosphere and to minimize coal loss, water spraying facilities should be used during coal unloading; coal piles should also be sprayed with water or covered with a covering agent. Some enterprises are experimenting with using dust nets and fences to prevent strong winds and dust from spreading ； The coal yard should also have a rainproof drainage system to remove water accumulation there promptly, and the wastewater from this drainage system should be treated to prevent environmental pollution. (2) Oxidation of coal: During storage, coal is inevitably in contact with air, which contains oxygen; this is the external factor that causes coal to oxidize. Coal itself undergoes a chemical reaction when in contact with oxygen, which is the fundamental cause of coal oxidation. When oxidized to a certain extent, spontaneous combustion may occur; spontaneous combustion is an inevitable consequence of the increasing degree of coal oxidation under certain conditions. Coal with a low degree of metamorphism has a high porosity and absorbs more oxygen, so it oxidizes rapidly. The lower the degree of metamorphism, the longer the storage time, and the higher the temperatures of the atmosphere and the coal pile, the faster the oxidation occurs. Coal newly extracted from the mine oxidizes the fastest. Take lignite as an example: it has a low degree of metamorphism. When freshly mined lignite is stored for half a month, many cracks appear in the lumps; with a little force, they can be broken into pieces. After coal is oxidized, it has a significant impact on its properties, mainly in the following aspects: 1) It deteriorates the caking property of the coal. After oxidation, coal with a uniform lithotype composition shows a reduction in the thickness of the plastic layer; whereas for coal with a non-uniform lithotype composition, the thickness of the plastic layer decreases only slightly after oxidation. However, the drum indices and properties of the resulting coke deteriorate. Therefore, the effect of oxidation on coal coking behavior can be determined using the thickness of the gel layer, the drum index, and fusibility. 2) It lowers the ignition point of coal. The deeper the oxidation, the more the ignition point decreases; therefore, the degree of oxidation of coal can be determined by the reduction in its ignition point. 3) It reduces the volatiles, carbon, hydrogen, and calorific value of coal, while increasing oxygen. Spontaneous combustion occurs as a result of coal oxidizing to a certain extent; therefore, reducing the oxidation of coal can prevent its spontaneous combustion. There are many measures to prevent oxidation, such as controlling the storage period of coal ； Adhere to the principle of using coal that arrives first, and reserve those that arrive later for later use, in order to avoid excessive storage time ； Regularly check the temperature of coal piles; when the temperature exceeds the specified limit, appropriate measures should be taken ； Avoid air circulation within the coal pile ； Remove residual coal in the coal yard, etc. Generally speaking, as long as one operates in accordance with the coal storage principles outlined earlier, it is possible to reduce coal oxidation, thereby ensuring that the caking property of the coal does not deteriorate due to severe oxidation. 3) Storage period of raw coal: The storage period for coal used in coking varies depending on the degree of coal transformation, as well as on the type of stacking method employed. Coal with a higher degree of metamorphism can be stored for a longer period than coal with a lower degree of metamorphism ； The storage period in winter is longer than in summer ； The storage periods for compacted and uncompacted materials also differ ； The South is also different from the North. (3) Storage period of raw coal: The storage period for coal used in coking varies depending on the degree of coal transformation, as well as on the type of stacking method employed. Coal with a higher degree of metamorphism can be stored for a longer period than coal with a lower degree of metamorphism ； The storage period in winter is longer than in summer ； The storage periods for compacted and uncompacted materials also differ ； The South is also different from the North. Table 2-3 Storage Period for Various Types of Coal Grade Coal-piling Season Coal-piling Type Storage Period/d Bituminous coal Summer Uncompacted 25~30 Summer Compacted 35 Winter Open-air 45 Gas coal Summer Open-air 30 Rich coal Summer Open-air 60 Winter Open-air 90 Coking coal Summer Uncompacted 60 Summer Compacted 60 Winter Compacted 90 Lean coal Summer Open-air 90 Winter Open-air 150

Winter, outdoor: 150. 6. Thawing and crushing of coal (1) Thawing of coal: In the northern regions of China, where winters are cold, bituminous coal with high moisture content often freezes during transportation, making unloading difficult. This not only requires more labor but also prolongs the unloading time, leading to vehicle backlogs and affecting the turnover rate of vehicles. To this end, a thawing warehouse needs to be built at the coking plant, where the frozen trucks are thawed before unloading. There are two common types of thawing chambers: one is the infrared thawing chamber, and the other is the hot air thawing chamber. 1) Infrared thawing chamber: The working principle of a gas-infrared thawing chamber is to use the infrared rays generated by gas infrared radiators as a heat source; it is a device that thaws materials through thermal radiation. Once the infrared rays emitted by this device are absorbed by the vehicle body, the heat spreads to the interior of the vehicle via its sides and bottom, thereby thawing the frozen coal layer. 2) Hot-air thawing warehouse: The working principle of a hot-air thawing warehouse is to mix the hot exhaust gas at 600–800°C generated by a gas combustion furnace with some cold air as well as exhaust gas circulated back within the warehouse, thereby reducing the temperature of the gas to 180–200°C. This gas is then sent into the enclosed thawing warehouse using a blower, and heat transfer by convection is utilized to thaw the frozen coal in the vehicles inside the warehouse. (2) Crusher chamber: The crusher is mainly used to break frozen lumps or large pieces of raw coal. Its location is generally between the coal storage yard and the coal blending room. Coking plants in northern regions experience cold winters, and generally have a crusher room, while those in southern regions usually do not. The main equipment in the crusher chamber is a double-roll crusher. During the frozen coal season, the coal containing ice crystals, which is transported from the coal storage area, passes through an iron removal device before entering the crusher; this device removes any foreign iron mixed in with the coal. After that, it goes through a grate screen on the crusher with an inclination of 45°, which filters out coal particles smaller than 80 mm, allowing them to flow directly onto the conveyor belt below without passing through the crusher, thereby reducing the load on the crusher. Frozen or lump coal with a particle size larger than 80 mm is fed into a crusher for crushing; the coal resulting from this crushing, along with the coal that passes through the sieve, is collected together and transported away via a belt conveyor. During the season when there is no frozen coal, the grating screen can be turned over, and the coal flows directly onto the lower belt conveyor without being screened or crushed.

7. Blending of raw coal The blending of raw coal is what is commonly referred to as coal blending; it involves combining coal of different grades in various proportions to produce coal that meets the required standards. Our country has abundant coal resources, and the quality of coal varies from region to region, with differences in ash and sulfur content as well. Therefore, it is necessary to take into account the characteristics of local coal sources when blending coal for coking, so as to make effective use of nearby coal resources, reduce the burden on railway transportation, and lower production costs. Coking plants, regardless of their size, should use coal blending for coking. The quality parameters of the blending coal (ash content, sulfur content, volatile matter, gum layer thickness, moisture, etc.) must meet the user’s requirements regarding the quality of coke. For the coal blending used in the production of metallurgical coke, the quality parameters of the blended coal and the corresponding quality parameters of the coke can be approximated using the following method. (1) Ash content: The ash content of the blended coal depends on the ash content of each individual coal type and their proportions in the blend. Once the coal blending ratio is determined, the ash content of that ratio can be calculated using the additive principle. The ash content of the blended coal is equal to the sum of the products of the ash content (on a dry basis) of each individual coal type and its weight percentage in the blend. For example, if coal types including gas coal, fat coal, coking coal, and lean coal are blended in the following proportions: 40% gas coal, 25% fat coal, 20% coking coal, and 15% lean coal, then… ； The ash content of various coals is as follows: bituminous coal 5.4%, semi-bituminous coal 9.5%, coking coal 12.8%, lean coal 11.2% ； The ash content Ad of the blended coal is calculated using the following formula: Ad = (5.4×40 + 9.5×25 + 12.8×20 + 11.2×15) ÷ 100 = 8.77%. The ash content criterion for the blended coal is determined based on the ash content criteria specified for coke. The ash content of the blended coal = ash content of coke × total coking ratio. For example, if the required ash content for coke is below 12.5% (the specified value), and the total coking ratio is 74%, then the ash content of the blended coal is: Ad = 12.5 × 74% = 9.25%. In other words, the ash content of the blended coal must be below 9.25%. Grade 1 metallurgical coke requires that the ash content of the coal blend be ≤9 %. (2) Sulfur content: The sulfur content in blended coal depends on the sulfur content of each individual coal type and their proportions in the blend. The calculation is also based on the principle of additivity; that is, the sulfur content of the blended coal equals the sum of the products of the sulfur content of each coal type and its weight percentage in the blend. Grade 1 metallurgical coke requires a sulfur content in the coal blend of ≤0.66 %. (3) Volatiles: The volatiles content of the coal blend has a direct impact on the final shrinkage amount of coke, its degree of cracking, as well as the yield and quality of chemical products. Generally, the volatiles content Vr of the coal blend is kept below 32% in order to obtain coke with larger particle sizes and higher strength. The specific indicators should be determined based on the specific requirements of the coal source and the using department. The volatiles of blended coal depend on the volatiles of individual coals and their proportions; it is also calculated using the additive principle, that is, the volatiles of blended coal equal the sum of the products of the volatiles of each individual coal and its weight percentage in the blend. Grade 1 metallurgical coke requires a volatile matter content in the coal blend of ≤26%. (4) Gel layer: The gel layer index of coal blending is an important indicator of its coking quality. The thickness of the coal blending gel layer is generally required to be greater than 14 mm; it is also calculated based on the principle of additivity, but the final shrinkage amount must be determined experimentally and does not follow additivity. (5) Moisture: The moisture content of the coal blend is generally required to be between 7% and 10%, and it should remain stable. For production, high moisture content will prolong coking time, reduce yield, and increase heat consumption. For each 1% increase in the moisture content of the coal blend, the coking time must be extended by 20 minutes; therefore, the moisture level should not be too high. The moisture content of the coal blend used in production should be kept as stable as possible to avoid affecting the stability of the coke oven heating regime. Secondly, too high a moisture content in the coal results in an increased amount of phenol produced. In addition, there are also certain requirements regarding the phosphorus content in the coal blend, as coke with high phosphorus levels increases the cold brittleness of pig iron. In production, the phosphorus content in the coal blend is required to be below 0.05%. Since all coal types in China are low-phosphorus coals, it is generally not necessary to measure the phosphorus content of the coal in normal production processes.