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Basic Knowledge of the Industrial Applications of Coal I. Chemical Composition of Coal The chemical composition of coal is complex, but it can be divided into two main categories: organic matter and inorganic matter, with organic matter being the dominant component. The organic matter in coal is primarily composed of five elements: carbon, hydrogen, oxygen, nitrogen, and organic sulfur. Among them, carbon, hydrogen, and oxygen account for over 95% of the organic matter. In addition, there are very small amounts of phosphorus and other elements. The elemental composition of the organic matter in coal changes regularly as the degree of coalification increases. Generally speaking, the higher the degree of coalification, the higher the carbon content, the lower the hydrogen and oxygen contents, and the nitrogen content also decreases slightly. The sulfur content, on the other hand, is related to the genetic type of the coal. Carbon and hydrogen are the key elements that generate heat during the combustion of coal, while oxygen is an element that facilitates combustion; together, these three constitute the main components of organic matter. When coal burns, nitrogen does not produce heat and is usually released in a free state. However, under high-temperature conditions, a portion of the nitrogen is converted into ammonia and other nitrogen-containing compounds, which can be recovered to produce ammonium sulfate, urea, and other nitrogen fertilizers. Sulfur, phosphorus, fluorine, chlorine, arsenic, etc. are harmful elements in coal. When coal with a high sulfur content is burned, sulfide gases are produced. These gases not only corrode metal equipment; they also react with water in the air to form acid rain, which pollutes the environment and harms plant growth. Furthermore, when such sulfur- and phosphorus-rich coal is used for metallurgical coking, most of the sulfur and phosphorus contained in the coal transfer into the coke, and during smelting, they further transfer into the steel. This significantly affects the quality of both coke and steel, thereby hindering their casting and machining processes. When coal containing fluorine and chlorine is burned or coked, various pipes and furnace walls suffer severe corrosion. Using arsenic-containing coal as fuel in the brewing and food industries, with excessive levels of arsenic, can increase the toxicity of the products and pose a threat to people’s health. The inorganic substances in coal are mainly water and minerals; their presence reduces the quality and utility value of coal, with the vast majority of them being harmful components in coal. In addition, there are some rare, dispersed, and radioactive elements such as germanium, gallium, indium, thorium, vanadium, titanium, uranium, etc., which exist in coal in the form of organic or inorganic compounds. The content of certain elements, once it reaches industrial grade or allows for comprehensive utilization, makes them important mineral resources. Elemental analysis can reveal the chemical composition and contents of coal, while industrial analysis allows for an initial understanding of its properties, enabling a rough determination of its type and uses. The industrial analysis of coal includes four components: the determination of moisture, ash, and volatile matter, as well as the calculation of fixed carbon. 1. Moisture refers to the amount of water contained in a unit weight of coal. The moisture in coal exists in three forms: external moisture, internal moisture, and crystalline water. The inherent moisture of coal is generally used as an indicator to assess its quality. The lower the degree of coalification, the greater the internal surface area of the coal, and the higher its moisture content. Moisture is a harmful substance in the processing and utilization of coal. During coal storage, it can accelerate weathering and cracking, and even lead to spontaneous combustion ； During transportation, it increases the volume of goods to be moved, wastes transport capacity, and raises shipping costs ； During coking, heat is consumed, the furnace temperature drops, the coking time lengthens, and production efficiency decreases ； When burning, it reduces the effective calorific value ； In the winters of cold regions, it can also cause coal to freeze, leading to difficulties in loading and unloading. Only when compressing coal bricks and coal balls is an appropriate amount of moisture required to give them shape. 2. Ash refers to the solid residue remaining after coal has been completely burned under specified conditions. It originates from the oxidation and decomposition of minerals in coal. Ash is extremely detrimental to the processing and utilization of coal. The higher the ash content, the lower the thermal efficiency ； During combustion, the melted ash also forms slag inside the furnace, which affects the gasification and combustion of coal and simultaneously makes slag removal difficult ； During coking, everything is converted into coke, which reduces the strength of the coke and severely affects its quality. The composition of coal ash is very complex, and different compositions directly affect the melting point of the ash. Coal with a low ash fusion temperature causes many difficulties in production operations during combustion and gasification. To this end, when evaluating the industrial uses of coal, it is necessary to analyze the ash composition and determine the ash fusion point. 3. Volatiles refer to the flammable gases produced by the thermal decomposition of organic substances in coal. It is the main indicator for classifying coal and is used to preliminarily determine its properties for processing and utilization. The volatile matter yield of coal is closely related to the degree of coalification; the lower the degree of coalification, the higher the volatiles, while as the degree of coalification increases, the volatiles gradually decrease. 4. Fixed carbon: When measuring the volatiles in coal, the remaining non-volatile substances are called cinders. Fixed carbon is defined as the residue after ash is removed. It is the non-volatile solid combustible material in coal, and can be calculated using mathematical methods. The appearance of cokery slag is closely related to the properties of the organic matter in coal; therefore, based on the visual characteristics of the cokery slag, it is possible to qualitatively assess the caking property and industrial applications of the coal. II. Process properties of coal: To enhance the comprehensive utilization value of coal, it is necessary to understand and study its process properties in order to meet the various requirements regarding coal quality. The technological properties of coal mainly include: caking and agglomeration properties, calorific value, chemical reactivity, thermal stability, light transmittance, mechanical strength, and selectivity. 1. Cohesiveness and coking property. Cohesiveness refers to the ability of coal, during the carbonization process, for its organic components to decompose and melt, thereby enabling the coal particles to stick together and form clumps. Coking property refers to the ability of coal to form coke during dry distillation. The caking property of coal is a necessary condition for its coking quality; coal with good coking properties must have good cohesion, but coal with good cohesion does not necessarily yield high-quality coke on its own. This is why coal blending for coking is necessary. Cohesion is the main indicator for the industrial classification of coal; it is generally expressed by the thickness of the colloid formed as a result of the thermal decomposition and softening of the organic matter in coal, and is often referred to as the thickness of the gum layer. The thicker the gel layer, the better the adhesiveness. There are many methods for determining adhesiveness and coking properties. Besides the gel layer determination method, there are also the Rugai index method, the Oya swelling test, and so on. Adhesiveness is influenced by various factors such as the degree of coalification, coal-rock composition, degree of oxidation, and mineral content. Coals with the highest and lowest degrees of coalification generally lack cohesion, and the thickness of their vitrinite layer is also very small. 2. Calorific value refers to the amount of heat generated when a unit weight of coal is completely burned; it is also known as heat value. It is commonly expressed in units of 10^6 J/kg. It is an important indicator for evaluating the quality of coal, especially thermal coal. In the international market, thermal coal is priced based on its calorific value. Since June 1985, our country has reformed the pricing system, shifting from one based on ash content to one based on calorific value, which had been in use for decades. The calorific value is mainly related to the content of combustible elements in coal and the degree of coalification. To facilitate the comparison of coal consumption, in industrial production, the actual amount of coal used is often converted into standard coal with a calorific value of 2.930368×107 J/kg for calculation purposes. 3. Chemical reactivity, also known as activity. It refers to the reactivity of coal when it interacts with carbon dioxide, oxygen, and water vapor at a certain temperature. It is an important indicator for evaluating coal used for gasification and coal used for power generation. The degree of reactivity directly affects the coal consumption and the effective components of the gas. The activity of coal generally decreases as the degree of coalification increases. 4. Thermal stability, also known as heat resistance. It refers to the property of coal to maintain its original particle size under high temperatures. It is another important indicator for evaluating coal used for gasification and coal used for power generation. The quality of thermal stability directly affects whether normal production can take place in the furnace, as well as the gasification and combustion efficiency of coal. 5. Transmittance refers to the ability of coals with a low degree of coalification (such as lignite and subbituminous coal) to allow light to pass through them; it is the transmittance of the solution obtained after treating these coals with a mixture of nitric acid and phosphoric acid under specified conditions. As the degree of coalification increases, the light transmittance gradually rises. Therefore, it is an important indicator for distinguishing lignite, bituminous coal, and gas coal. 6. Mechanical strength refers to the ease with which lump coal breaks under external forces. When coal with low mechanical strength is fed into the gasifier, it tends to break into small pieces and powder, affecting the normal operation of the gasifier. Therefore, coal used for gasification must have high mechanical strength. 7. Selectivity refers to the ease with which coal can be cleaned by removing impurities and minerals from it. China’s current coal washing methods are detailed in Section 4. III. Industrial classification of coal: In 1958, a classification system focused on coking coal was introduced, which created favorable conditions for industrial sectors to make rational use of coal resources; however, some problems arose in practice. On the basis of careful analysis, research, and adoption of advanced foreign classification methods, and in order to ensure that the technical and economic indicators used for classification best reflect the quality characteristics of coal and thus facilitate a more rational utilization of coal resources, comprehensive technical classification standards ranging from lignite to anthracite were **reissued in 1986**. Coal in nature was divided into 14 major categories, with lignite and anthracite further subdivided into 2 and 3 subcategories respectively (Table 2.2.1). This is our country’s current **standard for coal classification: the Chinese Standard for Coal Classification (GB5751-86)**. (1) Classification index and its symbol Vr represent volatile matter on a dry, ash-free basis (%) ； Hr is the hydrogen content on a dry, ash-free basis (%) ； GR.I (abbreviated as G) is the caking index of bituminous coal ； Y is the maximum thickness of the vitrinite layer in bituminous coal ； PM is the light transmittance (%) of the coal sample ； b is the Oya swelling degree (%) of bituminous coal ； Q-A.GNGW is the constant-humidity, ash-free higher heating value of coal (MJ/kg). (2) Coding of coals: Various types of coal are represented by two Arabic digits. The first digit indicates the volatiles content of coal; for anthracite and lignite, it represents the degree of coalification, while for bituminous coal it indicates its caking tendency. IV. Main characteristics and uses of various types of coal 1. Lignite It is the coal with the lowest degree of coalification. It is characterized by high moisture content, low specific gravity, high volatile matter, non-adhesiveness, strong chemical reactivity, poor thermal stability, low calorific value, and contains varying amounts of humic acid. It is often used as fuel, as a raw material for gasification or low-temperature carbonization, and can also be used to extract lignite wax and humic acid, as well as to produce sulfonated coal or activated carbon. Lignite No. 1 can also be used as organic fertilizer for farmland and orchards. 2. Long-flame coal: It has a high volatile matter content, little or no caking property, with a gum layer thickness of no more than 5 mm. It burns easily and produces long flames, which is why it is called long-flame coal. It can be used as a raw material for gasification and low-temperature carbonization, as well as as a fuel for domestic use and power generation. 3. Non-stick coal: It has a high moisture content and lacks cohesion; it produces little gelatinous substance when heated, has a low calorific value during combustion, and contains certain amounts of secondary humic acid. It is mainly used in the production of gas and as a fuel for domestic use or power generation. 4. Weakly caking coal has a high moisture content, weak cohesion, and a high volatile matter content; it produces less gum when heated. It can cake on its own, but the resulting coke pieces are small and fragile, resulting in a high rate of pulverized coke. This type of coal is mainly used as a raw material for gasification and as a fuel source. 5. Medium-caking coal in the 1/2 category: It has moderate caking properties and medium-to-high volatiles content. It can be used as raw material for coal blending in coking, as well as for gasification and as a fuel source. 6. Bituminous coal has a high volatilize content, a thicker resinous layer, and poor thermal stability. It can coking on its own, but the resulting coke is long and fragile, has a high shrinkage rate, numerous longitudinal cracks, and poor resistance to crushing and wear. Therefore, it can only be used for coal blending in coke production; it can also be used for oil refining, gas production, nitrogen fertilizer manufacturing, or as a fuel source. 7. Gas-rich coking coal: It has high volatility and cohesion, while its coking property lies between that of gas coal and rich coal; when used for coking alone, it produces large amounts of gaseous and liquid chemical substances. It is most suitable for producing gas through high-temperature dry distillation, and is also an excellent raw material for coal blending in coke production. 8. Fat coal possesses excellent cohesiveness and moderate to high volatiles; when heated, it produces a large amount of gum, forming a gum layer larger than 25 mm, and it has the strongest coking property. When this type of coal is used for coking, coke with excellent fluidity and wear resistance can be produced; however, this coke has many transverse cracks, and the root portion often features honeycomb structures, making it prone to breaking into small pieces. Due to its strong bonding properties, it is a major component in coal blending for coking. 9. 1/3 coking coal: It is a transitional coal type that lies between coking coal, fat coal, and gas coal. It possesses strong cohesiveness and moderate to high volatile matter content; when used alone for coking, it can produce coke with good fluidity and high strength. Therefore, it is an excellent base coal for coal blending in coking. 10. Coking coal has moderate to low volatiles and medium to high caking properties; when heated, it forms gelatinous substances with excellent stability. When used solely for coking, it produces coke that is dense in structure, large in size, high in strength, resistant to wear, has few cracks, and is not prone to breaking. However, due to its high expansion pressure, it can cause difficulties in pushing coke out and damage to the furnace, so it is generally used as a coal blend for coking. 11. Thin coal has low volatiles and moderate caking property. When coked alone, it produces coke with large lump sizes, few cracks, and good crush resistance, but poor wear resistance. Therefore, using it in coal blending for coking can increase the size and strength of the coke. 12. Thin coal has a low volatile content, weak cohesion, and poor coking properties. When coking is carried out separately, a large amount of coke dust is produced. But it can act as a slimming agent. Therefore, it can be used as coking coal, and at the same time, it is also an excellent fuel for domestic use and power generation. 13. Low-grade coal contains a certain amount of volatiles; it does not produce gums when heated, has no cohesion or only weak cohesion, results in a short burning flame, and does not form coke during coking. It is mainly used as power and domestic fuel. In areas lacking lean materials, it can also serve as a leaning agent for coal blending in coke production. 14. Anthracite is the coal with the highest degree of coalification. It has low volatiles, high specific gravity, high hardness; it produces little smoke and a short flame when burning, with strong burning power. It is usually used as a fuel for civilian and power applications. High-quality anthracite can be used as a raw material for gasification, as fuel for blast furnaces and in the sintering of iron ore, as well as for the production of calcium carbide, electrodes, and carbon materials. V. Quality requirements for industrial coal Coal has a wide range of industrial applications, which can be mainly categorized into metallurgy, chemicals, and power generation. At the same time, it also holds broad application prospects in fields such as oil refining, pharmaceuticals, precision casting, and the aerospace industry. Various industrial sectors have specific quality requirements and technical standards for the coal they use. A brief introduction is as follows: 1. Coal for coking. Coking involves heating coal in a dry distillation furnace; as the temperature rises (reaching around 1,000°C eventually), the organic substances in the coal gradually decompose. The volatile substances escape in gaseous or vapor form, forming coal gas and coal tar, while the remaining non-volatile substances become coke. Coke in ironmaking furnaces serves to reduce and melt the ore, provide thermal energy, support the charge, and maintain good air permeability in the charge. Therefore, the quality requirements for coking coal are aimed at producing high-quality metallurgical coke with high mechanical strength, uniform lump size, and low ash and sulfur content. **There are specific quality standards for coal used in metallurgical coke production («Quality Standards for Coal Used in Metallurgical Coke (GB397-65)»). 2 Coal for gasification: Coal gasification involves using gases such as oxygen, water, carbon dioxide, and hydrogen as reaction media, and through a thermochemical process, converting coal into gas suitable for various uses. The gaseous products obtained from gasification can be used as industrial and domestic fuels as well as raw materials for chemical synthesis. There are two common methods for gas production: ① Fixed-bed gasification. Currently, in China, bituminous coal and coke are mainly used as gasification feedstocks to produce raw gas for synthetic ammonia production. It is required that the fixed carbon content of the coal used as raw material be >80%, the ash content (Ag) be <25%, and the sulfur content (SgQ) be ≤2%. The particle size should be uniform, either within the range of 25–75 mm, or 19–50 mm, or 13–25 mm. The mechanical strength must be >65%, the thermal stability S+13 must be >60%, the ash fusion point (T2) must be >1,250°C, and the volatile matter content should not exceed 9%. The higher the chemical reactivity, the better. ②Boiling layer gasification method. The quality requirements for the raw coal are as follows: chemical reactivity must be greater than 60%, it should not be caking-prone or only slightly caking-prone, ash content (Ag) must be <25%, sulfur content (SgQ) must be <2%, moisture content (WQ) must be <10%, the ash fusion point (T2) must be >1,200°C, and the particle size must be <10 mm. Lignite, bituminous coal, and weakly caking coal are primarily used. 3. Coal used for refining is generally lignite and long-flame coal; weakly caking coal and bituminous coal can also be used, with the requirements depending on the refining method. ①The low-temperature carbonization method involves subjecting coal to carbonization at a temperature of around 550°C in order to produce low-temperature tar; simultaneously, semi-coke and low-temperature coke oven gas are also obtained. The types of coal include lignite, bituminous coal, subbituminous coal or weakly caking coal, and gas coal. The quality requirements for the raw coal are: tar yield (Tf) > 7%, gum layer thickness < 9 mm, thermal stability S+13 > 40%, particle size of 6–13 mm, with 20–80 mm being preferred. ②The hydroliquefaction method involves mixing coal, a catalyst, and heavy oil; under high temperature and pressure, the organic substances in the coal are broken down and reacted with hydrogen to produce low-molecular-weight liquid or gaseous products, which can be further processed to yield fuels such as gasoline and diesel. The raw coal mainly consists of lignite, bituminous coal, and gas coal. The carbon-to-hydrogen ratio (C/H) of the coal is required to be <16, the volatiles content to be >35%, the ash content (Ag) to be <5%, and the vitrinite content of the coal rock to be <2%. 4. Coal for fuel: Any type of coal can be used as fuel for industrial and domestic purposes. Different industrial sectors have varying quality requirements for coal used as fuel. High requirements are placed on the coal used for steam locomotives. **The specifications are as follows: volatile matter (Vr) ≥ 20%, ash content (Ag) ≤ 24%, ash fusion point (T2) ≥ 1,200°C; sulfur content (SgQ) in long tunnels and tunnel networks ≤ 1%, and the lower calorific value must be above 2.09312×107–2.51174×107 J/kg. Power plants should generally use low-quality coal with an ash content (Ag) of over 30%, while a few large boilers can use coal with an ash content (Ag) of around 20%. In order to use high-quality coal for the development of the metallurgical and chemical industries, China has made rapid progress in recent years in applying low-calorific-value coal. Many low-quality coals with a calorific value of only around 8,372.5 J/kg, as well as coal gangue, can now be used in ordinary factories; some power plants have even used coal gangue accounting for up to 30% of their fuel mix. Coal has many other uses as well. For example, lignite and oxidized coal can be used to produce humic acid fertilizers ； Lignite wax can be extracted from lignite for use in industries such as electricity, printing, precision casting, and the chemical industry ； High-quality anthracite can be used to produce silicon carbide, carbon granular sand, synthetic corundum, synthetic graphite, electrodes, calcium carbide, as well as materials for blast furnace injection or as fuel for casting ； Carbon fibers made from coal tar pitch have a tensile strength that is a thousand times greater than that of steel; they are also lightweight and resistant to high temperatures, making them important materials for the development of space technology ； Coal tar can also be used to produce needle coke, which is used in the manufacture of new types of electric furnace electrodes, thereby improving the efficiency of steel production in electric furnaces and so on. In short, as modern science and technology continue to advance, the technologies for the comprehensive utilization of coal are also developing rapidly, and the field of coal utilization will surely continue to expand.