Basic knowledge of coal

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Basic knowledge of coal: Composition of coal. The composition of coal is primarily based on organic matter, with elements such as carbon, hydrogen, oxygen, and nitrogen being the main components of these organic polymers. There are dozens of elements present in coal, but the elemental composition of coal generally refers to five main elements: carbon, hydrogen, oxygen, nitrogen, and sulfur. Other elements, which are present in small amounts in coal and come in various types, are generally not included in the elemental composition of coal; instead, they are regarded as accompanying elements or trace elements in coal. I. Carbon in coal It is generally believed that coal is composed of large aromatic rings and fused rings with fatty side chains. The skeletons of these fused rings are composed of carbon atoms. Therefore, carbon is the main element constituting the organic polymers in coal. At the same time, there is also a small amount of inorganic carbon in coal, which mainly comes from carbonate minerals such as limestone and calcite. The carbon content increases with the degree of coalification. In the peat of our country, the dry ash-free basic carbon content ranges from 55% to 62% ； After becoming lignite, the carbon content increases to 60–76.5% ； The carbon content of bituminous coal is 77–92.7% ； Up to highly metamorphosed anthracite, the carbon content is 88.98%. Some anthracites with a higher degree of coalification have a carbon content of over 90%; for example, the anthracites found in Beijing and Siwangfeng have a carbon content as high as 95–98%. Therefore, the entire coal-forming process can also be regarded as a carbon-increasing process. II. Hydrogen in coal Hydrogen is the second most important elemental component in coal. In addition to organic hydrogen, coal minerals also contain a small amount of inorganic hydrogen. It is primarily present in the crystalline water of minerals; substances such as kaolin (Al2O3•2SiO2•2H2O) and gypsum (CaSO4•2H2O) all contain crystalline water. Throughout the entire process of coal metamorphism, as the degree of coalification increases, the hydrogen content gradually decreases; coal with a lower degree of coalification has a higher hydrogen content ； Coal with a high degree of coalification has a low hydrogen content. The general rule is that the hydrogen content decreases as the carbon content increases. This is particularly evident during the anthracite stage. As the carbon content increases from 92% to 98%, the hydrogen content decreases from 2.1% to below 1%. Generally, the hydrogen content is highest when the carbon content is between 80% and 86%. That is, in the semi-bituminous and gas-rich bituminous coal categories of bituminous coal, the hydrogen content can reach as high as 6.5%. In the lignite and long-flame coal ranges with a carbon content of 65–80%, the hydrogen content is generally less than 6%. However, the general trend remains that as the carbon content increases, the hydrogen content decreases. 　 III. Oxygen in coal Oxygen is the third most important constituent element in coal. It exists in both organic and inorganic forms. Organic oxygen is primarily found in oxygen-containing functional groups, such as carboxyl groups (--COOH), hydroxyl groups (--OH), and methoxy groups (--OCH3) ； Inorganic oxygen is mainly present in coal in the form of moisture, silicates, carbonates, sulfates, and oxides. Organic oxygen in coal decreases as the degree of coalification increases, and even tends to disappear. When the ash-free carbon content of lignite is less than 70%, its oxygen content can be as high as over 20%. When the carbon content of bituminous coal is around 85%, its oxygen content is almost always less than 10%. When the carbon content of anthracite is above 92%, its oxygen content drops to below 5%. 　 IV. Nitrogen in coal　The nitrogen content in coal is relatively low, generally ranging from 0.5 to 3.0%. Nitrogen is the only element in coal that exists entirely in an organic state. Organic nitrides in coal are considered to be compounds with relatively stable heterocyclic and complex acyclic structures, whose precursors may be animal and plant fats. Alkaloids in plants, chlorophyll, and the cyclic structures in other tissues all contain nitrogen, which is quite stable and does not change during the coalification process, remaining as nitrogen compounds in coal. Nitrogen in the form of proteins is found only in peat and lignite; it is rare in bituminous coal and hardly ever found at all. The nitrogen content in coal decreases as the degree of coal metamorphism increases. Its relationship with hydrogen content is that it increases as the hydrogen content rises. 　 V. Sulfur in coal Sulfur contained in coal is a harmful impurity; it can cause steel to become thermally brittle, lead to corrosion of equipment, and the sulfur dioxide (SO2) produced during combustion contaminates the atmosphere, harming the growth of plants and animals as well as human health. Therefore, the sulfur content is one of the important indicators for evaluating coal quality. The sulfur content in coal does not seem to have a significant relationship with the degree of coalification; whether it is coal with a high degree of metamorphism or coal with a low degree of metamorphism, it always contains some amount of organic sulfur. The sulfur content in coal is closely related to the paleogeographical environment at the time of coal formation. Coal seams formed in inland environments or coastal delta plains, as well as those resulting from intertidal sedimentation or shallow marine sedimentation, have a relatively high sulfur content, with most of this sulfur being organic sulfur. Based on the occurrence form of sulfur in coal, it is generally divided into two major categories: organic sulfur and inorganic sulfur. The total of sulfur in all its forms is referred to as total sulfur content. So-called organic sulfur refers to sulfur that is combined with the organic structure of coal. Organic sulfur mainly comes from proteins in coal-forming plants and microbial proteins. Inorganic sulfur in coal mainly comes from various sulfur-containing compounds in minerals, and is generally divided into sulfide sulfur and sulfate sulfur; sometimes there are also trace amounts of elemental sulfur. In sulfides, sulfur is primarily present in the form of pyrite, followed by marcasite, magnetite ((Fe7S8)), sphalerite (ZnS), galena (PbS), and others. Sulfate sulfur is mainly present in the form of gypsum (CaSO4•2H2O), with small amounts of green vitriol (FeSO4•7H2O) etc. also present. The formation of coal: Coal is formed from plant remains through complex biochemical and physicochemical processes. This process of transformation is called the coalification of plants. It is generally believed that the coalification process is divided into two stages: the peatification stage and the coalification stage. The former are mainly biochemical processes, while the latter are physicochemical processes. During the peatification stage, plant remains both decompose and combine, ultimately forming peat or sapropele. Both peat and humus contain large amounts of humic acid, and their composition differs significantly from that of plants. The coalification stage involves two consecutive processes: In the first process, under the influence of geothermal heat and pressure, peat layers undergo various changes such as compaction, water loss, aging of organic matter, and hardening, thereby turning into lignite. The density of lignite is greater than that of peat, and its composition has also changed significantly: the carbon content has increased relatively, while the humic acid and oxygen contents have decreased. Since coal is an organic rock, this process is also known as diagenesis. The second process is the transformation of lignite into bituminous coal and anthracite. During this process, the properties of coal change, which is why this process is also called metamorphism. The Earth’s crust continued to sink, and the layer of lignite covering it grew thicker as a result. Under the influence of geothermal heat and static pressure, lignite continues to undergo physicochemical changes, resulting in compaction and loss of water. Its internal composition, structure, and properties undergo further changes. This process is the metamorphism in which lignite turns into bituminous coal. Bituminous coal has a higher carbon content and lower oxygen content compared to lignite; humic acid is no longer present in bituminous coal. Bituminous coal continues to undergo metamorphism. It changes from low degree of metamorphism to high degree of metamorphism. As a result, bituminous coals and gas coals with low degrees of metamorphism emerged, along with rich coals and coking coals with moderate degrees of metamorphism, and lean coals and poor coals with high degrees of metamorphism. The carbon content between them also increases as the degree of metamorphism increases. Temperature plays a decisive role in the chemical reactions during coal formation. As the strata deepen, the geothermal temperature rises, and the degree of coal metamorphism gradually increases. The longer the duration of high-temperature exposure, the greater the degree of coal metamorphism, and vice versa. Under the simultaneous effects of temperature and time, the coal metamorphism process is essentially a chemical change process. The chemical reactions that occur during its transformation are diverse, including dehydration, decarboxylation, demethanation, deoxygenation, and polycondensation. Pressure is also an important factor in the formation of coal. As gases are released and pressure increases during the coalification process, the reaction rate slows down progressively; however, this facilitates changes in the physical structure of the coal. It helps to reduce the porosity and moisture content of coal with a low degree of metamorphism, while increasing its density. Throughout different geological eras on Earth, as the climate and geographical environment changed, organisms continued to develop and evolve. As for plants, they have evolved from lifeless forms all the way to angiosperms. These plants produced large amounts of coal during the corresponding geological periods. Throughout geological history, there have been three major coal-forming periods on a global scale: (1) the Carboniferous and Permian periods of the Paleozoic Era; the coal-forming plants were mainly spore-bearing plants. The main types of coal are bituminous coal and anthracite. (2) During the Mesozoic Cretaceous and Jurassic periods, the coal-forming plants were mainly gymnosperms. The main types of coal are lignite and bituminous coal. (3) During the Tertiary period of the Cenozoic era, the coal-forming plants were mainly angiosperms. The main type of coal is lignite; peat comes next, and there is also some young bituminous coal. Industrial analysis of coal: Industrial analysis of coal is also known as technical analysis or practical analysis; it includes the determination of moisture, ash, and volatile matter in coal, as well as the calculation of fixed carbon. The industrial analysis of coal is a primary indicator for understanding the characteristics of coal and serves as a fundamental basis for evaluating its quality. Based on the various test results obtained from this analysis, one can preliminarily determine the properties and type of coal, as well as the effectiveness of its processing and utilization, along with its industrial applications. The various indicators in the industrial analysis of coal are as follows: 1. Moisture – an important indicator of coal quality, which plays a significant role in both the fundamental theoretical studies of coal and its processing and utilization. The moisture content in coal changes systematically with increasing degree of coal metamorphism: it gradually decreases from peat, lignite, bituminous coal to young anthracite; however, from young anthracite to old anthracite, the moisture content increases again. The moisture content of coal has a significant impact on its processing and utilization, as well as on its trade and storage and transportation. In boiler combustion, high moisture levels can affect combustion stability and heat transfer ； In the coking industry, high moisture content reduces coke yield, and the extensive evaporation of water carries away heat, thereby prolonging the coking cycle ； In the coal trade, the moisture content of coal is an important quality and measurement indicator. In modern coal processing and utilization, a high moisture content can sometimes be an advantage; for example, the moisture in coal can serve as a hydrogen source for hydrodesulfurization and hydrogasification. In coal quality analysis, the moisture content of coal is the fundamental data for converting the results of coal quality analysis based on different bases. 2. Ash content: Ash in coal is another indicator that plays an important role in the study of coal quality characteristics and its utilization. In coal quality research, since ash content has varying degrees of dependence on other properties such as carbon content, calorific value, slagging tendency, reactivity, and grindability, these properties can be studied through ash content. Since coal ash is a derivative of the minerals in coal, it can be used to calculate the mineral content in coal. Furthermore, since the determination of ash content in coal is simple, and its distribution in coal is not uniform, it is generally used in the study of coal sampling and sample preparation methods to assess the accuracy and precision of these methods. In the research on coal washing processes, coal ash is also generally used as an indicator of washing efficiency. In coal combustion and gasification, corrosion, fouling, and slagging issues that may arise during these processes are predicted based on the ash content of the coal as well as its properties such as melting point, viscosity, electrical conductivity, and chemical composition. Based on this, decisions are made regarding the type of furnace to be used, and research is conducted on the utilization of coal ash and slag. 3. Volatiles: The yield of volatiles in coal is closely related to the degree of coal metamorphism. As the degree of metamorphism increases, the volatiles in coal gradually decrease. In lignites with a low degree of coalification, the volatile matter yield ranges from 65% to >37% ； When the metamorphic stage reaches bituminous coal, the volatile matter content is 55% to >10% ； By reaching the anthracite stage, the volatiles drop to 10% or even below 3%. Therefore, the degree of coalification of coal can be roughly determined based on its volatile matter yield. In China’s coal classification system, as well as in the Soviet, American, British, French, Polish, and international coal classification systems, volatile matter is used as the primary classification criterion. The processing and utilization methods for coal can be preliminarily determined based on the volatiles yield and the characteristics of the cinder after volatiles measurement. Coals with high volatile matter, for example, produce high yields of chemical by-products during carbonization; they are suitable as raw materials for low-temperature carbonization or hydroliquefaction, as well as for gasification ； Bituminous coal with moderate volatiles has good caking properties and is suitable for coking. In coal blending for coking, the volatile matter content is used to determine the coal blending ratio, in order to keep the volatile matter of the blended coal within an appropriate range of 25% to 31%. Furthermore, the yields of coke, gas, tar, and other products during coking can be estimated based on the volatile matter. Among thermal coal, specific combustion equipment or coal sources for specific equipment can be selected based on the volatiles content. Volatile matter also plays an important role as a reference in the selection of conditions for gasification and liquefaction processes. In environmental protection, volatiles also serve as a basis for formulating smoke regulations. Furthermore, volatile matter has a good correlation with other medium property indicators such as calorific value, and carbon and hydrogen contents. The volatile matter can be used to calculate the calorific value of coal, as well as the contents of carbon, hydrogen, and chlorine, and the tar yield. 4. Fixed carbon: Fixed carbon is an important indicator in the classification, combustion, and coking of coal. The amount of fixed carbon in coal increases as the degree of metamorphism deepens. In coal combustion, the fixed carbon is used to calculate the efficiency of the combustion equipment ； In the coke-making industry, it is used to predict the yield of coke. Definition of “basis” in coal analysis: Since the contents of moisture and ash in coal are influenced by external conditions, the percentages of other components also change accordingly. Therefore, it is not sufficient to simply use the percentage composition to determine the type and certain characteristics of coal; it is necessary to specify what the basis for these percentages is. ““Base” indicates the state of the coal sample upon which the test results are based. The commonly used “bases” in coal quality analysis include air-dried basis, dry basis, as-received basis, dry ash-free basis, and dry mineral-matter-free basis. Its definition is as follows: 1. Air-dried basis: It is based on coal that has reached equilibrium with atmospheric humidity. The symbol used is ad (air dry basis). 2. Dry basis: It is based on coal in a hypothetical anhydrous state. The symbol used is d (dry basis). 3. As-received basis: It is based on coal in its as-received state. The notation is ar (as received). 4. Dry, ash-free basis: It uses coal in a hypothetical state with no water and no ash as the reference. The symbol used is: daf (dry ash free). 5. Dry, mineral-matter-free basis: It is based on coal in a supposedly anhydrous and mineral-matter-free state. The symbol used is dmmf (dry mineral matter free). Classification of coal: With the development of society and the progress of science, the uses of coal have become increasingly diverse. People’s understanding of the properties, compositional structure, and applications of coal has also deepened; it has gradually become apparent that different types of coal share certain similarities while also possessing distinct characteristics. Based on various needs, different types of coal are grouped and classified into several categories with similar properties. In this way, the concept of coal classification is formed. Depending on the focus, there are two methods of coal classification: 1. Genetic classification of coal: This involves classifying coal based on the original materials from which it formed and the depositional environment; this is known as genetic classification. 2. Scientific classification of coal: This involves classifying coal based on its basic properties such as elemental composition; this is referred to as scientific classification. 3. Practical classification of coal: The practical classification of coal is also known as the industrial classification of coal. Classification based on the technological properties and uses of coal is known as practical classification. The coal classification in China and that in major industrial countries all fall under practical classifications. Below, we provide a detailed introduction to the practical coal classification system in China. Based on the degree of coalification, all coals in China are classified into three main categories: lignite, bituminous coal, and anthracite. Furthermore, based on the degree of coalification and characteristics of industrial utilization, lignite is divided into 2 subcategories, and anthracite into 3 subcategories. Bituminous coal is relatively complex; based on its volatile matter content, it is classified into four categories: Vdaf >10–20%, >20–28%, >28–37%, and >37%. These correspond to low, medium, medium-high, and high-volatile bituminous coals, respectively. Based on adhesiveness, it can be divided into 5 or 6 grades; specifically, GR.I. ranges from 0 to 5, and coals in this range are referred to as non-adhesive or weakly adhesive coals ； GR.I. >5–20 is referred to as weakly caking coal ； GR.I. >20–50 is referred to as moderately weak-caking coal ； GR.I. >50–65 is referred to as moderately strong caking coal ； GR.I. >65 is termed strongly caking coal. In strongly caking coals, those with y > 25 mm or b > 150% (for fat coals with Vdaf > 28%, b > 220%) are referred to as extremely strongly caking coals. See GB5751-1986. The basic characteristics of various types of coal are as follows: (1) Anthracite (WY). Anthracite has a high fixed carbon content, low volatile matter yield, high density, great hardness, and a high ignition temperature; it produces no smoke when burned. Anthracite No. 01 is aged anthracite ； Anthracite No. 02 is typical anthracite ； No. 03 anthracite is young anthracite. For example, the anthracite from Beijing, Jincheng, and Yangquan is designated as No. 01, No. 02, and No. 03 respectively. (2) Poor coal (PM). Lean coal is a type of bituminous coal with the highest degree of coalification; it is non-caking or slightly caking. No coking occurs in layered coke ovens. It has a short flame when burning and is resistant to combustion. (3) Lean coal (PS). Lean coal is a type of bituminous coal with high metamorphism, low volatiles content, and weak caking properties. Coking quality is worse than that of typical lean coals; when coked alone, more coke dust is produced. (4) Thin coal (SM). Lean coal is a coking coal with low volatiles and moderate caking property. A certain amount of colloid is produced during coking. When coking is carried out separately, coke with large particle size, few cracks, and good crush resistance can be obtained, but its wear resistance is poor. (5) Coking coal (JM). Coking coal is a type of bituminous coal with medium and low volatile matter, as well as medium and high caking strength. When heated, it can produce colloids with high thermal stability. When coked separately, coke with large particle size, few cracks, high crush resistance can be obtained, and it also has good wear resistance. However, when coking is carried out alone, the resulting expansion pressure is high, making it difficult to push out the coke. (6) Fat coal (FM). Fatty coal is a strongly caking bituminous coal with low, medium, or high volatile matter content. A large amount of colloids are produced during heating. When coked separately, it can produce coke with good fluidity and high strength; its wear resistance is also sometimes superior to that of coke produced from bituminous coal. The disadvantage is that the coke produced separately has many transverse cracks, and beecomb structure is often found in the root portion of the coke. (7) 1/3 coking coal (1/3JM). 1/3 coking coal is a new type of coal; it is a type of bituminous coal with medium to high volatile matter and strong cohesiveness, and it serves as a transitional coal type between coking coal, fat coal, and gas coal. Coking alone can produce coke with better fluidity and higher strength. (8) Gas-fertilized coal (QF). Gas-fertilized coal is a type of highly cohesive fertilizing coal with very high volatile matter and gum layer content; it is sometimes referred to as liquid-fertilized coal. Its coking properties lie between those of fat coal and gas coal; when coked alone, it produces a large amount of gaseous and liquid chemical products. (9) Bituminous coal (QM). Bituminous coal is a type of coking coal with a low degree of coalification. When heated, it produces higher levels of volatiles and more tar. The thermal stability of colloids is lower than that of fat coal, allowing them to be coked separately. However, coke is usually in the form of long, thin strips and is fragile, with numerous longitudinal cracks; as a result, its resistance to crushing and wear is lower compared to that of other coking coals. (10) Medium-coal bituminous coal (1/2ZN). 1/2 medium caking coal is a medium-caking, medium-to-high volatile bituminous coal. Some of them can form coke with a certain strength when coked separately, and can be used as raw material for coke-making coal blends. Coal with poor cohesiveness, when coked separately, produces coke of low strength and a high proportion of fine coke. (11) Weakly caking coal (RN). Weakly caking coal is a type of bituminous coal with low to moderate degree of metamorphism and relatively weak caking properties. When heated, fewer colloids are produced. When coked separately, some form small coke lumps with very poor strength, while in others only a small portion condenses into fine coke particles, resulting in a high rate of powder coke. (12) Non-stick coal (BN). Non-stick coal is a bituminous coal with low to moderate degree of metamorphism that has undergone considerable oxidation during the early stages of coal formation. When heated, virtually no colloids are produced. Coal has a high moisture content; some also contain certain amounts of secondary humic acid, as well as a high oxygen content, which can exceed 10% in some cases. (13) Long-flame coal (CY). Long-flame coal is the type of bituminous coal with the lowest degree of metamorphism, ranging from non-caking to weakly caking. The youngest of them also contain a certain amount of humic acid. It is prone to weathering and cracking during storage. Older coals with a higher degree of coalification can produce a certain amount of colloid when heated. When coked alone, fine, elongated coke can also be formed, but its strength is extremely poor and the rate of powder formation is high. (14) Lignite (HM). Lignite is divided into two subcategories: young lignite with a light transmittance of Pm < 30%, and old lignite with Pm > 30–50%. Lignite is characterized by high moisture content, low density, lack of cohesion, varying amounts of humic acid, and a high oxygen content in the coal. It often reaches around 15–30%. It has strong chemical reactivity and poor thermal stability; the lump coal breaks down severely when heated. When stored in air, it tends to weather and deteriorate, breaking down into lumps or even powder. It has a low calorific value, and the ash melting point is also low; its ash contains more CaO and less Al2O3. Applications of coal for power generation and the requirements for its quality I. Coal for power generation Coal used for power generation accounts for the largest share nationwide; over one-third of all coal is utilized for this purpose. The amount of coal consumed for power generation during 1996–1997 was over 500 million tons each year. Currently, the average coal consumption for power generation is around 370 g of standard coal per (kW•h). Power plants use the calorific value of coal to convert thermal energy into electrical energy. Calorific value is a major factor affecting the technical and economic indicators of power plants. When designing and constructing a power plant, it is required to have a fixed coal supply source, and the boilers of the plant are designed based on the different qualities of coal. Once it is built and put into operation, and the coal quality meets the requirements of the boiler design, the boiler’s efficiency can be fully utilized. The power industry generally requires that the calorific value of coal (Qnet,ar) be greater than 20.9 lMJ/kg (excluding lignite), with a tolerance of 5%; the particle size should be fine coal of less than 25 mm or mixed coal of less than 50 mm. The volatile matter content (Vdaf) must be above 20%, sulfur content (St,d) below 1.0%, external moisture content (Mt) below 10%, and the softening temperature of the ash resulting from dry slag removal (ST) must be above 1250°C. The calorific value of coal affects various aspects such as the raw coal handling system, coal grinding system, furnace volume, flue ducts, ventilation volume, dust collection, and slag discharge in power plants, thereby having a direct impact on the construction costs and operating expenses of these plants. II. Coal for locomotives: Coal used for locomotives accounts for about 2% of the coal used for power generation, with an annual consumption of around 20 million tons. The average coal consumption rate for steam locomotive boilers is around 100 kg per 10,000 tons-km. The design of these boilers and the conditions under which they operate require the use of lump coal, which must be both easy to ignite and resistant to burning. The evaporation area of the boiler is generally 240 m2. The evaporation rate is 75 kg/(m2•h), with an average of 50 kg/(m2•h); trains can operate on schedule only if this requirement is met. When the track conditions are poor, it is necessary to quickly adjust the heating power to increase the evaporation rate. Therefore, high requirements are placed on the calorific value and volatile matter of coal: the calorific value (Qnet,ar) must be greater than 23.00 MJ/kg, and the volatile matter (Vdaf) must be above 30%. Due to the high temperature in the furnace, the softening temperature at melting point (ST) is required to be greater than 1350°C. The particle size should not exceed 50 mm; 50–25 mm or 25–13 mm is the ideal range. The less powder (<6 mm) present, the better. Coal dust can easily be carried away by the flue gases or fall through the grate of the furnace. According to tests, an increase of 1% in powder content results in a 0.4% increase in loss. The sulfur content should be less than 1%, otherwise it can easily cause SO2 poisoning in people when locomotives pass through long tunnels. III. Coal for building materials: Coal used in the building materials industry accounts for over 10% of the total coal consumption for power generation, with an annual consumption of over 100 million tons. Cement is used in the largest quantity, followed by glass, bricks, tiles, etc. Bricks and tiles do not have strict requirements regarding coal quality; a lower calorific value is sufficient. Coal gas used as fuel in glass production requires the same quality as coal used for fertilizer production. In the cement production process, coal serves not only as a fuel but also becomes a component of the cement; therefore, strict quality requirements are placed on coal. The coal consumption rate for rotary kilns is around 240 kg of standard coal per ton of sludge; bituminous coal is commonly used, with requirements that the ash content remain between 18% and 22%, and the volatile matter content be above 25%, using coal dust with a particle size of less than 25 mm. The coal consumption rate for vertical kilns is 220 kg per ton of cement; bituminous coal with an ash content of less than 20% and a volatile matter content of less than 10% is required. It is also desired that it have low moisture content, low sulfur content, and a calorific value of more than 20.90 MJ/kg. IV. Coal for general industrial boilers: Apart from thermal power plants and large-scale heating boilers, there are a wide variety of industrial boilers used in various enterprises and for heating purposes, with large numbers of them spread across different locations. The coal consumption accounts for about 30% of thermal coal, with an annual consumption of around 300 million tons. Most of these boilers use layer-bed combustion, resulting in lower efficiency; chain grate boilers have an efficiency of around 75%, while reciprocating and vibrating boilers have an efficiency of 65% to 70%. Industrial boilers are designed based on different types of coal, ash content, calorific value, and volatile matter, with layer-bed combustion being the primary method, while coal powder boilers are less common. The quality requirements for coal are relatively flexible; generally, the ash content is less than 40%, and the calorific value ranges from 14.63 to 20.90 MJ/kg. Layered bed furnaces have fixed and movable grates; the particle size should be less than 50 mm, with coal in lumps of 13–50 mm being preferred, and the proportion of particles smaller than 6 mm should be less than 35%. Less than 13-mm sized fine coal is mostly used in reciprocating furnaces and furnaces with stationary grates. Different particle sizes are suitable for different types of boilers; overly large particles can lead to incomplete combustion, while excessively small particles may result in coal leakage losses. Therefore, burning raw coal is very uneconomical. It contains many impurities, which also have a significant impact on the boiler, its operation, and thermal efficiency. Industrial boilers consume large amounts of coal, and also have great potential for energy conservation; therefore, it is essential to ensure a proper match between supply and demand, as well as to make effective use of and conserve coal. 　 V. Coal for domestic use The amount of coal used for domestic purposes is also significant, accounting for about 20% of the coal used as fuel, with an annual consumption of over 200 million tons. Most are directly for civilian use. The amount of coal used for urban gas is also increasing year by year. The quality requirements for domestic coal vary by region. For general civilian use, anthracite with low volatile matter should be used as fuel. Urban gas is produced from coal; to achieve a higher yield of gas, bituminous coal with a volatility content of 30%–40% or more, semi-coking coal, gas-rich coal, or other bituminesous coals with high volatility content are generally used for this purpose. A low ash content is required; washed coal is preferable. Different furnace types have different specific requirements regarding the quality and type of coal. Additionally, when using a Lurgi pressurized gasifier, the quality requirements for coal are low. The external moisture content is 6%–8%, DT is greater than 1,200°C, and the particle size is 50–6 mm. The use of gas allows for long-distance transportation, eliminating the need to transport coal and ash back and forth, improving environmental hygiene, and saving coal; it should be promoted and developed further. VI. Thermal coal for metallurgy Thermal coal for metallurgy is mainly anthracite used for sintering and blast furnace injection. Its usage accounts for less than 1% of the amount of coal used for power generation. The annual consumption is less than 10 million tons. Most of it is used for sintering, with injection accounting for about 20%. Blast furnace injection involves using coal instead of coke. It is calculated that 1 ton of anthracite powder used in injection can replace 0.8 tons of coke, which is equivalent to 1.2 tons of washed coking coal. It is evident that replacing coke with coal is the development direction for the rational utilization of coal resources. More than half of the blast furnaces in our country are equipped with injection facilities, and the coal consumption for injection can be gradually increased to over 100 kg per ton of pig iron. About 80 kg of anthracite powder is added per ton of sintered ore. Anthracite for sintering and blast furnace injection is required to have an ash content of less than 13%, a sulfur content of less than 0.80%, a phosphorus content of less than 0.02%, a moisture content of less than 8%, and a particle size of less than 25 mm. Given users’ requirements regarding the quality of commercial coal, the raw coal produced must be processed before it can be used. For users with less stringent requirements as regards coal quality, screening and grading are sufficient; however, for those with strict demands on both coal quality and variety, commercially processed coal that has been cleaned and refined must be supplied.