Prize-winning Quiz on Coal Chemical Engineering Knowledge 4

Coal is a biological rock (organic rock) formed from plants through natural coalification; it is considered coal when its ash content is less than 40%, and it is classified as carbonaceous rock when the ash content is greater than 40%. Formation of coal: 1. Peatification stage—under shallow surface waters, peat is formed ； 2. Coalification stage: Stage-wise lithification—lignite is formed in shallow strata ； Brown or dark brown, dull, not hard and even soft ； The pores are large. High volatile content. 3. Gradual metamorphism—deeper in the strata, lignite transforms into bituminous coal and anthracite. Bituminous coal---black and shiny, with alternating light and dark bands ； It has a high volatile content. 4. Anthracite – gray-black in color, with a metallic luster, and no distinct bands. It has small pores, low internal water content, low volatiles, and low chemical activity.

1. Briefly describe the differences in origin, structure, and properties among different coal lithological components? Answer: Rockological scientific research methods are used to study the origin, structure, and properties of coal, which is classified into vitrinite, bright coal, dark coal, and filamentous carbon on a macroscopic level ； Microscopically, coal is divided into vitrinite, inertinite, chitin, and minerals; among these, vitrinite, inertinite, and chitin are different organic components known as microcomponents. Relationship between microscopic components and macroscopic components of coal: Vitrine coal is mainly composed of vitrinite, bright coal is a mixture of vitrinite and chitin, dark coal is a mixture of chitin and inertinite, while thread coal is primarily composed of inertinite. In scientific research, the microscopic components of coal are commonly referred to as coal petrographic components. (1) Origin of coal lithographic components. The vitrinite fraction, also known as the gel component, is formed from plant residues through a process of gelation. The lignin fiber tissue of plant residues, when submerged in water and subjected to anaerobic microorganisms, gradually decomposes to form a structureless colloidal substance; after a long process of coalification, it then transforms into vitrinite. The inert component, also known as the silk carbonization component, is formed through silk carbonization. The lignin fiber tissue of plant residues is first exposed to air, placing it in an oxidizing environment; the protoplasm within the cell cavities is quickly destroyed by aerobic microorganisms, while the cell walls remain relatively stable and survive only through dehydration. Due to changes in geological conditions, the accumulation environment became reducing, and further degradation of the residues ceased, resulting in the formation of filamentous carbon with a certain cellular structure. Additionally, a portion of silk charcoal comes from the charcoal left behind by forest fires, and is known as fire-induced silk charcoal. The chitinous fraction is different from the previous two groups of microcomponents; it is formed from lipids found in plant remains, such as spores, resins, and the cutin layer. (2) Structure of coal rock components: Under transmitted light microscopy, the vitrinite appears orange-red to brownish-red in color and is transparent ； It appears gray under reflected light, with no protrusions. The inert group appears black and opaque under transmitted light ； It appears white to bright yellow under reflected light, with protrusions; the chitinous layer appears yellow to orange-yellow under transmitted light, and is semi-transparent ； Under reflected light, it appears black-gray to gray-black. (3) Properties of coal rock components: Optical properties: reflectivity, fluorescence characteristics, microstructure. Elemental composition: Vitrineinite fraction: relatively rich in oxygen; Inertinite fraction: rich in carbon and poor in hydrogen; Chitinite fraction: rich in hydrogen and poor in carbon. Process-related properties: cohesiveness, reactivity, calorific value. Evolution pattern: Vitrineinite fraction: changes relatively uniformly; Inertinite fraction: changes slowly; Chitinite fraction: sensitive to heat, becoming similar to the vitrineinite fraction at higher ranks of bituminous coal. Vitrineinite fraction, Chitinite fraction, Inertinite fraction; Volatiles: moderate, high, low. Cohesiveness: high, medium, low. Reflectivity: moderate, low, high. Carbon content: moderate, low, high. Hydrogen content: moderate, high, low. Fluorescence: weak to moderate, strong, weak

I. Formation of coal Coal is generally classified into three main categories based on its industrial uses: anthracite, bituminous coal, and lignite. Bituminous coal is further divided into 8 types: lean coal, thin coal, coking coal, rich coal, gas coal, weakly caking coal, non-caking coal, and long-flame coal. The coal quality indicators used to classify coal types are mainly: moisture (W), ash content (A), volatile matter (V), fixed carbon (C), gum layer thickness (Y), and calorific value (Q). Total sulfur (S) is also measured in industrial analysis. Coal is formed from the remains of plants over geological time. During the coal-forming period in geological history, the climate on Earth was warm and humid, allowing plants to thrive; in particular, dense forests or aquatic plants covered the lake and swamp areas. The dead plant remains accumulate at the bottom of lakes and swamps, and as the earth’s crust slowly sinks, they are gradually covered by water and isolated from air. Under the biochemical actions involving bacteria, plant remains begin to decay and break down; some turn into gases that escape, some become liquids that dissipate, and the remaining portion forms peat layers. The process by which plant remains transform into peat is called the peatification stage. Over time, the Earth’s crust continued to sink slowly; the peat layers were covered by sediments and other materials carried by water, and this covering layer gradually thickened. Under the influence of pressure and temperature, peat layers gradually lose moisture and become compact, at which point the peat turns into lignite. As the Earth’s crust continues to sink and the overlying layers grow thicker, lignite is subjected to high temperatures and pressures deep underground. As a result, the carbon content increases, while the amounts of oxygen and water decrease. The density rises, the color darkens, and the hardness increases, gradually transforming the lignite into bituminous coal. This process of coal transformation is known as the coalification stage. As the degree of metamorphism increases further, bituminous coal turns into anthracite. In certain cases, anthracite may further transform into an incombustible mineral—graphite. Lower plants form peat coal through stages of humification similar to those in peat, as well as a carbonization stage. The metamorphic process by which plant remains turn into coal is shown in Figure 1–4. II. The concept of coal seams: During the formation of coal, many rock layers are formed simultaneously above and below the coal seams. These rock layers containing coal seams were formed during the same coal-forming period, and are commonly referred to as coal-bearing strata of a particular geological era. A coal series refers to a series of sedimentary rocks from a particular geological era that contain coal seams; these layers were deposited sequentially one after another and are closely related in terms of their origin. Coal measures are also known as coal-bearing strata or coal-bearing formations. Coal formations are generally named after the geological era in which they were formed. For example, the Carboniferous and Permian coal formations in North China, the Jurassic coal formations in the Northeast, and the Late Permian coal formations in South China (which were studied earlier in places such as Longtan in Jiangsu and Leping in Jiangxi, and are therefore also known as the Longtan coal formations or Leping coal formations). Therefore, coal formations formed during the same geological period often have different regional names in various areas. Coal measures were formed under warm and humid climate conditions; they are rich in plant material, which is why the colors of coal measure rocks are often gray, gray-black, gray-green, or yellow-green. In coal formations, in addition to coal deposits, other sedimentary minerals are often found as well, such as oil shale, bauxite, siderite, hematite, limonite, pyrite, etc. Therefore, when developing coal fields, the possibility of comprehensively developing other minerals should also be considered. Coal-bearing strata are mainly composed of sedimentary rocks such as siltstone, sandstone, mudstone, and coal seams; sometimes there are also layers of limestone and conglomerate. If there is volcanic activity in the vicinity during the deposition of coal-bearing strata, pyroclastic rocks and volcanic lavas composed of volcanic ejecta may occur within those coal deposits, as is the case with the Jurassic coal formations in northeastern China. If the coal measures undergo metamorphism after their formation, then the coal-bearing strata in certain areas may turn into metamorphic rocks. For example, the Carboniferous-Permian coal seams in Zhoukoudian, Beijing, underwent local metamorphism as a result of magmatic intrusions. III. The main coal-forming periods in China Coal formation is conditional, with crustal movements playing a dominant role. Coal seams can only form during the historical development of the Earth’s crust, when ancient plants, climate, geography, and environment work together in harmony. Therefore, coal formation is time-bound and region-specific. China’s large-scale coal fields all appeared during the Carboniferous period and thereafter. Our country has three important coal-forming periods, namely: the Carboniferous–Permian period of the Paleozoic era ； Late Permian to Jurassic of the Mesozoic Era ； The Tertiary period of the Cenozoic era. During these periods, many **small coal fields** emerged in our country, and quite a few of them have become the main coal production areas in our country today. IV. Coal seam structure and burial characteristics Different formation conditions of coal seams result in significant variations in aspects such as the structure of the coal seams and their occurrence patterns, the lithology of the roof and floor rocks, and the degree of influence by geological structures. The geological conditions of these coal seams are closely related to coal mining operations. Factors such as the thickness, structure, inclination, stability, depth of burial of the coal seams, as well as the properties of the surrounding rock at the top and bottom, have a significant impact on determining the mining methods and selecting the appropriate coal extraction techniques. (1) Thickness, structure, and classification of coal seams 1. Thickness of coal seams The thickness of a coal seam refers to the vertical distance between the rock layers above and below the coal seam, also known as the true thickness. 2. Coal seam structure: The coal seam structure refers to whether there are intercalated seams of gangue in the coal seam. Based on the number of intercalated seams, coal seams are commonly divided into the following two types: Coal seams with a simple structure: such seams generally contain no intercalated seams, or may have 1–2 layers of stable intercalated seams. Complexly structured coal seams: These coal seams contain a large number of interbedded shale layers, with significant variations in the number of layers, their positions, thicknesses, and lithological properties. 3. Coal seam classification: There are various classification methods based on different criteria. (1) Classification by coal seam thickness: The thickness of coal seams varies greatly, with thinner ones being only a few centimeters (commonly referred to as coal veins), while thicker ones can reach over 200 meters. Based on the characteristics of mining technical conditions, coal seams can be classified into the following categories: extremely thin coal seams – 0.3 meters to 0.5 meters; thin coal seams – 0.5 meters to 1.3 meters; medium-thick coal seams – 1.3 meters to 3.5 meters; thick coal seams – 3.5 meters to 6.0 meters; and extremely thick coal seams – greater than 6.0 meters. Thick and medium-thick coal seams account for a large proportion in China’s coal fields. In terms of production, thick coal seams and medium-thick coal seams account for approximately 40% each, while thin coal seams account for only 20%. (2) Classification by coal seam inclination: Nearly horizontal coal seams have an inclination of 45°; the inclination of coal seams ranges from 0° to 90°, and the greater the inclination, the more difficult it is to extract them. (3) Classification by coal seam stability: Based on the thickness of the coal seam and the variations in its structure within the mining area, coal seams can generally be divided into four categories: stable, relatively stable, unstable, and extremely unstable. (2) Burial characteristics of coal seams: The roof and floor of a coal seam refer to the rock layers located at a certain distance above and below the coal seam. In accordance with the sequence of deposition, under normal conditions, the rock layer beneath the coal seam and formed before the coal was created is called the floor, while the rock layer above the coal seam and formed after its formation is called the roof. Due to differences in sedimentary materials and deposition environments, the properties and thickness of the roof and floor rock layers vary, which in turn results in different degrees of fragmentation and collapse during mining. Understanding the lithological characteristics, thickness, stratification, degree of joint development, strength, and water content of these rock strata is of great significance for determining roof management methods and roadway support approaches. 1. The roof and floor of coal seams: Depending on the position of the rock strata forming the roof and floor relative to the coal seam, as well as their characteristics such as collapse behavior and strength, the roof is divided into three parts from top to bottom: the fundamental roof, the immediate roof, and the false roof. The floor is divided into three parts: the false floor, the immediate floor, and the true floor. However, for a particular coal seam, these six components of its roof and floor are not necessarily all present; there may be strata that lack one or several of these components. False roof: It is a thin rock layer that lies right above the coal seam and is highly prone to collapsing along with the extraction of coal; its thickness is generally between 0.3 meters and 0.5 meters, and it is usually composed of shale or carbonaceous shale. Immediate roof: It is the rock stratum that lies directly above the false roof or the coal seam (if there is no false roof), and it often collapses as the supports are retracted. The thickness is generally 1 to 2 meters, and it is mostly composed of rocks such as mudstone, shale, and siltstone that are prone to collapse. Primary roof: It is a thick and hard rock layer that lies above the immediate roof, or directly above the coal seam (in which case there is no immediate roof nor false roof). It often remains suspended above the goaf for a period of time, collapsing only after reaching a sufficient size; it is usually composed of hard rocks such as sandstone, conglomerate, and limestone. Pseudo-bottom: A thin layer of weak rock situated directly beneath the coal seam, usually consisting of carbonaceous shale or mudstone, with a thickness generally ranging from 0.2 meters to 0.3 meters. Direct base: A rock layer with lower hardness that lies directly beneath the coal seam; its thickness ranges from several dozen centimeters to about 1 meter, and it is usually mudstone, shale, or claystone. If the direct floor is composed of clay rock, it tends to expand when exposed to water, which can cause the floor of the roadway to bulge and the supports to be pushed down. In mild cases, this affects transportation in the roadway and the support structure at the working face; in severe cases, it can lead to serious damage to the roadway. Bedrock: refers to the relatively hard rock layer located directly beneath the surface, often consisting of sandstone, limestone, etc. 2. Number of coal seams: The number of coal-bearing seams varies from one coalfield to another; coal seams are generally found in groups. Some coal fields have only a few layers of coal, while others have as many as a dozen or even dozens of layers. The normal distance between adjacent coal seams is called the seam spacing. The spacing between coal seams varies. When the spacing between layers is small, during mining the adjacent coal seams can be treated as a single layer, with the thin layers of rock in between becoming intercalated gangue. Generally speaking, a smaller layer spacing is advantageous for concentrated mining. 3. Burial depth: The burial depth of coal seams varies significantly across different coalfields. Even within the same coal seam, its depth can differ due to factors such as the inclination of the seam or geological structures. If buried too shallowly, the coal seam is prone to weathering, thereby losing its utility ； As the burial depth increases, factors such as mine pressure, ground temperature, gas emission, and water inflow also increase, thereby raising the complexity of mining conditions and the difficulty of mining techniques.