What is the density of coal?

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Density and specific density are numerically equal, but their physical meanings are different. Academically, density is generally used, while in industry, specific gravity (formerly known as relative density) is commonly employed. The relative density of coal is the ratio of the mass of coal at 20°C to the mass of water with the same volume. Coal is a porous material, with pores containing water, air, or other gases. Coal also contains mineral impurities. When coal is piled up, there are gaps between the particles, which are filled with air. To suit different applications, there are three ways to express the density of coal: the true density of coal, the apparent density of coal, and the bulk density of coal. True density of coal: The ratio of the mass of a single coal particle at 0°C to the volume of the solid material it contains (excluding the volume of the pores in the coal). Expressed in g/cm3. The true density of coal reflects the physical properties of its molecular spatial structure, and it is closely related to other properties of coal. Determining the true density of coal is necessary for studying its structure, its processing for refinement, and calculating the average quality of coal seams. The true density of coal is commonly determined using the pycnometer method, with water as the displacement medium. The weighed coal sample is immersed in water to fill the pores of the coal, and calculations are performed using Archimedes’ principle. Water is used as the displacement medium, making the operation convenient ； However, water molecules are relatively large in diameter and cannot enter very small capillaries and micropores; thus, the measured true density is only an approximate value. Various wetting agents have been used to improve water penetration into coal. Using helium as a displacement medium allows for a more accurate determination of the true density of coal. Helium has a very small molecular diameter (1.78 λ), allowing it to penetrate into tiny pores, and it does not adsorb onto the surface of coal. However, X-ray studies show that helium also does not easily enter some of the extremely small-pore spaces in anthracite; at room temperature, helium can sometimes be adsorbed by the coal as well. Therefore, the true density determined using helium is not entirely the true value either. The true density of minerals in coal is much higher than that of the organic matter in coal; for example, the true density of quartz is 2.15 g/cm3, that of clay is 2.40 g/cm3, and that of pyrite is 5.00 g/cm3. The average true density of minerals in coal is approximately 3.00 g/cm3. The higher the mineral content, the greater the true density of the coal. Based on empirical data, for every 1% increase in the ash content of coal, its true density increases by 0.01 g/cm3. In scientific research, the true density value of \"pure coal\" (coal organic matter) is sometimes required. Since it is actually impossible to determine the true density of pure coal after completely removing minerals, the measured value can only be corrected based on the content of mineral components and the true density. However, determining the content of mineral components is also relatively complex; therefore, ash content is often used as a substitute to roughly correct for the mineral components. The mineral content and density in coal are roughly equivalent to those of the ash yield and density of coal; therefore, when the mineral content in coal is low, the true density of pure coal can be determined by correcting the ash yield using the following formula. The true densities of coals with different degrees of coalification vary significantly, and exhibit a regular pattern as the degree of coalification increases. The true density of lignite ranges from 1.28 to 1.42 g/cm3, that of bituminous coal is between 1.27 and 1.33 g/cm3, while that of anthracite lies in the range of 1.40 to 1.8 g/cm3. The change in true density from lignite to bituminous coal is not very significant; bituminous coal with a carbon content of around 85% has the lowest true density (1.28–1.30 g/cm3). As the degree of coalification increases, the true density gradually rises, and at the anthracite stage, it increases sharply as the degree of coalification further increases. For coals of the same degree of coalification (as indicated by carbon content), the true densities of their various microstructural components also differ; the vitrine fraction (see inert fraction) has the highest true density, the kerinite fraction has a lower one, and the stable fraction has the lowest. (See figure) As the degree of coalification increases (with an increase in C content), this difference gradually disappears. Apparent density of coal: At 20°C, it is the ratio of the mass of a single coal particle (or lump) to its apparent volume (including the pores in the coal), expressed in g/cm3; it is also known as the pseudo-density of coal. Calculating the reserves of coal, as well as the processes of its transportation, crushing, and combustion, all require data on the apparent density of coal. There are various methods for determining visual density, with the waxing method (or vaseline application method) and the mercury method being commonly used. The waxing method involves applying a thin layer of wax to the outer surface of the coal particles, thereby sealing their pores and preventing the medium from entering. The wax-coated coal particles are immersed in water and weighed using a hydrometer; the apparent volume of the coal particles is determined based on Archimedes’ principle, thereby allowing the apparent density to be calculated. The mercury method involves immersing coal particles directly into a mercury medium. Due to the high surface tension of mercury, it cannot penetrate into the pores of the coal under normal pressure. The volume of mercury displaced by the coal particles corresponds to the apparent volume of those particles, including their pores, and this value can then be used to calculate the apparent density of the coal. Coals with different degrees of coalification exhibit significant differences in apparent density: lignite ranges from 1.05 to 1.30 g/cm3, bituminous coal ranges from 1.15 to 1.50 g/cm3, and anthracite ranges from 1.40 to 1.70 g/cm3. Bulk density of coal: The ratio of the mass of the coal particles filling a container to the volume of that container (including the spaces between the particles), expressed in t/m3; it is also known as the packed density of coal. Data on the bulk density of coal is required when designing coal silos, calculating the mass of coal piles and the loading capacity of vehicles and ships, as well as determining the amount of coal that can be loaded into coke ovens and gasification furnaces. Scatter density is measured directly in a specific container. The larger the container used for measurement, the higher the accuracy. The bulk density of coal is a conditional parameter, influenced by factors such as the size and shape of the container, the method of loading the coal, as well as the moisture and particle size of the coal. To achieve better comparability and come as close as possible to reality, strict regulations are set for the measurement conditions. The bulk density of coal increases as the coal particles grow larger. The more uniform the particle size, the lower the bulk density of the coal. The bulk density of coal gradually decreases until it reaches around 5% moisture content, after which it remains essentially stable. In practical production, the bulk density of coal is generally 0.50~0.75 t/m3. Based on the apparent density and bulk density of coal, the porosity or void fraction of the coal pile layer can be calculated, which is the ratio of the volume of voids between the coal particles to the volume of the coal particles themselves. Porosity can be used to determine the gas flow resistance in the pile layer.