Coal is a sedimentary rock primarily utilized as a fuel source for generating electricity and producing steel. Determining the weight of coal is complex because this fuel lacks a fixed, uniform density. The heaviness of any sample is a property determined by its geological history, chemical composition, and the amount of non-coal material it contains. Understanding coal’s weight requires examining its inherent structure and variable components.
Inherent Weight: The Ranks of Coal
The most fundamental factor determining coal’s inherent weight is its rank, which reflects the degree of change—or coalification—the original plant matter has undergone. This process involves increasing pressure and heat, which drives out moisture and volatile compounds, concentrating the fixed carbon content. As carbon concentration increases, the coal becomes denser and heavier per unit volume.
The ranks progress from Lignite (brown coal) to Anthracite (hard coal). Lignite is the lowest rank, geologically youngest, and has the lowest carbon content (25% to 35%). It is structurally less dense and often contains high inherent moisture. Subbituminous coal is next, possessing a higher carbon content (35% to 45%).
Bituminous coal is a middle-rank coal, with carbon content ranging from 45% to 86%. This type is denser and harder than the lower ranks, making it the most common form used for electricity generation. Anthracite is the highest rank, containing the highest fixed carbon (86% to 97%). Anthracite has been subjected to the greatest heat and pressure, resulting in the densest, hardest, and least porous form of coal.
Modifying Factors: Moisture and Ash Content
While the rank establishes the baseline density, two variable components—moisture and ash—significantly modify the actual weight of a coal shipment. Moisture content is water trapped within the coal’s porous structure or clinging to its surface, directly adding non-combustible weight. Lower-rank coals like Lignite can have very high moisture levels, sometimes exceeding 55% of the total weight.
Moisture exists as both surface water and water held within the coal’s microscopic pores. Even high-rank coals like Anthracite contain moisture, though typically less than 20%. Preparation processes are often employed to reduce this moisture, which increases the coal’s heating value per ton and decreases its shipping weight.
Ash content represents the inorganic, non-combustible mineral matter remaining after the coal is burned, such as rock and clay trapped within the structure. This mineral matter is generally denser than the organic carbon components, so a higher ash percentage directly increases the total weight without adding energy content. Washing the coal in a preparation plant can remove some non-coal material, reducing the overall weight and increasing the fuel’s quality.
Quantifying Coal’s Mass and Volume
Measuring the heaviness of coal involves three distinct concepts of density that account for the spaces within and between the coal particles. The most specific measure is True Density, which represents the density of the pure coal solid itself, excluding all pores and voids. This value is typically determined using specialized equipment like a gas pycnometer.
A second measure is Apparent Density, which includes the volume of the solid material plus the volume of the tiny, internal pores within each coal particle. Apparent density excludes the space between individual pieces of coal. The most practical measure in industry is Bulk Density, which is the weight of the coal when it is loaded into a container, such as a rail car or ship hold.
Bulk density is the figure that answers how heavy a volume of coal is, as it accounts for the weight of the solid coal, internal pores, moisture, ash, and the air-filled spaces between the pieces. This measurement is obtained by placing a broken coal sample into a container of a known volume and measuring the mass required to fill it. Bulk density is used for calculating shipping capacity and storage requirements.
Practical Implications of Coal Weight
The density and bulk weight of coal have direct consequences for its commercial value and logistics. Because coal is transported in massive quantities, bulk density dictates the required volume capacity of storage facilities and transport vessels. A lower bulk density means a greater volume is needed to move the same amount of weight, increasing the number of train cars or the size of the ship required.
Weight also ties directly into the energy content, often expressed in British thermal units (BTUs) per ton. Lower-rank coals, such as Lignite, have a lower energy density due to high moisture content. This means a greater weight of the fuel must be burned to produce the same amount of heat as a smaller weight of high-rank coal. Consequently, lower-energy coal is often uneconomical to transport over long distances, leading to power plants being built near the mines.