What are the types of coal?

Coal comes in four main types or ranks, lignite or brown coal, bituminous coal or black coal, anthracite and graphite. Each type of coal has a certain set of physical parameters which are mostly controlled by moisture, volatile content (in terms of aliphatic or aromatic hydrocarbons) and carbon content.

Moisture?

Moisture is an important property of coal, as all coals are mined wet. Groundwater and other extraneous moisture is known as adventitious moisture and is readily evaporated. Moisture held within the coal itself is known as inherent moisture and is analyzed. Moisture may occur in four possible forms within coal:

  • Surface moisture: water held on the surface of coal particles or minerals
  • Heat Capacitya
  • Hydroscopic moisture: water held by capillary action within the microstructures of the coal
  • Decomposition moisture: water held within the coal's decomposed organic compounds
  • Mineral moisture: water which comprises part of the crystal structure of hydrous silicates such as clays
  • Heating the coal within a solution of toluene
  • Drying in a minimum free-space oven at 150 °C within a nitrogen atmosphere/li>
  • Drying in air at 100-105 °C and relative loss of mass determined

Ash

Ash content of coal is the non-combustible residue left after coal is burnt. It represents the bulk mineral matter after carbon, oxygen, sulphur and water (including from clays) has been driven off during combustion. Analysis is fairly straightforward, with the coal thoroughly burnt and the ash material expressed as a percentage of the original weight.

Fixed Carbon

The fixed carbon content of the coal is the carbon found in the material which is left after volatile materials are driven off. This differs from the ultimate carbon content of the coal because some carbon is lost in hydrocarbons with the volatiles. Fixed carbon is used as an estimate of the amount of coke that will be yielded from a sample of coal. Fixed carbon is determined by removing the mass of volatiles determined by the volatility test, above, from the original mass of the coal sample.

Chemical Analysis?

Coal is also assayed for oxygen content, hydrogen content and sulphur. Sulphur is also analyzed to determine whether it is a sulfide mineral or in a sulfate form. This is achieved by dissolution of the sulfates in hydrochloric acid and precipitation as barium sulphate. Sulfide content is determined by measurement of iron content, as this will determine the amount of sulphur present as iron pyrite.
Carbonate minerals are analyzed similarly, by measurement of the amount of carbon dioxide emitted when the coal is treated with hydrochloric acid. Calcium is analyzed. The carbonate content is necessary to determine the combustible carbon content and incombustible (carbonate carbon) content.
Chlorine, phosphorus and iron are also determined to characterize the coal's suitability for steel manufacture.
An analysis of coal ash may also be carried out to determine not only the composition of coal ash, but also to determine the levels at which trace elements occur in ash. These data are useful for environmental impact modeling, and may be obtained by spectroscopic methods such as ICP-OES or AAS

Relative density

Relative density or specific gravity of the coal depends on the rank of the coal and degree of mineral impurity. Knowledge of the density of each coal ply is necessary to determine the properties of composites and blends. The density of the coal seam is necessary for conversion of resources into reserves.
Relative density is normally determined by the loss of a sample's weight in water. This is best achieved using finely ground coal, as bulk samples are quite porous.

Particle size distribution

The particle size distribution of milled coal depends partly on the rank of the coal, which determines its brittleness, and on the handling, crushing and milling it has undergone. Generally coal is utilized in furnaces and coking ovens at a certain size, so the crushability of the coal must be determined and its behavior quantified. It is necessary to know these data before coal is mined, so that suitable crushing machinery can be designed to optimize the particle size for transport and use.

Abrasion Testing

Abrasion is the property of the coal which describes its propensity and ability to wear away machinery and undergo autonomous grinding. While carbonaceous matter in coal is relatively soft, quartz and other mineral constituents in coal are quite abrasive. This is tested in a calibrated mill, containing four blades of known mass. The coal is agitated in the mill for 12,000 revolutions at a rate of 1,500 revolutions per minute. The abrasion index is determined by measuring the loss of mass of the four metal blades.

Specific Energy

Aside from physical or chemical analyses to determine the handling and pollutant profile of a coal, the energy output of a coal is determined using a bomb calorimeter which measures the specific energy output of a coal during complete combustion. This is required particularly for coals used in steam-raising.

Ash Fusion Test

The behavior of a coal's ash residue at high temperature is a critical factor in selecting coals for steam power generation. Most furnaces are designed to remove ash as a powdery residue. Coal which has ash that fuses into a hard glassy slag known as clinker is usually unsatisfactory in furnaces as it requires cleaning. However, furnaces can be designed to handle the clinker, generally by removing it as a molten liquid. .

  • Deformation temperature: This is reached when the corners of the mould first become rounded
  • Softening (sphere) temperature: This is reached when the top of the mould takes on a spherical shape.
  • Hemisphere temperature: This is reached when the entire mould takes on a hemisphere shape
  • Flow (fluid) temperature: This is reached when the molten ash collapses to a flattened button on the furnace floor.
Crucible swelling index (Free Swelling Index)

The simplest test to evaluate whether a coal is suitable for production of coke is the Free Swelling Index test. This involves heating a small sample of coal in a standardized crucible to around 800 degrees Celsius.
After heating for a specified time, or until all volatiles are driven off, a small coke button remains in the crucible. The cross sectional profile of this coke button compared to a set of standardized profiles determines the Free Swelling Index.