Coal is a readily combustible black or brownish-black sedimentary, organic rock. It is formed from plant remains that have been compacted, hardened, chemically altered, and metamorphosed by heat and pressure over 300 million years. Coal is composed mainly of carbon, hydrogen and oxygen, and smaller amounts of nitrogen, sulphur and other elements. It also contains a small amount of water, and grains of inorganic matter that remain as a residue known as ash when coal is burnt
Coal Formation
During the Pennsylvanian Period, the Earth was covered with large swampy forests of giant ferns, reeds and mosses, which grew taller than our tallest trees today. As these plants died and fell into the swamp water, new plants grew to take their place and when these plants died, still others grew. In time, there was a thick layer of dead, decaying plants in the water.
The surface of the earth also changed and sediments washed into the water, covering the dead plants and preventing them from completely decomposing. More plants grew, but they too died and fell into the water, forming a separate layer of decaying plants which over time were also covered by sediments. After millions of years, many layers of organic matter and sediments had formed one on top of the other.
The weight of the overlying layers compressed the lower layers of organic matter, forming peat. Heat and pressure caused by the overlying sediments produced chemical changes in the peat, forcing out oxygen and hydrogen andleaving behind rich carbon deposits, i.e. coal. Geologists estimate that a layer of plants 20 feet thick may have been required to form a one foot thick coal seam. Coal seams vary in thickness, ranging from only a few inches thick to more than 100 feet in thickness.
Coal Rank and Classification
Coal starts off as peat. After a considerable amount of time, heat, and burial pressure, it is metamorphosed from peat to lignite. Lignite is considered to be "immature" coal at this stage of development because it is still somewhat light in color and remains soft. As time passes, lignite increases in maturity by becoming darker and harder and is then classified as sub-bituminous coal. As this process of burial and alteration continues, more chemical and physical changes occur and the coal is classified as bituminous. At this point the coal is dark and hard. Anthracite is the last rank in the classification, and this term is used when the coal has reached ultimate maturation. Anthracite coal is very hard and shiny.
The degree of alteration (or metamorphism) that occurs as a coal matures from peat to anthracite is referred to as the "rank" of the coal. Low-rank coals include lignite and sub-bituminous coals. These coals have a lower energy content because they have a low carbon content. They are lighter (earthier) and have higher moisture levels. High-rank coals include bituminous coals and anthracite. They contain more carbon which results in a much higher energy content. They have a more vitreous (shiny) appearance and lower moisture content than low-rank coals.
All coal has carbon and sulfur to some degree. Coal that was formed in swamps covered by seawater contains a higher sulfur content; low sulfur coal was generally formed under freshwater conditions.
Lignite
Lignite, also known as brown coal, is a brownish-black coal with generally high moisture and ash content and lower heating value. Geologically, it is the youngest and the lowest ranked coal, containing 25 to 35% carbon and the lowest heating value - 4,000 to 8,300 Btus (British thermal units) per pound. However, it is an important form of energy for generating electricity and producing synthetic natural gas and liquids. About 9% of the United States’ coal reserves are lignite.
Sub-bituminous Coal
Under more pressure, some lignite was changed into the next rank of coal: sub-bituminous. This coal is a dull black coal with a higher heating value than lignite. It is used primarily for heating and generating electricity. It contains about 35 to 45% carbon and has a heating value between 8,300 to 11,500 Btus per pound. In the United States, sub-bituminous coal is predominately found in Montana, Wyoming, Utah, Colorado, New Mexico, Washington, and Alaska. It accounts for about 37% of the coal reserves in the US.
Bituminous Coal
This rank of coal, sometimes called "soft coal" or “black coal”, is the most common type of coal found in the United States. It is 45 to 86% carbon and has a heating value between 10,500 and 14,000 Btus per pound. This is the type most commonly used for electric power generation in the US and for producing coke for the steel industry. About 52 percent of the United States' coal reserves are bituminous coal.
Anthracite
Sometimes called “hard coal", anthracite was formed from bituminous coal when great pressures developed in folded rock strata during the creation of mountain ranges. Anthracite has the highest energy content of all coals. It contains 86 to 97% carbon, and has a heating value of nearly 15,000 Btus per pound. It is used for heating and generating electricity, and accounts for about 2% of the coal reserves in the United States.
Geologists also classify coal types according to the organic debris, called macerals, from which the coal is formed. Macerals are identified (microscopically) by reflected light, the reflective or translucent properties of the coal indicating the individual component macerals and the way they have combined to form the coal.
The purpose of classifying coal in this way is to determine its best uses. There is a finite supply of the resource and therefore type and chemical composition must be matched to the most suitable end use.
If you are not familiar with the terminology used in the coal industry, please visit the Energy Information Administration’s coal glossary.