The formation of coaloccurs over the course of a very long period. Most of the coal that people mine and use today was formed around 300 to 400 million years ago. Coal is basically the decayed remains of plant matter that was once thriving and has undergone a process sometimes referred to as coalification. During this process, there are many distinct stages that can result in several different types of coal depending on the conditions and how much time the material has had to change.
From the beginning as peat to the end of process which produces anthracite coal, coalificationis the result of many different factors that culminate for create the fossil fuel known as coal. The initial plant material, peat, will be subjected to pressure, heat, and decay which will eventually transform it into the four different types of coal. Depending on the exact circumstances, the process may halt at some point or continue on to the final form of coal.
Around the time of 300 to 400 million years ago, most of the Earths surface was a hot swampy area. The swamp had an incredible amount of plant matter and as it died or was covered up by other growth, the material was pushed down into the Earth’s surface where it became known as peat. The initial material that created peat could have been moss, trees, leaves, roots, and any plant matter. Peat is the initial state of coal before it undergoes the coalification process.
Covered by more plant matter, or eventually by sediments, if the correct factors were met, then the peat eventually began the coalification process. The pressure from being covered with material would have squeezed out all of the water from the peat. The plant material would have begun to decay due to anaerobic bacteria. As the material was pushed further beneath the surface, the temperature and pressure will increase and the organisms would have begun consuming materials altering the peat. Gases are formed and escape while the carbon material from the plants is left over.
The original amount of carbon present in the peat is low but as the other materials are consumed or leave the peat, the carbon content will increase. Should the process continue and not be halted, then the carbon content will continue to increase into the many stages or types of coal formation.
The first of the four stages of coal formation is lignite coal. The bacteria has removed a large portion of the other constituents of the plant matter and left around 25 to 34 percent. Lignite has a brownish coloring that is not characteristic of what people commonly associate coal to look like. The heating value in this type of coal is around 4,000 to 8,300 BTU (British Thermal Units which is the most common way to measure the heat value of coal) per pound. This type of coal makes up the vast majority of the coal found in the world.
As the coalification process continues, the lignite will form into the second stage which is sub-bituminous coal. The carbon content of this type of coal is approximately 35 to 45 percent while the heat value has also increased to between 8,300 and 13,000 BTU per pound. Sub-bituminous is sometimes more desirable than the other forms of coal because it has a lower sulfur content. The lower sulfur content means that it burns cleaner and less equipment is needed to capture emissions.
The third stage of the coalification process is bituminous coal. As per the course of the process, the carbon content of this type of coal is higher than the lower levels at around 45 to 86 percent. The heat value has also increased to between 10,500 and 15,500 BTU per pound. In the United States, this type of coal is the most abundant and used extensively in the power and steel industries.
The fourth and final stage of the coalification process is anthracite coal. The carbon content of anthracite coal is between 85 to 98 percent. Beyond 98 percent and the material becomes graphite which is not useful for power generation because it is quite difficult to burn. Anthracite coal doesn’t have the highest possible heat value but does have a consistently high heat value of 15,000 BTU per pound. This type of coal is most commonly used to heat the homes that still use coal heating.