Coal Formation and Coal Rank-The Buried Sunshine

April 22, 2011, (Coal Geology) Coal is known as “buried sunshine,” because the plants which formed coal captured energy from the sun through photosynthesis to create the compounds that make up plant tissues. The most important element in the plant material is carbon, which gives coal most of its energy. Coal is found in seams/coal bed which varies from a few inches to 100 feet or more in thickness. Coal can be expressed as an aggregate of variable organic constituents called macerals or lithotypes such as vitrain, fusain, durain and clarain along with some amount of mineral matter. It is important to note that coal is a “non-renewable” fossil fuel. In short, coal can not be reproduced in the vast mined out areas in our lifetime.

Coal is the most abundant energy source in the US. Nearly 1.7 Trillion tons of coal resources have been documented in the US. In terms of world coal reserve, the US has nearly 20% of the world coal deposit. At current domestic consumption rate, coal could provide energy for  at least another 300 years.

The coal can be mined by the surface method (complete removal of overburden) or deep (underground) method based on the amount of ground cover or over burden. In early years of coal mining, horses, mules and other animals pulled the cart from the miner working at the face of the mine to the adit (the entry portal of the mine). In places where a surface adit could not be made due to the depth of the coal seam,  a large elevator or a skip bucket was used to brought the coal to the surface. Advanced machines came to replace the miners in the mid-20th century. Mining methods have always been changing parallel to the growth of machinery and technology in the mining industry. Drag lines and power shovel can remove entire hilltops to uncover target seams today. The long wall mining machines needs just 3 miners to cut a 500 feet wide swath through the coal seam.  Today we can mine more tonnage of coals with less manpower because of the tremendous advancement of mining technology. In fact, present world use only about 10% of employees than the coal mining employees in 1940-50.

So, what is Coal?
We have discussed the definition of coal in earlier articles. In general, coal could be defined as a a readily combustible rock containing more than 50% by a weight of carbonaceous material, formed from compaction of variously altered plant remains. Peat is an unconsolidated deposit of plant remains from a water saturated environment such as a bog or mire, structure of vegetal matter can be seen and when dried peat burns freely. Peat is the early stage of coal formation.

Coal Formation:
Coal is formed by the physical and chemical alteration of peat by processes that involve bacterial decay, compaction, heat and time. The physical, chemical and petrographical nature of coal determines the rank and type of use.

In-situ Theory: Some scientists believe that the coal formation is always in-situ. The coal forming plants grew in the swamp area. Erosion, subsidence buried the plant debris and form coal bed over time. Humic coals fall under this group.  Humic coals form in the lakes, swamps, bogs and lagoons with dense forest. In-situ coals are thereby “autochthonous”. The great swamp of Georgia in the US is an example of such coal formation.

Drift Theory: Some scientists believe in drift theory for coal formation. According to this theory, the plants grew in different areas. The plant debris is carried to the depositional basin by current. Trees along the river channels were drifted downstream. The plant remains could then accumulate with other sediments on the sea floor.

In reality, the formation of coal involves both theory discussed above. The plant debris could accumulate in-situ at a location, but occasionally transported by water into depositional environment.

Factors affecting coal formation:

There are several factors that control the coal formation:

1. Transformation of plant debris to peat
2. Nature and chemical composition of source plants.
3. Climate
4. Paleogeography
5. Depositional environment
6. Tectonics
7. Time

COALIFICATION: Biochemical and geochemical stages of coal formation

The process of coal formation is known as coalification.

Peat is an accumulation of partially decayed vegetation matter. Peat deposits could contain everything from pristine plant parts (roots, barks, spores etc.) to decayed plants, decay products & even to charcoal if the peat caught fire. Peat deposits formed in a waterlogged environment (wet condition also reduces chemical decay of the plant materials) where plant debris is accumulated, for example, peat bogs and peat swamps. The accumulation of plant debris exceeds the rate of bacterial decay of the debris as the available oxygen in organic-rich water is completely used up by the decaying process. For the peat to become coal it has be protected from oxygen (oxidizing environment can’t form coal) and must buried by sediment. Burial promotes the compaction of peat and helps water to squeeze out during the early stages.

Formation of peat is considered the biochemical stage of coal formation. During this stage, plant residue partially decompose (degradation of plant cellulose materials). Some authors call the stage of peat formation as the “humification stage“.

Once the peat is buried under thick sediments, it is subject to high temperature and pressure over long period of geologic time. The geochemical stage is the irreversible and continuous process that turns the peat into sedimentary rock (conversion of the lignin into humic substance; and condensation of humic substance to form larger coal molecules). Diagenesis and metamorphism gradually turns the peat into lignite, sub-bituminous, bituminous and anthracite with increasing temperature. The geochemical process, due to diagenesis and metamorphism,  that changes the peat into humic coal series is called the “coalification stage”. During the coalification stage, physical properties such as color, lustre, hardness, specific gravity, porosity, vitrinite reflectance. texture and structure of the coal forming macerals are substantially changes.

httpv://www.youtube.com/watch?v=MBeXRRTGjNE

Phase III: Bituminazition: After the completion of initial bacterial decay stages, the peat must be buried under thick sediment that acts an insulating layer trapping the natural heat rising to the surface. The bituminization process begins when the temperature reaches 100°C, (212°F). Chemical reactions drive off water, oxygen and hydrogen which raises the percentage of carbon during this process.

As the burial continues, addition of heat and time causes the complex hydrocarbon compounds to breakdown and alter in various ways. The gaseous alteration products such as methane are typically expelled from the deposit making the formation richer in carbon content over time. It could take millions of years to convert peat to anthracite coal.

Both temperature and pressure play important role during the stages of geochemical process. There is some disagreement about temperature or pressure being the most important factor. One other factor is time. It could take million of years for the peat to turn into coal under right temperature and pressure.

The path of peat to coal could be shown as linear transformation as shown below:
plant debris -> peat -> lignite -> sub-bituminous coal -> bituminous coal -> anthracite coal -> graphite (a pure carbon mineral).

It is estimated that about 10 vertical feet of original peat material  is needed to produce 1 vertical foot of bituminous coal due to squeezing and compaction of peat material.

What would you find in Coal: The Maceral Groups
Coal an organic sedimentary deposit made up of organic grains called macerals. Coal petrographers study coal under the microscope and separate the macerals into three main groups, each subdivided into several macerals types. These groups are liptinite, vitrinite and inertinite and are defined according to their grayness in reflected light: Liptinites are dark gray, vitrinites are medium to light gray and inertinites are white and can be very bright.

• Liptinites are made up of hydrogen rich hydrocarbons derived from spores, pollens, cuticles and resins in the original plant materials.
• Vitrinites are made up of wood, bark, and roots and contained less hydrogen than the Liptinites.
• Inertinites are mainly oxidation products of the other macerals and are richer in carbon, it includes fusinite, most of which is fossil charcoal derived from ancient peat fires.

ANTHRACITE COAL
Anthracite is coal with the highest carbon content, between 86 and 98 percent and with heat value of nearly 15,000 BTUs-per-pound. Anthracite is most frequently associated with home heating. Anthracite is however covers only a very small segment of the world coal market. Highest Grade Coal in U.S with low sulfur content. Anthracite is very rare in the United States and make up only 2% of overall production. The only anthracite mines in the United States are located in northeastern Pennsylvania.

BITUMINOUS COAL
The Bituminous coal is the cost abundant form of coal in the world. Bituminous coal is primarily used to generate electricity and make coke for the steel industry. Bituminous coal has a carbon content ranging from 45 to 86 percent carbon and a heat value of 10,500 to 15,500 BTUs-per-pound. It is also the most abundant rank of coal found in the United States, accounting nearly half of U.S. coal production. However, Bituminous coal has a high Sulfur content and thus is the Lowest Grade U.S. coal deposit. (Example, Illinois Coal).

SUB-BITUMINOUS COAL
Sub-Bituminous coal ranks below the bituminous coal had has 35-45 percent carbon content and heat value varying between 8,300 and 13,000 BTUs-per-pound. Although the heat value is lower for the Sub-Bituminous coal, the sulfur content is typically less than other coal forms. The low sulfur content helps the coal to burn clean and makes attractive to many end users. Over 40 percent of the coal produced in the United States is Sub-Bituminous. Sub-Bituminous coal is mainly produced in the Western US. Central Illinois Coal Burning Power Plants mostly use the Sub-Bituminous coals.

LIGNITE COAL
Lignite is a geologically young coal with the lowest carbon content, 25-35 percent and heat value ranging between 4,000 and 8,300 BTUs-per-pound. Sometimes Peat is also called brown coal and mainly used for electric power generation. Mainly found in Western U.S. Lignite is crumbly and has high moisture content. About eight percent of the coal produced in the United States is lignite.

Use of coal:
The main use of coal is for electricity generation. Coal is also used to heat homes while coking coal is used for iron and steel making.

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