An Introduction to Coal: Its Classifications

We all know that usage of coal garnered lots of criticism especially in the field of mining and power generation but we cannot deny the fact that this organic sedimentary rock has played a very important role in helping mankind achieve industrialization for centuries and even up to this day especially to those developing countries. And with that, allow me to share on what is a coal and most importantly, its classifications.

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What is coal?

Coal is a combustible black or brownish-black sedimentary rock that has been formed by the accumulation of organic matter such as wood, grass and shrubs; and is extracted from the earth either by surface mining or underground mining. Those mentioned organic matter has undergone thousands or millions of years of intense heating and compression from the surface of the earth before becoming a rock that is composed mainly of carbon with a small percentage for other known elements such as hydrogen, oxygen, nitrogen and sulfur. The relatively high carbon content of coal earned its reputation as a great of source of energy and at the same time made it a key ingredient in steelmaking process.

How it is classified?

We've heard this phrase "not all eggs are created equal", in the same way this also applies to coals. Important factors that resulted in variations of coals include the following: extent and duration of compression and heating; and the vegetation of the location for which the coal has been extracted.

If the extracted coal has layers of charcoal, it is an indication that the location has experience a drought or simply was being burned all throughout its geological history. And if the extracted coal has a very fine structure, it is an indication that the location was once a vegetation for which grass and shrubs flourish; whereas if the structure of the extracted coal has traces of wood or logs, well, it simply means that the location was a forest for which there are many logs and woods.

The extent and duration of compression and heating that the coal has undergone greatly affects its composition and usability, since if it has been heated to a very high temperature and pressure for prolonged duration of time it may result to the disappearance of volatile matter that the coal once had thereby leaving only the minerals and carbon.

Because of the aforementioned factors that affect the composition of coal, scientists devised three ways of classifying coal which are as follows: by rank, by grade and by type. For this article, we shall only tackle the classification by rank since it is the most commonly used classification that is being used by the power generation and steelmaking industries. Another reason why I decided to just tackle the classification by rank is that my next series of posts will lead to the power plant engineering for which I will be starting with the fuel which is coal and additionally, I will tackle the other important roles of coal in other industries like steelmaking and cement manufacturing.

Classifications of Coal

The classification by rank of coal is derived by analyzing the chemical composition of coal specifically its carbon content (coined as fixed carbon), volatile matter and moisture content. Coals that have been subjected to longer and intense periods of metamorphism are classified as high ranking coals. In order of increasing ranks, coals are classified as peat, lignite, subbituminous, bituminous, semianthracite, anthracite and superanthracite.

Peat

Peat belongs to the lowest rank of coals wherein it is characterized by its high moisture content with only small percentages of volatile matter and fixed carbon. Although it has been used as a source of energy for thousands of years, the advancement of today's technology resulted unto the gradual decrease of usage of this coal as a fuel for power generating plants but in some places in the world especially in the Arctic, this coal is used for domestic purposes such as for heating and cooking.

Lignite

Lignite is the second classification by rank of coal which is a dark brown to black in color combustible rock. It has a higher heating value as compared to peat due to its fixed carbon and volatile matter content. After this rock is mined and dried, lignite disintegrates into small flakes or in other words slacks which often results to difficulty of transporting this coal from distant places and the main culprit for that is its moisture content. Fortunately, this coal is perfect for power generation specifically in steam power plants and that is the very reason why steam power plants that uses lignite as its fuel must ideally be situated near the mining site of lignite.

Subbituminous
Subbituminous coal is the lowest rank for bituminous coals and the very reason for that is it has the same problems with lignite which is due to slacking that adds difficulty in transporting this rock. And you might be wondering why it has properties found common to lignite, well it is because it falls between lignite and bituminous coal. Lastly, this rock is generally dark brown to black in color.

Bituminous

Bituminous coals which is also known as black coal that is characterized by low moisture content and non-slacking properties. This coal has lots of classification or grades for which those low-rank bituminous coals are rich in gas and other coal by-products wherein it is not suitable for domestic and powerplant usage since the emissions of being produced when this coals are used as fuels is so dark and forms soot when it is in contact with metals or solid materials specifically buildings. A remarkable property for bituminous coals is its caking ability which results to the formation of coke, which is a fuel with a high carbon content and few impurities made successfully without the aid of air.

Semibituminous
Semibituminous coals are characterized with its high-carbon content and most notably, the highest heating value of all known coals. Additionally,

Heating value or calorific value is simply defined as the energy that is being released by any fuel when it is completely burned and when the products of combustion are cooled to the original fuel temperature.^[1]

Due to its high heating value, this coal that is nearly smokeless when burned are one of the highly sought after fuel for power generation.

Semianthracite
Seminanthracite coal is much harder than semibituminous coal and although it has a very high fixed carbon content which translates to a heating value that is next to semibituminous coals, the price is way too high if it is used for power generation simply because this coal is scarce in numbers.

Anthracite

Anthracite coal is also scarce and although it has a much higher fixed carbon content than semianthritic coals, its heating value falls next to semianthracite coals since it has a much higher moisture content than the latter. Since this coal has the highest known fixed carbon content amongst all coals, when burned it produces a short and bluish flame.

Superanthracite
Well, this coal is relatively close to graphite and although it has a high heating value, superanthracite coals are very difficult to ignite for which it isn't considered for use in the field of power generation.

Furthermore, there is a table below, for which coals classified by rank are shown with its respective content (fixed carbon, volatile matter and moisture) which is expressed in percent and its heating value in British thermal unit per pound (Btu/lb_m); for which both are obtained using the ash-free basis analysis of coal. The data found in the table is obtained from the book, Power Plant Theory and Design 2^nd edition by P. Potter.

Coal	Fixed Carbon (in %)	Volatile Matter (in %)	Moisture (in %)	Heating Value (in Btu/lbm)
Peat	4.6	10.4	85	1,290
Lignite	28.7	25.8	45.5	6,640
Subbituminous	42.4	34.2	23.4	9,720
Bituminous	47.0-64.6	32.2-41.4	3.2-11.6	12,880-15,160
Semibituminous	75.0-83.4	11.6-22.0	3.0-5.0	15,360-15,480
Semianthracite	85.8	11.7	2.5	14,880
Anthracite	95.6	1.2	3.2	14,440
Superanthracite	80.5	3.1	16.4	11,480

The table above summarizes the classification by rank of coals and I guess that's all for this article. Stay tune for more articles relating to the importance of coal in power generation and steelmaking industries.

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References

1. Potter, P. (1959). Power Plant Theory and Design (2^nd ed.). New York: The Ronald Press Company.
2. https://en.wikipedia.org/wiki/Coal. Retrieved on July 25, 2018.
3. https://www.studentenergy.org/topics/coal. Retrieved on July 25, 2018.
4. https://www.eia.gov/energyexplained/index.php?page=coal_home. Retrieved on July 25, 2018.
5. https://en.wikipedia.org/wiki/Coal_mining. Retrieved on July 25, 2018.
6. https://www.britannica.com/topic/coal-classification-1703417. Retrieved on July 25, 2018.
7. https://en.wikipedia.org/wiki/Peat. Retrieved on July 25, 2018.
8. http://www.fao.org/docrep/x5872e/x5872e0b.htm. Retrieved on July 25, 2018.
9. https://en.wikipedia.org/wiki/Lignite. Retrieved on July 25, 2018.
10. https://en.wikipedia.org/wiki/Sub-bituminous_coal. Retrieved on July 25, 2018.
11. https://www.britannica.com/science/subbituminous-coal. Retrieved on July 25, 2018.
12. https://en.wikipedia.org/wiki/Coke_(fuel). Retrieved on July 25, 2018.
13. https://en.wikipedia.org/wiki/Anthracite. Retrieved on July 25, 2018.

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