Classification of coal

Categories of coal
Coal classification
Coals come into various categories depending on where they were produced. Their colors range from light brown to dark black.

They can be classified into two main categories: High Grade and Low Grade coals.This refers to the Energy content of the coal, which is the amount of energy that will be released by the combustion of 1 kg of it.

* High Grade coals may release 6 000 to 9 000 kcal / kg, while low grade coals will release between 1 000 to 6 000 kcal / kg.
* High grade coals are darker, contain more carbon, less water and less minerals (cinders) than low grade coals.


Among High Grade coals, we can find :

* Graphite (pure carbon, used to make pencils, or as a lubricant)

* Anthracite (black and shiny, used for domestic heating)

* Bituminous coals (black or dark brown, used for power generation and in the steel manufacturing process, after being converted into coke)

And low grade coals are classified as :

* Sub-bituminous coals (dark brown, used mostly in power plants, and by various industries such as Chemistry, Cement production etc...)

* Lignite (brown, used in power plants)

* Peat (brown, similar to compact mud, used for domestic heating)


Coal is a readily combustible rock containing more than 50 percent by weight of carbonaceous material formed from compaction and indurations of variously altered plant remains similar to those in 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 it remains soft.

* Lignite increases in maturity by becoming darker and harder and is then classified as sub-bituminous coal. After a continuous process of burial and alteration, chemical and physical changes occur until the coal is classified as bituminous - dark and hard coal.
* Bituminous coal ignites easily and burns long with a relatively long flame. If improperly fired bituminous coal is characterized with excess smoke and soot.
* Anthracite coal is the last classification, the ultimate maturation. Anthracite coal is very hard and shiny.

1) Volatile matter - dry mineral matter free basis. In coal, those products, exclusive of moisture, given off as gas and vapor determined analytically.

Anthracite coal creates a steady and clean flame and is preferred for domestic heating. Furthermore it burn longer with more heat than the other types.
Typical Sulfur Content in Coal

* Anthracite Coal : 0.6 - 0.77 weight %
* Bituminous Coal : 0.7 - 4.0 weight %
* Lignite Coal : 0.4 weight %

Typical Moisture Content in Coal

* Anthracite Coal : 2.8 - 16.3 weight %
* Bituminous Coal : 2.2 - 15.9 weight %
* Lignite Coal : 39 weight %

Typical Fixed Carbon Content in Coal

* Anthracite Coal : 80.5 - 85.7 weight %
* Bituminous Coal : 44.9-78.2 weight %
* Lignite Coal : 31.4 weight %

Typical Bulk Density of Coal

* Anthracite Coal : 50 - 58 (lb/ft3), 800 - 929 (kg/m3)
* Bituminous Coal : 42 - 57 (lb/ft3), 673 - 913 (kg/m3)
* Lignite Coal : 40 - 54 (lb/ft3), 641 - 865 (kg/m3)

Typical Ash Content in Coal

* Anthracite Coal : 9.7 - 20.2 weight %
* Bituminous Coal : 3.3-11.7 weight %
* Lignite Coal : 4.2 weight %

Coal Quality :

The amount of impurities (ash and trace elements) in the coal determines its GRADE:
1. Ash Ash is the unburnable part of coal. It is most often sand and clay blown into the swamp or brought in by river or tides. Most commercial coals range from 3% to 9% ash. A train car carrying 100 tons of coal may be actually carrying 91 tons of burnable coal. With nine tons of clay and sand along for the ride. After burning, the ash is either removed from the combustion chamber or it goes up the smokestacks as fly ash. With 100 tons of coal lasting only 20 minutes in a power plant, the amount of ash accumulating is considerable.

2. Trace Elements A number of elements can affect either the combustion process or add to possible atmospheric pollution through emissions from the smoke stacks of power plants including sodium, sulfur, phosphorous, chlorides, nitrates, sulfates, and arsenic.

Sodium in the coal causes ash to precipitate on the boilers reducing the efficiency of the boiler and raising the cost of generating electricity. In one test, a lignite with 8% sodium fouled the boiler such that it had to be shut down for cleaning after only three days.

Sulfur in the coal is released as sulfur dioxide (S02) upon burning. If it is not scrubbed out of the emissions, it will combine with moisture in the air producing sulfuric acid (H2SO4) which makes up some 60 % of acid rain. (The other 40 % is nitric acid from automobiles and trucks.) Coals range from 0.5% to 8% or more sulfur.

Phosphorus in coal causes slagging in boilers reducing their efficiency. In addition, steel made with a phosphorous rich coal as a heat source tends to be brittle.

Chlorides, Nitrates, and Sulfates cause corrosion in the boilers.

Arsenic is present in most coals at the ppb (parts per billion) level. The Appalachian Basin Coal Field averages 13.3 ppb, while the Rocky Mountain coal is 1.8 ppb.