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Lecture 22: Coal Processing Overview

Types of Coal

Coals are classified into different "ranks" based on the degree to which the original plant material has

been transformed into carbon. Rank is a rough indication of the age of the coal. Generally, the older the

coal, the higher its carbon content. Coal ranks (from most to least carbon content) are as follows:

- Anthracite

- Bituminous

- Sub-bituminous

- Lignite

Coal with the highest carbon content is the best and cleanest type.

Lignite coal

Lignite (sometimes called "brown coal" is used almost exclusively for electric power generation. Lignite

is a young coal. Lignite is brownish black, has a high moisture content (up to 45 %), and a high sulphur

content. Lignite is more like soil and tends to disintegrate when exposed to the weather. Lignite is also

called brown coal. It has a colorific value of less than 5 kW/kg.

Sub-bituminous coal

Sub-bituminous coal is referred to as black lignite. Sub-bituminous coal contains 20-30 % moisture and is

used for generating electricity and space heating. It has a calorific values ranging from 5 - 6.8 kW/kg.

Bituminous coal

Bituminous coal is a soft, dense, black coal. Bituminous coal often has bands of bright and dull material.

Bituminous coal is the most common coal with a moisture content below 20 %. It is used for generating

electricity, making coke, and for space heating. Their calorific value ranges from 6.8 - 9 kW/kg.

Anthracite coal

Often referred to as hard coal, anthracite is black and lustrous. It is low in sulfur and high in carbon. It is

the highest rank of coal with a moisture content below 15 %. Its calorific values is 9 kW/kg or above.

Fig. 1. Types of Coal and Requirements for formation

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Fig. 2. Ranks of Coal and Properties.

Fig. 2. Properties of different types of coal.

Coal is produced and marketed for two essential purposes:

Thermal coal for burning and producing heat and electricity

Metallurgical coal to produce coke and smelt metals (Fe, Pb, Zn, Cu).

Processing Coal is called Coal Preparation and the plants are referred to as Coal Washing Plants.

The objectives of Coal Preparation include:

(1) Grading into sizes for sale.(2) Removal of undesirable constituents Ash and Pyrite.

Coal may be mined from underground or surface operations. Surface operations include strip mining,

open pit, and mountain-top. When produced by mechanized mining, coal arrives on surface in an

assortment of sizes from dust to coarse lumps, with inclusions of chalk, clay, slate, ankerite

((Ca,Mg,Fe,Mn)CO3), and pyrite.

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Mined coal contains two kinds of ash-forming materials - "fixed" and "free". Fixed ash derives from

inorganic matter in the tissues of the original coal-forming plants together with fine silt entrained during

deposition of the seam. Free ash is extraneous to the true coal seam and comes from the adjacent rock.

The coal can include clay, shale, locked-gangue and other mined rock. Fixed ash is not removable by

standard cleaning methods, but free ash can be reduced by suitable treatment.

Sulphur also occurs in two forms: as carbonaceous sulphur intimately associated with the coal and as

Pyrite. Pyritic sulphur can be removed by flotation - chemical leaching methods have also been studied,

but are too expensive.

Eastern North American coals are predominantly mined from underground sources and generally have

high sulphur content (>2%) while Western coals in both the US and Canada derive from open pit

operations and have low sulphur content (

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