As geological processes apply pressure to dead biotic
material over time, under suitable conditions it is transformed successively
into:
● Peat, considered to be a precursor of
coal, has industrial importance as a fuel in some regions, for example, Ireland
and Finland. In its dehydrated form, peat is a highly effective absorbent for
fuel and oil spills on land and water
● Lignite, also referred to as brown coal,
is the lowest rank of coal and used almost exclusively as fuel for electric
power generation. Jet is a compact form of lignite that is sometimes polished
and has been used as an ornamental stone since the Upper Palaeolithic
● Sub-bituminous coal, whose properties
range from those of lignite to those of bituminous coal are used primarily as
fuel for steam-electric power generation. Additionally, it is an important
source of light aromatic hydrocarbons for the chemical synthesis industry.
● Bituminous coal, dense sedimentary rock,
black but sometimes dark brown, often with well-defined bands of bright and
dull material, used primarily as fuel in steam-electric power generation, with
substantial quantities also used for heat and power applications in
manufacturing and to make coke
● Steam coal is a grade between bituminous coal
and anthracite, once widely used as a fuel for steam locomotives. In this
specialized use it is sometimes known as sea-coal in the U.S.
Small steam coal (dry small steam nuts or DSSN) was used as a fuel
for domestic water heating
● Anthracite, the highest rank; a harder,
glossy, black coal used primarily for residential and commercial space heating.
It may be divided further into metamorphicallyaltered
bituminous coal and petrified oil, as from the deposits in
Pennsylvania
● Graphite, technically the highest rank,
but difficult to ignite and is not so commonly used as fuel: it is mostly used
in pencils and, when powdered, as a lubricant.
The classification of coal is generally based on the content
of volatiles. However, the exact classification varies between countries.
According to the German classification, coal is classified as follows:
|
NAME |
VOLATILES % |
C CARBON % |
H HYDROGEN % |
O OXYGEN % |
S SULFUR % |
HEAT CONTENT KJ/KG |
|
Braunkohle(Lignite) |
45-65 |
60-75 |
6.0-5.8 |
34-17 |
0.5-3 |
<28470 |
|
Flammkohle(Flame coal) |
40-45 |
75-82 |
6.0-5.8 |
>9.8 |
~1 |
<32870 |
|
Gasflammkohle(Gas flame coal) |
35-40 |
82-85 |
5.8-5.6 |
9.8-7.3 |
~1 |
<33910 |
|
Gaskohle (Gas coal) |
28-35 |
85-87.5 |
5.6-5.0 |
7.3-4.5 |
~1 |
<34960 |
|
Fettkohle (Fat coal) |
19-28 |
87.5-89.5 |
5.0-4.5 |
4.5-3.2 |
~1 |
<35380 |
|
Esskohle(Forge coal) |
14-19 |
89.5-90.5 |
4.5-4.0 |
3.2-2.8 |
~1 |
<35380 |
|
Magerkohle(Non baking coal) |
10-14 |
90.5-91.5 |
4.0-3.75 |
2.8-3.5 |
~1 |
35380 |
|
Anthrazit(Anthracite) |
7-12 |
>91.5 |
<3.75 |
<2.5 |
~1 |
<35300 |
|
Percent by weight |
||||||
The middle six grades in the table represent a progressive
transition from the English-language sub-bituminous to bituminous coal, while
the last class is an approximate equivalent to anthracite, but more inclusive
(the U.S. anthracite has < 6% volatiles).
Cannel coal (sometimes called "candle coal"), is a
variety of fine-grained, high-rank coal with significant hydrogen content. It
consists primarily of "exinite" macerals,
now termed "liptinite".