There are four main types of coal: lignite, sub-bituminous coal, bituminous coal, and anthracite. They differ in their carbon content, heat value, and weathering characteristics, ranging from lignite as the lowest form to anthracite as the highest form. Coal is a heterogeneous mixture that forms from the compression of peat and plant materials over millions of years. It varies in its calorific value and organic composition of elements like carbon, hydrogen, oxygen, sulfur, and nitrogen.

1. COAL

2. Four kinds of coal

3. Four kinds of coal
Peat while essential to the development of coal is
not classified as a coal itself. The four
classifications or ranks of coal are: lignite, sub-
bituminous coal, bituminous coal and anthracite.
Lignite: is the lowest form of coal. Often called
brown coal because of its colour, lignite is the
initial form of all higher grades of coal. It has a
high moisture content, relatively low heat value,
and tends to disintegrate when exposed to
weather.

4. Sub-bituminous : coal has about half as much
moisture as lignite, slightly higher heat value, and
weathering characteristics similar to lignite. Both
lignite and sub-bituminous coal are used some
areas as fuels for the generation of electricity.
Bituminous : coal has a higher percentage of fixed
carbon, less moisture, greater heat value, and
better weathering characteristics than lignite and
sub bituminous coals. It is the most useful and
abundant kind of coal. According to geological
estimates, seven feet or more of compacted plant
life was required to form one foot of bituminous
coal. All of the grades of coking coal used for
metallurgical purposes fall within the bituminous
classification. Most of the non-metallurgical grades
of bituminous coal are burnt in boilers or furnaces
to obtain thermal energy for generating electricity.

5. Anthracite: has a higher percentage of fixed
carbon and a lower percentage of volatile
matter than all other coals. It was subjected to
the highest pressures and temperatures, making
it the hardest and highest carbon of all types of
coals. While the use of anthracite as a fuel for
heating has declined over the years, its high
carbon and low sulphur content have increased
its use for chemical and metallurgical purposes.

6. A) Coal originates from vegetal matter and forms
from the compression of peat over millions of
years (Figure 1).
B) It is a stratified, organic, biochemical
sedimentary rock.
C) Coal is a heterogeneous mixture of plant
materials and minerals.
• -Ex. wood, bark, leaves and sap.
D) The calorific value (which is the calories or
thermal units contained in one unit of a
substance and released when the substance is
burned) of coal and lignite varies tremendously;
ranging from 15,000 BTUs/lbs to under 6,300
BTUs/lbs. These values, however, are the
extremes. Most ranks of coal are between these
two levels.

8. E) The organic component of coal consists of
chemical compounds from carbon, hydrogen,
oxygen, sulfur, and nitrogen. (Figure 2)
F) With the exception of nitrogen, these
elements are also found in the mineral species
that occur in coal.
- Ex. carbonates, sulfides, sulfates and
hydrous clay minerals.
- The ash in coal is made of decomposed
residues of silicate,
carbonates and sulfides.

9. Composition of coal

10. Coal is ranked in six different categories:
1. Peat - not really a coal; it is undeveloped, soft and spongy. It is 20-
25% carbon, and was deposited during the last 100 years by
decomposed plant remains.
2. Lignite- the first "real" coal. It has a low heating value and is often
found near the earth's surface. Lignite coal loses its moisture when
exposed to air.
3. Subbituminous - lower in carbon content than bituminous coal. It
also loses its moisture when exposed to air. When exposed to
wetness it
4. Bituminous - has a high carbon content (65-85%), and a low oxygen
content (5-15%). It usually burns with a yellow flame.
5. Semianthracite - a hard coal but softer than anthracite. It has a
fixed carbon percentage of 80-85%. It contains more volatile matter
than anthracite and also burns more freely.
6. Anthracite - hard, durable, high density. It has low volatile matter
and is 85-98% fixed carbon. It ignites slowly and burns with little
flame or smoke.

11. Hydrogen and Carbon Composition
1. Low-rank coals typically have relatively low carbon contents
and relatively high oxygen contents (Figure 3).
2. Lignite, subbituminous, and low rank bituminous coals have
hydrogen contents of 5-6% (Figure 3).
3. High rank bituminous and anthracite coal are 3-4% hydrogen
(Figure 3).
Figure 3: Percent Hydrogen, Percent Carbon