According to the International Coal Classification,
there are two types of coals: hard coals and low-rank coals.
Hard Coal
The term "hard coal" (as used in the international
classification system) is defined as coal with a gross calorific
value of more than 5,700 Kcal/kg (10,260 Btu/lb or 23.86 MJ/kg)
on a moisture, ash-free basis. The American Society of Mechanical
Engineers (ASME) classifies anthracite, bituminous, and higher
rank subbituminous in the hard-coal category.
Low-Rank Coal
The term "low-rank coal" (as used in the international
classification system) is defined as coal with a gross calorific
value of less than 5,700 Kcal/kg (10,260 Btu/lb or 23.86 MJ/kg)
on a moisture, ash-free basis. The American Society of Mechanical
Engineers (ASME) classifies medium and lower rank subbituminous,
lignities, and brown coals are included in the low-rank coal
category.
-- Peat
soil material consisting of partially decomposed organic matter;
found in swamps and bogs in various parts of the temperate zone.
One type of peat is fuel peat, which is most widely used in
regions where coal and wood are scarce, e.g., Ireland, Scandinavia,
and parts of Russia. Peat is the earliest stage of transition
from compressed plant growth to the formation of coal. Large
deposits of peat in the United States are found in Michigan,
California, and the Florida Everglades.
-- Lignite
- or brown coal, carbonaceous fuel intermediate between coal
and peat , brown or yellowish in color and woody in texture.
It contains more moisture than coal and tends to dry and crumble
when exposed to the air; the flame is long and smoky and the
heating power low. It is found in the United States, Canada,
Germany, and elsewhere chiefly in formations formed in the Tertiary
period.
- - Sub-bituminous
coal - is a coal whose properties range from those of
lignite to those of bituminous coal It may be dull, dark brown
to black, soft and crumbly at the lower end of the range, to
bright, jet-black, hard, and relatively strong at the upper
end. Subbituminous coal contains 20 to 30 percent inherent moisture
by weight. The heat content of sub-bituminous coal ranges from
17 to 24 million Btu per short ton (20 to 28 MJ/kg) on a moist,
mineral-matter-free basis. The heat content of sub-bituminous
coal consumed in the United States averages 17 to 18 million
Btu/ton (20 to 21 MJ/kg), on the as-received basis (i.e., containing
both inherent moisture and mineral matter). A major source of
sub-bituminous coal in the United States is the Powder River
Basin in Wyoming. Relatively low density and high water content
renders some types of sub-bituminous coal susceptible to spontaneous
combustion if not packed densely during storage in order to
exclude free air flow.
-- Bituminous
coal - is usually black, sometimes dark brown, often
with well-defined bands of bright and dull material.The carbon
content of bituminous coal is around 60-80%; the rest is composed
of water, air, hydrogen, and sulphur, which have not been driven
off from the macerals.
The heat content of bituminous coal ranges from 21 to 30 million
Btu/ton (24 to 35 MJ/kg) on a moist, mineral-matter-free basis.
-- Anthracite
- coal is the highest of the metamorphic rank, in which the
carbon content is between 92% and 98%. The term is applied to
those varieties of coal which do not give off tarry or other
hydrocarbon vapours when heated below their point of ignition.
Anthracite ignites with difficulty and burns with a short, blue,
and smokeless flame. It is the highest rank; a harder, glossy,
black coal used primarily for residential and commercial space
heating.
-- Briquette
A block of compressed coal dust. Some briquettes are compressed
and dried brown coal (lignite) extruded into hard blocks. This
is a common technique for low rank coals. They are typically
dried to 12-18% moisture, and are primarily used in household
and industry. In Ireland, peat briquettes are a common type
of solid fuel after coal. Although often used as the sole fuel
for a fire, they are also used to begin a coal fire quickly
without hassle. A fire burning peat briquettes is, similarly
to a turf fire, slow burning. For the CARBOROBOT the size
of the briquette is the most important parameter. Usable the
small size "perl" briquette or fraction max. piece
size 30mm.
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