Types of coal

1. Coal Classification
• There are two main ways for classifying coal
- by rank and by type.
• Coal Rank
• Coal Types
• Coal Rank : The degree of 'metamorphisrn'
or coalification undergone by a coal, as it
matures from peat to anthracite
• This has an important bearing on its
physical and chemical properties, and is
referred to as the 'rank' of the coal.
• Low rank coals, such as lignite and sub-bituminous coals, are typically softer,
friable materials with a dull, earthy appearance; they are characterised by
high moisture levels and a low carbon content, and hence a low energy
content.
• Higher rank coals are typically harder and stronger and often have a black
vitreous lustre.

3. Composition of Coals
• The natural constituents of coal can be divided into two
groups:
• (i) the organic fraction, which can be further subdivided
into microscopically identifiable macerals; and
• (ii) the inorganic fraction, which is commonly identified as
ash subsequent to combustion, but which may be isolated
in the form of mineral matter by low-temperature ashing
(LTA).
• The organic fraction can be further subdivided on the
basis of its rank or maturity.

4. Characteristics of Coal
• Sulfur Content : Coal with sulfur > 5% is not
recommended for combustion.
• Weatherability : Weathering or Slacking Index .
• An indication of size stability.
• Denotes the tendency to break on exposure to alternate
wet and dry periods.
• Weathering index is the percentage of coal passing
through a sieve having 170 mm2
openings.
• Grindability Index : A measure of relative ease of grinding
coals or the power required for grinding coals in a
pulverizer.
• G = 6.93 W + 13 -- W is the weight of sample passing
through 200 mesh size.

5. • Burning Characteristics of Coal :
• Free burning coals and Caking Coals.
• Caking index -- Pulverulent, sintered, weakly caked, caked
and strongly caked.
• Ash Fusion temperature -- The temperature where the ash
becomes very plastic.
• Design of ash handling system. -- Stoker furnace cannot use
low ash fusion temperature coals.
• Dirtiness of furnace walls.

6. • A typical coking operation produces 80% coke
by weight, 12% coke oven gas, 3% tar, and 1%
light oil consisting of crude benzene, toluene,
and xylenes as well as over 200 other
chemicals.

8. Products from Coal Gasification & F-T

11. Destructive distillation
• Destructive distillation is the chemical process involving the
decomposition of feedstock by heating to a high
temperature; the term generally applies to processing of
organic material in the absence of air or in the presence of
limited amounts of oxygen or other reagents, catalysts, or
solvents, such as steam or phenols. The process breaks up or
'cracks' large molecules, generally into a mixture of
hundreds of different compounds. The molecules distilled
off generally are smaller and more volatile than the
feedstock molecules, but some reactions polymerise small
molecules into larger molecules, including heat-stable tarry
substances and chars.

13. Coal tar is distilled to give four
fractions:
1. Light oils, boiling below 200°C. They are called light oils
because they float on water. They are crudely fractionated,
then agitated with concentrated sulfuric acid to remove
olefins. The hydrocarbons are washed with dilute sodium
hydroxide and redistilled to give benzene, toluene, xylenes
and “solvent naphtha,” a mixture of indene, coumarone and
their homologues.
• This is a powerful solvent especially for coatings containing
coal tar and pitch. Treated with a Friedel–Crafts catalyst such
as aluminum chloride, it gives coumarone–indene
thermoplastic resins, used for cheap floor tiles, varnishes, and
adhesives.

14. • The middle oils boil between 200 and 250–270°C. The most
abundant chemical is naphthalene and it occurs with
phenols, cresols, and pyridines in the tar.
• It crystallizes when the middle distillate from the tar is
allowed to cool and, even in this impure form, is suitable for
phthalic anhydride manufacture
• Alternatively, it may be purified by sublimation, a somewhat
unusual purification process. Extraction of the remaining tar
with aqueous sodium hydroxide takes the acidic phenols
and cresols into the aqueous layer as phenates and
cresylates. They are regenerated with carbon dioxide.

15. • Heavy oil comes off between 250 and 300°C if anthracene oil is
taken off as a separate fraction, but sometimes they are
combined. It is used for wood preservatives generally under the
name of creosote or (if the fractions are not separated)
anthracene oil.
• Anthracene oil comes off between 250 and 300–400°C or 350 and
400°C if taken off as a separate fraction. It contains anthracene,
phenanthrene, carbazole, and many other compounds in small
quantities. It makes up about 1% of coal tar.
• Some 60% of the tar remains as a residue called pitch. Its
production is driven by the aluminum industry, which thermally
polymerizes the pitch to make electrodes for the electrolysis of
molten alumina–cryolite mixtures to give aluminum.

17. GasificationGasification
• Breaks down coal into basic chemical constituents.
• Coal is exposed to hot steam and controlled amounts
of air or oxygen under high temperature and
pressures.
• Carbon molecules in coal break apart, setting off
chemical reactions that produce syn gas and other
gaseous compounds.
Integrated gasification combined-cycle (IGCC)
• Syn gas is burned in a combustion turbine which
drives an electric generator.
• The exhaust gases are used to heat steam.

18. SYNTHESIS GAS
• Synthesis gas is the name given to a variety of
mixtures of carbon monoxide and hydrogen or
nitrogen and hydrogen. It is made from
methane from natural gas.
• Coal was an important feedstock prior to
1960. During World War II, it was used in
Germany to provide synthesis gas for the
manufacture of fuel and chemicals by the
Fischer–Tropsch process

19. THE FISCHER–TROPSCH REACTION
• The Fischer–Tropsch reaction provides a route from coal to
hydrocarbons. When synthesis gas at near atmospheric
pressure is passed over an iron, nickel, or cobalt catalyst at
150–300°C, a mixture of alkanes and olefins with a broad
range of molecular weights is formed. The olefins are
formed first and they may be reduced to alkanes.
• If hydrogen-rich synthesis gas is used, made from naphtha or
methane instead of coal, alkanes may be the initial products

20. • The hydrocarbons are predominantly C5–C11 straight chain,
although methane, ethylene, and propylene are also
produced together with some higher molecular weight
Fischer–Tropsch waxes used for candles.
• There are also oxygenated compounds such as alcohols and
acids. The result is a petroleum-like mixture that can be used
both as a fuel and a chemical feedstock

25. Combined Cycle
• Combines gas turbine
and steam turbine.
• Exhaust energy from
gas section used in
steam system.
• High thermal efficiency.
• Small plants combined.
• High mobility.

26. Knocking the NOx out of coalKnocking the NOx out of coal
• NOx emissions reduced at low-combustion temperatures
and by use of low-nitrogen fuels, low- NOx burners and
fluidized-bed combustion.
• Particulate matter removed by fabric filters or electrostatic
precipitator.
• Membranes for separating gases.
• Selective removal of hydrogen from syngas.
• Flue gas desulfurisation units, selective catalytic control
systems and evaporative cooling towers.
• Sulfur extracted from coal converted into commercial-
grade sulfuric acid or elemental sulfur.
• Mercury controls - sorbents and oxidizing agents.

27. Steam Reforming of Methane
• The most widely used synthesis gas process is
the steam reforming of hydrocarbons,
• with partial oxidation of hydrocarbons as
another possibility.