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.

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.

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.

• 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.

• 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.

Products from Coal Gasification & F-T

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.

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.

• 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.

• 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.

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.

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

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

• 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

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.

Knocking the NOx out of Knocking the NOx out of coalcoal

• 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.

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.