Coal is formed from plant remains that have been compacted, hardened, chemically altered, and metamorphosed by heat and pressure over geologic time. It is suspected that coal was formed from prehistoric plants that grew in swamp ecosystems. When such plants died, their biomass was deposited in anaerobic, aquatic environments where low oxygen levels prevented their reduction (rotting and release of carbon dioxide). Successive generations of this type of plant growth and death formed deep deposits of unoxidized organic matter that were subsequently covered by sediments and compacted into carboniferous deposits such as peat or bituminous or anthracite coal. Evidence of the types of plants that contributed to carboniferous deposits can occasionally be found in the shale and sandstone sediments that overlie coal deposits. It is believed that most coal was formed during the carboniferous era (280 to 345 million years ago).

The continuing effects of temperature and pressure produced more changes in the lignite, progressively increasing its maturity and transforming it into sub-bituminous coals. As further chemical and physical changes occurred these coals became harder and more mature, to be classified as bituminous or hard coals. Under the right conditions, the progressive increase in the organic maturity continued, ultimately to form anthracite.

Coal classification depends on the nature of the original vegetation, its biochemical experiences, the length of the coalification process and most importantly the depth that the coal seam was buried. The proportion of hydrogen, oxygen and carbon also decides how a coal will be classified or ranked. Coal can be further analysed by its proportion of moisture, sulphur, volatile matter, fixed carbon, ash and physical properties.

Bituminous coals are typically softer, friable materials with a dull, earthy appearance. They have high moisture levels, a low carbon content and therefore a low energy content. Higher rank coals are typically harder and stronger and often have a black vitreous lustre. Increasing rank is accompanied by a rise in the carbon and energy contents and a decrease in the moisture content of the coal.

Contents: Coal contains many trace elements, including arsenic and mercury, which are dangerous if released into the environment. Coal also contains low levels of uranium, thorium, and other naturally-occurring radioactive isotopes. Chemical composition of the coal is defined in terms of its proximate and ultimate (elemental) analysis. The parameters of proximate analysis are moisture, volatile matter, ash, and fixed carbon. Elemental or Ultimate analysis encompasses the quantitative determination of carbon, hydrogen, nitrogen, sulfur, and oxygen.

Calorific Value

The calorific value or heat of combustion or heating value of a sample of fuel is defined as the amount of heat evolved when a unit weight ( or volume in the case of a sample of gaseous fuels ) of the fuel is completely burnt and the products of combustion cooled to a standard temperature of 298 degree K.

The quality of coal depends upon its rank and grade. The coal rank arranged in an ascending order of carbon contents is Lignite → sub_bituminous coal → bituminous coal → anthracite

Types of coal

Lignite coal is the lowest rank of coal, often referred to as brown coal, used almost exclusively as fuel for steam-electric power generation. It is brownish-black and has a high inherent moisture content, sometimes as high as 45 percent. The heat content of lignite ranges from 9 to 17 million Btu/ton (10 to 20 MJ/kg) on a moist, mineral-matter-free basis. The heat content of lignite consumed in the United States averages 13 million Btu/ton (15 MJ/kg), on the as-received basis (i.e., containing both inherent moisture and mineral matter).

Subbituminous coal is a coal whose properties range from those of lignite to those of bituminous coal and are used primarily as fuel for steam-electric power generation. 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 subbituminous coal ranges from 17 to 24 million Btu per ton on a moist, mineral-matter-free basis. The heat content of subbituminous 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).

Coke is a solid carbonaceous residue derived from low-ash, low-sulfur bituminous coal from which the volatile constituents are driven off by baking in an oven at temperatures as high as 2,000 F (1,000 C) so that the fixed carbon and residual ash are fused together. Coke is used as a fuel and as a reducing agent in smelting iron ore in a blast furnace. Coke from coal is grey, hard, and porous and has a heating value of 24.8 million Btu/ton (29 MJ/kg). Byproducts of this conversion of coal to coke include coal-tar, ammonia, light oils, and "coal-gas". (Coke can also be made from petroleum) .

Metallurgical coke, also known as “Met” coke, is a carbon material manufactured by the “destructive distillation” of various blends of bituminous coal. Met coke has a very low volatile content. However, the “ash” constituents, remain encapsulated in the resultant coke. Typical purities range from 88-92% fixed carbon. Metallurgical coke is used where a high quality, tough, resilient, wearing carbon is required. Applications include but are not limited to conductive flooring, friction materials, foundry coatings, foundry carbon raiser, corrosion materials, drilling applications, reducing agents, heat-treatment, ceramic packing media, electrolytic processes, and oxygen exclusion.
Blast furnace coke is used in the blast furnaces of steel mills. It is the largest traded coke by volume and is produced in several countries with China being the largest producer and exporter.

Petcoke is a solid carbonization by-product of high-boiling hydrocarbon fractions obtained in petroleum processing There are many different variations and consistencies of petroleum from which the coke is derived. It is the general term for all special petroleum coke products such as green, calcined and needle petroleum coke. There is over 60 Million tones of petcoke produced around the world annually.

INDIAN COAL

India is the world's third largest coal producer (after China and the United States), so most of the country's coal demand is satisfied by domestic supplies. Indian coal generally has a high ash content and low calorific value, so most coking coal must be imported. Major Indian coal fields are found in Bihar, West Bengal, and Madhya Pradesh.

The Indian government controls almost all coal production. Nearly all of India's 390 mines are under Coal India Ltd. (CIL), which accounts for about 90% of the country's coal production.

India has seven per cent of the world’s proven coal reserves. Coal meets approximately 63 % of the country’s total energy requirements. By current estimates the reserves are enough to meet India’s needs for at least another 100 years.

Coal (hard coal and Lignite) is the predominant primary commercial energy source in India. Its share in total commercial energy since 1970-71 is more or less consistent around 62% in spite of increasing share of Natural Gas. The ash/mineral matter of Indian Coal is of inherent nature i.e. intimately mixed with coal mass at the time of formation resulting in different cleaning characteristics.
 

Indian Coal, in spite of the handicap of high ash, has many positive characteristics particularly with respect to environmental aspects and end-use. These are :

In 1972-73, the Indian government nationalised the coal industry, primarily to develop the sector, since it was considered to be of strategic importance for rapid industrial development. Coal India Ltd (CIL) was incorporated as a holding company for seven coal producing subsidiaries and a planning and design-focused company.

Coal consuming sectors comprising:

ELECTRICITY

• Worldwide, coal is enormously important. It is the world's most abundant and widely distributed fossil fuel. it is economic.

• Coal is the major fuel for generating electricity worldwide. More than 45 percent of the world's electricity is generated from coal.

• Coal is used in at least three-quarters of all steel making and it has other industrial uses as well.

• Around four thousand million tones of coal are mined every year in more than proved environment in the developed world or an improved standard of living in the developing world, the fact is that 87% or more of the world's primary energy is derived at present from fossil fuels, oil, gas and coal. And the greatest of these three energy, coal is expected to continue its primary role in the world scenario in the near future also.

World Coal Reserves

Coal is one of the most significant natural resources in the world, with extensive reserves in almost 100 countries, estimated in 1996 at around one thousand billion (1 x 1012) tones of coal reserves economically accessible using current mining technology. The world’s major hard coal producers are China, the USA, India, South Africa, Australia, Russia, Poland, Kazakhstan and the Ukraine. Coal is mostly used in the region it is produced but about 12% is traded between countries. Australia, the USA and South Africa are the largest exporters of coal.

At current production levels, there is enough coal to last over 200 years, not taking in account other reserves which might be proved by on-going exploration or become accessible through improvements in mining technology. Known world oil and gas reserves will be largely exhausted within 45 to 60 years time. Growth in demand for coal for energy and steel making is expected to drive increased worldwide coal use from around 5.3 billion tonnes per annum (btpa) at present to 7.6 btpa by 2020.