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Coal Basics

What is coal?

Coal is a solid fossil hydrocarbon that represents the storage of solar energy from the past in the form of preserved plant material.  The plants during the preservation process have been broken down into organic matter that comprises moisture, ash, volatile matter (gases and liquids given off on heating) and fixed carbon.  The volatile matter and fixed carbon both contain energy and this is released on burning the coal.

In 2008 the world consumed some 4.76 billion tonnes of coal and this made up about 32.53% of all fossil fuel consumption in the world.

Coal Rank

Coal is made up plant material that has been initially broken down by bacteria and fungi in a peat swamp and then has been subjected to temperature for varying amount of time to produce a progression of coals Table 2.  In moving from peat through to anthracite the main driver is heat which is used for the coalification process.  This expels moisture, carboxyl groups (Carbon dioxide) and methane as rank increases.

      

Rank Description

Peat Wet plant material that has been subject to bacterial and fungal action, very low energy level, moisture level ~60% calorific value ~2,600 kcal/kg
Brown coal Peat that has had the water squeezed out, plant remains still visible moisture ~ 50%, calorific value 2,800 kcal/kg
Lignite Coal is hard and massive, black looking, moisture content 40-50%, calorific value about 4,000 kcal/kg
Sub-bituminous Coal is hard and brittle and black and shiny, moisture content is 20 – 40%, calorific value 4,000 kcal/kg to 5,800 kcal/kg
Bituminous Coal is softer and shiny, moisture content is 8 -20% calorific value is 5,800 to 8,000 kcal/kg, crucible swelling number from 2 – 9+ possible for coking coals, volatile matter from 16% - 40%
Anthracite Coal is very shiny, repels moisture, calorific value 7,800 – 8,000 kcal/kg, no coking properties.

 

 

Uses for Coal

 

  • Thermal Coal

Most coal is used for the energy content contained within the volatile matter and the fixed carbon.  These coals are generically termed thermal or steam coals.  These coals are mostly used for electricity generation.
A typical Australian thermal coal contains 6,080 kcal/kg of usable energy (net as-received energy) or 25.46 MegaJoules/kilogram (25.46 MJ/kg) of coal.  Electrical energy (power) is measured in Watts which are Joules per second, therefore 1 kilowatt hour of electricity (1 unit)converted from coal at 35% efficiency requires 10.286 MJ of coal energy every hour or 0.404 kg of coal.
Other thermal coal uses are the calcination (breakdown by heat) of limestone to form cement for construction industries or lime for agricultural purposes.
Hospitals and other institutions use coal for process heat as do meatworks, wool sours and timber drying processes.

 
  • Metallurgical Coal

Some special bituminous coals swell on heating above 350 degrees Celcius and release their volatile matter, leaving behind a hard porous carbon residue called coke. These coals are called coking coals and are limited in their occurrence around the world.  Coking coals are primarily used to make coke which under high temperatures reduces metal oxides to metals.  This process occurs when the coke is combined with the metal oxides at elevated temperatures.  The carbon from the coke combines with the oxygen from the metal oxides to produce carbon dioxide, liquid metal and residual ash (slag).

C (coke) + MO (metallic oxide)     Metal (liquid) + CO2 (gas) + slag (liquid)

 

Other coals used for the reduction of metallic oxides include non-coking coals that are used in direct reduction processes that rely on coal having very low ash contents and high reactivity of the carbon with carbon dioxide.  These processes are normally restricted to small electric arc furnaces or rotary kilns where no strength properties are required from the carbon.