Coal - Definitions and chemistry

Rank of coal

As the vegetable matter is put under heat and pressure it undergoes coalification. It loses water and methane, increasing the carbon content to become harder and increase it's rank. The rank is measured by it's carbon content
The heating values listed by various internet sources vary a great deal, so the values below are not consistent. A lot depends on the moisture content.
Coke is made by heating coal without air. The liquids and vapours are driven off leaving coke. Coking coal needs to produce coke strong enough to maintain porosity in a blast furnace filled with lumps of iron ore, limestone, and coke. If it crushed, and powder filled the gaps between the lumps, the hot CO would not be able to reach the iron ore to remove its oxygen.
RANK Uses % Water MJ/KG %Carbon
Peat 10-55% water 18-22 MJ/KG dry   6-15 wet 40-60 ???
Lignite (Brown) Power (dirty) 45 11-23 25-35
Sub bituminous Power   23-37 35-45
Bituminous Coke   30-44 45-86
Anthracite Clean burn   27-42 86-98

Australia exports mainly coking coal, also known as metallurgical coal. The coke from it does not crush and clog a blast furnace with powder.

Coalification - Lignin to coal        Coking coal in steel making


Chemical structure of coal

There are several different structures of coal reported in the literature.

The chemical formula of coal is reported as: C135H96O9NS.

Coal is usually taken as 85% carbon based on dry mass.

One tonne of oil equivalent is usually 1.5 t of black coal or 3 t of brown coal.

Lignin molecule (in wood)



Coalification is the process of fermenting and compressing wood (lignin). Under pressure, any reaction to reduce volume will be favoured, so the methane etc is ejected leaving benzene rings joined more closely together.

CO2 emissions

Peat:           106 g CO2/MJ,[12] 

Black coal    94.6 g CO2/MJ)

Methane       56.1) 

Figure 1. An example of the structure of coal.

Coal molecule


Coal molecule

Brown coal has 40-60% water and it takes a lot of energy to remove this. There are several processes for removing the water more efficiently.

De-watering brown coal

Edited from

Exergen has developed technology to deliver a 40% reduction in emissions. The technology is currently known as the Continuous Hydrothermal De-watering (CHTD) of brown coal (lignites) and high moisture subbituminous coal.

A vertical autoclave uses gravitational pressure and heat to squeeze the molecular structure of brown coal to remove up to 80% of its moisture content.

This reduces the power needed to dry fuel prior to combustion.

The hope is that the process will extend the ‘social license’ of brown coal-fired power generation.

The technology is ready for existing and new brown coal-fired power generation plant.

The Exergen CHTD process makes high moisture-stranded lignites suitable for export to the thermal coal market. At 10% moisture an Exergen CHTD coal ex Latrobe Valley has a GAR of 24.1 MJ/KG or 5700-5800 kcal/kg. (Gross As Received is the energy of the coal less the latent heat of water that needs to be evaporated)

The Coldry Process

Essentially, Coldry is patented process that changes the naturally porous form of brown coal to produce a dry, dense pellet, via a process call ‘brown coal densification’:

  • Brown coal is crushed to < 8mm coal and combined with up to 5% water.
  • The mixture is fed into an :"Attritioner" that rubs the coal faces together. The heat generated triggers an exothermic reaction which in turn triggers a "natural reaction that expels water from the coal" .
  • The now plasticised coal is extruded at low pressure.
  • The mixture is heated with air at 40oC for an hour.
  • As it cools it cracks. 
  • Dried to about 12% with waste heat.
  • Moist air from drier is cooled. The water is condensed for reuse.
  • Coldry pellets do not rehydrate

Heat of combustion = 23.1 MJ/KG = 5521 kCal/Kg



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