by Allan Taylor

Created on: September 19, 2010

What are fossil fuels?  They are carbon-based materials that are derived from formerly biological matter that has been compacted and  chemically or physically changed  into a convenient fuel now found  within or on the earth.  Conventional wisdom includes materials such as  peat, lignite, brown coal, black coal,  natural gas (methane) and petroleum.   Fossil fuels may take millions of years to form.   They are regarded as a finite resource and therefore “non-renewable”,  in contrast to “renewable” energy resources, which  include wind, solar, wave, hydro, geothermal  and biofuels.  

There is no doubt that coal in its various forms is a fossil biological material because it is found to contain plant and tree remains that are derived from ancient forests and peat bogs that have later become buried by sediments many millions of years ago.    In Europe the coal measures give name to the Carboniferous Period of geological time about 300 to 360 million years ago.  In Australia and New Zealand coal measures also occur in much younger rocks,  from Cretaceous through to Miocene Age (70 to 20  million years ago).  Lignite seams over 100 meters thick are open-cut mined in the La Trobe Valley,  Victoria, for electricity power production.

In Scotland and Ireland, crofters in the countryside  still cut out the surface peat and dry it for a domestic heating fuel.  When peat bogs subside and become buried under mud and sands the overlying pressure of the sediments compacts the organic matter into lignite and then coal.  It takes about 10 meters of peat to make 1 meter thickness of coal.   Coal increases in rank as water and gases are released during burial.   The sequence is peat, lignite, sub-bituminous, bituminous to anthracite coal, the later being almost pure carbon without water or volatiles, having been subjected to high pressures and temperatures of burial (or volcanic intrusion).  

The lower rank coals often contain appreciable methane, or “coal-seam” gas which is exploited by drilling into the seams, even though they may be uneconomic to mine by underground means.  Up to 10% of the energy can be extracted by this means.  In Australia, coal seam gas industry is substantial and supplies ca 30% of domestic and industrial gas supplies for the Eastern states of Queensland and

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