EGEE 101
Energy and the Environment

Coal Formation

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Coal Formation

Coal is one of the fossil fuels (along with crude oil, natural gas, oil shale, and tar sands). The name fossil fuel invokes the notion that at one point in time coal was alive. Well, almost, the coal precursors, mostly the plants, were alive growing in the sunshine. We know this because we can find fossil imprints of leaves and branches in coal, amber containing flies, and other organic material preserved, bark imprints in the coal, coalified trees, coalified roots, and biomarkers which are chemical compounds produced by living organisms, etc.

The Carbon Cycle - again

You've seen the carbon cycle once already in relation to Biomass back in Lesson 3. This time, focus on how the living matter (plants and animals) breaks down but the decay is limited, creating the organic basis for fossil fuel formation.

Photosynthesis & Decay Revisited

Recall that photosynthesis is the process by which plants absorb the sunlight (solar energy), store it, and convert it into energy to grow and survive. The plant takes in carbon dioxide (CO2) and water (H2O), stores and uses the glucose to grow and live, then releases oxygen (O2) back into the environment.

When plants die, this process simply works in reverse. Walking through almost any forest is the best way to witness the decaying process in action. The forest floor is generally strewn with dead and decaying leaves, branches, and sometimes entire trees.

Normally the process of growth and decay occurs but in the formation of the fossil fuels the normal decay path did not occur. Instead, the organic material is somewhat preserved. The key is the absence of oxygen, which is necessary for the normal decay process. Bogs are ideal for this to occur so in swamps (a particular type of bog) the plant material dies but is protected from the normal decay process by the stagnant water, which is low in oxygen. Decay still occurs but the bio-resistant material (the chemical structures which are resistant to the bugs dining in the swamp) is enriched. Over long time periods, considerable quantities of organic material might be buried in the swamp which might eventually form the material peat. As the peat is aged and buried deeper in the ground the slow coalification process (the maturation process for coal) continues and eventually transforms peat (a coal precursor) into low-rank lignite coal. This brown/black coal is typically a "young" coal (~60 million years old). With further maturation, long time periods (millions of years), warmer temperatures (within the earth), and higher pressure as the coal is buried deeper, other ranks of coal are produced: subbituminous, then bituminous coal. If there is uplifting, then anthracite coal can be formed because of the high temperatures and stresses involved. Magma can also bake some coals enhancing the rank in some locations. (refer back to the World Coal Organization PDF on the previous page for a refresher of coal rank: lignite, subbituminous, bituminous, and anthracite).

During the various stages, other materials such as minerals might be washed into the bogs, mud will form clay which will turn to shale over the years. The organic material will have contained inorganic material (it forms the ash left behind in a wood fire). Thus, coal is not pure organic material, and as the coalification process is over such long time periods we have different ranks of coal in the US coal of different qualities.

Coal is very old. The formation of coal spans the geologic ages and is still being formed today, just very slowly. Below, a coal slab shows the footprints of a giant armored salamander (the footprints were made during the peat stage but were preserved during the coalification process). 

dinosaur footprints in a slab of coal
Coal Footprints
Credit: JPM
see link in caption for a text description
Coal Formation Process
Credit: ND Geological Survey, Becky Barnes, artist.

The organic matter that falls or is washed into the swamp will be protected from the usual decay process because of the low oxygen concentration in the water. Decay will occur in the less bio-resistant material, leaving behind the bio-resistant organic material. Eventually, this material will form peat. With burial and the "cooking" of the earth (as you get deeper, it gets warmer) for geological time periods, low-rank coals will form such as the lignite shown here for North Dakota. The process of coal formation might repeat itself and so layers of coal and rock are present in many locations within the US. There the deeper you go the higher the coal rank. In Pennsylvania, we have bituminous and anthracite coal, the mountain formation transformed the bituminous coal into anthracite and it can occur at the surface.