The term biodegradable is used to describe materials that decompose through the actions of bacteria, fungi, and other living organisms, and coal is one of the end results of organic breakdown.
Coal has already undergone complete biodegradation produced from the decomposition of the remains of plant and animal material.
In its current state, coal starts to decompose and evolves a mixture of hydrogen and/or oxygen-rich products when heated progressively in an inert atmosphere.
Millions of years ago, dead plant matter fell into the swampy water and over the years, a thick layer of dead plants lay decaying at the bottom of the swamps. Over time, the surface and climate of the earth changed, and more water and dirt washed in, halting the decay process.
The weight of the top layers of water and dirt packed down the lower layers of plant matter. Under heat and pressure, this plant matter underwent chemical and physical changes, pushing out oxygen and leaving rich hydrocarbon deposits. What once had been plants gradually turned into coal.
For more info about coal, go to lsa.colorado.edu and www.uky.edu.