There's a new Palaeocast podcast, episode 86. It is called Coal. It discusses the origin and composition of Coal. The Carboniferous Period "coal bearing" is named for the sheer amount of coal produced during that time period. Current theories are discussed as to why this happened.
For this interview, we asked Standford University’s Prof. Kevin Boyce to introduce us to coal and to tell us how it’s formed and what it’s made of. We then concentrate on determining why the Carboniferous was the period with the largest coal deposits when we know that forests existed in other periods too. Finally, we look at the impact that coal production and subsequent exploitation has had on the planet.