Note: 1 million BTUs is approximately the same as 1 billion Joules.
Coal is about 30 GJ per tonne.
Wood averages 21 Million BTU per 4231 lbs (I averaged the data in one of the links.)
or 21GJ per 1.9 tonnes which is approximately 11GJ per tonne.
So in very approximate numbers 1 tonne of coal produces about as much heat as 3 tonnes of trees.
Also, wood is between 2 and 5 times less dense than coal. So it could be that one truck of coal would need to be replaced by between 6 and 15 trucks of wood.
(NB Please check my arithmetic!)
There are other things to take into account. A tree is called "green". That is, the wood is wet. You would probably not burn it like that so the wood will need to be dried out prior to burning. The drying process reduces the weight of the wood to between half and two-thirds of its wet weight, typically.
Simple answer, none, coal is a product formed over millions of years, by pressure, but not one you could make for current use unless you have millions of years to wait.
If you burn a tree you get charcoal, which is not quite the same thing, you only get (by weight) about 25% of the original mass of the tree (or trees) as charcoal.
http://forestry.about.com/od/alternative...
People have been making charcoal for a very long time, it's energy output is not as good as coal but for home fires it works well.
That depends on if it is first eaten by Panda Bears or not.
Why would you want to know?
