http://www.rocksandminerals.com/coal.htm
"Why is Coal Important?
BECAUSE 56% OF OUR ELECTRICITY COMES FROM COAL!
There is probably not a single person in the United States that is not dependent to some extent on coal. Coal is at work each time you turn on a switch that allows electricity to power up your lights, microwave, stereo, TV, computer and all those other electircal appliances and gadgets you have lying around the house.
And where does the rest of our electricity come from?
* Nuclear plants provide about 24%
* Oil fired plants 10%
* Hydroelectric dams 8%
* Alternative energy sources (wind, solar etc.) 2%"
"If we continue mining coal at current rates, the US has between 500 and 1,000 years of coal reserves remaining. Put another way, it means that, at a minimum, and IF we choose to do so, we have enough coal reserves to produce electricity at current consumption rates for a period equal to that time between the time Columbus first set foot in the New World and today."
What is overlooked by many, is that coal and oil do not exist in finite amounts as is commonly thought. Both are "renewable" resources. Granted it takes around 50,000,000 years for decaying vegetation to be transformed into coal, but the process does not stop.
Look at it this way: Let's say that it takes exactly 50,000,000 years to change vegetation to coal. That means that every leaf of vegetation that fell to the ground 50,000,000 years ago became coal last year. It follows then that every leaf that fell to the ground 49,999,999 years ago becomes coal this year, and every leaf that fell 49,999, 998 years ago will become coal next year, and on, and on. So every year the entire annual growth and death of the Earth's vegetation becomes coal 50,000,000 years later. As long as vegetation continues to grow and decay, coal will be produced.
Now, I have no idea of many pounds of vegetation it takes to produce a pound of coal, but it is estimated that the Earth's annual production of vegetation is about 100 billion tons. That's a lot of coal.
This is something that I have wondered about as well. maybe someday I'll find a source of info. that answers these questions.