"Energy contained in coal" doesn’t make any sense. Is it "energy we could get from burning coal if it was 100% efficient"? "Energy we could get from coal if we could use it in a nuclear reaction"?
Coal (anything) doesn’t "contain" energy. We can transform some things, and some transformations produce energy in some form or another.
The upper line of this graph should be labeled "total energy liberated by burning coal" and the lower one "useful energy liberated by burning coal".
"Energy contained in coal" doesn’t make any sense. Is it "energy we could get from burning coal if it was 100% efficient"? "Energy we could get from coal if we could use it in a nuclear reaction"?
Coal (anything) doesn’t "contain" energy. We can transform some things, and some transformations produce energy in some form or another.
The upper line of this graph should be labeled "total energy liberated by burning coal" and the lower one "useful energy liberated by burning coal".