Energy stored as coal

“The poorest lignites (lighter colored and crumbly) have an energy density lower than wood, because most of their mass is moisture and ash. In elemental composition, the poorest lignites are less than fifty per cent carbon, anthracites more than ninety per cent, and bituminous coals mostly between seventy to seventy-five per cent. This means that the most commonly used bituminous coals have energy densities about fifty per cent higher than air-dried wood.
Consequently, energy stored as coal will occupy less space, require less frequent stoking of stoves and furnaces, and untended fires will last longer. On the other hand, underground mining, costly and dangerous, is the most obvious disadvantage of coal extraction, while the presence of relatively large volumes of ash (bituminous coals typically contain about ten per cent of incombustibles, mostly oxides of iron, silica, and alkaline elements) and sulfur are its great­est environmental drawbacks.”

Excerpt From: Vaclav Smil. “Energy.”