After you burn coal, the leftover material is called fly ash. Same concept as wood. You burn cord wood or wood pellets, you have wood ash leftover. If you burn coal, you have coal ash leftover. Coal naturally contains some very nasty things. Some is released into the air when burned. What is left in the ash is the concentrated leftovers. These leftovers include: arsenic, beryllium, cadmium, barium, chromium, copper, lead, mercury, molybdenum, nickel, radium, selenium, thorium, uranium, vanadium, and zinc. A few are radioactive and bad for obvious reasons. Lead and mercury of course affect the brain/nervous system. Nothing I would want in my house and nothing I would want to be responsible for disposal of. Wood ash doesn't contain any of this nastiness. I've seen posts on that coal forum talking about how they dispose of their ash. The prevailing attitude seems to be "I seem OK, it must be OK!". The above mentioned materials are proven to be harmful to human and animal health. Some in varying doses, some in any dose. My advice would be to stay far away from coal.