The additional mass comes from oxygen from the air used to burn the fuel. Burn 1g of coal, to make it easy: C + O₂ → CO₂ Carbon has molecular mass 12g/mole

The additional mass comes from oxygen from the air used to burn the fuel. Burn 1g of coal, to make it easy: C + O₂ → CO₂ Carbon has molecular mass 12g/mole (carbon has atomic weight ~12), carbon dioxide 44g/mole (12 + 2×16). So burning 1g of carbon produces 3.67g of CO₂.