[Symbiote]

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.

[niloc132]

Ah thanks, that does indeed line up the numbers I had misunderstood.