Burning fuels is a major source of atmospheric pollutants. Most fuels, including coal, contain carbon and/or hydrogen, and many also contain some sulfur. The gases that leave a chimney or exhaust depend on what is in the fuel and on how much oxygen is available.
When there is plenty of oxygen the combustion is complete. The carbon forms carbon dioxide and the hydrogen forms water vapour. When the oxygen supply is limited the combustion is incomplete. Some carbon forms toxic carbon monoxide, CO, and some is left as soot, which is tiny solid particles of carbon. Unburned hydrocarbons can also escape, and with soot they form particulates in the air.
If the fuel contains sulfur, the sulfur burns to form sulfur dioxide, SO2. Oxides of nitrogen are different, because they do not come from the fuel. In the very high temperatures inside an engine, nitrogen and oxygen from the air react together to form oxides of nitrogen.
To predict the products of burning a fuel, look at which elements it contains and how much oxygen is available. For example, a fuel made only of carbon and hydrogen gives carbon dioxide and water with plenty of oxygen, and carbon monoxide, soot and water with limited oxygen.