What gas is produced during the complete combustion of fossil fuels?
If the fuel is pure carbon, carbon dioxide is produced. But fuels used in real life are different.
Pure natural gas (CH4 or methane) produces carbon dioxide and water vapor when it is combusted.
Coal (some include less than 50% carbon) produce carbon dioxide, water vapor and nitrogen oxide, sulfur dioxide, etc.
Some fuels and their combustion products (consumed in large boilers used for electricity production) are provided below (Nazaroff and Alvarez-Cohen, 2001).
Bituminous coal Residual fuel oil (#6) (pulverized, 1.8%S) (2% S)
Particles 31 kg/Mg 2.9 kg/cubic m)
sulfur dioxide 35 kg/Mg 38 kg/(cubic m)
nitrogen dioxide 10.5 kg/Mg 8 kg/(cubic m)
carbonmonoxide 0.3 kg/Mg 0.6 kg/(cubic m)
Nonmethane organics 0.04 kg/Mg 0.09 kg/(cubic m)
methane 0.015 kg/Mg 0.03 kg/(cubic m)
Reference: Nazaroff, W.W. and Alvarez-Cohen, L. (2001). Environmental Engineering Science. John Wiley and Sons, Inc. New York, NY USA.