Ideally, the flue gas resulting from high-temperature oxidation of hydrocarbons (HC) contains CO 2 , H 2 O, N 2 , O 2 and some acceptable levels of oxides of nitrogen (NOx) and oxides of sulfur (SOx). In reality, the flue gas from a combustion process contains CO 2 , H 2 O, N 2 , O 2 and some concentration of carbon monoxide (CO), unburned hydrocarbons (UHC), NOx and SOx .