Flue-gas emissions from fossil-fuel combustion refers to the combustion-product gas resulting from the burning of fossil fuels. Most fossil fuels are combusted with ambient air (as differentiated from combustion with pure oxygen). Since ambient air contains about 79 volume percent gaseous nitrogen (N2), which is essentially non-combustible, the largest part of the flue gas from most fossil-fuel combustion is uncombusted nitrogen. Carbon dioxide (CO2 A typical flue gas from the combustion of fossil fuels contains very small amounts of nitrogen oxides (NOx ), sulfur dioxide (SO2