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 (N
2), which is essentially non-combustible, the largest part of the
flue gas from most fossil-fuel combustion is uncombusted nitrogen.
Carbon dioxide (CO
2), the next largest part of flue gas, can be as much as 10-25 volume percent or more of the flue gas. This is closely followed in volume by water vapor (H
2O) created by the combustion of the hydrogen in the fuel with atmospheric oxygen. Much of the 'smoke' seen pouring from
flue gas stacks is this water vapor forming a cloud as it contacts cool air.