If an analysis of boiler flue gas determines there is no excess air for combustion, you should expect the nitrogen content of the flue gas to be approximately __________.

Correct Answer: Option C — 79.0%

Atmospheric air consists of approximately 79 percent nitrogen and 21 percent oxygen by volume. When fuel burns with zero excess air, the oxygen is completely consumed, leaving the inert nitrogen essentially unchanged, resulting in roughly 79 percent nitrogen in the flue gas.