What effect may reducing excess air have on flue gases?

Reducing excess air in combustion processes can lead to a decrease in nitrogen oxides (NOx) emissions. When excess air is reduced, the combustion process becomes more efficient, as there is a closer balance between the fuel and the oxygen required for combustion. This optimal combustion temperature can prevent the conditions that often lead to the formation of nitrogen oxides.

In general, nitrogen oxides are formed at high temperatures when combustion takes place in the presence of excess oxygen, which enhances the combustion temperature and facilitates the reaction of nitrogen and oxygen in the air. By minimizing excess air, you lower the combustion temperature and, as a result, reduce the formation of NOx. This is especially important in industrial processes where controlling emissions is critical for meeting environmental regulations.

Therefore, the action of reducing excess air directly contributes to a decrease in nitrogen oxides emitted in the flue gases, which is why the chosen answer is accurate in this context.