Alberta Boilers Safety Association (ABSA) Practice Test

How can nitrogen oxides in flue gas be reduced?

1. By employing a dilute sulphuric acid scrubbing system

2. By decreasing the size of the furnace

3. By scrubbing the flue gas with lime

4. By reducing the percentage of excess air

The correct answer is: By reducing the percentage of excess air

Reducing the percentage of excess air is an effective way to decrease nitrogen oxides (NOx) in flue gas. When combustion occurs, the presence of excess air leads to higher combustion temperatures, which can facilitate the formation of NOx through a reaction between nitrogen in the air and oxygen. By optimizing the combustion process to minimize excess air, the temperatures can be lowered, which in turn reduces the formation of NOx. This method not only improves efficiency by ensuring that fuel is burned more completely but also contributes to lowering the overall emissions from the system. While other options may involve some filtration or scrubbing techniques, they do not target the fundamental combustion process directly as effectively as adjusting the excess air does. For instance, scrubbing flue gas with lime primarily targets acid gases rather than directly addressing NOx formation, and the size of the furnace isn't directly related to NOx emissions in the context of combustion efficiency.