Minnesota Boiler License Practice Exam

Why is secondary air necessary in hand-fired boilers?

1. Necessary to burn the combustible gases

2. If primary air should fail

3. Used only if sulfur content is high

4. To increase the combustion rate

The correct answer is: Necessary to burn the combustible gases

Secondary air is crucial in hand-fired boilers primarily because it facilitates the effective combustion of combustible gases produced during the burning process. When fuel is burned, it generates gases that contain volatile compounds. If only primary air is supplied, there may not be enough oxygen to fully combust these gases, leading to incomplete combustion, which can produce pollutants like carbon monoxide and unburnt hydrocarbons. By introducing secondary air into the combustion zone, additional oxygen is provided, allowing for a more complete reaction of the combustible gases. This enhances the overall efficiency of the boiler and reduces emissions, contributing to better environmental performance as well as improved fuel utilization. The other options touch upon scenarios where secondary air might also play a role, but they do not emphasize its primary function. For instance, while it can support the system if primary air fails or can be important depending on the sulfur content, these situations are not the foundational reason for its necessity. Additionally, while increasing the combustion rate is a potential benefit of secondary air, its most critical role is to ensure that all combustible gases are adequately burned, thus optimizing the combustion process.