What are the recommended percentages of underfire air versus overfire air when using a stoker coal-fired boiler?

In a stoker coal-fired boiler, the distribution of underfire and overfire air is crucial for efficient combustion. The correct percentage of 65% underfire air and 35% overfire air is recommended because this balance optimizes the combustion process.

Underfire air is essential for the ignition of the coal and for sustaining the flame and the initial combustion. It helps maintain the desired temperature and efficiently burns volatile components of coal. On the other hand, overfire air is introduced to promote the complete combustion of remaining volatile gases and particles, ensuring that the combustion byproducts are minimized.

A ratio of 65% underfire and 35% overfire air allows for sufficient oxygen to be available at the beginning of the combustion process while providing enough mixing and additional oxygen to facilitate complete combustion. This distribution helps reduce unburned carbon and emissions, contributing to better energy efficiency and reduced environmental impact.

Options with higher percentages of underfire air would not provide enough oxygen for complete combustion of the volatiles, leading to inefficiency and higher emissions. Likewise, options with a more balanced or higher percentage of overfire air may lead to inadequate initial combustion, preventing optimal ignition and maintaining flame stability.