What is the secondary function of combustion air in black liquor combustion?

In black liquor combustion, the secondary function of combustion air is to help with combustion completion. This process involves ensuring that all the combustible components in the black liquor are fully oxidized during combustion. Providing adequate combustion air helps to maintain an efficient burning process, which is crucial for optimizing energy release and minimizing unburned residues.

While black liquor contains a significant amount of volatile organic components that require proper oxygen availability for complete combustion, the introduction of combustion air not only aids in this completion process but also supports the overall efficiency of the boiler operation. By ensuring that combustion is thorough, the generation of problematic emissions is diminished—though that would fall under the primary function of optimizing performance rather than being its sole purpose.

The other options focus on distinct aspects of the combustion process. Generating steam pertains to the primary purpose of the combustion process overall but does not specifically highlight the role of combustion air. Similarly, increasing pressure is related to the system's operation and the result of steam generation. Reducing emissions is an important outcome of efficient combustion but is not specifically tied to the secondary function of combustion air, which is more focused on ensuring that the burning process completes effectively.