Complex Combustion-Air/Flue-Gas Systems
This section uses the example of NOx technology to illustrate fundamental concepts.
Figure 600-4 is a typical schematic for a natural draft firebox. The flue gas from one or more radiant sections flows through a convection section and then up the stack. The stack dampers are manually adjusted to get the proper firebox drafts. (Alternatively, they may be adjusted automatically to hold the firebox(es) within a number of constraints including firebox draft, flue-gas oxygen and carbon monoxide contents, and tube skin temperatures.) The stack dampers are the only modulating control elements in the air/flue gas flow path.
Figure 600-5 is a typical schematic for the same fireboxes retrofitted with common convection, selective catalytic reduction (SCR) and economizer sections.
Each firebox has a pressure controller to maintain it’s normal air-flow pressure drop. Without these controllers, variations in the pressure drop across the three common sections could significantly vary the flow of air through the burners. Consider using an automatic shutdown (PDSHH—pressure differential shutdown high) to cut off all the fuel if the burner air-flow differential exceeds a safe value.
This maximum safe pressure drop of the air should be established by testing the burner, either at the burner vendor’s facility or in the field while commissioning the firebox.
There is also an anticipatory alarm (PDAH—pressure differential alarm high) to alarm the operator that the burner pressure drop for the air is getting outside the normal operating range. This should be set far enough from the normal operating drop to avoid nuisance alarms and far enough from the shutdown trip to avoid shutdowns without prior warning.
The alarms and shutdowns are needed to cover the operating cases when the pressure differential controller is in manual or is being repaired.
Note that the induced draft fan may need inlet louvers to keep the pressure differential controlling dampers in a good operating position.