OCR Text |
Show 3. NEW SYSTEM DESIGN (cont'd) Automatic Combustion Control (cont'd) Figure 3 - NOx. CO and Oxygen Relationship N O.~~ ______________________ ~ 'O~ M '0 > QE. • S: c i~ 8 c • 8 o u ~ . .c, NOx. co. and Oxygen Relationship co / dZ ••L -__ ~.~ ___ .~ ___ .~ ___ .~ __ ~ Excess Oxygen <" volume, dry) The concept here is that the stack damper is used for automatic firebox draft control and the burner registers are used to minimize both oxygen and CO. The only problem that arises is the control of many burner registers in unison. This can be accomplished by either linking the damper positioners together or installing a common windbox with single or dual dampers. When possible, the common windbox should be used as the linkage of many positioners becomes complicated. If an SCR unit is to be installed, the type of ACC may change depending on the equipment installed downstream of the SeR. Once an SeR unit is chosen, a induced draft fan will become a necessity (if not already existing) to overcome the flue gas pressure drop caused by the SeR. If simple waste heat recovery is chosen, such as boiler feed water preheat, then the combustion control would remain unchanged except the firebox draft would be controlled via the induced draft fan (Figure 4). If a combustion air preheater (APH) is more economical, a forced draft fan will also become a necessity to overcome the APH pressure drop on the air side. This forced draft can also be used to benefit the combustion control. 6 |