OCR Text |
Show 3. NEW SYSTEM DESIGN (cont'd) Automatic Combustion Control (cont'd) Figure 4 - Selective Catalytic Reduction with BFW Preheat ~ _._---- - The damper on the forced draft fan can be used to regulate the pressure of combustion air (hence the flow) to the burners. With this, "Iead/lag control II can be used for combustion. Lead/lag control is a name used for combustion control since it is designed so that the combustion air will always "Iead" or "Iag" the fuel so that the combustion zone can never be fuel rich. For an increase in heater load, it controls how much the fuel and air flows should be increased and ensures that the air is increased prior to an increase in fuel (air leads fuel). Similarly, for a decrease in heater load, it controls how much the fuel and air should be reduced and decreases the fuel flow prior to the air (air lags fuel). I n either case the heater is never fuel rich but, if anything, fuel lean. Fan Control and Reliability Fan control and reliability is one of the most important issues in the design of a NOx reduction system. It must be remembered that in many instances, where a SeR unit is required, we are revamping an existing natural draft heater (with no mechanical equipment) to one with either forced, induced, or balanced (forced and induced) draft. Once a fan(s) is installed on a heater the issue of fan or drive reliability is sure to arise. The combustion process will no longer be driven by stack height (inducing draft) alone, but will be fully dependent on the fan(s) operating. 7 |