Page 120 - Boiler plant and distribution system optimization manual
P. 120
Boiler Tuneup 105
2. Record boiler and stack data (pressure tem- in appearance and remember to observe
perature etc.), and observe flame conditions local air pollution ordinances.
after the boiler operation stabilizes at the • Incomplete burning of solid fuels. Rec-
particular firing rate selected. If you find ognize this by high carbon carryover to
that the amount of excess oxygen in the flue dust collectors or increased amounts of
gas is at the lower end of the range of typical combustibles in the ash.
minimum values and the CO and the smoke • Equipment-related limitations such as
are at acceptable levels, the boiler is already low windbox/furnace pressure differen-
operating at a near optimum air to fuel ratio. tial, built-in air flow limits, etc.
This may not be so at other firing rates. It
may still remaining portion of this proce- 6. Develop O /smoke or O /CO characteristic
2
2
dure, to practical. curves, similar to those shown in Figures
9.28 and 9.29 using the excess oxygen and
3. Increase air flow to the furnace until read- CO or smoke-spot number data obtained at
ings of excess oxygen at the stack increase each air-flow setting.
by 1-2%. Again, be sure to take readings
after boiler operation stabilizes and note any 7. Find the minimum excess-oxygen level for
changes in flame conditions. the boiler from the curves prepared in step
6, but do not adjust the burner controls to
4. Return air flow to normal level and begin to this value. Though this may be the point of
slowly reduce it further, in small increments. maximum efficiency, as well as minimal NO
x
Watch the stack for any signs of smoke and emissions, it usually is impractical to operate
constantly observe the flame and stack. Re- the boiler controls at this setting, because of
cord stack excess-oxygen reading, smoke the tendency to smoke or to increase CO to
spot number, the concentration of CO in the dangerously high levels as load changes.
flue gas and the stack temperature after each
change. Compare this minimum value of excess oxy-
gen to the expected value provided by the boiler
Do not reduce air flow by throttling the manufacturer. If the minimum level you found is
burner air registers, because this alters the fuel air substantially higher then the manufacturer’s re-
mixing characteristics and complicates the tests. pairs or parts replacement probably can improve
Also, if you run tests at low firing rates, which is fuel and air mixing, thereby allowing operation
not generally recommended, keep a close watch with less air.
on the windbox/furnace differential. If it drops
too low, a fuel trip may be activated by the burner 8. Establish the excess oxygen (buffer zone)
safeguard system. margin above the minimum value, required
for fuel variations, load changes, and atmo-
5. Continue to reduce the airflow step- wise spheric conditions. Add this to the minimum
until you reach one of theses limits: value and reset burner controls to operate
• Unacceptable flame conditions such as automatically at the higher level-the lowest
flame impingement on furnace walls or practical setting at the particular firing rate.
burner parts, excessive flame carryover,
or flame instability. 9. Repeat steps 1-8 for each firing rate being
• High level of CO in the flue gas. tested. For some control systems, it is not
• Smoking at the stack. Do not confuse possible to establish the optimum excess-ox-
smoke with water vapor, sulfur or dust ygen level at each firing rate. The reason is
plumes which are usually white or gray that control adjustments at one firing rate
