POWER June 2015 - 61

FUNDAMENTALS
7. Optimization overview. Getting these parameters in sync
is a good start to optimizing combustion and minimizing slagging and
fouling. Courtesy: Storm Technologies Inc.
Adaptive Brush
Seal Solutions for
Air Preheaters
fired boilers, contributes to poor fuel balance, poor fuel fineness, and
longer flame lengths. Primary air is basically transport air and provides
15% to 25% of the total air for combustion. Therefore, when the
primary airflow is very high, the fuel particles " outrun " the secondary
air and result in longer-than-optimum flames (Figure 5).
High primary airflow on nearly any modern low-NOx
burner will
drive the fuel deep into the furnace, thus outrunning the secondary
airflow. As a consequence, fuel-rich zones can form in the upper furnace,
resulting in secondary combustion, elevated temperatures, and
zones of localized reducing atmosphere-all of which contribute to
slagging and fouling.
Burner Damage and Deficient Mechanical Condition/
Tolerances
One of the 13 essentials of optimum combustion is burner tolerance
at plus or minus one-quarter inch. The photographs shown in Figure 6
offer some examples of typical as-found burners.
Most of the damage to burners results from overheating of idle
burners due to radiant furnace heat. One way to monitor for burner
overheating is to attach thermocouples to the burner nozzles and provide
cooling air through the burner nozzles to keep below 800F when
out of service. (Burner cooling is a topic for another day.)
Poor Coal Pulverizer Performance
The most frequent cause of extreme fuel imbalances at the furnace
exit is coal pulverizer performance. Poor fuel fineness nearly always
contributes to poor fuel balance. At best, pulverized fuel balance will
be in the range of plus or minus 5% to 15% deviation.
When classifiers are not set for best fineness (usually to substitute
more coal pulverizer throughput for reduced fineness), the fineness
may deteriorate to less than 70% passing 200 mesh. Along with the
reduced fineness, there will be less-uniform fuel balance. Poor fuel
fineness nearly always results in poor fuel distribution. It is not unusual
to find fuel deviations of plus or minus 25% when the pulverizers
are not optimized.
The coal pulverizers are the heart of a pulverized coal-fueled boiler.
About 75% of the opportunities for improvements in tuning are
with the coal mills, primary airflow, and fuel line balancing. Figure 7
shows the important points for achieving optimum combustion with
minimum slagging and fouling.
For additional articles on related topics-from fuel analysis to pulverizer
control-search powermag.com. ■
-Richard F. (Dick) Storm, PE is senior consultant for Storm
Technologies Inc. and a long-time POWER contributor. The staff
of Storm Technologies contributed to this article.
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June 2015 | POWER
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POWER June 2015

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