POWER April 2017 - 44

OPERATIONS & MAINTENANCE
Table 1. Benchmarking variables. This table shows variables evaluated through
Duke Energy's combustion and boiler performance program. Prior to data collection, all controls,
instrumentation, and process variables should be established, functionally checked, tuned, and
optimized. Source: Duke Energy
Item
Description
1 Excess air control
Subset
Parameter
2
Mill-burner performance
(and condition)
A Measured furnace exit O2
B Measure boiler outlet O2 actual
C Control indicated boiler outlet excess O2
D Deviation between boiler exit and actual furnace values
E Deviation between actual boiler exit and the indicated value
A Assess clean airflow imbalances
B Assess dirty airflow imbalances
C
Assess fuel flow distribution (test and/or permanent instrumentation)
D Collect representative coal fineness samples; (weighted values),
% through 200m
E Collect representative coal fineness samples; (weighted values),
% retained on 50m
F Assess primary air-fuel ratios
G Assess mechanical burner condition and tolerances (including
airflow damper controls)
3 Boiler heat transfer
optimization
A Assess furnace exit gas temperature (FEGT), O2, CO
B Evaluate superheat (SH) spray flow rates
C Evaluate reheat (RH) spray flow rates (% of feedwater flow)
D Assess air preheater (APH) efficiency
E Assess APH X-ratio
F Assess APH leakage rates
G Assess APH outlet temperatures (corrected to no-leakage)
H Assess sootblower availability-IK (long retracts)
I
Assess sootblower availability-IR (wall blowers)
J Assess boiler efficiency
4 Combustion byproducts A Evaluate losses due to unburned carbon in ash (weighted average for
fly ash and bottom ash)
B Evaluate boiler exit/selective catalytic reduction (SCR) inlet NOx
C Evaluate boiler exit CO (ppm)
D Evaluate fly ash particle sizing
5 Air-gas flow
A Evaluate forced draft fan dampers, % at full load
B Evaluate induced draft (ID) fan dampers, % at full load
C Evaluate total system air in-leakage (furnace to ID fan)
6 Fuels prep
A Evaluate top size of raw coal (gauging screen/crusher)
B Evaluate process for coal quality control
C Evaluate coal treatment systems; chemical consumption details
(fuel, SCR, dry sorbent injection, flue gas desulphurization)
7 Training
A Evaluate site-specific training needs
confused with the previously mentioned Boiler
Performance Program benchmarking assessment,
some examples of proactive actions
being deployed to enhance Duke Energy's
boiler defense strategy and prevent common
boiler failure mechanisms on its aging coal
fleet include the following:
■ Combustion optimization
■ Slag and clinker fall mitigation
■ Boiler tube overheat mitigation
44
■ Boiler water chemistry control
■ Fireside corrosion and thermal fatigue mitigation
■
Air in-leakage control
■ Sootblower and fly ash erosion mitigation
■ Air heater fouling mitigation and air-gas
management improvements
Combustion Optimization. As with
most large pulverized coal-fired boilers, boiler
efficiency losses account for about 10% to
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15% of total losses. However, if performance
is not optimal, the controllable losses can result
in an even greater degradation in boiler
and overall plant performance.
Controllable losses are often related to dry
flue gas and carbon losses, which are in turn
related to combustion. The best boiler efficiency
and reliability can be attained if losses
are kept to a minimum. Because half of the
losses are typically dependent on the fuel and
ambient condition, the best efficiency can be
achieved through optimal settings, tuning of
the boiler, and its auxiliary equipment.
Some combustion optimization steps include:
■ Monitoring air-fuel distribution with temporary
and/or real-time instrumentation to
provide feedback for air-fuel distribution
and control.
■ Monitoring mill horsepower per ton to detect
mill performance degradation after a
mill is rebuilt, tested, and proven.
■ Optimizing unit performance using feedback
from representative economizer outlet
O2 and CO measurement equipment.
■ Applying performance-driven maintenance,
modeling, tuning, and optimization
to achieve optimal performance.
■ Mechanically managing and optimizing
firing system components, using an internal
database for conditional assessment
of components. (This can also be used
for blueprinting milling and firing system
equipment to specifications.) With cyclic
operations, it is a challenge to sustain optimal
mechanical geometry and centering
of air zones with low-NOx burners that are
out of service and suffering from thermal
distortion. Closer visual inspection of burners
with drones should be considered for a
cost-effective approach to conducting burner
assessments (Figure 4).
Combustion and boiler performance assessments
should also include the following:
■ Assess firing conditions (for example, mills
in/out of service, damper stroking, out of
service burners, etcetera)
■ Ensure optimal fan performance and margins
exist for the as-fired fuels
■ Optimize furnace exit gas temperature
(FEGT) to ensure proper lower furnace
heat transfer
■ Validate boiler O2 representation and balance
■ Obtain representative boiler exit O2 and CO
measurements at the immediate boiler outlet
(These are key components for real-time
dynamic combustion optimization.)
■ Collect routine fly ash samples for consistent
loss on ignition (LOI) analyses
■ Optimize desuperheating spray flow rates
■ Conduct routine visual observations or,
POWER | April 2017
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POWER April 2017

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