POWER March 2021 - 29

COAL
In total, primary airflow typically accounts
for approximately 25% of the
total theoretical air required for combustion
for Powder River Basin fuels.
Initially, Storm found that PPES was
operating with approximately 33% of
total air being supplied as primary air
for the pulverizers at low load. This was
concerning as elevated burner nozzle
velocities can impact the combustion
zone negatively, leading to poor combustion,
higher formation of nitrogen
oxides (NOX
), undesirable upper furnace
conditions, poor fuel fineness/distribution,
slagging, excessive wear on fuel
lines and burners, and is also a stealth
heat rate penalty.
To improve upon this baseline, Storm
and plant personnel reduced the minimum
required airflow to each mill so
that the total primary airflow accounted
for 28.8%-an approximate reduction of
200,000 lb/hr-of the total airflow supplied
to the boiler at low load. After implementing
a new operating curve, each
mill was retested to ensure minimum
required velocities were maintained
and optimum performance achieved.
As a result of reducing the minimum
airflow, tempering airflow bypassing the
air heaters was reduced and directly resulted
in a small heat rate credit.
Pulverizers have proven to be a leading
factor to achieving great combustion
efficiency; however, pulverizers
can also dictate how responsive a unit
is to a change in demand. At PPES, a
goal was to increase the rate at which
the unit could ramp when a change in
demand was made. This was achieved
in conjunction with plant personnel and
Emerson Automation Solutions by adding
anticipatory signals to the control
logic. In short, the anticipatory signals
detect a change in demand, and immediately
signal the primary air fans to
increase slightly and remove additional
inventory from the pulverizers allowing
the unit to ramp quicker. These changes
were also aided by the calibration
of primary airflow and the adjustments
made to the air-to-fuel curve in the
logic. Following the load change, the
primary air then settles back to the desired
air-to-fuel curve setpoint to maintain
optimum performance.
Emissions Analysis Confirms the
Benefits
The second phase of tuning focused on
quantifying flue gas constituents including
oxygen (%), carbon monoxide (CO,
ppm), NOX
March 2021 | POWER
■ Series 1 ■ Series 2 ■ Series 3 ■ Series 4 ■ Series 5 ■ Series 6 ■ Series 7
2. The graph on the left shows the final superheater outlet tube metal temperatures over
a one-hour period before combustion tuning was performed, while the one on the right
shows the temperatures during a one-hour period after the unit was optimized. Courtesy:
Storm Technologies
at the economizer outlet across the entire
operating range of the boiler. The
goal of the tuning at the economizer
outlet was to balance the oxygen and
temperature evenly across both outlet
ducts, and reduce the amount of CO
and NOX
measured. After a series of
iterative tests utilizing Storm's SOAR
(Storm Optimization Automated Results)
gas sampling system to sample
gas constituents at 64 evenly spaced
points, Storm was able to reduce both
CO and NOX
across the operating range.
To achieve these goals, adjustments
were made to the compartmentalized
wind boxes controlling secondary airflow
to each burner elevation, as well
as overfire air ports located on both the
front and rear walls. This allowed Storm
to achieve a better oxygen distribution
in the backpass and subsequently led to
the ability to lower overall excess air.
Lowering excess air aided with the
staged-combustion, low-NOX
burner
performance; improved dry gas losses;
and resulted in a significant reduction in
NOX
measured at the economizer outlet.
A 30% reduction in NOX
on a lb/MMBtu
basis was calculated from a baseline of
0.15 lb/MMBtu to 0.10 lb/MMBtu at the
inlet of the selective catalytic reduction
(SCR) system.
To further validate the effects boiler
(ppm), and temperature (F)
tuning had on PPES, historian data was
analyzed for ammonia injection at low
load before and after the tuning efforts
made by Storm and plant personnel.
These results showed a significant decrease
in the ammonia injection rate on
both ducts in the boiler. Data collected
from the control room indicated that
spray flows were reduced by 22.83% at
full load and 54.89% at low load.
An added benefit of a balanced furnace
with regard to combustion and
oxygen distribution is also a temperature
balance that coincides with these
www.powermag.com
conditions. Initially at PPES, tube metal
temperatures were observed to have a
significant deviation from east to west.
After tuning, the tube metal temperatures
indicated a more even balance.
Furthermore, a decrease of 115,000 lb/
hr in superheat spray flow was observed
after the final adjustments were made.
Figure 2 depicts the final superheat
tube metal temperatures measured by
the distributed control system before
and after combustion tubing.
The result of PPES's efforts was naturally
apparent in the results of its heat
rate evaluation. After more than 10 years
of operation, PPES boasts an impressive
net heat rate of 9,231 Btu/kWh (37% efficiency)
at full load. The heat rate evaluation
was done in conjunction with Storm
Technologies, plant personnel, and several
other companies. Each heat rate test
was run to meet the ASME PTC 46 Overall
Plant Performance evaluation standard.
Plant reliability, efficiency, emissions,
and operating range are four of the main
factors affecting today's active coal fleet
in the U.S. and many other places in the
world. Each facet of operation was addressed
at PPES during Storm Technologies'
time onsite in 2020, and as result,
the plant saw improvements across the
board. Operating range of the boiler
improved as a result of the tuning by
achieving a safe low-load operating setpoint
of 290 MW (gross), approximately
60 MW less than the previous low-load
operating limit. This new operating
range along with the other factors addressed
during this project will help to
solidify PPES's position as one of the
most competitive coal-fired boilers in
the country for years to come. ■
-Teddy Cook, EI is project engineer
with Storm Technologies Inc.;
Bruce Partlow, PE and Robert Dickson
are plant engineers with NRG Energy
at Plum Point Energy Station.
29

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POWER March 2021

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