POWER December 2021 - 34

COAL POWER
Unit Flexibility and Asset
Optimization Technical Issues
Because there are several different types
of coal-fired units, each can present a different
set of problems and required solutions.
Technical issues will be addressed
below for the boiler, turbine, and BOP.
Boiler. Maintaining adequate steam
temperature is impacted by which pulverizers/cyclones
are put into operation. Operating
pulverizers/cyclones at the higher
elevations of the boiler will produce higher
steam temperatures, while operating
lower pulverizers/cyclones will improve
the heat rate but may cause issues with
holding the steam temperature.
Depending upon the boiler configuration
and the operating load, pulverizer/
cyclone operation is a mixture of upper,
middle, and lower elevations to achieve
the best results during low-load operation.
Multiple days of testing different
combinations throughout the entire load
range will help identify the best configuration
for pulverizer operation. Biasing
coal flow to the different pulverizers in
service is an effective method of boiler
operation. Increasing the excess air into
the boiler will also assist in maintaining
steam temperatures.
In pulverized coal-fired units, burner
low-load function may be impacted. At
full unit load, the secondary air (SA) to
primary air (PA) ratio at the burner is approximately
1.6. During load reduction,
the SA tends to reduce more than the
PA, and this causes the PA to outrun the
SA, and it mixes in the furnace instead of
in the burner, thus reducing combustion
efficiency, increasing slagging, and increasing
loss on ignition (LOI). Individual
burner adjustments may be required at
low loads. Installing dynamic classifiers
to improve coal fineness will reduce momentum
and thus lower the SA-PA ratio
at low load.
At very low loads, adequate coal drying
in the pulverizers may be impacted
by low hot air temperatures. As load is
reduced, exit gas temperatures will also
reduce, thus decreasing the temperature
exchange via the air heater. Operating one
or two upper pulverizers will assist in maintaining
adequate exit gas temperature.
In cyclone
fired
boilers,
the most
pressing technical issue with low-load
operation is being able to maintain proper
ash temperature and flow at the boiler
bottom slag taps. If the slag taps cool,
and then plug, the unit will have to be
shut down and the taps cleaned out.
Operating lower level cyclones will eliminate
or reduce this issue.
34
At times, an ignitor or two closest to
the slag tap will have to be operated intermittently
to heat up the area and get
the tap fully open and slagging properly.
Reducing the slag tank level, reducing
refractory cooling sprays, and reducing
seal trough make-up are other effective
steps to aid in keeping the slag taps
open and flowing.
Multiple days of testing different combinations
throughout the entire load
range will help identify the best configuration
for cyclone operation. Biasing
coal flow to the cyclones in service can
also be an effective method for boiler
operation. Increasing the excess air into
the boiler will also assist in maintaining
steam temperatures.
Increasing ramp rates to the 10-15
MW/min range requires a new control
tuning approach, as well as, a well-trained
operations staff to safely, efficiently, and
quickly place equipment in or out of operation.
On large units, this may require
going from several mills/cyclones down
to a few at minimum load, and then
transitioning back to all mills/cyclones
in service at full load. This requires that
all instrumentation and equipment function
as designed, and that the operations
staff has proper procedures in place and
is well trained.
To date, the largest challenges involved
having adequate steam drum level control,
and superheater (SH) and reheater
(RH) attemperator control while ramping
in the 10-15 MW/min range over the entire
load range of the unit. Attemperators
require new control tuning and it is essential
that there is adequate feed-forward input.
Also critical is that the attemperators
completely close and do not leak through,
as this will impact SH and RH temperature
due to excessive cooling, which in
turn will impact cyclical life of the steam
turbine and create pipe stress in the highenergy
piping system.
To achieve elevated ramp rates on
pulverized coal units, the PA curves to
each mill must be set correctly to avoid
sweeping the reserve fines out of the
mill. A properly configured and tuned mill
will always have a reserve of fines circulating
within the mill and its classifier.
During elevated ramp rates, the turbine
valves start to open to increase
steam flow, and thus, increase MW output.
On average, it takes five to seven
minutes from the time a coal feeder
increases coal flow to the mill, the mill
pulverizes the coal, and it's transported
to the boiler as additional fuel. Without
a reserve of fines in the mill, the turbine
www.powermag.com
throttle pressure will decay and the unit's
MW output will start to decrease. The reserve
fines prevent the throttle pressure
from decaying by providing additional
fuel to the boiler as the PA dampers on
the mills open and transport an increased
amount of coal to the boiler.
Steam Turbine. Maintaining stable
SH and RH temperatures across the entire
load range is critical to reduce the
impacts of cyclical life to the turbine and
thus minimize O&M costs. Most original
equipment manufacturers (OEMs) have
a startup and loading chart showing the
impacts of thermal stress caused by
exceeding stated degree/hr changes in
steam and turbine metal temperatures.
These stresses can occur when either
reducing or increasing load. Control system
tuning,
proper
pulverizer/cyclone
configuration and loading, ramp rate,
sliding pressure operation, and full-arc
operation can maintain cyclical life impacts
within an acceptable range.
Most steam turbine OEMs have water
induction protection logic that will
open the turbine drains at 20% of rated
full load. In many plants, this becomes
the new low-load minimum as the turbine
cannot be operated with all the
drains open. This logic is more intended
for unit startup to ensure proper heating
and condensate removal during the
early stages of startup. During low-load
operation, steam and turbine metal temperatures
can be maintained very near
OEM recommendations, thus reducing
any potential for a water induction event.
There are several plant owners that have
revised this 20% of rated full load logic
to only apply to startups, so they can operate
the units even lower during times
of low power demand and low prices.
Keep in mind the rest of the water induction
protection system is still active.
Operating at low load for extended periods
of time can create rotor expansion.
This occurs primarily in the intermediatepressure
(IP) section of the turbine. In
units that have a high-pressure (HP)/IP
cooling valve, the logic can be revised
to provide cooling at low loads. In tested
units, this has been effective. Reducing
the throttle pressure and getting turbine
valves open seems to have the greatest
effect at eliminating IP rotor expansion.
Potential low-load backpressure and laststage
blade erosion are some additional
areas of concern.
Increased ramp rates in the 10-15
MW/min range will impact the turbine
if SH and RH steam temperatures cannot
be controlled within an acceptPOWER
| December 2021
http://www.powermag.com
POWER December 2021

Table of Contents for the Digital Edition of POWER December 2021
