POWER February 2012 - 33

INSTRUMENTATION & CONTROL
such as windbox and coal mill pipe distribution
and an influence from sootblowing,
or lack thereof, from side to side. Some of
these can be controlled by operation; others
may take a longer-term approach to
rectify. The overriding plant requirements,
however, are to eliminate or significantly
reduce the temperature excursions as
quickly as possible.
Sootblowing is set up on an intelligent
model-based program that could help, but
it shuts off below a mid-generation point.
When generation is above this point, however,
sootblowing can augment the control,
decreasing the temperature difference
between the waterwall furnace outlets in
addition to maintaining furnace specific
main steam temperatures and reheat temperatures.
As
an intermediate step, the setpoint
was decreased on the desuperheater inlet
override for the waterwall temperature
control. This provided some improvement,
yet not to the extent desired for protecting
the component tubes of the platen superheater;
therefore, additional enhancement
was needed.
Unit Front-End Slowdown
To protect the unit precipitators and the
scrubber, which provides flue gas desulfurization
(FGD), a DCS function has been
implemented to slow down the unit ramp
rate. This helps if opacity to the FGD (outlet
of the precipitators) becomes too high.
The opacity occurs as a result of precipitators
experiencing unacceptable levels of
furnace ash loading. The opacity also results
from the high ash content loading to
the FGD.
The algorithm employs mechanisms to
ensure signal quality by a software type
" deadman " function. This function is programmed
without the use of an external
signal from the monitoring instrument, as
none is available. This program ensures
signal health and validity. If high opacity
is detected from the precipitator to the
SCR equipment, the unit front end control
will slow down the unit's ramp rate proportionally
in relation to the value of the
opacity level.
MPC Model Development
At higher generation loads, the burner tilts
and the waterwall outlet temperatures as
measured by the BT valve links exhibited
a significant consistent relationship in the
modeling tools. In essence, the model predictive
control (MPC) model identified
the influence of tilts on the distribution of
furnace heat input between the furnace radiation
area and the convection pass at the
February 2012 | POWER
higher load points.
Integrating the model into the predictive
control produced noteworthy results. The
burner tilts went under the management of
the MPC with major weight given to maintaining
the two furnace sides' waterwall
outlet temperatures and less weight given
to the reheat temperature control. The reheat
spray control within the DCS processor
was revamped to ensure control of the
reheat temperature should the new model
control cause higher-than-desired temperatures.
The reheat sprays functioned
to control this temperature and have not
become excessive. This change minimized
any heat rate penalty, as the MPC does not
allow the tilts to generate excessive reheat
temperature.
The MPC is utilized in conjunction with
previously implemented furnace-to-furnace
bias model control. The MPC manipulates
the secondary air bias between the furnaces
based upon the desuperheater temperatures
of each furnace side, along with carbon monoxide
delta between each furnace side.
The furnace-to-furnace model bias control
offsets the influence of the gas flow
imbalances, functioning as designed.
However, there is a limitation at lower
generation. To maintain cooling air on the
secondary air registers requires a minimum
opening. As the steam generator load
point is decreased and the secondary air
registers reach this minimum flow limit,
the furnace-to-furnace bias is reduced in a
proportional fashion to the point where it
reaches zero and can no longer influence
furnace-to-furnace distribution.
Additional predictive control utilizing
the desuperheater inlet temperature as a
modeled constraint on the manipulation of
the waterwall temperature setpoint is set
in place. This lowers the effect of firing
in reducing both furnaces' platen temperatures
and thereby significantly reduces the
platen outlet peak temperatures. In other
words, the predictive control " rounds-out "
the platen superheater temperature increase
before reaching undesirable values.
During the performance analysis of the
furnace, further furnace modeling needs
became apparent. The unit exhibited a
1. Pre-SCR boiler heat balance. Before installation of the selective catalytic reduction
(SCR) system, heat absorption in the economizer was fairly constant throughout the load range.
Courtesy: GenOn Energy
Enthalpy gain across economizer
Enthalpy gain center wall
Enthalpy gain platen superheater
Total economizer in-desuperheater in
550
Enthalpy difference due to waterwall recirculation
Enthalpy gain side wall
Enthalpy gain waterwalls
Waterwall out average temp. (F)
1,000
450
900
Waterwall outlet
temperature constant
over load range
350
250
Waterwall heat gain
moderate increase at
low loads
150
600
Economizer heat gain
slight increase at low load
800
700
50
500
-50
Load (MW)
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33
100
200
300
400
500
600
700
400
Energy change (Btu/lb)
Waterwall and total enthalpy gain (Btu/lb)

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POWER February 2012

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