POWER October 2010 - 73

PLANT CONTROLS
6. Put it all together. The complete control system for a biomass-fired boiler combines
coordinated control techniques, boiler optimization, and the fuel power compensator. Source:
Metso Automation USA Inc.
Calculated real-time
energy flow
Combustion
and bed
temperature
optimizer
Unit demand
calculation
Boiler
protection
DLR
Oxygen
correction
Fuel control
calculations
Airflow control
calculations
Bed level
Airflow measurements
secondary, overfire,
and underfire
it is only accurate at a defined fuel quality
for a specific fuel/air ratio. In the event that
fuel quality decreases, the pressure control
will increase the firing rate to maintain boiler
output, but the cross-limiting algorithms will
tend to decrease air, thus creating a potentially
dangerous condition.
Under those conditions, a protection soluA
B
Demand
to screw feeders
uses as many variables as are available within
the control system and, most importantly, the
process model is described linguistically.
Figure 5 is an overview of the information
flow used in this " fuzzy logic " -based
combustion optimization scheme. The fuzzy
logic combustion optimizer has been successfully
employed on more than 20 different
FBC boilers burning different combinations
of biomass and coal, including a BFB boiler
that burns peat, wood chips, and bark at a
Stora Enso paper mill in Finland.
We are now able to combine all the
pieces into a complete and coordinated design
that is capable of interacting with automatic
generation controls often required
by the local independent system operator
(ISO) for dispatch and coordination of
plants operating in a region on a common
grid. If this is a requirement, the controls
must also compensate for the variation in
fuel supply to meet the plant's electricity
supply commitments to the ISO. Figure
6 illustrates a comprehensive approach
to plant controls that combines the previously
described control designs.
Protect the Boiler from Unsafe Operating
Conditions. The fourth goal is to ensure
that the control system design protects
the boiler under all operating conditions,
specifically from dangerous fuel-rich conditions,
because National Fire Protection Association
85 regulations are applicable to even
the smallest biomass operations.
One method typically employed by many
control system designers is called " cross limiting. "
Simply stated, cross limiting means
that the metered fuel is continuously compared
to airflow demand, and the greater of
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C
Demand to
overfire air
dampers
Demand to FD
fan air damper
Demand to
underfire air
damper
the two is used as the demand signal for air.
Furthermore, metered airflow is compared to
the fuel firing rate demand, and the lower of
the two is used as the demand for fuel. This
method has several limitations; specifically,
tion is required. The demand limit regulator
(DLR), shown in Figure 6, monitors both the
fuel and airflow errors. If either should exceed
a predetermined limit, the firing rate demand
is changed in the direction that reduces
the error to an acceptable level. For example,
should airflow become limited because of
a stuck damper while increasing load, the
airflow error increases until it reaches a predetermined
limit that is unacceptable. The
boiler demand is then run back until the airflow
error is returned to normal. Note that

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POWER October 2010

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