POWER October 2010 - 71

PLANT CONTROLS
cient operation of a unit. In too many plants,
fuel delivery equipment failures are cited as
the number one reason for poor performance
and availability. Inconsistent fuel delivery
can lead to the inability to match boiler output
with turbine energy requirements, maintain
throttle pressure, or maintain emission
levels at or below what is required by environmental
authorities.
Biofuel delivery systems are usually
composed of some combination of
drag chains, silos, metering bins, feeding
screws, and/or gravity drop chutes-all of
which present opportunities for discontinuity
and/or unplanned stoppages. These systems
are prime candidates for machinery
condition monitoring. Combine delivery
system problems with varying fuel characteristics,
and you have a recipe for inconsistent
energy input, inconsistent energy
output, and inadvertent releases of hazardous
air pollutants.
Note that the new Boiler Maximum
Achievable Control Technology (Boiler
MACT) rule will no longer permit operating
above the limits for hazardous air pollutants
during start-up, shutdown, or emergency conditions.
The proposed new emissions standard
was signed by the Environmental Protection
Agency administrator on April 29, 2010, and
is expected to be promulgated on December
16, 2010. For more information on the Boiler
MACT rule, go to www.epa.gov/ttn/atw.
Control Objectives
We have discussed many of the concerns
associated with firing biomass. Please note
that all of these issues are relevant when
co-firing biomass and coal, whether it is in
an FBC boiler, grate boiler, or pulverized
coal-fired boiler. The following objectives
apply to any boiler-turbine control system,
but especially to systems that have boilers
that combust biomass fuels with much
variation in quality:
■ Maintain output to match demand.
■ Maintain boiler-turbine balance (throttle
pressure).
■ Optimize for emissions and efficiency.
■ Protect the boiler from unsafe operating
conditions, especially fuel-rich conditions.
We have chosen a coordinated approach
to meet these objectives. Note that we have
assumed in this discussion that the plant
uses a steam turbine. The turbine can be
fed by any number of steam sources (such
as multiple boilers) that are directly connected
to a common steam header.
Maintain Output to Match Demand.
First and foremost we must match generation
to meet demand. A closed-loop generation
control employing turbine first-stage
pressure, megawatts, frequency bias, and
operating modes must be employed. It must
have an interface with the steam turbine
control so that the coordinated control can
directly operate the governor valves. Runup/run-down
capability also must be provided.
This control function must linearize
the governor valves with the power output.
If there are multiple steam sources, each
must have a similar control arrangement.
Maintain Boiler-Turbine Balance. The
second objective, to fire the boiler to main2.
Stay in balance. The direct energy balance boiler demand calculation is nonregenerative
and self-calibrating. It is also a measure of turbine energy and provides the correct amount of
boiler over- or underfiring. Source: Metso Automation USA Inc.
Pressure
program
A
First-stage
pressure
P1
Ps
P1
PT
X
Energy
balance
signal
PT
Throttle
pressure
October 2010 | POWER
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Turbine energy input
requirement
X
Boiler energy
input demand
d/dt
Dynamic rate
X
F(x)
Feedforward for variable
pressure control
d/dt
PID
PT
Throttle
pressure
tain boiler energy output (or that of a combination
of steam clients) equal to turbine
demand, is far more difficult. Note that when
throttle pressure is at setpoint, the boiler and
turbine are in balance-an energy balance,
not steam flow balance. We are concerned
with the total energy output and total energy
input. The energy input must include energy
to accommodate boiler storage, the level of
which is nonlinear with steaming rate. To do
so, we will first develop a firing rate demand
signal and then the energy input into the boiler,
which must remain in balance for proper
plant operation.
The firing rate demand function computes
the overall turbine energy requirement.
It must be a linear calculation, be
correct under all conditions, and not be affected
by boiler upsets or changes in the
energy input (such as changing fuel quality).
The demand signal must also consider
multiple steam sources, if they are present.
The demand calculation is taken from
the well-known direct energy balance
(DEB) coordinated control system. DEB is
based on a normalized ratio of turbine firststage
pressure (P1
) and throttle pressure
(PT), which is called the energy balance
(EB) calculation. Next, dynamic compensation
for over/underfiring is added to the
EB calculation. Throttle pressure setpoint
(PS
/PT
) x PS
.
), also a normalized value, accounts for
variable pressure operation, although this
is uncommon in IPP installations. The EB
is found as (P1
The EB is equal to one (100%) when the
governor valves are wide open and zero
when they are fully closed. The dynamic
compensation is found as the derivative of
the EB times the EB. The complete firing
rate demand, including pressure correction,
is illustrated in Figure 2. This is a proven
application based upon more than 30 years
of industry experience and more than 1,000
installations on boilers of all sizes.
Now we must calculate the energy input to
the boiler. For most boilers, including FBC
units, a measure of the fuel delivery rate is
derived from the screw feeder. Unfortunately,
this approach is subject to error, as biomass
constituents and moisture change. However,
this is also true for plants that use gravimetric
belt feeders. We need a way to compensate
for the changes in fuel quality, density, moisture
content, and so on in order to have a true
measure of boiler energy input.
Over the years, several different methods
have been used for solid fuel firing
under these operating conditions. One
method that has worked well with fluidized
bed boilers is called the " fuel power
compensator. " It is a real-time calculation
that provides an accurate indication of the
71

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POWER October 2010

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