POWER February 2013 - 33

INSTRUMENTATION & CONTROL
■ Superheat sprays and reheat sprays cooling
steam to an operator-selected setpoint.
■ Gas path dampers or gas recirculation to
distribute gases to specific sections of
the boiler.
■ Burner tilt angle, which directs the location
of combustion within the furnace, thereby
influencing the energy distribution.
■ The number of burners in service at any
time, which affects the location of combustion
heat release.
However, these typical systems may be
inadequate to reach and maintain design superheat
steam temperatures at all operating
points due to other overriding factors, such
as fuel quality or flow, boiler design changes,
emissions limitations, boiler cleaning practices,
and the effects of cycling on furnace
heat release.
Fuel Quality Changes or Coal Mills
Out of Service Change Fuel Flow. Fuel
quality changes or coal mills out of service
can result in a shift of the energy distribution
within the furnace. For example, an
increase in fuel heating value or lower coal
mill out of service may raise the combustion
zone temperature and NOx
, causing
greater heat release in the waterwall (evaporator)
section of the boiler. The combination
of higher steam generation rate and
lower gas energy per unit of heat input to
the superheater may lower the attainable
superheat and reheat steam temperatures
to well below design.
Boiler Modifications Change Heat
Release. Over the course of the life of
the boiler, major modifications to boiler
sections may affect the boiler's ability to
achieve design steam temperatures. One
example is the replacement of the economizer
with a more effective heat exchanger
that increases the outlet water temperature
that is then sent to the steam drum. This
replacement reduces the evaporator energy
requirement to generate steam, increasing
steam production per unit of fuel. Higher
steam flow through the superheater per unit
of fuel, with no increase in gas energy to the
superheater, adversely affects the ability to
attain design steam temperatures.
Emissions May Limit Boiler Operation.
The amount of emissions allowed may limit
the range of in-furnace temperature controls.
In-furnace steam temperature controls, such
as burner tilt angle, shift the energy distribution
between sections of the boiler. However,
burner tilt angles may have adverse consequences
on the combustion process, air/fuel
mixing, and CO and NOx
emissions.
Sootblowers Are Not Effective. The
daily operating profile for traditional coalfired
power plants has shifted substantially
February 2013 | POWER
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Entropy (Btu F/lb)
33
in recent years due to the availability and
low cost of natural gas and increased utilization
of renewable power generators.
Nightly load shedding to minimum load
and unit shutdowns have a major impact on
the cleanliness of the furnace and energy
distribution. For example, the waterwall
section may completely shed its normal
fouling, resulting in excessive steam production
per unit of heat input on the following
startup or load ramp.
Boiler Cycling Changes Furnace
Fouling. Frequent load shedding or shutdowns
may change the pattern of furnace
fouling, changing the energy distribution
within the furnace and limiting steam temperatures.
Multivariable steam temperature
control systems have been commissioned
that maximize the integrated performance
of the available steam temperature control
systems and provide the platform for adding
additional control elements in the future
to further enhance unit performance.
HP FWH Energy Control
For the purposes of illustrating the usefulness
of FWH energy control, comparative
operating scenarios are presented. From
the design point systems described in Figure
2, assume steam temperatures below
design, which introduces a loss of cycle
efficiency. If steam temperatures were
970F, for example, then the cycle diagram
is as presented in red on Figure 2. Lower
steam temperatures reduce the work produced
by each turbine, indicated by the
enthalpy difference. Also, the LP turbine
exhaust is shifted toward higher moisture,
which is in practice a less-efficient operating
range for the last stage of a steam turbine.
The net impact is a loss of thermal
efficiency and power generation capability
when compared with design.
For drum boilers, boiler feedwater temperature
directly affects superheater steam
temperature. Cooler feedwater reduces the
steam generation per unit of energy sup2.
Steam cycle diagram. The typical regenerative steam cycle with reheat is shown on
a T-S diagram (green line). Q represents fuel burned to raise the water temperature to make
steam and to superheat and reheat the steam. W represents the work produced by the expansion
of steam in the various steam turbine stages. The red line illustrates suboptimal cycle
operating temperatures. Source: Invensys Operations Management
Q Reheating
Q Superheating
Q Evaporation
W HP
Turbine
Q Sub-cool heating
Q HP feedwater
heating
W LP
Turbine
Temperature (F)

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POWER February 2013

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