TM 5-810-15
(2) Combustion air control. Combustion air
Table 9-1. Combustion Control System Selection Guide.
flow in pulverized coal systems consists of primary
air flow and secondary air flow. Primary air is the
Boiler Type
air which transports the pulverized coal to the
Multiple
burner or burners. Secondary air is the air delivered
Gas/
Pulverized
Burner
by the FD fan to the boiler to support combustion.
Control System Oil
Stoker
Coal (PC)
PC
ACFB
Total air flow is the sum of secondary air flow and
all primary air flows. Secondary air is measured
Jackshaft
downstream of the FD fan and controlled by
positioning
X*
positioning FD fan inlet vanes or inlet damper.
Series control
X**
X**
Primary air can be supplied by the FD fan or by a
primary air fan. Primary air is normally measured
Parallel
X
positioning
X
on each pulverizer.
(3) Combustion air flow measurement. Accu-
Parallel
X
rate combustion air flow measurement is essential
positioning
for combustion control systems of pulverized coal
with steam
flow correction
fired boilers. Secondary air flow will be measured
of air
X
X
with a venturi section or air foil flow element in the
ductwork between the FD fan and the boiler
Parallel meter-
windbox. The flow element should be designed to
ing with cross
provide a design differential pressure across the
limiting and
flow tie-back
X
X
X
flow element of not less than 2-inches wg at full
load conditions. Primary air flow will be measured
X Recommended Application
on each pulverizer with a venturi section or pilot
*Restricted to use on boilers 70,000 pph and under.
tube between the primary air fan and the pulverizer.
**Restricted to use on boilers under 100,000 pph with constant
The design pressure differential pressure across the
load and constant Btu value fuel.
stream from the fan outlet. The relationship be-
primary air flow elements should not be less than 2-
tween inlet vane or damper position and air flow
inches wg at full load. The transmitters selected for
will be determined for use in the combustion
primary air flow and secondary air flow will be
control system or for characterizing the final con-
differential pressure transmitters that are accurate
trol element. Overfire air flow on stoker fired units
in the range of differential pressure developed by
normally is not measured. If overfire air is to be
the flow elements.
controlled the air flow demand signal will be used
(4) Pulverizer controls. Figure 9-4 shows one
to control overfire air in parallel with combustion
multiple pulverizer control scheme. In this ar-
air.
rangement the firing rate demand is compared to
(3) Combustion air flow measurement. Accu-
total fuel flow, which is the sum of all feeders to
rate combustion air flow measurement is extremely
develop the demand to the pulverizer master. The
important in combustion control systems for stoker
pulverizer master demand signal is applied in
fired boilers. A venturi section of air foil flow
parallel to all pulverizers which have duplicate
element should be provided in the ductwork
controls. If an upset occurs in the fuel/air ratio such
between the FD fan and the stoker or a Piezometer
that total air flow is low, an error signal from air
ring may be installed at the FD fan inlet. The flow
flow control reduces the firing rate demand to the
element will be designed to provide a design
pulverizer master to restore the proper fuel/air
differential pressure across the flow element of not
ratio. Since coal flow to the burner is a function of
less than 2 inches wg at full load conditions. The
primary air flow the primary air damper and coal
flow transmitter selected for combustion air flow
feeder speed control receive the same demand
will be a differential pressure transmitter that is
signal. If an error develops between demand and
accurate in the range of differential pressure
measured primary air flow or coal feeder speed, the
developed by the flow element.
controllers adjust the primary air flow or feeder
f. Pulverized coal system controls.
speed to eliminate the error. If primary air flow is
(1) Fuel flow control. Fuel flow in pulverized
less than feeder speed demand, the feeder speed
coal systems is established by coal feeder inputs to
demand is made equal to the primary air flow by
the pulverizer. Coal feeder speed controls the fuel
the low select auctioneer. A minimum pulverizer
flow to the pulverizer. The volumetric rate of coal
loading and a minimum primary air flow limit
flow delivered to the pulverizer is directly related
should be used to maintain the pulverizers above
to feeder speed. Feeder speed varies with fuel flow
the minimum safe operating load to maintain
demand.
adequate burner nozzle velocities and to maintain
9-5
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