Note: Descriptions are shown in the official language in which they were submitted.
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FURNACE WALL AS~ MONITORING SYSTEM
The present invention relates to the monitoring of
the deposition of ash in combustion operations,
particularly pulverized coal fired boilers and steam
generators used in power productions.
In the operation of a pulverized coal-fired boiler
a significant fraction of the ash contained in the coal
is deposited on the water walls of the combustion
chamber and on the tubes of the convection section of
the boiler. The ash deposits have a low thermal
conductivity, modify the radiative properties of the
surfaces and insulate the tubes from the flame. soth
of these effects interfere with the efficient flame and
gas-to-tube heat transfer.
Uncontrolled accumulation of ash often assumes
catastrophic proportions necessitating boiler
shutdowns. Often physical damage results to tubes in
the furnace hopper when large masses of ash detach and
fall there.
As a result of the generally decreased heat fluxes
a larger heat transfer surface is required than
otherwise would be the caseO Th2 increased use of
fouling-type coals has led to a substantial increase in
furnace size for a particular load, leading to an
increased initial capital cost.
Boiler operation is controlled by maintaining
superheat stem temperature and flow rate by use of the
available operating variables, such as, gas
recirculation, burner tilt, gas tempering and steam
atemperation. A need for frequent use of these
adjustments is indicative of uneconomic operation.
The build up of ash on the furnace walls is
controlled by the intermittent operation oE soot
blowers, which remove the built-up ash from the walls.
At the present time the boiler operator is not provided
with any direct measurement of the degree of fouling of
the combustion chamber and convection section.
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The degree of fouling, in general, however, is
random and unpredictable with respect to its
distribution on the various parts of the furnace walls,
and also as to its severity at any one point. In the
mode of operation, as practised according to the
presently available state of the art, soot blower
actuation is based on operator judgment of the indirect
evidence from superheated steam and economizer
temperature, burner position, and/or amount of gas
recirculation and steam atemperation.
Because the boiler response time (i.e., the time
where changes in degree of fouling are reflected in
these variables) is long, control is erratic.
Moreover, in order to avoid catastrophic loss of
control, boilers are designed with larger furnaces than
they otherwise might need to be. If methods and
instrumentation were provided to directly monitor the
degree and distribution of fouling, both the boiler
control would be improved, and smaller and therefore
less costly furnaces would prove adequate. As far as
the applicants are aware, there has been no development
to date of ash deposit-measuring instrumentation to
provide such means.
- In accordance with one aspect of the present
invention, there is provided a method of monitoring the
build up of deposits of ash from a product gas stream
produced by an ash-generating combustion operation on a
surface contacted by the product gas stream, which
comprises: simultaneously determining the actual heat
flux produced b~ the combustion operation and
theoretically capable of reaching the surface and the
heat flux reaching the surface, and determining the
difference in heat flux value as a measure of the build
up of ash on the surface.
Monitoring of the build up of ash may be utilized
in the control of a combustion process. Accordingly,
in another aspect of the invention there is provided a
method for the control of a combustion process, which
comprises: determi~ing the heat flux actually reaching
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surfaces contacted by products of combustion;
determining the heat flux theoretically reaching the
surfaces if free from deposits; recording the heat flux
values; and actuating at least one combustion process
control operation in response to the recorded heat flux
values.
The present invention uses flux meters, or similar
flux detectors. One flux meter, which is of unique
construction and directly views the furnace flame, is
always maintained free of any deposits, and hence
0 receives the full heat flux from the flame. That flux
is also equivalent to the flux to be received by a
perfectly clean water wall. Another flux meter is
permitted to become fouled by ash deposits in identical
manner to the water walls themselves, so that heat flux
which is received by the fouled flux meter is
equivalent to that received by the fouled wall.
The fluxes, detected simultaneously by each of the
two meters, are converted to electrical signals
indicating the detected fl~x values. The electrical
signals are either continuously displayed by separate
traces on a chart plotted by an electronic recorder, or
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combined electrically by special electronic circuitry
and displayed by a recorder as a signal proportional to
the degree of fouling of the furnace wall. Either or
both of these signals can be used by the operator in
boiler control. In practice, several fouled meters may
be combined with a single clean meter to indicate the
degree of fouling of a larger furnace wall area. Still
further, several sets of clean-and-fouled meter
combinations may be judiciously distributed over all
the furnace walls at various levels so as to obtain
separate but continuous indications of the degree of
fouling in these areas.
These signals, which indicate the degree of
furnace fouling, may then be used by the operator as a
basis of soot blower actuation. More importantly, he
may more judiciously actuate other suitable boiler
controls. Furthermore, the totality, or groups, of
these signals may be recorded and manipulated by
on-line use of digital computers (both macro and micro)
in order to assist the automatic control of the whole
boiler.
The invention is described further, by way of
illustration, with reference to the accompanying
drawings, in which:
Figure 1 is a schematic representation of one
typical embodiment of a pulverized coal-fired furnace
to which the present invention is applied;
Figure 2 is a sectional view of a flux meter which
is permitted to become fouled by ash deposits in this
invention;
3 Figure 3 is a perspective view of a flux meter
which is intended to be maintained free of ash deposits
in this invention;
Figure 4 is a schematic view of a typical
electrical circuit used in this invention; and
Figure 5 is a graphical representation of typical
experimental results obtained using the present
invention.
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Referring to the drawingsl a schematic
representation of one particular example of a
pulverized coal-fired furnace 10 is shown in Figure 1.
Pulverized coal and air are fed through burners 12 into
the interior
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13 of the furnace 10. As is well known, the furnace
walls 14 are comprised of a plurality of parallel tubes
wherein steam is generated for feed to a steam collection
system (not shown).
Combustion gases pass up into a chamber 16,
known as the boiler convection section, wherein may be
located superheating surfaces for the generated steam,
etc. The combustion gases then may pass over an
economizer and air heater before exhausting to atmosphere
through a stack.
During operation of the furnace 10, ash and
slag form on the furnace walls 14 sticking to the tubes,
decreasing heat absorption in the furnace and otherwise
causing operating difficulties. Furnace wall soot
blowers (not shown) are located throughout the furnace
walls and each soot blower is operative to clean a
section of the furnace wall.
In accordance with the present invention, a
plurality of flux meters are located in the walls 14 of
the furnace 10 directly facing the flame. In the
illustrated embodiment, a single flux meter 20 is
maintained clean at all times while three flux meters 22
are permitted to become fouled by ash and slag during
furnace operation. As stated above, any desired number
of flux meters 20 and 22, or other flux detectors, may be
used to achieve the desired monitoring.
The heat flux meters 22 are illustrated in
Figure 2 and include a meter disc 44 and a meter body 46
constructed of dissimilar metals and to which wire leads
48 and 50 are respectively connected. Heat flowing to
the surface of the disc 44 is conducted radially to the
meter body 46 and thence through an attachment mold 52 to
the boiler tubes. The thermal resistance of the disc 44
causes temperature difference between the centre of the
disc and its periphery. The dissimilar metal e.m.f.
generated by the flux meter is proportional to the
magnitude of the disc radial temperature difference and
hence to the heat flux to the meter.
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The specific construction of the flux meter 20
is shown in the perspective view of Figure 3. The flux
meter 20 includes a housing 24 which has a smaller
diameter portion 26 extending through the furnace wall 14
and terminating at the internal surface 28 of the wall 14
and an integral larger diameter portion 30 located
outside the furnace wall 14.
The housing 24 has an opening 32 in the end
plate 33 of the larger diameter portions 30 for feeding
air into the housing 24 and openings 34 in the end of the
smaller diameter portion 26 to permit air to pass
therethrough into the furnace interior 13.
Located in the interior of the housing 24 is an
evacuated elongate tube 36 filled to about 5% of its
volume with water. At one end of the tube 36, remote
from the end plate 33, an external copper sleeve 38 is
provided, acting as a heat sink. A flux meter 40, of any
convenient type, such as, that described above with
respect to Figure 2, is mounted on the forward end of the
copper sleeve 38. The lead wires to the flux meter 40
have been omitted for convenience of illustration.
The opposite end of the elongate tube 36 is
provided with a plurality of heat dissipating fins 42
located in the larger diameter housing portion 30. The
illustrated structure of the probe 20 enables the flux
meter 40 to be maintained at a relatively constant
temperature near the temperature of the wall tubes while
the air envelope emerging from the opening 34 prevents
build up of ash fouling on the flux meter probe 40.
The copper sleeve 38 acts as a heat sink which
removes heat from the flux meter 40. This heat causes
water present in the evacuated tube 36 to evaporate
thereby cooling the copper sleeve 38. The heat exchange
fins 42, which are cooled by pumping air over them
through the opening 32, cause the steam to recondense,
thereby removing heat from the system. Meanwhile, the
air flowing through the housing, from the inlet 32 to the
outlet 34, passes over the surface of the flux meter 40
and prevents the build up of ash on that surface.
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When both the flux meter 20 and the flux meter
22 are clean, on start up or after a soot removal
operation, the net voltage generated by the suitably
attenuated combination is zero. As the flux meter 22
becomes fouled while the flux meter 20 remains clean, the
difference in voltage is a measure of the extent of the
fouling of the flux meter 22. The electrical signals
produced by the flux meters 20 and 22 may be recorded and
continuously displayed as by separate traces on a chart
plotted by an electronic recorder. Alternatively, the
signals may be combined and displayed as a signal
proportional to the degree of fouling of the furnace
wall.
An electrical circuit suitable for monitoring
the voltage difference is illustrated in Figure 4. Input
amplifiers 54 and 56 respectively receive the voltages
produced by the flux meters 20 and 22 as a result of the
detected heat fluxes. The positive and negative
terminals of the flux meter 20 are connected to the
positive and negative inputs respectively of the
amplifier 54 to generate a positive output signal in wire
58 while the positive and negative terminals of the flux
meter 22 are connected to the negative and positive
inputs of the amplifier 56 to generate a negative output
signal in wire 60. The wires 58 and 60 join to form an
input to the positive terminal of an output gain
amplifier 64, thereby generating an output signal in wire
66. The output may be recorded and displayed on a
suitable recorder (not shown).
When the flux meters 20 and 22 are both clean,
the amplifiers 54 and 56 are adjusted to provide a zero
voltage output in wire 66, the positive potential in wire
58 balancing the negative potential in wire 60. As
fouling of flux meter 22 occurs, the negative potential
in wire 60 decreases and the potential in wire 62 becomes
more positive, thereby producing a positive potential in
wire 66 which is proportional to the decrease in heat
flux through the furnace water walls due to fouling of
the flux meter 22.
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In the above description, the flux meter which
is maintained clean at all times is essentially of the
same design as the one allowed to be fouled, except that
flux meter 40 is mounted in a special manner, as seen in
Figure 3. In another embodiment of this invention, in
place of heat flux meter 40, mounted as shown in Figure
3, a recording radiation pyrometer, sensitive in the
infra-red region of the electromagnetic spectrum, and/or
a total radiation pyrometer, can be used. Both the
latter instruments are commercially available and are
known to those familiar with temperature measurement.
They have not yet been put to use in such an application
as the present.
In this invention, therefore, heat flux
detectors, preferably in the form of flux meters, but
also including pyrometers and/or thermocouples, are
directly exposed to the heat flux within the furnace.
Two different types of detectors are used, one of which
is maintained free from contamination at all times and
the other of which is allowed to become contaminated in
the furnace. The voltages generated by the detectors are
simultaneously measured and compared to provide an
instantaneous indication of the heat flux loss due to
fouling of the probe 22 and hence fouling of the furnace
walls in the location of the probe 22.
The signal indicative of the degree of furnace
fouling may be used by a furnace operator as a
determination for initiation of soot blower operation
and/or other furnace control action known in the art.
Alternatively, the signal may be utilized for automatic
initiation of soot blower operation or automatic control
of the whole boiler.
EXAMP~E
A coal fired boiler of the type shown in Figure
l was operated for a two month period with probes 20 and
22 located therein. The boiler was a 150 ~J Combustion
Engineering Superheater corner-fired pulverized coal
boiler, operated at 1800 psi and a superheat steam
temperature of 1000F (585C).
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The value of the combined signal in wire 66 was
monitored as heat flux loss and the actual heat flux
values detected by the probes 20 and 22 were also
monitored separately. The results were recorded
graphically and the results for a typical six hour run
are shown in Figure 5.
As may be seen from Figure 5, the combined
output signal (curve III) expressed as heat flux loss,
was near zero immediately after a soot blow (shown at the
right of Figure 5). The heat flux loss slowly increased
as the ash deposit built up. The heat flux measured by
the clean probe (curve II) increased slowly as the
fouling of the furnace walls decreased the total rate of
heat transfer from the flame to the water walls, causing
the temperature of the latter to increase. The flux
measured by the fouled probe (çurve I) fall rapidly
initially, went through a vacillation phase due to ash
deposit consolidation, and then fell less rapidly.
SUMMARY OF DISCLOSURE
_
In summary of this disclosure, the present
invention provides a novel and accurate pulverized
coal-fired boiler furnace monitoring system which
utilizes the simultaneous measurement of heat flux within
the furnace by two probes. Modifications are possible
within the scope of this invention.
