Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates generally to heat
' exchangers and, more particularly, to an improved
combustion boiler ~or use with ~eedwater having a high
dissolved solids content.
One application of combustion ~oilers is in
connection with enhanced oil recovery systems. A steam
and water mixture from the boiler is injected into oil
bearing strata to displace the oil~ Since the feedwater
;~ 10 available to the boiler at oil fields is normally o~
poor quality with a ver,y high proportion of total
- dissolved solids, boilers for such applicati~ns
- ~enerall~ employ a single tube circuit throughout the
- unit. Moreover, the steam qualit~ at ~he boiler outlet
is usually limited to not greater than 80~ steam. By
using this level of resi~ual water at the outlet,
together with employing a high fluid velocity in the
tube circuit, salts and other dissolved solids are kept
in solution to prevent deposition in the boiler tubes.
Typical boilers utili~ed for enhanced oil
recover~ applications are oil burning and utili~e a
horizontal combustion chamber. The chamber is cooled
with tubes or pipes arranged in a horizontal serpentine
,f '' configuration around the combustion chamber. Although
' ' 25 meeting with some limited success, a prvblem exists in
such desi~ns in that the steam and water mixture in the
tubes may be suhjected to large variation in heat fluxes
', ' at diff~rent points in the t,ube circuit~ I a hi~h heat
flux coincides with a region of the circuit ~ontaining
rela~ively high quality steam, overheating may result in
tube failure.
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Accordin~ to the present invention, there is
provided a combustion boiler which has a substant-ially
vertically elongated combustion chamber with circulation
means -for circulating a combusting fluidi~ed bed upwardly
through the co~bustion chamber from the lower end thereof
to the upper end thereof, with tube means being arranged in
a single layer around the outer periphery of the combustion
chamber. The tube means is oF a con-figurAtion to provide
an asc.ending helical flow path For fluid passing there-
throu~h so that the region of higher steam quality in thetube means coincides ~Jith the region of lowest hea-t transfer
coefficients.
It is, therefore, an object of the invention to
provide an improved boiler construction that allows utiliz-
ation of water containing a ver~ high proportion of totaldissolved solids as the heat transFer fluid.
Another object of the invention is to provide
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an improved boiler construction particularly suited for
use in connection with enhanced oil recovery systems~
Another object of the invention is to provide
an improved boiler that allows the ut.ilization of water
containing a very high proportion of total dissolved
solids as the heat transfex fluid and which minimizes
the possibility of tube failure while at the same time
maximizing the heat transfer efficiency.
Other objects of the invention will become
apparent to those skilled in the art from the following
description, taken in connection with the accompanying
drawings wherein:
FIGURE 1 is a schematic view of a boiler system
employing the boiler of the invention,
FXGURE 2 is a schematic perspective view, with
parts broken away, of a portion of the system of FIGURE
l;
FIGURE 3 is a schematic view of a portion of a
further embodiment of the invention:
FIGURE 4 is a sectional view taken along the
line 4-4 of FIGURE 3; and
FIGURE 5 and 6 are schematic views of portions
of ~wo different further embodiments of the invention.
Very generally, the combustion boiler 10 of the
inventiQn comprises a substantially vertical elongated
combustion chamber 11. Circulation means 13 are
provided for circulating a combusting fluidized bed
upwardly through the combustion chamber from the lower
end thereof to the upper end thereofO Tub~ means 15 are
arranged in a single layer around the outer periphery of
the combustion chamber. The tube means are o a
configuration to provide an ascending helical flow path
for fluid passing therethrough.
Referring now in greater detail to the
drawings, in FIGURE 1 the overall schematic of the
boiler system is shown. The boiler 10 of the invention
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employs a fluidi2ed bed for high heat transfer
efficiency. Circulating fluidized bed combustion
boilers are able to utilize a wide variety of fuel and
fuel sizes. Combustion efficiencies of 95-98% are
common with inherent low generation of nitrous oxides
and retention of the order of 95~ of sulphur in the fuel
as part of the solid ash discharge~ Accordingly,
significant benefits from an air pollution standpoint
accrue.
The combusting fluidized bed is circulated
vertically from the lower end of the combustion chamber
11 to the upper end 19 thereof and is removed through a
duct 21 to a hot cyclone collector 13. The cyclone
collector may be, for example, a 10 foot diameter
cyclone into which the hot flue gas enters at a
temperature of typically 1600F with entrained ash,
limestone and unburned solid fuel. The solids are
separated from the hot flue gas and are gravity fed to a
lower chamber 23 from where they are returned to the
combustion chamber by means of a suitable system, not
shown.
The hot flue gas which exits the hot cyclone
collector 13 continues through an overhead duct ~5 to an
economizer 27. In the economizer, the hot flue gas
transfers i~s heat to the feedwater.
After leaving the steam generator 27, the ~lue
gas is transferred through a duct 33 to a centrifugal
type dust collector 35. The collector 35 may be of any
suitable design to remove large particles from the flue
gas. A duct 37 then passes the flue gas to a bag house
type dust collector 39 of a suitable design known in the
art for removal of fine particles in accordance with
published requirements of the Environmental Protection
Agency. The flue gas is then disoharged to the stack,
not shown, by means of an induced draft fan 41. Ash
removal from the dust collectors 35 and 39 is through a
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duct 43 and the ash is entrained by a fly ash blower 45
to be deposited in a fly ash storage silo 47.
The water steam path begins at a feedwater
inlet 49 from where it circulates upwardly through a
counterflow heat exchanger 51 and from there into the
feedwater preheater section 31. From there the
feedwater circulates through the final feedwater section
29 and then back through the tubing 53 and the
counterflow heat exchanger 51 to the main combustion
boiler 10 where the water is turned to steam at about
80% dryness. Steam then leaving the combustion boiler
at the outlet 55 is utilized fGr enhanced oil recovery.
Fuel for the fluidized bed in the combustion
boiler 10 is provided from a coal hopper 57 and a
limestone hopper 59 through gravimetric feeders 61 and
63, respectively, to a screw type conveyor 65. The
conveyor feeds the coal and limestone into the
combustion boiler at the same point that the hot solids
are returned from the hot cyclone collector 13. Air for
combustion and entrainment of solids to provide the
fluidized bed is provided from an air inlet 67 by means
of a primary air fan 69 and a secondary air fan 71.
Bottom ash is removed through a conduit 73 to a ~uitable
removal conveyor 75.
Referring now more particularly to FIGU~E 2,
the specific details of the construction of the
combustion boiler 10 may be more ~asily s~en. The lower
end of the combustion boiler 10 is provided with an ash
removal chamber 77 which encloses a hopper 78 which
communicates with the ash removal duct 73 tFIGURE 1) for
bottom ash removal from the combustion ~oiler 10. Above
the lower ash chamber 77 is a refractory chamber 79
which is lined with a wall of refractory bricks 81 or
other suitable material for withstanding high
temperature combusting materials. Recirculated solids
enter the lower refractory chamber 79 through a duct 83
which communicates with the collector 23.
The combustion chamber 10 is provided with an
outer wall 85 a-c tFIGURE 2) which encloses,
respectively, the refractor~ chamber 79, the combustion
chamber 11 and the tube means 15, and the upper end 19.
The fluidized bed, which is made up of circulating coal
or other solid fuels and limestone replenished from the
screw conveyor 65 (~IGURE 1), enters the lower
refractory chamber 7~ at a point not illustrated in
FIGURE 2 but level with the duct 83. These solids
entering the lower refractory chamber 83 are entrained
in the circulating gases and flow upwardly through the
combustion chamber 11. As a natural consequence of the
combustion pattern, the heat transfer coefficients to
the steam tubing are higher at the lower end of the
combustion chamber 11 than at the higher end of the
combustion chamber 11.
In operation of the combustion boiler, start-up
is accomplished by utilizing an oil or gas fired
start-up heating system, not shown, as is known in the
fluidized bed art. Once the bed rnaterial is heated to a
sufficiently ~igh temperature, for example 1000~F, the
feeding of coal and limestone is begun and the unit is
brought up to full load using the solid fuel. The oil
and gas firing for start-up purposes is then terminated
once stable combustion of the coal i5 achieved at a
suitable flow rate, for example 334 of full flow rate.
~ypically, fuel feed is controlled by the gravimetric
feeders 61 and 63 in FIGURE 1 as desired. The ratio of
limestone to coal may be maintained over all load ranges
as desired in response to changes in the content of
sulphuroxygen compounds in the exhaust gases.
As shown in FIGURE 2, the combustion chamber 11
is of generally square cross section. A circular cross
section, however, is also practical. The flow of
water-steam mixture through the combustion chamber for
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heat exchange with the fluidized bed is accomplished by
the tube means 15. The tube means comprise one or more
tubular conduits arranged in a single layer and having a
helical configuration so that the water-steam mixture
passes in a helical path around the combustion chamber
11 and upwardly toward the upper end thereof. The tube
means 15 form an envelope surrounding the outer
periphery of the combustion chamber 11 and form a wall
by being tangentailly engaged throughout substantially
the entire length of the tubes, as shown in FIGURE 2 and
also in the right-hand s~hematicized broken out portion
in FIGURE 1. Alternatively, the tubes may be joined by
webs 15a as shown in the left-hand schematicized broken
out portion of FIGURE 1. In either case, the tube means
form a wall representing a water-cooled envelope
surrounding the exterior or periphery of the comnbustion
chamber.
In FIGURES 3 and 4, ~ combustion boiler
comprising a further embodiment of the invention is
shown. Portions of the boiler halving similar function
to those portions of the boiler of the embodiment of
FIGURES 1 and 2 are given identical reference numbers
preceded by a 1. In the embodiment of FIGU~ES 3 and 4,
the tube means 115 are arranged at a location displaced
from the wall 85 of the combustion boiler 10. Moreover,
the convolutions of the tube means are spaced axially
from each other to permit the com~ustion gases with
entrained solids to flow along the outside as well as
the inside of the helical tube coil~ This provides a
high heat transfer efficiency.
In order to avoid disturbance of the flow of
material in the lower end of the combustion chamber 111,
the vertical distance or distance parallel with the axis
of the helix between adjacent convolutions may be
arranged to be larger in the lower part of the helix
than in the upper part. In this way, more heat will be
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absorbed by the fluid flowing through the c~nduit in the
upper part of the rombustion chamber. Thus~ the
distance between the convolutions o the tube means 115
may be selected to precisely match the heat distribution
in the flowing gases and entrained solids within the
combustion boiler.
In FIGURE ~, a still further embodiment of the
invention is shown. Those elements of FIGURE 5 which
are similar in function to corxesponding elements of
FIGURE 1 have been given identical reference nu~erals
preceded by a 2. In the embodiment of FIGURE 5, the
tube means 215 are arranged in three groups, 215',
215'', and 215'''. The tubes of the group ~15' and
215'' in the lower part of the combustion chamber are
arranged around its outer periphery in order to offer
less disturbance to the flow of the gases and entrained
solids. The conduit of the third group 215' " is
disposed at a location displaced from the wall of the
combustion chamber so that the gases with entrained
solids may flow along the outside as well as the inside
of the tube coils in this region. The groups 215' and
215'' are separated by a distance sufficient to allow
for the injection of secondary air through a duct 86.
similarly, the group 215''' is separated from the group
215'' by a space sufficient to permit insertion ~f
secondary air through a pair of ducts 87.
In FIGURE 6, a further embodiment of the
invention is shown in which elements having function
similar to those of the corresponding elements of FIGURE
l are given identical reference numeral preceded by a
3. In the embodiment of FIGURE 6, the tube means 315
are of decreasing diameter as the height of the helix
increases. Accordingly, the envelope of the helix is in
a genexally frustoconical shape. This provides for a
flow path for the gases and entrained solids to fully
surround the tubes, passing bet~een adjacent
convolutions as well as on the inside and the outside of
the helically arranged tube means. In the ~ube means
15, 115, etc. r the flow of the steam-water mixture is
helically upward. Thus, at the relatively higher heat
fluxes toward the lower end of the combustion chamber 11
the steam quality of the circulating water-steam mixture
is lower. Toward the higher end of the combustion
chamber, where the heat transfer coefficients to the
tube means are lower in the fluidized bed, the steam
quality in the water-steam mixture flowing through the
tube means is higher. This configuration in a single
pass flow mitigates against possible overheating of the
steam/water to produce deposition of the dissolved
solids on the tube and consequent tube failure. Typical
exiting steam quality of the boiler is limited to about
80% and this/ together with the maintenance of a
relatively high flow velocity in the tube means, is
adequate to keep fiolids in solution and prevent
deposition inside the tubes.
More particularly, typical steam-water flow
rates in the boiler of the invent~on may vary between
700 and 3,000 kilograms per second-square meter. The
boiler may be constructed with tube dimensions in the
range of about 12.5 millimeters to about 100 millimeters
diamter. The larger diameter tubes are normally used in
the single pass design on the water side. Smaller
diameter tubes are normally used for the multipass
designs. For a twin pass boiler of 50 million BTU per
hour capacity, the preferred diameter range is 40-65
millimeters inner diameter with a superficial velocity
of between 2 meters per second and 20 meters per
secondO The gas temperture at typical design conditions
is 850-950C plus or minus 100C in most cases~ Where
sulphur is present, a lower design temperature may be
used.
It may be seen, therefore, that the invention
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provides an improved combustion boiler which employs a
fluidized bed in combination with a vertical bed flow
and upward helical circulation of coolant. Despite high
dissolved solids content in the water-steam mixture,
5 efficient heating with minimal internal deposits
result. Moreover, the invention minimizes conditions
which lead to tube failure. This, in combination with
the fluidized bed, leads to high heating efficiency
despite the once ~hrough pass conditions required under
~uch circumstances.
Various modificatoins of the invention in
addition to those shown and described herein will become
apparent to those skilled in the art from the foregoing
description and accompanying drawings. Such
modifications are intended to fall within the scope of
the appended claims.
