Note: Descriptions are shown in the official language in which they were submitted.
4~l!66i6
METHOD AND APPARATUS FOR IMPROVING THE OPERATION
_ _ _
OF A HEAT GENER~TOR
Field of the Invention
This invention relates to heat generators in which
combustible fuels such as fossil fuels, refuse or other
materials are burned. More specifically, this invention
relates to a method and system for improving the ef-
ficiency of such heat generators and particularly for
better utilization of heat produced in the thermal section
for a large electric power plant using a combustible fuel.
Background of the Invention
Heat generators using combustible fuels such as
oil, coal, gas or refuse materials and the like, generate
a substantial quantity of waste materials.in the form of
pollutant gases and particulates. Federal and state
environmental requirements have imposed maximum emission
standards for these waste materials. Compliance with
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these emission standards involves substantial investments
for appropriate pollution control equipment, the costs
for which can be prohibitively high.
For example, large systems are available to remove
5 particulates using a dry flue gas treatment. Typical
devices used for this purpose may involve electrostatic
precipitators, bag houses and the like. These devices are
suitable for the removal of the particulates, but gaseous
pollutants are not removed and as can be appreciated, the
addition of these devices increases cost and reduces the
efficiency of the heat generator.
The magnitude of gaseous pollutants generated from
the combustion of fuel throughout the world is enormous.
As a result, many techniques have been described for the
removal of thesepollutants from flue gases exhausted from
- heat generators. A general statement of various wet
scrubbing processes for pollutant removal from flue gases
exhausted from large scale electric power plants can be
found in a chapter entitled "Wet Scrubbing Process - SOx
and NOX Removal Chemistry" by R. G. Nevill, at page 9-312
of "Energy Technology Handbook" edited by D. M. Considine
and published by McGraw-Hill Book Company.
Flue gas wet scrubbing techniques also involve
substantial investments with complex systems. For ex-
ample, in the U.S. patents 3,320,906 to Domahidy and3,733,777 to Huntington, wet scrubbers are described in
which flue gases are passed through a filter bed for
intimate contact with a wash liquiZ. The wash liquid may
be an aqueous bisulfite salt solution such as described in
the Huntington patent or such alkaline scrubbing liquors
indicated as useful with the wet scrubber described in
U.S. patent 4,049,399 to Teller.
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Since corrosive liquid droplets are likely to be
entrained by the scrubbed flue gas, special techniques
such as described by Teller or in the U.S. patent to Brandt
3,844,740 may be used to avoid corrosion on subsequent
equipment such as an induced draft fan located at the stack
where the flue gas is exhausted to atmosphere.
Another technique for the removal of pollutants
may involve cooling of the flue gas to such low tem-
peratures that gaseous pollutants such as SO2 and SO3
condense out. One such system is described in the U.S.
patent to Maniya 3,839,948, in which the flue gasis cooled
to about 10-C to condense out the sulfurous pollutants
after which the flue gas is reheated before discharge to
atmosphere.
lS These and other techniques for theremoval of waste
materials from flue gas involve a substantial amount of
energy, much of which is irretrievably lost. As a result,
the overall efficiency,i.e. the energy available for sale
from a power plant is significantly reduced.
Techniques for preheating of air have been known -
and used for many years in connection with boilers to
improve combustion. One such preheating technique em-~
ploys a Ljungstrom air preheater. This uses as shown in
Fig. 3 herein a rotor 2 through which on one side 3 flue
gas is passed while an inflow of combustion air is passed
through the other side 4, with the two gas flows being in
opposite directions. Air preheaters, however, are op-
erated at sufficiently high temperatures to avoid con-
densation inside the heat exchanger of pollutants such as
SO3 present in the flue gas.
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Summary of the_Invention
-
In a technique in accordance with theinvention for
the operation of a heat generator in which combustible
fuels are burned, the thermal efficiency is improved by
combining the preheating of the air for the heat generator
with the removal of pollutants.
For example, as described herein with respect to
one embodiment in accordance with the invention for the
operation of a heat generator using combustible fuels,
~ both an inflow of air and the flue gas from the combustion
are passed through a heat exchanger, which is simul-
taneously flooded with a scrubbing liquid for removal of
particulates and gaseous pollutants in the flue gas. Heat
from the flue gas is transferred through the heat ex-
changer to the inflow of air for its preheating while theflue gas pollutants are removed by collecting the liquid
after its passage through the heat exchanger.
It is well well known that below about 700-F SO3
will combine with water vapor molecules to form H2SO4. At
temperatures above approximately 300-F in the flue gas the
H2SO4 is a gas. The cooling of the flue gas can be carried
out to a temperature at which a pollutant may condense out.
For example, the flue gas may be cooled in the heat
exchanger to a temperature at which SO3 in the form of
H2SO4 condenses out. By employing a suitable neutralizing
scrubbing liquid, the corrosive effective of the con-
densed SO3 is avoided, yet a substantial part and even
- virtually all of the SO3 in its H2SO4 form in the flue gas
is removed.
With a technique in accordance with the invention
for operating a heat generator, its net thermal efficiency
can be significantly increased. The technique can be
applied to improve operating efficiencies of existing
heat generators such as may be used in electric power
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plants, steel manufacturing furnaces, sulfur producing
plants and the like.
It is, therefore, an object of the invention to
improve the thermal efficiency of a heat generator using
combustible fuels while removing pollutants from flue gas
generated from the generator. It is a further object of
the invention to enable the cleaning of an air preheater
during its use as a heat exchanger in a heat generator
using combustible fuels.
These and other advantages and objects of the
invention can be understood from the following descrip-
tion of one illustrative embodiment in accordance with the
invention and described in conjunction with the drawings.
Brief Description of Drawings
FIGURE 1 is a schematic representation of a con-
ventional thermal section for a power plant; and
FIGURE 2 is a schematic representation of a ther-
mal section improved in accordance with the invention.
FIGURE 3 is a schematic representation of a rotary
air preheater that has been modified in accordance with
the invention.
Detailed Description of Embodiment
With reference to FIGURE 1, the thermal section 10
of a conventional power plant is shown with a boiler 12 in
which a suitable fuel such as fossil fuel in the form of
coal, oil, or gas or other fuel such as a waste material
is burned. An inflow of combustion air is provided, as
suggested by arrows 16, through suitable ducts 1~ into the
boiler 12.
The boiler 12 includes suitable heat exchange
elements (not shown) in which a working fluid (water or
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-- 6
steam) is circulated for heating by the combustion gases
generated in the boiler 12. Flue gas, as suggested by
arrows 18, emerges at discharge 20 from the boiler 12 at
a high temperature, typically in the range of about 650-F,
and is passed through a heat exchanger 22 to preheat the
inflow of air 14. After passage through heat exchanger 22,
the flue gas 18 is discharged to atmosphere at a stack 24.
Air flow through the thermal section 10 is obtained with
a forced draft fan 26 and an induced draft fan 28.
The flue gas 18 may include pollutant materials in
the form of particulates such as fly ash and gases such as
SO2, SO3 and others. Techniques for removal of the
pollutants are usually a part of the thermal section 10,
though for purposes of simplicity of FIGURE 1, these
pollution controls have been left out of the schematic
representation. Suffice it to say that techniques and
devices for collecting particulates and pollutant gases
from flue gases have been extensively described in the
art.
It is generally recognized that, particularly in
large electric power plants, the exhaust temperature of
the flue gas should preferably be kept above the dew point
of the acid H2SO4 to avoid corrosive effects from contact
by precipitated SO3 with equipment such as the induced
draft fan 28. Hence, the amount of heat recaptured from
the flue gas is usually limited to maintain the flue gas
temperature above the acid (H2SO4) dew point, i.e. at
about 300-F. As a result, the temperature of the inflow
of air 16 at the boiler 12 is usually about 450-F and the
thermal efficiency of thermal section 10 is not as high as
it could theoretically be made.
With a technique for operating a heat generator in
accordance with the invention, a substantially greater
amount of heat from flue gas is recaptured to achieve a
high thermal efficiency while simultaneously extracting
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7 --
pollutants. This can be achieved with a thermal section
30 as described for a power plant as illustrated in FIGURE
2.
In FIGURE 2, the flue gas 18, after passage through
preheater 22, is passed through a heat exchanger 32 where
a substantial portion of the heat in the flue gas 18 is
extracted for transfer to the inflow of air 16.
The heat exchanger 32 operates with a working
liquid which is applied through an inlet 34 from a supply
- 10 (not ~hown) to sprayers 36 into the heat exchanger portion
38 through which the flue gas 18 is passed. 'rhe sprayers
36 flood portion 38 to enable intimate and direct contact
between the flue gas and the liquid. The liquid is applied
in such volume as to collect particulates in the flue gas
while also acting as a protective sheath for the heat
exchanger to prevent its damage from corrosive consti-
tuents in the flue gas.
Although the liquid could be water, the liquid is
preferably formed with ingredients suitable for absorbing
- 20 and neutralizing various pollutants in the flue gas. These
pollutants may be SO2, SO3 (in the form of H2SO4) and
others, for which absorption and neutralizing techniques
are well known, see for example, some of the aforementioned
prior art publications. An alkaline wash liquid may be used
to, for example, neutralize condensed SO3 (H2SO4) and
absorb SO2.
In some cases the heat exchanger 32 may be sprayed
with a powder at the same time that the liquid is applied.
The powder may be of a type which neutralizes corrosive
components. Use of such powders to protect heat exchangers
against corrosion is known in the art.
While recuperative, regenerative, and heat pump
types of heat exchangers 32 can be used, the heat exchanger
32 preferably is of the rotary regenerative type. The large
surface area in such heat exchangers enhances mixing and
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contact of flue gas pollutants with the absorbent alkaline
wash liquid. The flue gas 18 in such case is passed through
a hot zone portion of a rotor used such as 2 in Fig. 3 in
heat exchanger 32 and the liquid spray is directed at that
hot zone with a flow rate selected to prevent a build-up of
particulates and corrosive effects from condensed and
absorbed pollutants. The spent liquid is collected at a
drain 40 for processing in a suitable conventional scrub-
bing cycle.
Preferably the liquid is applied to the rotary heat
exchanger in a manner sufficient to timely remove condensed
corrosive constituents, i.e. before corrosive damage to
the heat exchanger occurs, while reducing the cooling
effect of the liquid on the heat exchange process. One
preferred technique for applying the liquid involves the
intermittent application of a flooding amount of liquid to
discreet portions of the rotary heat exchanger at suf-
ficiently short intervals to prevent corrosive damage. The
intervals may be related to the rotation of the rotary heat
exchanger such as by effectively flooding the entire rotor
2 once every several or more rotations of the rotary heat
exchanger 32. The liquid is preferably applied to that part
of the rotor where it is about to leave the "cold" region
where combustion air passes through to move into the "hot"
region where flue gas passes through.
Another technique for applying the liquid involves,
as shown in Fig. 3, the application of a high pressure
spray, through a nozzle 70 that reciprocates with the aid
of a drive 71 at a desired speed, along a generally radial
part or zone 72 of the rotor 2 between its inner and outer
diameters 74, 76. In this manner the entire rotor 2 is
cleansed of condensed pollutants along a spiral path as the
rotor 2 rotates in the direction of arrow 78 below the
reciprocating nozzle 70.
The application of liquid in accordance with the
invention to a preheater type rotary heat exchanger such as
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g
22 advantageously removes flue gas materials that clog the
preheater to thus reduce the load on combustion air fan 26.
As a result, the intervals between heat generator down
times for the cleaning of preheater 22 can be considerably
5 lengthened. Furthermore, in certain heat generators an
additional combustion air preheater that is located ahead
or upstream of the preheater 22 is used. Such additional
preheater serves to assure a minimum temperature for the
combustio~ air and thus avoid condensation of corrosive
10 constituents, such as H2SO4, in the flue gas in preheater
22 during cold ambient air temperatures. The application
of liquid to preheater 22 may thus allow a deletion of such
additional air preheater for an improvement in the heat
generator thermal efficiency.
Sufficient heat is transferred from the flue gas by
the rotor 2 in the rotary heat exchanger 32 to the inflow
of air 16 to increase the latter's temperature signifi-
cantly while the flue gas 18 is considerably cooled when it
emerges at the outlet 42 of heat exchanger 32.
The flue gas temperature may in fact be so low that
if discharged to atmosphere, the water vapor in the flue gas
would create a visible plume. Since this is undesirable,
a visibility suppression technique is used whereby the flue
gas 18 from heat exchanger 32 is reheated with a heat
25 exchanger 44, which thus also promotes rise of the flue gas
from the stack. Appropriate moisture separators 46, 48 are
placed at the outlets 42, 42' of heat exchanger 32 to
collect and enable removal of droplets entrained by the gas
flow through heat exchanger 32.
A significant improvement in the overall efficiency
of the thermal section 30 is obtained with a heat exchanger
such as 32 with which a substantial portion of heat in the
flue gas 18 is recovered while pollutants are removed and
the heat exchanger 32 is protected against corrosive ef-
35 fects of the removed pollutants. The thermal efficiency of
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-- 10 --
the heat generator may be increased by about three and a
half percent (3.5~). The net thermal efficiency, i.e.
after allowing for additional energy requirements to im-
plement the improvement of the invention, being about two
and sixteenth of a percent (2.6%). In addition, less costly
high sulfur containing fossil fuels may be used so that the
operating costs of the thermal section 30 can be sig-
nificantly reduced.
As a result of the thermal efficiency improvement,
the temperature of the inflow of air 16 to boiler 12 at 50
is increased. It is estimated that the air flow can be
raised to within 90-F or even less from the temperature of
the flue gas 18 at the outlet 52 of boiler 12.
The efficiency advantage of the invention can be
illustrated with the following table of normal temper-
atures encountered in the prior art system of FIGURE 1 in
comparison with temperatures estimated to be generated in
a system of FIGURE 2.
Table 1
Temperatures
Places FIGURE 1 FIGURE 2
At air inlet 54 70-F 70-F
At outlet 42' -na- 266-F
At boiler inlet 50 450-F 630'F
25 At boiler outlet 52 650~F 650-F
At reheater inlet 56 -na- 320-F
At reheater outlet 58 -na- 300-F
At outlet 42 -na- 120-F
At stack 24 300-F 140-F
The efficiency advantage of the invention can fur-
ther be illustrated with the following tables 2 and 3
normalized for a heat generator using one pound of com-
bustion air and assuming a mass of flue gas of 1.06 pounds
for a number 6 type fuel oil.
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Table 2
FIG 1 FIG. 2
Places Temperature Temperature
At air inlet 54 70-F 70-F
5 At outlet 42' -na- 253-F
At boiler inlet 50 450-F 611-F
At boiler outlet 52 650-F 650-F
At reheater inlet 56 -na- 320'F
At reheater outlet 58 -na- 300-F
10 At outlet 42 -na- 120-F
- At stack 24 300-F 139-F
Table 3
Heat Transfer BTU Recovered BTU Transferred
Efficiency From Flue Gas to Combustion Air
Or Reheated Flue Gas
FIG 1 FIG 2 FIG 1FIG 2 FIG 1 FIG 2
Air Pre-
heater 22 95% 95~ 95.8 90 391.2 85.8
Reheater
44 na 95% na 5.5 na 5.2
20 Condenser-
Heat Ex-
changer 32 na 89~ na 49.3 na 43 9
The incremental efficiency improvement achieved with
the invention using the data from Tables 2 and 3 is obtained
using the relationship of:
Efficiency increment % = 24(611-450) x 100 3 4
1130
where .24 is the specific heat of air, 611-450 reflects the
higher temperature of the flue gas into the boiler with the
invention over the embodiment of FIGURE 1, and 1130 is the
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- 12 -
amount of BTU released by the complete combustion of 0.06
pounds of #6 fuel oil with one pound of air to produce flue
gas with 15% excess air in the resulting flue gas.
Having thus described an illustrative embodiment in
accordance with the invention for improving the efficiency
of the thermal section for a power plant, the advantages of
the invention can be appreciated. The invention can be
advantageously used without a preheater 22 and reheater 44
and for different heat generators such as those used in blast
furnaces, municipal waste burning plants, chemical process-
es and the like. variations from the described embodiment
can be made, such as in the selection of the washing liquid
and the heat exchangers without departing from the scope of
the invention.
