Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
11777~9
IMPROVED HEAT GENERATOR
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 efficiency 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.
Heat generators using combustible fuels such as
oil, c~al, gas or refuse materials and the like, generate a
substantial quantity of waste materials in the form of pollu-
tant gases and particulates. Federal and state environmental
requirements have imposed maximum emission standards for
these waste materials. Compliance with these emission stand-
ards involves substantial investments for appropriate pollu-
tion control equipment, the costs for which can be prohibi-
tively high.
For example, large systems are available to remove
particulates using a dry flue gas treatment. Typical devices
used for this purpose may involve electrostatic precipita-
tors, bag houses and the like. These devices are suitable forthe 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.
25 The magnitude of saseous pollutants generated from
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the combustion of fuel throughout the world is enormous. As
a result, man~ techniques have been described for the removal
of these pollutants from flue gases exhausted from heat
generators. A general statement of various wet scrubbing
5 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 Chemis-
try" by R. G. Nevill, at page 9-312 of "Energy Technology
Handbook" edited by D. M. Considine and published by McGraw-
10 Hill Book Company.
Flue gas wet scrubbing techniques also involvesubstantial investments with complex systems. For example, in
the U.S. patents 3,320,906 to Domahidy and 3,733,777 to
Huntington, wet scrubbers are described in which flue gases
15 are passed through a filter bed for intimate contact with a
wash liquid. 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.
Since corrosive liquid droplets are iikely to be en-
trained 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
25 is exhausted to atmosphere.
Another technique for the removal of pollutants may
involve cooling of the flue gas to such low temperatures 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
30 which the flue gas is cooled to about 10-C to condense out the
sulfurous pollutants after which the flue gas is reheated
before discharge to atmosphere.
These and other techniques for the removal of waste
materials from flue gas involve a substantial amount of
35 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.
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Techniques for preheating of air have been known and
used for many years in connection with boilers to irnprove
combustion. One such preheating technique employs a Ljung-
strom air preheater. This uses a rotor through which on one
side flue gas is passed while an inflow of combustion air is
passed through the other side, with the two gas flows being in
opposite directions. Air preheaters, however, are operated at
sufficiently high temperatures to avoid condensation inside
the heat exchanger of pollutants such as SO3 present in the
flue gas.
In a technique in accordance with the invention 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 from the flue gas.
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 simultaneously flooded with a scrub-
bing liquid for removal of particulates and gaseous pollutants
in the flue gas. Heat from the flue gas is transferred through
the heat exchanger to the inflow of air for its preheating
while the flue gas pollutants are removed by collecting the
liquid after its passage through the heat exchanger.
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 is avoided, yet a substantial part
of the SO3 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 plants, steel manufac-
turing furnaces, sulfur producing plants and the like.
It is, therefore, an object of the invention to
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improve the thermal efficlency of a heat generator using
combustible ~uels while removing pollutants from flue gas
generated from the generator.
These and other advantages and objects of the in-
vention can be understood from the following description ofone illustrative embodiment in accordance with the invention
and described in conjunction with the drawings.
FIGURE 1 is a schematic representation of a conven-
tional thermal section for a power plant; and
FIGURE 2 is a schematic representation of a thermal
section improved in accordance with the invention.
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 14 into the boiler 12.
The boiler 12 includes suitable heat exchange ele-
ments (hOt shown) in which a working fluid (water or 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 dis-
charged 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 S02,
S03 and others. Techniques for removal of the pollutants are
usually a part of the thermal section 10, though for purposes
of simplicity of FIG~RE 1, these pollution controls have been
left out of the schematic representation. Suffice it to say
3s that techniques and devices for collecting particulates and
pollutant gases from flue gases have been extensively de-
scribed in the art.
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It is generally recognized that, particularly in
large electri`c power plants, the exhaust temperature of the
flue gas should preferably be kept above the dew point of the
SO3 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 SO3 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 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 (not shown)
to sprayers 36 into the heat exchanger portion 38 through which
the flue gas 18 is passed. The 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 constituents in the flue gas.
The liquid is preferably formed with ingredients
suitable for absorbing the neutralizing various pollutants in
the flue gas. These pollutants may be SO2, SO3 and others, for
which absorption and neutralizing techniques are well known,
see for example, some of the aforementioned prior art publica-
- tions. An alkaline wash liquid may be used to, for example,
neutralize condensed SO3 and absorb SO2.
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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 compon-
ents. 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
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 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 scrubbing cycle.
Sufficient heat is transferred from the flue gas by
the rotor in the rotary heat exchanger 32 to the inflow of air
16 to increase the latter's temperature significantly while
the flue gas 18 is considerab]y 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 undesir~ble, a
visibility suppression technique is used whereby the flue gas
18 from heat exchanger 32 is reheated with a heat 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 effects of the
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removed pollutants. The thermal efficiency of the heat
generator ma~ be increased by about three and a half percent
(3.5%). The net thermal efficiency, i.e. after allowing for
additional energy requirements to implement the improvement
of the invention, being about two and six-tenth 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 significantly 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 50-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 temperatures
encountered in the prior art system of Fig. 1 in comparison
with temperatures estimated to be generated in a system of
FIGURE 2.
Temperatures
20 Places FIGURE-l FIGURE 2
At air inlet 54 70 F 70 F
At outlet 42' -na- 266 F
At boiler inlet 50 450 F 630 F
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
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 advan-
tageously used for different heat generators such as those
used in blast furnaces, municipal waste burning plants, chem-
ical processes and the like. Variations from the describedembodiment can be made, such as in the selection of the washing
liquid and the heat exchangers without departing from the
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scope of the invention.
