Note: Descriptions are shown in the official language in which they were submitted.
~2~)74~
-- 2 --
Field of the Invention
This invention relates to heat generators in which
combustible fuels such as fossil fuels, refuse or other
materials are burned. Nore specifically, this invention
relate~ to a method and system for improving the efficiency
of such heat generators and particularly for better
utilization of hea~ produced in ~he thermal section for a
large electric power plant using a co~bustible fuel.
Background of the Invention
In United States Patent No. 4,444,128 by Richard J.
Monro, a technique i~ described for improving the efficiency
of a heat generator in which an inflow o~ combustion air is
preheated by the exhausting flue gas. An air preheater of the
rotary type is described which i5 operated with such a heat
exchange relationship that sufficient heat in the flue gas is
transferred to the combustion air that a gaseous pollutant
condenses out in the preheater. A liquid is simultaneously
applied to the heat exchanger so that a pollutant such as S03
and its condensed form of HzSO4 are washed away. A
neutrali~ing liquid may be used.
37~
- 3 -
Prior Ar-t Ba k~Lround
Heat generators using combustible fuels such as
oil, coal, gas or refuse materials and the like,
generate a subs-tantial quantity of waste materials in
the form of pollutant gases and particulates. Fed~ral
and state environrnental requirements have imposed
maximum emission standards for these waste materials.
Compliance with these emission standards involves
substantial investments for appropriate pollution
control equiprnent, the costs for which can be
prohibitively high.
For example, large systerns are available to remove
particlalates using a dry flue gas treatment. Typical
devices used for this purpose may involve electrostatic
precipitators, bag houses and the like. These devices
are suita~le 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 ~agnitude 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 these pollutants f.rom flue gases exhausted
from heat generators. A general statement of various
wet scrubbing processes for pollutan~ removal from flue
gases exhausted from large scale electric power plants
can be found in a chapter entitled "Wet Scrubbing
Process - Sx and Nx Removal Chemistry" by R.G. Nevill,
at pages 9~312 of "Ener~y Technology Handbook" edited by
D.M. Considine and published by McGraw-Hill Book
Company.
Flue gas wet scrubbiny techniques also involve
substantial investments with complex systems. For
example, in the U.S. patents 3,320,906 to Domahidy and
3,733,777 to Huntington, wet scrl~bers are described in
~ ~30 7 ~
which flue gases are passed through a filter bed for
intimate contact with a wash liquid. The wash liquid
rnay be an aqueous bisulfite salt solution such as
described in the Huntington patent or such alkaline
scrubbing liguors indicated as useful with the ~et
scrubber described in U.S. patent 4,049,399 to Teller.
Since corrosive li~uid droplets are likel~ to be
entrained by the scrubbed flue gas, special technigues
such as described by Teller or in the U.S. patent to
Brandt 3,844,740 may be used to avoid corrosion on
subsequent eguipment 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 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 which the flue gas is cooled to
about 10C 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 arnount of
energy, much of which is irretrievably lost. As a
result, the overall effici~ncy, i.e., the energy
available for sale from a power plant is significantly
reduced.
Techni~ues for preheating of air have been known
and used for many years in connection with boilers to
irnprove cornbustion. One such preheating technique
employs a Ljungstrom air preheater. This uses a rotor
through which on one side flue gas is passed while an
inflow of cornbustion air is passed through the other
side, with the two gas flows being in opposite
directions. Air pxeheaters, however, are operated at
~ 7 ~
sufficierl-tly high temperatures to avoid condensation
inside the heat exchanger of pollutants such as 5O3
present in the flue gas.
For example, in typical published tempera~ure guid~
lines for Ljungstrom air preheaters, the outlet flue gas
temperature is maintained at least at 350F. At this
temperature, gaseous SO3 does not condense and corrosive
effects on the prehea-ter are minimized.
TechIliqlles for cleanirlg rotary heat exchangers have
been described in the art. For example, khe U.S. Patent
~,812,923 to Schoenherr et al describes such an
apparatus which applies a cleaning liquid through ports
in a sector plate above the heat exchanger and withdraws
the liquid through slots in a sector plate located below
the heat exchanger.
Summary of the _ vention
With a system and method in accordance with the
invention, improvements in the operation of a rotary air
preheater as described in the aforementioned copending
patent application are obtained whereby significant fuel
savings are achieved.
With reference to one system and method for
operating a xotary preheater in accordance with the
invention, the washing liquid is applied to the rotary
pxeheater in such manner in conjunction with the
rotational speed of the rotor of the preheater that the
carry-over of liguid into the air flow side of the
preheater is substantially reduced and the wetting of
the preheater is controlled to a low level selected to
attain a high fuel saving. This involves a slow
rotational speed of the rotor used in the rotary heat
exchanger and the application of liquid in a strong
localized stream so as to wet a limited portion of the
air preheater at any one time. The stream is moved
~L~907~2
along a zone located at the hot axial end of the heat
exchanger ~here air exits and flue gas enters and in the
vicinity of the region where the rotor leaves the air
flow side to enter the flue gas side.
The thcrrnal efficiency of conven-tional rotary air
preheaters generally increases with rotational speed
with a maximum speed for large scale electricity
generating stations being of the order of about four
revolutions per minute (rpm). In a system and method in
accordance with the invention, high rotational speeds
have been found to reduce efficiency and a low speed is
used to yield desired net fuel savings. The rotor speed
is, therefore, selected commensurate with the time
needed for liquid to drain through the heat exchanger
prior to the re entry of wetted heat exchange elements
into the combustion air side. The rotor speed generally
is less than one rpm and preferably of the order of a
quarter of an rpm.
The mass flow of the air through the preheater
preferably is selected sufficiently high relative to the
heat capacity of the rotary air preheater to overcome
degradation effects due to the application of liquor to
remove particulates and the acid pollutants condensed
out in the preheater. The liquid flow preferably is
selected at the lowest level which is consistent with
corrosion protection and wheel cleaning capabllity.
With these considerations a wide range of operating
conditions can be set depending upon an acceptable level
of fuel savings with the invention. For high fuel
savings the ro-tary preheater should be sized so ~hat the
ratio of the heat capacity of the air Ha to the heat
capacity of the rotor, Hr, is greater than a
predetermined value while the ratio of the heat capacity
of the air Ha to that of the liguid, HL, should also be
greater than a preselected value.
74~
As described herein with reference to one method
for operating a thermal section in accordance with the
invention, a neutralizing liquid containing a high level
of alkaline material is applied to previously liquid
cleaned rotor parts that are preferably essentially dry.
The neutrali~ing liquid contains a sufficient amount of
alkaline material, such as NaOH, so that as this dries,
a thin dry layer is formed over the surfaces of the heat
exchanger elernents of the xotor. Condensed sulfuric
acid tends to preferentially react with this layer
instead of the base metal during operation -to form a
neutralized salt layer. With -this technique, protection
of the rotary heat exchanger against acid attack can be
significantly extended over long time periods. This in
lS turn advantageously enables a reduction of the amount of
liquid cleaning needed of the rotor.
As described herein, protection of the air
preheater against corrosion from condensed H2SO4 is
enhanced by orienting the neutralizing stream of liquid
in a preferred direction with respect to the preheater
media plates so as to assure neutralizer liquid impact
on -their surfaces. Additional coxrosion prote~tion is
obtained by establishing a small pressure in the plenum
of the preheater surrounding its rotor. This reduces
the escape of corrosive SO3 and condensed H2SO4 past the
rotor seals into the plenum. Corrosion protection is
further enhanced with an application of the wash or
neutralizing liquids or both from the cold end or flue
gas exit side of the preheater.
It is, therefore, an object of the invention to
improve the thermal efficiency of a h~at generator using
combustible fuels. It is another object of the
invention to obtain such efficiellcy improvement while
removing a pollutant in the flue gas from the heat
generator and cleaning an air preheater used to achieve
~ ~0~4~
the efficiency improvement. It is a further object of
the invention to protect a rotary air preheater against
at-tack from an acid condensed out from the extraction of
hea-t from a flue gas by neutralizing the condensed acid,
washing away salts and ash while maintaining a heat
transfer from the flue gas to the combus~ion air, and do
all this at relatively low costs in comparison with the
fuel saving benefits obtained.
These and other advantages and objects of the
invention can be understood from the following detailed
description of an embodiment in accordance with the
invention and described with reference to the drawings.
Brief Description of Draw~
Figure 1 is a schernatic perspective representation
of one heat generator employing air preheaters in
accordance with the invention;
Figure 2 is a schematic plan view of an air
preheater employed in accordance with the inventioni
Figure 2A is a heat exchange efficiency plot of a
rotary air preheater as a function of the percentage of
its wetter surfaces;
Figure 3 is a schematic vertical sectional view of
the air preheater shown in Figure 2;
Figure 3A is a partial sectional view of a notched
flat media plate used in a rotary air preheater used in
accordance with the invention;
Figure 3B is a partial, horizontal sectional view
of one form of a media assembly employed in accordance
with the invention;
Figure 4 is a broken-a~ay top plan view of a rotary
air preheater in accordance with the invention;
Figure 5 is a vertical section view of the air
preheater as shown in Figure 4 and is taken along the
line 5-5 i.n Figure 4;
~ 7 ~
Figure 6 is an enlarged partial top plan ~ie~t~ of
the air preheater shown in Figuxe 4;
Figure 7 is a vertical section view of the air
preh~ater of Figure 6 -taken along the line 7-7 therein;
Fiyure 8 is a perspec-tive view of a conventional
rotary air preheater that has been modified in
accordance with the invention;
Figure 9 is a partial vertical sectional and
schematic view of the air flow side of a rotary air
preheater modified in accordance with the inventioni and
Figure 10 is a schematic vertical section view of
the ai.r flow side on the left and the flue gas side on
the right of a modified rotary air preheater and wash
assembly in accordance with the invention;
Figure 11 is a partial side section view of an
apertured upper and radially outer seal taken along the
line 11-11 in Figure 10; and
Figure 12 is a schematic representation of a
technique to apply a scrubber to a flue gas pollutant
~r~ 2
condenslng hea~ exchange~. '
Detailed Description of Drawin~
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 suita~le
ducts 14 into -the boiler 12.
: The boiler 12 includes suitable heat exchange
elements (not shown) in which a working fluid (water or
steam) i.s circulated for heating by the combustion gases
generated in the boiler 12. Flue gas, as suggested by
arrows 18, emerges at a discharge 20 from the ~oiler 12
at a high temperature, typically in the range of about
~[)7~
-- 10 --
6$0F, and is passed through first and second rotary
heat exchangers 22, 24 to preheat the inflow of air 16.
Although two air preheaters are shown, a single suitably
sized unit could be used. After passage through heat
exchangers 22, 24 the flue gas 18 is discharged to
atmosphere at a stack 26 after passage through a
reheater 31. Air flow throuyh the thermal section 10 is
obtained with a forced draft fan 28 and an induced draft
fan 30. An air reheater 31 may be used to raise the
ternperature o~ the flue gas at the stack for plume
suppression.
The flue ~as 18 may include pollutant materials in
the form of particulates such as fly ash and gases such
as S02, SO3 and others. Technigues 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 H2S04 to avoid corrosive effects from
contact by condensed H2S04 with equipment such as the
air preheaters 22, 24 and the induced draft fan 30 as
well as the emission of corrosive particles from stack
26. 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., the
flue gas is kept above about 300F to 350F. As a
result, the temperature of the inflow of air 16 at the
boiler 12 is usually about 450F and the ther~al
efficiency of thermal section 10 in conventional systems
is not as high as it could ~heoretically be made.
~9~7d~2
As described in the aforementioned copending patent
application, a substantially greater amount of heat from
flue gas is recaptured to achieve a higher thermal
efficiency. Thls is advantageously simulkaneously
accompanied by the extraction of pollutants and cleaning
of the preheater. The recapture of heat is achieved by
passing the flue gas 18, after its passage through
preheater 22, through rotary heat exchanger 24 where an
additional portion of the heat in the flue gas lg is
extracted for transfer to the inflow of air 16.
I'he ro-tary heat exchanger 24 operates with a
working liquid, such as water, which is applied through
a conduit 33 mounted to a wash arm 32 from a pressurized
supply 35 (shown in Figure 2) into the heat exchanger.
The liquid is applied in a strong localized stream to
dislodge particulates adhering to heat exchange elements
while also washing away corrosive constituents such as
condensed H2S04 as well as salts formed by condensed
H2S04 with previously deposited alkaline neutralizing
materials. Although the wash liquid could contain all
of the neutralizing material to neutralize condensed
acid, it is in most cases more economical to supply the
neutralizing material with a separate smaller volume
stream through a conduit 33' and with a much higher
concentration. In such case the wash liguid is
primarily water with a small concentration of
neutralizing base material to keep its pH above seven,
preferably about 11.
The operation of thermal section 10 is as described
in the aforementioned copending patent application
whereby the rotary air preheater 22 operates with a flue
gas exit temperature that is above the acid dew point
level. The use of a wash-arm 32' with preheater 22
sexves to periodically clean the preheater while the
preheater is on-line wi~hout having to take the
preheater off-line.
1~9~37~
- 12 -
Air preheater 24, however, causes a heat exchange
from the flue gas 18 to the combustion air 16 so that
the flue gas temperature at outlet 36 is well below the
acid dew point. As a result, any gaseous H2SO~ formed
in the flue gas condenses out as acid H~SO4 in rotar~
heat exchanger 24. Accordingly, rotary heat exchanger
24 is further rinsed wi-th a separate neutralizing
liquid. This may be supplied through a bank of misters
38 located as shown in Figure 1 over a portion of the
side 40 of the rotor ~2 where flue gas enters. However,
pxeferably the neutralizing liquid is primarily supplied
through a separate conduit 33' in a manner as further
explained with reference to Figures 2 and 3.
The separate neutralizing stream of liguid or
powder is preferably formed with ingredients suitable
for absorbing 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, for example a water
based solution of NaOH, to neutralize acid ~H2SO4~
condensed from H2S04 gas. The neutralizing liguid
preferably is formed with a high concentration of
neutralizer material so that this will form a protective
layer when the neutralizer liquid dries. The
concentration of the neutralizer preferably is se-t
sufficiently high to deliver the layer to all preheater
surfaces and should be at least above the stoichiometric
level for neutralization of the condensed acid. In case
of a powder the neutralizer stream has a concentration
of about 100%. Preferably a neutralizer liquid with a
concentration of up to about 50% by weight such as a
water base soIution containing about 50% by weight of
NaOH can be used. Other neutralizer materials may be
~L~9~)7~
- 13 -
used such as KOH and CaO~ or MgOH or other basic
compounds depending upon the cos-ts as well as
compatibility of the neutralizer material with the
removal of salts formed by t~e reaction of condensed S33
(H2SO4) with the neutralizer and subseguent waste
material processing. Neutralizer materials capable of
forming highly soluble salts are preferred. ~rhe amount
of neutralizing liquid is selected sufficiently low to
minimize excessive wetting, yet sufficiently high to
accommodate water evaporation, which may be about 25%,
and still assure comple-te heat exchanger surac~
coverage.
The operation of rotary air preheater 24 has been
found dependent upon the exten-t of the two phase, wet
and dry, conditions arising from the application of
liquid to wash away particula-tes and pollutants. A
typical net fuel saving can be obtained with a dry
condition, about 4.36%, but in such case condensed
sulfuric acid unacceptably attacks the e~uipment. The
addition of an on-line liquid stream for protection of
heat exchanger 24 and the neutralization of condensed
acid to avoid its attack on down stream equipment
reduces theoretical maximum net fu~l savings to about
4.26% texclusive of reheating requirements). Generally,
the less liquid needed to protect heat exchanger 24, -the
easier it is to approach high fuel savings.
In another technigue fox protecting the rotary air
preheater against attack by acid, a neutralizing liquid
rnist is directed at the rotor 42 such as on its flue gas
side 40 by a bank of misters 38 extending across the
active region of the rotor. The neutralizing liquid
contains an amount of neutralizer material that is
sufficient to more than stoichiometrically neutralize
condensed acid. In a practical system the neutralizing
material is at least e~uivalent in weight, and usually
~LZ9074X
significantly higher, to the weight of H2S04 condensed
over the time period for which protection ayainst acid
attack is required and is a function of factors such as
the type of fuel and boiler load. Since the amount of
acid is a function of the acid dew point te~perature in
the flue yas, a suitable acid dew point temperature
sensor 39 is employed and applied to a control 41 for
regulating the amount of neutralizing liquid and assure
protection against corrosion attack.
The localized wash liquid stream is applied to the
rotary heat exchanger in a rnanner 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. The wash liquid is, therefore,
applied in a controlled manner whereby at any one time
the amount of wetted surfaces of the wheel in rotary air
preheater 24 is limited. The advan-tage of limiting
wetting of the rotor surfaces is illustrated in FIG. 2A
where a curve 43 approximately represents the efficiency
of rotary heat exchanger 24 as a function of the
percentage of wetted surface areas. Curve 43
illustrates that efficiency decreases to a minimum level
with increased wetness. Although the amount of wetted
surfaces can vary considerably depending upon the
particular application, a high heat exchange efficiency
can be retained by limiting the wetted surfaces to a
zone that occupies generally less than about ten ~10)
percent of the total heat exchange surface area of the
rotary heat exchanger 24. In a full scale plant, the
wetted axea of the rotary heat exchanger would
preferably be no more than about 4% of the surface area
of the heat exchange media.
one technique for applying the wash liquid stream
involves the periodic application of a flooding amount
74~
- lS -
of liquid to discrete portions of the rotary heat
exchanger at sufficiently short intervals to prevent
corrosive damage. The intervals may be related to the
rotation of the rotary heat exchanger such as b~
effectively flooding the rotor on,ce every several or
more rotations of the rotary heat exchanger 24. The
li~uid is preferably applied to that part of the rotor
where it is about to leave the region where combustion
air passes through to move into the region where flue
gas passes through.
With reference to Figures 2 and 3, the rotary heat
exchanger 24 has a rotor 42 within a rotor shell 44.
The rotary air preheater 24 includes a combustion air
side 46 that is separated from flue gas side 40 by a
dead~zone 48 where the rotor 42 has a rotor post 50 and
bearing (not shown). The rotary preheater 24 has,
except as modified for this invention, a conventional
construction as may be obtained, for example, from
companies such as Combustion Engineering or The Air
Preheater Company. The rotary air preheater 24 includes
heat exchange elements 52 (see Figure 5) that are
arranged in chordal directions relative to the axis of
rotation in sector baskets 54 (see Figure 4).
Returning to Figures 2 and 3, air preheater 24 is
provided with a pivotally mounted and driven wash arm 32
carrying a pair of conduits 33, 33', each of which is
respectively coupled to pressurized supplies 35, 35' of
liquid wash water and neutralizer liquid. Each conduit
; 33, 33' terminates in a nozzle 56, 56' respectively.
The nozæles 56, 56' are downwardly oriented to eject a
localized stream of liquid onto the rotor 42. Wash arm
32 is movably mounted at pivot 57 so that nozzles 56,
56' traverse a zone 58 that crosses the entire rotor 42.
The streams of liquids thus can, by passing along zone
58, treat the entire rotor 42 along a spiral path as the
~L~9()7~
- 16 -
ro-tor is rotated during operation of the rotary heat
exchanger 24.
Generally the neutralizing stream is so located
that the region of the rotor exposed to it is covered by
S a dry neu-tralizing material after the liquid has
evaporated. In the i]lustrated embodiment, the nozzles
are radially spaced from each other at a distance that
is so selected that the wash stream nozzle is
sufficiently spaced from the neutralizer liquid stream
so that the la-tter is applied to a rotor zone that is
almost dry and thus at a sufficiently low temperature to
prevent excessive evaporation of the neutralizing stream
before it has been applied through the axial extent of
the rokor. One such spacing between nozzles 56, 56' may
lS be such that the wash stream nozzle 56 wets the rotor
along a spiral path that is in effect several rotor
revolutions spaced from the spiral path traced by the
neutralizer liquid stream from nozzle 56'. With such
spacing the portion of the rotor to which the
neutralizer liquid is applied has at least almost
essentially dried. As the neutralizer liquid contacts
the rotor, its heat exchange surfaces are coated with an
initially liquid neutralizer film which, as it dries,
leaves a protective layer of neutralizing material.
When a powder neutralizer material is used the surfaces
are sufficiently wet, though preferably almost dry, to
aid in the adherence of the powder to the heat exchanger
surfaces.
For this purpose a high concentration of alkaline
neutralizing material is used, such as a solution in the
range from about 15% up to about 50% of NaOH. Generally
a concentration in the range from about 15% to about 30%
is used with higher concentrations such as 50% if these
37~Z
- 17 -
are deliverable to the rotary heater elernents. This
enables the formation of a layer that reacts with
condensed acid to form a protective salt layer. The
surface protection persists for a long time depending
inversely upon the amount of sulfur in the fuel and the
rate of conversion of SO2 to SO3. Generally it is
estimated that with a solution in the range from about
15% to about 50% of NaOEI, a protective layer is formed
that could protect the rotor for about 60 hours with a
flue gas formed with a 2% sulfur fuel and containing SO3
converted at a 3% rate. In practice the interval
between washes also is a function of the ~nount of
particulates that accumulate and tend to clog the rotary
preheater. Hence, the amount of sulfur in the fuel, the
amount of particulates accumulating in the rotary heat
exchanger and the concentration of the neutralizer
affect the interval between wash cycles. A rotary heat
exchanger in practice is preferably continuously washed
and coated with neutralized material so as to complete
the washing and coating of the entire unit in about
eight hours.
The traversal of wash arm 32 to clean and treat
rotor 42 may thus occur on an intermittent basis. In
such case the amount of wetted surfaces of rotor 42
aggregated over the interval between intermittent
washings can be a small percentage of the dry surfaces
in operation during an interval so that in eff~ct the
rotary heat exchanger i5 operated at a high efficiency
for most of the time. It is still desirable in such
case to prevent the carryover of liguid to the air flow
side 46.
Operation of the liquid streams in FIG. 2 occurs in
a single direction depending upon the relative positions
o the wash liquid nozzle 56 and neutralizing liquid
nozzle 56'. In the embodiment as shown in Figure 2, the
~Q~
- 18 -
liquid streams from nozzles 56, S6' are applied
simultaneously as the wash arm 32 moves outwardly in the
direction of arrow 70 from the central part of the rotor
42.
In the event the liquid streams must be operated in
both pivot directions o wash arm 32 a third
neutralizing liquid nozzle 56 " could be placed on the
other radial side of wash li~uid nozzle 56. In such
case, nozzle 56' would be operated duriny movement of
wash arm 32 in the direction of arrow 70 and nozzle 56 "
would be opexated during rnovement in the opposite
direction.
The rotation of rotor 42 is as shown in the
direction of arrow 60 a~out a vertical axis. The zone
58 is, therefore, located at the hot axial end 62 where
flue gas enters and combustion air exits while near the
region where the rotor leaves the combustion side 46 and
enters the flue side 40. The ~one 58 is preferably
located in the air side 46, though it may be in the dead
zone 48 or in the flue side 40. Hence, as the streams
of washing liquid and neutralizing liquor are applied to
the rotor 42, both li~lids are preferably given a
maximum available time to drain from the wetted heat
exchange elements 52 before these are again rotated into
the combustion air side 46.
The localized stream of washing liquid from nozzle
56 is a solid spray under high pressure. Pressures may
be in the range from about 200 to about 3000 psi,
preferably about a 1000 psi. The liquid volume should
be sufficient to cleanse the entire axial portion of the
heat exchange elements over which the nozzle 56 is
located. The cleansing includes the dislodgement of
particulates and condensed materials and of salts left
from previous exposure to the neutralizing stream.
~x~
- lg
With a powerful localized strearn of liquid, the
splashing effect from horizontal cross bars and other
structural members as these pass beneath the liquid
stream tends to cause severe wetting of heat exchange
elements intended to remain dry. Such splashing is
particularly undesirable when liquid is splashed into
the combustion air flow or interferes with the operation
of the nearby neutralizing liquid. One aspect of the
invention, therefore, involves, as shown in Figures 2
and 3, the addition of splash guards 66 that are mounted
in choxdal directions on each sector basket 54. The
splash guards 66 are formed of vertically oriented
plates that are spaced a small distance, of the order of
about a half inch, from each o-ther and reach up towards
the nozzles 56, 56' for a distance selected to inhibit
radial splashing of the liquid stream though permitting
splashing along chordal directions. Such splash guards
can be deleted when the media, i.e., the rotary he^t
exchanger, is designed to avoid structural members
directly below the liquid streams.
When a strong localized liquid stream is applied to
a zone such as 58, liquid may spill into the air side 46
at the lower axial end 62' of the preheater 24. A drain
trough 68 is, therefore, provided below zone 58 to
capture any liquid that may tend to flow back towards
the air side. This trough advantageously provides a
slightly extended dead-zone so that incoming co~ustion
air flows around the nozzles 56, 56' and en~rainment or
liquid therefrom into the boiler 12 is minimi~ed. The
liquid collected by trough 68 is passed through a trap
69 to a main liquid collector 71 located below preheater
side 40. Traps such as 69, 69' are sized to accommodate
gas pressure dif~erences between sides 40 and 46 of the
air preheaters.
3L~9(~7~
~ 20 -
In some air-preheaters the shaping of th~ heat
exchange elements (the media plates) ~ith double
undulations promote channeling of the wash liquid and
neutralizing liquid, thus tending to cause non-uniform
wet-ting of the media surfaces. When the media plates
are made notched flat as ~hown a~ 69 in Figure 3A, the
channeling can be eliminated.
Media d2sign, however, has been found to affect the
ability of rotary heat exchanger 24 to extract all of
-the condensed H2SO4 from the flue gas stream. This
problem arises when the hot flue gas, after it has
entered preheater 24, generates an aerosol of condensed
H2SO4 droplets which do not become attached to the media
plates and tend to pass through. Such aerosol condition
tends to arise when the flue gas mixes with eddy flows
of gas chilled by the media plates. This in turn cools
free flowing portions of the flue gas below the S03
condensation temperature and allows the formation of
H2SO~ droplets that are entrained right through the
preheater 24.
When the apertures or cross-sectional dimensions of
the passages between media plates are made sufficiently
small, how~ver, aerosol ormation of condensed S03 iS
avoided. In such case the flue gas is forced into
intimate contact with the media plates, eddy flows are
suppressed and virtually all of the S03 condenses out on
media surfaces.
Hence, as shown in Figure 3B the media plates 92,
93 are so designed that the cross sectional dimensions
of the passages g4 through which the flue gas 18 passes
are selected to suppress the formation of condensed S03
aerosol droplets. The dimension or specific shape of
passages 94 may be varied; however, aerosol conducive
conditions were suppressed when the ma~imum cross-
sectional dimensions be~ween main cooling surfaces such
~29~17~
- 21 -
as 95, 95' in passages 94 was held to about one tenth of
an inch (about 2.5 mm). A media plate configuration as
shown in Figu,re 3B was found effective. The size of
some passages may be increased such as at the
undulations 96 that are used to space the plates 92, 93.
However, generally the cross-sectional dimensions of the
passages are selected to promote laminar flow and are
generally less than about .2 inches (about 5 r~m) and
preferably about 0.1 inch.
With reference to Figures 4-7, further details of
rotary air preheater 24 are shown. In Figure 4 the wash
arm 32 is shown d,riven back and forth as suygested by
arrows 70-70' by a motor driven mechanism 72. This may
include a reversible motor that is reversed Pach time
washing arm 32 reaches a limit position as detected with
a limit switch. The speed of the ~ash arm movement is
selected sufficiently slow to assure complete rinsing of
rotor 42. Rotation of rotor 42 is obtained in a
we]l-known manner with a conventional variable speed
motor 73 whose output shaft has a gear that meshes a
ring gear located around the outer periphery of rotor
42.
In the embodiment shown the wash arm 32 is
pivotally mounted at a pivot 57 near the external wall.
This shortens the radius of curvature of the path
traversed by nozzles 56 and tends to increase the width
of zone 58 to assure complete coverage of the rotor 42
by the wash liguid. Another technique for moving the
nozzles 56 would involve, as shown in Figure 2, a
hydraulic or mechanical actuator 74 having a movable
piston 76 to which nozzles 56, 56' would be mounted and
moved along zone 58. The conduits 33, 33' could be made
of flexible hoses to accommodate such piston mounting of
the nozzles. Instead of actuator 74 a track could be
used.
1~530742
The motion of wash arm 32 or piston 74 is
controlled by a controller 77 (see Figure 4) to impart a
variable speed. Thus as nozzles 56, 56' are moved
radially outwardly, their speed reduces so as to deliver
a generally uniorm amount of wash liquid and
neutralizing liquid to the entire rotor 42. Such speed
is prefera~ly made inversely proportional to the radial
position of the nozæles 56, 56' relative to the center
of rotor 42. In one embodiment contemplated in
accordance with the invention the wash liquid is applied
by a plurality of radially closely spaced nozzles, each
of which operates under a high pressure, say a 1000 psi,
to deliver a solid stream that may be of the order of an
eighth of an inch wide when it first reaches the media.
These streams are ~adially adjacent to each other to
provide a somewhat radially elongated continuous spray
zone.
Nozzle 56' preferably is so oriented as to reduce
losses as may arise from a direct pass through and thus
assure impact of the neutralizing liquid on th~ media
plates 52 of rotor 42. This is obtained by orienting
nozzle, such as 56', at an angle relative to thP planes
of media plates 56, 56' as shown in Figure 5. Such
angle A may for example be of the order of about 30
re~ative to the orientation of the planes of the media
plates at the location where the nozzles 56' operate.
Preferably, however, th neutralizer spray has a
conical, hollow shape that is directed vertically down
onto the top of the media plates that need to be
protected to thus apply the spxay over a sufficiently
wide area. The cone angl~ preferably i~ greater than abou~
ten degr~es to minimize neutralizer liquid los~ through
media pa~ages.
Enhanced protection of the rotary heat exchanger
3~ a-~ainst corrosion from condensed aci~ is obtained by the
adoption of the features as shown in Figures 8-10. In
74~
- 23 -
Flgure ~ a conventional rotary air preheater 100 is
shown but modified in accordance with the invention.
The air preheater 100 is shown, for clarity, without the
wash and neutralizer liquid delivery system. The air
prehea-ter 100 includes a rotor 102 inside a rectangular
shell 104 so that a plenum space 105 between rotor 102
and shell 104 is formed. Such plenum 105 is isolated
from the combustion air and fllle gas flows ~y the use of
stationary seals, as shown in Fi~ure 9, such as inner
and outer circumferential seals 106, 106', 108, and 108'
mounted to shell 104.
The rotor 102 typically is made of different
segments such as the hot end heating el~ments 110 and
cold end basketed heating elements 112. The latter
basket elements 112 are replaceable through an opening
covered by a panel 114. As a result of such
construction there is likely to be some flue gas leakage
through apertures, gaps and the like as well as past
seals that are either worn out or do not provide a
proper seal. Hence, flue gas containing corrosiv~
constituents such as condensed acid may enter the plenum
105 and cause damage to the metal.
In the embodiment of Figure 8 the entry of flue gas
into the plenum is inhibited ~y pressurizing the plenum
2S by a small differential pressure above the highest flue
gas pressure at the rotary heat exchanger 100. This is
done by bleeding a small amount of the flow of air 16 on
the air side into the plenum 105 through a duct such as
116 that extends from the air discharge side to the
plenum 105.
The pressurization of the plenum 105 may be done in
different ways. One technigue as shown in connection
with Figure 9 utilizes a pair of pressure s~nsors 118,
120 located respectively in the plenum 105 and the flue
~o~
- 24 -
gas duc-t 122. The sensors 118, 120 may be of the
pneumatic type and coupled to a differential pressure
sensor 123 whose output is compared at 124 witn a
reference 126 to produce a control signal on output 128.
The contxol signal in turn is applied through an
appropriate actuator 130 to set a valve 132 inside duct
116 so that the pressure in plenum 105 is at t'ne
reference level 126. Electrical controls m~y be used
where ternperature conditions permit this.
The plenl1m gas pressure is kept small to reduce the
flow of air from the plenum into the flue gas stream and
thus reduce waste of fan power. Yet the pressure is
sufficiently high to prevent gaseous S03 or condensed
acid from entering the plen~n 105. The plenum air
pressure may be from one to several inches of water
above the flue gas pressure.
In a conventional rotary heat exchanger, the
distribution of the wash and neutralizer streams tends
to broaden as the liquids drain from one set of heating
elements 110 to the nex-t layer of elements 112. This
increases the wetted zone of the rotary heat exchanger
and may affect the even application of the neutralizer
liquid. Hence, it is preferred that a rotary heat
exchanger is used in which the wetted zone can be
confined to the smallest that is needed and in which
neutralizer material can be reliably applied to all
surfaces. One technique for accomplishing even wetting
involves the use of heating elernents 52 that extend, as
shown in Figures 5 and 9, in a continuous uninterrupted
manner frorn one axial end 62 to th~ other 62' of a rotor
42.
The application of the wash and neutralizing
li~uids from the hot end of the rotary air preheater may
be supplemenked with wash and neutralizing liquid
streams applied from the cold end as shown in Figure 10.
)742
- 25 -
A wash liquid stream is shown applied by a nozzle 140 to
cold end 62' in radial and circumferential alignment
with the wash liquid stream from nozzle 56. A
neutralizer stream from nozzle 142 is similarly aligned
with the stream from nozzle 56'. The nozzles 56, 140
and 56', 142 are xespectively moved in unison to limit
the size of the wetted zone while assuxing the
application of ~7ash and neutralizing liquids to all
heating elements 52 as well as other componenk surfaces
of rotor 42. Movement of nozzles 140, 142 relative to
rotor 42 may be done i.n synchronization with and in the
manner as described with reference to nozzles 56, 56' in
Figure 4. The application of the liquid streams to cold
end 62' enhances the protection of the rotor 42 against
acid corrosion without si~nificantly affecting the rotor
temperature. The amount of wash liquid and neutralizer
liquid applied by nozzles 140, 142 can differ from that
applied by nozzles 56, 56' depending upon the desired
depth of penetration,
Figures 10 and 11 further illustrate an alternate
technique for pressurizing the plenum 105. The upper
outer stationary circumferential seal 108 is
intentionally provided with an enlarged gap or slot 148
so as to provide an air passage 150 between the hot air
flow side and the plenum 105 to enable air to flow as
suggested by arroid 152. The size of gap 148 is
selected to establish the desired plenl~ pressure. The
size of gap 148 may be fixed or controlled with a slide
valve. Alternately, a duct can be provided between the
air flow side and plenum with a slide valve, not shown,
positioned to regulate plenum pressure.
A significant improvement in the overall eficiency
of the thermal section 10 is obtained with a heat
exchanger 24 with which a substantial portion o heat in
~2q~3~7~
26 -
the flue gas 18 is recovered while pollut~nts are
removed and the heat exchanger 24 is protected against
corrosive effects of the removed pollutants. The gross
fuel savings for a typical utility plant is about 4.36%
which is the equivalent of about 12.5% cycle efficiency
improvement for an electric generating station having a
current cycle efficiency of 35%. The net fuel saving of
the thermal section depends upon a number of factors
such as the type and nature of the fuel, the amount of
excess air in the combustion process and the temperature
difference between the co~ustion air and the flue gas
when it exits the air preheater 22. Typically, for 15%
excess air and a flue gas temperature difference of
200F across air preheater 24 a net fuel saving of a~out
4.26% for heat recovery alone can be obtained. Net fuel
savings means the efficiency improvement after the
energy requirements for operation of the additional
preheater 24 are taken into account. These generally
are about 0.1% when heat recovery alone is consider~d or
about 0.53% for energy needed for both heat recovery
with full pollution controls for S02 and particulates
removal at a 99% rate.
The -thermal efficiency depends upon a number of
factors such as the rotational speed of the rotor wheel
42, the air mass flow and the amount of liquid used in
the cleaning of the heat exchanger.
Rotor wheel speed has been found to strongly
influence the obtainable fuel savings. Thus, commonly
used speeds of the order of about 4 rpm have been found
to allow too little time to evaporate liquid and causes
excessive carry-over of liquid to the air flow side 46.
Too low a speed, however, may lead to inadeguate heat
transfer performance by the air preheater. The rotary
speed needs to be sufficiently low to enable a draining
of liquid introduced by the localized stream with drain
~9~
- 27 -
time being a function of axial size. Generally, the
rotary speed should be less than about one rpm with the
speed preferably selected in a range less than about
O.75 rpm. ~ wheel speed of about 0.25 rprn for a six
S foot axial length has been found to be optimum.
Another factor that si~nificantly affects the net
fuel savings obtained with a rotary air preheater 24 is
the hea-t capacity of the air, Ha, in relation to the
heat capacity of the wheel Hr and the heat capacity of
t'rle liquid, HL. These heat capacities are each variable
depending respectively upon the mass flow of the air,
the rotational speed, wheel depth and wheel diameter of
the rotor wheel 42 and the total amount of liquid
(including the wash and neutralizing liquids) applied to
the heat exchanger.
Net fuel savings can be achieved over a wide range
of conditions for rotor wheel speed, air mass flow and
liquid flow with the percentage of net fuel savings
varying as well. Although it might be desirable to
achieve a maximum net fuel saving, the particular
operating parameters for a heat generator may require
compromises that would limit fuel savings. A practical
limit of a least acceptable fuel saving would be
dictated by the capital costs needed to implement the
invention and the resulting pay-back time. This places
a practical minimum acceptable net fuel saving generally
at about two percent (2%) when considering the costs
involved in only a recovery of heat.
The air flow mass (not including the approximately
6% additional mass added by the combustion of fuel), and
the size of the rotary air preheater 24 are determined
according to standard well known principles applicable
to power plants. When an air preheater 24 for this
invention is to be used with a particular air flow, the
~g~7~X
- ~8 -
size of the air preheater is selected so that the
pressure drop is preferably held to a limit which should
not be exceeded (for example 15" of ~ater). As a
result, scaling of air-preheater 24 to different
requirements leaves the axial depth of the rotor 42
normally fixed regardless of size for most standard
applica-tions. The flow of wash liquid is selected in an
amount sufficient to wash away par-ticulates and salts
forrned on the heat exchanger elements. The amount of
wash liquid preferably is in the xange between about 0.3
to 5.0 gallons per minute for each rnegawatt of
electricity generating capacity (gpm/Mw).
The total liquid flow preferably is set at as low a
level as possible which level is a function of the
amount of evaporation of the wash stream. This may be
of the order of about 0.3 ~pm/Mw and optimum flow,
depending upon the size of the rotary heat e~changer is
likely to be between 0.5 and l gpm/Mw and probably
closer to 0.5 gpm/Mw. The amount of neutralizer liquid
applied may vary, but prefercibly is in the range from
a~out .05 to 1 gpm/Mw, depending upon the concentration
of the neutralizer agent in the stream. Generally, the
higher the concentration of neutralizer agent the lower
-the volume. A preferred flow rate is about .05 gpm/Mw
with a neutralizer concentration of 25% by weight.
Higher liquid flows can be used, particularly when
higher air flows are employed so that the ratio of heat
capacity of air Ha to the heat capacity of the liquid
remains at a high level or when the wash liquid can be
applied infrequently after long intervals. Too much
wash liguid should be avoided lest it would cause a head
of liquid to form in the space between heat exchange
elements.
In order to obtain a minimum acceptable fuel
saving, the ratio o~ the heat capacity of the air flow
to the heat capacity of the rotor ~2, Ha/Hr, should be
~9~74X
- 29 -
greater than about 0.02 and the ratio of the heat
capacity of the air to the heat capacity of the liguid,
Ha/HL should be greater than about 2, with the speed of
the rotor 42 generally less than ahout 0.75 rpm. At
lower rotor speeds khe ratio Ha~Hr should be increased
to a level in excess of a~out .14 with the rakio of
Ha/HL increased somewhat to at least greater than 2.5.
With a ratio Ha/Hr a~ove about .3 rnaximum gross
fuel savings can be obtai~ed by correspondingly assuring
that the ratio Ha/HL is sufficiently high, preferably
yreater than about 10.
For near maxlmum gross fuel saving operation, 'che
xatio Ha/Hr can be set greater than ~bout 0.43 with the
ratio Ha/HL somewhat lower, but still greater than about
ive (5). At higher values of Ha/Hr in excess of above
O.86 with Ha~HL greater than about 5, a maximum gross
fuel savings can be achieved. In summary, therefore,
the greater the maximum air flow, the higher the
resulting fuel saving though the maximum air flow cannot
be indefinitely increased lest the pressure drop across
the rotary air preheater 24 becomes too high.
The efficienc~r advantage of the invention can
further be illustrated with the following Table
normalized for a heat generator using one pound o
combustion air and assuming a mass of flue gas of 1.06
pounds for a number 6 type of fuel oil with 15% excess
air. It is ass~med that an air reheater 31 for plume
suppression is needed, thus reducing the fuel savings by
about .4% depending upon the water dew point temp~rature
in the flue gas.
Places in Fi ~ re 1 Temperature
At air inlet 80 70F
At outlet 82 297F
At boiler inlet 84 611F
At boiler outlet 20 650F
At reheat~r inlet 86 320F
At reheater outlet 88 300F
At preheater outlet 36 120F
At stack 24 110-140F depending
on water dew point
temperature
74~
30 -
The application of a wash liquid during operation
in accordance with the invention to the first rotary air
preheater 22 advantageously enables periodic cleaning of
materials that clog the preheater to thus reduce the
load on combustion air fan 28 and induced draft fan 30.
As a result, the intervals between heat generator down
times for the cleaning of preheater 22 can be
considerably lengthened. The advantage of such on-line
cleaning can be appreciated when one considers that
normal operation requires dropping a 300 megawatt
generator and a ccxresponding load every six weeks for
periods ranging from 8 to 24 hours to effect a cleaning
of the air preheaters. Cleaning often also uses
expensive high pressure steam for soot blowing. Since
dropping generator capacity affects peak reserve
capacity, electrical power may have to be purchased to
accommodate cleaning. A cleaning of air preheater 22
with a wash system as is used in air preheater 24
enables subs-tantial lengthening of intervals between the
times the air preheater is taken off-line for inspection
or other servicing.
Furthermore, in certain heat generators an
additional combustion air preheater that is located
ahead or upstream of the preheater 22 is used. Such
additional preAeater serves to assure a minim~m
temperature for the com~ustion air and thus avoid
condensation of corrosive constituents, such as H2SO4,
in the flue gas in the preheater 22 during cold ambient
air temperatures. The application of liquid to
preheater 22 will thus also allow a deletion of such
additional air preheater for an improvement in -the heat
generator thermal eficiency.
In conventional wet flue gas desulferization
systems problems are encountered at dry/wet interaces.
Such interfaces occur on surfaces inside ducts and the
- 31 -
like leading to or inside the scrubber and permit a
build-up of particulates to blockage levels. In
conventional wet scrubbers, therefore, a pre-quenching
of the flue gas is undertaken to create 2 ~ater-
saturated flue gas stream. The pre-quenching cools the
flue gas but adds a significant mass flow, in the form
of water vapor and droplets, to the flue gas. The
additional mass flow impcses additional power demands on
the fan 30.
An advantage ~ith an improved heat yenerator in
accordance with the invention is that flue gas can be
directly supplied to a wet flue gas desulferization
system without pre-quenching of the flue gas. For
example, with reference to Figure 12 the rotary heat
exchanger 24 is operated or selected so that the amount
of heat transferred from the flue gas is sufficient to
lower its temperature to a level where the flue gas is
water saturated at discharge end 36. This eliminates a
downstream dry/wet interface.
~0 The water saturated flue gas may then be directly
fed into the wet scrubber 180. This includes a suitable
spray system 182 to remove pollutants from the flue gas
which is then discharged to stack 26 by the action of
fan 30.
The water saturated flue gas uses the water inside
the gas stream itself and does not result in additional
loads on fan 30. Particulates build-up problems are
avoided since cooler duct surfaces will be wet and tend
to have water-running conditions. Standard corrosion
resistant materials used for typical scrubbers are used
on ducts guiding the saturated flue gas to the scru~ber
180. This protects against water aerosols in the
presence of CO2 and SO2 gases that combine to form
caxbonic acid and sulfurous acid.
- 32 -
Havi.ng thus described an illustrative embodiment in
accordance with the invention for improving the
efficiency of their thermal section for a power plant,
the advantages of the invention can be appreciated. The
invention can be advantageously used for different heat
yenerators such as those used in blast furnaces,
municipal waste burning plants, chemical processes and
the like. Vari.ations from the described embodiment can
be made such as in the selection of the washing liquid
without depaxting fxom t;he scope of the invention.
