U.S. patent application number 09/943302 was filed with the patent office on 2003-03-13 for minimizing corrosion and build-up in a flue-gas system including a desulfurizer.
Invention is credited to Kukin, Ira, Pepe, William Carmen.
Application Number | 20030049160 09/943302 |
Document ID | / |
Family ID | 25479403 |
Filed Date | 2003-03-13 |
United States Patent
Application |
20030049160 |
Kind Code |
A1 |
Kukin, Ira ; et al. |
March 13, 2003 |
Minimizing corrosion and build-up in a flue-gas system including a
desulfurizer
Abstract
A method for minimizing corrosion and the build-up of deposits
on surfaces of a flue-gas system exposed to moist substances and
elevated temperatures, and which includes a desulfurizer and heat
transfer means communicating with the desulfurizer, said method
involving adding to the system at the heat transfer means a readily
water soluble alkaline substance such as sodium hydroxide in
amounts sufficient to produce with said flue-gas in said heat
transfer means a minimum pH of about 5, preferably 7 or higher.
Inventors: |
Kukin, Ira; (West Orange,
NJ) ; Pepe, William Carmen; (Stanhope, NJ) |
Correspondence
Address: |
Harold James, Esq.
JAMES & FRANKLIN, LLP
Suite 2915
60 East 42nd Street
New York
NY
10165
US
|
Family ID: |
25479403 |
Appl. No.: |
09/943302 |
Filed: |
August 31, 2001 |
Current U.S.
Class: |
422/9 |
Current CPC
Class: |
B01D 53/501
20130101 |
Class at
Publication: |
422/9 |
International
Class: |
C23F 011/02 |
Claims
We claim:
1. In a flue-gas system in which the flue gases, at a temperature
above the boiling point of water, pass through a heat transfer
means on their way to a desulfurizer, a method of minimizing
corrosion and build-up by applying to said flue gases at the said
heat transfer means a readily water-soluble alkaline substance in
an amount sufficient to produce with said flue gases a pH of a
minimum of 5.
2. In a flue-gas system in which the flue gases, at a temperature
above the boiling point of water, pass through the hot side of a
heat transfer means having a cold side and a hot side on their way
to a desulfurizer and then pass from the desulfurizer through the
cold side of said heat transfer means, a method of minimizing
corrosion and build-up by applying to said flue gases at the said
hot side of said heat transfer means a readily water-soluble
alkaline substance in an amount sufficient to produce with said
flue gases a pH of a minimum of 5.
3. In a flue-gas system in which the flue gases, at a temperature
above the boiling point of water, pass through the hot side of a
heat transfer means having a cold side and a hot side on their way
to a desulfurizer and then pass from the desulfurizer through the
cold side of said heat transfer means, a method of minimizing
corrosion and build-up by applying to said flue gases at the said
hot side of said heat transfer means a readily water-soluble
alkaline substance in water solution in an amount sufficient to
produce with said flue gases a pH of a minimum of 5.
4. The method of any of claims 1-3, in which said alkaline
substance is a member of the group consisting of sodium hydroxide
and sodium carbonate.
5. The method of any of claims 1-3, in which said application to
said flue gases includes, in addition to said alkaline substance,
one or more of the following substances: water soluble sodium,
potassium, or cesium phosphates, silicates and borates and other
water soluble anionic salts of said elements having an initial pH
of a minimum of 5.
6. The method of any of claims 1-3 in which, as the flue-gas system
functions, the pH of the combination of flue gas and alkaline
substance is monitored and the rate of addition of said alkaline
substance to said flue gas is modified in accordance with that
monitoring in order to substantially maintain the desired pH value.
Description
[0001] The present invention minimizes corrosion and build-up in a
flue-gas system including a desulfurizer and heat transfer means
communicating with the desulfurizer where significant amounts of
moisture and/or sulfuric acid are present by adding to the flue-gas
while it is at a relatively high temperature a readily water
soluble alkaline substance such as sodium hydroxide in an amount
sufficient to produce with said flue-gases as they leave the heat
transfer means a pH of a minimum of about 5, preferably 7 or
higher.
BACKGROUND OF THE INVENTION
[0002] In most flue-gas systems, for safety and environmental
reasons, as a means of conserving heat, the flue-gas leaving the
furnace at relatively high temperatures is passed through a variety
of treatment devices before escaping into the atmosphere. Among
these devices are, usually in sequence, a boiler or heater, a
precipitator, a heat transfer device such as a gas/gas heater, and
a scrubber or desulfurizer, the flue-gas then returning to the
gas/gas heater on its way to the stack. The temperature of the
flue-gas decreases as the gas passes through the system, and in the
course of that temperature decrease moisture, originally in the
form of steam, and even containing sulfuric acid, comes into being
as a liquid which deposits on the surfaces of the system, and
particularly on the gas/gas heater surfaces, the sulfuric acid
content of that liquid producing corrosion and the solid content of
the flue-gas tending to deposit and build-up on exposed system
surfaces.
[0003] It has long been customary to add substances to the flue-gas
to minimize or prevent corrosion of the exposed surfaces of the
system. (My prior U.S. Pat. Nos. 4,842,617 of Jun. 27, 1989
entitled "Combustion Control By Addition of Magnesium Compounds of
Particular Particle Sizes", and 5,034,114 of Jul. 23, 1991 entitled
"Acid Neutralizing Composition Additive With Detergent Builder" are
representative of the use of such additives, as is a pending U.S.
application Serial No. 814,598 of Mar. 23, 2001 entitled "Use Of
Expanded Agents For Minimizing Corrosion And Build-Up Of Deposits
In Flue-Gas Systems", the invention of myself and William Carmen
Pepe.) The corrosive action of sulfuric acid on exposed surfaces of
the system is obviously undesirable and it is therefore common to
add such substances as limestone or magnesium oxide to the system
to neutralize the sulfuric acid. Because a solid/liquid reaction
rate is generally slow, relatively large amounts of such additives
must be provided. They are usually pneumatically injected into the
affected portion of the system through conduits, usually in the
form of pipes, using pressurized air as the vehicle to transport
the additives through the conduit to the injection location in the
system. The act of compressing air generates both heat and
moisture, and hence the pressurized air which does the conveying is
usually both moisture-laden and hot. Movement of the pressurized
additive through the conduits results in some condensation of the
moisture on the conduit surface and this enhances the tendency of
the solid additives to stick to and build-up on those surfaces. As
a result it is periodically necessary to take the injection
equipment off line for cleaning, a process which is itself costly
and time consuming, and while the injection equipment is off line
no anti-corrosion additive is fed to the system, thus increasing
the likelihood of corrosion.
[0004] When the system is provided with a scrubber or desulfurizer
the flue-gas emanating from the scrubber has a comparatively high
moisture content and a comparatively low temperature, thus leading
to the condensation of comparatively large volumes of moisture,
significantly including sulfuric acid in its liquid form because
its temperature is below its dew point. When, as is usually the
case, the output from the scrubber is fed back to the gas/gas
heater the moisture content of the flue-gas becomes a significant
corrosion-producing factor.
[0005] Previous attempts to solve this problem, as for example the
injection of lime, limestone, magnesium oxide, magnesium hydroxide,
or other basifying agents that are insoluble in water, have two
major drawbacks. Because these products are insoluble in water,
they are most unreactive or very slowly reactive, particularly
where the neutralization of the acidity takes place in the steam
which exists because of the high temperatures involved, and as a
result excess amounts of basifying agent relative to the amount of
SO.sub.3 that has to be neutralized are required, and the use of
such a large excess of these inefficient basifying agents results
in clogging and deposit build-up on the gas/gas heater inlet
plates, rendering the gas/gas heaters inoperative.
SUMMARY OF THE INVENTION
[0006] We have discovered that corrosion of the system surfaces,
and particularly the inlet surfaces of a scrubber and gas/gas
heater used in conjunction therewith, can be significantly reduced,
and the build-up on those surfaces of residue particularly from the
substances described above, which have been added to the system for
various purposes, can likewise be significantly reduced or even
substantially eliminated, by not using such substances at all, but
rather by applying to the flue-gas, while it is in the heat
transfer means upstream of said scrubber, where its temperature is
well above the boiling point of water, an alkaline substance which
is readily water-soluble in an amount sufficient to bring the
flue-gas-additive combination as it leaves the heat transfer means
to a minimum pH 5, preferably 7 or higher. In particular, the
alkaline material, preferably in the form of a water solution, is
added to the flue-gas while the moisture content of the flue-gas is
in the form of steam. Significantly, the amount of such alkaline
substance which must be added is in general that amount required to
neutralize the acid content of the flue-gas. This in itself is a
significant factor, since the prior art additives, generally
insoluble or difficultly soluble, must be added in amounts
substantially greater than the stoichiometric amounts in order to
obtain the desired neutralization but by the same token increasing
the build-up which is so undesirable. With the use of this
invention the SO.sub.3 acid dew point of the vapor stream from the
heat transfer means is reduced from above 300.degree. F. to
250.degree. F. or even lower which minimizes condensation of
sulfuric acid in the gas/gas heaters, even at the high humidities
present at the heater plates. The actual untreated SO.sub.3 dew
point is dependent upon the amount of SO.sub.3 present in the
flue-gases. A dew point of 300.degree. F. represents a
concentration of 61 parts per million of sulfuric acid while an
acid dew point of 250.degree. F. represents a concentration of only
2 parts per million.
[0007] The amount of basifying agent required can be determined by
measuring the acid dew point of the gases in or leaving the gas/gas
heater, which can be done manually, or preferably with a
continuous, on-line, automatic acid dew point meter, and the using
of the latter to regulate the feed rate of the aqueous basifying
agent.
BRIEF DESCRIPTION OF THE DRAWING
[0008] FIG. 1 discloses diagrammatically a typical flue-gas system
in which the method of the present invention is particularly
useful.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0009] A typical flue-gas system such as is shown in FIG. 1
comprises a furnace or boiler 2 where steam is generated. Ambient
air enters the system at 4 and passes through a primary air heater
6 in which it is heated to perhaps 150.degree. F. and it then
enters the furnace 2 to combine with fuel for combustion purposes.
A waste product from the combustion in the furnace 2 is the
flue-gas which exits the furnace at 8 at a temperature of perhaps
800.degree. F. The flue-gas passes through the air heater 6,
providing the means for the initial heating of the ambient air, and
the flue-gas which leaves the air heater 6, at 10, will have lost a
great deal of its heat and be at a temperature of about 350.degree.
F. to 400.degree. F. It then passes into an electrostatic
precipitator 12 in which certain impurities are removed, and it
escapes from the precipitator 12 at 14 at a further reduced
temperature of about 275.degree. to 300.degree. F. Because of its
reduced temperature the flue-gas may now have a significant
moisture content of perhaps 5-15%. The flue-gas goes into the upper
portion 16A of a heat transfer means, here shown as a gas/gas
heater 16, from which it escapes to point 18 at a temperature of
less than 250.degree. F. and it then passes through a desulfurizer
or scrubber 20 which it leaves at point 22 at a temperature of
perhaps 100.degree. F.-150.degree. F. and with a moisture content
of perhaps 40-50%. The gas is then fed back through the lower
portion 16B of the gas/gas heater 16 and escapes through the stack
at 24.
[0010] The gas/gas heater 16 has structural parts which move from
the upper hot portion of 16A to the lower or relatively cool
portion of 16B and back again. It will be apparent that exposed
surfaces of the gas/gas heater 16, and particularly those surfaces
thereof which at any given moment are in the lower portion 16B of
the heater, are very susceptible to acid corrosion because of the
high moisture content and low temperatures to which they are
subjected, so that the metal temperature is below the acid dew
point. From the point of view of minimizing corrosion in the
gas/gas heater 16 it is at the area 14 immediately up-stream of the
gas/gas heater 16 where the usual corrosion-minimizing additives
are injected into the system, as indicated by the arrow 26.
[0011] The susceptibility of the gas/gas heater 16 to corrosion can
perhaps be best appreciated by considering that scrubber 20 more
easily and effectively absorbs impurities from the flue-gas when
the flue-gas is at or below its dew point, and when the flue-gas
exits the scrubber 20 its temperature is below the dew point to an
even greater degree, thereby increasing its moisture content and
making corrosion more likely. Also, because structural parts of the
gas/gas heater 16 move sequentially through the upper and lower
portions 16A and 16B thereof, they are constantly subjected to
variations in temperature, and the constant heating and cooling of
the structural parts of the gas/gas heater 16, coupled with the
resultant high moisture content of the flue-gas as that passes
through the heater, produces a situation ideal for corrosion and
for deposit build-up.
[0012] Also, with the previous usage of lime, limestone or
magnesium, additive builds up in the conduit feeding those
additives to the system. The additives are preferably injected into
the system between the precipitator 12 and the gas/gas heater 16,
as indicated by the arrow 26, so that they can perform their
desired action where that action is most needed, to wit, in the
gas/gas heater 16.
[0013] The conventional, previously used, anti-corrosion additives,
such as calcium oxide, calcium hydroxide, calcium carbonate,
dolomite, dolomitic lime, lime, calcium hydrate, limestone,
magnesium oxide, magnesium hydroxide, magnesium carbonate, as well
as combinations thereof such as calcium/magnesium oxides and
hydroxides, are relatively ineffective because of the relative
slowness of the reaction between these basifying additives and the
sulfur trioxide that they are designed to neutralize, and those
additives must be provided in relatively large quantities, well in
excess of the stoichiometric amount required to neutralize the
acidic constituents. As a result the problem involved in preventing
build-up in the conduits through which those basifying agents are
fed is intensified, and the inevitable deposit build-up on the
gas/gas heaters results in a rapid shut down of the scrubber.
[0014] According to the present invention the build-up problem,
both in the additive conduit and on the perforated gas/gas heater
revolving plates, is eliminated and the corrosion problem,
particularly in the gas/gas heater 16, is minimized or virtually
eliminated with the use of the water-soluble basifying agents
described herein added to the system as shown.
[0015] In the embodiment of the present invention here specifically
disclosed the system comprises a desulfurizer or scrubber 20
functioning in combination with a heat transfer means here
specifically shown as a gas/gas heater 16. The upper portion 16A of
the gas/gas heater 16 is at a higher temperature than its lower
portion 16B. In particular, the temperature in the upper portion
16A is well above the boiling point of water. Hence the water
content of the flue-gas is in the form of steam. It is when the
flue-gas enters the lower portion 16B of the gas/gas heater 16
that, because of the lower temperature in the region, the sulfuric
acid content of the flue-gas is at or below its dew point, as a
result of which corrosive sulfuric acid, if present, tends to form
on the exposed surfaces of the gas/gas heater 16.
[0016] In accordance with the present invention, we take advantage
of the high temperature of the flue-gas at the upper portion 16A of
the gas/gas heater 16 to subject the flue-gas at that relatively
high temperature to the action of a readily water soluble alkaline
substance, preferably in the form of a water solution. Under those
conditions the alkaline substance reacts substantially immediately
and completely with the acid content of the flue-gas, neutralizing
that acid content so that, when the flue-gas exits from the
scrubber 20 and returns to the lower section 16B of the gas/gas
heater 16 in a condition below its dew point, there will be little
or no acid content available to produce corrosion. Thus by adding
the alkaline substance in an area where, because of the high
temperature, the liquid content from the flue-gas is in the form of
vapor, the sought-for neutralization action is effectively
achieved. Best results are obtained if the pH of the flue-gas after
it leaves gas/gas heater 16 is at a minimum of 5, and preferably 7
or greater, and the optimum amount of alkaline substance added is
that which produces the specified pH at that point in the system.
This can, of course, be readily monitored by available sensors and
control equipment.
[0017] Preferred alkaline substances for use in connection with the
present invention are sodium hydroxide and sodium carbonate
primarily because they are low cost materials, and the fact that
the corrosion and build-up inhibition can be achieved with
stoichiometric amounts of such low cost material is an exceedingly
important advantage of the present invention. However, other
substances having the same characteristics of alkalinity and ready
water solubility could be employed. When certain chemicals often
used in water treating systems are co-added with sodium hydroxide
or sodium carbonate, this enhanced corrosion protection, and
particularly helped to prevent clogging of the gas/gas heater
openings even beyond that which was achieved with the use of either
sodium hydroxide or sodium carbonate by itself. Particularly
effective as co-additives were trisodium polyphosphate, trisodium
phosphate, disodium monohydrogen phosphate, sodium borate, sodium
di and polyborates, sodium silicates and sodium polysilicates, and
in general water soluble sodium salts of the various phosphates,
silicates and borates.
[0018] The effectiveness of the use of these basifying products
having a high initial pH in minimizing corrosion or deposit
build-up is shown by the following laboratory demonstration. In
each of the following samples a mixture of 30 cc of water, 3 cc of
6 Normal sulfuric acid and 2 cc of the additive as described in
Table I was observed after incubation at 130.degree. C. for one
hour with the results as set forth in Table I.
1TABLE I Basifying Agents to Prevent Corrosion of Gas/Gas Heaters
Connected to a Desulfurizer Results After Incubating at 130.degree.
for 1.5 Hours with Steel Metal Strip Exposed to Condensate from
Sample Composition the Mixture of Dilute No. of Additive Sulfuric
Acid and Additive 1 No additive Heavy corrosion, 1/8" of a de-
posited, brown, layer on bottom of metal specimen. 2 14 cc of 1.2 N
No corrosion, very slight, caustic solution scattered, brownish
spots; trace of brown deposits on bot- tom 1/8" of metal specimen.
3 18 cc of 1.2 N No corrosion; deposit-free met- caustic solution
al specimen, trace spotting on bottom 1/8" of metal specimen.
[0019] Without the basifying additive, rapid corrosion of the steel
metal strips takes place. In each case, the additive increased the
pH of the solution from below 3 to over 8.
[0020] In another series of experiments as shown in Table II, use
of combining basifying chemicals with deposit modifying, or
anti-corrosion, enhancing chemicals show a further improvement when
this combination is used.
2TABLE II Use of Combined Basifying Chemicals and Deposit-Modifying
or Anti-Corrosion, or Enhancing Chemicals Are Used In Combination
Results After Incubating at 130.degree. for 1.5 Hours with Steel
Metal Strip Exposed to Condensate from Sample Composition the
Mixture of Dilute No. of Additive Sulfuric Acid and Additive 1A
None Metal completely covered with brown, rust-like, stain. Very
heavy brownish/black coating on bottom 1/4" of metal specimen. 1B
18 cc of 1.2 N Essentially clear, very slight, caustic solution
scattered, brown spots, trace deposit on bottom 1/8" of metal
specimen. 1C 15 cc of 1.2 N Totally clear, deposit-free, caustic
solution, metal strip, no trace of de- plus 0.70 cc of a posit on
bottom 1/8" of metal 3% solution of specimen. trisodium phosphate
1D 15 cc of 1.2 N Totally clear, deposit-free, caustic solution,
metal strip throughout. plus 0.70 cc of a 3% solution of sodium
tripolyphosphate
[0021] The total amount of additives required is based on the flow
rates of the flue-gas itself and the recirculating water solution
from the scrubber 20, as well as the acidity present in the system.
Basically, the feed rate of additive is determined primarily by the
acidity of the stream and that amount of basifying agent, or
basifying agent with modifier, that decrease the acid dew point of
the stream as it leaves the gas/gas heater 16 from plus 300.degree.
F. to 250.degree. F. or less.
[0022] With a boiler of 200 megawatts, an SO.sub.2 content of 6000
mg/Nm, and sulfuric acid content at the gas/gas heater of 30 ppm
(112 mg/m.sup.3) and with a treatment rate of 600 ppm of a 5%
solution of caustic, the following results were obtained. The
acidity was reduced to 5.0 mg/Nm, or less than 2 ppm or 7.5
mg/m.sup.3. With the additive combination as shown in Example 1C of
Table II, a treatment rate of 600 ppm reduced the acidity to 2.1
parts per million, or 7.8 mg/m.sup.3 without any corrosion or
traces of deposits on the gas/gas heater plates.
[0023] The most cost effective treatment rates may vary from boiler
to boiler and will depend upon the megawatts of the boiler, the
temperature at the inlet and outlet of the gas/gas heater, the
acidity of the return flow rate from the scrubber to the gas/gas
heater, the design of the gas/gas heater and the amount of sulfur
dioxide and sulfuric acid present and the amount of moisture in the
condensate. Measurement of the acid dew point of the stream
immediately prior to or after the gas/gas heater may be made by
employing an on-line automatic acid dew point test apparatus, such
as a Land Continuous Dewpoint Acid Monitor, which in turn can be
connected to an automatic feed adjusting system that monitors the
inlet feed rate of the aqueous basifying agents. The controlling
feed rate is that amount of additive that increases the acid dew
point of the circulating acid solution from the scrubber to below
250.degree. F.
[0024] While but a single preferred embodiment is here specifically
disclosed, it will be apparent that many variations may be made in
the details of the method here disclosed, all within the scope of
the instant invention as defined in the following claims.
* * * * *