PROCESS FOR CORROSION CONTROL IN BOILERS
A corrosion control process is described. The process is especially useful in the control of chloride corrosion in waste to energy boilers. Corrosion of high temperature surfaces can be assessed by the monitor and controlled introduction of treatment chemicals by targeted in furnace injection reduces corrosion while maximizing combustion efficiency. A corrosion monitor is also described. Before and following selection of corrosion control chemicals and the locations for targeted in furnace injection, injection regimen and chemical selection and introduction parameters are monitored with the aid of the method and apparatus of the invention to adjust one or more control parameters to reduce corrosion. A preferred method will employ a treatment chemical that comprises an SO2 or SO3 reagent, e.g., sulfuric acid, sulfur, a sulfate salt or a bisulfite salt.
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This application is related to and claims priority to prior U.S. Provisional Patent Application No. 60/681,786 filed May 17, 2005, the disclosure of which is incorporated herein by reference in its entirety.
BACKGROUND OF THE INVENTIONThe invention relates to a corrosion control process, which is especially useful in the control of chloride corrosion in boilers, particularly waste to energy boilers.
Over several recent years the literature has extensively reported that chloride induced corrosion of high temperature surfaces in waste to energy (WTE) boilers is one of the most costly problems in the industry. This problem can result in replacement of superheater pendants as often as annually in some units or the costly use of higher alloyed materials to either shield the metal surfaces or serve as replacement tube material.
The cost-effectiveness of the replacement alloys has not been proven in many cases, and the industry has been looking for alternative solutions. There is a need for chemical solutions to the problem of corrosion in boilers of all types and especially in the high temperature flue gas near WTE superheater pendants.
The problem is not limited to WTE boilers. In U.S. Pat. No. 6,478,948, Breen, et al., indicate that until recently, furnace boiler tubes corroded slowly and had a service life of 20 to 30 years, but the introduction of low NOx burners has increased the rate of boiler tube corrosion and can reduce their life expectancy to only 1 to 2 years. Breen, et al. point out that the corrosion of furnace wall tubes involves several mechanisms. First, the say that the removal of the protective oxide film allows further oxidation. Second, they say that if the oxide film is not present, the iron surface is attacked and pitted by condensed phase chlorides which may be present. They also point to a third mechanism which occurs when wet slag runs across the surface of the film. As that happens, iron from the tube goes into the slag solution which contains low fusion calcium-iron-silicate eutectics that are formed in the liquid slag under reducing conditions in the furnace. They state that reduced sulfur in the form of S, H2S, FeS or FeS2 can react with the oxygen of the tube scale depriving the tube metal of its protective layer.
While the problems of boiler corrosion are well documented and there is a growing understanding of the causes, the available solutions to these problems are not as easily facilitated or economical as would be desired. In a 2004 paper delivered at NAWTEC, Ken Robbins of Maine Recovery Company detailed attempts to use shielding, alternate metallurgies, and various soot blowing strategies to mitigate corrosion found in a WTE unit. The paper also discussed a proprietary chemical slag control program, which was found helpful in controlling slag and minimizing cleaning outages, but had no discernable effect on specific localized corrosion problems. In the case of isolated corrosion, especially on superheater pendant surfaces, which can experience corrosion rates ranging from 0.020 to 0.050 inches per month, tube failures can occur in as little as seven months and create a need for replacement of the entire pendant annually. A photo of a tube removed due to a failure is shown in
A TNO (Nederlandse Organisatie Toegepast—Voor Naturwetenschappelijk) report entitled “Review on Corrosion in Waste Incinerators and Possible Effect of Bromine” provides a mechanistic explanation for the severe corrosion suffered by WTE units. See Ir. P. Rademakers (TNO IND), Ing. W. Hesseling (TNO-MEP), Ir. J. van de Wetering (Akzo Nobel AMC) (July 2002). In addition to the overall analysis of the primary chemical components involved in this corrosion mechanism, it provides a series of equations that may explain why chloride corrosion occurs at the temperature and metallurgical conditions of a waste incinerator.
It is well known that corrosion by high CO levels and reducing atmospheres occurs in the first pass above the grate in-furnace. A refractory lining is often employed on the water walls in the first pass. A strong temperature gradient and condensing substances can also contribute to reducing conditions in these areas. Alkali metal chlorides have been found in deposits near the metal surface, and the high level of chlorides in the waste are strongly implicated with the problem.
Rademakers, et al., explain that high temperature corrosion in waste incinerators is caused by chlorine either in the form of HCl, Cl2, or combined with Na, K, Zn, Pb, Sn and other elements. Both gaseous HCl with and without a reducing atmosphere and molten chlorides within the deposit, are considered major factors. As with Breen, et al, they point out that sulfur compounds can be corrosive compounds under some circumstances and can influence the corrosion by chlorine.
Rademakers, et al., identify several factors as the most important in high temperature corrosion: the metal temperature and the temperature difference between gas and metal, the flue gas composition, deposits formation and reducing conditions, and the ratio of SO2/HCl. They indicate that following mechanisms can be distinguished:
-
- Corrosion by HCl/Cl2 or SO2/SO3 containing gas under oxidizing or oxidizing/reducing conditions, and
- Corrosion by solid or molten deposits of metal chlorides and sulfates.
Rademakers, et al., describe these mechanisms and refer to a schematic, inFIG. 3 , as drawn from Krause, 1986, 1993, and as set out below in various steps.
Corrosion caused by chlorine-containing gas at metal temperatures above about 450° C. is referred to as ‘active oxidation’. Alkali chlorides, such as NaCl, CaCl2 and KCl, can be present already or can be formed by the combustion and subsequent reaction of alkali oxides:
Na2O+2HCl=2NaCl+H2O [1]
Under ideal conditions (good mixing, sufficient residence time) alkali chlorides can be sulfated according to the following reaction, provided there is enough SO2 and O2:
2NaCl+SO2+½ O2+H2O═Na2SO4+2HCl [2]
This would result in formation of sulfates and volatile HCl. At the relatively low tube wall temperatures of most waste incinerators, the sulfates are not very harmful and the HCl formed will be transported to the flue gas clean up system. However, if the gas reaches the cooler tube walls before the reaction is completed, the alkali metals will tend to condense on the cooler metal. In this case, further sulfate formation can occur on the metal under the release of HCl, and that causes high chlorine partial pressures and enhanced corrosion.
Without SO2 at 500° C., NaCl and iron oxides can form Cl2:
2NaCl+Fe2O3½ O2═Na2Fe2O4+Cl2 [3]
6NaCl+2Fe3O4+2 O2=3Na2Fe2O4+3Cl2 [4]
Calculations of the dissociation constant of HCl as a function of temperature indicate that chlorine is present as Cl2 under oxidizing conditions up to gas temperatures of 600° C., whereas above 600° C. formation of HCl is enhanced in the presence of water vapor according to the reaction:
H2O+Cl2=2HCl+½ O2 [5]
Rademakers, et al., state that at about 500° C., Cl2 can penetrate pores or cracks in an oxide layer. At the low oxygen partial pressures that exist near the metal-oxide scale boundary, the metal chlorides are the more stable phase. Reactions 3 and 4 can result in a Cl2 partial pressure sufficiently high that it reacts directly with the steel to form FeCl2:
Fe+Cl2═FeCl2 (solid) [6]
The vapor pressures of metal chlorides will depend primarily on the temperature and the HCl content of the gas. In addition, the type of oxide (and alloy) can considerably influence the vapor pressure. The vapor pressure of FeCl2 is already relatively high at low temperatures. As a result, formation of FeCl2 can decrease the adherence of the oxide scale or can cause spallation of the oxide layer.
Rademakers, et al., explain that iron chlorides form and migrate out from the corrosion product due to their volatility. At higher oxygen partial pressures near the oxide-gas interface, these chlorides are then converted to oxides and liberate chlorine. These new oxides are not formed as a perfect layer and do not offer protection. Part of the liberated chlorine migrates back through the oxide/deposit to react with the metal at the oxide-metal interface, and form metal chlorides again:
FeCl2 (solid)=FeCl2 (gas) [7]
4FeCl2+3O2═Fe2O3+2Cl2 [8]
3FeCl2+2O2═Fe3O4+3Cl2 [9]
In this process, the chlorine has a catalytic effect on the oxidation of the metal resulting in enhanced corrosion.
The kinetics of active oxidation is mainly determined by the evaporation and outward diffusion of FeCl2. Similar chlorine corrosion and regeneration cycles may proceed via FeCl3 and it is possible for the ferrous iron to be oxidized to the ferric state, which liberates chlorine when oxidized.
4FeCl2+4HCl+O2=4FeCl3+2H2O [10]
4FcCl3+3O2=2Fc2O3+4Cl2 [11]
The volatility of different compounds can be compared based on the temperature T4 (temperature at which the vapor pressure reaches 10−4 bar), and vapor pressure values for some compounds are given in Table 1.
From the above, Rademakers, et al., conclude that low alloy steels and iron-base alloys have limited resistance against active oxidation. High alloyed materials, nickel base alloys in particular, have a much better resistance, which may be because chlorides are more difficult to form and, once formed, have a relatively low volatility. Except for the FeCl3, most T4 temperatures are well above 500° C. indicating that this mechanism is most relevant to superheaters and less to evaporators.
Corrosion of heat transfer surfaces in boilers has been a major problem, particularly WTE units which generate highly corrosive flue gases, and continues to trouble the industry.
There remains a present challenge to provide a process for taking necessary corrective action to address the corrosion boilers, particularly in WTE units, before damage becomes excessive and requires expensive shut down and repair.
SUMMARY OF THE INVENTIONIt is an object of the invention to provide a method for controlling corrosion of the high temperature surfaces of a boiler, particularly a waste to energy boiler under operating load.
It is another object of the invention to provide a method for reducing corrosion of the high temperature surfaces of a boiler, particularly a waste to energy boiler by the introduction of an inexpensive chemical treatment agent that can modify the corrosion process itself.
These and other objects of the invention are achieved by the invention, which provides a method for corrosion control in a boiler, particularly a waste to energy boiler which involves the introduction of treatment chemicals for the purpose of modulating or preventing the problems of high temperature corrosion of metal surfaces. The invention also provides a new constant temperature probe useful in such a process.
In one aspect the invention will comprise: monitoring the degree of corrosion in a boiler with a corrosion monitor and utilizing the information on corrosion to control introduction of corrosion control chemicals into the boiler. The effect of the chemical on corrosion is monitored and adjusted to provide effective corrosion control. The original placement of the monitor and/or its probes or electrodes and the original introduction parameters for the introduction of corrosion control chemicals are preferably evaluated through the use of computational fluid dynamics.
A preferred corrosion control process of the invention will comprise: disposing an apparatus comprising a constant temperature probe having a corrodible surface in a known position in a boiler; periodically removing the probe for visual and/or physical observation; based on the observations of the probe and comparison with data for boiler components such as tubes, calibrating the degree of corrosion on the probe with what could be expected of the boiler components; and utilizing observations of the probe as calibrated to control introduction of corrosion control chemicals into the boiler.
In the case where chloride has been identified as a cause of the problem, the preferred chemical treatment provided by the invention is to introduce SO3 or a precursor of it into the corrosive atmosphere in a manner as to most directly attack the problem, preferably in a targeted fashion. This process applies a source of sulfur directly to the flue gas stream in the location most ideal to drive the reaction toward the sulfate salts. This aspect of the invention provides for the addition of a sulfur compound capable of releasing SO2 or SO3, preferably in the form of a sulfate salt, bisulfite salt, sulfur or sulfuric acid, e.g., H2SO4, in amounts sufficient to interfere with the chloride chemistry as outlined above and help maintain the chloride in gaseous form.
In one preferred aspect, the method involves subjecting the corrodible surface of said probe to UT measurements, e.g., such as weekly; and then after a predetermined period of time, e.g., 25-30 days, obtaining a metallographic analysis and physical measurement of metal thickness remaining. A preferred apparatus for use in this process will comprise: a probe capable of fixing to a boiler exterior and extending into a boiler to an extent necessary to reach a suspected trouble point for corrosion; a source of cooling fluid and means for directing the fluid into the probe for cooling the probe; and temperature sensing means associated with the probe and control means for controlling supply of the fluid to the probe. The probe is adapted to permit insertion and withdrawal from the boiler for visual and physical observation.
One type of apparatus useful in the process of the invention comprises: a probe capable of fixing to a boiler exterior and extending into a boiler to an extent necessary to reach a suspected trouble point for corrosion; a source of cooling fluid and means for directing the fluid into the probe for cooling the probe; temperature sensing means associated with the probe; and control means for controlling supply of the fluid to the probe.
A preferred corrosion monitoring process of the invention will comprise: disposing an apparatus as just described in a predetermined operable position in a boiler; periodically removing the probe for visual and/or physical observation; based on the observations of the probe and comparison with data for boiler components such as tubes, calibrating the degree of corrosion on the probe with what could be expected of the boiler components; and utilizing observations of the probe as calibrated to determine the degree of corrosion of metal surfaces near the predetermined location of the probe to determine when boiler shutdown should be effected for repair and/or cleaning.
BRIEF DESCRIPTION OF THE DRAWINGSThe invention will be better understood and its advantages will become more apparent from the following detailed description, especially when taken with the accompanying drawings, wherein:
The invention provides processes for monitoring corrosion and for controlling corrosion, which are described below with reference to exemplary embodiments. Because the monitor and the processes are especially useful in the control of chloride corrosion in waste to energy boilers, they will be described in this context while those skilled in the art will see the application of the invention to other environments.
As discussed above, the problem of high temperature corrosion within waste to energy boilers by chlorides is one of the most costly in the industry. The invention enables assessment and control of corrosion of high temperature surfaces and preferably involves the controlled introduction of treatment chemicals by targeted in furnace injection to reduce corrosion while maximizing combustion efficiency. To illustrate the problem of chloride caused corrosion, applicants provide
The chemistry of the corrosion is explained above with reference to
Reference is now made to
Another type of corrosion monitor measures electrochemical noise occurring at the surface of the tubes while that surface is exposed to combustion products. U.S. Pat. No. 6,478,948 to Breen, et al., describes such a probe and discusses how it is employed for measuring electrochemical noise and then limiting corrosion by adjusting the ratio of fuel to oxygen—because their premise is that low NOx conditions can result in corrosion. We have determined that such a probe can be useful in corrosion control without altering NOx control measures. The details of the corrosion monitor of Breen, et al., are incorporated herein by reference. The probe is connected to a corrosion monitor having a computer and software which determines a corrosion rate from the measured electrochemical noise. That rate is compared to a standard to determine if the rate is within acceptable limits. If not, the rate and/or location of chemical addition can be changed. As with the probe 10 of the invention, the probe of Breen, et al., can be jacket to control its temperature, preferably to be of constant temperature.
In one preferred mode of operation, the probe 10 shown in
The probe 10 is shown to include three thermocouples. Thermocouple 18 is for the purpose of sensing the temperature of the outer tube 16, and is preferably welded to it. A second thermocouple 20 for the purpose of monitoring the temperature of the tip 21 of the outer tube 16 of the probe 10, e.g., for assuring that the tip 21 does not overheat, is positioned a suitable distance, e.g., 1-3 feet from the end of the probe 10. Also important and provided by the invention is a third thermocouple 22, which extends through the wall of the outer tube 16 of the probe 10 at a predetermined location to sense the temperature of the combustion gases in the boiler. The exact number, type and location of the thermocouples will be a matter of design for each individual unit, and the thermocouples can, where feasible, be replaced with other effective temperature sensing means and techniques.
As noted, the probe 10 is maintained at constant temperature. This can be achieved by providing cooling air from controllable source 14, and passing it to the interior of the probe 10 by means of a suitable conduit, e.g., flexible braded hose 24 and suitable couplings, and permitting it to exit the open end of the tube 16. Control valve 26 takes control signals from a controller 12 and supplies air as necessary to maintain the temperature of the probe 10 within a predetermined temperature range. In operation, thermocouples 18 and 20 sense the temperatures at their respective locations and send sensed signals representative of temperature to the controller 12 via suitable lines 28 (or by wireless means not shown). The controller 12 compares the sensed signals to reference values and sends a control signal to the control valve 26 in response to the comparison. Responsive to the control signals, the control valve 26 can adjust the feed of cooling air to provide the amount as needed to the probe 10 to maintain its desired temperature.
The valve 26 can be of any type suitable to provide the control desired, and is preferably an air flow control valve with a digital positioner. The outer tube 16 of probe 10 is attached to probe housing 30, preferably by means, e.g., threaded engagement, which permits easy removal for testing and replacement. The housing 30 also has a fitting 32 to permit ease of connection to air hose 24, preferably for quick coupling and removal. The housing 30 also includes a threaded fitting or other connection means to permit secure attachment to a support mechanism located on the boiler wall (not shown).
The apparatus of the invention as shown in
According to the invention, sulfur bearing materials in a water based mixture are targeted for injection to the trouble spots in the boiler and as close to the flame as practical in a form designed to maximize the conversion of the chloride salts to their sulfate forms. The primary chemical reaction is believed to be:
2XCl+SO3+H2O+→X2SO4+2HCl
-
- Where X can be a suitable metal anion, e.g., an alkali metal such as sodium, potassium or the like.
It is believed that without SO3 at 500° C., NaCl and iron oxides present in deposits can form Cl2 as discussed above. It is believed important to reduce the presence of Cl2 near the metal tubes at temperatures above about 500° C., and the introduction of an SO3 reagent is a preferred manner of corrosion control according to the invention. The preferred SO3 reagent for application to deposits as they form is a sulfate salt (e.g., ZnSO4) a bisulfite salt, e.g., sodium bisulfite, sulfur and/or sulfuric acid, or in-situ or on-line generated SO3 from a combustion of sulfur, at any concentration suitable for use. It is possible to use concentrated solutions of sodium bisulfite, ZnSO4 and/or sulfuric acid, but the reagent is generally diluted with sufficient water to permit application to the desired area in the boiler. Typically, concentrations of from about 10 to 90 weight percent will be employed at a molar ratio sulfur to chlorine in the combustion gases of from 1:3 to 3:1. Ratios near stoichiometric based on chlorine are preferred.
- Where X can be a suitable metal anion, e.g., an alkali metal such as sodium, potassium or the like.
Before and following selection of corrosion control chemicals and the locations for targeted in furnace injection, injection regimen and chemical selection and introduction parameters are monitored with the aid of the method and apparatus of the invention to adjust one or more control parameters to reduce corrosion. The processing and chemicals can be of the type described in U.S. Pat. Nos. 5,740,745 and 5,894,806 and U.S. patent application Ser. No. 10/754072, all to Smyrniotis, et al., which are incorporated herein by reference, to reduce the problems with chloride corrosion in waste to energy boilers.
The above description is for the purpose of teaching the person of ordinary skill in the art how to practice the invention. It is not intended to detail all of those obvious modifications and variations, which will become apparent to the skilled worker upon reading the description. It is intended, however, that all such obvious modifications and variations be included within the scope of the invention which is defined by the following claims. The claims are meant to cover the claimed components and steps in any sequence which is effective to meet the objectives there intended, unless the context specifically indicates the contrary.
Claims
1. A process for corrosion control in a boiler which comprises: monitoring corrosion within a boiler having a corrosive atmosphere and introducing treatment chemicals containing SO2 or SO3 or precursors therefor at positions selected in response to monitored corrosion, to thereby modulate or prevent the problems of high temperature corrosion of metal surfaces.
2. A process according to claim 1, wherein: a constant temperature probe having a corrodible surface is positioned in a known position in a boiler; the probe is periodically removing for visual and/or physical observation; based on the observations of the probe and comparison with data for boiler components such as tubes, the degree of corrosion on the probe is calibrated with what could be expected of the boiler components; and then the observations of the probe as calibrated are utilized to control introduction of corrosion control chemicals into the boiler.
3. A process according to claim 1, wherein: the degree of corrosion in a boiler is monitored utilizing a probe and signal processor for observing electrochemical noise in a probe within the furnace and comparing that information to known values to provide a signal that corresponds to corrosion rate.
4. A process according to claim 3, wherein: the probe is a constant temperature probe.
5. A process according to claim 1 wherein the boiler is a waste to energy boiler.
6. A process according to claim 1, wherein: probes for a corrosion monitor are placed in original positions and the original introduction parameters for the introduction of corrosion control chemicals are based on computational fluid dynamic determinations.
7. A process according to claim 1, wherein: a chemical composition comprising an SO2 or SO3 or precursor of either is introduced into the corrosive atmosphere in a manner effective to maintain chlorine gas in gaseous form and the sulfate salts of the metal being corroded in solid form.
8. A process according to claim 7, wherein: the chemical composition comprising SO2 or SO3 or precursor of either comprises a sulfate salt, bisulfite salt, sulfuric acid, or sulfur in amounts sufficient to reduce corrosion.
9. A process according to claim 8, wherein: the corrodible surface of said probe is subjected to periodic UT measurements and to metallographic analysis and physical measurement of metal thickness remaining.
10. A process according to claim 1, wherein: corrosion is monitored by use of a probe capable of fixing to a boiler exterior and extending into a boiler to an extent necessary to reach a suspected trouble point for corrosion; a source of cooling fluid and means for directing the fluid into the probe for cooling the probe; and temperature sensing means associated with the probe and control means for controlling supply of the fluid to the probe; wherein the probe is adapted to permit insertion and withdrawal from the boiler for visual and physical observation.
11. A method for monitoring corrosion on a real time basis and under operational conditions for boilers, where high temperature corrosion is a problem, comprising:
- disposing an apparatus comprising a cooled, probe having a corrodible portion with means to sense the temperature of the probe and the surroundings in a predetermined operable position in a boiler;
- periodically removing the probe for visual and/or physical observation;
- based on the observations of the probe and comparison with data for boiler components such as tubes, calibrating the degree of corrosion on the probe with what could be expected of the boiler components; and
- utilizing observations of the probe as calibrated to determine the degree of corrosion of metal surfaces near the predetermined location of the probe.
12. A method according to claim 11 wherein the boiler is a waste to energy boiler.
13. A method according to claim 11 wherein corrosion observed by use of the probe are used to set or adjust introduction of treatment chemicals for the purpose of modulating or preventing the problems of high temperature corrosion of metal surfaces.
14. A method according to claim 11 wherein the treatment chemical comprises SO2 or SO3 or precursor of either.
15. A process according to claim 14, wherein: the chemical composition comprising SO2 or SO3 or precursor of either comprises a sulfate salt, bisulfite salt, sulfur or sulfuric acid, in amounts sufficient to reduce corrosion.
16. An apparatus of the invention provides a constant temperature metal sample that can be exposed to operating conditions in a WTE boiler and removed on-line and be subjected to UT measurements frequently, e.g., such as weekly and then after a longer period of exposure, e.g., 25-30 days, be sent to a laboratory for detailed metallographic analysis and physical measurement of metal thickness remaining.
17. A corrosion control process for monitoring corrosion on a real time basis and under operational conditions for boilers, where high temperature corrosion is a problem, comprising:
- determining initial placement of one or more probes as described in claim 5 in a operable position in a boiler and initial introduction parameters for the introduction of corrosion control chemicals through the use of computational fluid dynamics;
- periodically removing the probe for visual and/or physical observation; based on the observations of the probe and comparison with data for boiler components such as tubes, calibrating the degree of corrosion on the probe with what could be expected of the boiler components; and
- utilizing observations of the probe as calibrated to control introduction of corrosion control chemicals into the boiler.
18. A method according to claim 17 wherein the treatment chemical comprises SO2 or SO3 or precursor of either.
19. A process according to claim 18, wherein: the chemical composition comprising SO2 or SO3 or precursor of either comprises a sulfate salt, bisulfite salt, sulfur, or sulfuric acid, in amounts sufficient to reduce corrosion.
20. An apparatus for determining corrosion in a boiler, comprising: a probe capable of being fixed to a boiler exterior and extending into a boiler to an extent necessary to reach a suspected trouble point for corrosion; a source of cooling fluid and means for directing the fluid into the probe for cooling the probe; temperature sensing means associated with the probe; and control means for controlling supply of the fluid to the probe.
Type: Application
Filed: May 16, 2006
Publication Date: Dec 28, 2006
Applicant: FUEL TECH, INC. (Batavia, IL)
Inventors: J. Martin (Elburn, IL), Christopher Smyrniotis (St. Charles, IL), Kent Schulz (Geneva, IL), William Sun (Lisle, IL), Scott Bohlen (Bucksport, ME)
Application Number: 11/383,646
International Classification: G01N 21/00 (20060101);