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Process for corrosion control in boilersProcess for corrosion control in boilers description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060290935, Process for corrosion control in boilers. Brief Patent Description - Full Patent Description - Patent Application Claims
RELATED APPLICATION AND PRIORITY CLAIM [0001] 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 INVENTION [0002] The invention relates to a corrosion control process, which is especially useful in the control of chloride corrosion in boilers, particularly waste to energy boilers. [0003] 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. [0004] 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. [0005] 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 NO.sub.x 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, H.sub.2S, FeS or FeS.sub.2 can react with the oxygen of the tube scale depriving the tube metal of its protective layer. [0006] 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 FIG. 1. An observation that can be made from FIG. 2 is that corrosion is strong on the sides tangential to flue gas flow and occurs on opposing sides of the tube. [0007] 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. [0008] 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. [0009] Rademakers, et al., explain that high temperature corrosion in waste incinerators is caused by chlorine either in the form of HCl, Cl.sub.2, 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. [0010] 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 SO.sub.2/HCl. They indicate that following mechanisms can be distinguished: [0011] Corrosion by HCl/Cl.sub.2 or SO.sub.2/SO.sub.3 containing gas under oxidizing or oxidizing/reducing conditions, and [0012] Corrosion by solid or molten deposits of metal chlorides and sulfates. Rademakers, et al., describe these mechanisms and refer to a schematic, in FIG. 3, as drawn from Krause, 1986, 1993, and as set out below in various steps. [0013] Corrosion caused by chlorine-containing gas at metal temperatures above about 450.degree. C. is referred to as `active oxidation`. Alkali chlorides, such as NaCl, CaCl.sub.2 and KCl, can be present already or can be formed by the combustion and subsequent reaction of alkali oxides: Na.sub.2O+2HCl=2NaCl+H.sub.2O [1] Under ideal conditions (good mixing, sufficient residence time) alkali chlorides can be sulfated according to the following reaction, provided there is enough SO.sub.2 and O.sub.2: 2NaCl+SO.sub.2+1/2 O.sub.2+H.sub.2O.dbd.Na.sub.2SO.sub.4+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. [0014] Without SO.sub.2 at 500.degree. C., NaCl and iron oxides can form Cl.sub.2: 2NaCl+Fe.sub.2O.sub.31/2 O.sub.2.dbd.Na.sub.2Fe.sub.2O.sub.4+Cl.sub.2 [3]6NaCl+2Fe.sub.3O.sub.4+2 O.sub.2=3Na.sub.2Fe.sub.2O.sub.4+3Cl.sub.2 [4] Calculations of the dissociation constant of HCl as a function of temperature indicate that chlorine is present as Cl.sub.2 under oxidizing conditions up to gas temperatures of 600.degree. C., whereas above 600.degree. C. formation of HCl is enhanced in the presence of water vapor according to the reaction: H.sub.2O+Cl.sub.2=2HCl+1/2 O.sub.2 [5] [0015] Rademakers, et al., state that at about 500.degree. C., Cl.sub.2 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 Cl.sub.2 partial pressure sufficiently high that it reacts directly with the steel to form FeCl.sub.2: Fe+Cl.sub.2.dbd.FeCl.sub.2 (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 FeCl.sub.2 is already relatively high at low temperatures. As a result, formation of FeCl.sub.2 can decrease the adherence of the oxide scale or can cause spallation of the oxide layer. [0016] 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: FeCl.sub.2 (solid)=FeCl.sub.2 (gas) [7]4FeCl.sub.2+3O.sub.2.dbd.Fe.sub.2O.sub.3+2Cl.sub.2 [8]3FeCl.sub.2+2O.sub.2.dbd.Fe.sub.3O.sub.4+3Cl.sub.2 [9] In this process, the chlorine has a catalytic effect on the oxidation of the metal resulting in enhanced corrosion. [0017] The kinetics of active oxidation is mainly determined by the evaporation and outward diffusion of FeCl.sub.2. Similar chlorine corrosion and regeneration cycles may proceed via FeCl.sub.3 and it is possible for the ferrous iron to be oxidized to the ferric state, which liberates chlorine when oxidized. 4FeCl.sub.2+4HCl+O.sub.2=4FeCl.sub.3+2H.sub.2O [10]4FcCl.sub.3+3O.sub.2=2Fc.sub.2O.sub.3+4Cl.sub.2 [11] [0018] The volatility of different compounds can be compared based on the temperature T4 (temperature at which the vapor pressure reaches 10.sup.-4 bar), and vapor pressure values for some compounds are given in Table 1. TABLE-US-00001 TABLE 1 T4 Temperatures of Metal Chlorides of Main Alloying Elements Metal chloride T4 (.degree. C.) FeCl.sub.2 536 FeCl.sub.3 167 CrCl.sub.2 741 CrCl.sub.3 611 NiCl.sub.2 607 [0019] 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 FeCl.sub.3, most T4 temperatures are well above 500.degree. C. indicating that this mechanism is most relevant to superheaters and less to evaporators. [0020] 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. [0021] 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 INVENTION [0022] It 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. [0023] 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. Continue reading about Process for corrosion control in boilers... Full patent description for Process for corrosion control in boilers Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Process for corrosion control in boilers patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. 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