Patent Application
20180112303
The present invention is directed to methods for coating articles and feedstocks for thermal spray processes. More particularly, the present invention is directed to methods for coating articles and feedstocks for thermal spray processes including fly ash in the feedstock.
Coal-fired power plants produce large volumes of fly ash throughout the world. Fly ash is the portion of ash formed while burning coal which is suspended in the flue gas exiting a coal combustion chamber, the major constituents of which are typically silica, alumina, iron oxide, and calcium oxide. While once considered to be mere refuse, fly ash has been utilized in a variety of applications, both for the purpose of reducing waste and also to make economically viable use of the combustion byproducts from burning coal. While upwards of 40% of fly ash is now recycled in the United States for beneficial use, this still leaves almost 60% of the fly ash produced in the United States as an available resource rich in useful elements and mineral compositions.
The boiler components of a coal-fired power plant, are subjected to harsh conditions, including chemical environments, elevated temperatures, and kinetic impacts which can cause erosion and corrosion, even for otherwise durable materials. In particular, the surface of such boiler components may become coated with slag deposits. Although coating systems to reduce erosion and corrosion of boiler components are known, such coating systems are typically expensive, and may form or liberate hexavalent chromium compounds.
In an exemplary embodiment, a method for coating an article includes applying a feedstock as a coating precursor by a thermal spray process. The feedstock includes fly ash. The fly ash preferentially forms a coating disposed on a substrate of the article.
In another exemplary embodiment, a feedstock for a thermal spray process includes fly ash derived from coal combustion.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention.
Wherever possible, the same reference numbers will be used throughout the drawings to represent the same parts.
Provided are methods for coating articles and feedstocks for thermal spray processes. Embodiments of the present disclosure, reduce or eliminate hexavalent chromium, reduce waste, reduce erosion, reduce corrosion, lower feedstock cost, or a combination thereof.
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The fly ash may be derived by any suitable method, including, but not limited to, the combustion of coal. Further, the coal combusted may be any suitable coal, originating in the United States of America or internationally, including, but not limited to, Western coal, Powder River Basin coal, Illinois Basin coal, or combinations thereof.
The fly ash may include any suitable chemical composition. The fly ash may include between about 15 wt. % to about 60 wt. % SiO2, alternatively between about 20 wt. % to about 60 wt. % SiO2, alternatively between about 30 wt. % to about 60 wt. % SiO2, alternatively between about 40 wt. % to about 60 wt. % SiO2, alternatively between about 15 wt. % to about 35 wt. % SiO2, alternatively between about 25 wt. % to about 45 wt. % SiO2. The fly ash may include between about 5 wt. % to about 35 wt. % Al2O3, alternatively between about 5 wt. % to about 15 wt. % Al2O3, alternatively between about 10 wt. % to about 20 wt. % Al2O3, alternatively between about 15 wt. % to about 25 wt. % Al2O3, alternatively between about 20 wt. % to about 30 wt. % Al2O3, alternatively between about 25 wt. % to about 35 wt. % Al2O3. The fly ash may include between about 4 wt. % to about 40 wt. % Fe2O3, alternatively between about 4 wt. % to about 20 wt. % Fe2O3, alternatively between about 10 wt. % to about 25 wt. % Fe2O3, alternatively between about 15 to about 30 wt. % Fe2O3, alternatively between about 20 wt. % to about 35 wt. % Fe2O3, alternatively between about 25 wt. % to about 40 wt. % Fe2O3. The fly ash may include between about 1 wt. % to about 40 wt. % CaO, alternatively between about 1 wt. % to about 15 wt. % CaO, alternatively between about 10 wt. % to about 25 wt. % CaO, alternatively between about 20 wt. % to about 35 wt. % CaO, alternatively between about 25 wt. % to about 40 wt. % CaO. The fly ash may include between about 0.1 wt. % to about 15 wt. % MgO. The fly ash may include between about 0.1 wt. % to about 15 wt. % Na2O. The fly ash may include between about 0.5 wt. % to about 10 wt. % K2O. The fly ash may include between about 0.5 wt. % to about 10 wt. % TiO2.
The fly ash may include any suitable particle size distribution, including, but not limited to, a particle distribution between about 200 nm to about 200 μm, alternatively between about 200 nm to about 100 μm, alternatively between about 500 nm to about 500 μm, alternatively between about 1 μm to about 1 mm, alternatively between about 500 μm to about 100 mm.
In one embodiment, the coating precursor further includes up to about 35 wt. % additional material, alternatively up to about 25 wt. % additional material, alternatively up to about 15 wt. % additional material, alternatively between about 5 wt. % to about 35 wt. % additional material, alternatively between about 5 wt. % to about 15 wt. % additional material, alternatively between about 10 wt. % to about 20 wt. % additional material, alternatively between about 15 wt. % to about 25 wt. % additional material, alternatively between about 20 wt. % to about 30 wt. % additional material, alternatively between about 25 wt. % to about 35 wt. % additional material. The additional materials may include any suitable materials, including, but not limited to, MgO, SO3, Na2O, K2O, TiO2, unburnt carbon, or combinations thereof.
The coating precursor may include a balancing composition. The balancing composition may be any suitable composition, including, but not limited to SiO2, Al2O3, Fe2O3, CaO, MgO, SO3, Na2O, K2O, TiO2, unburnt carbon, or combinations thereof
The thermal spray process may be any suitable spray process in which the coating precursor, including one or more feedstocks, is heated and then selectively deposited or sprayed on a surface to apply a coating on the surface. In one embodiment the thermal spray process includes plasma spraying, air plasma spraying, high-velocity air-fuel spraying (HVAF), high-velocity air plasma spraying (HV-AP), detonation spraying, flame spraying, high velocity oxy-fuel spraying (HVOF), cold (warm) spraying, or combinations thereof.
In one embodiment, wherein the thermal spray process includes air plasma spraying, applying the fly ash includes a fly ash flow rate of between about 1 g/min and about 100 g/min, and an energy input of between 30 kW and about 60 kW, alternatively about 30 kW to about 40 kW, alternatively about 35 kW to about 45 kW, alternatively about 40 kW to about 50 kW, alternatively about 45 kW to about 55 kW, alternatively about 50 kW to about 60 kW, alternatively about 35 kW, alternatively about 45 kW, alternatively about 55 kW. The processing gas may include any suitable gas, including, but not limited to, argon, hydrogen, and combinations thereof In one embodiment, argon is the primary component of the processing gas, and hydrogen is the secondary component of the processing gas.
The coating 202 may include any suitable coating thickness 206, including, but not limited to, a coating thickness 206 between about 25 μm and about 1,000 μm, alternatively between 50 μm and about 500 μm, alternatively between about 100 μm and about 400 μm, alternatively between about 100 μm and about 200 μm, alternatively between about 150 μm and about 250 μm, alternatively between about 200 μm and about 300 μm, alternatively between about 250 μm and about 350 μm, alternatively between about 300 μm and about 400 μm.
In one embodiment, the coating 202 is essentially free of hexavalent chromium. As used herein, “essentially free” may indicate that hexavalent chromium, if present, is only a trace element, alternatively is present in a concentration of less than about 100 ppm, alternatively less than about 10 ppm, alternatively less than about 1 ppm, alternatively less than about 100 ppb, alternatively less than about 10 ppb, alternatively less than about 1 ppb, alternatively less than about 100 ppt, alternatively less than about 10 ppt. In a further embodiment, the coating 202 is free of hexavalent chromium.
The coating 202 may include any suitable porosity, including, but not limited to a porosity of less than about 5%, alternatively less than about 3%, alternatively less than about 2%, alternatively less than about 1%, alternatively between about 0.1% and about 10%, alternatively between about 0.1% and about 5%, alternatively between about 0.1% and about 3%, alternatively between about 1% and about 3%, alternatively between about 2% and about 3%.
In one embodiment, the coating 202 includes an incompletely sintered ceramic phase. In another embodiment, the coating 202 includes an essentially completely sintered ceramic phase. In yet another embodiment, the coating 202 includes a completely sintered ceramic phase. As used herein, an incompletely sintered ceramic phase indicates up to 95% sintered, an essentially completely sintered ceramic phase indicates 95% sintered up to 99% sintered, and a completely sintered ceramic phase indicates more than 99% sintered.
The coating 202 may include any suitable hardness, including, but not limited to, a hardness between about 35 HRC to about 70 HRC, alternatively between about 35 HRC to about 60 HRC, alternatively between about 45 HRC to about 70 HRC, alternatively between about 40 HRC to about 65 HRC, alternatively between about 35 HRC to about 50 HRC, alternatively between about 45 HRC to about 60 HRC, alternatively between about 55 HRC to about 70 HRC.
The article 100 may be any suitable article, including but not limited to, a boiler component. Suitable boiler components, may include, but are not limited to, a coal-fired boiler component, a pulverized coal-fired boiler component, a steam locomotive boiler component, a boiler tube, or combinations thereof In one embodiment, wherein the article 100 is a boiler tube (shown schematically as an example in
In one embodiment, the method includes applying a bond coat material to the substrate 200, the bond coat material forming a bond coat 204 disposed between the substrate 200 and the coating 202. The bond coat 204 may include any suitable composition, including, but not limited to, nickel-aluminum alloy, MCrAlY (where M is a metal, which could be iron, nickel, or cobalt), and combinations thereof. The bond coat 204 may include any suitable bond coat thickness 210, including, but not limited to, a bond coat thickness 210 between about 5 μm and about 150 μm, alternatively between about 10 μm and about 100 μm, alternatively between about 10 μm and about 50 μm, alternatively between about 25 μm and about 75 μm, alternatively between about 40 μm and about 90 μm, alternatively between about 50 μm and about 100 μm, alternatively between about 70 μm and about 100 μm.
In one embodiment, the coating 202 reduces erosion, corrosion, or both of the article 100 under operating conditions for a boiler component, relative to a comparable article not including the coating 202. In another embodiment, the coating 202 reduces the deposition rate of slag on the article 100, increases the rate of slag shedding from the article 100, or both, under operating conditions for a boiler component, relative to a comparable article not including the coating 202.
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While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
