Claims:We claim:
1. A method for applying nano-structured coating on boiler heat transfer surface to prevent slagging and fouling, the method comprises:
synthesizing nano-crystalline feedstock powder ;
characterizing of nano-crystalline feedstock powder using SEM, XRD, HRTEM/EDS;
fabricating the characterized nano-crystalline feedstock powder on surface of steel and allow substrate using High Velocity Oxy Fuel (HVOF) system.
2. The method as claimed in claim 1, wherein the nano-crystalline feedstock powder is nickel based as-atomized thermal spray feedstock powder (NiCrFeSiB).
3. The method as claimed in claim 1, wherein the nano-crystalline feedstock powder NiCrFeSiB is in ratio (73.9/14.5/4.3/4.3/3).
4. The method as claimed in claim 1, wherein the surface of steel and allow substrate is grit blasted prior to coating process.
5. The method as claimed in claim 4, wherein the grit blasted surface of the steel and alloy has surface roughness to a Ra value of 10um.
, Description:NON-STICK NANOSTRUCTURED COATING FOR HEAT TRANSFER SURFACES USING HIGH VELOCITY OXY FUEL TECHNIQUE (HVOF)
FILED OF INVENTION:
[001] The present subject matter described herein relates to slagging/fouling on Boiler heat transfer surfaces by developing a nano-structured coating through mechanical milling and High Velocity Oxy Fuel (HVOF) spray technique is provided to Boiler heat transfer surfaces. The present Non-stick nanostructured coating is used to prevent slagging of Boiler heat transfer surfaces, which is not presently available in any system of Boiler heat transfer surfaces.
BACKGROUND OF INVENTION:
[002] In a typical coal fired thermal power plant, coals of different grades are being used. The coal has different quality with high ash content and higher moisture content. With this coal, different sources are also added or blended to form the fuel. Due to varying nature of the fuel, behavior of the fuel firing is different and does not deliver as per design. This sort of unsure nature of the coal, ash deposition, clinkering and other related problems prop up. One such problem of the ash deposition is very persistent in many boilers. A boiler which can accept any type, grade or impurities of fuel is required or a boiler which is resistant to ash deposition is required.
[003] The wall Blower is currently used to remove ash deposit on Boiler water wall surface to prevent slagging and similarly Soot blower to remove ash deposits from Super Heater/ Re-Heater (SH/RH) areas to prevent fouling. But, the steam released by Blowers hit on heat transfer surfaces will increase the chances of eroding of the heat transfer surfaces during absence of slagging/fouling on the heat transfer surfaces as well as the energy consumed by the Blowers is also added up. Hence, this invention on exploring a coating hard and non-stick in nature is developed to mitigate the ash deposition
[004] US patent 4456635 is development of Coating a thin film of magnesium hydroxide on the radiant section of the furnace walls of boilers prevents slagging and controls ash build up.
[005] US patent 20080291965 and WIPO Patent WO/2008/143962 is a method for measuring ash/slag deposition in an operating utility boiler. The method has the following steps: i) providing a probe for the boiler wherein the probe has at least one thermocouple therein or thereon for measuring temperature; ii) measuring the temperature at the thermocouple at a baseline time; iii) measuring the temperature at least one thermocouple at a pre-determined time later than the baseline time; and iv) comparing the temperature at the baseline time to the temperature at the pre-determined time to correlate to a level of deposition. There is also a utility boiler system.
[006] However none of these prior art inventions deal with nanostructure coating on the boiler.
[007] However there exists a need for nanostructure coating for preventing slag/foul on boiler heat transfer surface. The present invention describes a method to apply coating of nanostructure coating.
OBJECT OF THE INVENTION:
[008] An object of the present invention is develop a non-stick coating which reduces the quantity of ash deposition on the wall of the furnace, super heater, and other heat transfer surfaces of a boiler.
[009] A still another objective of the invention is to identify the material suitable for the above mentioned objective. Material such that it has low surface energy.
[0010] Synthesize nano-crystalline coating material from the parent material selected from the above objective reducing the grain size of the material to nano scale and the retaining the particle size.
[0011] This objective is required for the application of coating material on to the parent metal through High velocity oxy fuel spray technique. Since the High velocity oxy fuel gun requires particle size to be a minimum in the micron size range.
[0012] Reduce the erosion in high ash content coal Boiler at elevated temperature.
SUMMARY OF INVENTION:
[0013] The present subject matter relates to a coating of nano-crystalline material on to the steel and alloy steel material to mitigate the ash deposition faced in the boilers was developed. The application of the coating on the grit blasted surface – surface preparation technique, through High velocity oxy fuel spray system. This coating was evaluated using ash adhesion testing, hard ness tester and high temperature erosion tests. The performance of the coating based on the tests and HRTEM, SEM, XRD analytic tools demonstrate the objective of the invention, the non-stick nature of the coating, erosion resistant of the coatings, etc.
[0014] In order to further understand the characteristics and technical contents of the present subject matter, a description relating thereto will be made with reference to the accompanying drawings. However, the drawings are illustrative only but not used to limit scope of the present subject matter.
BRIEF DESCRIPTION OF THE DRAWING:
[0015] It is to be noted, however, that the appended drawings illustrate only typical embodiments of the present subject matter and are therefore not to be considered for limiting of its scope, for the invention may admit to other equally effective embodiments. The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the figures to reference like features and components. Some embodiments of system or methods in accordance with embodiments of the present subject matter are now described, by way of example, and with reference to the accompanying figures, in which:
[0016] FIGURE-1: illustrates process flow of non stick nano-coating, in accordance with an embodiment of the present subject matter;
[0017] FIGURE-2: illustrates graph of micro hardness along the Cross-Section of Coatings, in accordance with an embodiment of the present subject matter;
[0018] FIGURE-3 illustrates a graph to compare the Variation in ash adhesion force between a plain carbon steel surface and nanostructured coated surface, in accordance with an embodiment of the present subject matter;
[0019] The figures depict embodiments of the present subject matter for the purposes of illustration only. A person skilled in the art will easily recognize from the following description that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the disclosure described herein.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT:
[0020] It should be noted that the description and figures merely illustrate the principles of the present subject matter. It should be appreciated by those skilled in the art that conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present subject matter. It should also be appreciated by those skilled in the art that by devising various arrangements that, although not explicitly described or shown herein, embody the principles of the present subject matter and are included within its spirit and scope. Furthermore, all examples recited herein are principally intended expressly to be for pedagogical purposes to aid the reader in understanding the principles of the present subject matter and the concepts contributed by the inventor(s) to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions. The novel features which are believed to be characteristic of the present subject matter, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures.
[0021] These and other advantages of the present subject matter would be described in greater detail with reference to the following figures. It should be noted that the description merely illustrates the principles of the present subject matter. It will thus be appreciated that those skilled in the art will be able to devise various arrangements that, although not explicitly described herein, embody the principles of the present subject matter and are included within its scope.
[0022] Fig. 1 shows the process flow of the non-stick nanostructured coating. The milled feedstock powders were used to fabricate the nanostructured coatings respectively. The nanostructure feedstock powder is nickel based as-atomized thermal spray feedstock powder (NiCrFeSiB). The High Velocity Oxy-Fuel (HVOF) process is adopted to fabricate the above coating on the steel and alloy steel substrate. The steel surface is prepared for the application of the HVOF coating. These surfaces are grit blasted prior to the coating process in order to get good adhesive strength between the coating and the substrate. The substrates are grit blasted to improve the surface roughness to a Ra value of 10 µm. The spraying was carried out with the HVOF system. The standard parameter used to fabricate the HVOF coating is given in Table.1.
S. No: Parameter Value
1.
2.
3.
4.
5.
6.
7.
8. Oxygen flow rate
Fuel (LPG) flow rate
Air-flow rate
Spray distance
Powder feed rate
Fuel pressure
Oxygen pressure
Air pressure 250 l/min
60 l/min
700 l/min
200-250mm
15-20 g/min
588 kPa
883 kPa
588 kPa

[0023] The development of a nanostructured coating by mechanical milling technique involving repeated welding, fracturing, and re-welding of powdered micro particles in a planetary ball mill. Thermal spraying of milled powders was used to produce nano-crystalline coatings on steel tubes. The material NiCrFeSiB (73.9/14.5/4.3/4.3/3) is chosen such a way that the non-stick surface is obtained by the nanostructured material developed by the mechanical planetary ball mill. The material is applicable in high temperature application, since the materials have a low surface energy compared to the parent material, which are carbon steel and alloy steel. Due to the property of high hardness of the nanostructured coating, they reduce erosion at high temperature prevailing in applications like boilers etc. The nanostructured coatings exhibited higher hardness value compared with conventional coatings of the same composition.
[0024] Due to the high temperature the nanostructured coating did not exhibit any grain size change, i.e., growth of the grain size, which will indirectly affect the mechanical properties of the coating material. The material is applicable for high temperature application as the materials have low surface energy as compared to that of parent material i.e., carbon steel and alloy steel in boiler heat transfer applications. On the basis of the hardness property of nanostructured coating, it reduces erosion at high temperature prevailing in Boiler applications. The nanostructured coatings exhibited higher hardness value when compared with the conventional coatings of the same composition.
[0025] The nanostructured composite coatings are characterized using X-ray diffraction (XRD), Scanning Electron Microscope (SEM) and High Resolution Transmission Electron Microscope/ Energy Dispersive Spectroscopy (HRTEM/EDS). For HRTEM analysis, the disc shape sample of 3mm diameter was punched from the thermal spray coating of thickness 100µm using disc punch tool followed by the dimple grinding process to reduce the thickness to 20µm. The dimpled discs were then ion milled for obtaining electron transparent regions. The micro-hardness of the HVOF coated specimens was measured using Vickers hardness tester at a load of 500 g and the indenter was allowed for 20 seconds. All the tests were repeated for five times randomly and the average data were reported in Fig. 2.
[0026] The porosity and microstructure of nanostructured HVOF coated was investigated. Thermal cycle analysis was carried out at 650°C, in order to examine the thermal stability of coating for high temperature service because the working condition for most boiler tubes was between 550°C and 650°C. Thermal stability of the HVOF coated substrate material was investigated by heating it to 650°C and maintain at different soaking time and to explore the potential of applying nanostructured coatings for boiler tubes protection during high-temperature service.
[0027] Adhesion force between the ash particles and the steel substrate with coating were measured with respect to time, using a table type adhesion strength testing station. This facility is mainly composed of a built-in 1 kN load cell, an 8 kW electrical furnace and a provision for circulation unit of cooling water. Ash pellet was prepared and the pellet size is 25 mm in diameter and 8 mm high.
[0028] During the adhesion test, the steel substrate with and without coating, temperature was maintained between 600°C to 650°C and ash temperature at 900°C.The ash adhesion force for nanostructured coating was comparatively less than the parent material with specified surface area (4.91×10-6m2) as shown in Fig. 3.
[0029] The ash adhesion test, demonstrates the non-stick nature of the coating which meets our primary objective of the invention.
[0030] Due to high hardness – micro hardness shows that the coating is hard and will prevent erosion one of the invention’s objectives.
[0031] Although embodiments for the present subject matter have been described in language specific to structural features, it is to be understood that the present subject matter is not necessarily limited to the specific features described. Rather, the specific features and methods are disclosed as embodiments for the present subject matter. Numerous modifications and adaptations of the system/component of the present invention will be apparent to those skilled in the art, and thus it is intended by the appended claims to cover all such modifications and adaptations which fall within the scope of the present subject matter.