Coatings superheaters

At this time, 1977-1978, it appears that the current best boiler-furnace design in use for large, high pressure units is the completely water-tube-walled furnace and radiant section, studded and coated with thin plastic refractory in the intense burning zone, followed by one or more long, open, vertical radiation passes preceding a convention-type superheater and boiler-convection passes and economizer. 

The theory of bubble nucleation in a superheated liquid was first applied to the concept of thermal inkjet by Allen et al. [7]. They were able to determine the minimum cmiditions for the first bubble nucleation by applying Hsu s theory [10]. Time dependent temperature profiles above a heater surface were obtained. By superimposing the activation curve with the thermal boimdary layer, the initial bubble size and the minimum temperature for nucleation were determined. Based on a one-dimensional model and by assuming the nucleation temperature to be the superheat limit of the liquid at 330°C transient temperature profiles for the heater structure and the bubble surface after nucleation were obtained. It was noticed that the decay time to ambient temperature from its initial state was only several microseconds after 6 ps heating pulse. The thermal effects of the passivation (protective coating) layer on the heater surface were also analyzed. The results showed that the effective pulse energy required for bubble nucleation increases with the thickness of the passivation layer. 

Corrosion is mainly caused by oxides of sulfur but in certain parts of a combustion system, specifically on furnace wall tubes with metal temperature of 290 C-425°C (550°F-800°F) and superheater or reheater tubes with temperatures in the range 600 C-700°C (1110°F-1300°F), corrosion can be induced by tube deposits that destroy protective surface oxide coatings. 

Coal ash corrosion is a widespread problem for superheater and reheater tubes in coal fired power plants that bum high-sulfur coals. The accelerated corrosion is caused by liquid sulfates on the surface of the metal beneath an over-lying ash deposit. Coal ash corrosion is very severe between 540 and 740°C (1000°F and 1364°F) because of the formation of molten alkali iron-trisulfate. Considerable work has been done to predict corrosion rates based on the nature of the coal (its sulfur and ash content). This was accomplished by the exposure of various alloys to synthetic ash mixtures and synthetic flue gases. The corrosion rates of various alloys were repotted in the form of iso-corrosion curves for various sulfur dioxide, alkali sulfiite, and temperature combinations. An equation was developed to predict corrosion rates for selected alloys from details of the nature of ash by analyzing deposits removed from steam generator tubes and from test probes installed in a boiler [33]. Then laboratory tests were conducted using coupons of various tdloys coated with synthetic coal ash that was exposed to simulated combustion gas atmospheres. 

With a few exceptions, coatings and linings are not used on the water and steam sides. In an EPRI project, about 50 turbine blade coatings have been evaluated, but none of these are being routinely applied. To reduce steam side oxidation in reheaters and superheaters, chromizing and chromating have been developed but these treatments are also not routinely applied. There is little use of composite materials with the exception of condenser tube sheets, which could be made of explosively clad stainless steel or titanium on carbon steel, and of the surfaces in the primary cycles of nuclear units where carbon or low alloy steels are protected by weld-deposited stainless steels. In pulp mill black liquor recovery boilers, stainless steel clad boiler tubes are often used. 

Blough, J. L., Seitz, W. W. (1998), Fireside Corrosion Testing of Candidate Superheater Tube Alloys, Coatings, and Claddings, Paper No. 4-8 in Proc. Twelfth Annu. Conf. Fossil Energy Materials, CONF-980561, ORNL/FMP-98/1. 

In SHTs, the corrosion conditions are more severe than in WWTs due to higher metal temperatures and the presence of molten salts. Figure 19.7 shows the microstructures of the Ti02-Al203/625 cermet HVOF coating exposed for approximately two years in WTE superheaters result in remarkably... 

Cermet spray coating on SIOHCbN superheater after 13,800 hours exposure. 

Zr02/Alloy 625 on-site 100 kW plasma-jet spray coating for 500°C superheaters. 

Y. Kawahara, Y. Nakagawa, M. Kira, T. Sakurada, H. Kamakura, and Y. Imaizumi, Life Evaluation of Zr02/Ni Base Alloy Plasma-Jet Coating Systems in High Efficiency Waste-to-Energy Boiler Superheaters, Corrosion/2004, Paper No. 04535 (New Orleans, LA), NACE, 2004... 

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