Fireside Corrosion Behavior of HVOF and Plasma-Sprayed Coatings in Advanced Coal/Biomass Co-Fired Power Plants
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T. Hussain, T. Dudziak, N.J. Simms, and J.R. Nicholls (Submitted October 10, 2012; in revised form December 30, 2012) This article presents a systematic evaluation of coatings for advanced fossil fuel plants and addresses fireside corrosion in coal/biomass-derived flue gases. A selection of four candidate coatings: alloy 625, NiCr, FeCrAl and NiCrAlY were deposited onto superheaters/reheaters alloy (T91) using high-velocity oxy-fuel (HVOF) and plasma spraying. A series of laboratory-based fireside corrosion exposures were carried out on these coated samples in furnaces under controlled atmosphere for 1000 h at 650 C. The tests were carried out using the ‘‘deposit-recoat’’ test method to simulate the environment that was anticipated from air-firing 20 wt.% cereal co-product mixed with a UK coal. The exposures were carried out using a deposit containing Na2SO4, K2SO4, and Fe2O3 to produce alkali-iron tri-sulfates, which had been identified as the principal cause of fireside corrosion on superheaters/reheaters in pulverized coalfired power plants. The exposed samples were examined in an ESEM with EDX analysis to characterize the damage. Pre- and post-exposure dimensional metrologies were used to quantify the metal damage in terms of metal loss distributions. The thermally sprayed coatings suffered significant corrosion attack from a combination of aggressive combustion gases and deposit mixtures. In this study, all the four plasma-sprayed coatings studied performed better than the HVOF-sprayed coatings because of a lower level of porosity. NiCr was found to be the best performing coating material with a median metal loss of ~87 lm (HVOF sprayed) and ~13 lm (plasma sprayed). In general, the median metal damage for coatings had the following ranking (in the descending order: most to the least damage): NiCrAlY > alloy 625 > FeCrAl > NiCr.
Keywords
air-firing, alloy 625, biomass/coal co-firing, FeCrAl, fireside corrosion, HVOF coatings, NiCr, NiCrAlY, plasma spraying, superheater corrosion, Triplex
1. Introduction Conventional fossil fuel-fired power plants are considered to be a significant contributor to CO2 emissions and resulting in global warming. The UK government has an ambitious target of reducing CO2 emissions to 80% of their 1990 levels by 2050 and generating 20% of the energy from renewable sources by 2020 (Ref 1, 2). To reduce the emissions of greenhouse gases, the power generation industry is increasingly moving toward higher steam temperatures, hence higher efficiencies, and using more carbon-neutral biomass in the fuel mix. However, the heat exchangers (superheaters/reheaters) in the power plants may encounter very aggressive fireside corrosion at higher
T. Hussain, T. Dudziak, N.J. Simms, and J.R. Nicholls, Centre for Energy and Resource Technology, Cranfield University, Bedfordshire MK43 0AL, UK. Contact e-mail: t.hussain@cran field.ac.uk.
Journal of Thermal Spray Technology
steam operating temperatures coupled with biomassderived flue gases. Fireside corrosion is an on-going concern for the power ge
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