The effects of ash deposits on high-temperature corrosion of alloys in a fluidized bed combustion system

Fine 1,000-h combustion tests firing 5 different coals were conducted in a 0.1-MW_th fluidized bed combustion (FBC) facility at Western Kentucky University (WKU) with the operating conditions simulating those of the FBC unit at the Tennessee Valley Authority's (TVA) Shawnee Power Station. Five coals were selected with various S and Cl contents ranging from high S (4.48%) and high Cl (0.47%) to low S (0.97%) and low Cl (0.012%). Three types of uncooled steel coupons (Types 304 [UNS S30400], 309 [UNS S30900], and 347 [UNS S34700] stainless steel [SS]) were prepared and installed, based on the advice of TVA, to simulate the evaporator tubes in the superheater region (550°C to 600°C [1,020°F to 1,100°F]). Metal wastage of each coupon was determined by measuring the thickness before and after each combustion test. In order to study the relationship between metal corrosion and ash deposits, a total of 170 ash deposits were collected from the test coupon surfaces during the 5,000-h testing and analyzed. The major components in the deposits were Ca-based compounds, with S content ∼ 10% to 15%, and chloride content ∼ 0.5% to 5%. It was concluded that the Ca sorbent in the FBC system can capture not only the S but also the chloride effectively, and as a result decrease the gas phase chloride concentration in the FBC system down to a level of < 60 ppm. The concentration is unlikely to have contributed much to corrosion of the metal in the FBC system. Alkali metals (K and Na) were observed only on the surface of the alloys. High S contents were found in the outer part of the deposits and appeared to be associated with Ca and Mg, suggesting that the fly ash may have reacted further after being deposited on the surface of the metal. There was good agreement between locations of high-S concentrations and high-Cr content in the inner layers of the corrosion scale on the samples, indicating that corrosion involved sulfidatlon attack. S is the major element causing corrosion of the metal.