Erosion by Char and Ash and Deposition of Ash on Carbon Steel in the Convective Section of an Industrial Boiler

Deposition of ash and erosion of carbon (C) steel by unburned char and ash were measured in the convective section of an industrial boiler while cofiring coal-water fuel and natural gas. Erosion was enhanced by directing a small, high-velocity jet of gas toward a test coupon mounted on an air-cooled tube. The jet entrained surrounding flue gas and suspended particles, accelerating the particles toward the specimen surface. After exposure for 2 h, changes in the surface were evaluated using a surface profiler and an electron microscope. Ash deposition was measured as a function of the jet gas (nitrogen [N₂], air, and oxygen[O₂]) and specimen temperature (450 K, 550 K, and 650 K [350°F, 530°F, and 710°F, respectively]). Erosion of the sample was greatest at the highest temperature and lowest O₂ concentration. Erosion was least severe at low temperature, regardless of O₂ concentration. Erosion also was low at high temperatures in the presence of high O₂ concentrations. Ash deposition was greatest at the highest temperatures and highest O₂ concentrations. The oxidized surfaces may have been more resistant to erosion than the metal substrate. However, the correlation between high ash deposition and low erosion suggested changes in the adherence of ash to the surface and resistance of ash deposits to erosion also influenced metal loss.