ABSTRACT
Erosion of carbon steel by fly ash and unburned char particles was measured in the convection section of an industrial boiler cofiring coal-water fuel and natural gas. The rate of erosion was enhanced by directing a small jet of nitrogen, air, or oxygen toward the surface of a test coupon mounted on an air-cooled tube. Ash and char particles which entered the jet from the surrounding flue gas were accelerated toward the surface of the specimen. Samples were exposed for 2 hours with the metal temperature at 450, 550, and 650 K. Changes in the surface were measured using a surface profiler. Time-averaged maximum erosion rates were obtained from the differences between the original surface height and the minima in the profiles. At the lowest oxygen partial pressure, when the jet gas was nitrogen, erosion increased with increasing temperature over the range of temperatures investigated. In the presence of the air jet, erosion was greatest at the intermediate temperature. At the highest oxygen, under the pure oxygen jet, erosion was low at all three temperatures. The changes in erosion rate with temperature and oxygen concentration were consistent with an erosion-oxidation mechanism based on the following assumptions: (1) Metal was eroded as a ductile material, at a rate which increased with increasing temperature; (2) Oxide eroded as a brittle material, at a rate independent of temperature; (3) Erosion of oxide and metal occurred in series; (4) Oxide scale was strongly attached to the metal; (5) Oxide formed according to the parabolic rate law, with a rate coefficient proportional to the oxygen partial pressure; and (6) The effects of particles sticking to, or embedded in, the surface could be incorporated in the effective erosion rate coefficients.
