Erosion-corrosion of carbon steel by products of coal combustion

Abstract

Erosion of carbon steel by fly ash and unburned char particles was measured in the convection section of an industrial boiler firing micronized coal. The rate of erosion was enhanced by directing a small jet of nitrogen, 3 vol.% oxygen in nitrogen, or air toward the surface of a test coupon mounted on an air-cooled tube. Ash and char particles suspended in the flue gas entrained by the jet were accelerated toward the surface of the specimen. Samples were exposed for 2 h with metal temperature at 450, 550, and 650 K (350, 530, and 710°F). Changes in the surface were measured using a surface profiler. Erosion was slowest at the lowest metal temperature, regardless of the jet gas composition. Under the nitrogen jet, erosion increased with increasing temperature over the range of temperatures investigated. In the presence of 3% oxygen, erosion was most rapid at the intermediate temperature. At the highest oxygen concentration, in the air jet, the erosion rate was low at all three temperatures. The temperature and oxygen dependences of the erosion rate were consistent with a model for simultaneous erosion and oxidation. Extrapolation of the results to lower velocity, using experimentally determined coefficients for metal and oxide erosion, provided estimates of erosion of a tube, as a function of impaction angle and gas velocity. Under the conditions of metal temperature, oxygen concentration, particle size, particle loading, and particle composition investigated, erosion of carbon steel tubes is expected to be slower than 0.05 μm h −1 when the gas velocity in the convection section of the boiler is less than approximately 10 m s −1.