EROSION-CORROSION OF CARBON-STEEL BY PRODUCTS OF COAL COMBUSTION

Erosion of carbon steel by fly ash and unburned char particles was mea sured in the convection section of an industrial boiler firing microni zed 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 susp ended in the flue gas entrained by the jet were accelerated toward the surface of the specimen. Samples were exposed for 2 h with metal temp erature at 450, 550, and 650 K (350, 530, and 710 degrees F). Changes in the surface were measured using a surface profiler. Erosion was slo west at the lowest metal temperature, regardless of the jet gas compos ition. Under the nitrogen jet, erosion increased with increasing tempe rature 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 wa s low at all three temperatures. The temperature and oxygen dependence s of the erosion rate were consistent with a model for simultaneous er osion and oxidation. Extrapolation of the results to lower velocity, u sing experimentally determined coefficients for metal and oxide erosio n, provided estimates of erosion of a tube, as a function of impaction angle and gas velocity. Under the conditions of metal temperature, ox ygen concentration, particle size, particle loading, and particle comp osition investigated, erosion of carbon steel tubes is expected to be slower than 0.05 mu m h(-1) when the gas velocity in the convection se ction of the boiler is less than approximately 10 m s(-1).