Fireside corrosion of selected alloys by ash recovered from coal-water slurry combustion

The corrosion behavior of five selected stainless steels was evaluated in a simulated fireside corrosion atmosphere for a coal-water slurry system. Two types of ash, generated from actual combustion tests of coal-water slurries, were used as the fireside test deposits. The ashes differed in the amount of unburned carbon in each. The alloys selected were Types 304, 316, 321, 347 stainless steels and high-nickel 800 M. Tests were conducted for 48 h at temperatures of 600, 700, 800 degrees C in a simulated flue gas, representative of the atmosphere expected in coal-water slurry combustion, and at 1000 degrees C in inert atmosphere. Analyses of the test coupons after removal from the furnace included scanning electron microscopy, optical microscopy, and weight-loss measurements. Scanning electron microscopy showed the presence of fused corrosion deposits, proving that liquid phases were involved in the corrosion process. Microprobe analysis of selected samples detected no sulfur in the corrosion products, suggesting that complex sulfates were not being formed. The low-carbon ash was more corrosive toward the alloys than the high-carbon ash. The high-carbon ash tended to shift the temperature of maximum corrosion toward higher temperatures. Type 316 stainless steel showed the highest corrosion losses among the five alloys tested with the low-carbon ash at 600-800 degrees C. For tests with the high-carbon ash, the corrosion losses for all the alloys, except for Type 347, were low. The differences in corrosion losses between the types of ashes relate to the differences in carbon and calcium contents of the two ashes. The overall corrosion losses were small when compared to literature values for fireside corrosion losses in pulverized coal combustion. Published by Elsevier Science B.V.