EROSION CORROSION OF STEEL TUBES IN THE LOOP SEAL OF A BIOFUEL FIRED CFB PLANT

摘要

Erosion corrosion during biomass combustion was studied through installation of a test superheater in the loop seal of the 30MW, Nassjo CFB boiler. The superheater tubes considered were made from the following alloys; X10 (Fe8.8Cr), Esshete 1250 (Fel5Cr9Ni6Mn), 347H (Fe17Cr11Ni) and AC66 (Fe27Cr32Ni). After one firing season (exposure time; 5242h) the tube bundle was removed from the boiler. However, some tubes were reinstalled for a further firing season (total exposure time; 11392h). Using metallography and Auger spectroscopy the exposed tubes were analysed with regard to material loss and corrosion products. The degradation of the tube surface depended on material, temperature and position in the superheater. However, extending the exposure time with one firing season did not give a linear increase in material loss. The material suffering the hardest corrosion attack was X10 on which a continuous oxide scale formed around the periphery of the tube. On the other materials the corresponding oxide layer was thinner. The oxide structure on all four materials was the same consisting of one inner layer of Fe- and Cr-oxide and one outer layer of Fe-oxide. The three most alloyed materials also suffered from internal oxidation the degree of which was largest on the AC66 alloy.