Behavior of tubular ceramic heat exchanger materials in acidic coal ash from coal-oil-mixture combustion

Tubes of five commercially available structural ceramics, including three types of SiC, a multiphase sialon, and a dense Al/sub 2/O/sub 3/ were exposed to the combustion products of an acidic ash coal. During the 496-h test, the tubular specimens exhibiting a wall temperature of 1230/sup 0/C experienced severe slagging conditions. The corrosion processes occurring in these materials were characterized using ceramographic and X-ray diffraction analyses. Reaction between the coal slag and SiC ceramics was generally quite limited. However, strong bonds were formed between the slag and both the alumina and sialon tubes. The extensive cracking observed in these materials apparently resulted from thermal expansion differences between these two ceramics and the coal slag.