The relative amounts of SiO2, CaO, CaCO3,CaSO4, and alkali compounds in the bed materials of circulating fluidized bed combustors determined the metal wastage of heat exchanger tubing alloys exposed to the particles. The ratio of SiO2 to CaO and to CaCO3 is especially important in determining the erosivity of the bed materials. It was found that increasing this ratio increased the metal wastage. A sharp transition ratio occurred on either side of which the metal wastages were linear. The slope of the linear curve was low for the lower sand content mixtures and higher for higher sand content mixtures. The transition ratio required a higher CaO content for higher velocity particles.

The transition ratio of SiO2 to CaCO3 is related to the formation or lack of formation of a protective layer on the metal surface. The transition ratio of SiO2 to CaO is related to the formation of a thick, protective layer or a thin, less protective layer on the metal surface. In the thick layer, CaO mechanically mixed with iron oxide to form an intimate mixture whose CaO content decreased inward from the gas-scale layer surface.

Subject
Limestone, Materials, Sand, Metal surfaces, Oxide layers, Metal wastage, Calcium, Metal loss, Mixtures, Thickness loss, Metals, Surface layers, Steel
Keywords:
Fluidized bed combustors, bed materials, erosivity, erosion-corrosion, plain carbon steel, elevated temperature, oxidation, sand-limestone, metal wastage
© 1990 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
1990
Association for Materials Protection and Performance (AMPP)
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