Influence of deposit formation on corrosion at a straw-fired boiler

Straw-fired boilers generally experience severe problems with deposit forma tion and are expected to suffer from severe superheater corrosion at high s team temperatures due to the large alkali and chlorine content in straw. In this study, deposits collected (1) on air-cooled probes and (2) directly a t the existing heat transfer surfaces of a straw-fired boiler have been exa mined. Deposits collected on air-cooled probes were found to consist of an inner layer of KCl and an outer layer of sintered fly ash. Ash deposits for med on the heat transfer surfaces all had a characteristic layered structur e, with a dense layer of K2SO4 present adjacent to the metal surface. It is argued that the K2SO4 layer present adjacent to the metal surface may lead to reduced corrosion rates at this boiler. A discussion of the deposit str ucture, the K2SO4 layer formation mechanism, and the influence of the inner layer composition on the corrosion of the superheaters is provided. (C) 20 00 Elsevier Science B.V. All rights reserved.