Corrosion tests in combustion of recovered fuels in a modern CFB boiler

Combustion of biofuels, recycled fuels (RF), and residue derived fuels (RDF) rouses the boilermakers to create new, cost efficient designs to ensure environmentally benign and reliable operation. Usually, these fuels are non-homogenous and their average particle size is coarse. This makes the fuel feeding more demanding than that of fossil fuels. Coarse particle size and non-uniform heating value cause additional fluctuations in the stability of the boiler operation. Recovered fuels also generally increase the risk of slagging and fouling, as well as corrosion of boiler parts. Thus, the steam pressure and finishing temperature must be selected according to the fuel type combusted. Special attention must be paid to the design of superheaters, and in some cases, also the firewall area must be separately protected. These facts cause significant differences in the boiler design compared to conventional units combusting fossil fuels. A new CFB boiler designed especially for the combustion of recovered fuels has started operation in fall of 1999, in Stockholm, Sweden. This paper presents the special characteristics of this boiler, and shows some of the measured results during a three-week combustion test period. During this test period, four different recovered fuel types were burnt. Corrosion characteristics of these fuels were measured by using a novel electric on-line corrosion probe. To ensure the results achieved from the combustion tests, several materials were exposed to corrosive environment for two years. These materials have now been inspected and analyzed, and the results of the analyses are included in this paper.