In 1998, during maintenance of the biomass-fired Masnedø Combined Heat and Power (CHP) Plant, serious corrosion problems were revealed in the cooler part of the flue gas path, i.e. the area just outside the air pre-heater. An investigation, caused by irregularly operational conditions in January 1999, revealed an air leakage in the air pre-heater. Most of the tubes in the bottom (coldest) section of the air pre-heater were significantly corroded.
A tube from the bottom section of the air pre-heater, was mounted in epoxy, cross sectioned, polished without access to water and investigated in a Scanning Electron Microscope (SEM). This analysis showed a heavily corroded tube, covered by a 3-layer oxide scale structure, rich in Fe and Cl in the innermost layer (closest to the tube), FeXOY in the middle layer, and K and Cl, with minor amounts of Ca and S in the outer layer. On the top of the oxide scale, the remnants of an ash layer rich in K, Cl, Si, Ca, P, and S was seen.
A bank of four St37 cylindrical probes, having metal temperatures of app. 60, 70, 80, and 87 ºC was installed just outside the bottom section of the air pre-heater. The probe bank was exposed to intermitting plant operation in 7 weeks, withdrawn, mounted in epoxy, cross sectioned, polished without access to water and investigated by SEM. The 60ºC probe was thoroughly corroded during this relatively short exposure time. The other probes were oxidized, i.e. an oxide scale was formed on each probe, but they were not nearly as corroded as the 60ºC probe.
This suggest that the corrosion observed in the air pre-heater is caused by a combination of the presence of hygroscopic (i.e. water-absorbing) species in the deposits, particularly iron chlorides, and temperature variations due to intermitting plant operation. The latter may cause the flue gas temperature to oscillate around the water and hydrochloric acid dew points.
This paper provides an outline of the observations made as well as a discussion of the practical implications of low-temperature ash deposit formation and corrosion in biomass-fired boilers.