How to control superheater tube corrosion in FB boilers which use wood and wood waste as fuel

The objective of this study is to determine the safe combustion conditions and optimised material technologies in circulating (CFB) as well as bubbling fluidised-bed (BFB) boilers using wood chips as fuel. The combustion and co-firing properties of fuels produced by different forest chains were studied first in VTT's test facilities and later in industrial-scale power plant boilers. The formation of alkaline and chlorine compounds and their effect on boiler fouling and corrosion were also studied also by cooled deposit and material probes. In addition to analysis of material exposures in boilers, materials embedded in different mixtures of wood and peat ashes with or without chloride were tested in the laboratory. Deposits and changes in their chemical phase composition compounds and the corrosion of materials were analysed by SEM-EDX and optical microscopy. Solutions to control superheater tube corrosion optimally were considered with reference to the experimental results and to model calculations performed using the F.A.C.T program. Utilisation of logging residue chips can cause deposition of potassium and sodium chloride on the heat exchanger surfaces and enhanced corrosion due to the chlorine. The harmful formation of alkaline and chlorine compounds on boiler surfaces could be prevented by co-firing sulphur-containing fuel. However, to suppress corrosion that is not directly related to alkali chloride deposition on superheater tube surfaces, optimised material solutions have to be seeked.