REDUCTION OF FIRESIDE CORROSION OF SUPERHEATER MATERIALS IN A BIOMASS-FIRED CIRCULATING FLUIDISED BED BOILER

摘要

Corrosion testing of candidate superheater materials has been performed using cooled probes in a 30MWth wood-fired circulating fluidised bed (CFB) boiler. The chromium content of the steels ranged from 2% to 28%. Two probes, temperatures 530°C and 600°C, were exposed at the final superheater in a convective pass and therefore heated by the flue gas. Two probes, temperatures 600°C and 650°C, were placed in the cyclone loop seal and were heated by the sand, which is at the same temperature as the flue gas, and which is then circulated back into the bottom of the furnace. As the heat transfer coefficient between sand and superheater tube wall is greater than that between the flue gas and the tube wall, the temperature of the probes in the loop seal was set at 50-70°C higher than the other probes. This was done to simulate the same steam temperature. The chemical composition of the flue gas at the superheaters and the environment in the cyclone loop seal were determined by Fourier Transform InfraRed Spectroscopy. Deposits were collected on probes and analysed by energy dispersive X-ray analysis in a scanning electron microscope. In spite of a higher metal temperature the corrosion rate in the loop seal was at least 50% lower than that in the convective pass, for the same materials. For some materials, the corrosion resistance improved 5-fold if the steel was positioned in the loop seal. The improvement in the corrosion resistance was explained on the basis that the water vapour content in the loop seal is much lower than in the convective pass and that the deposits formed were much thinner. Austenitic steels containing 15-18% Cr performed much better than 2-12% Cr ferritic/martensitic steels and marginally better than high Cr/Ni alloys, when positioned in the convective pass.