Investigation of fouling and corrosion of low-temperature reheater in a CFBC boiler

Abstract

A remarkable feature of circulating fluidized bed combustion technology is the adeptness to handle a wide variety of solid fuels and their mixtures with high availability and low emissions. The fuels mainly used in fluidized bed boilers are various types of coal, peat, refuse-derived fuels, wood and other biomass. Nonetheless, the properties and behavior of some solid fuel ashes present challenges in power production regardless of the combustion technology and may even limit the use of these fuels. Biomass-based fuels typically contain high contents of easily vaporizable alkalis and chlorine, as also oil shale does. Extensive vaporization of alkalis or chlorine from the fuel increases the risk for ash-related problems such as fouling, slagging and corrosion.Co-utilization of biomass and retort gas in oil shale-fired power stations is an additional process option that increases the fuel flexibility but at the same it also increases operational risks, especially fouling and corrosion.Operational experience from Foster Wheeler two circulating fluidized bed combustion units, which were commissioned in 2004 and 2005, have proven that the above-mentioned fouling or slagging issues have been avoided. Even with different fuel mixtures. This was one of the main reasons why 300MWel Auvere power plant was acquired with circulating fluidized bed combustion technology in 2011. The plant was due to be handed over to Eesti Energia by General Electric (former Alstom) in November 2015. It was delayed since emission limits were not achieved. Therefore, additional flue gas cleaning devices were needed to be installed. Meanwhile, the power plant continued operating at loads where emissions standards were met.The 300MWel Auvere power plant was finally commissioned in 2018. Since then, the circulating fluidized bed combustion boiler has been facing severe operational problems such as fouling and corrosion of the low-temperature reheater.The failures of low-temperature reheater tubes have been occurred as a result of operation conditions.Three different reasons for low-temperature reheater malfunctions have been determined: erosion, changes in metal structure and high temperature chlorine corrosion.