Metal wastage in fluidized-bed combustors

The fluidized-bed combustion of coal is finding increasing applications in industry. By employing heat exchange surfaces located in and above the fluidized bed, it is possible to generate hot water, steam or hot gas which can be used for a range of heating and process applications. For some types of combustor design, it has been observed that unacceptably high rates of metal wastage can occur on heat exchanger surfaces which come into contact with the fluidized bed. The reason why only some units exhibit wastage is unclear and is probably masked by the wide range of operating conditions and boiler designs employed. Additionally, the precise wastage mechanism is unknown; however, it is widely believed that the wearing action of the fluidized bed on metallic surfaces is largely responsible. Whether wear occurs as a result of erosion and/or abrasion, and what role is played by corrosion, is open to question. In an attempt to resolve these issues, the metallographic nature of heat exchanger surfaces removed from a number of fluidized-bed combustors, some of which have suffered wear, has been studied using microscopic analysis. Also, laboratory-based experiments have been devised in order to simulate the type of wastage experienced in industrial units. A laboratory rig has been constructed, which allows erosion tests to be performed at low particle velocities and at elevated temperatures. Erosion tests at a low velocity and at ambient temperature are reported. Erosion rates and morphologies produced in the laboratory are compared with those observed in commercial fluidized-bed combustors. Areas of future work are also identified.