The air distribution system in a fluidised bed combustor is usually required to provide a reasonably uniform distribution of combustion air over the cross-sectional area of the entire bed. Various designs of distributor have been employed and one of the simplest and cheapest constructions is the so-called sparge pipe system, in which an array of horizontal pipes is fitted near the base of the bed. Combustion air is then supplied to one end of each pipe and enters the bed through a series of downward facing holes positioned along the pipe length. This paper describes the re-design and subsequent modification of an existing sparge pipe distributor for a large coal fired fluidised bed combustor which produced hot exhaust gases for drying of pressed sugar beet pulp. The air flow out of the holes in the existing pipes varied by a factor of approximately 3.8:1 along the length and moreover the overall flow was limited by the high pressure drop within the system. As a result the thermal output of the combustor cannot always meet the demands of the drying process. Excessive erosion and wear of the walls of the pipes near some of the holes can also be a problem. A computational fluid dynamics (CFD) study was undertaken of the flow characteristics of different designs of sparge pipe and the results validated by flow and pressure measurements on full scale laboratory models. The flow distribution was substantially improved and the overall flow rate increased by approximately 7% by varying the hole diameters and spacings between adjacent holes. In addition much greater increases in predicted overall flow rates can be achieved by reducing the thickness of the pipe wall (whilst maintaining a constant outer diameter) although this may reduce the operating life of the pipe because of erosion and excessive wear. Erosion of the outside of the pipes was studied in near ambient temperature fluidised beds using multiple thin layers of different coloured paints on the outside of the pipes to assess the wear patterns. These patterns were found to be similar to those observed on actual sparge pipes at the end of an operating campaign. Quantitative measurements of the rate of wear of the paint layers indicated that pipe wall erosion can be substantially reduced by reducing the angle of inclination of the downward facing holes in the pipes.

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