An Improved Sparge Pipe Air Distribution System for a Fluidised Bed Combustor

The purpose of the air distribution system in a fluidised bed combustor is to provide a reasonably uniform spread of combustion air over the entire horizontal cross-sectional area of the bed. One of the simplest, and hence cheapest, forms of air distributor is the sparge pipe design, in which an array of horizontal pipes is fitted at the base of the bed. Combustion air is then supplied to one end of each pipe and is subsequently distributed to the bed through a series of downward facing holes. The paper describes the development of an improved sparge pipe system for a large coal fired fluidised bed producing hot combustion gases for drying of pressed sugar beet pulp. The air flow out of the holes varied by a factor of three along the length of the pipes with the existing system and in addition the overall flow and hence the thermal output of the bed was unduly restricted. Excessive erosion of the pipe walls near some of the holes was also a problem. A computational fluid dynamics (CFD) study was undertaken of the flows along the pipes using FLUENT 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 9% by varying the hole diameters and pitches. Wear tests were also undertaken in near ambient temperature fluidised beds using multiple thin layers of different coloured paints on the outside of the pipes to assess the erosion patterns. These patterns were found to be similar to those observed on actual sparge pipes removed from the combustor at the end of an operating campaign. The rate of wear can be substantially reduced by reducing the angle of inclination of the downward facing holes in the pipes. Consequently an improved sparge pipe air distributor was designed for this large fluidised bed combustor.