Fluidized bed combustion of waste material

The behaviour of waste biomass materials, specifically bagasse* and sawdust, in deep fluidized beds was investigated. The bagasse used was dry (less than 1% moisture by mass). Sawdust was from mixed eucalyptus hardwoods with moisture contents up to 25% by mass. A series of cold flow visualisation tests were completed in a bed of 190mm diameter using graded river sand of surface mean particle diameters of 180 and 490 microns. Bagasse was added to the bed in various quantities and the ingestion and mixing phenomena observed. The influence of distributor design, cones, and draft tubes on mixing rates were investigated for use in the combustor design. Combustion Tests using Sawdust and Bagasse were completed in a Combustor of 489mm diameter with graded river sands of surface mean particle diameters of 300,490 and 530 microns. Various configurations were tested including a shallow bed of depth 130mm, deep beds of depths up to 460mm, a Reverse Circulation Bed, and Modified Spouted Beds of depths up to 740mm. Fuel feeding systems included above bed chutes, an ingestor tube, a direct bed wall screw feeder, and a pressurised screw feeder fitted to the air supply of a Modified Spouted Bed. Bagasse was not successfully fed through the screw feeder systems used. Sawdust, which has similar fluidized bed combustion characteristics to bagasse, was used in screw feeders to indicate the possible results that could be obtained from bagasse using below bed feed systems. Configurations utilising direct below bed surface screw feed, Ingestor tube feed, and pressurised screw feed to the fluidizing air were all successful in increasing the percentage of combustion occurring below the bed surface. The best results were obtained from pre-mixed air and fuel particles entering the modified spouted bed giving combustion efficiencies of up to 60% comparable to coal. Higher efficiencies would be possible with further optimisation of the design. The results of the investigation open several avenues of development including partial gasification/combustion systems and further development of the ingestor tube, reverse circulation bed and modified spouted bed concepts. The problems encountered with the combustion of lightweight, particulate biomass fuels are now reduced to finding practical methods of fuel feeding and rate control. *Bagasse is the cellulose residue from sugar cane stalks which remains after crushing.It is particulate, fibrous, tangled and irregular in size, length and aspect ratio.