The schematic diagram of a circulating bed boiler system is shown in Fig. 6.4.
The combustion of biomass in a fluidised bed requires that the bed be initially heated by means of a gas burner to a temperature of approximately 400°C. The gas burner is then shut off and biomass feed is started. The fuel ignites and from then on sustains combustion. In some cases, the bed remains hot enough to ignite the fuel directly without gas assistance even after a shutdown of 24 hours coarse materials burn within the bed.
Finer portions are carried through the bed and burn in the hot gases over the bed. Rocks and other non-combustible foreign materials are automatically removed from the bed on a continuous basis by extracting a portion of the bed material, screening out foreign matter, and reintroducing the bed material to the combustion unit. Biogas can be easily made available for initial heating of the bed.
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Analysis of the overall heat transfer process for a tube immersed in a fluidised bed shows that the main resistance to heat flow is on the fluidised bed side of the tube. Combination of fin designs and operating parameters have been evolved that lead to optimum performance. Ceramic tubes of silicon carbide in both plain and finned configuration are now commercially available. The bed material may by any clean, non-combustible material of uniform size. The system will require air clean-up equipment to satisfy an quality requirements of Air Pollution Control Board.