Fluidized Bed Combustion of Scrap Tires

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Further increase of fluidization velocity will cause the incipient or minimally fluidized bed to expand in volume. This is known as particulate fluidization. Further increase of the fluidization velocity will cause vigorous agitation inside the fluidized bed. Big bubbles are formed, which is called bubble fluidization. Particles move randomly upward inside the fluidized bed as the fluidization velocity increases. Further increase of fluidization velocity in a fluidized bed results in larger bubbles, comparable in size to the bed width, and may cause slugs of solids to move upward intermittently. This is known as a slugging bed. As the fluidization velocity increases beyond the bubbling or slug­ging fluidization velocity, the bed material will be transported; when the fluidization velocity exceeds or equals the terminal velocity of the largest solid particle, the bed will completely empty unless the bed material is constantly replenished. This state of fluidiza­tion is known as an entrained or transported bed, and is similar to the action of a pneumatic conveyor in industrial practice. A plot of pressure drop across the bed versus both increasing and decreasing fluidization velocity is shown in figure 2. When fluidiza­tion velocity is increased, a bump in the pressure drop versus velocity plot appears near the velocity of incipient fluidization. When fluidi­zation velocity is decreased, no bump appears when passing through incipient fluidization. This behavior is primarily due to the interlocking nature of the particles inside the packed bed. 2.2 History of Fluidized-Bed Combustion Contrary to many beliefs, fluidized-bed combustion (FBC) is not a new technology; it is an evolving technology, with a fascinating history of development. The technology originated with the demand for high octane aviation gasoline during World War II. The accumula­tion of carbon deposits on the petroleum cracking catalysts retarded their activity and decreased the gasoline yield. These carbon deposits had to be removed by burning to restore the activity of the spent cracking catalyst. The regeneration of these spent cracking catalysts by combustion in a packed bed is difficult, if not impossi­ble, because the excessive heat generated by high-temperature combus­tion is slow to dissipate from a packed bed and can result in local hot spots or temperature excursions, causing catalyst damage such as thermal breakage, particle fusion, etc. Experts were asked to devise a means of coping with this problem. A group of engineers at Massachusetts Institute of Technology, headed by Professor E. R. Gilliand, conducted the research which gave birth to the fluidized-bed reactor in the catalytic regeneration process. Inside the fluidized-bed regenerator, heat transfer surfaces were provided to recover the heat of combustion resulting from carbon burn-off. Modern petroleum refineries even conduct the catalyst regeneration under moderately elevated pressure to improve process 11-3