OCR Text |
Show Page 5 . When hydrocarbons, in fuel or a constltuent of a waste material are injected into a conventional FBe , bubbles of combustible gas rise through the bed improperly mixed with the requisite combustion air and burn above the bed, sometimes with considerable violence. To overcome this, deep beds can be used, but this requires high differential pressure, resulting in high horsepower. The SFBC on the other hand does not suffer from poor mixing and shallower beds can be used with reduced fluidizing blower horsepower. Probably the most serious limitation of conventional FBC operation is the limitation on turndown. Reports vary, but it is widely held that 3:1 is hard to get, and some statements say that 2:1 is found to be a practical limit. The problem is, of course, not being neglected and, for example, sectional segregation of the bed to allow partial deactivation is being tried. A difficulty is that deactivation of a separate section entails reheating of the sand when it is to be brought on again, which is an unwanted complication and slow in response. The spouted fluidized bed system lends itself uniquely to high turndown. At least 6:1 turndown on maximum effluent disposal rates is available under automatic control, Higher turndown to more than 10:1 is available by supplying spouting/combustion air through multiple plenums. Care must only be taken that fuel air ratios are maintained and that adequate bed temperature is sustained. In conventional FBC operation, the particles comprising the bed move mainly up and down, wlth little lateral movement. Consequently, particulate solids, waste slurries or sludges, do not disperse rapidly across the bed. Fuel distribution is, therefore, uneven and combustion air is deficient in some areas, and grossly excessive in others. One incidental result of this uneven combustion is the passage into the space above the bed of the pockets of unburnt gas mentioned earlier. In a spouted fluid bed, vigorous lateral circulation starts around each cone generated by every spout, and this can be extended, if desired, to a continuous interchange between the cones of adjacent spouts. |