| OCR Text |
Show nitrogen will relax back to their normal equilibrium (lower) concentrations . Particulate emissions must be controlled in fluidized-bed combustion, as in conventional processes. Control of particulate emissions normally entails different considerations depending on whether the process is atmospheric (AFBC) or pressurized (PFBC) fluidized-bed combustion. In atmospheric fluidized-bed combustion, there is no provision for recovery of mechanical energy from the gas effluent. The effluent flue gas is cooled by a heat recovery system to a temperature moderately above the dew point of the flue gas. At these temperature levels, conventional techniques can then be used to control the particulate emissions from AFBC systems. Caution must be exercised to maintain the flue gas temperature above its dew point, although in FBC the dew point of flue gas is typically lower than in conventional boilers. The conventional, industrially-accepted, particulate control processes are inertial separators, electrostatic precipitators, filtration, and wet scrubbing. In pressurized fluidized-bed combustion (PFBC), a gas turbine is typically used to recover mechanical energy from the pressurized flue gas. The role of the clean-up system consequently becomes one of hot-gas cleaning at high pressure to lower dust (and/or alkali) concentrations to levels which can be tolerated by the gas turbine. Gas treatment following expansion parallels the AFBC approach with heat recovery units, etc. Carbon monoxide in fluidized-bed combustion system flue gas is present in amounts which are on the order of one-tenth of 1 percent by volume. It is suspected that carbon monoxide is formed by the reduction of carbon dioxide by carbon particles inside the fluidized bed and the over-bed space. The EPA has not yet specified an acceptable CO emissions limit which is applicable to FBC systems. Reduction of carbon monoxide emissions from a fluidized-bed combustor may be achieved by over-bed injection of secondary air. REVIEW OF TIRE DISPOSAL PROCESSES In order to make clear the merits of fluidized-bed combustion as applied to tire disposal, it is necessary to review current disposal processes in these categories: (1) physical disposal, (2) chemical applications, (3) pyrolysis, and (4) incineration for heat recovery. All of these disposal processes should have desirable attributes when evaluated against criteria of: (a) environmental acceptability, (b) conservation of resources, (c) impact on existing industry, and (d) operational feasibility. 3.1 Physical Disposal Methods for physical disposal of scrap tires can be classified as either whole tire or shredded tire processes. Judged by environmental acceptability criteria, neither classification is acceptable. As far as conservation of a resource is concerned, whole-tire disposal is highly ineffective. Whole-tire disposal is passive; it is as if a scrap tire "mine" was being created to supply future generations 11-9 |
