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Show INTRODUCTION This nation disposes of 200,000,000 passenger tires every year. Traditionally, these used tires are disposed of by: (1) open-air incineration, (2) chemical reclamation, and (3) discarding in a dump. As the result of enforcement of clean air laws, open-air incineration of scrap tires has been outlawed. The chemical reclamation process has also suffered a setback as the current environmental regulations on air and water pollution have taken effect. In addition, the presence of synthetic rubbers like styrene butadiene rubber (SBR) has made the reclamation process more complex. The discarding of scrap tires in a dump can cause many problems; buried tires tend to wiggle their way to the top and present fire hazards, which makes dump or landfill disposal difficult and unsafe. The passage of a Federal solid waste disposal act on October 21, 1976, put the scrap tire disposal problem at the focus of national attention. In this report, scrap tire disposal will be reviewed in the categories of (1) physical applications, (2) chemical applications, and (3) incineration for thermal energy recovery. Scrap tire disposal will be reviewed on the basis of (1) environmental acceptability, (2) conservation of resources, (3) impact on existing industries, (4) operational feasibility, and (5) special features. The focus of this report will be the fluidized-bed incineration of scrap tires for thermal energy recovery. The factors which affect scrap tire combustion will be discussed. These factors are: (1) agitation, (2) temperature, (3) excess air, (4) residence time, (5) feed uniformity, (6) solid waste handling, and (7) pollutants emission control. In reviewing these incineration processes, the criteria of (1) fuel flexibility, (2) environmental acceptability, (3) combustion efficiency, and (4) operational reliability will be discussed. WHAT IS A FLUIDIZED-BED COMBUSTOR? A fluidized-bed combustor is a solid fuel burner which burns fuel in suspension in an environmentally acceptable manner. It can also burn gaseous and liquid fuels. 2.1 Fluidized Bed Figure 1 shows the fluidizing process. In the picture to the far left, fuel particles and inert bed materials have been stacked inside a vertical cylinder, resting on a perforated plate distributor. The upper surface of the bed is deliberately slanted to rest at an angle to the horizontal plane. The maximum angle or slope at which the solid particles can be stacked and remain stationary is known as the angle of repose. The angle of repose for a batch of given particles is a function of many factors, such as surface roughness, moisture content, size distribution, and other physical properties. Figure 1 shows that as the velocity of fluid (in this case, air) increases gradually, the particulate bed's angle of repose disappears when a certain air velocity is achieved, which is known as the minimum, or incipient, fluidization velocity. At the instant when this angle of repose disappears, the top surface is leveled as if it were a liquid. 11-1 |