| OCR Text |
Show The fluidized-bed combustion process can be counted on to yield uniform temperature distribution by its vigorous in-bed mixing. Consequently, even and uniform mixing can sufficiently reduce the maximum combustion temperature required to insure adequately fast reaction rates and yet avoid formation of excessively high concentrations of nitrogen oxides. Excess Air Ferrous metal surfaces alternately exposed to oxidizing and reducing conditions degrade readily, which can lead to equipment failures. Conventional furnaces, operated at 80 percent excess air, have on occasion been found to produce alternating oxidizing and reducing conditions. Maintaining oxidative conditions in all parts of the incinerator will insure that no volatile ferrous chloride, FeCl2, will form and evaporate(4). In existing conventional tire incinerators, the amount of excess air used is on the order of 80 to 140 percent. Unfortunately, these high levels of excess air result in large volumes of flue gas which must be be cleaned up before being released to the atmosphere. The cost of flue gas cleaning is directly proportional to the volume of flue gas to be cleaned. In industrial practice, it is common to put a mechanical collector in front of an electrostatic precipitator or a bag filter house as primary particulate removal equipment. Schweiger(9) gave the cost of some applicable particulate collection systems and components: a two-stage mechanical collector, such as a cyclone, will cost $1.20 to $1.50 per actual cubic foot of gas throughput per minute. In addition, the pressure drop of a single-stage cyclone is 3 inches of water and the collection efficiency ranges from 70 to 90 percent. Electrostatic precipitators typically cost about $4.05 to $7.50 per actual cubic foot of gas throughput per minute, have a pressure drop of about a half-inch of water, and produce a collection efficiency of 99 percent. The bag filter house costs $5.25 per actual cubic foot of gas throughput per minute with a collection efficiency of 99 percent and a pressure drop on the order of 2.5 to 3.5 inches of water. The consequence of the large amounts of excess air used in conventional processes is that cleaning up the resulting amount of flue gas to meet environmental standards costs a significant amount of money in terms of equipment cost and of parasitic power consumption, and both directly and indirectly in terms of maintenance. Because of the vigorous mixing and resulting uniformity which occur inside a fluidized-bed combustion boiler, 15 to 20 percent excess air has been found acceptable for high-suifur coal applications. Since coal and rubber are very much alike in chemical composition and some of their combustion characteristics are almost identical, lower excess air percentages will be practical for tires. Residence Time Every chemical reaction takes time to occur. The residence time required for efficient tire incineration is a function of many variables, primarily furnace temperature, oxygen level, scrap tire particle 11-15 |
