Air Pollution Control Systems for Boiler and Incinerators.Unique control problems_9

Tham khảo tài liệu air pollution control systems for boiler and incinerators.unique control problems_9, kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả
Nội dung trích xuất từ tài liệu:
Air Pollution Control Systems for Boiler and Incinerators.Unique control problems_9TM 5-815-1/AFR 19-6Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.comparticles leave the combustion chamber with the flue Desulfurization efficiency of a shallow bed is poor,gases so that solids recirculation is necessary to main- with only about 60 to 80 percent removal, because SO2 does not have adequate time to react with the limestonetain the bed solids. This type of fluidization is calledcirculating fluidized bed. before moving out of the shallow bed. The shallow bed e. The mean solids velocity increases at a slower rate fluidized boiler is of the bubbling bed design. The shal-than does the gas velocity, as illustrated in figure 13-3. low bed will be of very limited use because of its poorTherefore, a maximum slip velocity between the solids sulfur dioxide removal.and the gas can be achieved resulting in good heat g. A deep fluidized bed boiler is a bubbling bedtransfer and contact time with the limestone, for sulfur design.dioxide removal. When gas velocity is further (1) The bed depth is usually 3 feet to 5 feet deepincreased, the mean slip velocity decreases again. and the pressure drop averages about oneThese are the operating conditions for transport reactor inch of water per inch of bed depth. The bulkor pulverized coal boiler. The design of the fluidized of the bed consists of limestone, sand, ash, orbed falls between the stoker fired boiler and the pul- other material and a small amount of fuel.verized coal boiler using the bed expansion. The rate at which air is blown through the bed f. The shallow fluidized bed boiler operates with a determines the amount of fuel that can besingle bed at a low gas velocity. A shallow bed mini- reacted. There are limits to the amount of airmizes fan horsepower and limits the free-board space. that can be blown through before the bedThe bed depth is usually about 6 inches to 9 inches and material and fuel are entrained and blown outthe free-board heights are only four to five feet.13-2Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com 5-815-1/AFR 19-6 TM of the furnace. Conversely, when air flow is continuous stopping of sections is required to reduced below the minimum fluidizing control load for extended periods, the velocity, the bed slumps and fluidization fluidized bed boiler may become a big user of stops. auxiliary fuel to maintain bed temperature. (2) The fuel feed systems available are either (4) Major limitations of the bubbling bed design under-bed feed system or over-the-bed feed are high calcium/sulfur ratios, low system. The under-bed feed system is quite combustion efficiency, limited turndown complex. It requires coal at less than 8 without sectionalization of the furnace bottom percent surface moisture and crushed to and complexity of the under bed feed system about 6 MM top size to minimize plugging required to minimize elutriation of unburned the coal pipes. Operating and maintenance fines. Typical fluidized bed combustors of costs are usually high for the under-bed feed this type are shown in figures 13-4 and 13-5. system. The major advantage of the under- h. In the circulating fluidized bed boiler, the fuel is bed feed system is that with use of recycle fed into the lower combustion chamber and primary air combustion efficiency approaches 99 percent. is introduced under the bed. The over-bed feed system is an adaptation of (1) Because of the turbulence and velocity in the the spreader stoker system for conventional circulating bed, the fuel mixes with the bed boilers. This system has a potential problem material quickly and uniformly. Since there is of effective carbon utilization. Carbon not a definite bed depth when operating, the elutriation can be as high as 10 percent. density of the bed varies throughout the sys- (3) Some bubbling bed units have sectionalized tem, with the highest density at the level or modular design for turndown or load where the fuel is introduced. Secondary air is response. This allows a section to be cut in or introduced at various levels to ensure solids out as required. Some are actually divided circulation, provide stage combustion for NOx with water cooled or refractory walls. This reduction, and supply air for continuous fines type unit should be matched to the facility combustion in the upper part of the combus- demand pro-file to avoid continual bed tion chamber. slumping and operator attention. When (2) Combustion takes place at about 1600 13-3TM 5-815-1/AFR 19-6Simpo PDF Merge and Split Unregistered Version - http://www.simpopdf.com degrees Fahrenheit for maximum sulfur throughout t ...