Combustion, fluid-bed

FIGURE 23.14 Schematic diagram of pulse combustion fluid bed dryer. (From Lockwood, R.M., Pulse combustion fluidizing dryer, U.S. Patent No. 4395830, 1983). 

The only design of the pulse combustion fluid bed dryer appears to be the one patented by Lockwood (1983). To avoid attenuation of the pulsating gas stream by a perforated gas distributor, the flue gases from a pulse combustor enter the bed of particulate material just above the solid floor, which rotates under a plurality of ducting blades adjacent to the floor. The radially spaced blades are fixed to the central hub at one end and to the inner annular baffle at the other one (Figure 14.11). The space between the baffle and dryer wall forms a gas manifold connected to the tailpipe of a pulse combustor. Each... 

RGURE 14.11 Pulse combustion fluid bed dryer (a) design principle, (b) partial perspective of the ducting blade. (From Lockwood, 1983.)... 

H. E. Bamer, H. Beisswenger, and K. E. Bamer, "Chemical Equilibrium Relationships AppHcable in Fluid Bed Combustion," Proceedings of the Ninth International Conference on Fluidi d-Bed Combustion, Boston, Mass., May 4—7,1987. 

W. Wein, Flow Dynamics of Atmospheric Fluid Bed Combustion Systems and their Effect on SO Capture and NO Suppression, trans. Lurgi from UGB Magafine, Feb. 1985, pp. 119-123. 

The first of these reactions takes place at temperatures of about 150°C, the second reaction proceeds at about 550—660°C. Typical furnaces used to carry out the reaction include cast-iron retorts the Mannheim mechanical furnace, which consists of an enclosed stationary circular muffle having a concave bottom pan and a domed cover and the Laury furnace, which employs a horizontal two-chambered rotating cylinder for the reaction vessel. The most recent design is the Cannon fluid-bed reactor in which the sulfuric acid vapor is injected with the combustion gases into a fluidized bed of salts. The Mannaheim furnace has also been used with potassium chloride as the feed. 

Hydrocarbon fumes evolved during anode baking are generally disposed of by burning. Additional fuel is required to support combustion because the hydrocarbon concentration is low. In some plants these products are now absorbed in a fluid bed of alurnina for burning in a concentrated form. This treatment also catches the fluoride evolved during anode baking. 

Significant waste heat may be recovered from the high (about 600°C) kiln off-gas. Pre-heating combustion air or feed ore improves the energy efficiency of the process. Reduction of barite in a fluid bed with CO and/or hydrogen has been performed on an experimental scale. 

While the rotary dryer shown is commonly used for grains and minerals, this system has been successfully applied to fluid-bed diying of plastic pellets, air-hft diying of wood fibers, and spray drying of milk solids. The air may be steam-heated as shown or heated By direct combustion of fuel, provided that a representative measurement of inlet air temperature can be made. If it cannot, then evaporative load can be inferred from a measurement of fuel flow, replacing AT in the set point calculation. 

A fluid-bed incinerator uses hot sand as a heat reservoir for dewatering the sludge and combusting the organics. The turbulence created By the incoming air and the sand suspension requires the effluent gases to be treated in a wet scrubber prior to final discharge. The ash is removed from the scrubber water by a cyclone separator. The scrubber water is normally returned to the treatment process and diluted with the total plant effluent. The ash is normally buried. 

Fluiaized-Bea Boilers As explained in the earlier discussion of coal combustion equipment, the furnace of a fluid-bed boiler has a unique design. The system as a whole, however, consists mainly of standard equipment items, adapted to suit process requirements. The... 

Fluid bed boilers have also been applied as a cure to sulfur dioxide air pollution from power plants. Various schemes have been developed in which combustion of a sulfur containing fuel takes place in a fluidized bed of particles which absorb or react with sulfur dioxide. The particles are usually regenerated to recover sulfur, which often has enough by-product value to make a significant contribution to process economics. 

The advantages of fluid bed combustion over the more traditional technology arise from the increased turbulence provided by the bed particle action. This fluidization increases the interaction of the fuel particles with the combustion air and creates a veiy accelerated combustion environment for the incoming fuel. Additionally, the sand, initially heated to an ignition temperature for the incoming fuel, provides a... 

The chamber is externally insulated and clad. Combustion equipment for solid fuel may be spreader or traveling-grate stokers or by pulverized fuel or fluid bed. Oil and gas burners may be fitted either as main or auxiliary firing equipment. The boilers will incorporate superheaters, economizers and, where necessary, air preheaters, grit arresters, and gas-cleaning equipment to meet clean air legislation. 

In the composite boiler, a watertube chamber directly connected to a single-pass shell boiler forms the combustion space housing the fluid bed. In order to fluidize the bed the fan power required would be greater than that with other forms of firing equipment. 

To its advantage, the fluid bed may utilize fuels with high ash contents, which affect the availability of other systems. It is also possible to control the acid emissions by additions to the bed during combustion. They are also less selective in fuels and can cope with a wide range of solid-fuel characteristics. 

Figure 24.18 Fluid-bed combustion system with sectioned bed...

PF burners and fluid beds best meet requirements for dual- and triple-fuel firing including solid fuel as one option. PF burners are particularly suitable, as no static grate exists to compromise the design. They also have a combustion geometry which is similar to gas and oil, and therefore the flame can be arranged to allow full development of flame shape and maximum radiant heat transfer surface utilization. 

Econ-Abator A process for oxidizing hydrogen sulfide and other sulfur compounds in waste gases by fluid bed combustion in the presence of an oxide catalyst. Licensed by ARI Technologies. In 1992 there were 90 installations. 

Grate-fired combustors are in use for old biomass-fired plant, while fluid bed combustors are rapidly becoming the preferred technology for biomass combustion because of their low NOx emissions. 

Sulfur dioxide is manufactured mostly by combustion of sulfur or its iron sulfide mineral, pyrite, FeS2, in air. The flame temperatures for such combustion of sulfur in the air are usually in the range 1,200 to 1,600°C. Many types of sulfur burners are available and are used to produce sulfur dioxide. They include rotary-kiln, spray, spinning-cup and air-atomizing sulfur burners. Selection and design of burners depend on quality of sulfur to be burned, and rate and concentration of sulfur dioxide to be generated. Pyrites or other metal sulfides may be burned in air in fluid-bed roasters to form sulfur dioxide. 

Chemical prqperties are also used in the largest field of application for the rare earth elements as catalysts. Most important are the cracking catalysts for the petroleum industry. The rare earth elements are combined into molecular sieves (Y-Zeolite) and serve in fluid bed or fixed bed reactors to increase the yield of gasoline. In addition thereto, there are the combustion catalysts for automobiles and for air pollution control. 

The fluid-bed combustion method (2) has been chosen, however, for process development in the regeneration of spent melts from the hydrocracking of coal. In this method, from one to two parts by weight of spent melt is generated for each part of coal fed to the hydrocracking process. The carbonaceous residue, sulfur, and ammonia retained in the melt are burned out with air in a fluidized bed of inert solids. The zinc chloride is simultaneously vaporized, the ash separated from the overhead vapors, and the zinc chloride vapor is condensed as pure liquid for return to the process. 

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