Furnaces – Refuse incinerator – Refuse suspended in or supported by a fluid medium
Patent
1994-11-04
1997-02-11
Denion, Thomas E.
Furnaces
Refuse incinerator
Refuse suspended in or supported by a fluid medium
122 4D, 110216, 55444, 55465, F23G 500
Patent
active
056010397
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method and apparatus for providing a gas seal in a return duct and/or controlling the circulating mass flow in a circulating fluidized bed reactor, which is provided with a slot-shaped, vertical return duct defined by two, mainly vertical plane wall panels and ends joining these.
Circulating fluidized bed reactors are used, to an ever increasing extent, for combusting and gasifying various fuels, and as reactors in diverse chemical processes. They provide efficient mixing of gaseous and solid particles, which results in a uniform temperature of the process and faultless process control. In circulating fluidized bed reactors, the gas flow rate is maintained so high in the reaction or combustion chamber that a considerable portion of the bed material, entrained with the gases, flows out of the chamber. The major part of this solid material, i.e. the circulating mass, is separated from the gases in a particle separator connected with the chamber and is returned to the lower section of the combustion chamber via a return duct.
In circulating fluidized bed reactors such as PYROFLOW boilers, cyclone separators are used for separating circulating bed material from the gas. The circulating material is in this case returned via a return duct from the lower section of the cyclone to the lower section of the combustion chamber. The lower part of the return duct is provided with a member which serves as a gas seal preventing the gas from flowing via the return duct to the separator.
Fuel feed in the circulating fluidized bed reactors is often arranged in the return duct, where the fuel efficiently mixes with the circulating mass. The fuels generally contain some volatile substances which are separated from the solid fuel already in the return duct. Therefore, the fuel feed has to be arranged in the return duct below the loop seal so that these volatile substances are introduced into the combustion chamber thereby not causing any trouble, which would be the case if they flowed upwardly in the return duct.
Heat recovery from the circulating mass is awkward to arrange in a conventional loop seal construction. For regulating the circulating mass temperature in the return duct, the return duct is equipped with a separate heat exchanger, e.g., such as is provided with a fluidized bed. However, such an arrangement takes a lot of space, it is complicated and, naturally expensive.
In circulating fluidized bed boilers, heat is generally recovered by water walls of the combustion chamber and by heat transfer surfaces disposed in the upper section of the boiler. In some cases, however, it is desirable for the temperature regulation that heat could be recovered also from the circulating mass before returning the material from the particle separator to the lower section of the combustion chamber. In respect of optimum combustion, regulation of temperature is desirable in the combustion chamber, especially if several fuels having different heat values are combusted in the same combustion chamber. In order to achieve optimum sulphur absorption, the desired temperature of the combustion chamber is in the range of 800.degree. to 950.degree. C. In the earlier known methods, regulation of the combustion temperature is problematic, especially if the heat value of the fuel or the load of the boiler vary greatly.
Temperature regulation in the boilers of prior art is effected, for example, by changing the air excess in the combustion chamber, by recirculating flue gases to the combustion chamber, by altering the suspension density in the combustion chamber or by dividing the bed into various operational sections. Lowering of the combustion temperature by increasing the air excess lowers the boiler efficiency because the flue gas losses will increase and the power requirement of the air blower will grow. Recirculation of flue gases increases the volume of the gas flowing through the boiler, thereby growing the power requirement of the boiler and raising the investment and operating costs.
According to prior
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Denion Thomas E.
Foster Wheeler Energia OY
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