MXPA00010674A - Fine solids recycle in a circulating fluidized bed - Google Patents

Abstract

A CFB furnace or reactor unit is provided having enhanced circulation of the reagents finest particles. The improved circulation is achieved by recycling gas having entrained fine particles from a solids collection hopper connected to the solids separator back into the reactor. A system of one or more conduits connects the upper portion of the solids collection hopper with the reactor. The conduit system is equipped with means for recycling gas from the hopper to the reactor. The invention is an inexpensive system which permits recycling of the finest particles that would otherwise be carried over with the gas flow exiting the separator.

Description

RECYCLING OF FINE SOLIDS IN A CIRCULATING FLUIDIZED BED DESCRIPTION OF THE INVENTION The present invention relates generally to the field of reactors, combustors or circulating fluidized bed (CFB) boilers and, in particular, to a simple system that allows recycling of the finer particles that would otherwise be transported with the gas flow coming out of a separator used in combination with such reactors, combustors or CFB boilers. The invention therefore allows for the improved utilization of reagents in the CFB equipment. A necessary condition for the efficient utilization of reagents in a CFB reactor, combustor or boiler, such as the combustion of a liquid fuel and / or the sulphation of a sorbent in a CFB boiler, is the circulation of the reagent particle. in the unit that provides enough time for the reactions to be completed. This is achieved by separating solids from the gases leaving the reactor and recycling these solids back to the unit. Solid recycling systems can be single-stage or multi-stage. For a CFB boiler, such as that shown in Figure 1, a one-stage system typically includes a cyclone separator 10 located downstream of the furnace 20 and a cycle for the recirculation of the solids comprised of a vertical pipe 30 connected to the a lower portion 35 of the cyclone 10 and a non-mechanical device 40 for sealing against the gas by diverting the separator. For example, the non-mechanical device 40 can be a siphon valve. A two-stage system may include two cyclones connected in series (not shown in the Figures), each having its own recirculation cycle, or an impact type particle separator 11, as shown in Figure 2. The separator 11 of Impact-type particles typically have an arrangement of U-shaped joists or similarly shaped elements disposed at the outlet 21 of the furnace. A secondary particle collection device 31 is placed after the impact type particle separator 11 (downstream with respect to the flow of gases and particles entrained through the CFB reactor). A common secondary device 31 particle collector is a mechanical dust collector, such as a polycyclic or multiciclonic dust collector (MDC). In this type of system, most of the solids leaving the furnace 20 are collected and recycled by the separator 11 of particles of the first stage, while in the second stage most of the fine particles that are collected and returned are collected and returned. they pass through the primary particle separator 11 towards the oven 20.

The CFB process could benefit if the previously identified particle separation / collection devices were more effective in collecting fine particulates from the combustion gases. The effect is that less fine particles are recycled before leaving the CFB unit and therefore, less time is available for the reaction of the particles. Although fine particles require less reaction time, most unreacted material leaving the system, such as unburned carbon and unsulfated sorbent in CFB boilers, is concentrated in the fine particles. These fine particles usually have diameters less than 50-70 microns. Fine particles of this size are commonly collected in a powder precipitator or electrostatic precipitator. U.S. Patent No. 5,343,830 to Alexander et al. describes a recycling method that recycles fine particles collected in the powder precipitator or the electrostatic precipitator back to the reactor. However, this method requires the installation of a complex solid recycling system. Any noticeable improvement in the collection of fine particles in the cyclone or any other inertial separation device, known up to now, that it uses the increase of the turbulence or the velocity of the gas of exit, if it is possible, will result in a high pressure drop of prohibitive and in an expense of the parts increased. Alternatively, a mechanical dust collector can be used to increase the collection of fine particles, as taught by the Russian publication Aerodynamic Calculation of Boiler Units (Standard Method), Edited by S. I. Mochan, 3rd. Ed., Leningrad, "Energy", 1977. As shown on page 87 of the same, the gas is removed from the mechanical dust collecting and recycled ash hopper to the mechanical dust collection inlet pipe using a dedicated fan for it. The recycled gas stream is cleaned of ash using high efficiency cyclones placed in the cycle of recycling circulation. The flow of gas from the separator in the same direction in which the solids are collected drags some of the finer particles that would otherwise be carried by the flow of the gas leaving the separator, therefore improving the efficiency of the collection of these particles. This method does not cause an increase in the velocity of the gas in the outlet ducts of the collected elements, which normally contributes greatly to the pressure drop and the erosion potential of the elements. It is an object of the present invention to improve the use of reagents in a CFB reactor unit with a simple and inexpensive method and apparatus. The present invention uses a concept similar to that described immediately above not only in the context of a CFB reactor unit, but also with a simpler installation that allows lower operating and capital costs. Accordingly, a CFB reactor unit or furnace is provided, which has an improved circulation of the finest reactive particles. Improved circulation is achieved by recycling the gas that has entrained fine particles, from a solid collection hopper connected to the solids separator, back to the reactor. A system of one or more conduction tubes that connects the upper portion of the solid collection hopper with the reactor. The pipeline system is equipped with means to recycle the gas from the hopper to the reactor. The invention is a non-expensive system that allows the recycling of fine particles that would otherwise be carried with the flow of gas leaving the separator. The various features of the novelty characterizing the invention are pointed out particularly in the appended claims and form a part of this description. For a better understanding of the invention, its operational advantages and its specific objects obtained by its uses, reference is made to the appended drawings and descriptive matter in which a preferred embodiment of the invention is illustrated. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an elevation view in side section of a CFB boiler of the prior art; Figure 2 is an elevation view in side section of another CFB boiler of the prior art; Figure 3 is an elevation view in side section of a CFB boiler according to the invention; Figure 4 is an elevation view in side section of a second embodiment of a CFB boiler according to the invention; Figure 5 is an elevation view in side section of a solids collection hopper of the invention; Figure 6 is an elevation view in side section of a solid collection hopper according to another embodiment of the invention; Figure 7 is an elevation view in side section of yet another embodiment of a solid collection hopper according to the invention; Figure 8 is an elevation view in side section of yet another embodiment of a solid collection hopper according to the invention; and Figure 9 is an elevation view in side section of yet another embodiment of a solid collection hopper according to the invention. As used herein, the term "CFB combustor" refers to a type of CFB reactor in where a combustion process takes place. Since the present invention is particularly directed to steam boilers or generators that employ CFB combustors as the means by which heat is produced, it should be understood that the present invention can be easily employed in a different class of CFB reactor. For example, the invention could be applied in a reactor that is used for different chemical reactions to combustion processes, or where a gas / solids mixture from a combustion process that occurs elsewhere is provided to the reactor for processing additional, or where the reactor merely provides an enclosure in which the particles or solids are entrained in a gas that is not necessarily a by-product of a combustion process. Referring now to the drawings, in which like reference numbers are used to refer to the same or functionally similar elements, Figure 3 illustrates a CFB unit with a one-stage solids recycling system, similar to that shown in the prior art system of Figure 1. As shown in Figure 3, the improvement of the invention comprises the conduit tube 150 which connects the hopper 160 of the cyclone with the lower end of the furnace 120. A device 170 for recycling the gas from the hopper 160 to the furnace 120 is provided as part of the connection pipe 150. The device 170 can be a fan, ejector or similar device. The one-stage system of Figure 3 which includes a cyclone 100 located downstream of the furnace 120 and a cycle 175 of recirculation of solids constituted by a vertical pipe 130 and a non-mechanical device 140 for sealing against the gas by diverting the separator. For example, the non-mechanical device 140 may be a siphon valve. Figure 4 shows a two-stage system of the type described in Figure 2 having the invention incorporated therein. In Figure 4, a conduit tube 150 has a recycling device 170 connected in series between the hopper 160 and the furnace 120. The conduit tube 150 is in communication through the wall of the hopper 160 at a point below of the secondary collection device 131, which is illustrated as a multicyclone. The recycling device 170 used in this embodiment of the invention may comprise a fan, an ejector or the like. Probably, the conduit tube 150 can be arranged as a conduit from 8"to 24" in diameter (depending on the capacity unit). The use of a "dirty" fan as a recycling device will allow for less auxiliary energy consumption while the use of an air or steam expeller can be more attractive from the installation and maintenance points of view. Impact-type separator 110 is typically an arrangement of U-shaped joists or elements similarly configured in the outlet 210 of the furnace. In each of the modalities shown in the Figures 3 and 4, the larger separated solids pass back to the furnace 120 by a cycle of solids recirculation 175 located below the conduit tube 150 in the bottom portion of the hopper 160. The gas with the finer entrained particles is recycles the hopper 160 from collection of solids back to the furnace 120 via the conduit tube 150. Figures 5-9 show five different configurations or modalities of how the connection of the conduit tube 150 can be made in the upper part of the hopper 160. In Figure 5, the end of the conduit tube 150 is flush with the side of the hopper 160 and ends in the side wall 230 of the hopper. In Figure 6, the end of the conduit tube 150 curves as it leaves the hopper 160, ending in the roof of the hopper 235. In the other embodiments shown in Figures 7-9, at least a portion of the tube 150 or an extension thereof extends through the side wall 230 into the hopper 160. These embodiments are preferred for use when they are recycled from the cyclone hopper 160 of Figure 3, as their constructions reduce the potential for dragging coarse particles in the recycled gas. The conduit tube 150 of Figure 7 extends in a straight line towards the hopper 160 beyond the side wall 230 without bending or changing angles. In Figure 8, the main conduit tube 150 terminates in the side wall, but a plate 155 extends over the opening of the conduit tube 150 within the hopper 160 from the interior of the side wall 230. The plate 155 could be flat or curved. Finally, in Figure 9, the end portion 157 of the conduit tube 150 that extends into the hopper 160 is curved downward. Using the invention to recycle gas flow improves the recirculation of the finer solids back to furnace 120 by entraining particles that would otherwise be entrained and carried with the gas leaving the separator. The solid charged gas flows back into the kiln that is added to the overall recycling capacity where the invention has been installed.

Recycling fine particles using line pipe 150 simplifies the recycling process and reduces the cost of increasing the efficiency of using reagents in a CFB reactor or boiler unit. Since the specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, those skilled in the art will appreciate that changes can be made in the form of the invention covered by the following claims without departing from of such principles. For example, the present invention can be applied to a new construction involving circulating fluidized bed reactors or combustors, or to the replacement, repair or modification of existing circulating fluidized bed reactors or combustors. In some embodiments of the invention, certain features of the invention can be used to advantage without corresponding use of other features. Accordingly, all changes and modalities properly fall within the scope and equivalents of the following claims.

Claims (7)

1. CLAIMS 1. A circulating fluidized bed unit having a reactor, at least one solids separator placed downstream of the reactor and a solid circulation cycle, the circulating fluidized bed unit that has increased the recovery of the reactive particles more thin, is characterized in that it comprises: a solid collection hopper connected to at least one solids separator and connected to the solid recirculation cycle; at least one pipe that connects the solid waste collection hopper to the reactor, the hopper end of the pipe under the connection of the solid collection hopper to the solid recirculation cycle; and gas recirculating means provided for at least one line pipe for recycling gas that carries finer reactive particles from the solid waste collection hopper to the furnace.
2. 2. The circulating fluidized bed unit according to claim 1, characterized in that at least one conduit tube terminates in the enclosure of the solids collection hopper.
3. 3. The circulating fluidized bed unit according to claim 1, characterized in that at least one pipeline extends through the enclosure of the solid waste collection hopper into the solid waste collection hopper.
4. The circulating fluidized bed unit according to claim 3, characterized in that at least one conduit tube extends directly into the solids collection hopper.
5. 5. The circulating fluidized bed unit according to claim 3, characterized in that one end of the hopper of the at least one guide tube is folded down into the solid waste collection hopper.
6. The circulating fluidized bed unit according to claim 2, characterized in that the end of the hopper of the at least one guide tube comprises a plate extending down from the inside of the hopper enclosure on the position where at least a driving tube is attached.
7. 7. A method for increasing the recovery and return of the finer reactive particles circulating in a fluidized bed unit to a circulating fluidized bed unit reactor, the circulating fluidized bed unit has a reactor, at least one separator of solids placed downstream of the reactor and a cycle of recirculation of solids, characterized in that it comprises: providing a solids collection hopper connected to at least one solids separator and connected to the solid recirculation cycle; providing at least one line pipe that connects the solid waste collection hopper to the reactor; positioning the hopper end of at least one pipeline above the connection of the solids collection hopper to the solids recirculation cycle; and recirculating gas from and through at least one line pipe to recycle the finest reactive particles from the solids collection hopper to the furnace.