METHOD AND APPARATUS FOR TREATING GASES AND/OR SOLID MATERIAL IN A CIRCULATING FLUIDIZED BED REACTOR

Abstract: A method and apparatus for treating gases and/or solid material in a circulating fluidized bed reactor, which is provided with a mixing chamber (12) for introducing gases into and mixing them with particulate solid material, an uptake shaft (14) for leading the gas suspension from the mixing chamber into a particle separator (16) and a duct (18) for returning the separated solid material into the mixing chamber. The uptake shaft is provided with heat transfer surfaces (52, 54) for cooling the gas suspension. The reaction time of the solid material is extended in the reactor by arranging the uptake shaft with at least one enlargement section (36, 38) where the flow rate of the gas suspension decreases.

Description: METHOD AND APPARATUS FOR TREATING GASES AND/OR SOLID MATERIAL IN A CIRCULATING FLUIDIZED BED REACTOR The present invention relates to a method and apparatus for treating gases and/or solid material in a circulating fluidized bed reactor. The circulating fluidized bed reactor according to the invention comprises firstly, a reactor with - means for introducing gas into the reactor; - a mixing chamber for mixing solid material with the gases; and - an uptake shaft for transferring a solids-containing gas suspension from the mixing chamber into a particle separator; and secondly, - the particle separator for separating solid material from the solids-containing gas suspension; - a gas outlet duct for discharging gases from the particle separator; and - a return duct for returning separated solids from the particle separator to the mixing chamber. The circulating fluidized bed reactor is also provided with inlet and outlet conduits for solid material. The gases are introduced as fluidizing gas to the lower section of the reactor, for fluidizing the solid material and for conveying it as a gas suspension from the mixing chamber via the uptake shaft to the particle separator. Solids separated in the particle separator are returned to the mixing chamber of the reactor. The above described gas cooler FLUXFLOW™ of a circulating fluidized bed type provided with cooling surfaces is ap licable for dry cleaning of, for example, gases which are produced in partial oxidation of bio masses, peat, or coal and which contain dust and tar and other condensable components. A large amount of solids flowing from the particle separator into the mixing chamber rapidly cools the gas introduced into the mixing chamber to a temperature level at which the harmful gaseous or fluid components contained in the gases condense and the tar-like substances are converted into dry solids. Thereafter, the solids are readily separable from the cooled gas. The FLUXFLOW™ gas cooler is also suitable for chemical processing, e.g., reduction of iron concentrate. Due to limitations in mass transfer, many processes, however, require a longer retention time for solids (circulating mass) than what is normally provided by FLUXFLOW mass circulation in order to induce desired reactions. It has been established that, in circulating fluidized bed reactors, about 80 to 90 % of the mass circulation continuously stays in the mixing chamber and only about 10 to 20 % is actually circulating in the transfer system composed of a gas duct, particle separator such as a cyclone and a solids return duct. The total retention time, however, normally takes less than a minute to some minutes only. A need exists for extending the retention time. It has been established that if the amount of circulating solids is continuously increased in order to provide a larger contact surface between gas and solids, a limit will be met, beyond which the function of the mixing chamber is disturbed and it will, e.g. drop solids downwards and further out through the gas inlet at the bottom of the mixing chamber, thereby overloading the system. An object of the present invention is therefore to provide a method and apparatus for extending the retention time of the mass circulation in the circulating fluidized bed reactor. Another object of the invention is to provide a method and apparatus for increasing the reactive surface area between the solid material and the gas in the circulating fluidized bed reactor. A further object of the invention is to provide a cooling structure of a circulating fluidized bed reactor type, which is provided with a smaller heating surface than the previously known corresponding structures. A still further object of the invention is to provide a cooling structure of a circulating fluidized bed reactor type, which is better protected against erosion than the conventional convection type structures. A characteristic of the method according to the invention for providing the objects of the invention is that the retention time of the solids in the circulating fluidized bed reactor is extended by arranging the uptake shaft between the mixing section and the particle separator with at least one zone having a decelerated, vertical flow rate. The zone having the decelerated flow rate is conveniently provided by increasing the cross-sectional area of the uptake shaft in this area. If necessary, the uptake shaft may be provided with a plurality of zones having a decelerated flow rate. In practice, the cross-sectional area of the uptake shaft is increased by providing the uptake shaft with, e.g., a fluidizing chamber of a mixing chamber type, in which fluidizing chamber the flow rate of the gas suspension is lower than in the section of the uptake shaft preceding the chamber and in the section of the uptkae shaft subsequent to the chamber. The cross-sectional area of the uptake shaft is smaller than the cross-sectional area of the chambers. The apparatus according to the invention for treating gases and/or solids in a circulating fluidized bed reactor is characterized in that the uptake shaft between the mixing chamber and the particle separator is provided with at least one enlargement section, where the flow rate of the gas suspension flowing in the uptake shaft slows down. The uptake shaft is mainly formed of a duct narrower than the cross-sectional area of the mixing chamber, said duct being provided with at least one enlargement section, the cross-sectional area thereof being of the order of the cross-sectional area of the mixing chamber. The enlargment sections are e.g. fluidizing chambers, preferably of a spouted bed type. The spouted bed type fluidizing chamber comprises, e.g., an upwardly enlarging, tapered lower section, a cylindrical middle section and an upwardly contracting, tapered upper section. The lower section of the chamber is provided with a simple inlet for introducing gases into the chamber. The spouted bed type chamber arrangement is especially applicable to arrangements for treating hot, dusty gases, containing solids which are in an at least partly molten state and which therefore tend to clog other types of inlet systems, such as conventional fluidized bed grids. This is also an important reason why the mixing chamber of the FLUXFLOW™ reactor is of a spouted bed type. In the arrangement according to the invention, the walls of the fluidizing chamber which constitutes the mixing chamber and/or the enlargement section are conveniently partly or completely constructed of heating surfaces in order to regulate the temperature in the circulating fluidized bed reactor. In accordance of a preferred arrangment, the tapered lower sections of the chambers are defined by heating surfaces. The present invention provides a simple arrangement for slowing down the vertical flow of both gas and especially solids in the uptake shaft of the circulating fluidized bed reactor, thereby extending the retention time and the mutual reactive surface area of the gas and the solids. The method of the invention is suitable for treating all such particulate materials to which a circulating fluidized bed reactor is applicable and in which a retention time/reaction surface area is needed for processing. The method according to the invention is applicable to treatment of both solids and gases. In the circulating fluidized bed reactor, reactions may take place in solids or in gas or simultaneously in both of them. For example, hot gas may induce desired reactions in a solid material, and cold gas may correspondingly prevent or slow down undesirable reactions in a solid material or possibly induce desired reactions. A solid material cooler than gas may cool a hot process gas for stopping undesirable reactions in the gases. In the circulating fluidized bed reactor, it is also possible to circulate two different solids which react with each other, e.g., when affected by hot gases. The method according to the invention is applicable to such reaction processes as reduction and oxidation; but it is also applicable to catalytic reactions in which the processing object is the gas itself. In this case, the mass circulation is formed by the catalyte used in the process. A preferred construction according to the invention is a circulating fludized bed reactor where the reaction temperature may be set by means of a heating surface or heating surfaces disposed therein. The temperature may be regulated by heating surfaces disposed in the mixing chamber or the uptake shaft for inducing desired reactions in the gas and/or solids circulating in the circulating fluidized bed reactor. In conditions defined by the composition, pressure and temperature of the process gas, the circulating fluidized bed reactor is capable of processing such solids as also constitute the mass circulation of the circulating fluidized bed reactor. As an example of such arrangement is preheating and prereduction of iron concentrate, in which process the concentrate is preheated by hot process gas obtained by the iron melting process. The reduction temperature being preferably 800 to 950°C, the circulating fluidized bed reactor has to be provided with a cooling surface for maintaining this temperature because preheating and potential prereduction of iron concentrate do not provide a sufficient cooling effect. The calculations indicate, however, that in this case the heat transfer surface is only needed in one unit, e.g. the lowermost unit, which is the mixing chamber. Necessary cooling surfaces are conveniently wall surfaces, preferably in the bottom tapers of the spouted bed units. The cooled, tapered parts of the mixing chamber or the enlargement sections provide a remarkably good heat transfer effect per each heating surface when compared with the cooling effect of a conventional convection heating surface with a close spacing of tubes disposed in the gas duct. Heat transfer to the walls of the spouted bed type fluidizing chamber is effective because the suspension density is high in the vicinity of the walls and, on the other hand, a high turbulence prevails in the chamber. In conventional heat exchangers, for example in the uptake shafts of circulating fluidized bed reactors, the particle density is lower in the vicinity of the heating surfaces and the particles tend to flow along the surfaces because the turbulence is lower there. Thus, a very good cooling effect is achieved by several superimposed, cooled spouted bed type enlargement sections. A great advantage of the preferred structure according to the invention is a smaller heating surface when compared with prior- art arrangements. When the heating surface is located on the wall of the bottom taper of a spouted bed type chamber, it is also much easier to prevent erosion than with convection heating surfaces disposed inside the chamber. In accordance with an alternative structure according to the invention, a connecting part betwen the superimposed fluidizing chambers is formed of a tubular heating surface so that the gas which is cooling is forced through a plurality of adjacent vertical cooling tubes, whereby heat is transferred to a cooling medium. In this manner, additonal heating surface is received for the structure. The grain size of the solids may be adjusted, for example, by grinding so that they are suitable for processing, according to the particle density, in the circulating fluidized bed reactor by selecting suitable vertical flow rates of the gas. A typical grain size ranges from 10 to 500 u. Typical flow velocities at the inlets of the spouted bed unit are 5 to 100 m/s and in the larger cross- sectional area of the fluidized beds from 1 to 10 m/s. The arrangement of the invention provides, e.g. the following advantages: - the solids retention time may be extended in the mass circulation; - the solids reaction surface area in contact with gas may be increased; - the solids is in proper contact with the gas in the uptake shaft after the mixing chamber; - when wall heating surfaces are used in the enlargement section, the total need of the heating surfaces is reduced; and - inexpensive wall heating surfaces may be used to replace convection surfaces susceptible to erosion. The invention is further described in the following, by way of example, with reference to the accompanying drawings, in which Fig. 1 is a schematic illustration of an apparatus according to the invention, Fig. 2 is a schematic illustration of a second embodiment of the appratus according to the invention, and Fig. 3 is a cross section of the apparatus of Fig. 2 taken along section A— . Fig. 1 illustrates a circulating fluidized bed reactor 10, comprising a mixing chamber 12, gas duct 14, particle separator 16 and solids return duct 18. In this case, the separator is a cyclone. The mixing chamber is provided with a gas inlet 20 and a feed conduit 22 for solids. The mixing chamber is also provided with an opening 24 for returning solids from return duct 18. The upper part 26 of the gas duct is connected to the cyclone 16 by a conduit 28. The cyclone is in communication with a gas outlet duct 30 and the upper part of the return duct 18. The return duct is provided with an outlet conduit 32 for solids, wherethrough solids treated in the reactor are discharged from the mass circulation. The gas duct is formed of uptake shafts 34 and 35 narrower than the mixing chamber and of two spouted bed type fluidizing chambers 36 and 38. The fluidizing chambers form two enlargement sections of the gas duct, where especially the vertical flow rate of the solids is slower in comparison with the flow rate in uptake shafts 34 and 35 leading to the enlargement sections. The fluidizing chambers comprise the following superimposed sections: upwardly enlarging, tapered lower sections 40 and 42, cylindrical middle sections 44 and 46 and upwardly contracting, tapered upper sections 48 and 50. In this arrangement, the tapered lower sections are comprised of cooling surfaces 52 and 54. The mixing chamber is also formed of a tapered lower section 56, a cylindrical middle section 58 and tapered upper section 60. The walls of the tapered lower section are formed of heating surfaces 62. When solids are treated with hot process gas in a reactor in accordance with Fig. 1, the hot process gas is introduced via inlet 20 into the mixing chamber 12, where gas is mixed with cooled solids issuing from the return duct 18. The temperature of the process gas decreases fast because heat is transferred to the solid material. Part of the heat is also efficiently transferred to heating surfaces 62, whereby it may be recovered. If necessary, untreated solids from outside the mass circulation may be added to the system via conduit 22 into the mixing chamber. Correspondingly, treated solids may be removed by conduit 32. Feed and discharge of solid material may naturally be arranged elsewhere. From the mixing chamber, the suspension of gas and solids rises via duct 34 into a first enlargement section 36 of the gas duct or uptake shaft. The enlargement section is a spouted bed type fluidizing chamber. The mixing chamber is of the same type. The hot gas suspension penetrates deep into the chamber in the middle thereof. Simultaneously, the flow rate slows down as the cross-sectional area increases. In addition to the vertical flow, also lateral flow as well as downwardly directed flow, especially of solids, start to occur, the downwardly directing flow occurring in the peripheral areas of the fluidizing chamber. The downwardly flowing solid material flows towards the inlet and meets the gas suspension coming in therethrough, which gas suspension refluidizes the solid material directing it upwards. In this manner, an internal solids circulation is created in the chamber. A portion of the solid material flows upwards via conduit 35 into the second fluidizing chamber 38 where internal solids circulation similar to that in the previous chamber takes place. The fluidizing chambers considerably extend the retention time of solids in the uptake shaft 14. Furthermore, the temperature regulation of the mass circulation is facilitated by the heating surfaces 52 and 54 arranged in the chambers. The flow rate of the gas suspension coming from the chamber 38 is increased in the narrow duct 28 to the level required by the operation of the cyclone separator 16. Duct 28 leads the gas suspension tangentially to the vortex chamber of the cyclone. Fig. 2 discloses a second embodiment of the invention. Items corresponding to those of Fig. 1 are denoted with the same reference numerals. The circulating fluidized bed reactor of Fig. 2 deviates from the reactor shown in Fig. 1 in that, instead of the second fluidizing chamber 38, a tubular heat exchanger 64 is arranged above the first fluidizing chamber 36 in the gas duct. The tubular heat exchanger where gas flows in tubes is directly arranged in the upper section of the fluidizing chamber 36 so as to lead the gas suspension from said chamber 36 via tubes 66 into the duct 28 which leads the gas suspension into the cyclone. In the tubular heat exchanger, the gas suspension emits heat to a cooling medium 68 which may be e.g. water and which surrounds the tubes 66. Fig. 3 shows a cross section of the tubular heat exchanger 64. It is not an intention to limit the invention to the embodiments as described hereinabove, but it may be applied within the inventive idea defined by the accompanying claims. Thus, for example heat transfer surfaces are not necessary in all processes. An appropriate number of enlargement sections may be chosen according to process parameters. In treatment of cooler and less contaminating gases, conventional fluidized bed grids may also be used instead of spouted bed arrangements.

Claims: Claims 1. A method of treating gases and/or solid material in a circulating fluidized bed reactor, firstly comprising a reactor with - means for introducing gas into the reactor; - a mixing chamber for mixing solid material with gases; and - an uptake shaft for transferring the solids-containing gas suspension from the mixing chamber into a particle separator; and secondly - a particle separator for separating solid material from the solids-containing gas suspension; - a gas outlet duct for discharging gases from the particle separator; and - a duct for returning the separated solid material from the particle separator into the mixing chamber of the reactor, and which method comprises leading the gases as fluidizing gas to the lower section of the mixing chamber for fluidizing the solid material and carrying said solid material as a gas suspension from the mixing chamber via the uptake shaft into the particle separator, characterized in that the retention time of the solid material in the circulating fluidized bed reactor is extended by arranging at least one zone having a decelerated, vertical flow in the uptake shaft between the mixing chamber and the particle separator. 2. A method as claimed in claim 1, characterized in that the uptake shaft between the mixing chamber and the particle separator is provided with a zone having a decelerated flow rate by increasing the cross-sectional area pf the uptake shaft in this zone. 3. A method as claimed in claim 1, characterized in that the retention time of the solid material in the circulating fluidized bed reactor is extended by arranging two or more zones having decelerated flow rates in the uptake shaft between the mixing chamber and the particle separator. 4. A method as claimed in claim 1, characterized in that the retention time of the solid material in the circulating fluidized bed reactor is extended by leading the solids- containing gas suspension from the mixing chamber to the particle separator via at least one fluidizing chamber system, whereby the gas suspension is first conducted from the mixing chamber into an inlet duct, the cross-sectional area of said inlet duct being narrower than that of the mixing chamber, in which inlet duct the flow rate of the gas is accelerated; and therefrom the gas suspension is further conducted into a chamber, the cross-sectional area of which is larger than that of the inlet duct, in which chamber t e flow rate of the gases and especially that of the solids contained in the gas suspension is decelerated and the retention time of the solid matrial is extended in the fluidizing chamber. 5. A method as claimed in claim 4, characterized in that the fluidizing chamber is of a spouted bed type, where the inlet of the gas into the fluidizing chamber brings about a spouted bed type internal flow, which extends the retention time of the solid material in the fluidizing chamber. 6. A method as claimed in claim 5, characterized in that the solids-containing hot gases are conducted from the mixing chamber via two spouted bed type chambers into a particle separator. 7. A method as claimed in claim 1, characterized in that the temperature of the circulating fluidized bed reactor is regulated, by means of heating surfaces arranged in the mixing chamber and/or uptake the shaft, suitable for inducing desired reactions in the gas and/or solid material. 8. A method as claimed in claim 1, characterized in that hot process gases are treated in the circulating fluidized bed reactor. 9. A method as claimed in claim 8, characterized in that metal concentrate is preheated and/or prereduced in the circulating fluidized bed reactor by hot process gases issuing from a metal melting process. 10. A method as claimed in claim 8, characterized in that process gases are cooled in a circulating fluidized bed reactor for recovering heat from said process gases. 11. A method as claimed in claim 10, characterized in that hot gases are cooled by heating surfaces arranged in the uptake shaft and/or mixing chamber. 12. A method as claimed in claim 1, characterized in that hot gases are cooled in a mixing chamber by cooled solids returning form the particle separator. 13. A method as claimed in claim 1, characterized in that hot gases are introduced into the mixing chamber for raising the temperature of the solids in the circulating fluidized bed reactor to a level suitable for inducing desired reactions in the solids. 14. A method as claimed in claim 1, characterized in that hot gases are introduced into the mixing chamber for inducing reactions between the solid material and the gases. 15. A method as claimed in claim 1, characterized in that solid material is circulated in the circulating fluidized bed reactor, said solid material functioning as a catalyst in reactions taking place in hot gases. 16. A method as claimed in claim 1, characterized in that two different solids are circulated in the circulating fluidized bed reactor, which react with each other as a result of the effect by hot gases. 17. An apparatus for treating gases and/or solid material in a circulating fluidized bed reactor, comprising - a mixing chamber (12) for mixing solid material with gases; - an uptake shaft (14) for leading a solids-containing gas suspension from the mixing chamber into the particle separator (16), - a particle separator (16) for separating solid material from the solids-containing gas suspension; - a gas outlet duct (30) for discharging gases from the particle separator; - a duct (18) for returning the separated solid material from the particle separator into the mixing chamber, and - inlet and outlet conduits (22, 32) for solids, characterized in that - the uptake shaft (14) between the mixing chamber (12) and the particle separator (16) is provided with at least one enlargement section (36, 38) where the flow rate of the gas suspension flowing in the uptake shaft decelerates. 18. An apparatus as claimed in claim 17, characterized in that the uptake shaft (14) is composed of a duct narrower than the mixing chamber (12), said duct being provided with at least one enlargement section (36), the cross- sectional area thereof being substantially the same as that of the mixing chamber. 19. An apparatus as claimed in claim 17, characterized in that the uptake shaft (14) is formed of a duct narrower than the mixing chamber, said duct being provided with two enlargement sections (36, 38). 20. An apparatus as claimed in claim 17, characterized in that the uptake shaft (14) connected with the mixing chamber is provided with at least one spouted bed type fluidizing chamber where the cross-sectional area of the duct is enlarged and the flow rate of the solids decelerates. 21. An apparatus as claimed in claim 20, characterized in that the spouted bed type fluidizing chamber (36, 38) comprises - a lower section (40, 42) in the shape of an upwardly enlarging taper, - a cylindrical middle section (44, 46), and - an upper section (48, 50) in the shape of an upwardly contracting taper. 22. An apparatus as claimed in claim 17, characterized in that the walls of the mixing chamber (12) and/or spouted bed type fluidizing chamber (36, 38) are formed of heat transfer surfaces (52, 54, 62). 23. An apparatus ass claimed in claim 17, characterized in that the tapered lower section of the mixing chamber and/or spouted bed type fluidizing chamber is formed of heat transfer Surfaces (52, 54, 62).

Inventors: ARPALAHTI OLLI, WESTERLUND KURT, ARPALAHTI, OLLI, WESTERLUND, KURT
Applicants: AHLSTROEM OY, A. AHLSTROM CORPORATION
Application year: 1991
Main classification: B01J8
IPC codes: B01J8, B04C9, C10J3, C21B13, C21D1, F27B15