Patent Publication Number: US-6214065-B1

Title: Method of operating a fluidized bed reactor system, and fluidized bed reactor system

Description:
The present invention refers to a method and system of operating a fluidized bed reactor system. 
     Fluidized bed reactors, particularly circulating fluidized bed (CFB) reactors, are extremely useful in practicing a wide variety of reactions, such as combustion and gasification of fuel material, in atmospheric or pressurized conditions. Gasification in a fluidized bed reactor is an attractive way to convert energy of fuel material into a more useful form, producing combustible gas. Combustion of fuel in a fluidized bed reactor may produce steam to drive a steam turbine. However, under many circumstances, the gas discharged from the reactor (e.g., fuel product gas) may contain undesirable substances such as extremely fine dust and tar-like condensable compounds. These substances tend to turn sticky especially below certain temperatures, and therefore deposit or accumulate on surrounding surfaces, in particular surfaces of gas cooling devices, having an adverse effect on the surfaces and heat transfer. 
     When the hot gas coming from the gasification/combustion reactor is introduced into a gas, the cooler above mentioned undesirable substances easily block the inlet of the gas cooler or heat transfer surfaces disposed therein. Especially, very fine carbon (soot), fine ash particles, alkali fumes, alkali oxides or liquid compounds tend to form deposits in the gas cooler. 
     In the gasification processes, the gas has to be cleaned before further use. The carbon particles (soot) contained in the gas is very fine, has typically a grain size of 0.1-5 μm, and is sticky. Such sticky fine material is difficult to separate by filtration. The gas can be filtrated by adding into the gas coarser non-sticky particles, having a grain size distribution of -200 μm. Those particles together with fine soot are able to form a filter cake on filter elements. Filtration properties will be further improved if the added particles are porous. 
     The problem of fouling of gas cooling surfaces has been addressed by using a direct heat transfer system, such as suggested in U.S. Pat. Nos. 4,412,848 and 4,936,872. In these patents, product gas is led into fluidized bed gas coolers, and the fouling components are captured by particles of the fluidized bed. 
     The use of a separate fluidized bed, as described above, is hardly an ideal solution to the problem, however, since the additional bed consumes space and requires construction and maintenance of different components, which can make costs prohibitive. Using indirect recuperator heat exchangers has also been found to be unacceptable, however, due to exhaust fouling difficulties. 
     The fouling problem described above is particularly acute under pressurized conditions, e.g., superatmospheric pressure of about 2-50 bars. Under such pressurized conditions, conventional steam soot blowers do not work properly. 
     The problems as indicated above do not exist solely during gasification, but also during combustion of a number of different types of fuel in a fluidized bed. For example, when brown coal is burned, the flue gases contain alkali species which condense on cooling surfaces, accumulating on the surfaces, fouling them, and causing corrosion of surrounding surfaces. Difficulties also occur particularly in the combustion of municipal waste or sludge. 
     It is, therefore, the primary object of the present invention to provide a method and system which minimize the problem of gas particles depositing on gas cooling surfaces. 
     It is also the object of the present invention to provide a method and system which minimize the fouling and corroding of cooling surfaces. 
     It is further the object of the present invention to provide a method and system which improve heat transfer from gas containing very fine particles or tar-like condensable compounds. 
     The above mentioned objects are achieved in accordance with the present invention by a method and system including the features recited in the pending claims. 
     The basic concept behind the invention thereby is to utilize the very same solids which are used as bed material (e.g., inert bed material such as sand and/or reactive bed material such as limestone) in fluidized bed reactors to mechanically scrub the gas cooler&#39;s cooling surfaces so as to prevent accumulation of deposits thereon, and or to remove deposits therefrom. 
     It has earlier been suggested in applicant&#39;s co-pending patent application PCT/FI95/00438 that bed material is introduced from a separate bed material supply source into the gas cooler for cleaning the cooling surfaces. Alternatively, in circulating fluidized bed reactors where the main part of the solid bed material is separated from gases exhausted from the reactor chamber in a separator (e.g., a cyclone separator or similar device) before introducing the thus cleaned gas into the gas cooler, it was suggested to periodically decrease the efficiency of the separator (cyclone) and allow non-separated particles to flow with the gas into the gas cooler. 
     The present invention also solves the above mentioned problems of particles depositing on gas cooling surfaces, and it does so in a very simple and easily controllable manner. The present invention provides an alternative method to supply easily controlled amounts of bed particles, without the need to transport the particles from distant supplies. 
     The present invention is also applicable to all types of fluidized bed reactors and reactor systems, and is particularly applicable to circulating fluidized bed reactors, and to pressurized systems (that is, operating at a pressure of about 2-50 bar, preferably, 2-30 bar). 
     According to one aspect of the present invention, a method of operating a fluidized bed reactor system for reacting fuel is provided, said reactor system comprising: 
     a fluidized bed reactor chamber having a reactor chamber outlet for gas produced during fuel reaction (combustion, gasification, etc.) 
     a particle separator, such as a cyclone separator, connected to the reactor chamber outlet for separating solid material from gas exhausted from the reactor chamber, said particle separator having a solid particle outlet and a gas outlet, and 
     a gas cooler having cooling surfaces (heat transfer surfaces) and being connected to the gas outlet of the particle separator. 
     The method comprises the steps of: 
     (a) introducing solid material particles, fluidization medium and fuel into the reactor chamber to provide a fluidized bed therewithin; 
     (b) reacting the fuel material within the fluidized bed to produce exhaust gas and discharging the exhaust gas from the reactor chamber outlet; 
     (c) introducing the exhaust gas into the particle separator and separating solid particles from the gas in said particle separator; 
     (d) discharging from the separator a first flow of separated solid particles through the solid particle outlet and gas through the gas outlet and 
     (e) cooling the gas discharged from the separator in the gas cooler. 
     The method is characterized by the additional steps of: 
     (f) branching off from the first flow of solid particles, before or after discharging said first flow of solid particles from the particle separator, a second flow of solid particles; 
     (g) introducing said second flow of particles into the gas discharged from the separator during, or before step (e), so that the particles mechanically dislodge deposits from, and thereby clean, the cooling surfaces, and 
     (h) removing the particles from the gas after step (g). 
     Step (f) is practiced to provide a sufficient concentration and size of separated solid particles into the gas for cleaning the cooling surfaces or keeping the cooling surfaces clean. 
     Steps (f) to (g) are preferably practiced only at spaced intervals (e.g., intermittently or periodically, or in response to sensing of a decrease in cooling efficiency), but may be practiced continuously. Step (g) is typically practiced by introducing particles separated in step (c) into the gas just before the gas cooler. 
     Typically, step (b) is practiced to produce gas at a temperature above 600° C. and step (e) is practiced to cool the gas to about 400° C. 
     According to another aspect of the present invention a circulating fluidized bed reactor system is provided, comprising the following elements: 
     a fluidized bed reactor chamber having a bed material inlet, an exhaust gas outlet and a fluidizing gas inlet; 
     a cyclone separator connected to the exhaust gas outlet, said separator having a gas outlet and a particle outlet for returning separated solid bed material to the reactor chamber; 
     a return conduit connecting the particle outlet of the separator to the reactor chamber; 
     a gas cooler connected to the separator gas outlet, the gas cooler having cooling surfaces and 
     means for branching off a flow of solid bed material from the separated solid bed material and introducing said branched off flow of bed material into the gas cooler. 
     The means for branching off a flow of bed material and introducing it into the gas cooler typically comprises an opening in the bottom of the separator and a by-pass conduit connecting said opening with the gas cooler or the inlet thereto. The means comprises according to another typical embodiment a branch conduit connecting the return conduit with the gas cooler or the inlet thereto. 
     According to the present invention, in a circulating fluidized bed reactor, the gas cooler may be kept clean by means of a portion of the circulating bed material itself. The main portion of the circulating bed material is typically returned from the separator (e.g., cyclone separator) to the dense bed in the reactor chamber, whereas a typically minor portion of the circulating bed material is branched off the main portion and introduced into the gas cooler for cleaning the cooling surfaces therein. A gas flow may be utilized to transport the minor portion of bed material to the gas cooler. 
     The solids needed for cleaning of the gas cooler are typically gathered from the bottom of the particle separator, but can alternatively be gathered from the wall of the particle separator or from the return conduit. The particles gathered in a cyclone separator are led through a separate conduit into the gas cooler. In embodiments where gas is discharged from the cyclone through a center pipe in the bottom thereof, the separate particle conduit by-passes the gas center pipe of the cyclone. A gas flow may be introduced into this by-pass conduit in order to help to carry the particles and prevent blocking of the by-pass conduit. 
     The mass flow of solids flowing to the gas cooler can be controlled e.g., by means of a plate which can be placed to cover wholly or partly the inlet opening into the by-pass conduit. The position of the cover plate may be controlled and operated outside the cyclone enclosure so that the plate opens or closes the inlet into the by-pass conduit for introducing sufficient amounts of particles to clean the cooling surfaces. 
     The system preferably further comprises one common or two or more separate pressure vessels for surrounding the reactor, separator and cooler for maintaining them at superatmospheric pressure (e.g., 2-50 bar). A second separator is preferably provided downstream of the gas cooler for separating bed particles from gas discharged from the cooler. 
     It is the primary object of the present invention to avoid the problem of gas cooler surface fouling in fluidized bed reactor systems in a simple yet effective manner. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of the first exemplary embodiment of a circulating fluidized bed reactor system according to the present invention; 
     FIG. 2 is a schematic view of the second exemplary embodiment of a circulating fluidized bed reactor system according to the present invention, and 
     FIG. 3 is a schematic view of the third exemplary embodiment of a circulating fluidized bed reactor system according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates a circulating fluidized bed (CFB) gasification reactor system  10  according to the present invention, including a circulating fluidized bed reactor  12  and a gas cooler  14 . Gasification is performed in the reactor  12  by introducing fluidizing gas through plenum  16  at the bottom of the reactor chamber  18 . Solid fuel material is introduced into the reactor chamber  18  via an inlet  20  and solid bed material is introduced via inlet  22 . The solid bed material may be an inert material such as sand, and may comprise additives, such as material active in the gasification process, e.g., limestone or other sulfur oxide reducing agents. 
     The fuel material introduced at  20  is reacted (gasified in the case of FIG. 1, but combusted or otherwise reacted in other reactor systems which also are within the scope of the invention) to produce an exhaust gas which is discharged from an outlet  24  adjacent the top of the reactor chamber  18  and connected to a cyclone separator  26 . 
     In the FIG. 1 embodiment, the cyclone separator  26  comprises a gas outlet  28  forming the inlet end of a gas discharge conduit  32  arranged to go through the bottom  34  of the separator  26 . The gas discharge conduit  32  protrudes into the cyclone separator  26 , so as to place the gas outlet at a distance above the bottom  34  and so as to form a center piece within the vortex chamber of the cyclone separator  26 . Hot gas is introduced through reactor outlet  24  into the cyclone separator  26  so as to form a vortex flow therein, whereby solid particles are separated and gather on the bottom  34 . Being inclined, the bottom  34  of the cyclone separator  26  causes the separated solid material to flow downwards towards a solid material outlet  36 , disposed in the lowermost part of the bottom  34 . The solid material outlet  36  is connected through a solid material return conduit  38  with the bottom region of the reactor chamber  18 , for recycling separated solid material into the reactor chamber  18 . 
     The gas produced during the reaction in reactor chamber  18  and discharged through reactor outlet  24  includes in it entrained particulates, such as inert solid bed particles, additives and un-reacted fuel material, including some fine carbon material. The vast majority of the particles, particularly the large particles, are separated from the exhaust gas by the separator  26 , and are returned by return conduit  38  to the lower part of the reactor chamber  18 , as is conventional per se. 
     The product gas which exhausts the separator  26  passes to the gas cooler  14 . Typically, the exhaust gas from the reactor chamber  18  and separator  26  has a temperature above 600° C., and the cooler  14  is typically designed to cool the gases to about 400° C. In the FIG. 1 embodiment, the gas cooler  14  includes a heat exchanger  30  formed of heat transfer surfaces, hot gas flowing on the outside of the heat transfer surfaces. The heat transfer surfaces may be made of water tubes, typically for producing steam to drive a steam turbine. Another heat exchanger or more may, if desired, be provided, connected to a turbine, other heat exchangers or the like. 
     Instead of a cooler  14  as shown in FIG. 1, a fire-tube cooler in which hot gas flows inside a plurality of spaced tubes could be used. In a fire-tube cooler, the space between the tubes is used as a conduit for a heat transfer medium to extract heat from the gases. 
     As the gas in the gas cooler  14  is cooled, tar-like substances condense or turn sticky and, therefore, tend to accumulate on the surfaces of the cooler. According to the present invention, the surfaces are kept clean, or cleaned after accumulation of deposits, by introducing solid particles into the gas flow in, or just before, the cooler  14 . This, for example, may be accomplished by injecting coarse particles using by-pass conduit  40 , the coarse particles being provided from particles being separated from the gas in the cyclone separator  26 . Such particles include e.g., sand, additives and/or un-reacted fuel. 
     While injection can take place continuously, it is preferred that it be at spaced time intervals, for example, either intermittently or periodically, when it is expected that the layer of condensed and/or sticky material has deposited on clean surfaces. Alternatively, control may be automatic, e.g., in response to sensing of a decrease in cooling efficiency as a result of depositing or condensing of sticky substances. 
     A second cyclone separator  100  may be provided downstream of the gas cooler  14 . The second separator may operate continuously, but is particularly necessary when particles are introduced (e.g., through by-pass conduit  40 ) to effect cleaning. Particles separated by the second separator may either be returned to the reactor chamber  18  or may be disposed of. The thus cleaned product gas, discharged from the second separator through gas dischage  27 , may be filtered, and acted upon, or may be used directly, depending upon the desired use and the gas&#39;s composition. 
     In the FIG. 1 embodiment, the by-pass conduit  40  is controlled by a cover plate  42  being able to partly or wholly cover the inlet  44  into the by-pass conduit  40 . The cover plate may be operated by a handle  46  by hand from outside the cyclone enclosure  48 , or the cover plate  42  may be automatically operated by suitable automatic control means  50 , such as a conventional computer controller, for controlling the flow of particles introduced for cleaning. 
     Typically, during a normal operation, only a very limited amount of solid particles, if any, flows from the particle separator  26  through the by-pass conduit  40  into the gas cooler, the cover plate  42  covering the inlet  44 . At intervals, the cover plate  42  is pulled away to allow a sufficient amount of particles, to effect cleaning of the cooler surfaces by mechanically dislodging deposits therefrom, to pass the by-pass conduit  40 . 
     FIG. 2 illustrates a system substantially the same as that shown in FIG. 1, in which the same reference numbers as those in FIG. 1 are used, preceded by a “2”. In the FIG. 2 embodiment, the by-pass conduit  240  is connected to the return conduit  238  and solid particles are introduced directly into the gas cooler  214 , not into the gas discharge conduit (or center pipe)  232 . Several heat exchanger packages  230  are provided in the gas cooler. Fluidizing gas may be used to transport particles in the by-pass conduit  240 . The reactor  218 , cyclone  226  and gas cooler  214  are enclosed in a pressure vessel  52  for maintaining them at superatmospheric pressure. 
     FIG. 3 illustrates a further system substantially the same as that shown in FIGS. 1 and 2, in which the same reference numbers are used, preceded by a “3”. The particle separator is a conventional cyclone  326  having its gas outlet  328  in the upper part thereof. Solid particles are gathered from the wall  348  of the cyclone and led through a by-pass conduit  340  into the gas cooler  314 . The by-pass conduit  340  is divided into two conduits  340 ′ and  340 ″ introducing solid particles at different vertical levels in the gas cooler  314  to mainly effect cleaning of different heat exchanger packages  330  and  330 ′. 
     While the invention has been described above with respect to the use of a conventional generally circular configuration cyclone separator, it is to be understood that other cyclone separators can also be utilized, such as the type shown in U.S. Pat. No. 5,281,398. Also, other types of separators besides cyclone separators may be utilized. 
     Also, while the invention has been described particularly with reference to circulating fluidized beds, which are the preferred embodiments, under some circumstances, bubbling beds may be utilized instead. 
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention is not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.