Patent Publication Number: US-6699444-B1

Title: Fluidized bed reactor

Description:
This application is a continuation-in-part of application Ser. No. 08/888,790, filed Jul. 7, 1997, now U.S. Pat. No. 6,029,612. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a fluidized bed reactor having in its lower part a furnace section, delimited by side walls and a bottom grid, and supplying means, for introducing a gas, such as partial combustion air, into a bed of fluidized particles in the furnace section. Such supplying means may include a gas source chamber, such as a windbox, and at least one nozzle or conduit connected to a respective opening in the side wall, for introducing gas from the gas source chamber to the furnace section. 
     This invention is particularly applicable to large scale circulating fluidized bed (CFB) boilers having a thermal effect of, e.g., 200-400 MWe, or more, in which boilers the lower section of the boiler furnace and the bottom grid may, if desired, be divided in two or more furnace sections, e.g., by a dual wall partition structure. The dual wall partition structure may be a complete partition wall reaching in the furnace from one wall to the opposite wall or a partial wall, i.e., the dual wall construction may consist of a continuous or a discontinuous wall between two opposite furnace walls. The partition wall structure, which typically is of a dual wall construction, may be made by a refractory wall or a cooled wall connected to a cooling water circulation system of the boiler. 
     Accordingly, in the large scale boilers to which the present invention is applicable, the partial combustion air may be distributed through one or more gas source chambers connected to the external side walls and/or connected to the partition wall structure, if such a wall structure is utilized. 
     2. Related Background 
     Optimized emission control and maximum fuel burn-up are decisive qualifications for a successful furnace design. Thus, they must especially be taken into consideration in circulating fluidized bed scale-up. A simple proportional scaling up of designs used in smaller systems may easily lead to problems in attempting to provide for a good mixing of fuel, combustion air and fluidized bed solids. Additionally, such designs may suffer from not being capable of providing a uniform furnace temperature within the optimum range and a sufficient heat transfer area. All these problems, which may cause enhanced emissions and less than optimal fuel burn-up, have led to a desire to find alternative solutions. Such solutions have, e.g., included designs with multiple furnaces with a common back pass, providing heat transfer panels and/or partial or full division walls within the furnace, or dividing the lower part of the furnace and the bottom grid with, e.g., a dual wall structure. 
     Different solutions for sectioning the bottom area of a fluidized bed boiler furnace are known in the prior art. U.S. Pat. No. 4,864,944 discloses a division of a fluidized bed reactor into compartments by partition walls having openings for secondary gas to be distributed in a desired manner into the reactor. The partition walls have ducts which are connected to air supply sources and lead to discharge openings at different heights in the partition walls. 
     Correspondingly, U.S. Pat. No. 4,817,563 discloses a fluidized bed system provided with one or more displacement bodies, which may be provided with lines and inlet openings for introducing secondary gas to segmented sections in the lower reactor. 
     U.S. Pat. No. 5,370,084 discloses different configurations for effective mixing of fuel in a partitioned circulating fluidized bed boiler, including ducts which feed air into the boiler on the interior walls. U.S. Pat. No. 5,215,042 discloses a CFB reactor divided into compartments by at least one vertical, substantially gas-tight partition in the upper part of the combustion chamber. The partition wall comprises cooling tubes and is provided with at least one line with a distributing manifold to feed combustion air into the compartments. 
     U.S. Pat. No. 4,545,959 discloses a chamber for the treatment of particulate matter in a fluidized bed, comprising a duct with a triangular cross section on the bottom of the chamber, and an arrangement of holes or slots in each of the upwardly sloping side walls of the duct for directing an ancillary gas from the duct into the chamber. 
     The above-mentioned publications suggest introduction of gas into a reactor chamber, e.g., furnace chamber, through a partition wall within the chamber. A problem arises, however, as the ducting from the air or gas source chamber to the air or gas injection point may be rather long and cause a high pressure drop. A problem arises also in these conventional supply duct constructions due to solids backsifting, i.e., the problems with solid particles from the furnace tending to flow into the gas supply ducts and an increase in the pressure drop over the gas supply ducts. The increase in pressure drop may be very difficult to attend to or to take into consideration when controlling the gas supply. 
     Conventional bottom grid nozzle constructions, e.g., those equipped with bubble caps normally reaching upward from the bottom grid, would be exposed to heavy erosion if installed on a vertical partition wall within a fluidized bed, due to very high erosive forces caused by the downward flowing solid particle layers in the vicinity of the wall. In fluidized bed reactor furnaces, solid particles tend to flow upward in the middle of each furnace section and downward along its vertical side walls. Such downward flowing particles come in the lower part of the furnace sections, where the cross-sectional area of the furnace sections typically abruptly decreases, into intense turbulent motion which may locally lead to very strong erosive forces, e.g., also in the regions of secondary gas inlets. In the prior art, no special solution for preventing backsifting into gas nozzles or conduits arranged, for example, on furnace side walls, such as partition walls or exterior side walls has been disclosed. 
     It is, therefore, an object of the present invention to provide a fluidized bed reactor with a furnace construction having an improved gas supply configuration. 
     It is particularly an object of the present invention to provide an improved gas supply configuration suitable for large scale circulating fluidized bed (CFB) boilers. 
     It is, then, more specifically an object of the present invention to provide an improved secondary gas supply configuration arranged in an exterior side wall and/or a partition wall within the lower part of a furnace. 
     It is more specifically an object of the present invention to provide a fluidized bed reactor with improved gas supply means, with minimized backsifting of solid particles into gas supply conduits therein. 
     It is thereby also an object of the present invention to provide a fluidized bed reactor with improved gas supply means with decreased pressure losses in the gas supply means. 
     SUMMARY OF THE INVENTION 
     These and other objects of the present invention are achieved by providing a fluidized bed reactor that includes at least one furnace section delimited by sidewalls and a bottom grid, the at least one furnace section being provided for containing a bed of fluidized solid particles therein, and supplying means for introducing a gas into the at least one furnace section at a level above the bottom grid. The supplying means includes (i) a gas source chamber, (ii) at least one opening in at least one of the side walls at a level above the bottom grid, and (iii) at least one conduit, having a first end connected to the at least one opening at a first vertical level and a second end connected to the gas source chamber, for introducing gas from the gas source chamber to the at least one furnace section. The at least one conduit provides a solid flow preventing element for preventing solid particles from flowing backward from the at least one furnace section into the at least one conduit. As used herein, the term “sidewalls” can refer to exterior side walls of the furnace and/or partition walls of the furnace, whether such partition walls are partial walls or complete walls. 
     In those large scale fluidized bed reactors to which the present invention can be applied, which are divided by dual-wall partitions into separate furnace sections, at least a portion of the free internal space between the partition walls may, according to one preferred embodiment of the present invention, constitute the gas source chamber or windbox, providing secondary or other gas to the furnace sections. 
     The gas source chamber may, on the other hand, if desired according to another preferred embodiment of the present invention be formed at another location, e.g., connected to an external side wall(s) or to the bottom grid. 
     Still further, the gas source chamber may be connected to at least one of the external side wall(s), the bottom grid and the partition walls (if such walls are so utilized). 
     Secondary gas or other similar gas is typically introduced into furnace sections through a plurality of gas injecting openings formed in the side walls delimiting the furnace sections. The openings may be arranged in a single row at the same vertical level in each wall, or the openings may, if desired, be arranged in some other configuration and at several different vertical levels in the walls. A conduit, such as a standpipe or a bent pipe construction, is according to the present invention disposed between each of the openings and the gas source chamber, for introducing gas from the gas source chamber through the openings into the furnace sections. 
     A solid flow seal is formed in the conduits so as to prevent solid particles from flowing backward into the conduit in a manner preventing or noticeably decreasing the introduction of gas from the gas source chamber to the furnace sections. Some minor back and forth flow of solid particles within the conduits close to the openings may be tolerable. The solid flow seals may be formed in different ways, e.g., depending on the location of the gas source chamber. 
     In a fluidized bed reactor in which the gas source chamber is formed in the space between two partition walls forming a partition on the bottom grid and/or in which the gas source chamber is attached to the external walls of the furnace, secondary gas/air nozzles or conduits in the form of open-ended standpipes may preferably be used. The standpipes may have a first open end connected to an opening in one of the partition walls and/or exterior side walls at a first vertical level l 1 , e.g., at the secondary air injection level, and a second open end opening into the gas source chamber at a second vertical level l 2  which is at a higher level than the first vertical level. This construction may be used when at least a portion of the gas source chamber reaches to a vertical level above the injection level of the gas, e.g., the injection level of secondary air. 
     The standpipe preferably has a circular cross section, but other forms are possible, such as slot-like cross sections. The vertical extent of the standpipe, i.e., the difference l 2 −l 1 , has to be big enough to generally prevent solid particles from backsifting therethrough from the furnace section to the gas source chamber. 
     The standpipe may be bent at its lower end, such that the lower end thereof may be fastened more easily to a vertical or only slightly inclined side wall construction. The standpipe may even have a short, nearly horizontal lower portion in order to bring the standpipe out from the side wall construction. Preferably, a minimum distance or clearance is provided between the side wall and the standpipe along the entire length of the standpipe, i.e., also when the side wall is inclined and approaches the standpipe at the upper end thereof. Another solution would be to make the standpipe slightly inclined. 
     The standpipe is, however, preferably substantially upright, but may, due to constructional reasons and as discussed above, have a lowermost portion, forming a &lt;90°, typically about 45°, but always ≧30° angle with the horizontal plane. The rest of the standpipe, i.e., the upper portion of the standpipe, is mainly upright forming a ≧30° angle with the horizontal plane. 
     In a fluidized bed reactor having a gas source chamber at a substantially different location, e.g., partly or totally above or below the grid level, another conduit or nozzle construction may be used in order to bring up gas from the gas source chamber to, e.g., the secondary gas level. The conduit, which may be formed of a pipe or other similar element, has according to a preferred embodiment of the present invention the form of an upside down U-bend. A first end of the conduit is connected to an opening at a first vertical level l 1  in one of the side walls and a second end of the conduit is connected at a third vertical level l 3  to an opening in an enclosure delimiting the gas source chamber. The conduit has between its first and second ends an upward bent portion, having its highest point at a second vertical level l 2 , which is at a higher level than the first l 1  and third l 3  vertical levels. The first level, i.e., the secondary air injection level, typically is at a higher level than the third level, which may be, e.g., at the bottom grid level or below or above the grid level. 
     The vertical extent of an upright standpipe, or the height of the first portion of a bent conduit, correlates to the solid flow backsifting preventing ability of the conduit. The height difference Δl between the first l 1  and second l 2  vertical levels is directly related to the pressure required to move solid particles through the standpipe, e.g., the larger the Δl the longer the standpipe, and the less solid particles are able to backsift through the conduit. 
     Typically, a vertical column Δl of about 1.0 meter may be needed for providing an efficient solid flow seal against normal furnace pressure variations. 
     The constructions described above may be used, as discussed earlier, in fluidized bed reactors having the lower part of the furnace section divided by a dual-wall partition. Such a partition may, if desired, reach from the bottom grid up to the roof of the furnace, dividing the entire furnace chamber in two separate sections. Such furnace dividing walls preferably include at least one opening in their upper part to allow horizontal mixing of the gases and fluidized particles in the separate furnace sections. 
     The partition walls dividing the lower part of the furnace or the divisional walls dividing the entire furnace into two parts or sections may preferably be constructed of finned tube panels, where the flow direction of the cooling medium is upwards from a header on the level of or below the furnace bottom. The cooling tubes of a partition wall may extend substantially vertically up to the roof of the furnace thus forming a divisional wall within the furnace, the tubes providing an additional cooling surface area within the furnace. 
     In many known fluidized bed reactor constructions, the interior of the dual wall partitions contains various ducts for different purposes, but the interior space formed between the partition walls has not been otherwise utilized. When using, according to one aspect of the present invention, at least a portion of the interior of the dual wall partition as a gas source chamber such as a windbox for air or gas, which is to be distributed into the furnace above the primary air grid, space is correspondingly spared below the main furnace grid. Moreover, the required length of ducting between the windbox and air/gas introduction point in the furnace is minimized, which leads to decreased pressure losses, i.e., lower cost, compared to conventional constructions. The present invention then provides, due to the decreased pressure losses, a better air/gas distribution and hence, more optimal reaction conditions within the furnace. Also, by locating structures preventing backsifting of solid particles into the interior of a dual wall partition, the structures are protected from the erosive forces of moving solids in the vicinity of the partition. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above brief description, as well as further objects, features and advantages of the present invention will be more fully appreciated by reference to the following detailed description of the presently preferred, but nonetheless illustrative, embodiments in accordance with the present invention when taken in conjunction with the accompanying drawings in which: 
     FIG. 1 schematically shows a vertical cross section of a first exemplary fluidized bed reactor according to the present invention; 
     FIG. 2 schematically shows a vertical and partly axonometrical cross section of the lower part of the fluidized bed reactor shown in FIG. 1; 
     FIG. 3 schematically shows a vertical cross section of a second fluidized bed reactor according to the present invention; 
     FIG. 4 schematically shows a vertical cross section of the lower part of the second fluidized bed reactor shown in FIG. 3, as well as an auxiliary gas source chamber attached to the exterior furnace walls, which gas source chamber may be used in addition to or instead of the gas source chamber attached to the partition walls; 
     FIG. 5 schematically shows an enlargement of a cross section of a standpipe connected to a side wall according to the present invention shown in FIG. 4; 
     FIG. 6A schematically shows a vertical cross section of another exemplary fluidized bed reactor according to the present invention; 
     FIG. 6B schematically shows a vertical cross section of yet another exemplary fluidized bed reactor according to the present invention; and 
     FIG. 7 schematically shows a vertical cross section of still another exemplary fluidized bed reactor according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     Referring now specifically to FIG.  1  and FIG. 2 of the drawings, reference numeral  10  refers, in general, to a fluidized bed reactor, having a furnace  12 , the lower part of which is divided in two furnace sections  14  and  16  by a partition  18 , having a dual wall construction. The partition  18  is in FIG. 2 shown as being a discontinuous partition consisting of partial partitions  18 ′ and  18 ″ separated by an intermediate free portion  19  allowing solids and gas flow from one furnace section  14 ,  16  to the other  16 ,  14 . The discontinuous partition shown in FIG. 2 is one example of a solids and gas flow path between furnace sections  14 ,  16 . Other embodiments not shown in these example drawings include one or more conduits through the partition wall; a partial partition dual wall construction; and others. 
     A fluidized bed of solid particles  20  is maintained in the furnace  12 . The furnace has external side walls  22  and  24 , a roof  26  and a bottom grid  28 . Fluidizing air or gas is introduced into the furnace sections  14  and  16  through grid parts  28 ′ and  28 ″ from gas sources such as, for example, windboxes  30  and  32 . 
     The partition  18 , i.e., the partial partitions  18 ′ and  18 ″, dividing the lower part of the furnace  12 , is of a dual wall construction, i.e., formed of two inclined partition walls, i.e., a first  34  and a second  36  partition wall. Thereby, a partition space  38 , or an internal space of the partition, is delimited by the partition walls  34  and  36  and a bottom  40  covered by the partition. The bottom  40  is, in FIG. 2, shown to be disposed slightly below the grid  28  level, but could be formed at the same level as the grid or even above the grid level. A free space is formed between the windboxes  30  and  32  which can be used for other purposes. The gas space  38  between the partition walls  34  and  36  is divided by a horizontal nozzle supporting partition  41  into an upper  38 ′ and a lower  38 ″ gas space. 
     Nozzles or conduits  42  and  44  according to this aspect of the invention are disposed in two rows in the partition space  38 ′ on the nozzle supporting partition or plate  41 . In this embodiment, the conduits  42  and  44  are made of tubes or pipes formed as upside down U-bends, one leg being longer than the other. The first conduits  42  are connected by their shorter legs  46 , i.e., the first ends of the conduits, to openings  48  in the partition wall  34  at a first vertical level l 1 . The shorter legs  46  reach within the partition space  38 ′ upward from the openings  48  to a second vertical level l 2 , i.e., the highest point of the U-bend. The first conduits  42  are further connected by their longer legs  50 , i.e., the second ends of the conduits, at a third vertical level l 3  to openings  52  in the nozzle supporting partition  41 , the openings opening into a gas source chamber or windbox formed in the gas space  38 ″ between the bottom  40  and the nozzle support partition  41 . Similarly, the other bent conduits  44  are connected to openings in partition wall  36  and nozzle supporting partition  41 . 
     The height difference Δl=l 2 −l 1  between the first ends of conduits  42  or  44  and the highest points of the conduits, i.e., of the U-bends, which corresponds to the vertical extension of the shorter legs  46  of the conduits, provides a solid flow seal. The pressure provided by the leg of solids against the counterflowing gas stream within the conduit then prevents particles from flowing from the furnace sections  14  and  16  upward into the conduits in such a manner that a severe pressure drop affecting gas flow through the conduits would arise. The solid flow seal also prevents backsifting of solid particles through the entire conduits  42 ,  44  from the furnace to the windbox  38 ″. 
     Thereby, in the FIG.  1  and FIG. 2 embodiment, openings  48 , conduits  42 ,  44 , including first legs  46  and second legs  50 , as well as a windbox  38 ″ constitute, e.g., a secondary gas supplying means for the fluidized bed reactor. 
     FIGS. 3,  4  and  5  show another preferred embodiment of the present invention. The same reference numerals as those in FIGS. 1 and 2 have been used where applicable. In this embodiment, a partition  18  reaches from the bottom grid  28  to the roof  26  dividing the entire furnace into two sections  14  and  16 . A discontinuous partition, as indicated by reference numeral  19  in FIG. 2, or other similar solids and gas communication conduit between the furnace sections  14  and  16  may also be provided. The lowermost portion of the partition  18  comprises two partition walls  34 ,  36 , forming a pyramidal free space  39  between the partition walls  34  and  36 . The space  39  between partition walls  34  and  36  and a bottom plate  56  is used as a gas source chamber or windbox for the gas supplying means. The gas source chamber may be divided by a horizontal partition  54 , as shown in FIG. 4, into an upper  39 ′ and a lower  39 ″ windbox. 
     The bottom plate  56  is disposed at the bottom grid level  28 , but could be disposed above or below said level. A free space  58  is, due to this construction, formed below the grid level between the fluidizing air windboxes  30 ,  32 , which space may be used for locating ancillary elements which otherwise would have to be located on the periphery of the reactor. The reactor&#39;s total footprint area may thus be used more efficiently. 
     In this embodiment, the gas injecting conduits  60 ,  62  are simple upright open ended standpipes located within the lower partition space  39 ″, the space thus forming a windbox. The standpipes are connected by their lower ends  64  at a vertical level  11  to openings  48  in the partition walls  34 ,  36 . The upper free ends  66  of the conduits reach upward within the partition space  39  to a vertical level l 2 . The difference Δl in height between levels l 1  and l 2  provides the solid flow seal preventing solid flow upward in the conduits  60 ,  62  and into the partition space  39 ″. 
     Air is supplied from the free gas space or windbox  39 ″ through conduits  60 ,  62 , e.g., as secondary air into the furnace sections  14  and  16 . The air flows from the windbox  39 ″ into the standpipes  60  and  62  at their upper open ends  66  and further downward through the standpipes via a bend  63  at the lower end of the standpipes and through openings  48  into the furnace. 
     The lower end of the standpipes is bent for better enabling a fixing of the standpipes to the openings  48  in the generally vertical walls  34 ,  36 . 
     FIG. 5 shows more clearly an exemplary position of a standpipe  60 , connected to opening  48  in partition wall  34 . The lower end  64  of the standpipe is disposed almost horizontally, upwardly inclined in an angle α≧30° but &lt;90° to the horizontal plane, in order for the standpipe to be able to stand out from the wall. The upper or main part  66  of the standpipe is almost vertical, inclined in an angle β&gt;45° to the horizontal plane. 
     Typically, all secondary air or gas conduits are arranged to introduce air or gas at a certain predetermined level. There may, however, be conduits at different levels, as well. Thus, conduits  60 ′ and  62 ′ (in FIG. 4) may be used to introduce tertiary air at a higher level than conduits  60  and  62 . The tertiary air conduits  60 ′ and  62 ′ are as shown in FIG. 4 located in the separate upper portion  39 ′ of the free gas space  39 . The horizontal partition  54  dividing the free gas space into separate lower and upper gas spaces enables separate control of, e.g., secondary and tertiary air injection. Vertical partition walls may also be used (not shown in the drawings) to divide the free gas space further and to enable separate control of gas injected to the separate furnace sections  14  and  16 . 
     Also, there may be conduits connected to openings in the external side walls  22  and  24 . An exemplary conduit  68  is depicted in FIG.  4 . The conduit  68  is located in a gas source chamber  70  connected to the external side wall  22 . The external gas source chamber  70  and associated conduit(s)  68  may be used in addition to or instead of the internal gas source chamber(s)  39 ′ and  39 ″ and associated conduits  60 ,  62 . Also, one or more external gas source chambers  70  and associated conduits  68  may be provided as desired. 
     FIG. 6A shows an arrangement in which a gas source chamber  71  is located below a free gas space  70 ′, which is provided externally of the furnace  12 . The free gas space  70 ′ includes an associated conduit(s)  68 ′ arranged in the manner discussed above with respect to conduit  46 , for example, shown in FIG. 2. A respective conduit  68 ′ supplies gas from a respective gas source chamber  71  to the furnace  12 . As discussed above with respect to FIG. 4, the external gas source chamber  71  and associated conduit(s)  68 ′ may be used in addition to or instead of the internal gas source chamber  38  and associated conduits. In this embodiment, a portion of the external wall of the furnace  12  thus forms a partition wall between the furnace  12  and the free gas space  70 ′. Further, one or more gas source chambers  71  and associated conduits  68 ′ can be provided as desired. 
     FIG. 6B shows an embodiment in which a free gas space  138 ′ and an associated gas source chamber  138 ″ are provided only externally of the furnace  112 . A portion  134  of the external wall  122  of the furnace  112  thus forms a partition wall between the furnace  112  and the free gas space  138 ′. 
     In this embodiment, the gas source chamber  138 ″ is formed below the free gas space  138 ′. A conduit  142  is arranged within the free gas space  138 ′ in a manner similar to that discussed above with respect to conduit  68 ′ shown in FIG.  6 A. In more detail, the conduit  142  is connected by its first end to the furnace  112  through an opening in the wall portion  134  and by its other end to the gas source chamber  138 ″ through an opening in the horizontal (bottom) partition between the gas space  138 ′ and the gas source chamber  138 ″. One or more gas source chambers  138 ″ can be provided as desired. 
     One having ordinary skill in the art would recognize that a similar arrangement to that shown in FIG. 6B could be provided based on the embodiment shown in FIG. 4, in which only an external gas space is provided (instead of an internal gas source chamber), in which the external wall of the furnace forms a partition wall between the furnace and the free gas space (which, in that embodiment, also serves as the gas source chamber). Of course, one or more gas source chambers can be provided as desired. 
     FIG. 7 shows yet another embodiment of the present invention, in which a furnace  212  has partition walls  234 ,  236  therein. A generally empty space  238 ′ is formed between the partition walls. 
     According to this aspect of the invention, a horizontal gas pipe forming a gas source chamber  238 ″ is disposed within the empty space. Conduits  246 , which provide a solid flow preventing element, i.e., a solid flow seal, are disposed within the gas space  238 ′, so as to connect the gas space within the gas pipe  238 ″ with the gas spaces  214 ,  216  of the furnace  212 . This aspect of the present invention provides a compact solution, as the gas pipe can be located in the empty space  238 ′ between the partition walls, which space  238 ′ usually is not used efficiently. 
     The horizontal gas pipe  238 ″ typically has an inlet for gas in either end, but could have inlets in both ends or even in other locations thereof. The conduits  246  may easily be connected to the gas pipe  238 ″ prior to the inserting of the gas pipe  238 ″ between the partition walls. 
     While the invention has been described in connection with what are 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. 
     Therefore, even if the present invention has mainly been described in connection with large scale fluidized bed boilers having a partition dividing the furnace into two or more sections, the conduit constructions according to the present invention may (as discussed above) be applied to non-divided furnace reactors as well. In such an instance, the upright conduits are connected to external walls of the furnace and gas source chambers utilized in connection therewith. 
     Also, the conduit construction of this invention may, of course, be used to feed other suitable fluid, such as some ancillary fluid or air and fuel mixtures, into a furnace. 
     Still further, the present invention has been explained by using the same types of conduits in each respective embodiment. This, however, is merely exemplary. Thus, any arrangement of conduits (of either the upside down U-bend type or the standpipe type) may be used in the present invention in any combination. Still further, the present invention is not limited to these types of conduits.