Abstract:
In a method for operating a steam generator, a transport reactor is provided. Only a substantially pure oxygen feed stream is introduced into the transport reactor in an amount sufficient to maintain the transport reactor at or above a specific system load. The specific load is the system load when only the substantially pure oxygen feed stream is provided to the transport reactor at a minimum flow velocity for operating the transport reactor. A fuel is combusted in the presence of the substantially pure oxygen feed stream to produce a flue gas, which contains solid material. The solid material is separated from the flue gas and passed to a heat exchanger. The heat exchange may be one of a moving bed heat exchanger or a fluidized bed heat exchanger. The solid material is directed to the transport reactor to contribute to the combustion process.

Description:
FIELD 
       [0001]    The specification herein generally relates to steam generators and is more particularly related to steam generators wherein substantially pure oxygen is fed into a transport reactor. 
       BACKGROUND 
       [0002]    Steam generators, particularly those of the coal fired type, can generate harmful emissions. Recent efforts have focused on oxygen firing which, due to the elimination of the inherent nitrogen that occurs with air firing, results in an essentially pure carbon dioxide product gas which can be more efficiently sequestered. Most oxygen fired steam generators utilize significant gas recirculation in order to maintain the required mass flow through the steam generator to support the heat transfer processes. Gas recirculation at high rates adds considerable cost, complexity, and increases the need for auxiliary power. 
         [0003]    Pulverized coal steam generators rely on furnace area for controlling heat transfer. These systems inherently also require substantial gas recirculation in order to maintain furnace and convective surface velocities. 
       SUMMARY  
       [0004]    In one aspect, a method for operating a steam generator includes employing a transport reactor wherein substantially pure oxygen feed stream is the only externally supplied gas introduced during the combustion process in an amount sufficient to raise the flow velocity through the reactor to a desired flow velocity above a minimum threshold flow velocity. Therefore, the amount of substantially pure oxygen added will vary depending on the desired flow rate through the transport reactor. As used herein, the term “substantially pure oxygen” should be construed to mean a gas comprising 95-100% oxygen. During operation, when the transparent reactor is operating at substantially full load, the fuel and substantially pure oxygen are combusted to generate flue gas having ash and other hot solids forming a part thereof. Subsequent to combustion, the flue gas is separated into an end product portion and a recycled product portion. The recycled product portion is passed through a heat exchanger of the moving bed or fluidized bed type. Heat from the hot flue gas is transferred to a working fluid such as, but not limited to, water, water vapor or steam, forming part of the heat exchanger. The recycled product portion is then directed to the transport reactor to contribute to the combustion process. 
         [0005]    In an embodiment, the separator is a cyclone and the transport reactor operates at a flow rate of between about 30 ft/sec. to about 50 ft/sec. In this embodiment, the heat exchanger is a moving bed heat exchanger (MBHE) in fluid communication with an outlet of the separator. The moving bed heat exchanger includes a bypass that allows a portion of the recycled product portion to bypass the tubular heat exchangers that form part of the MBHE, thereby providing a greater degree of temperature control in the transport reactor by allowing a portion of the hot combustion products to bypass the heat exchanger and be returned to the transport reactor. The transport reactor can also include an inlet through which supplemental air or supplemental inert gas can be introduced into the transport reactor. The supplemental air or inert gas is introduced into the when the flow velocity through the reactor is below a minimum threshold level. Adding the supplemental air or inert gas raises the flow velocity through the transport reactor to at least the threshold velocity at which point the substantially pure oxygen can be introduced. 
         [0006]    A steam generator is also described and includes a transport reactor that defines an inlet and an outlet. A separator is provided and has an inlet in fluid communication with the outlet of the transport reactor for receiving and separating flue gas produced thereby into an end product portion that passes through a first outlet defined by the separator and a recycled product portion that passes through a second outlet defined by the separator. An MBHE or a fluid bed heat exchanger is positioned downstream of the second outlet of the separator and has an inlet in fluid communication therewith. The heat exchanger also includes an outlet in fluid communication with the inlet of the transport reactor. The transport reactor can include an inlet through which supplemental air or supplemental inert gas can be provided to maintain the transport reactor throughput under less than substantially full load conditions. Once substantially full load conditions are achieved, the addition of the supplemental air or inert gas can be discontinued and at this point, only substantially pure oxygen can be fed to the transport reactor. When operating at substantially full load, the substantially pure oxygen is the only gas externally supplied to the transport reactor. 
         [0007]    In one embodiment, prior to entering the transport reactor, the substantially pure oxygen feed stream passes through an oxygen preheater positioned downstream of the first outlet of the separator. The oxygen preheater is in fluid communication with the end product portion and the substantially pure oxygen feed stream, so that heat is transferred from the end product portion of the flue gas to the substantially pure oxygen feed stream. 
         [0008]    The above-described and other features are exemplified by the following figures and detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Referring now to the figures, which are exemplary embodiments, and wherein like elements are numbered alike: 
           [0010]      FIG. 1  is a schematic illustration of a steam generator that employs a moving bed heat exchanger; and 
           [0011]      FIG. 2  is a schematic illustration of a steam generator that employs a fluidized bed heat exchanger. 
       
    
    
     DETAILED DESCRIPTION 
       [0012]    As shown in  FIG. 1 , a transport reactor generally designated by the reference number  10  operates as a combustor and defines an inlet  12  and an outlet  14 . Solid fuel  64 , such as, but not limited to, coal, is fed into the transport reactor. When the transport reactor  10  is operating at or above a specific system load, substantially all the transport gas is provided by conduits  16  and  48 . The specific system load is defined by the condition when all or substantially all the transport gas is provided by conduits  16  and  48  at a minimum flow velocity for operating the transport reactor  10 . The minimum flow velocity is defined by the design and parameters of the transport reactor and solid fuel, such as the dimensions of the transport reactor and solid fuel. Prior to being fed into the combustor, a portion or substantially all of the transport gas can be heated in an oxygen preheater  18  (explained in greater detail below). The combustor outlet  14  is coupled to an inlet  20  of a separator  22  via a conduit  24 . As shown in the illustrated embodiment, the separator  22  is a cyclone having an upper outlet  26  for discharging an end product portion of the flue gas and a lower outlet  28  for discharging a recycled product portion of the flue gas that includes heated solids that were not consumed during residence in the combustor  10  as well as ash generated during the combustion process. The lower outlet  28  of the cyclone  22  is in fluid communication with an inlet  29  of a heat exchanger  30 , shown in the illustrated embodiment as a “Moving Bed Heat Exchanger” (MBHE), explained in greater detail below. While an MBHE has been shown, the present invention is not limited in this regard as other types of heat exchangers known to those skilled in the pertinent art to which the present invention pertains, such as, but not limited to, a “Fluid Bed Heat Exchanger” (FBHE), as shown in  FIG. 2  and explained in detail below, can be substituted. 
         [0013]    The transport gas flowing through conduits  16  and  18  is a substantially pure oxygen feed stream, which comprises substantially pure oxygen, provided by an oxygen source  46 , and possibly other residual gas infiltrated from the heat exchanger  30 , the solid fuel source, and/or the oxygen preheater  18 . The substantially pure oxygen feed stream comprises of approximately 90-100% of oxygen. As suggested, the oxygen purity of the substantially pure oxygen feed stream can be affected by the addition of fluidizing gas, such as air, into heat exchangers forming part of the embodiments described herein. While air has been described as the fluidizing gas for the heat exchangers, the present invention is not limited in this regard as other gases such as, but not limited to, flue gas and substantially pure oxygen can also be employed. 
         [0014]    Still referring to  FIG. 1 , a supplemental gas line  31  is coupled to an inlet  27  to the transport reactor  10 . During operation, if the transport reactor  10  is operating at less than the minimum flow velocity for a desired system load, supplemental air or inert gas, such as, but not limited to, flue gas can be introduced into the transport reactor via the gas line  31  in order to raise the flow velocity through the transport reactor to at least the minimum flow velocity. At this point the substantially pure oxygen feed stream can be introduced into the transport reactor. The amount of substantially pure oxygen feed stream introduced into the transport reactor will be governed by the desired system load through the transport reactor. 
         [0015]    An outlet  32  of the MBHE  30  is in fluid communication with the conduit  16  so that after passing through the MBHE, the above-described recycled product portion is mixed in the conduit with the substantially pure oxygen feed stream and the fuel being fed therein and is transported into the combustor  10  through the combustor inlet  12 . 
         [0016]    The end product portion of the flue gas exits the cyclone  22  via the upper exit  26  and flows into a backpass volume  33  defined by a backpass duct  35 . Heat exchangers  34  are in communication with the backpass duct  35  and employ a working fluid such as steam, water, or water vapor that is fed through tubes and/or coils forming part of the heat exchangers. Via conduction and convection, the hot end product portion of the flue gas flows past the heat exchangers  34 , thereby heating the working fluid to the desired temperatures. 
         [0017]    Subsequent to the end product portion of the flue gas passing through the backpass volume  33 , the end product portion flows through the oxygen preheater  18 , as explained in greater detail below, to preheat the substantially pure oxygen prior to its introduction into the combustor  10 . 
         [0018]    As described above, the lower outlet  28  of the cyclone  22  is in fluid communication with the MBHE  30 . The MBHE  30  defines an interior area  40  into which the recycled product portion of the flue gas flows after being discharged from the lower cyclone outlet  28 . The MBHE  30  is a counterflow direct contact heat exchanger that employs a mass flow of solids that move downward through a series of tubular heat exchangers  42 . Heat from the combustion products is transferred to the tubular heat exchangers to heat a working fluid, such as, but not limited to, steam, to the desired process temperatures. The tubular heat exchangers  42  can employ fins to increase the heat transfer area. The heat transfer mechanism of the MBHE  30  is dominated by conduction and convection with the heat transfer rates being higher than those achieved using gas-only convection. While the MBHE  30  has been described as a counterflow direct contact heat exchanger, it is not limited in this regard as the heat exchanger can also be of the parallel flow type. 
         [0019]    The MBHE  30  shown in the illustrated embodiment is equipped with a bypass  44  to allow some of the recycled product portion of the flue gas to bypass the tubular heat exchangers  42 . This provides for a greater degree of temperature control in the combustor  10  by allowing a portion of the hot combustion products to bypass the heat exchanger and be returned to the combustor. 
         [0020]    During operation of the transport reactor  10  at or above the specific system load, the substantially pure oxygen feed stream is all or substantially all the gas introduced into the transport reactor. Due to its low gas weight and higher combustor velocity, using substantially pure oxygen in this manner allows for combustor/furnace areas to be drastically reduced, in some instances by over 80% compared to air fired “Circulating Fluidized Beds” (CFB). Moreover, gas recirculation is not employed and instead the discharge pressure of the oxygen source  46  (explained below) that feeds the substantially pure oxygen into the conduit  16  operates as the primary draft source. 
         [0021]    The substantially pure oxygen is supplied to the transport reactor  10  from an oxygen source  46  via the oxygen introduction elements  48 . The substantially pure oxygen supplied by the oxygen source  46  is preheated upstream of the oxygen introduction elements  48 , by the oxygen preheater  18 . The oxygen preheater  18  includes a cold side inlet  50  in communication with an exit  52  defined by the oxygen source  46 . The oxygen preheater  18  includes an oxygen outlet  54  in fluid communication with the oxygen introduction elements  48  for introducing heated substantially pure oxygen thereto. The end product portion of the flue gas supplied from the backpass volume  33  flows through a conduit  56  that communicates with a hot side inlet  58  of the oxygen preheater. 
         [0022]    The substantially pure oxygen supplied by the oxygen source  46  can be generated by an air separation unit that separates oxygen from an ambient air feed stream. Accordingly, the oxygen source  46  can be configured by a cryogenic plant. The oxygen source  46  can also be configured as an apparatus comprising an oxygen transport membrane. 
         [0023]    As shown in  FIG. 1 , a mechanism  62  for capturing byproducts of the combustion process such as, but not limited to, particulate and sulfur dioxide, is positioned downstream of the oxygen preheater  18  and in the illustrated embodiment is in communication with the conduit  56 . 
         [0024]    The combustor  10  shown in the illustrated embodiment is a transport reactor wherein the flow rate of combustion gases and solids (ash and unconsumed fuel) is preferably between about 30 ft/sec. to about 50 ft/sec. However, the present invention is not limited in this regard as any effective flow rate known to those skilled in the pertinent art to which the present description pertains may be employed. 
         [0025]    The transport reactor  10  operates at higher temperatures than conventional reactors. Typically, the transport reactor is operated at temperatures just below the ash fusion temperature of the fuel being used. This temperature is generally about 2000° F.; however, depending on the fuel fed into the combustor, this temperature can vary somewhat. Due to these high operating temperatures, N 2 O is typically destroyed and is not a byproduct of the combustion process. However, NO x  is generated during the combustion process. Accordingly, a selective catalytic reactor (SCR)  60  is positioned in the bypass duct  33  downstream of the heat exchangers  34  to remove NO x  from the end product portion of the flue gas. A capture mechanism  62  is positioned downstream of the oxygen preheater  18  for capturing sulfur dioxide (SO 2 ) and particulate present in the flue gas. In the illustrated embodiment, the capture mechanism is in communication with the conduit  56 . 
         [0026]    Still referring to  FIG. 1 , if needed, an induced draft (ID) fan  65  can be provided. 
         [0027]    Turning to  FIG. 2 , the embodiment shown therein is similar to that shown in  FIG. 1 . Accordingly, like elements will be given like reference numbers preceded by the numeral  1 . The embodiment of  FIG. 2  differs from that shown in  FIG. 1  in that instead of the MBHE  30 , the system of  FIG. 2  employs a fluid bed heat exchanger (FBHE)  130  and a throttle valve  131 . The FBHE  130 , shown in the illustrated embodiment, employs a bypass  144 . The bypass  144  has a first exit  164  in fluid communication with the conduit  145  that in turn is in fluid communication with conduit  116  and a second exit  166  that is in fluid communication with the throttle valve  131 . The throttle valve  136  is also in fluid communication with the conduit  116  so that during operation, the throttle valve can be operated to allow more or less of the hot combustion products to flow back into the combustor  110 , thereby controlling the temperature therein. 
         [0028]    The FBHE  130  is in gaseous communication with a fluidizing gas supply line  168 . The supply line  168  is employed to introduce a fluidizing gas, such as, but not limited to air, flue gas, or substantially pure oxygen into a fluidized bed  170  forming part of the FBHE. 
         [0029]    The embodiments described herein employ the use of substantially pure oxygen being fed into the transport reactor  10 . An advantage of configuring a steam generator in this manner is that the complexity of the steam generator is reduced. Due to the low gas weight and higher combustor velocity, the combustor furnace area can be significantly reduced over that required by conventional air-fired CFB steam generators. 
         [0030]    The embodiments described herein separate the combustion and the heat transfer processes, thereby allowing for more effective heat transfer in the heat exchangers. 
         [0031]    While the invention has been described with reference to various exemplary embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to a particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.