Abstract:
A once-through boiler system for use in conjunction with a combustion chamber includes a water inlet through which water having a high total dissolved solids content is supplied to the system, at least one tubular preheating surface for preheating the water as the water flows through the preheating surface, and at least one tubular evaporation surface for further heating the water flowing therein to produce a steam/water mixture. The preheating surface is disposed downstream from the inlet and encloses at least part of the combustion chamber, and the evaporation surface is disposed within the combustion chamber, downstream from the preheating surface. Also, a method of producing a steam/water mixture from water having a high total dissolved solids content by using a once-through boiler system provided in conjunction with a combustion chamber includes steps of supplying water having a high total dissolved solids content to the boiler system, preheating the water by directing the water through at least one tubular preheating surface that encloses at least part of the combustion chamber, and further heating the water to produce a steam/water mixture by directing the preheated water through at least one tubular evaporation surface disposed within the combustion chamber.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Our invention relates generally to a system for and a method of generating steam for use in enhanced oil recovery processes. More particularly, our invention relates to a system for and a method of producing a steam/water mixture from feedwater having a high total dissolved solids content. 
     2. Description of the Related Art 
     Steam injection is used in the oil industry to promote the flow of viscous, heavy oils or liquid hydrocarbons from tar sands. Because the feedwater available to boilers at oil fields is normally of poor quality, having a very high proportion of total dissolved solids (TDS), boilers for such applications usually employ a single-tube flow path throughout the unit. A very high proportion of total dissolved solids (TDS) in this application is intended to mean an amount above about 2,000 ppm, especially, above about 5,000 ppm. The quality of the produced steam, i.e., the ratio of the mass flow rate of the gas phase to the total mass flow rate, is usually limited to not greater than about 80% steam. By maintaining at least this level of residual water throughout the flow path, and by employing a high fluid velocity along the flow path, salts and other dissolved solids are kept in solution to prevent their deposition inside the boiler tubes. 
     Typical boilers utilized for enhanced oil recovery applications are small-scale, once-through boilers fired with oil or gas. Usually, a single large diameter tube or a few parallel tubes are configured in a helical or serpentine arrangement to form the furnace or combustion chamber enclosure. These tubes then extend into a heat recovery area of an exhaust gas passage to further cool the flue gas and to complete the generation of 80% quality steam. 
     A natural circulation boiler with a steam drum has also been used for enhanced oil recovery applications. Saturated steam leaving the steam drum is mixed with drum blowdown water to provide 80% quality steam. As steam is generated, the TDS concentration of the water in the boiler increases. With high-TDS feedwater, the tendency for foam formation may become severe, which can cause drum level control problems as well as increased potential for tube failure due to dynamic instability and/or dryout. Therefore, anti-foaming chemicals must be added to the boiler water to minimize foam formation. 
     For large boiler applications, i.e., when production of more than about 100 tons per hour of the steam/water mixture is required, it is mechanically difficult to design the furnace enclosure to be a once-through configuration. A drum-type boiler simplifies the configuration, but does not eliminate the concerns noted above with respect to high-TDS feedwater. 
     SUMMARY OF THE INVENTION 
     Our invention provides an improved steam generation system and method suited for use in connection with enhanced oil recovery processes. Our invention is particularly suited for producing a steam/water mixture from water having a high total dissolved solids content, i.e., an amount above about 2,000 ppm, especially, above about 5,000 ppm. 
     In one aspect, our invention relates to a once-through boiler system for use in conjunction with a combustion chamber. The system includes a water inlet through which water having a high total dissolved solids content is supplied, at least one tubular preheating surface for preheating the water as the water flows through the preheating surface, and at least one tubular evaporation surface for further heating the water flowing therein to produce a steam/water mixture. The preheating surface is disposed downstream from the inlet and encloses at least part of the combustion chamber. Meanwhile, the evaporation surface is disposed within the combustion chamber, downstream from the preheating surface. 
     Such a boiler system thus differs from conventional systems in that the combustion chamber is enclosed at least in part by one or more preheating surfaces, instead of evaporation surfaces. A benefit of using water to cool the combustion chamber enclosure—as opposed to a steam/water mixture—is that relatively small diameter tubes can be used to form the enclosure, thereby providing more efficient cooling of the enclosure while reducing the likelihood of deposit buildups inside the tubes. In a preferred embodiment, for example, the combustion chamber is enclosed at least in part by a plurality of tubular preheating surfaces, and each of the preheating surfaces comprises a tube panel having a plurality of individual tubes. Preferably, each of the individual tubes has an outer diameter of less than about 50 mm, more preferably less than about 40 mm. 
     The plurality of preheating surfaces preferably is arranged in a multiple-pass configuration. That is, the preheating surfaces are arranged so that the water makes multiple passes over the combustion chamber enclosure before moving on to the next stage. The multiple-pass configuration permits a relatively high flow velocity to be maintained through the preheating tubes, which further reduces the likelihood of deposit buildups inside the tubes. The multiple-pass configuration also limits the temperature pickup per pass so that temperature unbalances are minimized. Complete mixing between passes further minimizes any unbalances. Preferably, the mass flux of water flowing through the preheating tubes is at least about 1000 kg/m 2 s, more preferably at least about 1300 kg/m 2 s. 
     Meanwhile, the evaporation surface within the combustion chamber preferably comprises a wingwall panel including a plurality of individual tubes. Preferably, each of the individual tubes has an outer diameter of at least about 70 mm, more preferably at least about 90 mm. Preferably, the mass flux of water flowing through the wingwall panel tubes is at least about 1000 kg/m 2 s, more preferably at least about 1300 kg/m 2 s. 
     Preferably, the system further comprises at least one additional tubular preheating surface that encloses at least part of a heat recovery area of an exhaust passage through which exhaust gases are discharged from the combustion chamber. This preheating surface preferably is disposed downstream from the one or more preheating surfaces that enclose at least part of the combustion chamber, but upstream from the one or more evaporation surfaces within the combustion chamber. Preferably, at least part of the heat recovery area is enclosed by a plurality of tubular preheating surfaces that is arranged in a multiple-pass configuration. 
     Optionally, the system may further comprise at least one more additional tubular preheating surface disposed within the heat recovery area. This preheating surface may comprise, for example, a stringer-type support tube, an economizer, or the like. 
     Preferably, the system also comprises at least one additional tubular evaporation surface disposed within the heat recovery area, downstream from the evaporation surface within the combustion chamber. In a particularly preferred embodiment, the evaporation surface within the combustion chamber includes an outlet header that is divided into one or more outlet sections, and the evaporation surface within the heat recovery area comprises a corresponding number of individual tubes, each tube being in flow communication with a different one of the outlet sections. Preferably, these individual tubes do not interconnect with each other, thereby reducing the risk of uneven flow distribution through the individual tubes of this evaporation surface. 
     In another aspect, our invention relates to a method of producing a steam/water mixture from water having a high total dissolved solids content by using a once-through boiler system provided in conjunction with a combustion chamber. The method includes the steps of (i) supplying water having a high total dissolved solids content to the boiler system, (ii) preheating the water by directing the water through at least one tubular preheating surface that encloses at least part of the combustion chamber, and (iii) further heating the water to produce a steam/water mixture by directing the preheated water through at least one tubular evaporation surface disposed within the combustion chamber. 
     Our invention thus enables the design of a large-scale, once-through boiler that is capable of reliably meeting the requirements for enhanced oil recovery in an efficient and economical way. The concept, however, is also applicable to small size boilers. The invention can be applied to suspension-fired or circulating fluidized bed boilers utilizing a variety of low cost fuels and feedstocks. Compared to conventional boilers having a natural circulation drum-type design, our invention eliminates the need for several pressure components, making our system much more cost effective. Additionally, a boiler system constructed in accordance with our invention is simple, practical, and easy to repair and maintain. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above brief description, as well as further features and advantages of our invention, will be more fully appreciated by reference to the following detailed description of a presently preferred, but merely illustrative, embodiment of the invention, taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 shows a schematic layout of a boiler plant according to a preferred embodiment of our invention; and 
     FIG. 2 schematically illustrates a preferred steam/water flow path through the boiler plant shown in FIG.  1 . 
    
    
     Except as otherwise disclosed herein, the various components shown in outline or block form in the figures are individually well known and their internal construction and operation are not critical either to the making or using of this invention or to a description of the best mode of the invention. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a schematic layout of a boiler plant  10  according to a preferred embodiment of our invention. Reference numeral  12  generally denotes a circulating fluidized bed (CFB) combustor  12 , in which fuel, bed material, and possibly also a sorbent material are fluidized in a furnace (i.e., combustion chamber)  14  using fluidizing air introduced into the furnace  14  by conventional combustion air introduction means, which typically include a windbox  16  and fluidizing air nozzles  17 . Combustion air usually is introduced into the furnace  14  at different levels, but for clarity, FIG. 1 shows the air introduction means only at the bottom of the furnace  14 . 
     Exhaust gases produced in the furnace  14  and particles entrained therein are discharged through a channel  18  extending from the upper part of the furnace  14  to a solids separator  20 , which is preferably a cyclone-type separator. In the solids separator  20 , most of the entrained particles are separated from the exhaust gases and returned to the furnace  14  via a return duct  22 . 
     The furnace  14  is enclosed at least in part by one or more tubular preheating surfaces. In the preferred embodiment shown in FIG. 1, the furnace  14  is enclosed by a front wall  24 , two side walls  26  (of which only one is seen in FIG.  1 ), and a rear wall  28 , which are formed of conventional tube panels. As feedwater flows through the tube panels that form the furnace enclosure, heat from within the furnace preheats the feedwater. The tube panels preferably are constructed of vertical tubes  30  welded together by fins and arranged in parallel between inlet headers  32   a,    32   b,  and  32   c  of the tube panels of the front wall  24 , side walls  26 , and rear wall  28 , respectively, and corresponding outlet headers  34   a,    34   b,  and  34   c.    
     The tube panels of the front wall  24 , the side walls  26 , and the rear wall  28  preferably are connected in a multiple-pass configuration. That is, the tube panels are connected in series so that feedwater introduced into the inlet header  32   a  of the tube panel of the front wall  24  flows through that tube panel and exits from outlet header  34   a.  The feedwater then flows from outlet header  34   a  into inlet headers  32   b  of the tube panels of the two side walls  26 . After flowing through the tube panels of the two side walls  26 , the feedwater exits from outlet headers  34   b  and flows into inlet header  32   c  of the tube panel of the rear wall  28 . After flowing through the tube panel of the rear wall  28 , the heated (but not evaporating) feedwater exits from the outlet header  34   c  and is directed to the next heat transfer stage, described below. 
     Because only water—as opposed to a steam/water mixture—flows through the tubes  30  that enclose the furnace  14 , relatively small tubes having preferably less than about a 50 mm outer diameter, more preferably less than about a 40 mm outer diameter, may be used to form the furnace enclosure. Due to the multiple-pass configuration, such tubes are capable of achieving the required mass flow for cooling the furnace walls while also preventing the deposition of dissolved solids inside the tubes  30 . Preferably, the mass flux in the preheating tubes  30  is at least about 1000 kg/m 2 s, most preferably at least about 1300 kg/m 2 s. Furnace width and depth preferably are selected so that an equal number of tubes are utilized for each pass. 
     A first set of tubular evaporation surfaces is provided within the furnace enclosure. In the preferred embodiment shown in FIG. 1, these internal evaporation surfaces are formed as one or more wingwall panels  36 , i.e., tube panels suspended from the furnace roof, having outlet headers  42  provided above the furnace roof and inlet headers  40  provided outside the front wall  24  of the furnace  14 . Each wingwall panel  36  comprises one or more evaporation tubes  38 , which may have a larger diameter than the preheating tubes  30  that form the furnace enclosure. Preferably, the evaporation tubes  38  have an outer diameter of at least about 70 mm, more preferably at least about 90 mm. The number and size of the evaporation tubes  38  are selected to provide a sufficient flow velocity. Preferably, the mass flux in the evaporation tubes  38  is at least about 1000 kg/m 2 s, most preferably at least about 1300 kg/m 2 s. Due to the high flow velocity and relatively large tube size, the solids are kept in the water solution and deposition inside the tubes  38  is substantially prevented. 
     Water is distributed among the evaporation tubes  38  of each wingwall panel  36  by an inlet header  40 . Meanwhile, an outlet header  42  of each wingwall panel  36  is divided into sections  42   a,    42   b,  and  42   c,  which are connected to outlet pipes  44   a,    44   b,  and  44   c , respectively. Each evaporation tube  38  is in flow communication with only one outlet section  42   a,    42   b,  or  42   c,  and, consequently, with only one outlet pipe  44   a,    44   b,  or  44   c . This reduces the chance that water and steam will be distributed unevenly among the different outlet pipes  44   a,    44   b,  and  44   c.  Although the outlet header  42  shown in FIG. 1 is divided into three outlet sections, the number of the outlet sections may also be other than three. When using narrow wingwall panels, for instance, it may even be desirable to have an unsectioned outlet header connected to a single outlet pipe. 
     As an alternative to wingwall panels, the internal evaporation surfaces could be constructed as full or partial division walls, evaporation columns, or other known evaporation tube structures within the furnace  14 . 
     The exhaust gases are directed away from the solids separator  20  through an exhaust passage  46  that includes a heat recovery area (HRA)  48  in which an air heater  50 , an economizer  52 , and a second set of tubular evaporation surfaces comprising first and second tube bundles  54  and  56  are provided. The first and second tube bundles  54  and  56  are preferably connected in the steam/water mixture flow path in series and downstream from the wingwall panels  36 . They are preferably arranged as serpentine coil-like structures within the HRA  48 . Cooled exhaust gases are directed from the downstream end  76  of the HRA  48  via conventional dust and emission reduction means (not shown) to a stack (not shown), from where the exhaust gases are released to the environment. 
     Preferably, at least part of the HRA  48  is enclosed by one or more tubular preheating surfaces. In the preferred embodiment shown in FIG. 1, the HRA  48  is enclosed by a front wall  58 , side walls  60  (of which only one is seen in FIG.  1 ), and rear wall  62 , which are formed of tube panels comprising vertical tubes connected in parallel between inlet and outlet headers  64  and  66 . The tube panels that form the walls  58 ,  60 , and  62  of the HRA enclosure preferably are connected in series, i.e., in a multiple-pass configuration, similar to the tube panels that form the walls  24 ,  26 , and  28  of the furnace enclosure. 
     In the preferred embodiment shown in FIG. 1, feedwater enters the system through the economizer inlet  68 , after which it passes through the economizer  52  and out of the economizer outlet  70 . The outlet  70 , in turn, is in flow communication with the inlet header  32   a  of the tube panel that forms the front wall  24  of the furnace  14 , i.e., the first of the multiple-pass preheating surfaces that form the furnace enclosure. After flowing through each of the furnace preheating surfaces, the heated (but not evaporating) water is directed, from the outlet header  34   c  to the inlet  64  of the multiple-pass preheating surfaces that form the HRA enclosure. Locating the furnace preheating surfaces upstream from those of the HRA preheating surfaces allows for relatively cold water to be introduced into the tubes  30  of the furnace  14 , thereby promoting efficient cooling of the furnace  14 . 
     After flowing through each of the HRA preheating surfaces, the further heated (but still not evaporating) water is directed from the outlet header  66  to the inlet headers  40  of the wingwall panels  36 . Optionally, one or more stringer-type support tubes (not shown) may be provided between the outlet header  66  and the inlet headers  40  for providing additional preheating, and also for supporting the tube bundles  54  and  56 , for example. The water reaches the inlets  40  of the wingwall panels  36  in a heated but non-evaporating state. This allows the water flow to be evenly split among the parallel evaporation tubes  38 . 
     The water begins to evaporate once inside the wingwall panels  36 , thereby generating a mixture of water and steam within the wingwall panels  36 . To avoid an uneven distribution of the steam/water mixture, the outlet header  42  of each wingwall panel  36  is divided into sections  42   a,    42   b,  and  42   c,  each of which is connected to a respective outlet pipe  44   a,    44   b,  or  44   c.    
     Each of the outlet pipes  44   a,    44   b,  and  44   c,  in turn, is in flow communication with an inlet connection  72  of an evaporation tube of the first tube bundle  54 . Preferably, each of the outlet pipes  44   a,    44   b,  and  44   c  is connected to a different inlet connection  72 , but in some applications it may be advantageous to have multiple outlet pipes connected to a common inlet connection. Each evaporation tube of the first tube bundle  54  is preferably connected on a one-to-one basis to an evaporation tube of the second tube bundle  56 . In some applications, however, it may be advantageous to have fewer evaporation tubes in the second bundle  56  than in the first bundle  54 . For example, multiple evaporation tubes of the first bundle  54  could be connected to a single evaporation tube of the second bundle  56 . In order to avoid splitting of the steam water mixture flow, no one tube of the first bundle  54  should be connected to multiple tubes of the second bundle  56 . It also is possible to use just one tube bundle, or more than two tube bundles, in which case, corresponding evaporation tubes of each tube bundle would be connected similarly, preferably on a one-to-one basis. 
     In the second tube bundle  56 , the steam generation is completed so that steam of about 80% quality is produced. At the downstream end of the second tube bundle  56 , all of the evaporation tubes of that bundle are connected to a common outlet header  74 . The steam leaving the system at the outlet header  74  may be utilized for enhanced oil recovery. If desired, the system may comprise several outlet headers  74  for distributing the steam to multiple locations. 
     In the preferred embodiment shown in FIG. 1, the multiple water flow paths between the inlet headers  40  of the wingwalls  36  and the outlet header  74  of the second evaporation tube bundle  56  do not split into multiple separate paths. Thus, the steam/water mixture flows from the evaporation surfaces within the furnace  14  through the evaporation surfaces within the HRA in a plurality of non-splitting, continuous streams. If, for example, the first set of evaporation surfaces comprises eight wingwall panels, each having three outlet sections, then the first and second tube bundles  54  and  56  would preferably comprise a serpentine coil of twenty-four evaporation tubes running from the inlet connections  72  to the outlet header  74 . 
     The preferred embodiment shown in FIG. 1 utilizes a conventional CFB boiler with uncooled plate-type cyclones discharging into a conventional HRA. However, the CFB boiler may also take on other configurations, such as, for example, a cooled plane-walled cyclone, where the walls of the cyclone are preferably also used as further preheating surfaces. The exhaust passage  46  may also be directed over the top of the furnace  14 , and the HRA enclosure may be integrated with the furnace construction. The boiler may also be of a type other than a CFB boiler, e.g., a suspension-fired boiler. 
     In an example in which petroleum coke is used as a fuel to generate 450 tons per hour of 80% quality steam at a pressure of 150 bar, the approximate exhaust gas temperatures in selected locations of the exhaust passage  46  are as follows: 884° C. at the inlet of the HRA; 480° C. at the outlet of the first evaporation tube bundle  56 ; 400° C. at the outlet of the second evaporation tube bundle  54 ; 230° C. at the outlet of the economizer  52 ; and 150° C. at the outlet of the air heater  50 . 
     FIG. 2 schematically illustrates a preferred steam/water flow path through the boiler plant  10  shown in FIG.  1 . The same reference numerals are used in both FIGS. 1 and 2 to identify the same parts of the system. A vertical dashed line separates the parts of the steam/water flow path located in the HRA and the furnace. 
     Cold feedwater first enters the system through the economizer inlet  68 . The economizer  52 , which is disposed within the HRA, may either cool the flue gases to the stack temperature or discharge the flue gases into an air heater (designated by reference numeral  50  in FIG. 1) for further cooling. The feedwater exits the economizer  52  through economizer outlet  70 . 
     The feedwater then flows to the preheating surfaces that form the furnace enclosure for further preheating. There, the still relatively cold water is heated as it flows through the series of tube panels that form the different walls  24 ,  26 , and  28  of the furnace. According to a preferred embodiment of the present invention, the inlet header  32   a  directs the water flow into parallel tubes on the front wall  24  of the furnace. The water is heated as it flows upward through these tubes, and, upon reaching the outlet header  34   a , preferably is combined back info two streams that are directed to the inlet headers  32   b  on the lower edge of the two side walls  26  (shown as one in FIG. 2) of the furnace. There, the water is further split into multiple streams which are directed upward through the side wall tubes, where the water is further heated. Upon reaching the outlet headers  34   b,  the multiple streams preferably are combined back into a single stream that is directed to the inlet header  32   c  at the lower edge of the rear wall  28  of the furnace. Once again, the water is split into multiple streams which are directed upward through the rear wall tubes for further heating. 
     FIG. 2 shows a preferred multiple-pass flow path, but the order of the passes and the water flow direction may be different than what is shown in FIG.  2 . The multiple-pass configuration provides for efficient tube cooling and high mass flow rates, which contributes to a reduction in the deposition of dissolved solids on the inside of the tubes. The multiple-pass configuration also limits the temperature pickup per pass so that temperature unbalances are minimized. Complete mixing between passes further minimizes any unbalances. 
     After exiting through the outlet header  34   c  of the rear wall  28  of the furnace, the subcooled feedwater is directed to the preheating surfaces that form the HRA enclosure for further preheating. The flow path of the water through the various tube panels of the HRA enclosure is similar to the multiple-pass flow path through the furnace tube panels, except that in the preferred embodiment shown, the water flows downward through the tube panels that form the front and rear walls  58  and  62  of the HRA enclosure. Those skilled in the art will appreciate, however, that the direction of water flow and the order of the passes can be varied. 
     The subcooled feedwater is directed from the last outlet header  66  of the HRA tube panels to the evaporation surfaces within the furnace, which, in this preferred embodiment, comprise wingwall panels  36  or other suitable evaporation structures. Optionally, one or more stringer-type support tubes (not shown in FIG. 2) may be interposed along the flow path between the outlet header  66  and the wingwall panels  36 . 
     Each wingwall panel  36  preferably comprises a plurality of parallel evaporation tubes connected between inlet and outlet headers  40  and  42 , respectively. It is within the wingwall panels  36  that the feedwater reaches the saturation temperature and steam formation begins. One or more pipes  44  are connected to each wingwall outlet header  42  to direct the steam/water mixture again back to the HRA. If more than one outlet pipe is utilized per wingwall panel, the outlet header is partitioned into separate outlet sections  42   a,    42   b,  and  42   c  equal to the number of outlet pipes  44 . Each of the outlet header sections  42   a,    42   b,  and  42   c  is connected to a different one of the outlet pipes  44  so that the two-place steam/water mixture generated within a particular evaporation tube of the wingwall panel  36  is not distributed to multiple outlet pipes. 
     The steam/water mixture entering each pipe  44  continues without splitting to one or more sequential evaporative, serpentine-like tube bundles within the HRA, where the steam/water mixture is further heated until a mixture of required steam quality, e.g. 80%, is achieved. The number of tubes extending from each wingwall panel  36  is selected to provide the necessary mass flow rate within the near horizontal HRA tube bundle(s). Individual tubes within the HRA tube bundle(s) preferably are inclined for drainage purposes. 
     The system is configured to ensure that a two-phase steam/water mixture does not enter the wingwall panels  36 . Further, the individual tubes within the wingwall panels are grouped so that flow entering a particular outlet header section  42   a,    42   b,  or  42   c  feeds only one outlet pipe  44  and tube bundle evaporation tube within the HRA. Thus, the number of outlet pipes  44  from the evaporative wingwall panels  36  is equal to the number of tubes that form the tube bundles  54  and  56  in the HRA. That way, the steam/water mixture does not have to be apportioned in its two-phase state among multiple tubes. 
     Preferably, all tube panels and tube bundles are drainable. Therefore, we prefer that the outlet headers  42   a,    42   b,  and  42   c  of each wingwall panel  36  be elevated with respect to the inlet connections  72  to the HRA tube bundles. At the low point in the piping between the outlet headers  42   a,    42   b,  and  42   c,  and the inlet connections, drains are provided in the outlet pipes  44 . 
     While the invention has been herein described by way of examples in connection with what are at present considered to be the most preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but is intended to cover various combinations or modifications of its features and several other applications included within the scope of the invention as defined in the appended claims.