| System for recovering water from flue gas -> Monitor Keywords |
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  System for recovering water from flue gasRelated Patent Categories: Gas Separation: Apparatus, With Gas And Liquid Contact ApparatusThe Patent Description & Claims data below is from USPTO Patent Application 20070175333. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] This invention relates generally to the field of recovering water from a flue gas and more particularly to recovering water from a flue gas produced by the combustion of a fossil fuel. BACKGROUND OF THE INVENTION [0002] Water is a natural byproduct of the combustion of hydrocarbon or fossil fuels. Permits for water are becoming increasingly difficult to obtain for power plants, which consume relatively large volumes of water during operation. In some cases, the difficulty with obtaining water permits for wells, or use of surface water may preclude construction of a needed power plant. Thus, recovering water from power plants is desirable to obviate the need of obtaining water permits. [0003] Fossil fuel exhaust or flue gas, such as that exhausted from a combustion turbine engine, or downstream of a coal-fired boiler, can contain varying concentrations of water. Water concentration may depend on ambient conditions, fuel composition, inlet air treatment, fuel treatment, flue gas treatment and other factors. If the flue gas exhaust stream were cooled, a portion of that water could be recovered. It is known that cooling an exhaust stream in a condenser to below the precipitation temperature of the moisture in the exhaust gas will result in the condensation of a portion of that moisture. The quantity and percentage of recovered moisture depends on the temperature to which the exhaust can be cooled by the condenser. [0004] Ambient air is commonly the ultimate heat sink for condensers, and the ambient air temperature thus determines the amount of moisture that can be removed by a condenser. In an arid desert environment, for example, the effectiveness of water removal by an ambient air-cooled condenser is limited. Given such high ambient temperatures and the limits of heat exchange equipment, direct condensation alone becomes technically untenable. BRIEF DESCRIPTION OF THE DRAWINGS [0005] FIG. 1 is a schematic of an exemplary embodiment of a system for removing water from a flue gas and recovering water from a desiccant stream. [0006] FIG. 2 is a schematic of another exemplary embodiment of the system of FIG. 1. [0007] FIG. 3 is a schematic of an exemplary embodiment of a system for removing water from a flue gas and recovering water from a desiccant stream. [0008] FIG. 4 is a schematic of another exemplary embodiment of the system of FIG. 3. [0009] FIG. 5 is a schematic of an exemplary embodiment of a system for removing water from a flue gas and recovering water from a desiccant stream. [0010] FIG. 6 is a schematic of another exemplary embodiment of the system of FIG. 5. DETAILED DESCRIPTION OF THE INVENTION [0011] FIG. 1 is a schematic of an exemplary embodiment of a water recovery system 10 for recovering water from a flue or exhaust gas 12 and removing water from a desiccant stream. System 10 may be used to recover water from a flue gas produced using a fossil fuel to generate power such as a combustion turbine power plant. One such power plant is a Model SGT5-5000F sold by Siemens Power Corporation, the assignee of the present invention. It will be appreciated that embodiments of system 10 may be used with various types of plants combusting fossil fuels in a combustion apparatus or furnace such as coal-fired, oil-fired or biomass-fired plants. Examples of combustion turbine power plants are disclosed in U.S. Pat. No. 6,804,964, which is specifically incorporated herein by reference in its entirety. Embodiments of the present invention provide lower capital costs and improved water recovery rates compared to conventional water recovery systems such as those relying on large quantities of heat for evaporation. [0012] Before flue gas 12 is released to the ambient atmosphere 13, it is first treated by system 10. FIG. 1 illustrates that flue gas 12 exiting a combustion apparatus 11 may be directed to a water stripper or absorber 20. Absorber 20 may define an interior portion or plenum containing a fill material or media 22. Media 22 may be a packed media based system using polyethylene, ceramic, metal or other suitable materials. Media 22 provides surface area contact between flue gas 12 and a flow of aqueous desiccant solution, for example, entering absorber 20 through inlet connection 24. Other desiccant solutions may be used comprising solvents and desiccant solutes recognized by those skilled in the art. Embodiments of system 10 may be adapted to use a solid form of desiccant, such as a desiccant wheel exposing the desiccant to flue gas 12 in absorber 20. Absorber 20 may be one disclosed in pending application having application Ser. No. 11/183,696 filed Jul. 18, 2005, which is specifically incorporated herein by reference in its entirety. [0013] In an exemplary embodiment, flue gas 12 passes into absorber 20 through a flue gas inlet 26. Flue gas 12 may enter absorber 20 at approximately 200.degree. F.-300.degree. F., or hotter and contain approximately 5%-10% by volume of moisture, or more. It will be appreciated that the flue gas temperature and moisture content may vary as a function of ambient conditions, performance objectives of the fossil fuel combustor and other operating parameters of a fossil fuel burning plant. The desiccant solution may flow into absorber 20 through inlet connection 24. Water is chemically absorbed from flue gas 12 by the desiccant solution. The desiccant solution may contain various desiccant compounds such as calcium chloride (CaCl.sub.2), bromide, lithium chloride, various hydroxides such as lithium hydroxide or sodium hydroxide, or organic liquids such as polypropylene glycol, or mixtures thereof, for example. [0014] Moisture removal from flue gas 12 in absorber 20 is a highly exothermic process. This process causes the desiccant solution temperature, such as a CaCl.sub.2 aqueous solution, for example, to increase and the concentration of CaCl.sub.2 in the solution to decrease by weight. As the moisture content in the desiccant solution increases, moisture in flue gas 13 exhausting to atmosphere decreases. The temperature and concentration of CaCl.sub.2 in the desiccant solution exiting absorber 20 depend on the relative quantity and inlet temperature of the CaCl.sub.2 desiccant solution, and the moisture content and temperature of flue gas 12 entering absorber 20. [0015] The desiccant solution may exit absorber 20 through outlet connection 28 and be pumped to a means for dehydrating the desiccant solution while maintaining the water in a liquid phase, such as reverse osmosis circuit 29. Reverse osmosis circuit 29 may include a primary or first reverse osmosis apparatus 30 comprising a membrane porous to water, but not to desiccant to separate at least a portion of water from desiccant. The flow of desiccant solution exiting absorber 20 may have no head pressure so pressurization pump 32 may be provided to increase the pressure to that required by primary reverse osmosis apparatus 30. [0016] The heated flow of desiccant solution flowing into primary reverse osmosis apparatus 30 will have a lower concentration of CaCl.sub.2 than that of the desiccant solution entering absorber 20 through inlet connection 24. This is due to the absorption of moisture into the desiccant solution in absorber 20. The concentration of desiccant within the desiccant solution within system 10 may be referred to herein in relativistic terms as being "weak" or "strong" but is not intended to imply specific concentrations. [0017] Primary reverse osmosis apparatus 30 may be configured with internal modules containing membranes that allow water to pass there through while retaining desiccant materials at the molecular level. This may be accomplished when pressure is applied to the desiccant feed solution stream flowing through inlet fluid connection 34 by a high-pressure pump such as pump 32. In an exemplary embodiment, a secondary or second reverse osmosis apparatus 40 may be provided that operates in conjunction with primary reverse osmosis apparatus 30. [0018] It will be appreciated that the employment of secondary reverse osmosis apparatus 40 may be predicated on the efficiency of absorber 20 and/or primary reverse osmosis apparatus 30. In this respect, the type of desiccant used, the rate of desiccant solution recirculation through absorber 20, contaminant level in the desiccant feed solution stream and/or the design specifications of reverse osmosis apparatus 30, 40 may influence the desirability of using secondary reverse osmosis apparatus 40. [0019] In an embodiment, first reverse osmosis system 30 may produce an intermediate stream of desiccant solution that flows to the second reverse osmosis apparatus 40. The intermediate stream may have an intermediate concentration of desiccant that is greater than the concentration of desiccant in the desiccant solution entering the first reverse osmosis apparatus 30. The intermediate stream may flow into second reverse osmosis apparatus 40 through fluid connection 42. The desiccant solution flowing out of second reverse osmosis apparatus 40 may have a concentration of desiccant that is greater than the concentration of desiccant in the desiccant solution flowing into the first reverse osmosis apparatus 30 from absorber 20. [0020] Additional reverse osmosis apparatus may be used as desired to perform the separation of desiccant from liquid water in sequential or parallel stages. For example, a third reverse osmosis apparatus 41 and respective fluid connections 43 are shown in phantom in FIGS. 1 & 2. Reverse osmosis systems or apparatus 30, 40, 41 may be commercially available ones such as suitably adapted FlowMAX reverse osmosis systems available from USFilter. Continue reading... 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