Abstract:
A method and system for dehumidifying flue gas from a flue gas-generating process that supplies the flue gas to a wet flue gas processor. A wet cooling tower supplies water to a wet flue gas processor to condense water from the flue gas and form a liquid mixture in the wet flue gas processor.

Description:
[0001]    This application claims the benefit of U.S. Provisional Application No. 61/363,299 filed Jul. 12, 2010. U.S. Provisional Application No. 61/363,299 filed Jul. 12, 2010 is incorporated herein by reference in its entirety. 
     
    
     FIELD AND BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to the field of flue gas treatment for boilers, and in particular to a new and useful method and apparatus for more efficient wet cooling of boiler exhaust gases, and more particularly to providing cooling water for dehumidification of boiler flue gasses even in areas where water is scarce. 
         [0003]    In order to remove moisture from flue gas, for example for oxycombustion, regenerable solvent advanced technology (RSAT™) scrubber, or other carbonaceous fuel burning or flue gas treatment process, one method is to use quench cooling and to control the spray water temperature to achieve the desired outlet gas saturation temperature. One such method is taught in U.S. Pat. No. 7,585,476 but this concept could be applied to any method that cools flue gas to remove a constituent such as water by controlling the saturation temperature. U.S. Pat. No. 7,585,476 is incorporated herein by reference 
         [0004]    In current approaches, a cooling tower is used to cool the water used within a quench cooler cooling surface. It is known in the industry that wet evaporative cooling is less costly and more effective than dry cooling, but it requires a significant amount of water which is evaporated to dissipate the heat removed from the cooling water. The present invention takes advantage of the water condensed from the flue gas within the quench cooler by using it in the wet cooling tower as make-up for evaporation. 
         [0005]    Prior art solutions have also been limited in that they require a cooling coil in the wet flue gas desulfurization scrubber (“WFGD”). One such solution is taught in U.S. patent application Ser. No. 12/830,850, which is herein incorporated by reference. 
       SUMMARY OF THE INVENTION 
       [0006]    It is an object of the present invention to provide a method and system for dehumidifying flue gas from a flue gas-generating process that supplies the flue gas to a wet flue gas processor. 
         [0007]    A wet cooling tower supplies water to the wet flue gas processor to condense water from the flue gas and form a liquid mixture in the wet flue gas processor. 
         [0008]    The invention provides a more effective, i.e. lower cost and higher performance, dehumidification system than currently possible and eliminates the need for significant fresh water while permitting more efficient and less costly wet-cooling to be used in conjunction with flue gas dehumidification by quench-cooling. This invention is applicable in contexts where flue gas dehumidification is needed and in one embodiment enhances current oxy-combustion and in another embodiment advances post-combustion CO 2  technology. 
         [0009]    The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawing and descriptive matter in which a preferred embodiment of the invention is illustrated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0010]    In the drawing: 
           [0011]      FIG. 1  is a schematic representation of the direct flue gas dehumidification and wet cooling tower system according to the present invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0012]    Referring now to the  FIG. 1 , this illustration shows a system for dehumidifying flue gases  12  from a boiler  10 , that uses much less water than was previously needed. The invention, in fact, almost eliminates the need for fresh water make-up to a wet cooling tower  14 . A large amount of make-up water would otherwise be needed and supplied into the system at water line  18 . By implementing the present invention, only make-up water for water losses or water that is used to purge solids at purge outlet  22 , is needed. This permits wet evaporative cooling to be used, even in climates where water is scarce. The present invention also accomplishes this without using a closed loop cooling system with a cooling coil located in the WFGD or quench tower  20 . Because the source of the water is the combustion of the fuel (primarily natural gas or coal), only a very small amount of fresh water make-up is required at water line  18 . 
         [0013]    One example of how the boiler  10  is fueled is by coal supplied at  28  to a pulverizer  32  and, once pulverized, by feed lines  34  to burners  36  of the boiler  10 . For an oxy-combustion boiler, CO 2 -rich flue gas can be recirculated to the boiler and supplied along with oxygen rich gas to burn the fuel in the boiler  10 . 
         [0014]    In some combustion processes such as oxy-combustion and postcombustion systems, moisture must be removed from the flue gas  12  before sending it to further processing, such as in a compression unit. In oxy-combustion it is also advantageous to remove moisture from some or all of the flue gas that is recycled to improve combustion in the boiler  10 . This function can be accomplished by controlling the gas temperature leaving a wet scrubber such as a WFGD or controlling the water temperature in a quench type cooler. The WFGD or the quench-type cooler is schematically shown at  20  in  FIG. 1  and is generically referred to herein as a wet flue gas processor. In either case, the objective is to control the flue gas temperature leaving the flue gas generating device, which flue gas will be saturated, and thus control the moisture content in the flue gas. 
         [0015]    In prior art methods, to control the flue gas temperature, water or, in a wet scrubber, slurry which may also remove other constituents, is cooled by a heat exchanger to the desired temperature before spraying into the flue gas stream. 
         [0016]    As shown in  FIG. 1 , the present invention takes advantage of the water condensed from the flue gas within the quench cooler by using it in the wet cooling tower as make-up for evaporation. Thus, the present invention is an improvement on the prior art in that it does not require a cooling coil in the WFGD but ties the WFGD directly to the wet cooling tower. 
         [0017]    In embodiments of the present invention, cool water or slurry  30  from a wet cooling tower  14  is pumped directly into the spray headers within the quench cooling tower or WFGD  20 . The cool water or slurry is sprayed into the gas stream  12  reducing the gas temperature to approximately the same temperature as the cool water or slurry  30 . The temperature of the cool water or slurry  30  is controlled by bypassing  24  some of the warm water or slurry  26  around the WCT  14  and mixing it with the cool water or slurry  30  to achieve the desired temperature. 
         [0018]    The cool water or slurry  30  absorbs heat from the incoming flue gas  12  to the desired saturated temperature, resulting in condensation of moisture from the flue gas  12 . The water or slurry  30  along with the condensed water from the flue gas  12  is pumped to the wet cooling tower  14  where it is cooled by evaporative cooling  16  before recirculating. 
         [0019]    The amount of water condensed in the quench cooler or WFGD  20  is determined by how much the adiabatic saturation temperature is reduced. The amount of heat that must be removed to achieve the reduced temperature is the latent heat of vaporization plus the sensible heat in the gas. Thus, the amount of water condensed from the flue gas in the quench cooler or WFGD  20  defines the temperature of the cool water or slurry  6  sprayed into the flue gas  12  and the circulation rate is determined by the quantity of cool water or slurry  6  required to absorb the heat to reduce the gas temperature to the same temperature as the cool water or slurry  30 . That temperature is set by the desired amount of condensation since the flue gas leaving the device will be saturated. 
         [0020]    Since the heat absorbed in the quench cooler or WFGD  20  is theoretically exactly the same amount of heat that must be removed in the wet cooling tower  14  to return the water or slurry to its cooled temperature, the amount of water condensed in the quench cooler or WFGD  20  is also theoretically equal to the amount of water that will be evaporated  16  in the wet cooling tower. By pumping the water condensed in the quench cooler or WFGD  20  to the WCT  14 , the water condensed in the quench cooler or WFGD  20  would evaporate in the WCT  14  and in theory no additional fresh water would be required. Since there is likely to be some solid in the stream the WCT  14  will require a purge  22  to control solids concentration. To compensate for the purge stream and any losses, some fresh water make-up will be needed but much less than would otherwise be necessary. 
         [0021]    In embodiments of the invention in which the system employs a quench cooler  20  using water, the solids will be negligible and the fresh make-up very low. 
         [0022]    In embodiments wherein a WFGD  20  is used, the amount of wet cooling tower purge  22  will depend upon the reagent being used. In addition, with a WFGD  20 , raw slurry must be added to control the pH (and removal of targeted constituent such as SO 2 ) in the WFGD  20 . It is likely that the pH returning to the WCT will be alkaline, so some additives will be necessary in the WCT to control biological growth resulting in a slightly acidic stream returning to the WFGD. This factor will slightly increase the raw slurry  30  requirement for the WFGD, adding some operating cost. However, compared to prior art systems having a cooling coil, the present direct system eliminates the cost and power consumption of one large recirculating pump. 
         [0023]    In embodiments of the present invention, the slurry or water at  30  is pumped by pump  38  to the top of the WFGD or quench cooler  20  and sprayed into the incoming gas from flue gas line  12  and collected in the bottom of the WFGD  20 . 
         [0024]    The amount of water condensed in the WFGD or quench cooler  20  is determined by how much the adiabatic saturation temperature is reduced. The amount of heat that must be removed to achieve the reduced temperature is the latent heat of vaporization plus the sensible heat in the flue gas  12 . Thus the amount of water condensed from the flue gas  12  in the WFGD or quench cooler  20  defines the amount of heat that must be removed from the flue gas  12  to achieve that degree of condensation. Likewise, the amount of heat that must be removed from the cooling water in the wet cooling tower defines the amount of water evaporated at  16  to achieve that degree of cooling. Since the heat being absorbed in the WFGD or quench cooler  20  is essentially exactly the same amount of heat being removed in the wet cooling tower  14 , the amount of water condensed on line  50  from the WFGD or quench cooler  20 , will be essentially equal to the amount of water that will be evaporated at  16  in the wet cooling tower  14 . By pumping the water by pump  42  on line  40  condensed in the WFGD or quench cooler  20  to the fresh water make-up line  18  for the WCT  14 , no additional fresh water would be required, ideally. Since there are likely to be some solids in the stream, the WCT  14  will require a water containing purge  22  to control solids concentration. To compensate for the purge stream and any other possible water losses such as by evaporation at  16 , some fresh water make-up will likely be introduced at water line  18  but much less than would otherwise be necessary. 
         [0025]    Dehumidified flue gas leaves flue gas processor  20  on line  44  and is supplied to a downstream unit  46  that may be a CO 2  compression and purification unit (CPU) if the boiler  10  is operated as an oxy-combustion boiler, or a post combustion CO 2  capture unit, and then is supplied on line  48  to EOR or storage. 
         [0026]    While a specific embodiment of the invention has been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles.