Abstract:
One aspect of the invention relates to a method for removing contaminants from a flue gas stream ( 22 ). The method includes: removing fly ash from a flue gas stream ( 22 ) utilizing a particle collector ( 24 ); contacting the flue gas stream with an alkaline reagent in a wet scrubber ( 26 ); discharging a purge liquid ( 30 ) from the wet scrubber ( 26 ); combining at least a portion of the purge liquid ( 30 ) with at least a portion of the fly ash ( 48 ) to form moistened fly ash ( 60 ); and injecting at least a portion of the moistened fly ash ( 60 ) into the flue gas stream ( 22 ) upstream of the particle collector ( 24 ), whereby the moistened fly ash ( 60 ) removes at least a portion of contaminants present in the flue gas stream ( 22 ).

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a wet flue gas desulfurization process. More particularly, this invention relates to reducing the amount of purge liquid discharged to a wastewater treatment system from a wet flue gas desulfurization system. 
     2. Description of the Related Art 
     Federal, state and even some local governments have laws regulating the emission of particulates, gases and other contaminants present in gas produced in coal combustion. To comply with these laws, industries must implement systems that reduce or eliminate emissions of particulates and/or gases that have been deemed harmful to the environment. 
     Several technologies and processes have been developed to reduce emissions of such elements. These technologies include desulfurization systems that employ fabric filters, electrostatic precipitators, and wet scrubbers. Desulfurization systems have shown sufficient efficiency in the removal of particulate and gases. 
     A particularly useful desulfurization system is the wet flue gas desulfurization system. Wet flue gas desulfurization systems (WFGD) purify flue gas which is produced by coal combustion. There are several known designs for WFGD systems. One example of a WFGD system uses small droplets of slurry that contain water and alkaline material, such as lime or limestone, which is sprayed into the flue gas. Another example of a WFGD system bubbles the flue gas through a bed of slurry to remove pollutants. Regardless of the design of the WFGD system, the slurry reacts with sulfur oxides (SO x ) present in the flue gas and removes them from the flue gas stream as precipitated compounds. 
     Besides the removal of SO x  from the flue gas stream, the WFGD system also captures HCl and HF gases, which are removed from the flue gas stream and become water soluble salts: CaCl 2  and CaF 2 , respectively. These salts dissociate and yield free Cl −  and F −  ions which build up in the WFGD system. This buildup can cause corrosion and other damage in the WFGD system, and can negatively affect SO x  removal. 
     Typically, a stream of water or other liquid, or a slurry containing liquid and particles, referred to as a purge liquid, is used to purge chlorides and other unwanted compounds from the WFGD system. The purge liquid helps maintain a desired chloride concentration, which in turn, helps to protect the equipment of the WFGD system from corrosion. The purge liquid is typically diverted to a wastewater treatment facility. 
     Typically, wastewater treatment facilities used in conjunction with WFGD systems are expensive. The design and supply cost of such a facility can exceed the cost of other systems used in connection with the WFGD plant. The cost of the wastewater treatment facility is even more pronounced when organic acids are used in the WFGD system. 
     In addition to operating expense, the wastewater treatment facilities require large portions of land, additional equipment, and several buildings. The capital and operating costs of a wastewater facility are dramatically increased when a biological reactor is required to remove organic acids or other constituents that may be used in or captured by the WFGD system. 
     BRIEF SUMMARY OF THE INVENTION 
     One aspect of the invention relates to a method for removing contaminants from a flue gas stream. The method includes: removing fly ash from a flue gas stream utilizing a particle collector; contacting the flue gas stream with an alkaline reagent in a wet scrubber; discharging a purge liquid from the wet scrubber; combining at least a portion of the purge liquid with at least a portion of the fly ash to form moistened fly ash; and injecting at least a portion of the moistened fly ash into the flue gas stream upstream of the particle collector, whereby the moistened fly ash removes at least a portion of contaminants present in the flue gas stream. 
     Another aspect of the present invention relates to a method of reusing an amount of purge liquid in a flue gas stream treatment system. The method includes the steps of: removing fly ash from a flue gas stream and transporting the fly ash to a solids mixer; combining the fly ash with a purge liquid which has been directed from a wet scrubber to the solids mixer; forming a moistened fly ash from the fly ash and purge liquid; and injecting the moistened fly ash mixture into the flue gas stream, thereby reducing the amount of purge liquid sent to a wastewater treatment plant. 
     Another aspect of the invention relates to a system for reducing or eliminating an amount of purge liquid sent to a wastewater treatment plant in a flue gas treatment system. The system includes: means for removing fly ash from a flue gas stream and diverting the fly ash to a solids mixer; means for discharging a purge liquid from a wet scrubber and diverting the purge liquid to the solids mixer; means for combining the removed fly ash with at least a portion of the purge liquid to form a moistened fly ash; and means for injecting the moistened fly ash into the flue gas stream. 
     The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects and advantages of the invention will be apparent from the description and drawings, and from the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purpose of illustrating the invention, the drawings show a form of the invention that is presently preferred. However it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein: 
         FIG. 1 : shows one embodiment of a WFGD system of the present invention; 
         FIG. 2 : shows one embodiment of a WFGD system of the present invention; 
         FIG. 3 : shows the injection of moistened fly ash into flue gas produced by a boiler; 
         FIG. 3A : shows a portion of  FIG. 3  in more detail; and 
         FIG. 4 : shows a flowchart of one embodiment of the present invention. 
     
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     The processes and systems described herein are typically used in coal-combustion systems; however it is foreseeable to use such processes and systems in waste-to-energy plants, and other facilities that produce a flue gas stream. 
     Flue gas streams contain, among other things: ash particles, noxious substances and other impurities that are considered to be environmental contaminants. Prior to being emitted into the atmosphere via a smoke stack (“stack”), the flue gas stream undergoes a cleansing or purification process. In coal combustion, this purification process is normally a desulfurization system. 
     Now referring to  FIGS. 1-3 , in which like numerals correspond to like parts, in the WFGD system  20 , a flue gas stream  22  leaves a boiler and travels to a particle collector  24 . Particle collector  24  may be a baghouse, an electrostatic precipitator, a venturi-type scrubber or any similar apparatus that can facilitate the removal of particles from flue gas stream  22 . 
     Ash and other particulate contaminants (collectively referred to hereinafter as “fly ash”) present in flue gas stream  22  are collected in particle collector  24 . The collected particulate contaminants may be disposed of or may be recycled within WFGD system  20 . 
     After passing through particle collector  24 , flue gas stream  22  travels to a wet scrubber  26 . Wet scrubber  26  removes acidic gases such as SO 2  from the flue gas by exposing the flue gas to an alkaline reagent. The alkaline reagent can be limestone, lime, or any other alkaline compound, in a slurry, which is sprayed into flue gas stream  22  as droplets. The unreacted alkaline reagent can be recirculated within wet scrubber  26  by utilizing a slurry recirculating line  27  to introduce the alkaline reagent to wet scrubber  26 . The reacted alkaline reagent is fully oxidized to form gypsum by exposing it to air from oxidation air supply line  23 . 
     After contacting the flue gas stream  22  with the alkaline reagent, the flue gas stream is transported to a stack  28  for release into the atmosphere. Flue gas stream  22  may be subjected to other reagents, or one or more devices, that facilitate the removal of contaminants prior to being released into the atmosphere via stack  28 . Other treatments include, but are not limited to, mercury removal via contact with a reagent such as activated carbon, additional particulate collection, and the like. 
     The reaction of the alkaline reagent with the acidic components of flue gas stream  22  in wet scrubber  26  produces salt and water. In addition, HCl and HF gases react to form water soluble salts that dissociate to form free Cl −  and F −  ions in the alkaline reagent. As discussed above, the increase of chloride ions tends to corrode the system and results in a chloride concentration in the alkaline reagent, which hinders the removal of acidic components from flue gas stream  22 . 
     To prevent an unacceptable chlorine ion (Cl − ) concentration, and to alleviate the corrosive tendencies of Cl −  and the negative effect Cl −  has on SO x  removal within wet scrubber  26 , most WFGD systems use a purge liquid  30  to control the chloride level within the system. Purge liquid  30  typically also removes fine particulate material that is present in wet scrubber  26 . Purge liquid  30  is typically sent to a wastewater treatment system or plant before it is discharged into the environment. 
     Purge liquid  30  is discharged from wet scrubber  26  at point  32 . After it is discharged from wet scrubber  26 , purge liquid  30  is then sent to a holding tank or other system component that diverts the purge liquid to be transported to a wastewater treatment plant. Purge liquid  30  is mainly water, reacted and unreacted alkaline reagent and contains water soluble ions such as Cl − , however some particulates, known as “fines” may be in the purge liquid as well. 
     In one embodiment of the present invention, purge liquid  30  is transported from wet scrubber  26  to hydrocyclone  34 . Hydrocyclone  34  utilizes centrifugal force to separate particulate matter from the purge liquid. The particulate matter, together with a small amount of purge liquid  30 , referred together as underflow liquid, drops down into a tank, a reservoir, or a vacuum filter  36  and can then be transported through process stream  38  to a separate system or apparatus used to collect gypsum. 
     Purge liquid  30  purified by hydrocyclone  34 , and not containing particulate matter or containing a small fraction of particulate matter, and referred to as overflow, travels by gravity through pipework to a storage tank  40 . Substantially all of purge liquid  30  from hydrocyclone  34  is transported to storage tank  40 . Any purge liquid  30  not diverted to storage tank  40  is typically sent to vacuum filter  36  along with any particulate matter. However, in one embodiment, a portion of purge liquid  30  that has been purified by hydrocyclone  34  can be recycled to wet scrubber  26 . Purge liquid  30  is not sent to a wastewater treatment plant. 
     After purge liquid  30  reaches storage tank  40 , the purge liquid travels via a pump  42  to a solids mixer  44 . Optionally, a process heater  46  may be installed to be in contact with purge liquid  30  as it is transported by pump  42  to solids mixer  44 . Process heater  46 , which can be electric or steam driven, is used to heat the purge liquid prior to delivering it to solids mixer  44 . 
     A portion of collected fly ash  48  from particle collector  24  is transported to the solids mixer  44  as well. Fly ash  48  may be directed to solids mixer  44  from particle collector  24  by fluidized troughs, pneumatic transfer, or by any other means for transporting dry ash known by those skilled in the art. 
     As shown particularly in  FIG. 2 , the amount of purge liquid  30  entering solids mixer  44  may be regulated by a control valve  50 . Control valve  50  is optionally connected to a user interface  52 , which may be, for example, a keyboard, a monitor or a computer, or other device that allows a user to increase, decrease or stop the transportation of purge liquid  30  into solids mixer  44 . Additionally, as shown in  FIG. 3 , control valve  50  may optionally be connected to a device  51 , which measures the temperature at a point  53  located behind particle collector  24 . 
     Also as shown in  FIG. 2 , a sliding gate, rotary valve or other control mechanism  54 , may be used to regulate the amount of fly ash  48  that enters solids mixer  44 . Likewise, control mechanism  54  may be connected to a user interface  56  that allows a user to increase, decrease or stop the transportation of fly ash  48  to solids mixer  44 . Excess fly ash  48  not directed to solids mixer  44  may be brought to an ash storage facility or an ash disposal system by process stream  58 . 
     Within solids mixer  44 , purge liquid  30  moistens fly ash  48  to form a moistened fly ash  60 . Moistened fly ash  60  can be in a dry free flowing form or can be in a slurry form. 
     When fly ash  48  is moistened, any unreacted alkaline particles (i.e. calcium and/or magnesium) present in the fly ash will become activated. The activated alkaline particles in moistened fly ash  60  are utilized to remove contaminants from flue gas stream  22 . 
     In one embodiment, the proportions of fly ash  48  and purge liquid  30  are controlled to ensure that moistened fly ash  60  remains dry, fluffy and free flowing. The ratio of fly ash  48  to purge liquid  30  is maintained sufficiently high to keep moistened fly ash  60  free flowing. In this embodiment the moistened fly ash contains between about 94% to about 98% by weight of fly ash based on the total weight of moistened fly ash  60 . However, the amount of fly ash in moistened fly ash  60  is typically between about 94% and about 95% by weight of the total weight of the moistened fly ash. To ensure the correct ratio, the moisture of the moistened fly ash  60  in solids mixer  44  is monitored by a humidistat  62  which is operatively connected to solids mixer  44 . 
     Alternatively, in another embodiment, the moistened fly ash  60  contains more purge liquid  30  making the moistened fly ash more slurry-like. Moistened fly ash  60  of this embodiment is formed by mixing purge liquid  30  with fly ash  48  in solids mixer  44 . To form the slurry-like moistened fly ash  60 , the moistened fly ash contains about 30% to about 50% by weight of fly ash based on the total weight of the moistened fly ash. 
     Utilizing moistened fly ash  60  in a dry free-flowing form is advantageous in systems that have limited space or have limited access to water sources. Utilizing moistened fly ash  60  in a slurry-like form is advantageous in systems that have flue gas streams with very high temperatures or contain a high amount of sulfur therein. 
     Once moistened fly ash  60  is formed, either in its substantially free flowing or slurry form, it is then introduced to newly produced flue gas  22 . Moistened fly ash  60  is typically injected into flue gas stream  22  at a position  64  upstream of particle collector  24 . However, moistened fly ash  60  can be injected into flue gas stream  22  at any position that allows the moistened fly ash to contact and remove contaminants from the flue gas stream. 
     Moistened fly ash  60  may be transported from solids mixer  44  to position  64  by a pump  61 . Optionally, air from pump  65  may be introduced to moistened fly ash  60  before it is introduced to flue gas stream  22 . 
     As shown particularly in  FIGS. 3 and 3A , moistened ash  60  flows out of solids mixer  44  through mixer outlets  66  and down an ash injection chute  68 . Ash injection chute  68  can be a simple flat plate or it may have a perforated or sawtooth design. Ash injection chute  68  promotes even distribution of the moistened fly ash across the width of the duct flue gas stream  22  travels in. 
     Below ash injection chute  68  may be a false wall  70 , which accelerates the speed of flue gas stream  22  to the proper velocity at which moistened fly ash  60  can be entrained into the flue gas stream. 
     Once moistened fly ash  60  is introduced to flue gas stream  22 , the moisture is evaporated therefrom and humidifies the flue gas stream. During the evaporation, moistened fly ash  60  acts as a reagent to remove SO 2 , HCl and other acidic components in flue gas stream  22 . After moistened fly ash  60  is dehumidified, it is brought back to particulate collector  24  where it is collected or removed as discussed in more detail above. If organic acids such as dibasic acid (DBA) or adipic acid are used in system  20 , they are evaporated and captured in particle collector  24 . If a fabric filter is used in the system, even greater SO 2  and HCl removal is achieved. 
     The embodiments described herein decrease the amount of fresh alkaline reagent needed by the system overall. Further, the embodiments described herein reduce or eliminate transporting used purge liquid  30  to a wastewater treatment plant. 
     As illustrated in the process sequence of  FIG. 4 , in step  72 , at least a portion of the fly ash present in flue gas stream  22  is removed from the flue gas stream. The fly ash is removed by introducing flue gas stream  22  into a particle collector  24 . 
     In step  74 , flue gas stream  22  is then introduced to a wet scrubber  26 , where in step  76  it is contacted with an alkaline reagent. As described in more detail above, the alkaline reagent is typically an alkaline compound that reacts with any acidic components present in flue gas stream  22 . The reaction of the alkaline reagent with the acidic components results in the production of corrosive chloride ions. 
     To remove the corrosive chloride ions, in step  78 , a purge liquid  30  removes corrosive chloride ions from wet scrubber  26 . After collecting at least a portion of the chloride ions from wet scrubber  26 , purge liquid  30  is discharged from the wet scrubber and sent to hydrocyclone  34 . 
     After the particulate material is removed or reduced from purge liquid  30 , in step  80  the purge liquid is combined with at least a portion of fly ash  48  collected from particle collector  24 . The combination of purge liquid  30  and fly ash  48  form a moistened fly ash  60 . 
     As shown in step  82 , moistened fly ash  60  is then injected into flue gas stream  22  at a point  64  upstream of particle collector  24 . The moisture from moistened fly ash  60  is evaporated therefrom, thereby humidifying flue gas stream  22  and dehumidifying the fly ash. During evaporation moistened fly ash  60  serves as a reactant to collect and remove contaminants from the flue gas stream. 
     One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.