Abstract:
A process for cleaning a flue gas stream ( 10 ) in a flue gas cleaning system ( 2 ) to remove contaminants such as particulates and acidic components therefrom. The process includes admitting the flue gas stream to a particle collection device ( 18 ) to deposit the contaminants onto collector surfaces ( 19 ) therein, contacting the collector surfaces ( 19 ) with a wash water ( 20 ) to remove the contaminants therefrom, and discharging the wash water from the particle collection device ( 18 ) to a position downstream of the particle collection device ( 18 ). The discharged wash water is neutralized and circulated within the flue gas cleaning system ( 2 ).

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a system and method for cleaning a flue gas stream to remove contaminants, and more particularly to a system and process for cleaning a flue gas stream in which a wet electrostatic precipitator (ESP) is employed. 
         [0003]    2. Description of the Related Art 
         [0004]    Laws and regulations governing air quality continue to focus on small particle emissions, such as particulate matter less than 2.5 microns in diameter as well as acid mist emissions from utility coal-fired power stations. Even moderate concentrations of acid mist, e.g., H 2 SO 4 , in a flue gas stream generated by combustion of coal may cause problems in the downstream equipment, such as corrosion of fans, flues, internal support structures, and other equipment exposed to the acid mist. Additionally, human or animal exposure to acid mist and other small particle emissions is a health and safety concern. 
         [0005]    One known approach to effectively capture the small particle emissions and acid mist from a flue gas stream is the use of a wet electrostatic precipitator (ESP). Wet ESPs are commonly installed downstream of a wet flue gas desulfurization (WFGD) system as a “polishing unit” for the small amount of fine particles and sulfur trioxide (SO 3 ) mist in the flue gas that escapes removal in the WFGD. Specifically, a wet ESP can be mounted directly on top of the WFGD absorber (i.e. vertical flow configuration) or installed downstream of the absorber (i.e. horizontal or vertical flow configuration). 
         [0006]    A wet ESP includes a system of grounded collector plate surfaces forming passages with discharge electrodes located between the collector surfaces. Flue gas that contains small particulates, acid mist, and moisture droplets, among other compounds, is passed between the collector surfaces. A negative voltage is applied to the discharge electrodes creating an electrical field. At a certain potential, corona discharge occurs and negative ions are generated which migrate toward the collector surface. As they pass across the inter-electrode space, charges are imparted to the small particles and moisture droplets in the flue gas. The small particles, acid mist droplets and moisture droplets then move under the electric field to the collector surface where they are deposited. The collected small particles and droplets are then flushed down the collector surfaces to the precipitator bottom by spraying or irrigating with water. 
         [0007]    There are numerous variations to the design of a wet ESP. However, common features among all designs include the washing of electrodes and collector plates and the exposure of internal surfaces to acid mist. The water that is used to spray the collector plates and electrodes, known as wash water, keeps the devices and surfaces clean, which allows optimal performance of the wet ESP. This water is collected regardless of the wet ESP&#39;s vertical or horizontal orientation or system-specific design. 
         [0008]    The discharged wash water contains high levels of acidic ions, which result in extremely low pH levels (below 1.0 in some cases). Such acidic wash water is highly corrosive. To prevent damage to internal equipment of the wet ESP from this highly acidic wash water, the equipment needs to be built of high-grade alloys that will not deteriorate rapidly when exposed to the corrosive wash water, or alternatively, there needs to be a high wash rate to keep the acid diluted. Both of these solutions may greatly increase capital and operation costs of the power plant. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    One aspect of the present invention relates to a process for cleaning a flue gas stream in a flue gas cleaning system to remove contaminants comprising particulates and acidic components therefrom, the process including: admitting the flue gas stream to a particle collection device to deposit the contaminants onto collector surfaces therein; contacting the collector surfaces with a wash water to remove the contaminants therefrom; discharging the wash water from the particle collection device to a position downstream of the particle collection device, wherein the wash water contains the contaminants and is acidified thereby; neutralizing the discharged wash water containing the contaminants to form a neutralized wash water; and circulating the neutralized wash water within the flue gas cleaning system. 
         [0010]    Another aspect of the present invention relates to a system for cleaning a flue gas stream to remove contaminants comprising particulates and acidic components therefrom, comprising: a particle collection device for receiving the flue gas and for depositing the contaminants onto collector surfaces therein; conduit means for admitting a wash water to the particle collection device for contacting the collector surfaces and thereby removing the contaminants therefrom; conduit means for discharging from the wash water from the particle collection device, wherein the wash water contains the contaminants and is acidified thereby; neutralizing means for neutralizing the discharged wash water containing the contaminants; and circulating means for circulating the neutralized wash water within the system. 
         [0011]    A further aspect of the present invention relates to a system for circulating and treating a wash water in a flue gas stream cleaning system, comprising: a particle collection device comprising collector surfaces, wherein contaminants from a flue gas stream are deposited on the collector surfaces; a wash water for contacting the collector surfaces and removing the contaminants therefrom, wherein the wash water is acidified thereby; means for discharging the acidified wash water from the collector surfaces to a recirculation tank; means for neutralizing the acidified wash water in the recirculation tank to form a neutralized wash water; and means for circulating the neutralized wash water to a location within the flue gas cleaning system, the location selected from a group consisting of the particle collection device and a position upstream of the particle collection device. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    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: 
           [0013]      FIG. 1  is a schematic view of a system according to one embodiment of the present invention; 
           [0014]      FIG. 2  is a schematic view of a system according to one embodiment of the present invention; and 
           [0015]      FIG. 3  is a flowchart of a process according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The present invention includes a process and system for cleaning a flue gas stream produced by a combuster, e.g., a boiler. The process also includes neutralizing and circulating wash water discharged from a particle collection device, which is operatively connected to a WFGD system. 
         [0017]    Referring now to  FIGS. 1-3 , and in particular to  FIG. 1 , one embodiment of the present invention includes a flue gas stream cleaning system and process  2 . A flue gas stream  10  is released from a boiler  11  and travels to a WFGD system  12 . WFGD system  12  employs an aqueous alkaline slurry  14  for the removal of contaminants, particularly sulfur dioxide, from flue gas stream  10 . Aqueous alkaline slurry  14  is typically transported to WFGD system  12  from a tank  16 . While  FIG. 1  shows aqueous alkaline slurry  14  and tank  16  located in one position, it is contemplated that the slurry and the tank can be located at any position that allows the slurry to be introduced to WFGD system  12 . 
         [0018]    Flue gas stream  10  also travels through a particle collection device  18  that removes particles from the flue gas stream by using electric forces. Particle collection device  18  is typically a wet electrostatic precipitator (wet ESP), which is a self-washing, self-cleaning device that utilizes a liquid, referred to as “wash water,” to clean the collector surfaces  19  and other internal structures of the wet ESP. In most systems, the wash water is added as fresh wash water  20 , which is added to particle collection device  18  via a conduit, such as pipework, that connects a water holding tank  22  to the particle collection device. 
         [0019]    Particle collection device  18  is in fluid communication with a stack  24 , which emits flue gas stream  10  into the atmosphere. Particle collection device  18  is also in fluid communication with a water recirculation tank  26 . 
         [0020]    Typically, water recirculation tank  26  is downstream of particle collection device  18  and is in communication with the particle collection device either by means of ducts or pipes made of metal or any other material that permits an acidified wash water  28  to flow from the particle collection device to the water recirculation tank. Acidified wash water  28  is the resulting liquid that was used to clean collecting surfaces  19  and internal structures of particle collection device  18 . Acidified wash water  28  typically contains water, gypsum particles, fly ash particles, chloride ions and other constituents typically found in treated or fresh water supply sources. 
         [0021]    Prior to, or upon entering water recirculation tank  26 , the pH of acidified wash water  28  may be measured. Since acidified wash water  28  is neutralized by a neutralizing material  30  in water recirculation tank  26 , measurement of the pH will allow for a more effective neutralization of the acidified wash water. 
         [0022]    As shown in  FIG. 1 , neutralizing material  30  is typically discharged from a mix tank  32  and added to water recirculation tank  26 . A fresh water  34  may also be added to water recirculation tank  26 . The combination of acidified wash water  28 , neutralizing material  30 , and fresh water  34  form a neutralized wash water  38 . 
         [0023]    Neutralizing material  30  is made in mix tank  32  and typically contains an alkaline material  35  and a water  40 . Alkaline material  35  may be any alkaline material such as lime, limestone, magnesium, sodium, or a mixture thereof. Alkaline material  35  may be in any form suitable for use in a flue gas stream cleaning system. For example, alkaline material  35  may be in the form of a powder. Typically, alkaline material  35  is discharged from a neutralizing material tank  36  and combined with water  40  in mix tank  32 , thereby forming neutralizing material  30 . Neutralizing material  30  neutralizes, i.e., increases the pH, of the acidified wash water  28 . Alternatively, as indicated by the dashed line in  FIG. 1 , alkaline material  35  can be added directly to water recirculation tank  26 , along with water  34 . In this embodiment, acidified wash water  28  is neutralized by the direct addition of alkaline material  35  to the acidified wash water present in water recirculation tank  26 . 
         [0024]    The basic ions in alkaline material  35  or neutralizing material  30 , will combine with acidic species present in acidified wash water  28 , such as: sulphuric acid, sulphurous acid, hydrochloric acid, and hydrofluoric acid to form stable, water-soluble compounds. 
         [0025]    Still referring to  FIG. 1 , a control device  42  may be placed between neutralizing material tank  36  and mix tank  32  to control the amount of alkaline material  35  transported to the mix tank. Additionally, a monitoring device  44  may be placed between mix tank  32  and neutralizing material tank  36  which would allow a user to monitor the amount of alkaline material  35  transported to the mix tank. Monitoring device  44  may be a meter, a computer or any other instrument that allows a user to monitor the amount of alkaline material  35 . After combining alkaline material  35  and water  40  in mix tank  32  to produce neutralizing material  30 , the neutralizing material is transported to water recirculation tank  26  by a pump  46 . 
         [0026]    Neutralizing material  30  is typically added to water recirculation tank  26  on a continuous basis in an amount effective to achieve a neutral or slightly acidic (i.e., between about 5-7 pH) wash water  38  in the water recirculation tank. 
         [0027]    A control valve  48  or other control device such as a valve, gauge, lever, and the like, may be arranged between mix tank  32  and water recirculation tank  26 . The control valve may be connected to a pH instrument located within water recirculation tank  26 . Optionally, a monitoring device  50  may also be placed between mix tank  32  and water recirculation tank  26  to allow a user to monitor the amount of a neutralization material  30  sent to the recirculation tank. 
         [0028]    Monitoring device  50  allows a user to monitor the amount of neutralizing material  30  transported to water recirculation tank  26 . Monitoring device  50  may be a meter, a computer or any other instrument that allows a user to monitor the amount of neutralizing material  30 . 
         [0029]    Still referring to  FIG. 1 , water recirculation tank  26  is also fluidly connected to particle collection device  18  in a manner that allows neutralized wash water  38  to be circulated back to the particle collection device. Neutralized wash water  38  is transported by pump  52  to particle collection device  18 . Neutralized wash water  38  washes over collector surfaces  19  and other internal structures (not shown) of particle collection device  18  and is discharged from the particle collection device as acidified wash water  28 . Acidified wash water  28  follows the process stream as discussed previously. 
         [0030]    Referring now to  FIG. 2 , another embodiment of the present invention includes a system and process  120 . With the exception of the differences described below, system and process  120  is substantially similar to or identical to the process described above as indicated by similar or identical element numbers. As in the description of the process described in  FIG. 1 , with respect to system and process  120 , flue gas stream  10  refers generally to any flue gas and the particular constituents that make up the flue gas are expected to vary as the flue gas is treated. 
         [0031]    One way system and process  120  differs from system and process  2  of  FIG. 1  is that neutralized wash water  38  is circulated to any location within system  120  instead of circulated to particle collection device  18 . Neutralized wash water  38  can be circulated to any place upstream of particle collection device  18 , such as to a reagent preparation system  122  or directly to an absorber reaction tank of WFGD system  12 . Neutralized wash water  38  may also be circulated to a gypsum dewatering area (not shown). 
         [0032]    Still referring to  FIG. 2 , neutralized wash water  38  may be sent via a conduit  54  to reagent preparation system  122 . Reagent preparation system  122  includes tank  16 . In tank  16 , neutralized wash water  38  is mixed with an alkaline material  124  from a tank  126  to form aqueous alkaline slurry  14 . Aqueous alkaline slurry  14  is then transported to WFGD system  12  to facilitate the removal of contaminants from flue gas stream  10 . 
         [0033]    Now referring to  FIG. 3 , one embodiment of the present invention is a process  130  for cleaning a flue gas stream in a flue gas cleaning system. As shown in step  80 , acidified wash water  28  is discharged from particle collection device  18  to water recirculation tank  26 . 
         [0034]    Next, at step  82 , the pH of acidified wash water  28  may be measured. Measurement of the pH can occur anytime after the acidified wash water has passed through particle collection device  18 . Here, it is shown that the measurement occurs after wash water  28  has been discharged from particle collection device  18 . 
         [0035]    After the pH has been measured, neutralizing material  30  or alkaline material  35  is added to acidified wash water  28  in step  84 . Addition of neutralizing material  30  or alkaline material  35  neutralizes at least a portion of acidified wash water  28 . Typically, the amount of neutralizing material  30  or alkaline material  35  added to acidified wash water  28  is effective to neutralize most of the acid present in the acidified wash water. 
         [0036]    Then, in step  86 , neutralized wash water  38  is produced after the neutralizing material  30  or alkaline material  35  is added to acidified wash water  28 . Next, as shown in step  88 , neutralized wash water  38  may then be circulated to various locations throughout flue gas cleaning systems  2  and  120 . 
         [0037]    Systems  2  and  120  allow water purged from the flue gas cleaning system to come from one source, namely a discharge stream from particle collection device  18 . Accordingly, the interface between the plant&#39;s waste water treatment plant and the wet ESP/WFGD systems is simplified since only one feed line would be required between the systems. 
         [0038]    The neutralization of the acidic wash water is important to reduce cost of the internal equipment used in a particle collection device such as a wet ESP. Generally, the overall structure and internal equipment of the wet ESP must be constructed of material resistant to the highly acidic conditions within the wet ESP. Typically these materials are higher grade alloys like C22 and C276, which are orders of magnitude more expensive than traditional carbon steel or even stainless steel. The present invention allows the wet ESP components, equipment, and surfaces exposed to the flue gas and wash water streams to be constructed of lower grade materials, with stainless steel being the most preferable choice. It has been found that the capital cost of the wet ESP can be reduced by a factor of three when the lower grade materials are used. However, the invention also contemplates retrofitting existing wet ESP systems with the mixing tank and recirculation tank to extend the life of the structures, equipment, and surfaces of the wet ESP systems. 
         [0039]    The present invention offers advantages over prior art systems. For instance, the present invention is inexpensive, not time-consuming, and is easily retrofitted in already existing systems. Additionally, the present invention advantageously allows the control of dissolved and suspended solids sent to a waste water treatment plant. Instead of sending these solids to the waste water treatment plant where it is costly to remove them, the solids are removed via the wet FGD equipment (i.e., gypsum dewatering filters) at no additional expense. 
         [0040]    As shown in  FIGS. 1-3 , the wash water introduced to particle collection device  18  can be neutralized and re-used within the flue gas stream cleaning system, thus avoiding discharging acidified wash water  28  into the environment or into a wastewater treatment plant. 
         [0041]    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.