Abstract:
Contaminant laden gas streams from wood product dryer operations, and other sources, are purified. The gas stream first is saturated with moisture by contacting the gas stream with fine liquid droplets which entrain particulates. The gas stream then is subjected to a plurality of separate particulate and liquid droplet removal steps before a droplet-free gas stream having substantially reduced particulate contaminant levels is passed to a burner for removal of volatile organic compounds from the gas stream.

Description:
FIELD OF INVENTION 
       [0001]    The present invention is concerned with pollution control in the wood products industry. 
       BACKGROUND OF THE INVENTION 
       [0002]    The wood products industry has been beset with many problems with pollution control equipment, including maintenance problems, unplanned downtime due to material build up in scrubbers and wet electrostatic precipitators (WESP) and in thermal oxidizer systems, including regenerative thermal oxidizer (RTO) and regenerative catalytic oxidizer (RCO) degradation caused by deposition of particulates and inorganic salts on ceramic media, causing plugging and deterioration of the media from chemical attack. 
         [0003]    The applicants have found that such problems are caused by lack of pre-cleaning prior to RTO/RCO, lack of focus on pre-scrubbing ahead of WESP, WESP design and operation, and poor mist elimination and droplet carryover into the RTO/RCO. 
         [0004]    The present invention seeks to solve such problems by modification to the pollution control equipment and procedures used in the wood products industry. The invention is illustrated, as disclosed below, by reference to pollution control of sawdust and wood chip dryer emissions, but the principles are applicable to pollution control to other wood products dryers and other sources of gas-borne pollution. 
       SUMMARY OF INVENTION 
       [0005]    In the present invention, existing pollution control equipment associated with a wood products dryer has been modified to reduce particulate emissions. The modifications which were effected are as follows:
       a flue gas recycle duct was installed from the cyclone outlet to the dryer inlet, reducing gas flow to the RTO and WESP.   the existing pre-scrubber was modified to provide an inertial separation baffle in the drop-out chamber and to provide an air atomizing nozzle immediately upstream of the venturi scrubber.   the existing WESP was modified by replacing the conventional transformer rectifier sets with high frequency transformer rectifier units (HF TR-Set) and to replace the mesh pad mist eliminator at the WESP outlet by a proprietary hood, as more fully described in U.S. patent application Ser. No. 10/892,124 filed Jul. 16, 2004 (WO 2005/007295), the disclosures of which are incorporated herein by reference.
 
Combinations of one, two or more of these modifications may be made, as desired or needed in any particular wood products dryer installation.
       
 
         [0009]    Modifications to the scrubber and WESP and addition of flue gas recirculation resulted in:
       a clear relationship was measured between front-half outlet loading and energy input to the air atomizing nozzle. Using both the HF TR-Set and medium energy input to the air atomizing nozzle, a 40% reduction in outlet emissions can be achieved.   the HF R-Set improved operating voltage by 24% and overall power input by 386%.   average emission reduced by 21% using the HF TR-set in addition to other modifications.   the effectiveness of the air atomizing nozzle was more pronounced using the high frequency TR-Set. By improving discharge electrode design, it is anticipated that a greater improvement can be achieved.   installation of the proprietary hood in place of the mesh mist eliminator pads not only eliminated servicing problems but measurements showed no droplet carryover from the WESP.       
 
         [0015]    The use of effective prescrubbing reduces the particulates, tars, pitch, sub-micron salts and other condensibles in the gas stream and minimizing their carryover into the WESP, thereby improving WESP performance and maintenance requirements. 
         [0016]    The improvements in performance achieved herein serve to extend the life of the RTO ceramic media, with specific tests showing that the RTO may be converted with catalyst into a Regenerative Catalytic Oxidizer (RCO), resulting in a significant reduction in the required energy. 
         [0017]    Accordingly, in one aspect of the present invention, there is provided a method for purifying a gas stream containing gas-borne contaminants, which comprises contacting said gas stream with fine liquid droplets to saturate the gas stream with moisture and to entrain particulates in the liquid droplets, subjecting said gas stream to wet electrostatic precipitation to electrostatically separate liquid droplets from the gas stream, and subjecting said gas stream to mist elimination to remove residual liquid droplets to provide a product gas stream. 
         [0018]    In another aspect of the present invention, there is provided an apparatus for purifying a gas stream containing gas-borne contaminants, which comprises a scrubbing duct for conveying the gas stream from an inlet end to an outlet end, an air-water dual fluid nozzle located in the scrubbing duct for producing liquid droplets having a Sauter size distribution of less that 500 microns, preferably less than 200 microns and more preferably less than 100 microns, an electrostatic precipitator downstream of the outlet of the scrubbing duct for electrostatically separating liquid droplets from the gas stream, and a mist elimination device for removing residual droplets from the gas stream. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0019]      FIG. 1  is a schematic representative of a pollution control system for a wood products dryer provided in accordance with one embodiment of the invention; 
           [0020]      FIG. 2  is a perspective, cut-away view of a dual-fluid spray nozzle; 
           [0021]      FIG. 3  is a perspective, cut-away view of a modified drop-out chamber, 
           [0022]      FIG. 4  is a perspective view (f a venturi scrubber and separator; 
           [0023]      FIG. 5  is a perspective view of a WESP incorporating a novel form of mist eliminator; 
           [0024]      FIGS. 6 ,  6 A and  6 B are close-up view of the mist eliminator of  FIG. 5 ; 
           [0025]      FIG. 7  is a detail of a ring channel of the mist eliminator of  FIG. 5 ; 
           [0026]      FIG. 8  is a graphical representation of the front half outlet loading vs. specific air consumption by the dual-fluid spray nozzle; 
           [0027]      FIG. 9  is a graphical representation of the reduction in emission achieved using the dual-fluid spray nozzle modification both with a conventional TR set and a high frequency TR set for the WESP; and 
           [0028]      FIG. 10  is a graphical representation of reduction of percentage in front half loading vs. specific air consumption by the dual-fluid spray nozzle. 
       
    
    
     DESCRIPTION OF INVENTION 
       [0029]    The problems of the wood products industry are indicated above. Areas for improvement include prescrubbing, gas flow distribution, insulators and insulator design, high frequency TR sets, electrodes, mist elimination, water treatment, dryer recirculation and catalyst use. The present invention seeks to address these issues, both individually and collectively. 
         [0030]    In providing the improvements of the present invention, certain performance goals were set, including gas saturation, particulate and condensable removal both by pre-scrubber optimization and WESP optimization, mist elimination with no droplet carryover, minimum maintenance, highest overall efficiency and low installation cost. 
         [0031]    The prescrubber operation requires saturation of the gas stream with moisture, condensation of volatile compounds and capture of particulates. We have found that optimum particle capture is achieved using water droplets which are sized about 15 to 20 times the size of the particles to be captured. 
         [0032]    The provision of droplets of an appropriate size is achieved by using a dual-fluid gas atomizer of the type described in U.S. Pat. No. 4,893,752, the disclosure of which is incorporated herein by reference. Such atomizing nozzles are erosion and plug-resistant, minimizing downtime and are easy to access and maintain. Large orifice diameters permit recirculation of high solids liquids. 
         [0033]    The nozzles are able to provide a droplet distribution as small as 5 microns, with over 50 trillion droplets being produced from a single US gallon of liquid representing a surface area of 50,000 ft 2 . Scrubber operations are able to achieve over 80% removal of organic and inorganic condensibles, up to 79 wt % removal of particulates and over 80% removal of inorganic salts. With more pollutants removed by the scrubber operation, less pollutants pass on to the WESP, improving overall efficiency and significantly reducing WESP build up and maintenance requirements. 
         [0034]    Mist elimination in WESP operation is often provided by mesh pads or chevrons located at the outlet from the WESP. Both have problems associated with them. Mesh pads are best suited for the removal of entrained droplets from particulate-free gas streams, with high removals being achieved at low micron sizes. However, pads often suffer plugging problem where fiber, particulates and volatile organic compounds (VOCs), (such as tars and pitch) are present in the gas stream. Chevrons provide a high efficiency entrainment separation with limit drop sizes of 15 to 25 microns, depending on gas velocity and blade spacing, but can also suffer from plugging problems. 
         [0035]    The present invention overcomes the inherent limitations and drawbacks of mesh pads and chevrons by the use of a hood arrangement as described in the aforementioned U.S. patent application Ser. No. 10/892,124. A schematic of the hood arrangement in relation to a WESP is seen in  FIGS. 5 to 7 . This hood arrangement prevents liquid droplet carryover, even during flushing of the WESP, and no additional mist elimination is required. The hood arrangement requires no maintenance. 
         [0036]    Catalyst can be used to lower the activation energy for a given reaction. To oxidize organic compounds which may be present in the gas stream, heat is the activation energy necessary to complete the reaction. RTO operates at about 1450° to 1550° F. while RCO operates at about 800° F., representing a considerable energy savings where a catalyst can be used. Catalyst operation results in little or no carbon monoxide and very low NO x  emissions. The much lower operating temperature leads to longer oxidizer life. Removal of the organic and inorganic particulates in the pre-scrubber, WESP and mist eliminator, as provided herein, eliminates these potential contaminants from reaching the inlet to the RTO, enabling catalytic operation to be achieved without interruption to clear fouled catalyst and the potential deleterious effects of masking or poisoning the catalyst. 
       DESCRIPTION OF PREFERRED EMBODIMENT 
       [0037]      FIG. 1  is a schematic flow sheet of a pollution control system  10  associated with a wood products dryer operation modified in accordance with one embodiment of the invention. 
         [0038]    Moist wood chips, sawdust, wood fiber, or other wood products are fed by line  12  to a dryer  14  wherein they are dried by a heated air stream  16  and the dried chips are transported in the air stream by line  18  to a cyclone separator  20 , wherein the dried chips are separated from the air stream and collected by line  22 . These elements are standard elements of a wood particle or wood chips dryer operation. 
         [0039]    A portion of the air stream from the cyclone separator is recycled by line  24  to the air inlet  16  to the dryer  14 . This modification to the standard arrangement reduces the gas flow in the remaining elements of the system, including the WESP and RTO. 
         [0040]    The air stream exiting the cyclone separator in line  26  contains a variety of pollutants, including organic and inorganic particulates, hazardous air pollutants (HASP), and volatile organic compounds, which must be removed before the air stream can be vented from the system. 
         [0041]    The air stream first is passed through a quench duct  28 , wherein the temperature of the gas stream is cooled by water fed by hydraulic quench nozzles  30  and also intended to saturate the air stream with moisture, which is necessary for the WESP to operate efficiently. It has been found that existing quench nozzles are not efficient in saturating the gas stream with moisture, or in scrubbing and condensing contaminants and pollutants. 
         [0042]    To remedy this defect, a TurboSonic air-atomizing two-phase spray nozzle  32  of the type described in U.S. Pat. No. 4,893,752 was used to replace one existing quench nozzle at the downstream end of the tube  28 , spraying water cocurrently to the air flow in the quench tube  28 . The characteristics of this nozzle are such that the energy level, micron size and liquid-to-gas ratio (l/g) can be varied at will and may be selected for optimum performance. The nozzle provides a spray of fine water droplets in the gas stream which remove particulates and soluble gas, condenses VOC&#39;s and saturates the gas stream with moisture. 
         [0043]    Any other nozzle able to provide a Sauter particle size distribution less than 500 microns, preferably less than 200 microns and optionally less than 100 microns may be used as the nozzle  32 . These droplet sizes compare to the approximately 800 microns that are the smallest droplet size produced by conventional hydraulic nozzles. 
         [0044]    The fine liquid droplets entrain particulates from the gas stream. A detail of the dual-fluid spray nozzle is shown in  FIG. 2 . 
         [0045]    The use of the dual-fluid spray nozzle provides a significant boost to prescrubber efficiency. With more material removed in the downstream venturi, drop-out chamber and cyclonic separator described below, less material enters WESP, reducing the possibility of build up on WESP internals. The complete humidification also discourages build up in the WESP in that the drying effect caused by incomplete saturation is no longer present, resulting in the complete irrigation of WESP internal surfaces. There is a light constant washdown of the WESP internal surfaces which does not affect the electrical performance of the WESP. 
         [0046]    The gas stream, now saturated with moisture and at its adiabatic dew point, is passed by line  34  through a venturi  36  and then by line  38  to a drop-out chamber  40  to remove large particulates by lines  37  and  41  respectively. As can be seen from  FIG. 3 , the drop-out chamber  40  is modified to add a baffle  42  which extends from the upper closure downwardly to below the height of the outlet, so as to provide more effective removal of large droplets of moisture and particulates from the gas stream by inertial means, thereby reducing the particulates load on the downstream elements. 
         [0047]    The gas stream next is passed by line  44  to a cyclonic separator  46 . The cyclonic separator removes entrained particulate-laden droplets from the gas stream by line  47 . 
         [0048]    The cyclonic flow of the gas stream in the cyclonic separator results in particulates and droplets impinging on the wall of the cyclonic separator and running down the wall to a lower outlet. 
         [0049]    The gas stream then is passed by line  48  to a WESP  50 . The low frequency rectifier set for the WESP  50  was replaced by a high frequency transformer rectifier set. Tests were carried out with respect to the modified installation with or without the TurboSonic nozzle  32  at three energy levels and with and without the high frequency rectifier set. When the TurboSonic nozzle  32  was operated, the hydraulic quench nozzle  30  immediately upstream was turned off so as not to interfere with the TurboSonic nozzle spray pattern. The dryer was operated at continuous steady state operation for all tests. The WESP was flushed only before the commencement of each test. The results obtained are discussed below. 
         [0050]    The conventional mesh pad located at the outlet from the VESP  50  was replaced by a discharge hood plenum  52  constructed as shown in  FIGS. 5 to 7 . The purpose of the hood  52  is to remove droplet carryover from the gas stream of the downflow WESP during both normal operation and flushing of the unit, as described in the aforementioned U.S. patent application Ser. No. 10/892,124. 
         [0051]    In  FIGS. 5 to 7  are shown a WESP installation  50  modified to include a mist eliminator  52 . The WESP installation  50  is of conventional construction comprising vertically-arranged discharge electrodes and collecting electrodes. Any desired arrangement of such elements may be employed, including square tube, round tube, hexagonal tube or plate. The moisture-laden gas stream to be treated is fed through an inlet header  54  to the upper inlet to the WESP down through the tubes containing the electrodes to the lower outlet  56 . 
         [0052]    Connected to the lower outlet  56  is the mist eliminator device  52  which includes a chamber  58  having a sloped bottom wall  60  to a moisture outlet  62 . Inside the chamber  58  is a hood  64  comprising upwardly sloping walls  66 . 
         [0053]    The mist eliminating device  52  accelerates the gas flow entering the device, causing droplets of free moisture to accelerate downward and then break free of the gas stream as the gas stream turns approximately 180 degrees into the interior of the hood  64  and from there to the gas outlet  68  from the chamber. 
         [0054]    A problem of moisture moving down the outer wall  66  of the hood  64  and dripping off the edge of the hood  64  to be swept into the exiting gas stream, is eliminated by providing a series of drip rings or guides or gutters  70  on the external walls  66  of the hood  64 . The gutters  70  are downwardly sloping from an apex to guide the moisture on the hood to the sides of the walls and then to drains  72  from which the accumulated moisture drops into the lower portion of the chamber  58  to the moisture outlet  62 . 
         [0055]    To capture any moisture which may be swept into the gas stream, further gutters  74  are provided on the interior wall of the hood  64 . In order to capture any moisture which may remain or accumulate on the inner walls of the gas outlet  68  and which is swept along by the gas stream, a ring channel  76  may be provided on the inner wall of the outlet  68  with any collected moisture draining through drain  78  to the moisture outlet. 
         [0056]    The gutters  70  may also provide structural support to the hood  68 . 
         [0057]    The gas stream finally is passed by line  80  to a thermal oxidizer  82  before being discharged to stack by line  84 . 
         [0058]    In another embodiment of the invention, the quench duct, venturi, drop-out chamber and cyclonic separator are replaced by a venturi scrubber  100  as seen in  FIG. 4 . The gas stream  26  enters the mouth of the venturi  102  which is maintained wet by a swirl of water  104  to avoid a wet dry interface where build-up may occur. The gas stream then enters the venturi throat  106  to which water is fed by line  108 . 
         [0059]    The gas stream, saturated and cooled to the adiabatic dew point exits the downstream end of the venturi  110  and passes tangentially into a separation chamber  112 . The cyclonic flow of the gas stream in the separation chamber  112  results in particulates and particulate-laden droplets impinging on the wall of the separation chamber and running down the wall to a lower outlet  114 . 
         [0060]    The air stream  48  then passes to the WESP  50 . 
       EXAMPLES 
     Example 1 
       [0061]    This Example described test data from tests conducted on the modified system described above with respect to  FIGS. 1 to 7 . 
         [0062]    Tests were carried out to ascertain the effects of the modifications discussed above to the efficiency of removal of contaminants from the air stream from the dryer. The results obtained are depicted in  FIGS. 8 to 10 . 
         [0063]      FIG. 8  shows the effect of air pressure at the dual-fluid spray nozzle on outlet particulate loading from the mist eliminator hood. 
         [0064]      FIG. 9  shows the combined effect of the use of the dual-fluid spray nozzle in combination with the conventional TR set and the high frequency T/R set. 
         [0065]      FIG. 10  shows the effect of the air pressure on percent reduction of outlet particulate loading. 
         [0066]    Overall, there was achieved over 80% removal of organic and inorganic condensibles, up to 79% removal of particulates and over 80% removal of inorganic fraction while there was no detected moisture carry-over from the mist elimination hood. 
       SUMMARY OF DISCLOSURE 
       [0067]    In summary of this disclosure, particulates and other contaminants produced in wood product dryers are reduced by a variety of measures. Modifications are possible within the scope of the invention.