Patent Publication Number: US-2002009793-A1

Title: Method of and apparatus for purifying an exhaust gas

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001] This application claims the priority of Swiss patent application 1427/00, filed on Jul. 19, 2000, for which priority is claimed and the disclosure of which shall be considered as disclosed herein by reference thereto.  
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] The present invention relates to a method of purifying an exhaust gas which includes impurities in form of aerosols, dust and organic gases. It also relates to an apparatus for purifying an exhaust gas which includes impurities in form of aerosols, dust and organic gases for practicing above method.  
       [0004] 2. Description of the Related Art  
       [0005] Gases as described above are produced in a great many of industrial thermal conversion processes, such as typically at a mixed combustion, a hot compressing or also a thermal drying. As not limiting example wood pressing plants for the production of for instance chipboards can be mentioned.  
       [0006] The impurities to which reference is made herein are among others aerosols with a typical particle size of 0.1 μm. Such aerosols occur as so-called blue smoke. Further impurities encompass mists of liquids which are paraffin and oil mists with a typical particle size of 1 μm. These liquid mists occur as so-called white smoke. Still further impurities or contaminants, respectively, occur as flue dust with a typical particle size of 1-3 μm.  
       [0007] The fine particles are quite often nonhygroscopic, that is water repellent and are highly electrically loaded and, therefore, can only be separated and removed in wet elecrofilters. Conversely, organic gases can not be separated and removed in wet electrofilters.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008] Hence, it is a general object of the present invention to provide a method of purifying an exhaust gas by means of which fine particles and also organic gases can be simultaneously completely and perfectly separated and removed at low costs in an apparatus of a simple and compact design.  
       [0009] A further general object of the present invention is to provide an apparatus for purifying an exhaust gas by means of which fine particles and also organic gases can be simultaneously completely and perfectly separated and removed at low costs in an apparatus of a simple and compact design.  
       [0010] Still a further object of the invention is to provide a method wherein a separation of the impurities present in the exhaust gas is realized within one single operating cycle by a biological washing reactor and a wet electrofilter, onto which biological washing reactor process water is continuously sprinkled, and which wet electrofilter is intermittently cleaned by process water, which process water which is fed to the biological washing reactor and to the wet electrofilter is circulated in a closed loop which includes a biotank.  
       [0011] Yet a further object of the invention is to provide an apparatus which includes a container, a biological washing reactor and a wet electrofilter, which biological washing reactor and which wet electrofilter are both located in mentioned container.  
       [0012] The advantages gained by the invention may be seen specifically in that two different purifying procedures are united in one single cycle in a compact designed container, so that the purifying of the exhaust gas may be executed at low costs in one single cycle. For a treating of all operating units, may these be acted upon by process water continuously or intermitted for various reasons, only one single water circuit is needed. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0013] The invention will be better understood and objects other than those set forth above, will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings, wherein:  
     [0014]FIG. 1 illustrates schematically a first embodiment of an apparatus for practicing the method according to the present invention; and  
     [0015]FIG. 2 illustrates schematically a second embodiment of an apparatus for practicing the method according to the present invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0016] A biotank  2  which contains process water which is circulated in a closed loop is located at the lower portion of a container  1 . The biotank  2  is equipped with a bottom aerator  3  which communicates with a source  4  of fresh air.  
     [0017] The biotank  2  communicates, furthermore, with a source  5  of a nutrient which will be described further below.  
     [0018] The exhaust gas to be purified enters the container  1  of the exhaust gas inlet  6 .  
     [0019] This exhaust gas inlet  6  is located at a preliminary separator  7  which also functions to ensure an even distribution of the exhaust gas over the cross-section of the container  1 . A sprinkling apparatus  8  is located above the preliminary separator  7 . This sprinkling apparatus  8  communicates through a branch conduit  9  with an outflow conduit  10  of the biotank  2 .  
     [0020] At the embodiment according to FIG. 1 a biological washing reactor  11  with a sprinkling apparatus  12  is located above the preliminary separator  7 .  
     [0021] The feeding of the process water to the sprinkling apparatus  12  will be described further below.  
     [0022] A wet electrofilter  13  with a sprinkling apparatus  14  is located above the biological-washing reactor  11 . This sprinkling apparatus  14  communicates through its feeding conduit  15  with the outflow conduit  10  of the biotank  2 .  
     [0023] A droplets remover  17  is located at the purified gas outlet  16  on top of the container  1 , and a sprinkling apparatus  18  is located above the droplets remover  17 . The feeding of process water to this sprinkling apparatus  18  will also be described further below.  
     [0024] The design of the embodiment illustrated in FIG. 2 differs insofar from the design illustrated in FIG. 1 in that the biological washing reactor  11  is located above the wet electrofilter  13 . This embodiment includes a partition wall  19  which is inserted between the biological washing reactor  11  and the wet electrofilter  13 . An outflow conduit  20  runs directly from this partition wall  19  to the biotank  2 , which outflow conduit  20  circumnavigates, runs around the wet electrofilter  13 .  
     [0025] A further outflow conduit  21  extends at both embodiments from the biotank  2  to a solids removing apparatus  22 . This outflow line  21  communicates with a source  23  of a flocculation agent. The reference numeral  24  designates the draining line for the separated solid matter.  
     [0026] A conduit  25  extends from the solids removing apparatus  22  to a heat exchanger  26 .  
     [0027] At the embodiment according to FIG. 1 the feeding conduit  28  running to the sprinkling apparatus  12  of the biological washing reactor  11  is branched off the outflow conduit  27  of the heat exchanger  26 .  
     [0028] The reference numeral  29  designates an outlet conduit for a draining off of superfluous water.  
     [0029] Finally, the outflow conduit  27  runs to the feeding conduit  30  of the sprinkling apparatus  18  of the droplet remover  17 .  
     [0030] At the embodiment according to FIG. 2, the feeding conduit  28  for the sprinkling apparatus  12  of the biological washing reactor  11  branches off not earlier than after the outlet conduit  29 . Also illustrated is again the feeding conduit  30  to the sprinkling apparatus  18  of the droplets remover  17 .  
     [0031] As generally can be seen, the disclosed alternatives differ from each other substantially in that according to the embodiment of FIG. 1 the biological washing reactor  11  is located under the wet electrofilter  13 , and according to the embodiment of FIG. 2 the wet electrofilter  13  is located under the biological washing reactor  11 .  
     [0032] At both embodiments the exhaust gas to be purified enters the container  1  of the exhaust gas inlet  6  and is fed through the preliminary separator  7 . Process water is continuously sprinkled over this preliminary separator, which process water is fed to the sprinkling apparatus  8  from the biotank  2  through the conduits  9  and  10  and drops in a closed circuit or loop, respectively, back into the biotank  2 . In the preliminary separator  7  there occurs at the one hand the distribution of the gas over the cross-section of the container  1 , but at the other hand, such as will be describe further below, a cooling of the exhaust gas and a preliminary separating and removing of coarse constituents of the exhaust gas.  
     [0033] At the embodiment according to FIG. 1 the exhaust gas, after leaving the preliminary separator  7 , enters the biological washing reactor  11  which is filled by a structured biomass.  
     [0034] At the embodiment according to FIG. 1 the exhaust gas, after leaving the preliminary separator  7 , enters a honeycomb-like designed wet electrofilter  13 .  
     [0035] The flow of the gas proceeds in the biological washing reactor  11  and in the wet electrofilter upwards from the bottom to the top.  
     [0036] In the embodiment according to FIG. 2 the wet electrofilter  13  is protected by the partition wall  19  from the sprinkled water dropping off the biological washing reactor  11 , which sprinkling water is returned directly to the biotank  2  through the outflow conduit  20 .  
     [0037] The wet electrofilter  13  is cleaned intermittently by circulating water which is fed untreated from the biotank  2  through the conduits  10  and  15  to the sprinkling apparatus  14 .  
     [0038] The biological washing reactor  11  is continuously sprinkled by pre-treated process water which flows from the biotank  2  through the outflow conduit  21  to the solids removing apparatus  22 , thereafter through the heat exchanger  26  and finally through the conduits  27  and  28  to the sprinkling apparatus  12 . The film of bacteria which has grown in the biological-washing reactor  11  absorbs the organic gases and oxidizes these gases.  
     [0039] The purified exhaust gas flows finally out of the container  12  through the purified gas outlet  16  and flows beforehand through the droplets remover  17  which is also intermittently cleaned by process water which has been pre-treated in the solids removing apparatus  22  and the heat exchanger  26 . This process water flows from the heat exchanger  26  through the conduits  27  and  30  to the sprinkling apparatus  18  which is located above the droplets remover  17 .  
     [0040] Fresh air is fed from the source  4  for fresh air into the biotank  4 . Nutrients for the supply of nitrogen and phosphorus are fed from the source of nutrients  5  into the biotank  4 .  
     [0041] Polyelectrolyte acting as flocculation agent is added from the flocculation agent source  23  to the process water which flows to the solids removing apparatus  22 . The solid matter which has been separated in the solids removing apparatus  22  is drained off or discharged, respectively, through the draining line  24 .  
     [0042] Excess waste water which stems from the partial condensation of the humidity of the raw gas, that is the humidity of the fed exhaust gas to be purified, is drained off the circuit through the outlet conduit  29  which branches off from the outflow conduit  27 .  
     [0043] The selection between the embodiments according to FIG. 1 and FIG. 2 depends from the composition of the exhaust gas which is to be purified.  
     [0044] When the exhaust gas which is to be purified has a low content of aerosols, the arrangement of the wet electrofilter  13  over the biological washing reactor  11  according to FIG. 1 is selected, in which embodiment there is no partition wall  19 , so that the apparatus is of a somewhat more simple design.  
     [0045] In case of a high content of aerosols, the arrangement according to FIG. 2 is to be selected, because due to this design an additional contamination of the biological washing reactor  11  by the cleaning water coming from the wet electrofilter  13  is avoided.  
     [0046] It is to be noted that the apparatus is designed in such a manner, that the speed of flow of the exhaust gas amounts in both the wet electrofilter  13  and the biological washing reactor  11  11-13 meters per second, and that the dwell time in both the wet electrofilter  13  and the biological washing reactor  11  amounts to 1-16 seconds.  
     [0047] Below, the individual steps of the treatment of the exhaust gas will now be described.  
     [0048] The exhaust gas which enters the apparatus through the exhaust gas inlet  6  has a temperature in the range of 60°-200° C. This exhaust gas is initially cooled by the sprinkling water flowing out of the preliminary separator  7  by a evaporative cooling down to the primary point of condensation amounting to 50°-70° C.  
     [0049] The exhaust gas flows thereafter through the preliminary separator  7  which is composed of trickling plates and causes additionally the distribution of the gas over the cross-section of the container  1  and accordingly the biological washing reactor  11  and the wet electrofilter  13 . The preliminary separator allows the passage of coarse, wettable particles with a size &gt;10 μm and also the absorption of easily water-soluble gas constituents. The process, that is sprinkling water which is fed through the sprinkling apparatus  8  to the preliminary separator  7  has previously been regenerated in the biotank  2  and can accordingly absorb impurities.  
     [0050] The exhaust gas flows thereafter through the biological washing reactor  11  followed by the wet electrofilter  13  according to the embodiment of FIG. 1, or through the wet electrofilter  13  followed by the biological washing reactor  11  according to the embodiment of FIG. 2.  
     [0051] The biological washing reactor  11  is composed of structured growth elements (packing material) which allow at the one hand a growth of the biological mass and at the other hand a easy rinsing of the grown film. The biological washing reactor  11  is continuously sprinkled upon. The absorption of the organic gases and the bio-catalytic oxidation proceed by the film of water present and the layer of bacteriae located under this film. The bacteriae is accordingly supplied from the gas with substrate and from the process water, that is the sprinkled water.  
     [0052] This process water is supplied from the biotank  2 . The process water in the biotank  2  is supplied from the source  5  with nutrients for the bacteriae, such as typically urea and phosphoric acid for the supply of Nitrogen N and phosphorous P. In the solids removing apparatus  22  solid water is separated and removed from the process water which has been drawn off the biotank  2  and to which a flocculation agent for supporting the flocculation has been added from the source  23 . The flocculation agent is obviously added for a improving of the separating capacity of the solids removing apparatus  22 , because the constituents of the process water which flows out of the biotank can be fine disperse. Depending from the prevailing constituents the solids removing apparatus  22  can be designed as a filter, a flotation apparatus or a sedimentation apparatus. In the heat exchanger  26  which follows the solids removing apparatus  22  the process water, which is now a clear water, is cooled 5°-20° C. down to a temperature range of 35°-55° C. This cooling causes a further lowering of the primary dew point, which allows at the one hand the maintaining of the optimal temperature range of 35°-55° C. for the growth of the bacteriae, and produces by condensation waste water which is drawn off through the outlet conduit  29 .  
     [0053] Accordingly, the process temperature can be optimized and held at a constant value depending from the composition of the gas and the humidity and the necessary degree of the decomposition.  
     [0054] The cooling water supply for the heat exchanger  26 , see connections  31 ,  32  may come from a closed cooling tower loop, from ground or surface water or also from a heat pump which feeds the waste heat into a district heating network.  
     [0055] By the process water which is treated as set forth above and which is sprinkled over the biological washing reactor optimal conditions for the disintegration of the organic constituents of the exhaust gas which is to be purified are reached. The biological mass optimized itself from the mixed population on its own. When starting the apparatus up a balanced state between the growing and the rinsed off mass of the bacteriae is produced, that is a newly installed biological washing reactor is completely operative after about 4 weeks.  
     [0056] The wet electrofilter  13  which is located either above or below the biological washing reactor  11  is composed of hexagonal, tube-shaped honeycomb elements and has a typical width over flats of the hexagon of 200-400 mm of a length of 2-6 meters. A electrical discharge wire, which is mounted to a upper and a lower mounting frame, is located at the center of each tube which forms a single honeycomb element. The upper mounting frame is held in insulating chambers and is supplied externally with a negative direct voltage of 50-110 kV. The voltage and the current are controlled automatically so that minor or no flashovers occur. The negative corona at the electrical discharge wire causes an electric wind towards the hexagonal inner surface of the tube which operates as a precipitation electrode and the emitted electrons ionize the constituents of the exhaust gas, predominantly the aerosols by a ionization by impact. When now the fine particles are loaded electrically, they move corresponding to the direction of the electrical wind perpendicularly to the direction of flow of the gas in the corresponding tube onto the inner wall of the tube were they a separated out as a coating, are deposited and accordingly are removed out of the gas flow.  
     [0057] This coating is intermittently rinsed off to flow downwards by means of circulated process water and enters, finally, the biotank  2 .  
     [0058] The partition wall  19  which in the embodiment of FIG. 2 is located above the wet electrofilter  13  prevents the process water which is dripping off the biological washing reactor  11  to cause a voltage drop in the wet electrofilter  13 .  
     [0059] A droplets remover  17  is located ahead of the purified gas outlet  16 , that is the chimney, which droplet remover  17  prevents droplet which are entrained in the gas exiting the biological washing reactor  11  or the wet electrofilter  13  from flowing into the chimney. This droplets remover  17  is intermittently rinsed automatically by purified process water.  
     [0060] The process water which is circulated in a loop and which flows continuously specifically from the biological washing reactor  11  and from the preliminary separator  7  into the biotank  2 , and which flows intermittently from the wet electrofilter  13  and from the droplets remover  17  into the biotank  2  is collected in the biotank  2 . Therefore, the biotank  2  contains all solid matter which has been separated and removed and contains activated sludge and is, such as has been already mentioned, aerated from the source  4  of fresh air through the bottom aerator  3 . Correspondingly, the further oxidation of the organic constituents of the exhaust gas takes place in the biotank  2 .  
     [0061] While there are shown and described present preferred embodiments of the invention, it is to be distinctly understood that the invention is not limited thereto, but may be otherwise variously embodied and practiced within the scope of the following claims.