Abstract:
A method is provided for cleaning a combustion exhaust gas containing impurities, such as sulfur oxides and hydrochloric acid, with an additive, such as calcium hydroxide, in a reactor. The combustion gas is passed into a wetting zone where water is injected into the gas. The wet combustion gas is then passed through additive injection zone where the additive is co-currently injected into the combustion gas at a location near the bottom of the injection zone. The additive injection zone is connected to the top of the wetting zone and expands conically outward from the gas discharge outlet of the wetting zone so that as the combustion gas (and additive) travel upward through the additive injection zone the velocity of the combustion gas (and additive) is decreased. The combustion gas and additive are then passed through a cylindrical section having a uniform diameter of a given height and connected to the top of the additive injection zone. The combustion gas is then discharged from the reactor and passed through a filtering unit for removal of solids.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application is a 371 of PCT/FI98/00258, filed Mar. 24, 1998. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method for cleaning of combustion gas containing impurities in a reactor plant. 
     BACKGROUND OF THE INVENTION 
     The present invention provides that combustion gas to be cleaned is supplied to a lower part of a reactor vault, wherefrom it 
     is directed to flow substantially vertically upwards, and 
     to exit at the upper part of the reactor vault to an after-treatment or the like, wherein 
     the combustion gas is wetted with a wetting agent in the first phase of the cleaning process of the combustion gas, 
     in the second phase of the cleaning process of the combustion gas at least one additive reacting with impurities of the combustion gas, particularly with sulphur compounds and sulphur oxides, is supplied to the combustion gas that is wetted with a wetting agent, and 
     in the third phase of the process, the speed of the upwards directed flow of the combustion gas that has been treated with a wetting agent and an additive is retarded in the retardation zone of the reactor vault by enlarging the horizontal cross-section area of the reactor vault in the flow direction of the combustion gas. 
     A method of the above-mentioned kind is known from publication EP-121431. In the method of the publication the supply of combustion gases is carried out to the lower part of the reactor vault, whereafter wetting of combustion gases is performed and alkali used as an additive and reacting with sulphur compounds of combustion gas is added. The combustion gas exits at the upper part of the reactor vault. However, publication EP 121431 presents no disclosure in what manner it is attended to in the process and the construction of the reactor vault that the combustion gas handled with the wetting agent and the additive reacting with impurities would have a sufficient retention time in the reactor vault. A sufficient retention time ensures for the first that the wetting of the combustion gas will be succeeded. Second, mixing of the wetted combustion gas and the additive as well as chemical reactions require a certain time. Further, sufficient drying of exiting combustion gases (removal of wetting agent drops) should be secured before the combustion gas is removed from the reactor vault. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an aim of the present invention to present a method by which it is possible to a great extent to eliminate the drawbacks of prior art in a manner that the impurities of the combustion gas exiting the reactor vault of the reactor plant, particularly sulphur oxides that have reacted with water in connection with wetting, have reacted with the additive, and the combustion gas is free from wetting agent drops to the extent that it is possible to avoid harmful chemical additional reactions from taking place in the after-treatment of combustion gases, particularly in a filter. For attaining this purpose, the method of the present invention is primarily characterised in that 
     in the fourth phase of the cleaning process of the combustion gases, the average flowing speed of the combustion gas flow is standardised to correspond substantially to the end speed of the combustion gas flow of the third phase of the process by forming in the reactor vault a retention zone having a substantially standardised horizontal cross section after the retardation zone to form an extension to the same, and that 
     the combustion gases are removed from the reactor vault after the retention zone. 
     By using the above-described solution, advantageously in combination with a filter, preferably a fibre filter, a method is obtained by which it is possible to economically resolve simultaneous removal of sulphur compounds and particles particularly, but not exclusively, in small and medium sized combustion plants (1 to 50 MW). Treating combustion gas containing sulphur oxides (SO x ) with alkali compounds, particularly with calcium hydroxides is a generally known method for binding harmful sulphur to an alkali compound. However, the reactions require that the sulphur oxides and alkali oxides, in particular calcium hydroxide, are in an aqueous solution. The structure of the reactor vault has to be designed in a manner that there is sufficient time also for these reactions to take place. On the other hand, it is not sensible to discharge combustion gas containing wetting agent drops from the reactor vault, since it is advantageous in combustion plants to add to the treatment of combustion gas a filter positioned after the reactor plant and intended for dust removal, in which filter the occurrence of acidic sulphur compounds dissolved in water is very harmful. The method according to the present invention resolves the above-described problem complex in a manner that in the retardation zone which enlarges upwards in the flow direction of the combustion gases brings about a sufficient retention time for the combustion gas and thus, at the same time, a sufficient reaction time, since with an appropriate selection for the drop size the retention of the wetting agent drops is longer than the retention of the combustion gas. The retention zone forming an extension to the retardation zone ensures that the combustion gases leaving the upper part of the reactor vault do not contain wetting agent drops, so harmful sulphur compounds dissolved in water exist neither after the reactor vault in the combustion gas nor thus in the filter intended for dust removal of the combustion gas. 
     The accompanying dependent claims describe some advantageous embodiments of the method in accordance with the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 Vertical cross-section schematic of the reactor plant of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will be illustrated in more detail in the following specification with reference made to the enclosed drawing presenting schematically a vertical cross section of an advantageous embodiment of a reactor plant employing the method of the invention. 
     Referring to FIG. 1, a reactor plant employing a method of the invention comprises as its main parts a reactor vault  1  having in the lower part thereof an input channel  2  for the combustion gas and in the upper part an outlet channel  3  for the combustion gas. Input channel  2  has a diameter d 1  and output channel  3  has a diameter d 4 . Further, the reactor plant comprises a supply arrangement  4  for wetting agent of the combustion gas, said arrangement comprising a wetting agent line  5  and a supply line  6  for compressed air, these having a connection with nozzles  7  in the reactor vault. Further, the lower part of the reactor vault has a rinsing water aggregate  8 , a mixer  9  with an actuator  9   a  thereto, as well as an outlet aggregate  10  for removal of a liquid phase situated on the bottom of the reactor vault. The reactor device includes further a supply unit  11  for the additive reacting with the wetted combustion gas, said unit comprising a supply tube  12  in combination with a storage container  13  for the additive. To the supply tube  12  is connected a supply arrangement for the additive, e.g. an input for compressed air  14 . 
     The reactor vault  1  is divided into three main parts, the lowermost of which being formed by a wetting zone  1   a  having a connection with the input channel  2  for the combustion gas, the middle one being formed by a retardation zone  1   b  and the uppermost by a retention zone  1   c  connected to the outlet channel  3  related to a conical top  15  of the reactor vault  1 . H 2  is the height of retention zone  1   c.    
     The wetting zone  1   a  is formed by means of the walls of the reactor vault to have a substantially cylindrical form, wherein the wetting nozzles  7  are placed in the upper part of the wetting part to surround a horizontal aperture  16  having a contact with the retardation zone  1   b . The retardation zone  1   b  starts at the level of said aperture  16  and continues upwards as a conically enlarging part. As can be discovered in the drawing, the horizontal cross-section area of the aperture  16  is smaller than the horizontal cross-section area of the wetting vault  1   a , wherein in the upper part of the wetting zone  1   a , around the aperture  16  is formed an annular cam structure  16   a , having said wetting nozzles  7  placed in its lower surface, wherein they are positioned covered in a manner that layers K possibly draining down the walls of the retardation zone  1   b  pass the wetting nozzles  7  without damaging them when falling to the bottom of the wetting zone  1   b  to be removed via the outlet aggregate  10 . 
     In the described embodiment, the retardation zone  1   b  is formed to open upwards in a conical manner, but it is substantial to the general structure of the retardation zone  1   b  in the purpose of the invention that the horizontal cross-section area of the retardation zone  1   b  enlarges in the flow direction of the combustion gas upwards. In the combustion gas wetted with wetting agent, the additive is supplied centrally at the level of the aperture  16  or directly thereabove substantially downstream in the flow direction of the combustion gas, i.e. upwards in a manner that the additive mixes with the wetted combustion gas as evenly as possible. 
     As an extension to the retardation zone in the upper part of the reactor vault there is a retention zone  1   c , having a substantially constant horizontal cross-section area. The retention zone  1   c  is advantageously formed to be a cylindrical part having a diameter corresponding to the upper diameter of the conical form piece of the retardation zone  1   b , i.e. the size and form of the aperture  17 . Subsequent to the retention zone  1   c  the top  15  of the reactor vault  1  is formed by a gathering cone in which the outlet channel  3  for the combustion gases is coupled in a manner that it is directed preferably upwards and centrally with the centre line of the retardation zone  1   b  and retention zone  1   c.    
     The method operates according to the following manner. In the first phase of the cleaning process the combustion gas that comes to the reactor vault  1  via the input channel is wetted with wetting agent particularly with water brought about into shower form by compressed air. The size range of the water drops is medium, which means that the drops cannot be too small, wherein the wetting (absorption) of the combustion gases is of too brief duration the combustion gas dries too fast in the retardation zone  1   b . On the other hand, the drop size cannot be too large, wherein the drops remain in the wetting zone and drain or fall to its bottom not finding their way either to the aperture  16  or upwards therefrom. By appropriate selection of drop size such composition for the combustion gas is obtained in the entire area of the retardation zone  1   b  of the reactor vault that humidity conditions necessary for attaining chemical reactions exist in the whole humidity measure h 1  of the retardation zone  1   b.    
     In the lower part of the retardation zone, the second phase of the cleaning process takes place, wherein to the combustion gas wetted with the wetting agent a powder like additive is supplied from the additive-supply unit  11 , e.g. by means of compressed air. The additive is usually a suitable alkali compound, a particularly suitable compound is calcium hydroxide reacting with sulphur oxides and hydrocloric acid and other acidifying compounds. 
     In the retardation zone, the flowing speed of the combustion gas is retarded in a manner that the ratio V SA =the initial average speed of combustion gas/V SL =the final average speed is 10 to 40. This lowering of flowing speed of the combustion gas in the retardation zone results, in most practical embodiments where the retardation zone  1   b  is conical, in the fact that an upwards opening conical angle of the conical form piece of the retardation zone is 10° to 20°, preferably about 15°, wherein the definition formula used for the angle and/or the height h 1  of the retardation zone  1   b  is                  tan                 α     =       (       d   3     -     d   2       )       2   ·     h   1           ,           (   1   )                                
     wherein in the formula 
     α=upwards opening conical angle, 
     h 1 =height of the conical retardation zone  1   b,    
     d 2 =diameter of the aperture  16  in the lower part of the retardation zone  1   b , and 
     d 3 =diameter of the aperture  17  in the upper part of the retardation zone  1   b.    
     The retarding flow of the wetted combustion gas that has been treated with an additive takes place in the retardation zone  1   b , upwards, wherein a long retention time is ensured for the combustion gases and thus, at the same time, a long reaction time for the reactions taking place in moist conditions required between the additive and the impurities, mainly sulphur oxides and possibly hydrochloric acid. At the same time the size of the drops is decreasing due to drying and in the retardation zone  1   b , in the upwards moving combustion gas flow, the drop size is decreasing. 
     The final drying of wetting agent drops in the combustion gas takes place in the retention zone  1   c , wherein the combustion gas leaving the outlet channel  3  contains no wetting agent drops. Significant layers K are not generated in the walls of the retention zone. In case these are generated, they drain downwards along the walls of the retardation zone  1   b  and fall on the bottom of the reactor vault. Further, the draining slows down the downwards directing speed of the layers to the extent that they do not cause harm when falling on the bottom of the reactor vault  1 . 
     The diameters of the apertures  16 ,  17  in the bottom part and upper part of the retardation zone  1   b  are defined in the formulas:                  d   2     =   0     ,     075   ·         m   ·     (     273   +     θ   1       )         p   ·     (       π   +   0     ,     0628   ·     D   50         )             ,              and           (   2   )                   d   3     =   0     ,     075   ·         m   p     ·     (     273   +     θ   2       )     ·     (     0   ,     7692   +         θ   1     -     θ   2         1718   +     1.299   ·     θ   2               )           ,           (   3   )                                
     wherein 
     d 2 =diameter of the aperture ( 16 ) in the lower part of the retardation zone ( 1   b ), 
     d 3 =diameter of the aperture ( 17 ) in the upper part of the retardation zone ( 1   b ), 
     m=mass flow (kg/s) of the combustion gas, 
     p=total pressure (bar) of the combustion gas, 
     θ 1 =initial temperature (° C.) of the combustion gas, 
     θ 2 =end temperature (° C.) of the combustion gas, 
     D 50 =median diameter (μm) of the drops. 
     The height of the retention zone  1   c  is defined by using the formula                  h   2     =       4   ·   G2   ·   G3       π   ·     d   1   2           ,           (   4   )                                
     wherein the terms G 2  and G 3  are defined in the following manner:                F1   =           π   ·     d   2   2       +     π   ·     d   1   2         8     ·     h   1         ,           (   5   )                 F2   =         θ   1     -     θ   2         ln          θ   1       θ   2             ,           (   6   )                 F3   =       m   p     ·   2       ,     858   ·     (     273   +   F2     )     ·     10     -   3         ,           (   7   )                                G 1= F 1 /F 3  (8), 
     
       
           G 2=5.0− G 1  (9), 
       
     
     
       
         
           
             
               
                 
                   
                     G3 
                     = 
                     
                       F3 
                       · 
                       
                         
                           ( 
                           
                             273 
                             + 
                             
                               θ 
                               2 
                             
                           
                           ) 
                         
                         
                           ( 
                           
                             273 
                             + 
                             F2 
                           
                           ) 
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
           
         
                 
         
             
         
      
     
     wherein in the formulas 
     h 1 =height of the conical retardation zone ( 1   b ), 
     d 2 =diameter of the aperture ( 16 ) in the lower part of the retardation zone ( 1   b ), 
     d 3 =diameter of the aperture ( 17 ) in the upper part of the retardation zone ( 1   b ), 
     m=mass flow (kg/s) of the combustion gas, 
     p=total pressure (bar) of the combustion gas, 
     θ 1 =initial temperature (° C.) of the combustion gas, 
     θ 2 =end temperature (° C.) of the combustion gas, 
     D 50 =median diameter (μm) of the drops. 
     The cleaned combustion gas is directed to a filter arranged for dust removal of the combustion gas, particularly to a fibre filter (not shown), where the filtering of particulate dust of the combustion gas, which does not contain wetting agent in drop form, takes place. 
     Thus, the method contains an operational entirety fulfilling the following operational requirements: 
     I) Large wetting agent drops do not reach the retardation zone  1   b,    
     II) Medium size wetting agent drops join the combustion gas flow, wet the combustion gas and confront the additive, 
     III) The combustion gas in the upper part of the reactor vault contains no wetting agent drops. 
     As to the requirements I to Ill, the requirement I is fulfilled by the measuring formula (2) for the aperture  16  in the lower part of the retardation zone  1   b.  In a corresponding manner, II is fulfilled by the measuring formula (3) for the aperture  17  in the upper part of the retardation zone  1   b.  The requirements I and II involve also the formula (1), on the basis of which, after the measuring of the apertures (i.e. d 2  and d 3 ), the facients of the product h 1 ·tan α, i.e. h 1  and α are defined. Further, the definition formula for the height of the retention zone  1   c  fulfils the requirement III.