Patent Publication Number: US-9895643-B2

Title: Wet scrubber and a method of cleaning a process gas

Description:
This is a divisional application claiming priority to co-pending U.S. application Ser. No. 13/992,822, having a filing date of Aug. 14, 2013, entitled “WET SCRUBBER AND A METHOD OF CLEANING A PROCESS GAS”, now U.S. Pat. No. 9,079,131, which claims priority to International Application No. PCT/1B2011/002745 having an International Filing Date of Nov. 16, 2011, which in turn claims priority to EP Application No. 10194445.2 having a filing date of Dec. 10, 2010, with each application incorporated herein in its entirety by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method of cleaning a process gas by means of a wet scrubber comprising a wet scrubber tower, at least a first spray level system within the wet scrubber tower, with nozzles to which an absorption liquid is supplied for atomization by the nozzles, and a second spray level system arranged above the first spray level system in the wet scrubber tower to which an absorption liquid is supplied for atomization by nozzles comprised in the second spray level system. 
     The present invention further relates to a wet scrubber useful for cleaning a process gas. 
     BACKGROUND OF THE INVENTION 
     Combustion of a fuel, such as coal, oil, peat, waste, etc., in a combustion plant, such as a power plant, generates a hot process gas containing, among other components, sulphur oxides, SO x , such as sulphur dioxide, SO 2 , and carbon dioxide, CO 2 . Sulphur dioxide is an environmental pollutant. Hence, it is necessary to remove at least a portion of the sulphur dioxide contained in a process gas before releasing the process gas into the atmosphere. Furthermore, with increasing focus on the negative environmental impacts of carbon dioxide gas, it has become important to remove also carbon dioxide from process gases before releasing them to the atmosphere. 
     WO 2008/042554 describes a wet scrubber in which a perforated plate is arranged adjacent to a number of atomizing nozzles. Each of the perforations in the perforated plate is aligned with a cone of absorption liquid sprayed from an atomizing nozzle. 
     The perforated plate illustrated in WO 2008/042554 may in some cases yield a rather high process gas pressure drop, which is not necessarily indicative of an increase in sulphur dioxide removal efficiency. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a wet scrubber, and a method of using the wet scrubber to clean a process gas, more efficient than the prior art wet scrubber and prior art method of using the wet scrubber. 
     This object is achieved by means of the subject method of cleaning a process gas by means of a wet scrubber comprising a wet scrubber tower, at least a first spray level system within the wet scrubber tower, with nozzles to which an absorption liquid is supplied for atomization by the nozzles, and a second spray level system arranged above the first spray level system in the wet scrubber tower to which an absorption liquid is supplied for atomization by nozzles comprised in the second spray level system, the method comprising 
     deflecting absorption liquid atomized by means of nozzles of the second spray level system flowing downwards in the wet scrubber tower, from the vicinity of at least one nozzle of the first spray level system by means of a gas-liquid contacting plate located vertically above the at least one nozzle of the first spray level system, to bring the deflected absorption liquid into contact with process gas that has been contacted by the absorption liquid atomized by the at least one nozzle of the first spray level system. 
     An advantage of the subject method is that contact between deflected absorption liquid and process gas contacted with absorption liquid atomized by the at least one nozzle of the first spray level system causes an intense liquid/gas intermixing, producing what could be referred to as a “cloud” of absorption liquid and process gas. This cloud of absorption liquid and process gas yields very efficient absorption of contaminants, such as sulphur dioxide and carbon dioxide, by the absorption liquid in the wet scrubber tower. 
     According to one embodiment the process gas flowing upwardly through the wet scrubber tower is contacted with absorption liquid atomized by first spray level system nozzles. 
     According to one method embodiment, an individual gas-liquid contacting plate is arranged in the wet scrubber tower vertically above each of at least half of all first spray level system nozzles. The gas-liquid contacting plates deflect absorption liquid from each of at least half of all the first spray level system nozzles positioned beneath gas-liquid contacting plates. An advantage of this embodiment is that removal of contaminants from the process gas becomes very efficient when a number of adjacent gas-liquid collecting plates arranged with open space therebetween cause absorption liquid deflection and intermixing with process gas. 
     According to one method embodiment, open space between adjacent gas-liquid contacting plates arranged within the same horizontal plane within the wet scrubber tower, allows the upward flow of process gas through the open spaces at a vertical process gas velocity of 5-15 m/s. With vertical process gas velocities less than 5 m/s, absorption liquid and process gas intermixing tends to be less efficient. With vertical process gas velocities greater than 15 m/s, the process gas pressure drop within the wet scrubber tower tends to increase to unacceptably high levels. Such high pressure drop levels are unacceptable due to the large amount of energy required to pass process gas through and out of the wet scrubber tower. Also, at greater vertical process gas velocities, a large portion of deflected absorption liquid becomes entrained within the process gas, causing increased liquid loads on wet scrubber tower mist eliminators, potentially causing an increased loss of absorption liquid from the wet scrubber. 
     According to one method embodiment, at least one damper is arranged adjacent to the at least one gas-liquid contacting plate. The subject method further comprising controlling the vertical velocity of the process gas contacting the deflected absorption liquid by the at least one gas-liquid contacting plate, by adjusting the damper. An advantage of this embodiment is that the vertical process gas velocity of the process gas contacting the deflected absorption liquid may be adjusted to a suitable velocity, such as 5-15 m/s, by adjusting the positioning of the damper. 
     A further object of the present invention is to provide a wet scrubber more efficient in removing contaminants from a process gas than the wet scrubber of the prior art. 
     This object is achieved by means of a wet scrubber useful for cleaning a process gas. The subject wet scrubber comprises at least a first spray level system with nozzles in a wet scrubber tower to which an absorption liquid may be supplied for atomization by the nozzles, a second spray level system arranged within the wet scrubber tower above the first spray level system, to which an absorption liquid may be supplied for atomization by the nozzles comprised in the second spray level system, and a wet scrubber tower housing the first and second spray level systems and comprising a process gas inlet arranged in a base portion of the wet scrubber tower, and a process gas outlet arranged in an upper portion of the wet scrubber tower. The first spray level system comprises at least one gas-liquid contacting plate located above at least one of the first spray level system nozzles. The at least one gas-liquid contacting plate deflecting absorption liquid flowing downwardly within the wet scrubber from atomizing nozzles of the second spray level system, away from at least one first spray level system nozzle, such that the deflected absorption liquid may contact process gas previously contacted by absorption liquid atomized by the at least one first spray level system nozzle. 
     An advantage of this wet scrubber embodiment is that it efficiently forces downwardly flowing absorption liquid originating from the second spray level system into contact with process gas just previously in contact with absorption liquid originating from first spray level system atomizing nozzles. Accordingly, the absorption liquid atomized by second spray level system nozzles first efficiently contacts process gas adjacent to second spray level system nozzles, and is then efficiently diverted to contact the process gas once more, adjacent the first spray level system. 
     According to one embodiment, the subject first spray level system comprises a number of separate gas-liquid contacting plates arranged within the wet scrubber tower above each of at least half of all of the first spray level system nozzles. The more nozzles equipped with a corresponding gas-liquid contacting plate, the more efficient the intermixing of deflected absorption liquid and process gas. 
     According to one embodiment, the total combined horizontal surface area of all gas-liquid contacting plates within the first spray level system equals approximately 30 to 75% of the internal wet scrubber horizontal cross-sectional area when measured in the same horizontal plane as the at least one gas-liquid contacting plate. It is preferable that the total combined horizontal surface area of all gas-liquid contacting plates covers at least 30% of the internal wet scrubber horizontal cross-sectional area, since a reduction in the total combined horizontal surface area of all the gas-liquid contacting plates reduces the deflection of absorption liquid flowing downwardly inside the wet scrubber tower. Reduced deflection equates with reduced efficiency in intermixing the deflected absorption liquid and process gas. Preferably, the total combined horizontal surface area of all gas-liquid contacting plates covers not more than approximately 75% of the internal wet scrubber horizontal cross-sectional area when measured in the same plane as that of the gas-liquid contacting plates. Greater gas-liquid contacting plate coverage equates with an increase in the process gas pressure drop, thus requiring a large amount of energy to pass process gas through and out of the wet scrubber tower. Coverage greater than 75% may also substantially increase the amount of absorption liquid entrained within the upwardly flowing process gas thus increasing the risk of losing absorption liquid from the wet scrubber tower. 
     According to one embodiment, the shortest distance from a point on a bottom surface of the at least one gas-liquid contacting plate to a spray opening of a nozzle therebelow is approximately 0.1 to 0.9 m. Preferably, the shortest distance from a point on a bottom surface of the at least one gas-liquid contacting plate to a spray opening of a nozzle therebelow is at least 0.1 m, since lesser distances may force process gas out of the sprayed absorption liquid prematurely. Preferably, the shortest distance from a point on a bottom surface of the at least one gas-liquid contacting plate to a spray opening of a nozzle therebelow, is no more than approximately 0.9 m. Greater distances tend to unduly increase the required height of the wet scrubber tower, thus increasing investment and maintenance costs associated therewith. According to a further embodiment, the shortest distance from a point on a bottom surface of the at least one gas-liquid contacting plate to a spray opening of a nozzle therebelow is approximately 0.1 to 0.6 m. 
     According to one embodiment, the first spray level system comprises at least one adjustable damper located adjacent to the at least one gas-liquid contacting plate for controlling the vertical velocity of the process gas contacted by the deflected absorption liquid. An advantage of a damper is that contaminant removal efficiency may be achieved at various process gas flows, by adjusting the positioning of the damper to control and maintain the vertical process gas velocity within a suitable range. According to a further embodiment, the first spray level system comprises at least one damper arranged between at least two adjacent gas-liquid contacting plates, and in substantially the same horizontal plane as the at least two adjacent gas-liquid contacting plates. The subject damper may be used to adjust the size of the open space between the at least two adjacent gas-liquid contacting plates to control the vertical velocity of the process gas contacted by absorption liquid deflected by the two adjacent gas-liquid contacting plates. An advantage of this embodiment is that the damper enables efficient control of the vertical process gas velocity in the open space between two adjacent gas-liquid contacting plates. Furthermore, the subject damper may even be used to close at least some open spaces at times of very low process gas flow. 
     According to one embodiment, the total combined top surface area of all gas-liquid contacting plates of one spray level system is smaller than the total combined top surface area of all gas-liquid contacting plates of another spray level system arranged above the first-mentioned spray level system within the same wet scrubber tower. This embodiment takes into account the fact that absorption liquid volumes flowing downwardly through the wet scrubber tower are greater in the base portion of the wet scrubber tower than in the upper portion of the wet scrubber tower. Hence, an advantage of this embodiment is that the gas-liquid contacting plates may have a larger total combined top surface area when arranged in the upper portion of the wet scrubber tower, where the volume of absorption liquid is less. 
     According to one embodiment, at least the first spray level system nozzles spray at least half of the absorption liquid supplied thereto in a downward direction. Spraying at least half of the absorption liquid in a downward direction separates process gas contaminant removal by the first spray level system into two distinct and separate zones. A first zone of contaminant removal occurs when the process gas contacts absorption liquid sprayed by the first spray level system nozzles. A second zone of contaminant removal occurs, after that of the first zone, when deflected absorption liquid from spray level systems arranged within the wet scrubber tower above the first spray level system contacts process gas flowing from the first zone. 
     Further objects and features of the present invention will be apparent from the following detailed description and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject invention is described in more detail below with reference to the appended drawings in which: 
         FIG. 1  is a schematic side view of a wet scrubber in accordance with a first embodiment. 
         FIG. 2  is a schematic top view of a spray level system of the wet scrubber of  FIG. 1  taken along line II-II. 
         FIG. 3  is an enlarged schematic side view of area III of  FIG. 1  illustrating detailed features of a spray level system. 
         FIG. 4  is an enlarged schematic perspective view of two spray level systems in operation. 
         FIG. 5 a    is a schematic top view of a spray level system in accordance with an alternative embodiment. 
         FIG. 5 b    is a side view of the spray level system of  FIG. 5 a    taken along line Vb-Vb. 
         FIG. 6  is a side view of a spray level system in accordance with yet another alternative embodiment. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a wet scrubber  1 . The wet scrubber  1  is operative for removing at least a portion of the sulphur dioxide content of a process gas, in the form of a flue gas, F, generated in a boiler (not shown) which is operative for combusting a fuel, such as coal, oil, peat, waste and the like. 
     Wet scrubber  1  comprises a vertical open wet scrubber tower  2  with interior  2   a , a process gas inlet  4  arranged in a base portion  2   b  of wet scrubber tower  2  for flow of flue gas, F, to be cleaned into fluidly connected interior  2   a , and a process gas outlet  6  arranged in an upper portion  2   c  of wet scrubber tower  2  for flow of cleaned flue gas, CF, from which at least a portion of the sulphur dioxide content has been removed, out of fluidly connected interior  2   a . As illustrated in  FIG. 1  as a vertical, upwardly pointed arrow, flue gas F travels substantially vertically upward inside interior  2   a  of wet scrubber tower  2 . 
     An absorption liquid tank  8  is arranged at the bottom  2   d  of base portion  2   b  of wet scrubber tower  2 . Absorption liquid tank  8  is equipped with a fluidly connected oxidation arrangement  10 . An absorbent, such as fresh limestone, CaCO 3 , is supplied to absorption liquid tank  8  from a fluidly connected absorbent supply device  12  comprising a limestone storage area  14  and a limestone supply pipe  14   a . It will be appreciated that absorption liquid tank  8  may, as an alternative, be positioned outside of wet scrubber tower  2 , and that the supply of limestone could, as an alternative, enter wet scrubber  1  at other locations, as a dry powder, a slurry or both. 
     Wet scrubber  1  further comprises a first circulation pump  16  which circulates in a fluidly connected absorption liquid circulation pipe  18 , a limestone absorption liquid, sometimes referred to as a limestone slurry. Absorption liquid is pumped by first circulation pump  16  from absorption liquid tank  8  through fluidly connected circulation pipe  18  to a fluidly connected first spray level system  20  arranged horizontally across interior  2   a  in mid portion  2   e  of wet scrubber tower  2  adjacent base portion  2   b . Wet scrubber  1  further comprises a second circulation pump  22  which circulates in a fluidly connected absorption liquid circulation pipe  24 , a limestone absorption liquid from fluidly connected absorption liquid tank  8 . Absorption liquid is pumped by second circulation pump  22  through fluidly connected circulation pipe  24  to a fluidly connected second spray level system  26  arranged horizontally across interior  2   a  in mid portion  2   e  of wet scrubber tower  2  above first spray level system  20 . Wet scrubber  1  further comprises a third circulation pump  28  which circulates in a fluidly connected absorption liquid circulation pipe  30 , a limestone absorption liquid from fluidly connected absorption liquid tank  8 . Absorption liquid is pumped by third circulation pump  28  through fluidly connected circulation pipe  30  to a fluidly connected third spray level system  32  arranged horizontally across interior  2   a  in mid portion  2   e  of wet scrubber tower  2  above second spray level system  26 . Distance CC illustrated in  FIG. 1 , is determined by measuring from the mid-point of one spray level system, e.g., the first spray level system  20 , to the mid-point of an adjacent spray level system, e.g., the second spray level system  26 . Distance CC is preferably approximately 1.25 m to 3 m. A distance CC less than 1.25 m is less preferable, due to unwanted absorption liquid spray interference between adjacent spray level systems, resulting in decreased sulphur dioxide removal efficiency. A distance CC more than 3 m is less preferable, since greater distances CC require increases in the overall height of wet scrubber tower  2 , thus increasing investment and operating costs. It will be appreciated that a wet scrubber  1  could comprise more or less than three spray level systems, for example 2 to 10 spray level systems arranged in interior  2   a  of wet scrubber tower  2 . 
     First spray level system  20  comprises a tubular portion  34  equipped with a number of fluidly connected atomizing nozzles  36  to finely distribute limestone absorption liquid supplied thereto by pump  16 . Absorption liquid is finely distributed by nozzles  36  to achieve effective contact between the absorption liquid and the flue gas flowing upwardly through interior  2   a  of wet scrubber tower  2 . All or some of nozzles  36  may, for example, be of the same type as Model 22298-2CF-SILCNB75-120, available from Spraying Systems Co, Wheaton, Ill., USA. This type of atomizing nozzle is operative for a liquid flow of about 17 m 3 /hour, corresponding to 17000 liters/hour, or 280 liters/minute, at a spraying pressure measured using water, of about 0.5 bar. 
     First spray level system  20  further comprises a number of gas-liquid contacting plates  38 . Each gas-liquid contacting plate  38  is located vertically above a nozzle  36 , as described in more detail hereinafter. 
     Second spray level system  26  is similar if not identical to first spray level system  20  and comprises a tubular portion  40  equipped with a number of fluidly connected atomizing nozzles  36  and a number of gas-liquid contacting plates  39 . Gas-liquid contacting plates  39  of second spray level system  26  may be of the same shape and size as gas-liquid contacting plates  38  of first spray level system  20 , or may have a different shape and/or size. In accordance with one alternative, top side  39   a  of at least some of gas-liquid contacting plates  39  may have a larger surface area than that of top side  38   a  of at least some of gas-liquid contacting plates  38 . 
     Third spray level system  32  comprises a tubular portion  42  equipped with a number of atomizing nozzles  36 . The third, and uppermost, spray level system  32  may be arranged without any gas-liquid contacting plates. 
     A mist eliminator  44  is located above third spray level system  32 . Mist eliminator  44  arranged horizontally across interior  2   a  in upper portion  2   c  adjacent to process gas outlet  6 , removes at least a portion of the absorption liquid droplets entrained by cleaned flue gas, CF. Hence, absorption liquid droplets are removed from cleaned flue gas CF as the cleaned flue gas flows upwardly through interior  2   a  of wet scrubber tower  2 , prior to exiting wet scrubber  1  via process gas outlet  6 . 
     In wet scrubber  1 , sulphur dioxide, SO 2 , in the flue gas reacts with limestone, CaCO 3 , in the absorption liquid to form calcium sulphite, CaSO 3 , which is subsequently oxidized to form gypsum, CaSO 4 . The oxidation of calcium sulphite is preferably performed by bubbling air or oxygen gas through the absorption liquid using oxidation arrangement  10 . Hence, the absorption liquid comprises, in addition to the limestone, also small amounts of calcium sulphite and, as a major constituent, gypsum. Gypsum formed through this process is removed from wet scrubber  1  via a fluidly connected disposal pipe  46  to a fluidly connected gypsum dewatering unit, schematically illustrated as belt filter  48 . The dewatered gypsum may be commercially used, for example in wallboard production. 
     In addition to sulphur dioxide, SO 2 , wet scrubber  1  removes, at least partly, other contaminants from the flue gas also. Examples of such other contaminants include sulphur trioxide, SO 3 , hydrochloric acid, HCl, hydrofluoric acid, HF, and other acidic process contaminants. Still further, wet scrubber  1  may also remove, at least partly, other types of contaminants from the flue gas, such as for example dust particles and mercury. 
       FIG. 2  illustrates an enlarged top view of first spray level system  20  taken along line II-II of  FIG. 1 . The second spray level system  26  (not illustrated) has the same principal design as illustrated first spray level system  20 . As illustrated in  FIG. 2 , tubular portion  34  is equipped with a number of fluidly connected perpendicular tubular extensions  50  forming a “grid” with and extending from tubular portion  34 . Atomizing nozzles  36  fluidly connect to tubular portion  34  and tubular extensions  50  so that nozzles  36  of first spray level system  20  are rather evenly distributed over the entire horizontal cross-section of wet scrubber tower  2 . 
     In the embodiment illustrated in  FIG. 2  each nozzle  36  is covered by a single, corresponding gas-liquid contacting plate  38 . In the embodiment illustrated in  FIG. 2 , the total number of nozzles  36  of first spray level system  20  is 13. It will be appreciated that any suitable number of nozzles  36  could be utilized. Furthermore, a nozzle  36  could be connected directly to tubular portion  34  or tubular extension  50 , as the case may be, as is illustrated in  FIG. 2 , or could be connected to portion  34  or extension  50  via a fluidly connected connecting pipe in a per se known manner. Typically, the number of nozzles  36  in each spray level system  20  would range from about 4 to about 500. Furthermore, it is not necessary to arrange a single, corresponding gas-liquid contacting plate  38  above each nozzle  36 . It may, for example, be sufficient to arrange gas-liquid contacting plates  38  above some of the nozzles  36 , for example above 5 to 10 of the 13 nozzles  36  illustrated in  FIG. 2 . It may be preferred to arrange single, corresponding gas-liquid contacting plates  38  above each of at least half of nozzles  36  of a spray level system  20 . More preferably, at least 75% of all of the nozzles  36  of a spray level system  20  may be equipped with single, corresponding gas-liquid contacting plates  38  thereabove. 
     The internal diameter, di, of interior  2   a  of wet scrubber tower  2 , measured at the level of the gas-liquid contacting plates  38  of spray level system  20 , is 5 m in the example depicted in  FIG. 2 . Thus, the interior  2   a  of wet scrubber tower  2  horizontal cross-sectional area measured at the level of the gas-liquid contacting plates  38  of spray level system  20 , is about 20 m 2  (πr 2 =3.14×5/2×5/2=19.6 m 2 ). In the embodiment of  FIG. 2 , each gas-liquid contacting plate  38  is substantially square in top view, with a planar top surface  38   a  bounded by four free edges  38   b . Each of the four free edges  38   b  has a length L of approximately 0.85 m. Hence, the area of top surface  38   a  is 0.72 m 2  (L 2 =0.85×0.85=0.72 m 2 ). The total combined area of all gas-liquid contacting plates  38  of the spray level system  20  is 13×0.72 m 2 =9.36 m 2 . Hence, the gas-liquid contacting plates  38  cover, in total, 9.36 m 2 /20 m 2 =47% of interior  2   a  wet scrubber tower  2  horizontal cross-sectional area measured at the level of gas-liquid contacting plates  38  of spray level system  20 . It has been found that gas-liquid contacting plates  38  of spray level system  20  may in combination constitute 30 to 75% of the interior  2   a  wet scrubber tower  2  horizontal cross-sectional area measured at the level of gas-liquid contacting plates  38  of spray level system  20 . 
     In accordance with one embodiment, the total combined area of all top surfaces  38   a  of a particular spray level system increases, moving from spray level system to spray level system with flue gas flow through wet scrubber tower  2 . Hence, for example, the top surfaces  39   a  of gas liquid contacting plates  39  of second spray level system  26  may have a total combined area of 11 m 2 . Accordingly, gas-liquid contacting plates  39  occupy 11 m 2 /20 m 2 =55% of the interior  2   a  wet scrubber tower  2  horizontal cross-sectional area measured at the level of the gas liquid contacting plates  39  of the second spray level system  26 . Such compares to the 47% occupied by gas-liquid contacting plates  38  of first spay level system  20 . The reason for such design is that the closer to base portion  2   b  a spray level system is arranged, the more absorbent liquid is present, creating an increased need for open space. 
       FIG. 3  is an enlarged side view of area III of  FIG. 1  illustrating in more detail spray level system  20 . Each gas-liquid contacting plate  38  is centred over a corresponding nozzle  36  fluidly connected to tubular extension  50 . The vertical distance, HP, is the vertical distance from a nozzle  36  spray opening  52  to a closest point on bottom surface  38   d  of the corresponding gas-liquid contacting plate  38 . Distance HP is typically approximately 0.1 to 0.9 m, often approximately 0.1 to 0.6 m. Nozzles  36  are arranged for spraying at least a portion of the absorption liquid supplied thereto in a downward direction within interior  2   a  of wet scrubber tower  2 . In accordance with one embodiment, at least half of the absorption liquid supplied to nozzles  36  is sprayed in a downward direction. 
     In accordance with one embodiment, substantially all of the absorption liquid supplied to nozzles  36  is sprayed in a downward direction. 
       FIG. 4  illustrates in perspective view spray level systems  20 ,  26  when in operation. Nozzles  36  of first spray level system  20  atomize absorption liquid supplied thereto and generate a spray cloud SC of absorption liquid. Typically, nozzles  36  provide an absorption liquid spray angle α of approximately 60° to 180°, and more typically approximately 90° to 130°. As illustrated in  FIG. 4 , under the momentum of atomized absorption liquid from nozzles  36  of first spray level system  20 , flue gas F is forced away from nozzles  36  toward open spaces between adjacent nozzles  36 . Hence, flue gas F flows as far away from nozzles  36  as possible. Absorption liquid, illustrated in  FIG. 4  with shaded arrows AL, atomized by nozzles  36  of second and third spray level systems  26  and  32 , flows downwardly in wet scrubber tower  2  contacting, at least partly, top surface  38   a  of gas-liquid contacting plates  38 . Absorption liquid AL then flows towards open spaces OS between gas-liquid contacting plates  38 . 
     The open spaces OS coincide with the flow path of flue gas F due to the momentum of atomized absorption liquid from nozzles  36  of first spray level system  20 . In open spaces OS an intense intermixing of flue gas F and absorption liquid AL occurs. Such intense intermixing leads to the formation of what could be regarded as “clouds” C adjacent to open spaces OS. In open spaces OS, flue gas F has a relatively high vertical flue gas velocity, due to a substantial portion of the interior  2   a  cross-sectional area of wet scrubber tower  2  being occupied by gas-liquid contacting plates  38 . Typically, vertical flue gas velocity may be approximately 5 to 15 m/s, often 6 to 10 m/s, in open spaces OS. The vertical flue gas velocity in the open spaces OS may, for example, be calculated from the measured gas flow in tower  2 , measured for example at a point P just below the spray level system  20 , or obtained from sensors in the general plant control system, and dividing the thus measured gas flow, in m 3 /s, by the total open interior  2   a  wet scrubber tower  2  horizontal cross-sectional area measured at the level of the gas-liquid contacting plates  38 . The vertical flue gas velocity is the actual gas velocity measured at the actual gas temperature, actual gas pressure, and actual gas composition prevailing just below the spray level system  20  at point P. The point P is suitably located on a vertical level where no portion of interior  2   a  cross-sectional area of wet scrubber tower  2  is occupied by tubular extensions  50 , nozzles  36 , plates  38 , or the like. Hence, point P is located in what could be referred to as an “empty floor” of interior  2   a , meaning that the flue gas velocity measured in point P reflects the flue gas velocity that would prevail in wet scrubber tower  2  if it was empty of all internal structures. Using the example provided above, the total open interior  2   a  wet scrubber tower  2  horizontal cross-sectional area at the level of the plates  38  is calculated as the internal wet scrubber horizontal cross-sectional area minus the total horizontal surface area of top surface  38   a  of gas-liquid contacting plates  38 . In the present example, the total open internal wet scrubber horizontal cross-sectional area is 10.64 m 2 , i.e., 20 m 2 −9.36 m 2 =10.64 m 2 . With a total gas flow through tower  2  of 80 m 3 /s, as measured, for example, at point P, the vertical flue gas velocity in the open spaces OS is 7.5 m/s, i.e., 80 m 3 /s/10.64 m 2 =7.5 m/s. 
     With such a high vertical flue gas velocity in the open spaces OS, the absorption liquid AL entering open spaces OS from second and third spray level systems  26 ,  32  dissipates, clears or drains from the flue gas F quite slowly. Slow absorption liquid AL dissipation results in the formation of the “clouds” C adjacent to and above open spaces OS. Clouds C comprise flue gas F mixed with absorption liquid AL that cannot easily drain or dissipate. Hence, clouds C almost resemble bubbling beds of turbulence within wet scrubber tower  2 . The intense mixing of absorption liquid AL and flue gas F in clouds C results in increased sulphur dioxide absorption levels and thereby efficient removal of sulphur dioxide from flue gas F. Absorption liquid AL eventually drains from cloud C, illustrated in  FIG. 4  as flow D. However, absorption liquid AL has a relatively long average residence time in clouds C before draining therefrom. 
     Within wet scrubber tower  2 , nozzles  36  are arranged so that at least half of the absorption liquid supplied thereto is sprayed in a downward direction. In fact, as illustrated in  FIG. 4 , all of the absorption liquid may be sprayed in a generally downward direction. Spraying absorption liquid in spray cloud SC in a generally downward direction from first spray level system  20  nozzles  36  separates the removal of sulphur dioxide from the flue gas into two distinct and separate zones of sulphur dioxide absorption of the first spray level system  20 . A first zone is the spray cloud SC formed upon flue gas F contacting absorption liquid sprayed from nozzles  36  of the first spray level system  20 . A second zone is the cloud C formed from the deflected absorption liquid AL from spray level systems  26 ,  32  contacting flue gas F flowing from the first zone, i.e., flue gas F just previously contacted with absorption liquid in spray cloud SC from nozzles  36  of the first spray level system  20 . 
       FIGS. 5 a  and 5 b    depict a spray level system  120  in accordance with an alternative embodiment. Features of  FIGS. 5 a  and 5 b    that are similar to features of the spray level system  20  described hereinbefore with reference to  FIGS. 1-4 , have been given the same reference numerals.  FIG. 5 a    illustrates a spray level system  120  in top view. A tubular portion  34  is provided with a number of fluidly connected tubular extensions  50 ,  151  forming a “grid” extending from tubular portion  34 . A number of atomizing nozzles  36  are fluidly connected to tubular portion  34  and tubular extensions  50 ,  151  in such a manner that nozzles  36  become rather evenly distributed over the entire horizontal cross-section of wet scrubber tower  2 . Each nozzle  36  is arranged beneath a gas-liquid contacting plate  38 , in a similar manner as described hereinbefore with reference to  FIG. 2 . 
     Spray level system  120  is provided with a first damper arrangement  152  and a second damper arrangement  154 . Each damper arrangement  152 ,  154  extends horizontally across wet scrubber tower  2 . The first and second damper arrangements  152 ,  154  extend on either side of a central tubular extension  151 . Each first and second damper arrangement  152 ,  154  comprises within wet scrubber tower  2  a damper in the form of a damper blade  156  and a horizontal damper shaft  158 . Also, each first and second damper arrangement  152 ,  154  comprises a damper motor  160  arranged outside of wet scrubber tower  2  for rotating horizontal damper shaft  158  to position damper blade  156  as desired. 
       FIG. 5 b   , illustrating a side view of spray level system  120  taken in the direction of arrows Vb-Vb of  FIG. 5 a   , depicts first and second damper arrangements  152 ,  154  arranged on either side of central tubular extension  151 . The respective horizontal damper shafts  158  of arrangements  152 ,  154  are arranged in substantially the same horizontal plane across wet scrubber tower  2 , as the gas-liquid contacting plates  38 . By rotating damper shafts  158  using motors  160  of  FIG. 5 a   , the angle β of damper blades  156  with respect to its horizontal plane may be adjusted. Angle β of damper blades  156  influences the width of open spaces OS between the gas-liquid contacting plates  38 . Hence, when angle β is about 90°, damper blades  156  have very little influence on flue gas F flow through open spaces OS. An angle β of about 90° would typically be utilized when a wet scrubber tower  2  is operated at full flue gas F flow load. When angle β is reduced, for example, to about 40° as depicted in  FIG. 5 b   , open spaces OS are substantially reduced, resulting in an increased flue gas F flow velocity through open spaces OS located adjacent damper blades  156 . Hence, an angle β of less than 90° may be used when wet scrubber tower  2  is operated at a reduced flow load, for example at 75% of full flue gas F flow load. By adjusting angle β, the vertical gas flow velocity through open spaces OS may be adjusted within a desired range of about 5 m/s to about 15 m/s. Also, when the wet scrubber tower  2  is operated at a reduced load, angle β may be adjusted accordingly. Angle β may even be reduced to 0°. In such an event, open spaces OS adjacent to damper blades  156  are completely or are almost completely blocked by damper blades  156 . However, as illustrated in  FIG. 5 a   , the two damper arrangements  152 ,  154  even if adjusted to have an angle β of 0°, do not serve to block all open spaces OS surrounding gas-liquid contacting plates  38 . Open spaces OS through which flue gas F may flow still remain. The vertical flue gas velocity in the remaining open spaces OS may be within the desired range of 5 m/s to 15 m/s. If the vertical flue gas velocity in the remaining open spaces OS is not within the desired range of 5 m/s to 15 m/s, damper arrangements  152 ,  154  may be adjusted to achieve the same. Hence, with the damper arrangements  152 ,  154  it becomes possible to generate the desired “clouds” C described hereinbefore with reference to  FIG. 4 . Also, when the flue gas flow load is lower than full operating gas flow, damper arrangements  152 ,  154  may be adjusted to maintain the desired clouds C. It will be appreciated that damper arrangements  152 ,  154  may be utilized for blocking or partially blocking open spaces OS even in situations when the flue gas flow load is at full operating gas flow. Blocking or partially blocking open spaces OS in conditions of full operating gas flow may be beneficial if the amount of sulphur dioxide requiring removal is temporarily increased. An increased vertical flue gas velocity through the open spaces OS increases under such conditions the sulphur dioxide removal efficiency. 
     It will be appreciated that other types of dampers may also be useful, including dampers in which horizontal plates are made to slide over open spaces OS when needed. 
       FIG. 6  depict a spray level system  220  in accordance with a further alternative embodiment. Features of  FIG. 6  that are similar to features of the spray level system  20  described hereinbefore with reference to  FIGS. 1-4 , have been given the same reference numerals.  FIG. 6  illustrates spray level system  220  in a side view. A tubular portion  34 , which is similar to tubular portion  34  illustrated in  FIG. 2 , is provided with a number of fluidly connected tubular extensions  50 . A number of atomizing nozzles  236  are fluidly connected to tubular extensions  50 . Each nozzle  236  is arranged beneath a gas-liquid contacting plate  238 , in a similar manner as described hereinbefore with reference to  FIG. 2 . 
     The nozzles  236  are so-called dual orifice nozzles, for example Dual Orifice WhirlJet Nozzles, that are available from Spraying Systems Co, Wheaton, Ill., USA. The nozzles  236  eject absorption liquid in two directions. A first portion of the amount of liquid supplied to nozzle  236  is atomized and ejected from first, lower, nozzle  236  spray opening  252  and generate a spray cloud SC 1  of absorption liquid. The spray cloud SC 1  is generally downwardly directed. A second portion of the amount of liquid supplied to nozzle  236  is atomized and ejected from second, upper, nozzle  236  spray opening  253  and generate a spray cloud SC 2  of absorption liquid. The spray cloud SC 2  is generally upwardly directed. Typically, nozzles  236  provide an absorption liquid spray angle α in each direction of approximately 60° to 180°. Often at least 50% of the absorption liquid supplied to nozzle  236  is ejected from first, lower, spray opening  252 . 
     Gas-liquid contacting plates  238  serve to deflect absorption liquid coming from a spray level system located vertically above spray level system  220  to open spaces OS between gas-liquid contacting plates  238  in accordance with similar principles as described hereinbefore with reference to  FIG. 4 . The vertical distance, HP 1 , HP 2 , respectively, is the vertical distance from a respective nozzle  236  spray opening  252 ,  253  to a closest point on bottom surface  238   d  of the corresponding gas-liquid contacting plate  238 . Distance HP 1 , HP 2  is typically approximately 0.1 to 0.9 m. With dual orifice type nozzles  236  arranged in the manner illustrated in  FIG. 6 , HP 1  will be larger than HP 2 . In the embodiment illustrated in  FIG. 6 , the vertical distance HP 1  from lower spray opening  252  to a closest point on bottom surface  238   d  may be 0.2 to 0.9 m. The vertical distance HP 2  from upper spray opening  253  to a closest point on bottom surface  238   d  may be 0.1 to 0.8 m. 
     It will be appreciated that numerous variants of the embodiments described above are possible within the scope of the appended claims. 
     Hereinbefore it has been described that each spray level system is equipped with gas-liquid contacting plates that have, as seen from a top view thereof, the shape of squares. It will be appreciated that the gas-liquid contacting plates could also have shapes other than squares, including such shapes as circles, triangles, ovals, rectangles, other polygon shapes, irregular shapes or combinations thereof. A spray level arrangement may in itself comprise gas-liquid contacting plates of different shapes. For example, some gas-liquid contacting plates of a spray level system could be of a special shape to coincide with an interior circular wall forming wet scrubber tower  2 . Likewise, the gas-liquid contacting plates need not be centred above a respective nozzle, but such a centred arrangement may be preferable. It is also preferable that, when in top view such as depicted in  FIG. 2 , each gas-liquid contacting plate  38  should completely obscure the entire nozzle  36  associated therewith. The gas-liquid contacting plates  38 ,  39  could be manufactured from any suitable material, including metal, such as stainless steel, various plastic materials, including fibre reinforced plastic, etc. The plate thickness of each gas-liquid contacting plate  38 ,  39  will depend on the mechanical strength of the material selected. Often a plate thickness of 2-15 mm will be suitable. 
     In the embodiment illustrated with reference to  FIGS. 1-4 , the two lower spray level systems  20 ,  26 , of a total number of three spray level systems  20 ,  26 ,  32  arranged in “stacked” vertical alignment within wet scrubber tower  2 , are provided with gas-liquid contacting plates  38 ,  39 . It will be appreciated that wet scrubber tower  2  could comprise any number from two to twenty spray level systems arranged in stacked vertical alignment. Any number of these spray level systems could be provided with gas-liquid contacting plates. Hence, for example, in a wet scrubber tower  2  comprising five spray level systems, only one may be provided with gas-liquid contacting plates, and that one spray level system need not be the spray level system closest to the base of wet scrubber tower  2 . Rather, for example, the one spray level system provided with gas-liquid contacting plates may be the central spray level system, i.e. the third spray level system from the base of wet scrubber tower  2 . Furthermore, with the wet scrubber tower  2  comprising five spray level systems it would also be possible to provide all five spray level systems with gas-liquid contacting plates. However, the effect of using gas-liquid contacting plates in conjunction with the uppermost spray level system would be rather limited. Hence, gas-liquid contacting plates could be installed for one or more of all of the spray level systems of a wet scrubber tower  2 , and for one or more of all of the nozzles of each spray level system, depending on the required efficiency for sulphur dioxide removal required of the wet scrubber tower  2 . Hence, when a limited increase in removal efficiency is sufficient, then gas-liquid contacting plates could be installed in only one spray level system, and maybe only for some of the nozzles of that spray level system, while in other cases, where a large increase in sulphur dioxide removal is needed, it might be preferable to install gas-liquid contacting plates for all, or almost all, spray level systems, and for most, or even all, of the nozzles of those spray level systems. 
     It will be appreciated that the gas-liquid contacting plates could be used both when building a new wet scrubber installation, and when retrofitting an existing wet scrubber installation. 
     Hereinbefore, a method and a wet scrubber for removing sulphur dioxide from a process gas have been described. It will be appreciated that the method and wet scrubber may also be utilized for removing other contaminants from a process gas. For example, the method and wet scrubber could be utilized for removing carbon dioxide from a process gas. The removal of carbon dioxide from the process gas may, in such a case, often be conducted in a wet scrubber which is of a similar same type as the wet scrubber operating for removing sulphur dioxide, but which is located downstream, with respect to the direction of forwarding the process gas, of the wet scrubber in which sulphur dioxide is removed. Furthermore, while limestone may often be part of the absorption liquid in a sulphur dioxide removing wet scrubber, a carbon dioxide removing wet scrubber may utilize another type of absorption liquid, for example an absorption liquid comprising an ammoniated solution or an amine solution. 
     To summarize, a wet scrubber tower  2  cleaning a process gas comprises a first spray level system  20  and a second spray level system  26  which is arranged vertically above the first spray level system  20  within wet scrubber tower  2 , each spray level system  20 ,  26  comprising nozzles  36  operative for atomizing absorption liquid. The first spray level system  20  comprises at least one gas-liquid contacting plate  38  located vertically above at least one of the nozzles  36  of the first spray level system  20 . The gas-liquid contacting plate  38  deflects absorption liquid atomized by means of the second spray level system  26  from the vicinity of the at least one nozzle  36  of the first spray level system  20 . The deflected absorption liquid may be brought into contact with process gas F previously brought into contact with the absorption liquid atomized by the first spray level system  20 . 
     While the invention has been described with reference to a number of preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the present invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. Moreover, the use of the terms first, second, etc. do not denote any order or importance, but rather the terms first, second, etc. are used to distinguish one element from another.