Abstract:
Methods and systems for the removal of SO2 from waste combustion flue gas are described herein. The subject methods and systems entail one step, two steps or three steps to produce cleaned flue gas for release to the atmosphere.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/769,982; filed on Feb. 27, 2013, which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present disclosure relates, in general, to the removal of contaminants from flue gas produced by the burning of waste as a combustion fuel and, in particular, to a new and useful method and system for removing SO 2  from the flue gas produced by waste fuel combustion. 
       BACKGROUND OF THE INVENTION 
       [0003]    In the pollution control field, several approaches are used to remove sulfur oxides and other contaminants from a flue gas produced by the burning of a fossil fuel in order to comply with both European and U.S. emissions requirements. 
         [0004]    One approach involves locating and utilizing fossil fuels lower in sulfur content and/or other contaminants. A second approach involves removing or reducing the sulfur content and/or other contaminants in the fuel prior to combustion of the fuel. Such may be achieved through mechanical and/or chemical processes. A major disadvantage to this second approach is the limited cost effectiveness of the mechanical and/or chemical processes required to achieve the mandated levels of reduction of sulfur oxides and/or other contaminants. 
         [0005]    By and large, the most widely used approaches to removing sulfur oxides and/or other contaminants from flue gas involve post-combustion clean up of the flue gas. Several methods have been developed to remove SO 2  species from flue gases. 
         [0006]    One method for removing SO 2  from flue gas involves either mixing dry alkali material with the fuel prior to combustion, or injection of pulverized alkali material directly into the hot combustion gases to remove sulfur oxides and other contaminants via absorption or absorption followed by oxidation. Disadvantages of this method include: fouling of heat transfer surfaces; low to moderate removal efficiencies; poor reagent utilization; and increased particulate loadings in the combustion gases which may require additional conditioning of the gas if an electrostatic precipitator is used for down stream particulate collection. 
         [0007]    Another method for removing SO 2  from flue gas, collectively referred to as wet chemical absorption processes and also known as wet scrubbing, involves “washing” the hot flue gases with an aqueous alkaline solution or slurry in an up-flow, gas-liquid contact device to remove sulfur oxides and other contaminants. Disadvantages associated with wet scrubbing processes include: the loss of liquid both to the atmosphere and to the sludge produced in the process; and the economics associated with the construction materials for the absorber module itself and all related auxiliary downstream equipment, such as primary/secondary dewatering and waste water treatment subsystems. 
         [0008]    Still another method for removing SO 2 , collectively referred to as spray drying chemical absorption processes and also known as dry scrubbing, involves spraying an aqueous alkaline solution or slurry which has been finely atomized via mechanical, dual-fluid or rotary cup-type atomizers, into the hot flue gases to remove sulfur oxides and other contaminants. Disadvantages associated with these dry scrubbing processes include: moderate to high gas-side pressure drop across the spray dryer gas inlet distribution device; and limitations on the spray down temperature required to maintain controlled operations. 
         [0009]    Other known systems for SO 2  removal from flue gas require additional equipment, are very complicated in design and operation, and/or provide a very costly removal method. It is thus apparent that a simple and economical method and system is needed to remove SO 2  from flue gas in general, and particularly to remove SO 2  from flue gas produced by the burning of waste, that overcomes the disadvantages of these prior approaches used in fossil fuel combustion fields. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention is directed to a system and process for removing SO 2  from flue gases produced by waste combustion through absorption of SO 2  to form SO 3   2−  and SO 4   2−  for efficient and cost effective capture. 
         [0011]    SO 2  removal in wet scrubbers and condensers is facilitated by absorption of SO 2  into the liquid phase according to Reaction 1 provided below. 
         [0000]      SO 2  (g)+H 2 O (l)&lt;--&gt;H 2 SO 3  (aq)  Reaction 1
 
         [0000]    Since the equilibrium of Reaction 1 rapidly saturates the liquid, the sulfur transforms into an ionic form according to Reaction 2 and Reaction 3 provided below. 
         [0000]      H 2 SO 3 &lt;--&gt;HSO 3   − +H +   Reaction 2
 
         [0000]      HSO 3   − &lt;--&gt;SO 3   2− +H +   Reaction 3
 
         [0000]    Transformation of the sulfur to the ionic form is facilitated by increasing the pH to thereby push the equilibrium toward the SO 3   2−  form. An increase in pH however also increases CO 2  dissolution into the liquid, which in turn increases the consumption of NaOH, or other alkali sorbent used in the system, making the system more costly. The methods of the present disclosure therefore are based on oxidizing SO 3   2−  according to Reaction 4 provided below. 
         [0000]      SO 3   2−  (aq)+½O 2  (aq)--&gt;SO 4   2−  (aq)  Reaction 4
 
         [0000]    Reaction 4 is quite slow and thereby inadequate for commercial purposes. However, with the addition of a catalyst, the reaction rate can be adequately increased. As such, sulfur removal by capture through Reaction 4 requires only compensation of the loss of catalyst, as no sorbent is consumed. 
         [0012]    In fields using fossil fuel combustion and limestone as a cleaning system absorbent, sulfur removal is accomplished using separate oxidation tanks where oxygen is continuously added by an air blower or similar means. However, due to differences in cleaning systems for waste combustion as opposed to fossil fuel combustion, sulfur removal or capture from waste combustion flue gas can be significantly more difficult than from that of its counterpart. Based on experience in fossil fuel combustion fields, oxidation of sulfite to sulfate is expected to occur in the flue gas treatment system condenser. Unexpectedly, when attempted, such did not occur. Faced with this unexpected problem, addition of a catalyst to the system condenser solves the problem. By placing a catalyst in the system condenser, oxidation is achieved for successful and efficient removal of SO 2  therefrom in accordance with Reactions 1-4 above. 
         [0013]    Based on the above, systems and methods for the removal of SO 2  from flue gas produced by waste combustion to obtain cleaned flue gas were developed. In general, the present system comprises: a desulfurization stage, a quench stage and a condenser stage for the treatment of waste combustion flue gas to produce cleaned flue gas for release to the atmosphere. Likewise, steps of the present method comprise in general: providing waste combustion flue gas to a system with a desulfurization stage, a quench stage, and a condenser stage to produce a cleaned flue gas for release to the atmosphere through a stack. 
         [0014]    The various novel features that characterize the subject systems and methods, and advantages related thereto are specified in the accompanying drawings and detailed description provided below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]      FIG. 1  is a schematic view illustrating one embodiment of a system for removing SO 2  from a waste combustion flue gas according to the present invention. 
           [0016]      FIG. 2  is a schematic view illustrating another embodiment of a system for removing SO 2  from a waste combustion flue gas according to the present invention. 
           [0017]      FIG. 3  is a schematic view illustrating still another embodiment of a system for removing SO 2  from a waste combustion flue gas according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    The combustion of waste produces flue gas comprising SO 2 . SO 2  from the flue gas is poorly absorbed into solution at a pH below 5 due to the dissociation equilibrium as described with regard to Reactions 1 through 3 above. At higher pH, SO 2  is dissociated and forms bisulphite, which contributes to a much better absorption. If a bisulphite solution is acidified, the absorbed SO 2  desorbs and again escapes the solution. However, if the sulphite in solution, whether dissociated or not, is oxidized to sulphate, i.e., SO 4   2− , it will be dissociated also at low pH since the corresponding sulphuric acid, unlike suphurous acid, is a very strong acid. Hence, the sulphate is “permanently” absorbed and removed from the system sulphite equilibrium. 
         [0019]    With the above in mind, the subject waste combustion flue gas treatment system  10  is described. As best illustrated in  FIG. 1 , in the subject waste combustion flue gas treatment system  10 , waste  12  is combusted in boiler  14 . Flue gas “FG” produced in the boiler  14  from the combustion of waste  12  flows from the boiler  14  at a temperature of approximately 125° C. to 175° C. with approximately 1000 to 1500 mg/Nm 3  HCl, approximately 300 to 500 mg/Nm 3  SO 2 , approximately 10 to 25 mg/Nm 3  NH 3  and approximately 1000 to 3000 mg/Nm 3  fly ash. The quantity of flue gas FG flowing from boiler  14  depends on boiler  14  size. The boiler  14  is typically sized to where approximately 50000 to 300000 Nm 3 /hour of flue gas FG is produced. 
         [0020]    The subject treatment system  10  has essentially three flue gas cleaning stages. The first cleaning stage of the subject treatment system  10  is a semidry desulfurization system  16 . The semidry desulfurization system  16  is fluidly connected via duct  14   a  to boiler  14  for flue gas flow from opening  36  of the boiler  14  into the semidry desulfurization system  16 . Semidry desulfurization system  16  comprises an absorption material  38 , typically lime, supplied within a hydration chamber  20 . The hydration chamber  20  is supplied solvent  22   a,  typically water, from a fluidly connected solvent source  22 , an absorption material  24   a,  typically lime, from a fluidly connected material source  24  and optionally a recycled absorption material  26   a  from a fluidly connected recycle source  26  fluidly connected to collection tank  92 . The hydration chamber  20  is fluidly connected to a mixer  28 . Fluidly connected to mixer  28  is reactor  30  housed within a reaction vessel  32 . Reaction vessel  32  is equipped with opening  34  through which flue gas FG flows into reaction vessel  32  and reactor  30  therein from boiler  14 . 
         [0021]    Hydration chamber  20  is generally a chamber of any commercially useful configuration. Within hydration chamber  20 , an absorption material  24   a,  such as lime from an absorption material source  24  and optionally a recycled absorption material  26   a  from recycle source  26 , such as recycled lime from collection tank  92 , are combined to form reaction material  38 . As needed for efficient operation of reactor  30 , reaction material  38  is mechanically and/or gravity fed into mixer  28  via mixer opening  40 . Mixer opening  40  fluidly connects hydration chamber  20  and mixer  28 . Prior to reaction material  38  passing through mixer opening  40  and into mixer  28 , reaction material  38  is sprayed with a predetermined amount of a solvent  22   a  such as water from a solvent source  22  so as to hydrate reaction material  38 . 
         [0022]    Mixer  28  is generally a mixer of any commercially useful configuration. Within mixer  28 , hydrated reaction material  38  is mixed for approximately 15 to 20 seconds to achieve a moisture content throughout of approximately 5%. Once the reaction material  38  is thoroughly mixed within mixer  28  to achieve the desired moisture content throughout reaction material  38 , reaction material  38  is mechanically and/or gravity fed out of mixer  28  and into reaction vessel  32  through exit opening  44 . Exit opening  44  fluidly connects mixer  28  and reaction vessel  32 . 
         [0023]    As noted previously, reaction vessel  32  houses reactor  30 . Reactor  30  is that portion of reaction vessel  32  where reaction material  38  enters reaction vessel  32  passing through exit opening  44  to be dispersed from dispersal ring or plate  46 . Dispersal ring or plate  46  is located within reactor  30  and disperses reaction material  38  therein. It is in reactor  30  where reaction material  38  contacts, commingles and reacts with flue gas FG laden with fly ash particulates and contaminants as noted previously. Thus, it is within reactor  30  where one or more of the following exemplificative reaction(s) occur to form dry particulates, DP. 
         [0000]      SO 2 : SO 2 +Ca(OH) 2 =CaSO 3 +H 2 O  Reaction 5
 
         [0000]      SO 3 : SO 3 +Ca(OH) 2 =CaSO 4 +H 2 O  Reaction 6
 
         [0000]    With the flow of flue gas FG out of outlet  48  of reaction vessel  32  the first stage of flue gas FG cleaning is completed. Flue gas FG flowing from outlet  48  is typically approximately 125° C. to 150° C., most typically approximately 140° C., and has contaminant levels or “emissions” below the European Union norms, i.e., HCl&lt;10 mg/Nm 3 , SO 2 &lt;50 mg/Nm 3 , and particulates &lt;5 mg/Nm 3 . 
         [0024]    Reaction vessel  32  is fluidly connected via duct  16   a  to a quench  50  by means of inlet opening  52 . The second stage of flue gas FG cleaning takes place in quench  50 . Within quench  50 , the flue gas FG is sprayed with recirculated water  56  from nozzles  58  to fully humidify the flue gas FG and fully wet all surfaces  54  within quench  50 . This stage of flue gas cleaning is operated at a relatively low pH in the range of approximately 0 to 4, or a pH of approximately 1. The reason for operating the quench  50  at a relatively low pH is for efficient absorption of ammonia and improved mercury removal. As a result of this relatively low pH, one of the primary flue gas FG contaminants, e.g., HCl, is still removed, although SO 2  removal is poor. In quench  50 , water  56  consumption is significant due to evaporation. Also, in order to bleed off removed impurities, a relatively small liquid flow “P” is purged from quench  50 . Impurities in liquid flow P may be optionally circulated back (not illustrated) to boiler  14  for combustion therein. From quench  50 , flue gas FG flows through exit opening  60  to a fluidly connected condenser  62 . Upon flue gas FG exit from quench  50 , the second stage of flue gas FG cleaning is complete. Flue gas FG exiting quench  50  through exit opening  60  has approximately 100 percent humidity at approximately 65° C., and has impurity levels comprising HCl&lt;2 mg/Nm 3  and NH 3 &lt;5 mg/Nm 3 . 
         [0025]    From quench  50 , flue gas FG flows through duct  50   a  and into fluidly connected condenser  62  via opening  64 . The subject condenser  62  may be of any of a variety of known types, such as for example a direct contact condenser, e.g., a packed tower or a spray scrubber, or an indirect condenser, e.g., tube and shell heat exchangers. Regardless of which type condenser  62  is used, a water recirculation spray  66  through nozzles  68  is always included in condenser  62  to keep condenser surfaces  70  wet and to further clean the flue gas FG flowing therethrough. This, the third stage of flue gas FG cleaning, taking place in condenser  62  is used to remove remaining SO 2  from the flue gas FG. As such, NaOH  72   a  is added from a NaOH supply source  72  to maintain a pH of approximately 5.0 to 7.5, or approximately 6.0 to 6.5, in the condenser  62 . Cooling of the flue gas FG that occurs in condenser  62  results in a significant flow of excess water or condensate  74  produced from the humidity of the entering flue gas FG. Condenser  62  produces approximately 5 to 15 m 3 /hour of condensate  74 . Condensate  74  so produced contains sodium and SO 2  as the primary impurities. 
         [0026]    Condensate  74  is cleaned in a membrane reverse osmosis system  76  or similar purification system. In system  76 , heavy metals, sulphurous compounds, chloride compounds and the like are removed in a concentrate water flow  78 . Approximately 20 to 30 percent of condensate  74  forms concentrate water flow  78 , which carries approximately &gt;95 percent of the impurities entering system  76 . The resultant concentrated water flow  78  is circulated to collection tank  92  of quench  50  for use as make-up water. Clean water  80  is also produced by system  76 , which is useful for other purposes. 
         [0027]    It is at this point in the subject system and process where a significant problem becomes evident. The problem is that since SO 2  captured in condenser  62  will be acidified in quench  50  upon circulation of the concentrated water flow  78  thereto, SO 2  will be emitted from the concentrate water flow  78  to the flue gas FG unless it has been oxidized. This leads to an increase of SO 2 /sulphite concentrations in condenser  62  and quench  50 . Experience in wet flue gas desulfurization (WFGD) of flue gas from fossil fuel combustion proves that oxidation is typically sufficient to oxidize SO 2  and thus avoid SO 2 /sulphite increases. 
         [0028]    Without being bound to any one mechanism, it appears that the subject system fails and results in an increase of SO 2 /sulphite concentrations in condenser  62  and quench  50  due to the operation being too clean. Iron and manganese radicals are active in promoting the reactions described above, and are abundant in WFGD. As such, to solve the problem of increasing of SO 2 /sulphite concentrations in condenser  62  and quench  50 , a relatively small amount, such as approximately 0.0001 to 0.200 mM, or approximately 0.020 mM of an oxidation catalyst  82 , such as FeSO4 or the like, is added to condenser  62 . Optionally, in addition to or in place of adding an oxidation catalyst  82  from a catalyst supply source  84 , air and/or oxygen  86  from an oxygen supply source  88  may be injected into and optionally mechanically stirred (not shown) into collection tank  90  of condenser  62  for forced oxidation therein. 
         [0029]    With the addition of an oxidation catalyst  82  and/or oxygen  86  to condenser  62 , flue gas FG flowing from opening  94  of condenser  62  meets and/or exceeds emissions standards and considered “clean” for release to the atmosphere by release through a stack  96 . As such, flue gas FG flows out of opening  94  of condenser  62  via duct  62   a  and into fluidly connected stack  96  though opening  98  for release therefrom into the atmosphere via opening  96   a.    
         [0030]    An additional benefit of the subject system  10  and method is that heat  100  recovered from the cooling of the flue gas FG in condenser  62  may be used in a district heating system  102 . As such, heat  100  from condenser  62  is used in the district heating system  102  and then returned to condenser  62  as coolant  104  for cooling the flue gas FG in condenser  62 . 
         [0031]    Schematically illustrated in  FIG. 2 , is another embodiment of the subject system and method. The system illustrated in  FIG. 2  has features in common with those illustrated in  FIG. 1 . As such, features illustrated in  FIG. 2  common to those of  FIG. 1  are signified using the same numbers but with the number “2” preceding them. 
         [0032]    Now referring to  FIG. 2 , in this embodiment of the subject waste combustion flue gas treatment system  210 , waste  212  is combusted in boiler  214 . Flue gas FG produced in the boiler  214  from the combustion of waste  212  flows from the boiler  214  at a temperature of approximately 125° C. to 175° C. with approximately 1000 to 1500 mg/Nm 3  HCl, approximately 300 to 500 mg/Nm 3  SO 2 , approximately 10 to 25 mg/Nm 3  NH 3  and approximately 1000 to 3000 mg/Nm 3  fly ash. The quantity of flue gas FG flowing from boiler  214  depends on boiler  214  size. The boiler  214  is typically sized to where approximately 50000 to 300000 Nm 3 /hour of flue gas FG is produced. 
         [0033]    The subject treatment system  210  has essentially two flue gas cleaning stages. The first cleaning stage of the subject treatment system  210  is a combination wet desulfurization system and quench  211 . The combination wet desulfurization system and quench  211  is fluidly connected to boiler  214  via duct  211   a  for flue gas flow from opening  236  of boiler  214  into opening  219  of combination wet desulfurization system and quench  211 . The combination wet desulphurization system and quench  211  is supplied solvent  222   a,  typically water, from a fluidly connected solvent source  222 , an absorption material  224   a,  typically lime, from a fluidly connected material source  224  and optionally a recycled absorption material  226   a  from a fluidly connected recycle source  226 . In collection tank  213  of combination wet desulphurization system and quench  211 , solvent  222   a,  absorption material  224   a  and recycled absorption material  226   a  are combined to form a slurry  215 . Slurry  215  is sprayed from nozzles  217  for contract with flue gas FG flowing therethrough thereby cleaning flue gas FG flowing therethrough. As such, flue gas FG flows from boiler  214  into fluidly connected combination wet desulphurization system and quench  211  through opening  219 , is cleaned, and exits through opening  221 . 
         [0034]    Combination wet desulphurization system and quench  211  is generally a chamber of any commercially useful configuration. In combination wet desulphurization system and quench  211 , an absorption material  224   a,  such as lime from an absorption material source  224  and optionally a recycled absorption material  226   a  from recycle source  226 , such as recycled lime from collection tank  213 , are combined with a solvent  222   a,  such as water, from a solvent source  222  to form a slurry  215 . As needed for efficient operation of combination wet desulphurization system and quench  211 , slurry  215  is sprayed from nozzles  217  within combination wet desulphurization system and quench  211 . It is in combination wet desulphurization system and quench  211  where slurry  215  contacts, commingles and reacts with flue gas FG laden with fly ash particulates and contaminants as noted previously. Thus, it is within combination wet desulphurization system and quench  211  where one or more of Reactions 5 and 6 above occur. 
         [0035]    Within combination wet desulphurization system and quench  211 , flue gas FG is sprayed with slurry  215  including solvent  222   a  from nozzles  217  to fully humidify the flue gas FG and fully wet all surfaces  223  within combination wet desulphurization system and quench  211 . This first stage of flue gas cleaning is operated at a relatively low pH in the range of approximately 0 to 4, or a pH of approximately 1. The reason for operating the combination wet desulphurization system and quench  211  at a relatively low pH is for efficient absorption of ammonia and improved mercury removal. As a result of this relatively low pH, one of the primary flue gas FG contaminants, e.g., HCl, is still removed, although SO 2  removal is poor. In combination wet desulphurization system and quench  211 , solvent  222   a  consumption is significant due to evaporation. Also, in order to bleed off removed impurities, a relatively small liquid flow “P” is purged from combination wet desulphurization system and quench  211 . Optionally, impurities in liquid flow P may be circulated (not shown) to boiler  214  for combustion therein. With the flow of flue gas FG out of opening  221  of combination wet desulphurization system and quench  211 , the first stage of flue gas FG cleaning is completed. Flue gas FG flowing from opening  221  typically has a humidity of approximately 100 percent at approximately 65° C., and has impurity levels comprising HCl&lt;2 mg/Nm 3  and NH 3 &lt;5 mg/Nm 3 . 
         [0036]    From combination wet desulphurization system and quench  211 , flue gas FG enters fluidly connected combination wet desulphurization system and condenser  263  via duct  263   a  and opening  265 . The subject combination wet desulphurization system and condenser  263  may include any of a variety of known condenser types, such as for example a direct contact condenser, e.g., a packed tower or a spray scrubber ( 266 ), or an indirect condenser, e.g., tube and shell heat exchangers. Regardless of which type condenser  266  is used in combination wet desulphurization system and condenser  263 , a water recirculation spray  267  through nozzles  269  is always included in condenser  266  to keep combination wet desulphurization system and condenser  263  surfaces  271  wet and to further clean the flue gas FG flowing therethrough. This, the second stage of flue gas FG cleaning, taking place in combination wet desulphurization system and condenser  263  is used to remove remaining SO 2  from the flue gas FG. As such, NaOH  272   a  is added from a NaOH supply source  272  to maintain a pH of approximately 5.0 to 7.5, or approximately 6.0 to 6.5, in the combination wet desulphurization system and condenser  263 . Cooling of the flue gas FG that occurs in combination wet desulphurization system and condenser  263  results in a significant flow of excess water or condensate  274  produced from the humidity of the entering flue gas FG. Combination wet desulphurization system and condenser  263  produces approximately 5 to 15 m 3 /hour of condensate  274 . Condensate  274  so produced contains some sodium and SO 2  as the primary impurities. 
         [0037]    Condensate  274  is optionally cleaned in a membrane reverse osmosis system  276  or similar purification system. In system  276 , heavy metals, sulphurous compounds, chloride compounds and the like are removed in a concentrate water flow  278 . Approximately 20 to 30 percent of condensate  274  forms concentrate water flow  278 , which carries approximately &gt;95 percent of the impurities entering system  276 . The resultant concentrated water flow  278  is circulated to collection tank  213  of combination wet desulphurization system and quench  211  for use as make-up water. Clean water  280  is also produced by system  276 , which is useful for other purposes. 
         [0038]    As noted above, it is at this point in the subject system  210  and process where a significant problem becomes evident. The problem is that since SO 2  captured in combination wet desulphurization system and condenser  263  will be acidified in combination wet desulphurization system and quench  211  upon circulation of the concentrated water flow  278  thereto, SO 2  will be emitted from the concentrate water flow  278  to the flue gas FG unless it has been oxidized. This leads to an increase of SO 2 /sulphite concentrations in combination wet desulphurization system and condenser  263  and combination wet desulphurization system and quench  211 . Experience in wet flue gas desulfurization (WFGD) of flue gas from fossil fuel combustion proves that oxidation is typically sufficient to oxidize SO 2  and thus avoid SO 2 /sulphite increases. 
         [0039]    Without being bound to any one mechanism, it appears that the subject system fails and results in an increase of SO 2 /sulphite concentrations due to the fact the operation is too clean. Iron and manganese radicals are active in promoting the reactions described above, and are abundant in WFGD. As such, to solve the problem of increasing of SO 2 /sulphite concentrations, a relatively small amount, such as approximately 0.0001 to 0.200 mM, or approximately 0.020 mM of an oxidation catalyst  282 , such as FeSO 4  or the like from a catalyst supply source  284  is added to combination wet desulphurization system and condenser  263 . Optionally, in addition to or in place of adding an oxidation catalyst  282  from a catalyst supply source  284 , air and/or oxygen  286  from an oxygen supply source  288  may be injected into and/or optionally mechanically stirred (not shown) into collection tank  290  of combination wet desulphurization system and condenser  263  for forced oxidation therein. 
         [0040]    Optionally if desired but not illustrated in  FIG. 2 , NaOH  272   a  from a NaOH supply source  272 , oxidation catalyst  282  from a catalyst supply source  284 , and/or air and/or oxygen  286  from an oxygen supply source  288 , may likewise be added to the solvent  222   a  spray of combination wet desulphurization system and quench  211 . 
         [0041]    With the addition of an oxidation catalyst  282  and/or oxygen  286  to combination wet desulphurization system and condenser  263 , flue gas FG flowing outwardly from opening  293  of combination wet desulphurization system and condenser  263  meets and/or exceeds emissions standards and considered “clean” for release to the atmosphere by release through a stack  296 . As such, flue gas FG flows out of opening  293  of combination wet desulphurization system and condenser  263  and into fluidly connected stack  296  via duct  299  and opening  298  for release therefrom into the atmosphere through opening  296   a.    
         [0042]    An additional benefit of the subject system and method is that optionally, heat  200  recovered from the cooling of the flue gas FG in the combination wet desulphurization system and condenser  263  may be used in a district heating system  202 . As such, heat  200  from combination wet desulphurization system and condenser  263  is used in the district heating system  202  and then returned to combination wet desulphurization system and condenser  263  as coolant  204  for cooling the flue gas FG in the combination wet desulphurization system and condenser  263 . 
         [0043]    Schematically illustrated in  FIG. 3 , is still another embodiment of the subject system and method. The system illustrated in  FIG. 3  has features in common with those illustrated in  FIG. 1 . As such, features illustrated in  FIG. 3  common to those of  FIG. 1  are signified using the same numbers but with the number “3” preceding them. 
         [0044]    Now referring to  FIG. 3 , in this embodiment of the subject waste combustion flue gas treatment system  310 , waste  312  is combusted in boiler  314 . Flue gas FG produced in the boiler  314  from the combustion of waste  312  flows from opening  336  of boiler  314  at a temperature of approximately 125° C. to 175° C. with approximately 1000 to 1500 mg/Nm 3  HCl, approximately 300 to 500 mg/Nm 3  SO 2 , approximately 10 to 25 mg/Nm 3  NH 3  and approximately 1000 to 3000 mg/Nm 3  fly ash. The quantity of flue gas FG flowing from boiler  314  depends on boiler  314  size. The boiler  314  is typically sized to where approximately 50000 to 300000 Nm 3 /hour of flue gas FG is produced. 
         [0045]    The subject treatment system  310  has essentially one flue gas cleaning stage. This one cleaning stage of the subject treatment system  310  takes place in a combination wet desulfurization system and condenser  311 . The combination wet desulfurization system and condenser  311  is fluidly connected to boiler  314  via duct  311   a  for flue gas flow from the boiler  314  into the combination wet desulfurization system and condenser  311  via opening  319 . The combination wet desulphurization system and condenser  311  is supplied solvent  322   a,  typically water, from a fluidly connected solvent source  322 , an absorption material  324   a , typically lime, from a fluidly connected material source  324  and optionally a recycled absorption material  326   a  from a fluidly connected recycle source  326 . In collection tank  313  of combination wet desulphurization system and condenser  311 , solvent  322   a,  absorption material  324   a  and recycled absorption material  326   a  are combined to form a slurry  315 . Slurry  315  is sprayed from nozzles  317  for contract with flue gas FG flowing therethrough thereby cleaning flue gas FG flowing therethrough. As such, flue gas FG flows from boiler  314  into fluidly connected combination wet desulphurization system and condenser  311  through opening  319  and exits through opening  321 . 
         [0046]    Combination wet desulphurization system and condenser  311  is generally a chamber of any commercially useful configuration. Combination wet desulphurization system and condenser  311 , an absorption material  324   a,  such as lime from an absorption material source  324  and optionally a recycled absorption material  326   a  from recycle source  326 , such as recycled lime from collection tank  313 , are combined with a solvent  322   a,  such as water, from a solvent source  322  to form a slurry  315 . As needed for efficient operation of combination wet desulphurization system and condenser  311 , slurry  315  is sprayed from nozzles  317  within combination wet desulphurization system and condenser  311 . It is in combination wet desulphurization system and condenser  311  where slurry  315  contacts, commingles and reacts with flue gas FG laden with fly ash particulates and contaminants as noted previously. Thus, it is within combination wet desulphurization system and condenser  311  where one or more of Reactions 5 and 6 above occur. Within combination wet desulphurization system and condenser  311 , the flue gas FG is sprayed with slurry  315  including solvent  322   a  from nozzles  317  to fully humidify the flue gas FG and fully wet all surfaces  323  within combination wet desulphurization system and condenser  311 . 
         [0047]    The subject combination wet desulphurization system and condenser  311  may include any of a variety of known condenser types, such as for example a direct contact condenser, e.g., a packed tower or a spray scrubber ( 365 ), or an indirect condenser, e.g., tube and shell heat exchangers. Regardless of which type condenser  365  is used in combination wet desulphurization system and condenser  311 , a recirculation spray  367  through nozzles  317  is always included in condenser  365  to keep combination wet desulphurization system and condenser  311  surfaces  323  wet and to further clean the flue gas FG flowing therethrough. The flue gas FG cleaning taking place in combination wet desulphurization system and condenser  311  is used to remove remaining SO 2  from the flue gas FG. As such, NaOH  372   a  is added from a NaOH supply source  372  to maintain a pH of approximately 5.0 to 7.5, or approximately 6.0 to 6.5, in the combination wet desulphurization system and condenser  311 . Cooling of the flue gas FG that occurs in combination wet desulphurization system and condenser  311  results in a significant flow of excess water or condensate  374  produced from the humidity of the entering flue gas FG. Combination wet desulphurization system and condenser  311  produces approximately 5 to 15 m 3 /hour of condensate  374 . Condensate  374  so produced contains Na 2 SO 4  as the primary impurity. 
         [0048]    Condensate  374  is optionally cleaned in a membrane reverse osmosis system  376  or similar purification system. In system  376 , heavy metals, sulphurous compounds, chloride compounds and the like are removed in a concentrate water flow  378 . Approximately 20 to 30 percent of condensate  374  forms concentrate water flow  378 , which carries approximately &gt;95 percent of the impurities entering system  376 . The resultant concentrated water flow  378  is circulated to collection tank  313  of combination wet desulphurization system and condenser  311  for use as make-up water. Clean water  380  is also produced by system  376 , which is useful for other purposes. 
         [0049]    As noted above, it is at this point in the subject system and process where a significant problem may be encountered. The problem encountered may be an increase of SO 2 /sulphite concentrations in the combination wet desulphurization system and condenser  311 . To solve the problem of increasing of SO 2 /sulphite concentrations, a relatively small amount, such as approximately 0.0001 to 0.200 mM, or approximately 0.020 mM of an oxidation catalyst  382 , such as FeSO 4  or the like is added to combination wet desulphurization system and condenser  311 . Optionally, in addition to or in place of adding an oxidation catalyst  382  from a catalyst supply source  384 , air and/or oxygen  386  from an oxygen supply source  388  may be injected into and/or optionally mechanically stirred (not shown) into collection tank  313  of combination wet desulphurization system and condenser  311  for forced oxidation therein. 
         [0050]    With the addition of an oxidation catalyst  382  and/or oxygen  386  to combination wet desulphurization system and condenser  311 , flue gas FG flowing outwardly from opening  321  of combination wet desulphurization system and condenser  311  meets and/or exceeds emissions standards and considered “clean” for release to the atmosphere by release through a stack  396 . As such, flue gas FG flows out of opening  321  of combination wet desulphurization system and condenser  311  and into fluidly connected duct  396   a  and fluidly connected opening  398  of stack  396  for release therefrom into the atmosphere via opening  399 . 
         [0051]    An additional benefit of the subject system  310  and method is that optionally, heat  300  recovered from the cooling of the flue gas FG in combination wet desulphurization system and condenser  311  may be used in a district heating system  302 . As such, heat  300  from combination wet desulphurization system and condenser  311  is used in the district heating system  302  and then returned to combination wet desulphurization system and condenser  311  as coolant  304  for cooling the flue gas FG in combination wet desulphurization system and condenser  311 . 
         [0052]    Methods of using the system embodiments illustrated in  FIGS. 1-3  and described above are useful for removing SO 2  from flue gas produced by waste combustion. One such method of using the subject system entails flowing waste combustion flue gas through three cleaning stages to remove SO 2  from the flue gas comprising SO 2  to produce cleaned flue gas for release of the cleaned flue gas to the atmosphere via a stack. As such, waste combustion flue gas is passed through a semi-dry flue gas desulfurization system, a quench and a condenser to produce cleaned flue gas. In the semi-dry flue gas desulfurization system, the waste combustion flue gas is contacted with a solvent moistened absorption material and/or recycled absorption material to form dry particulates in accordance with Reactions 5 and 6 above. The waste combustion flue gas then flows through a quench where it is sprayed with recirculated water to fully humidify the waste combustion flue gas under relatively low pH conditions in the range of approximately 0 to 4, or a pH of approximately 1. From the quench, the waste combustion flue gas flows through a condenser where it is sprayed with a water recirculation spray, NaOH from an NaOH supply source to maintain a pH of approximately 5.0 to 7.5, or approximately 6.0 to 6.5, in the condenser, and a relatively small amount of an oxidation catalyst, such as FeSO 4 . Through this three step cleaning method, waste combustion flue gas is cleaned to produce cleaned flue gas for release to the atmosphere via an associated stack. An additional benefit to the method so described is the optional use of heat energy from the system condenser in a district heating system and/or optional cleaning of condensation water from the condenser by a reverse osmosis system for uses of the so produced water outside the subject system. 
         [0053]    Another method of using the subject system entails flowing waste combustion flue gas through two cleaning stages to remove SO 2  from the flue gas comprising SO 2  to produce cleaned flue gas for release of the cleaned flue gas to the atmosphere via a stack. As such, waste combustion flue gas is passed through a combination wet desulfurization system and quench and a combination wet desulfurization system and condenser to produce cleaned flue gas. In the combined wet desulfurization system and quench, the waste combustion flue gas is contacted with a solvent and absorption material slurry, which may or may not include recycled absorption material. As the waste combustion flue gas flows through combination wet desulfurization system and quench it is sprayed with the slurry and recirculated water to fully humidify the waste combustion flue gas under relatively low pH conditions in the range of approximately 0 to 4, or a pH of approximately 1. From the combination wet desulfurization system and quench, the waste combustion flue gas flows through a wet desulfurization system and condenser where it is sprayed with a water recirculation spray, NaOH from an NaOH supply source to maintain a pH of approximately 5.0 to 7.5, or approximately 6.0 to 6.5, in the condenser, and a relatively small amount of an oxidation catalyst, such as FeSO 4 . Through this two step cleaning method, waste combustion flue gas is cleaned to produce cleaned flue gas for release to the atmosphere via an associated stack. An additional benefit to the method so described is the optional use of heat energy from the combination wet desulfurization system and condenser in a district heating system and/or optional cleaning of condensation water from the combined wet desulfurization system and condenser by a reverse osmosis system for uses of the so produced water outside the subject system. 
         [0054]    Still another method of using the subject system entails flowing waste combustion flue gas through one cleaning stage to remove SO 2  from the flue gas comprising SO 2  to produce cleaned flue gas for release of the cleaned flue gas to the atmosphere via a stack. As such, waste combustion flue gas is passed through a combination wet desulfurization system and condenser to produce cleaned flue gas. In the combined wet desulfurization system and condenser, the waste combustion flue gas is contacted with a solvent and absorption material slurry, which may or may not include recycled absorption material. As the waste combustion flue gas flows through combination wet desulfurization system and condenser, it is sprayed with the slurry and recirculated water to fully humidify the waste combustion flue gas. The waste combustion flue gas flowing through the wet desulfurization system and condenser also is sprayed with NaOH from an NaOH supply source to maintain a pH of approximately 5.0 to 7.5, or approximately 6.0 to 6.5 in the condenser, and a relatively small amount of an oxidation catalyst, such as FeSO 4 . Through this one step cleaning method, waste combustion flue gas is cleaned to produce cleaned flue gas for release to the atmosphere via an associated stack. An additional benefit to the method so described is the optional use of heat energy from the combination wet desulfurization system and condenser in a district heating system and/or optional cleaning of condensation water from the combined wet desulfurization system and condenser by a reverse osmosis system for uses of the so produced water outside the subject system. 
         [0055]    While the present 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 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.