smelting Absorbing SO2 to make H2SO4 Low concentration

							                                       SO2
        Reading: Chap 15

        • Strategies for SO2 removal
        • Formation Prevention
            – Low sulfur fuel, Fuel desulfurization,
        • Flue Gas Treatment
            – Wet scrubbing, Dry scrubbing, Spray drying, Dual
              Alkali, Wellman-Lord Process




                           Aerosol & Particulate Research Laboratory
2012/5/24                                                              1
                     Strategies for SO2 Removal
       Main Option       Suboption                    Examples of Processes
       Do not create SO2 Low-sulfur fuel
                         Desulfurize the fuel         Oil desulfurization, Coal
                                                      cleaning
       SO2 scrubbing:
         Throwaway         Wet scrubbing              Lime, Limestone, Dual alkali,
                                                      Misubishi, Bischoff, Forced
                                                      oxidation (w/ gypsum disposal)
                           Dry scrubbing              Lime spray drying, Lime
                                                      injection, Trona, Nahcolite
            Regenerative   Wet processes              Absorption with water
                                                      (smelters), Wellman-Lord,
                                                      MgO, Citrate, Aqueous
                                                      carbonate, SULF-x,
                                                      CONOSOx, Forced oxidation
                                                      (w/ gympsum sales)
                           Dry processes              Activated carbon adsorption,
                                                      Copper oxide adsorption

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                        Fuel Desulfurization
   Oil & natural gas                                               Coal
                    
     R  S  H 2 cataly ticreaction H 2 S  R     Mineral sulfate
    Claus process                                        --> wash away
                                                    Organic sulfur ???
     H 2 S  3 O2  H 2O  SO2
              2                                          --> SO2 removal
    2H 2 S  SO2 cataly tic  2 H 2O  3S IGCC – Integrated
                    reaction
                                    (for sale; a commodity, Gasification
                                            not a pollutant)
                                                              Combined
                       SO2 Removal                            Cycle
            High concentration (e.g. smelting)
                Absorbing SO2 to make H2SO4

            Low concentration (< 2000 ppm)
                 Flue gas desulfurization
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                      Limestone (Wet) Scrubbing
 CaCO3 ( s )  H 2O  2 SO2  Ca 2  2 HSO3  CO2 ( g ) (packed tower)
 CaCO3 ( s )  2 HSO3  Ca  2  2CaSO3  CO2 ( g )  H 2O (effluent
                                                                                      hold tank)
      Q: Advantages and disadvantages compared to water?
        Mass transfer in liquid film
                       
        J   k L CSO2 , I  CSO2 , L                   : enhancement factor

        Overall mass transfer coefficient
                             1
            K OG                                                  Q: Does mass transfer
                     1 / kG  H /  k L                            increase as  increases?
        Main reactions converting SO2 to bisulfite in the liquid film
                                              
            SO2  H 2O  H   HSO3
                        2                           
            SO2  SO3  H 2O  2HSO3                       precipitate
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     Q: Is it better to have a high pH?




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                   Lime (Wet) Scrubbing
      CaO  H 2O  Ca OH 2
      SO2  H 2O  H 2 SO3
      H 2 SO3  Ca (OH ) 2  CaSO3  2 H 2O
      CaSO3  2 H 2O  0.5O2  CaSO4  2 H 2O
      Q: How does it compare to limestone scrubbing?


                             Dual Alkali
                                                               2
                                        
      Na2 SO3 / NaOH  SO2  Na   xSO4 2  ySO3  0.5H 2O
                                 2
                        
      CaO / CaCO3  xSO4 2  ySO3  Na   0.5H 2O
        xCaSO4  yCaSO3  NaOH (recycled)
       Q: Advantages and disadvantages?

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                          Lime-Spray Drying
        Same as lime scrubbing except the water evaporates
        before the droplets reach the bottom of the tower

                             Dry scrubbing
            Direct injection of pulverized lime or limestone, also
            trona (natural Na2CO3) or nahcolite (natural NaHCO3)

                                    Ca2+                                 CaO
              Ca2+

            SO2                                                    CaO
                                    CaSO4
                                                          CaSO4
             Q: Advantage of dry methods over wet methods?
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                    Wellman-Lord (W-L) process
       (1) Flue gas pretreatment: (venturi prescrubber) to remove
           particulates, SO3 and HCl
       (2) SO2 absorption by Na2SO3 solution
             Na2 SO3  SO2  H 2O  2 NaHSO3
                                                 No further SO2 absorption
             Na2 SO3  0.5O2  Na2 SO4
             2 Na2 SO3  SO3  H 2O  Na2 SO4  2 NaHSO3
       (3) Purge treatment: centrifuge the slurry to remove solids
       (4) Na2SO3 regeneration
                                                 (concentrated, 85%)    Claus process
                         
                         
              2 NaHSO3 heat Na2 SO3  SO2  H 2O
              Na2CO3  SO2  Na2 SO3  CO2
            (make-up sodium)              (averagely, 1 mole Na2SO3 for 42 moles of SO2)
                               Westvaco process
                                               carbon
                                                    
                    SO2  H 2O  0.5O2 activated  H 2 SO4
                                        carbon
                    H 2 SO4  3H 2 S activated  4 S  4 H 2O
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            Quick Reflection




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