ENVE 4003

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ENVE 4003

• The Sulfur problem and SO2 Control

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• H2S S SO2 SO3
•  
• H2S From natural gas
• From anaerobic systems
•   Toxic, Highly odorous
•  
• SO2 Fossil Fuel combustion, S (in fuel) O2 ?
  SO2
•   Base metal smelting, e.g.
• CuFeS2 5/2 O2 ? Cu FeO 2 SO2
• SO3 From oxidation of SO2, normally requires V2O5
  or other catalyst
•  
• Effects Acid rain, sulfate aerosols (PM10)
  visibility problems

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• S content in fossil fuels 0-4
• ? SO2 in flue gas 5000 ppm
•  
• S content in mineral ores, e.g. CuFeS2 ,
• Cu 63.5 Fe 55.8 S 2 X 32
• Thus, 64/183.3 33 S in ore
• Correspondingly higher concentration in roaster
  gas

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Table (11.2) de Nevers
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Sulfur Control options

•  H2S Recover elemental S at high
  concentrations,
• burn to SO2 at low concentrations
•  
• SO2 Recover as H2SO4 at high concentrations
• (more than a few )
•  
• Capture as CaSO4 at low concentrations
• (1000 5000 ppm)
•  

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H2S removal from petroleum and natural gas

• NG, absorption with monoethanolamine
  stripping, Fig. 11.1,
•  
• S recovery from H2S by controlled catalytic
  oxidation H2S ½ O2 ? S H2O CLAUS
  PROCESS
•  
• S removal from petroleum fractions by
  hydrodesulfurization
• S (in HC) H2 ? H2S (Ni, Co, Mo, W catalysts)
• H2S recovery follows as above

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Figure 11.1 (10.15) de Nevers
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SO2 from rich gases

•  Fig. 11.4 (11.3) catalyst beds and absorption
  tower,
• SO2 ½ O2 ? SO3
• SO3 H2O ? H2SO4
•  
• H2SO4 used in fertilizer production
• phospate rock H2SO4 ? phosphoric acid ?
  phosphate fertilizer.
• As SO2 strength decreases, acid production
  becomes uneconomical,
• but we still want to limit SO2 emissions to the
  atmosphere
• - treat lean gases for SO2 removal (as CaSO4)
• - modify process to reduce or eliminate lean
  gases in favor of rich gases suitable for acid
  production.
• (e.g. Inco-Sudbury)

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Figure 11.4 (11.3) de Nevers
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SO2 removal from lean gases Flue Gas
Desulphurization, FGD

• Gas scrubbing,
• Fig. 11.5 (11.4) shows alternate arrangements
•  
• Example 11.6 (11.4) looks at scrubbing 106 scfm
  of flue gas with 1000 ppm SO2, using H2O,
• SO2 H2O ? H2SO3
• Governed by the solubility of SO2 in water.
  19,900 kg/s of water to achieve 90 removal!
  Very large amount of water. We end up with a
  large quantity of acidic water.

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Figure 11.5 (11.4) de Nevers

• Three arrangements for scrubbing gas with a liquid

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SO2 removal from lean gases Flue Gas
Desulphurization, FGD

• Example 11.7 (11.5) looks at using a dilute
  solution of NaOH instead of straight water 
• 2NaOH SO2 ½ O2 ? Na2SO4
• This overall reaction proceeds via the
  dissolution of SO2 first to make H2SO3, then
  neutralization.
• From stoichiometry, 49,200 t/yr NaOH required
  34 million/yr! A lot of money. The resulting
  effluent is not acidic but contains a large
  amount of salt.

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SO2 removal from lean gases Flue Gas
Desulphurization, FGD

•  
• A complicating factor CO2 also dissolves in
  water,
• yCO2 / ySO2 20 in flue gas,
• solubility of CO2 is much lower
• but we still get H2CO3 / H2SO3 3
•  
• Thus we end up using NaOH also for the reaction
• 2NaOH CO2 ? Na2CO3 H2O

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SO2 removal from lean gases Flue Gas
Desulphurization, FGD

• We can try to adjust pH such that we dissolve
  SO2 but not CO2
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• CO2 (g) ? CO2 (aq) H2O ? H2CO3
  ? H HCO3-
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• SO2 (g) ? SO2 (aq) H2O ? H2SO3
  ? H HSO3-
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• Based on the multiple simultaneous equilibria in
  water, find H concentration that forces first
  equation to left, second equation to right 4 lt
  pH lt 6
• But, this is acidic, not alkaline. So it is not
  possible to avoid the additional cost for NaOH.
  We need a cheaper reagent.

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SO2 removal from lean gases Flue Gas
Desulphurization, FGD

• Limestone scrubbing Fig. 11.6 (11.5 11.6)
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• (1) SO2 H2O ? H2SO3
• (2)       CaCO3 H2SO3 ? CaSO3 CO2 H2O
• (3) 2 CaSO3 O2 ? 2 CaSO4
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• Reactions (1) and (2) in scrubber, (2) and (3)
  in holding tank.
• In fact, the aqueous chemistry of these systems
  is quite complex (Figure 11.13 (11.12) de Nevers)
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Figure 11.13 (11.12) de Nevers

• Principal chemical equilibria in a limestone
  scrubber

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Fig. 11.6 (11.5 11.6) de Nevers

• Flue gas desulfurization details

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SO2 removal from lean gases Flue Gas
Desulphurization, FGD

• Example 11.8 (11.6), demonstrates the application
  of familiar mass balance and fluid mechanics
  principles to the scrubber system in Fig. 11.6
• settling velocity of a spherical water droplet in
  air
• fraction of water evaporated to saturate the gas
• fraction of CaCO3 reacting per pass through
  scrubber

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SO2 removal from lean gases Flue Gas
Desulphurization, FGD

• Problems
• (largely overcome by recent systems)
• corrosion due to Cl- buildup
• solids deposition
• poor reagent utilization
• poor solid-liquid separation

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Figure (11.6) de Nevers
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Ca/S ratio in sulfur capture

• Most common final form is CaSO4 Ca/S 1
• Typically 90-95 removal of SO2 is aimed at.
  Practice shows Ca/S 1.5 2 required higher
  reagent and solids handling costs.
• Main reason
• CaCO3 (solid) ? CaO (porous solid) CO2 (gas)
• CaO (porous solid) SO3 (gas) ? CaSO4 (solid)
• Molar volume of CaSO4 is greater than the molar
  volume of original CaCO3, pores plug up before
  all the CaO is accessed by SO2.

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Figure 11.10 (11.9) de Nevers
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Figure 13.8 after Shearer

• Schematic model of the enhancement of limestone
  sulphation by hydration.

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Alternative scrubbing/capture systems

•  Lime scrubbing i.e. use Ca(OH)2 instead of
  CaCO3
• More reactive, but also more expensive
•  
• Double alkali, Fig. 11.7
• Scrub with soluble Na alkali
• Na2CO3 SO2 ? Na2SO3 CO2
• Then convert to insoluble Ca species in holding
  tank
• Na2SO3 CaCO3 ½ O2 ? CaSO4 Na2CO3
• Dry systems, Fig. 11.8 (11.7), once through
•  
• Wet-dry system, Fig. 11.9 (11.8) spray dryer
•  
• Regenerative systems, various chemical based
  systems for capturing S as relatively pure SO2 or
  H2SO4

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Figure 11.7 de Nevers

• Double alkali scrubber

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Figure 11.8 (11.7) de Nevers

• Once through dry solids addition

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Fig. 11.9 (11.8) de Nevers

• Spray dryer

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SO2 removal from lean gases

• Alternatives to scrubbing
• Use low S fuel
• Capture during combustion
• Fluidized bed combustion
• Circulating fluidized bed combustion
• Coal gasification, IGCC
• (integrated gasification combined cycle)
• Dont burn fuel with S, (burn less)

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Fuel S

• Coal high (3-4), low (lt1)
• Diesel high (0.5 1.5), low (0.1), now going
  down to 500, 50 ppm?
•  
• Petroleum S can be reduced by hydrodesulfurization
  
•  
• Coal S
• - Pyritic, FeS2, can be removed by crushing down
  to 100 microns and flotation, washing
• Organic, bonded to coal matrix, cannot be removed
  

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Fig. (11.10) de Nevers
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