1. Control of Sulfur Oxides
CHAPTER 11
Control of Sulfur Oxides
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2. Control of Sulfur Oxides
Unlike PM and VOCs, control of sulfur oxides are largely chemical rather than physical (cyclones, ESPs, filters, vapor capture, condensation, adsorption).
Sulfur and nitrogen oxides have many sources and they are respiratory irritants, principal causes of acid rain.
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3. Elementary oxidation and reduction of S and N
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4. An Overwiew of Sulfur problem
S is the sixteenth most abundant element in the earth crust (260 ppm)
Sulfur in the earth crust exist mostly in the form of sulfates
CaSO4.2H2O: gypsum dihydrate
Chemically inert, slightly water soluble, nontoxic
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5. Sulfur in the fuel Coal (0.5 to 3 %) Oil Natural gas Peat
Wood (<0.1 %)
Other organic matter
S+O SO H2SO4
Acid rain
(in fuel)
(in the atmosphere)
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6. Fuel oil 4 (kalorifer yakıtı) 1.5 % Fuel Oil 5 2.0-2.8 % Fuel Oil 6
S content
Fuel Oil 3
1.0 %
Fuel oil 4 (kalorifer yakıtı)
1.5 %
Fuel Oil 5 %
Fuel Oil 6 %
Unleaded gasoline 95 octane
10 mg/kg
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7. Sulfur in the environment as a result of human activities
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8. Control of S compounds
Removal of reduced S compounds from petroleum and natural gas
Removal of SO2 from Rich waste gases
Removal of SO2 from lean waste gases
Alternatives to Burn and then scrub
Flue gas desulfurization
Forced oxidation limestone wet scrubbers
Other approaches
Wet
systems
Dry
systems
Wet-Dry
systems
regenerative
systems
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9. Basis of SO2 control CaCO3 +SO2+0.5 O2 CaSO4 +CO2
limestone
Gypsum
Forms of sulfur in the fuels
Natural gas
H2S
Oil (liquid petroleum)
S is bonded to hydrocarbons
Coal
Chemically bound S or pyritic (crystals of iron pyrite, FeS2
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10. Mining as a source of SO2 Processing of sulfur bearing ores
To obtain cupper from ore, high temperature smelting reaction occur
CuFeS2 +5/2 O Cu +FeO +2SO2
Chalcopyrite, CuFeS2
principal copper ore
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11. 18:52

12. 18:52

13. Which sectors contribute more?
Coal combustion is major contributor of SO2 emissions
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14. Absorption (Scrubbing)
The removal of reduced S compounds from petroleum and natural gas streams
Absorption (Scrubbing)
Solute gas out
Gas liquid separator
Alkali scrubbing solution
Weak alkaline
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15. Natural gas containing 1%H2S
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16. H2S, SO2, SO3, NO2, HCl, CO2 are acid gases
If absorption is used for removal of H2S fom a gas stream using water, the required flow rate is 148 kg/s too much
H2S, SO2, SO3, NO2, HCl, CO2 are acid gases
H2S(gas)  H2S (dissolved in water) H+ +HS-
Solubility of H2S increase
If we add OH- into the scrubbing soln
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17. OR
Another solvent in which H2S dissolve more readily can be used such as
Monoethanolamine (liquid flow rate=6.1 kg/s)
Dithanolamine
Triethanolamine
Potassium carbonate
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18. Uses and limitations of Absorbers and strippers in APC
SO2 can also be removed by absorption using alkali solns
BUT
If there are other gases present such as NO and NO2…..
3NO2 +H2O HNO3 +NO (NO2 is an acid gas but the rxn rate is slow)
NO is not an acid gas
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19. Sulfur Removal from Hydrocarbons
Once H2S has been separated from the other components of the gas
H2S +1/2 O S+H2O
H2S +3/2 O SO2+H2O
Claus Process
Sold for use for the H2SO4 production
Further oxidation yields SO2
Sulfur in hyrocarbons can be reduced to H2S but it is not economical,H2 is expensive, Oxidation is more feasible, air is free.
Sulfur in hydrocarbon mixtures is converted to SO2 during combustion
H2S concentration in natural gas should not exceed 4 ppm.
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20. S in fuel for use in vehicles
S in fuels normally converted to SO2 during combustion.
Large oil burning facilities have equipment to capture SO2 but vehicles do not.
Reduction in fuel S content is a major way for reducing SO2 emissions, OR
catalyst
(hydrocarbon containing S) +H hydrocarbon+H2S
Further removal by absorption
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21. Removal of SO2 from rich waste gases
During smelting SO2 emissions may result from 2 to 40 %.
Sulfuric acid production is an economical solution
SO2 +1/2 O SO3
SO3 +1/2 H2O H2SO4
catalyst
Exothermic reaction
catalyst
catalyst
catalyst
catalyst
Exothermic reaction
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22. App. 98% of SO2 is converted to H2SO4 in a sulfuric acid plant
catalyst
SO2 +1/2 O SO3
Exothermic reaction
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23. Single and double absorption sulfuric acid plants
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Phosphate fertilizer plants use sulfuric acid to produce phosphoric acid.

24. The large scale production of H2SO4 from smelter gases is only economical if there is a nearyby market (fertilizer plant)
Production of sulfuric acid is not economical when the SO2 concentration is low.
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25. Removal of SO2 from Lean waste gases
Major source of SO2 in developing countries are coal and oil burning facilities (power plants)
Typical SO2 content of the exhaust gas is 0.1%
FGD(flue gas desulfurization) is widely used.
Major drawbacks of absorption especially used in power generation
High amount of water flow
Waste water stream contains high amount of SO2
Aqueous phase SO2 reacts with dissolved O2 in river water consuming O2 needed for fishes.
Oxidizing SO2 to SO42- before discharging is a solution.
Or NaOH can be used in the scrubbing solution
2NaOH+SO2+1/2 O Na2SO4 +H2O (Not economical, NaOH is expensive)
If we regenarate SO2 it is also not economical
High flow rate causes much more pressure drop, you need power to reduce it, not economical
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26. 18:52

27. Three plausible arrangement for scrubbing gas with a liquid
bubbler
Spray chamber.Pressure drop is small, contact area is small
Packed column
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Pressure drop

28. 2NaOH+SO2+1/2 O2 Na2SO4 +H2O (Not economical)
Na2SO4 needs to be very pure for selling it to a detergent manusfactuting plant or paper making.
Na2SO4 produced always contaminated with fly ash.
Usually ESP follows absorption equipment.
Market is not large
Na2SO4 is soluble that you can not send it to a landfill.
CO2 Problem
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29. 2 NaOH +CO Na2CO3 +H2O
CO2 (gas)  CO2(dissolved); +H2O H2CO3 H+ +HCO3-
SO2 (gas)  SO2(dissolved); +H2O H2SO3 H+ +HSO3-
pH between 4-6 is needed to precipitate Na2SO4
Even pH adjustment still it is problematic
pH adjustment
Amount of alkali neededn is high, costly
Waste product, Na2SO4
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30. Forced-oxidation Limestone Wet Scrubbers
CaCO3 +SO2+0.5 O CaSO4 +CO2
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hydroclone

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32. Problems with limestone scrubbers
Corrosion: Chlorides in the exhaust gases are corrosive
Solids deposition, scaling and plugging: precipitatipn of calcium salts on solid surfaces
Entrainment separator plugging: Spray nozzles do not ptoduce uniform drops, some small drops might be carried out by the gas and result corrosion
Poor reagent utilization: Sulfates and sulfites can precipitate on the limestone particles reducing absorption efficiency of SOx
Poor solid-liquid separation: CaSO3.0.5H2O, tootpaste like structure formation, retain water.
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33. Solutions
corrosion:Chloride content of the circulating liquid is controlled.
Solids deposition, scaling and plugging,Enough gypsum is kept in the ciculating slurry to prevent supersaturation.
Entrainment separator plugging: chevron type
Poor reagent utilization: larger tanks, oxidation air
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34. Other approaches Other wet systems Dry systems Wet dry systems
Regenarative systems
Tomorrow’s limestone control devices
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35. Other wet systems
Ca(OH)2 (quicklime) is an alternative of CaCO3 in thwrowaway processes.
CaO is oxidized in the ozidation tank to Ca(OH)2.
Ca(OH)2 is more reactive than limestone.
Prepartion of CaO needs extra process.
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37. Double alkali throwaway systems
They were the early systems designed because of solid deposition, scaling and plugging problems due to use of CaCO3
Na2CO3+SO Na2SO3+CO2 in the scrubber (liquid and gas)
Na2SO3+CaCO3+1/2 O2+2H2O CaSO4.2H2O+Na2CO3
Small amount
Out of the scrubber
Na2CO3 is regenerated
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38. Dry systems CaO+SO2 CaSO3 Excess alkaline reagent (CaO, CaCO3
But more reactive reagent such as Na2CO3 can be used to reduce amount of alkaline
Dry alkaline particles are injected
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39. Wet dry systems
Slurry of basic reagents were introduced via atomizer.10-50u drops
Water evaporates, particlees formed
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40. Other alternatives Change to a lower sulfur content fuel
Remove sulfur from fuel
Solvent refined coal
Modify the combustion process (Fluidized bed combustion)
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41. Alternatives to “Burn and then scrub”
Change to a lower sulfur content fuel
Remove sulfur from the fuel (coal cleaning and solvent refined coal)
Modify the combustion process
Do not burn at all
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42. Modify the combustion process
Fludized bed combustion
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43. SUMMARY
Major sources of SO2 are combustion of S containing fuels and smelting metal sulfide ores
Major control strategy of SO2: conversion to CaSO4. 2H2O and return to ground (landfill)
For fuels containing reduced sulfur: claus process or H2SO4 production
Metal sulfide ores containing high amounts of sulfur: H2SO4 and marketing
For large power plants: CaSO4. 2H2O and transfer landfill or a wallboard plant
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