Sulfur oxides are produced from the burning of fossil fuels, mainly coal and oil, and the smelting of metal ores that contain sulfur.
Emissions of sulfur oxides cause serious impacts on human health and the environment, both directly and as a result of the way they react with other substances in the air.
Sulfur oxides are main precursors of atmospheric acidification, aerosol generation, and acidic dry and wet deposition.
There are many methods available for controlling the emission of SO2. Such as:
extraction of sulfur from fuel oils.
Sulfur reduction within combustion chamber.
Treating of flue gases.
DRY METHODS:
Mainly in industries dry, elevated temperature removal processes are used as cold plume is not formed and problem of handling large amount of slurry in flue gases is avoided.
But there are technical issues resulting in such method making wet method more applicable in industries.
Adsorption of SO2 by metal oxides to from stable sulphites or sulphates with subsequent regeneration.
-Alkalized Alumina Process
-Manganese Oxide Process
Adsorption on activated carbon followed by regeneration and conversion of concentrated SO2 to sulphuric acid or elemental sulphur.
-The Reinluft Process
ALKALIZED ALUMINA PROCESS:
Also called as Cyclic Adsorption Process.
It was developed by U.S Bureau of Mines.
Adsorbent used : Sodium Aluminate (Na2O.Al2O3)-it is porous form.
This process uses Sodium Aluminate (Na2O.Al2O3) to remove SO2 in fluidized bed at 315°C.
Na2O.Al2O3 + SO2 + ½ O2 → Na2SO4 + Al2SO3
The product of above reaction is then contacted with a reducing gas such as H2 in a regenerator at 680°C to produce H2S.
Na2SO4 + Al2O3 + 4H2 → Na2O.Al2O3 + H2S + 3H2O
Sodium Aluminate is recycled back and H2S is sent to Claus Process for producing Sulphur.
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INTRODUCTION:
•Sulfur oxides are produced from the burning of fossil fuels, mainly coal and oil, and the smelting of
metal ores that contain sulfur.
•Emissions of sulfur oxides cause serious impacts on human health and the environment, both
directly and as a result of the way they react with other substances in the air.
•Sulfur oxides are main precursors of atmospheric acidification, aerosol generation, and acidic dry
and wet deposition.
•There are many methods available for controlling the emission of SO2. Such as:
1. extraction of sulfur from fuel oils.
2. Sulfur reduction within combustion chamber.
3. Treating of flue gases.
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DRY METHODS:
•Mainly in industries dry, elevated temperature removal processes are used as cold plume is not
formed and problem of handling large amount of slurry in flue gases is avoided.
•But there are technical issues resulting in such method making wet method more applicable in
industries.
•Adsorption of SO2 by metal oxides to from stable sulphites or sulphates with subsequent
regeneration.
-Alkalized Alumina Process
-Manganese Oxide Process
•Adsorption on activated carbon followed by regeneration and conversion of concentrated SO2 to
sulphuric acid or elemental sulphur.
-The Reinluft Process
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1)PROCESS USING METAL OXIDES:
• Many oxides and oxide mixtures are identified as adsorbents and catalysts for SO2 in flue gases.
• Aluminum sodium oxide and manganese oxide have been found to be better adsorbents. Cobalt
and copper are also active.
• A metal oxide with a support exhibit enhanced reactivity compared to that of bulk oxides.
• Oxides of Na, K, Sr, Cr, Cu supported on alumina are rated as superior catalysts.
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ALKALIZED ALUMINA PROCESS:
•Also called as Cyclic Adsorption Process.
• It was developed by U.S Bureau of Mines.
•Adsorbent used : Sodium Aluminate (Na2O.Al2O3)-it is porous form.
•This process uses Sodium Aluminate (Na2O.Al2O3) to remove SO2 in fluidized bed at 315°C.
Na2O.Al2O3 + SO2 + ½ O2 → Na2SO4 + Al2SO3
•The product of above reaction is then contacted with a reducing gas such as H2 in a regenerator at
680°C to produce H2S.
Na2SO4 + Al2O3 + 4H2 → Na2O.Al2O3 + H2S + 3H2O
•Sodium Aluminate is recycled back and H2S is sent to Claus Process for producing Sulphur.
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MANGANESE OXIDE PROCESS:
•Developed by Mitsubishi Heavy Industries, Japan.
•The powdered adsorbent is fed into the reactor wherein the Manganese Oxide reacts with SO2 to produce
Manganese Sulphate (MnSO4) :
MnOx.yH2O + SO2 + ½ (2-x) O2 → MnSO4 + YH2O
•Where x ranges from 1.5 to 1.8, and y ranges from 0.1 to 1.0.
•The spent adsorbent is regenerated by reacting with Manganese Sulphate with Air and Ammonia to produce
Ammonium Sulphate:
MnSO4 + 2NH3 + 2H2O + (y-1) H2O + ½ (x-1) O2 → (NH4)2SO4 + MnOx.yH2O
•The activated manganese oxide is filtered from the Ammonium Sulphate (NH4)2SO4 Solution and the filtrate is
passed through a crystallizer where Ammonium Sulphate (NH4)2SO4 is Recovered.
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PROCESS USING ACTIVATED CHARCOAL:
•This is an attractive method for continuous removal of SO2 from flue gases due to high surface area
and low cost of activated carbon.
•It can be produces from variety of sources such as wood, coconut, or nut shells, or petroleum feed
stocks.
•Its has many limitations: - its attrition
- tendency towards spontaneous ignition
- problems of thermal regeneration.
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THE REINLUFT PROCESS:
•Uses cheap semi coke of peat, carbonized under vacuum at 600°C as a adsorbent.
•The flue gas enters the lower section of the adsorber at 150-200°C and after partial adsorption of
the SO2 gas it is withdrawn and then passed through the cooler where it is cooled to about 110°C.
• After cooling it is returned to the upper section of the adsorber and leaves the top of adsorber at
temperature 100°C.
•The spent carbon is dropped into a regenerator where it is heated up to 380-450°C by an inert gas
stream which removes the adsorbed acidic materials and produces SO2 of 10 – 15% concentration
which can be used as a feed gas for contact acid plant.
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WET PROCESS:
•Several wet flue gases desulfurization methods are available using solutions and slurries of
compounds of calcium, magnesium, sodium.
•Some process require fly ash removal before scrubbing while others remove SO2 and ash
simultaneously.
•Disadvantage : -temperature reduced to 30-50 oC.
•Reheat of the effluent gases, consuming 3% more heat of fuel than combustion chamber.
•Absorption of SO2 to from stable sulphites or sulphates with subsequent regeneration.
•Process such as:
-Lime Stone Scrubbing Process (Calsox Process)
-Magnesium Oxide Scrubbing Process (Chemico Process)
-Welman Lord Process (Single Alkali Process)
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LIME STONE SCRUBBING(CALCOX):
•Uses slurry of Lime or Lime stone as scrubbing liquid.
•Waste gases are first made free from any objectionable materials like ash in a ash collector and then
introduced into Scrubber.
•The flue gas is scrubbed within 5 to 15% slurry of Lime and Limes stone through which SO2 is
absorbed forming Calcium Sulphate & Sulphite. (CaSO4 , CaSO3)
•The solids are continuously separated from the slurry and are discharges into settling pond.
•The remaining liquor of pH 6 to 8 is recycled back to the scrubbing tower after fresh lime or limestone
has been added.
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Removal Efficiency:
Generally can remove 80-90% of SO2 . However if designed properly can remove upto 99% of SO2.
Removal Efficiency :
CaO + H2O → Ca (OH)2
Ca (OH)2 + CO2 → CaCO3 + H2O
CaCO3 + CO2 + H2O → Ca (HCO3)2
Ca (HCO3)2 + SO2 + H2O → CaSO3.2H2O + 2CO2
CaSO3.2H2O + ½ O2 → CaSO4.2H2O
2CaCO3 + 2SO2 + ½ O2 → CaSO3 + CaSO4+ 2CO2
2CaOH2 + 2SO2 + ½ O2 → CaSO3 + CaSO4+ 2H2O
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MAGNESIUM OXIDE SCRUBBING:
•Also known as Chemico Process.
•Uses Slurry of Magnesium Oxide to scrub the flue gas in a Venturi Scrubber.
•The flue gas is scrubbed with a slurry of magnesium oxide in venture scrubber which absorbs SO2
and yields Magnesium Sulphite and Sulphate according to the following reaction:
MgO + SO2 → MgSO3
MgSO3 + SO2 + H2O → Mg (HSO3)2
Mg (HSO3)2 + MgO → 2MgSO3 + H2O
MgSO3 + ½ O2 → MgSO4
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•The mixed sulphite/sulphate slurry along with unreacted MgO is separated from the liquid phase in a
Centrifuge and the mother liquor is recycled to the absorber.
•The centrifuged wet cake is dried and regenerated in a calciner.
•Carbon is added in the calcining step to reduce any MgSO4 to MgO and SO2:
MgSO4 + ½ C → MgO + SO2 + ½ CO2
•MgO is returned to the process, while the concentrated SO2 can be reprocessed into H2SO4 or elemental
sulphur.
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WELMAN-LORD PROCESS:
•Also known as Single Alkali Process.
•Uses Slurry of aqueous solution of sodium sulphite to scrub the flue gas in a Absorber.
•In this process sulphite is converted into bisulphite as the SO2 from the flue gases is absorbed by
saturated solution of sodium sulphite according to the following reaction:
Na2SO3 + SO2 + H2O → 2NaHSO3
•The sodium bisulphite formed is led to a double effect evaporator cum crystallizer where it decomposed
into sodium sulphite at a temperature 100°C. This results in ejection of SO2 and steam.
2NaHSO3 → 100°C → Na2SO3 + SO2 + H2O
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•Fly ash is removed before the SO2 scrubbing to keep the rate of its accumulation in the scrubbing
liquid low. SO2 and water vapour released from the evaporators are passed into a condenser and the
product goes to the dissolving tank for dissolution of Na2SO3 crystals and the rich SO2 gas is
processed.
•Sodium sulphate is produced in this reaction which is removed and substituted by an equivalent
amount of NaOH.
•Clear solutions of either sodium or ammonia are excellent absorbers of SO2. The regeneration step
can be carried out a low temperature in liquid system.
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REFERENCES:
1) ENVIRONMENTAL POLLUTION CONTROL ENGINEERING BY C.S. RAO
2) AIR POLLUTION CONTROL BY DHAVAL N YADAV
3) CONTROL OF SULPHUR OXIDES BY S. ZANDARYAA AND A. BEUKENS