Oxides of Sulfur
SOx
SO2: largest amount
SO3 very small amount
2SO2 + O2 SO3
Sulfuric acid
H2SO4 : Acid rain
Sulfuric acid : uses
• Batteries in cars
• drain cleaners
• Synthesize more than 100 million tons of sulfuric acid each year!
SOx
• SO2 : produced in largest amount
• Slowly oxidized to SO3
• Reaction is catalyzed by :– heavy metal pollutants/finely divided particles– hydroxyl free radical /OH– Ozone, and sunlight
• 2SO2(g) + O2 (g) 2 SO3(g)
Principal Sources:
• Anthropogenic : man made: 150 million tons– Coal, low grade, 7% S ( 1-5 % S)
– As iron pyrite FeS2 and
– Organic sulfur / proteins– Combustion of sulfur to sulfur dioxide
– 4FeS2 + 11 O2 2 Fe2O3 + 8 SO2
– Petroleum refining removes sulfur impurities by bubbling Hydrogen gas in presence of a catalyst
– Heavy fuel used by ships/ high sulfur content
SO2
• Smelting Plants– Oxidize sulfide ore to metal oxide
– Cu2S(s) +2 O2 2CuO(s) + SO2(g)
– Sulfuric acid plants: acid is produced by the oxidation of SO2
• Reacts with water to form sulfuric acid
• O2 + SO2(g) SO3(g)
• H2O(l) + SO3 H2 SO4(l)
SO2 Natural Sources
• Volcanic and bacterial activities
• Atmospheric oxidation of Hydrogen sulfide gas ( SO2 as secondary pollutant)
• H2S is produced by the decay of organic matter ( swamps, ocean)
• 2H2S(g) + 3O2(g) 2 SO2 + 2 H2O(l)
Effect of SO2 on Health
• Acidic oxide
• Breathing aerosol irritates respiratory tract
• Greatest effect on elderly and young and those suffering from asthma
Methods of controlling Sox pollution
• Converting high sulfur coal to Synthetic natural gas
• Requires 30 % of energy for the conversion P 608
• Sulfur is converted to H2S gas
• S + H2 H2S
4 steps
• 1.Hydrogasification: synthesis gas
• C(s) + H2O(g) CO(g) + H2(g) + impurities
• Ho = 131.3 kJ/mol endothermic
• Synthesis gas is used as a fuel
• 2.catalytic increase of hydrogen gas• CO(g) + H2O(g) CO2(g) + H2(g)
• more steam more H2
heat
SNG
• 3.Removal of impurities:• CO2, water, and hydrogen sulfide gas
– H2S sulfur and sold as by-product
4. Catalytic Methanation: water gas with more H2 is passed over heated catalyst
3H2 + CO CH4 + H2O water vapor is removed
This reaction is similar but opposite for synthesis of ammonia
oxidized
catalyst
SNG Summary
• 1. Hydrogasification: high temp/pressure• C(s) + H2O(g) CO(g) + H2(g) + impurities
• 2.Catalytic increase of H2• CO + H2O (g) CO2 + H2
• 3.Removal of CO2 and H2O, and H2S
• 4. Catalytic Methanation3H2 + CO CH4 + H2O water vapor is removed
More H2 gas
Methods of controlling Sox pollution
• Using cleaner burning coal• Controlling before burning fossil fuel
a. removing S before burning using H2/catalyst
b. coal washing/finely ground coal and wash with water/iron pyrites removed
Post combustion method
a. limestone fluidized beds or injection method
b. Wet scrubber method
3.Pre- Combustion Method
• Controlling before burning fossil fuel : • 2 ways• A. bubble H2 gas in presence of catalyst
– remove sulfur as H2S during refining process– B: desulfurize / finely ground the coal and wash with
water– FeS2/ iron pyrite is dense; – settles down– Remove it. Organic sulfur can not be removed
4. Post Combustion Method
• Removal of S from exhaust gases after burning the coal
• but before releasing to the atmosphere
• a) Limestone fluidized beds or injection systems– Add powdered limestone, CaCO3 with coal in
the combustion process– CaCO3 CaO + CO2heat
Sulfur
• SO2 can be removed by injecting powdered limestone which is converted to calcium oxide.
• The CaO reacts with SO2 to form a precipitate of calcium sulfite.
a) Limestone fluidized beds or injection systems
• CaO + SO2 CaSO3
• 2CaO + 2SO2 + O2 2 CaSO4
• Calcium sulfate, un-reacted CaO, SO2 and particulates are absorbed into water in a wet-scrubber.
b. Wet scrubber
• Usually water based
• Removes particulates/ contaminants
• Alkaline liquid is sprayed downward while the gas stream moves upward
• Effectiveness depends upon the contact between the alkaline liquid and SO2
• CaO + SO2 CaSO3
• Slurry goes to ponds
b. Wet scrubber
• Drawbacks:
• Deposition of CaSO3 on the scrubber surface
• Need extremely large amounts of calcium carbonate
• SO2 is replaced by greenhouse gas
b. Wet scrubber with Mg(OH)2
• Mg(OH)2 + SO2 MgSO3 + H2O
• MgSO3 MgO + SO2
• SO2 is used in the manufacturing of sulfuric acid
• MgO is recycled as Mg(OH)2
• MgO + H2O Mg(OH)2
• Drawback: heating requires lot of energy / expensive
heat
2. Using low sulfur, cleaner burning coal
• Hard coal / anthracite
• CO(g) + H2O(g) CO2(g) + H2(g)
• Ho = -41.2 kJ/molrxn
• C(s) + CO2(g) 2 CO(g)– Ho = 172.5 kJ/molrxn
• N2(g) + 3 H2(g) 2 NH3(g)
• It can also be used to make methyl alcohol, or methanol.
• CO(g) + 2 H2(g) CH3OH(l)
• Methanol can then be used as a starting material for the synthesis of alkenes, aromatic compounds, acetic acid, formaldehyde, and ethyl alcohol (ethanol). Synthesis gas can also be used to produce methane, or synthetic natural gas (SNG).
• CO(g) + 3 H2(g) CH4(g) + H2O(g) • 2 CO(g) + 2 H2(g) CH4(g) + CO2(g)