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)