study guide
EHST 3600 1
CONTROL OF SULFUR OXIDES AND OTHER ACID GASES
EHST 3600: Air Pollution
SESSION OBJECTIVES
• To present the chemical kinetics of the formation of sulfur oxides
• To review technologies used to control SO2 emissions from combustion systems
SESSION OUTLINE
• Thermodynamics and kinetics of sulfur oxide formation
• General control methods
• Flue-gas desulfurization processes
INTRODUCTION
• Sulfur dioxide as one of the most abundant air pollutants in the U.S.
• Sulfur is a component of all natural oil and coal (0.1 – >5%)
• Coal
• Average: 2.0-2.5 % sulfur
• Very low sulfur content: 0.5% sulfur
• Fuel oils
• Range: 1-4% sulfur
Fossil-fired power plants as major sources of sulfur dioxide
SULFUR DIOXIDE FORMATION
• High exothermic
• Small quantity of sulfur trioxide (SO3) formed in the combustion reaction
• SO2/SO3 ratio: 40:1 to 80:1
• Reducing agent
• Oxidizing agent: 2𝐻2𝑆+ 𝑆𝑂2 𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡
3𝑆+2𝐻2𝑂
S + O2 SO2
SULFUR DIOXIDE REACTION
• Reacting either photochemically or catalytically with other atmospheric contaminants sulfur trioxide, sulfuric acid, various salt of sulfuric acid
𝑆𝑂2 + 1 2 𝑂2 𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡
𝑆𝑂3
• Metal oxides oxidizing SO2 directly to sulfate
4𝑀𝑔𝑂+4𝑆𝑂2 →3𝑀𝑔𝑆𝑂4 +𝑀𝑔𝑆
• Reactions in high humidity
2𝑆𝑂2 +2𝐻2𝑂+𝑂2 𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡
2𝐻2𝑆𝑂4
𝑆𝑂3 +𝐻2𝑂 →𝐻2𝑆𝑂4
EHST 3600 2
SULFUR OXIDE FORMATION
𝑆+𝑂2 →𝑆𝑂2
𝑆𝑂2 + 1 2 𝑂2 𝑆𝑂3
GENERAL CONTROL METHODS
• Change to low-sulfur fuel
• Natural gas
• Liquefied natural gas
• Low-sulfur oil
• Low-sulfur coal
• Use desulfurized coal and oil
• Build tall stacks to increase atmospheric dispersion
• Use flue-gas desulfurization systems
LOW-SULFUR FUELS
• 40% of crude oil: 0-0.25% S
• Residual fuel oil: ~2% S
• Low-sulfur coal (1% S or
less)
• >1 trillion tons in reserve
• 62% of coal is west of Mississippi.
• 90% of coal- consuming utility power plants are east
of Mississippi.
DESULFURIZATION PROCESSES FOR COAL AND OIL
• Organic sulfur
• Chemically bound in the coal
• More complex and costly
chemical process required for removal
• Coal gasification
• Conversion of coal to a synthetic oil or solid material
• Inorganic sulfur
• Iron pyrite (FeS2)
• Present as discrete particles
• Amenable to physical removal by gravity washing Iron pyrite (FeS2) in coal
COAL GASIFICATION
• Coal is converted into gaseous component by applying heat under pressure in the presence of steam and oxygen
http://energy.gov /fe/science-innov ation/clean-coal-research/gasification
http://energy.gov /fe/how-coal-gasification-power-plants-work
• Rather than burning coal directly, it breaks down coal into its basic chemical constituents CO,
hydrogen and other gaseous compounds
• Sulfur impurities converted to H2S and carbonyl sulfide sulfur
extracted as elemental sulfur or sulfuric acid (valuable byproducts)
COAL LIQUEFACTION
• Process of converting coal into liquid hydrocarbons (liquid fuels, petrochemicals)
• C to H ratio in coal is much higher
than in oil.
• Addition H must be added to the process if a liquid product is to result.
• Involves in the production of hydrogen form coal by gasification as an intermediate step
EHST 3600 3
http://energy.gov /fe/science-innov ation/clean-coal-research
TALL STACK DISPERSION
• Dispersion of sulfur dioxide emissions from tall stacks
• Based on the natural dispersion at high
elevation so that ground-level concentrations are acceptable at all times
• Current regulations no longer recognize this as an acceptable alternative.
• “Dilution is not a solution to pollution.”
• Overall sulfur emissions are not reduced
• Dispersed at lower concentration over a much larger area, but still contributes to
the overall sulfur emissions
LONG DISTANCE TRANSPORT
• Some pollutants can remain in the air for a long period of time, and can be carried on prevailing winds across geographical and political boundaries, far from their place of origin
• Pollution control strategies of the 1970’s developed the “tall stack strategy” where smokestacks were built 500 feet
tall to reduce local air pollution. This ironically contributed to long-distance pollution.
• Example: Much of the acid rain damaging vegetation in Japan originated in China.
CONVENTION ON LONG-RANGE TRANSBOUNDARY AIR POLLUTION
• Agreement that required the signatory nations to achieve a 30% reduction in SO2 emissions from 1980 levels by 1993
• Took place in Europe and involved 35 countries in 1979
FLUE-GAS DESULFURIZATION (FGD)
• A set of technologies use to remove sulfur dioxide from exhaust flue gas of fossil-fuel power plants, and from the emissions of
other sulfur oxide emitting processes
• Wet scrubbers (>90% removal efficiencies)
• Spray dry scrubbers • Dry scrubbers (<80%)
• Involves the absorption and reaction using an alkaline reagent to produce a solid compound
• Primary means of SO2 emission reduction from coal-fired boilers (70-90% removal eff.)
https://www3.epa.gov /ttncatc1/dir1/ffdg.pdf
FLUE-GAS DESULFURIZATION (FGD)
• Typically uses a calcium or sodium based alkaline reagent
• Reagent injected in the flue gas in a spray tower or directly into the
duct
• SO2 absorbed, neutralized and/or oxidized by reagent solid compound (either calcium or
sodium sulfate)
• Solid is removed from the waste gas stream using downstream
equipment.
https://www3.epa.gov /ttncatc1/dir1/ffdg.pdf
EHST 3600 4
WET SCRUBBER SYSTEM
http://chemwiki.ucdav is.edu/?title=Textbook_Maps/General_Chemistry_Textbook_Maps/Map:_Chemistry_%28Av erill_%26_Eldredge%29/05:_Energy_Changes_in_Chemical_Reactions/5.7:_Acid_Rain
FLUE GAS DESULFURIZATION PROCESSES
• Throwaway process
• Resulting products are disposed
in landfill
• Fresh chemicals must be
continually added
• Can be used to remove PM
• Regenerative process
• Removal agents can be
continually regenerated
• Recovers saleable products
• Must be preceded by a
particulate collector in high PM
emissions
FLUE GAS DESULFURIZATION PROCESSES
• Wet FGD
• Active removal agent is
contained in a liquid solution
• Dry FGD
• Active removal agent is not
contained in a liquid solution
• Wet/dry FGD
• Spray dryer – starts out as wet
FGD but ends up as a dry FGD
• Reaction products are dried during the process
FLUE GAS DESULFURIZATION PROCESSES
• 3 most common processes in the U.S.
• Wet lime scrubbing
• Wet limestone scrubbing
• Lime spray drying (wet/dry FGD)
• All 3 are throwaway processes
• Saleable product processes have
not become widely acceptable
• Cost
• Complexity
• Lack of commercial market for products
FLUE GAS DESULFURIZATION PROCESSES
1. Dry limestone scrubbing in fluidized beds
2. Limestone and lime scrubbing
3. The use of magnesium as an additive in
limestone and lime wet scrubbers
4. Magnesium oxide scrubbing
5. Single alkali scrubbing
6. Double alkali scrubbing
7. Dry scrubbing
LIMESTONE AND LIME SCRUBBING
• 2 most common FGD systems
• Removal efficiencies of >90%
• Flue gas is scrubbed with a 5-15% slurry of
calcium sulfite/ sulfate salts, and added amount of calcium hydroxide (Ca(OH)2) or
limestone (CaCO3).
• Calcium hydroxide is formed by slaking lime (CaO) in water.
• CaO + H2O Ca(OH)2 + heat
• “Limestone” or “lime” scrubber
• Alkaline input material
• Calcium calcium sulfites/ sulfates
EHST 3600 5
COAL-FIRED POWER PLANT
• https://www.youtube.com/watch?v=rEJKiUYjW1E
SUMMARY
• Thermodynamics and kinetics of sulfur oxide formation
• General control methods
• Change to low-sulfur fuel
• Use desulfurized coal and oil
• Build tall stacks to increase atmospheric dispersion
• Use flue-gas desulfurization systems
• Flue-gas desulfurization processes
• Throwaway, regenerative
• Wet FGD, dry FGD, wet/dry FGD