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SOx Control processes
AE/CE 524BJ. (Hans) van Leeuwen
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Legislation
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In March 2005, the U.S. EPA finalized the Clean Air Interstate Rule (CAIR). CAIR caps emissions of sulfur dioxide (SO2) and nitrogen oxides (NOx) in the eastern United States.[1] The rule includes three separate cap and trade programs, including annual SO2 and NOx reduction programs to limit fine particulate matter formation (PM2.5), and a seasonal NOx reduction program to decrease ozone formation.[2]
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Sources of SOx
• The majority of sulfur oxides come from power generation sources:
- coal- oil
S + O2 SO2
Other sources include:- other industry such as paper mills- mobile sources- natural sources (e.g. volcanoes)
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Specific Pollutants Involved
While SO2 is the main sulfur-containing pollutant, there are various others:
• SO3 (the other “oxide”) SO2 + ½ O2 SO3
• H2S (rotten egg smell)
• Various mercaptans (decaying garbage) H - S - R or R’- S - R
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Effects
• Human Health (as well as animals)
• Plants
• Corrosion
• Acid Rain
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Human Health Effects
• Lung and throat irritation• Worse symptoms for those with bronchitis• Weakening of the immune system
• Sulfur oxides in combination with other air pollutants seem affect health even more:
€ SOx with acid aerosols can product chronic cough and difficulty breathing € SOx with PM has produced some of the worst air pollution disasters (Meuse Valley 1930, Donora 1948, London 1952)
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Effects on Plants
• Necrosis – morphological changes due to cell death (produces brown or black spots)
• Chlorosis – “bleaching” of leaves, producing a yellow or white discoloration
Necrosis (above)Chlorosis (below)
Sulfur dioxide* can enter leaf stomata, causing:
*These effects can happen with other air pollutants as well
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Corrosion• Apart from acid rain, sulfur-containing gases can directly corrode metals
• H2S is most notorious for corrosion, but sulfur oxides and mercaptans can also corrode. Many times, corrosion is actually due to microbial activity acting on the sulfur
Gas pipeline
Stainless Steel
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Acid Rain
Sulfur oxides are not the only contributor to acid rain, but they are a primary cause. The chemistry is:
SO2 + hν SO2*SO2* +O2 SO3 + OSO3 + H2O H2SO4
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Acid Rain – The process
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Problems with Acid Rain
• Increased acidity in bodies of water
• Destruction of vegetation in forests
• Corrosion of paints
• Deterioration of building materials • Deterioration decorative materials (e.g. stone on statues)
• Decreased visibility
• Deterioration of human health
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Control Measures for Sulfur Air Pollutants
Main Option Suboption Examples of Processes Do not create SO2 Desulfurize the fuel Oil desulfurization, Coal
cleaning Low-sulfur fuel Dispersion Build tall stacks SO2 scrubbing:
Throwaway Wet scrubbing Lime, Limestone, Dual alkali, Mitsubishi, Bischoff, Forced oxidation (w/ gypsum disposal)
Dry scrubbing Lime spray drying, Lime injection, Trona, Nahcolite
Regenerative Wet processes Absorption with water (smelters), Wellman-Lord, MgO, Citrate, Aqueous carbonate, SULF-x, CONOSOx, Forced oxidation (w/ gympsum sales)
Dry processes Activated carbon adsorption, Copper oxide adsorption
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Fuel Desulfurization
Oil & natural gas
RSHHSR 2reaction catalytic
2
Claus process
SOHSOSH
SOOHOSH
322
23
2reaction catalytic
22
2222
CoalMineral sulfate wash away Organic sulfur + O2 SO2 SO2 removal
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SO2 Removal
High concentration (e.g. smelting)Absorbing SO2 to make H2SO4
Low concentration (< 2000 ppm)
Flue gas desulfurization
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Limestone Scrubbing
OHgCOCaSOCaHSOsCaCO
gCOHSOCaSOOHsCaCO
2232
33
232
223
)(22)(
)(22)(
Lime Scrubbing
OHCaSOOOHCaSOOHCaSOOHCaSOH
SOHOHSOOHCaOHCaO
24223
23232
3222
22
25.022)(
Dual Alkali
NaOHyCaSOxCaSOOHNaySOxSOCaCOCaO
OHySOxSONaSONaOHSONa
34
22
32
43
22
32
4232
5.0/
5.0/
(recycled)
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Scrubbing with a basic solid or solution
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SO2 is an acid gas – sorbent slurries used to remove SO2 from flue gases alkaline.
Wet scrubbing using a CaCO3 (limestone) slurry produces CaSO3 (calcium sulfite):
CaCO3 (solid) + SO2 (gas) → CaSO3 (solid) + CO2 (gas)
Ca(OH)2 (lime) slurry, the reaction also produces CaSO3 (calcium sulfite):
Ca(OH)2 (solid) + SO2 (gas) → CaSO3 (solid) + H2O (liquid)
Mg(OH)2 (magnesium hydroxide) slurry produces MgSO3 (magnesium sulfite):
Mg(OH)2 (solid) + SO2 (gas) → MgSO3 (solid) + H2O (liquid)
To partially offset the cost of the FGD, in some designs, the CaSO3 (calcium sulfite)
is oxidized to produce marketable CaSO4·2H2O (gypsum) by forced oxidation:
CaSO3 (solid) + H2O (liquid) + ½O2 (gas) → CaSO4 (solid) + H2O
A natural alkaline usable to absorb SO2 is seawater. The SO2 is absorbed in the
water, and oxygen is added to react to form sulfate ions SO4- and free H+.
The surplus of H+ is offset by the carbonates in seawater pushing the carbonate equilibrium to release CO2 gas:
SO2 (gas) + H2O + ½O2 (gas)→ SO42- (solid) + 2H+
HCO3- + H+ → H2O + CO2 (gas)
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Lime-spray dryingSame as lime scrubbing except the water evaporates before the droplets reach the bottom of the tower
Dry scrubbingDirect injection of pulverized lime or limestone, also trona (natural Na2CO3) or nahcolite (natural NaHCO3)
Ca2+
SO2
Ca2+
CaSO4
CaO
CaSO4
CaO
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Wellman-Lord (W-L) process(1) Flue gas pretreatment: (venturi prescrubber) to remove particulates, SO3 and HCl
(2) SO2 absorption by Na2SO3 solution
3422332
42232
32232
225.0
2
NaHSOSONaOHSOSONaSONaOSONaNaHSOOHSOSONa
(3) Purge treatment: centrifuge the slurry to remove solids
(4) Na2SO3 regeneration
232232
223232COSONaSOCONa
OHSOSONaNaHSOheat
MgO processSimilar to lime/limestone scrubbing
(average, 1 mole Na2SO3 for 42 moles of SO2)
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Citrate scrubbing process
CiHHHCiCiHCiHCi
HSOHSOHSOHOHgSO
22
323332
3222
ion citrate:
)(
US Bureau of Mines process
SO2 is reduced with H2S to S in a liquid phase reaction
Flakt-Boliden process
Use steam to strip SO2 off the liquid. The SO2 can then be processed further to S via a Claus rdn or to H2SO4 by oxidation and absorption
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Westvaco process
OHSSHSOH
SOHOOHSO
2carbon activated
242
42carbon activated
222
443
5.0
CuO process
422 5.0 CuSOOSOCuO
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AIR POLLUTION CONTROL WITH LIME SLUDGE
SO2 + CaCO3 CaSO3 + CO2
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Water SofteningWater Softening
The removal of divalent metal ions Precipitation/settling or IX resins
Ca(HCO3)2 + Ca(OH)2 2CaCO3 + 2H2O
MgCO3 + Ca(OH)2 Mg(OH)2 + CaCO3
Note that the CaCO3 precipitated is chemically identical to limestone
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Typical water treatment process with lime softening
Lime sludge
Ca(OH)2
Lime
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SETTLINGSETTLING
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LAGOON DEWATERING
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STOCKPILING
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ISU Power Plant
Reuse Applications in Power Plants
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CONTROLCONTROL OF POWEROF POWER PLANT ANDPLANT AND EMISSIONSEMISSIONS
RESULTS:ENCOURAGING
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Flue Gas Scrubbing with Fly Ash and Oxidant with Production of a New Wastewater Treatment Chemical
NaClO3 + 3H2O + 3SO2 3SO42- + 6H+ + Na+ + Cl-
Fe2O3 + 6H+ + 6e 3H2O + 2Fe3+
Al2O3 + 6H+ + 6e 3H2O + 2Al3+
2Fe3+ + (3-n/2)(SO42-) + n (OH-) Fe2(OH)n(SO4)3-n/2
2Al3+ + (3-n/2)(SO42-) + m (OH-) Al2(OH)m(SO4)3-m/2