Transcript SOx - Iowa State University

AE/CE 524B
J. (Hans) van Leeuwen
SOx Control processes
1
Legislation
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]
2
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)
3
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
4
Effects
• Human Health (as well as animals)
• Plants
• Corrosion
• Acid Rain
5
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)
6
Effects on Plants
Sulfur dioxide* can enter leaf
stomata, causing:
• Necrosis – morphological
changes due to cell death
(produces brown or black spots)
Necrosis (above)
Chlorosis (below)
• Chlorosis – “bleaching” of
leaves, producing a yellow or
white discoloration
*These effects can happen with other air pollutants as well
7
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
8
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 + O
SO3 + H2O  H2SO4
9
Acid Rain –
The process
10
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
11
Control Measures for Sulfur Air Pollutants
Main Option
Suboption
Do not create SO2 Desulfurize the fuel
Dispersion
SO2 scrubbing:
Throwaway
Low-sulfur fuel
Build tall stacks
Wet scrubbing
Dry scrubbing
Regenerative
Examples of Processes
Oil desulfurization, Coal
cleaning
Wet processes
Dry processes
Lime, Limestone, Dual alkali,
Mitsubishi, Bischoff, Forced
oxidation (w/ gypsum
disposal)
Lime spray drying, Lime
injection, Trona, Nahcolite
Absorption with water
(smelters), Wellman-Lord,
MgO, Citrate, Aqueous
carbonate, SULF-x,
CONOSOx, Forced oxidation
(w/ gympsum sales)
Activated carbon adsorption,
Copper oxide adsorption
12
Fuel Desulfurization
Coal
Mineral sulfate  wash away
Organic sulfur + O2  SO2  SO2 removal
Oil & natural gas
R  S  H 2 catalytic
reaction
 H 2 S  R
Claus process
H 2 S  3 O2  H 2O  SO2
2
reaction
2 H 2 S  SO2 catalytic

 2 H 2O  3S
13
SO2 Removal
High concentration (e.g. smelting)
Absorbing SO2 to make H2SO4
Low concentration (< 2000 ppm)
Flue gas desulfurization
14
Limestone Scrubbing
CaCO3 ( s)  H 2O  2SO2  Ca 2  2 HSO3  CO2 ( g )
CaCO3 ( s)  2 HSO3  Ca  2  2CaSO3  CO2 ( g )  H 2O
Lime Scrubbing
CaO  H 2O  Ca OH 2
SO2  H 2O  H 2 SO3
H 2 SO3  Ca (OH ) 2  CaSO3  2 H 2O
CaSO3  2 H 2O  0.5O2  CaSO4  2 H 2O
Dual Alkali
2
Na2 SO3 / NaOH  SO2  Na   xSO42  ySO3  0.5H 2O
2
2
CaO / CaCO3  xSO4  ySO3  Na   0.5H 2O
 xCaSO4  yCaSO3  NaOH (recycled)
15
Scrubbing with a basic solid or solution
SO2 is an acid gas – alkaline sorbent slurries used to remove SO2 from flue gas
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)
16
Lime-spray drying
Same as lime scrubbing except the water evaporates
before the droplets reach the bottom of the tower
Dry scrubbing
Direct injection of pulverized lime or limestone, also
trona (natural Na2CO3) or nahcolite (natural NaHCO3)
Ca2+
CaO
Ca2+
SO2
CaO
CaSO4
CaSO4
17
Wellman-Lord (W-L) process
(1) Flue gas pretreatment: (venturi prescrubber) to remove
particulates, SO3 and HCl
(2) SO2 absorption by Na2SO3 solution
Na2 SO3  SO2  H 2O  2 NaHSO3
Na2 SO3  0.5O2  Na2 SO4
2 Na2 SO3  SO3  H 2O  Na2 SO4  2 NaHSO3
(3) Purge treatment: centrifuge the slurry to remove solids
(4) Na2SO3 regeneration
2 NaHSO3 heat

 Na2 SO3  SO2  H 2O
Na2CO3  SO2  Na2 SO3  CO2
MgO process
Similar to lime/limestone scrubbing
18
Citrate scrubbing process
SO2 ( g )  H 2O  H 2 SO3


3
H 2 SO3  H  HSO
Ci 3  H   HCi 2
Ci 3 : citrateion
HCi  2  H   H 2Ci 
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
19
Westvaco process
SO2  H 2O  0.5O2  H 2 SO4
activatedcarbon
H 2 SO4  3H 2 S  4S  4 H 2O
activatedcarbon
CuO process
CuO  SO2  0.5O2  CuSO4
20
AIR POLLUTION CONTROL WITH LIME SLUDGE
SO2 + CaCO3  CaSO3 + CO2
Typical water treatment
process with lime softening
Ca(OH)2
Lime



Lime sludge
SETTLING
LAGOON DEWATERING
STOCKPILING
Reuse applications in power plants
ISU Power Plant
CONTROL
OF POWER
PLANT AND
EMISSIONS
RESULTS:
ENCOURAGING
Flue gas scrubbing with fly ash and oxidant
with production of a new wastewater
treatment chemical
NaClO3 + 3H2O + 3SO2
Fe2O3 + 6H+ + 6eAl2O3 + 6H+ + 6e2Fe3+ + (3-n/2)(SO42-) + n (OH-)
2Al3+ + (3-n/2)(SO42-) + m (OH-)





3SO42- + 6H+ + Na+ + Cl3H2O + 2Fe3+
3H2O + 2Al3+
Fe2(OH)n(SO4)3-n/2
Al2(OH)m(SO4)3-m/2
30