1 SO x Control processes AE/CE 524B J. (Hans) van Leeuwen

Legislation 2 In March 2005, the U.S. EPA finalized the Clean Air Interstate Rule (CAIR). CAIR caps emissions of sulfur dioxide (SO 2 ) and nitrogen oxides (NOx) in the eastern United States. [1] The rule includes three separate cap and trade programs, including annual SO 2 and NOx reduction programs to limit fine particulate matter formation (PM2.5), and a seasonal NOx reduction program to decrease ozone formation. [2]sulfur dioxide [1] [2]

3 Sources of SO x The majority of sulfur oxides come from power generation sources: - coal - oil S + O 2  SO 2 Other sources include: - other industry such as paper mills - mobile sources - natural sources (e.g. volcanoes)

4 Specific Pollutants Involved While SO 2 is the main sulfur-containing pollutant, there are various others: SO 3 (the other “ oxide ” ) SO 2 + ½ O 2  SO 3 H 2 S (rotten egg smell) Various mercaptans (decaying garbage) H - S - R or R ’ - S - R

5 Effects Human Health (as well as animals) Plants Corrosion Acid Rain

6 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)

7 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

8 Corrosion Apart from acid rain, sulfur-containing gases can directly corrode metals H 2 S 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

9 Acid Rain Sulfur oxides are not the only contributor to acid rain, but they are a primary cause. The chemistry is: SO 2 + hν  SO 2 * SO 2 * +O 2  SO 3 + O SO 3 + H 2 O  H 2 SO 4

10 Acid Rain – The process

11 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

12 Control Measures for Sulfur Air Pollutants

13 Fuel Desulfurization Oil & natural gas Claus process Coal Mineral sulfate  wash away Organic sulfur + O 2  SO 2  SO 2 removal

14 SO 2 Removal High concentration (e.g. smelting) Absorbing SO 2 to make H 2 SO 4 Low concentration (< 2000 ppm) Flue gas desulfurization

15 Limestone Scrubbing Lime Scrubbing Dual Alkali (recycled)

Scrubbing with a basic solid or solution 16 SO 2 is an acid gas – sorbent slurries used to remove SO 2 from flue gases alkaline. Wet scrubbing using a CaCO 3 (limestone) slurry produces CaSO 3 (calcium sulfite):limestone CaCO 3 (solid) + SO 2 (gas) → CaSO 3 (solid) + CO 2 (gas) Ca(OH) 2 (lime) slurry, the reaction also produces CaSO 3 (calcium sulfite):limecalcium sulfite Ca(OH) 2 (solid) + SO 2 (gas) → CaSO 3 (solid) + H 2 O (liquid) Mg(OH) 2 (magnesium hydroxide) slurry produces MgSO 3 (magnesium sulfite):magnesium hydroxidemagnesium sulfite Mg(OH) 2 (solid) + SO 2 (gas) → MgSO 3 (solid) + H 2 O (liquid) To partially offset the cost of the FGD, in some designs, the CaSO 3 (calcium sulfite) is oxidized to produce marketable CaSO 4 ·2H 2 O (gypsum) by forced oxidation:gypsum CaSO 3 (solid) + H 2 O (liquid) + ½O 2 (gas) → CaSO 4 (solid) + H 2 O A natural alkaline usable to absorb SO 2 is seawater. The SO 2 is absorbed in the water, and oxygen is added to react to form sulfate ions SO 4 - and free H +. The surplus of H + is offset by the carbonates in seawater pushing the carbonate equilibrium to release CO 2 gas: SO 2 (gas) + H 2 O + ½O 2 (gas)→ SO 4 2- (solid) + 2H + HCO H + → H 2 O + CO 2 (gas)

17 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 Na 2 CO 3 ) or nahcolite (natural NaHCO 3 ) Ca 2+ SO 2 Ca 2+ CaSO 4 CaO CaSO 4 CaO

18 Wellman-Lord (W-L) process (1) Flue gas pretreatment: (venturi prescrubber) to remove particulates, SO 3 and HCl (2) SO 2 absorption by Na 2 SO 3 solution (3) Purge treatment: centrifuge the slurry to remove solids (4) Na 2 SO 3 regeneration MgO process Similar to lime/limestone scrubbing (average, 1 mole Na 2 SO 3 for 42 moles of SO 2 )

19 Citrate scrubbing process US Bureau of Mines process SO 2 is reduced with H 2 S to S in a liquid phase reaction Flakt-Boliden process Use steam to strip SO 2 off the liquid. The SO 2 can then be processed further to S via a Claus rdn or to H 2 SO 4 by oxidation and absorption

20 Westvaco process CuO process

AIR POLLUTION CONTROL WITH LIME SLUDGE SO 2 + CaCO 3  CaSO 3 + CO 2

Typical water treatment process with lime softening Lime sludge Ca(OH) 2 Lime   

SETTLING

LAGOON DEWATERING

STOCKPILING

ISU Power Plant Reuse Applications in Power Plants

CONTROL OF POWER OF POWER PLANT AND PLANT AND EMISSIONS EMISSIONS RESULTS: ENCOURAGING

30 Flue Gas Scrubbing with Fly Ash and Oxidant with Production of a New Wastewater Treatment Chemical NaClO 3 + 3H 2 O + 3SO 2  3SO H + + Na + + Cl - Fe2O3 + 6H + + 6e  3H 2 O + 2Fe 3+ Al2O3 + 6H + + 6e  3H 2 O + 2Al 3+ 2Fe 3+ + (3-n/2)(SO 4 2- ) + n (OH - )  Fe 2 (OH) n (SO 4 ) 3-n/2 2Al 3+ + (3-n/2)(SO 4 2- ) + m (OH - )  Al 2 (OH) m (SO 4 ) 3-m/2