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SOx Control processes

AE/CE 524BJ. (Hans) van Leeuwen

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

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Claus process

SOHSOSH

SOOHOSH

322

23

2reaction catalytic

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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)

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

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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

AIR POLLUTION CONTROL WITH LIME SLUDGE

SO2 + CaCO3 CaSO3 + CO2

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

Typical water treatment process with lime softening

Lime sludge

Ca(OH)2

Lime

SETTLINGSETTLING

LAGOON DEWATERING

STOCKPILING

ISU Power Plant

Reuse Applications in Power Plants

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