Sulfur oxidesSulfur oxides

authors: Dr. Bajnóczy Gábor

Kiss Bernadett

BUDAPEST UNIVERSITY OF TECHNOLOGY AND ECONOMICS

DEPARTMENT OF CHEMICAL AND ENVIRONMENTAL PROCESS ENGINEERING

FACULTY OF CHEMICAL AND BIOCHEMICAL ENGINEERING

The pictures and drawings The pictures and drawings of this presentation can be of this presentation can be used only for education !used only for education !

Any commercial use is Any commercial use is prohibited !prohibited !

Physical properties of Physical properties of atmospheric sulfur oxidesatmospheric sulfur oxides

Sulfur dioxide

SO2

Sulfur trioxideSO3

Molecular mass 64 80

Melting point oC -75 16,8

Boiling point oC -10 43,7

density0 0C, 101.3 kPa

20 0C, 101.3 kPa

1.250 g/dm3

2.93 g/dm3

2.052 g/dm3

1,916 g/dm3

Solubility in water0 0C 101.3 kPa

80 dm3/ dm3 (97.7 ppmm)** decay

Conversion factors

0 0C, 101.3 kPa

1 mg/m3 = 2.663 ppmv***

1 ppmv = 0.376 mg/m3

Sulfur dioxide:• colorless• irritating odor (odor threshold 0,3-1 ppm)• soluble in water• natural background~1 ppb

Sulfur trioxide: • highly reactive => short lifetime in the atmosphere, sulfuric acid formation with water

Direct emission is mainly sulfur dioxide, only some percent of sulfur trioxide

Natural sources of sulfur Natural sources of sulfur

dioxidedioxide Volcanic action.

A volcanic gases: 90% water and

CO2, SO2 content 1-10 % .

Sulfur compounds from biological

activity

(CH3SCH3, H2S, CS2, COS) will be

oxidized in the atmosphere.

Sulfur emission in highest amount:

dimethyl-szulfide from the oceans →

biological activity of fitoplankton.

Sulfur dioxide from human activitySulfur dioxide from human activity

Main source: combustion of fossile fuels

Sulfur content of coal and crude oil differs.

Crude oil: mainly organic, (sulfides, mercaptenes, bisulfides,

tiofenes) => can be removed by simple technology.

Coal:

• pyritic (FeS), removable by physical method ,

• sulfates (CaSO4, FeSO4) removable by physical method (no decay at

combustion temperature to SO2)

• matrix sulfur a incorporated in polymer molecule

Atmospheric sulfur content is mainly anthropogenic origin.

Sulfur dioxide from human activitySulfur dioxide from human activityAnother significant source: smelter operation

Cu, Zn, Cd, Pb occur in the nature mainly in sulfide ore form. Can

not be reduced directly.

Roasting of sulfide ores on air produces oxides (ZnO, CdO, CuO….)

→ the oxides can be reduced

Cu, Ni:

roasting: only the metal content can be concentrated

Final sulfur elimination: air blowing through the melted sulfur

contaminated ore

Sulfur content is transformed to sulfur dioxide => the capture of SO2

depends on the level of technology.

Sulfur dioxide from human activitySulfur dioxide from human activity

smelter operation : far from built-up area.

Not a solution due to the global character of sulfur

oxide pollution !

Chemistry of sulfur oxide Chemistry of sulfur oxide

formationformation

O + S2 = SO + S

O + SO = SO2

Tüzelőanyag-S│

hő hatásra ▼ termikus bomlás┌─────────────────────┼─────────────────────┐

▼ ▼ ▼ H2S COS koksz-S

H2S + O = •OH + SH O + COS = SO + CO koksz-S + O → SO

SH + O = SO + H SO + O = SO2 koksz-S + CO2 → COS

SO + O = SO2 koksz-S + H2O → H2S

Burning of elemental sulfur: oxygen atom starts the reaction,

transition compound: sulfur monoxide.

Combustion of sulfur containing materials (fuel-S) :

thermal decay → char-S, (shrunken solid fuel) hydrogen sulfide (H2S) and carbonyl sulfide (COS) form.

All of the three products is oxidized to sulfur dioxideFuel-S

Heat effect Thermal decay

char-S

char-S

char-S

char-S

Chemistry of sulfur oxide Chemistry of sulfur oxide formationformation

O + SO2 <=> SO3

Further oxidation of sulfur dioxide → sulfur trioxide:

• oxygen atoms and hydroxyl radicals play a significant role.

• equilibrium shifts left with temperature increase

• the reaction rate is slow

• only 0.5-2.5% of SO2 is converted to SO3

• the time to reach the equilibrium depends of the catalytic effect of the metal content (W, Mo, V, Cr, Ni, Fe oxides) in ash

Chemistry of sulfur oxide Chemistry of sulfur oxide formationformation

The dew point of sulfuric acid depends on the SO3 and water content of the stack gas.

Dew point of sulfuric acid in function of SO3 and water concentration in stack gas ..and the damage

• Sulfur trioxide at 482 oC transforms to sulfuric acid. Under the dew point sulfuric acid condensates on the structure materials (heat exchanger, stack wall ).

stack gas temperature

Dimethyl sulfide, hydrogen sulfide, carbonyl Dimethyl sulfide, hydrogen sulfide, carbonyl sulfide, carbon disulfide in the atmospheresulfide, carbon disulfide in the atmosphere

Oxidized to sulfur dioxide

Oxidizing agent: hydroxyl radicals

Oxidation of dimethyl-sulfide → methane sulfonic acid aerosol (CH3SO3H) → serves as nuclei for cloud formation

carbonyl-sulfide:

Slow oxidation by hydroxyl radicals => oxidation in the stratosphere by atomic oxygen

Lifetime in years.

Hydrogen sulfide, carbonyl disulfide quick oxidation to SO2 in the troposphere

Catalytic transformation of sulfur Catalytic transformation of sulfur dioxide to sulfuric aciddioxide to sulfuric acid

SO2 dissolution

O2 dissolution Ash particles

catalyst

Photochemical transformation of Photochemical transformation of sulfur dioxide to sulfuric acidsulfur dioxide to sulfuric acid

Hydroxyl radicals oxidize the sulfur dioxide to sulfur trioxide.

O3 + light = O + O2

O + H2O = 2 OH•

OH• + SO2 + M = HSO3• + M*

HSO3• + O2 = HO2• + SO3

Sulfur trioxide and water → quick reaction → sulfuric acid

SO3 + H2O = H2SO4

▼

Acidic rain

Effects of acidic rainEffects of acidic rain

pH of rain: adjusted by the rate of natural acidic and basic materials

Usually acidic: solution of carbon dioxide (pH=5.56)

Generally accepted: pH under 5 is due to human activity .

The acidity surplus :

60-70% sulfuric acid from sulfur dioxide

The rest comes from nitric acid formed from nitric oxide

Some percent hydrochloric acid

pH of rain > pH of fog particles

Effects of acidic rain on plantsEffects of acidic rain on plants

Three stages can be distinguished

1. Mobilization of soluble plant nutrients, e.g. nitrogen compounds

2. Nutrient wash out by the rain water → nutrient shortage

3. Al 3+ ion liberation from the clay minerals due to the decreasing pH in the soil.

The free aluminum ion is toxic to the roots, weakens the immunizing system → secondary infections

toxic Non toxic▐

Soil pH

Effect of acidic rain on natural Effect of acidic rain on natural water I.water I.

The excess of H+ ion in rain water shifts the hydro carbonate equilibrium towards the formation of free carbon dioxide.

H+ + HCO3 ─ <=> H2O + CO 2

• The physically dissolved CO2 inhibit the OO22 ↔↔ CO CO22 exchange in living organisms : e.g. fishexchange in living organisms : e.g. fish). ).

• occurs at spring when the melted acidic snow flows suddenly into the rivers of catchments area.

• If natural water is in contact with limestone, dolomite, the pH does not change → buffer effect. The living organisms are killed by the increased CO2 content

• In case of week buffer effect (small Ca- and Mg-hydro carbonate content) the living organisms are killed by the decreased pH

Effect of acidic rain on natural water Effect of acidic rain on natural water II.II.

pH tolerance of waterborne organisms

The lack of mussels in natural waters may indicate the change of pH, they can not change theirs position

quickly

Effect of acidic rain on calcium carbonate Effect of acidic rain on calcium carbonate containing materials I.containing materials I.

• calcium carbonate containing materials: marble, limestone, plaster, concrete → sensitive to acidic rain

• in the last fifty year the weathering of open air ancient monuments speeded up

Effect of acidic rain on calcium Effect of acidic rain on calcium carbonate containing materialscarbonate containing materials II. II.

Effect: The infiltrating acidic rainwater contaminated by sulfuric acid changes the crystals of calcium carbonate to calcium sulfate

The solubility of calcium sulfate > solubility of calcium carbonate.

Crystal volume of CaSO4 > crystal volume of CaCO3

stress in the material structure → crack

CaCO3 + H2SO4 = CaSO4 + H2O + CO2

Effect of acidic rain on metal constructions Effect of acidic rain on metal constructions

I.I. Preliminary conditions of the electrochemical corrosion:

1. two metallic material with different electrochemical potential in metallic contact.

2. electrolyte cover on the metallic contact, (e.g.. water solutions of acids, salts)

3. presence of electron uptake material (H+ O2 Cl2 )

Electrochemical corrosion of metal:

Oxidation: the metal transform to ion and free electrons release.

Fe = Fe 2+ + 2 e-

Reduction: uptake of free electrons by

2H+ + 2e- = H2

O2 + 4H+ + 4e- = 2 H2O

H2O + CO2 + e- = H + CO32-

Cl2 + 2e- = 2 Cl-

Effect of acidic rain on metal Effect of acidic rain on metal constructions II.constructions II.Fe ─────> Fe2+ + 2 e-

2H+ + 2e- = H2

Air pollution induced electrochemical corrosion resulted in the collapse of Silver bridge over Ohio river on 15-th. Dec. 1967.

Acid rain: electrolyte and the hydrogen ion serves the reduction (electron uptake)

Effect of atmospheric SOEffect of atmospheric SO22 on on paperspapers

Paper surface H2SO4 formation on the surface The result

The paper gets yellow and brittle.

No damag

e

Destroyed surface

adsorption

desorption

Fe catalyst

Anthropogenic effects of SOAnthropogenic effects of SO22

Good solubility in water → the effect on the upper part of the respiratory system.

Irritating effect over 10 ppm

Nonstop irritation of mucous lining in urban air results in frequent colds, flu

Control of sulfur dioxide Control of sulfur dioxide emissionemission

Power plants based on fossil fuels produce sulfur oxide emission

Transportation is based on fossil fuel but the emission is not so serious

The sulfur can be removed easily from liquid material

The anthropogenic emission decreased half of the original one since.

Sulfur dioxide easily can be eliminated from the stack gas

Problems to be solved: fate of the sulfur containing product

SO2 emission controls are divided into two categories:

Reduction of the sulfur content of the fuel,

The sulfur dioxide is removed from the exhaust gas.

Power plantsPower plants Simplest way: replacement of high sulfur content

fuel with low sulfur content fuel

e.g. coal fired power station → gas fired power plant.

Not a final solution: the available gas resources are restricted

Coal might be the fuel of future again

Sulfur content reduction of solid Sulfur content reduction of solid

fuelfuel Sulfur content of the coal: <1% . . . 10-12%

Form of sulfur:

1. pyritic sulfur (FeS2)

2. sulfate sulfur (e.g.. iron- and calcium sulfate)

3. matrix sulfur. (sulfur in organic bond ) Extraction of sulfur:

1.and 2. by simple technology (e.g.. flotation) , based on density difference of coal and pyrite, sulfate

3. chemically bonded => physical methods can not be applied.

Direct chemical treatment e.g. NaOH is not economical

Gasification of coal by air and/or water. Removal of the formed hydrogen sulfide H2S from the gas.

Sulfur content reduction of liquid and Sulfur content reduction of liquid and gas fuelgas fuel

Desulphurization:

Hydro processing: evaporated fractions of crude oil and hydrogen is in contact with catalyst Co/Mo to transform organic sulfur to hydrogen sulfide at high pressure.

Hydro processing might be

Destructive: carbon chain crack and sulfur transformation

Nondestructive: to improve the quality of oil fractions

Sulfur content → hydrogen sulfide

Nitrogen content → ammonia

Hydrogen sulfide removalHydrogen sulfide removal

Physical absorption at low temperature minus 30-120 °C (methanol, dimethyl ether)

Chemical adsorption (organic amines, metal oxides)

Reversible processes → the absorbed and adsorbed hydrogen sulfide can be recovered in concentrated form

The concentrated hydrogen sulfide → Claus plant

Hidrogen sulfide treatment by Claus Hidrogen sulfide treatment by Claus

processprocess Concentrated gas (> 50..60 vol % H2S) is partially

oxidized to sulfur and water2 H2S + O2 = S + 2

H2O• 2/3 part of the H2S concentrated gas is oxidized and combined with the rest

2 H2S + 2 O2 = SO2 + 2 H2O

2 H2S + SO2 = 2 S + 2 H2O

After cooling the gas mixture is feed into the Claus reactor at 200-350 0C(catalist: aluminum oxide) to increase the conversion.

→ 1000-1400 0C

→ after cooling the sulfur can be separated

Sulfur dioxide removal from the flue Sulfur dioxide removal from the flue gas gas

by wet processby wet process

Füstgázkezelés vizes mész szuszpenzióvalCaO + H2O = Ca(OH)2

SO2 + Ca(OH)2 + H2O = CaSO3∙2H2O

CaSO3∙2H2O + ½ O2 = CaSO4∙2H2O

-----------------------------------------------------------Füstgázkezelés vizes mészkő szuszpenzióval

CaCO3 + H2O + 2 SO2 = Ca(HSO3)2 + CO2

CaCO3 + Ca(HSO3)2 + H2O = CaSO3∙2H2O + CO2

CaSO3∙2H2O + ½ O2 = CaSO4∙2H2O

reactant: lime CaO / limestone CaCO3 → product: CaSO4 (gypsum)

Cheap raw material → 80 % of SO2 cleaning technologies are based on this one Wet scrubbers, at 70 – 90

0C

efficiency 90-95%

Flue gas treatment by lime suspension in water

Flue gas treatment by limestone suspension in water

Wet scrubber for SOWet scrubber for SO22 removal removal

Flue gas desulphurization (FGD) Flue gas desulphurization (FGD) regenerable regenerable adsorbentadsorbent

ScrubberNa2SO3 + SO2 + H2O = 2 NaHSO3

Na2SO3 + ½ O2 = Na2SO4

-------------------------------------------Regenerator

2 NaHSO3 = Na2SO3 + SO2 + H2O

-------------------------------------------Solution from the make up in the

scrubberNa2CO3 + SO2 = 2 Na2SO3 + CO2

2 NaOH + SO2 = 2 Na2SO3 + H2O

Wellman-Lord technology, advantages: Not too much byproduct Extracted sulfur dioxide in concentrated form