Transcript 1. dia

BUDAPEST
FACULTY
UNIVERSITY OF TECHNOLOGY AND ECONOMICS
OF CHEMICAL AND
BIOCHEMICAL ENGINEERING
DEPARTMENT
OF CHEMICAL AND
PROCESS
ENVIRONMENTAL
ENGINEERING
Sulfur oxides
authors: Dr. Bajnóczy Gábor
Kiss Bernadett
The pictures and drawings of this
presentation can be used only for
education !
Any commercial use is prohibited !
Physical properties of atmospheric
sulfur oxides
Sulfur dioxide:
• colorless
• irritating odor (odor threshold 0,3-1
ppm)
• soluble in water
• natural background~1 ppb
Sulfur dioxide Sulfur trioxide
SO2
SO3
Molecular mass
Melting point
oC
Boiling point
Sulfur trioxide:
• highly reactive => short lifetime in
the atmosphere, sulfuric acid
formation with water
oC
density
0
101.3 kPa
20 0C, 101.3 kPa
0C,
64
80
-75
16,8
-10
43,7
1.250 g/dm3
2.93 g/dm3
Solubility in water
0 0C 101.3 kPa
80 dm3/ dm3 (97.7
ppmm)**
Conversion
factors
0
0 C, 101.3 kPa
1 mg/m3 = 2.663
ppmv***
1 ppmv = 0.376
mg/m3
2.052 g/dm3
1,916 g/dm3
decay
Direct emission is mainly sulfur dioxide, only some percent of
sulfur trioxide
Natural sources of sulfur dioxide

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 activity
Atmospheric sulfur content is mainly anthropogenic origin.
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
Sulfur dioxide from human activity
Another 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 activity
smelter operation : far from built-up area.
Not a solution due to the global character of sulfur
oxide pollution !
Chemistry of sulfur oxide formation
Burning of elemental sulfur: oxygen atom starts the reaction,
transition compound: sulfur monoxide.
O + S2 = SO + S
O + SO = SO2
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 dioxide
Tüzelőanyag-S
Fuel-S
│
hőeffect
hatásra ▼ termikus
bomlás
Heat
Thermal
decay
┌─────────────────────┼─────────────────────┐
▼
▼
▼
char-S
H2S
COS
koksz-S
H2S + O = •OH + SH
O + COS = SO + CO
koksz-S
char-S + O → SO
SH + O = SO + H
SO + O = SO2
koksz-S
char-S + CO2 → COS
SO + O = SO2
koksz-S
char-S + H2O → H2S
Chemistry of sulfur oxide formation
Further oxidation of sulfur dioxide → sulfur trioxide:
O + SO2 <=> SO3
• 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 formation
• Sulfur trioxide at 482 oC transforms to sulfuric acid. Under the dew point sulfuric
acid condensates on the structure materials (heat exchanger, stack wall ).
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
stack gas temperature
..and the damage
Dimethyl sulfide, hydrogen sulfide, carbonyl sulfide,
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 dioxide to
sulfuric acid
SO2
dissolution
catalyst
O2
dissolution
Ash particles
Photochemical transformation of sulfur
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 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 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 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 O2 ↔ CO2 exchange 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 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
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 carbonate
containing materials II.

Effect: The infiltrating acidic rainwater contaminated
by sulfuric acid changes the crystals of calcium
carbonate to calcium sulfate
CaCO3 + H2SO4 = CaSO4 + H2O + CO2


The solubility of calcium sulfate > solubility of
calcium carbonate.
Crystal volume of CaSO4 > crystal volume of CaCO3
stress in the material structure → crack
Effect of acidic rain on metal constructions 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 constructions II.
Fe ─────> Fe2+ + 2 e2H+ + 2e- = H2
Acid rain: electrolyte and the hydrogen ion serves the reduction (electron uptake)
Air pollution induced electrochemical corrosion resulted in the
collapse of Silver bridge over Ohio river on 15-th. Dec. 1967.
Effect of atmospheric SO2 on papers
The paper gets yellow and brittle.
Paper surface
H2SO4 formation on the surface
adsorption
Fe catalyst
desorption
Destroyed
surface
No
damage
The result
Anthropogenic effects of SO2

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 emission

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 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 fuel

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 gas 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 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 process

Concentrated gas (> 50..60 vol % H2S) is partially oxidized to
sulfur and water
2 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
→ 1000-1400 0C
→ after cooling the sulfur can be
separated
After cooling the gas mixture is feed into the Claus reactor at 200-350 0C(catalist:
aluminum oxide) to increase the conversion.
Sulfur dioxide removal from the flue gas
by wet process
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
Flue
gas treatmentvizes
by lime
suspension
in water
Füstgázkezelés
mész
szuszpenzióval
CaO + H2O = Ca(OH)2
SO2 + Ca(OH)2 + H2O = CaSO3∙2H2O
CaSO3∙2H2O + ½ O2 = CaSO4∙2H2O
----------------------------------------------------------Flue
gas treatment vizes
by limestone
suspension
in
Füstgázkezelés
mészkő
szuszpenzióval
water
CaCO3 + H2O + 2 SO2 = Ca(HSO3)2 + CO2
CaCO3 + Ca(HSO3)2 + H2O = CaSO3∙2H2O +
CO2
CaSO3∙2H2O + ½ O2 = CaSO4∙2H2O
efficiency 90-95%
Wet scrubber for SO2 removal
Flue gas desulphurization (FGD) regenerable
adsorbent

Wellman-Lord technology, advantages:


Not too much byproduct
Extracted sulfur dioxide in concentrated form
Scrubber
Na2SO3 + SO2 + H2O = 2 NaHSO3
Na2SO3 + ½ O2 = Na2SO4
------------------------------------------Regenerator
2 NaHSO3 = Na2SO3 + SO2 + H2O
------------------------------------------Solution from the make up in the
scrubber
Na2CO3 + SO2 = 2 Na2SO3 + CO2
2 NaOH + SO2 = 2 Na2SO3 + H2O