pollution
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Reducing ground level air pollution
In order to improve the air quality in urban environments - subjected to air pollution
The amount of reactants must be reduced
Such reactants:
SO2, NOx and hydrocarbons containing C=C bonds plus other VOCs
Most common strategy has been to reduce hydrocarbon emissions.
Control of emissions of all VOCs is required in areas with serious photochemical smog problems
Gasolines - complex mixture of hydrocarbons – contribute significantly to atmospheric concentrations of hydrocarbons.
They are now formulated in order to reduce their evaporation
New regulations – limiting the use of hydrocarbons-containing products e.g.,
The household aerosol sprays and
Oil-based paints that consist partially of a hydrocarbon solvent that evaporates into the air as the paints dries
For example, the air quality in California and Los Angeles, has improved because of current emission controls
The creation of nitric oxides can be reduced by lowering the temperature of the flame
In recent years a more complete control of NOx emissions from gasoline-powered cars and trucks
Attempted using catalytic converters placed just ahead of the mufflers in the exhaust system.
The original two-way converters dealt only with the carbon-based gases
Including carbon monoxide CO completing their combustion to the end product, carbon dioxide (CO2)
However, the modern three-way converter – via a rhodium catalyst
First changes nitrogen oxides back to elemental nitrogen and oxygen, using unburnt hydrocarbons and CO and H2 as reducing agents
2NO. →N2 + O2
Then with a palladium and/or platinum catalyst
The carbon-containing gases are oxidized almost completely to CO2 and water
2 CO + O2 → 2CO2
An oxygen sensor in the exhaust system is monitored by a computer chip that controls the air/fuel ratio of the engine to ensure a high level of conversion of the pollutants
The whole process is illustrated in Figure 1, next slide
Fig. 1 Schematic representation of the three-way catalytic converter
Properly working three-way catalysts eliminate 80-90% of the
Hydrocarbons
CO and
NOx from the engine before the exhaust gases are released into the atmosphere.
Note: Before the engine has warmed up & episodes of sudden acceleration or deceleration
The converters cannot operate effectively and there are burst of emissions from the tailpipe
In additions, older cars - with no converters or just two-way converters continue to pollute the atmosphere with nitrogen oxides even during normal operation
In some countries, the governments have instituted mandatory inspections of exhaust systems to control the amount of emissions
Tanzania?????
In North America, approximately equal amounts of NOx are emitted from vehicles and from electric power plants
To reduce their NOx production, some power plants in USA use special burners designed to lower the temperature of the flame
Other plants have been fitted with large-scale versions of catalytic converters
To change NOx back to N2 before the release of stack gases into air
The reduction of NOx to N2 in some of this catalytic systems is accomplished by adding ammonia, NH3, to the gas stream
This highly reduced compound of nitrogen combines with the partially oxidized compound NO
To produce N2 gas in the presence of O2
4NH3 + 4 NO + O2→ 4N2 + 6 H2O
Tight control is needed to regulate the addition of ammonia in order to prevent its inadvertent oxidation to NOx.
In a related technology
Reduced nitrogen in the form of the compound Urea, CO(NH2)2 is injected directly into the combustion flame to combine there, rather than later in the presence of a catalyst, with NO to produce N2.
2CO(NH2)2 + NO • + O2 →4N2 + 6H2O
SO2 emission from metal smelting and coal combustion have also been controlled recently
Possible strategies to prevent environmental damage by a pollutant includes
(a) minimizing the production of the pollutant
(b) dilution of the pollutant so that it is no longer harmful
(c) Conversion of the pollutant to a harmless or better, useable substance.
Abatement measure have been directed toward
dilution, or
Preventing emission by removal of sulfur - either from the unburned coal or from the effluent gases
Dilution consists of building tall stacks at the power plant, so as to disperse the effluent gases.
While this spares the immediate locale from the most serious effects of SO2 pollution
The gas must ultimately be oxidized and deposited somewhere else
Coal cleaning
Minimizing the production of SO2
By taking advantage of the different density of coal (2.3 g cm-3) and the major sulphide mineral iron pyrite, FeS2 (4.5 g cm-3)
If the coal is finely ground, most of the particles of coal will be physically distinct from the particles of pyrites.
They may then be separated on the basis of density, using the technique of oil flotation
In which the particles are vigorously agitated with air and water, to which has been added oil and a small amount of surfactant.
Oil flotation is generally useful for the separation of large quantities of solids having different densities
The composition of the water/oil/surfactant phase is adjusted
So that the less dense particles are carried by surface tension at the air/liquid interface
And the denser particles are allowed to sink
N.B. Flotation – Only effective at removing inorganic sulfur (e.g., iron pyrites, from coal, but it leaves behind any sulfur that is organically bound.
Chemical conversion
This involves the capture of SO2 before it is released from the stack
Since SO2 is an acidic gas, the method of choice is to trap it with base, such as lime (Ca(OH)2) or limestone (CaCO3)
Bases such as NaOH or KOH which we might choose in the laboratory are too expensive for industrial use.E.g., Flue Desulphurization method
“scrubbing” of the stack gases with base by passing a fine spray of lime or limestone slurry in water down the stack as the hot combustion gases are passing upwards:
SO2 + Ca(OH)2 (g) → CaSO3 (s) + H2O(l)
SO2 + CaCO3 (s) → CaSO3 (s) +CO2(g)
Scrubbing equipment is expensive to install and maintain, but the cost of lime or limestone is considerable
Only 70-90% of the SO2 is removed
An additional problem is the disposal of the thousands of tonnes per year of the CaSO3
By-product which is obtained as an aqueous slurry
Perhaps 98% + water,This must be “dewatered” in huge holding tanks and eventually land-filled
It represent a serious environmental problem itself.
Metal Extraction
SO2 emission result when sulfides ores are roasted in air
- Sulfide ores – E.g., FeS2, CuFeS2, NiS and CuS
The size of smelter determines its environmental impact
- For example, the Inco Ltd smelter at Sudbury, Ontario processes about 35, 000 tonnes of ores per day
The ore is initially concentrated by oil flotation to remove the waste rock (SiO2, Al2O3 etc), known as gangue
The various sulfides – nickel, copper, and iron - can also be separated by flotation on the basis of their densities
The production of metals involves roasting the separated sulfide ores.
Sulfur dioxide results from roasting the concentrated ore as shown below
2NiS (s) + 3O2 → 2NiO (s) + 2SO2 (g)
The roasting of ores with pure oxygen, rather than with air
Has facilitated the subsequent recovery of SO2
Recovered as liquid SO2 for resale and
It is also oxidized to sulfuric acid
- Most of which is used elsewhere in the production of fertilizers
SO2 (g) + ½ O2 SO3 (g)
SO3 (g) H2SO4. SO3(L) H2SO4
The substitution of oxygen for air in the roasting process enhances the conversion of SO2 to SO3, by providing a “higher strength” SO2 gas stream
The environmental considerations dictate that the conversion of SO3 to H2SO4 is carried out in two steps
The SO3 is dissolved in concentrated acid to give “oleum”
- This reaction occurs rapidly and efficiently
Water is then added to produce the conventional concentrated sulfuric acid (93% H2SO4)
The dissolution of SO3 in water to give sulfuric acid directly is rather slow
- And it is difficult to ensure that all the SO3 is trapped by the water
Principally, sulfuric acid manufacture is attractive in that it converts a polluting waste product (SO2) into a useful commodity (H2SO4)
However, the economics of the process are very unfavourable
- First, the capital costs of the sulfuric acid manufacturing part of the operation are very expensive
The greater the proportion of the sulfur dioxide that is to be trapped, the more expensive the facility
Second, the selling price of smelter grade sulfuric acid is very low
Thus there is a dilemma of a high manufacturing cost combined with a low selling price for the product
Smelter grade sulphuric acid must compete with purer sulfuric acid
Manufactured from elemental sulfur
- Elemental sulfur is very cheaply available.
The other pollutant which is associated with SO2 is H2S
The substantial amounts of H2S can be converted to solid, elemental sulphur, an environmentally benign substance, using the gas-phase process called the Claus reaction:
2H2S +SO2 3S + 2H2O
Control of particles
Control of particulate emissions is most practical for industrial facilities
E.g., coal-burning power plants
cement manufacturing
Metal smelters, and
Incinerators
The two leading technologies for removing particles from effluent gas stream are
Woven fabric bags and
Electrostatic precipitators.
The Woven fabric bags method is similar to the system used to collect dust in a household vacuum cleaner
Finely woven containers are used to trap the particles while allowing the passage of air
Electrostatic precipitators work on the principle of charge neutralization
When particle rub together, their surfaces acquire an electrostatic charge
Chemically similar particles gain charges of like sign and hence will not coagulate.
In the electrostatic precipitator, the gas stream travels down a tube containing a central rod
Charged particles are attracted to the electrode of unlike relative charge, where they are discharged
Once neutral, the particles are large enough to settle, and to be collected
Reducing ground level air pollution
In order to improve the air quality in urban environments - subjected to air pollution
The amount of reactants must be reduced
Such reactants:
SO2, NOx and hydrocarbons containing C=C bonds plus other VOCs
Most common strategy has been to reduce hydrocarbon emissions.
Control of emissions of all VOCs is required in areas with serious photochemical smog problems
Gasolines - complex mixture of hydrocarbons – contribute significantly to atmospheric concentrations of hydrocarbons.
They are now formulated in order to reduce their evaporation
New regulations – limiting the use of hydrocarbons-containing products e.g.,
The household aerosol sprays and
Oil-based paints that consist partially of a hydrocarbon solvent that evaporates into the air as the paints dries
For example, the air quality in California and Los Angeles, has improved because of current emission controls
The creation of nitric oxides can be reduced by lowering the temperature of the flame
In recent years a more complete control of NOx emissions from gasoline-powered cars and trucks
Attempted using catalytic converters placed just ahead of the mufflers in the exhaust system.
The original two-way converters dealt only with the carbon-based gases
Including carbon monoxide CO completing their combustion to the end product, carbon dioxide (CO2)
However, the modern three-way converter – via a rhodium catalyst
First changes nitrogen oxides back to elemental nitrogen and oxygen, using unburnt hydrocarbons and CO and H2 as reducing agents
2NO. →N2 + O2
Then with a palladium and/or platinum catalyst
The carbon-containing gases are oxidized almost completely to CO2 and water
2 CO + O2 → 2CO2
An oxygen sensor in the exhaust system is monitored by a computer chip that controls the air/fuel ratio of the engine to ensure a high level of conversion of the pollutants
The whole process is illustrated in Figure 1, next slide
Fig. 1 Schematic representation of the three-way catalytic converter
Properly working three-way catalysts eliminate 80-90% of the
Hydrocarbons
CO and
NOx from the engine before the exhaust gases are released into the atmosphere.
Note: Before the engine has warmed up & episodes of sudden acceleration or deceleration
The converters cannot operate effectively and there are burst of emissions from the tailpipe
In additions, older cars - with no converters or just two-way converters continue to pollute the atmosphere with nitrogen oxides even during normal operation
In some countries, the governments have instituted mandatory inspections of exhaust systems to control the amount of emissions
Tanzania?????
In North America, approximately equal amounts of NOx are emitted from vehicles and from electric power plants
To reduce their NOx production, some power plants in USA use special burners designed to lower the temperature of the flame
Other plants have been fitted with large-scale versions of catalytic converters
To change NOx back to N2 before the release of stack gases into air
The reduction of NOx to N2 in some of this catalytic systems is accomplished by adding ammonia, NH3, to the gas stream
This highly reduced compound of nitrogen combines with the partially oxidized compound NO
To produce N2 gas in the presence of O2
4NH3 + 4 NO + O2→ 4N2 + 6 H2O
Tight control is needed to regulate the addition of ammonia in order to prevent its inadvertent oxidation to NOx.
In a related technology
Reduced nitrogen in the form of the compound Urea, CO(NH2)2 is injected directly into the combustion flame to combine there, rather than later in the presence of a catalyst, with NO to produce N2.
2CO(NH2)2 + NO • + O2 →4N2 + 6H2O
SO2 emission from metal smelting and coal combustion have also been controlled recently
Possible strategies to prevent environmental damage by a pollutant includes
(a) minimizing the production of the pollutant
(b) dilution of the pollutant so that it is no longer harmful
(c) Conversion of the pollutant to a harmless or better, useable substance.
Abatement measure have been directed toward
dilution, or
Preventing emission by removal of sulfur - either from the unburned coal or from the effluent gases
Dilution consists of building tall stacks at the power plant, so as to disperse the effluent gases.
While this spares the immediate locale from the most serious effects of SO2 pollution
The gas must ultimately be oxidized and deposited somewhere else
Coal cleaning
Minimizing the production of SO2
By taking advantage of the different density of coal (2.3 g cm-3) and the major sulphide mineral iron pyrite, FeS2 (4.5 g cm-3)
If the coal is finely ground, most of the particles of coal will be physically distinct from the particles of pyrites.
They may then be separated on the basis of density, using the technique of oil flotation
In which the particles are vigorously agitated with air and water, to which has been added oil and a small amount of surfactant.
Oil flotation is generally useful for the separation of large quantities of solids having different densities
The composition of the water/oil/surfactant phase is adjusted
So that the less dense particles are carried by surface tension at the air/liquid interface
And the denser particles are allowed to sink
N.B. Flotation – Only effective at removing inorganic sulfur (e.g., iron pyrites, from coal, but it leaves behind any sulfur that is organically bound.
Chemical conversion
This involves the capture of SO2 before it is released from the stack
Since SO2 is an acidic gas, the method of choice is to trap it with base, such as lime (Ca(OH)2) or limestone (CaCO3)
Bases such as NaOH or KOH which we might choose in the laboratory are too expensive for industrial use.E.g., Flue Desulphurization method
“scrubbing” of the stack gases with base by passing a fine spray of lime or limestone slurry in water down the stack as the hot combustion gases are passing upwards:
SO2 + Ca(OH)2 (g) → CaSO3 (s) + H2O(l)
SO2 + CaCO3 (s) → CaSO3 (s) +CO2(g)
Scrubbing equipment is expensive to install and maintain, but the cost of lime or limestone is considerable
Only 70-90% of the SO2 is removed
An additional problem is the disposal of the thousands of tonnes per year of the CaSO3
By-product which is obtained as an aqueous slurry
Perhaps 98% + water,This must be “dewatered” in huge holding tanks and eventually land-filled
It represent a serious environmental problem itself.
Metal Extraction
SO2 emission result when sulfides ores are roasted in air
- Sulfide ores – E.g., FeS2, CuFeS2, NiS and CuS
The size of smelter determines its environmental impact
- For example, the Inco Ltd smelter at Sudbury, Ontario processes about 35, 000 tonnes of ores per day
The ore is initially concentrated by oil flotation to remove the waste rock (SiO2, Al2O3 etc), known as gangue
The various sulfides – nickel, copper, and iron - can also be separated by flotation on the basis of their densities
The production of metals involves roasting the separated sulfide ores.
Sulfur dioxide results from roasting the concentrated ore as shown below
2NiS (s) + 3O2 → 2NiO (s) + 2SO2 (g)
The roasting of ores with pure oxygen, rather than with air
Has facilitated the subsequent recovery of SO2
Recovered as liquid SO2 for resale and
It is also oxidized to sulfuric acid
- Most of which is used elsewhere in the production of fertilizers
SO2 (g) + ½ O2 SO3 (g)
SO3 (g) H2SO4. SO3(L) H2SO4
The substitution of oxygen for air in the roasting process enhances the conversion of SO2 to SO3, by providing a “higher strength” SO2 gas stream
The environmental considerations dictate that the conversion of SO3 to H2SO4 is carried out in two steps
The SO3 is dissolved in concentrated acid to give “oleum”
- This reaction occurs rapidly and efficiently
Water is then added to produce the conventional concentrated sulfuric acid (93% H2SO4)
The dissolution of SO3 in water to give sulfuric acid directly is rather slow
- And it is difficult to ensure that all the SO3 is trapped by the water
Principally, sulfuric acid manufacture is attractive in that it converts a polluting waste product (SO2) into a useful commodity (H2SO4)
However, the economics of the process are very unfavourable
- First, the capital costs of the sulfuric acid manufacturing part of the operation are very expensive
The greater the proportion of the sulfur dioxide that is to be trapped, the more expensive the facility
Second, the selling price of smelter grade sulfuric acid is very low
Thus there is a dilemma of a high manufacturing cost combined with a low selling price for the product
Smelter grade sulphuric acid must compete with purer sulfuric acid
Manufactured from elemental sulfur
- Elemental sulfur is very cheaply available.
The other pollutant which is associated with SO2 is H2S
The substantial amounts of H2S can be converted to solid, elemental sulphur, an environmentally benign substance, using the gas-phase process called the Claus reaction:
2H2S +SO2 3S + 2H2O
Control of particles
Control of particulate emissions is most practical for industrial facilities
E.g., coal-burning power plants
cement manufacturing
Metal smelters, and
Incinerators
The two leading technologies for removing particles from effluent gas stream are
Woven fabric bags and
Electrostatic precipitators.
The Woven fabric bags method is similar to the system used to collect dust in a household vacuum cleaner
Finely woven containers are used to trap the particles while allowing the passage of air
Electrostatic precipitators work on the principle of charge neutralization
When particle rub together, their surfaces acquire an electrostatic charge
Chemically similar particles gain charges of like sign and hence will not coagulate.
In the electrostatic precipitator, the gas stream travels down a tube containing a central rod
Charged particles are attracted to the electrode of unlike relative charge, where they are discharged
Once neutral, the particles are large enough to settle, and to be collected
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