Control of Gaseous pollutants
M.Sc. Environmental Science (II Semester)
Course Name: Air pollution: causes, consequences and control
Course coordinator: Dr.N.L.Devi
Department of Environmental Science
Central University of South Bihar, Gaya
Note: Class PPT for M.Sc. Environmental Sc. Student.
Materials compiled from different sources for the teaching purposes only
Adsorption methods for VOC and HAP
• The adsorption method applied in solids, usually in granular form, are
brought in contact with gaseous or liquid mixtures.
• In this process the molecules of a fluid gas adhere to the surface of a
solid. The material being adsorbed is called the adsorbate and the
adsorption system is called the adsorber.
• Adsorption method can be used effectively to separate gases from
gases, solids from liquid, ions from liquid, and dissolved gases from
liquid
The contaminated gaseous constituents should be adequately removed
from airstreams for air pollution control :-
Different adsorbents
Activated carbon: solvent recovery, elimination of odors, purification
of gases
Alumina: drying of gases, air, and liquids
Bauxite: treatment of petroleum fractions; drying of gases and liquids
Bone char: decolorizing of sugar solutions
Decolorizing adsorbents: decolorizing of oils, fats, and waxes;
deodorizing of domestic water
Fuller’s earth: refining of lube oils and vegetable and animals oils,
fats, and waxes
Magnesia: treatment of gasoline and solvents; removal of metallic
impurities from caustic solutions
Silica gel: drying and purification of gases
Strontium sulfate: removal of iron from caustic solutions
Different adsorbents
Note: Class PPT for M.Sc. Environmental Sc. Student.
Materials compiled from different sources for the teaching purposes only
• The exothermic process is applied in Carbon adsorption. It is a
physicochemical process during which heat is liberated and the
temperature of the adsorbent bed increases. As a result, it may be
necessary to provide cooling for the carbon bed.
• Adsorption power of activated charcoal is mainly the result of
molecular capillary condensation, whereas the adsorption power of
silica gel is mainly the result of capillary condensation
Adsorption methods for VOC and HAP
Note: Class PPT for M.Sc. Environmental Sc. Student.
Materials compiled from different sources for the teaching purposes only
• VOCs with lower vapor pressures are more easily adsorbed than those
with higher vapor pressures. the vapor pressure of VOC is inversely
proportional to the molecular weight of the compound.
• the heavier VOCs will tend to be more easily adsorbed than the lighter
VOCs
Adsorption methods for VOC and HAP
Adsorption temperature for VOCs
Adsorption methods for VOC and HAP
Adsorption methods for VOC and HAP
Fixed- bed adsorber
Adsorption methods for VOC and HAP
Lower temperatures provide for a more favorable condition for
adsorption of VOCs. When emission stream temperatures are
significantly higher than 130ºF, a heat exchanger may be used to
lower the temperature of the emission stream to 130ºF or less
Cooling
Pre-treatment for adsorption
Adsorption methods for VOC and HAP
1
Emitted gas streams may contain both water vapor and VOCs.
When the humidity level exceeds 50% (relative humidity) in the
emission stream, the efficiency of the adsorption may be limited
for a dilute emission stream.
Dehumidif
ication 2
In absorption process soluble gaseous component is removed from a gas
stream by dissolution in a solvent liquid
It is one of the most convenient methods for removal of water-soluble
gases and effective recovery method in the chemical and petroleum
industry
Absorption methods for VOC and HAP
Note: Class PPT for M.Sc. Environmental Sc. Student.
Materials compiled from different sources for the teaching purposes only
Control technique employed for inorganic vapors.
• Hydrochloric acid vapor in water
• Mercury vapor in brine and hypochlorite solution
• Hydrogen sulfide vapor in sodium carbonate and water
• Hydrofluoric acid vapor in water
• Chlorine gas in alkali solution
Absorption methods
Absorption column
Thermal Oxidation for VOC Control
thermal
oxidation —
flares
thermal
oxidation
and
incineration
catalytic
oxidation
VOC control by Thermal Oxidation
Note: Class PPT for M.Sc. Environmental Sc. Student.
Materials compiled from different sources for the teaching purposes only
Thermal oxidation-Flaring is a incineration process in
which VOCs are piped to a remote location and burned in
either an open or an enclosed flame.
It can be used to control a wide variety of flammable VOC
streams, and can handle large fluctuations in VOC
concentration, flow rate, and heating value
thermal
oxidation —
flares
VOC control by Thermal Oxidation
• Thermal oxidation in an incinerator is the high destruction
efficiency that can be obtained by proper control of the
combustion chamber design
and operation.
• Temperatures are maintained above 1800°F, greater than
99% hydrocarbon destruction is routinely achievable
VOC control by Thermal Oxidation
thermal
oxidation
and incineration
catalytic
oxidation
• It destroy hydrocarbon vapors at lower temperatures and can
operates in temp. ranges from 400 to 650°F
• It incorporate a recuperative heat exchanger for heat
recovery to save additional fuel cost and system consists of a
hot gas heat exchanger, a thermal preheat zone with a
standard burner
• Metals such as platinum and palladium may be used as
catalysts for VOC oxidation
VOC control by Thermal Oxidation
Catalytic oxidation system
• Biofilter consists of a bed of soil or compost beneath provide environmental
media to survive microorganism
• Effective systems for removing pollutants from gaseous streams, VOCs range
of 65 to 99%
• Microorganism such as fungi, bacteria, and actinomycetes are used and
organic substrate provides the salts and trace elements for the bacteria, and
the VOC provides the food source
VOC and HAP control by biofiltration
Biofiler
VOC and HAP control by biofiltration
• Flue gas flows upward through the bed, VOCs sorb onto the organic surface of
the soil or compost. The sorbed gases are oxidized by the microorganisms to
CO2. The volatile inorganics are also sorbed and oxidized to form calcium
salts.
• The pH in the biofilter should remain near neutral, in the range of 7 to 8
• Microorganisms’ activity and growth is optimal in a temperature range of 10 to
40°C
• Microorganisms require nutrients to growth and perform the removal activity
such as nitrogen, phosphorous, and some trace metals
VOC and HAP control by biofiltration
Note: Class PPT for M.Sc. Environmental Sc. Student.
Materials compiled from different sources for the teaching purposes only
SOx control
• Sulfur occurs naturally in fuels ,it bound as iron pyrite in coal, FeS2,
mineral sulfates, elemental sulfur, and in organic compounds and
mercaptans. High sulfur coals typically contain 2 to 5% sulfur.
• sulfur emissions include petroleum refining, oil and gas production,
sulfur and sulfuric acid manufacturing, ore smelting, waste
incineration, and petroleum coke calcining.
Flue Gas Desulfurization Processes
The major flue gas desulfurization ( FGD ),
processes are :
Limestone Scrubbing
Lime Scrubbing
Dual Alkali Processes
Lime Spray Drying
Wellman-Lord Process
SOx control
Wet limestone scrubbing is the workhorse process for coal-fired electric utility power
plants. The high capital cost and the cost of operating and maintaining a complex system is
offset by the low cost of limestone used to remove very large quantities of SO2
Limestone slurry is sprayed on the incoming flue gas. The sulfur dioxide gets absorbed
The limestone and the sulfur dioxide react as follows :
CaCO3 + H2O + 2SO2 ----> Ca+2 + 2HSO3-+ CO2
CaCO3 + 2HSO3-+ Ca+2 ----> 2CaSO3 + CO2 + H2O
SOx control
Limestone Scrubbing
The lime scrubbing processes are similar to those in limestone scrubbing Lime
Scrubbing offers better utilization of the reagent. The operation is more flexible.
The major disadvantage is the high cost of lime compared to limestone.
The reactions occurring during lime scrubbing are :
CaO + H2O -----> Ca(OH)2
SO2 + H2O <----> H2SO3
H2SO3 + Ca(OH)2 -----> CaSO3.2 H2O
CaSO3.2 H2O + (1/2)O2 -----> CaSO4.2 H2O
SOx control
Lime Scrubbing
The limestone and lime scrubbing technique leads to deposits inside spray tower.
The deposits can lead to plugging of the nozzles through which the scrubbing slurry is
sprayed.
The Dual Alkali system uses two regents to remove the sulfur dioxide.
Sodium sulfite / Sodium hydroxide are used for the absorption of sulfur dioxide inside the
spray chamber.
The resulting sodium salts are soluble in water,so no deposits are formed.
The spray water is treated with lime or limestone, along with make-up sodium hydroxide or
sodium carbonate.
The sulfite / sulfate ions are precipitated, and the sodium hydroxide is regenerated.
Dual Alkali Processes
SOx control
• Spray drying with slaked lime slurry is relatively simple compared to a wet
scrubber
• SO2 removal performance is enhanced by the presence of surface moisture on the
solid lime particles, but excess moisture can result in accumulation of deposits in the
spray dryer vessel.
• The liquid-to-gas ratio is maintained such that the spray dries before it reaches the
bottom of the chamber
• The optimum temperature is maintained by control of the approach-to-adiabatic
saturation (approach).
• SO2 removal efficiency of 90% is attainable with a lime spray dryer under good
operating conditions
Lime Spray Drying
SOx control
In this regenerable process, sulfur dioxide (SO2) is removed from flue gases with
a sodium sulfite scrubbing solution
Wellman-Lord Process
SOx control
sodium sulfite neutralizes the sulfur dioxide
Na2SO3 + SO2 + H2O -----> 2NaHSO3
Some of the Na2SO3 reacts with O2 and the SO3 present in the flue gas to
form Na2SO4 and NaHSO3.
Sodium sulfate does not help in the removal of sulfur dioxide, and is removed. Part of
the bisulfate stream is chilled to precipitate the remaining bisulfate. The remaining
bisulfate stream is evaporated to release the sulfur dioxide, and regenerate the bisulfite
• Oxides of nitrogen, including nitrous oxide (N2O), nitric oxide (NO), nitrogen dioxide
(NO2), nitrogen trioxide (N2O3), and nitrogen pentoxide (N2O5)
• NO is a colorless gas
• NO2 is a reddish brown gas that gives color to smog and can contribute to opacity in
flue gas plumes from stacks
NOx control
COMBUSTION CONTROL TECHNIQUES
NOx control
Low-Excess Air Firing: The lower oxygen supply in the flame zone reduces NOx
production , low excess air in the resulting flame may be longer and less stable, and
carbon monoxide emissions may increase
Overfire Air: The overfire air technique provides oxygen to complete combustion of
unburned fuel and oxidizes carbon monoxide to carbon
dioxide, than creating a second combustion zone. So that the peak flame temperature is
low. NOx formation is inhibited in both the primary and overfire combustion zones
Flue Gas Recirculation: Recirculation of cool flue gas (10-20%) in to the chamber,
effective in reducing NOx formation
NOx control
Two stage combustion
Most effective control methods of nitrogen oxides
Fuel is fired with 90-95%,supplied secondary to complete the combustion,
primary zone reduction of nitrogen oxide is notice, reduction in the emission of
Nox by 38% for coal & oil firing,50% for natural gas.
Reduce Air Preheat: Combustion air often is preheated in a recuperator with the heat from
the flue gas. This conserves energy by recovering the heat in the flue gas and it also raises the
peak flame temperature
NOx control
Reduce Firing Rate: Reducing both air and fuel proportionately would result in the same
flame temperature reducing fuel and air in a fixed size chamber results in a proportionately
larger heat loss to the chamber walls and peak flame temperature is reduced
Water/Steam Injection: In this process heat sink that reduces peak flame temperature.
In natural gas-fired burners, up to 50% NOx reduction can be achieved by injecting steam
at a rate up to 20 to 30% of the fuel weight.
NOx control
Selective Non-catalytic Reduction (SNCR):
Selective non-catalytic reduction uses ammonia (NH3) or urea (H2NCONH2) to reduce
NO x to nitrogen and water.
NOx control
Selective Catalytic Reduction (SCR):
• catalyst bed can be used with ammonia as a reducing agent to promote the reduction
reaction and to lower the effective temperature
• Vanadium pentoxide supported on titanium dioxide is a common catalyst for the
temperature range of 500 to 800°F
• Zeolites, which are various alumino silicates, are used as high temperature catalysts
in the range of 850 to 1100°F.
Note: Class PPT for M.Sc. Environmental Sc. Student.
Materials compiled from different sources for the teaching purposes only
NOx control
Leite, O. C., Safety, noise, and emissions elements round out flare guidelines, Oil
Gas J., 24, 68, 1992.
Leite, O. C., Design alternatives, components key to optimum flares, Oil Gas J., 23,
70, 1992.
American Petroleum Institute, Guide for Pressure-Relieving and Depressuring Systems, API Recommended
Practice 521, 4th ed., Washington, D.C., 1997.
U.S. Environmental Protection Agency, Handbook — Control Technologies for Hazardous Air Pollutants,
EPA-625-6-91-014, Research Triangle Park, NC, 1991.
Williams, T. O. and Miller, F. C., Odor control using biofiltration, BioCycle, 72–77,
October 1992.
Leson, G. and Winer, A. M., Biofiltration: An innovative air pollution control technology for VOC emissions,
J. Air Waste Manage. Assoc., 41(8), 1045–1054, 1991.
Speece, R. E., Biofiltration of gaseous contaminants, State-of-the-art report to Weyerhauser Corporation, May
24, 1995.
Kampbell, D. H. et al., Removal of volatile aliphatic hydrocarbons in a soil bioreactor,
J. Air Poll. Contr. Assoc., 37(10), 1236–1240, 1987.
Bibliography
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