Control of Gaseous pollutants · M.Sc. Environmental Science (II Semester) Course Name: Air...

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



• 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




• 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 temperature for VOCs


Fixed- bed adsorber


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


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



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



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


• 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


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


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


• 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




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.



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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