7. air pollution control

Air Pollution Control

1.Mobile sources-line2.Stationary sources-point

Mobile SourcesCleaner/Alternative Fuels/Handling

• Vaporization of gasoline should be reduced-techniques available• Low sulphur Diesel-India 50 ppm, USA 15 ppm

• Oxygen containing additives reduce air requirement e.g., ethanol, MTBE (Methyl

tertiary butyl ether, 110 octane rating, hazardous).– Methanol: (Less photochemically reactive VOC, but emits HCHO (eye irritant),

difficult to start in winters: Can be overcome by M85 (85% methanol, 15% gasoline)

– Ethanol: GASOHOL (10% ethanol & 90% Gasoline)

– CNG: Low HC, NOx high, inconvenient refueling, leakage hazard.

– LPG: Propane and Butane, NOx high

Two–Way and Three-Way Catalytic Converters

Reduction of nitrogen oxides to nitrogen and oxygen

Oxidation of carbon monoxide to carbon dioxide

Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water

Emissions in Internal Combustion Engines

Rich Mixture

Two Way

Catalytic Converter

Two pollutants:COHC

Leaded gasoline spoils converters

A two-way catalytic converter has two simultaneous tasks:

Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2

Oxidation of unburnt hydrocarbons (unburnt and partially-burnt fuel) to carbon dioxide and water: 2CxHy + (2x+y/2)O2 → 2xCO2 + yH2O

Three Way

Catalytic ConverterThree pollutants:COHC&NOx


A three-way catalytic converter has three simultaneous tasks:

Reduction of nitrogen oxides to nitrogen and oxygen: 2NOx → xO2 + N2

Oxidation of carbon monoxide to carbon dioxide: 2CO + O2 → 2CO2

Oxidation of unburnt hydrocarbons (HC) to carbon dioxide and water: 2CxHy + (2x+y/2)O2 → 2xCO2 + yH2O

Three Way

Catalytic ConverterThree pollutants:COHC&NOx


Catalytic Converters

usePlatinum/Palladium/Rhodiumcatalysts

Research work going on to reduce the costs- new catalysts development

Stationary SourcesPre-combustion Control

– Switching to less sulphur and nitrogen fuels- R&D for the removal of sulphur

(Diesel:15ppm, Removal from coal etc.)

Combustion Control

– Improving the combustion process- Diesel with nano droplets of water

– New burners to reduce NOx

– Integrated gasification combined cycle - Gasification + gasturbine + steam

turbine route

Post-Combustion Control

– Particulate collection devices

– Flue gas desulphurization/other gases removal

Control of Particulate Matter

Device Selection Depends on• Particle size• Concentration/loading• Corrosivity• Volumetric flow rate• Required collection efficiency• Cost

PARTICULATE EMISSION CONTROL BY MECHANICAL SEPARATION

The basic mechanisms of removing particulate matter from gas stream :

1. gravitational settling

2. centrifugal impaction

3. inertial impaction

4. direct interception

5. diffusion

6. electrostatic precipitation

Equipment presently available, which make use of one or more of the above

mechanisms, fall into the following broad categories:

1) Gravitational settling chambers

2) Cyclone separators/multiclones

3) Fabric filters

4) Electrostatic precipitator

5) Wet collectors

Gravitational Settling ChamberGravitational settling chamber is generally used to remove large, abrasive particles (usually >50 µm) from gas stream. It provides enlarged areas to minimize horizontal velocities and allow time for the vertical velocity to carry the particle to the

bottom. The usual velocity through settling chamber is between 0.5 to 2.5 m/s.

GAS INLET

GAS OUTLET

CHAMBER

DUST HOPPER

Advantage low pressure drop simplicity of design low maintenance costcan be used as pre-cleaner devicedry and continuous disposal of particles

Disadvantage larger space low efficiency larger sized particles separation only

Lvh

Hvt

Design of Gravitational Settling ChamberIf we assume that Stokes law applies we can derive a formula for calculating the

minimum diameter of a particle collected at 100% theoretical efficiency in a chamber

of length L.

ht vL

vH

Where vt=terminal settling

velocity, m/s a

papt

dgv

18

2

Where, g=gravitational constant, m/s2; ρp=density of particle, kg/m3; ρa=density of

flue gas, kg/m3; dp=diameter of particle, m; µa=viscosity of air, kg/m s; H=height of

settling chamber, m; vh=horizontal flow velocity, m/s; and L=length of settling

chamber, m.

Solving for dp gives an equation that predicts the largest-size particle that

can be removed with 100% efficiency from a settling chamber of given

dimension.

ap

ha

p gLHvd

18

1. Horizontal trays are used to reduce H2. 10-30 mm spacing3. Cleaning mechanism required for trays

TRAY ARRANGEMENTS

Cyclone Separators

The magnitude of the centrifugal force generated depends on particle mass, velocity and radius

2i

c pv

F MR

MECHANISM

Advantage Relatively inexpensive

Simple to design and maintain

Requires less floor area

Low to moderate pressure drop No moving parts

Continuous disposal of solid particulates

Disadvantage Low collection efficiency for particles below 5-10 µm size

Requires much head room

Abrasion problem

Sensitive to variable dust loading and flow rates

Specific types include

a. Large-diameter cyclones

b. Vane-axial centrifugal separator

Small-diameter multi-cyclones

c. Bottom entry cyclone separator

DESIGN OF CYCLONESShepherd and Lapple (1939,1940)

Cyclones - A small-diameter vane- axial centrifugal separator (Multiclone)

Multiclone unit

gpie

cgpc vN

Bd

29

Where Bc=inlet width, vi=gas inlet velocity and Ne=effective number of turns a gas

makes in traversing the cyclone (5 to 10 in most cases).

Lapple (1951) correlated collection efficiency in terms of the cut size dpc which is the

size of those particle that are collected with 50% efficiency. Particle larger than dpc will

have collection efficiency greater than 50% while the smaller particle will be collected with

lesser efficiency. The cut size is given by:

2

2

2 e

ccig

DBHvK

P

P g

Where, K=a constant, which averages 13 and ranges from 7.5 to 18.4

= pressure drop, Hc, Bc and De=cyclone dimensions, vi=inlet gas velocity and =gas density

Pressure drop The pressure drop may be estimated according to the following equation,

Pressure drop rangeLow efficiency cyclone = 5-10 cm of H2O Medium efficiency cyclone = 8-10 cm of H2OHigh efficiency cyclones = 10-15 cm of H2O

SERIES COMBINATION OF CYCLONES

1. Bigger sized2. Smaller sized

REMOVAL MECHANISM OF PARTICULATES

FABRIC FILTER/BAGHOUSE

Why Bag/Baghouse Filter ?

Very high efficiency down to submicron particlesFilter press- chemical process industries

A baghouse (BH, B/H) or fabric filter (FF) is an air pollution control device that removes particulates out of air or gas released from commercial processes or combustion devices.

Power plants, steel mills, pharmaceutical producers, food manufacturers, chemical producers and other industrial companies often use baghouses to control emission of air pollutants.

Baghouses came into widespread use in the late 1970s after the invention of high-temperature fabrics (for use in the filter media) capable of withstanding temperatures over 180 oC.

Functioning baghouses typically have a particulate collection efficiency of 99.5%+ or better, even when particle size is submicron.

•Inertial collection - Dust particles strike the fibers placed perpendicular to the gas-flow direction.

•Interception - Particles that do not cross the fluid streamlines come in contact with fibers because of the fiber size.

•Submicrometre particles are diffused, increasing the probability of contact between the particles and collecting surfaces.

•Electrostatic forces - The presence of an electrostatic charge on the particles and the filter can increase dust capture. Charged fabrics are in market for this purpose.

A combination of these mechanisms results in formation of the dust cake on the filter, which eventually increases the resistance to gas flow. The filter must be cleaned then periodically.

MECHANISM OF DUST COLLECTION

Maximum temperature at bag house

Inlet for continuous duty

Chemical Resistance

Fabric Summary of published data

(0C)

Recommended

maximum (0C)

Acid Alkali

Cotton 70-90 80 poor Fair

Dynel 65-80 80 Good Good

Wool 80-110 105 Good Poor

Nylon 90-140 105 Good Poor

Orlon 90-175 135 Good Fair

Dacron 120-175 135 Excellent Good

Glass 260-370 290 Excellent Excellent

Summary of data on the common filter media used in industrial baghouses

Dynel: A copolymer of acrylonitrile and vinyl chloride (flame resistant). It is an acrylic resin. Dynel was originally produced by Union Carbide Corporation.

Orlon: Acrylic fiber, Du Pont 1941

Dacron: Polyethylene terephthalate (PET)

GAS FEEDING ARRANGEMENTS

DIFFERENT FILTER BAG CLEANING ARRANGEMENTS

1. Mechanical Shaker Baghouses

2. Reverse Air (R/A) Baghouses

3. Pulse Jet Baghouses

Types of Baghouses

Mechanical Shaker Baghouse

Advantages Disadvantages

Have high collection efficiency for respirable dust Have low air-to-cloth ratio (1.5 to 2 ft/min)

Can use strong woven bags, which can withstand intensified cleaning cycle to reduce residual dust buildup

Cannot be used in high temperatures

Simple to operate Require large amounts of space

Have low pressure drop for equivalent collection efficiencies Need large number of filter bags

Consist of many moving parts and require frequent maintenance

Personnel must enter baghouse to replace bags, creating potential for exposure to toxic dust

Can result in reduced cleaning efficiency if even a slight positive pressure exists outside bags

Mechanical Shaker Baghouses

Reverse Air Baghouses

Have high collection efficiency for respirable dust

Have low air-to-cloth ratio (1 to 2 ft/min)

Are preferred for high temperatures due to gentle cleaning action

Require frequent cleaning because of gentle cleaning action

Have low pressure drop for equivalent collection efficiencies

Have no effective way to remove residual dust buildup

Cleaning air must be filtered

Require personnel to enter baghouse to replace bags which creates potential for toxic dust exposure

Advantages DisadvantagesReverse Air Baghouses

Pulse Jet (Reversed Jet) Baghouses


0.1 s air pulseDigital sequencing

Have high collection efficiency for respirable dust Require use of dry compressed air

Can have high air-to-cloth ratio (6 to 10 ft/min)

May not be used readily in high temperatures unless special fabrics are used

Have increased efficiency and minimal residual dust buildup due to aggressive cleaning action

Cannot be used if high moisture content or humidity levels are present in the exhaust gases

Can clean continuouslyCan use strong woven bagsHave lower bag wearHave small size and fewer bags because of high air-to-cloth ratioSome designs allow bag changing without entering baghouseHave low pressure drop for equivalent collection efficiencies

Advantages Disadvantages


DESIGN OF FABRIC FILTER

Pressure drop and air-to-cloth ratio are the major design parameters in bag-house design.

Higher pressure drops implies that more energy is required to pull the gas through the system.

Air-to-cloth ratio, also referred to as the face velocity, is the volumetric flow of gas received by a bag-house divided by the total area of the filtering cloth.

The air-to-cloth ratio is typically between 1.5 to 14 ft/min depending on the dust and type of baghouse cloth.

• The air-to-cloth ratio determines the unit size and thus the capital cost of installation.

Higher air-to-cloth ratio mean less fabric, therefore less capital cost. However, higher ratio can lead to high pressure drop thus requiring higher energy. Also, more frequent bag cleanings may be required, thus increasing the downtime.

Pressure drop is limited to 10-20 cm of water depending upon the bag material

Suppliers quote actual values based on experiments/installations

ELECTROSTATIC PRECIPITATORS (ESP)

Fredrick Cottrell – early twentieth century for recovery of process materials.

Presently gas/air cleaning is prime objective.

Mechanism involves a high voltage (30 kV or more) DC field creating corona making the particles charged and subsequent attraction on an electrode and dislodging.

Electrostatic Precipitator

Electrode 50000-70000 v

DC

Clean air and gases out

Charged particle attracted to earthed

platesCorona discharge

Dust return to process or dumped

Dust laden air and gases in

Movement of dust and air/gas in a ESP

Advantages DisadvantagesHigh collection efficiency, 99.99%

High initial cost

Low maintenance and operating costs

More space requirement

Handles large volume of high temperature gas, 200-450 oC

Possible explosion hazardPersonnel safety

Negligible treatment time Production of poisonous gas

Easy cleaning Not suitable for combustible particles

Negligible pressure drops Ozone formation may take place

Advantages and Disadvantages of ESP

REQUIREMENT OF ELECTROSTATIC PRECIPITATION PROCESS

Source of high voltage (DC)

Discharge and collecting electrodes

A means for disposal of collected material

Cleaning system

STEPS IN ELECTROSTATIC PRECIPITATION Generation of high voltage 30-100 kV

Generation of electric charges

Transfer of electric charge to a dust particle

Movement of the charge dust particle in an electric field to the collection

electrodes

Adhesion of the charged dust particles to the surface of the collection

electrode

Dislodging of dust layer from collection electrode

Collection of dust layer in a hopper

Removal of the dust from the hopper

TYPES OF ELECTROSTATIC PRECIPITATORS

A. SINGLE STAGE PRECIPITATORS

B. TWO STAGE PRECIPITATORS

SINGLE STAGE PRECIPITATORS

Plate-Wire Precipitators

In a plate-wire ESP, gas flows between parallel plates of sheet metal and high-

voltage electrodes.

These electrodes are long wires weighted and hanging between the plates or are

supported there by mast-like structures (rigid frames).

Within each flow path, gas flow must pass each wire in sequence as flows through

the unit.

Plate-wire ESPs are used in a wide variety of industrial applications, including

coal-fired boilers, cement kilns, solid waste incinerators, paper mill recovery

boilers, petroleum refining catalytic cracking units, sinter plants, basic oxygen

furnaces, open hearth furnaces, electric arc furnaces, coke oven batteries, and

glass furnaces.

Schematic of plate-wire electrostatic precipitator

Plate-Wire electrostatic precipitator

Tubular Precipitators The original ESPs were tubular like the smokestacks they were placed

on, with the high-voltage electrode running along the axis of the tube.

Tubular precipitators have typical applications in sulfuric acid plants,

coke oven by-product gas cleaning (tar removal) and iron & steel

sinter plants.

Tubular Electrostatic Precipitator

Close Look at Tubular Electrostatic Precipitator

TWO-STAGE PRECIPITATORS

The previously described precipitators are all parallel in nature, i.e., the

discharge and collecting electrodes are side by side.

Two-stage precipitators are considered to be separate and distinct types of

devices compared to large, high-gas-volume, single-stage ESPs.

The two-stage precipitator invented by Penney is a series device with the

discharge electrode, or ionizer, preceding the collector electrodes.

Advantages of this configuration include more time for particle charging, less

propensity for back corona, and economical construction for small sizes.

Two stage electrostatic precipitator

Wet Precipitators Any of the precipitator configurations discussed above may be operated

with wet walls instead of dry.

Inlet gas is saturated with water

The water flow may be applied intermittently or continuously to wash

the collected particles into a sump for disposal.

The advantage of the wet wall precipitator is that it has no problems with

rapping re-entrainment or with back coronas.

The disadvantage is the increased complexity of the wash and the fact

that the collected slurry must be handled more carefully than a dry

product, adding to the expense of disposal.

Collection efficiency mathematically expressed as

QAV cpmexp1

=Fractional collection efficiency Ac =Area of the collection electrode Vpm =Particle migration velocity Q =Volumetric flow rate of gas

MIGRATION VELOCITY

The velocity of charged particle suspended in a gas under the influence of an

electric field is known as migration velocity. The particle migration velocity

is the most important parameter and is function of a large number of

operation quantities such as- Electric field strength, particle size, gas

viscosity, properties of the dust .

Dust Migration Velocity (m/s)

Zinc Oxide 0.02-0.03

Sulfuric Acid 0.08-0.16

Metal Oxides 0.02-0.03

Calcium Carbonate 0.04-0.05

Smoke Fume pit coal furnace 0.02-0.11

Fly ash from lignite furnace 0.18-0.25

Smelter dust 0.07-0.09

Blast furnace dust 0.05-0.07

Effective migration velocity (m/s) for various type of dusts

Consumer-oriented electrostatic air cleaners

Particulate matter

Generation of 1. NOX

2. Ozone

First introduced in1954 by Raytheon

7. air pollution control

Environment

Transcript of 7. air pollution control