air pollution management.pptx

7/30/2019 Air Pollution Management.pptx

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Air Pollution Management: Air

Pollution Control DevicesM.Phil-Ph.D.

Centre for Environmental Science and Technology

Central University of Punjab,Bathinda


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Control Techniques

Fabric filters

Gravity settling chamber

Mechanical collectors

Particulate wet scrubbers

Electrostatic precipitators


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Fabric Filters

Operating Principles

Fabric filters collect particulate matter on the surfaces of filter bags.

Most of the particles are captured by inertial impaction,interception, Brownian diffusion, and sieving on already collectedparticles that have formed a dust layer on the bags.

The fabric material itself can capture particles that have penetratedthrough the dust layers.

Electrostatic attraction may also contribute to particle capture in thedust layer and in the fabric itself.


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Due to the multiple mechanisms of particle capture possible, fabric filters can

be highly efficient for the entire particle size range of interest in air pollution

control.

Types of Fabric Filters

A reverse-air-type fabric filter, shown in Figure 13, is one of the major categories of fabric

filters. It is used mainly for large industrial sources.

In this type, the particle-laden gas stream enters from the bottom and passes into theinside of the bags.

The dust cake accumulates on the inside surfaces of the bags.

Filtered gas passes through the bags and is exhausted from the unit.


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When cleaning is necessary, dampers are used to isolate a

compartment of bags from the inlet gas flow.

Then, some of the filtered gas passes in the reverse direction

(from the outside of the bag to the inside) in order to remove

some of the dust cake.

The gas used for reverse air cleaning is re-filtered and released.

Fabric Filters


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Another common type of fabric filter is the pulse jet shownin Figure 14.

In this type, the bags are supported on metal wire cages

that are suspended from the top of the unit.

Particulate-laden gas flows around the outside of the bags,and a dust cake accumulates on the exterior surfaces.

When cleaning is needed, a very-short-duration pulse ofcompressed air is injected at the top inside part of eachbag in the row of bags being cleaned.

Fabric Filters


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The compressed air pulse generates a pressure wave that moves

down each bag and, in the process, dislodges some of the dust

cake from the bag.


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Advantages and Disadvantages of Fabric Filters

Fabric filters are used in a wide variety of applications

where high efficiency particulate collection is needed.

The control efficiencies usually range from 99% to greater

than 99.5% depending on the characteristics of the

particulate matter and the fabric filter design.

Fabric filters can be very efficient at collecting particles in

the entire size range of interest in air pollution control.


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The performance of fabric filters is usually independent ofthe chemical composition of the particulate matter.

They are not used when the gas stream generated by the

process equipment includes corrosive materials that couldchemically attack the filter media.

Fabric filters are also not used when there are sticky or wet

particles in the gas stream.

These materials accumulate on the filter media surface andblock gas movement.



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Fabric filters must be designed carefully if there are

potentially combustible or explosive particulate matter,

gases, or vapors in the gas stream being treated.

If these conditions are severe, alternative control

techniques, such as wet scrubbers, are often used.



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Gravity Settling Chambers

This category of control devices relies upon gravity settling toremove particles from the gas stream.

Gravity settling chambers are used only for very largeparticles in the upper end of the supercoarse size range

(approximately 75 micrometers and larger).

The very low terminal settling velocities of most particlesencountered in the field of air pollution limit the usefulness

of gravity settling chambers.

The stringent control requirements adopted in the late 1960sthrough early 1970s have resulted in a sharp decline in theuse of this type of collector.


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Mechanical Collectors

The particulate-laden gas stream is forced to spin in a cyclonic

manner.

The mass of the particles causes them to move toward the

outside of the vortex.

Most of the large-diameter particles enter a hopper below the

cyclonic tubes while the gas stream turns and exits the tube.


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There are two main types of mechanical collectors: (1)large-diameter cyclones, and (2) small-diameter multi-cyclones.

Large-diameter cyclones are usually one to six feet indiameter; while small-diameter multi-cyclones usually havediameters between 3 and 12 inches.

A typical large-diameter cyclone system is shown in Figure1.

The gas stream enters the cyclone tangentially and creates aweak vortex of spinning gas in the cyclone body.



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Large-diameter particles move toward the cyclone body

wall and then settle into the hopper of the cyclone.

The cleaned gas turns and exits the cyclone.

Large-diameter cyclones are used to collect particles down

to 1/16 inch (1.5 mm) diameter and above.



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In systems where the large-diameter cyclone is locatedafter the fan (positive pressure), the treated gas is usuallydischarged directly from the cyclone.

In systems where the cyclone is located before the fan(negative pressure), the gas stream is either exhaustedfrom a separate stack or from the discharge of the fan itself.

In negative pressure systems, a solids discharge valve isused to prevent air infiltration up through the hopper area.



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A small-diameter cyclone tube is shown in Figure 2.

Vanes located on the inlet of each of the tubescreate the spinning movement of the gas stream.

Most of the commercial tubes are six, nine, or twelveinches in diameter.

Due to the limited gas handling capacity of eachtube, large numbers of tubes are mounted in parallelin a single collector.



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The small-diameter of the cyclone tube creates more rapid spinning of

the gas stream than in large-diameter cyclones.

The particles moving outward in the spinning gas stream have a relatively

shorter distance to travel in a small-diameter multi-cyclone tube before

they reach the cyclone body wall.

These features allow small-diameter multi-cyclones to collect

considerably smaller particles than large-diameter cyclones can.



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Small-diameter multi-cyclones, such as the one shown in Figure 2 arecapable of removing particles having diameters down to 5 micrometers.

Small-diameter multi-cyclones are not generally used for very large

diameter material, such as 3 mm and above, because large particles may

plug the spinner vanes in the multi-cyclone tubes.

Some mechanical collectors are specially designed to provide high-

efficiency PM collection down to a particle size of one micrometer.



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These have higher gas velocities within the cyclone tubes and different

cyclone geometries than those shown in Figure 2.

A typical application of a conventional multi-cyclone collector is shown

in Figure 3.

In this example, the multi-cyclone is located after a small, wood-fired

boiler and is used as a pre-collector for the fabric filter.



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Mechanical collectors are used whenever the particle size relatively large (> 5

micrometers) and/or the control efficiency requirements are in the low-to-moderate

range of 50 to 90%.

They are also used as the pre-collector of large-diameter embers generated in some

combustion systems.

Removal of the embers is necessary to protect high-efficiency particulate control systems

downstream from the mechanical collectors.

Most mechanical collectors are not applicable to industrial sources that generate sticky

and/or wet particulate matter.

These materials can accumulate on the cyclone body wall or the inlet spinner vanes of

conventional multi-cyclone collectors.


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Cyclone

2

1

2

5.0

9

gpQ

HBd

212 LL

H

De

L3D2

L1

L2

Dd

H

B

d0.5 = cut diameter at 50% removal

= dynamic viscosity of gas, Pa-s

B = width, m

H = height, m

p = particle density, kg/m3

Qg = gas flow rate, m3/s = effective number of turns


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For standard Cyclone:


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Ex. 6-9

Given:

D2 = 0.5 m

Qg = 4 m3

/sT = 25 oC

p = 800 kg/m3

For standard Cyclone:

B = 0.25 D2 = 0.13 m

H = 0.5 D2 = 0.25 m

L1 = L2 = 2 D2 = 1 m

Q = What is the removal efficiencyfor particles with ave

diameter of 10 m?

7.371)1(225.0

)(41.2

1041.2)7.37)(4)(800(

)25.0()13.0)(105.18(96

5.026

5.0

m

xx

d

@ d =10 m 15.441.2

10

5.0

d

d

h = 0.95


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Electrostatic Precipitators

An electrostatic precipitator (ESP) uses non-uniform, high-voltage fields to apply large electrical charges to particlesmoving through the field.

The charged particles move toward an oppositely charged

collection surface, where they accumulate.

There are three main styles of electrostatic precipitators:

(1) negatively charged dry precipitators, (2) negatively

charged wetted-wall precipitators, and (3) positively

charged two-stage precipitators.


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The negatively charged dry precipitators are thetype most frequently used on large applicationssuch as coal-fired boilers, cement kilns, and kraftpulp mills.

Wetted-wall precipitators (wet precipitators) areoften used to collect mist and/or solid material

that is moderately sticky.

The positively charged two-stage precipitators are

used only for the removal of mists.


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Electrostatic precipitation has been a reliable technology

since the early 1900's. Originally developed to abate serious

smoke nuisances. Zinc, copper, and lead industries found ESP a

cost efficient way to recover valuable product. Today ESP are

found mainly on large power plants, incinerators, cementplants,

In wood products industry, ESP preceded by multi clones is

considered the best available control technology for wood

fired boiler emissions. Wet ESP have found renewed interest

from particle board, and plywood veneer manufactures for

controlling dryer exhaust.

An ESP can consistently provide 99%+ removal reducing

emissions levels to 0.002 - 0.015 grains per dry standard cubic

foot of exhaust gas.

Precipitators are designed to handle flow form 10,000 cfm to

300,000 cfm and can operate at temperatures as high as 750

degrees F. Normal gas flow through a precipitator is 2-5 feet

per second, consequently, the pressure drop is only 0.5" wc.


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Many countries around the world depend on coal and other fossil fuels to produce

electricity. A natural result from the burning of fossil fuels, particularly coal, is the

emission of flyash. Ash is mineral matter present in the fuel. For a pulverized coal unit,

60-80% of ash leaves with the flue gas. Flyash emissions have received the greatestattention since they are easily seen leaving smokestacks.

Two emission control devices for flyash are

The traditional fabric filters - The fabric filters are large baghouse filters having a high

maintenance cost (the cloth bags have a life of 18 to 36 months, but can be temporarily cleaned

by shaking or back flushing with air). These fabric filters are inherently large structures resulting in

a large pressure drop, which reduces the plant efficiency

The electrostatic precipitators -ESP have collection efficiency of 99%, but do not work well for

flyash with a high electrical resistivity (as commonly results from combustion of low-sulfur coal).

In addition, the designer must avoid allowing unburned gas to enter the electrostatic precipitator

since the gas could be ignited.


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DESIGN AND OPERATION

A precipitator is a relatively simple device.

The main components are as followsAn insulated and lagged shell

Collection plates or tubes

Discharge electrodes

Collection Plate Rappers/Electrode Vibrators

Hoppers


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Insulated Steel Housing: The development of modular, factory built units has significantly lowered

the installed cost of precipitators. Dry precipitators are normally fabricated from 3/16" thick steel

plate, insulated and lagged with aluminum. The electrodes are made of steel tubing and the

collection plates are made of rolled steel. The housing can last 15-20 years.

Wet precipitators are traditionally fabricated of stainless steel for corrosion resistance.

Discharge Electrodes: In the past dust particles were charged by a series of small diameter wires

which were suspended from a ceiling rack and weights at the bottom. This maze of electrodes was

subject to electric erosion. Today, discharge electrodes are rigid and constructed of 2" steel tubing.

Ten years of continuous service is the expected norm.

Rappers and Vibrators: Heavy duty rappers are used in the wood industry. They consist of 30 poundpiston hammers designed to rap small sections of collection plates. A timer periodically releases the

rapper to transfer the dust on the collection plates to the hopper. Electric vibrators are placed on

the electrode rack to transfer any collected dust to the hopper and are operated by a timer.

Power: ESP will take 480 volt AC and transform / convert the power to 55-70,000 Volt DC.

Electrostatic precipitator use the lowest power to accomplish the job. Electrostatic forces are

applied directly to the particles and not the entire gas stream. Combining this feature with the low

pressure drop (0.5" wc) across the system results in power requirements approximately 50% of

comparable wet systems and 25% of equivalent bag filter systems.


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Efficiency of Electrostatic Precipitators

The efficiency of removal of particles by

an Electrostatic Precipitator is given by

= fractional collection efficiencyw = drift velocity, m/min.

A = available collection area, m2

Q = volumetric flow rate m3/min


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Migration velocity

Where,q = charge (columbos)

Ep = collection field intensity (volts/m)

r = particle radius (m)

= dynamic viscosity of gas (Pa-S)

c = cunningham correction factor


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Cunningham correction factor

where,T = absolute temperature (k)dp = diameter of particle (m)


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Collection Efficiency of ESPs

The efficiency is usually at a minimum in the range of 0.1 to0.5 micrometers.

The shape of the efficiency curve is the combined effect oftwo particle electrical charging mechanisms, neither of

which is highly effective in this particle size range.

It should be noted that this decrease in efficiency occurs inthe same particle size range as for particulate wet

scrubbers.

However, the reason for this decreased efficiency zone isentirely different than that for particulate wet scrubbers.


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Factors that influence precipitator sizing are:

gas volume

precipitator inlet loading

precipitator outlet loading required

outlet opacity required

particulate resistivity

particle size

Particle size of the incoming particulate has a dramatic impact on the sizing of an

electrostatic precipitator.

Fluid Catalytic Crackers and Recovery Boilers, which have particle resistivities in the

medium range, exhibit very fine particulate. The size of the precipitator must be

increased in these cases because the fine particulate is easily re-entrained into the

gas stream.In the power industry, generally the higher the fuel ash content, the larger the ash

particle size.
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Resistivity is the resistance of a medium to the flow of an electrical current. By definition,

resistivity, which has units of ohm-cm, is the electrical resistance of a dust sample 1 cm2 in cross

sectional area and 1 cm thick.

Resistivity levels are generally broken down into three categories:

Low 10 11 ohm-cm

Particles with medium resistivity are the most acceptable for electrostatic precipitators. Particles

in the low range are easily charged, however upon contact with the collecting electrodes, they

rapidly loss their negative charge and are repelled by the collecting electrodes back into the gasstream. Particles in the high resistivity category may cause back corona which is a localized

discharge at the collecting electrode due to the surface being coated by a layer of non-conductive

material.

Resistivity is influenced by flue gas temperature and conditioning agents, such as flue gas moisture

and ash chemistry. Conductive chemical species, such as sulfur and sodium will tend to reduce

resistivity levels while insulating species, such as SiO2, Al2O3 and Ca will tend to increase resistivity

(ash from low sulfur coal is). Flue gas conditioning with SO3 can reduce resistivity to a more

optimum value thus reducing the size of the precipitator needed.


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ESP Advantages

They have high efficiencies (exceeds 99.9% in some applications)

Fine dust particles are collected efficientlyCan function at high temperatures (as high as 700 degree F 1300 degree F)

Pressure and temperature changes are small

Difficult material like acid and tars can be collected

They withstand extremely corrosive material

Low power requirement for cleaning

Dry dust is collected making recovery of lost product easyLarge flow rates are possible

ESP Disadvantages

High initial cost

Materials with very high or low resistivity are difficult to collectInefficiencies could arise in the system due to variable condition of airflow (though

automatic voltage control improves collector efficiency)

They can be larger than baghouses (fabric collectors) and cartridge units, and can occupy

greater space

Material in gaseous phase cannot be removed by electrostatic method

Dust loads may be needed to be reduced before precipitation process (precleaner may be

needed)


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When are Electrostatic Precipitators not a suitable solution?

As the size of the required precipitator increases, other technologies become more

cost effective. For low sulfur utility applications, fabric filters are an attractive

alternative. As part of the overall precipitator/fabric filter cost evaluation, operating

costs need to be included. Typically, the pressure drop across a flange to flange fabric

filter will be in the 6 to 8" w.c. range whereas an electrostatic precipitator will have

approximately a 0.5 to 1" w.c. pressure drop. This pressure drop penalty for a fabric

filter will be somewhat offset by its lower power consumption.

Another benefit of a fabric filter is high acid gas, SO2, chlorides, fluorides and Hg

removal capability.

When operating downstream of a spray dryer absorber, removal efficiencies of 90% or

greater can be attained for some species when operating in conjunction with a fabric

filter. The fabric filter dust layer acts as a fixed bed where high acid gas removal

efficiency can take place. Since most of the particulate is removed from the collecting

electrodes of a precipitator during normal operation, acid gas removal capability ismuch reduced.
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Wet Electrostatic Precipitators

Wet electrostatic precipitators (WESPs)

operate in the same three-step process asdry ESPs charging, collecting and finally

cleaning of the particles.

However, cleaning the collecting electrode is

performed by washing the collection surface

with liquid rather than mechanically rappingthe collection plates.

While the cleaning mechanism would not be

thought to have any impact upon

performance, it significantly affects the

nature of the particles that can be captured,

the performance efficiencies that can be

achieved, and the design parameters and

operating maintenance of the equipment.

Th l f t i l l t t ti i it t d


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The scale of a typical electrostatic precipitator usedat a coal-fired boiler.


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Essentially all of these units are divided into a number ofseparately energized areas that are termed fields (see Figure10).

Most precipitators have between three and ten fields in

series along the gas flow path.

On large units, the precipitators are divided into a number ofseparate, parallel chambers, each of which has an equal

number of fields in series.

There is a solid partition or physical separation between the2 to 8 chambers that are present on the large systems.


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Advantages and Disadvantages of ESPs

Electrostatic precipitators can have very high efficiencies due tothe strong electrical forces applied to the small particles.

These types of collectors can be used when the gas stream is not

explosive and does not contain entrained droplets or other stickymaterial.

The composition of the particulate matter is very important

because it influences the electrical conductivity within the dustlayers on the collection plate.


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Resistivity, an important concept associated withelectrostatic precipitators, is a measure of the ability of theparticulate matter to conduct electricity and is expressedin units of ohm-cm.

As the resistivity increases, the ability of the particulatematter to conduct electricity decreases.

Precipitators can be designed to work in any resistivityrange; however, they usually work best when the resistivityis in the moderate range (108 to 1010 ohms-cm).


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Particulate Wet Scrubbers

There are a number of major categories of particulate wetscrubbers:

- Venturis -Impingement and Sieve Plates

- Spray Towers -Mechanically Aided

- Condensation Growth -Packed Beds

- Ejector -Mobile Bed

- Caternary Grid -Froth Tower

-Oriented Fiber Pad -Wetted Mist Eliminators

We will discuss three of the above types of scrubbers:venturis, impingement plate scrubbers, and spray towers.


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Venturi Scrubbers

A typical venturi throat is shown in Figure 4.

Particulate matter, which accelerates as it enters thethroat, is driven into the slow moving, large water

droplets that are introduced near the high velocitypoint at the inlet of the venturi throat.

The adjustable dampers in the unit illustrated are

used to adjust the open cross-sectional area andthereby affect the speed of the particles entrained inthe inlet gas stream.


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Impingement Plate Scrubbers

An impingement plate scrubber is shown in Figure 5.

These scrubbers usually have one to three horizontal

plates, each of which has a large number of small holes.

The gas stream accelerating through the holes atomizessome water droplets in the water layer above the plate.

Particles impact into these water droplets.


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Spray Tower Scrubbers

A typical spray tower scrubber is shown in Figure 6.

This is the simplest type of particulate wet scrubber in

commercial service.

Sets of spray nozzles located near the top of the scrubbervessel generate water droplets that impact with particles

in the gas stream as the gas stream moves upwards.


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Each of the categories of particulate wet scrubbers listed earlier has a large

number of different design types.

For example, venturi scrubbers include the following different design types: (1)

fixed throat, (2) adjustable throat.

Spray tower scrubbers include these design types: (1) open, (2) cyclonic.

The scrubber categories listed above comprise more than fifty different types of

scrubbers in common commercial use.

Scrubbers are by far the most diverse group of air pollution control devices used

for particulate control.

bb


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Wet Scrubbing Systems

Each particulate wet scrubber vessel is part of a large,and sometimes complex, wet scrubbing system.

For example, Figure 7 illustrates a venturi scrubber in ascrubbing system.

The evaporative cooler, located before the venturiscrubber in the system, cools the gas stream, whichserves the following purpose:

1- It protects the construction materials of the venturithroat.

2- It helps to homogeneously and heterogeneously


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2 It helps to homogeneously and heterogeneouslynucleate vapor phase material emitted from theprocess before it reaches the scrubbing system.

3-It prevents the water droplets from evaporating andinhibiting inertial impaction.


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Located after the venturi scrubber, the cyclonic separator removes entrained

water droplets from the gas stream leaving the venturi.

The cyclonic separator consists of a cyclonic vessel and a horizontal mist

eliminator.

The overall scrubbing system includes pumps for liquid recirculation, a tank to

treat the liquid being recirculated, an alkali addition unit to control the liquid pH,

a purged liquid treatment unit, a fan for gas movement, and a stack.

There are a wide variety of wet scrubber system designs; however, thesecomponents are present in many systems, regardless of which type of

particulate matter scrubber is used.


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The ability of a particulate wet scrubber to remove particles

depends on two or more of the following variables:

The size (aerodynamic diameter) of the particle

The velocity of the particle

The velocity of the droplet.

Scrubber Operating Principles

C ll ti Effi i f W t S bb


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Collection Efficiency of Wet Scrubbers

The velocities of the particle-laden gas stream and the

liquid targets vary substantially.

There are substantial differences in the ability of particulate

wet scrubbers to collect particles less than approximately 5

micrometers.

This is illustrated in Figure 8.


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If a significant portion of the particulate matter mass is composed of particles


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Scrubber

Efficiency

where,

k = Scrubber coefficient (m3 of gas/ m3 of liquid)

R = Liquid-to-gas flow rate (QL/QG)

= internal impaction parameter

Internal impaction parameter

where,

c = cunningham correction factor

p = particle density (kg/m3)Vg = speed of gas at throat (m/sec)

dp = diameter of particle (m)

dd = diameter of droplet (m)

= dynamic viscosity of gas, (Pa-S)

Ad d i d f S bb


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Advantages and Disadvantages of Scrubbers

Many types of particulate wet scrubbers can provide highefficiency control of particulate matter.

One of the main advantages of particulate wet scrubbers is

that they are often able to simultaneously collectparticulate matter and gaseous pollutants.

Also, wet scrubbers can often be used on sources that have

potentially explosive gases or particulate matter.

They are compact and can often be retrofitted into existingplants with very limited space.


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One of the main disadvantages of particulate wet scrubbers is that they

require make-up water to replace the water vaporized into the gas stream

and lost to purge liquid and sludge removed from the scrubber system.

Wet scrubbers generate a waste stream that must be treated properly.

General Applicability of Particulate


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General Applicability of Particulate

Control Systems

Particulate matter control systems are often selectedbased on the general criteria listed in Figure 15.

If there is a high concentration of wet and/or sticky particulate matter, either a


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particulate wet scrubber or a wet electrostatic precipitator is used.

If wet or sticky materials are present with combustible materials or explosive

gases or vapors, the particulate wet scrubber is most appropriate.

If the particulate matter is primarily dry, mechanical collectors, particulate wet

scrubbers, conventional electrostatic precipitators, and fabric filters can be used.

The next step in the selection process is to determine if the particulate matter

and/or gases and vapors in the gas stream are combustible or explosive.

If so, then mechanical collectors or particulate wet scrubbers can be used

because both of these categories of systems can be designed to minimize therisks of ignition.

In some cases, a fabric filter can also be used if it includes the appropriate safety

equipment.


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An electrostatic precipitator is not used due to the risk of

ignition caused by electrical sparking in the precipitator fields.

When selecting between mechanical collectors and wetscrubbers, mechanical collectors are the more economicalchoice.

They have a lower purchase cost and a lower operating costthan wet scrubbers.

If the dry particulate matter is present in a gas stream that isnot combustible or explosive, the selection depends on theparticle size range and the control efficiency requirements.


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If a significant portion of the gas stream is in the less than0.5-micrometer size range, and high efficiency control isneeded, a fabric filter is the most common choice.

If a significant portion of the particulate matter is in the

0.5- to 5-micrometer size range, and high efficiency controlis needed, fabric filters, electrostatic precipitators, orparticulate wet scrubbers (certain types) could be used.

If most of the particulate matter is larger than 5micrometers, any of the four main types of particulatecontrol systems could be used.


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There are numerous exceptions to the general applicability

information presented above due to site-specific process

conditions and unique particulate matter control systems.

Nevertheless, this chart provides a general indication of the

uses and limitations of many commercially available

particulate matter control systems.

The applicability of VOC control systems for high concentration


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The applicability of VOC control systems for high concentrationsystems also depends, in part, on the number of separate VOCcompounds present in the gas stream and the economicincentives for recovery and reuse.

Thermal oxidizers can be used in all cases in which recovery andreuse are not desired or economically feasible.

Catalytic oxidizers can be used in these same situations if thereare no gas stream components that would poison, mask, or foulthe catalyst.

Adsorbers can also be used for this service as long as there are

environmentally acceptable means for disposal of the collectedorganics.


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If recovery and reuse are desired, either adsorbers orcondenser/refrigeration systems can be used.

These systems are limited to gas streams containing

at most three organic compounds due to the costsassociated with separating the recovered materialinto individual components.

If the process can reuse a multi-component organicstream, both adsorbers and condenser/refrigerationsystems can be used without the costs of recoveredmaterial purification and reprocessing.


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There are a number of commercial VOC controlsystems that fall outside the general pattern of

applicability indicated in Figures 5 and 6.

These figures provide a very general indication of the

uses and limitations of the five main types of VOCcontrol systems.

GENERAL METHODS FOR CONTROL OF SO


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GENERAL METHODS FOR CONTROL OF SO2

EMISSIONS

Change to Low Sulfur Fuel Natural Gas

Liquefied Natural Gas

Low Sulfur Oil

Low Sulfur Coal

Use Desulfurized Coal and Oil Increase

Effective Stack Height Build Tall Stacks

Redistribution of Stack Gas Velocity Profile

Modification of Plume Buoyancy

General Methods for Control of SO


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General Methods for Control of SO2

Emissions (contd.)

Use Flue Gas Desulfurization Systems

Use Alternative Energy Sources, such as Hydro-

Power or Nuclear-Power

l lf


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Flue Gas Desulfurization

SO2 scrubbing, or Flue Gas Desulfurization processes can beclassified as:

Throwaway or Regenerative, depending upon whether the recovered sulfur

is discarded or recycled.

Wet or Dry, depending upon whether the scrubber is a liquid or a solid.

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

i S bbi


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Limestone Scrubbing

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

Li S bbi


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Lime Scrubbing

The equipment and the processes are similar to thosein 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

D l Alk li S


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Dual Alkali System

Lime and Limestone scrubbing lead to deposits inside spray tower.

The deposits can lead to plugging of the nozzles through which thescrubbing slurry is sprayed.

The Dual Alkali system uses two regents to remove the sulfurdioxide.

Sodium sulfite / Sodium hydroxide are used for the absorption ofsulfur dioxide inside the spray chamber.

The resulting sodium salts are soluble in water, so no deposits areformed.

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 hydroxideis regenerated.

Li S D i


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Lime Spray Drying

Lime Slurry is sprayed into the chamber

The sulfur dioxide is absorbed by the slurry

The liquid-to-gas ratio is maintained such that the spray driesbefore it reaches the bottom of the chamber

The dry solids are carried out with the gas, and are collectedin fabric filtration unit

This system needs lower maintenance, lower capital costs,and lower energy usage

W ll L d P


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Wellman Lord Process

This process consists of the followingsubprocesses: Flue gas pre-treatment.

Sulfur dioxide absorption by sodium sulfite

Purge treatment

Sodium sulfite regeneration.

The concentrated sulfur dioxide stream is processed to amarketable product.

The flue gas is pre - treated to remove the particulate. The sodium

sulfite neutralizes the sulfur dioxide :

Na2SO3 + SO2 + H2O -----> 2NaHSO3

W ll L d P ( td )


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Wellman Lord Process (contd.)

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.

B k d Nit O id


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Background on Nitrogen Oxides

There are seven known oxides of nitrogen :

NO

NO2

NO3

N2O

N2O3

N2O4

N2O5

NO and NO2 are the most common of the seven oxides listed

above. NOx released from stationary sources is of two types

General Methods For Control Of Nox


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Emissions

NOx control can be achieved by: Fuel Denitrogenation

Combustion Modification

Modification of operating conditions

Tail-end control equipment

Selective Catalytic Reduction

Selective Non - Catalytic Reduction

Electron Beam Radiation

Staged Combustion

F l D it ti


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Fuel Denitrogenation

o One approach of fuel denitrogenation is to remove a large part of

the nitrogen contained in the fuels. Nitrogen is removed from liquid

fuels by mixing the fuels with hydrogen gas, heating the mixture

and using a catalyst to cause nitrogen in the fuel and gaseoushydrogen to unite. This produces ammonia and cleaner fuel.

This technology can reduce the nitrogen contained in both

naturally occurring and synthetic fuels.

C b ti M difi ti


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Combustion Modification

Combustion control uses one of thefollowing strategies: Reduce peak temperatures of the flame zone. The methods are :

increase the rate of flame cooling

decrease the adiabatic flame temperature by dilution

Reduce residence time in the flame zone. For this we,

change the shape of the flame zone

Reduce Oxygen concentration in the flame one. This can be

accomplished by: decreasing the excess air

controlled mixing of fuel and air

using a fuel rich primary flame zone

M difi ti Of O ti C diti


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Modification Of Operating Conditions

The operating conditions can be modified to

achieve significant reductions in the rate of

thermal NOx production. the various methods

are:

Low-excess firing

Off-stoichiometric combustion ( staged combustion )

Flue gas recirculation Reduced air preheat

Reduced firing rates

Water Injection

Tail end Control Processes


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Tail-end Control Processes

o Combustion modification and modification of operating

conditions provide significant reductions in NOx, but not

enough to meet regulations.

For further reduction in emissions, tail-end control equipment is

required.

Some of the control processes are:

Selective Catalytic Reduction

Selective Non-catalytic Reduction


Staged Combustion

Selective Catalytic Reduction (SCR)


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Selective Catalytic Reduction (SCR)

In this process, the nitrogen oxides in the flue gases are reduced to

nitrogen

During this process, only the NOx species are reduced

NH3 is used as a reducing gas

The catalyst is a combination of titanium and vanadium oxides. The

reactions are given below :

4 NO + 4 NH3 + O2 -----> 4N2 + 6H2O

2NO2 + 4 NH3+ O2 -----> 3N2 + 6H2O

Selective catalytic reduction catalyst is best at around 300 to 400 oC.

Typical efficiencies are around 80 %



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Irradiation of flue gases containing NOx or SOxproduce nitrate and sulfate ions.

The addition of water and ammonia produces NH4NO3,

and (NH4)2SO4

The solids are removed from the gas, and are sold asfertilizers.

Staged Combustion


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Staged Combustion

PRINCIPLE

Initially, less air is supplied to bring about incomplete

combustion

Nitrogen is not oxidized. Carbon particles and CO are released.

In the second stage, more air is supplied to complete the

combustion of carbon and carbon monoxide.

30% to 50% reductions in NOx emissions are achieved.


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CARBON MONOXIDE CONTROL

Formation Of Carbon Monoxide


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Formation Of Carbon Monoxide

Due to insufficient oxygen

Factors affecting Carbon monoxide

formation:

Fuel-air ratio

Degree of mixing

Temperature

General Methods For Control of CO


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Emissions

Control carbon monoxide formation.Note : CO & NOx control strategies are in conflict.

Stationary Sources

Proper Design

Installation

Operation

Maintenance

Process Industries

Burn in furnaces or waste heat boilers.


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CONTROL OF MERCURY EMISSIONS

Mercury Emissions


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Mercury Emissions

Mercury exists in trace amounts in Fossil fuels such as Coal, Oil, and Natural Gas

Vegetation

Waste products

Mercury is released to the atmosphere throughcombustion or natural processes

It creates both human and environmental risks

Fish consumption is the primary pathway for humanand wildlife exposure

United states is the first country in the world toregulate mercury emissions from coal-fired powerplants (March 15, 2005).

Types of Sources


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Source: Seingeur, 2004 and Mason and Sheu, 2002.

Source: Presentation by J. Pacyna and J. Munthe at mercury workshop in Brussels,

March 29-30, 2004

Worldwide Distribution of Emissions

Control Technologies for Mercury


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Emissions

Currently installed control devices for SO2, NOX,and particulates, in apower plant, remove some of the mercury before releasing from the

stack

Activated Carbon Injection:

Particles of activated carbon are injected into the exit gas flow, downstreamof the boiler. The mercury attaches to the carbon particles and is removed in

a particle control device

Thief process for the removal of mercury from flue gas:

It is a process which extracts partially burned coal from a pulverized coal-

fired combustor using a suction pipe, or "thief," and injects the resulting

sorbent into the flue gas to capture the mercury.

Control Techniques For VOCs


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The dominant source of VOC emissions is the vaporization of

organic compounds used in industrial processes.

A variety of techniques can be used to reduce VOC emissions.

Using material containing an inherently low quantity of VOCcompounds will reduce the release of VOCs.

Also, the processes can be redesigned to reduce the quantitiesthat are lost as fugitive emissions.


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When these techniques are inapplicable or insufficient,

add-on control systems, such as the techniques listedbelow, can be used:

Thermal oxidation

Catalytic oxidation

Adsorption

Condensation and refrigeration

Biological oxidation

Thermal Oxidation


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In a thermal oxidizer, the VOC-laden air stream isheated to gas temperatures several hundred degreesFahrenheit above the autoignition temperatures ofthe organic compounds that need to be oxidized.

Due to these very high temperatures, thermaloxidizers have refractory-lined combustion chambers(also called fume incinerators), which increase their

weight and size considerably.

A sketch of a thermal oxidizer is shown in Figure 1.


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The VOC-laden gas stream is held at this temperature

for residence times ranging from a fraction of asecond to more than two seconds.

Temperatures of the exhaust gas from the refractory-lined combustion chambers are often 1,000 to2,000F.

Thermal oxidizers usually provide VOC destructionefficiencies that exceed 95% and often exceed 99%.


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One of the main limitations of thermal oxidizers is the

large amount of fuel required to heat the gas stream to thetemperature necessary for high-efficiency VOCdestruction.

Heat exchangers are used to recover some of this heat.

The heat exchanger shown in Figure 1 is sometimes calleda recuperative heat exchanger.

This type of heat exchanger has a heat recovery efficiencyranging from 30 to 60% depending on the size of the unit.


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Some types of thermal oxidizers use large regenerative

beds for heat exchange.

These beds have heat recovery efficiencies up to 95%.

Due to the amount of heat that can be recovered andreturned to the inlet gas stream,

these units, termed regenerative thermal oxidizers (RTOs)require less fuel to maintain the combustion chamber atthe necessary temperature.


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Thermal oxidizers have the broadest applicability of all the

VOC control devices.

They can be used for almost any VOC compound.

Thermal oxidizers can also be used for gas streams havingVOC concentrations at the very low concentration range ofless than 10 ppm up to the very high concentrationsapproaching 10,000 ppm.

Thermal oxidizers are rarely used on gas streams havingVOC concentrations exceeding approximately 25% of thelower explosive limit (LEL).

This limit is imposed by safety constraints due to the possibilitythat a short-term concentration spike would exceed the LEL, andth t ld l d


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the gas stream would explode.

The 25% LEL limit depends on the actual gas constituentsand usually is in the 10,000 to 20,000 ppm range.

Thermal oxidizers handling VOC materials that contain

chlorine, fluorine, or bromine atoms generate HCl, Cl2, HF,and HBr as additional reaction products during oxidation.

A gaseous absorber (scrubber) is used as part of the airpollution control system to collect these contaminantsprior to gas stream release to the atmosphere.

Catalytic Oxidation


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y

Catalytic oxidizers operate at substantially lower temperatures

than thermal oxidizers.

Due to the presence of the catalyst, oxidation reactions can beperformed at temperatures in the range of 500 to 1000F.

Common types of catalysts include noble metals (i.e. platinumand palladium) and ceramic materials.

VOC destruction by catalytic oxidizers usually exceeds 95% and

often exceeds 99%.

A sketch of a catalytic oxidizer is shown in Figure 2.


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Due to the relatively low gas temperatures in the

combustion chamber, there is no need for a refractorylining to protect the oxidizer shell.

This minimizes the overall weight of catalytic oxidizers andprovides an option for mounting the units on roofs close tothe point of VOC generation.

This placement can reduce the overall cost of the system bylimiting the distance the VOC-laden stream must betransported in ductwork.


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Catalytic oxidizers are also applicable to a wide range ofVOC-laden streams;

however, they cannot be used on sources that alsogenerate small quantities of catalyst poisons.

Catalyst poisons are compounds that react chemically in anirreversible manner with the catalyst.

Common catalyst poisons include phosphorus, tin, andzinc.

Another potential operating problem associated with


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Another potential operating problem associated withcatalytic oxidizers is their vulnerability to chemicals and/or

particulate matter that masks or fouls the surface of thecatalyst.

(Masking is the reversible reaction of a chemical with thecatalyst and fouling is the coating of the catalyst with a

deposited material.)

If the conditions are potentially severe, catalytic units arenot installed.

As with thermal oxidizers, catalytic oxidizers should notexceed 25% of the LEL, a value that is often equivalent to aVOC concentration of 10,000 to 20,000 ppm.

Adsorption


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Adsorption systems beds are generally used in thefollowing two quite different situations:

1- When the VOC-laden gas stream only contains one to three

organic solvent compounds, and it is economical to recoverand reuse these compounds, or

2- When the VOC-laden gas stream contains a large number

of organic compounds at low concentration, and it isnecessary to preconcentrate these organics prior tothermal or catalytic oxidation.


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A flowchart for a multi-bed adsorber system used for

collection and recovery of organic solventcompounds is shown in Figure 3.


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The VOC-laden gas is often cooled prior to entry into the

adsorption system because the effectiveness of adsorptionimproves at cold temperatures.

As the gas stream passes through the bed, the organiccompounds adsorb weakly onto the surfaces of theactivated carbon, zeolite, or organic polymer used as theadsorbent.

Essentially all of the commercially used adsorbents have a

very high surface area per gram of material.

When the adsorbent is approaching saturation with organicvapor, a bed is isolated from the gas stream and desorbed.


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Low-pressure steam or hot nitrogen gas is often used toremove the weakly adsorbed organics.

The concentrated stream from the desorption cycle istreated to recover the organic compounds.

After desorption, the adsorption bed is returned to service,and another bed in the system is isolated and desorbed.


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An adsorption system used for preconcentration is smaller

than a system similar to the one in Figure 3 for solventrecovery.

In preconcentrator systems, the VOC-laden stream passesthrough a rotary wheel containing zeolite or carbon-basedadsorbents.

Approximately 75-90% of the wheel is in adsorption servicewhile the remaining portion of the adsorbent passesthrough an area where the organics are desorbed into avery small, moderately hot gas stream.

The concentrated organic vapors are then transported to athermal or catalytic oxidizer for destruction.


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The preconcentration step substantially reduces the fuelrequirements for the thermal or catalytic oxidizer.


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Adsorption systems (in general) are usually limited to

sources generating organic compounds having a molecularweight of more than 50 and less than approximately 200.

The low molecular weight organics usually do not adsorbsufficiently.

The high molecular weight compounds adsorb so stronglythat is it is difficult to remove these materials from theadsorbent during the desorption cycle.


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These molecular weights are provided as a guideline and

the suitability of an adsorption system for a particularsituation should be considered on a case-by-case basis.

Adsorption systems can be used for a wide range of VOCconcentrations from less than 10 ppm to approximately10,000 ppm.

The upper concentration limit is due to the potentialexplosion hazards when the total VOC concentrationexceeds 25% of the LEL.


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Adsorption systems are not recommended for gas

streams that contain particulate matter and/or highmoisture concentrations, because the particulatematter and moisture compete with the gaseouspollutants for pore space on the adsorbent material.

The adsorption removal efficiency usually exceeds95% and is often in the 98% to 99% range for bothsolvent recovery and preconcentrator type systems.

In both types of units, the removal efficiencyincreases with reduced gas temperatures.

Condensation, Refrigeration, and Cryogenics


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Condensation, refrigeration, and cryogenic systemsremove organic vapor by making them condense on cold

surfaces.

These cold conditions can be created by passing cold waterthrough an indirect heat exchanger, by spraying cold liquidinto an open chamber with the gas stream,

- by using a freon-based refrigerant to create very cold coils,or by injecting cryogenic gases such as liquid nitrogen intothe gas stream.


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The concentration of VOCs is reduced to the level

equivalent to the vapor pressures of the compounds at theoperating temperature.

Condensation and refrigeration systems are usually used

on high concentration, low gas flow rate sources.

Typical applications include gasoline loading terminals andchemical reaction vessels.

The removal efficiencies attainable with this approachdepend strongly on the outlet gas temperature


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For cold-water-based condensation systems, the outlet gas

temperature is usually in the 40 to 50F range,

- and the VOC removal efficiencies are in the 90 to 99% rangedepending on the vapor pressures of the specificcompounds.

For refrigerant and cryogenic systems, the removalefficiencies can be considerably above 99%,

- due to the extremely low vapor pressures of essentially allVOC compounds at the very low operating temperatures of-70F to less than -200F.


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Condensation, refrigeration, and cryogenic systemsare usually used on gas streams that contain onlyVOC compounds.

High particulate concentrations are rare in the typesof applications that can usually apply this type ofVOC control system.

However, if particulate matter is present, it couldaccumulate on heat exchange surfaces and reduceheat transfer efficiency.

Biological Oxidation


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Biological systems are a relatively new control device in the

air pollution control field.

VOCs can be removed by forcing them to absorb into anaqueous liquid or moist media inoculated withmicroorganisms that consume the dissolved and/oradsorbed organic compounds.

The control systems usually consist of an irrigated packedbed that hosts the microorganisms (biofilters).

A presaturator is often placed ahead of the biologicalsystem to increase the gas stream relative humidity to morethan 95%.


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The gas stream temperatures are maintained at less than

approximately 105F to avoid harming the organisms and to

prevent excessive moisture loss from the media.

Biological oxidation systems are used primarily for very low

concentration VOC-laden streams.

The VOC inlet concentrations are often less than 500 ppm and

sometimes less than 100 ppm.


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The overall VOC destruction efficiencies are oftenabove 95%.

Biological oxidation systems are used for a wide

variety of organic compounds; however, there arecertain materials that are toxic to the organisms.

In these cases, an alternative type of VOC controlsystem is needed.

General Applicability of VOC Control Systems


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Figures 5 and 6 summarize the general applicability of VOC

control systems.

These two charts apply to gas streams having total VOCconcentrations less than approximately 25% of the LEL.

If the concentrations are above this value, units such asflares (not discussed) are used for control.

Control system applicability has been divided into twoseparate groups: low VOC concentration and high VOCconcentration.


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There is no generally accepted distinction between these

two groups.

For the purposes of Basic Concepts in Environmental

Sciences, total VOC concentrations less than 500 ppm areconsidered low.

The low concentration group is further divided into threemain categories depending on the number of different VOCcompounds in the gas stream and the value of recoveringthese compounds for re-use.


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If there are a large number of separate VOC compounds, it

is usually not economically feasible to recover and reusethe captured organics.

In this case, thermal or catalytic oxidizers are used to

oxidize the VOC compounds.

Adsorbers can also be used as independent control systemsor as preconcentrators for the oxidizers.

If there are a very limited number of VOC compounds (lessthan or equal to 3), it is usually possible to use eitheradsorbers or biological oxidation systems


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It is necessary to confirm that the compounds can be

desorbed from regenerative-type adsorbers and that thespecific organics are not toxic to the microorganisms inbiological oxidation systems.

Both thermal and catalytic oxidizers can also be used forthese types of gas streams.

If recovery and reuse are necessary, an adsorber system isgenerally used as the control technique.

Due to the low VOC concentrations, the cost of organiccompound recovery can be quite high.


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air pollution management.pptx

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