Download - Air Control Devices
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AIR POLLUTION CONTROL DEVICES: for CRITERIA
POLLUTANTS including VOCs
1. Ground Level O32. CO
3. SO24. NO2
5. Pb6. Particulate Matter
a) PM10b) Total Suspended Particulate Matter (TSP)
Particles ranging in size from 0.1 micrometer to about 30
micrometer in diameter are referred to as total
suspended particulate matter (TSP). TSP includes a broad
range of particle sizes including fine, coarse, and
supercoarse particles.
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National Ambient Air Quality Guideline for Criteria Pollutants:
Philippine Clean Air Act of 1999 (R.A. 8749)
utants
Short Terma Long Termb
m g/Ncm ppm
Averaging
Time m g/Ncm ppm Averaging Time
Suspended Particulate
Matterc TSP
PM-10
230d150f 24 hours24
hours
9060 1 yeare1 yeare
Sulfur Dioxidec 180 0.07 24 hours 80 0.03 1 year
Nitrogen Dioxide 150 0.08 24 hours
Photochemical
Oxidants as Ozone
14060 0.070.03 1 hour8
hours
Carbon Monoxide 35
mg/Ncm10
mg/Ncm
309 1 hour8
hours
Leadg 1.5 3 monthsg 1.0 1 year
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Good 0.000 0.034
Fair 0.035 0.144
Unhealthy for
sensitive groups
0.145 0.224
Very Unhealthy 0.225 0.304
Acutely unhealthy 0.305 0.604
Emergency 0.605 0.804
Sulfur Dioxide (ppm) [24-hour]
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Good None
Fair None
Unhealthy for sensitive
groups
People with respiratory disease, such as asthma, should
limit outdoor exertion.
Very unhealthy Pedestrians should avoid heavy traffic areas. People with
heart or respiratory disease, such as asthma, should stay
indoors and rest as much as possible. Unnecessary trips
should be postponed. People should voluntarily restrict
the use of vehicles.
Acutely unhealthy People, should limit outdoor exertion. People with heart
or respiratory disease, such as asthma, should stay indoors
and rest as much as possible. Unnecessary trips should be
postponed. Motor vehicle use may be restricted. Industrial
activities may be curtailed.
Emergency Everyone should remain indoors, (keeping windows and
doors closed unless heat stress is possible). Motor vehicle
use should be prohibited except for emergency situations.
Industrial activities, except that which is vital for public
safety and health, should be curtailed.
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AIR POLLUTION CONTROL DEVICES
1. NOx CONTROL and REMOVAL
Fuel switching
Conversion to a fuel with a lower nitrogen content or one that
burns at a lower temperature may result in a reduction ofNOx
emissions.
Combustion of natural gas or distillate oils tends to result in lowerNOx emissions than is the case for coal or heavy fuel oils.
technical constraints and the availability and costs of alternative
fuels are major considerations in determining the viability of fuel
switching.
fuel switching may result in greater emissions of other criteriapollutants.
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Fuel DenitrificationFuel denitrification of coal or heavy oils could, in principle, be used to
control fuelNox formation.
Denitrification currently occurs as a side benefit of fuel pretreatment to
remove other pollutants, such as pretreatment of oil by desulfurization
and chemical cleaning, or solvent refining of coal for ash and sulfur
removal.
The low denitrification efficiency
and high costs of these processes do
not make them attractive on the
basis ofNOx control alone, but they
may prove cost effective on the basisof total environmental impact.
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PROCESS MODIFICATION POST PROCESS MODIFICATION
Low NOx burners Selective catalytic reduction
Natural gas burner/reburn Selective noncatalytic reduction
Water/stream injection Nonselective catalytic reduction
Staged combustion
Flue gas recirculation
Low excess air
Staged overfire air
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NOx
NOx
NH3NOx
NH3NOx
N2H2O
N2H2O
FLUE
GAS
CLEAN
FLUE GAS
CATALYST BED
Removal of NOx by SCR
SELECTIVE CATALYTIC REACTOR (SCR)
4NO + 4NH3
+ O2
4N2
+ 6H2
O
2NO2 + 4NH3 + O2 3N2 + 6H2O
2NO + (NH2)2CO + O2 2N2 + 2H2O + CO2
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CONTROL OF SULFUR DIOXIDE EMISSIONS
1. PROCESS MOFIFICATION OPTIONSFuel Switching
Coal Washing
Coal Gasification and Liquefaction
Fuel Switching
Many coal-fired facilities attempt to reduce these emissions byswitching to coal with a lower sulfur content, such as subbituminous
coal which generally contains less sulfur than bituminous coal.
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Coal Gasification and Liquefaction
Organic sulfur, which is part of themolecular structure of the coal,
cannot be removed by washing or
other physical cleaning processes.
Chemical desulfurization of
organic sulfur from coal is extremely
expensive.
Coal gasification and liquefaction
can remove much of the organic
sulfur, but results in a substantial loss
of total available heating value.
Coal Washing
Much of the sulfur in coal is in pyrite (FeS2) or in mineral sulfate
form, much of which can be removed by washing or other physicalcleaning processes.
However, disposal of the solid or liquid wastes formed during these
processes can be difficult and/or expensive.
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Desulfurization of Oil and Natural Gas
The sulfur in crude oils and natural gas can be removedeasily and economically and the elemental sulfur recovered
as a by-product can be sold as a raw material.
The steps in the desulfurization of oil or natural gas are:
R-S + H2 H2S + R (where R represents any organic
group)
H2S + 3/2 O2 H2O + SO2
2H2S + SO2 2H2O + 3S2H2S + SO2 2H2O + 3S
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Fluidized Bed Combustion
Also, combustion of crushed coal in a bed of a sorbent material
(fluidized-bed combustion) can reduce SO2 emissions. Sulfur dioxide inthe coal reacts with limestone or dolomite in the bed to form gypsum.
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2. POST PROCESS MODIFICATION
Dry and wet scrubbing
are the most common
technologies to
desulfurize flue gas.
Slurries of sorbent and
water react with SO2 inthe flue gas.
SO2 emission control contd..
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Particulate Matter CONTROL
1. PROCESS MODIFICATIONS
Process controls typically used to control particulate
matter emissions include fuel switching, coal cleaning,
and good combustion practices.
Particulate matter," also known as particle pollution or PM, isa complex mixture of extremely small particles and liquid
droplets. Particle pollution is made up of a number of
components, including acids (such as nitrates and sulfates),
organic chemicals, metals, and soil or dust particles.
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2. POST-PROCESS AIR POLLUTION CONTROL DEVICESFour classes of control equipment are used to remove
PM from gas streams:
a. Mechanical collectors such as cyclones
b. Electrostatic precipitators
c. Fabric filters, also referred to as baghouses
d. Wet PM scrubbers
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CYCLONE
Cyclones are essentially cylinders
with inlet and outlet ducts for theair stream. A vortex is created in the
cylindrical section of the cyclone
either by injecting the air stream
tangentially or by passing the gas
through a series of vanes. As the
particulate-laden gas is forced to
change
direction in the vortex, the inertia of
the particles forces them tocontinue in the original direction,
collide with the outer wall, and slide
downward to the bottom of the
device to be collected in a hopper.
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ELECTORSTATIC PRECIPITATORS
Electrostatic precipitators use an electrostatic field to chargeparticulate matter in the flue gas stream. The charged
particles then migrate to a grounded collection surface. The
collected particles are periodically dislodged from the
collection surface by vibration or rapping.
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FABRIC FILTERS
* also referred to as baghouses* capable of achieving the highest particulate removal
efficiencies of all the particulate control devices.
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A fabric filter system consists of several filtering elements
(bags), a bag cleaning system, and dust hoppers contained
in a main shell structure. Fabric filters remove dust from agas stream by passing thestream through a porous
fabric. The fabric does some
of the filtering, but plays a
more important role by
acting as a support medium
for the layer of dust that
quickly accumulates on it.
The dust layer (cake) isresponsible for the highly
efficient filtering of small
particles, but also increases
the resistance to gas flow.
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PM WET SCRUBBER
Wet PM scrubbers control PM and acid gases, with somecontrol of organics. Wet PM scrubbers are applied as a
post-process technique to:
1. Scrub particulates from incinerator exhausts;
2.Control particulate and gaseous emissions
simultaneously;3. Control acid gases;
4. Control sticky emissions that would otherwise plug
filter-type collectors;
5. Recover soluble dusts and powders; and6. Control metallic powders such as aluminum dust that
tend to explode if handled dry.
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How does WET PM Scrubbers work?
Wet PM scrubbers remove particles from gas by capturing
the particles in liquid droplets (usually water) and
separating the droplets from the gas stream. The goal is to
cause the tiny pollutant particle to be lodged inside the
collecting droplet and then to remove the larger dropletfrom the gas stream. In general, the smaller the target
droplet, the smaller the size of particulate that can be
captured and the more densely the droplets are packed, the
greater the probability of capture.
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CONTROL OF VOLATILE ORGANIC COMPOUNDS
1. PROCESS MODIFICATION STRATEGIES
Typical strategies are:
Change of coating formulation, such as conversion to water-based
paint;
Change from a VOC-based coating to a non-liquid coating such aspowder coat; and
Change to coating methods that increase transfer efficiency and
reduce total coatings used per application.
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VOCs POST-PROCESS AIR POLLUTION CONTROL DEVICES
Typical post-process control devices ofVOC are:1. Carbon adsorber
2. Incinerator
3. Floating-roof storage tank
4. Vapor capture device during tank filling; and
5. Fluid capture, recycle, and reuse.
Many VOC emission sources are processes that are not enclosed or
contained and the VOC are emitted into the ambient work area. Before
the emissions can be routed to a control device, they must first be
captured.
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Incinerators are used to control emission from curing
ovens, dryers, and other sources of organic vapors.Incinerators are
installed to allow
compliance with
regulatoryrequirements.
The ultimate function
of incineration is to
achieve complete
combustion. Incomplete
combustion can form
new, potentially more
toxic compounds.
INCINERATORS
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TwoMajor types: Thermal and catalytic incinerators
Major Components of an incinerator:
1. Burner
- supplies air and fuel needed to provide heat.
- provides turbulent mixing and hot gas which are
necessary to oxidize the VOCs.
2. Heating/oxidizing Chamber
- acts as a holding space acts as a holding space that allows
time for all the vapors to oxidize
3. Heat recovery device
Incinerators include a recovery system to recover heat fromthe ho exhaust gases.
4. Gas stack
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Non refractory linedCatalyst poisons P, Sn, Zn,Masking and fouling
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Operating Principles of Incinerator Systems:
1. Proper Operating Temperature
The burner flame in the main chamber generates hot
combustion gas that heats the relatively cool VOC-
containing gas stream to the combustion temperature.
Combustion happens only when the auto-ignition
temperature.Auto-ignition temperature is the minimum temperature that
must be reached before combustion can occur (800 to
1400oF).
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2. TURBULENCE
Turbulence provides the
mixing needed to bring the
organic material and the O2
together and helps transferheat between the hot
combustion gases and the
other VOC material
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3. OXYGEN
- excess air is used to ensure complete combustion.-Excess air is kept low, heating the excess air requires
additional fuel.
- extreme levels of excess air can reduce incinerator
temperatures, thus reduce VOC removal efficiency
4. Residence Time
Residence time in an incinerator is measured from the time the
waste gas stream reaches the operating temperature, until the
time the waste gas leaves the combustion chamber.
It is determined by the size of the combustion chamber and the
gas flow rate through the chamber.
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5. VOC Concentration
Inlet gas stream VOC concentration are limited to 500-7500
ppm.VOC concentrations are kept below 25% of the Lower
Explosive Limit (LEL) so that the incinerator flame does not
flash back to the process equipment.