AIR POLLUTION EXAM Flashcards
2 Concepts of Control
- Preventative measures
- Control measures using equipment
- Selection of suitable fuel (Low sulphur coal in power plant, use of CNG)
- Modification in industrial process
- Selection of suitable site and zoning for industrial unit
Preventative Measures (Source Control)
MTBE
Methyl tert-Butyl Ether
Any vehicle propelled by or through combustion of carbon-based or other
fuel, constructed and operated principally for the conveyance of persons
or the transportation of property or goods in a public highway or street
open to public use
Mobile source
Any building or immobile structure, facility or installation which emits or may emit any pollutant
Stationary source
It has three simultaneous tasks:
* 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.
Three Way Catalytic Converter
Level of control (Stationary sources):
* Switching to less sulphur and nitrogen fuel
Pre-Combustion Control
Level of control (Stationary sources):
* Improving the combustion process
* New burners to reduce NOx
* New fluidized bed boilers
* Integrated gasification combined cycle
Combustion Control
Level of control (Stationary sources):
* Particulate collection devices
* Flue gas desulphurization
Post-Combustion Control
The Philippine
Clean Air Act, 1999
RA 8749
sets the tools to control air pollutant emissions
Air quality management
The selection of control technologies depends on
environmental, engineering,
economic factors, and pollutant type
- _______ use the force of gravity to remove solid particles.
- The gas stream enters a chamber where the velocity of the gas is reduced.
- Large particles drop out of the gas and are recollected in hoppers.
- Because settling chambers are effective in removing only larger particles, they are
used in conjunction with a more efficient control device.
Settling Chambers
- The general principle of inertia separation is that the particulate laden gas is forced to change direction.
- As gas changes direction, the inertia of the particles causes them to continue in the original direction and be separated from the gas stream.
- The walls of the ____ narrow toward the bottom of the unit, allowing the particles to be collected in a hopper.
- The cleaner air leaves the ____ through the top of the
chamber, flowing upward in a spiral vortex, formed within a downward moving spiral. - ____ are efficient in removing large particles but are not as efficient with smaller particles. For this reason, they are used with other particulate control devices.
Cyclones
- ____ use a liquid stream to remove solid particles.
- In the ____, gas laden with particulate matter passes
through a short tube with flared ends and a constricted middle. - This constriction causes the gas stream to speed up when the pressure is increased.
Venturi Scrubbers
- An ____ is a particle control device that uses electrical forces to move the particles out of the flowing gas stream and onto collector plates.
- The ____ places electrical charges on the particles, causing them to be attracted to oppositely charged metal plates located in the equipment.
- The particles are removed from the plates by “rapping” and collected in a hopper located below the unit.
Electrostatic Precipitators (ESPs)
Particulates-Controlling Equipment
- Gravitational Settling Chamber
- Cyclone Separator
- Electrostatic Precipitators
- Fabric Filters
- Wet Gas Scrubbers
- ____ are 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
floor. - The usual velocity is between 0.5 to 2.5 m/s.
- Velocity of flue gas reduced in large chamber.
- Particles settle under gravitational force.
Gravitational Settling Chamber
Advantages
* Low initial cost.
* Easy to design.
* Low pressure drop.
* Low maintenance cost.
* Dry and continuous disposal of solid particulates.
Disadvantages
* Require large space.
* Less collection efficiency.
* Only larger size particles can be collected.
Gravitational Settling Chamber
- Centrifugal force is utilized to separate the particulate matter.
- It can remove 10 to 50 μm particle size.
- Used mostly in industries
- A ____ consists of a cylindrical shell, conical base, dust
hopper and an inlet where the dust-laden gas enters tangentially. - Under the influence of the centrifugal force generated by the spinning gas, the solid particles are thrown to the wall of the equipment as the gas spirals upward at the inside of the cone.
- The particles slide down the walls of the cone and into the hopper. The operating efficiency depends on the magnitude of the centrifugal force exerted on the particles.
- The greater the centrifugal force, the greater the spreading efficiency.
cyclone separator
Advantages
* Low initial cost.
* Require less floor area.
* Simple construction and maintenance.
* Can handle large volume of gas at high temp.
Disadvantages
* Requires large head room.
* Less efficiency for smaller particles (<10μm).
* Sensitive to variable dust load and flow rate.
cyclone separator
- One of the most widely used device for
controlling particulate emission at industrial
installations ranging from power plants,
cement, and paper mills to oil refineries
*A physical process by which particles
suspended in gas stream are charged
electrically and, under the influence of the
electrical field, separated from the gas
stream
- Works on the principle of electrical charging of particulate matter (negative) and collecting it in a positively -charged surface
.
Electrostatic Precipitators
- At a very high DC voltage, of the order of 50kV, a corona discharge occurs close to the negative electrode, setting up an electric field between the emitted and the grounded surface.
- 99% efficiency.
- Can remove particle size range of 0.1 μm to 1 μm.
Electrostatic Precipitators
Advantages
* High collection efficiency.
* Particles may be collected dry or wet.
* Can be operated at high temp. (300-450˚c).
* Maintenance is normal.
* Few moving parts.
Disadvantages
* High initial cost.
* Require high voltage.
* Collection efficiency reduce with time.
* Space requirement is more.
* Possible of explosion during collection of combustible gases or particulates
Electrostatic Precipitators
- Flue gas is allowed to pass
through a woven fabric, which
filters out particulate matter. - Small particles are retained on
the fabric. - Remove particles up to 1 μm.
- Its efficiency up to 99%.
Fabric Filters
Advantages
* Higher collection efficiency for smaller than
10 μm particle size
* Performance decrease becomes visible, giving prewarning.
* Normal power consumption
Disadvantages
* High temperature gases need to be cooled
* High maintenance and fabric replacement cost
* Large size equipment
* Fabric is liable to chemical attack
Fabric Filters
- ____ removes particulate matter from gas streams by incorporating the particles into liquid droplets directly on contact.
- The basic function is to provide contact between the scrubbing liquid, usually water and, the particulate to be collected.
- This contact can be achieved in a variety of ways as the particles are confronted with so-called impaction target, which can be wetted surface.
Wet Gas Scrubbers
Major Types of Wet Gas Scrubbers
- Plate/Tray Tower Scrubber
- Packed-Bed Scrubber
- Spray Scrubber
- Venturi Scrubber
- Cyclone Scrubber
- Baffle Scrubber
- Impingement-Entrainment Scrubber
- Fluidized-Bed Scrubber
Type of Wet Gas Scrubbers:
- It contains a vertical tower containing
one or more horizontal plates (trays). - Gas enters the bottom of the tower
and must pass through perforations in
each plate as it flows countercurrent
to the descending water stream. - Collection efficiency increases as the
diameter of the perforations
decreases. - A cut diameter, that collected with
50% efficiency, of about μm
aerodynamic diameter can be
achieved with 3.2-mm-diameter holes
in a sieve plate.
Plate or Tray Tower Scrubber
Type of Wet Gas Scrubbers:
- Operates similarly to packed-bed gas absorber.
- Collection efficiency increases as packing size
decreases. - A cut diameter of 1.5 μm aerodynamic diameter
can be attained in columns packed with 2.5 cm elements.
Packed – Bed Scrubber
Type of Wet Gas Scrubbers:
A moving gas stream
is used to atomize
liquids into droplets.
High gas velocities
(60 to 120 m/s) lead
to high relative
velocities between
gas and particles and
promote collection.
Venturi Scrubber
Type of Wet Gas Scrubbers:
- Drops can be introduced
into the gas stream of a
cyclone to collect particles. - The spray can be directed
outward from a central
manifold or inward from
the collector wall.
Cyclone Scrubber
Type of Wet Gas Scrubbers:
- Particles are collected by liquid drops
that have been atomized by spray
nozzles - Horizontal and vertical gas flows are
used, as well as spray introduced cocurrent, countercurrent, or cross-flow to the gas
Spray Scrubber
Type of Wet Gas Scrubbers:
- The gas is forced to
impinge on a liquid surface
to reach a gas exit. Some
of the liquid atomizes into
drops that are entrained
by the gas. - The gas exit is designed so
as to minimize the loss of
entrained droplets.
Impingement-Entrainment
Scrubber
Type of Wet Gas Scrubbers:
- A zone of fluidized
packing is provided
where gas and liquid can
mix intimately. - Gas passes upward
through the packing,
while liquid is sprayed up
from the bottom and/or
flows down over the top
of the fluidized layer of
packing
Fluidized-Bed Scrubber
Selection Criteria for Equipment
- Environmental
- Engineering
- Economic
Commonly used control technologies for gaseous pollutants
- Absorption
- Adsorption
- Condensation
- Incineration (Combustion)
The removal of one or more selected components from a gas mixture and probably the most important operation in the control of gaseous pollutant emissions.
Absorption
is a process in which a gaseous pollutant is dissolved in a liquid
Absorption
When a gas or vapor is brought into contact with a solid, part of it is taken up
by the solid.
Adsorption
The most common industrial adsorbents are activated carbon, silica gel, and
alumina, because they have enormous surface areas per unit weight.
Adsorption
- Usually contain more than one carbon bed.
- As one bed actively removes pollutants, another bed is being regenerated for future
use.
Regenerative Systems
the spent carbon is disposed of when it becomes
saturated with the pollutant
Non-Regenerative Systems
The process of converting a gas or vapor to liquid
Condensation
In a ___ condenser, the gas comes into contact with cold liquid.
contact
In a ____ condenser, the gas contacts a cooled surface in which cooled liquid or gas is circulated, such as the
outside of the tube.
surface
A device in which air and all the combustible waste gases react at the burner
Direct combustor
In _____ incinerators the combustible waste gases pass over or around a
burner flame into a residence chamber where oxidation of the waste gases
is completed.
thermal
_____ incinerators are very similar to thermal incinerators. The main difference
is that after passing through the flame area, the gases pass over a catalyst bed.
Catalytic
The water systems of a production or processing site (tanks, ponds,
sewer system drains, are the main source of
secondary emissions
used to reduce the mineral and ash content in coal
Pre-combustion cleaning
second largest producer of cement after China
India