ENSC 412612 Week 3 Engineering control of pollutants

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ENSC 412/612 Week 3Engineering control of
pollutants

• READINGS FOR THIS WEEK
• Text CHAPTERS 28, 29, 30, 31
• Millar (2007) paper

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Concepts

• Some sources are uncontrollable (e.g. volcano).
• Cost of controlling a source is usually an
  exponential function of the percentage of control
  
• important consideration in defining the level of
  control required.
• This can change if material recovered has an
  economic value OH Fig 28-2 .

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Control of pollutants

• Designers must be aware of chemical and physical
  properties of effluent wrt changes in
  temperature, volume, moisture, velocity, as well
  as regulatory issues. Control can either be
  removal of pollutant, or conversion to a less
  polluting form.
• Three general methods of control
• process change to a less polluting process or a
  change in the existing process operation.
  pollution prevention'' is often the best and
  most economic solution.
• fuel change - often used in episode management
  (eg switch to natural gas or low sulfur coal when
  stagnation conditions are likely). May be
  difficult since low polluting fuel in high demand
  and may be expensive, and supply hard to obtain.
• installation of control equipment between the
  point of generation and the point of its emission
  into the atmosphere. Efficiency is expressed as a
  percentage of removal on a weight basis.

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• E 100(C/A) but since ABC
• E 100(C/(BC)) 100(A-B)/A
• 100(A- B)/ (BC)
• (ignoring hold-up and loss)

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• If a pollutant is removed, it must be disposed
  of.
• This can create other problems, especially if the
  pollutant is a toxic (now solid) waste.
• To remove a pollutant from the carrying stream,
  some property of the pollutant that is different
  from the carrier must be exploited. (eg. size,
  inertia, electrical or absorption properties)

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Devices and systems

• Control device is specific for pollutant. eg
  control of aerosols different from gases dry
  aerosols different from liquid droplets.
• There are a large number of characteristics which
  must be considered in designing control devices.
• Once equipment is installed, operation and
  maintenance become important. Most regulations
  for new facilities require use of the Best
  Available Control Technology (BACT).

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Control device characteristics
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Removal of dry particulate matter

• Dry aerosols differ so much from the carrying gas
  that their removal can be done in many different
  ways physically, inertially, chemically,
  electrically.

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Filters

• Particles impinges on and adheres to filter
  material, as deposit accumulates it also acts as
  a filtering material. When deposit becomes heavy
  it reduces the flow and must be cleaned or
  replaced. Filter material can be fibrous (eg
  cloth), granular (eg sand), rigid (eg a screen)
  or a mat (eg felt). Removal occurs by
• direct impaction
• inertial impaction (streamline misses filter by
  inertia of particle resists change in direction
  and adheres)
• electrostatic attraction

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• Industrial baghouses are the most common form of
  industrial filtration.
• Pollutant stream is passed through a series of
  bags.
• Filter ratio is expressed as gas to cloth ratio
  (0.6 to 1.5 m3 of gas per minute per m2 fabric).
• Sections of the bags are cleaned periodically by
  shaking, reverse air flow, etc.
• Other methods include
• passing gases through beds of sand or coke, mats
  (throw away filters),
• paper filters,
• rigid porous beds,
• porous ceramics,
• fluidized beds in which the granular material
  forced to act as a fluid by the gas passing
  through it.

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Electrostatic precipitators

• Widely used since start of century,
• very efficient, but more expensive typically than
  filtration.
• Works by charging dust with ions and then
  collecting charged particles on either a tubular
  or flat plate surface.
• Plates are cleaned usually by rapping the surface
  after power is shut off.
• A high voltage (30kV or more) DC field set up
  between a central wire electrode and the grounded
  collecting surface
• cascade of negative ions in the gap bombards and
  charges the particles which then migrate to the
  collecting surface because of both the
  bombardment and the charge attraction.

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Advantages

• Can handle high temperature gases which are often
  a problem for filtration methods.
• small pressure drop so fan costs are minimized
• high collection efficiency if used on proper
  pollutants
• handles a wide range of particle sizes and
  concentrations
• operating and maintenance costs are lower than
  other types

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Disadvantages

• initial cost is the highest
• take up lots of space
• not suitable for combustible particles like grain
  or wood dust

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Inertial collectors

• Operate on principle that aerosol material in the
  carrying gas has a greater inertia than the gas.
• Drag on a particle scales as r2 while inertia
  increases as r3 so that inertial collectors
  become more efficient for larger particles,
  especially those which are dense (since mass as
  well as velocity determine inertia).
• Essential idea is that the stream is forced to
  change direction dramatically - the particles
  because of their inertia resist the change in
  direction, impact, and are removed from the gas
  stream.

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• Types include cyclones, baffles, louvers or
  rotating impellers. OH Fig 29-6 For a cyclone,
  the centrifugal force acting on a particle is
• F mA m V2/R
• so that the inertial impaction increases with
  mass and velocity, and decreases with radius of
  curvature.

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Scrubbers

• Wet collectors are commonly used.
• First particles are wetted, either by bubbling
  the gas through a liquid, or by spraying the gas
  with an atomized liquid.
• Next the particles are removed from the liquid on
  a collecting surface (on a bed, or using an
  inertial collector.
• Efficiency increases with smaller bubbles or
  droplets since the surface area for contact is
  larger.
• Dry scrubbers are gravel bed filters that are
  recirculated and washed.
• Can remove large quantities, but stream may need
  further treatment.

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Removal of liquid droplets

• Similar to dry particles.
• Filters need to be looser,
• ESPs are usually wetted wall tube types
• Inertial collectors are common and have similar
  efficiency as with dry collectors.
• Scrubbers also work effectively (venturi
  scrubbers etc.).
• One problem with venturi scrubbers is that
  cooling can condense some gases resulting in a
  plume with high opacity.

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Removal of gaseous pollutants

• If gases are reactive to other chemicals, this
  property can be used to remove them.
• absorption in a liquid (wet scrubbers)
• used for sulfur dioxide, hydrogen sulfide,
  hydrogen chloride, chlorine, ammonia, oxides of
  nitrogen, and some HC.
• Gas to be removed must be soluble in scrubbing
  liquid.
• Water possibly enhanced with acid or base is most
  commonly used.
• Gases recovered can have economic value when
  removed from the scrubbing solvent.
• adsorption on a solid surface
• gases may be selectively retained on surface of a
  solid,
• can be a surface phenomenon or combined with a
  chemical reaction (chemisorption).
• Porous materials used eg activated carbon,
  alumina, silica gel.
• Materials must be capable of being regenerated
  and reused after saturation with gas molecules.
  Can have efficiencies near 100 until adsorber
  becomes saturated.

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• 3. condensation to a liquid condensers may be
  used first to remove condensible components.
  Accomplished either by reducing T or increasing P
  - usually T drop is used. In contact condensers
  the coolant, vapors and condensate are mixed, in
  surface condensers, coolant does not contact
  vapors or condensate.
• 4. conversion to non-pollutant eg organic gases
  are oxidized by combustion. H2S SO2 and H2O
  even though SO2 is still considered a pollutant
  its odour threshold is 3 orders of magnitude
  higher than for H2S. Usually an open flare is
  used (eg Husky Oil), which may have a pilot or
  afterburner. Afterburners can be direct flame
  (oxidation in a combustion chamber above ignition
  T), or catalytic (oxidation below ignition T).

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• Reasons for condensing
• Recovery of valuable products
• Removal of corrosive or damaging components
• Reduction of effluent gas volume

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Odour removal

• Any objectionable odour above the detection
  threshold will cause complaints.
• Control involves reduction to less than the odour
  threshold, or conversion to a substance that is
  not as odourous. Can be done by

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• dilution the solution to pollution is infinite
  dilution - tall stacks, or by adding air to the
  stream
• removal change in process (eg replace
  formaldehyde based resin with another),
  adsorption, etc.
• conversion eg oxidation of H2S

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Control of stationary sources

• Read text chapter 30, focus here on forest
  production and oil refining.
• Major sources are open burning of forests, wood
  fired power boilers which burn waste materials.

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• For Kraft pulp mills, control is accomplished by
• ESPs on the recovery furnace,
• collection of noncondensible gases from vent
  points (digesters, blow tanks, washers) which are
  ducted to the lime kiln where they are burned,
  and
• wet scrubbers on the lime kiln exhaust to remove
  PM and sulfurous gases

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Control of mobile sources

• Automobiles are the main source.
• Since 1960's crankcase emissions have been
  controlled by PCVs which take gases from the
  crankcase through a control valve into the
  combustion chamber where they are burned.
• evaporative emissions from the fuel tank
  controlled since the 1970s by either a vapour
  recovery system or an adsorption system using
  activated carbon canister.
• exhaust emissions are hardest to control.
• exhaust depends on fuel type, air-fuel ratio,
  ignition timing, compression, engine speed,
  engine condition, coolant temperature, etc.
• focus is on elimination of unburned HC, CO and
  NOx.

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• Control of exhaust for unburned HC and CO
  involves
• fuel modifications
• minimizing pollutants from the combustion chamber
  - better engineering of motors
• oxidation of pollutants outside the combustion
  chamber - either by normal combustion, or by
  catalytic oxidation. Requires pumping of air to
  the exhaust stream.

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• To lower NOx, two systems are used
• exhaust gas recirculation sends part of the
  exhaust stream back into the intake manifold
  which reduces the combustion temperature and
  decreases NOx production (tolerated when
  power'' is not required)
• a second catalytic converter can be used in
  series with the HC/CO converter to decompose NOx
  to O and N.

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• Best options are switching to less polluting
  transportation systems!
• Readings for next week Text Chapters 7,8
• Anderson (2009) and Ashmore Dimitroulopoulou
  (2009) papers