TECHNIQUES%20WITHOUT%20USING%20EMISSION%20CONTROL%20DEVICES

1

• TECHNIQUES WITHOUT USING EMISSION CONTROL DEVICES
  
• Process Change
• Wind, Geothermal, Hydroelectric, or Solar Unit
  instead of Fossil fired Unit.
• Change in Fuel
• e.g. Use of Low Sulfur Fuel, instead of High
  Sulfur fuel.
• Good Operating Practices
• Good Housekeeping
• Maintenance
• Plant Shutdown

2

• COMMONLY USED METHODS FOR AIR POLLUTION CONTROL
• PARTICULATE
• Cyclones
• Electrostatic Precipitators
• Fabric Filter
• Wet Scrubbers
• GASES
• Adsorption Towers
• Thermal Incernation
• Catalytic Combustion

3

• SOx CONTROL

4

• 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

5

• GENERAL METHODS FOR CONTROL OF SO2 EMISSIONS
  (contd.)
• Use Flue Gas Desulfurization Systems
• Use Alternative Energy Sources, such as
  Hydro-Power
• or Nuclear-Power

6

• FLUE GAS DESULFURIZATION
• SO2 scrubbing, or Flue Gas Desulfurization
  processes can be classified 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

7

• 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 ----gt Ca2 2HSO3- CO2
• CaCO3 2HSO3- Ca2 ----gt 2CaSO3 CO2 H2O

8

• LIME SCRUBBING
• The equipment and the processes are similar to
  those in 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 -----gt Ca(OH)2
• SO2 H2O lt----gt H2SO3
• H2SO3 Ca(OH)2 -----gt CaSO3.2 H2O
• CaSO3.2 H2O (1/2)O2 -----gt CaSO4.2 H2O

9

• DUAL ALKALI SYSTEM
• Lime and Limestone scrubbing lead to deposits
  inside spray tower.
• The deposits can lead to plugging of the nozzles
  through which the scrubbing slurry is sprayed.
• The Dual Alkali system uses two regents to remove
  the sulfur dioxide.
• Sodium sulfite / Sodium hydroxide are used for
  the absorption of sulfur dioxide inside the spray
  chamber.
• The resulting sodium salts are soluble in
  water,so no deposits are formed.
• 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 hydroxide is regenerated.

10

• 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 dries before it reaches the bottom of
  the chamber
• The dry solids are carried out with the gas, and
  are collected in fabric filtration unit
• This system needs lower maintenance, lower
  capital costs, and lower energy usage

11

• WELLMAN - LORD PROCESS
• This process consists of the following
  subprocesses
• Flue gas pre-treatment.
• Sulfur dioxide absorption by sodium sulfite
• Purge treatment
• Sodium sulfite regeneration.
• The concentrated sulfur dioxide stream is
  processed to a marketable product.
• The flue gas is pre - treated to remove the
  particulate. The sodium sulfite neutralizes the
  sulfur dioxide
• Na2SO3 SO2 H2O -----gt 2NaHSO3

12

• 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.

13

• NOx CONTROL

14

• 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

15

• GENERAL METHODS FOR CONTROL OF NOx 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

16

• FUEL DENITROGENATION
• 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 gaseous hydrogen to unite. This
  produces ammonia and cleaner fuel.
• This technology can reduce the nitrogen contained
  in both naturally occurring and synthetic fuels.

17

• COMBUSTION MODIFICATION
• Combustion control uses one of the following
  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

18

• 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

19

• TAIL-END CONTROL PROCESSES
• 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
• Electron Beam Radiation
• Staged Combustion

20

• 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 -----gt 4N2 6H2O
• 2NO2 4 NH3 O2 -----gt 3N2 6H2O
• Selective catalytic reduction catalyst is best at
  around 300 too 400 oC.
• Typical efficiencies are around 80

21

• ELECTRON BEAM RADIATION
• This treatment process is under development, and
  is not widely used. Work is underway to determine
  the feasibility of electron beam radiation for
  neutralizing hazardous wastes and air toxics.
• Irradiation of flue gases containing NOx or SOx
  produce 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
  as fertilizers.

22

• 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.

23

• CARBON MONOXIDE CONTROL

24

• GENERAL METHODS FOR CONTROL OF CO 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.

25

• FORMATION OF CARBON MONOXIDE
• Due to insufficient oxygen
• Factors affecting Carbon monoxide formation
• fuel-air ratio
• degree of mixing
• temperature

26

• PARTICULATE MATTER CONTROL

27

• GENERAL METHODS FOR CONTROL OF PARTICULATE
  EMISSIONS
• Five Basic Types of Dust Collectors
• Gravity and Momentum collectors
• settling chambers, louvers, baffle chambers
• Centrifugal Collectors
• cyclones
• mechanical centrifugal collectors
• Fabric Filters
• baghouses
• fabric collectors

28

• GENERAL METHODS FOR CONTROL OF PARTICULATE
  EMISSIONS (contd.)
• Electrostatic Precipitators
• tubular
• plate
• wet
• dry
• Wet Collectors
• spray towers
• impingement scrubbers
• wet cyclones
• peaked towers
• mobile bed scrubbers

29

• PARTICULATE COLLECTION MECHANISM
• Gravity Settling
• Centrifugal Impaction
• Inertial Impaction
• Direct Interception
• Diffusion
• Electrostatic Effects

30

• INDUSTRIAL SOURCES OF PARTICULATE EMISSIONS
• Iron Steel Mills, the blast furnaces, steel
  making furnaces.
• Petroleum Refineries, the catalyst regenerators,
  air-blown asphalt stills, and sludge burners.
• Portland cement industry
• Asphalt batching plants
• Production of sulfuric acid
• Production of phosphoric acid
• Soap and Synthetic detergent manufacturing
• Glass glass fiber industry
• Instant coffee plants

31

• EFFECTS  OF  PARTICULATE  EMISSIONS
• Primary Effects
• Reduction of visibility
• size distribution and refractive index of the
  particles
• direct absorption of light by particles
• direct light scattering by particles
• 150 micro g / m3 concentration average
  visibility of 5 miles
• ( satisfactory for air and ground transportation
  )
• Soiling of nuisance
• increase cost of building maintenance, cleaning
  of furnishings, and households
• threshold limit is 200 - 250 micro g / m3 ( dust
  )
• levels of 400 - 500 micro g / m3 considered as
  nuisance

32

• CYCLONES
• Principle
• The particles are removed by the application of a
  centrifugal force. The polluted gas stream is
  forced into a vortex. the motion of the gas
  exerts a centrifugal force on the particles, and
  they get deposited on the inner surface of the
  cyclones
• Construction and Operation
• The gas enters through the inlet, and is forced
  into a spiral.
• At the bottom, the gas reverses direction and
  flows upwards.
• To prevent particles in the incoming stream from
  contaminating the clean gas, a vortex finder is
  provided to separate them. the cleaned gas flows
  out through the vortex finder.
• .

33

• CYCLONES (contd.)
• Advantages of Cyclones
• Cyclones have a lost capital cost
• Reasonable high efficiency for specially designed
  cyclones.
• They can be used under almost any operating
  condition.
• Cyclones can be constructed of a wide variety of
  materials.
• There are no moving parts, so there are no
  maintenance requirements.
• Disadvantages of Cyclones
• They can be used for small particles
• High pressure drops contribute to increased costs
  of operation.

34

• FABRIC FILTERS
• Principle
• The filters retain particles larger than the mesh
  size
• Air and most of the smaller particles flow
  through. Some of the smaller particles are
  retained due to interception and diffusion.
• The retained particles cause a reduction in the
  mesh size.
• The primary collection is on the layer of
  previously deposited particles
• Advantages of Fabric Filters
• Very high collection efficiency
• They can operate over a wide range of volumetric
  flow rates
• The pressure drops are reasonably low.
• Fabric Filter houses are modular in design, and
  can be pre-assembled at the factory

35

• FABRIC FILTERS (contd.)
• Disadvantages of Fabric Filters
• Fabric Filters require a large floor area.
• The fabric is damaged at high temperature.
• Ordinary fabrics cannot handle corrosive gases.
• Fabric Filters cannot handle moist gas streams
• A fabric filtration unit is a potential fire
  hazard

36

• ELECTROSTATIC PRECIPITATOR
• Principle
• The particles in a polluted gas stream are
  charged by passing them through an electric
  field.
• The charged particles are led through collector
  plates
• The collector plates carry charges opposite to
  that on the particles
• The particles are attracted to these collector
  plates and are thus removed from the gas steam
• Construction and Operation of Electrostatic
  Precipitator
• Charging Electrodes in the form of thin wires are
  placed in the path of the influent gas.
• The charging electrodes generate a strong
  electric field, which charges the particles as
  they flow through it.
• The collector plates get deposited with the
  particles. the particles are occasionally removed
  either by rapping or by washing the collector
  plates.

37

• ELECTROSTATIC PRECIPITATOR (contd.)
• Advantages of Electrostatic Precipitators
• Electrostatic precipitators are capable very high
  efficiency, generally of the order of 99.5-99.9.
  
• Since the electrostatic precipitators act on the
  particles and not on the air, they can handle
  higher loads with lower pressure drops.
• They can operate at higher temperatures.
• The operating costs are generally low.
• Disadvantages of Electrostatic Precipitators
• The initial capital costs are high.
• Although they can be designed for a variety of
  operating conditions, they are not very flexible
  to changes in the operating conditions, once
  installed.
• Particulate with high resistivity may go
  uncollected.

38

• WET SCRUBBERS
• Principle
• Wet scrubbers are used for removal of particles
  which have a diameter of the order of 0.2 mm or
  higher.
• Wet scrubbers work by spraying a stream of fine
  liquid droplets on the incoming stream.
• The droplets capture the particles
• The liquid is subsequently removed for treatment.
  
• Construction and Operation
• A wet scrubber consists of a rectangular or
  circular chamber in which nozzles are mounted.
• The nozzles spray a stream of droplets on the
  incoming gas stream
• The droplets contact the particulate matter, and
  the particles get sorbed.
• The droplet size has to be optimized.

39

• WET SCRUBBERS (contd.)
• Construction and Operation (contd.)
• Smaller droplets provide better cleaning, but are
  more difficult to remove from the cleaned stream.
  
• The polluted spray is collected.
• Particles are settled out or otherwise removed
  from the liquid.
• The liquid is recycled.
• Wet scrubbers are also used for the removal of
  gases from the air streams.

40

• WET SCRUBBERS (contd.)
• Advantages of Wet Scrubbers
• Wet Scrubbers can handle incoming streams at high
  temperature, thus removing the need for
  temperature control equipment.
• Wet scrubbers can handle high particle loading.
• Loading fluctuations do not affect the removal
  efficiency.
• They can handle explosive gases with little risk.
  
• Gas adsorption and dust collection are handled in
  one unit.
• Corrosive gases and dusts are neutralized.
• Disadvantages of Wet Scrubbers
• High potential for corrosive problems
• Effluent scrubbing liquid poses a water pollution
  problem.

41

• CYCLONE SPRAY CHAMBERS
• These scrubbers combine a cyclone with a spray
  nozzle.
• The added centrifugal force permits good
  separation of the droplets, hence a smaller
  droplet size can be used.
• Cyclone spray chambers provide up to 95 removal
  of particles gt 5 micron.

42

• ORIFICE SCRUBBERS
• The gas is impacted onto a layer of the scrubbing
  liquid.
• The gas passes through the liquid, thus removing
  almost all the particulate matter, and a large
  portion of the probable gases.
• After coming out of the liquid, the gas is passed
  through baffles to remove the liquid droplets.

43

• IMPINGEMENT SCRUBBERS
• In Impingement scrubbers, the gas impacts a layer
  of liquid/froth through a perforated tray.
• Passing through this layer removes the
  particulate matter.
• The wet gas stream is then passed through a mist
  collector.

44

• VENTURI SCRUBBERS
• The dirty gas is led in to the chamber at high
  inlet velocities.
• At the inlet throat, liquid at low pressure is
  added to the gas stream
• This increases the relative velocity between the
  gas and the droplets, thus increasing the
  efficiency of removal.
• Efficiencies of the range of 95 for particles
  larger than 0.2 mm have been obtained.

45

• HYDROCARBON CONTROL

46

• GENERAL METHODS FOR CONTROL OF HYDROCARBON
  EMISSIONS
• Incineration or after burning
• Direct flame incineration
• Thermal incineration
• Catalytic incineration

47

• VOC INCINERATORS
• Principle
• VOC incinerators thermally oxidize the effluent
  stream, in the presence of excess air.
• The complete oxidation of the VOC results in the
  formation of carbon monoxide and water. The
  reaction proceeds as follows
• CxHy ( x y/4 ) O2 x CO2 (y/2) H2O
• Operation
• The most important parameters in the design and
  operation of an incineration system are what are
  called the
• ' three T's ' Temperature, Turbulence, and
  residence Time.

48

• VOC INCINERATORS (contd.)
• Temperature
• The reaction kinetics are very sensitive to
  temperature
• The higher the temperature, the faster the
  reaction
• Timing
• A certain time has to be provided for the
  reaction to proceed
• Turbulence
• Turbulence promotes mixing between the VOC's and
  oxygen
• Proper mixing helps the reaction to proceed to
  completion in the given time.

49

• VOC INCINERATORS (contd.)
• The various methods for incineration are
• Elevated fires, for concentrated streams
• Direct thermal oxidation, for dilute streams
• Catalytic oxidation, for dilute streams.

50

• GASES

51

• AIR POLLUTION CONTROL FOR GASES
• Adsorption Towers
• Thermal Incernation
• Catalytic Combustion

52

• ADSORPTION TOWERS
• Principle
• Adsorption towers use adsorbents to remove the
  impurities from the gas stream.
• The impurities bind either physically or
  chemically to the adsorbing material.
• The impurities can be recovered by regenerating
  the adsorbent.
• Adsorption towers can remove low concentrations
  of impurities from the flue gas stream.

53

• ADSORPTION TOWERS (contd.)
• Construction and Operation
• Adsorption towers consist of cylinders packed
  with the adsorbent.
• The adsorbent is supported on a heavy screen
• Since adsorption is temperature dependent, the
  flue gas is temperature conditioned.
• Vapor monitors are provided to detect for large
  concentrations in the effluent. Large
  concentrations of the pollutant in the effluent
  indicate that the adsorbent needs to be
  regenerated.
• Advantages of Adsorption Towers
• Very low concentrations of pollutants can be
  removed.
• Energy consumption is low.
• Do not need much maintenance.
• Economically valuable material can be recovered
  during regeneration.

54

• ADSORPTION TOWERS (contd.)
• Disadvantages of adsorption Towers
• Operation is not continuous.
• They can only be used for specific pollutants.
• Extensive temperature pre-conditioning equipment
  to be installed.
• Despite regeneration, the capacity of the
  adsorbent decreases with use.