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Air pollution prevention and control lecture 2018

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Air pollution prevention and control lecture 2018

  1. 1. AIR POLLUTION PREVENTION AND CONTROL Mr. William Kitagwa Department of Environmental Health School of Science and Technology University of Kabianga
  2. 2. Pollution Prevention Strategies • Pollution prevention [vs. control] offers important economic benefits and at the same time allows continued protection of the environment. • While most pollution control strategies cost money, pollution prevention has saved many firms thousands of dollars in treatment and disposal costs. • More importantly, pollution prevention should be viewed as a means to increase company productivity. • By reducing the amount of raw materials that are wasted and disposed of; manufacturing processes become more efficient, resulting in cost savings to the company.
  3. 3. • Pollution prevention should be the first consideration in planning for processes that emit air contaminants. • Undertaking pollution prevention practices may reduce air emissions enough to allow a business or industry to avoid classification as a major air emission source.
  4. 4. What is Pollution Prevention? • Pollution prevention is the elimination or prevention of wastes (air emissions, water discharges, or solid/hazardous waste) at the source. In other words, pollution prevention is eliminating wastes before they are generated. • Pollution prevention approaches can be applied to all pollution generating activity: hazardous and nonhazardous, regulated and unregulated. Pollution prevention does not include practices that create new risks of concern.
  5. 5. Pollution Prevention Act In 1990, the US Congress established federal policy on pollution prevention by passing the Pollution Prevention Act. The Act states: 1. pollution should be prevented or reduced at the source whenever feasible (i.e., source reduction), 2. pollution that cannot be prevented should be recycled in an environmentally safe manner whenever feasible,
  6. 6. 3. pollution that cannot be prevented or recycled should be treated in an environmentally safe manner whenever feasible, and 4. disposal or other release into the environment should be employed only as last resort and should be conducted in an environmentally safe manner.
  7. 7. The Pollution Prevention Act defines pollution prevention as source reduction. Recycling, energy recovery, treatment and disposal are not considered pollution prevention under the Act.
  8. 8. SOURCE REDUCTION • Product Changes • Designing and producing a product that has less environmental impact • Changing the composition of a product so that less hazardous chemicals are used in, and result from, production • Using recycled materials in the product • Reusing the generated scrap and excess raw materials back in the process • Minimizing product filler and packaging • Producing goods and packaging reusable by the consumer • Producing more durable products
  9. 9. Techniques Without Using Emissions 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
  10. 10. Commonly Used Methods For Air Pollution Control PARTICULATE  Cyclones  Electrostatic Precipitators  Fabric Filter  Wet Scrubbers GASES  Adsorption Towers  Thermal Incernation  Catalytic Combustion
  11. 11. • Input Material Changes • Material substitution Using a less hazardous or toxic solvent for cleaning or as coating • Purchasing raw materials that are free of trace quantities of hazardous or toxic impurities
  12. 12. Equipment and Process Modifications •Changing the production process or flow of materials through the process. •Replacing or modifying the process equipment, piping or layout. •Using automation. •Changing process operating conditions such as flow rates, temperatures, pressures and residence times. •Implementing new technologies
  13. 13. Good Operating Practices • Instituting management and personnel programs such as employee training or employee incentive programs that encourage employees to reduce waste. • Performing good material handling and inventory control practices that reduce loss of materials due to mishandling, expired shelf life, or improper storage. • Preventing loss of materials from equipment leaks and spills. • Segregating hazardous waste from non-hazardous waste to reduce the volume of hazardous waste disposed.
  14. 14. • Using standard operating procedures for process operation and maintenance tasks • Performing preventative maintenance checks to avoid unexpected problems with equipment. • Turning off equipment when not in use. • Improving or increasing insulation on heating or cooling lines. • Environmentally Sound Reuse and Recycling
  15. 15. Control of Gaseous Pollutants • Absorption • Adsorption • Oxidation • Reduction
  16. 16. Absorption Primary application: inorganic gases Example: SO2 Mass transfer from gas to liquid Contaminant is dissolved in liquid Liquid must be treated
  17. 17. Adsorption Primary application: organic gases Example: trichloroethylene Mass transfer from gas to solid Contaminant is ‘bound’ to solid Adsorbent may be regenerated
  18. 18. Common Adsorbents Activated carbon Silica gel Activated alumina Zeolites (molecular sieves)
  19. 19. Oxidation • Thermal Oxidation • Catalytic Oxidation
  20. 20. • A thermal oxidizer (or thermal oxidiser) is a process unit for air pollution control in many chemical plants that decomposes hazardous gases at a high temperature and releases them into the atmosphere. • Thermal Oxidizers are typically used to destroy Hazardous Air Pollutants (HAPs) and Volatile Organic Compounds (VOCs) from industrial air streams. • These pollutants are generally hydrocarbon based and when destroyed via thermal combustion they are chemically changed to form CO and H O.
  21. 21. Thermal Oxidation Application: organic gases Autogenous gases = 7 MJ/kg (heat value) Operating temperatures: 700 - 1300 oC Efficiency = 95 - 99% By-products must not be more hazardous Heat recovery is economical necessity
  22. 22. • Catalytic oxidation is a relatively recently applied alternative for the treatment of VOCs in air streams resulting from remedial operations. • The addition of a catalyst accelerates the rate of oxidation by adsorbing the oxygen and the contaminant on the catalyst surface where they react to form carbon dioxide, water, and hydrochloric gas. • The catalyst enables the oxidation reaction to occur at much lower temperatures than required by a conventional thermal oxidation Catalytic Oxidation
  23. 23. Catalytic Oxidation Application: organic gases Non-autogenous gases < 7 MJ/kg Operating temperatures: 250 - 425 oC Efficiency = 90 - 98% Catalyst may be poisoned Heat recovery is not normal
  24. 24. Selective Catalytic Reduction (SCR) Application: NOx control Ammonia is reducing agent injected into exhaust NOx is reduced to N2 in a separate reactor containing catalyst Reactions: 4NO + 4NH3 + O2 --> 4N2 + 6H2O 2NO2 + 4NH3 + O2 --> 3N2 + 6H2O
  25. 25. Control of Particulate Pollutants • Spray chamber • Cyclone • Bag house • Venturi • Electrostatic Precipitator (ESP)
  26. 26. Spray Chamber
  27. 27. Spray Chamber Primary collection mechanism: Inertial impaction of particle into water droplet Efficiency: < 1% for < 1 um diameter >90% for > 5 um diameter Pressure drop: 0.5 to 1.5 cm of H2O Water droplet size range: 50 - 200 um
  28. 28. Spray Chamber Applications: 1. Sticky, wet corrosive or liquid particles Examples: chrome plating bath paint booth over spray 2. Explosive or combustible particles 3. Simultaneous particle/gas removal
  29. 29. Cyclone
  30. 30. Cyclone (Multi-clones for high gas volumes) Primary collection mechanism: Centrifugal force carries particle to wall Efficiency: <50% for <1 um diameter >95% for >5 um diameter
  31. 31. Cyclone (Multi-clones for high gas volumes) Pressure drop: 8-12 cm of H2O Applications: 1. Dry particles Examples: fly ash pre-cleaner saw dust 2. Liquid particles Examples: following venturi
  32. 32. Bag House
  33. 33. Bag House Particle Collection Mechanisms Screening Impaction + - Electrostatic
  34. 34. Bag House Efficiency: >99.5% for <1 um diameter >99.8% for >5 um diameter Fabric filter materials: 1. Natural fibers (cotton & wool) Temperature limit: 80 oC 2. Synthetics (acetates, acrylics, etc.) Temperature limit: 90 oC 3. Fiberglass Temperature limit: 260 oC
  35. 35. Bag House Bag dimensions: 15 to 30 cm diameter ~10 m in length Pressure drop: 10-15 cm of H2O Cleaning: 1. Shaker 2. Reverse air 3. Pulse jet
  36. 36. Bag House Applications: Dry collection Fly ash Grain dust Fertilizer May be combined with dry adsorption media to control gaseous emission (e.g. SO2)
  37. 37. Venturi
  38. 38. Venturi Primary collection mechanism: Inertial impaction of particle into water droplet Water droplet size: 50 to 100 um Water drop and collected particle are removed by cyclone
  39. 39. Venturi Efficiency: >98% for >1 um diameter >99.9% for > 5 um diameter Very high pressure drop: 60 to 120 cm of H2O Liquid/gas ratios: 1.4 - 32 gal/1000 ft3 of gas
  40. 40. Venturi Applications: Phosphoric acid mist Open hearth steel (metal fume) Ferro-silicon furnace
  41. 41. Electrostatic Precipitator (ESP)
  42. 42. ESP Tube (a) and Plate (b) collectors
  43. 43. ESP Collection Mechanism
  44. 44. Electrostatic Precipitator (ESP) Efficiency: >95% for >1 um diameter >99.5% for > 5 um diameter Pressure drop: 0.5 to 1.5 cm of H2O Voltage: 20 to 100 kV dc Plate spacing: 30 cm Plate dimensions: 10-12 m high x 8-10 m long Gas velocity: 1 to 1.5 m/s Cleaning: rapping plates
  45. 45. Electrostatic Precipitator (ESP) Applications (non-explosive): 1. Fly ash 2. Cement dust 3. Iron/steel sinter
  46. 46. Flue Gas Desulfurization (FGD) Predominant Processes (all non-regenerative): 1. Limestone wet scrubbing 2. Lime wet scrubbing 3. Lime spray drying Typical scrubbers: venturi, packed bed and plate towers and spray towers
  47. 47. Flue Gas Desulfurization (FGD) Spray dryer systems include a spray dryer absorber and a particle-collection system (either a bag house or an ESP) In 1990 the average design efficiency for new and retrofit systems was 82% and 76% respectively
  48. 48. Flue Gas Desulfurization (FGD) Overall reactions: Limestone: SO2 + CaCO3 --> CaSO3 + CO2 Lime: SO2 + Ca(OH)2 --> CaSO3 + H2O
  49. 49. SOX CONTROL
  50. 50. 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
  51. 51. General Methods for Control of SO2 Emissions (contd.) • Use Flue Gas Desulfurization Systems • Use Alternative Energy Sources, such as Hydro-Power or Nuclear-Power
  52. 52. 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
  53. 53. Limestone Scrubbing
  54. 54. 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
  55. 55. Lime Scrubbing
  56. 56. 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.  .
  57. 57. Dual Alkali System con`t  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.
  58. 58. 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
  59. 59. Wellman – Lord Process Schematic process flow diagram – SO2 scrubbing and recovery system
  60. 60. Wellman – Lord Process • This process consists of the following sub processes:  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 -----> 2NaHSO3
  61. 61. 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.
  62. 62. NOX CONTROL
  63. 63. 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
  64. 64. 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
  65. 65. 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 gaseous hydrogen to unite. This produces ammonia and cleaner fuel.  This technology can reduce the nitrogen contained in both naturally occurring and synthetic fuels.
  66. 66. 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
  67. 67. Catalytic Combustion
  68. 68. Catalytic Emission Control
  69. 69. 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
  70. 70. 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  Electron Beam Radiation  Staged Combustion
  71. 71. Selective Catalytic Reduction (SCR) Schematic process flow diagram – NOX control
  72. 72. 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 too 400 oC.  Typical efficiencies are around 80 %
  73. 73. Selective Non-catalytic Reduction (SNR) • At higher temperatures (900-1000oC), NH3 will reduce NOX to nitrogen without a catalyst. • At NH3 : NOX molar ratios 1:1 to 2:1, about 40-60%reduction is obtained. • SNR is cheaper than SCR in terms of operation cost and capital cost.
  74. 74. 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.
  75. 75. Staged Combustion
  76. 76. 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.