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Technology for Air Pollution Control

Technology for Air Pollution Control. 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

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Technology for Air Pollution Control

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  1. Technology for Air Pollution Control

  2. 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

  3. Commonly Used Methods For Air Pollution Control PARTICULATE • Cyclones • Electrostatic Precipitators • Fabric Filter • Wet Scrubbers GASES • Adsorption Towers • Thermal Incernation • Catalytic Combustion

  4. SOx CONTROL

  5. 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

  6. General Methods for Control of SO2 Emissions (contd.) • Use Flue Gas Desulfurization Systems • Use Alternative Energy Sources, such as Hydro-Power or Nuclear-Power

  7. 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

  8. 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

  9. 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 -----> Ca(OH)2 SO2 + H2O <----> H2SO3 H2SO3 + Ca(OH)2 -----> CaSO3.2 H2O CaSO3.2 H2O + (1/2)O2 -----> CaSO4.2 H2O

  10. 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.

  11. 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

  12. 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 -----> 2NaHSO3

  13. 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.

  14. NOX CONTROL

  15. 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

  16. 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

  17. 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.

  18. 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

  19. 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

  20. 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

  21. 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 %

  22. 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.

  23. 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.

  24. CARBON MONOXIDE CONTROL

  25. Formation Of Carbon Monoxide • Due to insufficient oxygen • Factors affecting Carbon monoxide formation: • Fuel-air ratio • Degree of mixing • Temperature

  26. 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.

  27. CARBON DIOXIDE CONTROL

  28. Sources of Carbon Dioxide Human-Related Sources • Combustion of fossil fuels: Coal, Oil, and Natural Gas in power plants, automobiles, and industrial facilities • Use of petroleum-based products • Industrial processes: Iron and steel production, cement, lime, and aluminum manufactures Natural Sources • Volcanic eruptions • Ocean-atmosphere exchange • Plant photosynthesis

  29. Sources of CO2 Emissions in the U.S. Source: USEPA (x-axis units are teragrams of CO2 equivalent)

  30. CO2 Emissions from Fossil Fuel Combustion by Sector and Fuel Type (y-axis units are teragrams of CO2 equivalent) Source: USEPA

  31. General Methods For Control of CO2 Emissions • Reducing energy consumption, increasing the efficiency of energy conversion • Switching to less carbon intensive fuels • Increasing the use of renewable sources • Sequestering CO2 through biological, chemical, or physical processes

  32. CONTROL OF MERCURY EMISSIONS

  33. Mercury Emissions • Mercury exists in trace amounts in • Fossil fuels such as Coal, Oil, and Natural Gas • Vegetation • Waste products • Mercury is released to the atmosphere through combustion or natural processes • It creates both human and environmental risks • Fish consumption is the primary pathway for human and wildlife exposure • United states is the first country in the world to regulate mercury emissions from coal-fired power plants (March 15, 2005).

  34. Types of Sources Source: Seingeur, 2004 and Mason and Sheu, 2002. Worldwide Distribution of Emissions Source: Presentation by J. Pacyna and J. Munthe at mercury workshop in Brussels, March 29-30, 2004

  35. Control Technologies for Mercury Emissions • Currently installed control devices for SO2, NOX,and particulates, in a power plant, remove some of the mercury before releasing from the stack • Activated Carbon Injection: Particles of activated carbon are injected into the exit gas flow, downstream of the boiler. The mercury attaches to the carbon particles and is removed in a particle control device • Thief process for the removal of mercury from flue gas: It is a process which extracts partially burned coal from a pulverized coal-fired combustor using a suction pipe, or "thief," and injects the resulting sorbent into the flue gas to capture the mercury.

  36. PARTICULATE MATTER CONTROL

  37. 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

  38. 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

  39. Particulate Collection Mechanism • Gravity Settling • Centrifugal Impaction • Inertial Impaction • Direct Interception • Diffusion • Electrostatic Effects

  40. 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

  41. 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

  42. 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 Overall collection η Ci inlet concentration Co outlet concentration

  43. Cyclones (contd.) 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.

  44. 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.

  45. 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.

  46. Design of Fabric Filters • The equation for fabric filters is based on Darcy’s law for flow through porous media. • Fabric filtration can be represented by the following equation: S = Ke + Ksw Where, S = filter drag, N-min/m3 Ke = extrapolated clean filter drag, N-min/m3 Ks = slope constant. Varies with the dust, gas and fabric, N-min/kg-m W= Areal dust density = LVt, where L = dust loading (g/m3), V = velocity (m/s) • Both Ke and Ks are determined empirically from pilot tests.

  47. Fabric Filters Δ P Total pressure drop Δ Pf Pressure drop due to the fabric Δ Pp Pressure drop due to the particulate layer Δ Ps Pressure drop due to the bag house structure

  48. 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

  49. 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

  50. Darcy’s equation ΔPf Pressure drop N/m2 ΔPp Pressure drop N/m2 Df Depth of filter in the direction of flow (m) Dp Depth of particulate layer in the direction of flow (m) μ Gas viscosity kg/m-s V superficial filtering velocity m/min Kf, Kp Permeability (filter & particulate layer m2) 60 Conversion factor δ/min V = Q/A Q volumetric gas flow rate m3/min A cloth area m2

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