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,COMMONLY USED M
2、ETHODS FOR AIR POLLUTION CONTROL PARTICULATE Cyclones Electrostatic Precipitators Fabric Filter Wet Scrubbers GASES Adsorption Towers Thermal Incernation Catalytic Combustion,SOx CONTROL,GENERAL METHODS FOR CONTROL OF SO2 EMISSIONS Change to Low Sulfur Fuel Natural Gas Liquefied Natural Gas Low Sulf
3、ur 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,GENERAL METHODS FOR CONTROL OF SO2 EMISSIONS (contd.) Use Flue Gas Desulfurization Systems Use Alternative Energy Sources
4、, such as Hydro-Power or Nuclear-Power,FLUE GAS DESULFURIZATIONSO2 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
5、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,LIMESTONE SCRUBBINGLimestone slurry is sprayed on the incoming flue gas. The sulfur dioxide gets absor
6、bed The limestone and the sulfur dioxide react as follows : CaCO3 + H2O + 2SO2 - Ca+2 + 2HSO3-+ CO2 CaCO3 + 2HSO3-+ Ca+2 - 2CaSO3 + CO2 + H2O,LIME SCRUBBING The equipment and the processes are similar to those in limestone scrubbing Lime Scrubbing offers better utilization of the reagent. The operat
7、ion 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)2SO2 + H2O H2SO3H2SO3 + Ca(OH)2 - CaSO3.2 H2OCaSO3.2 H2O + (1/2)O2 - CaSO4.2 H2O,DUAL ALKALI SYSTEM Lime and Limestone scrubbing lead to d
8、eposits 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.
9、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.,LIME - SPRAY DRYING Lime Slurry
10、 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 chamberThe dry solids are carried out with the gas, and are collected in fabric filtration unitThis system needs lower mainten
11、ance, lower capital costs, and lower energy usage,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 mark
12、etable product. The flue gas is pre - treated to remove the particulate. The sodium sulfite neutralizes the sulfur dioxide : Na2SO3 + SO2 + H2O - 2NaHSO3,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 doe
13、s 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.,NOx CONTROL,BACKGROUND ON NITROGEN OXIDES There are s
14、even 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,GENERAL METHODS FOR CONTROL OF NOx EMISSIONS NOx control can be achieved by: Fuel Denitrogenation Combustion Modificatio
15、n Modification of operating conditions Tail-end control equipment Selective Catalytic Reduction Selective Non - Catalytic Reduction Electron Beam Radiation Staged Combustion,FUEL DENITROGENATION One approach of fuel denitrogenation is to remove a large part of the nitrogen contained in the fuels. Ni
16、trogen 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 s
17、ynthetic fuels.,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,
18、 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,MODIFICATION OF OPERATING CONDITIONS The operating conditions can be modified to achieve sig
19、nificant 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,TAIL-END CONTROL PROCESSES Combustion modification and modificatio
20、n 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
21、Staged Combustion,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 g
22、iven 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 %,ELECTRON BEAM RADIATIONThis treatment process is under development, and is not widely used. Work is underway to dete
23、rmine 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 f
24、ertilizers.,STAGED COMBUSTIONPRINCIPLE 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
25、 emissions are achieved.,CARBON MONOXIDE CONTROL,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 bo
26、ilers.,FORMATION OF CARBON MONOXIDE Due to insufficient oxygen Factors affecting Carbon monoxide formation: fuel-air ratio degree of mixing temperature,PARTICULATE MATTER CONTROL,GENERAL METHODS FOR CONTROL OF PARTICULATE EMISSIONS Five Basic Types of Dust Collectors :Gravity and Momentum collectors
27、 settling chambers, louvers, baffle chambers Centrifugal Collectors cyclones mechanical centrifugal collectors Fabric Filters baghouses fabric collectors,GENERAL METHODS FOR CONTROL OF PARTICULATE EMISSIONS (contd.)Electrostatic Precipitators tubular plate wet dry Wet Collectors spray towers impinge
28、ment scrubbers wet cyclones peaked towers mobile bed scrubbers,PARTICULATE COLLECTION MECHANISM Gravity Settling Centrifugal Impaction Inertial Impaction Direct Interception Diffusion Electrostatic Effects,INDUSTRIAL SOURCES OF PARTICULATE EMISSIONS Iron & Steel Mills, the blast furnaces, steel maki
29、ng 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 coffe
30、e plants,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 an
31、d 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,CYCLONES Principle The particles are removed by the application of a
32、 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,
33、 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. .,CYCLONES (contd.)Advantages of Cyclones Cyclones have a lost capital cost
34、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 par
35、ticles High pressure drops contribute to increased costs of operation.,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 partic
36、les 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 mo
37、dular in design, and can be pre-assembled at the factory,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 fabri
38、c filtration unit is a potential fire hazard,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
39、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, w
40、hich 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.,ELECTROSTATIC PRECIPITATOR (contd.)Advantages of Electrostatic Precipitators Electrostatic precipita
41、tors 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. Disadvan
42、tages 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.,WET SCRUBBERS Princip
43、le 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 Ope
44、ration 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.,WET SCRUBBERS (contd.)C
45、onstruction 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
46、of gases from the air streams.,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 eff
47、iciency. 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.,CYCLONE SP
48、RAY 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 5 micron.,ORIFICE SCRUBBERS The gas is impacted onto a laye
49、r 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.,IMPINGEMENT SCRUBBERS In Impingement scrubbers, t
50、he 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.,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.,
