epc_fa2011_lecture_10

epc_fa2011_lecture_10 - Air Pollutant Removal...

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Unformatted text preview: Air Pollutant Removal “Particulate” Air Pollutants vs. “Gaseous” Air Pollutants 1 “Particulate” Air Pollutant Removal • Gravitational Settling Chamber • Centrifugal Collector -- Cyclones -- Dynamic Precipitators • Wet Collector (= Liquid Scrubbing) • Fabric Collector -- Deep Bed Filter -- Baghouse • Electrostatic Precipitators (ESP) • HEPA Filters 2 “Particulate” Air Pollutant Removal Gravitational Settling Chamber Operating principle -- Create localized velocity gradients inside of the chamber so that gravitational forces dominate horizontal velocity of particles, and they can settle from the emission gas stream. -- Based on a simple principle of MB, Q1=Q2 where Q1= u1A1 and Q2= u2A2. As A2 increases, u2 decreases - as a result, particulates will 'fall out' and settle down. Performance -- Simple and inexpensive -- Low efficiency (less than 50% removal rate) -- Ideal for removing any large solid and liquid particles -- Can withstand high temperature and corrosive particles/gases 3 “Particulate” Air Pollutant Removal Centrifugal Collector -- 1 Operating principle -- Centrifugal force is provided by the flow rate of gas stream. -- Ideal for removing particles greater than 10 µm in diameter. -- Centrifugal force is used to impact particles against the collector wall where they slide down into a hopper. 1) Cyclones -- Inertial collector with no moving parts -- Ideal for removing particles greater than 10 µm in diameter 4 “Particulate” Air Pollutant Removal Centrifugal Collector -- 2 2) Dynamic Precipitators -- Motorized, similar to exhaust fan. -- Ideal for removing particles greater than 10 µm in diameter (a high-efficiency model might have a collection efficiency around 80%) -- Rotating blades impart centrifugal force to particles which will and hit wall and slide into a hopper. 5 “Particulate” Air Pollutant Removal Wet Collector (= Liquid Scrubbing) -- 1 -- Wet collectors, or Wet scrubbers are designed to increase particle size in the gas stream by spraying water or slurry droplets. (i.e., larger particles are easier to settle out and are easier to collect). -- Used when the target particulate matter is wet, corrosive or very hot. -- Control of emission of talc, dust, phosphoric acid mist, steel furnace fumes. -- Varied configurations, but two types are most common -Conventional scrubbers and Venturi scrubbers. (to collect small particles generated in steel making or smelting operations) -- Operating costs are relatively higher than other types of collector, but capital cost is lower than other collectors of equivalent performance. 6 “Particulate” Air Pollutant Removal Wet Collector (= Liquid Scrubbing) -- 2 Operating principle -- Remove particulates from a gas stream by incorporating them into water (= liquid scrubbing). -- Water can contain reagents such as lime (CaO), quicklime (CaCO3), and caustic soda (NaOH) for neutralization and stabilization. SO 2 + H 2 O → H 2 SO 4 H 2 SO 3 + CaO → CaSO 3 + H 2 O H 2 SO 3 + 2 NaOH → Na 2 SO 3 + 2 H 2 O H2SO3 (Sulfurous acid / Dihydrogen sulfur trioxide) Na2SO3 (Sodium sulfite) *Reagent --a substance/compound that is added to a system to bring about a chemical reaction *Reactant -- a substance that is consumed in the course of a chemical reaction 7 “Particulate” Air Pollutant Removal Wet Collector (= Liquid Scrubbing) -- 3 -- Water can contain reagents such as lime (CaO), quicklime (CaCO3), and caustic soda (NaOH) for neutralization and stabilization. 8 “Particulate” Air Pollutant Removal Fabric Collector -- 1 Operating principle -- Fabric filters are either woven (most popular) or made of felt. -- The same principle to a vacuum cleaner. -- Gas stream passes through a fabric filter where particles are filtered out. -- For removing dry particles from dry, low-temperature gas streams (0 to 275ºC). -- Used to collect particles sizes smaller than 5 µm in diameter. -- Fabric materials include cotton, wool, synthetic (handling gas streams upto 100ºC) and fiber glass (handling gas streams upto 275ºC) depending on temperature, physical and chemical characteristic of particles to be removed. 9 “Particulate” Air Pollutant Removal Fabric Collector -- 2 1) Deep Bed Filter -- Effective for relatively clean gas and low volume like A/C systems. -- Typically disposable, although many may be washed and reused several times. -- Used extensively in air-conditioning, hot-air domestic heating systems, and filtration of air entering internal combustion engines. 10 “Particulate” Air Pollutant Removal Fabric Collector -- 3 2) Baghouse -- Effective for dirty gas and high volume like industrial gas. -- Made of cloth fabric, used in controlling dirty industrial gas. -- Particles are retained on inside of bags. -- Bags are routinely shaken to remove particles to a hopper below. -- Extensively used in carbon black industry, cement crushing, feed and grain handling, gypsum and limestone crushing. 11 “Particulate” Air Pollutant Removal Fabric Collector -- 4 2) Baghouse – Cont.d 12 “Particulate” Air Pollutant Removal Electrostatic Precipitators (ESP) -- 1 Operating principle -- Particles in a gas stream are electrically charged and then removed by attraction to an oppositely charged electrode. -- Ideal for dry collection of particles from hot gas stream such as fly ash collections. (i.e., particulate matter from furnaces burning fossil fuels) -- Constructed of alternating wires and plates, and a direct current potential is established between them. -- As the gas stream passes between the wire and the plate, ions attach to the particles and give particles a net negative charge. -- Particles then migrate and stick to positively charged plate --- Used extensively in air-conditioning, hot-air domestic heating systems, and filtration of air entering internal combustion engines. 13 “Particulate” Air Pollutant Removal Electrostatic Precipitators (ESP) -- 2 1) Electrostatic Precipitators (ESP) -- After being neutralized, the particles either fall by gravity or constantly scrapped by a mechanical device or washed down by flowing the plates with liquid to prevent the formation of a layer of particle called a blanket. -- Very high efficiency (greater than 99% removal of particles). 14 “Particulate” Air Pollutant Removal Electrostatic Precipitators (ESP) -- 3 1) Electrostatic Precipitators (ESP) – Cont.d 15 “Particulate” Air Pollutant Removal HEPA Filters -- HEPA stands for High Efficiency Particle Air filter (HEPA). -- Expensive -- Used when 99.9% efficient in removal rate at 0.1 µm diameter of very hazardous or toxic particulate matter is required. -- Precleaned gas is forced or sucked by vacuum through a single stage of HEPA micropore or glass frit fibers filtering for 99.9% removal. -- Four to six HEPA filter stages in series can archive 99.9999% removal of particles from the gas stream. -- Normally used in 'clean room' air filtration control and in radioactive particle emission controls. -- Ultra High Efficiency Particle Air filter (ULPA) can achieve 99.9999% removal of particles at 0.1 µm diameter from the gas stream in a single stage. 16 RECAP -- “Particulate” Air Pollutant Removal • Gravitational Settling Chamber • Centrifugal Collector -- Cyclones -- Dynamic Precipitators • Wet Collector (= Liquid Scrubbing) • Fabric Collector -- Deep Bed Filter -- Baghouse • Electrostatic Precipitators (ESP) • HEPA Filters 17 “Gaseous” Air Pollutant Removal -- 1 -- Four fundamental ways to reduce emission of undesirable gases; 1) Reduce/eliminate the production of the undesirable gases at source level (most effective measure). 2) Right after production, induce the gases to react in chemical processes to produce different, less objectionable type of emissions. 3) Selectively remove the undesirable product from a gas stream by absorption, a transfer of gas molecules into a liquid. 4) Selectively remove the undesirable product from a gas stream by adsorption, a deposition of gas molecules on a solid surface. 18 “Gaseous” Air Pollutant Removal -- 2 -- Where the gas is recovered from the absorbing liquid or the adsorbing solid, this gas pollutant removal process is called regenerative because the liquid or solid is usually used repeatedly in the same process over and over again. -- In such regenerative case, the gas is frequently processed further, making it clean enough to be a sellable as by-product gas -- so that part or all of the treatment costs can be recouped. 19 “Gaseous” Air Pollutant Removal -- 3 • Absorption • Adsorption • Combustion -- Direct flame combustion -- Thermal combustion (afterburners) -- Catalytic combustion • Flue Gas Desulfurization (FGD) -- Dry scrubbing (or Spray drying/dry FGD) 20 “Gaseous” Air Pollutant Removal Absorption Operating principle -- Removal of gas contaminants by absorption into a liquid. -- Mixtures of water and dissolved substances that react with gas contaminants are often used. For example, MgOH slurry is often used to remove SO2. 21 “Gaseous” Air Pollutant Removal Adsorption Operating principle -- Removal of gas contaminants by deposition on a solid surface such as organic contaminant removal using activated carbon (GAC). -- Widely used among industries involved on handling manufacturing or using volatile organics (i.e., chemical manufacturers, pharmaceutical, petroleum refining). 22 “Gaseous” Air Pollutant Removal Combustion -- 1 -- Applicable when air contaminants (HC/Hydrocarbon and CO) are oxidizable to inert CO2 and water. -- For efficient combustion to occur, it is necessary to maintain the four basic elements of combustion -- oxygen, temperature, turbulence and time. 1) Direct flame combustion -- If the energy concentration of the gas stream is greater than 3.7 megajoules (MJ/m3), then by-products of the combustion will be nontoxic. -- Since the gas stream already has a high heating value, it will just burn by itself in a flame combustor. -- Examples include meat smokehouse, paint bake oven emissions 23 “Gaseous” Air Pollutant Removal Combustion -- 2 2) Thermal combustion (afterburners) -- Gas stream has a low heating value so that it needs supplementary fuels to burn completely and cleanly. -- Gas stream is preheated by a heat exchanger or recuperator before entering combustor. 24 “Gaseous” Air Pollutant Removal Combustion -- 3 3) Catalytic combustion -- Use of catalysts such as Pt (platinum) and Pd (palladium) compounds, that are substances that speed up the reaction without having themselves consumed. -- Reaction occurs at a much lower temperature. -- Catalyst draws the reactants to its surface by diffusion and adsorption. -- Examples include varnish cooking and asphalt oxidation emissions. -- Exiting gas may contain sulfur and lead in trace amounts. 25 “Gaseous” Air Pollutant Removal Flue Gas Desulfurization (FGD) -- 1 -- Chemical reagents added to remove the pollutants, predominantly sulfur oxides, from the gas stream, mainly from coal-burning boilers. (=power generator) -- Based on the recyclability of reagent, FGD systems can be categorized into a non-regenerative and a regenerative. -- Non-regenerative system installation is more common. -- For regenerative system, typical reagents includes; Lime (CaO) Caustic soda (NaOH) Soda ash (Na2CO3) Ammonia (NH3) 26 “Gaseous” Air Pollutant Removal Flue Gas Desulfurization (FGD) -- 2 Dry scrubbing (or Spray drying/dry FGD) -- Reagent is injected in a form of droplets into the flue gas in the spray dryer. -- Reagent droplets absorb SO2 while simultaneously drying up before impacting the wall of a dryer vessel and hopper. 27 RECAP -- “Gaseous” Air Pollutant Removal • Absorption • Adsorption • Combustion -- Direct flame combustion -- Thermal combustion (afterburners) -- Catalytic combustion • Flue Gas Desulfurization (FGD) -- Dry scrubbing (or Spray drying/dry FGD) 28 ...
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This note was uploaded on 11/14/2011 for the course CEE 350 taught by Professor Jaewanyoon during the Fall '10 term at Old Dominion.

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