Air Pollution Control

Air pollution control Control strategies •

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Complete source shutdown: most effective strategy, but only practical in emergency situations. It causes economic loss and can only be a temporary solution. Source location: locate the source of pollution where fewer people will be affected. This may protect local air quality, but the pollutants are still produced and can be transported to neighboring communities by air. Tall smoke stacks: take pollution high into the atmosphere, allowing mixing and dispersal to dilute them. However, what goes up must come down… Encourage or require industries to make fuel substitutions or process changes with less pollution emissions (e.g. new energy sources like solar power, hydroelectric and so on) Use fuel substitutions in machines, like electric cars to work towards zero emission.

Settling chamber with enlarged flue section Settling chambers use the force of gravity to remove solid particles. The gas stream enters a chamber where the velocity of the gas is reduced. Large particles drop out of the gas and are recollected in hoppers. Because settling chambers are effective in removing only larger particles (d>40µm), they are used in combination with a more efficient control device.

Cyclone collector Cyclones provide a low-cost, low-maintenance method of removing larger particulates from a gas stream. The general principle of inertia separation is that the particulate-laden gas is forced to change direction. As gas changes direction, the inertia of the particles causes them to continue in the original direction and be separated from the gas stream. The walls of the cyclone narrow toward the bottom of the unit, allowing the particles to be collected in a hopper. The cleaner air leaves the cyclone through the top of the chamber, flowing upward in a spiral vortex, formed within a downward moving spiral. Cyclones are efficient in removing large particles but are not as efficient with smaller particles (d>15µm). For this reason, they are used with other particulate control devices. The settling chamber and the cyclone only capture the pollutants but don't destroy them, therefore proper disposal of the collected material is needed. Collected solid particles are most often disposed of in a landfill. Wastewater generated by scrubber must be sent to a wastewater treatment facility. When possible, collected particle matter is recycled and reused.

Spray tower Effective for particles of d>8µm. A spray tower is a cylinder where dirty gas is usually injected at the bottom. As the gas moves upwards the cylinder, many small nozzles spray water or some other fluid down towards the gas. Theoretically, the smaller the droplets formed, the higher the collection efficiency achieved for both gaseous and particulate pollutants. However, the liquid droplets must be large enough not to be carried out of the scrubber by the scrubbed outlet gas stream. Therefore, spray towers use nozzles to produce droplets that are usually 500 to 1,000 µm in diameter. The reason for using many nozzles is to maximize the number of fine droplets impacting the pollutant particles and to provide a large surface area for absorbing gas. The pollutants get attached to the droplets, which sinks to the bottom of the cylinder due to gravity. The dirty wastewater is then collected and disposed.

Fabric filters (bag house) Effective for particles of diameter larger than 1µm. Dust enters the baghouse compartment through hoppers. Larger particles drop out while smaller dust particles collect on filter bags. When the dust layer thickness reaches a level where flow through the system is restricted (called pressure drop or delta P), the bag cleaning process is initiated. Cleaning can be done while the baghouse is still online (filtering) or in isolation (offline). Once cleaned, the compartment is placed back in service and the filtering process starts over.

Electrostatic precipitator Effective for particles of d>1µm. Removes particles from a flowing gas using the force of an induced electrostatic charge. Electrostatic precipitators are highly efficient filtration devices that minimally impede the flow of gases through the device, and can easily remove fine particulate matter such as dust and smoke from the air stream. In contrast to wet scrubbers which apply energy directly to the flowing fluid medium, an ESP applies energy only to the particulate matter being collected and therefore is very efficient in its consumption of energy. The most basic precipitator contains a row of thin vertical wires, and followed by a stack of large flat metal plates oriented vertically, with the plates typically spaced about 1 cm to 18 cm apart, depending on the application. The air or gas stream flows horizontally through the spaces between the wires, and then passes through the stack of plates. Today, the use of discharge electrodes is more widely used. A negative voltage of several thousand volts is applied and an electric discharge ionizes the gas around the electrodes. Negative ions flow to the plates and charge the gas-flow particles. The ionized particles, following the negative electric field created by the power supply, move to the grounded plates. Particles build up on the collection plates and form a layer. The layer does not collapse, thanks to electrostatic pressure. The collection efficiency of an electrostatic precipitator is strongly dependent on the electrical properties of the particles. Automatic plate-rapping systems and

hopper-evacuation systems remove the collected particulate matter while on line, theoretically allowing ESPs to stay in operation for years at a time.

Wet scrubbing process In a wet scrubber, the polluted gas stream is brought into contact with the scrubbing liquid, by spraying it with the liquid, by forcing it through a pool of liquid, or by some other contact method, so as to remove the pollutants. Wet scrubbers remove dust particles by capturing them in liquid droplets. Wet scrubbers remove pollutant gases by dissolving or absorbing them into the liquid. Any droplets that are in the scrubber inlet gas must be separated from the outlet gas stream by means of another device referred to as a mist eliminator or entrainment separator (these terms are interchangeable). Also, the resultant scrubbing liquid must be treated prior to any ultimate discharge or being reused in the plant. The wet scrubber typically consists of a tank in which gases are allowed to mix with liquid. Most scrubbers are used to remove sulfur dioxide compounds, which is not sufficiently soluble in water. Thus, the liquid used in such cases is one that will react chemically with the sulfur dioxide, like sodium carbonate or lime. The reaction between sulfur dioxide and these liquids forms sodium sulfite or calcium sulfate, which can be drawn off at the bottom of the tank. Advantages and disadvantages of wet scrubbers compared to fabric filters and electrostatic precipitators Advantages Disadvantages Small space requirements Corrosion problems Scrubbers reduce the temperature Water and dissolved pollutants can and volume of the unsaturated form highly corrosive acid solutions. exhaust stream.  Smaller machines Proper construction materials are than those of other control devices.  very important. Also, wet-dry Lower capital costs and more interface areas can result in corrosion. flexibility in site location of the scrubber. High power requirements No secondary dust sources High collection efficiencies for Once particulate matter is collected, it particulate matter require high cannot escape from hoppers or pressure drops high operating during transport. costs. Handles high-temperature, highhumidity gas streams No temperature limits or Water-disposal problems condensation problems can occur as in baghouses or ESPs. Minimal fire and explosion hazards Difficult product recovery Various dry dusts are flammable. Dewatering and drying of scrubber Using water eliminates the possibility sludge make recovery of any dust for of explosions. reuse very expensive and difficult.

Ability to collect both gases and particulate matter

Dry scrubbing process (Integrated dry scrubbing plus baghouse process) A Dry Scrubber System is comprised of three (3) primary components: • A Gas Cooling System • A Reagent Injection System • A Fabric Filter (Baghouse) System A dry or semi-dry scrubbing system, unlike the wet scrubber, does not saturate the flue gas stream that is being treated with moisture. In some cases no moisture is added; while in other designs only the amount of moisture that can be evaporated in the flue gas without condensing is added. Therefore, dry scrubbers do generally not have a stack steam plume or wastewater handling/disposal requirements. Dry scrubbing systems are used to remove acid gases (such as SO2 and HCl) primarily from combustion sources.

Activated carbon filters Carbon filtering is a method of filtering that uses a piece of activated carbon to remove contaminants and impurities, utilizing chemical adsorption. Each piece of carbon is designed to provide a large section of surface area, in order to allow contaminants the most possible exposure to the filter media. 454g of activated carbon contains a surface area of approximately 100 acres (1 km2/kg). This carbon is generally activated with a positive charge and is designed to attract negatively charged water contaminants. Carbon filtering is commonly used for water purification, but is also used in air purifiers. Carbon filters are most effective at removing chlorine, sediment, and volatile organic compounds (VOCs) from water or air. They are not effective at removing minerals, salts, and dissolved inorganic compounds. Typical particle sizes that can be removed by carbon filters range from 0.5 to 50 micrometres. The particle size will be used as part of the filter description. The efficacy of a carbon filter is also based upon the flow rate regulation. When the water or air is allowed to flow through the filter at a slower rate, the contaminants are exposed to the filter media for a longer amount of time.