Air Pollution Control Methods and Devices

Air Pollution Control of Stationary Sources

• Stationary sources of air pollution result from incomplete fuel combustion or industrial processes.
• Pollutants emitted depend on the specific process operations.
• Examples include ash, sulfur dioxide, nitrogen oxides, mercury, and vanadium (in fuel). Metallurgical plants may emit metal dusts (like iron oxides), fluorides, and chlorides. Industrial plants manufacturing inorganic chemicals emit various waste gases dependent on their product. Odorous organic waste gases may also come from organochemical and petrochemical plants.

Industrial Pollutant Sources Categorization

• Process Operations: Incomplete chemical reactions (combustion due to unconverted reactants, or reaction yield less than theoretical conversion).
• Atmospheric Releases: Process secondary components or impurities of raw materials.
• Auxiliary Losses: Compounds like volatile organic solvents from fugitive sources, inorganics like carbon disulfide and hydrogen sulfide, and fluorine compounds (in aluminum production) in rayon production.

Waste Emissions

• Emissions of malodorous and oxidation compounds from oxidation, heating, drying processes.
• Releases may originate from various emission points and not be centrally collected.
• Air release emission points from industrial processes: stack, duct, vent, fugitive, and area.
• Fugitive emission points are unconfined in a stack or duct before reaching the atmosphere.

Industrial Process Operation Air Emission Points and Categories

• Process Operations: Reactors vents, Distillation systems, Vacuum systems, Combustion stacks, Blow molding, Spray drying and booths, Extrusion machines
• Surface Area Sources: Pond evaporation, Cooling tower evaporation, Wastewater treatment, Land disposal
• Fugitive Sources: Valves, Pump seals, Flanges/connectors, Compressors, Open ended lines, Pressure relief devices, Equipment cleaning/maintenance, Handling, Storage, Loading, Storage tank breathing losses, Loading/unloading, Line venting, Packaging and container loading

Pollution Control

• Effective pollution control achieved by common sense solutions: installation of effective control technology, changes in production processes, and pollution prevention techniques.
• Compliance with emission standards depends largely on the application of appropriate stationary source control measures.

PM Control Procedures

• Control procedures include using tall smokestacks, changes in plant operations, and installing effective control devices.
• Industrial environmental impact control strategy is a four-step process: elimination of the problem source, modification of the source operation, relocation of the source, and selection/application of appropriate control technology.

Exhaust Stacks

• Exhaust stacks elevate the exhaust stream.
• This reduces the local effects of pollution by dispersing the emission more effectively.
• High stacks were an inexpensive solution, reducing near-community impact.
• However, it may transfer the problem to another location (like acid rain).

Plant Operations

• Compliance with emission standards may require control technology.
• Many industrial operations have reduced emissions by changing operational methods.
• Pre-treating process materials/fuel, material substitution, changes in manufacturing, and coal washing to reduce particulate/sulfur emissions.
• Substituting cleaner fuels during refining (like natural gas or low-sulfur fuel oil) can decrease pollution from combustion.

Plant Maintenance

• Reduced emissions from stationary sources may be achieved through increased attention to plant maintenance.
• Improperly maintained equipment (especially combustion equipment) can release significant pollutants into the environment..
• Adequately scheduled maintenance for exhausts and fugitive emissions from vats, valves, and transmission lines.

Control Technology

• Advanced add-on control technologies can destroy/recover gaseous/particulate matter for proper disposal or re-use.
• Pollution control operations include combustion, adsorption, absorption and condensation.
• Control devices include thermal incinerators, catalytic incinerators, flares, boilers, process heaters, carbon absorbers, spray towers, and surface condensers.
• Important parameters for selecting equipment: exhaust gas characteristics (from emissions tests), and process/site characteristics (from field surveys).

Exhaust Gas Characteristics

• Total exhaust gas flow rate, exhaust gas temperature, required control efficiency, particle size distribution, particle resistivity, composition of emissions, corrosiveness of exhaust gas over operating range, moisture content, stack pressure, exhaust gas combustibility/flammability properties.

Process or Site Characteristics

• Reuse/recycling of collected emissions, availability of space, availability of additional electrical power, availability of water, availability of wastewater facilities, frequency of startup and shutdowns, environmental conditions, anticipated changes in control regulations/raw materials, plant type (stationary/mobile).

Control Devices for Particulate Emissions

• Technologies for removing particles from effluent gas streams.
• Factors determining the appropriate device: particle size and chemical characteristics.
• Common devices: gravity settlers, mechanical collectors (cyclones), electrostatic precipitators (ESPs), scrubbers, fabric filters, and hybrid systems.
• Combinations of devices improve collection efficiency.

Equipment: Gravity Settlers/Chambers

• Industrial use for removing solid/liquid waste materials from gaseous streams.
• Advantages: Simple construction, low initial cost, low pressure losses, easy waste disposal.

Equipment: Mechanical Collectors (Cyclones)

• Centrifugal separators widely used for removing solid/liquid particles (particulates) from gas streams.

Equipment: Fabric Filters (Bag Houses)

• Filtration processes of varying fabric types, cleaning mechanisms and equipment.
• Gases can be "pushed" or "pulled" through the bag house.

Baghouse Filter

• Efficiency: >99.5% for <1µm diameter particles, >99.8% for >5µm.
• Similar to conventional home vacuum cleaners.

Fabric Filter Materials

• Natural fibers (cotton, wool); synthetics (acetates, acrylics); fiberglass
• Temperature limitations
• Cannot be used for wet air systems, corrosive gases.

Hybrid Systems

• Defined as using combinations of control mechanisms, often fabric filtration combined with electrostatic precipitation.
• Major types include: wet electrostatic precipitators, ionizing wet scrubbers, dry scrubbers, and electrostatically augmented fabric filtration.

Cyclones

• Used for 50-100 µm size particles and down to 10 µm.
• Simple, economical, no moving parts, relies on inertial effects.

Electrostatic Precipitator (ESP)

• High efficiency, dry collectors of particulates.
• High electrical direct current potential (30-75 kV).
• Tubular and plate configurations.

Electrostatic Precipitators (ESPs)

• Satisfactory devices for removing small particles from gas streams with high collection efficiency.
• Commonly used in power plants for removing fly ash from gases before discharge, also to remove chlorinated compounds

Wet Scrubbers

• Suitable for wet environments, corrosive gases and high temperatures.
• Combines a venturi scrubber, cyclone to achieve higher efficiency.

Scrubbers (Venturi Scrubbers)

• Involves bringing contaminated gas streams into close contact with a liquid (usually water).
• Used for removing particulate matter from a carrier gas stream.

Control Devices for Gaseous Emissions:

• Thermal incinerators, catalytic incinerators, flares, boilers and process heaters, carbon adsorbers, absorbers, condensers.

Thermal Incinerator

• Destroys volatile organic compounds (VOC) by controlled high-temperature burning of liquid, solid or gaseous waste.
• High efficiency (up to 99.9% possible).

Applicable Pollutants

• Primarily volatile organic compounds (VOC), some particulate matter (soot).
• Efficiency results from design criteria (reaction temperature, time, inlet VOC concentration).

Catalyst Temperatures

• Required temperatures (in °F) for 99.99% destruction of various compounds.

Flares

• Last resort for disposing of gases with low value or flammability.
• Used in refineries, production, and chemical industry.

Boilers and Process Heaters

• Commonly used by production facilities for heat and power.
• Recycling pollutants as fuel can help control emissions.
• Only pollutants that don't affect burner performance can be used for fuel.