Air Pollution Control Techniques

Air Pollution: Control of Stationary Sources

• Stationary sources of air pollution primarily stem from incomplete fuel combustion or industrial processing.
• The specific pollutants emitted depend greatly on the industrial process.
• Examples of pollutants include ash, sulfur dioxide, nitrogen oxides, mercury, and various metal dusts (like iron oxides). Other sources may emit fluorides, chlorides, and organic waste gases.
• Stationary industrial pollution sources can be grouped into categories based on their specific process operations:
  • Process Operations: These involve incomplete chemical reactions, e.g., combustion with unconverted reactants, or reactions yielding less product than expected.
  • Atmospheric Releases: These are the secondary components or impurities from raw materials released into the atmosphere.
  • Auxiliary Losses: This pertains to the loss of compounds (e.g., volatile organic solvents, carbon disulfide, hydrogen sulfide, fluorine compounds) during various industrial processes.
• Waste Emissions: Exhausts from oxidation, heating, or drying processes can contain malodorous or oxidation compounds.
• Emission points originate from various locations and may not be collected centrally before entering the atmosphere. These include stack, duct, vent, fugitive and area release points.
• Whether an emission source is classified as "point" or "fugitive" is determined by whether the release is confined in a duct or stack prior to release into the atmosphere.
• Table of industrial process operation air emission points and categories:
  • Process Operations: Examples include reactors vents, distillation systems, vacuum systems, combustion stacks, blow molding, spray drying and booths, and extrusion machines.
  • Categories (Fugitive Sources) Examples: Valves, pump seals, flanges/connectors, compressors, open ended lines, pressure relief devices, equipment cleaning, handling storage loading, storage tank breathing losses, loading/unloading, line venting, packaging and container loading.
    • Surface Area Sources: Examples: Pond evaporation, cooling tower evaporation, wastewater treatment, and land disposal.

Pollution Control

• Effective pollution control involves common-sense solutions:
  • Installation of effective control technology.
  • Modifications to production processes.
  • Implementation of pollution prevention techniques.
• Compliance with emission standards hinges on implementing appropriate stationary source control measures.

PM Control Procedures

• General control procedures for stationary pollution sources:
  • Using tall smokestacks.
  • Modifying plant operations.
  • Installing effective control devices.
• The control strategy for industrial environmental impacts involves four steps:
  • Eliminating the source causing the problem
  • Modifying the operations of the source.
  • Relocating the source.
  • Selecting and applying the appropriate control technology.

Exhaust Stacks

• Exhaust stacks elevate pollutant emissions, reducing local impact by dispersing them over a larger area.
• In the past, tall stacks were a common, inexpensive solution but posed regional concerns, and transboundary issues like acid rain.
• Tall stacks transferred issues to different locations rather than solving them.

Plant Operations

• Meeting emission standards sometimes necessitates control technology implementation.
• Pre-treating raw materials, changing manufacturing processes, or changing fuels can all reduce emissions. Coal washing is one example of pre-treating.
• Substituting cleaner fuels (such as natural gas or low-sulfur fuels) is another viable emission control method.

Plant Maintenance

• Reducing emissions also involves increased attention to plant maintenance.
• Improperly maintained equipment, often combustion equipment, is a common cause of significant pollutant releases.
• Regular maintenance of equipment (vats, valves, and transmission lines) helps reduce fugitive emissions and the risk of spills.

Control Technology

• Advanced, add-on control technology plays a key role in pollution control.
• Control devices can either destroy or recover pollutants. Techniques include combustion, adsorption, absorption, and condensation.
• Various devices implemented for these processes are thermal incinerators, catalytic incinerators, flares, boilers, process heaters, carbon absorbers, spray towers, and surface condensers.
• Exhaust gas characteristics (flow rate, temperature, efficiency requirements), and site characteristics (space, power availability, water, regulations) are crucial for selecting air pollution control equipment.

Exhaust Gas Characteristics

• Total exhaust gas flow rate.
• Exhaust gas temperature.
• Required emission control efficiency.
• Particle size distribution.
• Particle resistivity.
• Composition of exhaust gases over operating range.
• Moisture content of the exhaust gases.
• Stack pressure.
• Exhaust gas combustibility and flammability properties.

Process or Site Characteristics

• Reuse/recycling of collected emissions.
• Availability of space.
• Availability of additional electrical power.
• Availability of water.
• Wastewater treatment facilities.
• Frequency of startup and shutdowns.
• Environmental conditions.
• Anticipated changes in control regulations.
• Anticipated changes in raw materials.
• Plant type (stationary or mobile).

Control Devices for Particulate Emissions

• Technologies for controlling particulate matter aim at removing particles from the gas stream.
• Various factors affect the optimal design (particle size, chemical characteristics).
• Common particulate control devices include gravity settlers (settling chambers), mechanical collectors (cyclones), electrostatic precipitators (ESPs), scrubbers, fabric filters, and hybrid systems.
• A combination of techniques is sometimes the most effective solution, e.g., a settling chamber could be used to remove large particles before smaller particles are managed by an ESP.

Equipment

• Gravity Settlers (Settling Chambers): Used to remove solids and liquids from gas streams. Characteristics include simple construction, low initial cost, low maintenance, low pressure losses, and easy waste disposal.
• Mechanical Collectors (Cyclones): Wide industrial use; centrifugal separators.

Fabric Filters (Bag Houses)

• Fabric filters (bag houses) perform filtration.
• Material types (natural fibers, synthetics, fiberglass) influence maximum operating temperatures.
• Cleaning mechanisms like shakers, reverse air, and pulse jet techniques are implemented.

Baghouse Filter

• Similar in efficiency to domestic vacuum cleaners.
• High efficiency for fine particle, removal (>99%).

Hybrid Systems

• Hybrid systems combine multiple control mechanisms (e.g., fabric filters with electrostatic precipitation).
• Wet electrostatic precipitators.
• Ionizing wet scrubbers.
• Dry scrubbers.
• Electrostatically augmented fabric filtration are examples.

Cyclones

• Used for particles between 10 and 100 microns.
• Simple, no moving parts, reliant on inertial forces.

Electrostatic Precipitators (ESPs)

• High efficiency, dry collectors of particulate matter. Use high voltage direct current (30–75 kV).
• Two major types: tubular and plate. Plate type is more common.

Wet Scrubbers

• Used in wet conditions, corrosive environments, or high temperatures where baghouses aren't suitable.
• High efficiency frequently achieved using a venturi scrubber in combination with a cyclone.

Scrubbers (Venturi Scrubbers)

• Wet scrubbing involves bringing the contaminated gas streams into close contact with a liquid (usually water).
• Wet scrubbers are part of a broader category of gas absorption equipment.

Comparison of Air Pollution Control Devices

• Graph illustrates the comparative removal efficiency of various air pollution control devices across different particle sizes.

Effectiveness of Air Pollution Control Devices

• Chart shows effectiveness of various air pollution control devices across different particle sizes and pollutant diameters.

Gaseous Emissions

• Control of gaseous pollutants usually involves add-on control devices for either destruction or recovery of the pollutant.
  • Control Equipment: Includes thermal incinerators, catalytic incinerators, flares, boilers and process heaters, carbon adsorbers, absorbers, and condensers.

Thermal Incinerator

• Used for destroying volatile organic compounds (VOCs).
• Controlled burn at high temperatures.
• High efficiency (up to 99.9%).

Applicable Pollutants

• Primarily VOCs.
• Some particulate matter (PM).

Catalytic Incinerators

• Similar to thermal incinerators but uses catalysts to increase oxidation reaction rates and lower temperatures.
• Catalysts commonly used include platinum and palladium.

Flares

• Primarily used as a last resort for gases of limited recyclability or those not easily combustible.
• Burning gas in an open flame.

Boilers and Process Heaters

• Usually used for generating heat and power.
• Can recycle pollutants to be used as fuel, if suitable to the burner.