4.3 Air pollution control technologies

Air pollution control technologies are crucial for managing harmful emissions. These methods, ranging from pre-combustion modifications to post-combustion treatments, target various pollutants like particulate matter, gases, and specific compounds such as NOx and SO2.

Effectiveness of these technologies depends on factors like removal efficiency and cost-effectiveness. Engineers must consider source characteristics, pollutant properties, and desired outcomes when designing control systems. Unintended consequences, including water pollution and increased energy use, also need careful evaluation.

Air Pollution Control Technologies

Classification and Mechanisms of Action

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• Classify air pollution control technologies into two main categories: pre-combustion and post-combustion controls
  • Pre-combustion controls reduce emissions by modifying the fuel or combustion process
  • Post-combustion controls remove pollutants from the exhaust gas stream
• Particulate matter control technologies use physical processes to remove particles from the gas stream
  • Cyclones utilize centrifugal force
  • Electrostatic precipitators (ESPs) employ electrostatic attraction
  • Fabric filters (baghouses) use filtration
  • Wet scrubbers rely on liquid absorption
• Gaseous pollutant control technologies use chemical and physical processes to remove or convert gaseous pollutants
  • Adsorption systems (activated carbon) adsorb pollutants onto a solid surface
  • Absorption systems (wet scrubbers) dissolve pollutants into a liquid
  • Condensation systems cool the gas stream to condense pollutants
  • Thermal oxidation (incineration) oxidizes pollutants at high temperatures
• Nitrogen oxide (NOx) control technologies reduce NOx formation or convert NOx to nitrogen and water
  • Low NOx burners minimize NOx formation during combustion
  • Selective catalytic reduction (SCR) uses catalysts to convert NOx
  • Selective non-catalytic reduction (SNCR) uses reagents to convert NOx without a catalyst
• Sulfur dioxide (SO2) control technologies use alkaline materials to react with SO2 and form solid compounds
  • Flue gas desulfurization (FGD) systems, such as wet scrubbers, use limestone or lime slurries
  • Dry sorbent injection systems inject alkaline materials directly into the flue gas
• Mercury control technologies adsorb or oxidize mercury and capture it in downstream devices
  • Activated carbon injection adsorbs mercury onto carbon particles
  • Oxidation catalysts convert elemental mercury to oxidized forms
  • Fabric filters with sorbent injection capture mercury-containing particles

Factors Influencing Performance and Cost-Effectiveness

• Removal efficiency measures the percentage of the target pollutant removed from the gas stream
  • Higher removal efficiencies indicate better performance but may increase costs
• Cost-effectiveness metrics assess the cost per unit of pollutant removed or health benefit achieved
  • Cost per ton of pollutant removed
  • Cost per unit of reduced mortality or morbidity
• Factors influencing cost-effectiveness include:
  • Capital costs for equipment and installation
  • Operating and maintenance costs (energy, labor, replacement parts)
  • Reagent costs (limestone, activated carbon, catalysts)
  • Value of recovered byproducts (sulfuric acid from FGD systems)
• Life cycle assessment (LCA) evaluates overall environmental impacts and trade-offs
  • Considers resource consumption, greenhouse gas emissions, and waste generation
• Multi-pollutant control strategies can be more cost-effective than separate control technologies for each pollutant
  • Simultaneously reduce emissions of NOx, SO2, and particulate matter

Performance and Cost-Effectiveness of Air Pollution Control Strategies

Measuring Performance and Cost-Effectiveness

• Removal efficiency is the percentage of the target pollutant removed from the gas stream
  • Higher removal efficiencies generally indicate better performance
  • Increasing removal efficiency may come at a higher cost
• Cost-effectiveness metrics assess the cost per unit of benefit achieved
  • Cost per ton of pollutant removed
  • Cost per unit of health benefit (reduced mortality or morbidity)
• Life cycle assessment (LCA) evaluates overall environmental impacts and trade-offs
  • Considers factors such as resource consumption, greenhouse gas emissions, and waste generation
  • Helps identify potential unintended consequences and environmental burdens

Factors Influencing Cost-Effectiveness

• Capital costs for equipment purchase and installation
  • Higher capital costs can increase the overall cost of the control strategy
• Operating and maintenance costs
  • Energy consumption for running the control devices
  • Labor costs for operation and maintenance
  • Replacement parts and consumables (filters, catalysts)
• Reagent costs for chemical-based control technologies
  • Limestone or lime for flue gas desulfurization (FGD) systems
  • Activated carbon for mercury adsorption
  • Ammonia or urea for selective catalytic reduction (SCR)
• Value of recovered byproducts
  • Sulfuric acid from FGD systems can be sold as a commodity
  • Fly ash from particulate control devices can be used in cement production
• Multi-pollutant control strategies can improve cost-effectiveness
  • Simultaneously reducing emissions of multiple pollutants (NOx, SO2, particulate matter)
  • Avoids the need for separate control technologies for each pollutant

Engineering Principles for Air Pollution Control Systems

Design Considerations and Parameters

• Characteristics of the emission source
  • Flow rate, temperature, and pressure of the gas stream
  • Pollutant concentrations and variability
• Properties of the target pollutants
  • Particle size distribution for particulate matter
  • Solubility and reactivity of gaseous pollutants
• Desired removal efficiency and outlet concentrations
  • Regulatory requirements and emission limits
  • Environmental and health impact goals
• Fluid mechanics principles for sizing and design
  • Pressure drop calculations to ensure adequate gas flow
  • Gas velocity profiles to optimize contact time with control devices
• Mass and energy balance calculations
  • Determining the required capacity of control systems
  • Estimating reagent consumption and heat input requirements

Modeling and Optimization Techniques

• Adsorption and absorption process modeling
  • Adsorption isotherms to predict the performance of activated carbon systems
  • Mass transfer coefficients to design wet scrubbers and absorbers
• Reaction kinetics and catalysis principles
  • Designing and optimizing selective catalytic reduction (SCR) systems
  • Selecting catalysts and determining optimal temperature ranges
• Optimization techniques for cost-effective design
  • Mathematical programming to minimize costs while meeting performance targets
  • Process simulation to evaluate different design scenarios and trade-offs
• Pilot-scale testing and validation
  • Verifying the performance of control technologies under real-world conditions
  • Fine-tuning design parameters based on experimental data

Unintended Consequences of Air Pollution Control Measures

Environmental and Health Impacts

• Water pollution from wet scrubber effluents
  • Discharge of acidic or metal-laden wastewater
  • Potential impacts on aquatic ecosystems and drinking water sources
• Solid waste generation from spent sorbents and catalysts
  • Disposal of hazardous or non-hazardous waste in landfills
  • Potential for leaching of contaminants into soil and groundwater
• Increased greenhouse gas emissions from energy consumption
  • Operating air pollution control devices can be energy-intensive
  • Contribution to climate change if energy source is carbon-intensive
• Formation of secondary pollutants
  • Ammonia slip from selective catalytic reduction (SCR) systems
  • Oxidation of mercury to more soluble and bioavailable forms

Economic and Social Considerations

• Increased costs for industries and consumers
  • Higher prices for goods and services due to compliance costs
  • Potential impacts on competitiveness and profitability
• Job losses in affected sectors
  • Closure of older, less efficient facilities unable to meet new standards
  • Shifts in employment towards cleaner industries and technologies
• Changes in market demand and innovation
  • Incentives for the development and adoption of cleaner technologies
  • Potential barriers to entry for new firms due to high compliance costs
• Equity and environmental justice concerns
  • Disproportionate impacts on low-income and minority communities
  • Unequal distribution of costs and benefits across different regions and populations

Policy and Regulatory Implications

• Emissions trading programs and market-based incentives
  • Creation of markets for pollution allowances and credits
  • Potential for gaming the system or creating perverse incentives
• Technology standards and best available control technology (BACT) requirements
  • Encouraging the adoption of state-of-the-art control technologies
  • Potential for technology lock-in and reduced innovation incentives
• Interaction with other environmental policies and regulations
  • Conflicts or synergies with climate change mitigation efforts
  • Impacts on the implementation of renewable energy and energy efficiency measures