Air Pollution Control Devices – Comprehensive Review

Particulate-Phase Emission Control

Electrostatic Precipitators (ESPs)

• Core principle: apply an external electric field so particles pick up a charge and migrate toward oppositely charged collector plates.

  • Negatively charged discharge wires (corona wires) ionise the passing gas stream → particles attain a negative charge.

  • Charged particles drift to the grounded, positively charged plates and accumulate as a dust layer.

  • Periodic “rapping” dislodges the layer; dust falls into a hopper for removal.

• Types:
  • Dry ESP (most common)
  • Wet ESP (collector surfaces continually or intermittently washed).

• Performance: removal efficiency for fine particles typically ≈ 99\%; actual results vary with particle resistivity, gas flow, moisture, power input, and plate spacing.

• Operating envelope: flue-gas temperatures up to \approx 700\,^{\circ}!\text{C} (coal plants) with specialised alloys; best with moderate humidity to reduce electrical resistivity.

• Typical industrial uses:
  • Utility boilers (fly-ash)
  • Cement kilns (raw-meal dust)
  • Pulp & paper recovery furnaces (salt-cake and lime dust)
  • Petrochemical plants (\ce{H2SO4} mist)
  • Steel & foundry shops (metal fumes).

• Advantages: very low pressure drop, heat recovery possible, capable of continuous operation.

• Limitations: high capital cost, performance sensitive to dust resistivity, ozone generation in high-voltage zone, bulky footprint.

Fabric Filters (Baghouses)

• Mechanism: dirty gas passes through porous fabric → dust accumulates on the filter cake; filtration is dominated by cake resistance rather than fabric pores once a layer forms.

• Filtration mechanisms: interception, inertial impaction, diffusion, electrostatic attraction, and sieving.

• Removal efficiency: generally >99\% even for sub-micron particles.

• Key design variables: cloth area/air-flow ratio (air-to-cloth ratio), fabric type (cotton, polyester, aramid, PTFE, glass, etc.), cleaning method (pulse-jet, reverse-air, shaker), maximum service temperature, chemical compatibility.

• Temperature constraint: gases often cooled to <260\,^{\circ}!\text{C} to protect fabrics → may require dilution air or heat exchangers.

• Pros: high efficiency, modular, collects dry dust (easy reuse/disposal).

• Cons: sensitive to moisture (clogging), flammable dust hazards, higher pressure drop than ESPs.

Wet Scrubbers – Venturi Type

• Geometry: converging-throat-diverging nozzle accelerates gas to high velocity (typically 60{-}120\,\text{m s}^{-1}) while liquid is injected axially or tangentially.

• At throat, atomisation produces fine droplets; collisions/coalescence capture particles.

• Exit diffuser slows gas, allowing droplet–particle agglomerates to disengage and fall into separator.

• Typical particulate removal: \le 1\,\mu\text{m} particles captured with efficiencies up to 99\% when pressure drop >25\,\text{cm H}_2\text{O}.

• Trade-offs: very effective for PM and simultaneous gas absorption, but generates wastewater, imposes high pressure drop (⇒ high fan energy), and requires corrosion-resistant materials.

Cyclone (and Multiclone) Separators

• Utilise centrifugal force: swirling gas creates radial acceleration ar=\tfrac{v{\theta}^2}{r} → particles migrate to wall, lose momentum, and fall to hopper.

• Gas path: descending outer vortex + ascending inner vortex; cleaned gas exits through central outlet tube.

• Typical efficiency curve:
  • >90\% for particles >10\,\mu\text{m}
  • Drops sharply below 5\,\mu\text{m}.

• Often deployed as a pre-cleaner upstream of ESP or baghouse to reduce dust loading.

• Advantages: simple, no moving parts, operates at high temperatures/pressures, low capital cost.

• Disadvantages: poor fine-particle capture, erosion at high inlet velocities.

Settling (Gravity) Chambers

• Large, low-velocity boxes allowing residence time so particles settle by gravity: terminal settling velocity vt = \tfrac{(\rhop-\rhog) g dp^2}{18\mu} (Stokes’ law for small Re).

• Effective only for coarse particles (>75\,\mu\text{m}); removal efficiencies modest (10–50 %).

• Widely used as the first stage to protect downstream equipment.

Gas- & Vapor-Phase Emission Control

Thermal Oxidizers (Afterburners)

• Objective: convert combustible pollutants (\ce{CO}, \ce{VOC}, \ce{HAP}) to \ce{CO2} and \ce{H2O}.

• Governed by the “3 T’s”:
  • Temperature – typically 760\text{–}1200\,^{\circ}!\text{C}
  • Time (residence) – 0.5\text{–}2\,\text{s}
  • Turbulence – ensures thorough mixing.

• Sub-categories:
  • Recuperative (heat exchanger recovers outlet heat)
  • Regenerative (RTO – ceramic beds store/release heat; >95\% heat recovery)
  • Direct-flame (enclosed flare).

• Regulatory aspects: continuous emission monitors (CEMs) for \ce{CO}, \ce{O2}, hydrocarbons; safety interlocks; auxiliary fuel consumption tracked.

• Pros: very high destruction efficiency (>99\% for VOCs).

• Cons: high fuel cost, NOx formation, possible dioxin/furan generation if chlorine present.

Catalytic Reactors – Selective Catalytic Reduction (SCR)

• Purpose: reduce \text{NO}_x (\ce{NO} + \ce{NO2}) to \ce{N2} and \ce{H2O} using a reducing agent (commonly ammonia or urea-derived \ce{NH3}).

• Global application: power plants, gas turbines, refinery heaters.

• Stoichiometric reactions:
  4\,\ce{NO} + 4\,\ce{NH3} + \ce{O2} \;\to\; 4\,\ce{N2} + 6\,\ce{H2O}
  6\,\ce{NO2} + 8\,\ce{NH3} \;\to\; 7\,\ce{N2} + 12\,\ce{H2O}

• Catalyst: vanadia–titania, zeolite, or precious-metal formulations; optimal 275\text{–}425\,^{\circ}!\text{C}.

• Typical overall \text{NO}_x reduction: 70\text{–}90\%; higher possible with fresh catalyst and optimised ammonia mixing.

• Side reactions/concerns:
  • \ce{SO2 + \tfrac{1}{2} O2 \to SO3} (acid mist formation).
  • Ammonia slip leading to \ce{NH4HSO4} deposition (sticky fouling).
  • Catalyst deactivation by arsenic, alkalis, dust.

Adsorption Systems

• Fundamental distinction:
  • Adsorption – molecules adhere to solid surface.
  • Absorption – molecules dissolve into bulk liquid.

• Commercial adsorbents: activated carbon (surface area 800\text{–}1200\,\text{m}^2\,\text{g}^{-1}), silica gel, activated alumina, molecular sieve zeolites.

• Configurations:
  • Regenerative – twin-or-multiple beds; whilst one adsorbs, another undergoes thermal or steam desorption.
  • Non-regenerative (throw-away) – thin beds; saturated media sent for re-activation or disposal.

• Use cases: VOC solvent recovery, odour control, mercury capture, solvent loading docks.

• Governing isotherms: Freundlich q = K P^{1/n}, Langmuir q = \tfrac{q_\text{max} bP}{1 + bP} (design for breakthrough time).

Absorbers (Gas Scrubbers)

• Gas species are transferred into a liquid phase; overall rate governed by two-film theory: NA = KL a (C{Ai} - C{Al}).

• Liquid of choice: water (cheap, non-toxic) or specialised solutions (alkaline, oxidising, chelating).

• Equipment families:
  • Spray tower (co-current or counter-current)
  • Packed column (random or structured packing)
  • Plate column
  • Venturi scrubber (dual PM & gas removal).

• Typical pollutant targets: \ce{SO2}, \ce{HCl}, \ce{NH3}, water-soluble VOCs.

• Removal efficiency: >95\% achievable with adequate liquid-to-gas ratio.

• Caveat: creates contaminated liquid effluent → must be treated (chemical neutralisation, biological treatment, crystallisation) to avoid cross-media pollution.

Biofilters

• Engineered soil-based or synthetic media beds (compost, wood chips, lava rock, plastic saddles) harbouring mixed microbial consortia.

• Polluted air (generally warm & humid, low VOC load, odours) flows upward; compounds diffuse into biofilm and are metabolised mainly to \ce{CO2}, \ce{H2O}, biomass.

• Operating conditions:
  • Temperature 10\text{–}40\,^{\circ}!\text{C}
  • Moisture >40\% wet basis
  • pH controlled via nutrient irrigation.

• Advantages: low energy and O&M cost, minimal secondary waste.

• Limitations: large footprint, sensitive to toxic shocks and drying, limited to water-soluble/biodegradable pollutants.

Other Control Technologies

Condensation

• Physics: condense vapour when T < T{\text{dew}} or P > P{\text{dew}} (Clausius–Clapeyron relation \tfrac{dP}{dT}=\tfrac{L}{T \Delta v}).

• Devices:
  • Contact condensers – intimate gas/liquid contact (spray, quench).
  • Surface condensers – shell-and-tube or plate heat exchangers (no mixing).

• Typical removal: 50\% to >95\% depending on coolant temperature and vapour partial pressure.

• Often installed as front-end pre-treatment to lower VOC load or recover valuable solvents (e.g.
  cryogenic condensation for \ce{CFC} recovery).

Incineration (Combustion)

• Complete oxidation: fuel/air ratio at or above stoichiometric; products ideally \ce{CO2} + \ce{H2O} (plus \ce{N2}).

• Classifications:

  1. Direct combustion / Flaring
     • No residence chamber; flame open to atmosphere (elevated or ground flare).
     • EPA studies → destruction removal efficiency (DRE) \approx 98\% for hydrocarbons if flame stability and mixing maintained.

  2. Thermal incinerator
     • Burner + insulated residence chamber (0.5–1.5 s).
     • DRE >99\%; supplementary fuel needed if waste gas heating value <930\,\text{kJ m}^{-3}.

  3. Catalytic incinerator
     • Downstream catalyst bed (Pt–Pd, Mn–Cu oxides).
     • Allows 150\text{–}300\,^{\circ}!\text{C} lower temperature than thermal firing for same DRE (>95\%).
     • Susceptible to catalyst poisoning (\ce{Si}, \ce{P}, heavy metals).

• Important design metrics:
  • Lower heating value (LHV) of waste gas
  • Minimum ignition temperature
  • Oxygen availability
  • Exit \ce{CO} / unburned hydrocarbon levels to satisfy 40 CFR 60.

Practical Cross-links & Exam Tips

• Pre-cleaners (settling chamber, cyclone) often precede high-efficiency collectors (baghouse, ESP) to minimise wear/fouling and lower pressure drop.

• Wet scrubbers simultaneously address PM and soluble gas pollutants (e.g.
  \ce{SO2}) but create liquid effluent → remember the “pollution shifting” concept.

• The 3 T’s (Temperature, Time, Turbulence) principle re-appears in incineration design – link related calculation problems.

• SCR relies on stoichiometric excess ammonia (NH₃/NOₓ ratio 0.9–1.1); know how to compute urea consumption: \ce{(NH2)2CO} + \tfrac{3}{2}\ce{O2} \to 2\ce{CO2} + 2\ce{NH3}.

• Condensation vs. Adsorption – both recover solvents; for high concentrations (> several g m⁻³) condensation first, adsorption polishes residual VOC (<100 mg m⁻³).

• Efficiency hierarchy for PM (coarse→fine): Settling < Cyclone < Venturi Scrubber ≈ Fabric Filter ≈ ESP.

• Always quote efficiency values and governing limitations in design/essay questions.