AP-42 Factors & Air Pollution Control Technologies

Quick Answer: Estimate emissions with AP-42 factors (mass per fuel or activity), then size/control with ESP/baghouse for PM, scrubbers for acid gas, SCR/SNCR for NOx, and thermal/catalytic oxidizers for VOCs. Control efficiency enters as Cout = Cin × (1 − η) or mass balance across the device.

Source testing and emission inventories underpin permits. The FE exam pairs factor-based emission estimates with control device selection and simple efficiency arithmetic.

AP-42 Emission Factors

EPA's AP-42 Compilation of Air Pollutant Emission Factors provides default mass emitted per unit activity:

Emission rate calculation:

[ \dot{E} = \text{Activity rate} \times \text{Emission factor} \times (1 - \eta_{control}) ]

Worked example: Natural gas boiler consumes 500 MMBtu/hr; uncontrolled NOx factor 0.15 lb/MMBtu; low-NOx burner η = 60% for NOx.

[ \dot{E}_{NOx} = 500 \times 0.15 \times (1 - 0.60) = 30 \text{ lb/hr} ]

AP-42 values are industry averages — permitted sources often use stack test or CEMS data instead for compliance.

Particulate Control

Electrostatic Precipitator (ESP)

• Charges particles in flue gas; collects on plates removed by rapping.
• Excellent for high-temperature coal flue gas.
• Sensitive to resistivity — very high or low resistivity reduces collection.
• Typical efficiencies 99%+ for PM when designed well.

Baghouse (Fabric Filter)

• Gas passes through fabric bags; dust cake aids filtration.
• Very high efficiency for fine PM including PM2.5.
• Requires temperature compatible with bag material; acid gas and moisture affect bag life.
• Pressure drop increases as cake builds; periodic pulse-jet cleaning.

Wet Scrubbers

Venturi, packed-bed, and spray-tower scrubbers contact gas with water or reagent solution:

• Remove SO2, HCl, PM simultaneously in some designs.
• Liquid-to-gas ratio and pH control absorption.
• Wastewater blowdown must be treated — cross-link to water exam topics.

[ \eta_{SO2} \approx 1 - \frac{C_{out}}{C_{in}} ]

Nitrogen Oxides Control

SCR requires catalyst bed and ammonia handling; watch for ammonia slip and SO2 to SO3 oxidation concerns on exams conceptually.

VOC and Hazardous Air Pollutant Control

• Thermal oxidizer — high temperature (~1400°F+) combusts VOCs to CO2 and H2O.
• Catalytic oxidizer — lower temperature with catalyst; sensitive to poisons (silicon, heavy metals).
• Carbon adsorption — regenerable GAC for low concentrations.
• Condensers and membranes — recovery when economics favor reuse.

Destruction efficiency 95–99% common for RACT/MACT contexts in problems.

Control Train Thinking

Exam scenarios may present a sequence:

1. Cyclone → coarse removal protecting downstream baghouse.
2. Baghouse → fine PM.
3. Dry sorbent injection + fabric filter → acid gas and metals.

Overall efficiency for series PM devices:

[ \eta_{total} = 1 - (1-\eta_1)(1-\eta_2) ]

Worked example: Cyclone 70%, baghouse 95% on remaining dust.

[ \eta_{total} = 1 - (0.30)(0.05) = 1 - 0.015 = 98.5% ]

Continuous Emission Monitoring

CEMS measure opacity, SO2, NOx, CO2 for large sources. Relative Accuracy Test Audits (RATA) verify CEMS against reference methods.

FE Exam Patterns

• Pick best control for pollutant class (PM vs. NOx vs. VOC).
• Apply factor × activity × (1 − η).
• Combine series efficiencies.
• Distinguish prevention (low-sulfur fuel) from end-of-pipe control.

Exam trap: Applying 95% control to already-controlled emissions without clarifying whether efficiency is overall or incremental on the uncontrolled rate.

AP-42 arithmetic plus control-device matching is high-yield FE air quality material — practice unit conversions between lb/hr, tpy, and ppm in the stack.

MACT, RACT, and Technology Standards Context

Beyond end-of-pipe devices, Maximum Achievable Control Technology (MACT) for HAPs and Reasonably Available Control Technology (RACT) in SIPs set technology-based limits. Compare destruction efficiency (oxidizers) versus collection efficiency (particulate devices). For VOC capture plus control, overall removal is the product of capture fraction and control device efficiency if uncollected VOC vents uncontrolled.

Continuous emission monitoring (CEMS) verifies compliance with permit limits; stack tests provide periodic verification when CEMS not required. Know destruction removal efficiency (DRE) for incinerators treating hazardous waste — often 99.99% requirement in rules cited conceptually on exams.

Selective Non-Catalytic Reduction Details

SNCR injects ammonia or urea into the furnace at 1600–2000°F where NOx forms; temperature window is narrow — too cold yields poor reduction, too hot decomposes reagent. SCR uses catalyst at lower flue gas temperature (~600–750°F) achieving higher NOx removal. Ammonia slip and SO2 oxidation to SO3 are operational concerns on coal units with SCR — conceptual tradeoffs for multiple-choice items.

Activated Carbon and Dry Sorbents

Activated carbon injection (ACI) ahead of fabric filters captures mercury and dioxins in coal combustion. Dry sorbent injection (lime, trona) removes acid gases in ductwork before particulate collection. These trains illustrate multi-pollutant control design common in modern power and industrial boilers.