Smog, Flue Gas Desulfurization and Gas-Phase Control Mechanisms: 18-Env-A5 Air Quality and Pollution Control Engineering, Undated paper, Q5

Problem 5: Smog, Flue Gas Desulfurization and Gas-Phase Control Mechanisms (25 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Check: the exam's own printed numbering skips directly from part (i) to part (iii), with no part (ii) printed anywhere on the page; the three sub-marks shown (5+10+10=25) fully account for the 25-mark question with no content gap.

Part (i) — smog: definition, formation, causal chemicals. Photochemical smog is a secondary form of urban air pollution — a hazy mixture of ground-level ozone, peroxyacetyl nitrate (PAN) and fine secondary organic aerosol — formed not by direct emission but through sunlight-driven reactions between nitrogen oxides and volatile organic compounds in the lower atmosphere. The cycle begins with photolysis of NO₂ ($NO_2+h\nu\rightarrow NO+O$); the O atom combines with O₂ to form ozone, and reactive hydrocarbon (VOC) radicals oxidise NO back to NO₂ without consuming an ozone molecule, breaking the NO/O₃ titration cycle that would otherwise hold ozone at a low steady state — letting ozone accumulate through the day. It is most intense on warm, sunny, low-wind days that trap and concentrate precursors near the surface. Two smog-causing chemicals: nitrogen oxides (NOₓ), from vehicle exhaust and stationary combustion, and volatile organic compounds (VOCs), from vehicle exhaust, fuel evaporation and solvent/paint use.

Part (iii) — flue gas desulfurization (FGD) schematic. The most common FGD technology on large coal-fired boilers is the wet limestone scrubber: flue gas from the boiler passes upward through a spray-tower absorber, contacting a downward-falling limestone (CaCO₃) slurry; SO₂ in the gas dissolves into the slurry droplets and reacts, $SO_2+CaCO_3+\tfrac12 H_2O\rightarrow CaSO_3\!\cdot\!\tfrac12H_2O+CO_2$. Air is sparged into a reaction tank to fully oxidise the sulphite to sulphate, $CaSO_3\!\cdot\!\tfrac12H_2O+\tfrac12O_2+\tfrac32H_2O\rightarrow CaSO_4\!\cdot\!2H_2O$ (gypsum), which is dewatered to a saleable by-product (wallboard-grade gypsum) rather than landfilled. Scrubbed gas passes through a mist eliminator to remove entrained slurry droplets before exiting to the stack.

Wet limestone FGD process schematic: flue gas contacts limestone slurry in the spray-tower absorber; spent (sulphite) slurry is oxidised in a reaction tank and dewatered to gypsum by-product; make-up limestone slurry is prepared from fresh limestone and water; scrubbed gas passes a mist eliminator before the stack.

Part (iv) — adsorption, absorption, combustion, incineration: definitions and application examples.

Mechanism	Definition	Example application
Adsorption	A gaseous pollutant molecule adheres to the surface (and internal pore structure) of a solid sorbent by physical (van der Waals) or chemical bonding, without changing phase or reacting in the bulk gas.	Activated-carbon canister capturing solvent VOC vapours from a paint-booth exhaust for later thermal regeneration/recovery.
Absorption	A gaseous pollutant is transferred from the gas phase into a contacting liquid, driven by solubility (Henry's law) and the concentration gradient across the gas–liquid interface; the liquid may be chosen to chemically react with and consume the dissolved species.	The wet limestone FGD scrubber of part (iii) — SO₂ absorbed into and chemically consumed by the CaCO₃ slurry; a packed-bed caustic (NaOH) scrubber similarly absorbs and neutralises acid gases such as HCl.
Combustion	A pollutant (typically a fuel-value organic compound) is oxidised at high temperature in a flame, converting it to CO₂ and H₂O with the release of heat, which may be recovered.	A flare combusting waste refinery/landfill gas at the point of release rather than venting it uncontrolled.
Incineration	Thermal oxidation of a waste gas stream in a dedicated combustion chamber (with or without a catalyst) at a controlled temperature and residence time, specifically to destroy the pollutant rather than to recover useful heat as the primary purpose.	A regenerative thermal oxidiser (RTO) destroying dilute solvent VOC emissions from an industrial coating/printing process to meet an emission-destruction-efficiency limit.

Check: this Problem is entirely qualitative — the reaction stoichiometry shown for the FGD chemistry is standard textbook chemistry, not derived from any numeric data supplied in the question.

Topic: Photochemical smog formation, flue-gas desulfurization, and the four generic gas-phase pollutant control mechanisms.

Key relations: NO₂ photolysis / ozone titration cycle and its VOC-mediated breaking; wet limestone FGD absorption and oxidation reactions; Henry's-law-driven gas absorption; adsorption isotherm capacity vs. sorbent surface area.

Why this works: smog formation and gas-phase pollutant control are two sides of the same reactive-chemistry coin — understanding which species drives a gas-phase or gas–liquid/gas–solid equilibrium forward (VOCs breaking the NO/O₃ titration cycle; CaCO₃ consuming dissolved SO₂; a sorbent's affinity capturing a VOC) is what lets a designer predict or engineer the outcome rather than memorise it, and the same four mechanisms (adsorption/absorption/combustion/incineration) recur across almost every gas-phase control problem in this subject.

Common pitfall: describing photochemical smog as if NOₓ alone produced net ozone build-up — without VOCs to convert NO back to NO₂ without consuming O₃, the NOₓ-only cycle is nearly self-cancelling; confusing incineration (destruction-focused, dedicated combustion chamber) with simple combustion/flaring (heat-recovery- or disposal-focused, often open or semi-open).

Source: Wark, Warner & Davis, Air Pollution: Its Origin and Control, 3rd ed., photochemical smog chapter; Cooper & Alley, Air Pollution Control: A Design Approach, 4th ed., Ch. 7–9 (adsorption, absorption, FGD, thermal oxidation).

Question 5 of 5: Smog, Flue Gas Desulfurization and Gas-Phase Control Mechanisms

Problem 5: Smog, Flue Gas Desulfurization and Gas-Phase Control Mechanisms (25 marks)