Question 6 of 7: Photochemical smog, chlorinated-hydrocarbon destruction, and industrial odour control

Question 6: Photochemical smog, chlorinated-hydrocarbon destruction, and industrial odour control (20 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.

(i) Sunlight-driven formation of photochemical smog

Physical atmospheric conditions. Smog accumulates when pollutants are trapped near ground level beneath a stagnant, temperature-inverted air mass (warm air aloft over cooler surface air, suppressing vertical mixing), typically within a topographically or thermally bounded airshed (a valley or coastal basin), and requires strong, sustained sunlight to drive the photochemistry — conditions most common on clear, calm, sunny days with light wind.

Chemical reactions. Sunlight (UV) photolyses NO₂ ($\text{NO}_2+h\nu\rightarrow\text{NO}+\text{O}$); the resulting atomic oxygen combines with O₂ to form ozone ($\text{O}+\text{O}_2\rightarrow\text{O}_3$), which in a clean NOx-only atmosphere would simply react back with NO to regenerate NO₂ (a null cycle, no net ozone build-up). In the presence of reactive VOCs, hydroxyl-radical-initiated VOC oxidation instead produces peroxy radicals (RO₂•) that oxidize NO to NO₂ without consuming ozone, breaking the null cycle and allowing ozone (and secondary products such as PAN and fine secondary particulate) to accumulate through the day — the VOC + NOx + sunlight combination is what distinguishes true photochemical smog from simple primary-pollutant haze.

(ii) 99.5% reduction of chlorinated-hydrocarbon emissions — process design

Stated assumptions. (1) The emission is a dilute, combustible vapour stream (below its lower flammable limit), not a liquid/solid waste requiring separate feed handling; (2) sufficient auxiliary natural-gas fuel is available to sustain the required combustion temperature independent of the pollutant's own heat of combustion; (3) chlorine content is low enough that HCl, not the more complex Cl₂/dioxin-precursor chemistry of high-chlorine waste incineration, is the principal combustion by-product; (4) the compound's destruction kinetics at the chosen design temperature/residence time are adequate to reach 99.5% destruction/removal efficiency (DRE), consistent with published thermal-oxidation performance for chlorinated hydrocarbons at 980–1,100 °C, ≥1 s residence time.

1 — Thermal oxidizer sized to the standard 99.5%-DRE envelope. A combustion chamber operated at 980–1,100 °C with a minimum 1-second residence time and adequate turbulence/mixing (the "three T's": temperature, time, turbulence) is the standard design point that achieves 99.5% DRE for chlorinated hydrocarbons — sized with margin (upper end of the temperature range, slightly more than 1 s) rather than the bare minimum, to absorb normal process variability.

2 — Adequate mixing/turbulence control. Proper burner and chamber aerodynamic design (baffled or swirl-induced mixing) ensures the pollutant-laden gas is uniformly exposed to the design temperature for the full residence time, since local under-mixed pockets that bypass the hot zone are the dominant cause of a thermal oxidizer under-performing its nominal DRE.

3 — Acid-gas quench and scrubbing. The hot, HCl-laden combustion gas is quenched and passed through a caustic (NaOH) wet scrubber to meet acid-gas emission limits and protect downstream equipment from corrosion.

4 — Continuous temperature monitoring with feed interlock. A continuous chamber-temperature monitor is interlocked to automatically divert or shut off feed if conditions drop below the validated 99.5%-DRE setpoint, since destruction efficiency falls rapidly below the design temperature — this closed-loop control is what actually guarantees the 99.5% figure is maintained continuously.

5 — Periodic stack testing. Confirmatory stack testing at commissioning and on a periodic schedule thereafter verifies the achieved DRE against the 99.5% target using an analytical method with sufficient sensitivity for the small residual outlet concentration.

(iii) Engineered odour-control technology

Technology: wet chemical scrubbing (oxidative scrubber, e.g. sodium hypochlorite or hydrogen peroxide). Foul air is drawn through a packed-tower scrubber where it contacts a recirculating oxidant solution counter-currently; reduced-sulfur odorants (H₂S, mercaptans) and other odour-causing compounds transfer from the gas phase into the liquid and are chemically oxidized (destroyed, not merely absorbed) by the hypochlorite/peroxide, converting them to non-odorous sulfate/sulfonate species. Design principle: the packing surface area and gas/liquid contact time (empty-bed residence time) are sized against the design odour loading and required removal efficiency, with the oxidant feed rate/concentration controlled (often via an oxidation-reduction potential, ORP, set point) to maintain sufficient residual oxidant in the recirculating liquor. Operational requirements: continuous ORP and pH monitoring/control of the recirculating scrub liquor, since insufficient residual oxidant lets odorant pass through untreated while excess oxidant wastes reagent and can create its own by-product/corrosion issues; periodic liquor blowdown/makeup to control dissolved-salt build-up. Maintenance requirements: periodic packing inspection/cleaning to prevent fouling or channeling that would reduce gas-liquid contact efficiency; mist-eliminator and pump maintenance; safe handling and storage of the oxidant chemical (hypochlorite degrades over time and must be replenished; peroxide requires careful materials compatibility).