Question 6 of 7: Photochemical smog, halogenated-hydrocarbon control, and odour control

Question 6: Photochemical smog, halogenated-hydrocarbon control, and 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) Engineering approaches to minimize photochemical smog

Approach 1 — reduce VOC and NOx emissions at the source (mobile and stationary). Photochemical smog forms from sunlight-driven reactions between volatile organic compounds (VOCs) and nitrogen oxides (NOx); cutting either precursor at source — catalytic converters and evaporative-emission controls on vehicles, Stage I/II vapour-recovery at fuel stations, low-NOx burners and SCR on stationary combustion sources, and VOC-content limits on paints/solvents/consumer products — directly reduces the raw material available for smog chemistry. Because the VOC/NOx ratio determines whether a given airshed is VOC-limited or NOx-limited, effective control requires knowing which regime the city is in (photochemical grid modelling), otherwise reducing the wrong precursor can locally increase ozone.

Approach 2 — land-use and transportation planning to reduce trapped emissions. Smog is worst where emissions accumulate under stagnant, temperature-inverted conditions in a topographically or thermally bounded airshed (e.g. a valley or coastal basin, as in the Lower Fraser Valley); engineering/planning measures that reduce vehicle-kilometres travelled (transit-oriented development, congestion pricing, industrial siting away from population centres and prevailing-wind fetch to residential areas) reduce the emission density feeding the trapped air mass, complementing per-source emission control.

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

A 99.5% destruction/removal efficiency (DRE) on a chlorinated-hydrocarbon (e.g. a chlorinated-solvent) emission stream is squarely in the range achieved by thermal oxidation with acid-gas scrubbing, and the design proceeds as follows.

Stated assumptions. (1) The emission is a dilute, combustible vapour stream (below its lower flammable limit) amenable to thermal oxidation rather than a liquid/solid waste requiring incineration with feed handling; (2) sufficient auxiliary fuel (natural gas) is available to sustain combustion temperature independent of the pollutant's own heating value; (3) the stream carries no particulate loading that would foul heat-recovery surfaces; (4) chlorine content is low enough that HCl (not the more difficult-to-control Cl₂/dioxin-precursor chemistry of high-chlorine chlorinated waste) is the principal combustion by-product.

1 — Thermal oxidizer sizing. Specify a combustion chamber temperature and residence time sufficient for ≥99.5% destruction of the specific chlorinated compound (chlorinated hydrocarbons typically need higher temperature/longer residence than simple hydrocarbons because C–Cl bonds are harder to fully oxidize) — typically 980–1,100 °C with ≥1 s residence time, verified against published DRE data or a trial burn for the specific compound, since destruction efficiency is compound-specific, not a single universal number.

2 — Acid-gas quench and scrubbing. Combustion of a chlorinated compound converts essentially all the chlorine to HCl; the hot combustion gas is quenched and passed through a caustic (NaOH) wet scrubber to neutralize HCl before discharge, both to meet acid-gas emission limits and to protect downstream ductwork/stack from corrosion.

3 — Heat recovery. A recuperative or regenerative heat exchanger preheats incoming waste-gas/combustion air with the hot oxidizer exhaust, reducing auxiliary fuel consumption — important because sustaining ≥980 °C continuously is otherwise fuel-intensive for a dilute vapour stream.

4 — Continuous emissions monitoring and interlock. A continuous temperature (and, where required, THC or CO surrogate) monitor is interlocked to automatically divert/shut off feed if the chamber temperature drops below the validated DRE setpoint, since destruction efficiency collapses rapidly below the design temperature — this closed-loop control is what actually guarantees the 99.5% figure is met continuously rather than only during a compliance test.

5 — Stack testing to confirm DRE. A compliance stack test measures inlet and outlet concentration of the target compound(s) to confirm $\text{DRE}=(1-C_{out}/C_{in})\times100\%\ge99.5\%$ under representative operating conditions, closing the design loop.

(iii) Physical-chemical odour control technology

Technology: chemical (wet) scrubbing with sodium hypochlorite / caustic solution, widely used for headworks and sludge-handling odour control at wastewater plants. Design principle: foul air is drawn from the odour-generating enclosure (e.g. headworks, digester, biosolids building) and contacted counter-currently with a recirculating hypochlorite/caustic solution in a packed tower; H₂S and reduced-sulfur odorants are oxidized and/or absorbed (H₂S is acidic and absorbs readily into the alkaline liquid, then is oxidized by the hypochlorite to sulfate), destroying rather than merely diluting the odour. Operational requirements: continuous or automated ORP/pH-trimmed dosing of hypochlorite and caustic to maintain oxidation capacity and pH as the odour load varies through the day; adequate exhaust-fan capacity to maintain negative pressure in the source enclosure so odours do not escape uncaptured. Maintenance requirements: periodic cleaning of packing media to prevent scaling/biological fouling that would reduce gas–liquid contact area; blowdown and replacement of recirculating scrubbing liquor as dissolved-solids/salt content builds up; chemical (hypochlorite, caustic) storage and feed-pump upkeep, including managing the safety hazards of on-site hypochlorite storage.