18-Env-A5 Air Quality and Pollution Control Engineering · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2014 — 04-Env-A5 / Air Quality and Pollution Control Engineering. 3 hours duration; closed book with a candidate-prepared 8.5×11 in double-sided aid sheet; Casio or Sharp approved calculator only. Any five (5) questions constitute a complete paper (the first five answers as they appear are marked); all seven are solved below for completeness. Each question is worth 20 marks with section marks shown in brackets.
Reference texts. Cooper & Alley, Air Pollution Control: A Design Approach (4th ed.); Wark, Warner & Davis, Air Pollution: Its Origin and Control (3rd ed.); Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); Canadian Environmental Protection Act, 1999 (CEPA) and the Canadian Ambient Air Quality Standards (CAAQS) administered by Environment and Climate Change Canada.
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.
Asphalt and concrete batch plants emit two distinct gas/vapour contaminant families: fine mineral/cement particulate (dust) from aggregate handling and drying, and odorous, condensable organic vapour (aldehydes and light hydrocarbons) driven off the heated asphalt binder. A venturi scrubber controls both in a single device. Dust-laden gas is accelerated through a converging throat to a high velocity (typically 60–120 m/s); scrubbing liquid injected at or just upstream of the throat is atomized by the high-velocity gas into fine droplets, and particles are captured predominantly by inertial impaction — their momentum carries them across gas streamlines and into the droplets while the lighter gas molecules deflect around them. The same intense gas–liquid contact area also drives absorption of the soluble/condensable organic vapour into the liquid phase, so the venturi controls the particulate and the odorous vapour together, with the pressure drop across the throat (set by the throat gas velocity) the primary design lever for collection efficiency on both.
Combustion-based: Regenerative Thermal Oxidizer (RTO). Example — a printing/coating line exhausting solvent-laden VOC-contaminated air. The exhaust stream is preheated by passing through a bed of ceramic heat-exchange media, then raised to 760–870 °C in a combustion chamber (with auxiliary natural-gas burners as needed) for a residence time of roughly 0.5–1 second, oxidizing the VOCs to CO2 and H2O. The hot, cleaned gas is then routed back through a second ceramic bed to recover its heat before exhaust, and the beds cycle roles (regenerative), giving thermal efficiencies above 95% and VOC destruction efficiencies typically above 98%.
Absorption-based: caustic packed-bed scrubber. Example — acid-gas exhaust (HCl or SO2) from a chemical-process vent or semiconductor fabrication exhaust. The gas is contacted counter-currently with a recirculated dilute sodium hydroxide (NaOH) solution flowing over structured or random packing (e.g., pall rings), which maximizes gas–liquid interfacial area; the acid gas dissolves into the liquid film and is neutralized (e.g., $\text{HCl}+\text{NaOH}\rightarrow \text{NaCl}+\text{H}_2\text{O}$), continuously regenerating the driving concentration gradient that sustains mass transfer. Removal efficiency is controlled by the liquid-to-gas ratio, packing height and the caustic concentration maintained in the recirculating sump.
The figure below shows a typical two-stage industrial-waste incineration train: waste and combustion air enter a primary combustion chamber, the resulting flue gas is held at high temperature in a secondary combustion chamber (afterburner) fed with auxiliary fuel and air, then passed through heat recovery/quench and an air-pollution-control device before release.
Three key design/operating principles (the "3 T's" plus excess air):
(1) Temperature. The secondary chamber must be held above the auto-ignition/destruction temperature of the target contaminants (typically 980–1200 °C for hazardous organics) — too low a temperature leaves partially-oxidized products (including products of incomplete combustion such as CO and dioxins/furans) in the exhaust.
(2) Residence time. Gas must remain in the high-temperature zone long enough (typically ≥1–2 seconds) for the oxidation reactions to go to completion; residence time is set by chamber volume divided by the actual (hot) volumetric gas flow, so under-sizing the chamber for a given flow silently erodes destruction efficiency even if the temperature target is met.
(3) Turbulence/mixing. Waste, fuel and combustion air must be thoroughly mixed (via burner design, baffles or swirl) so that every parcel of gas actually reaches the target temperature for the full residence time — poor mixing creates cooler "dead zones" that bypass complete destruction even when the bulk average temperature and residence time both look adequate. A closely related operating condition is maintaining adequate excess air (typically 20–100% above stoichiometric) to guarantee oxygen is never the limiting reactant in any parcel, which works together with turbulence to avoid localized fuel-rich pockets.
