18-Env-A5 Air Quality and Pollution Control Engineering · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2013 — 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.
| Strategy | Mechanism | Typical NOx reduction | Advantages | Costs / drawbacks |
|---|---|---|---|---|
| Combustion modification (staged/low-NOx burners, flue-gas recirculation) | Lowers peak flame temperature and local O2 availability, suppressing thermal-NOx formation at its source (Question 1(ii)) | ≈ 30–50% | No consumable reagent; low incremental operating cost; can often retrofit existing burners | Cannot reach the very low NOx limits some jurisdictions require; some loss of combustion efficiency/flame stability if over-applied |
| Selective catalytic reduction (SCR) | Injects NH3/urea ahead of a catalyst bed to reduce NOx to N2 and H2O after it has already formed | ≈ 80–90%+ | Reaches the deepest NOx reduction of any single technology; effective regardless of how NOx formed | High capital cost (catalyst, reactor, reagent injection/storage); ongoing reagent and catalyst-replacement cost; risk of ammonia slip if over-dosed |
The two strategies are complementary rather than substitutes: combustion modification is the low-cost first step that reduces the load the (much more expensive) SCR system must then polish to meet a strict emission limit, which is why most modern coal-fired stations use both together.
Principle 1 — liquid-to-gas ratio and gas–liquid contact time. In a wet limestone/lime scrubber, SO2 must diffuse from the bulk gas into the alkaline slurry droplets/film and react before the gas exits the absorber; effectiveness rises with a higher liquid-to-gas (L/G) flow ratio and longer residence time in the spray/packed zone, because both increase the cumulative mass-transfer area and contact time available for absorption, up to the point where gas-side pressure drop or flooding becomes limiting.
Principle 2 — slurry alkalinity (stoichiometric ratio) and pH control. SO2 absorption and its conversion to a stable sulfite/sulfate product depend on maintaining sufficient dissolved alkalinity (limestone/lime fed at a stoichiometric ratio typically 1.02–1.10 relative to the SO2 load) and a controlled slurry pH (typically 5.5–6.0 for limestone systems); too little alkalinity or too low a pH starves the absorption reaction and lets SO2 slip through, while too high a pH risks scaling/plugging from calcium sulfite/sulfate precipitation on the internals.
Condition 1 — an adequate supply of NOx and reactive VOC (hydrocarbon) precursors. Photochemical smog is the product of a radical chain reaction between NOx (chiefly from combustion, especially vehicle exhaust) and reactive volatile organic compounds (from vehicle exhaust, solvent evaporation, and biogenic sources); without both precursor classes present together in sufficient concentration the ozone-forming cycle has no chain to propagate — NOx alone or VOC alone cannot sustain net ozone production.
Condition 2 — strong solar UV radiation (and typically a temperature inversion trapping the precursors). The chain is initiated by photolysis of NO2 ($\text{NO}_2 + h\nu \rightarrow \text{NO} + \text{O}$), which requires sunlight of sufficient intensity and UV content — this is why smog episodes peak on clear, sunny, warm afternoons rather than at night or under heavy overcast. A low-level temperature inversion (common with light wind and strong daytime heating trapped under a stable layer aloft) is typically also present, because it confines the precursors near ground level long enough for the sunlight-driven chain reaction to build ozone and other oxidants to smog-episode concentrations rather than letting them disperse and dilute before reacting.
