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23-Chem-B2 Environmental Engineering · December 2019

Question 5 of 7: Photochemical Reactions, Noxious Pollutants and Odour Control

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Exam 16-Chem-B2, Environmental Engineering — December 2019. 3 hours, Closed-Book Exam with a candidate-prepared 8½×11" double-sided aid sheet. Any five (5) of the seven questions constitute a complete paper (100 marks); all seven are solved below for completeness.

Reference texts: Metcalf & Eddy (Tchobanoglous, Burton, Stensel), Wastewater Engineering: Treatment and Reuse, 4th ed.; Davis & Cornwell, Introduction to Environmental Engineering, 5th ed.; Turner, Workbook of Atmospheric Dispersion Estimates, 2nd ed.; Cooper & Alley, Air Pollution Control: A Design Approach, 4th ed.

Problem 5: Photochemical Reactions, Noxious Pollutants 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) Photochemical smog formation

Photochemical smog forms when primary emissions of nitrogen oxides (NOₓ) and volatile organic compounds (VOCs), mainly from vehicle and industrial combustion, accumulate under a temperature inversion (a stable, stagnant air mass that traps pollutants near ground level and prevents vertical dispersion) and are exposed to strong solar ultraviolet radiation. UV photolysis of NO₂ produces atomic oxygen (NO₂+hν→NO+O), which combines with O₂ to form ozone (O+O₂→O₃); simultaneously, reactive VOCs react with hydroxyl radicals and NO to regenerate NO₂ without consuming ozone, driving a photochemical cycle that builds up ground-level ozone and secondary pollutants (peroxyacetyl nitrate, aldehydes, fine particulate) through the afternoon as solar intensity peaks. The physical trapping (inversion, low wind speed, valley/basin topography) is just as essential as the chemistry: without a stagnant air mass to concentrate the precursors, the same emissions would disperse before the photochemical cycle could build up significant ozone.

(ii) Three noxious pollutants from fixed sources and engineering solutions

PollutantFixed-source originEngineering solution
Sulfur dioxide (SO₂)Combustion of sulfur-bearing fuels (coal, heavy fuel oil) in boilers/power plants. Wet flue-gas desulfurization (limestone slurry scrubber) absorbs SO₂ and converts it to gypsum, or switch to lower-sulfur fuel to cut the emission at the source.
Nitrogen oxides (NOₓ)High-temperature combustion in boilers and furnaces (thermal NOₓ formation). Low-NOₓ burners / staged combustion reduce peak flame temperature and available oxygen at the flame front; selective catalytic reduction (SCR) removes remaining NOₓ post-combustion with ammonia/urea injection over a catalyst.
Particulate matter (PM)Fly ash and unburned carbon from solid/liquid fuel combustion, and process dust from material handling. Electrostatic precipitator (ESP) or fabric filter (baghouse) captures fine particulate before the flue gas exits the stack, achieving >99% collection efficiency on well-maintained units.

(iii) Odour control for water-soluble solvent vapours — venturi wet scrubber

A venturi wet scrubber is an effective odour-control technology for water-soluble solvent vapours (e.g. alcohols, ketones, amines from a printing or coating operation), because the high aqueous solubility of these compounds means gas-liquid contact alone (no biological or adsorptive step) can achieve high removal.

  1. High relative gas-liquid velocity at the throat. The gas stream is accelerated through a converging throat where liquid is injected, atomizing it into a fine droplet spray; the large relative velocity between gas and droplets maximizes the mass-transfer coefficient for absorption of the water-soluble contaminant into the liquid phase.
  2. Liquid-to-gas ratio matched to the contaminant loading. The scrubbing liquid flow rate is sized against the contaminant's Henry's law constant and inlet loading so the liquid never approaches saturation with the absorbed solvent within the contact zone, which would otherwise stall the concentration- driving-force for further absorption.
  3. Adequate pressure drop / residence time in the contact zone. A higher throat pressure drop produces finer droplets and more total gas-liquid interfacial area, directly increasing removal efficiency, but is balanced against the fan energy cost, so throat geometry is selected to the target removal efficiency rather than maximized indiscriminately.