18-Env-A5 Air Quality and Pollution Control Engineering · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2016 — 04-Env-A5 / Air Quality and Pollution Control Engineering. 3 hours duration, closed book; Casio or Sharp approved calculator only. Any five (5) questions constitute a complete paper (only the first five answered, as they appear in the workbook, are marked) — all seven Problems are answered in full below as a complete study resource.
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.
What it is and when it occurs. Photochemical smog is a secondary air-pollution mixture — dominated by ground-level ozone and other oxidants (e.g. peroxyacetyl nitrate, PAN) — that forms in urban air on hot, sunny, low-wind days, typically in summer afternoons when solar UV intensity and precursor emissions (traffic, industry) are both high and dispersion is limited (often under a subsidence inversion trapping the precursors near the surface).
Role of nitrogen (NOx) and hydrocarbons (VOCs). NO2 photolyzes in sunlight, $\text{NO}_2\xrightarrow{h\nu}\text{NO}+\text{O}$, and the free O atom reacts with O2 to form O3. On its own this reaches a low-level photostationary equilibrium, because the NO produced rapidly titrates the O3 back to NO2 ($\text{NO}+\text{O}_3\to\text{NO}_2+\text{O}_2$). VOCs break this cycle: OH-radical oxidation of hydrocarbons generates peroxy radicals (RO2, HO2) that convert NO to NO2 without consuming ozone, so ozone accumulates net rather than cycling back down — both NOx and VOCs are therefore required precursors for smog to build up.
Engineering approach. Control both precursor classes at their major sources: catalytic converters and lean-burn/SCR technology on vehicles and stationary combustion sources to cut NOx, and vapour-recovery/VOC controls at fuel-dispensing, solvent-use and industrial sources. In a VOC-limited urban core (common downtown, where NOx is already abundant), prioritizing VOC control is more effective than NOx control alone, since cutting NOx without cutting VOCs can locally increase ozone by removing the NO available to titrate it back down.
(1) Pre-combustion fuel switching/desulphurization. Burning a low-sulphur coal, or physically washing/beneficiating coal to remove pyritic sulphur before it is fired, reduces the sulphur available to oxidize to SO2 in the first place, cutting emissions without any post-combustion equipment.
(2) In-furnace sorbent injection / fluidized-bed combustion. Injecting a calcium-based sorbent (limestone or dolomite) directly into the furnace, or using a fluidized bed with limestone as the bed material, captures SO2 as it forms: $\text{CaO}+\text{SO}_2+\tfrac12\text{O}_2\to\text{CaSO}_4$, removing sulphur inside the combustion process itself rather than downstream.
The most commonly used FGD process is wet limestone scrubbing. Flue gas leaving the boiler (already passed through particulate control, e.g. an ESP or baghouse) enters an absorber tower where it contacts a finely ground limestone slurry sprayed countercurrent to the gas flow. SO2 is absorbed into the slurry droplets and reacts: $\text{CaCO}_3+\text{SO}_2+\tfrac12\text{O}_2+2\text{H}_2\text{O}\to\text{CaSO}_4\cdot2\text{H}_2\text{O}+\text{CO}_2$. The cleaned gas passes through a mist eliminator before exiting the stack, while the resulting gypsum (CaSO4·2H2O) slurry is dewatered — the recovered gypsum is often sold as a wallboard-manufacturing feedstock, turning a waste stream into a saleable byproduct.
