Question 6 of 7: Smog, Photochemical Reactions and Flue Gas Desulfurization

Problem 6: Smog, Photochemical Reactions and Flue Gas Desulfurization (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.

Part (i) — smog. "Smog" (smoke + fog) denotes visible, health-degrading atmospheric haze, of which there are two classic types. Photochemical (Los Angeles-type) smog forms when nitrogen oxides (NOₓ) and volatile organic compounds (VOCs), emitted mainly by vehicle exhaust and industrial solvent use, react under strong sunlight to build up ground-level ozone and peroxyacetyl nitrate (PAN), typically on hot, sunny, low-wind days. Industrial (London-type) smog forms from sulphur dioxide and particulate matter emitted by coal combustion, combined with fog/high humidity and a temperature inversion that traps the pollutants near the ground, historically the deadlier of the two (e.g. the 1952 London smog event). Sources: motor vehicles and other combustion engines, coal- and oil-fired power plants, industrial solvent and fuel evaporation, and residential heating.

Part (ii) — photochemical reactions; roles of nitrogen and hydrocarbons. A photochemical reaction is one driven by the absorption of light (here, solar UV/visible radiation) rather than by thermal energy alone — the photon energy breaks a chemical bond or excites a molecule into a reactive state. Nitrogen oxides initiate the cycle: NO₂ photolyses ($\text{NO}_2 + h\nu \rightarrow \text{NO} + \text{O}$), the resulting atomic oxygen combines with O₂ to form ozone, and in a "clean" NOₓ-only cycle that ozone is quickly consumed again by reacting with NO back to NO₂ — a self-limiting null cycle. Hydrocarbons (VOCs) break that null cycle: they react with the hydroxyl radical and atomic oxygen to form organic peroxy radicals, which oxidise NO to NO₂ without consuming an ozone molecule; this frees up more NO₂ to photolyse and produce additional ozone, so hydrocarbons are the catalytic ingredient that lets ozone accumulate to unhealthy levels rather than cycling in a steady low-level balance.

Part (iii) — Flue Gas Desulfurization (FGD). The most common technology is wet limestone scrubbing: raw flue gas from the boiler/kiln is ducted into a spray-tower absorber where it contacts a re-circulated limestone (CaCO₃) slurry. SO₂ is absorbed into the slurry droplets and reacts to form calcium sulphite, which is then oxidised (often by forced air) to calcium sulphate dihydrate (gypsum): $$\text{SO}_2 + \text{CaCO}_3 + \tfrac12\text{O}_2 + 2\text{H}_2\text{O} \rightarrow \text{CaSO}_4\cdot2\text{H}_2\text{O} + \text{CO}_2$$ The scrubbed, now-saturated gas passes through a mist eliminator to remove entrained droplets before exiting the stack, while the gypsum slurry is drawn off to a dewatering/thickening circuit (gypsum by-product can be sold to wallboard manufacturing).