Question 6 of 7: Control of SOx and NOx, Desulfurisation and Photochemical Smog

Question 6: Control of SOx and NOx, Desulfurisation and Photochemical Smog (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) SOx Control — Pre- and Post-Combustion Strategies

Pre-combustion (fuel switching / fuel desulfurization). Because SO₂ formation is stoichiometrically tied to the sulphur content of the fuel (Question 1(i)), the most direct control is to reduce the sulphur entering the flame: switching to a lower-sulphur coal or fuel oil, or physically/chemically cleaning coal (froth flotation removes pyritic sulphur before combustion) or hydrodesulfurizing liquid fuels, reduces the SO₂ generated per unit of energy produced without needing any downstream capture equipment. Post-combustion (flue gas desulfurization). Where high-sulphur fuel must still be burned, the SO₂ already formed in the flue gas is captured after combustion, most commonly by wet limestone scrubbing (Part (ii) below), which reacts SO₂ with an alkaline sorbent slurry to precipitate it as a solid (gypsum) product; this can be added to an existing plant without changing the fuel supply chain, at the cost of a large capital/operating burden and a solid or liquid waste/byproduct stream to manage.

(ii) Flue Gas Desulfurisation (FGD) — Wet Limestone Scrubbing

The most widely used FGD configuration is the wet limestone (lime/limestone) scrubber. Flue gas leaving the boiler first passes through an electrostatic precipitator (ESP) to remove fly ash (protecting the scrubber and keeping the byproduct gypsum saleable), then enters an absorber tower where it flows countercurrent to a finely ground limestone (CaCO₃) slurry sprayed from banks of nozzles. SO₂ dissolves into the slurry droplets and reacts:

$$\text{CaCO}_3 + \text{SO}_2 + \tfrac12\text{O}_2 + 2\text{H}_2\text{O} \rightarrow \text{CaSO}_4\cdot 2\text{H}_2\text{O}\ (\text{gypsum}) + \text{CO}_2.$$ Clean, cooled gas passes through a mist eliminator to remove entrained slurry droplets before exiting the stack, while the reacted slurry is drawn off, thickened and dewatered; the recovered gypsum by-product is often sold for wallboard manufacture, and clarified water is recycled back to the slurry make-up tank. A well-operated wet limestone FGD system commonly achieves 90–95%+ SO₂ removal.

(iii) Role of Nitrogen and Hydrocarbons in Smog Formation

Photochemical smog forms through a chain reaction driven by sunlight acting on NO_x and reactive hydrocarbons (VOCs) emitted primarily by combustion and vehicle traffic. Nitrogen's role: NO emitted at the source is rapidly oxidized to NO₂; sunlight then photolyzes NO₂ ($\text{NO}_2 + h\nu \rightarrow \text{NO} + \text{O}$), and the resulting atomic oxygen combines with atmospheric O₂ to form ozone, $\text{O}+\text{O}_2\rightarrow\text{O}_3$. In a "clean" NO_x-only atmosphere this ozone would simply react back with NO to regenerate NO₂ (a null cycle with no net O₃ accumulation). Hydrocarbons' role: reactive VOCs break that null cycle — hydroxyl-radical attack on hydrocarbons generates organic peroxy radicals (RO₂•) that oxidize NO to NO₂ WITHOUT consuming an ozone molecule, so NO₂ (and hence O₃, via its photolysis) accumulates faster than it can be destroyed. The organic radicals also combine with NO₂ to form peroxyacetyl nitrate (PAN), a potent eye and respiratory irritant and secondary pollutant in its own right. Because both precursors are required simultaneously, smog severity is controlled by whichever precursor is locally limiting (VOC-limited vs. NO_x-limited regimes), which is why urban smog-control strategy must manage NO_x and VOC emissions together rather than either one alone.