Question 6 of 7: Control of SO x and NO x , and Photochemical Smog

Question 6: Control of SO_x and NO_x, 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) NO_x Control Strategies — Pre- and Post-Combustion

A pre-combustion / combustion-modification strategy is low-NO_x burner (LNB) design with staged combustion: fuel and air are introduced in stages so the primary flame zone burns fuel-rich (starving the Zeldovich reactions of O₂ and lowering peak flame temperature), with the remaining air added downstream to complete burnout at lower temperature; this directly attacks both combustion parameters identified in Question 5(i) (peak temperature and local O₂ availability) and typically cuts thermal NO_x by 40–60% at a fraction of the capital cost of add-on control. A complementary post-combustion strategy is selective catalytic reduction (SCR): ammonia (or urea, hydrolyzed on-site to ammonia) is injected into the flue gas upstream of a catalyst bed (typically vanadium/titanium oxide) operating around 300–400 °C, where NO_x is reduced to N₂ and H₂O via reactions such as $4\text{NO}+4\text{NH}_3+\text{O}_2\rightarrow4\text{N}_2+6\text{H}_2\text{O}$, achieving 80–90%+ NO_x removal independent of what combustion modification alone can reach. The two are complementary rather than substitutes: combustion modification is cheap but plateaus well short of stringent limits, while SCR is capital-intensive but reaches deep removal on top of whatever the combustion stage already achieved.

(ii) Flue Gas Desulfurisation — Schematic and Principle

The most common FGD process, shown above, is wet limestone scrubbing. Flue gas rises through a spray tower where it is contacted countercurrently with a recirculated slurry of finely ground limestone (CaCO₃) in water. SO₂ in the gas dissolves into the slurry droplets and reacts acid–base with the dissolved calcium carbonate: $$\text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3, \qquad \text{CaCO}_3 + \text{H}_2\text{SO}_3 \rightarrow \text{CaSO}_3 + \text{CO}_2 + \text{H}_2\text{O}.$$ The calcium sulfite is typically further oxidized (forced-oxidation air sparged into the sump) to calcium sulfate dihydrate (gypsum, CaSO₄·2H₂O), a marketable byproduct used in wallboard manufacture rather than a waste requiring landfill. A mist eliminator downstream of the spray zone removes entrained slurry droplets before the scrubbed, humidified gas is reheated (in some designs) and released through the stack. Removal efficiency of 90–95%+ SO₂ is routinely achieved because the process is limited by gas–liquid mass transfer and reagent stoichiometry, both of which are controlled directly by slurry recirculation rate and limestone feed rate — the design levers a plant operator adjusts to hold a permitted SO₂ removal target.

(iii) Photochemical Smog Formation and an Engineering Control Approach

Photochemical smog forms through a sunlight-driven catalytic cycle involving NO_x and reactive hydrocarbons (VOCs). NO₂ photolyzes under UV sunlight, $\text{NO}_2+h\nu\rightarrow\text{NO}+\text{O}$, and the resulting atomic oxygen combines with O₂ to form ozone, $\text{O}+\text{O}_2\rightarrow\text{O}_3$; in a "clean" NO_x-only atmosphere, O₃ is then rapidly consumed by reaction with the NO produced in the first step, $\text{NO}+\text{O}_3\rightarrow\text{NO}_2+\text{O}_2$, closing a null cycle that builds no net ozone. Reactive VOCs break this null cycle: hydroxyl-radical-initiated oxidation of VOCs generates peroxy radicals (RO₂•, HO₂•) that oxidize NO back to NO₂ without consuming ozone, so NO is regenerated as NO₂ by two competing pathways (VOC-driven and O₃-driven) and net ozone accumulates over the course of a sunlit day; the same VOC-oxidation chain also produces peroxyacetyl nitrate (PAN) and secondary organic aerosol, the other defining smog products. Because the cycle needs both NO_x and VOCs simultaneously, and because urban smog is frequently VOC-limited (ozone production is more sensitive to VOC than to further NO_x reduction in dense city cores), a targeted engineering approach is aggressive control of reactive VOC emissions — vapour recovery at fuel-dispensing and storage facilities, low-solvent/low-VOC coatings and consumer-product formulations, and evaporative-emission control on vehicles (Question 4(i)) — combined with continued NO_x reduction (Question 6(i)) so that as a city's VOC-to-NO_x ratio shifts over time, the smog-formation regime does not simply move from VOC-limited back to NO_x-limited without a matching NO_x reduction already in place.