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

Question 6: Control of SO_x and NO_x, and Photochemical Smog Formation (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) Primary NO_x Reduction Methods

Combustion (in-furnace) control — staged combustion / low-NO_x burners. Air or fuel staging reduces peak flame temperature and/or local oxygen availability in the primary combustion zone, suppressing thermal-NO_x formation (the strongly temperature-sensitive Zel'dovich mechanism). Advantage: comparatively low capital and operating cost, and can often be retrofitted into an existing burner without a large add-on system. Disadvantage: aggressive staging can reduce combustion efficiency and increase CO or unburned carbon in the ash, and has a practical reduction ceiling (typically 40–60%) that is insufficient alone for the tightest emission limits.

Post-combustion (flue-gas) control — Selective Catalytic Reduction (SCR). Ammonia or urea is injected into the flue gas upstream of a catalyst bed, reducing NO_x to N₂ and water. Advantage: very high removal efficiency (80–90%+), effective across a wide range of inlet NO_x concentrations without modifying the combustion process itself. Disadvantage: high capital cost, catalyst activity degrades over time from fouling or poisoning and requires periodic replacement, and unreacted ammonia ("ammonia slip") can itself become an emission if the reagent dose is not well controlled.

(ii) Desulfurisation Technique

Hydrodesulfurization (HDS). The fuel or refinery intermediate stream is contacted with hydrogen gas over a cobalt–molybdenum or nickel–molybdenum catalyst (typically on an alumina support) at elevated temperature and pressure, converting organically-bound sulfur compounds to H₂S. The H₂S is then stripped from the product and typically recovered as elemental sulfur via a Claus process, leaving a low-sulfur fuel.

Two reasons to remove sulfur. (1) Combustion of sulfur-bearing fuel produces SO₂/SO₃ emissions — precursors to acid deposition and secondary sulfate PM2.5; removing sulfur upstream at the refinery avoids the need for costly flue-gas desulfurization equipment at every downstream combustion source and directly reduces regional SO_x loading. (2) Sulfur compounds poison downstream catalytic systems — most directly, sulfur in gasoline fouls and deactivates a vehicle's own three-way catalytic converter, reducing its NO_x/HC/CO conversion efficiency and shortening its service life, so low-sulfur fuel is essential for modern emission-control-equipped vehicles to actually achieve their designed tailpipe emission reductions.

(iii) Photochemical Reactions and Smog Formation

Reaction 1 — NO₂ photolysis. $\text{NO}_2 + h\nu \rightarrow \text{NO} + \text{O}$; the resulting atomic oxygen rapidly combines with molecular oxygen, $\text{O} + \text{O}_2 + M \rightarrow \text{O}_3 + M$, forming ozone.

Reaction 2 — the "null cycle" broken by hydrocarbons. $\text{NO} + \text{O}_3 \rightarrow \text{NO}_2 + \text{O}_2$ would, on its own, simply consume the ozone just formed, holding O₃ at a low steady state. Reactive hydrocarbons intervene: VOCs react with hydroxyl radicals to form organic peroxy radicals ($\text{RO}_2\cdot$), which oxidize NO to NO₂ without consuming ozone — breaking the null cycle and allowing O₃ to accumulate through repeated cycling of reaction 1.

Reaction 3 — PAN formation. Peroxyacetyl radicals, formed from aldehyde/hydrocarbon oxidation, react with NO₂: $\text{CH}_3\text{C(O)OO}\cdot + \text{NO}_2 \rightarrow \text{CH}_3\text{C(O)OONO}_2$ (peroxyacetyl nitrate, PAN) — a secondary eye irritant and phytotoxic pollutant, and a thermally unstable temporary NO_x reservoir that can decompose and re-release NO_x further downwind.

Environmental conditions contributing to smog. Strong sunlight/UV drives reaction 1 and the OH-radical chemistry that sustains reaction 2, and is most intense at midday, in summer and at lower latitude; high ambient temperature accelerates reaction rates and increases both biogenic and evaporative VOC emissions; and a stagnant, calm wind field combined with a shallow mixing height or a temperature inversion traps the NO_x and VOC precursors together long enough for the multi-hour reaction sequence to build up ozone and PAN before the air mass disperses. Classically, the worst photochemical smog episodes occur in a topographic basin or valley (e.g. the Los Angeles basin, or the Lower Fraser Valley in Canada) under a summertime high-pressure system with light wind and a persistent inversion.