Question 7 of 7: Gas/Vapour Control Technologies and Photochemical Smog Formation

Problem 7: Gas/Vapour Control Technologies 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.

Part (i) — adsorption vs. absorption. (1) Phase of the capturing medium: adsorption captures the pollutant on the surface of a solid sorbent (e.g. activated carbon, zeolite) through physical (van der Waals) or weak chemical bonding, whereas absorption transfers the pollutant into the bulk of a liquid solvent (water, caustic or amine solution) across a gas–liquid interface. (2) Applicable pollutant loading and mechanism: adsorption is best suited to dilute organic vapours (VOCs) at low-to-moderate concentration, and the sorbent has a finite capacity that saturates and must be thermally/steam-regenerated or replaced; absorption is typically paired with a chemical reaction in the liquid (e.g. SO₂ into an alkaline slurry, HCl or NH₃ into water) that continuously regenerates capacity, so it handles higher-concentration, water- or reactive-soluble gas streams (as in an FGD scrubber) without the same saturation limit.

Part (ii) — SOₓ formation and a control example. Coal contains sulphur in both organic (bound in the fuel matrix) and inorganic/pyritic (FeS₂) forms; during combustion, essentially all of this fuel-bound sulphur oxidizes rapidly at flame temperature, $S+O_2\rightarrow SO_2$, with a small fraction (typically 1–5%) further oxidizing to SO₃, which readily hydrates to H₂SO₄ aerosol in the cooling flue gas or atmosphere. Example control technology: post-combustion wet limestone flue gas desulphurisation (as described in Problem 4(ii)), absorbing SO₂ into a CaCO₃ slurry and oxidizing it to gypsum; alternatively, a pre-combustion option is physical coal cleaning/washing, which mechanically separates and removes a portion of the denser pyritic sulphur from the coal before it is burned.

Part (iii) — photochemical smog formation. The cycle begins with NO₂ photolysis by ultraviolet sunlight: $NO_2+h\nu\rightarrow NO+O$, followed by $O+O_2+M\rightarrow O_3+M$ forming ozone. In an atmosphere containing only NOₓ (no reactive hydrocarbons), this ozone is promptly consumed by the reverse titration reaction $O_3+NO\rightarrow NO_2+O_2$, establishing a photostationary steady state with no net ozone accumulation. When reactive hydrocarbons (VOCs) are also present, hydroxyl-radical oxidation of the VOC generates peroxy radicals ($RO_2\cdot$, $HO_2\cdot$) that oxidize NO to NO₂ without consuming ozone ($RO_2\cdot+NO\rightarrow RO\cdot+NO_2$), breaking the photostationary cycle: NO is converted to NO₂ by the hydrocarbon-radical pathway instead of by ozone, so ozone is no longer titrated away and accumulates through the day. The same peroxy-radical chemistry also produces peroxyacetyl nitrate, $CH_3C(O)O_2\cdot+NO_2\rightarrow CH_3C(O)O_2NO_2$ (PAN), a potent eye and respiratory irritant. This full cycle — NO₂ photolysis, VOC/OH radical initiation, NO-to-NO₂ conversion via peroxy radicals, and O₃/PAN accumulation — requires strong sunlight, an adequate supply of both NOₓ and reactive hydrocarbons, and stagnant, warm conditions that allow precursor concentrations to build over the course of a day, which is why photochemical smog is worst in sunny, high-traffic urban basins with limited ventilation (e.g. Los Angeles-type smog episodes).