Question 5 of 5: Smog, Photochemical Reactions and Flue Gas Desulfurization

Problem 5: Smog, Photochemical Reactions and Flue Gas Desulfurization (25 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: definition, formation, sources. Photochemical smog is a secondary form of urban air pollution — a hazy mixture of ground-level ozone, peroxyacetyl nitrate (PAN), and fine secondary organic aerosol — formed not by direct emission but through sunlight-driven reactions between nitrogen oxides and volatile organic compounds in the lower atmosphere, most intense on warm, sunny, low-wind days that trap and concentrate precursors near the surface. Sources emitting the precursor chemicals: motor vehicle exhaust (NOₓ and unburned hydrocarbons), fuel evaporation and solvent/paint/consumer-product use (VOCs), and stationary combustion sources (utility boilers, industrial furnaces) contributing further NOₓ.

Part (ii) — photochemical reaction and the roles of NOₓ and hydrocarbons. A photochemical reaction is one initiated or driven by the absorption of light (here, solar UV/visible radiation) rather than by thermal energy alone. The cycle begins with photolysis of NO₂ ($NO_2+h\nu\rightarrow NO+O$), and the resulting O atom combines with O₂ to form ozone; on its own this reaches a low-level steady state because NO reacts back with O₃ to regenerate NO₂ ($NO+O_3\rightarrow NO_2+O_2$), with no net ozone build-up. Role of hydrocarbons (VOCs): they are attacked by hydroxyl radicals to form organic peroxy radicals, which oxidise NO to NO₂ without consuming an ozone molecule — this breaks the NO/O₃ titration cycle and lets ozone accumulate. Role of nitrogen (as NOₓ): NOₓ supplies both the initiating NO₂ photolysis step and, once converted, the source of PAN and other nitrogenous secondary products; the ratio of VOC to NOₓ concentration determines whether ozone formation is VOC-limited or NOₓ-limited in a given airshed.

Part (iii) — flue gas desulfurization (FGD) schematic. The most common FGD technology on large coal-fired boilers is the wet limestone scrubber: flue gas from the boiler passes upward through a spray-tower absorber, contacting a downward-falling limestone (CaCO₃) slurry; SO₂ in the gas dissolves into the slurry droplets and reacts, $SO_2+CaCO_3+\tfrac12 H_2O\rightarrow CaSO_3\!\cdot\!\tfrac12H_2O+CO_2$. Air is sparged into a reaction tank to fully oxidise the sulphite to sulphate, $CaSO_3\!\cdot\!\tfrac12H_2O+\tfrac12O_2+\tfrac32H_2O\rightarrow CaSO_4\!\cdot\!2H_2O$ (gypsum), which is dewatered to a saleable by-product (wallboard-grade gypsum) rather than landfilled. Scrubbed gas passes through a mist eliminator to remove entrained slurry droplets before exiting to the stack.

Part (iv) — absorption: description and application example. Absorption is a mass-transfer control technique in which a gaseous pollutant is dissolved from the gas phase into a contacting liquid, driven by the pollutant's solubility (Henry's law) and the concentration gradient across the gas–liquid interface; the liquid is chosen (and often chemically reactive, as with the limestone slurry above) to keep the dissolved-phase concentration low at the interface, maximising the driving force and hence the overall transfer rate. Example: the wet limestone FGD scrubber described in part (iii) is itself a reactive absorption process (SO₂ absorbed into and chemically consumed by the CaCO₃ slurry); a second common example is a packed-bed caustic (NaOH) scrubber used to absorb and neutralise acid gases such as HCl from a chemical-process vent or hazardous-waste incinerator exhaust.