Question 1 of 7: NOx Control, Flue Gas Desulfurisation and Photochemical Smog

Question 1: NOx Control, Flue Gas 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) Two NOx Control Strategies: Pre-Combustion and Post-Combustion

Pre-combustion / in-furnace: staged combustion with low-NOx burners. Coal-fired NOx forms by three competing mechanisms — thermal NOx (Zeldovich chain, $\text{O}+\text{N}_2\rightarrow\text{NO}+\text{N}$, exponentially sensitive to peak flame temperature above roughly 1300 °C), fuel NOx (oxidation of the 1–2% organically bound nitrogen already in coal, dominant in coal flames), and a minor prompt-NOx contribution. A low-NOx burner splits combustion air into a fuel-rich primary zone followed by a fuel-lean burnout zone; the fuel-rich zone starves the flame of O₂ exactly when fuel nitrogen would otherwise oxidize, and the staged, cooler flame front also suppresses the thermal pathway. This is attractive because it needs no consumable reagent and typically cuts NOx by 30–50% at modest capital cost, but it cannot reach the very low limits a post-combustion system can.

Post-combustion: selective catalytic reduction (SCR). Ammonia or urea is injected into the flue gas ahead of a vanadium/titanium-oxide catalyst bed operating near 300–400 °C, where NOx is reduced to nitrogen and water: $4\text{NO}+4\text{NH}_3+\text{O}_2\rightarrow 4\text{N}_2+6\text{H}_2\text{O}$. SCR routinely achieves 80–90%+ NOx removal, far beyond combustion modification alone, but at a substantially higher capital and operating cost (catalyst replacement, reagent handling, and risk of unreacted ammonia "slip" into the flue gas). In practice utilities combine both: low-NOx burners handle the bulk reduction cheaply, and SCR polishes the remainder to meet a stringent emission limit.

(ii) Black-Box Schematic of a Wet Limestone FGD Process

Boiler/combustion — generates raw flue gas carrying SO2, NOx, CO2, H2O and fly ash. ESP/baghouse — removes particulate matter first, protecting the downstream absorber's spray nozzles and slurry chemistry from ash fouling. FGD absorber — the flue gas contacts an atomized limestone (CaCO₃) slurry; SO2 dissolves and reacts, $\text{SO}_2+\text{CaCO}_3+\tfrac12\text{O}_2+2\text{H}_2\text{O}\rightarrow\text{CaSO}_4\cdot2\text{H}_2\text{O}+\text{CO}_2$, producing a saleable gypsum byproduct while stripping 90%+ of the SO2. Mist eliminator — baffled chevron vanes strip entrained slurry droplets from the gas stream before it is reheated and released to the stack, preventing visible plume droplet carryover and downstream corrosion.

(iii) Photochemical Smog: Governing Equations and Conditions for Maximum Production

The NO₂ photolytic cycle (the "null cycle" without hydrocarbons):

Sunlight photolyzes NO₂ to NO and a ground-state oxygen atom. This reaction requires strong UV/visible solar radiation, so it is fastest at midday under clear skies and essentially stops at night — the trigger step for the whole smog cycle.

The atomic oxygen combines with molecular oxygen (M is any third body that carries off the excess energy) to form ozone. This step is fast and essentially temperature/pressure independent once O atoms are present, so it tracks the NO₂ photolysis rate directly.

Freshly formed ozone is consumed almost as fast as it forms by reacting with NO (itself just produced in the first step), regenerating NO₂ — in a NOx-only atmosphere this closes a null cycle with no net ozone accumulation, only a small steady-state O3 concentration.

This is the pathway that breaks the null cycle. Reactive hydrocarbons (VOCs) are attacked by hydroxyl radicals to form organic peroxy radicals, which oxidize NO to NO₂ without consuming an ozone molecule. NO is thereby converted to NO2 by a route that competes with reaction 3, so less NO remains to destroy the ozone made in reaction 2 — ozone is free to accumulate net, and the peroxy/NO2 chemistry also produces secondary irritants such as peroxyacetyl nitrate (PAN). Conditions that maximize this pathway (and hence smog/ozone production) are: (a) strong solar UV radiation (clear skies, high sun angle, summer midday) to drive reaction 1 and OH-radical generation; (b) a high VOC/NOx ratio, since abundant reactive hydrocarbons are what diverts NO away from ozone-destroying reaction 3; (c) stagnant, low-wind conditions, often under a temperature inversion, that trap precursor emissions in a shallow mixed layer for hours, allowing the radical chain to build rather than disperse; and (d) warm ambient temperature, which accelerates most of the underlying gas-phase reaction rate constants.