Question 5 of 5: Gas-Cleaning Equipment, SOₓ/NOₓ Control, and Atmospheric Behaviour

Problem 5: Gas-Cleaning Equipment, SOₓ/NOₓ Control, and Atmospheric Behaviour (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) — two gas-cleaning devices. Cyclone. (a) Appropriate for moderate-to-coarse particulate (efficient above roughly 10–15 µm, poor below about 5 µm) at moderate dust loading, and especially for hot, abrasive, or high-moisture gas streams because it has no filter media to blind or foul; commonly used as a pre-cleaner ahead of a finer downstream device. (b) Example: wood-products/sawmill pneumatic-conveying exhaust (coarse sawdust and chips) or grain-handling dust. (c) Design aspects: (1) body diameter and inlet velocity together set the cut diameter d₅₀ — a smaller-diameter cyclone or higher inlet velocity improves fine-particle collection but increases pressure drop, so the two trade directly against each other; (2) dust-hopper and discharge design must prevent re-entrainment of already-collected dust back into the vortex, typically via a properly sealed rotary airlock. Electrostatic precipitator (ESP). (a) Appropriate for fine particulate (sub-micron up to a few hundred microns, e.g. fly ash) at high gas volumes, provided the particle electrical resistivity falls in a workable range (roughly 10⁴–10¹⁰ Ω·cm — too low causes re-entrainment on the collecting plate, too high causes back-corona that limits current). (b) Example: fly-ash collection on a coal-fired power-plant boiler. (c) Design aspects: (1) specific collection area (plate area per unit gas flow) and migration velocity, sized via the Deutsch–Anderson relation $\eta=1-e^{-wA/Q}$ — more plate area raises efficiency but raises capital cost; (2) rappers/vibrators and hopper design to dislodge collected dust from the plates and remove it without excessive re-entrainment, plus corona-electrode spacing and voltage tuned to sustain stable corona without sparkover.

Part (ii) — SOₓ and NOₓ control on a fuel-fired industrial dryer. Three options to reduce SOₓ: (1) fuel switching to a lower-sulphur fuel (natural gas, or a low-sulphur fuel oil/coal), which directly cuts SO₂ formation since it is stoichiometrically tied to fuel sulphur content; (2) pre-combustion fuel cleaning, such as physical coal washing to remove pyritic sulphur, or hydrodesulphurisation of a liquid fuel, lowering the sulphur burned in the first place; (3) post-combustion flue-gas desulphurisation (FGD), wet (limestone/lime slurry scrubbing) or dry (hydrated-lime or sodium-bicarbonate sorbent injection), reacting an alkaline sorbent with SO₂ in the flue gas ahead of the stack, typically achieving >90% removal. One way to reduce NOₓ: low-NOₓ burner design, using staged (air- or fuel-staged) combustion that lowers peak flame temperature and delays fuel–air mixing, suppressing thermal NOₓ formation at the source (a combustion-side control, in contrast to a post-combustion treatment like SCR).

Part (iii) — atmospheric behaviour of two gaseous pollutants. Carbon monoxide (CO) is chemically relatively unreactive and has a comparatively long atmospheric lifetime (on the order of weeks to a couple of months); its principal removal pathway is slow oxidation by the hydroxyl radical (OH) to CO₂, so it can be transported over moderate-to-regional distances before being removed, and it does not readily form secondary particulate or acidic species — its main hazard remains direct inhalation toxicity rather than downstream atmospheric transformation. Sulphur dioxide (SO₂) is far more reactive: it oxidises in the atmosphere over hours to days, either by gas-phase reaction with OH or by aqueous-phase oxidation inside cloud droplets (catalysed by trace metals, H₂O₂, or O₃), forming sulphuric acid and sulphate aerosol. That sulphate aerosol is a major contributor to PM₂.₅ mass and regional haze, and — via the same oxidation pathway — drives acid deposition (acid rain) affecting soils, lakes, and forests often well downwind of the emitting source through long-range transboundary transport.