Question 5 of 6: Gaseous Pollutant Behaviour and Particulate Emission Control

Problem 5: Gaseous Pollutant Behaviour and Particulate Emission Control (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) — stack-emission monitoring techniques. Particulate: an in-stack opacity monitor (transmissometer) shines a light beam across the stack and measures attenuation; it gives a true continuous, real-time compliance signal, though it must be calibrated on-site against gravimetric mass and is insensitive to fine, weakly light-scattering particles. CO: a non-dispersive infrared (NDIR) analyzer extracts a slipstream of stack gas and measures the characteristic infrared absorption of CO at its specific wavelength; it is fast, continuous and highly selective, but requires a conditioned sample (particulate filter, moisture removal) to protect the optical cell. SOₓ: a pulsed-fluorescence analyzer excites SO₂ molecules in the extracted sample with UV light and measures the resulting fluorescence, which is proportional to SO₂ concentration; it is the EPA/ECCC reference continuous-emission-monitoring (CEM) method for SO₂, offering high sensitivity but requiring routine span/zero calibration with certified gas standards.

Part (ii) — behaviour of two gaseous pollutants. Carbon monoxide (CO) is chemically fairly inert in the lower atmosphere on short timescales — it is primarily a direct exposure hazard (binding haemoglobin, reducing oxygen transport) rather than a precursor to secondary chemistry at ground level; over weeks it is slowly oxidised to CO₂ by reaction with the hydroxyl radical (OH), and it also participates indirectly in tropospheric ozone formation by competing for OH with VOCs. Because it does not readily deposit or react quickly, CO behaves almost as a conservative (non-reactive) tracer over urban-to-regional distances, accumulating under stable, low-wind conditions (e.g. winter inversions in a valley). Sulphur dioxide (SO₂), by contrast, is atmospherically reactive: it oxidises (catalysed by OH in the gas phase, or by H₂O₂/O₃ in cloud droplets) to sulphuric acid and then to sulphate aerosol over hours to a few days, the timescale depending strongly on humidity, sunlight and the presence of oxidant/catalyst species. This secondary sulphate is a major PM_2.5 component and, dissolved in cloud/rain droplets, the principal chemical driver of acid rain — so unlike CO, SO₂'s dominant atmospheric fate is transformation into a different pollutant rather than simple dilution.

Part (iii) — two particulate control technologies with schematics. (1) Cyclone separator — dirty gas enters tangentially near the top of a conical vessel, spinning into a vortex; centrifugal force throws coarser particles outward to the wall, where they lose momentum, fall, and are collected in a hopper at the bottom, while the cleaned gas exits upward through a central vortex finder. Cyclones are cheap, robust and low-maintenance (no moving parts, no filter media), but efficiency drops sharply below roughly 10 µm particle diameter.

(2) Fabric filter (baghouse) — dirty gas is drawn through an array of long fabric bags (or cartridges); particles are captured on the bag surface (and, once a dust cake builds up, by the cake itself, which further improves fine-particle efficiency) while clean gas passes through the fabric and exits the top plenum. Periodically (pulse-jet compressed air, or reverse-air/shaker cleaning) the accumulated cake is dislodged and falls into a hopper. Baghouses exceed 99% efficiency across essentially the whole particle-size range, including sub-micron particles, at the cost of higher pressure drop, fabric wear, and sensitivity to gas moisture/temperature (bag blinding or thermal damage).