Question 5 of 5: Gaseous Pollutant Behaviour and Particulate Emission Monitoring/Control

Question 5: Gaseous Pollutant Behaviour and Particulate Emission Monitoring/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.

(i) Baghouse, Electrostatic Precipitator and Cyclone

Cyclone. Spins the dust-laden gas stream in a vortex inside a conical vessel; centrifugal force drives particles outward to the wall, where they lose momentum, slide down and are collected in a hopper, while the cleaned gas exits through a central vortex finder. Best applied as a low-cost, low-pressure-drop pre-cleaner for coarse particulate (typically effective above ~10–20 µm), ahead of a higher-efficiency final control device, or wherever high dust loading, high temperature or abrasive particulate makes a fabric or electrical device impractical as the first stage.

Baghouse (fabric filter). Draws the gas stream through woven or felted fabric bags; particles are captured on the fabric surface and build a "dust cake" that itself becomes an increasingly effective filtration medium, periodically cleaned by pulse-jet, shaker or reverse-air mechanisms. Best applied where very high collection efficiency (>99%, including fine PM) is required on a dry, non-sticky, non-condensing dust stream at a temperature and moisture content compatible with the bag material — a common final control for cement kilns, grain handling and combustion sources burning a wide range of fuels.

Electrostatic precipitator (ESP). Charges particles as they pass through a corona discharge field between discharge and collection electrodes; the charged particles migrate to and adhere on grounded collection plates under the electric field, and are periodically removed by rapping the plates. Best applied for very large gas volumes at very high collection efficiency and comparatively low pressure drop/energy cost per unit volume treated (e.g. utility coal-fired boilers), but performance is sensitive to the electrical resistivity of the dust (very high- or very low-resistivity dust degrades collection) and capital cost is high relative to the other two options.

(ii) Monitoring Techniques for Stack Emissions

Particulate — opacity monitor (transmissometer). A continuous light-transmission instrument spans the stack, measuring the fraction of a light beam attenuated by particulate in the gas path; the measured opacity is used as a continuous, indirect surrogate for particulate loading and, most importantly, as an immediate indicator of control-device malfunction (e.g. a torn baghouse bag) between periodic reference-method stack tests.

CO — nondispersive infrared (NDIR) continuous analyzer. As described in Question 3(i): CO's characteristic ~4.6 µm IR absorption band is measured continuously via Beer–Lambert attenuation, giving a real-time stack CO concentration used both for combustion-efficiency control and regulatory compliance monitoring.

SO_x — pulsed UV fluorescence analyzer. Sample gas is irradiated with UV light at a wavelength that excites SO₂ molecules; the SO₂ re-emits (fluoresces) light at a longer wavelength in proportion to its concentration, detected by a photomultiplier tube. This is the standard continuous reference method for SO₂, offering high sensitivity and selectivity with minimal cross-interference from other stack gases when properly filtered.

(iii) Atmospheric Behaviour of Three Gaseous Pollutants

CO. Comparatively unreactive in the lower troposphere; its principal removal pathway is slow oxidation by the hydroxyl radical (CO + OH → CO₂ + H), giving an atmospheric residence time on the order of one to a few months. This is long enough for CO to become fairly well mixed over a regional to hemispheric scale, and because CO consumes OH radicals, elevated CO can indirectly extend the atmospheric lifetime of other OH-scavenged pollutants (an effect on the atmosphere's overall oxidative capacity, distinct from CO's own direct toxicity at the point of emission).

SO₂. Oxidizes in the atmosphere — via gas-phase OH-radical reaction, or faster via aqueous-phase reaction inside cloud/fog droplets — to SO₃ and ultimately sulfuric acid/sulfate aerosol, over a residence time of roughly a few days. This conversion is the direct chemical basis of acid rain (wet and dry sulfate deposition) and also of secondary sulfate particulate, a major contributor to regional PM_2.5 and haze; because its lifetime is on the order of days, SO₂'s impact is felt at a regional (hundreds of km) rather than purely local scale.

NO_x. Freshly emitted NO oxidizes rapidly (within minutes to hours) to NO₂, which under sunlight drives the photolytic NO₂ + hν → NO + O cycle that is the entry point of the photochemical smog cycle producing ground-level ozone and peroxyacetyl nitrate (PAN) in the presence of reactive VOCs. NO₂ is further oxidized to nitric acid (HNO₃), an acid-rain precursor that also deposits as nitrate aerosol. NO_x's atmospheric lifetime (hours to about a day) is much shorter than SO₂'s, so its photochemical and acidifying effects are felt more locally to sub-regionally, concentrated downwind of urban/industrial NO_x sources on sunny, stagnant days.