Question 7 of 7: Formation and Monitoring of Gaseous Pollutants and Gravity Settlers

Question 7: Formation and Monitoring of Gaseous Pollutants and Gravity Settlers (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) SOx Formation from Coal Combustion and a Pre-Combustion Control Example

Coal contains sulfur in two principal forms: organic sulfur bound within the coal's carbon matrix, and pyritic sulfur present as discrete iron-disulfide (FeS₂, pyrite) mineral inclusions. During combustion, both forms oxidize essentially completely at flame temperature, releasing sulfur as SO₂ ($\text{S}+\text{O}_2\rightarrow\text{SO}_2$, and $4\text{FeS}_2+11\text{O}_2\rightarrow2\text{Fe}_2\text{O}_3+8\text{SO}_2$ for the pyritic fraction); a small additional fraction (typically 1–5% of the SO₂ formed) further oxidizes in the flame/flue-gas path to SO₃, which condenses with moisture to form corrosive, fine sulfuric-acid mist and particulate sulfate downstream. Pre-combustion control example: physical coal cleaning (washing). Because pyrite is denser than the coal matrix, crushed run-of-mine coal is processed through a gravity-based washery (e.g., a dense-medium cyclone or jig) that separates and rejects the higher-density pyritic fraction before the coal is burned, typically removing 30–50% of a coal's pyritic sulfur (though not the organically-bound fraction) and correspondingly lowering the SOx generated per tonne burned — a control applied entirely upstream of the boiler, unlike the post-combustion FGD scrubbing already detailed in Question 6(ii).

(ii) Two PM Monitoring Technologies

Beta attenuation monitor (BAM). Ambient air is drawn through a spot on a moving filter tape at a constant flow rate; a beta-particle source (typically ¹⁴C) on one side of the tape and a detector on the other continuously measure the attenuation of the beta radiation passing through the accumulating particulate deposit. Because beta attenuation follows a Beer–Lambert-type relation that depends on the deposited mass per unit area (largely independent of particle chemistry), the instrument converts the measured attenuation directly into a near-real-time PM mass concentration, and the tape advances periodically to present a fresh filter spot.

Light scattering (nephelometry). Sampled air passes continuously through a small optical chamber illuminated by a laser or LED; particles in the beam scatter light (predominantly, for particles near the light wavelength, forward and at roughly 90°), and a photodetector positioned off the direct beam path measures the scattered-light intensity, which correlates with particle number and, after calibration against particle size/refractive index, with mass concentration. Nephelometers give very fast (sub-minute) response suited to trend and event detection, but the scattering signal is sensitive to particle size distribution, shape and humidity (hygroscopic growth), so they are normally co-located with, and periodically calibrated against, a gravimetric reference method rather than used alone for regulatory compliance.

(iii) Gravity Settler: Design Principles and Operational Factors

The figure below (as given in the source exam) shows the characteristic gravity-settler geometry: a chamber that widens sharply from the narrow inlet/outlet ducts so the gas slows down, allowing entrained particles time to settle by gravity into hoppers below before the cleaned gas exits.

Two key design principles. (1) Settling-time versus residence-time sizing. A particle only reaches the hopper before the gas exits if its Stokes terminal settling velocity $v_t$ times the gas residence time in the chamber exceeds the vertical drop distance to the floor; equivalently, for a chamber of length $L$, width $W$ and horizontal gas velocity $V$, a particle is captured if $v_t \geq \dfrac{Q_{gas}}{L\,W}$ — this is the governing design equation, and it shows removal efficiency depends only on chamber floor AREA ($L\times W$) for a given gas flow, not on chamber height. (2) Low, uniform gas velocity to avoid re-entrainment. The chamber is deliberately widened (per the figure) to drop the gas velocity, typically to below about 3 m/s, since settled dust on the floor or in the hopper throat can be picked back up by the gas stream if local velocity is too high; some designs add horizontal trays/shelves (a multi-tray "Howard" settling chamber) to shorten the vertical settling distance for each layer and improve capture of smaller particles within the same footprint.

Two operational factors. (1) Gas flow-rate control. Since capture depends on $v_t \geq Q_{gas}/(LW)$, any upstream process surge that raises $Q_{gas}$ above the design flow directly and immediately reduces the smallest particle size still captured, so flow must be kept near the design value (or the unit derated) to hold the intended efficiency. (2) Regular hopper discharge. Collected dust must be removed from the hoppers on a schedule that prevents the pile from building up into the active gas stream (which would both re-entrain settled material and reduce the effective settling height); hopper discharge angles must exceed the dust's angle of repose and airlocks must prevent false air in-leakage, which would otherwise increase the in-chamber gas velocity above design and undermine the low-velocity principle the whole unit relies on.