18-Env-A5 Air Quality and Pollution Control Engineering · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2015 — 04-Env-A5 / Air Quality and Pollution Control Engineering. 3 hours duration; closed book with a candidate-prepared 8½×11 in double-sided aid sheet; Casio or Sharp approved calculator only. Any five (5) questions constitute a complete paper (the first five answers as they appear are marked); all seven are solved below for completeness. Each question is worth 20 marks with section marks shown in brackets.
Reference texts. Cooper & Alley, Air Pollution Control: A Design Approach (4th ed.); Wark, Warner & Davis, Air Pollution: Its Origin and Control (3rd ed.); Davis & Cornwell, Introduction to Environmental Engineering (6th ed.); Canadian Environmental Protection Act, 1999 (CEPA) and the Canadian Ambient Air Quality Standards (CAAQS) administered by Environment and Climate Change Canada.
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.
Selecting NOx (NO and NO2): (a) Kinetics of formation. The dominant pathway in most fossil-fuel combustion is thermal NOx, described by the Zeldovich mechanism — atmospheric N2 and O2 dissociate at high flame temperature and recombine via the chain $\text{O}+\text{N}_2\rightleftharpoons \text{NO}+\text{N}$, $\text{N}+\text{O}_2\rightleftharpoons \text{NO}+\text{O}$; because the rate-limiting first step has a very high activation energy, the formation rate is strongly (exponentially) temperature-dependent and only becomes significant above roughly 1600–1800 °C. A secondary, faster-acting prompt NOx pathway forms via hydrocarbon radicals (CH, CH2) attacking N2 in the fuel-rich flame front, independent of peak temperature. (b) Two important combustion parameters: peak flame temperature (controls thermal NOx exponentially via the Zeldovich rate) and excess air / local O2 availability (more excess air raises both flame temperature in some regimes and the O2 available to the Zeldovich reactions; conversely staged/fuel-rich primary combustion, i.e. low-NOx burner design, suppresses it) — together with residence time at peak temperature, these are the levers combustion (rather than post-combustion) NOx control manipulates (Question 6(i)). (c) Atmospheric behaviour: NO emitted from the stack oxidizes in the atmosphere (by O3 or peroxy radicals) to NO2, which under sunlight photolyzes and, together with reactive VOCs, drives the photochemical cycle that produces ground-level ozone and PAN (Question 6(iii)); NOx also converts to nitric acid/nitrate aerosol, contributing to acid deposition and to the fine-particulate nitrate fraction — making it an environmental concern through both direct toxicity and its role as the key precursor of two major secondary pollutant systems (smog and acid rain).
Given. A spherical particle settles under gravity in still air:
| Quantity | Symbol | Value |
|---|---|---|
| Particle diameter | $d_p$ | 20 µm $= 20\times10^{-6}$ m |
| Particle density | $\rho_p$ | 3 g/cm3 $=3000$ kg/m3 |
| Air temperature | $T$ | 25 °C |
| Air dynamic viscosity (25 °C) | $\mu_g$ | $1.81\times10^{-5}$ Pa·s |
Find. The terminal settling velocity $v_t$.
Approach. Apply the given Stokes' law terminal-velocity equation directly with SI units, then verify $Re_p\ll1$ to confirm laminar (Stokes-regime) settling was the correct assumption.
| Quantity | Value |
|---|---|
| Terminal settling velocity, $v_t$ | 0.0361 m/s (3.61 cm/s) |
| Particle Reynolds number, $Re_p$ | 0.047 (Stokes regime confirmed) |
Gravity settling chambers rely purely on this settling velocity acting over the residence time available while the gas traverses the chamber, and $v_t\propto d_p^2$ means collection efficiency falls off extremely steeply as particle size decreases — halving the diameter cuts $v_t$, and therefore the achievable collection efficiency at fixed chamber size, to one-quarter. A chamber sized to capture the 20 µm particle above at 3.6 cm/s would need roughly sixteen times the residence time (or floor area, at fixed gas flow and chamber height) to capture a 5 µm particle at the same efficiency, which is rarely practical at real gas flow rates. Gravity settlers are therefore used only as coarse pre-cleaners: they cheaply strip out the large-particle fraction (which would otherwise erode fan blades, abrade ductwork, or overload a downstream cyclone/baghouse/ESP) at very low pressure drop, while the fine-particle fraction that actually drives most health and visibility impacts (Question 3(iii)) is left to a downstream device — a cyclone (inertial impaction, still limited to roughly $d_p>10\ \mu\text{m}$, Question 7(i)), a fabric filter, or an electrostatic precipitator — that exploits a different, size-independent or sub-micron-capable capture mechanism.
