18-Env-A5 Air Quality and Pollution Control Engineering · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — December 2013 — 04-Env-A5 / Air Quality and Pollution Control Engineering. 3 hours duration; closed book with a candidate-prepared 8.5×11 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.
Given. A utility ESP serving a coal-fired boiler:
| Quantity | Symbol | Value |
|---|---|---|
| Gas flow rate | $Q$ | 250,000 ACFM |
| Total collection plate area | $A$ | 20,000 m² |
| Effective migration velocity | $\omega$ | 0.05 m/s |
Find. The expected particulate collection efficiency $\eta$.
Approach. Convert the gas flow rate to SI (m³/s) to match the plate-area and migration-velocity units, then substitute directly into the Deutsch-Anderson equation.
| Quantity | Value |
|---|---|
| Gas flow rate, $Q$ | 118.0 m³/s |
| Specific collection area, $A/Q$ | 169.5 s/m |
| Collection efficiency, $\eta$ | ≈ 99.98% |
Because $A/Q$ sits inside a negative exponential, efficiency is extremely sensitive to plate area and migration velocity at the high-efficiency end: even a 10% shortfall in plate area (down to 18,000 m²) would drop $\eta$ only to about 99.94% at this same $\omega$, illustrating why utility ESPs are typically sized with generous margin — the exponential means the LAST fraction of a percent of efficiency is disproportionately expensive to buy in added plate area, which is exactly the design trade-off a plant engineer must balance against the emission limit actually required.
(1) Mass-transfer-driven sizing. Both technologies remove a gas/vapour by transferring it across a phase boundary — onto a solid sorbent surface (adsorption) or into a liquid solvent (absorption) — so both are designed around a mass-transfer-zone concept: sufficient contact area and residence time must be provided for the pollutant to diffuse to and across the interface before the gas exits, and both are characterized by a breakthrough curve (adsorption bed saturation) or an equilibrium/operating-line approach-to-saturation (absorption column) that governs when the bed/solvent must be regenerated or replaced.
(2) Countercurrent contacting geometry. Both equipment types are conventionally arranged countercurrent — contaminated gas enters where the sorbent/solvent is most "used up" (nearest exhaustion) and exits where it meets the freshest sorbent/solvent — because this configuration maximizes the driving-force (concentration gradient) available at every point along the bed/column, achieving a given removal efficiency with the smallest possible sorbent inventory or solvent flow, compared with a co-current arrangement.
Complete destruction of an organic pollutant by incineration requires the classic "3 T's," each addressing a different way partial combustion can leave unburned or partially oxidized product behind.
Temperature. Combustion must be sustained above the auto-ignition/destruction temperature of the specific compound being incinerated (typically 760–1100 °C for hazardous organics) for the oxidation reaction to proceed essentially to completion rather than stalling at intermediate, often more toxic, partial-oxidation products (e.g., incomplete combustion of chlorinated organics can form dioxins/furans if temperature is insufficient).
Residence time. Even at adequate temperature, the reacting gas must remain in the hot combustion zone long enough (typically ≥1–2 seconds for hazardous-waste incineration) for the oxidation kinetics to run to completion; a chamber sized for a shorter residence time will pass unreacted or partially reacted compound straight to the stack regardless of how hot the chamber nominally runs, since temperature and time trade off together in the underlying reaction-rate kinetics.
Combustion air distribution (turbulence/mixing). Fuel/waste and combustion air must be thoroughly mixed so every parcel of waste gas actually contacts sufficient oxygen at the required temperature; poor air distribution creates locally fuel-rich (oxygen-starved) pockets that survive even a nominally hot, long-residence-time chamber, producing soot, CO and unburned hydrocarbons in the exhaust despite an adequate bulk average temperature and time. Properly staged/distributed combustion air (often introduced in multiple zones) is therefore what makes the temperature and residence-time design targets actually achievable in practice, rather than being met only on a chamber-average basis while local pockets fail.
