Sources and Classification of Atmospheric Pollutants: 18-Env-A5 Air Quality and Pollution Control Engineering, May 2017, Q1

Problem 1: Sources and Classification of Atmospheric Pollutants (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) — classification of atmospheric pollutants. Atmospheric pollutants are classified along two independent axes. By origin: a primary pollutant is emitted directly from a source (e.g. sulphur dioxide from coal combustion; carbon monoxide from incomplete combustion in vehicle engines), while a secondary pollutant forms in the atmosphere from chemical reaction between precursors (e.g. ground-level ozone from NOₓ+VOC photochemistry; sulphate aerosol from SO₂ oxidation). By state: a gaseous pollutant is a true vapour-phase species (e.g. NO₂; SO₂), while a particulate pollutant is a suspended solid or liquid droplet (e.g. fly ash from coal combustion; diesel soot). A third useful axis is source type: stationary sources (e.g. a power-plant stack; a cement kiln) versus mobile sources (e.g. a highway's vehicle fleet; marine vessel exhaust). Classifying a pollutant this way determines the control strategy: primary/stationary emissions are controlled at the source (scrubbers, filters), while secondary pollutants require precursor control spread across many sources.

Part (ii) — combustion air demand.

Given. A 10,000 MW plant burning approximately 80,000 US tons of coal per day; the exam supplies the single reaction $\text{C}+\text{O}_2\rightarrow\text{CO}_2$.

Quantity	Value
Coal consumption	80,000 US tons/day
1 US ton	907.185 kg
Molar mass, C	12.011 kg/kmol
O₂ in air (by mole)	21%
Molar mass, air	28.97 kg/kmol

Find. The theoretical (stoichiometric) volumetric and mass flow rate of combustion air.

Approach. Treat the coal as the pure carbon implied by the exam's own reaction (no elemental analysis is given, and the plant's 10,000 MW rating is context, not a term in the mass balance), convert the daily coal mass to moles of C, apply 1:1 stoichiometry to get moles of O₂, then divide by air's 21% O₂ mole fraction to get moles of air; convert to volume at STP and cross-check by mass.

Mass of coal (as carbon). $$\dot m_C = 80{,}000\ \text{ton/day}\times 907.185\ \dfrac{\text{kg}}{\text{ton}} = 7.257\times10^{7}\ \text{kg/day}$$
Moles of carbon, then stoichiometric O₂. $$n_C=\dfrac{7.257\times10^{7}}{12.011}=6.042\times10^{6}\ \text{kmol/day}=n_{O_2}\quad(1{:}1\ \text{from } \text{C}+\text{O}_2\rightarrow\text{CO}_2)$$
Moles of air (21% O₂ by volume). $$n_{air}=\dfrac{n_{O_2}}{0.21}=\dfrac{6.042\times10^{6}}{0.21}=2.877\times10^{7}\ \text{kmol/day}$$
Volumetric and mass air rate. At STP ($22.414\ \text{m}^3/\text{kmol}$): $$\boxed{V_{air}=2.877\times10^{7}\times22.414 = 6.45\times10^{8}\ \text{m}^3/\text{day} \approx 7{,}460\ \text{m}^3/\text{s}}$$ $$\dot m_{air}=n_{air}\times28.97 = 8.34\times10^{5}\ \text{tonnes/day}$$

Cross-check on a mass basis (O₂ is 23.2% of air by mass, and 1 kg C needs $32.00/12.011=2.664$ kg O₂): $\dot m_{air}= \dot m_C\times2.664/0.232$ reproduces the same $8.33\times10^5$ tonnes/day to within 0.02%, and gives an air-to-fuel mass ratio of 11.5 kg air per kg coal — the same textbook-typical ratio as a smaller plant burning the same fuel, since the calculation is purely stoichiometric per unit mass of coal and does not depend on the plant's rated output.

Quantity	Result
Theoretical O₂ required	$6.04\times10^{6}$ kmol/day
Theoretical combustion air (volume)	$6.45\times10^{8}$ m³/day $\approx$ 7,460 m³/s
Theoretical combustion air (mass)	$8.34\times10^{5}$ tonnes/day
Air-to-fuel mass ratio	11.5 kg air / kg coal

Check: assumes (1) coal is idealised as pure carbon per the exam's own single reaction (real bituminous coal is only ~65–85% C by mass, with H, S, ash and moisture making up the rest, so an elemental-analysis-based estimate would be somewhat lower); (2) purely theoretical (stoichiometric) air — no excess-air margin, though real boilers run 15–40% excess air for complete combustion; (3) dry air at 21% O₂ by volume; (4) the stated 10,000 MW rating and 80,000 US tons/day are both steady-state, average daily figures (no start-up/shutdown swings); the 10,000 MW value is not itself used in the air-demand calculation because the fuel-consumption rate already fixes the combustion stoichiometry.

Part (iii) — one indoor and one outdoor pollutant. Outdoor: sulphur dioxide (SO₂) is a primary pollutant emitted chiefly from combustion of sulphur-bearing fossil fuels (coal, heavy fuel oil) in power plants and smelters. Health impact: it irritates the respiratory tract and aggravates asthma at short-term peak exposures. Ecological impact: it oxidises in the atmosphere to sulphuric acid/sulphate aerosol, the principal driver of acid rain, which acidifies lakes and soils and damages forests and aquatic ecosystems far downwind of the source. Indoor: radon (²²²Rn) is a naturally occurring radioactive soil gas that seeps into basements through foundation cracks and sumps. Health impact: it is the second-leading cause of lung cancer (after smoking), from inhaled alpha-emitting decay products lodging in lung tissue. Ecological impact: essentially confined to the indoor exposure pathway — radon decays with a short half-life and disperses harmlessly outdoors, so unlike SO₂ it has almost no broader ecological footprint, illustrating how the health/ecology balance of a pollutant depends entirely on where it accumulates.

Topic: Classification of atmospheric pollutants; theoretical combustion air demand.

Key relations: primary vs. secondary, gaseous vs. particulate, stationary vs. mobile source classification; stoichiometric air demand $n_{air}=n_{fuel}\times(\text{O}_2\ \text{stoich. ratio})/0.21$.

Why this works: Classifying a pollutant by origin and state determines which engineering lever controls it — source treatment for primary emissions, precursor control for secondary pollutants; the combustion-air calculation is a straightforward mole balance on the given reaction once the fuel is idealised to its dominant element.

Common pitfall: using the plant's MW rating in the air-demand calculation (it is context, not a balance input — only the fuel mass rate matters here) and forgetting to convert US tons to kg before the mole balance.

Source: Wark, Warner & Davis, Air Pollution: Its Origin and Control, 3rd ed., Ch. 1–3 (Pollutant Sources and Classification) & Ch. 8 (Combustion); Health Canada indoor air quality guidelines.

Question 1 of 6: Sources and Classification of Atmospheric Pollutants

Problem 1: Sources and Classification of Atmospheric Pollutants (25 marks)