Sources and Classification of Atmospheric Pollutants: 18-Env-A5 Air Quality and Pollution Control Engineering, May 2016, Q1

Question 1: Sources and Classification of Atmospheric Pollutants (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) Two Outdoor Air Pollutants

Ground-level ozone (O₃). A secondary pollutant — it is not emitted directly, but forms downwind of urban/industrial areas when NO_x and volatile organic compounds (VOCs) from vehicle exhaust, fuel combustion and solvent use react in sunlight. Health impacts: it is a strong respiratory irritant that inflames airway tissue, reduces lung function and aggravates asthma and COPD, with elevated hospital admissions on high-ozone summer days. Engineering method: selective catalytic reduction (SCR) and low-NO_x combustion (staged combustion, flue-gas recirculation) on major stationary NO_x sources removes one of the two precursors needed for ozone formation.

Sulphur dioxide (SO₂). A primary pollutant emitted directly from combustion of sulphur-bearing fossil fuels (coal-fired power plants, smelters, marine bunker fuel). Health impacts: it irritates the respiratory tract and causes bronchoconstriction, particularly in asthmatics, and it is a precursor to fine sulfate particulate and acid deposition, which carries its own respiratory and ecosystem effects. Engineering method: wet limestone flue-gas desulfurization (FGD), which scrubs SO₂ from the flue gas before it reaches the stack (see Question 6(iii) for the process schematic).

(ii) Combustion Air Requirement

Given. A 2,200 MW power plant burning coal:

Given data
Quantity	Symbol	Value
Plant capacity	—	2,200 MW
Coal consumption	—	21,000 US tons/day

Find. The mass (and volume) of air required to supply the combustion.

Check: the question gives only the plant's electrical capacity and coal consumption, with no ultimate (elemental) analysis of the coal and no combustion-air formula supplied (unlike Questions 5(i) and 7(ii), which do supply their formulas). The plant capacity is therefore only used to confirm the coal-burn rate is of the right order for a large baseload station; the air requirement follows directly from the coal mass and an assumed coal composition. A representative bituminous-coal ultimate analysis is assumed: 75% C, 5% H, 6% O, 1.5% N, 2.5% S, 10% ash and moisture (mass basis), consistent with typical utility bituminous coal (Cooper & Alley Table 2.x). The calculation gives the theoretical (stoichiometric) air; real boilers run 15–30% excess air for complete combustion, shown separately below.

Approach. Convert the daily coal tonnage to SI mass, apply the standard elemental theoretical-air formula to the assumed ultimate analysis to get a mass ratio (kg air/kg coal), then multiply by the daily coal mass.

Daily coal mass. $21{,}000\ \text{US tons/day} \times 907.185\ \text{kg/ton} = \boxed{1.905\times10^{7}\ \text{kg/day}}$.
Theoretical air-fuel ratio. Using the standard combustion-engineering formula for theoretical air per unit mass of fuel, $A/F = 11.53\,C + 34.34\left(H-\dfrac{O}{8}\right) + 4.29\,S$ (mass fractions): $$A/F = 11.53(0.75) + 34.34\left(0.05-\frac{0.06}{8}\right) + 4.29(0.025) = 8.65+1.46+0.11 = \boxed{10.21\ \text{kg air/kg coal}}.$$
Air required. $\dot m_{air} = (1.905\times10^{7})(10.21) = 1.946\times10^{8}\ \text{kg/day} = \boxed{194{,}600\ \text{tonnes/day}}$. At a standard air density of 1.20 kg/m³, this is $\approx 162$ million m³/day of air. Allowing a typical 20% excess-air margin for complete, reliable combustion raises the actual air supplied to $\approx 233{,}500$ tonnes/day.

Quantity	Value
Daily coal consumption	1.905 × 10⁷ kg/day
Theoretical air-fuel ratio	10.21 kg air/kg coal
Theoretical air required	≈ 194,600 tonnes/day (≈ 162 million m³/day)
Air required with 20% excess air	≈ 233,500 tonnes/day

(iii) Two Indoor Air Pollutants

Radon (²²²Rn). A naturally occurring radioactive noble gas that seeps from uranium-bearing soil and rock into buildings through foundation cracks and sumps, common across parts of Canada (e.g. the Canadian Shield). Health impacts: (1) it is the second leading cause of lung cancer after smoking, since its short-lived decay progeny attach to airborne dust and irradiate lung tissue with alpha particles when inhaled; (2) chronic low-level exposure is linked to elevated respiratory-tract inflammation. Ecological impacts: (1) elevated background radioactivity in soil gas and groundwater around uranium-bearing geology can affect nearby aquatic and soil ecosystems; (2) sub-slab depressurization systems installed to protect occupants simply relocate the radon to outdoor ambient air, adding (a small, regulated) radioactive loading to the immediate local outdoor environment.

Volatile organic compounds — formaldehyde. Off-gassed indoors from pressed-wood products, adhesives, insulation and paints. Health impacts: (1) it is a potent eye, nose and respiratory irritant even at low ppm concentrations, aggravating asthma; (2) it is classified as a human carcinogen with prolonged exposure. Ecological impacts: (1) when building ventilation exhausts formaldehyde and other VOCs outdoors, they react with NO_x in sunlight to form ground-level ozone and photochemical smog, damaging vegetation downwind; (2) VOCs contribute to secondary organic aerosol formation, adding to regional haze and its associated ecosystem effects.

Concept & Theory

Topic. Primary vs. secondary air pollutants (outdoor), source-specific indoor pollutants, and stoichiometric combustion-air demand.

Key relations. Photochemical ozone formation NO₂ $\xrightarrow{h\nu}$ NO + O, O + O₂ → O₃ (VOC oxidation breaks the NO–O₃ titration cycle, allowing net accumulation); theoretical combustion air $A/F = 11.53C+34.34(H-O/8)+4.29S$ (mass basis).

Why this works. Classifying a pollutant as primary (emitted) or secondary (formed downwind) determines whether control is applied at the source (SO₂ scrubbing) or at a precursor (NO_x/VOC control for ozone). The elemental theoretical-air formula is simply the oxygen stoichiometry of C, H and S combustion, expressed per unit fuel mass and converted to air via air's 23.2% O₂ mass fraction.

Common pitfall. Treating "air required for combustion" as automatically meaning the actual air supplied — real boilers always run excess air above the theoretical minimum to ensure complete combustion, and the two figures should be reported separately, not conflated.

Source. Cooper & Alley, Air Pollution Control, ch. 2 (combustion stoichiometry); Davis & Cornwell, Introduction to Environmental Engineering, ch. 8 (indoor air quality, secondary pollutant formation).

Question 1 of 7: Sources and Classification of Atmospheric Pollutants

Question 1: Sources and Classification of Atmospheric Pollutants (20 marks)

(i) Two Outdoor Air Pollutants

(ii) Combustion Air Requirement

(iii) Two Indoor Air Pollutants