Question 1 of 5: Sources, Classification and Combustion Air Demand of Atmospheric Pollutants

Problem 1: Sources, Classification and Combustion Air Demand 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) — five outdoor air pollutants: source, health impact, one engineering control each.

1. Particulate matter (PM₂.₅/PM₁₀). Source: incomplete combustion (diesel exhaust, wood burning), industrial process emissions (cement kilns, smelters), and re-entrained road/construction dust. Health impact: fine particles penetrate deep into the alveolar region, aggravating asthma and cardiovascular disease and are classified a Group 1 carcinogen. Engineering control: a fabric-filter baghouse (or electrostatic precipitator for very fine fume) removes particulate from the flue gas stream before stack discharge.

2. Sulphur dioxide (SO₂). Source: oxidation of fuel-bound sulphur during combustion of coal or heavy fuel oil, and non-ferrous metal smelting. Health impact: bronchoconstriction and aggravated asthma on acute exposure, and a precursor to acid deposition that damages ecosystems and infrastructure. Engineering control: wet limestone flue-gas desulfurization (FGD), absorbing SO₂ into a CaCO₃ slurry.

3. Nitrogen oxides (NOₓ). Source: high-temperature combustion in vehicle engines and utility boilers, via thermal NOₓ (Zeldovich mechanism, N₂+O₂ dissociation above ∼1,300 °C) and fuel NOₓ (oxidation of fuel-bound nitrogen). Health impact: airway inflammation and, as an ozone/smog precursor, indirect respiratory harm across a wider population. Engineering control: selective catalytic reduction (SCR), injecting NH₃ over a catalyst to reduce NOₓ to N₂ and H₂O.

4. Carbon monoxide (CO). Source: incomplete combustion under locally fuel-rich conditions — cold-start vehicle engines and poorly-maintained residential heating appliances. Health impact: binds haemoglobin roughly 200× more strongly than oxygen, reducing the blood's oxygen-carrying capacity and causing hypoxia, headache, and at high concentration death. Engineering control: an oxidation catalytic converter completes combustion of CO to CO₂ in the vehicle exhaust stream.

5. Ground-level ozone (O₃). Source: a secondary pollutant, not directly emitted, formed photochemically from NOₓ and volatile organic compounds (VOCs) in sunlight. Health impact: airway irritation, reduced lung function on exertion, and crop/vegetation yield loss. Engineering control: VOC vapour-recovery systems at fuel-storage and dispensing facilities cut the hydrocarbon precursor available for the photochemical cycle.

Part (ii) — combustion air demand for the coal-fired plant.

Given. The plant burns 55,000 US tons of coal per day; the supplied reaction is the stoichiometric oxidation of carbon, $C+O_2\rightarrow CO_2$.

Find. The mass and volumetric flow rate of air required to stoichiometrically combust the daily coal feed.

Approach. Convert the coal feed to a molar carbon flow, apply the 1:1 stoichiometry of $C+O_2\rightarrow CO_2$ to get the theoretical O₂ demand, then scale to air using the 21% O₂ mole fraction of standard dry air.

1. Coal feed rate, SI units. $\dot m_{coal}=55{,}000\ \text{ton}\times 907.185\ \tfrac{\text{kg}}{\text{ton}}=4.99\times10^{7}\ \text{kg/day}$.
2. Molar carbon flow (coal ≈ pure C). $\dot n_C=\dfrac{\dot m_{coal}}{M_C}=\dfrac{4.99\times10^{7}}{12.011}=4.154\times10^{6}\ \text{kmol/day}$.
3. Stoichiometric oxygen demand. From $C+O_2\rightarrow CO_2$, $\dot n_{O_2}=\dot n_C$, so $$\dot m_{O_2}=\dot n_{O_2}\,M_{O_2}=4.154\times10^{6}\times32.00=\boxed{1.329\times10^{8}\ \text{kg/day}}$$
4. Scale oxygen to air (21% O₂ by mole). $\dot n_{air}=\dot n_{O_2}/0.21=1.978\times10^{7}\ \text{kmol/day}$, so $$\dot m_{air}=\dot n_{air}\,M_{air}=1.978\times10^{7}\times28.97=\boxed{5.73\times10^{8}\ \text{kg/day}\ (\approx 6{,}630\ \text{kg/s})}$$
5. Express as a volumetric flow (STP, 0 °C, 101.325 kPa, 22.414 m³/kmol). $$\dot V_{air}=\dot n_{air}\times22.414=\boxed{4.43\times10^{8}\ \text{m}^3/\text{day}\ (\approx 5{,}130\ \text{m}^3/\text{s})}$$