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18-Env-A5 Air Quality and Pollution Control Engineering · December 2016

Question 2 of 7: Sources of Atmospheric Pollutants and Combustion Air Demand

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

04-Env-A5 / 18-Env-A5, Air Quality and Pollution Control Engineering — National Exam, December 2016. 3 hours, open book. Question 1 is compulsory; any other four (4) of Questions 2–7 complete the 100-mark paper (only the first five (5) answers in the work book are marked). All seven Problems are answered below.

Reference texts

confirmed against the printed paper; all 7 Problems and every (i)/(ii)/(iii) sub-part are answered in full below.

Problem 2: Sources of Atmospheric Pollutants and Combustion Air Demand (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.

Part (i) — two outdoor air pollutants. (1) Sulphur dioxide (SO₂) originates chiefly from combustion of sulphur-bearing fossil fuels (coal, heavy fuel oil) in power plants and smelters; it irritates the respiratory tract, aggravates asthma, and oxidises to sulphate aerosol that contributes to cardiopulmonary mortality. Engineering method: wet flue-gas desulphurisation (limestone scrubbing, Problem 6(iii)) removes SO₂ before the stack. (2) Ground-level ozone (O₃) is a secondary pollutant formed photochemically from NOₓ and volatile organic compounds emitted by vehicles and industry; it damages lung tissue, reduces lung function, and aggravates asthma/COPD. Engineering method: selective catalytic reduction (SCR) and low-NOₓ combustion on the NOₓ source reduce the ozone-forming precursor at the point of generation.

Part (ii) — combustion air demand.

Given. A 4,000 MW plant burning approximately 50,000 US tons of coal per day; the exam supplies the single reaction $\text{C}+\text{O}_2\rightarrow\text{CO}_2$.

QuantityValue
Coal consumption50,000 US tons/day
1 US ton907.185 kg
Molar mass, C12.011 kg/kmol
O₂ in air (by mole)21%
Molar mass, air28.97 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 other elemental composition is given), convert to moles of C, apply the 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.

  1. Mass of coal (as carbon). $$\dot m_C = 50{,}000\ \text{ton/day}\times 907.185\ \dfrac{\text{kg}}{\text{ton}} = 4.536\times10^{7}\ \text{kg/day}$$
  2. Moles of carbon, then stoichiometric O₂. $$n_C=\dfrac{4.536\times10^{7}}{12.011}=3.776\times10^{6}\ \text{kmol/day}=n_{O_2}\quad(1{:}1\ \text{from } \text{C}+\text{O}_2\rightarrow\text{CO}_2)$$
  3. Moles of air (21% O₂ by volume). $$n_{air}=\dfrac{n_{O_2}}{0.21}=\dfrac{3.776\times10^{6}}{0.21}=1.798\times10^{7}\ \text{kmol/day}$$
  4. Volumetric and mass air rate. At STP ($22.414\ \text{m}^3/\text{kmol}$): $$\boxed{V_{air}=1.798\times10^{7}\times22.414 = 4.03\times10^{8}\ \text{m}^3/\text{day} \approx 4{,}670\ \text{m}^3/\text{s}}$$ $$\dot m_{air}=n_{air}\times28.97 = 5.21\times10^{5}\ \text{tonnes/day}$$

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 $5.21\times10^5$ tonnes/day to within 1%, and gives an air-to-fuel mass ratio of 11.5 kg air per kg coal — a textbook-typical value for coal combustion, confirming the result.

QuantityResult
Theoretical O₂ required$3.78\times10^{6}$ kmol/day
Theoretical combustion air (volume)$4.03\times10^{8}$ m³/day $\approx$ 4,670 m³/s
Theoretical combustion air (mass)$5.21\times10^{5}$ tonnes/day
Air-to-fuel mass ratio11.5 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 would lower this estimate somewhat); (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) all 50,000 US tons/day is attributable to steady-state generation (no start-up/shutdown swings).

Part (iii) — three indoor air pollutants.

PollutantHealth impactsEcological impacts
Radon (²²²Rn), a naturally occurring radioactive soil gas that seeps into basements1) lung cancer from inhaled alpha-emitting decay products; 2) cumulative dose risk rises with basement occupancy time1) radon progeny deposit on outdoor surfaces/soil near vented sub-slab systems; 2) negligible ambient ecological footprint outdoors — its impact is almost entirely an indoor-exposure pathway, illustrating how "ecological" impact can be minimal even when health risk is high
Formaldehyde / VOCs off-gassed from pressed-wood furniture, adhesives and new carpet1) eye/throat/respiratory irritation and headaches at typical indoor levels; 2) IARC Group 1 carcinogen (nasopharyngeal cancer) at higher chronic exposure1) exhaust/exfiltrated VOCs are ozone precursors that add to outdoor photochemical smog; 2) VOC-laden building exhaust contributes to regional volatile-organic loading and secondary organic aerosol formation
Environmental tobacco smoke / combustion by-products (CO, NO₂, PM) from unvented gas stoves and ETS1) childhood asthma and respiratory infection from PM/NO₂ exposure; 2) acute CO poisoning risk from incomplete combustion in poorly ventilated spaces1) exhausted combustion gases add to local outdoor NO₂ and PM burden; 2) black-carbon component of the PM contributes, in aggregate, to regional haze and short-lived climate forcing