Question 4 of 7: Air Toxics, Odour Control and Emission Trading

Problem 4: Air Toxics, Odour Control and Emission Trading (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) — air toxics. Air toxics (hazardous air pollutants, HAPs) are pollutants known or suspected to cause serious chronic health effects — cancer, reproductive/developmental harm, or other severe illness — at exposures below the levels normally used to regulate the six "criteria" pollutants, and are typically regulated individually rather than as a mass/opacity limit. Three examples: (1) benzene, a known human carcinogen, from vehicle exhaust, fuel evaporation and petrochemical processing; (2) mercury (Hg), a neurotoxin that bioaccumulates as methylmercury in fish, emitted mainly from coal combustion and metal smelting; (3) polycyclic aromatic hydrocarbons (PAHs), carcinogenic products of incomplete combustion, from coke ovens, wood burning and diesel exhaust.

Part (ii) — odour control for a pet-food-like process odour. Food-processing odours are typically complex mixtures of volatile organic and sulphur/nitrogen compounds at low concentration but very low human odour-detection thresholds, so the standard control technology is a regenerative thermal oxidiser (RTO) (or, for lower-strength/continuous streams, a biofilter). Two fundamental design principles: (1) sufficient temperature, residence time and turbulence ("the three T's") in the oxidiser — typically 760–820 °C for ≥0.5 s with good mixing — to destroy the odorous compounds by combustion rather than merely diluting them; (2) ceramic heat-exchange media that regenerate (store combustion heat and pre-heat the next incoming air stream), which is what makes an RTO thermally self-sustaining (often >95% thermal recovery) for a large, dilute, continuous kitchen/rendering exhaust stream instead of requiring continuous auxiliary fuel.

Part (iii) — emission trading. Emission (cap-and-trade) trading is a market-based regulatory instrument: a regulator sets an aggregate emissions cap for a pollutant (e.g. CO₂e) across a sector, issues (or auctions) tradable allowances/credits totalling that cap, and lets individual emitters buy and sell allowances among themselves. A facility that can reduce emissions cheaply sells its surplus allowances; a facility facing high abatement cost buys allowances instead of over-investing in controls — the same total reduction is achieved, but at the lowest aggregate cost across the whole economy, and the cap can be ratcheted down over successive periods to drive continued improvement. Canadian example: Québec's cap-and-trade system (linked with California under the Western Climate Initiative) auctions CO₂ allowances to large emitters and lets covered facilities purchase offset credits from verified emission-reduction projects (e.g. landfill methane capture) instead of reducing on-site.