Question 4 of 7: Control of NOₓ and SOₓ, and Biofiltration for Odour Control

Problem 4: Control of NOₓ and SOₓ, and Biofiltration for Odour Control (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) — three NOₓ control strategies. (1) Low-NOₓ burners with staged combustion (pre/in-combustion): air and/or fuel are introduced in stages rather than all at once, lowering the peak flame temperature and the local O₂ availability at that peak temperature, both of which suppress the Zeldovich thermal-NOₓ mechanism ($N_2+O_2\rightleftharpoons 2NO$, strongly temperature-dependent above roughly 1,300 °C). (2) Flue gas recirculation (FGR) (in-combustion): a fraction of the cooled flue gas is reintroduced into the combustion air, diluting the O₂ concentration and lowering the adiabatic flame temperature, further suppressing thermal NOₓ. (3) Selective catalytic reduction (SCR) (post-combustion): ammonia (or urea) is injected into the flue gas upstream of a catalyst bed (typically V₂O₅/TiO₂) operating near 300–400 °C, where $4NO+4NH_3+O_2\rightarrow 4N_2+6H_2O$ reduces NOₓ to harmless N₂ and water with 80–90%+ removal efficiency.

Part (ii) — three FGD design/operational principles. A wet limestone scrubber is the most common FGD technology. (1) Countercurrent gas–liquid contact: SO₂-laden flue gas rises through a spray tower while an alkaline limestone (CaCO₃) slurry is sprayed downward, maximising the gas–liquid interfacial area and contact time for SO₂ absorption: $SO_2+CaCO_3+\tfrac12 O_2+2H_2O\rightarrow CaSO_4\cdot2H_2O+CO_2$. (2) Adequate liquid-to-gas ratio (L/G) and residence time: the scrubber is sized so that L/G and gas residence time in the spray zone are sufficient to achieve the target removal efficiency, typically 90–95%+ for a well-designed unit. (3) Reagent stoichiometry and forced oxidation control: limestone feed rate is controlled (via pH or SO₂ removal setpoint) to balance removal efficiency against reagent consumption and scaling risk, and forced-air oxidation converts the intermediate calcium sulphite to gypsum (CaSO₄·2H₂O), a saleable by-product rather than a difficult-to-dewater waste; downstream mist eliminators remove entrained slurry droplets before stack discharge.

Part (iii) — biofiltration for odour control. A biofilter passes the odorous off-gas from a rendering/food-processing cooker or digester through a bed of moist, porous organic packing media (compost, wood chips, bark or peat) colonised by a mixed microbial population that metabolically oxidises the odorous compounds (H₂S, mercaptans, amines, VOCs) to CO₂, water and biomass. Three fundamental principles: (1) gas-to-liquid mass transfer — odour compounds must first partition from the gas phase into the aqueous biofilm coating the media, so the media must be kept adequately moist (typically 40–60% moisture content) or mass transfer, and hence removal, stalls; (2) biodegradation kinetics and empty-bed residence time (EBRT) — the gas must remain in contact with the acclimated biomass long enough (typically 30–60 s EBRT) for the microorganisms to fully oxidise the pollutant before the gas exits the bed; (3) nutrient supply and pH/buffering control — sulphur-containing odorants oxidise to sulphuric acid within the biofilm, progressively acidifying the media, so the packing must supply buffering capacity (e.g. periodic lime addition) or be replaced on a maintenance cycle to sustain microbial activity.