MECH6019 Sources and control of air pollution
The Complete Exam Bible · MECH6019 Overview
- The University of Hong Kong
- First Semester, 2026-27
- Postgraduate engineering course
- Exam-based assessment
What this course asks you to connect
MECH6019 links the origin of an air pollutant to the calculation used to quantify it and the physical or chemical mechanism used to control it.
- Assessment Mid-term 15%, quiz 15%, final exam 70%.
- Calculation habit State the gas basis before using any concentration.
- Control habit Match each device to the pollutant formation or capture mechanism.
- Final details Confirm duration, format and permitted materials on Moodle.
How MECH6019 is assessed
| Component | Weight | Format |
|---|---|---|
| Mid-term | 15% | Mid-term assessment |
| Quiz | 15% | Quiz |
| Final Exam | 70% | Final examination; format and permitted materials are not stated in the captured outline |
The published teaching schedule totals 100%. It does not state a hurdle, final-exam duration, section structure, question count or permitted materials; confirm those operational details on Moodle.
What MECH6019 covers
Ten chapters follow the teaching sequence from concentration units and combustion calculations to particulate, gaseous and industrial VOC control.
Pollutant Sources and Concentration Units
primary and secondary pollutants, mole fraction, ppm and reference conditionsPrimary pollutants leave a source directly, while secondary pollutants form through later atmospheric reactions. That classification identifies where a control can act.
- Primary and secondary pollutants
- Gas mole and volume fractions
- Parts per million by volume
- Mass concentration
- Wet and dry reporting
- Reference conditions
Combustion Stoichiometry and Fuels
atom balances, oxygen demand, excess air and equivalence ratioStoichiometric combustion provides the theoretical oxygen demand used to define excess air, equivalence ratio and exhaust composition. Start by choosing a fuel basis.
- Fuel basis
- Elemental atom balance
- Theoretical oxygen
- Air nitrogen
- Excess air
- Equivalence ratio
Chemical Kinetics and Pollutant Formation
reaction rate, equilibrium and thermal, prompt and fuel NOx pathwaysKinetics explains how quickly pollutant-forming reactions proceed, while equilibrium describes the composition approached if a reversible system has enough time. A reaction may be thermodynamically favoured but too slow during the available residence time.
- Reaction rate
- Reaction order
- Rate constant
- Arrhenius behaviour
- Chemical equilibrium
- NOx formation routes
Combustion Pollutant Concentration
wet and dry exhaust, dilution and oxygen-reference correctionCombustion concentration calculations build the exhaust denominator from a fuel and air balance. Begin on a stated fuel basis, calculate theoretical oxygen and actual air, and list every product species.
- Exhaust product balance
- Wet concentration
- Dry concentration
- Excess-air dilution
- Reference oxygen
- Mass emission rate
Combustion Emission Controls
formation modification, fuel intervention and exhaust after-treatmentCombustion emissions can be controlled by modifying formation inside the flame, changing or cleaning fuel, treating pollutants after formation and reducing the carbon intensity of useful energy. These options act at different boundaries.
- Combustion modification
- Cleaner fuels
- Fuel cleaning
- Exhaust after-treatment
- Carbon controls
- Multi-pollutant trade-offs
Particle Behaviour and Size Distributions
settling, drag regimes, aerodynamic diameter and distribution weightingParticle motion depends on diameter, density, shape and the surrounding gas. A settling calculation balances gravitational force, buoyancy and drag. After release, a particle accelerates until drag equals effective weight and terminal velocity is reached.
- Terminal settling
- Stokes law
- Reynolds number
- Drag coefficient
- Aerodynamic diameter
- Size-distribution weighting
Particulate Collection Equipment
gravity settlers, cyclones, electrostatic precipitation and grade efficiencyGravity settlers, cyclones and electrostatic precipitators capture particles through different physical mechanisms. A gravity chamber slows gas and provides time for particles to fall.
- Gravity settling chambers
- Cyclone separators
- Electrostatic precipitators
- Dividing collectors
- Grade efficiency
- Overall mass efficiency
Industrial VOC Sources and Treatment
industrial streams, treatment mechanisms, removal performance and cost basisIndustrial VOC streams arise from processes such as melting, printing, coating, paint spraying, drying and solvent handling. The process defines the compound mixture, concentration, gas flow, temperature and variability.
- Industrial VOC sources
- Waste-gas characterisation
- Adsorption
- Thermal and catalytic oxidation
- Biological treatment
- Removal-cost comparison
Absorber Design and Sulphur Control
countercurrent mass transfer, solute balance and sulphur removal pathwaysGas absorption transfers a soluble pollutant from gas into liquid across an interface. Solubility is necessary, but useful equipment also needs interfacial area, a concentration driving force and time for transfer.
- Gas solubility
- Countercurrent packed towers
- Operating line
- Mass-transfer driving force
- Sulphur oxidation state
- Wet scrubbing
Nitrogen Oxide Formation and Control
atmospheric reactions, combustion modification and selective reductionNitrogen oxide control connects formation in combustion equipment, transformation after release and chemical treatment in exhaust.
- NO and nitrogen dioxide
- Atmospheric reactions
- Thermal NOx
- Prompt and fuel NOx
- Combustion modification
- Selective reduction
The first half builds the quantitative spine: concentration units, fuel and oxidant balances, reaction kinetics, pollutant formation and exhaust correction. The second half applies that spine to combustion modification, particulate behaviour and collection, industrial volatile organic compound treatment, gas absorption, sulphur chemistry and nitrogen oxide control. A strong answer does more than name equipment.
It states the stream basis, closes the material balance, explains why the mechanism fits and checks what happens to the removed material.
How the chapters fit together
Chapters on combustion and concentration should be worked in sequence because later emission calculations inherit the earlier atom balance.
Particle chapters pair behaviour with equipment selection: settling and drag explain why gravity, inertia or electrical migration dominates. The final chapters compare gas-treatment pathways and require mass or molar flow rather than concentration alone.
Throughout the guide, original numerical exercises reproduce the methods and question types visible in the teaching materials without copying University questions.
How to read a control problem
Begin by identifying whether the question concerns formation, transport, capture or chemical conversion. Fix the system boundary and reporting basis before selecting an equation.
Then calculate the inlet quantity, apply the relevant mechanism and close the outlet balance. This sequence keeps a concentration change from being mistaken for a mass reduction and keeps captured material visible in the liquid or solid stream that receives it.
Finish by naming the operating constraint that could invalidate the result, such as a drag regime, flame-stability limit, absorber equilibrium or reagent temperature window.
Worked example · free
Convert a dry gas fraction into ppmv
- 1Keep the mole fraction on a dry basis.
- 1Multiply by 10⁶.
- 1Report 40 ppmv dry.
Key terms
- Absorption
- Transfer of a soluble gas constituent into a liquid phase.
- Adsorption
- Accumulation of molecules on a solid surface.
- Equivalence ratio
- Actual fuel-air ratio divided by the stoichiometric fuel-air ratio.
- Excess air
- Air supplied above the theoretical amount required for complete combustion.
- Grade efficiency
- Collection efficiency stated as a function of particle size.
- Mole fraction
- Moles of one species divided by total moles on the stated basis.
- Primary pollutant
- A pollutant emitted directly from a source.
- Secondary pollutant
- A pollutant formed after emission through atmospheric reactions.
- Stokes regime
- Low-Reynolds-number particle motion where viscous drag is linear.
- Thermal NOx
- Nitrogen oxides formed mainly from atmospheric nitrogen at high temperature.
- Volatile organic compound
- An organic compound whose volatility permits emission to air.
- Wet basis
- A gas-composition basis that retains water vapour in the total.
MECH6019 FAQ
How is the course assessed?
The teaching schedule assigns 15% to the mid-term, 15% to the quiz and 70% to the final exam. These numeric rows total 100%. The captured materials do not state a hurdle or separate pass threshold, so no hurdle badge is asserted here.
What is known about the final exam?
The source confirms that a final exam exists and carries 70% of the course mark. Its duration, sections, question count and permitted materials are not stated in the captured outline. Confirm those operational details on the course Moodle page.
Which calculation skill matters most?
Keep the reporting basis visible. State wet or dry gas, temperature, pressure, reference oxygen and whether the quantity is a concentration, mass flow or molar flow. Most large errors begin when a denominator changes silently.
How should combustion problems be approached?
Choose a fuel basis, balance carbon, hydrogen, sulphur and oxygen, calculate theoretical oxygen, add excess air and accompanying nitrogen, then construct wet or dry products. Finish by checking atom totals and whether product percentages sum correctly.
How are particulate-control devices compared?
Start with particle size, density and distribution weighting, then identify the capture mechanism. Gravity settlers use settling, cyclones use inertia and electrostatic precipitators use electrical migration. Pressure drop, loading and maintenance also shape the decision.
How should VOC and absorber calculations be checked?
Convert inlet and outlet values to mass or molar flow on a common basis. Removed pollutant must reappear in captured media, liquid, reaction product or another measured stream. A concentration reduction alone does not close the balance.
Questions students ask in each MECH6019 chapter
- Why must the gas basis be stated?
- Wet or dry reporting changes the denominator, while temperature and pressure change the volume occupied by a fixed amount of gas. A bare concentration cannot be reconstructed or compared safely. The sampling location and averaging period also remain part of the quantity, so matching units alone does not make two readings comparable. Chapter 01 · Pollutant Sources and Concentration Units
- Why choose a fuel basis first?
- A fixed basis converts composition data into definite moles or masses and makes atom conservation checkable. Without it, ratios can be mixed accidentally. One kilomole suits a pure fuel, while one hundred kilograms makes an ultimate analysis easy to translate before oxygen demand is calculated. Chapter 02 · Combustion Stoichiometry and Fuels
- Are kinetics and equilibrium interchangeable?
- No. Kinetics addresses speed, while equilibrium addresses the limiting composition. A favoured reaction may still be too slow in the available residence time. Rapid cooling can also preserve a composition formed upstream because reverse reactions become slow before equilibrium is restored. Chapter 03 · Chemical Kinetics and Pollutant Formation
- Why is dry concentration usually higher?
- Removing water shrinks the denominator while pollutant moles remain unchanged. The dry fraction therefore rises for the same products. Water from fuel moisture and chemically formed water both belong in the wet total, but neither remains when the stated result is converted to dry basis. Chapter 04 · Combustion Pollutant Concentration
- Why is prevention placed above treatment?
- Preventing formation avoids creating a pollutant load, while downstream treatment must handle material already produced and may create captured waste. Prevention can still shift combustion conditions or useful output, so the hierarchy does not remove the need for multi-pollutant and process verification. Chapter 05 · Combustion Emission Controls
- Why does diameter matter so strongly?
- In the Stokes regime settling velocity scales with diameter squared, while particle mass scales approximately with diameter cubed. Small sizing errors can therefore change predictions greatly. Convert diameter units before applying the power and confirm that both compared particles remain within the same drag regime. Chapter 06 · Particle Behaviour and Size Distributions
- Why is one efficiency number insufficient?
- The same device captures different particle sizes at different rates. Overall performance changes when the inlet size distribution changes. Grade efficiency must be weighted by inlet mass in each band, and the residual distribution should be carried forward when another collector follows. Chapter 07 · Particulate Collection Equipment
- Why start with the industrial process?
- The process determines concentration, flow, variability and compound mixture, which control whether recovery, destruction or biological treatment is practical. Temperature, moisture, particles and peak loading can also create pretreatment needs or push a technology outside its stable operating range. Chapter 08 · Industrial VOC Sources and Treatment
- Why use countercurrent contact?
- Fresh solvent meets the cleanest gas while progressively richer liquid meets stronger gas, preserving a useful driving force along the tower. The arrangement does not remove equilibrium or hydraulic limits, so solvent rate, packing and approach to flooding must still be checked together. Chapter 09 · Absorber Design and Sulphur Control
- How do formation and treatment controls differ?
- Staging and recirculation alter flame conditions before NOx forms, while selective reduction reacts with NOx downstream after formation. The first group can change carbon monoxide or efficiency; the second introduces reagent, temperature-window, slip and catalyst considerations that require separate verification. Chapter 10 · Nitrogen Oxide Formation and Control
How to study for the exam
Work the numerical sequence rather than memorising isolated equations. Begin with units and stoichiometry, because concentration correction and device loading depend on those balances. For each worked problem, draw a boundary, write the basis, calculate the inlet quantity, apply formation or removal physics, and close the outlet balance.
Build a one-page error log with direction checks: dry concentration exceeds wet concentration for unchanged pollutant moles; lean combustion has equivalence ratio below one under the course convention; Stokes settling velocity rises with diameter squared; and overall collector efficiency must be weighted by the inlet distribution. In the final week, alternate a calculation with a technology-selection prompt.
This forces the equation and the mechanism to stay connected. Use the revision lecture and tutorial answers to identify methods, but write fresh values and explain each assumption instead of memorising a particular stem. Confirm the final exam's actual arrangements on Moodle because the captured outline does not publish them.
