Sources and References

• Cooper and Alley, Air Pollution Control: A Design Approach, 4th ed., Waveland Press.
• de Nevers, Air Pollution Control Engineering, 3rd ed., Waveland Press.
• Davis and Cornwell, Introduction to Environmental Engineering, 5th ed., McGraw-Hill.
• Wark, Warner, and Davis, Air Pollution: Its Origin and Control, 3rd ed., Addison-Wesley.
• IPCC, Climate Change 2021: The Physical Science Basis, Cambridge University Press.
• IEA, CCUS in Clean Energy Transitions (reports, current edition).

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Chapter 1: Sources, Species, and Effects of Atmospheric Emissions

Clean-air engineering addresses emissions from stationary and mobile sources that affect human health, ecosystems, and climate. The field divides into pollutant-focused control of conventional emissions, greenhouse-gas mitigation, and the emerging area of carbon capture, utilization, and storage (CCUS). Modern practice increasingly considers these together, because control technologies and siting decisions cut across all three.

1.1 Conventional Pollutants

Criteria pollutants regulated in most jurisdictions include particulate matter (PM₁₀, PM₂.₅), nitrogen oxides (NOₓ: NO, NO₂), sulphur oxides (SOₓ), carbon monoxide (CO), ozone (O₃, a secondary pollutant), and lead. Hazardous air pollutants cover a longer list of volatile organics (benzene, formaldehyde) and trace metals (mercury). Each originates from specific sources — combustion for NOₓ, SO₂, and CO; solvent use for VOCs; ore processing for metals.

1.2 Greenhouse Gases

Carbon dioxide is dominant by emission mass; methane, nitrous oxide, and fluorinated gases have lower concentrations but much higher global warming potential. Radiative forcing (W/m²) links concentration change to climate response; global warming potential (GWP) integrates forcing over a time horizon relative to CO₂. Engineering decisions on fuel mix, efficiency, and capture directly influence these integrated quantities.

1.3 Effects on Health and Environment

PM₂.₅ penetrates deep into lungs and is associated with cardiovascular and respiratory disease. Ozone damages lung tissue and agricultural yields. Acid deposition from SO₂ and NOₓ acidifies soils and surface waters. Nitrogen deposition perturbs nutrient cycles. Biological effects of nanoparticles and ultrafines remain under active investigation, with mounting epidemiological evidence of harm at low exposure levels.

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Chapter 2: Thermodynamics and Chemistry of Emissions

2.1 Combustion Stoichiometry

A hydrocarbon \( C_x H_y \) burnt in dry air \( (O_2 + 3.76 N_2) \) requires

\[ \left(x + \frac{y}{4}\right) (O_2 + 3.76 N_2) \]

per mole of fuel at stoichiometry. Excess air increases oxygen and temperature; deficient air produces CO and unburned hydrocarbons. Adiabatic flame temperature \( T_{ad} \) follows from energy balance at constant pressure:

\[ \sum_{reactants} n_i h_i(T_r) = \sum_{products} n_j h_j(T_{ad}). \]

2.2 NOₓ Formation

Thermal NOₓ forms via the Zeldovich mechanism at temperatures above about 1800 K; its rate is strongly Arrhenius. Prompt NOₓ forms through radicals in the flame front. Fuel NOₓ arises from nitrogen in the fuel (coal, heavy oil). Each mechanism dictates a distinct control strategy: thermal NOₓ yields to temperature reduction through flue-gas recirculation or lean premixing; fuel NOₓ responds to staged combustion.

2.3 Equilibria and Kinetics

Species concentrations at equilibrium follow from minimization of Gibbs energy with elemental mass constraints. Real combustion is kinetically limited: residence time controls CO burnout, soot oxidation, and NOₓ formation. Chemical-kinetic solvers (Cantera, Chemkin) coupled to CFD predict both.

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Chapter 3: Particulate Control

3.1 Particle Properties

Particle size distribution spans orders of magnitude from tens of nanometres (nucleation mode) to tens of micrometres. Aerodynamic diameter combines geometric size and density into a single parameter governing collection physics. Collection efficiency of any device is strongly size-dependent.

3.2 Collection Devices

Gravitational settling chambers capture only coarse particles (> 50 μm) and are used as pre-cleaners. Cyclones apply centrifugal separation; their cut diameter is approximately

\[ d_{50} = \left(\frac{9 \mu W}{2 \pi N_e v_g (\rho_p - \rho_g)}\right)^{1/2}, \]

with \( W \) gas-inlet width, \( N_e \) effective turns, and \( v_g \) gas velocity. Wet scrubbers capture fine particles by inertial impaction on droplets, at the cost of handling wastewater.

Fabric filters (baghouses) collect particles by forming a dust cake that acts as a self-cleaning filter. Cleaning is by reverse air, shake, or pulse jet. Collection efficiencies exceed 99.9 percent for modern units. Electrostatic precipitators charge particles in a corona and collect them on grounded plates. Efficiency follows Deutsch–Anderson:

\[ \eta = 1 - \exp\!\left(-\frac{A w}{Q}\right), \]

with \( A \) collecting area, \( w \) migration velocity, and \( Q \) gas flow.

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Chapter 4: Gaseous Pollutant Control

4.1 Absorption

Gaseous pollutants soluble in a liquid absorb across a gas–liquid interface. Packed columns with structured or random packing present contact area; the design problem is the height of a transfer unit times the number of transfer units:

\[ Z = H_{OG} N_{OG}. \]

Flue-gas desulphurization (FGD) uses limestone slurry to absorb SO₂; wet and dry variants trade water use for byproduct quality. Amine scrubbing captures CO₂ in post-combustion capture, at the cost of regeneration energy.

4.2 Adsorption

Activated carbon, zeolites, and metal–organic frameworks adsorb VOCs and trace pollutants. Breakthrough curves are modelled with the linear driving-force approximation; pressure- and temperature-swing cycles regenerate the sorbent. Industrial applications include VOC recovery, mercury control, and direct air capture of CO₂.

4.3 Catalytic Reduction and Oxidation

Selective catalytic reduction (SCR) reduces NOₓ with ammonia over vanadia–titania or zeolite catalysts:

\[ 4 NO + 4 NH_3 + O_2 \rightarrow 4 N_2 + 6 H_2 O. \]

Three-way catalysts in vehicle exhausts simultaneously oxidize CO and hydrocarbons while reducing NOₓ near stoichiometric air–fuel ratio. Oxidation catalysts eliminate CO and hydrocarbons in diesel and lean-burn exhausts. Thermal oxidation and regenerative thermal oxidation destroy VOCs at 700–1000 °C.

4.4 Continuous Emission Monitoring

Compliance demands continuous measurement of flue-gas species. Non-dispersive infrared analysers cover CO and CO₂; chemiluminescence measures NO/NOx; UV fluorescence measures SO₂. Opacity monitors and particulate CEMS give PM metrics. Calibration, drift correction, and data-availability reporting are regulated under jurisdictional protocols.

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Chapter 5: Climate Technologies

5.1 Carbon Capture

Post-combustion capture uses amine or advanced solvent scrubbing on flue gas. Pre-combustion capture converts fuel to syngas and CO₂ before combustion in integrated gasification combined cycle plants. Oxy-fuel combustion produces a flue gas of CO₂ and water, facilitating capture.

Energy penalty — the loss in plant net output per unit CO₂ captured — is the key metric. Representative values are 20–30 percent of gross output for mature post-combustion capture, trending down with process intensification and next-generation solvents. Direct air capture uses sorbent beds or solvent contactors to extract CO₂ from ambient air; its energy requirements are higher because of the much lower concentration, but siting is flexible.

5.2 Transport, Utilization, and Storage

Captured CO₂ is compressed and transported by pipeline or ship. Utilization converts CO₂ into fuels, chemicals, or building materials; storage injects supercritical CO₂ into saline aquifers, depleted hydrocarbon reservoirs, or basalt formations. Monitoring, verification, and accounting protocols document storage integrity.

5.3 Low-Carbon Technologies

Beyond capture, decarbonization draws on electrification, renewable electricity, hydrogen (blue, green, and pink), bioenergy with carbon capture (BECCS), and ammonia as an energy carrier. Each carries its own engineering challenges — electrolyser efficiency, hydrogen storage, ammonia cracking, biomass supply chains — and its own thermodynamic limits. A clean-air engineer evaluates these on the basis of lifecycle greenhouse-gas emissions, conventional-pollutant implications, and cost per tonne of avoided CO₂.