week 9

what is acid pollution

atmopsheric emmision, transport, chemical tansformation and deposition of acidying species

eg SO2, Nox, H2SO4

what does acid pollution manifest as

acid rain

acid deposition

acid mist

acid aerosols

acid precipitation

precipitation with ph <5.6

acid mist

fog/mist with acidity <2.5

acid deposition

total wet + dry deposition of acidifying species

why is normal rain slightly acidic

cabonic acid fomation

co2 dissolves into ain

critical load

deposition below which harmful effects do not occur

Critical level

atmospheric concentration above which direct adverse effects occur.

principal natural phenomena

emmisions from volcanoes, biological processes, soil microbes ect

principal human sources

industrial and power generating plants

transportation

factory farming

specific S pollutants

SO2

SO3

H2S

mercaptans (Foul odour)

how does glacial ice record the industrial pollution

decrease in pH

start of industrial revolution of 6 to 4.5

why has so2 emmisions decreased since 1989 in uk

changed from coal to gas in power station

• natural gas has lot less sulphur

where are sox and nox conc concentrated

over urban and industrial areas

why do tall smoke sacks cause bad effects

gases chucked high up into the atmopshere

• above inversion layer

• where there are high winds

• transports long distances→ cross boundary

only significant gas phase equation of so2 to acid

HO⋅+SO2(+M)→HSO3⋅(+M)

• hydroxyl radical → highly reactive, photochemical decomposition of ozone

aqueous phase conversion of sulphur dioxide to acid

So2 dissolves and dissociates

main equation for conversion of nitrogen oxides to acid

HO⋅+NO2(+M)→HNO3(+M)

gas phase

effects of so2 on plants

so2 can enter leaf stomata casuing

1. necrosis- cell death- black spots

2. chlorosis- bleaching of leaves- white and yellow

spring acid shock

snow and ice contain frozen acid

melts and transports frozen acid to lakes and river in high concentrations

• spring is time of new plants and reproductive cycle

• kills everything

• damage to fish

vegetation impacts

• SO₂ enters stomata → necrosis and chlorosis

• Reduced photosynthesis

• Increased susceptibility to frost, pests, disease

aquatic impacts

• Spring acid shock

• Mobilisation of Al³⁺ → fish toxicity

• Loss of sensitive species (mayfly, trout, salmonids)

soil impacts

leaching of nutrients

plants can no longer grow

acid impact on built environment

• Acid dissolution of carbonate stone (CaCO₃ → Ca²⁺ + CO₂)

• Damage to limestone, marble, metals, paints

• Examples: St Paul’s Cathedral, Acropolis

why is mercaptan added to natural gas

natural gas is odorless

add small amounts so people can smell leaks

what is the convention of long range trans boundary pollution convention

1979

• Framework for transboundary pollution control

• control emmisions though legally binding protocols

why do we want to enhance dispersion of emmisions

lowers pollutants conc to an acceptable threshold

methods for dispersion enhancement

• Taller stacks- above inversion layer

• Production curtailment- stop or reduce sulphur
  Not a sustainable solution.

process change

modification of production process to reduce emissions

methods for process change

• Fuel switching (coal → natural gas)

• Efficiency improvements

• Waste minimisation
  Most economical option.

intervention

downstream pollution control

remove or destroy contaminants through chemical and mechanical processes

end of pipe intervention methods

• Absorption (wet limestone scrubbing)

• Adsorption

• Low‑sulphur fuels

principal of absorption

relies on solubility of so2 in water

so2 dissolves and forms sulphurous acid (H2SO3)

forms sulfites and sulfates

prevents so2 from re-emitting into gas stream

most common so2 control technology

wet limestone scrubbing

absorption

• so2 removed by aqueous limestone (CaCO3) slurry

• limestone neutralises acid and forms calcium sulfite and sulfate

wet limestone scrubber chemistry

why is regenerative processes good

waste poducts can be comerically useful

eg gypsum

wall plaster

what is adsorption

• Adsorption removes SO₂ by binding it onto the surface of a solid sorbent.

• Typical sorbents include:

  • Activated carbon

  • Metal oxides

• used for lower conc

limitations of adsorbent

• Sorbents must be regenerated or replaced.

• More expensive than wet scrubbing for large flue‑gas volumes.

low sulphur fuel control technology

pollution prevention by process change

most economical and preferred option

types of low sulphur fuels

• Natural gas

• Low‑sulphur coal

• Low‑sulphur fuel oils
  reduces SO₂ formation at source.

advantages of low sulphur fuels

• Avoids the need for large end‑of‑pipe systems.

• Reduces other pollutants (e.g., particulates, metals).

• Aligns with modern environmental policy favouring prevention over intervention.

disadvantages of low sulphur fuels

• Availability and cost of low‑sulphur fuels.

• Not always feasible for legacy industrial infrastructure.

• Does not address NOx or other pollutants.

Flue Gas Desulfurisation (FGD)

end‑of‑pipe pollution control systems

• remove SO₂ from flue gases using absorption, adsorption, or chemical reaction

• commonly through wet limestone scrubbing, producing sulphite/sulphate by‑products.

schematic of spray drier absorbing system

Wellman–Lord SO₂ Recovery Process

regenerative SO₂ removal system

• Uses sodium sulfite (Na₂SO₃) solution to absorb SO₂

• Produces sodium bisulfite (NaHSO₃)

• Regenerated by heating to release pure SO₂, which can be used to make sulphuric acid- can have useful use

major industrial sources of pm

• iron and steel mills

• cement and asphalt

• sulphuic and phosphoic acid

• detergent

• glass

• instant coffee

collection efficiency equation

conc of exhaust gas/ conc of inlet gas

gravitational settling chambers

slows gas flow to allow large particles to settle under gravity

• made of metals that can withstand high temps and corrosion

• solid and liquid

• used for dirty gases- smelters and metallurgical process

adv of gravitational settlers

simple

low cost

low maintenance

effective for big particles

disadv of gravitational settlers

ineffective for fine pm

schematic of gravitational settling chamber

cyclone (centrifugal) collectors

gas spirals downwards

spiral diameter reduces

until gas tuns and spirals upward

particles driven by centrifugal force to wall

cannot turn fast enough

form lager agglomerates collect in gas hopper

• multi-clones used for finer PM

advantages of cyclone collectors

• more powerful than gravity

• conc cyclones - 50% efficiency for 20um

• high efficiency cyclones- 80% for >10um

disadvantages of cyclone collectors

poor for sticky particles (tar)

schematic of cyclone collector

electrostatic precipitators

• corona discharge (electrons) charges particles

• charged particles migrate to electrodes forming cake

• removed by rapping (tapping) or washing with liquid

• conditioning agents (NH3) can improve performance

advantages of ESPs

high efficiency- 99% for >2um

well for medium-resistivity solid

widely used

disadvantages for ESPs

works poorly for low or high resistivity

schematic diagram of ESPs

conventional scrubbers

falling water droplets collide with particles from upward flowing gases

liquid drains through disc- shower

liquid containing particles is collected

liquid recirculated; demister removes droplets

schematic of conventional scrubber

venturi scrubbers

particle laden gas accelerates in throat of device- narrows

liquid injected into throat

particles collide with liquid and form drops

drops containing particles removed in demister

• used in steelmaking and smelting

• efficiency up to 100% for 1um

schematic of venturi scrubber

surface filters (baghouse)

fabric bags trap particles on surface

clean air goes out

bags get shaken to clean

advantages of surface baghouse filters

high efficiency for small pm

inexpensive

disadvantages of surface baghouse filters

not suitable for wet or high temperature streams

depth filters

thick mats of tangled fibres capture particles throughout entire filter body

particles have many chances to impact upon individual fibres

liquid and solid particles

very fine droplets - sulphuric acid mist

emerging technologies

Fly ash leaching- FLUWA

• acidic leaching of fly ash to remove contaminants

FLUREC

• recovers valuable metals- Zn, Cu, Cd, Pb

• waste incineration and wastewater

life cycle assessment

• ISO-aligned method assessing environmental impacts across full life cycles

• compare device impact, energy use, filter lifetimes and disposal routes

Material Flow Analysis (MFA)

• quantifies inflows, outflows and stocks of materials

• identifies emission hotspot and informs PM control strategies

• combined with LCA for integrated assessment

dry flue gas desulphurisation

• inject dry alkaline sorbents (hydrated lime) into flue gas

• produces dry solid by-products

• requires less water than wet scrubber

• usually low efficiency

dry sorbent injection

• simple and low cost

• powdered sorbents injected into flue gas stream

• moderate so2 removal

regenerative FGD system

• uses regenerable sorbents→ activated carbon

• sorbent regenerated

• so2 recovered as concentrated stream and converted to sulphuric acid

• high removal efficiency + minimal waste

• high cost

post combustion techniques consideration

• wet FGD produces wastewater→ requires treatment

• dry systems avoid wastewater → produce more solid waste

• regenerative systems minimise waste→ require more energy