Gas separation technologies

Question 1

Types of pollutants in the atmosphere (and should be removed during gas treatment)

Answer 1

Carbon dioxide (CO2)
GHGs (CO2, CH4)
Nitrogen oxides (NOx of various kinds)
Ozone and photochemical smog (w. hydrocarbons) ,ODP
Laughing gas (N2O)
Sulphur oxides (SOx)-acidification
Dust, soot, particles (PM)
VOCs (solvents, petrol, etc.) photochemical smog
Ozone depleting substances-chlorofluorocarbons
Harmful/heavy metals (lead Pb, mercury Hg, cadmium Cd, chromium Cr, Arsenic As, etc.)
Smelly substances

Question 2

How does a wet scrubber work? What does it remove?

Answer 2

Pollutants in the chamber react with water or another liquid. Pollutants transfer to water, and are drained through the bottom. Clean air exits through the top.

Removes SOx, and very small PMs like PM 2.5 and 10

Question 3

What is the mass transfer equation?

Answer 3

Mass transfer=coefficient x Area x driving force (concentration difference)

m= C x A x Δconc

Question 4

Advantages and disadvantages of a wet scrubber

Answer 4

Advantages: High removal potential for small PMs and SOx gas

Disadvantages: High water and energy use, size (it’s big)

Question 5

How do gas cyclones work, and what do they remove?

Answer 5

Flue gas enters the container, where it turns into an outer and inner vortex. The outer vortex flows down, and the inner vortex flows up. Particles with higher inertia (larger) resist the vortex, hit the walls, and fall into the collection chamber at the bottom. The cleaned air forms the inner vortex, which flows out the top of the container.

Removes particulates down to PM 10.

Question 6

What are some advantages and disadvantages of gas cyclones?

Answer 6

Advantages: low energy use

Disadvantages: standard models don’t remove particles smaller than PM 10

Question 7

How does a gas fabric/bag filter work, and what does it remove?

Answer 7

Gas flows through porous materials, and any materials larger than the pores get stuck and drop to the collection chamber at the bottom. Removes very small PMs, down to 1. Also can neutralize SOx and HCl if sorbacal and lime are added.

Question 8

What are the advantages and disadvantages of a gas fabric/bag filter?

Answer 8

Advantages: can remove very small particles, and can reuse collected dust/ash

Disadvantages: temperature limit at 350 degrees, combustion potential

Question 9

What is Selective Catalytic Reduction? How does it work and what does it remove?

Answer 9

Fue gas enters a chamber operating at 300-400 degrees celsius. Ammonia is injected, and with the help of a noble metal catalyst, NOx is neutralized and N2 and H2O are released.

Removes NOx.

Question 10

What are the advantages and disadvantages of SCRs?

Answer 10

Advantage: high removal of NOx: up to 95%

Drawbacks: Temp. sensitive, can release ammonia into environment

Question 11

Where are SCRs typically used?

Answer 11

Diesel engine exhaust

Question 12

What are the benefits and drawbacks to SNCRs?

Answer 12

Benefits: Cheaper installation than SCRs

Drawbacks: removes only 30-70% of NOx

Question 12

How does Selective Non-Catalytic Reduction work?

Answer 12

NOx reacts with Ammonia or urea at the end of the combustion chamber, where it is injected at an operational temperature of 950 degrees. It removes NOx.

Question 13

How does an electrostatic precipitator work? And what does it remove?

Answer 13

Gas flows through an electrical field, and becomes negatively charged. These negatively charged particles are then attracted to positively charged collector plates. Clean air then flows out, and collector plates must be cleaned constantly or periodically.

Removes PMs from 1-10 micrometers

Question 14

What are the advantages and disadvantages of electrostatic precipitators?

Answer 14

Advantages: high collection efficiency– up to 99%
Low pressure drop

Disadvantages: high initial cost
not effective for all particles

Question 15

What are some ways to remove sulfur oxides?

Answer 15

Source reduction: Fluidised bed combustion (FBC): adding lime
already in the boiler so it reacts with sulphur

End-of-pipe:
Flue Gas Desulfurization: adding alkaline substances such as
lime to counteract acidity (Wet and dry scrubbing)

Question 16

What are some reacting agents that neutralize SOx?

Answer 16

lime, limestone, ammonia, magnesium and sodium hydroxide (alkaline substances to neutralize the acidity)

Question 17

What are some ways to reduce NOx?

Answer 17

Combustion optimisation: controlling temperature to reduce formation of NOx; reducing air to react with NOx

1. SNCR: ammonia / urea added in the combustion chamber and
   reacts with NOx

ii. End-of-pipe:
1. SCR: ammonia reacts with NOx in the presence of a noble
catalyst