Air pollution Flashcards
How does a bag filter work?
A bag filter collects particles from the flue gas stream by diffusion, interception, inertial impaction, and electrostatic precipitation which builds up a filter cake. The filter cake provides high efficiency removal of PM through sieving action. Can generally capture very small particles. However need to ensure this is cleaned to avoid high P drop, bag life is 1-3 years
How does an EP work?
Particles are charged by corona discharge as they enter EP in an electrical field between positive and negative electrodes. The charged particles are attracted to a collection plate and away from the discharge electrode due to electrostatic force. The particles are periodically removed by rapping - there are inevitable losses during each rapping event and losses from last collector.
How does venturi scrubber work?
They are used to remove both particulates and soluble gases, and are very effective at removing fine particulates from an exhaust stream. Gas enters vena contractor which sprays the gas with water increasing the particle size, then a cyclonic seperator separates the particles from the clean gas stream. This has a high operating cost and creates a water-based waste stream so not commonly used unless there are strict PM requirements.
How does a settling chamber work?
Slow gas velocities enter chamber, the PM settles to bottom of chamber into collection hoppers. The rate at which particles settle is governed by stokes law.
How does a cyclone separator work?
Use centripetal force where gas follows cylindrical path in a chamber and inertia forces particles to the wall. Particles fall down into collection chamber and clean gas exits from the top. Has better removal eff than settling chamber.
Practical limits on radius and inlet velocity causing pressure losses is what limits efficiency.
Typically used for separating particles > 2 micron, not very fine, often used as pretreatment for EP.
What is wet scrubbing?
Wet scrubbing uses absorption where the pollutant leaves the gaseous phase and enters the liquid phase. Removal efficiency is dependent on solubility of gas in contacting liquid.
Can add lime for desulphurisation.
What is dry scrubbing?
Uses adsorption where gaseous component adheres to surface of a solid, solid is adsorbent, gas is adsorbate.
Usually single-use bed if removing trace quantities of pollutants.
Can use a regenerating bed with PSA
Some gases poison the adsorbent and gas needs to be particulate free.
How to reduce NOx?
Reduce excess air (O2 available in combustion zone)
Stage the combustion
Flue gas recirculation
Dilute (flameless or spacious) combustion
What is flame staging?
Combustion or flame staging is where you don’t have a stoichiometric flowrate going into one port within the burner but rather separate it. You have a primary flame which is exceptionally fuel rich (low excess air) and lower temperature, because its so fuel rich NOx is highly unlikely to form. And then secondary air is fed in later and flame is distributed over a much larger volume which helps to keep it cooler and also burn out soot/carbons formed in fuel rich zone. Keeping the flame temperature low is the most effective way of reducing NOx. By doing flame staging you can also create a reducing environment which helps to reduce NOx back to N2 and O2.
What is wrong with Gaussian dispersion model? (3)
Assumes:
Wind is ‘unidirectional’ and constant velocity
Doesnt account for gas density differences eg heavy gases, light gases
Environmental stability not well accounted for eg adiabatic lapse rate from earlier.
How does an inversion layer form?
Normally, air temperature decreases at a rate of 3.5°F for every 1,000 feet (or roughly 6.4°C for every kilometer) you climb into the atmosphere. When this normal cycle is present, it is considered an unstable air mass, and air constantly flows between the warm and cool areas. The air is better able to mix and spread around pollutants.
During an inversion episode, temperatures increase with increasing altitude. The warm inversion layer then acts as a cap and stops atmospheric mixing. This is why inversion layers are called stable air masses.
Temperature inversions are a result of other weather conditions in an area. They occur most often when a warm, less dense air mass moves over a dense, cold air mass.
This can happen, for example, when the air near the ground rapidly loses its heat on a clear night. The ground becomes cooled quickly while the air above it retains the heat the ground was holding during the day.
What favours soot formation?
Lack of oxygen, inadequate mixing of fuels and air, longer chain fuels
Yellow flame indicates the presence of soot bc carbon formed inside of flame lumineses
What affects adiabatic flame temperature?
Oxy fuel - higher
Excess fuel - decreases as not extracting every unit of energy of that fuel
Excess air - decreases, extra N2 and O2 has to be heated
Flue gas recycle - decreases - acts as dilutent, suppresses flame temp
Why is adiabatic flame temp important?
After this temp there is dissociation of products from combustion and high enough temp to oxidise N2 to NO
How to control SOX?
Source low sulphur fuels
Avoid conditions that lead to formation of SO3 (eg vanadium)
Injection of lime (forms calcium sulphate)
