Airbourne Pollutants Flashcards
Within the adiabatic lapse rate, what happens when (think of the graph with two lines):
a. a particle gains potential energy
b. a particle loses potential energy
Gain PE:
- loss in temp
- less dense than atmosphere
- rises as gas is buoyant
Lose PE:
- gains temp
- higher density than atmosphere
- gas falls/sinks
What are the three types of formation of NOx?
- Thermal NOx
– Controlled by N2 and O2 concentrations and temperature of the reaction. Temp less than 1300degC forms much smaller concentrations of NOx
Favoured by perfect mixing (S =0 ) and stoichiometric ratioϕ = 1 where ϕ>1 fuel is rich and ϕ<1 fuel is lean. - Fuel NOx
– Fuels that contain nitrogen and therefore resulting from oxidation of the already ionized nitrogen contained in the fuel.
Lower atomizing pressure = less mixing
Higher ϕ –> more fuel rich operation (but not too high)
Combustion gases maintained fuel-rich long enough for NO to be reduced to N2
- Prompt NOx
– Molecular nitrogen in the air combining with fuel in fuel rich conditions. Just like Fuel NOx the nitrogen oxidises with the fuel
Occurs at low temperature, fuel rich and short resisdence times
Occur with HC and do not occur with pure CO and H2
Typically forms within the flame and NO increases as combustion becomes more rich and residence times are decreased
Define PM2.5 and PM10
PM2.5 is considered to be fine particles (2.5 micro metres)
PM10 is considered to be fine and coarse particles (also emcompasses PM2.5)
Between 2.5-10 particles are considered coarse.
When is Ozone good or bad?
Ozone in the troposphere is bad, breathable environment is very thin (we are in the troposphere).
Stratosphere contains good ozone (this is above us, where ozone should be), this is where you want ozone, it is required here as it eliminates a lot of ultraviolet radiation to protect us from the sun.
What is a Dobson unit (DU)?
What value is considered to be normal and to be creating a ozone hole?
Dobson unit represents 0.01 mm of pure ozone at 1 atm at 0˚C.
200 DU = for the hole over antarctica (naturally occurring over the poles)
200 DU > Normal Ozone
200 DU< “Ozone hole” just less ozone then the rest of planet
Why is there an ozone hole over the pole?
Strong winds encircle the south pole called a “polar vortex” for long periods over winter. Very little mixing of air outside these poles.
Polar stratospheric cloud (PSC) particles react with the CLONO2 and releases the chlorine atoms, which sunlight breaks up into radicals.
These Cl radicals then react with O3 and O3 and the ozone depletion begins again.
Pre Combustion techniques of reduction of NOx (1)
- Switching to a fuel with a low-nitrogen content
Pre Combustion techniques of reduction of SOx
Switching to a low-sulphur fuel is an effective technique for reducing SO2 emissions.
Performing beneficiation before the combustion process is an option. This improves the fuel quality, removing the sulphur content.
Pre Combustion techniques of reduction of PM
Switching from solid fuel such as coal, which has a high-ash content, to oil or natural gas is an effective technique for reducing PM emissions.
Performing beneficiation before the combustion process is an option. This improves the fuel quality, removing ash prior to combustion.
What happens to the products of combustion when there is too little or too much excess air?
Too little excess air:
inefficient burning of the fuel, soot build up and unnecessary amounts of carbon monoxide emissions. This is wasted fuel.
Too much excess air:
the concentration of nitrogen oxide and sulphur dioxide increases. Carbon dioxide decreases as it is diluted the flue gas by the excess air.
There also gets to a point when, if you had a significant amount of excess air the flame cools down and you get incomplete combustion as heavy oil requires very high temperature to achieve combustion.
What happens when flue gases are recycled and exchange heat?
Fuel coming into the chamber is hotter that the recycled flue gas, therefore the peak flame temperature will decreased.
Why is it important to consider adiabatic flame temperature?
The dissociation of fully oxide species become significant, which is an endothermic reaction which become more favour at higher temperatures. Some of the fully oxides species can reduce. Can oxide nitrogen which performs NOx, so need to consider how hot your flame is as you do not want to produce NOx as it makes acid rain.
The 6 main factors contributing to complete combustion of carbon?
- Fuel reactivity
- Fuel fineness and particle size
- Efficiency of fuel-air mixing
- Excess air available for complete combustion
- Residence time
- Temperature profile within the boiler
What are products of fuel impurities?
SOx, H2S, NOx and fly ash
The 4 methods to deduce the formation of NOx during combustion?
- Reducing the conc of oxygen in the combustion zone
- Reducing the conc of nitrogen in the combustion zone
- Reducing the peak combustion temperature
- Reducing the amount of time that the combustion gases remain at the high temperatures
What is a large consideration to take onboard when reducing excess oxygen and residence time?
This will reduce the overall performance of the combustion, which can impact the presents of PM. Goal should be to reduce NOx formation as much as possible while impacting efficiency as little as possible.
Reducing SOx from combustion reactions?
Controlling Sulphur dioxide emissions is best achieved by removing it from the fuel before or removing it from the flue gas after combustion.
SO3 should be avoided as it leads to corrosion of the boiler and equipment.
Why would you recycle the flue gas around?
Particularly good when lower temperature as this will dilute the flame temperature. This will also reduce NOx and reduce the number of fine particles coming out of the flue stack.
