In this article, I will share several methods of controlling gaseous pollutants and how they work.
When a fuel is burnt gasses will be produced and disposed to the atmosphere.
These gasses usually contain different pollutants depending on the type of fuel being burnt.
In most cases, you will find that gasses contain carbon monoxide, nitrogen and sulphur.
If these types of elements are left to escape to the atmosphere they will increase the effect of greenhouse gasses and this will increase global warming.
The same gaseous pollutants are not good for the ozone layer.
We also have respiratory problems and acid rain, these issues are going to make the earth inhabitable.
And that is why it’s necessary to clean the gasses that are produced from combustion before we dump them into the atmosphere.
Without wasting time, let me share with you what you need to know about the methods of controlling these gaseous pollutants in the atmosphere.
Methods of controlling gaseous pollutants in the biosphere
There are many methods used to control gaseous pollutants in the biosphere but I will talk about the common ones.
These are the methods of controlling gaseous pollutants in the biosphere;
- Absorption.
- Adsorption.
- Condensation.
- Incineration(combustion)
1. Absorption method of controlling gaseous pollutants
Absorption happens when the gasses are completely dissolved in another matter, in this case, liquid.
The liquid absorbing the gaseous pollutants may either be reactive or non-reactive.
That is why this method of controlling gaseous pollutants is further divided into two types namely;
- Physical absorption.
- Chemical absorption.
Physical absorption doesn’t involve chemical reactions for the pollutants to be absorbed. A good example is when oxygen is dissolved in water.
This doesn’t require any chemical reaction for the absorption to happen.
The main aspect that is considered here is the solubility of the pollutants to the liquid, temperature and pressure.
Chemical absorption on the other hand depends on chemical reactions for the absorption to occur.
Bringing the gaseous pollutants in contact with the liquid is not going to have any effect unless there is a reaction.
A good example is carbon dioxide (CO2) dissolving in a solvent forming a rich liquid.
Carbon monoxide dissolves in water forming carbonic acid.
The liquid absorbing the gasses is the absorbent while the gasses being absorbed (pollutants) act as absorbates.
This is how the absorption method of gaseous pollutant control happens; when the gaseous pollutants are passed through the liquid (absorbent) the gaseous components that contain the pollutants are dissolved completely in the liquid while clean gas (without contaminants) leaves the liquid.
Here are ways to increase the efficiency of absorption of gaseous pollutants in the absorbers;
- Large surface contact area: the larger the surface contact area the greater the absorption rate of the gaseous contaminants in the absorber. This means that the absorber should be wide enough for a large volume of gasses to be absorbed.
- The time of contact: the longer the gaseous pollutants are in contact with the absorbents the higher the rate of pollutants absorption will be absorbed.
- The concentration of the absorbing liquid: the higher the concentration of the liquid absorbing the gaseous pollutants the higher the rate of absorption. Liquid with a low concentration will not allow maximum absorption of the gaseous pollutants.
- The speed of the reaction: if it’s chemical absorption, the higher the rate of reaction between the gaseous pollutants and the liquid the higher the rate of absorption.
These are some of the equipment used in controlling gaseous pollutants by the method of absorption;
1. Packed tower.
The packed tower has a counter flow of gasses and liquids just like in cooling towers.
The gasses are fed into the tower through a bottom inlet and the liquid trickles from the top of the tower.
As the gasses rise up while the liquid trickles down, the gaseous pollutants are absorbed by the liquid falling down the tower.
The clean air leaves the packed tower through the top vent while the liquid that has absorbed the gaseous pollutants is released from the tower as wastewater.
2. Plate tower.
This tower has plates that are perforated and fitted at several distances in the tower.
The perforation on these plates is to allow the gasses to pass through them while heading to the top of the tower.
These plates contain liquid that is used to absorb the pollutants from the gasses.
The gas is fed from the bottom of the tower as usual and as it passes through these plates, it comes in contact with the liquid within the plates and the gaseous pollutants are absorbed.
The gasses still proceed to the next plate and the process is repeated within the plates until they finally leave the tower.
You may ask me, why does the liquid in the perforated plates doesn’t trickle down the tower? The liquid doesn’t trickle down the tower since there is the pressure created by the gasses flowing via the plates at a very high velocity.
This equipment is suitable for insoluble gaseous contaminants.
3. Spray tower.
This tower works on the same principle as the packed tower but the only difference is that the spray tower has a nozzle that sprays the liquid (absorbent).
The gasses are fed into the tower through the bottom inlet and as they raise up they come into contact with the sprayed liquid which absorbs the gaseous pollutants and leaves clean air leaving the tower.
The spray tower is not that effective since there is less area of contact between the liquid and the gaseous pollutants.
2. Adsorption method of controlling gaseous pollutants
Adsorption is simply the assimilation of one state of matter on the surface of another matter.
The gaseous pollutants in this method are adsorbed on the surface or inside the adsorbents.
Solids such as activated carbon, silica gel, zeolite, aluminium etc are used in this process of adsorption.
The treatment units used in the adsorption process are called adsorbers.
Adsorption also may be physical or chemical as in the absorption method.
This process of adsorption to happen effectively depends on the available surface area of the absorbent and chemical reactivity of the gasses and their bond-forming capacity with the surface of the solid adsorbent.
The adsorption units are of two types:
- Regenerative
- Non-generative
1. Regenerative adsorption unit.
It has two cylinders that are connected and both have a bed of adsorbent (carbon).
The gasses pass through the first tank, the gaseous pollutants are adsorbed in the carbon bed and the remaining volume of gasses is directed to the next cylinder to be cleaned too.
This ensures that the air is sufficiently cleaned of contaminants. Only the treated air will leave the last cylinder.
When the carbon beds are saturated with contaminants they are removed and replaced.
3. The combustion method of controlling gaseous pollutants
This method involves heating up the gasses at a rapid rate that increases their temperatures.
The main reason why these gaseous pollutants are heated is to chemically oxidize them into inert gasses such as carbon dioxide and hydrocarbons.
Combustion of gaseous pollutants removes odour, toxic substances from the gasses and reactive materials.
One important requirement that the combustion method should satisfy is; complete combustion.
Complete combustion of these gaseous contaminants is necessary to prevent the formation of intermediate products which will also need to be handled.
Plenty of oxygen is needed to come in contact with the combustible materials at high temperatures, long residence time and enough turbulence.
There are three types of combustion processes that you should know;
- Direct combustion.
- Thermal incineration.
- Catalytic combustion.
Direct combustion/direct incineration; Direct combustor is used to facilitate the combustible waste gasses to react at the burner. This device (direct combustor) offers complete combustion since there is no residence chamber. Flare is used to control the emissions containing volatile organic compounds.
Thermal incineration; The waste gasses are passed around the burner first before they are directed to the residence chamber where oxidation happens. Since the waste gasses are passed around or over the burner this increases the oxidation rate in this type of combustion and that is why it has high efficiency of 99%.
Catalytic combustion; This type uses catalysts such as Vanadium, Platinum, Palladium and their oxides to alter the rate of chemical reaction that results in combustion. A mixture of diluted waste gasses and oxygen is exposed to a catalytic surface where the chemical reaction reduces the rate of temperature required to oxide the organic compounds.
Catalytic combustion is highly preferable because it doesn’t require much energy to heat the gaseous pollutants.
4. Condensation method of controlling gaseous pollutants
This method of controlling gaseous pollutants is based on reducing the temperature of the gasses.
When the temperatures of the gasses are reduced they will turn into liquid.
This method is used when the pollutants from the gas steam has a higher dew point than the non-pollutant gas.
This will enable the gaseous pollutants to turn into a liquid while the clean gasses are taken to the atmosphere. The liquid will be disposed of as wastewater.
Condensers are used in this method of controlling gaseous pollutants.
This is how the condensers work when controlling gaseous pollutants; when the warmer waste gasses come in contact with the cooling medium in the condenser, the heat is transferred to the cooling medium while the gaseous pollutants cool down.
The moment the gaseous pollutants cool down in the condenser the kinetic energy of these gasses is reduced, this causes the pollutants to slow down and clump together to form the condensate.
Only the clean gas will leave the condenser, while the condensate of the gaseous pollutants will be disposed of from the condenser as wastewater.
The heat transferred to the cooling medium can be used to heat the feed water or any other important uses.
With this method of controlling gaseous contaminants, you get to get recover energy and at the same time clean the gasses leaving the stack.
You might like this: Type Of Heat Losses In a Steam Plant (+How To Minimize Them)
There are two types of condensers used in the condensation method of controlling gaseous pollutants;
Direct contact condensers; the gaseous pollutants are mixed with the cooling medium. This makes the whole cooling medium contaminant. The cooling medium will have to be replaced.
The advantage of this type of condenser is that it’s cheaper and the disadvantage is that it requires a large volume of cooling medium since replacement has to be done every time the condenser cleans gasses and also has disposal issues of the contaminated cooling medium.
Indirect condenser (surface condensers); the cooling medium doesn’t mix with the contaminated gasses. An indirect condenser just works like an economizer or radiator.
This type of condenser is widely used since it doesn’t need much attention compared to the direct contact condenser.
But an indirect condenser can’t be used as stand-alone equipment for controlling gaseous pollutants because it’s not that effective.
The selection of the condenser depends on the goal, composition type of coolants available and condensate recovery.
Final thoughts
These are the common types of methods used to control gaseous pollutants.
It’s essential that the air is cleaned to keep the biosphere safe and sound for the human population and animals.
There is one method that I haven’t touched on, which is carbon sequestration. Carbon sequestration is used in gasses that contain carbon dioxide as their main pollutant.
This method involves long-term storage of carbon dioxide underground or on the surface of the earth to be used by trees instead of being left in the atmosphere.
This method is expensive and that is why most plants rarely use it.
Anyway, what I have shared in this article is enough to give you an idea of what happens when it comes to the control of gaseous pollutants.
If you want to know more about other methods used you should do more research I’m sure there are more methods I haven’t covered in this article.
Thanks for reading this article, if you have any questions you should leave a comment.
See you in my next article.
