Advantages and Disadvantages of Underfeed Stoker

In this article, I will share the advantages and disadvantages of different types of underfeed stokers.


If you want to know more about underfeed stoker you should read the article below.


You might like this: Overfeed & Underfeed Stokers: Classification and Working Principles


In underfeed stokers, the fuel is fed from the bottom while the air is fed from the top or sides depending on the type of stoker used.


Without wasting time, let me share what you need to know about the advantages and disadvantages of underfeed stokers.


Advantages of single retort stoker

1. It produces very minimal smoke.

In any industrial production, we are required to take care of the environment by minimizing emissions to the atmosphere.


You might like this: Methods Of Controlling Gaseous Pollutants (Ultimate Guide)


If the stoker used in a boiler produces a huge amount of emissions it’s going to cost the production processes even more since there will be a need to deal with these gaseous pollutants.


But with a single retort stoker, the amount of smoke produced is very minimal and the need of controlling gaseous pollutants won’t be of any need.


Single retort stoker produces minimal smoke since the coal is fed from the bottom which ensures that the volatile matter, moisture and air must pass through the active burning nozzle.


This action tends to facilitate complete combustion and that is why very minimal smoke is produced during combustion.


2. High thermal efficiency.

This is another advantage of single retort stoker that makes it amazing for use in boilers.


High thermal efficiency is needed for the generation of steam in any steam boiler.


It’s advantageous to use a single retort stoker since it produces a very hot flame that leads to high thermal efficiency.


The high thermal efficiency in the single retort stoker is due to the complete combustion that happens at the grate.


The air supplied from the sides or bottom of the retort goes through all the layers of coal facilitating complete combustion.


3. Suitable for variable loads.

Single retort stoker is suitable for variable loads of fuel.


This is advantageous since one can use any amount of coal in this stoker without any issues.


The fuel is fed from the bottom, and a small volume of the coal is fed by the feed worm on the grate from the hopper or fuel supply cylinder.


The hopper may be filled with coal and this may not affect the working of the stoker since it’s not the whole coal that is fed on the grate.


The coal may be of a large volume or small, the single retort stoker will just work fine.


Disadvantages of single retort stoker

1. It causes the segregation of coal.

Single retort stoker causes segregation of coal and this problem will end up affecting the quality of heat produced.


Segregation happens when the fine coal is made from big chunks of coal during movement along the length of the retort.


When the coal is being taken to the grate, there is a change of direction several times along the retort until it reaches the grate, this is enough to cause segregation.


Also, the feed worm, as it pushes the coal to the grate, will increase the friction between the coal and the feed worm, and segregation will eventually happen.


Segregation reduces the quality of the heat produced, it will contain smoke and the thermal energy will be lower.


So, when one wants to use a single retort stoker this is an issue that one should put in mind.


It’s almost impossible to prevent segregation in a single retort stoker.


2. The distribution of coal on the grate is hard.

The distribution of coal on the grate is hard because of the feeding mechanism of the single retort stoker.


The coal is fed from the bottom, and it moves through a single retort to the grate.


The coal that is pushed to the grate will be accumulated at the centre point of where the coal was fed.


There is no action of distributing the coal on the grate. The heat produced will be mainly from the centre point, where the coal is highly accumulated.


Even quantity distribution of coal is essential for the heat to be produced evenly around the grate so that the heat is not focused only on one point of the boiler.


Apart from the grate issues other issues that cause distribution problems in single-retort stokers are the moisture in coal and some of the characteristics of coal such as size.


This is a problem that one should expect when using a single retort stoker.


3. Can’t sufficiently handle fine coal.

The coal used in the single retort stoker has to be of a particular size that can be conveyed by the feed worm.


If the coal used is fine then it will be hard for the coal to reach the grate since it will slip between the feed worm as it moves along the length of the retort.


The result is that only a small volume of coal will reach the grate.


And as I shared with you earlier on the point of segregation; fine coal produces poor-quality flame in this type of underfeed stoker.


So, the coal selected to be used in this type of stoker has to be verified if it’s efficient enough for use.


Advantages of multi-retort stoker

1. Self-cleaning stoker.

This is one of the advantages of multi-retort stoker.


You may ask me how this stoker cleans itself. The issue here is not about cleaning itself, it’s the movement of the coal along the pushes that makes this possible.


As long as the pushers keep on distributing the coal along the length of the stoker, the pushers will always be clean due to the friction between the coal and the pusher.


The other thing that contributes to the self-cleaning aspect is that the tuyers and retorts aren’t exposed to high temperatures as they remain in contact with fresh coal every time they work.


2. Automatic control can be adopted.

Automatic control of the feeding mechanism can be adopted in this type of stoker.


This is very essential since there won’t be the extra work of checking the stoker.


In multi retort stoker, there is the stoker ram that is connected to the pusher.


Automating the stoker ram will enable the control of the feeding mechanisms of the ram and also the amount of energy produced by the stoker.


You have to understand that the amount of energy produced by the stoker is directly proportional to the amount of coal being supplied inside the stoker.


3. Responds quickly to changes in demand.

Whenever there is an increase in demand for the amount of energy needed, the multi-retort can easily increase the feed of coal in the stoker.


The only thing that will be needed when the demand is higher, is just the stoker ram to increase the load that it supplies to the stoker.


This is different from different stokers; when you compare this situation to a chain grate stoker, it’s going to be hard for the chain grate stoker to respond quickly when there is a demand.


An increase in the coal load on the fuel bed of the coal will only result in insufficient combustion since air supply will be limited in a deep bed of coal.


With a multi-retort stoker, it’s easier because of the pushers that offer uniform distribution of coal along the length of the retort.


4. It can use coal directly from the mines.

The coal that is used in multi retort stoker doesn’t have to be checked very keenly like the one used in single retort stoker.


This is possible because it pushes and spreads the load without issues.


The coal size used in the stoker is not a strict consideration and this makes it easier when it comes to feeding the stoker.


But if the coal is too big, it will have to be crushed to a considerable size.


5. High combustion efficiency.

High combustion efficiency is another advantage of a multi-retort stoker.


The high combustion efficiency is brought about by the ability of the pushers to perfectly distribute the coal which results in sufficient circulation of air.


With enough air supplied through the tuyers, the combustion rate increases thus the high efficiency.


This advantage makes it suitable for middle-size plant applications.


Disadvantages of multi-retort stoker

1. Operation and maintenance are expensive.

The operation of the multi-retort stoker is expensive since it mostly operates successfully on graded coals.


When it comes to the use of graded coals, makes the operation of this stoker expensive.


Much money will be used on the purchase of this coal which will add up to the cost of operation.


The maintenance of the multi-retort is also expensive since many parts are involved in the movement of the fuel.


When you compare it to a single-retort stoker, a multi-retort stoker has many parts that will need to be replaced.


For instance, when we look at pushers that are worn out they will have to be replaced, and it’s not only one pusher that will be replaced.


This makes the maintenance expensive.


2. The initial cost is higher compared to a single retort stoker.

The design of multi retort stoker makes it expensive when it comes to initial cost.


The fact that it has higher thermal efficiency compared to a single retort stoker, makes it relatively expensive when it comes to purchasing.


The complicated construction that multi-retort has also makes it expensive when it comes to its initial set-up.


3. Not suitable for low-quality fuels.

Multi-retort stoker works successfully with graded coal. Low-quality coal will not burn sufficiently in this type of stoker.


This is a disadvantage because the choice of coal used in the stoker will affect the operation process if not enough resources are available.


If low-quality fuels are used in multi retort stoker the quality of heat produced will be low and this will affect the steam generation process.


4. Wear and tear of pushers.

Wear and tear of pushers is a problem that can’t be avoided in this type of stoker.


Since the pushers are constantly used to distribute the coal along the length of the stoker, they are fond to wear and tear due to friction.


Especially, when the demand is high, the speed of the feed and distribution increases and this will end up overloading the pushers.


Final thoughts


From what I have shared in this article, I hope you have a grasp of which stoker you should go for depending on your needs.


If you want to know more about the working principle of the underfeed stokers you should read the article below.


You might like this: Overfeed & Underfeed Stokers: Classification and Working Principles


Before choosing any stoker, you have to ensure that it can handle the load that you require, know its limitation and how effective it is.


This is what will save you from the headache when it comes to steam generation.


For the boiler to be effective, you need a good heating system.


Thanks for reading, leave a comment if you have a question. See you in my next article.


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  5. Type Of Pipe Joint Selection In a Steam Distribution (Considerations)

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