Overfeed & Underfeed Stokers: Classification and Working Principles

In this article, I will share the classification of overfeed and underfeed stokers and their working principle.


There are two common methods used in burning coal to provide heat energy to steam boilers or any other processes that require heat.


The first one is stoker firing which I’m going to be talking about in this article and the second one is pulverized fuel firing.


If you want to know more about pulverized fuel firing you should read the article below.


You might like this: Pulverized Fuel Burner: Components, Types, Requirements and Working Principle


Without wasting time let me share what you need to know about overfeed and underfeed stokers.


The main difference between overfeed and underfeed stokers is the feeding mechanism.


In overfeed stoker, the fuel is fed from the top while the air is fed from the bottom by a draught fan while in underfeed the fuel is fed from the bottom and the air is fed from the top or side depending on the stoker in use.


1. Overfeed Stoker

In overfeed stoker as the name suggests, the fuel is fed by the hopper from the top and falls on the grate.


The grate has air holes in which the primary air is blown through from the bottom by a draught fan and passes through the grate via the holes.


The draught fan is used to increase the turbulence and allow combustion at a higher rate.


The high the velocity of air through the air holes under the great the higher the combustion rate.


Secondary air is supplied over the surface of the fuel bed to increase the combustion of the fuel at the top.


The primary air that is blown from the bottom of the grate is preheated as it passes through the grate openings from the bottom, this increases the thermal energy thus facilitating proper combustion.


The ash that is produced settles at the bottom of the grate and dust will have to be collected.


Overfeed stokers are divided into two;

Travelling grate stokers.

  1. Chain grate stoker.
  2. Grate bar stoker.

Spreader stoker.

  1. Single retort.
  2. Multi retort.


Travelling chain grate stoker.

The grate surface is made of a chain that continuously travels with the aid of two sprocket wheels.


The chain is fitted on these sprocket wheels and the front one is driven by a variable mechanism.


This variable mechanism controls the speed of the chain, which controls the amount of heat produced as the chain rotates around the sprocket wheels.


The primary air is fed through the bottom of the grate while the secondary air is fed over the fuel through the secondary airports.


Working principle of travelling chain grate stoker

The fuel falls by gravity from the hopper and is fed on the top of the travelling chain.

The bed of the coal is adjusted by an adjustable fuel gate to ensure the bed is of the required depth.


If the depth of the coal bed is deeper then complete combustion will be impossible since the amount of air supplied will be limited.


The primary air enters the grate from the bottom with the help of a draught fan.


This air volume in the grate is divided into several sections to ensure that it’s distributed evenly over the whole chain bed.


Secondary air is then fed over the fuel through the secondary airports to facilitate fuel combustion at the top part of the bed.


The ash produced falls from the chain when it reaches the end of the sprocket will.


Travelling bar grate stoker

This type of stoker has moving bars that carry the fuel instead of the chain when compared to the chain grate stoker.


The bars have gaps in which the fuel is carried. Between the bars, some spaces allow the primary air to pass through from the bottom.


These bars are made of alloy structures depending on the kind of fuel they use.


The bars are of different shapes and sizes and are usually replaced once they wear out.


There are different types of grate bars used in travelling bar grate stokers;

  1. Pinhole grate bars.
  2. Four jaw-long hole grate bars.
  3. Live core fire grate bar.
  4. Large grate fire grate bar.


Working principle of travelling bar grate stoker

The bar grate stoker works under the same principle as the travelling chain grate stoker.


The only difference is that instead of the surface being a chain, the bar grate stoker has bars that carry the fuel to be burnt.


The gaps between the bars allow primary air to be supplied from the bottom of the grate and the initial heat from the bar is used to preheat the air entering the fuel bed.


Don’t forget that the air is blown by a draught fan to create proper turbulence that will facilitate proper combustion.


The secondary air is fed from the secondary airports on top of the fuel to allow proper burning of the fuel on the top of the grate.


Spread stoker.

The spread spreads fuel on the top of the fuel bed hence the name spreader.


It has quite a unique design when you compare it with the other stoker.


Don’t forget that it’s still an overfeed stoker.


It has a feeder and spreader which looks like a sprocket wheel since it has blades that aid it in moving the coal from the hopper.


The feeder feeds the fuel to the spreader and the spreader spreads it with force to the fuel bed.


The spread action by the spreader allows proper distribution of the fuel on the coal.


The grate still has air holes that allow the primary air from the bottom to reach the fuel bed.


Working principle of spreader stoker

The coal is fed by gravity from the hopper, but it doesn’t fall directly on the fuel bed.


The feeder will feed the coal to the spreader when it starts rotating. The spreader will spread the coal from the feeder as it’s rotating fast.


The fuel will be spread to the fuel bed, part of the fuel will be burnt while still suspended in the air as it’s being spread and the remaining coal will be burnt on the grate.


Primary air as usual is forced by a draught fan from the bottom, preheated in the grate before it reaches the fuel bed.


A stream of secondary air is fed from the top as the coal is being spread by the spreader which facilitates the combustion of the suspended coal in the air before it falls on the grate.


2. Underfeed stoker.

The underfeed stoker works differently compared to the overfeed stoker.


In underfeed stoker, the fuel is fed from the bottom and pushed up by the feed worm which forces the fuel up from the bottom of the retort in which it’s burned.


The air comes in contact with the raw coal and is mixed then the mixture is passed through the ignition and enters the incandescent coke region.


The gases flow out through the incandescent ash region to the surface of the fuel and are discharged to the furnace.


The primary air is fed either from the top or the side depending on the type of underfeed stoker being used.


These are two types of underfeed stoker;

  • Single retort stoker.
  • Multi-retort stoker.


Single retort stoker

It has a hopper which feeds the coal to the furnace, a cylinder fitted with a ram that supplies the coal, pushers that distributes the coal and tuyers that allow air to flow in the fuel.


Working principle of single retort stoker

The coal is fed into the cylinder and then forced into the bottom by the reciprocating ram with the feed worm. As the fuel moves up from the retort, it reaches the inclined grate.


The air is supplied through the tuyers on the sides of the cylinder and then taken to the grate as it passes through the bed coal until it reaches the top.


As the coal is being burnt ash is formed, and as the coal is being fed to the grate, the ash on the grate will fall into the ash pit since the grate is inclined.


Multi retort stoker

It has more than one stoker and hence the name “Multi” and this makes it preferable to a single retort.


It has a stoker ram that supplies the fuel from the hopper, retorts that are fitted with pushers to facilitate the uniform distribution of coal, tuyers are present to supply primary air and an extension gate that is used to provide a passage of the ash to the collecting pit.


The grate is inclined to allow easy movement of the coal along the length of the grate.


Working principle of multi-retort stoker

The fuel is fed by a hopper under gravity and the stoker ram pushes the coal forward the pushers. The pushers force the coal downwards the inclined grate through the length of the stoker evenly.


This allows every part of the grate to receive the fuel, reduces the bed depth of the coal and facilitates easy mixing of the coal with air.


Primary air is supplied through the tuyers to facilitate combustion.


The ashes are discharged along the extension grate.


Final thoughts


For you to be able to master all these types of stokers, you have to know their feeding mechanisms and how air is supplied.


The rest will be easier for you. I have shared the summary of how these stokers work but there is still more to learn about them.


We have the design requirements and advantages and disadvantages that I will share in my next article.


If you have any questions don’t be afraid to leave a comment.


Thanks for reading, see you in my next article.

  1. Advantages And Disadvantages Of Overfeed Stoker
  2. Advantages and Disadvantages of Underfeed Stoker
  3. Boiler Economizers: Function, Types, Design And Operation
  4. Effects of Impurities of Feed Water In Boiler (+Prevention)
  5. Steam Separators: Types, Working Principle And Advantages

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