Boiler Grate Stoker: How Do They Work In A Boiler?

coal fire grate bars

Do you want to know everything there is to know about boiler grate stoker? What are boilers, how does a boiler work, and how to repair it when the boiler grate burnout? If that is the case, then congratulations, you’ve come to the correct place. You might be surprised to learn that boiler grate stoker not only plays an essential role in industries but also plays an important role in our day-to-day activities.

Yes, boilers have an effect on the way we eat, the outfits we wear, and even the water we consume. But still, confused about what exactly grate stoker is?  Relax as now you no longer need to burn your 100 calories and spend several hours searching for a grate stoker. Because we will answer all your questions in this article. So, a 10 minute read to this article will be well worth your time. Continue reading to find out more about boiler grate stoker.

1.    What is the Grate Stoker in a Boiler?

A grate stoker is a form of mechanical stoker. It is built out of an unending chain that rotates and draws solid fuel into the boiler. Its main function is to support solid fuel in a boiler.

The boiler grate stoker is built in such a manner that air can enter the solid fuel for combustion.  It is primarily responsible for transferring an equivalent quantity of gas into the burners. Plus, it has certain size specifications dependent on its model and type. The thickness of the fuel must range from 1 to 10 inches. But the velocity of travel must be between 6 and 10 inches per minute.

Moreover, the boiler grate stoker also acts as a filter, preventing large particles from passing while allowing small particles to pass through the boiler. But the grate apertures should not be too large to enable unburned fuel partials to fall through. Still not too small to prevent enough air from passing through the fuel.

2.    What Are The Existing Types Of Boiler Grate Stoker?

Boiler grate stoker is classified according to the type of grate and the burning capacities.

The following are its most common types:

  • Stoker with a chain grate or a moving grate
  • Boiler stoker distributor
  • Stoker with a Chain Grate or a Moving Grate

(1)  Stoker With a Chain Grate or a Moving Grate

fire bar for boiler

In this grate stoker type, the Coal supplies through one side of a rotating steel chain grate. As the grate runs along with the height of the grate stoker burner, the coal burns before dropping off as ash. To achieve combustion with the least amount of carbon, some expertise is necessary. Especially when constructing the grate, air absorbers, and baffles.

(2)  Boiler Stoker Distributor

This type of boiler grate stoker uses a mixture of suspension and grate combustion techniques. The coal drops into the burner, which comes above a coal-burning bed. The coal particles burn in suspension. The ignition is almost instantaneous when the firing rate goes up. So this form of firing gives good versatility to match load changes. That’s why industries prefer the boiler grate stoker distributor over the other types of stokers.

grate stoker

(3) Travelling Grate Stoker

It’s a type of boiler grate stoker in which the coal is gravity-fed into a hopper at one end of a moving travelling grate. As the grate travels, it delivers a layer of clean fuel toward the burner. Yet, a solid fuel boiler travelling grate stoker must maintain consistent tension for trouble-free operation.

wood burning steam boiler

Moreover, it must avoid grate and grate chain obstruction. Additionally, gravity controls the pressure in the moving grate. The gravity pulls the grate’s weight which keeps the tension in place. The travelling grate discharges ash since it runs in a short period.

It’s best for medium-sized granulated fuels with a high ash percentage. Moreover, it is also chosen by processing factories that operate often. It allows for high levels of automated cleaning and fuel loading. Besides this, it reduced emissions and improved boiler efficiency.

3. Boiler grate stoker Types Based on Coal-Burning Capability

●      Class 1 Stoker

The coal-burning capability of this boiler grate stoker ranges from 10 to 100 lbs coal per hour.

●      Class 2 Stoker

The fuel-burning capacity of this stoker ranges between 100 and 300 per hour.

●      Class 3 Stoker

Their Coal-burning is from 300 to 1200 lbs per hour, 135 to 540 kg per hour.

●      Class 4 Stoker

This boiler Grate Stoker class has the highest burning capacity, over 1200 lbs each hour and over 540 kilograms.

So these grate stokers are designed to burn anthracite, bituminous, semi-bituminous, and lignite coal. However, its most common type used in household heating systems is a class 1 stoker.

Moreover, the other three types of boiler grate stoker are used in commercial and industrial heating systems. Subsequently, these strokers can easily remove ash, either mechanically or manually, depending upon which route you prefer.

4. Can You Describe the Boiler Manufacturing Process?

Different sheets, levers, and other raw materials are used to produce boilers. Aside from that, the boiler’s main manufacturing process breaks down into several parts. So, we’ve outlined the following stages in detail below for your convenience. Scroll down to learn about the boiler grate stoker manufacturing process.


The metal sheets, or any other material plates, are first chopped down into a flat shape. It is the most fundamental and critical phase in the manufacturing process. If it is not completed correctly, the entire process may get ruined. The metal goes through the rolling process after it cuts down into the proper shape.


In this phase, the work of a hydraulic roller is to roll flattened metal sheets, crush raw material, or slag. It also handles dry feed materials without the need for water during processing. In this step, the hydraulic roller rolls down the cutting flat sheet.


Now, it’s time to weld the rolled sheets. Moreover, we use X-rays in this process to inspect plate welds. Now, these welded sheets are sent to step 5 for tube alignment.


After that, we used a computer-controlled longitudinal milling machine to create the tube plate for perfect hole tube alignment. Welders then do hand welding with the relevant codes.


After that, all under-pressure welds are subjected to a non-destructive test (NDT). The furnace, combustor, and front tube sheet are installed in the boiler casing.


Then all the pieces are stacked on top of each other. After finishing the tubing process, the boiler grate stoker shell goes through a hydraulic test and examination.


After the examination, the workers connect the grate stoker to an oil or gas burner. The smokebox doors are then installed. Now the safety valves for steam exporting have been installed. After installing all internal components, it’s time to wire the external electrical components. Finally, the boiler grate stoker is now ready for use.

5. What Boiler Auxiliary & Boiler Accessories are Included in the Boiler System?

Economizer, deaerator, condensate, and damper are examples of boiler grate stoker equipment. A different instrument can be used depending on the job.

●      Economizers

A significant quantity of heat is included in the combustion gases from boilers of furnaces. By increasing the temperature of the material in the boiler, the heat in the flue gas is used to save fuel. The performance of the grate stoker increased from 3% to 8.5%. However, the performance is based on the boiler flue gas, temperature, and fuel type. ​

●      Deaerator

The two mixed gasses, o2 and co2, are usually found in boiler feedwater that is harmful to boilers. The deaerator is the only area for an extensive boiler feedwater system that removes these harmful gasses. It removes these gasses before they may escape into the boiler. This results in a reduction of corrosion in the furnace, steam tubes, condensation routes, and heat transfer equipment.

●      Condensate Vessel

It’s the container where the condensation from the grate stoker chamber and the process are collected. Moreover, clean water is added and kept for pumping into the deaerator as needed.

The delivered condensate is pumped into the container using a specifically built condensation system. This system allows it to safely and uniformly penetrate the tank. As a result, the amount of Flash steam reduced to some extent.

●      Damper

Dampers in gas streamlines are often two-way or three-way systems. Depending on the process needs, they’re regulated appropriately or turned on/off. It is commonly used for monitoring and management to prevent overheating of equipment. Plus, it provides smooth and efficient services. It must be built of proper equipment, with the required size and intermolecular properties of the fluid, so that the flow in the tubes is not blocked.

●      Feed Pump

In boiler grate stoker, feed pumps are used to provide high-pressure water. Because of the significant pressure differential inside the boiler, the feedwater must be heated before entering the grate stoker to ensure proper operation. The feed water pressure is typically 20% higher than the boiler’s pressure. The feed pump may be rotational or oscillatory.

●      Injector

The injectors serve the same purpose as feed pumps. They’re utilized when there isn’t enough room for a feed pump to be installed. In a converging-diverging valve, the kinetic energy of steam in an injector is employed to raise the pressure and velocity of feed water.

●      Superheater

The superheater boosts the temperature of saturated steam while keeping the pressure constant. It’s made up of a tiny tube bundle. It is positioned in the furnace’s hot flue gas route. Saturated steam passes through the heating tubes, while hot flue gases pass outside the tubes.

As a result, heat is transferred from hot flue gases to saturated steam, boosting its temperature without increasing steam pressure in a  grate stoker

6. How Come the Boiler Efficiency on an LHV Basis is Higher?

To understand how LHV affects boiler grate stoker efficiency, you must first understand the basics, such as what is LHV ?. What is the efficiency of a boiler, and how does LHV affect it? So, for your ease, we’ve discussed it below.

●      What is Boiler Efficiency?

Boiler efficiency refers to how well a boiler’s whole output of its variable components works. It’s the ratio of outlet steam’s total absorption heating value to the entire supply heating value. In other words, it’s a measure of how well a grate stoker is at a given stage and how efficient it is at completing tasks at various levels of thermal efficiency.

Moreover, older grate stoker systems have efficiencies in the range of 56 to 70 percent. Whereas modern conventional heating systems can attain efficiencies as high as 98.5 percent, turning nearly all of the fuel into practical heat for your desired work.

●      What Exactly is LHV?

What exactly is LHV? The quantity of heat radiated by burning a specific amount of gas and regaining the temperature of the combustion gases is referred to as LHV. It is defined as the amount of heat available from fuel after the latent heat of vaporization.

LHV can be calculated by subtracting the heat of water vaporization from the higher heating value. The temperature used to determine the volume corresponding to one mole, however, is 20 degrees Celsius.

●      How Is Boiler Efficiency On An LHV Basis Higher?

The efficiency of the boiler grate stoker elements has an impact on the overall efficiency. As discussed earlier, the LHV quantifies the enthalpy change of combustion with and without water vapor, respectively. So, the amount of moisture and other elements in the air impacts the heating value (LHV).

As a result, the LHV increases as the moisture content of the fuel decreases. Thus, in the burning, the specific heat capacity of water vaporization is lost. Moreover, the LHV of fuel falls as the oxygen percentage rises in the boiler grate stoker.

However, on the other hand, the LHV of fuel rises as the hydrogen concentration increases due to water combustion. Subsequently, the LHV of fuel increases as the sulfur concentration rises because sulfur dioxide gasses are easily absorbed by water.

Thus, LHV indicates the highest amount of recoverable heat, except in specialized equipment manufactured using unordinary materials. As a result, many practitioners choose LHV-based efficiency boilers.

7. What is the Difference Between an AFBC and CFBC Boiler?

According to reports, CFBC is becoming more common in the industry and commercial settings. First of all, it stands for circulating fluidized bed combustion boiler for those newbies who don’t know what CFBC is.

In CFBC, the burner is compressed, and the furnace air is recycled back to collect unburned carbon and improve the boiler grate stoker’s thermal efficiency. Whereas the term “atmospheric fluidized bed combustion”, AFBC refers to a process in which the burner pressure is equal to atmospheric pressure. As a result, CFBC is considered a better version of AFBC.

Moreover, some people look for an AFBC boiler, while others look for a CFBC boiler. But all folks are puzzled about one thing: are AFBC and CFBC the same thing, or what’s the difference between the two? So, let’s see how these two differ from one another. Moreover, When comparing the differences between an AFBC and a CFBC boiler, we can pay attention to the following factors:


AFBC  Boiler CFBC Boiler
Velocity 1.2 m/sec to 3.7 m/sec 3.7 to 9 m/sec
Heat Transfer Surface Heat transfer occurs within the combustion zone. Heat transmission takes place outside the combustion zone.
Coal’s Size 6-10mm 6-12mm
Air Input for Combustion 3-5 PSIG 1.5-2 PSIG
Coal Firing Rate 6000kg/hr 6250 kg/hr

After comparing the two, We may conclude that the CFBC boiler is more competitive than the AFBC boiler.

8. Why is Steel Used for Making a Boiler?

wood pellet fired steam boiler

Steel or traditionally cast iron is utilized in fire bars for boiler production because of the factors mentioned below.

They are:

  • Affordable
  • Widely available
  • Quickly molded
  • Welded to the necessary structure
  • Oxidation and corrosion-resistant
  • Use for long-term service

Stainless steel, especially iron alloy forms, is not utilized in moistened portions of boiler grate stoker due to rust and stress corrosion cracking. However, ferritic stainless steel is commonly found in overheated places that are not exposed to boiling water and electrical heating stainless steel casings. But, one thing that everyone wonders about is why steel is used instead of copper.

So, yes, the heat pipes in the boiler grate bar are composed of steel rather than copper. Although we know copper has high thermal conduction than steel, steel is more potent and can more quickly resist high pressure. One thing more, we know a steel-tube boiler will have more metal, but since steel is less expensive than copper, this isn’t a big deal?


As the number of installations of different types of Mechanical Stokers continues to grow, the necessity for specific literature for each type of boiler grate stoker becomes increasingly essential. We hope that after reading our article, all of your concepts about grate stoker are clear now.

As we’ve covered everything, you need to know about boiler grate stoker, from its basic definition to its functioning and types, and much more. Contact us if you have any further questions about it. We will surely assist you and get back to you as soon as possible.

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