Pea coal—what fuel parameters are important for proper boiler operation?

Pea coal—what fuel parameters are important for proper boiler operation?

Pea coal is not a fuel with a single, consistent set of properties. Two batches may differ in particle size, moisture content, ash content, caking tendency, or calorific value. In a boiler with automatic fuel feeding, these differences affect not only the amount of energy produced but also the stability of combustion, the operation of the burner and feeder, and the amount of combustion residue.

Therefore, the documentation for the specific unit should serve as the primary point of reference. A good example is the Eko Vegas II boiler’s technical manual (DTR), which specifies both the type of fuel and its selected parameters. It is the fuel’s compliance with the boiler’s requirements that is more important than searching for a single “best” parameter.

Key Information

• First, check the boiler’s technical data sheet (TDS). Particle size range, moisture content, and other requirements may vary between units; therefore, fuel parameters should not be selected based solely on general guidelines.
• Particle size has mechanical implications. In a boiler with a fuel feeder, the fuel must be compatible with the fuel conveyance system and the burner design.
• Moisture content affects the combustion process. The Eko Vegas II Technical Data Sheet (DTR) indicates that wet fuel can cause problems with automatic ignition.
• Ash content and caking tendency affect furnace operating conditions. A high tendency to form clumps or a larger amount of combustion residue can make it difficult to maintain stable operation.
• Calorific value is important, but it is not sufficient for evaluating fuel. A high calorific value does not guarantee that a given batch of fuel will work well with a specific burner and feeder.

Why are the characteristics of pea coal important for a boiler with a feeder?

In an automatic boiler, the fuel goes through several successive stages: it is drawn from the hopper, transported by a feeder, fed into the furnace, and then burned with a controlled air supply. Each of these stages may be affected by different fuel properties.

A publication by the Institute of Mineral Resources and Energy Management of the Polish Academy of Sciences (PAN) on qualified coal fuels highlights the need for consistent physicochemical parameters as well as low ash and sulfur content and low caking tendency in fuels intended for modern retort boilers. In turn, current data from the Southern Coal Group show that even fuels belonging to the same commercial group may differ in calorific value, caking properties, moisture content, or ash content.

In practice, not every pea coal will be suitable for every boiler, as its specific parameters are what matter. A good point of reference is pea coal boilers with a feeder, but the requirements of a specific model must always be compared with its documentation.

Particle size – the fuel must be compatible with the feeder and the burner

Grain size determines the size of the fuel particles. In a boiler with automatic feeding, it has a direct impact on the transport of fuel from the hopper to the furnace. The range suitable for one unit is not necessarily appropriate for another.

The Technical Data Sheet Eko Vegas II specifies type 31.2 hard coal as the primary fuel, with a particle size range of 5–32 mm. By comparison, current PKW product data sheets list pea coal with a particle size of 5–25 mm. These values clearly illustrate why it is not advisable to assume a single universal range for all devices and all products.

Particles that are too large, an excess of fine material, or foreign objects can interfere with the smooth operation of the feeding system. If the mechanism begins to operate abnormally, it is advisable to check not only the settings but also the fuel and the mechanical condition of the equipment. One sign that an inspection is needed is when [the screw feeder squeaks.] (https://stalmark.pl/en/tips/screw-feeder-squeaks-causes-and-effective-solutions)

Moisture—Why can wet pea coal impair boiler performance?

When heating coal, one of the first steps is to evaporate the moisture it contains. This means that the water present in the fuel must first be heated and removed as steam before the combustion process can proceed to the next stages. The more moisture the fuel contains, the more difficult it is to maintain consistent operating conditions.

The Eko Vegas II technical documentation states that fuel moisture content should not exceed 11%, and that the fuel loaded into the hopper should be dry. The documentation also warns that wet fuel may cause problems with automatic ignition.

This shows that moisture content is not merely a laboratory statistic. It can have a direct impact on ignition and the stability of the combustion process. However, this parameter must always be considered in relation to the requirements of a specific boiler, rather than a single value deemed universal for the entire market.

Ash content—What does it reveal about the fuel and boiler maintenance?

Ash is a non-combustible residue left over from the combustion process. From the user’s perspective, the amount of ash primarily affects how often the ash pan needs to be emptied and the unit cleaned. More ash means more residue that needs to be dealt with later.

The DTR Eko Vegas II specifies the ash content for approved fuel at 2–7%. The documentation also indicates that the amount of ash and the rate at which the ash pan fills depend on the quality of the fuel being burned. In turn, a publication by the Polish Academy of Sciences (PAN) identifies low ash content as one of the characteristics of qualified coal fuels intended for retort boilers.

However, this does not mean that pea coal should be evaluated based on a single parameter. A low ash content will not compensate for fuel with an inappropriate particle size, excessively high moisture content, or properties unsuitable for the burner in use.

RI Sinterability – Why can hard sintered deposits form in the burner?

Sinterability describes the ability of coal to form compact structures when heated. In technical literature, this parameter is often described by the Rogi number, or RI. The greater the fuel’s tendency to sinter, the more important the burner design and the combustion process control become.

Grzegorz Ojczyk’s doctoral dissertation from the Krakow University of Technology shows that different retort burner designs have varying tolerances for sintering fuels. In the case of conventional burners, the requirements are more restrictive, whereas solutions featuring a different fuel feed method or a moving grate may be better suited to handle fuels with higher sintering potential. Therefore, there is no single RI value that can safely be considered appropriate for every boiler.

Research by the Central Mining Institute further showed that, in the tested system, certain types of eco-pea coal—despite their good energy properties—caused slag formation and operational problems in the hopper and the fuel-feeding mechanism to the retort burner. This is an important point: high energy quality does not rule out problems arising from other fuel characteristics.

The Eko Vegas II Technical Data Sheet (DTR) does not specify a separate RI limit. Therefore, such a requirement should not be added based on data from other burners. If clinkers appear in the furnace, the fuel properties, boiler settings, and combustion conditions must all be analyzed simultaneously.

The caloric content of pea coal—important, but not the most important factor in and of itself

The calorific value indicates how much energy is contained in a unit of fuel mass. The Eko Vegas II Technical Data Sheet specifies a calorific value of over 28 MJ/kg for approved fuel. However, this does not mean that a high MJ/kg value alone guarantees proper boiler operation.

An example from GIG research shows that good energy properties can coexist with undesirable fuel behavior in the burner and the feeding system. Therefore, calorific value must be considered in conjunction with the device’s other properties and requirements. Detailed information on what coal calorific value is and how it affects fuel consumption can be found in a separate guide.

Consistency of parameters – Why doesn't the same fuel name mean identical properties?

One of the goals of certified fuels was to ensure physical and chemical properties that were as consistent as possible. This is important in automated boilers, because fuel and air feed control works best when the fuel’s behavior does not vary drastically from batch to batch.

Current data from PKW provide a good example. The pellets offered by PKW have a uniform particle size of 5–25 mm, but differ in terms of, among other things, their declared calorific value, caking resistance, and sulfur content. Moisture content and ash content are also specified as ranges. This shows that the term “peas” describes a group of fuels, not a single, identical product.

Therefore, when changing suppliers or switching to a specific fuel, it is advisable to monitor the boiler’s performance and compare the parameters of the new batch with the equipment’s requirements. Do not assume that the new fuel will behave exactly the same simply because it has the same trade name.

How do fuel parameters interact with the boiler's automatic control system?

In the Eko Vegas II, the combustion process takes place in a cast-iron retort burner, to which fuel is transported by a screw feeder. The controller manages the operation of the feeder and the air supply, and the settings must be adjusted individually based on, among other things, the type of fuel used and the boiler’s output.

This is an important relationship: changes in fuel parameters can alter the behavior of the combustion chamber even when the boiler itself remains the same. Therefore, combustion problems should not be diagnosed based solely on a single number from the fuel data sheet. Fuel feeding, air supply, and the condition of the combustion chamber must also be taken into account.

The practical guidelines described in the guide “How to Burn Eco-Coal Properly” are also helpful when setting up the boiler. However, they do not replace the instructions for a specific unit.

Example: What type of fuel is specified in the Eko Vegas II documentation?

The documentation for a specific unit best illustrates why it is not worth looking for a universal definition of “good pea coal.” The Eko Vegas II documentation specifies Type 31.2 hard coal as the fuel, specifically the “pea coal” grade with the following parameters:

• particle size: 5–32 mm,
• moisture content: no more than 11%,
• calorific value: above 28 MJ/kg,
• ash content: 2–7%,
• volatile matter content: above 15%.

These values should not be automatically applied to every pea coal boiler. They are requirements for a specific model, and this is precisely how the Technical Operating Manual (DTR) should be used: as a specification for that particular unit.

The Eko Vegas is also available. Before using a specific fuel in this or any other model, you must refer to the documentation specific to that device, not to the parameters provided for the Eko Vegas II.

How do you evaluate pea coal before using it in a boiler?

Instead of looking for a single parameter to determine fuel quality, it’s better to perform a quick compliance check. The most important thing is to ensure that the fuel’s overall set of properties matches the equipment’s requirements.

1. Check the fuel types approved in the operating manual. Start with the type of coal and the range of parameters specified in the boiler’s documentation.
2. Compare the particle size. In an automatic boiler, this affects the fuel’s flow to the burner.
3. Check the moisture content. Wet fuel can make it difficult to ignite and disrupt consistent combustion.
4. Pay attention to ash content and caking tendency. These characteristics affect the amount of residue and how the fuel behaves in the furnace.
5. Evaluate the calorific value along with other parameters. More energy per kilogram will not solve the problem of fuel that is not suited to the burner.
6. Monitor the boiler’s operation after changing the fuel. Switching to a new batch may require adjusting the settings according to the controller manual and technical documentation.

The most common errors in assessing pea coal

• Selecting fuel based solely on calorific value. A high MJ/kg value does not tell the whole story about how the fuel behaves in a retort burner.
• Assuming a single “ideal” RI for all boilers. Tolerance for caking depends on the burner design; therefore, the equipment documentation must serve as the reference point.
• Ignoring moisture content. Wet fuel can hinder automatic ignition and alter the combustion process.
• Assuming that every pellet has the same properties. Market data show differences between products belonging to the same fuel group.
• Changing the fuel without monitoring the boiler’s operation. Automation helps stabilize the process, but the settings must still be tailored to the specific fuel and equipment.

FAQ – Pea coal and fuel specifications

Is every type of pea coal suitable for every boiler with a feeder?

No. Pea coal can vary in particle size, moisture content, ash content, caking properties, and calorific value. The fuel must be compared with the technical specifications (DTR) of the specific boiler.

Is a lower RI (caking index) always better?

There is no single answer that applies to all devices. Technical literature shows that different retort burner designs tolerate different fuel properties. The requirements of a specific burner are more important than a universal number.

Can wet pea coal cause problems with the boiler?

Yes. The technical documentation for the Eko Vegas II specifies that the fuel should be dry and warns that wet fuel can cause problems with automatic ignition.

Does a high calorific value mean the fuel will burn well?

Not always. The calorific value describes the amount of energy in the fuel, but it does not tell the whole story about its caking tendency, moisture content, ash content, or compatibility with a specific burner. That is why the parameters must be analyzed together.

What should you do if clumping or incomplete combustion occurs after switching fuels?

First, compare the new fuel’s parameters with the boiler’s technical documentation, inspect the condition of the furnace and the feeder, and check the controller settings. Do not assume that every problem stems solely from a single fuel parameter.

Summary

Pea coal should be evaluated based on a set of parameters that collectively influence its behavior in the boiler. Particle size determines how well it works with the feeder, moisture content can make ignition difficult, ash content affects the amount of residue, and caking tendency is important for how the fuel behaves in the retort. Calorific value is important, but it does not replace the other data.

The safest rule is simple: first check the boiler’s technical specifications (DTR), then compare the parameters of a specific fuel. This way, instead of searching abstractly for the “best pea coal,” you can select a fuel compatible with the equipment and more easily maintain stable operation of the entire system.

Sources cited in this article

1. STALMARK – Technical and Operational Documentation for Eko Vegas II
2. K. Stala-Szlugaj, “The Development of the Market for Qualified Coal Fuels in Poland,” Gospodarka Surowcami Mineralnymi – Mineral Resources Management, Polish Academy of Sciences (PAN), 2017
3. E. Orszulik, L. Dudek, “The Use of Solutions to Aid the Combustion of Eco-Coal in Grate Boilers Equipped with a Retort Burner,” Central Mining Institute, 2010
4. G. Ojczyk, “Research and Modeling of the Operation of a Multi-Fuel Boiler Fired with Wood Pellets,” doctoral dissertation, Kraków University of Technology, 2023
5. Południowy Koncern Węglowy S.A. – Groszki Plus Fuels: Current Parameters of the Offered Fuels