Today, the growing demand for energy, coupled with environmental concerns, has made the optimization of combustion methods increasingly important. Therefore, premixed burners are known as an innovative solution, playing a crucial role in enhancing efficiency and reducing pollutants.
These burners are designed to uniformly mix fuel and air before entering the combustion chamber. This process leads to a more complete and efficient combustion that can help reduce fuel consumption and the emission of harmful gases. In this article, the structure, operation, advantages, and applications of these burners will be examined, along with their impact on various industries.
Combustion
The combustion process is scientifically and technologically categorized into three types:
- Premixed Combustion: In this method, fuel and air are mixed before the burner head. A chemical reaction occurs between fuel and air at the flame formation place. This technology is used in premixed burners.
- Nozzle mix Combustion: In nozzle mix or non-premixed combustion, fuel and air are completely separate before they reach the burner head. They come into contact and react at the flame formation place. The flame formed by this method is also known as a diffusion flame. This method is used in nozzle mix burners for burning fuel.
- Partially-Premixed Combustion: This method lies between the two previous types, whereby part of the combustion occurs as premixed and part occurs as non-premixed.
Advantages and Disadvantages of Premixed Combustion
Lean premixed combustion is a relatively new combustion technology, notable for its primary benefit of significantly reducing the emission of harmful pollutants like NOx and CO. This method is used in gas turbines and premixed burners. The term “lean” is used because, in this method, the air/fuel ratio is set to be higher than the stoichiometric air/fuel ratio. This adjustment leads to a complete combustion while simultaneously reduces the flame temperature and NOx emission.
One of the main disadvantages of premixed combustion is the increased likelihood of flame instability, which not only poses safety hazards but can also result in increased emission levels. One of the common types of instability unique to premixed combustion is the phenomenon referred to as flashback. In premixed combustion, where fuel and air are mixed before the flame, the mixture can ignite before combustion chamber. It caused the flame to return into the burner. This phenomenon occurs when the flame speed exceeds the flow velocity of the air/fuel mixture.
Packman Premixed Burners
Packman’s premixed burners are all classified as metal fiber burners and are manufactured in two types, which are named as follows:
- Premixed Burners (PE Series)
- Post mixed Burners (PM Series)
It is important to emphasize that this naming convention is used only to differentiate between the two categories of Packman burners, both of which belong to the premixed metal fiber burner type. The difference between the lies in the location and method of mixing air and fuel, which is schematically shown in the figure below. In the following sections, more detailed specifications for each of these two categories of burners will be provided.
Metal fiber burners generally have a combustion head as illustrated below. This combustion head consists of a metallic cylinder with numerous very small holes along its side surface. Additionally, the side surface is covered with a woven layer of metallic fibers.
This unique structure facilitates the distribution of the fuel/air mixture along the combustion head and allows for a nearly uniform exit from the holes in the tube. Furthermore, the metallic woven layer enhances heat distribution throughout the burner head and increases radiant heat transfer from the flame to the boiler. This process lowers the flame temperature, thereby significantly reducing NOx emissions.
One of the limitations of these burners is that the inlet air must be largely free of dust, as dust particles can clog the combustion head holes and impair the burner operation. Therefore, the inlet air must pass through a filter to remove dust particles before it enters the burner. Additionally, liquid fuel cannot be used in these burners.
1- raadman Premixed Burners (PE Series)
In premixed burners by Packman, the fuel is injected into a venturi put at the inlet of the fan. The venturi helps in drawing the fuel into the airflow. As a result, both fuel and air enter the fan together and are well mixed as they pass through the fan blades. In these burners, flashback can be even more harmful because it can damage the fan and electric motor. Additionally, the flame may escape from the combustion chamber, posing potential safety hazards.
To prevent unwanted ignition of the air and fuel mixture, the fans in these burners are made of spark-proof components and use brushless motors. The figure below shows a premixed burner by Packman and its components.
2- raadman Post mixed Burner (PM Series)
The post mixed burner is another variant of Packman’s premixed burners, primarily distinguished by its location and mechanism of mixing air and fuel. In post mixed burners, the outlet air from the fan first passes through a fuel and air mixer, and then the fuel-air mixture enters the combustion head.
In the mixer, gaseous fuel is injected into the airflow, allowing the fuel and air to mix together. This process ensures that the mixing of fuel and air occurs after the fan, thereby eliminating the risk of ignition within the fan, which means that usual fans can be used. However, this provides less time for the fuel and air to mix completely and uniformly, making the design of the mixer particularly important.
An improperly designed fuel and air mixer leads to poor mixing quality, which can result in localized combustion in excessively lean or rich conditions in certain areas. This creates high- and low-temperature zones on the combustion surface. In these zones, pollutants such as NOx and CO may be produced, and flame flashback could occur, introducing various risks. Consequently, poor mixer operation can decrease the benefits of using premixed combustion and metal fiber burners.
The mixers in raadman’s PM series burners are specifically designed to blend fuel and air optimally, ensuring stable flame operation with very low emission levels.
Supplementary Components of Packman Premixed and Post mixed Burners
In previous sections, the main components of Packman premixed and post-mixed burners were introduced, which include:
- combustion head made by fiber metal
- Fan
- Electromotor
- Air/fuel mixer in post mixed burners
- Gas injection venturi in premixed burners
However, these burners also have other components, which are briefly explained in this section:
- Gas Train: A specialized type of multiblock gas train is used in premixed burners, equipped with a zero-governor regulator. In these systems, a shutter is used to regulate the maximum gas flow based on the burner’s maximum capacity, while additional adjusting screw, known as the Offset, is used to set the minimum capacity. The gas train in post mixed burners is similar to those used in nozzle mix burners.
- Flame Detector: Both premixed and post mixed burners use an ionization rod to ensure the flame is properly formed.
- Air Filter: In post mixed burners, an air filter is installed at the air inlet to prevent dust particles from entering. However, in premixed burners, there is no air filter at the fan inlet, so the air must be filtered at the burner’s installation location.
Applications of Premixed Burners
Today, premixed combustion is widely used in various applications including gas turbines, internal combustion engines, and premixed burners. The most important application of metal fiber burners—such as Packman’s premixed and post-mixed burners, which are produced in capacities ranging from 500 to 4000 kW—is in condensing boilers. These boilers have relatively small combustion chambers, making it impossible to use nozzle mix burners with large flames.
Instead, premixed fiber metal burners with confined flames within the combustion head are used. Furthermore, condensing boilers employ the maximum energy released during the combustion process. So, using these boilers together with premixed or post mixed burners can be an ideal package which offers an efficient and clean combustion in heating systems.
Comparison of Premixed, Post mixed, and Nozzle mix Burners
The table below provides the specifications of premixed, post mixed, and nozzle mix burners, allowing for a clear comparison between these types of burners.
Limitations of Metal Fiber Burners
Despite the many advantages of metal fiber burners, they do have some notable limitations, including:
- The potential for flame flashback, which must be carefully considered during the burner design process and when selecting its components.
- Sensitivity to airborne dust, requiring the installation of filters at the air intake. Gas and air filters must also be regularly maintained and inspected.
- Capacity constraints: Metal fiber burners are typically suited for medium-capacity applications and are not suitable for large boilers or high-capacity operations such as power plants.
- Fuel limitations: These burners are not compatible with liquid fuels and can only operate with gaseous fuels like natural gas or LPG.
Fuel Consumption Optimization and Emission Reduction with Premixed Burners
Premixed burners are a type of industrial burner in which fuel and air are mixed before entering the combustion head where the chemical combustion process takes place. The advantage of these burners lies in their high efficiency and very low CO and NOx emissions. Packman’s premixed (PE series) and post mixed (PM series) burners are examples of premixed metal fiber burners, available in capacities ranging from 500 to 4000 kW, with their primary application being in condensing boilers.
