Nozzle-mix burners play a key role in various processes. Choosing the right burner improves efficiency and lowers environmental impact. To select the best burner, users must understand its characteristics and operation. Important factors include the industrial process type, emission limits, and flame features. Nozzle-mix burners are widely used across industries. Knowing their components, structure, and applications helps users make better choices. This article offers a brief overview of nozzle-mix burners, covering their components, emissions, and flame type.
What is a nozzle-mix burner?
Burners are categorized into two types based on how fuel and air mix: nozzle-mix burners and premixed burners.
- Nozzle-mix burner: In this type of burner, fuel and air are separate until they exit the nozzle. Air and fuel are mixed when they reach or exit the nozzle, then combustion occurs.
- Premixed burner: In this type, air and fuel are mixed before reaching the nozzle and then move toward the burner head.
Simply put, nozzle-mix burners combine air and fuel at combustion, while premixed burners mix them before the nozzle.
The fuel is injected via fuel nozzles while the air is supplied separately by a blower in nozzle-mix burners. The fuel and air mixture occurs by turbulence and rotation in the flow. This turbulence is formed using equipment such as diffusers and blades in the flow path.
To reduce emissions and increase the efficiency of burners, fuel and air must be uniformly combined at all points of the flame with proper ratio. So, nozzles are designed to synchronize the speed and angle of gas outlet with the speed and direction of airflow. This synchronization ensures optimal combustion and minimizes emissions. On the other hand, if the air speed exceeds the gas speed, the gas may not mix properly with the air and leave the chamber unburned, leading to the production of toxic carbon monoxide.
Additionally, if the angle of gas outlet is not aligned with the air flow direction, for instance when both move in parallel, fuel and air will not mix effectively, resulting in incomplete combustion and flame instability.
Emission of Nozzle-Mix Burners
Environmental pollutants in nozzle-mix burners, depending on the type of fuel, include carbon monoxide, nitrogen oxides (NOx), and sulfur oxides (SOx). To reduce carbon monoxide emissions, it is essential to have complete mixing of fuel and air in the burner. Technologies such as Low-NOx burners, Flue Gas Recirculation, and staging the fuel or air are used to minimize NOx emissions. In liquid fuel processes, the presence of sulfur in the fuel makes the emission of sulfur oxides unavoidable. To minimize sulfur oxide emissions, it is essential to refine the sulfur content in the fuel.
The level of pollutant emissions in nozzle-mix burners is largely depends on their design. raadman burners are specifically designed to meet the emission standards of BS EN 676 and BS EN 267, achieving up to NOx Class 3 emission. Moreover, the integration of a Flue Gas Recirculation (FGR) system further reduces NOx emissions, a feature included in raadman burners.
Application of Nozzle-Mix Burners
Burners play a critical role in industrial combustion systems. To design an effective burner, it is crucial to have a full understanding of the process in which the burner will be used. Nozzle-mix burners with different structures are used in various industries, depending on the type of process, the required temperature in furnace, furnace geometry, and other parameters. This section will briefly introduce the most common applications of burners in various industries.
1- Hot Water or Steam Boilers
Boilers are used for a wide range of purposes and applications. For example, boilers are used to produce hot water for heating purposes, to generate steam for use in driving industries or sterilizing materials in industry, to produce hot air, and more. Burner applications in boilers cover a wide range from single-burner boilers used in hospitals, schools, and small businesses to multi-burner boilers used in power plants.
2- Metal Production
In the production of magnetic metals, such as iron, and non-magnetic metals, like aluminum, burners are used as a heat source throughout the entire process, from melting to shaping. High-velocity burners, recuperative burners, and radiant tube burners are commonly used in these industries.
3- Mineral Production
Common mineral production processes include the manufacturing of glass, cement, bricks, refractory materials, and ceramics. These processes typically require high-temperature heat for melting. In these industries, high-velocity burners, flameless burners, oxy-fuel burners, and rotary kiln burners are used to provide the high heat efficiently.
4- Chemical Industries
The fuel compositions used for combustion in furnaces and boilers within the chemical industry are significantly more varied compared to other sectors. Typically, gaseous fuels consist of defined proportions of hydrogen, methane, and propane. Nitrogen and carbon dioxide are also present in these fuels. A boiler or furnace may require combustion with multiple fuels, highlighting the importance of multi-fuel burners. Another unique feature in this industry is the use of natural draft burners. These burners do not need blowers to supply combustion air and are more sensitive to environmental changes, such as temperature, humidity, and wind speed.
5- Industrial Incinerators
While waste materials typically lack high thermal value and cannot serve as fuel independently, the products derived from these materials can be used as fuel for heat production.
Components of Nozzle-Mix Burners
The main components of nozzle-mix burners include the air supply system, fuel supply system, combustion head, and electrical system, which consists of the power circuit and control circuit. Here is an example of the components found in a burner used in a boiler. While all burners incorporate these essential elements, their design and configuration can vary based on specific applications and operating conditions. These components will be described in the following section.
1- Air Supply System
This includes the inlet air damper, silencer, fan, and electromotor, which work together to provide combustion air and control airflow.
2- Burner Combustion Head
It consists of pipes for transferring the fuel within the burner, gas or liquid fuel nozzles, a diffuser or swirler, a flame stabilizer, and an igniter. The combustion head is responsible for ensuring the proper mixing of fuel and air to form a stable flame with minimal environmental pollutants, such as nitrogen oxides (NOx) and carbon monoxide (CO).
3- External Flame Cover
The external flame cover of a burner plays a crucial role in shaping and controlling the flame speed. Adjusting the flame speed helps prevent flashback. Depending on the burner application, the flame cover can have various designs. An example of such flame covers is shown below.
4- Fuel Supply System
The fuel supply system includes all components necessary for delivering and transferring liquid or gaseous fuel to the burners. The design of these systems is based on specific standards to ensure optimal operation and high safety. For gaseous fuels, the system comprises a filter, regulator, safety and main valves, pressure gauges, relief valves, and other necessary components. In contrast, the liquid fuel system includes a pump and electromotor, regulator, lance, safety valves, and fuel piping.
5- Safety System
The safety system installed on the burner includes a flame detector and minimum and maximum air pressure switches. The flame detector ensures the safety of both the burner and the boiler by detecting the presence or absence of a flame. If the flame is not detected, the detector sends a shutdown command to the control system to prevent explosions. Flame detectors can be categorized into four types: ionization detectors, ultraviolet flame detectors, infrared flame detectors, and frequency detectors. Both ultraviolet and infrared flame detectors monitor different aspects of the flame.
If the air pressure drops below a specified level for any reason, such as a fan failure, this change must be reported to the control system. In this case, the burner will shut down, and the gas train will be closed to maintain environment safety. This is done by the minimum pressure switch. Additionally, if the combustion chamber pressure increases due to reasons such as a blockage of chimney, it is essential to prevent the return of combustion products and flashback. In this case, the maximum pressure switch sends a shutdown command to the system.
6- Burner Control System
This system varies depending on the burner’s operational type and includes actuators installed on the air damper path and the gas or liquid fuel train, a central processing unit, load control, and a user interface.
7- Burner Electrical System
This system comprises the control circuit, power circuit, wiring, and protective electrical devices for the burner. The power circuit is responsible for starting the burner’s electromotors (including the fan motor and liquid fuel pump).
The Flame of Nozzle-Mix Burners
The fuel and oxidizer enter the burner separately in the nozzle mix burners. Then, they are mixed in the combustion head and being transferred to the combustion chamber. Combustion begins as soon as the fuel/air mixture enters the chamber, with the molecules of fuel penetrating into the oxidizer. The intensity of combustion in these burners depends on the mixing ratio of fuel and oxidizer. The combustion reaction in diffusion flames initiates at the upper limit of the fuel’s flammability and continues to lower limits.
This type of flame is highly important because of its industrial applications since many industrial furnaces and heating systems use this flame type. Therefore, controlling parameters such as NOx emissions, flame dimensions, and stability is crucial. Various methods can be used to manage the flame, including adjusting flow rates, dilution, preheating, burner geometry, pressure, and gravitational effects.
Heating systems must have specific burners based on the particular process, combustion chamber geometry, process temperature, oxygen availability in the combustion chamber, environmental requirements, and other necessary conditions. For example, if a furnace requires a uniform temperature pattern, combustion products must enter the furnace at high speed, which makes high-velocity burners a recommended choice for such applications.
Using the burners with long flame and narrow diameter are necessary in fire tube boilers. In these burners, a diffuser is usually used in the combustion head. On the other hand, water tube boilers require burners with short flames and larger diameters due to their combustion chamber structure. These burners often have a swirler in the combustion head to convert axial velocity into radial and tangential velocities. This design reduces flame length while increasing its diameter. Below are various types of flames commonly used in burners for different applications.
Advantages and Disadvantages of Using Nozzle Mix Burners Compared to Other Types of Burners
Nozzle mix burners have their own unique advantages and disadvantages due to their special design, which should be considered depending on industrial applications.
Advantages of Nozzle Mix Burners
Nozzle mix burners, with their special design, offer high efficiency and flexibility across various industries.
Flame Stability: In nozzle mix burners, the process of mixing fuel and air occurs at the moment of combustion. This creates a stable and controllable flame, which is essential for many industrial applications, including large boilers.
Fuel Flexibility: Nozzle mix burners can operate with various gaseous or liquid fuels and under different industrial conditions. This flexibility makes them suitable for a wide range of applications.
No Flashback or Explosion Risk: Since nozzle mix burners do not mix fuel and air within the burner itself, the risk of flashback does not exist. As a result, there is no risk of explosions. These burners do not require special anti-sparking devices or explosion-proof equipment.
Application Across Various Industries: These burners are used in all industries requiring high combustion temperatures or special designs, such as industries involved in steam and hot water production, metals, ceramics, mining, chemical production, and waste incineration.
Disadvantages of Nozzle Mix Burners
Nozzle mix burners require precise and accurate equipment, as well as proper maintenance.
High Emissions: Compared to premixed burners, nozzle mix burners have higher levels of emissions.
Sensitivity to Environmental Conditions: The operation of these burners may be affected by changes in pressure, temperature, or fuel quality. Therefore, they require careful and ongoing monitoring and maintenance.
Higher Repair and Maintenance Costs: Due to their complex design and use of advanced components, such as diffusers and specialized nozzles, these burners have higher repair and maintenance costs compared to some other types of burners.
Nozzle-Mix Burners Reduce Emissions Using Innovations in Combustion
Nozzle-mix burners are truly important in combustion processes. The main difference between nozzle-mix and premixed burners is the timing that fuel and air mixed. In these burners, fuel and air move separately and mix only at the moment of combustion, leading to a stable flame and reduced emissions. They are used in various industries, including hot water and steam boilers, metal production, mineral processing, chemical industries, and waste incineration.
The nozzle-mix burners may have various designs according to industrial processes, required temperatures, and furnace geometry. One of the key advantages of these burners is their ability to decrease pollutants such as carbon monoxide, nitrogen oxides, and sulfur oxides. Ultimately, nozzle-mix burners offer flexibility in design and the capability to reduce emissions, making them an excellent choice for industrial combustion. They play a vital role in enhancing energy efficiency and mitigating environmental impacts.
raadman Company, with its high technical expertise, specializes in designing and manufacturing nozzle-mix burners in accordance with national and international standards. Raadman burners are engineered to focus on reducing emissions and optimizing energy consumption. So, they can improve combustion processes in various industries.
