Industrial burners using heavy fuels such as mazut, are extensively used in many heavy industries, including power plants, petrochemical facilities, and cement industries. Mazut-fired burners due to their high heat output and capability to operate at high temperatures, are suitable for long-term, intensive industrial processes. However, using heavy fuels requires specialized equipment, advanced combustion methods, and regular maintenance. This article explores different types of heavy fuels, their physical properties, and various combustion methods.
Before reading this article and to better understand the concept of fuel and its types, it is advised to first read a detailed article titled “Burner Fuel Selection: A Review of Different Fuels in Industrial Burners“.
Definition of Heavy Fuel
Heavy fuel, such as mazut, is derived from crude oil and consists of long-chain carbon rings, which increase its density and viscosity. While it is an economical choice for large-scale energy generation compared to light fuels like natural gas, its high impurity content—such as sulfur and heavy metals—results in greater environmental pollution.
Heavy liquid fuel, such as mazut, has high viscosity, density, and significant levels of sulfur and impurities, making its combustion process more complex. Using this fuel in combustion processes requires purification to remove impurities, as well as preheating to reduce viscosity and improve flowability. Despite these characteristics, mazut offers high energy output, making it particularly suitable for industrial applications.
Classification of Heavy Liquid Fuels According to Viscosity and Temperature
Heavy fuels are categorized according to their viscosity and the temperatures at which they are used. Viscosity has an inverse correlation with temperature, meaning that as temperature increases, viscosity decreases. The diagram below shows the relationship between temperature and the viscosity of heavy fuel, showing that as the temperature rises, viscosity decreases and the fuel’s fluidity improves, facilitating its pumping and spraying.
This diagram highlights different ranges, each representing a distinct classification of fuels.
Range A: The minimum and maximum viscosity for light fuels, which remain liquid at lower temperatures.
Range B: The range defined for heavy fuels with high flowability. In this range, heavy fuels do not require additional preheating and are easily sprayable in their liquid form.
Ranges C and D: These zones are dedicated to heavy fuels with high density and increased viscosity. To achieve suitable flow properties, the temperature must be sufficiently raised to lower viscosity.
Range E: The suggested viscosity range at the nozzle, where fuel can be sprayed easily for optimal combustion. Maintaining the right fuel temperature within this range helps avoid incomplete combustion and minimizes emissions.
Range F: This range represents the optimal viscosity limit for pumping heavy fuel.
Heavy Fuel Circuit
To optimize the use of heavy fuel such as mazut, preheating is essential to reduce its viscosity and bring it to the appropriate temperature for pumping and combustion. This preheating can be achieved using heaters. The heater can be electric or operate with steam or hot oil. The mazut pump, lance, and pre-heater must be equipped with heaters to prevent the fuel from cooling down and to maintain its viscosity within the suitable range for pumping and combustion.
The fuel circuit of heavy fuel oil burners is composed of two sections: primary/transfer and secondary/main. In the primary circuit, the conditions for storage and preparation of mazut are ensured until it reaches the appropriate conditions for entering the secondary circuit. The suitable viscosity of the fuel at the output of the primary circuit should be less than 75 centistokes. Therefore, mazut must be heated to a temperature of around 70-80°C. Additionally, to supply the required input pressure for the main burner pump, which is in the secondary circuit, mazut at the output of the primary circuit must have a pressure of 3 to 4 bar.
In the primary fuel circuit, the fuel passes from the main source through a valve and is filtered through a coarse mesh filter to remove large impurities. It then enters a positive displacement pump and, under a pressure of 3 to 4 bar, is transferred into the secondary circuit.
In the secondary circuit, the fuel passes through a fine mesh filter before entering the secondary circuit pump. The fuel pressure is increased to 25 bar in this pump and then moves into the preheater. In the preheater, the fuel is heated to reduce its viscosity to a suitable level for spraying. After passing through the preheater, the fuel enters the burner.
Fuel Circuit Components
The fuel circuit consists of a series of critical components designed for the storage, filtration, transfer, and preparation of heavy fuels like mazut, aimed at efficient combustion in furnaces and burners. These components work together to create optimal conditions for spraying, mixing with air, and ignition of the fuel. The following sections introduce and examine each of these components.
Daily Storage Tank
This tank is designed for storing fuel. The temperature of mazut in this tank should be at a pumpable level, depending on the type of heavy fuel.
Filter
Given the high impurity content in heavy fuels (like mazut), the fuel undergoes several filtration stages to ensure that impurities do not reach the nozzles and lance.
Primary Pump
This pump is a positive displacement type and is part of the primary circuit. It raises the fuel pressure to 4 bar. After passing through this pump, the fuel moves into the secondary circuit.
Secondary Pump
Located in the secondary circuit, this pump raises the fuel pressure to 25 bar. Pumps used for heavy fuels, like mazut, are usually equipped with internal heaters. These heaters are mounted directly inside the pump and assist in raising the fuel temperature to the correct level before it is pumped into the combustion system. They are particularly effective when the burner is off, ensuring that the fuel temperature remains constant. Without the heater, the fuel temperature would drop, causing its viscosity to rise. This increased viscosity could cause the fuel to stay inside the pumps, leading to system malfunction.
Heavy Oil Preheater
A preheater is a type of heat exchanger that transfers heat from various sources to mazut. Heat sources can include electricity, steam, or thermal oil. In heavy oil preheaters with an electric source, electric elements are used.. These elements are located inside the preheater chamber, and by receiving an electric current, they heat up and generate heat. This heat is transferred to the fuel, raising its temperature to a level where its viscosity is sufficiently reduced, ensuring the necessary conditions for combustion.
Heater-equipped Lance
The lance in burners serves as a component of the combustion system, with its primary role being the delivery of fuel to the combustion head. Lances used for heavy fuels, such as mazut, must be equipped with heaters. A heating element is coiled shape and continuously heats the fuel inside the lance., continuously warming the fuel inside. The heater around the lance ensures that the fuel’s temperature remains within the optimal range, reducing its viscosity and making fuel spraying easier.
Combustion Methods of Heavy Fuel
To achieve optimal combustion of heavy fuels like mazut, it is essential to first evaporate the fuel and convert it into a gaseous phase. This process involves turning the liquid fuel into extremely fine droplets to increase its surface area in contact with air, facilitating complete combustion. This is achieved through fuel atomization. Common methods of mazut atomization include pressure atomization, air/steam atomization, and rotary cup atomization, each of which is explained below.
Pressure Atomization
Pressure atomization is a common method for converting liquid fuel into fine droplets in the combustion process. In this method, the fuel is subjected to high pressure (up to 25 bar) and passed through a small nozzle. As the fuel passes through the nozzle, its velocity increases, causing the fuel to break into tiny droplets. These fine droplets, due to their low pressure, evaporate and then mix with the combustion air for ignition. For optimal performance of this method, preheating the fuel to reduce its viscosity and ensure easy passage through the nozzle is essential. Additionally, providing high pressure plays a key role in producing uniform droplets and maintaining the stability of the atomization process
Air/Steam Atomization
Air or steam atomization is an efficient method for converting liquid fuel into fine droplets using a jet of compressed air or steam. In this method, the fuel is supplied at a lower pressure than in pressure atomization (typically up to 10 bar), while the air or steam is also supplied at a similar pressure (up to 10 bar). Inside the nozzle, both the fuel and air or steam pass through a small orifice, and the high shear force of the air or steam stream breaks the fuel into very fine droplets. These small droplets are evenly distributed throughout the combustion chamber, rapidly evaporate, and mix with the combustion air.
Air or steam atomization is a critical technique in the combustion process that helps improve fuel spray quality, increase combustion efficiency, and reduce pollutant emissions. This method is particularly effective for heavy, viscous fuels such as mazut and plays a crucial role in improving the efficiency of combustion systems.
Rotary Cup Atomization
Rotary cup atomization is an efficient technique for converting liquid fuel into fine droplets. In this method, fuel is introduced into a rotating vessel known as the “rotary cup,” which spins at high speeds. The centrifugal force created by the rotation causes the fuel to break into small droplets and be ejected from the cup’s edges. When the fuel exits the rotating cup, a high-speed air jet collides with the fuel stream, generating shear forces that break the fuel into even finer droplets.
The rotary cup method is effective for heavy fuels like mazut, as it breaks the fuel into fine droplets and, with the help of a high-speed air jet, facilitates better evaporation. Due to the impurities present in heavy oil, the rotary cup method helps avoid problems like nozzle blockages. However, it may result in the generation of higher pollutant emissions.
Compared to the other two methods, the rotary cup method requires more maintenance due to the use of electromotors and rotating components like belts and pulleys. These parts, operating at high speeds, are more vulnerable to damage and wear, which makes regular maintenance and repairs more necessary.
Improving Heavy Fuel Efficiency
In conclusion, industrial burners that use heavy fuels like mazut are particularly applied in heavy industries such as power plants, petrochemical facilities, and cement industries. Mazut-fired burners are suitable for producing high heat and operating at high temperatures. However, due to their high density and viscosity, they require preheating and specialized equipment for optimal pumping and combustion. Additionally, heavy fuels contain high amounts of sulfur and impurities, which can lead to the production of pollutants; therefore, the use of filtration systems to reduce these pollutants is essential.
For the optimal combustion of mazut, various methods such as pressure atomization, air or steam atomization, and rotary cup atomization are used, each with its own specific characteristics and benefits in improving efficiency and reducing environmental issues.
The pressure atomization method employed in the mazut-fired burners manufactured by Radman Industrial Group serves as an advanced and efficient solution, playing a key role in improving combustion efficiency, reducing energy consumption, and lowering environmental pollutants.
