Module 5: Combustion Technology. Lecture 38: Combustion Burners

Transcription

1 1 P age Module 5: Combustion Technology Lecture 38: Combustion Burners

2 2 P age Keywords : Atmospheric burner, atomizer, rotating cup burner, swirl type burner, nozzle design 5.7 Combustion Burners Types of Burner The basic combustion theory and laws are applied for the design of burning equipments depending on the type of fuels, such as gaseous or liquid fuels. Theses burning equipments are called burners. There are different liquid fuel and gaseous fuel burners. There are some similarities in their design but it depends on the type and properties of fuel in addition to their type of applications. Although many designs are available, but they are common in one aspect of requiring proper mixing of air and fuel prior to their combustion. These oil or gas burners are used in furnaces. Gas Burners Gas burners are usually classified based on their operating gas pressure. They are operated both atmospheric and high-pressure conditions. The gases are supplied in different ways depending on the pressure. In low pressure burner, the gas pressure varies from 1 to 4 kpa. Whereas, high pressure burners the pressure varies from 7 to 70 kpa. Oil Burners The oldest burning process of fuel oil is the pot type burner in which the oil is first vaporized by applying heat. After vaporization, the oil vapors are mixed with excess air and then burned. As oil is vaporized at a slower rate than the rate of combustion reaction, the process is modified by using atomizer to form tiny droplets of oil before its ignition. The droplets can be vaporized easily. The atomization is carried out by either a mechanical device or applying a fuel jet. The nozzles are designed in such a way so that, it can be used to atomize oil at an elevated oil

3 3 P age pressure at about 100 psi. The process can produce oil droplets in the range from inch to inch. Quick ignition of relatively smaller droplets results in the rapid flame formation. Proper designing of nozzle is required to deliver droplets at a uniform rate for a particular burner. Oil is also atomized by using a rotating cup, rotating disc and swirling method. The examples of some oil burners are described in this section. Swirl Oil Burner. The liquid oil is first pressurized at about psi by a pump. In the swirl oil burner, the pressurized liquid fuel enters tangentially through the slots at a high velocity in the oil swirl chamber. It flows in the form of a vortex and escapes through a nozzle at the other end of the chamber. The sketch of a swirl type burner is shown in Fig.1. The centrifugal force is exerted on to the oil and it moves forward in the shape of a hollow tube. Air enters to the annular space and moves forward. Then the fine droplets emerge at the exit of the chamber in the form of a spray. Fig. 1 Swirl oil burner Nozzle Design

4 4 P age The nozzle design is mainly based on mass balance in the burner.the diameter of oil tube may be determined from the equation (1) diameter of oil tube in mm = mass flow rate of oil in kg/hr. = allowable velocity through the tube in m/s = oil density in kg/m 3 The velocity through the orifice is related with oil pressure by the following equation (2) = oil pressure in pascal = nozzle area in mm 2 = flow coefficient. (3) The diameter of nozzle, 1.128

5 5 P age Rotary Cup Burner The rotary cup burners are usually used in industry as well as for domestic purpose. A sketch of the rotary cup burner is shown in Fig.2. The capacity of fuel varies in wide range. The device consists of a rotating cup and a fan. Both the cup and the fan are rotated by an electric motor( not shown in figure) with a shaft. Oil is fed to the oil distributer to throw at the inner surface of the cup. The cup is rotated at a speed of about 3600 rpm. Then oil flows in a swirling motion and is thrown as fine droplets at the other end. The primary air is supplied by a fan or blower in a whirling motion opposite to the oil motion. This helps to further disintegrate the oil particles and finally to reach the combustion chamber. Fig.2 Rotary cup burner

6 6 P age Gas Burners Gaseous fuel burners are classified based on the air and fuel mixing phenomena. They are either premixed type or diffusion type. If the fuel and gas are premixed before passing through the burner nozzle it is called premixed type. In diffusion type, a small quantity of air is mixed in the flow of gas and there will be diffusion between them. Industrial burners for gaseous fuel are diffusive type. In diffusion burner air and gaseous fuel are supplied separately in the furnace. These burners are also used as for domestic purposes. The mixing rate between air and fuel controls the combustion process. The burners are also categorized based on the applied gas pressure. The gas burners may be either atmospheric or high pressure type. In the atmospheric type burner, the gas pressure varies from 2 to 12 in water gauge. Whereas, in the high pressure type, 0.5 to 40 psi gas pressure is used with a large combustion chamber. The construction of an atmospheric burner is described in Fig.3. The design and operation is similar to the Bunsen burner. Fig.3 Atmospheric gas burner

7 7 P age Air for combustion is supplied in the furnace chamber. In these burners small portion of air is mixed with fuel as primary air and the rest amount, known as secondary air, is supplied above the burner port. Figure 3 shows an atmospheric gas burner where, the mass of air is about 10 times than that of fuel. Some fraction of total air is mixed with the fuel and this air supplied with the fuel is called the primary air. The secondary air is further added in the combustion point in the furnace. Mixing and combustion take place simultaneously. A free jet is produced in the downstream side of the burner during the discharge of fuel. The gas velocity at the orifice will be proportional to the square root of the differential pressure across the orifice. The venture tube is provided for better mixing. When a mixture of air and gaseous fuel passes through the nozzle of the burner, they mixed thoroughly in the divergent section of the venture. In this section pressure increases gradually as the velocity head is converted to the pressure head. The secondary air is supplied to the flame to complete combustion of the fuel.

8 8 P age References 1. Fuels, combustion, and furnaces, John Griswold, McGraw-Hill Book Company, Inc., Fuels and Combustion, Marion L. Smith and Karl W. Stinson, 1 st Edition, New York, Combustion Equipment, Colin French, Book/Engineer/Power%20Plant/Plant%20Engineer%27s%20Reference%20Book/44524_ 11.pdf.