Boiler Burners and Types of Burners

Types of boiler burners:

Boiler burners are the central element of effective combustion system design which includes fuel preparation, air fuel distribution, furnace design and combustion control. A burner is a mechanical device that supplies required amount of fuel and air and creates a condition of rapid mixing and produce flame. The mixing rate of air and fuel directly affects the flame stability, shape and emission.

Features of good burners

1.    Stable and proper operation in the range of design parameters
2.    Low pollution
3.    Low level of noise
4.    Longer life time
5.    Security of operation

Turndown Ratio:

It is defined as the ratio of minimum heat input rate to maximum heat input rate. Turndown ratio is important from economic and legislative perspective so that the burner provides efficient and proper combustion. Some burners can have as much as a 50:1 or more turn down while other types of burners are designed for only 2 or 3:1 turndown.
The turndown ratio for various burners are-
Burner type    Turndown ratio
Pressure jet    2:1
Rotary cup    4:1
Gas    5:1
Table 1

Depending on the type of fuel, design and application there are different types of burners.

Gas Burners:

These are type of burners that are classified on the basis of the air and fuel mixing phenomenon as- premixed and diffusion type. In premix type, fuel and gas are premixed before passing through the burner nozzle.  In diffusion type, a small quantity of air is mixed in the flow of gas and there will be diffusion between them. Air and gaseous fuels are supplied separately in the furnace. The rate of mixing of air and fuel decides the combustion process. Diffusion type of burners are used in industrial as well as domestic application.
Gas burners are also classified on the basis of pressure. They are operated on atmospheric as well as high pressure condition. Low pressure burners vary from 1 to 4 kpa. While high pressure burners vary from 7 to 70 kpa.

In atmospheric type of burners air is supplied in the furnace chamber. Small portion of air is mixed with the fuel as primary air and the rest amount known as secondary air is supplied above the burner port.
The mass of air is about 10 times 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

Oil burners:

The pot type of burners is the oldest type of burner. In this type, the oil is first vaporized by applying heat. The oil vapours are mixed with excess air and then burned. Oil is vaporized at a slower rate than the rate of combustion reaction. This process is modified by using atomizers to form tiny droplets of oil before its ignition. The droplets can be vaporized easily. The oil droplets produced are in the range of 0.0002 to 0.010 inch. The small size of droplets results in quick ignition and rapid flame formation. Designing of nozzle is important so that the droplets produced are delivered at uniform rate.
Oil is also atomised by using rotary cup and swirling method.

Pressure jet burners:

These burners consist of an orifice at the end of the pressurised tube. The pressure of the fuel oil ranges from 7 to 15 bar. These burners are easy to maintain and have less cost.
Substantial pressure drop is developed over the orifice in the operating range when fuel is discharged into the furnace leads to atomisation of the fuel. Pressure of the fuel oil is directly proportional to the flowrate of the fuel.
P  ∝  F2.
 The turndown ratio is restricted to 2:1. Pressure jet burners are supplied with variety of interchangeable nozzles to adjust different boiler loads.
 As the load in the boiler is not constant, the boiler will have to be taken off-line to change the nozzle. .

Characteristics of jet:

A jet is produced when a fluid is discharged through the nozzle. In the jet the velocity of the fluid is accelerated. Free jet is produced when the fluid is discharged in the surrounding with no confinement.

A jet is said to be confined when the fluid is discharged in the container. The characteristic feature of the jet (whether free of confined) is that it spreads due to the difference in the density of the jet and the surrounding.  A hot jet in the cold surrounding spreads faster than a cold jet in the same surrounding. Spreading of the jet is due to entrainment of the surrounding.

Due to entrainment of the surrounding, the axial velocity of the jet decreases. For any downstream axial distance, the maximum velocity is at the centre and minimum at the periphery such that a parabolic profile is developed.

Swirl oil burner:

In swirl oil burner, oil is pressurized to about 7-10 bar. This pressurized fuel enters tangentially through the slot at a high velocity in the oil swirl chamber. It flows in the form of vortex and escapes through a nozzle at the other end of the chamber.  Centrifugal force is exerted on to the oil and it moves forward in the shape of 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

Rotary cup Burner:

This burner has a rotary cup and a fan.  The cup and the fan are rotated by an electric motor with a shaft. Oil is feed to the oil distributer to throw at the inner surface of the cup. The cup rotates at the speed of about 3600 rpm. Then oil flows in a swirling motion and is thrown as fine droplets at the other end. Fan or blower supplies primary air in whirling motion opposite to oil motion. This helps to further disintegrate the oil particles and reach combustion chamber.

Dual fuel burners:   

These are burners designed with gas as main fuel and have additional facility for burning fuel oil. The changeover of the fuel from gas to oil should be rapid as there should not be any interruption in the supply.

While changing the supply from gas to oil, the gas lines should be first isolated, and oil supply should be switched on. The boiler should then be re-fired. Fuel oil is a stand by and used for short periods. Oil firing capacity may be basic.

Selection of burner:

 Combustion volume:

 The space occupied by the fuel and the products of combustion varies considerably with the burner design, upstream pressures and flow rates.

Stability :

 Turndown ratio is defined as the ratio of minimum heat input rate to maximum heat input rate. Minimum heat input rate is controlled by the phenomenon “back fire” whereas maximum heat input is controlled by the extinguishing of the flame.

If the velocity of mixture of fuel +air is greater than flame velocity, flame will extinguish. The back fire will occur when the velocity of mixture of fuel +air is lower than flame velocity,

 Burner control:

Combustion controls assist the burner in regulation of fuel supply, air supply, and removal of gases of combustion to get optimum boiler efficiency. The fuel supplied must be in proportion to the steam pressure and the quantity of steam required.
The various types of combustion controls used are:
1.    On-off control
2.    High/ low/ off control
3.    Modulating control
Following table gives us the information of the type of burner and control used for particular pressure in shell and tube boiler.

Pressure    Under 500 kg/h    500 to 2000 kg/h    2000 to 5000 kg/h    Over 5000kg/h
Control    On/ off control    High/low/ off    High/low/ off    Modulating control
Burner type    Pressure jet burner    Pressure jet burner    Pressure jet or rotary cup burner    Pressure jet or rotary cup burner
Table 2

1.    On- off control:
It is the simplest type of control. Burner’s fire in full rate or are in off condition. This type of control can be used for small type of burners. Every time the boiler is fired it undergoes thermal shock. When large load comes to boiler after the burner is switched off, the steam available is reduced. It also cause priming and locking out of boiler.
2.    High/ low/ off control
It is complex than on –off control and hence expensive. The burner has two firing stages, the first stage is operated at low firing rate and then moves to full firing. This switching eliminates thermal shock. The burner can also revert to low fire position at lower loads If the large load is applied when the burner is on 'low fire', it can immediately respond by increasing the firing rate to 'high fire', the purge cycle hence can be eliminated.

3.    Modulating control:

The modulating control operates on the principle of matching the steam pressure demand by altering the firing rate over the entire operating range of the boiler. Each time the burner is shut and re-started, system is purged by blowing cold air through the boiler passage, this reduces the efficiency. Modulating motors are conventional mechanical linkage or electric valves to regulate the primary air, secondary air and fuel supplied to the boiler. At full modulation the boiler keeps firing, and fuel and air are carefully matched over the whole firing range to maximize thermal efficiency. This type of control is applicable for burners with high pressure loads.