Chapter 8 Fuel Firing System

BOILER AND AUXILIARIES

FUEL FIRING SYSTEM

CHAPTER 8 FUEL FIRING SYSTEM The fuel firing system of boiler provide controlled, efficient conversion of the chemical energy of the fuel into heat energy which in turn is transferred to heat absorbing surfaces.

8.1 CHARACTER CHARACTERISTICS ISTICS OF IDEAL IDEAL FIRING SYSTEM SYSTEM •

No excess excess oxygen oxygen or unburn unburned ed combustibl combustibles es in the end product productss of combustion combustion..

•

A low rate of auxiliary ignition energy input to initiate combustion.

•

An economic reaction rate between fuel and oxygen compatible with acceptable NOx & SOx formation. formation.

•

An effective method of handling and disposing of the solid impurities introduced with the fuel.

•

Uniform distribution of product weight and temperature in relation to the parallel circuits of heat absorbing surface.

•

Fast responses to changes in firing rate.

•

High equipment availability with low maintenance.

COMBUSTION REACTION

Combustion is the chemical reaction between the combustibles in a fuel and oxygen of air. These are termed as the reactants. The reaction takes place only when there is physical physical contact between the reacting molecules. The rate and degree degree of completion completion of combustion reaction is mainly influenced by •

Temperature, concentration, preparation and distribution of the reactants

•

Mechanical turbulence which increase contact between molecules of the reactants.

8.2 FIRING FIRING SYSTEM SYSTEM CONCEPT CONCEPTS S Success Successful ful molecul molecular ar contact contact of reactant reactantss of combus combustion tion through through turbule turbulence nce can be achieved by producing two methods of flow pattern in the furnace.

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FUEL FIRING SYSTEM

In the first concept, the fuel and air are divided and distributed into many similar streams. Each stream is treated independently to provide multiple flame envelopes called multi flame envelope concept.

In the second concept a single flame envelope is produced, by providing interaction between all streams of air and fuel introduced into the furnace. This is called single flame envelope concept.

8.2.1

COMPARISON OF SINGLE AND MULTI FLAME CONCEPT

The single flame envelope provides interaction between all streams of fuel and air introduced into the furnace and so precise subdivision of fuel and air at each point of admission is not required. Allows more time for contact between all fuel and air molecules and mechanical turbulence is sustained throughout the furnace.

The multi flame envelope requires accurate subdivision fuel and air supplied to the furnace. This concept limits the opportunity for sustained mechanical turbulence partially in the early stages of combustion.

8.3 TYPES OF FIRING SYSTEM According to single/multi flame concept three major types of firing systems are adopted in boilers to produce the turbulence:

a.

Horizontally fired system or

-

Multi flame envelope concept

b.

Wall fired system Tangentially fired system or

-

Single flame envelope concept

Corner fired system Vertical firing system or Down

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Combination of both concepts

c.

shot firing system

8.3.1 HORIZONTALLY FIRED SYSTEM

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FUEL FIRING SYSTEM

In horizontal fired system the fuel is mixed with combustion air in individual burner registers. The burners are located in rows through the wind box receiving hot secondary air either on the front wall or both of the front and rear walls. The coal and primary air are introduced tangentially to the coal nozzle, thus imparting strong rotation with in the nozzle. Secondary air from the wind box is admitted to burner through adjustable radial or axial flow swirl vanes called air registers. Once the coal is ignited hot products of combustion are directed back towards the nozzle to provide the ignition energy necessary for stable combustion. This type of coal burners is fitted with an oil burner mounted in a central support tube for coal burner ignition.

8.3.2 TANGENTIALLY FIRED SYSTEM The tangentially fired system is based on the concept of single flame envelope. Both fuel & air are projected from the corners of the furnace along a line tangent to a small circle lying in the horizontal plane at the centre of the furnace. A rotative motion similar to that of a cyclone is imparted to the flame body, which spreads out and fills the furnace area.

In this system at each corner of the furnace a wind box assembly is installed. The wind box is vertically divided into number of compartments each compartment receives hot secondary air from the secondary air duct through a damper called secondary air damper. Alternate compartments of the wind box are provided with coal nozzles through which the pulverized coal from the mill is delivered to the furnace. In these compartments secondary air is admitted to the furnace surrounding the coal nozzles. In this arrangement there will be a facility for tilting the fuel and the air nozzles of the wind box compartments. All these nozzles tilt in unison to raise and lower the flame in the furnace to control furnace heat absorption and thus heat absorption in the super heater and reheater section.

8.3.3 VERTICALLY FIRED SYSTEM This system is used only to fire solid fuels that are difficult to ignite such as coals with moisture ash free volatile matter less than 13 percent. The y require less supplementary fuel than the horizontal or tangential fired system but have more complex firing equipment and therefore more complex operating characteristics.

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FUEL FIRING SYSTEM

Pulverized coal is discharged through the nozzles in the furnace arch. A portion of the heated combustion air is introduced around the fuel nozzles and through adjacent auxiliary parts. This firing system produces a long looping flame in the lower furnace, with the hot gasses discharging up the centre. A portion of the total combustion air is with held from the fuel stream until it projects well down into the furnace. This arrangement has the advantage of heating the fuel stream separately from the significant portion of its combustion air to provide good ignition stability.

8.4 COMPONENTS OF FUEL FIRING SYSTEM The essential components for a pulverised coal fired boiler firing system are:

8.4.1

IGNITION SYSTEM

Any boiler firing system needs ignition system to provide ignition energy to the flammable mixture of the fuel & air introduced to the furnace. Combustion starts only when the mixture is heated to its ignition temperature. To initiate combustion of any fuel and to keep the flame stable, continuous supply of ignition energy is required.

Total Ignition Energy = Inherent Ignition Energy + Auxiliary Ignition Energy

The ignition energy required at any given instant depend on many factors such as •

Location

•

Fuel quality

•

Fuel parameters

•

Combustion air parameters

•

Fuel air distribution

•

Total fuel air ratio

•

Mass burner flow rate

Some of the oil Ignitors are: a)

Eddy Plate Ignitor

b)

Ion Flame Monitoring Ignitor (IFM)

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c)

FUEL FIRING SYSTEM

High Energy Arc Ignitors (HEA)

8.4.2

OIL GUNS AND ATOMISERS

Fuel oils like LDO, HFO or LSHS are used in the boilers either as main fuel or supplement fuel. These fuel oils are burnt by spray combustion method wherein the oil is split into the fine droplets and distributed into the furnace in a spray form in a controlled manner. Oil guns either concentric tubes or parallel tubes are installed through the furnace walls or wind box with atomizer mounted at there tips provide this oil spray to furnace. The oil guns at their rear end are connected through flexible hoses to the oil pipe & atomizing agent (either steam or compressed air) pipelines.

The oil guns may be provided with a pneumatic gun advance & retract mechanism. With this the oil gun can be retracted in its housing when it is not in service. By this the atomizer will be kept at a safe distance from the furnace heat radiation thereby ensuring long life and reliability of the atomizer

Atomization

The exact mechanism of atomization depends on the type of atomizer used and the nature of the liquid being atomized but the basic mechanism involves the formation of unstable columns of liquid which breakdown into rows of droplets. The process of atomization can be accomplished in a number of ways in practice, which are usually grouped according to the source of energy used. There are mainly two methods for atomization: a)

mechanical atomization or pressure atomization

b)

twin fluid atomization

a). Mechanical / Pressure Atomization

In a typical pressure atomizer, oil at a high pressure flows in the centre tube and is discharged through tangential slots in the sprayer plate swirling chamber where the oil rotate at high speed. The swirling oil then passes with undiminished energy through the sprayer plate orifice and escapes as a spray. In this case the hollow conical sheet that is produced emerges from the orifice with a tangential velocity which is sufficient enough to cause an air core throughout the nozzle so produce a hollow cone spray. In power plants these atomizers are used for light oil start up burners only. ET – ’08

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FUEL FIRING SYSTEM

b). Twin Fluid Atomizer

In this type of atomizers, the atomization consists of the following stages -

Formation of thin liquid sheet along the inner walls of the internal mixed atomizer or of free sheets.

-

Disintegration of these sheets by aerodynamic forces to form ligaments and large droplets to form spray.

In Indian power stations normally compressed air is used for atomizing light oil and steam is used for heavy furnace oil.

8.4.3

FLAME SCANNER

In any boiler, a large quantity of fuel is being admitted to the furnace, the flammable mixture formed in the furnace must be converted into inert at the earliest available time before any appreciable quantity builds up in the furnace. Admission of fuel without flame in furnace leads to furnace explosions. Hence detection of absence of flame is required for any control action to prevent furnace explosion.

Fundamental Requirement of Flame Detection System

Any flame detection system provided in the boiler must meet the following requirements. -

Must be reliable

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Sensible to discern the minimum flame envelope

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Fail safe characteristics to avoid frequent trips

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Reaction time must be minimum

There are basically three types of flame scanners 1.

ultraviolet scanners

2.

visible light scanners 3. infrared scanners

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Chapter 8 Fuel Firing System

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