Burners & Refractory

Combustion of fuel in furnace and burner design

Process

The heat producing constituents of the fuel are hydrogen, carbon and sulphur.

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  • Carbon to carbon dioxide - 34
  • Hydrogen to water - 120.5 ( assuming the water vapor is not allowed to condense)
  • Sulphur to sulphur dioxide - 9.3
The calorific value of the combustion processes measured in mega joules for each Kg of fuel burnt

The main cause of heat loss with the process is that taken away by nitrogen. Therefore, to achieve maximum efficiency the excess air should be kept to a minimum. However there is a limit to the reduction in the excess air in that the combustion process must be fully completed within the furnace and within a finite time.
The main type of combustion process is called the suspended flame. The flame front remains in the same position relative to the burner and quarl.. The fuel particles pass through the flame completeing their combustion process and exiting at the same rate as the fuel entering.

Primary Flame-To burn oil the temperature must be raised to vaporisation temperature, this can not be done in heaters due to gassing but is done by radiant heat in the flame. The lighter hydrocarbons in the atomised spray are rapidly heated and burnt in the primary flame. The heavier fractions pass through this achieving their vaporisation temperature. The primary flame is essential to good combustion. By design the primary flame exists where it receives maximum reflected heat from the shape of the quarl. The size of the primary flame ( shown smaller than actual in drawing) just fills the quarl space. Too large and impingement leads to carbon deposits building up. Too small unheated secondary air reduces combustion efficiency. The tip plate creates vortices reducing the mixing time for the air/fuel and reduces the forward speed of the flame

Secondary Flame-Here the heavier fractions are burnt. The velocity of the air and fuel must be matched to the required flame propogation rate.

Combustion in furnace space

For proper combustion of fuel in the furnace and adequate supply of air must be supplied and intimately mixed with a supply of combustible material which has been presented in the correct condition.

Air- it is the purpose of the register, swirler vanes and (vortice) plates, and quarl to supply the correct quantity of air for efficient combustion suitably agitated to allow proper mixing.

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  • prevent thermal shocking
  • improve the combustion process
  • improve plant efficiency (bled steam and regenerative)
The air is generally heated on larger plant to;

Fuel It is the purpose of the burner to present the fuel in suitable condition for proper combustion. Generally this means atomising the fuel and giving it some axial (for penetration) and angular (for mixing) velocity. For effective atomisation the viscosity of the fuel is critical, for fuels heavier than gas or diesel oils some degree of heating is required. It should be noted that the temperature of the fuel should not be allowed to raise too high as this can not only cause problem with fuel booster pumps but also can cause flame instability due to premature excessive gassification (is that a real word-answers to the normal address)
The smaller the droplet size the greater the surface areas/volume ratio is, this increases evaporation, heating and combustion rate.

Combustion zones

Register- supplies the correct quantity of excess air. Too little allows incomplete combustion, smoking, soot deposits and flame instability. Too much excess air reduces combustion efficiency by removing heat from the furnace space, may cause 'white' smoking and promote sulphurous deposits. In addition too much excess air increases the proportion of sulphur trioxide to dioxide promoting increase acid corrosion attack in the upper regions.
The register and to some extent the quarl determine the shape of the flame, short and fat for side fired boilers, long and thin for roof fired.

Flame burning off the tip- may occur after initial ignition or after a period of high excess air. The effect of this is to move the primary flame away from the quarl thereby effecting the combustion process leading to black smoke and flame instability. Two methods of bringing the flame back are to reduce excess air and introduce a hand ignitor to ignite the fuel correctly, or to rapidly close then open the register damper

Types

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  • Pressure jet
  • Spill type pressure jet
  • Variable orifice pressure jet
  • Spinning cup
  • Steam assisted
  • Ultrasonic
There are six main types of burner in common use;

Turndown ratio ratio of minimum to maximum flow ( roughly the square root of the ratio of maximum to minimum pressure)

Pressure jet

This is the simplest and oldest design of burner. Atomisation of the fuel is achieved by forcing the fuel under pressure through an orifice at the end of the burner, the pressure energy in the fuel is converted to velocity. Spin is given to the fuel prior to the orifice imparting centrigual force on the spray of fuel causing it to atomise.The disadvantage of this burner is its low 'Turn-Down' ratio (in the region of 3.5). The advantage is that it does not require any assistance other than supplying the fuel at the correct pressure. Due to this it is still seen even on larger plant were it is used as a first start or emergency burner.
Anouther disadvantage over assisted atomisation burners is the lack of cooling from stam or air means the burner must be removed when not in use from lit boilers to prevent carbonising in the tube

Spill type pressure jet

The method of atomisation is the same as for simple pressure jet type. The burner differs in that a proportion of the supplied fuel may be spilled off. This allows for increased turn down ratio

Variable orifice pressure jet

Fuel Pressure entering the burner acts against a spring loaded piston arrangement. Increasing pressure causes the piston to pull a spindle away from the tip, this has the effect of enlarging a closed swirl chamber and uncovering ports. In this way atomisation efficiency is maintained over a greater fuel supply pressure range

Steam assisted

Steam assisted atomisers. This can refer to both external and Internal steam/fuel mixing although conventionally they refer to external mix. In these no mixing of the steam and fuel occurs within the burner itself.
Fuel is suplied to a standard pressure tip atomiser. Steam passes around the fuel passage and exists through an open annulus having being given an angle of swirl to match the fuel spray. At low fuel pressure the steam, supplied at constant pressure throughout turndown, provides for good atomisation. At higher fure pressure the pressure tip provides for the atomisation.
For first start arrangements compressed air may be used.

Steam atomisation

The two main types of internal mixing (the most common) ar the 'Y' jet and the Skew jet .

Y- Jet

Here the steam anf fuel are mixed into an emulsion and expanded in the holes before emision creating good atomisation. This design is tolerant of viscosity changes and is frugal on steam consumption and require reduced fuel pump pressures .

Skew Jet

The main advantage of this design over the 'Y' jet is the reduced 'bluff' zone due the reduced pitch diameter of the exit holes.
Matched to a venturi register, a very stable efficient flame is formed. The Fuel/Steam mix exits the nozzle in a series of conic tangents, fuel reversals inside the fuel cone allow efficient mixing with air over a wide 'Turn-Down ratio (20:1). In addition this type of nozzle is associated with reduced atomising steam consumption (0.02Kg per Kg fuel burnt) Venturi and conventional register throat design

Ultrasonic

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  • Wider turn down ratio with lower excess air (15 :1)
  • Low O₂ levels
  • Simplified operation
  • Reduced acid corrosion problems
Manufactured by Kawasaki is said to offer the following advantages;

Atomisation is achieved primarily by the energy of ultrasonic waves imparted onto the fuel by the resonator tip which vibrates at a frequency of 5 MHz to 20 MHz under the influence of high speed steam or air impinging on it. Extremely small droplet sizes result which allow for a very stable flame.

Spinning Cup

Fuel is introduced onto the inner running surface of a highly polished fast spinning cup (3 to 7000 rpm). Under centrifugal force this fuel forms a thin film.
Due to the conical shape of the cup the fuel flows to the outer edge spilling into the primary atomising air stream. The fuel is broken into small droplets and mixed with the primary air supplied by the shaft mounted fan. Secondary air is supplied by an external fan for larger units.
Packaged units of this design have the air flow valve controlled by the fuel supply pressure to the distribution manifold.

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  • Wider turn down ratio with lower excess air
  • Low O₂ levels
  • No requirement for atomising air or steam
  • Low fuel pressure requirements to an extent that gravity flow is sufficient
  • stable flames achievable with very low fuel flows although maximum flow limited by size of cup. This, allied to being limited to side firing making the design more suitable for smaller installations.
The spinning cup offers the following advantages;

Blue Flame

This highly efficient and claen burning method is very close to stoichiometric combustion. Under normal conditions a portion of the hot gasses from the combustion process is recirculated. Fuel is introduced into the gas were it is vaporised. The resultant flame is blue with little or no smoke

REFRACTORY

Refractories

A material in solid form which is capable of maintaining its shape at high tempo (furnace tempo as high as 1650^oC) have been recorded.

Purpose

1.  
   1. To protect blr casing from overheating and distortion and the possible resulting leakage of gasses into the machinery space.
   2. To reduce heat loss and ensure acceptable cold faced temperature for operating personnel
   3. To protect exposed parts of drum and headers which would otherwise become overheated. Some tubes are similarly protected.
   4. Act as a heat reservoir.
   5. To be used to form baffles for protective purposes or for directing gas flow.

1.  

   Properties

   1. Must have good insulating properties.
   2. Must be able to withstand high tempo's
   3. Must have the mechanical strength to resist the forces set up by the adjacent refractory.
   4. Must be able to withstand vibration.
   5. Must be able to withstand the cutting and abrasive action of the flame and dust
   6. Must be able to expand and contract without cracking Note: no one refractory can be used economically throughout the boiler

Types

1.  
   1. Acid materials- clay, silica, quartz , sandstone etc
   2. Neutral materials-chromite, graphite, plumbago, alumina
   3. Alkaline or base materials- lime, magnesia, zirconia

Note that acid and alkaline refractories must be sepperated

Forms

1.  
   1. Firebricks- these are made from natural clay containing alumina , silica and quartz. They are shaped into bricks and fired in a kiln
   2. Monolithic refractories- These are supplied in the unfired state, installed in the boiler and fired in situ when the boiler is commissioned.
   3. Mouldable refractory- This is used where direct exposure to radiant heat takes place. It must be pounded into place during installation . It is made from natural clay with added calcided fire clay which has been chrushed and graded.
   4. Plastic chrome ore- This is bonded with clay and used for studded walls. It has little strength and hence stud provides the support and it is pounded inot place.It is able to resist high temperatures
   5. Castable refractory-This is placed over water walls and other parts of the boiler were it is protected from radiant heat . It is installed in a manner similar to concreting in building
   6. Insulating materials- Blocks, bricks , sheets and powder are usually second line refractories. I.E. Behind the furnace refractory which is exposed to the flame. Material; asbestos millboard, magnesia , calcined magnesia block, diatomite blocks, vermiculite etc. all having very low heat conductivity.

Furnace linings

Studded tubes

- these are lined with plastic chrome ore The amount of studding and the extent of tube surface covered with chrome ore is varied to suit the heat absorption rate required in the various zones of the boiler furnace.
Floor tubes may be situated beneath a 3" layer of brickwork, the tubes are embedded in chrushed insulating material below which is a layer of solid insulation and then layers of asbestos millboard and magnessia.

PRESENT DAY TYPES

TANGENT WALL.

Membrane wall

Furnace floors

- Two layers of 50 mm firebrick above the tubes and 100 mm slab insulation below. Tubes in castable insulation are covered with crushed firebrick. Note; Before castable insulation applied ,tubes coated with bitumen to allow expansion clearance when tubes are at working tempo

Front walls

- In front fired boilers these need additional insulation (200 mm) made up of 125 mm mouldable refractory backed by 50 mm castable or slab and 25 mm of asbestos millboard.

Burner openings

- These have specially shaped bricks called quarls or have plastic refractory

Brick bolts

1.  
   1. using a hole right through the brick
   2. Using a recess in the back of the brick.
There are two basic types;

A source of weakness is where bricks crack, bolts will be exposed to the direct heat which leads to failure.
Adequate expansion arrangements must be provided. For floor tubes a coating of bitumastic is first applied before the castable refractory is applied. When the boiler is fired the bitumastic is burnt away then a space is left for expansion

Refractory failure

This is one of the major items of maintenance costs in older types of boiler

SPALLING

This is the breaking away of layers of the brick surface. It can be caused by fluctuating temperature under flame impingement or firing a boiler too soon after waterwashing or brick work repair.May also be caused by failure to close off air from register outlet causing cool air to impinge on hot refractory.

SLAGGING

This is the softening of the bricks to a liquid state due to the prescience of vanadium or sodium ( ex sea water ) in the fuel. This acts as fluxes and lowers the melting point of the bricks which run to form a liquid pool in the furnace Eyebrows may form above quarls and attachment arrangements may become exposed Material falling to floor may critically reduce burner clearance and reduce efficiencyFlame impingement may lead to carbon penetrating refractory.

SHRINKAGE CRACKING

Refractories are weaker in tension than in compression or shear thus, if compression takes place due to the expansion of the brick at high temperature , if suddenly cooled cracking may occur.