DieselShip

Boiler burners

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 completing their combustion process and exiting at the same rate as the fuel entering.

Primary Flame

• 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 just fills the quarl space

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.

• The air is generally heated on larger plant to;

–      prevent thermal shocking

–      improve the combustion process

–      improve plant efficiency (bled steam and regenerative)

• 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.
• 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

Turndown ratio

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

Types of burners

–      Pressure jet

–      Spill type pressure jet

–      Variable orifice pressure jet

–      Spinning cup

–      Steam assisted

–      Ultrasonic

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).
• Another disadvantage over assisted atomisation burners is the lack of cooling from steam or air means the burner must be removed when not in use from lit boilers to prevent carbonising in the tube

• The advantage is that it does not require any assistance other than supplying the fuel at the correct pressure.

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 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 supplied 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 fuel pressure the pressure tip provides for the atomisation

Steam atomisation

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

Y- Jet

• Here the steam and fuel are mixed into an emulsion and expanded in the holes before emission 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

venturi register

• In 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

–      Wider turn down ratio with lower excess air (15 :1)

–      Low O₂ levels

–      Simplified operation

–      Reduced acid corrosion problems

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.
• The spinning cup offers the following advantages;
  • 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.

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