Liquid Fuel Burners

Oils may be burnt in two ways it is vaporized before ignition so that

it burns like a gas(vaporising burners) it is converted into droplets which are

injected into hot air so that they evaporate while burning (atomising burners)

Liquid Fuel Burners

Atomising burners: On industrial scale, most commonly used burners are atomising burners

Oil is heated to low viscosity and atomised(i) Mechanically by means of a rotating disc

or cup with a uniform droplet size (50 microns)

(ii) By a high pressure ejection from a fine orifice which gives a conical spray

Types of Atomising Oil Burners

There are three types which differ on the principal of atomising

Pressure Jet Atomising Burners Blast Atomising Burners Rotary Atomising Burners

Pressure Jet Atomising Burners Oil enters the circular swirl chamber through

tangentially spaced slots Oil will rotate in the chamber around the air The rotating mass is passed through an orifice

resulting in the formation of spray of drops The viscosity should be 70 Redwood I seconds

for small nozzles and 100 Redwood I seconds for large nozzles

These burners have low operating cost and most widely used

These type of burners have limited turndown ratio

Turndown ratio can be increased by design modifications e.g. by increasing the number of tangential slots

Blast Atomising Burners

These burners use air or steam to atomize the oil

Oil flow through a central tube at a controlled rate mixed with mixed with air as it emerges from the tube

Depending upon the pressure they may be classified as Low pressure, Medium pressure or high pressure

Depending upon the mixing system they may be classified as inside-mix type or outside-mix type

Blast Atomising Burners

High pressure burners have high turndown ratio (10:1)

Inside-mix type commonly provide more eficiency

Blast Atomising Burners

Rotary Atomising Burners These burners have a centeral stationary fuel line which

delivers the oil to the inner surface of rotating hollow cup The cup is rotated at 3600- 10,000 rpm Centrifugal force causes the oil to flow towards the brim of

the cup in the form of thin film which disintegrates into small droplets

A fan attached to the rotating shaft provides primary air These burners can be used for more viscous fuels Low viscosity may cause the oil to slip within the cup

resulting in low atomizing efficiency

These burners can have high turn down ratio but low capacity

Rotary Atomising Burners

Adiabatic Flame Temperature

For adiabatic flame temperature following assumptions are made

No heat loss to the surroundings Combustion is complete No thermal dissociation A reference/datum temperature is

selected

Adiabatic Flame Temperature

Adiabatic flame

Fuel

oxident

diluent

Combustion products

Adiabatic Flame Temperature

A fuel gas containing 20 % CO and 80 % N2 is

burned with 150 % excess air (both air and gas

being at 25 C). Calculate the theoretical flame

temperaure of the gas.

Following data is available

CO2 O2 N2 CO

Av. Sp. Heat kcal/mole K

12.10 7.90 7.55 -

Heat of formation at 25 C kcal/kg mole

-94052 -26412

Material BalanceBasis: 100 kg moles of fuel gas

Material enteringKg moles

Materials leavingKg moles

CON2O2CO2

2080 + 174.0525-

-80 +174.051520

total 299.05 289.05

Energy Balance: assuming reference temp. 25 CHeat of reaction = -94052 –(-26412)= -67640 kcal / kg moleHeat produced by combustion = 20 x -67640 = -1352800

Cp kcal / kg mole K

Amount kg mole

mCp dT

CO2 12.10 20 20x12.10xdT

O2 7.90 15 7.90 x 15 x dT

N2 7.55 174.05 7.55 x 174.05 x dT

Total=1674.58 dT

1674.58(Tf -298)= 1352800Tf= 832.8 C

Adiabatic Flame Temperature

NCV+∆hf +A∆Ha=V∆Hfg +qd + ql

A= air supplied m3/m3 fuel

V=flue gases produced m3/m3 of fuel

∆Hf=enthalpy of fuel above reference temperature

∆Hfg=tf. Cpfg(0-tf) –trCpfg(0-tr)

Tf=(NCV+∆hf +A∆Ha -qd– ql +V. trCpfg(0-tr))/ V. Cpfg(0-tf)

Adiabatic Flame TemperatureCalculate the theoretical flame temperature for a fuel

gas under the following conditions:(i) Both fuel and theoretical air are at 15 C(ii) 50 % excess air at 15 C and fuel gas at 15 C(iii) Theoretical air at 60 C and gas at 400 C(iv) Theoretical oxygen at 15 C and fuel gas at 15 CData:Fuel gas: CO : 22% CO2 : 18% H2 : 2% N2 : 58%NCV: 719 kcal/m3Mean sp. Heat of fuel gas at 600 C= 0.342 kcal/m3 C