A Cause – Effect Analysis of Furnace Heat Transfer

BY

P M V SubbaraoAssociate Professor

Mechanical Engineering DepartmentI I T Delhi

Closed form solutions for performance analysis of complex heat transfer devices…..

Cause – Effect Analysis• Combustion is a primary cause

• Steam Generation is an ultimate effect.

• Heat transfer is a mediation.

• Combustion causes the generation of heat in side furnace volume.

• Heat generation causes the production of high temperature gases.

• These high temperature gases cause the Radiation and convection heat transfer processes.

• Heat Transfer processes carry the thermal energy to furnace wall & steam tubes.

• Conduction through the tubes and walls causes the convection inside the tubes.

• Convection Causes the generation of steam.

• A cause effect analysis can simplify the design analysis of a furnace.

Analysis of the Primary Cause

• Reactants ↔ Products i j

jjii PYRX

•At a given temperature the Gibbs free energy of products is less than Reactants.•Depending on the effectiveness of heat release rate, the sensible energy of products will be higher than sensible energy of reactants.•Hence, the temperature of products of combustion is very high.

• Chemical Energy ↔ Thermal (Sensible) Energy

exchangei j

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The temperature of the gases in an adiabatic furnace attain a maximum temperature called adiabatic flame temperature.

General Design Principles

• The effective heat release rate is depends on the size of furnace.• The furnace should provide the required physical environment and the

time to complete the combustion of fuel.• The furnace should have adequate radiative heating surfaces to cool

the flue gas sufficiently to ensure safe operation of the downstream convective heating surface.

• Aerodynamics in the furnace should prevent impingement of flames on the water wall and ensure uniform distribution of heat flux on the water wall.

• The furnace should provide conditions favoring reliable natural circulation of water through water wall tubes.

• The configuration of the furnace should be compact enough to minimize the amount of steel and other construction material.

Determination of Furnace Size

• What is the boundary of a furnace?

• The boundary of a furnace is defined by– Central horizontal plane of water wall

and roof tubes

– Central horizontal lines of the first set of super heater panels.

= 30 to 50O

> 30O

= 50 to 55O

• E = 0.8 to 1.6 m

• d = 0.25 b to 0.33 b

Design Constrains:Heat Release Rate

• Heat Release Rate per Unit Volume, qv, kW/m3

• Heat Release Rate per Unit Cross Sectional Area,qa, kW/m2

• Heat Release Rate per Unit Wall Area of the Burner Region, qb, kW/m2

• The maximum allowable heat flux of the water wall is restricted by its water-side burnout (dryout) heat flux.

Heat Release Rate per Unit Volume, qv

• The amount of heat generated by combustion of fuel in a unit effective volume of the furnace.

3/ mkWV

LHVmq

c

v

r

cv Vt

LHVmq

• Where, mc = Design fuel(coal) consumption rate, kg/s.• V = Furnace volume, Cu. m.• LHV= Lower heating value of fuel kJ/kg.

• A proper choice of volumetric heat release rate ensures the critical fuel residence time.

• Fuel particles are burnt substantially• The flue gas is cooled to the required safe temperature.

Heat Release Rate per Unit Cross Sectional Area,qa

• The amount of heat released per unit cross section of the furnace.

• Also called as Grate heat release rate.

2/ mkWA

LHVmq

grate

c

A

• Agrate is the cross sectional area or grate area of the furnace, Sq. m.

• This indicates the temperature levels in the furnace.

• An increase in qa, leads to a rise in temperature in burner region.

• This helps in the stability of flame

• Increases the possibility of slagging.

A

Heat Release Rate per Unit Wall Area of the Burner Region

• The burner region of the furnace is the most intense heat zone.

• The amount of heat released per unit water wall area in the burner region.

2/

2mkW

Hba

LHVmq

bb

• a and b are width and depth of furnace, and Hb is the height of burner region.

• This represents the temperature level and heat flux in the burner region.

• Used to judge the general condition of the burner region.• Its value depends on Fuel ignition characteristics, ash characteristics,

firing method and arrangement of the burners.

Furnace Depth & Height

• Depth to breadth ratio is an important parameter from both combustion and heat absorption standpoint.

• Following factors influence the minimum value of breadth.– Capacity of the boiler

– Type of fuel

– Arrangement of burners

– Heat release rate per unit furnace area

– Capacity of each burner

• The furnace should be sufficiently high so that the flame does not hit the super heater tubes.

• The minimum height depends on type of coal and capacity of burner.

• Lower the value of height the worse the natural circulation.

Furnace Depth & Height

Basic Geometry of A Furnace

safev

c

q

LHVmV

,

safeA

c

grate q

LHVmbaA

,

safeb

b q

LHVmHba

,

2

safeff hh ,

safebb

Dimensions of A 500 MW(e) Plant Furnace

HT Areas of A 500 MW(e) Plant Furnace

Analysis of the Secondary Cause

• Emissive power of flame: kWTAQ flflameemi 4

How to find the area of a Flame ?

• Where flame is the emissivity of flame.

How to Find the Emissivity of A Flame

Flame Length, m

Analysis of the Tertiary Cause

• Radiation heat transfer

• Where eff is the emissivity of flame and water wall system.

kWTTAFQ wafleffrad 44

wafl

wafleff

111

•Heat flux is non uniform.•Wall temperature is non uniform.•This effect is another cause for further analysis.

Distribution of Heat Flux on Furnace Walls kWTTFtzyxq wafleff ):,,('' 44

Analysis of the Last but One Effect

• Final effect : Tfl gets changed to Furnace Exit GasTemperature.

• Due to energy lost by hot gases. – Loss due to Environment– Energy absorbed by water walls

• Energy lost by hot gasses from flame to exit.

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kWTTCF

AmQ FEGTadp

act

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Tflame

Tfe

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Mixed Adiabatic Temperature of the Gases

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ref

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CT

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210