Design Steps : Furnace Of A Steam Generator

P M V SubbaraoProfessor

Mechanical Engineering Department

Selection of Geometric Parameters….

Heat available to the furnace

• Incomplete combustion loss

• Unburned Carbon loss

• Loss due to slag

• Energy brought in by preheated air & fuel.

• A part of this total heat should be absorbed in furnace.• The designer should provide an environment for the same.

aislagCCOcfu QQQQLHVmQ

COQ

slagQ

CQ

aiQ

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

burningrr

cv tt

Vt

LHVmq

*

• Where, mc = Design fuel 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 completely.• 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.

Selection of Furnace Design Parameters

• A suitable value for ;

• the rate of heat generated by combustion of fuel in a unit effective volume of the furnace, qv.

• the rate of heat released per unit cross section of the furnace, qA.

• The rate of heat released per unit wall area of burner section of the furnace, qb.

General Guide Lines for Design

• 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.

• Furnace should proved an exit and path for free fall of ash, without major heat loss.

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

Basic Geometry of A Furnace

v

c

q

LHVmV

A

c

grate q

LHVmbaA

b

cb q

LHVmHba

2

Any limit on minimum height of furnace?

Any limit on depth of furnace C.S.?

Furnace Depth & Height

• Depth (a) to breadth (b)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.

Modfications in Geometry of A Furnace

Boiling process in Tubular Geometries

Water

Heat Input

Hea

t Inp

utWater

Water

Steam

Steam

Partial Steam GenerationComplete or Once-through Generation

Further Geometrical Details of A Furnace

Determination of Furnace Size

• What is the boundary of a furnace?

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

tubes

– Central lines of the first row super heater tubes.

= 30 to 50O

> 30O

= 50 to 55O

• E = 0.8 to 1.6 m

• d = 0.25 b to 0.33 b

Heat Transfer in A Furnace

• The flame transfers its heat energy to the water walls in the furnace by Radiation.

• Convective Heat Transfer < 5%.• Only Radiation Heat Transfer is Considered!• Complexities:• Non uniform temperature of tubes.• Fouling of surfaces of tubes.• Variation of furnace temperature along its

– Height– Width– Depth.

Non uniform Heat Flux !!!!!

Simplified Approach

• Emitted Radiation heat flux of flames:

• Emitted Radiation = Available Heat

• Heat flux absorbed by walls :

• Thermal efficiency factor,

• The rate of heat absorption

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flabs Jq

kWTAQ flflabs 4

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Coal fired furnace

Structure of water walls*

Hot Exhaust gases

Burner

Flame

Furnace Exit

Heat Radiation & Convection

*www.directindustry.com

• Two functions of coal fired

furnace: Release of chemical energy by

combustion of fuel

Transfer of heat from flame

to water walls

• Combustion space

surrounded by water walls