A Cause – Effect Analysis of Furnace Heat Transfer

1. A Cause – Effect Analysis of Furnace Heat Transfer BY P M V Subbarao Associate Professor Mechanical Engineering Department I I T Delhi Closed form solutions for performance analysis of complex heat transfer devices…..

2. 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.

3. Analysis of the Primary Cause • Chemical Energy ↔ Thermal (Sensible) Energy • Reactants ↔ Products • 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. The temperature of the gases in an adiabatic furnace attain a maximum temperature called adiabatic flame temperature.

4. 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.

5. 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

6. 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.

7. Heat Release Rate per Unit Volume, qv • The amount of heat generated by combustion of fuel in a unit effective volume of the furnace. • 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.

9. 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. • 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.

10. A

11. 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. • 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.

13. 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.

14. Furnace Depth & Height

16. Basic Geometry of A Furnace

17. Dimensions of A 500 MW(e) Plant Furnace

18. HT Areas of A 500 MW(e) Plant Furnace

19. Analysis of the Secondary Cause • Emissive power of flame: • Where eflame is the emissivity of flame. How to find the area of a Flame ?

20. How to Find the Emissivity of A Flame Flame Length, m

21. Analysis of the Tertiary Cause • Radiation heat transfer • Where eeff is the emissivity of flame and water wall system. • Heat flux is non uniform. • Wall temperature is non uniform. • This effect is another cause for further analysis.

22. Distribution of Heat Flux on Furnace Walls

23. Analysis of the Last but One Effect Tfe • 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. Tflame

24. Mixed Adiabatic Temperature of the Gases