1. Phases in Combustion of Travelling Coal Particles
P M V Subbarao
Professor
Mechanical Engineering Department
Timing Processes for A True Open System…..

2. Mill Mass Balance : A SSSF Open System
Flow of
Crushed Coal
~20 mm
Tempering Air, Tatm
Low Moisture pulverized coal
+ Air
+ Moisture
Flow of Hot air

3. Mill Energy Balance Hot air Heat loss Puliverizer frictional
Coal
Dry pulverized coal +
Air + Moisture
Puliverizer frictional
dissipation
Motor Power Input
Heat loss
Tempering Air, Tatm
Energy Balance for Drying of Coal:
Heat Transfer to coal particle due to convection = The rate of Change of internal Energy of the particle + Rate of Energy used for evaporation of moisture in coal.

4. Drying of Coal Particle: Energy Balance
The rate of change of internal energy of the coal particle
Moisture

5. Drying of Coal Particle
Energy loss due to evaporation =
Instantaneous rate of evaporation:

6. Moisture Content in Coal at Mill Exit
Moisture removed in Mill:

7. Mill Operation Window Erosion Limit PC Transport Limit
Tampering Limit
Erosion Limit
PC Transport
Limit
Drying Limit
Milling Capacity Limit

8. Suggested Moisture Level in Coal at Mill Exit
Min  Mexit

9. Pulverized Coal Distribution System

10. Transport of coal particles from Mill to Furnace
Pneumatic conveying of coal particles are known as flows with low-solids concentration ("dilute-phase flow").
The ratio of coal to carrying gas is determined by systems and combustion considerations and is usually in the range of kg of coal/kg of air.
Assuming a coal density rc = 1.5 x 103 kg/m3, and the density of the carrying gas as rg = 0.9 kg/m3, the volume fraction of the coal can be shown to be very small, %.

11. Coal Particle Transport Velocity
An important aerodynamic characteristic of the particles is their terminal velocity.
This is defined as the free-fall velocity in stagnant air.
For a spherical particle of d = 100 m has an approximate value of Vt = 0.3m/sec.
Local velocities of air flow must impart a velocity to coal particles.
If Vparticle < Vterminal : Falling of particles
If Vparticle = Vterminal : Floating of particles
If Vparticle > Vterminal : Transport of particles
Due to non-uniformities of flow behind bends, and to avoid settling of solids in horizontal sections of the transport line, a minimum air velocity of V = m/sec is recommended.

12. Introduction of Coal Particles into Furnace ????

13. Introduction of Coal Particles into Furnace ????
Combustion is Chemistry
Fuel transport is fluid mechanics
Calculations for Chemistry ????
Robert Bunsen
Bunsen’s developed a gas burner.
By introducing air into the gas in the correct proportion before it burns, a clean, soot-free, almost colorless flame is produced.
Using his burner, Bunsen used flame tests to analyze substances much more reliably than ever before.
The burners he designed were made by Peter Desaga, his laboratory assistant.

14. The Burners
Bunsen published the design of the burner in 1857, but did not patent his design.
He did not wish to make profits from science; he believed the intellectual rewards were more than enough.
His burner is now used not only for flame tests.
It is used to heat samples and to sterilize equipment in medical laboratories all over the world.
Burner Governs:
Fuel Ignition
Aerodynamics of Fuel air mixture
Generation of combustion conditions.

15. Simple Bunsen Burner
Burning Velocity
Flow velocity
Air
Fuel

16. Positioning of Flame in A Furnace

17. Velocity of Planar Laminar Flames

18. Flame Speed & Rate of Combustion
Laminar
Turbulent

19. Stable Flame, Flame Speed & Rate of Combustion : Coal Quality
VM=30% & A=5%
VM=20% & A=5%
VM=15% & A=5%

20. Stable Flame, Flame Speed & Rate of Combustion
VM=30% & A=15%
VM=30% & A=5%
VM=30% & A=30%
VM=30% & A=40%

21. Stability & Flammability Limits
Burning Velocity > Flow Velocity : Flash Back Limit
Burning Velocity < flow Velocity : Blow Off Limit
Burning Velocity = Flow Velocity : Stable Flame.
Rich Mixture
Fuel Flow rate
Flash Back
Stable Flame
Blow off
Lean Mixture
Air Flow rate

22. Burning Velocity & Residence Time
Quality of Fuel & Fuel Chemistry.
Air-fuel ratio
Turbulence level
Time to be spent by fuel particle in the furnace before it burns completely.
Residence time is inversely proportional to burning velocity.
Fuel particle is continuously moving.
The distance traveled by the fuel particle should be much larger than furnace height.
Swirl motion will ensure the required residence time.
Internally generated swirl : Swirl Burners.
Externally generated swirl: Direct Burners.

23. Frugal Solutions for Turbulent Combustion

24. Tangentially Fired Burnes
External Swirl……. A tiny Tornado …
Generation of a Whirling Fireball …

25. Anatomy of Whirling Fireball
Vortex core – Quasi-solid Zone.
Vortex annulus – Quasi-equivalence zone.

26. Anatomy of Fireball
Tangential Velocity in the furnace

27. Stream line in Furnace Cross-section

28. Three dimensional Fire Ball

29. Burner Parameters
Primary and secondary air velocities should be decided to locate the point of ignition from the exit plane of burner.
Low (16-20 m/s) are not suitable for high volatile coals.
Low velocities are recommended for low volatile coals.
Slagging and thermal distortion of the burners can result in variations in velocities.
Large particles throw active combustion region into the wall region.
This increases slagging and increase in Unburned carbon losses.
For bituminous secondary : primary air velocity ratios are
This gives sufficient reserve for load decrease without significantly impairing the aerodynamics of P.F. Jet.

30. Particle size distribution
Particles less than 60 mm burn before entering the cyclone.
Particles 60 mm to 100 mm are likely to be carried inside the cyclone.
The majority of the particles larger than 150 mm will be flung to the wall and will start to burn there.

31. Thermo-physics of Traveling Coal Particle in A Furnace
Coal Particles are dragged into furnace by air and continuously moving towards furnace exit.
The hot gas environment changes the thermo-physics of the coal particle.
Last stage Drying
Devolatization & Pyrolysis
Char Ignition and combustion.

32. Drying of Coal Particle: Energy Balance
The rate of Change of internal Energy of the particle + Rate of Energy loss due to evaporation of moisture =
Energy gain due to convection +Radiation
Energy gain: Qin = Qconv + Qrad
Qconv
Qrad
Moisture

33. Drying Time for Coal Particle & Furnace Size
Qconv
Qrad
Moisture