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. 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.

4. 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.

5. Simple Bunsen Burner
Burning Velocity
Flow velocity
Air
Fuel

6. Positioning of Flame in A Furnace

7. Velocity of Planar Laminar Flames

8. Simple Bunsen Burner
Burning Velocity
Flow velocity
Air
Fuel

9. Flame Speed & Rate of Combustion
Laminar
Turbulent

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

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

12. 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

13. 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.

14. Frugal Solutions for Turbulent Combustion

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

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

17. Anatomy of Fireball
Tangential Velocity in the furnace

18. Stream line in Furnace Cross-section

19. Three dimensional Fire Ball

20. 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.

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

22. 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 of Drying
Devolatization & Pyrolysis
Char Ignition and combustion.

23. 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

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

25. Post Drying Process in PC Furnace

26. Timings & Phases of pulverized coal burning
The time of pulverized coal-air mixture in a furnace can be divided into three periods:
1. Drying, Devolatilization and ignition of char particles, which require 0.2–0.3 s.
2. Intensive mixing and burning of pulverized coal-air jet during 0.5–1.5s.
Leads to formation of with formation of flame kernel with a temperature 1500–1600 °C at a distance of 1–5 m.
3. Burnout of larger coal particles and cooling of flue gas during1–3 s over a distance 2/3 of the furnace height.

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

28. Onset of Devolatization & Pyrolysis
Temperature of the particle rises fast after the completion of particle drying.
As the dried particle heats up, volatile gases containing hydrocarbons, CO, CH4 and other gaseous components are released.
Start of Pyrolysis:
Terpens : 225 0C
Hemi cellulose : 225 – 325 0C
Cellulose : 325 – 375 0C
Lignin : 300 – 500 0C

29. Time Taken by Devolatization & Pyrolysis
The rate of devolatization/pyrolysis of solid fuel
Where
The released combustible gases get ignite and burn outside the particle.
In a combustion process, these gases contribute about 70% of the heating value of the biomass.

30. Char Combustion Char is a highly porous mixture solid carbon and ash.
Wood char , f = 0.9
Coal char, f = 0.7
Internal surface area : 100 sq. m. per gm. – coal char.
: 10,000 sq.m per gm – Wood char.
Oxygen is first absorbed from the gas volume on the surface of particles.
Absorbed oxygen reacts with carbon to from complex carbon-oxygen compounds : CxOy.
These complex compounds dissociated into CO2 & CO.

31. Mechanism of Char Combustion
Oxygen reacts with char to produce CO in the lower portion of the furnace.
The CO reacts rapidly inn the gas to form CO2.
The CO2 in turn is reduced by the char.
The latter reaction causes CO buildup when oxygen is depleted.
The resulting reactions:
C +1/2 O2 → CO
CO+ 1/2O2 → CO2
C+CO2 → 2CO
C+H2O → H2O + CO

32. Combustion of Char Cloud
Furnace Exit
Plug Flow Zone
Heat release Zone
Recirculation Zone
Elevation Z=0

33. The Process of PC Cloud Combustion
The heat release zone comprises of the "firing" slices.
Each slice is capable of releasing a certain percentage of the available calorific value.
The mass of products "created" by the firing slices directly proportional to the percentage of heat release.
The flow of reactants originates in the heat release zone, circulates through the hopper region.
The contribution from each "firing" slice to the recirculation or "back-mix" flow is assumed proportional to the mass of combustion products generated in the firing slice.

34. Generation & Exchange of Microscopic Kinetic Energy
Popularly known as the Sensible Heat Inventory of A slice.
SHI of ith Slice:
Sensible Heat Leaving Slice "i"
ith Slice
Heat Release in Slice "i"
Sensible Heat Entering Slice "i"

35. Distribution of Heat Release Rate

36. Energy Balance
The rate of change of enthalpy of gas is equal to rate of generation of thermal energy due to combustion of several volatile hydro carbon compounds and solid carbon.
Furnace

37. Distribution of Heat Flux
Average Heat flux can be calculated using:
As the temperature and emissivity of flame is not uniform in the furnace volume, the local heat flux is not uniform.
Actual local wall temperature depends on the value of local heat flux.
Special experiments are carried out to find the heat flux distribution on water wall.

38. Control of Local Heat Flux Distribution

39. Diabatic Furnace Wall : Water Wall
Furnace Exit
Hot Exhaust gases
Flame
Heat Radiation & Convection
Burners

40. The Basics of Flow Boiling
Supercritical Steam Generation
Subcritical Flow Boiling

41. Religious to Secular Attitude of Water