1. Mechanisms of Fuel CombustionBY
P M V Subbarao
Associate Professor
Mechanical Engineering Department
I I T Delhi
An essential items for Reliability of A Combustion System .

2. Anatomy of Combustion Mechanism
Fuel Handling Equipment – To ensure reliable supply of fuel at any given rate of requirement.
Fuel Preparation Equipment – To ensure all the requirements of efficient combustion.
Fuel Firing Equipment – To make sure that combustion will reach its culmination.
Fuels – solids, liquids & gases.

3. Basic Mechanisms of Solid Fuel Combustion

4. Major Components of Present Coal Fired Steam Generator

5. Basic Features of solid Combustion
Green Coal
Grate
Primary Air

6. Basic Features of solid Combustion
O2+CO2+N2+H2O
Flame
Secondary Air
VM+CO+CO2+N2+H2
Green Coal
Incandescent coke
CO+CO2+N2+H2
Grate
Primary Air

7. Basic Features of solid Combustion
O2+CO2+N2+H2O
Flame
Secondary Air
VM+CO+CO2+N2+H2
Green Coal
Incandescent coke
ASH
CO+CO2+N2+H2
Grate
Primary Air

8. Flow Through Porous Beds
Floating Bed
Dynamic Bed
Static Bed Dp
Velocity

10. Fuel Bed Combustion O2+CO2+N2+H2O Secondary Air Flame Green Coal
VM+CO+CO2+N2+H2
Green Coal
Incandescent coke
ASH
CO+CO2+N2+H2
Grate
Primary Air

11. Salient Features of Stokers
The grate heat release rate is < 1340 kW/sq.m.
Size of coal 19 mm to 38 mm.
Only suitable for non caking or free burning coal.
Max. ash allowed is 20%.
Largest capacity possible : 155MW.
Maximum steaming rate: 50kg/s.
Pressure drop
m U (1- e) rg U2 (1 – e)
Dp =
(fdp)2e fdpe3

12. Automation in Fuel Bed Combustion
Also called Mechanical Stokers.
Travelling Grate Stoker
Chain Grate Stoker
Spreader Stoker
Vibrating Stoker
Underfeed Stoker

13. Travelling Grate Stoker
Boiler type: natural circulation
Firing method: stoker fired
Max. continuous rating:9,7 kg/s
Steam temperature outlet: 450°C
Steam pressure outlet: 4,0 Mpa
Gross efficiency: 87%
Fuel type: Bituminous coal
Net calorific value: 23 MJ/kg

14. SPREADER-STOKER FIRED BOILER
                                                                                                                                                            

15. Modeling of Fixed Bed Combustion
A bed of stoker-sized coal particles is supported by a grate.
Air flows upwards through the grate and the fuel bed.
Upon heating, coal particles first undergo a stage of Thermal preparation.
Evaporation of Moisture drying.
Distillation of VM (Pyrolosis)
Production of enough VM to start ignition.
Combustion of char.
The coal flows slowly downward at a velocity Vs, as the coal burns out in the lower layer of the bed.

16. Drying of Coal in a stoker
Moisture exists in solid fuel in two forms.
Free moisture
Bound moisture
WOOD ~ 45% : Lignite ~ 40% : Bituminous < 10%
During combustion, the biomass first loses its moisture at temperatures up to 100°C, using heat from other particles that release their heat value.
In a large fuel particles, temperature gradients exist within the particle.
Moisture evolves from inside the particle, while volatiles are being driven off near the outer shell of the particle.
Release of volatiles in outer shell develop high pressure in the fuel pores.
This high pressure in outer pores, some of the moisture released in inner layers is forced toward the center of the particle until the pressure builds up throughout the particle.

17. Hence, drying of large solid fuel particle initially involve migration of the water vapor as well as the outward flow.
A pyrolysis layer starts at the outer edge of the particle and gradually moves inward, releasing volatiles and forming char.
The moisture release reduces the heat and mass transfer to the particle surface so that the mass loss of the particle is reduced.
As the moisture and volatile release is reduced, the char surface begins to react.

19. 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

21. Heating & Devolatization (Pyrolysis)
Temperature of the particle rises fast after the completion of particle drying.
Start of Pyrolysis:
Terpens : 225 0C
Hemi cellulose : 225 – 325 0C
Cellulose : 325 – 375 0C
Lignin : 300 – 500 0C

22. The rate of devolatization of solid fuel
Where
As the dried particle heats up, volatile gases containing hydrocarbons, CO, CH4 and other gaseous components are released.
In a combustion process, these gases contribute about 70% of the heating value of the biomass.
Finally, char oxidizes and ash remains.
Completion of volatization generates char.

23. Char Combustion Char is a highly porous solid carbon.
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.

24. C +1/2 O2 → CO CO+ 1/2O2 → CO2 C+CO2 → 2CO C+H2O → H2O + CO
Oxygen reacts with char to produce CO in the lower portion of the bed.
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

25. The char moves down because the char is shrinking as it burns.
Consider a one-dimensional slice dz thick through the bed of corss-sectional area A.
The rate at which char particles subside toward the grate equals the rate at which the char is consumed:
The char moves down because the char is shrinking as it burns.
If bed void fraction is e,

26. Where

27. Energy Balance
The rate of change of enthalpy of gas is equal to rate of generation of thermal energy due to combustion.

28. Flame in fixed Bed Combustion

30. Pulverized Fuel Combustion
Invented in 1920.
An universal choice for power plants till 1990.
Fine particles of coal ~ 75 microns.
Surface area : 150 m2/kg.
Huge heat release per unit area : 5 – 10 MW/m2.
Steam generation : 5000 tons/hour.

31. Flow Path of Fuel

33. Introduction of fuel in A PC Furnace

34. Pulverizers
Normally, from 2 to 8 pulverizers may be installed at each steam generator, and often one pulverizer is kept out of service as a spare.
Each pulverizer normally supplies coal to one level of burners.

35. Ball Mill

36. Ball TubeMill The oldest pulverizer design still in frequent use.
25% to 30% of cylinder volume is filled with wear resistant balls of 30 – 60mm.
The cylinder is rotated at a speed of about 20 rpm.
Specific power consumption 22 kWh per Ton.
Suitable for hard coals.
Highly reliable in requires low maintenance.
Bulky and heavy in construction.

37. Ball and Race Mill It is a high speed pulverizser.
The speed of lower race is > 225 rpm.
It is a compact mill.
Specific power consumption 14 kWh per ton.

38. Bowl Mill The most widely used mill for grinding coal.
This is an intermediate speed pulverizer.
The vertical shaft rotates at a speed 65 – 100 rpm.
Specific power consumption 12 kWh/ton.

39. Mill Design Parameters
The number and/or size of the pulverizer train components.
The pulverizer capacity, feeder capacity, and burner heat input,
The capability of the pulverizer train equipment
Auxiliary power requirements.
Maintenance and availability impacts.
Adequacy of the primary air heating system to supply enough heat to achieve the desired pulverizer outlet coal/air temperatures.

40. Energy Balance across pulverizer is very critical for satisfactory
operation of Steam Generator.

41. Hot air
Heat loss
Puliverizer frictional
dissipation
Dry pulverized coal +
Air + Moisture
Coal
Motor Power Input

42. Prediction of Coal Drying
For predicting the amount of coal drying which is needed from the pulverizers, the following methods were accepted.
For very high rank coals (fixed carbon greater than 93 percent), an outlet temperature of 75 to 80° C appeared most valid.
For low- and medium-volatile bituminous coals, an outlet temperature of ° C appeared most valid.
Bituminous B and C coals appear to have good outlet moisture an outlet temperature of 55 to 60° C is valid.
For low-rank coals, subbituminous through lignite (less than 69 percent fixed carbon, all of the surface moisture and one-third of the equilibrium moisture is driven off in the mills.

44. Fluidized Bed Combustion
A True Appropriate Technology for Effficient Solid Fuel Combustion

47. Liquid Fuel Combustion Systems

48. Oil Supply System
A typical oil supply system includes an oil tank, oil strainer, oil supply pump, heater and connecting pipelines to the boiler.

49. Design of Oil Supply Systems
General considerations in the design of the oil supply system are safety, cost, convenience and emergency provision.
Heating Loop: Heavy oils are economic but difficult to atomize owing its high viscosity at atmospheric temperatures.
The oil is heated prior to burning.
The heating loop consists of strainer, pump and heater.
Classification of Heating Systems:
Single unit heating loop
Centralized heating loop
Oil pumps are selected as several in number with smaller capacity: