1. PULVERISED COMBUSTION SYSTEM1

2. CONTENTS PULVERISATION FUELS COAL COAL APPLICATIONS COAL COMBUSTION
COMBUSTION OF FUELS- SOLID, LIQUID & GAS
EMERGING TRENDS
Nox FORMATION AND CONTROL

3. PULVERSIATION Coal dust is a fine powdered form of coal
More surface area per unit weight than lumps of coal
More susceptible to spontaneous combustion
Pulverised coal has significant dust explosion hazard

4. FUELS Solid Fuels Coal Peat Lignite Bituminous Anthracite Liquid Fuels
HSD
LFO
HFO
Gaseous Fuels
BFG
Producer Gas
LPG
Natural Gas

5. Evolution Time, Pres.&Temp.
COAL
Heterogeneous organic fuel formed mainly from decomposed plant matter.
Over 1200 coals have been classified.
40-90% C
2-7% H
<18-30% O
<6% S
1-5% N
20-70%
Char
5-45%
Ash
2-20%
H2O
15-30% VM
Ultimate Analysis
Coalification forms different Coal Types: Peat Lignite Bituminous coal Anthracite
Evolution Time, Pres.&Temp.
Coal Rank
Proximate Analysis

6. Coal

7. COAL APPLICATIONS Home – heating and cooking
Transportation – steam engines
Industry – Iron & steel
Electricity – power plants

8. COMBUSTION
Oxidation
oxygen combines with other elements and forms oxidies.
Combustion,
A special form of oxidation
Oxygen combines with fuels; coal, oil, gas
substantial amounts of heat is liberated.
The Degree of Flammability depends
convertibility to a gas,-nothing truly burns until it is a gas.
Nature of fuel
Quantity of the fuel,

9. Stages of Combustion Ignition Combustion Stability
Completion of Combustion
Combustion process, ignition occurs in vapour phase.
Solid and liquid fuels get ignited from their vapours. 9

10. Combustion Stability:
Ignited flame shall be sustained further so that the process of combustion would be continuous.
Completion of Combustion:
Fuel staying for the minimum period (residence time)
Completely oxidizing the combustible.
Considerations of combustion:
Safety consideration during fuel handling
Generation of Pollutants viz. Nox and Sox .

12. Coal Combustion Processes
homogeneous
combustion
CO2, H2O, …
volatiles
coal particle
p-coal, d=30-70m
heterogeneous
combustion
char
CO2, H2O, …
devolatilization
tdevolatile=1-5ms
tvolatiles=50-100ms
tchar=1-2sec t

13. Converting the complex fuel into elementary fuels;
Combustion of Fuels in furnace
Preparing the fuel and air ;
Converting the complex fuel into elementary fuels;
Right fuel and air mixture
Transferring heat from the products of combustion to the boiler or other surfaces.

15. The physical processes influencing pulverized coal combustion
Turbulent/Swirling flow of air and coal.
Turbulent/Convective/molecular diffusion of gaseous reactants and products.
Convective heat transfer through the gas and between the gas and coal particles.
Radiative heat transfer between the gas and coal particles and between the coal/air mixture and the furnace walls.

16. COMBUSTION OF DIFFERENT FUELS
Solid Fuel
Volatile matter is released over a temp. of deg. C.
The volatile matter is first ignited.
The coal particle upon releasing the volatile matter become a char. The char slowly burns out.
Liquid Fuel
Boils and releases volatile matter and gets ignited
The balance char completes the combustion. 7

17. COAL - AIR BALANCING IN FUEL PIPING
Un balance in Coal- air flow into the furnace results
uneven heat release
uneven distribution of excess air
unpredictable Nox formation
Remedies
Selection of Orifices for Coal air two phase flow regime.
Provision of on -line adjustment dampers in coal air flow path.
On-line measurement of coal -air flow using microwave techniques

20. OIL FIRING SYSTEM FEATURES
Fuel Oil Preparation
Pumping the oil and heating it are the major preparatory functions.
Filtration of oil to remove any dust, dirt, sediments, sledge etc.
This renders long trouble free service life to pumps, valves, atomisers etc
Maintaining the HFO temperature constant, corresponding to the atomizing viscosity of 15 to 20 centistokes, is essential for better fuel oil atomization.
A lower temperature of fuel oil impairs the burner performance and a higher temperature causes oil cracking.

21. Fuel Oil Atomisation
Atomisation
spraying the fuel oil into fine mist
Better mixing of the fuel with the combustion air
Pressure and viscosity influence atomisation
Pressure energy of the steam to velocity energy, which breaks up the oil stream into fine particles
Poorly atomisation results in
Bigger spray particles
Longer burning time
Carryovers of carbon and
flame instability due to low rate of heat liberation
Incomplete combustion and smoke

22. Oil Recirculation
To warm up the oil supply lines
To maintain correct atomising temperature
System Vents
Fuel oil heaters
Oil strainers
oil & steam lines - get rid of air locks.
System Drains
Oil lines with a drain valve at the lowest point.

23. BURNERS:
To deliver coal , oil and air in a proper proportion
To facilitate ignition energy to the coal air stream
To sustain the ignition
To provide a stable flame during the operation
Types of Burners
Tangential Burners
Wall Burners, Ex: Low Nox R burners
Down shot or fan tail burners

24. Modern Burners are equipped with:
Separate flame envelope ports for coal, oil and gas
Secondary air control to adjust the flame envelops
Ignitors
Flame Scanners - detect the distinct flames in an enclosure
Flame Stabilisers
Flame Analysers

25. Burner Arrangement
Tangential firing:
Four tall windboxes (combustion air boxes) one at each corner of the furnace.
The oil and gas burners are located at different levels or elevations of the windboxes.
The coal , oil and gas burners are sandwiched between air nozzles or air compartments.
That is, air nozzles are arranged between gas spuds, one below the bottom gas spud and one above the top gas spud.

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27. Burner Tilt:
The burners are tiltable +/- 30o about horizontal,
To shifts the flame zone across the furnace height
To control over steam temperature
Combustion Air Distribution
The Combustion air
Primary Air (PA) and Secondary Air (SA)
Secondary Air(SA) provided from FD Fans
Primary Air(PA) provided from PA Fans

29. Ignitors
Oil and gas are ignited by a pilot flame.
Type of Ignitors
Oil ignitor
Gas ignitor
High Energy Arc ( HEA) ignitor

30. Flame Sensing
Devices
Flame sensing devices are broadly grouped in to
Infrared flame sensors
UV flame scanners
Visible light scanners
Recent development
Flame analysers for multiple fuels

31. Emissions of Combustion - Pollutants
1. Nox emission
2. Sox emission
3.CO 2 emission (Green House Gases)
4. CO emission
5. Particulate emission

32. combustion system Design
Emerging Trends in
combustion system Design

33. Emerging trends in combustion system Design
Multiple fuel Burners
Low Emission Burner
- Technology development
Longer guarantee period for high ash coals
- Material selection,
- Improved design features
New devices such as thermal analysers.,

34. Emerging trends in combustion system Design
Cont.……
Micro processor based on line measurement and control
Computer simulations using software tools viz., ANSYS, CFD.
Virtual assembly using CAD tools

35. NOx Formation
and
Control Strategies

36. NOX FORMATION
Thermal NOx Formation
Nox Formation from Fuel Nitrogen

37. NOx Control options Control Technique NOx Reduction Potential(%)
Over fire air (OFA)
20-30
Low Nox Burners (LNB)
35-55
LNB + OFA
40-60
Re-burn
50-60
SNCR ( Selective Non Catalytic Reduction)
30-60
SCR (Selective Catalytic Reduction)
75-85
LNB with SCR
50-80
LNB with OFA and SCR
85-95

38. Selective Catalytic Reduction (SCR) Reactions

41. OVER FIRE AIR (OFA)
Through additional air compartments, to handle 15 percent of total wind box air flow.
Inhibits formation of both fuel Nox and thermal Nox as an Oxygen Deficient environment is established in the primary combustion zone
20 to 30% reduction in Nox formation

44. While Nox emission decrease linearly with increasing over fire air, Excess air rises (i.e., More Air Is Needed To Complete The Combustion).
If over fire air is increased beyond 15%. This decreases boiler efficiency due to the heating of extra air

45. Nox Reduction Techniques
STAGED COMBUSTION:
Fuel Bound Nitrogen to be regulated
High-Temperature Formation must be curtailed.
Withholding of Some O2 from primary flame zone
Air staging

46. LOW NOx BURNERs
Wall burners (oil and gas firing )
Low NOx R-burner is capable of emitting NOx at a level of 150 ppm on oil firing.
Tangential firing ( Coal firing)
CCOFA(Close Coupled Over Fire Air) feature.
Separate Over Fire Air( SOFA ) .

47. THANK YOU ALL