air & draft
TRANSCRIPT
11 October 2011 2
Need of Draft System
BoilerCombustion AirFlue Gases
• Air needed for combustion
• Flue are needed to be evacuated
• Losses due to flow need to be overcome
The Three Ts & One S Practice
Technology Time Temperature Turbulence Size
Stoker large Medium Low Big
Pulverized Short High Medium Tiny
Cyclone Short+ V High High Medium
Fluid Bed Medium Low High Medium
Size the Coal and Add the Air !!!
11 October 2011 3
11 October 2011 T K Ray NTPC 4
Coal – an organic complex polymer
Sequential events
• Devolatilization (Pyrolysis)
− Heating causes its structure to
decompose
− Weaker chemical bonds break at
lower temperatures
− Stronger ones at higher
temperatures
− Volatile yield can be up to 50%
greater than indicated by proximate
analysis
SEM of a de-volatilizing coal particle
11 October 2011 T K Ray NTPC 5
• Burning of volatile matters
− Homogeneous gas phase reaction
around the particle
• Char burning
− Heterogeneous combustion
− Occurs at the char surface and pore
surfaces (porosity~0.7; surface area
~100 m2/g)
− Guided through competing effects
of heat and mass transfers to the
char surface and chemical reaction
Char particle at the early
stage of burning
11 October 2011 6
Coal Combustion
coal particlep-coal, d=30-70µm
De-volatilization
volatiles
char
homogeneous
combustion
heterogeneous
combustion
CO2, H2O, …
CO2, H2O, …
tchar
=1-2sectvolatiles
=50-100mstdevolatile
=1-5ms
t
Temperature, Turbulence, Time and Size
Heating
11 October 2011 7T K Ray PMI
• Vol. combustion intensity, Iv is
in the range 150-250 kW/m3.
• FEGT<IDT
• For a desired Iv, one can select
tr (~ 2 sec)
• The combustion time, tc (~ 1
sec) <tr by a significant margin.
• Low VM coal; more fineness required
VM, Fineness, Combustion & Residence Time
• If 1-2% is +50 in BS mesh sieve (300 µm), most of
this coarse coal will not burn and end up in C in BA
− It also frequently causes slagging around the burners
• If -200 mesh (75 µm) fineness is poor, results in high
C in FA
• If the coal is not properly ground, the distribution to
the burners may not be even
− Resulting in air-fuel imbalances at one or more burners
Coal particle size distribution
11 October 2011 8
More Details of Pulverized Fuel fired SG
Furnace Wall
Sequence of Energy Exchange from Flue Gas to
Steam
FLUE GAS
PLATEN SH
PENDENT SH
COVECTIVE SH
ECONOMIZER
RHEVAPORATOR
Fuel Power
Furnace absorption
Platen SH
Final SH
LTSH
Reheater
Economizer
Combustion Losses C & R lossesHot Exhaust Gas
losses
~4000C
Air Heater in Fossil Fired Plant
October 11, 2011 T K Ray PMI NTPC
�The accumulated heat is released to the
incoming air as the same surfaces pass through
other half.
�The heat transfer cycle is continuous as the
surfaces are alternately exposed to the
outgoing gas and incoming air.
�The rotating cylinder packed with
thousands of sq m of specially formed
sheets of heat transfer surfaces.
�As it revolves, heat of FG is absorbed
through one half.
Air Pre-Heater
• An air pre-heater heats the combustion air where it is economically feasible.
• Reducing the stack gas temperature and increasing the boiler efficiency.
• The pre-heating helps the following:
• Igniting the fuel.
• Improving combustion.
• Drying the pulverized coal in pulverizer.
11 October 2011 16
How to select a fan
• Air or Gas flow -Kg/hr
• Density (function of temperature and pressure)
• System, resistance (losses)
Major Fans in Boiler Draft System
• PA Fans
• FD Fans
• ID Fans
11 October 2011 17
Arrangement of Boiler Auxiliaries
11 October 2011 18
PRIMARY AIR/MILL SEAL AIR
SYSTEM
• Ambient air is drawn into the primary air ducting by two 50% duty, motor driven axial reaction fans.
• Air discharging from each fan is divided into two parts, one passes first through a air pre-heater then through a gate into the P.A bus duct. The second goes to the cold air duct. The mix of both is used to carry the pulverized coal to the boiler.
11 October 2011 19
Components of PA Fan
• Suction Bend With volume Measurement instruments
• Fan housing with Guide Vanes
• Main Bearings (Anti Friction)
• Rotor with impeller with adjustable blade with pitch control
• Guide vane housing with guide vanes
• Diffuser with pressure measurement instruments
11 October 2011 20
SECONDARY AIR SYSTEM
• Ambient air is drawn into the secondary air system by two 50% duty, motor driven axial reaction forced draft fans with variable pitch control.
• Air discharging from each fan passes first through a air preheated then through a isolating damper into the secondary air bust duct.
• The cross over duct extends around to each side of the boiler furnace to form two secondary air to burner ducts.
• At the sides of the furnace, the ducts split to supply air to two corners. Then split again to supply air to each of nineteen burner/air nozzle elevations in the burner box.
11 October 2011 21
Burner Box Dampers
• Coal/Air Dampers
• Secondary Air Dampers
• Oil/Secondary Air Dampers
• Bottom Tier Secondary Air Damper
• Over Fire Damper
11 October 2011 22
Primary and Secondary air System
in the Boiler
11 October 2011 23
Induced Draft System
• There are three induced draught fans per boiler, two operating and one standby
• In 500 MW fans are single-stage, double-inlet centrifugal fans. Principal fan elements of the fan are:
• Housing
• Inlet dampers
• Rotor with Bearing
• Shaft Seal
11 October 2011 24
ID Fan
• The rotor consists of shaft and assembled impeller and runs in two sleeve bearings that are arranged outside of the housing.
• The impeller consists of a centre disc and two cover discs that are reinforced by forged rings. The bent blades are welded into position between the impeller discs.
• The blades are protected by screwed - on wear plates.
• The shaft is of hollow design. The fan shaft has been rated so that max. operating speed is below the critical speed. Impeller and shaft are connected by means of a flange. This screwing is protected by wear plates.
• The fan housing is sealed by means of two-part labyrinth seals.
• Bearings are lubricated with oil.
11 October 2011 25
Other Fans in The System
• Ignitor Air Fan: Provide combustion air to the ignitors. Take suction air from the atmosphere and supplies air to the ignitor wind box.
• Scanner Air Fan: Supplies cooling air to flame scanners. Normally there are two fans taking suction from FD Fan discharge duct.
• Mill Seal air fan: Seal air fans provide air for the sealing of Mill bearing. Suction is from cold Primary air and pressure is boosted up to maintain the differential pressure
11 October 2011 26
PA
PH
-AS
AP
H-A
SA
PH
-BP
AP
H-B
FD FAN -B
FD FAN-A
PA FAN -A
PA FAN -A
SC
AP
H-B
PR
IMA
RY
SC
AP
H-B
SE
CO
ND
SC
AP
H-A
PR
IMA
RY
SC
AP
H-A
SE
CO
ND
TO MILLS
TO MILLS
HO
T P
A H
DR
HO
T
S
A H
DR
ECO BYPASS
ECO BYPASS
EC
ON
OM
ISE
R-3
EC
ON
OM
ISE
R -
2
EC
ON
OM
ISE
R -
1
HO
RIZ
ON
TA
L S
.H
RE
HE
AT
ER
PL
AT
EN
S.H
DIV
ISIO
N
PA
NE
LE
TT
E S
.H
COMBUSTION
CHAMBER
AEROFOIL
AEROFOIL
COLD SEC. AIR
COLD PRIM AIR
HOT PRIMARY AIR
HOT SEC. AIR
FLUE GAS
PNEUMATICALLY O/P
KNIFE EDGE GATE
MOTOR O/P LOUVER
DAMPER
PNEUMATICALLY O/P
LOUVER DAMPER
MOTOR O/P GATE
BIPLANE DAMPER
DIVERTER DAMPER
AA201
AA202AA203
AA204
AA205
AA201
AA202AA203AA205
AA204
AIR AND FLUE GAS PATH TYPICAL 500 MW BOILER
TO ESP
TO ESP
11 October 2011 27
-300
-250
-200
-150
-100
-50
0
50
100
150
200
250
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Fu
rna
ce
Pre
ss
ure
Points in Boiler
1 FD Fan Inlet
2FD Fan Outlet
3Airheater Inlet
4Airheater Outlet
5Windbox Pressure
6 Furnace
7
Superheater Platen
Inlet
8Reheater Inlet
9 LTSH Inlet
10 Economiser Inlet
11Airheater Inlet
12 E.P. Inlet
13 I.D. Fan Inlet
14I.D. Fan Outlet
Pressure at key points in air and FG path
