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Performance Analysis and
Optimisation of Boiler System forImproved Heat Rate
by
Dr.N.S.Murty
Formerly AGM and Head ofThermal Systems and Plant Performance Group
BHEL (R&D) , Hyderabad .
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The complexity of the system
The boiler system is required to supply theneeded quality steam as per the demand
It broadly consists of
Coal supply system
Air supply system
Steam generating system
Draught generating systemAsh removing system
With all controls and interlocks
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Coal Supply System
The coal must continuously be driven to the
furnace as per the requirement
It must be pulverised and pre-heated
From yard Bins
Bins Feeders
Feeders Mills
Mills Furnace
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Air supply system
This consists of
PA and SA Fan systems
Air Preheating system
cold air duct
PA Mills Furnace
APH hot PA duct
SA APH Hot Duct Furnace
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Steam Generating system
This consists of
Furnace
Economiser
Drum
Water wall
LTSH
Platen SHFSH
RH
SH Spray
RH Spray
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Draught Generation system
This is accomplished by ID fan system
The flue gas path from furnace to
ESP is maintained under vacuum to
ensure that no hot flue comes out of
its path and endanger the surrounding
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Ash Removal
From
Furnace bottom
ESP
The functioning of Draught generation
as well as Ash removal System has no
role in the Unit optimisation. Howevertheir healthy running is essential for the
running of the Unit.
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All these systems are
independent but are interconnected by control
loops.
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Scope for Optimisation
Heat is produced in the furnace system to
generate the needed steam at the specified
parameters. If more heat is generated in the
system than the requirement; then one can
expect that losses to become more. Thusfor the best conditions of operation
Qin = Qreq + Qlos
And for any other condition of operation
Qin+ = Qreq + (Qlos + )
Hence it is important to constantly monitor
the system to bring down to zero.
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Cost consideration
Let best
= 1 QL/ Q
in
For normal working let
b= 1 (QL + ) /( Qin + )
= bestbest /QinThis implies that if bestis 0.88 and when it falls to
0.8712 then the coal consumption increases by 1%.
For a 100 ton per hour coal consumption unit this
amounts to 1 ton per hour or 7200 tons of extra coal
for 300 days of unit operation. All this amounts to
an annual loss of 1.08 crore rupeesat a price of
1500 rupees per ton of coal.
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Statement of the problem
Since the heat is supplied by burning
of coal ; the problem is reduced to
fix the MINIMUM coal needed forthe given load under the prevailing
constraints of the systems of the Unit.
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The Boiler system must supply at any given
load (W) the needed flow rate (ms) of
superheat steam at the prescribed pressure (Ps)
and temperature (Ts).
Also, the system must maintain the HRH
temperature (Thrh) at the prescribed value.
Thus the boiler system must maintain a set of 4
operating parameters ms,Ps, Tsand Thrh at any
given load W.
Hence one must have at least 4 independent
parameters for controlling and maintaining the
unique set of above 4 operating parameters.
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The 4 independent parameters are
Coal flow rate
Excess Air
SH spray
RH spray.
Mill elevation and burner tilt
only provide supplementarysupport.
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Performance Prediction Model
This model predicts all the thermodynamic
parameters online for the boiler system at all
important locations of its sub systems.
Thus, the performance of each sub system isworked out and a global equation for efficiency
of the boiler system is obtained.
The unique set of parameters for the best boiler
efficiency can then be found out
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The following boiler sub systems
are modeled
A. Air Supply system
B. Coal Supply systemC. Steam generating system
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Air Supply system
P A Fan systemF D Fan system
I D Fan system
Air Heater System
Coal Supply system
Coal Mill system
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Steam generating system
Furnace System
Platen Super Heater System
Final Super Heater System
Re heater SystemLTSH System
Economizer System
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Each sub system is analysed by
appropriate mathematical model and the
outlet parameters are predicted based on
the conservation equations and known
basic transfer equations of the system
These models give
yi= F(x1,x2,.xn, 1, 2, 3. m)
IN
P
U
T
O
U
T
P
U
T
xiyiSub system
Where 1, 2, 3. mare system constants
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The following data is assumed
to be known
CBD
Radiation losses
Ultimate Analysis of Coal
Carbon Loss
Furnace Air in leak
Low and high limits of the
parametersxmin,ixixmax,i
Where xiare W, Ps, Ts, Trh and mair
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PA Fan System
FAN B
FAN A
APH A
APH B
M
I
L
LS
M
I
L
L
S
In this model the following are predicted
PA
Cold header pressure
Hot header pressure
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S A Fan System
FAN B
FAN A
APH A
APH B
F
U
R
NC
E
In this model the following are predicted
SA
SA header pressure
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I D Fan System
Knowing the I D fan characteristics one can
arrive at the needed pressure by controlling
the fan inlet aero foils.
FAN B
FAN A
S
T
A
CK
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Air Heater System
Pressure drop along
PA, SA & Flue paths
Temperature rise forPA &SA
Temperature drop for
Flue gas
LeakagesPA to SA
PA to Flue
SA to Flue
In this model the following
are predicted
PA SA
Flue
PA SA
Flue
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Coal Mill System
In this model the
following are predicted
Hot PA Cold PA
Coal Pulverised coal
to furnace
MillPressure drop
Fineness level
depending on the
condition of the
classifier
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Furnace system
In this model thefollowing are predicted
Coal Flow rateFlue gas at FOT
Heat transferred to
water wallFOT
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Platen Super Heater System
From LTSH
SH Spray
In this model the
following are predicted
SH spray
Platen Heater outlettemperature
Flue gas temperature
drop across heaterSteam side pressure drop
NTU
1.0 C=0.5
0.60.7
= f(NTU,C)
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Final Super Heater System
In this model the
Following are predicted
Flue gas temperature drop
Steam side pressure drop
From Platen
Super heaterHPT
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Re-Heater System
In this model the
Following are predictedFrom H PTurbine
IPT
RH Spray
Flue gas temperature drop
RH Spray
Steam side Pressure drop
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LTSH System
In this model thefollowing are predicted
From drum
Platen SH
Steam side temperature riseSteam side pressure drop
Gas side temperature drop
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Economiser system
From Feed
Control station
Drum
In this model the
Following are predicted
FW temperature rise
Flue temperature drop
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General Assembly
The boiler system
is modeled and
identified by 23
StationsThe parameters
such as Pressure,
Temperature and
Flow rate arepredicted at each
station
1
2
3 4 5
6
7
AH
8
9
10
11
12
13
14
15 1617
18
19
20
21
Air
Coal
22
23
ESP
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Furnace system heat transfer to
water wall is predicted based on themill elevations and burner tilt.
Platen heater and Re heater are
modeled for radiation and convection
heat transfer and other heaters are
modeled for convection heat transfer.
All needed data are continuously
generated by PEM.
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Heat Loss Calculation
All losses are calculated as per the
PTC code on Boiler testing
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Performance Evaluation Module
( P E M )
Boiler PEM is the online performance calculation
package based on the field data. The field data is
processed and validated using appropriate techniques.
in case some field data is found to be erroneous ormissing the same is appropriately interpolated from
the design data with proper corrections.
The PEM displays all the calculations for each sub
system and also for the entire system on demand at
any desired fixed intervals of time.
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System configuration
Plantglobal
data
base
Prediction andOptimisation
Module
Boiler
PEM
Graphic
Display
Field
signals
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Boiler Efficiency
b= ( Qin
QL) / Qin
Where Qin is the product of coal flow rate
and its calorific value
QL is a function of all the inputparameters and system state
conditions
Hence b is a function of all the inputparameters and system state
conditions
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Optimisation
The boiler system efficiency can be expressedfor the given W, Ps, Ts, Trh as
b= ( QinQL) / Qin
= F(mair, mcoal,msh,spr,mrh,spr
1, 2, 3. m)
From this one can arrive at the maximum value
of the bby usual computational methods.
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Effect of parameters on b
Coal flow rate: As coal flow rate increases
heat input to the boiler system increases. At
the optimum state the heat losses will beminimum and beyond that any increase in
coal flow increases the heat losses and hence
efficiency comes down.
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Air flow rate: As air flow rate increases
beyond chemically needed combustionquantity the bincreases up to certain
range because un burnt carbon loss reduces
and beyond that range any increase in airreduces the bbecause the dry gas heat
loss increases more than the heat added
due to complete combustion of coal.
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SH spray: The increase in SH spray
indicates that platen and Final SH heat
pick up is high. SH spray does not
effect the cycle efficiency. However
any increase in SH spray slightlyreduces the dry gas heat loss .
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RH Spray: RH spray is needed when
the RH super heater heat pick up ishigh. Any quantity of RH spray
slightly reduces the cycle efficiency.
Hence this must be used at its minimumfor a limited time onlymay be before
the water wall steam blowing.
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The overall study indicates that one
must watch two parameters namely;
Air flow rate (Excess Air )
RH spray.
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Summary
Optimisation was defined and its importance for
power generation boiler system was explained.
Evaluation of online performance of the boilersystem, methodology for predicting the
performance and arriving at the optimised state
of the system were discussed and explained
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JAIHIND