Boiler HR Improvement

8/13/2019 Boiler HR Improvement

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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

Boiler HR Improvement

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