Lanco Operation and Maintenance Company Ltd.

Revision History

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A 27-12-2011 D V Deshpande N/A P K Majumdar First Issue

Description latest Revision:

Legal Owner

Document Type

Guideline Document Status :

First Issue

Title, Subtitle

Guidelines on O2 measurement in

Boiler Flue Gas Path

Identification number

L(O&M)/COSG/E/01 Rev.

A

Sheet

1 of 6

Company Name

Lanco Operation and Maintenance Company Ltd.

Document Number

L(O&M)/COSG/E/01

Rev.

A Sheet

2 of 7

Index

1.0 Introduction: Boiler Efficiency 3

1.1 Direct Method 3

1.2 Indirect Method 3

1.3 Boiler Losses 3

1.4 Dry Flue Gas Loss 4

2.1 Importance of O2 and its measurement 4

2.2 Minimum Air Required for Combustion (Stiochometric Air) 5

3.0 Desired Location of Measurement 6

4.0 Present Status 6

5.0 Guideline 7

Company Name

Lanco Operation and Maintenance Company Ltd.

Document Number

L(O&M)/COSG/E/01

Rev.

A Sheet

3 of 7

Guidelines on O2 measurement in Boiler Flue Gas Path

1.0 Introduction: Boiler efficiency

Boiler efficiency is the ratio of heat absorbed by the steam from the boiler per unit time to the heat

liberated by the combustion of fuel in furnace during the same time.

Boiler Efficiency can be calculated by

• Direct Method (also called as input – output method )

• Indirect method (also called as Heat loss method )

1.1 Direct Method

This is also known as ‘input - output method' due to the fact that it needs only useful output

(steam) and the heat input (fuel) for evaluating the efficiency. The boiler efficiency as per this

method is calculated as below

Heat Output

Boiler Efficiency = --------------------- * 100 %

Heat Input

Steam flow rate *(steam enthalpy – FW enthalpy)

Boiler Efficiency = ------------------------------------------------------------------ * 100 %

fuel firing rate * GCV of Coal

1.2 Indirect Method

The Boiler efficiency can be measured easily by measuring all the losses occurring in the boiler.

The efficiency can be arrived at, by subtracting the heat losses from 100. An important advantage

of this method is that the errors in measurement do not make significant change in efficiency.

Thus if boiler efficiency is 90%, an error of 1% in direct method will result in significant change in

efficiency i.e. 90 ± 0.9 = 89.1 to 90.9.

In indirect method, 1% error in measurement of losses will result in

Efficiency = 100 – (10 ± 0.1) = 90 ± 0.1 = 89.9 to 90.1

Boiler efficiency (by indirect method) = 100 – Various Losses in boiler

1.3 Boiler Losses

Various losses in boiler are as below

• Loss Due To Un-burnt Carbon

• Dry Flue Gas Loss

• Wet Flue Gas Loss

• Surface Radiation & Convection Loss

• Sensible Heat Of Fly Ash

• Sensible Heat Of Bottom Ash

Company Name

Lanco Operation and Maintenance Company Ltd.

Document Number

L(O&M)/COSG/E/01

Rev.

A Sheet

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• Radiation To Furnace Bottom

• Carbon Monoxide

1.4 Dry Flue Gas Loss:

The heat loss due to dry flue gases is a very significant component in boiler efficiency; this loss is

calculated as below

Sensible Heat x 100

• DFG LOSS % = -------------------------------- %

GCV of Coal x 4.186

• Sensible Heat of Dry Flue Gas = WD x Cp x (Tg – Ta)

• WD = WT. of Dry Flue Gas per kg of “As Fired Fuel” , it is calculated as below

(C + S/2.67 – 100 x U) (40.60 + 0.39/2.67 – 100 x 0.00485)

= -------------------------------- = ------------------------------------------------

12 x (CO2 + CO) 12 x (15.48 + 0.01 )

Where,

C = Carbon in Coal

S = Sulphur in Coal

U = Total Un-burnt Carbon in Ash

Where Co and CO2 is calculated as below

CO AT APH O/L % = -------------------- x 100 %

1000000

CO2 AT APH O/L = 19 – (% of O2 AT APH O/L + % of CO AT APH O/L)

The dry flue gas loss is dependent upon the combustion performance in the boiler. Combustion

refers to rapid oxidation of fuel accompanied by the production of heat. Complete combustion of

fuel is possible only in the presence of an adequate supply of oxygen. The objective of good

combustion is to release all of the heat in the fuel. This is possible only when there e is adequate

supply of O2 in the form of secondary air and proper air to fuel ratio.

2.1 Importance of O2 & its measurement

As explained above Oxygen is required to burn combustible elements like Hydrogen, Carbon &

Sulphur present in fuel

Company Name

Lanco Operation and Maintenance Company Ltd.

Document Number

L(O&M)/COSG/E/01

Rev.

A Sheet

5 of 7

The reaction taking place during combustion processes are as below

• Hydrogen:

2H2 + O2 = 2H2O + 28922 kcal/kg of H2

(2 * 2 ) + 32 = 36

1 + 8 = 9 ------------- in Kg

Thus, one 1 kg of hydrogen requires 8 kg of oxygen to form 9 kg of water vapour. Further, to

supply 8 kg of oxygen it is necessary to supply 8 * 4.31 = 34.48 kg of air and the nitrogen

supplied with that air will be 8 * 3.31 = 26.48 kg.

• Carbon:

C + O2 =CO2 + 8084 kcal/kg of carbon

12 + 32 = 44

1 + 32/12 = 44/12

1 + 2.67 = 3.67

So, burning of 1 kg of carbon to carbon dioxide requires 2.67 kg of oxygen, and forms 3.67 kg of

carbon dioxides. Further to supply 2.67 kg oxygen it is necessary to supply 2.67 * 4.31 = 11.49

kg of air and nitrogen supply with that air will be 2.67 * 3.31 = 8.82 kg

• Sulphur:

S + O2 = SO2 + 2224 kcal/kg of Sulphur

32 + 32 =64

1 + 1 =2

So, 1 kg of Sulphur requires 1 kg of oxygen for complete combustion and forms 2 kg of Sulphur

dioxide. The air required is 4.31 kg, which contains 3.31 kg of nitrogen

• Carbon monoxide:

2C + O2 = 2 CO + 2430 kcal/kg of carbon

8084 – 2430 = 5654 kcal

Each kg of CO formed means a loss of 5654 kcal of heat.

2.2 Minimum Air required for Combustion (Stiochometric Air)

Minimum air required

}100

%)

800

%

100

%(8

100

%67.2{31.4

SOHC+−×+

××= kg/kg of coal

By measuring O2 % before APH inlet, the excess air is calculated as

)123200

%

3200

%

400

%

1200

%(2

CinashOSHCyrequiredMinimumO −−++==

By using this equation we can find excess air %, if x is excess air, and total air required as

Company Name

Lanco Operation and Maintenance Company Ltd.

Document Number

L(O&M)/COSG/E/01

Rev.

A Sheet

6 of 7

)100

%1(ReRe

xquiredMinimumAirquiredTotalAir +×=

quiredMinimumAirquiredTotalAirExcessAir ReRe −=

Further, excess air can also be calculated by using formulae,

%221

100%2%

O

OExcessAir

−

×=

Boilers are designed to operate at excess air of 20%. If the excess air is not maintained at desired

level, then there will increase in dry flue gas loss. Further if excess air is reduced then there will be

improper combustion, this will result in increase in un-burnt carbon in Bottom ash and fly ash.

So there has to be optimization of excess air. For which CO analyzers are very useful .The

incomplete combustion can be noticed by increase in CO level .The excess air can be reduced

slowly till there is marginal increase in CO level. So It is required to have CO analyzer also at

APH inlet along with O2 analyzer at Inlet and outlet of APH.

From it is clear that accurate measurement of O2 and CO is essential to ensure complete

combustion of fuel.

3.0 Desired Location of Measurement

The combustion performance in boiler can be judged by O2% and CO in flue gas leaving boiler.

But the measurement of O2 at Eco Inlet with single point measurement is not much useful as gas

stratification takes place. Hence O2 is to be measured at APH inlet with grid measurement. As

there is possibility of Gas stratification at air pre heater inlet also. Since grid measurement is not

possible every time, it is essential to have at least two measurements for O2 at APH inlet

Further as explained above, while calculating boiler efficiency, Dry flue gas loss is calculated

considering measurement of O2 %at Air- preheater out let. Measurement of O2 at APH outlet is

also useful for measurement APH seal leakage

4.0 Present status

In view of this requirement for monitoring & improving boiler efficiency, the present arrangement

of O2 analyzers in our plant is reviewed and the status of this is as below.

Sr.

No. Plant

O2 Analyzers

Make Type

CO Analyzers

Type Eco

I/L

(L

&R)

APH

-

A&B

I/L

APH-

A&B

O/L

APH-

(A

&B)I/

L

Stack

1 Udupi 2 2 2 AMETEK Zirconia 2 1 IR

2 Amarkantak 2 2 NA AMETEK Zirconia 2 1 IR

3 Anpara NA 2 2 AMETEK Zirconia 2 1 IR

Company Name

Lanco Operation and Maintenance Company Ltd.

Document Number

L(O&M)/COSG/E/01

Rev.

A Sheet

7 of 7

5.0 Guideline

The guideline on provision of O2 analyzers is as below.

All plants to have Two Numbers of O2 analyzers at Air preheater inlet & outlet, along- with CO

analyzers at Air Preheater inlet & at Stack.O2 analyzer should be of Zirconia type with

calibration facilities. CO analyzer should be IR type with 0-1000 PPM with calibration facilities

with least count 1ppm. For better understanding the location should be as shown in figure.

Online data of all analyzer should be available in DCS.

O2 Measurement

O2 Measurement O2 / CO Measurement

O2 Measurement

APH