Fired Heaters

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FUEL FIRED EQUIPMENT MODULE 13 Implemented by AGRA Monenco Atlantic Limited for the Canadian International Development Agency SADC Industrial Energy Management Project LEARNING OBJECTIVES In this module you will learn about: General Objectives: L Fuel Fired Equipment Specific Objectives: L The importance of Fuel Fired Equipment in Industry, L Principles of Combustion, L Characteristics of Various Fuels, L Types and Applications of Fuel Fired Equipment L Burners, L Combustion Testing Procedures (Flue Gas Analysis), L Efficiency Improvement of Fired Equipment. Performance Objectives: L Perform Flue Gas Analysis, L Calculate Thermal and Combustion Efficiencies, L Implement a Performance Testing Schedule in Your Plant.

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All about industrial furnaces

Transcript of Fired Heaters

Page 1: Fired Heaters

FU

EL

FIR

ED

EQ

UIP

ME

NT

MODULE 13

Implemented by AGRA Monenco Atlantic Limited for the Canadian International Development Agency

SADC Industrial Energy Management Project

LEARNING OBJECTIVES

In this module you will learn about:

General Objectives:

L Fuel Fired Equipment

Specific Objectives:

L The importance of Fuel Fired Equipment in Industry,

L Principles of Combustion,

L Characteristics of Various Fuels,

L Types and Applications of Fuel Fired Equipment

L Burners,

L Combustion Testing Procedures (Flue Gas Analysis),

L Efficiency Improvement of Fired Equipment.

Performance Objectives:

L Perform Flue Gas Analysis,

L Calculate Thermal and Combustion Efficiencies,

L Implement a Performance Testing Schedule in Your Plant.

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Module 13Fuel Fired Equipment

TABLE OF CONTENTS

1.0 INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2.0 FUEL FIRED SYSTEMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

3.0 PROPERTIES OF FUELS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

3.1 Properties of Solid Fuels . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23.2 Properties of Liquid Fuels (Oil) . . . . . . . . . . . . . . . . . . . . . . . 23.3 Properties of Gaseous Fuels . . . . . . . . . . . . . . . . . . . . . . . . . 5

4.0 COMBUSTION PROCESS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

4.1 Combustion Fundamentals . . . . . . . . . . . . . . . . . . . . . . . . . . 64.2 Combustion Testing - Flue Gas Analysis . . . . . . . . . . . . . . . . 94.3 Flue Gas & Other Losses in Process Furnaces, Dryers & Kilns 114.4 Thermal Efficiencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.5 Burners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 164.6 Air Pollution Control - Process & Equipment . . . . . . . . . . . . . 19

5.0 FUEL FIRED EQUIPMENT & APPLICATIONS . . . . . . . . . . . . . . . . . 20

6.0 ENERGY MANAGEMENT OPPORTUNITIES . . . . . . . . . . . . . . . . . . 22

6.1 Housekeeping Opportunities . . . . . . . . . . . . . . . . . . . . . . . . . 236.2 Low Cost Opportunities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246.3 Retrofit Opportunities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

7.0 WORKED EXAMPLES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

7.1 Example 1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257.2 Example 2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

8.0 ASSIGNMENT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

9.0 SUMMARY - Module 13 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

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Figure 13.1 FUEL TYPES & USES

MODULE 13FUEL FIRED EQUIPMENT

1.0 INTRODUCTION

The standard of living in the majority of countries in the world largely depends onthe use of fossil fuels. Any time the supply of the fossil fuels is endangered, amajor economic crisis follows. It would seem logical that every country should tryto reduce its dependence on fossil fuels by better utilization of the resource. So farthe primary method of using fossil fuel is by burning, which is not the best way toutilize such a valuable source of energy. However, since combustion is the mostpopular way of fuel conversion, it is important for the technical personnel, whohandle energy conversion equipment such as boilers, furnaces and kilns tounderstand the basic principles of combustion process.

2.0 FUEL FIRED SYSTEMS

Furnaces, dryers, boilers and kilns are used extensively in industry for diverseapplications such as melting and heating metals, evaporating water or solvents andmanufacturing lime for cement and in the pulp industries. Much of this equipmentwas installed when fuel was relatively cheap and little or no consideration wasgiven to energy management. Even today, first cost and production capability arefrequently the prime criteria for the selection of equipment, with energymanagement being relegated to a secondary role. The high cost of the fuels todaydemands a greater awareness for energy management techniques which can beapplied to existing and new installations. Substantial savings in energy and cost

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can be realized by the application of these techniques. In many instances thereturn on invested capital make the application of energy management one of themost attractive investment opportunities available to industry. Figure 13.1 showstypical types of fuels and their industrial applications.

3.0 PROPERTIES OF FUELS The most important characteristics of the fuels is their calorific or heating value.Each fuel has a certain range of heating values depending on its origin. In the caseof wood, bagasse and other biomass, the moisture content will determine the rangeof heating values. All fuels contain hydrogen which burns and produces water.This water normally leaves the plant as hot vapour at the temperature of exit gases.This loss is significant because even small quantities of water absorb largequantities of heat when it evaporates. The net calorific value or Low HeatingValue ( LHV) is the gross calorific value or High Heating Value (HHV) less thisloss. The difference between these two values is about 4% for coal, 5% for oilsand 11% for natural gas. When comparing the efficiencies of different fuel burningequipment, it is important to establish the heating value of the fuels used during thetests.

3.1 Properties of Solid Fuels

Fuel fired equipment using solid fuels must be carefully designed for the fuelproperties. Among these are calorific value, volatile content, ash content, moisturecontent, ash fusion temperature, grindability and agglomerating characteristics. Formore information about these factors, consult reference manuals that dealspecifically in various solid fuels.

3.2 Properties of Liquid Fuels (Oil)

Fuel oil is classified by its viscosity, sulphur content, heating value, pour point, flashpoint and specific gravity. Figure 13.2 gives characteristics of typically availablefuels, together with data on combustion air requirements and storage temperature.

! Viscosity

Viscosity, or resistance to flow, is expressed in the number of seconds it takesa litre of fuel to pass through a certain size hole at a certain temperature. Thescales used are Redwood, Sybolt or Centistokes. Viscosity may be specifiedas maximum for Residual Fuel Oil (RFO) at 50EC as follows:

< 125 centistokes ( 1000 sec Redwood)< 180 centistokes ( 1500 sec Redwood)< 280 centistokes ( 2500 sec Redwood)

The most widely used grade is 125 centistokes.

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! Flash Point

Flash point is a measure of fire hazard of bulk storage. Flash point is usuallycontrolled to a minimum of 65.5EC for the following reasons:

< For handling this category of product, a minimum flash point isspecified. The product is not expected to be volatile.

< If the flash point is lower than the specified value, then the viscosity maybe too low and this could make the product unsuitable.

< Addition of distillates such as kerosene with flash point of 38EC toheavier oils considerably increases the fire hazard.

! Pour Point

Pour point indicates the lowest temperature at which the fuel can be pumped.It is the temperature slightly above the solidification point.

! Sulphur Content

Upon combustion, the sulphur in fuel is converted to sulphur dioxide andultimately to sulphur trioxide. On cooling, sulphur trioxide combines with waterto form sulphuric acid which is destructive to the chimneys. For this reason thestack temperature should not fall below 150EC.

Typically, maximum sulphur content is 3.7% for 125 and 180 centistokes and4.0% max for 280 centistokes.

TYPICAL SPECIFICATION FOR INDUSTRIAL DIESEL OIL (IDO)

Description Specification Typical Value

Density at 20EC Max 0.920 0.855 Diesel Index Min 51 55 Viscosity, Redwood (sec) Max 55 45 High Heating Value (MJ/kg) 45,000 45,680 Pour Point (EC) Max 10 5 Sulphur Content (%wt) Max 1.8 1.5 Water (%vol) Max 0.25 0.05 Sediment (% wt) Max 0.02 0.01 Ash (% wt) Max 0.02 0.01 Flash Point (EC) Min 66 96 Ashfaltenes (% wt) Max 0.3 0.20

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Fig

ure

13.

2S

toic

hio

met

ric

Co

mb

ust

ion

Dat

a F

or

Typ

ical

Fu

els

Fuel

Typ

e U

SA C

oal

Bot

swan

a C

oal

Zim

babw

e C

oal

No.

2 O

il N

o. 6

Oil

Nat

ural

Gas

LPG

Bag

asse

Pin

e W

ood

Oak

Woo

dU

ltim

ate

Ana

lysi

sF

uel (

%b.

w.)

Car

bon

(C)

80.3

1%59

.71%

70.5

3%87

.20%

85.6

0%69

.26%

81.8

2%23

.40%

29.3

7%27

.34%

H

ydro

gen

(H)

4.47

%3.

30%

3.94

%12

.50%

9.70

%22

.68%

18.1

8%2.

80%

3.08

%2.

97%

N

itrog

en (

N2)

1.38

%1.

36%

1.54

%--

1.

50%

8.06

%--

0.

10%

0.06

%0.

11%

O

xyge

n (O

2)2.

85%

10.2

8%2.

96%

--

0.50

%--

--

20

.00%

20.8

5%21

.62%

Su

lphu

r (S

2)1.

54%

1.75

%2.

03%

0.30

%2.

30%

--

--

--

0.06

%0.

06%

M

oist

ure

(H2O

)2.

90%

5.10

%6.

00%

--

0.28

%--

--

52

.00%

45.0

0% 4

5.00

%

Ash

6.55

%18

.50%

13.0

0%--

0.

12%

--

--

1.70

%1.

60%

2.92

%

Tot

al F

uel

100.

00%

100.

00%

100.

00%

100.

00%

100.

00%

100.

00%

100.

00%

100.

00%

100.

00%

100.

00%

C

ombu

stio

n A

ir (

%b.

w.)

Oxy

gen

(O2)

23.3

1%23

.31%

23.3

1%23

.31%

23.3

1%23

.31%

23.3

1%23

.31%

23.3

1%23

.31%

N

itrog

en (

N2)

76.6

9%76

.69%

76.6

9%76

.69%

76.6

9%76

.69%

76.6

9%76

.69%

76.6

9%76

.69%

M

oist

ure

(H2O

)--

--

--

--

--

--

--

--

--

--

Tot

al C

ombu

stio

n A

ir10

0.00

%10

0.00

%10

0.00

%10

0.00

%10

0.00

%10

0.00

%10

0.00

%10

0.00

%10

0.00

%10

0.00

%

Stoi

chio

met

ric

Flu

e G

as (

%b.

w.)

Car

bon

Dio

xide

(C

O2)

25.3

9%26

.03%

25.2

3%20

.93%

22.1

1%15

.20%

18.0

7%22

.80%

23.8

8%23

.91%

N

itrog

en (

N2)

70.6

3%69

.42%

70.3

3%71

.67%

71.4

0%72

.58%

72.0

7%56

.68%

59.9

7%58

.96%

Su

lfur

Dio

xide

(SO

2)0.

27%

0.42

%0.

40%

0.04

%0.

32%

--

--

--

0.02

%0.

03%

M

oist

ure

(H2O

)3.

72%

4.14

%4.

04%

7.36

%6.

17%

12.2

2%9.

86%

20.5

2%16

.13%

17.1

1%

Tot

al F

lue

Gas

100.

00%

100.

00%

100.

00%

100.

00%

100.

00%

100.

00%

100.

00%

100.

00%

100.

00%

100.

00%

M

ass

Rat

ios

Fuel

(ne

t)0.

9055

0.

7640

0.

8100

1.

0000

0.

9960

1.

0000

1.

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0.

4630

0.

5341

0.

5209

Fu

el M

oist

ure

0.02

90

0.05

10

0.06

00

--

0.00

28

--

--

0.52

00

0.45

00

0.45

00

Fuel

Ash

0.06

55

0.18

50

0.13

00

--

0.00

12

--

--

0.01

70

0.01

60

0.02

92

Fuel

(gr

oss)

1.00

00

1.00

00

1.00

00

1.00

00

1.00

00

1.00

00

1.00

00

1.00

00

1.00

00

1.00

00

Stoi

chio

met

ric

Air

(dr

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4 7.

5974

9.

3809

14

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6 13

.198

9 15

.707

1 15

.599

6 2.

7799

3.

5251

3.

2215

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oich

iom

etri

c A

ir (

moi

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--

--

--

--

--

--

--

--

--

Tot

al S

toic

hiom

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c A

ir10

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5974

9.

3809

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1 15

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3.

5251

3.

2215

Fl

ue G

as (

dry)

11.1

686

8.06

44

9.83

63

14.1

536

13.3

219

14.6

659

14.9

634

2.99

09

3.78

19

3.47

51

Flue

Gas

(m

oist

ure)

0.43

13

0.34

80

0.41

46

1.12

50

0.87

58

2.04

12

1.63

62

0.77

20

0.72

72

0.71

73

Tot

al F

lue

Gas

11.5

999

8.41

24

10.2

509

15.2

786

14.1

977

16.7

071

16.5

996

3.76

29

4.50

91

4.19

24

Fuel

HH

V (

kJ/k

g)32

,800

24

,000

30

,000

45

,200

42

,570

50

,770

50

,390

9,

300

11,5

50

10,7

00

Fuel

Spe

cifi

c G

ravi

tyn/

an/

an/

a0.

87

0.98

0.

13

n/a

n/a

0.73

0.

85

Fuel

Spe

cifi

c H

eat (

kJ/k

gC)

0.83

0.

83

0.83

2.

01

2.01

n/

an/

an/

a2.

93

2.58

Fl

ue G

as S

peci

fic

Hea

t (kJ

/kgC

)1.

02

1.01

1.

02

1.02

1.

02

1.03

1.

03

1.02

1.

02

1.02

Sp

ecif

ic H

eat C

onst

ants

: D

ry A

ir =

1.0

2 kJ

/kgC

, M

oist

ure

(liq

uid)

= 4

.19

kJ/k

gC,

Moi

stur

e (v

apou

r) =

1.8

kJ/

kgC

, L

aten

t Hea

t of M

oist

ure

= 2

,500

kJ/

kg

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3.3 Properties of Gaseous Fuels

Gaseous fuels may be analyzed in terms of the chemical compounds they contain.Other properties by which the fuels are identified are:

! Gas Gravity

Gas gravity is a convenient measure of specific gravity of a gas relative to thatof air (1.225 kg/m ).3

! Heating Value

Although the heating value can be calculated from gas analysis, it is frequentlymeasured by means of steady flow, constant pressure calorimeter in which thegas is burned in a water jacketed combustion chamber. The temperature risein the water is a measure of the heat given off by the fuel.

! Condensible Hydrocarbon Content

The term wet or dry as applied to natural gases indicates whether the quantityof contained condensible hydrocarbons (usually natural gasoline) is greater orless than 0.13 litres per cubic meter (0.1 gallon per 1000 cubic feet) of gas,respectively.

! Sulphur Content

The term sweet and sour refers to the sulphur or hydrogen sulfide content ofthe gas; sour gas being that which contains large proportion of sulphurcompounds.

4.0 COMBUSTION PROCESS

The combustion process is the cornerstone to development in our civilization. Fromburning wood for warmth and cooking, to modern transportation which burnspetroleum products, to generating electricity by burning solid fossil fuels, ourmodern world would collapse without conversion of fossil fuels to heat.

Combustion is a complex subject and any substantive changes to the processshould only be contemplated after consultation with the regulating bodies havingjurisdiction, the manufacturer of the fuel burning equipment, the control systemsupplier and other trained specialists.

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Figure 13.3INCOMPLETE COMBUSTION LOSS

4.1 Combustion Fundamentals

Combustion or burning by definition is a process of conversion of chemical energyto thermal energy by very rapid oxidation of the component elements in fuels. Thethree main elements of fuels are: carbon, hydrogen and sulphur.

Oxygen is obtained from combustion air which contains: 21% oxygen by volume(23% by weight) and 79% nitrogen by volume.

During combustion, these elements are oxidized into carbon dioxide (CO ), water2

vapour (H O) and sulphur dioxide (SO ) accompanied by the release of heat and2 2

light.

!! Combustion of Carbon

Carbon can produce two compounds depending on the availability of the airsupply.

< If enough air is supplied, carbon dioxide is produced. If the air is exactlyright (stoichiometric conditions), the gaseous products equal the airquantity, i.e. 21% CO and 79% nitrogen, plus release of heat.2

< With a starved air supply, the carbon is partially burnt to carbon monoxideand the full calorific value of the fuel is not released. This is known asincomplete combustion; a dangerous condition in any fuel burningequipment. Figure 13.3 provides an estimate of combustion loss due toincomplete combustion which is indicated by the presence of CO in the fluegas. Note the loss indicated in this chart is in addition to normalcombustion losses.

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! Combustion Air Requirement

Stoichiometric air is the theoretical amount of air required for completecombustion. In actual applications, however, it is impossible to get perfectmixing of the fuel and air. Thus additional air, termed excess air, is required toburn the fuel safely and completely. The more refined the fuel, the less excessair is needed. Typical excess air values are:

Natural gas 5 - 10%IDO (No.2 oil ) 10 - 20%RFO (No.6 oil) 10 - 25%Coal 20 - 40%Biomass (bagasse) 30 - 50%

The effect of excess air on burning of oils is shown below. It can be seen thatthe CO content is reduced from the stoichiometric 16% for perfect combustion2

to 12% at 30% excess air on dry basis (i.e. water vapour removed).

% Excess Air % CO % O2 2

Nil 16% 0%30% 12% 5%50% 11% 7%75% 9% 9.5%120% 7% 12%

For more common fuels, the typical target values are:

Fuel Max CO Target CO Target O2 2 2

Coal 19% 14% 6%Fuel Oils 16% 13% 4%Natural Gas 12% 11% 2%

Although minimum quantities of excess air are required to ensure goodcombustion, too much excess air leads to lowered thermal efficiency as largerquantities of heated flue gases are produced and discharged to theatmosphere.

Simple instruments such as a Bacharach one-bulb Orsat unit, filled with liquidwhich absorbs CO and O , can be used to give a quick assessment of the2 2

combustion efficiency.

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Figure 13.4TEMPERATURE MEASUREMENT POINTS

(For Boilers)

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4.2 Combustion Testing - Flue Gas Analysis

! Instrument Requirement

The following instruments are recommended for assessing the combustionefficiency in most fuel fired equipment:

< Stack thermometer, to measure the flue gas temperatures,< Digital thermometer, to measure the ambient and equipment surface

temperatures, < Smoke pump, to establish the flue gas conditions,

< Combustion testing kit, to measure oxygen (%O ) and/or carbon dioxide2

(%CO ) readings to calculate excess air and combustion efficiency, and2

< ,Psychrometer, to measure the quality of the incoming combustion air.

The use of an electronic combustion tester, either hand-held or continuous typeis an alternative.

! Flue Gas Analysis

To obtain reasonably good data the equipment undergoing the test should bein continuous operation for at least 20 minutes to reach stable conditions.

1. Take and record Bacharach smoke spot reading. If the smoke spotreading is too high for the fuel used, say above 6 , have the air-fuel ratioadjusted and repeat the reading. Proceed to number 2.

2. Measure and record the combustion air and stack temperatures asindicated in Figure 13.4. Where air preheaters or economizers are used,the stack and combustion air temperatures must be taken as indicated.

3. Using gas analyzer, read and record percentage of O and/or CO .2 2

4. Using the value of O or CO read the excess air from the appropriate2 2

nomograph in Figure 13.6.

5. Calculate flue gas loss using Seigert's formula or read flue gas loss aspercentage of fuel input from the appropriate nomograph in Figure 13.7.Approximate flue gas losses can also be obtained by simply measuring the%C0 or %0 and using nomographs based on Seigert's Formula shown in2 2

Figure 13.5.

6. Calculate heat losses in the exhaust air and gas mixture by an alternatemethod using the following formula and the mass conservation law.

The mass of fuel plus the mass of air entering the furnace must be thesame as the mass of flue gases leaving the stack, assuming no infiltrationand negligible ash. The temperature and volume will change.

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%Loss 'K x )T%CO2

% C

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where:% Loss = total flue gas loss as % of the fuel's

gross energy (HHV),K, C = constants for fuel type (see table

below),%CO = CO as percent (by volume) of dry gas2 2

in flue gas,)T = temperature difference (EC) between

flue gas and combustion air (refer toFigure 13.4)

SEIGERT CONSTANTS

Fuel Type K C

Fuel OilCoalNatural Gas

0.560.630.38

6.5 5.0

11.0

Figure 13.5SEIGERT FORMULA

Q = M x C x )Tp

where:Q = heat loss flow (kJ/h)C = specific heat of mixture (1.01 kJ/kgEC for air)p

)T = temperature difference (EC) between incoming and exhaust airM = mass flow of mixture (kg/h), where:

M = fuel input + (fuel input x CV x SA x %EA)

where:fuel input is in Kg/h or l/h or m /sec 3

CV = calorific value of fuel in MJ/kg, MJ/l, etcSA = stoichiometric air requirement for specific fuels in kg/ GJ as

in table below or in Kg /kg as in Figure 13.2%EA = excess air percentage obtained from the flue gas analysis

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COMBUSTION AIR REQUIREMENTS

Fuel Theoretical Air Minimum Air Total Air Mass(kg/GJ as fired) (kg/GJ as fired) (kg/GJ as fired)

Natural Gas 309 10% 340No.2 Oil (IDO) 316 15% 363No.6 Oil (RFO) 310 20% 372Zimbabwe Coal 313 30% 407Biomass (wood) 305 50% 458Bagasse (50% 299 40% 418mc)

4.3 Flue Gas and Other Losses in Process Furnaces, Dryers andKilns

Process requirements for some furnaces and dryers require high excess air valueswhich cannot be reduced. Thus, flue gas heat loss is high, and cannot be reducedby lowering the excess air quantity. It is often possible in these applications toinstall a heat exchanger to preheat the incoming air with the flue gases leaving thefurnace or dryer. The heat loss is then the heat in the flue gas after the heatrecovery equipment. Flue gas analysis and temperature should be measureddownstream of this equipment.

! Example

A furnace burns natural gas and the excess air is determined to be 77%. Thetemperature of the gas leaving the furnace is 850EC.

From Figure 13.7 the heat loss is 65%. This is the per cent heat loss to thestack. There are additional losses through the furnace walls and roof, whichmay be as high as 20% of the fuel heat value. As a result, only 15% of the heatinput ends up as useful heat to the product.

There is good potential for improved energy management in this instance.Either recover some of the heat by preheating combustion air as discussedabove and find more applications for recovered hot air, or consider a moreadvanced technology such as induction heating where applicable.

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Figure 13.6%O & CO vs EXCESS AIR2 2

SCALE FOR EXTREME EXCESS AIR

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Figure 13.6 (cont'd)O & CO vs EXCESS AIR2 2

SCALE FOR EXTREME EXCESS AIR

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Figure 13.7FLUE GAS LOSSES

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ThermalEfficiency 'UsefulEnergyOutputHeatSuppliedtoPlant

x 100

BoilerPlantEfficiency 'HeatValueofSteam

WeightofFuelUsed x HHVx 100

Furnace/KilnEfficiency 'HeatContentofProduct

WeightofFuelUsed x HHVx 100

DieselGeneratorEfficiency 'kWhConvertedtoHeatUnitsWeightofFuelUsed x HHV

x 100

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4.4 Thermal Efficiencies

Thermal efficiency is the ratio of useful heat output to the heat supplied to the plant.It is necessary to convert the units of output to the same units as the energy input.

Thermal efficiency can also be defined as total energy input minus losses. In boilerplants and furnaces, these losses are mainly due to flue gas losses and radiationfrom the plant. Since boilers and furnaces are normally kept at constanttemperatures, the radiation losses should be fairly constant. If a value of radiationis assumed, the Seigert formula can be used to quickly obtain the thermal efficiencyto take into account of air fuel ratio and exhaust temperature.

Thermal Efficiency = Total Input - Total Losses

Flue gas losses - use nomograph or Seigert formulaRadiation losses - use standard values

- 2 - 5% for boilers- 10% for furnaces and kilns

Figure 13.8AIR DENSITY CORRECTION FACTORS

Altitude (m) SeaLevel 250 500 750 1000 1250 1500 1750 2000 2500 3000

Barometer (kPa) 101.3 98.3 96.3 93.2 90.2 88.2 85.1 83.1 80.0 76.0 71.9

Air 0 1.08 1.05 1.02 0.99 0.96 0.93 0.91 0.88 0.86 0.81 0.76Temp. 20 1.00 0.97 0.95 0.92 0.89 0.87 0.84 0.82 0.79 0.75 0.71(EC) 50 0.91 0.89 0.86 0.84 0.81 0.79 0.77 0.75 0.72 0.68 0.64

100 0.79 0.77 0.75 0.72 0.70 0.68 0.66 0.65 0.63 0.59 0.56

150 0.70 0.68 0.66 0.64 0.62 0.60 0.59 0.57 0.55 0.52 0.49200 0.62 0.61 0.59 0.57 0.56 0.54 0.52 0.51 0.49 0.47 0.44250 0.56 0.55 0.53 0.52 0.50 0.49 0.47 0.46 0.45 0.42 0.40300 0.51 0.50 0.49 0.47 0.46 0.45 0.43 0.42 0.41 0.38 0.36

350 0.47 0.46 0.45 0.43 0.42 0.41 0.40 0.39 0.38 0.35 0.33400 0.44 0.43 0.41 0.40 0.39 0.38 0.37 0.36 0.35 0.33 0.31450 0.41 0.40 0.38 0.37 0.36 0.35 0.34 0.33 0.32 0.31 0.39500 0.38 0.37 0.36 0.35 0.34 0.33 0.32 0.31 0.30 0.28 0.27

Standard Air Density, Sea Level, 20EC = 1.2041 kg/m at 101.325 kPa3

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4.5 Burners

!! Liquid Fuel Combustion

To burn oils, particularly the heavier grades efficiently, it is first necessary tobreak down the fuel into small droplets which can be quickly heated and mixedwith air. A fuel droplet of lighter oils is vaporized by heat from the downstreamflame and produces gases which readily react with oxygen.

A fuel droplet of the residual oils partially vaporizes and the gases burn readily,leaving a shell of liquid. The shell cracks with further heat leaving an empty ashshell which eventually breaks down. The whole process takes less than 2seconds.

To atomise oil satisfactorily, it is necessary to control the viscosity of the oil. Ifthe oil is too thick, large droplets will form and will not burn fully. If the oil is toothin, the droplets will be too small and evaporate too quickly, causing lift offfrom the burner, pulsations, etc.

!! Pressure Jet Burners

The pressure jet burner is essentially a nozzle through which the oil is pumpedat high pressure (4 to 10 bars). The oil is introduced tangentially into achamber through slots which cause the oil to spin through a small outlet orificein a hollow cone. Different nozzles can be used to give varying outputs andflame shapes. Normally these burners are restricted to oil of less than 1000secs viscosity, usually in an "On/Off" or "High/Low" mode. The maincharacteristics are:

< cheap to install,< oilways are fine and must be cleaned,< very sensitive to oil viscosity limited to 1000 secs,< heat soak-back can cause coking up around the nozzle, and< sensitive to draft variations.

! Rotary Cup Burners

In this type, the oil is pumped into a tapered cup which is rotating at about 6000rpm. The oil film flows to the tip where it is thrown off. Primary air is introducedat high velocity and atomises the film into droplets. The main characteristicsare:

< high turn down ration (4:1) making the burner ideal for the fluctuatingloads,

< moderate cost,< not too sensitive to oil viscosity, and< easy to clean.

These burners are widely used on boiler applications.

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! Air Blast Burners

Atomising is achieved by introducing high velocity swirling air onto a stream ofoil. With low air pressure burners, 20 to 30% of the combustion air is requiredfor atomising, with the remainder being introduced through different ports. Theturn down ratio is usually about 4 to 2.1. Medium and high pressure burners (7bar air pressure) use less than 10% of the combustion air for atomising, hencethe turn down ration of 5:1 are easily achieved.

This type of burner is mainly used for furnace work where preheatedcombustion air can be used. Low pressure air 200EC, high pressure 400EC.The main characteristics are:

< good turn down ratio,< easy to maintain, the high pressure burners are almost self cleaning,< insensitive to draught, and< flame shape controllable.

!! Common Problems in Burners

CONDITION CAUSE ACTION

Sparky Flame Atomization Check & Clean Nozzles

Flame Incorrect Air Supply Check Control AdjustmentsImpingement

Flame Too High Oil Temperature Adjust PreheatPulsates Too High Air Velocity

Smoke Too Little Air Adjust Air/OilSeal Air Leaks

High Atomization Check Nozzle PreheatParticulate Fuel Input Too High Reduce Fuel

Check Design

!! Solid Fuel Combustion

In a bed of burning solid fuel (wood, coal, peat, etc) under-grate air combineswith the carbon to produce C0 and C0. These hot gases rise through the bed2

and drive off the volatiles of the fuel (Hydrocarbons such as methane). Abovethe bed, secondary air is admitted which burns off the C0 and the volatiles.

!! Optimizing Combustion Conditions

In order to burn fuels efficiently, it is necessary to introduce optimum quantitiesof air for combustion. To little air will cause smoking with consequent loss dueto unburnt fuel. Because of visible smoke, this problem is usually correctedquickly.

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The introduction of too much combustion air is more common in boilers,furnaces and vehicles, but less apparent, and therefore can continueundetected for long periods. The use of excessive quantities of air leads tosubstantial energy losses and can also cause operation problems, i.e. scalingin furnaces.

Control of the air/fuel ratio is very important particularly in high temperatureexhausts i.e. furnaces and kilns, where stack losses can be up to 60% of thefuel input. A simple oxygen analyzer and high temperature thermometer candetect high excess air quantities which can often be rectified by simpleadjustment of the fuel control or the air fans.

! Control of Thermal Input

‚ Overfiring

Losses can also occur due to the use of excessive amounts of fuel inputinto the furnaces and boilers, i.e. overfiring. This leads to high stacktemperatures and avoidable energy losses. Overfiring is generallyassociated with incorrectly adjusted burners and/or with fouled heat transfersurfaces.

‚ Underfiring

Low thermal inputs are easily detected because the boiler or furnaceoutputs will be low. However, overfiring and therefore excessive losses, arenot apparent. A regular check of stack temperatures can ensure that theburner outputs are optimized.

‚ Fuel air ratio

Experience has shown that many burners are incorrectly adjusted,particularly under low load conditions. Wear on cams, linkages, fuel pumpadjustments affect the performance of the energy conversion equipment.Regular combustion checks can identify any shortcomings in maintenance,cleanliness etc.

‚ Flue gas temperature

High flue gas temperatures are associated with the following conditions:

< too high firing rate, usually due to incorrect setting of controls,< fouled heating surfaces - in boilers it could be fouling of surfaces on

fireside or scaling on surfaces on the water side or both.

Fouled heating surfaces impede the heat transfer resulting in more heatbeing rejected to the stack in form of higher flue gas temperature.

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4.6 Air Pollution Control - Process and Equipment

The combustion processes for heat generation, transportation and chemicalprocesses emit pollutants that are harmful to the environment. The three mostcommon effects of the air pollution are:

< Greenhouse effect< Acid rain< Ground level ozone

! Greenhouse Gas Effect

Sun's short wave radiation penetrates the atmosphere and heats up the earth.The warmed earth radiates back the excess heat in form of long wave lengthsradiation because of much lower surface temperatures. Water vapour andgreenhouse gases such as carbon dioxide, nitrous oxides and methane absorbthe infrared radiation, thus heating the atmosphere and the earth's surface.The heating of the atmosphere by blocking the escape of infrared radiation isknown as greenhouse gas effect which is responsible for global warming.

! Acid Rain

Acid rain results from combining of nitrogen and sulphur oxides withatmospheric water vapour. These pollutants originate from coal burning, metalsmelting, vehicles and all other fuel burning activities. Nitric oxide and sulphuricoxides, when combined with water vapour, form nitric and sulphuric acids thatreturn to the earth as acid rain, snow or fog that leads to acidification of lakesand other surface waters.

! Ground Level Ozone

Ground level ozone is produced by the chemical reaction between nitrogenoxides and volatile organic compounds and is the key NOx and VOC related airquality problem. NOx is formed by burning fossil fuels. VOCs are formedmainly from the evaporation of liquid fuels, solvents and organic chemicals.Ozone damage to crops and vegetation can be significant. Ozone sensitivecrops include beans, tomatoes, potatoes , soybeans and wheat.

! Reduction of Pollutant Emissions From Combustion Process

The emission of pollutants from combustion processes can be reduced by fourdifferent methods:

< Energy efficiency improvements< Refinements and modifications to the combustion process< Flue gas treatment< Switching to cleaner fuels or alternative energy source.

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! Energy Efficiency Improvements

Changes to the combustion system that reduce fuel usage have the additionalbenefit of reducing pollutant emissions. Measures to reduce fuel consumptionare desirable because cost savings accrue as fuel usage is reduced.

! Refinement to the Combustion Process

Modifications can be made to the combustion process for the purpose ofreducing the pollutant emissions. However, the changes may have a little or noeffect on combustion efficiency. Some of the methods used include flue gasrecirculation, staged air combustion and staged fuel combustion. All threemethods are designed to delay the availability of oxygen to the fuel. Flue gasrecirculation has added benefit of cooling the flame below the temperaturewhere most NOx formation takes place.

! Flue Gas Treatment

Flue gas treatment equipment is available that can remove NOx and SOx fromthe flue gas stream, but it is quite expensive. SOx can be removed from theflue gas through use of a chemical scrubber that works by spraying a solutionthrough flue gas stream. The spray chemically neutralizes the SO in the gas2

and removes it from the stream before releasing it to the atmosphere.

! Fuel Switching

Different fuels have significantly different emission characteristics. Wherecircumstances warrant, emissions can be reduced by switching to lower sulphurfuels or by changing fuels altogether.

Note:For more discussion of the environmental impact of fuel combustion, refer toModule 1, Section 9, Environmental Issues.

5.0 FUEL FIRED EQUIPMENT AND APPLICATIONS

!! Furnaces

The purpose of a process furnace is to supply heat to the contents in controlledmanner. The furnace may be used for heating metals to a precisely controlledtemperature for heat treatment or for melting. The furnaces are manufacturedin many different types and sizes, some of which are described in this section.

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Furnaces may be batch or continuous type. Furnaces, which generate heat byburning fuels, may be of the direct or indirect fired types. Furnaces are alsoheated by electric resistance or induction heaters.

! Batch Furnaces

The batch furnaces process the product in batches, which means that thefurnace doors must be opened and closed at the beginning and end of eachbatch cycle. Since this is a significant source of energy loss, the loading andunloading times should be minimized. It is also important to load the furnacecompletely to minimize the energy loss per unit of product.

! Continuous Furnaces

Continuous furnaces process the product continually by moving it through theheating zones on chains or conveyors. Since the loading and unloading doorsare open all or most of the operating time, there is a significant heat lossthrough these openings.

Continuous furnaces also may have a significant heat loss because of theconveying mechanism, which is heated to the operating temperature of theproduct. If the conveyor cools off outside the furnace before re-entering theloading area, the energy required to heat the conveyor is not used productively.Thus it is better if the conveyor stays within the heated furnace area.

! Direct Fired Furnaces

The products of combustion are in direct contact with the product being heatedin a direct fired furnace. The heat transfer process from the flame to theproduct is more effective than with the indirect heated furnace. The higher rateof heat transfer which can be achieved with direct fired furnaces can lead to alocal surface overheating of the product, unless the furnace temperature isproperly controlled.

! Indirect Heated Furnaces

In indirect heated furnaces the heat is transferred through some form of heatexchanger. This type of furnace may be used to provide a controlledenvironment for oxidizing or reducing, by introducing an artificial atmosphereindependent of the combustion process. Since the heat transfer from the flameto the product is not as effective as with the direct fired furnace, it can beexpected that the flue gas temperature will be higher, resulting in higher heatlosses unless heat recovery is used.

There are few special considerations for indirect fired furnaces which affect theheat balance calculations. If the controlled atmosphere is maintained inside thefurnace, the heat input and output of the gas entering and leaving the furnace

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must be included in the heat balance. If heat is required for the preparation ofthe atmosphere, the energy required in the gas generator must be included aspart of the total heat input to the furnace. Electrical energy used forrefrigeration or other purposes in the gas generator must also be included.

! Dryers

Dryers use heat to evaporate water or solvents from materials such as lumber,grain, ceramics, paints and carbon electrodes. The same principles of energymanagement described for furnaces also apply to dryers and much of theequipment is similar in concept. A major difference is in operating temperature,which is generally much lower than furnaces, as this avoids damage to theproduct. As a result the direct fired heaters must operate with very highpercentage of excess air. This means that excess air cannot be reduced toachieve the energy savings. Indirect fired dryers can operate at normal valuesof excess air within the combustion chamber. With direct and indirect firedheaters there is a large amount of heat in exhausted air in the form ofevaporated water or solvent. Often the solvents must be incinerated beforedischarge to the atmosphere by burning additional fuel in the dryer dischargeand raising the temperature to about 900EC. Recovery of the heat in the dryerexhaust can be achieved by a heat exchanger which is used to preheat theincoming air for drying with indirect fired dryers or the combustion air for firingin the direct dryers.

! Kilns

There is no fundamental difference between furnaces and kilns from the energymanagement viewpoint. The ceramic and brick industries use stationary kilns.The rotary kilns are used by the cement and pulp industries. Some rotary kilnsburn pulverized coal or refuse-derived fuel. The large heat input to the rotarykilns provides opportunities for the insulation of heat exchangers to recover fluegas heat.

6.0 ENERGY MANAGEMENT OPPORTUNITIES

Energy Management Opportunities is a term used to represent the way that energycan be used wisely to save money. It is intended to provide management,operating and maintenance personnel with ideas to identify the opportunities.Energy Management Opportunities are subdivided into Housekeeping, Low-Costand Retrofit categories.

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6.1 Housekeeping Opportunities

!! Maintain Proper Burner Adjustment

It is a good practice to have an experienced burner manufacturer'srepresentative set up burner adjustments. Furnace operators can then identifythe appearance of a proper burner flame for future reference. The flameshould be checked frequently and always after a significant change in operatingconditions affecting the fuel, combustion air flow or furnace pressure.

!! Check Excess Air and Combustibles in the Flue Gas

A continuous O and combustibles analyzer is the best arrangement, but cost2

is high. Sampling tests with an Orsat or by other chemical means can be areliable guide for proper combustion conditions. Re-adjustment of the fuel/airratio control should be done promptly if required.

! Keep Heat Exchange Surfaces Clean

This is required more frequently with oil fired furnaces and for theseapplications, the use of permanently installed steam or air sootblower may bejustified.

!! Replace/Repair Missing and Damaged Insulation

Heat radiation from a furnace with inadequate insulation can be easily detectedduring the plant survey.

!! Check Furnace Pressure Regularly

Air leakage into or gas leaking out of a furnace can be controlled by maintaininga slight positive furnace pressure. The control dampers in the furnace flue gasducting or related controls should be readjusted if the furnace pressure is notat a correct value.

!! Schedule Production to Operate Furnaces At Or Near Maximum Output

It may be possible to operate the furnace at maximum load every other day,instead of at 50% load continuously. Alternatively, the work may be switchedto a smaller furnace which can operate near full load continuously.

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6.2 Low Cost Opportunities

! Replace Damaged Furnace Doors Or Covers

Furnace doors or covers which are warped or damaged can be a source ofconsiderable leakage of air into or gas out of the furnace. These should bereplaced by doors or covers with tight fitting seals.

! Install Adequate Monitoring Instrumentation

The minimum requirement is to have the ability to determine the energy usedper unit of output, so that significant deviations from this can be identified andcorrective action taken. The fuel or watt meter may be a portable instrumentwhich can then be used on several furnaces. Additional instrumentation will berequired to identify individual losses. Measurements of flue gas temperatureand oxygen content can be used to indicate the flue gas loss. If a heatexchanger is used to recover the heat from the flue gas, the temperature of thegas and air in and out of the heat exchanger can be used to check theperformance.

! Recover Heat From Equipment Cooling Water

It is often possible to use the warm water discharge from equipment coolers forthe purposes such as process washing. In some systems the water dischargemay be too cool to be useful. In these instances the installation of a water flowcontrol valve and temperature controller may be helpful. The water flow iscontrolled automatically from the water temperature at the cooler outlet so thatthe water temperature is high enough to be useful, while maintaining propercooling. The control system will also reduce water use.

6.3 Retrofit Opportunities

!! Install A Heat Exchanger in the Flue Gas Outlet

The cost of a heat exchanger is significantly affected by the temperature of thegas entering the unit. Careful consideration should be given to introducing coldair into the gas stream, if required, to lower the gas temperature enough to useeconomic materials. Stainless steels or alloys cannot be used for temperaturesabove 950EC.

If the recovered heat is to be used to preheat combustion air, the burnermanufacturer should be consulted to determine the maximum allowabletemperature. Frequently it will be as low as 250EC. It is unlikely that it will behigher than 400EC since that would require alloy steels instead of carbon steel.If it is not practical to preheat the combustion air it may be possible to heat theprocess water or to install a waste heat boiler to utilize the heat energy in theflue gas.

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%ofHeatInput '6,121MJ/h

700L/h x 41.7MJ/L' 21%

'700L/h x 41.7MJ/L x 310kg/GJ x 1.15

1,000MJ/GJ

' 10,406kg/h or 10,406

1.204kg/m 3' 8,643m 3/h

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7.0 WORKED EXAMPLES

7.1 Example 1

A furnace uses 700 L/h of RFO (# 6) oil at 50% excess air. Ambient temperatureis 25EC and stack temperature is 450EC. The RFO oil, with 2.5% sulphur, has acalorific value of 41.7 MJ/L and theoretical air requirement of 310 kg/GJ.

Calculate the heat loss as percentage of heat input.

Heat loss Q = M x C x )T p

where:M = fuel input + (fuel input x CV x Stoichiometric Air x %Excess Air)

= 700 L/h x 0.98 kg/L + (700 L/h x 41.7 MJ/L x 310 kg/GJ x 1.5 x 0.001 GJ/MJ)

= 14,259 kg/hCp = 1.01 kJ/kgEC)T = (450 - 25)EC = 425EC

ˆ Q = M x C x )Tp

= 14,259 kg/h x 1.01 kJ/kgEC x 425EC= 6,121 MJ/h

7.2 Example 2

Volumetric Combustion Air RequirementsAt Higher Elevations and Temperatures

Determine combustion air requirements for a furnace using 700 L/h of RFO oil with15% excess air at sea level conditions. Calculate the volumetric air requirementsfor an altitude of 2,000 m and temperature of 20EC.

! Combustion Air Requirements At Sea Level:

The blower has to deliver 8,643 m /h of combustion air at 1.204 kg/ m density3 3

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'10,406kg/h

1.204kg/m 3 x 0.79' 10,940m 3/h

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! Combustion Air Requirements At Other Conditions:

From Figure 13.8, the air density correction factor at 2,000 m altitude and 20ECtemperature is 0.79. Thus the combustion air requirement is:

To deliver an equal mass of air the blower must deliver 10,940 m /h at an3

altitude of 2,000 m and 20EC.

8.0 ASSIGNMENT

The purpose of this assignment is to assess the operating conditions of existingfuel-fired equipment in the facility (including the steam boilers) and bring them totheir design operating level. The following procedure is suggested. Use the "FuelFired Equipment - Data Sheet and Test Results" form in Figure 13.9 for recordinginformation.

! Review equipment installation and operating manuals and record bothequipment and burner data. (Note equipment manufacturers do not necessarilyproduce burners.)

!! Analyze the flue gas, using the procedure outlined in this module. Measureand/or calculate the following data:

.1 Bacharach smoke test number (if applicable),

.2 Percentage Oxygen and/or Carbon Dioxide reading,

.3 Excess air used by the unit,

.4 Flue gas loss using Seigert's formula or nomographs,

.5 Flue gas loss using mass law conservation formula (for comparison),

.6 Combustion efficiency of the unit.

!! Calculate the annual energy cost due to combustion loss.

!! Calculate radiation loss from the unit based on measured surface temperaturesand areas and estimated annual operating hours. (Part of Module 8assignment.)

!! Evaluate possibilities for reducing the operating cost of the unit.

! Prepare a one-page proposal suggesting recommended improvements,potential benefits, cost of implementation and simple payback. (Use EnergyManagement Opportunities Form from Module 3 for each proposal.)

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In this module you learned about:

L Properties of Solid Fuels,

L Properties of Liquid Fuels,

L Properties of Gaseous Fuels,

L The Combustion Process,

L Flue Gas Analysis,

L Losses in Fuel Fired Equipment,

L Burners,

L Energy Management Opportunities.

You should now be able to perform the following tasks:

L Assess the operating conditions of existing fuel firedequipment in your plant.

L Prepare a report identifying potential improvements.

L Indicate the cost of implementing improvements, including apayback schedule.

9.0 SUMMARY - Module 13

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Figure 13.9FUEL FIRED EQUIPMENT

DATA SHEET & TEST RESULTS

1. Unit Data

Plant Name:

Type: Manufacturer:

Model #: Serial #:

Manufacturer's Ratings:

Fuel Type: 1. 2.

Rated Capacity:

Minimum Fuel Input:

Maximum Fuel Input:

Rated Efficiency:

2. Test Results

Date: Fuel Type:

Unit Load Comb Air Stack Flue Gas Analysis Combustion ThermalTemp Temp Efficiency Efficiency(EC) (EC) (%) (%)(kg/h) % of Max %O %CO %CO2 2

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FUEL FIRED EQUIPMENTDATA SHEET & TEST RESULTS

1. Unit Data

Plant Name:

Type: Manufacturer:

Model #: Serial #:

Manufacturer's Ratings:

Fuel Type: 1. 2.

Rated Capacity:

Minimum Fuel Input:

Maximum Fuel Input:

Rated Efficiency:

2. Test Results

Date: Fuel Type:

Unit Load Comb Air Stack Flue Gas Analysis Combustion ThermalTemp Temp Efficiency Efficiency(EC) (EC) (%) (%)(kg/h) % of Max %O %CO %CO2 2