Int. J. Exergy, Vol. 11, No. 1, 2012 119

Copyright © 2012 Inderscience Enterprises Ltd.

Cumulative exergetic assessment of LPFO utilised steam boilers

I.O. Ohijeagbon*Mechanical Engineering Department,University of Ilorin,P.M.B. 1515, Ilorin, NigeriaE-mail: idehaiohi@yahoo.comE-mail: ohijeagbon.io@unilorin.edu.ng*Corresponding author

S.O. JekayinfaAgricultural Engineering Department,Ladoke Akintola University of Technology,Ogbomoso, NigeriaE-mail: jekaysol@yahoo.com

M.A. WaheedMechanical Engineering Department,University of Agriculture,Abeokuta, NigeriaE-mail: akindoye@yahoo.com

Abstract: This paper relates to a study on the energy and exergy resources, transfers, consumption and losses in the combustion and heat exchanging units of industrial steam boilers. Mass, energy and exergy analyses, which included the physical and chemical exergies of material streams, were used to investigate the energy resource utilisation of Low Pour Fuel Oil (LPFO) in industrial steam boilers. The chemical exergies of LPFO and the exhaust fl ue gases were determined as 44,566.66 and 147.97 kJ/kg, respectively, for LPFO undergoing complete combustion process. Higher evaporation ratios and energy consumption by steam were associated with reduction in heat loss. Lower exergy destruction was due to lower rapid changes in temperature potential for the generation of steam in the heat exchanging unit. The average energy and exergy effi ciencies of the boilers are obtained as 69.54 and 38.5%, respectively. The exergy effi ciencies obtained for the combustion and heat exchanging units for the boilers investigated are given as 55.35, 49.06 and 58.69 and 63.80%, respectively. The results obtained in this study could be applied for energy resources management and control of oil-fi red industrial steam boilers.

Keywords: exergy; energy; combustion; steam; boilers; heat-loss; exergy-destruction.

Reference to this paper should be made as follows: Ohijeagbon, I.O., Jekayinfa, S.O. and Waheed, M.A .(2012) ‘Cumulative exergetic assessment of LPFO utilised steam boilers’, Int. J. Exergy, Vol. 11, No. 1, pp.119–135.

120 I.O. Ohijeagbon et al.

Biographical notes: Idehai O. Ohijeagbon is currently Lecturing in the Department of Mechanical Engineering, University of Ilorin, Nigeria. He specialised in energy studies and sustainability in his doctorate studies.

Simeon O. Jekayinfa is a Professor in the Department of Agricultural Engineering, Ladoke Akintola University of Technology, Nigeria and specializes in the area of energetics.

M. Adekojo Waheed is a Professor of Thermofl uids Engineering at the Federal University of Agriculture Abeokuta, Nigeria. His discipline is Mechanical Engineering. His research focus is in heat and mass transfer, computational fl uid dynamics, (CFD) and energy studies.

1 Introduction

Steam boilers remain inevitably important in several industrial, domestic and manufacturing applications for heating, processing and power generation, amongst others. A boiler is an enclosed vessel that provides a means for combustion heat to be transferred into water until it becomes heated water or steam (UNEP, 2010). Steam production is basically an energy conversion process in which fuel energy is converted into energy resident in steam. Several factors are key ingredients in boiler performance. A steam system analysis investigates the energy transfer of the fuel to the steam and the steam to other processes (Harrell, 2002).

The energetic process fundamentally allows a systematic analysis of energy and exergy methodologies to be conducted on systems, operations or processes during energy transport and transformations. Energetics is the studies of energy transfers, conversions and dynamic equilibrium in relation to energy fl ows. It is a concept that emphasises the construction of predictive models of energy fl ow. In fact, all energy transfer and conversion processes are accompanied by changes in both energy quantity and quality (Zhou et al., 1996). Different energies have different qualities, indicating to what extent they are theoretically convertible to mechanical work. This limitation, ‘a Law of Nature’, implies that the total energy quality always decreases in each conversion (the second law of thermodynamics). The quality of energy is described by the concept of entropy. High entropy is equal to low quality of energy (Wall, 1998).

Valero (2006) had proposed that stakeholders such as scientists, engineers and industrialists should be interested to know the physical cost measured in exergy units of all manufactured products as well as the exergy needed for its use. Exergy as a measure of waste residuals reveals that exergy is not only a natural measure of the resource inputs to an economic system, it is also a measure of the material outputs. Exergy is lost in all processes (Ayres et al., 1998). Exergy is, therefore, presented as a means to determine the useful part of a given amount of any energy form, such as heat or enthalpy. Hence, exergy is the maximum possible work for a reversible process from the specifi ed initial state to the state of its environment. In an exergy analysis, a form of reversible process is not needed, like other thermodynamic analysis (Ayhan and Demirtas, 2001). Knowing the conditions of the initial and fi nal states of the process is enough to calculate the exergy (Ayhan and Demirtas, 2001). The concept of exergy is crucial not only to effi ciency studies, but also to cost accounting and economic analyses. Costs should refl ect value, since the value is not in energy but in exergy and assignment of cost to energy leads to misappropriations, which are common and often gross (Wall, 1986). The scarcity and

Cumulative exergetic assessment of LPFO utilised steam boilers 121

undesirable side effects of careless utilisation of energy resources on economics and ecology require careful analysis and planning for proper energy consumption (Dincer et al., 2003).

The present study considers the cumulative energy and exergy assessment of Low Pour Fuel Oil (LPFO)-operated steam boilers. The energy resource utilisation of LPFO in the combustion and heat exchanging units of industrial steam boilers was investigated by considering both the physical and chemical exergies of the material streams in boiler operations used in Nigeria. The scope of steam boilers investigated was limited to Nigeria; however, the principles herein contained are universally applicable.

2 Method

2.1 Thermodynamic properties of material streamsThe stoichiometric (AFRst) and Actual Air-Fuel ratio (AAF) for LPFO undergoing complete combustion was determined. An appropriate mass and energy balance was applied to the boiler system with the evaporation ratio and the air fuel ratio and all were used to determine masses of substances of all material streams in the boiler, such as; air, am� , fuel, fm� , hot products, pm� , feed water, wm� , steam, sm� and exhaust fl ue gas, . gm�

Computed mass fl ow rates of material streams of the investigated boiler operations were used to determine other thermodynamic properties of materials streams, such as enthalpy and entropy at different states. The determined thermodynamic properties were used to further compute various exergetic values which were used to obtain the energy and exergy fl ows of the boiler operations.

2.2 Schematic description of a boiler plant The schematic diagram of a boiler presented in Figure 1 shows that it is a thermodynamic system which comprises of the combustion unit and the heat exchanging unit (Saidur et al., 2010). The two units of the boiler mentioned are separately analysed to establish the mass fl ow rate of the material streams in a boiler system. Combustion of fuel takes place in the combustion unit of the boiler, while the heat carried by the hot fl ue gas from the combustion unit is used to transform liquid water to steam in the heat exchanging unit.

Figure 1 Schematic diagram of combustion and heat exchanging units in a boiler

Source: (Saidur et al., 2010)

3 Analysis

3.1 Mass, energy and exergy balanceAn exergetic analysis involves mass, energy and exergy balance for each plant unit, as well as exergetic effi ciency for each of them (Modesto and Nebra, 2009). The mass, energy and

122 I.O. Ohijeagbon et al.

exergy balances at steady state for the material fl ows in a boiler system with negligible potential and kinetic energy changes can be expressed, respectively, by equations (1) to (3) (Aljundi, 2009; Modesto and Nebra, 2009; Saidur et al., 2010).

,i em m=∑ ∑� � (1)

,e e i iQ m h m h= −∑ ∑� � � (2)

,i i e eI m mε ε= −∑ ∑� � � (3)

where

im� = inlet mass fl ow rate, kg/s

em� = exit mass fl ow rate, kg/s

Q� = heat transfer rate to the system, kJ/s

I� = exergy destruction rate, kJ/s

iε = inlet specifi c fl ow exergy of material streams, kJ/s

eε = exit specifi c fl ow exergy of material streams, kJ/s.

3.2 Physical and chemical exergyThe total exergy transfer associated with material streams for a fl ow process comprises of physical and chemical exergy (Cornelissen, 1997; Dincer and Rosen, 2007; Talens et al., 2007; Chemical Exergy, 2011). The fl ow exergy of a substance is the theoretically obtainable work when the substance is brought into total equilibrium with the local environment. It can be split into chemical exergy and thermomechanical (physical) fl ow exergy, which is represented by equation (4) (Ertesvag, 2007).

.ph chε ε ε= + (4)

The specifi c physical fl ow exergy is expressed by equation (5):

0 0 0( ),ph h h T s sε = − − − (5)

where

ε = specifi c fl ow exergy of material streams, kJ/kg phε = specifi c physical fl ow exergy, kJ/kgchε = specifi c chemical fl ow exergy, kJ/kg

h = specifi c enthalpy, kJ/kg

s = specifi c entropy, kJ/kg K

h0 = h (T0, P0)

s0 = s (T0, P0)

T0, P0 = reference state temperature and pressure.

Cumulative exergetic assessment of LPFO utilised steam boilers 123

Most analyses often involve the use of the natural environment-subsystem model described in Table 1. A distinguishing difference between exergy and other thermodynamic properties is that the reference state for exergy is determined by the surroundings. In fact, exergy of matter will change if the state of the surroundings changes, even when no changes occur in the system itself. A temperature of 25°C (298.15 K), a pressure of 101.323 kPa and zero values for the height z0 and velocity v0 of the earth surface are typically taken as reference state (Jorge Luis Hau, 2005).

Table 1 A reference-environment model

Temperature: T0 = 298.15 K

Pressure: P0 = 1 atm (1 bar) = 101.325 kPa

Composition (i)Atmospheric air saturated with H2O at T0 and P0 having the following composition:Air constituents Mole fraction

N2 0.7565 O2 0.2035 H2O 0.0303 Ar 0.0091 CO2 0.0003 H2 0.0001

(ii) The following condensed phases at T0 and P0: Water (H2O) Limestone (CaCO3) Gypsum (CaSO4.2H2O)

Source: Adapted from Dincer and Rosen (2007)

3.2.1 Chemical exergy of fuelsComplete combustion of a general hydrocarbon with atmospheric air is written as (Combustion in Power Generation, 2010):

2 2 2 ,4 2x yy yC H x O xCO H O + + → + (6)

where x and y are constant coeffi cients that characterise the hydrocarbon and combustion process. Assuming no irreversibility, the molar chemical exergy of reactants and products can be developed as expressed by equation (7) (Chemical Exergy, 2011).

2 2 2( ) ,

4 2ch ch ch ch

x fuel xO xCO xH Oy ye x e g xe e+ + → −∆ + + (7)

wherech

x fuele = molar chemical exergy of fuel, kJ/kmol

2

chxOe = molar chemical exergy of oxygen (O2), kJ/kmol

2

chxCOe = molar chemical exergy of carbon dioxide (CO2), kJ/kmol

124 I.O. Ohijeagbon et al.

2

chxH Oe = molar chemical exergy of water vapour (H2O), kJ/kmol

g∆ = the change in the standard Gibbs function, kJ/kmol.

The chemical exergy of fuel can thus be obtained by equation (8),

2

2 2

( /4)

0 /2ln .x y

Ochx fuel x y

CO H O

ne g RT

n n

+ = −∆ +

(8)

The mole fractions of oxygen (2On ), carbon dioxide (

2COn ) and water (2H On ) in a standard

environment are given in Table 1.Where

R = 8.3144 kJ/kmol K (molar or universal gas constant).

The standard Gibbs functions of formation of other components in equation (8) are given as follows (Roger and Mayhew, 1992):

2COg = –394,390 kJ/kmol

2 ( )H O vg = –228,590 kJ/kmol

2Og = 0.

The change in standard Gibbs function can be determined by equation (9),

2 2 2( ) .2 4CO H O v Oy yg xg g x g ∆ = + − + (9)

The specifi c chemical exergy can be obtained by equation

,ch

ch xeM

ε = (10)

where M = molar mass of chemical substance, kg/kmol.

3.2.2 Analysis of the combustion unitApplying the energy balance (equation (2)) to the combustion unit as shown in Figure 1, the energy input in the combustion unit can be determined by

,in f f a aE m h m h= +� � (11)

where and f ah h are the specifi c enthalpies of combustion fuel and air.The combustion effi ciency of an adiabatic combustor is usually equal to unity (Saidur

et al., 2010) and may be represented by the fi rst law effi ciency given by

,p pC

f

m hm HHV

η =×�

� (12)

where HHV = high or gross heating value of fuel.The combustion unit is assumed to operate in a steady-fl ow adiabatic process, whereby

the change of mass and energy of the control volume is zero. It is also assumed that the

Cumulative exergetic assessment of LPFO utilised steam boilers 125

kinetic and potential energies are negligible and that there is no work transfer involved. Applying the exergy balance (equation (3)) to the combustion unit, the exergy destruction in the combustion unit can be determined by

1 1 3 2 2 3[ ] [ ],ph ch ph ph ch ch

C a a a p p f f f fI m m mε ε ε ε ε ε= + − + + −� � � � (13)

where 1 2 3

, and ph ph pha f pε ε ε are the specifi c physical exergies of air and fuel at the inlet and of

hot products at the exit, while 1

chaε ,

2( )ch

f HHVε , are the specifi c chemical exergies of air and the combustion fuel at the inlet and

3

chfε is the specifi c chemical exergy of the combustion

fuel at the exit.The exergy effi ciency of the combustion unit is given by

3

32 2

.[ ]

php p

C ph ch chf f f f f

mm m

εψ

ε ε ε=

+ −

�

� � (14)

3.2.3 Analysis of the heat exchanging unitPerforming an energy balance on the heat exchanging unit as shown in Figure 1 and noting that p gm m=� � and w sm m=� � , the heat loss can be determined by

( ) ( ) ( ),H loss p p g w s wQ m h h m h h= − − −� � (15)

where , and g s wh h h are the specifi c enthalpies of exhaust fl ue gas, steam and feed water, respectively.

The fi rst law effi ciency of the heat exchanging unit is given by

( ).

( )w s w

Hp p g

m h hm h h

η −=

−�� (16)

The exergy destruction in the heat exchanging unit can be determined by

3 4 4 5 5 6 6[ ] [ ] [ ],ph ph ch ph ch ph ch

H p p w w w s s s g g gI m m m mε ε ε ε ε ε ε= + + − + − +� � � � � (17)

where 3 4 5 6

, , and , ph ph ph php w s gε ε ε ε are the specifi c physical exergies of products of combustion,

feed water, steam and exhaust fl ue gas, respectively, while 4 5 6, and ch ch ch

w s gε ε ε are the chemical exergies of the feed water, steam and exhaust fl ue gases, respectively. The specifi c chemical exergies of steam and liquid water are respectively given as 526.33 kJ/kg (9474 kJ/kmol) and 49.12 kJ/kg (884.20 kJ/kmol) (Ertesvag, 2007; Szargut et al., 2011; Talens, et al., 2007).

The exergy effi ciency of the heat exchanging unit is given by

5 5 4 4

3 6 6

[ ] [ ].

[ )]

ph ch ph chs s s w w w

H ph ph chp p g g g

m mm mε ε ε ε

ψε ε ε

+ − +=

− +

� �

� � (18)

3.2.4 Analysis of the entire boilerThe overall boiler energy effi ciency can be determined by

( ).s s w

Bf f

m h hm h

η −=�� (19)

126 I.O. Ohijeagbon et al.

The overall exergy destruction of the boiler is given by the sum of exergy destruction in the combustion chamber and the heat exchanger. That is,

.B C HI I I= +� � � (20)

The overall boiler exergy effi ciency can be determined as the rational effi ciency of the entire boiler, which is the ratio of the desired exergy output, desired outputE� to the exergy used, usedE�(Cornelissen, 1997). The desired outputE� was the net exergy for the transformation of water, while usedE� was the net energy input into the system. The exergy effi ciency of the boiler is therefore given by equation (21).

.desired outputB

used

EE

ψ =�

� (22)

Equation (22) may be expressed as follows in equation (23)

5 4 5 4

2 32

[ ][ ]

ph ph ch chs s w s w

B ph ch chf f f f

mmε ε ε ε

ψε ε ε− + −

=+ −

�

� (23)

3.2.5 Average operational parameters of steam boilers Weekly average operational parameters of two different steam boilers operation using LPFO were obtained, as presented in Table 2, which include the following; air temperature (0C), steam pressure (kN/m2), product temperature (0C), fi ring rate (litres/hr) and evaporation ratio. The specifi cations of LPFO used in the following analysis are presented in Table 3. The determined thermodynamic properties were used to compute the exergetic input and output used in obtaining the energy and exergy fl ow diagrams, shown in Figures 2 to 5.

Typical specifi cation for No. 6 oil representing LPFO is given in Table 3.

Table 2 Average operational parameters of steam boiler investigated

Boiler operationAir temperature

(0C)Steam pressure

(kN/m2)Product

temperature (0C)Firing rate (litres/hr)

Evaporation ratio

1 80.63 772.00 275.73 799.08 12.002 64.68 747.00 229.83 589.79 12.00

Table 3 Fuel specifi cations for No. 6 fuel oil

Basic constituent of No.6 fuel oil Maximum CO2 in product

HHV (Gross heating value)

LHV (Net heating value)

%Carbon %Hydrogen %Sulfur %Oxygen (Btu/lb) kJ/kg (Btu/lb) kJ/kg

87.49 9.92 1.40 1.19 16.50 18300.00 42657.34 17381.00 40515.15

Source: TSI Incorporated (2004)

Cumulative exergetic assessment of LPFO utilised steam boilers 127

Figure 2 Cumulative energy fl ow of Boiler operation 1

Figure 3 Cumulative energy fl ow of boiler operation 2

Figure 4 Cumulative exergy fl ow of boiler operation 1

3.2.6 Stoichiometric and actual air-fuel ratioThe composition of LPFO (No. 6 fuel oil) was given in Table 3 and this was used to compute the amount of oxygen required, as presented in Table 4 and other products released during the combustion are as presented in Table 5. Hence, the ultimate and proximate analysis of the products of combustion for complete combustion were then determined.

128 I.O. Ohijeagbon et al.

Table 4 Oxygen required and products per kg of fuel for complete combustion

ReactantMass per kg of fuel Reaction

O2 required per kg of fuel

Products per kg of fuel

C 0.8749 2 2

12 kg 32 kg 44 kgC O CO+ →

+ →

320.874912

2.333 kg

×

=2

440.874912

3.208 kg CO

×

=

H 0.09922 2 2

12

2 kg 16 kg 18 kg

H O H O+ →

+ →

160.09922

0.7936 kg

×

= 2

180.09922

0.8928 kg H O

×

=

S 0.01402 2

32 kg 32 kg 64 kgS O SO+ →

+ →

320.01432

0.014 kg

×

=

O 0.0119 – –0.0119 –1.0000 3.1287 kg

Table 5 Ultimate and proximate analysis of the products of combustion for complete combustion of LPFO

Product

Mass of Wet Flue Gas

(kg/kg of Oil)

Mass of Dry Flue Gas

(kg/kg of Oil)

Ultimate Analysis (%) Proximate Analysis (%)

Wet Dry Wet Dry

CO2 3.2080 3.2080 18.740 19.780 12.4840 13.6430H2O 0.8928 – 5.220 – 8.4940 –SO2 0.0280 0.0280 0.164 0.173 0.0754 0.0823O2 0.6263 0.6263 3.660 3.860 3.3570 3.6680N2 12.3600 12.3600 72.220 76.190 75.590 82.6070

17.1151 16.2223 100.00 100.00 100.00 100.00

Figure 5 Cumulative exergy fl ow of boiler operation 2

Cumulative exergetic assessment of LPFO utilised steam boilers 129

Since air contained 23.3% O2 by mass,

Air required/kg of fuel = 3.1287 13.4280.233

=

Hence, the stoichiome tric air – fuel ratio = 13.428

1 by mass

That is, AFRst = 13.428 kg of air/kg of fuel

The AAF is given by (Process Heating, 2010):

AAF = (1 + EA) × AFRst (24)

in which, EA = excess air.Recommended excess air levels for fuel oil burners are: register type, 15–20%;

multi-fuel burners and fl at-fl ame type, 20–30% (Bureau of Energy Effi ciency, 2010a). The boiler investigated is a multi-fuel burner type; hence, 20% excess air was selected.

AAF = (1 + EA) × AFRst =(1 + 0.2) × 13.428 = 16.114 kg of air/kg of fuel.

Since air contained 76.7% of N2 by mass,

2N supplied = 0.767 × 16.114 = 12.36 kg/kg of fuel.

That is, N2 contained in the products of combustion = 12.36 kg/kg of fuel.Similarly,

• O2 supplied = 0.233 × 16.114 = 3.755 kg/kg of fuel

• the excess air = 3.755 − 3.1287 = 0.6263 kg/kg of fuel

• that is, O2 contained in the products of combustion = 0.6263 kg/kg of fuel.

The above computed values were used to calculate the ultimate and proximate analysis presented in Table 5. The proximate analysis values were determined by dividing the mass per kg of fuel for each product by its molar mass, which was then summed up to obtain individual ratio distribution for the wet and dry analysis presented in Table 5.

3.2.7 Material streams propertiesThe operational variables of steam boilers given in Table 2 were used to compute other thermodynamic properties of materials streams which may not be measured directly in a boiler and they include: mass fl ow rates, temperature, enthalpy and entropy of material streams in boilers investigated, as presented in Table 6.

3.2.8 Chemical exergy of exhaust fl ue gasesThe chemical exergy of exhaust fl ue gases was determined from the mole fractions of the of the products of combustion presented in Table 5 and their respective chemical exergies given by Ertesvag (2007) and the associated carbon monoxide was determined as 0.336%. Hence,

( )( )1 ,

100 i

ch chx g i x g

e n e= ∑ (25)

130 I.O. Ohijeagbon et al.

Poin

tSu

bsta

nces

Mas

s fl o

w ra

te (k

g/s)

Tem

pera

ture

(0 C)

Enth

alpy

(kJ/

kg)

Entro

py (k

J/kg

K)

Boile

r O

pera

tion

1Bo

iler

Ope

ratio

n 2

Boile

r O

pera

tion

1Bo

iler

Ope

ratio

n 2

Boile

r O

pera

tion

1Bo

iler

Ope

ratio

n 2

Boile

r O

pera

tion

1Bo

iler

Ope

ratio

n 2

1A

ir,

am�

,3.

3744

2.48

9680

.63

64.6

835

4.53

338.

081.

8684

1.82

092

Fuel

, f

m�0.

2109

0.15

561,

084.

821,

068.

8740

,515

.15

40,5

15.1

52.

0009

2.01

323

Hot

pro

duct

s, 3.

5853

2.64

5227

5.73

229.

832,

716.

922,

701.

444.

9513

5.37

254

Feed

wat

er,

pm�

2.53

081.

8672

100.

0010

0.00

419.

1041

9.10

1.30

701.

3070

5St

eam

, w

m�2.

5308

1.86

7216

8.89

167.

542,

767.

602,

766.

356.

6759

6.68

746

Exha

ust fl

ue

gas,

3.58

532.

6452

152.

9513

7.00

231.

0020

1.80

2.05

162.

0134

Table 6 Computed thermodynamic properties of materials streams in investigated boilers

Cumulative exergetic assessment of LPFO utilised steam boilers 131

( ) 4,350.36 kJ/kmol.chx ge =

The molar mass of the exhaust gas from the combustion of LPFO was also determined from the mole fractions of the products of combustion and the respective molar mass of each constituent member contained in the gas. Hence,

( ) ( )( )LPFOg i i g LPFOM n M= ∑ (26)

( ) 29.40 kg /kmol.

LPFOgM =

Hence,

( )

6

( )

147.97 kJ/kg.g

LPFO

chxch

gg

e

Mε = =

4 Results and discussions

4.1 Chemical exergy of material streamsThe chemical exergy of atmospheric air in a standard environment was determined as 1702.65 kJ/kmol. Using the molar mass of air as Mair = 28.96 kg/kmol, the chemical exergy of air can thus be obtained as 58.79 kJ/kg. Similarly, the chemical exergy of LPFO was determined as 44,566.66 kJ/kg. The chemical exergy was slightly higher than the gross heating value (HHV) of the fuel, which is given as 42,657.34 kJ/kg. The chemical exergy of the exhaust fl ue gases was determined as 147.97 kJ/kg for LPFO undergoing complete combustion process.

4.2 Exergetic effi ciencies of the boiler unitsThe exergy effi ciencies obtained for the combustion/heat exchanging units for boiler operations 1 and 2 are given as 55.35/49.06 and 58.69/63.80% respectively as presented in Table 7. These were generally higher than the fi gures obtained by Saidur et al. (2010), which were 52% exergy destruction in both the heat exchanging unit and the combustion unit.

Table 7 Summary of exergetic effi ciencies in investigated boilers

Exergetic Values and Effi ciencies Boiler operation 1 Boiler operation 2

Adiabatic energy effi ciency of combustion unit (%) 100.00 100.00Exergy effi ciency of combustion unit (%) 55.35 49.06Energy effi ciency of heat exchanging unit (%) 66.69 66.28Exergy effi ciency of heat exchanging unit (%) 58.69 63.80Energy effi ciency of entire boiler (%) 69.56 69.52Exergy effi ciency of entire boiler (%) 38.57 38.43

132 I.O. Ohijeagbon et al.

4.3 Exergetic fl ow diagrams of boilers operation The results of the analysis of energy and exergy balance of the investigated boilers using LPFO are shown in ures 2 to 5. The energetics of an entire boiler system was sub-divided into two major groups, namely, the combustion unit and the heat exchanging unit and was inclusive of the energy analysis (energy quantity) and exergy analysis or exergy (energy quality and quantity).

4.3.1 Energy fl ow of boilers operationThe cumulatve energy fl ows in the boilers are shown in Figures 2 and 3. The energy quantity contributed by fuel in the combustion unit was between 87.72 and 88.22%, in which the higher value of 8545 kJ/s was obtained in boiler operation 1, while the lower value of 6304 kJ/s was obtained in boiler operation 2. The fi ring rates presented in Table 1, which were higher for boiler operation 1 (799.08 litres/hr) compared with that of boiler operation 2, given as 589.79 litres/hr, accounted for the values of energy contribution experienced.

The energy value consumed by steam, fl ue gas and heat loss in the heat exchanging unit was 64.84, 7.67 and 27.49% respectively for boiler operations 1, while those for boiler operation 2 are given as 65.15, 6.73 and 28.12 respectively. The average heat loss was 27.81% for the two boilers. Saidur et al. (2010) recorded a heat loss of 22.5% in the methane-operated boiler investigated, in which steam consumed 68.3% of energy. This showed that higher energy consumption by steam was associated with reduction in heat loss. The study of Saidur et al. (2010) revealed that an evaporation ratio of 15.35 was used, while an evaporation ratio of 12 was used in this study. This is an indication that a higher evaporation ratio accounts for reduced energy loss in boilers. Further, the temperatures of the products of combustion presented in Table 3 were 275.73 and 229.830C for boilers operations 1 and 2. This shows that a higher temperature of the products of combustion also contributed to the reduction in heat loss in the heat exchanging unit, since the rate at which water is converted to steam is higher at higher temperatures.

4.3.2 Exergy fl ow of boilers operationThe exergy of the hot products in the combustion unit was higher for boiler operation 1 (58.90%) than for boiler operation 2 (52.26%), as revealed by the cumulative exergy fl ow shown in Figures 4 and 5. This indicates that greater exergy was destroyed in the combustion unit of boiler operation 2, with a value of 47.74%, than of boiler operation 1, given as 41.10%. The higher exergy destruction experienced in the combustion unit of boiler operation 2 was due to higher entropies of the hot products in Table 3, 5.3725 kJ/kg K, while that of boiler operation 1 was 4.9513 kJ/kg K.

The exergies destroyed in the heat exchanging unit were given as 46.96 and 42.32% for boiler operations 1 and 2; although percentage energy loss was slightly higher in boiler operation 2, the lower exergy destruction experienced was due to lower rapid changes in temperature potential for the generation of steam. Also, the combined exergy consumption of steam and fl ue gas in the heat exchanger was higher for boiler operation 2, which was obtained as 57.68%, as against 53.04% for boiler operation 1, as shown in Figures 4 and 5. The higher exergy consumption by steam observed in boiler operation 2 was attributed to the fact that the investigated boiler operations were analysed for a similar evaporation ratio of 12.

This study, which included the chemical exergy of the material streams of the boilers investigated, reveals that inclusion of chemical exergy in the analysis contributed to

Cumulative exergetic assessment of LPFO utilised steam boilers 133

reduction in exergy destruction in the heat exchanging unit, in comparison with the study of Saidur et al. (2010), which obtained 52% exergy destruction in both the heat exchanging unit and the combustion unit.

5 Conclusion

The stoichiometric (AFRst) and actual air-fuel ratio (AAF) for LPFO were determined as 13.428 and 16.114 kg of air/kg of fuel respectively. The mass fl ow rate of fuel and steam were determined as 0.2109 and 2.5308 respectively for boiler operation 1 and 0.1556 and 1.8672 respectively for boiler operation 2.

The chemical exergy of atmospheric air in a standard environment was determined as 58.79 kJ/kg (1702.65 kJ/kmol). The chemical exergies of LPFO and the exhaust fl ue gases were determined as 44,566.66 kJ/kg and 147.97 kJ/kg for LPFO undergoing complete combustion process.

The energy quantity contributed by fuel was higher for boiler operation 1, given as 8545 kJ/s, compared with the lower value of 6304 kJ/s for boiler operation 2. The fi ring rates accounted for the values of energy contribution by fuel in the combustion unit. This showed that higher energy consumption by steam was associated with reduction in heat loss. The higher evaporation ratio accounts for reduced energy loss in boilers. The higher temperature of the products of combustion also contributed to the reduction in heat loss in the heat exchanging unit, since the rate at which water is converted to steam is higher at higher temperatures.

The higher exergy destruction experienced in the combustion unit of boiler operation 2 was due to higher entropy of the hot products, which is given as 5.3725 kJ/kg K, while that of boiler operation 1 was obtained as 4.9513 kJ/kg K. Even though the percentage energy loss was slightly higher in boiler operation 2, the lower exergy destruction experienced was due to lower rapid changes in temperature potential for the generation of steam. The exergy consumption by steam was higher in boiler operation 2 due to the constant evaporation ratio used in the analysis. The inclusion of chemical exergy in the analysis was found to contribute generally to reduction in exergy destruction of the entire boiler, thus enhancing exergy effi ciency.

With the exception of the adiabatic energy effi ciency of the combustion unit, which was unity and the same for the boiler operations, the energy and exergy effi ciencies were generally higher for boiler operation 1 compared with those of boiler operation 2. The exergy effi ciency of the heat exchanging unit was, however, higher in boiler operation 2. Increased evaporation ratio is expected to increase exergy effi ciency in the heat exchanging unit.

The exergy effi ciencies obtained for the combustion/heat exchanging units for boiler operations 1 and 2 are given as 55.35/49.06 and 58.69/63.80%, respectively. Exergetic analysis, which considers the chemical exergy of the material streams involved in a boiler system, offers more comprehensive details on the logical energy and exergy allocation and losses of the processes in the system.

The analysis of the products of combustion results in 18.74 and 19.78% carbon dioxide, CO2., for the ultimate analysis and 12.48 and 13.64 for the proximate analysis of the complete combustion of LPFO.

References Aljundi, I.H. (2009) ‘Energy and exergy analysis of a steam power plant in Jordan’, Applied Thermal

Engineering, Vol. 29, pp.324–328.Ayhan, B. and Demirtas, C. (2001) ‘Investigation of turbulators for fi re tube boilers using exergy

analysis’, Turk. J. Engin. Environ. Sci., TUBITAK Vol. 25, pp.249–258.

134 I.O. Ohijeagbon et al.

Ayres, R.U., Ayres, L.W. and Martinas K. (1998) ‘Energy, waste accounting and life-cycle analysis’, Energy, Vol. 23, No. 5, pp.355–363.

Bureau of Energy Effi ciency (2010) ‘Boilers: types, combustion in boilers, performances evaluation, analysis of losses, feed water treatment, blow down’, Energy Conservation Opportunities, [online], www.emt-india.com/BEE-Exam/GuideBooks/2Ch2.pdf (Accessed: 19/10/2010).

Chemical Exergy (2011) Lecture 7.5 [online], www.oocities.org/pldhar/Lecture 7_5.ppt (Accessed: 4/07/2011).

Combustion in Power Generation (2010) Introduction to Combustion, fl ame temperature, governing equations, [online], Available at:www-diva.eng.cam.ac.uk/energy/NondasTeaching/3A5/Lecture1.pdf (Accessed: 16/10/2010).

Cornelissen, R.L. (1997) Thermodynamics and Sustainable Development – The Use of Exergy Analysis and the Reduction of Irreversibility, Enschede, The Netherlands, Vols. 2–5, pp.11–14.

Dincer, I. and Rosen, M.A. (2007) Exergy: Energy, Environment and Sustainable Development, Elsevier, London.

Dincer, I. Hussain, M.M. and Al-Zaharnah, I. (2003) ‘Energy and exergy use in the industrial sector of Saudi Arabia’, Proc. Instn. Mech. Engrs., Part A: J. Power and Energy, A02603 IMechE, Vol. 217, pp.481–492.

Ertesvag, I.S. (2007) ‘Sensitivity of the chemical exergy for atmospheric gases and gaseous fuels to variations in ambient conditions’, Energy Conservation and Management, Vol. 48, No. 7, pp.1983–1995.

Harrell, G. (2002) Steam System Survey Guide, U.S. Department of Energy Best Practices Steam Program, U.S. Department of Energy, www1.eere.energy.gov/manufacturing/tech_deployment/pdfs/steam_survey_guide.pdf

Jorge Luis Hau, M.S. (2005) ‘Toward environmentally conscious process systems engineering via joint thermodynamic accounting of industrial and ecological systems’ Unpublished Ph.D Dissertation, Ohio State University, USA, http://etd.ohiolink.edu/send-pdf.cgi/Hau Jorge L.pdf?osu1117650243

Modesto, M. and Nebra, S.A. (2009) ‘Exergoeconomic analysis of the power generation system using blast furnace and coke oven gas in a brazilian steel mill’, Applied Thermal Engineering, Vol. 29, pp.2127–2136.

Process Heating (2010) Combustion Effi ciency, [online], Available at: www.docstoc.com/docs/.../Process-Heating—Combustion-Effi ciency (Accessed: 14/10/2010).

Roger, G.F.C. and Mayhew, Y.R. (1992) Engineering Thermodynamics: Work and Heat Transfer, 4th Ed., Longman Group Limited, UK.

Saidur, R., Ahamed, J.U. and Masjuki, H.H. (2010) ‘Energy, exergy and economic analysis of industrial boilers’, Energy Policy, Vol. 38, pp.2188–2197.

Szargut, J., Valero, A., Stanek, W. and Valero, A. (2011). ‘Towards an international reference environment of chemical exergy’ [online]. www.exergoecology.com/papers/towards_int_re.pdf (Accessed: 20/05/2011).

Talens, L., Villalba, G. and Gabarrell, X. (2007) ‘Exergy analysis applied to biodiesel production’, Elsevier (Resources, Conservation and Recycling), Vol. 51, pp.397–407.

TSI Incorporated (2004) Combustion Analysis Basics: An Overview of Measurements, Methods and Calculations Used in Combustion Analysis, www.tsi.com

United Nations Environment Programme (UNEP) (2010) Division of Technology, Industry and Economics, [online]. Energy Effi ciency Guide for Industry in Asia, www.greenbiz.com/sites/default/fi les/document/CustomO16C45F67124.pdf [Accessed 20th October].

Valero, A. (2006) ‘Exergy accounting: capabilities and drawbacks’, Energy, Vol. 31, pp.164–180.Wall, G. (1986) ‘Exergy - a useful concept’, Physical Resource Theory Group, Chalmers University

of Technology, Göteborg, Sweden, 1st and 2nd Eds., ISBN 91-7032-269-4, Chalmers Biblioteks Tryckeri, Göteborg.

Cumulative exergetic assessment of LPFO utilised steam boilers 135

Wall, G. (1998) ‘Exergetics’, Exergy Ecology Democracy, Molndal, Sweden.Zhou, J., Shijun Ma, S. and Hinman, G.W. (1996) ‘Ecological exergy analysis: a new method for

ecological energetics research’, Ecological Modelling, Vol. 84, pp.29–303.

Nomenclature

LPFO low pour fuel oil Chemical symbolsstAFR stoichiometric air-fuel ratio

(kg of air/kg of fuel)x yC H hydrocarbon (fossil

fuel) moleculesAAF actual air-fuel ratio (kg of air/kg of

fuel)C Carbon

im� inlet mass fl ow rate, kg/s H Hydrogenem� exit mass fl ow rate, kg/s O Oxygen

Q� heat transfer rate to the system, kJ/s CO2 carbon dioxideI� exergy destruction rate, kJ/s H2O water

iε inlet specifi c fl ow exergy of material streams, kJ/s

N2 nitrogen

eε exit specifi c fl ow exergy of material streams, kJ/s

Ar argon

phε specifi c physical fl ow exergy, kJ/kg Sub- and Superscriptschε specifi c chemical fl ow exergy, kJ/kg a air

H specifi c enthalpy, kJ/kg f fuelS specifi c entropy, kJ/kg K p hot productsT temperature, 0C, K w feed waterP pressure, N/m2 s steam

chfuelxe molar chemical exergy of fuel,

kJ/kmolg exhaust fl ue gas

2

chxOe molar chemical exergy of oxygen

(O2), kJ/kmol0a reference state of air

2

chxCOe molar chemical exergy of carbon

dioxide (CO2), kJ/kmol0 reference state

2

chxH Oe molar chemical exergy of water

vapour (H2O), kJ/kmolatm atmospheric

g∆ the change in the standard Gibbs function, kJ/kmol

in in

R molar or universal gas constant, kJ/kmol K

out out

M molar mass of chemical substance, kg/kmol

C combustion unit

Ein the energy input in the combustion unit, kJ/s

H heat exchanging unit

HHV high or gross heating value of fuel, kJ/kg

B boiler

LHV low or net heating value of fuel, kJ/kg ph physical(loss)HQ Heat loss in heat exchanging unit, kJ/s ch chemical

Greek lettersη energy effi ciency (%)ψ exergy effi ciency (%)ε specifi c exergy (kJ/kg)