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Exergy analysis as a tool for energy efficiency improvements in the Tanzanian
and Zambian industries
P.P.A.J. van Schijndel, J. den Boer and F.J.J.G. Janssen, Centre for Environmental
Technology, Eindhoven University of Technology, The Netherlands;
M.G. Mwaba, Department of Mechanical Engineering, University of Zambia, Lusaka,
Zambia;
G.D. Mrema, Department of Chemical and Process Engineering, University of Dar es
Salaam, Tanzania;
Abstract
The story behind energy efficiency in industrial processes is productivity, industrial
competitiveness, jobs, and a clean environment.
A method for energy efficiency evaluation is to carry out an energy analysis, a so
called First-Law analysis. With such an energy-audit, however, it is impossible to
compare different kinds of energies like electrical energy, mechanical work, high and
low temperature streams, etc. A better tool for comparing different kinds or qualities
of energy is exergy analysis in which the quality or exergy of all energy streams is
calculated. In this way the energies are calculated and compared in a more
scientifically correct and accurate manner. Advantages of the exergy analysis is to
get a better understanding of the energy losses in, parts of, an industrial process,
which means it is easier to find out in what part of a process energy use can be
decreased.
In this paper two analyses of energy intensive processes in Tanzania and Zambia
are described. First, the energy and exergy analyses of the Tanzania Portland
Cement Co., Ltd., at Wazo Hill in Dar es Salaam are presented. It was possible to
perform an exergy analysis using available energy and mass balance data and some
basic thermodynamic data like enthalpies and Gibbs Free energies. The analysis
predicted that the energy use in the cement production could be decreased by 15%
and higher by improving the kiln process and by installing new equipment (pre-
calciners). Secondly, energy evaluations concerning sugar production were
performed. For heat-exchangers, exergy analysis identified temperature difference
between the hot and the cold streams as a critical parameter in equipment efficiency
calculations. A big problem affecting this temperature difference in heat exchangers
is fouling, the formation of solid deposits on heat exchanger surfaces. Elimination or
minimising fouling can lead to low exergy losses in heat exchangers and hence high
equipment effectiveness. A research project formulated to investigate the problem of
fouling in the sugar industry in Zambia is described.
Generally it can be concluded that an exergy analysis is more accurate than a
thermal analysis because not only quantity but also quality of the energy used is
calculated.
1. Introduction
Since the start of the industrialisation, some fifty years ago, the use of fossil fuels
has increased in a rapid growing ‘scheme’. Fuels are used for power generation,
mechanical work, transportation and heating. In the industrialised countries fossil
fuels are the main source of energy. Up to 50% of the total energy use accounts for

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industrial use in production and transportation. The percentage of fossil fuel usage
by industry is even higher in the less developed countries.
From a sustainable point of view this is not justified because the use of fossil fuels
causes depletion of oil, coal and gas which will not be available for next generations.
Another problem is damage to the environment by greenhouse effect, acid rain and
ozone depletion because of CO, CO2, SOx, NOx and particles emissions. The policy
at this moment is, therefore, to decrease energy use by both industry and consumers
and to increase the percentage of renewable energy sources, i.e. wind, solar,
biomass, waste and water.
The most-commonly used method for efficiency evaluation of industrial processes is
first-law-analysis. However, there is increasing interest in the combined utilisation of
first and second law thermodynamics, using concepts like exergy (quality of energy,
availability, available energy), entropy generation and irreversibility in order to
evaluate the efficiency with which the available energy is used.
The concept of exergy provides an estimation of the minimum theoretical resource
requirements for energy and raw materials of a given process. This also provides
information on maximum savings which can be achieved by making use of new
technology and new processes or process improvements. Therefore, exergy analysis
is expected to be a useful tool to determine the real efficiency and the potential for
efficiency improvements of any given process.
This article provides methods to decrease energy use in industrial production
processes of cement and sugar in Tanzania and Zambia by performing an energy
and exergy analysis. In chapter 2 some basic knowledge about the concept of
exergy is explained. In chapter 3a, the exergy analysis of a cement production plant
in Tanzania is shown. In chapter 3b the influence of heat exchanger-fouling on
energy use in the sugar production in Zambia is explained using exergy analysis.
2. Exergy analysis; basic concepts
According to Wall
1
energy and exergy concepts may be expressed in the following
simple terms: (1) energy is motion or the ability to produce motion (2) exergy is work
or the ability to produce work. Accordingly the first and second laws of
thermodynamics may be formulated as: (1) energy is always conserved in a process
(law of energy conservation) and (2) exergy is always conserved in a reversible
process but is always consumed in an irreversible process (law of exergy).
The concept of exergy is extensively reported in the books of Szargut and Kotas
2,3
.
A basic example is the possibility of converting mechanical work into heat with 100%
efficiency. Heat has a lower exergy, or quality of energy, compared with work.
Therefore, heat cannot be converted into work by 100% efficiency. Some examples
of the difference between energy and exergy are shown in Table 1. From this Table
hot water and steam with the same enthalpy have different exergy values. Steam has
a higher quality than hot water. Fuels like natural gas and gasoline have exergetic
values comparable to their net combustion value. Work or electricity has the same
exergy as enthalpy.
Table 1. Examples of energy and exergy of different matter

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Material Energy
[J]
Exergy
[J]
Quality
[-]
Water 80°C 100 16 0.16
Steam 1 bar and 120°C 100 24 0.24
Natural Gas 100 94 0.94
Electricity / work 100 100 1.00
A definition of exergy can be the potential to produce work from an energy form or a
material. This work potential can be obtained by processes which are irreversible in
practice and therefore there are always losses when getting work out of energy.
Some background information on exergy and formulas necessary to calculate exergy
are shown in the Appendix.
3a. Exergy analysis of the cement industry (Wazo Hill)
The process of making Portland cement is shown in Figure 1. The three basic
chemicals needed are limestone (CaCO3), red soil (mixture of silicium, aluminium
and iron oxides) and gypsum (CaSO4). The first two are ground, dried and blended
into a mixture called raw meal. This mixture is then preheated and burned in a kiln
during which the clinker formation takes place. Energy for burning can be provided
by fossil fuels, like oil, coal and gas or by waste fuels. After cooling and grinding,
gypsum is added and the product is ready for use.
limestone
crushing
drying
grinding
weighing
blending
burning
cooling
grinding
blending
Portland cement
gypsum
red soil
Figure 1. Cement production scheme for kiln 3, Wazo Hill
During the burning process two types of reaction are important. The first reaction at
temperatures ranging from 450 to 1100°C involves the transition of calcium
carbonate into calcium oxide and CO2 (calcination). Second reaction set is the real
clinker formation and this takes place at 1000 -1600°C. During clinker formation the

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calcium oxide reacts with the other raw materials resulting in the formation of
tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetra calcium alumino-
ferrite.
Of course the boundaries of the process or system have to be chosen before starting
with an exergy analysis. Because the kiln section has the highest energy use this
study focuses on the kiln section of the cement production. A mass and energy
balance have to be performed before performing an exergy analysis.
Energy and exergy analysis of kiln section
The Wazo Hill, cement factory has several kilns. For the energy and exergy
analysis, kiln 3 was chosen because it has the highest capacity (800 tons/day). The
data used for the exergy analysis are based on an energy research at Wazo Hill by
C. Samplonius
4
in 1994.
Kiln
Cooler
Pre-
Heater
Fuel
Exhaust
Gas
Raw meal
Hot meal
Air
Hot
Clinker
Clinker
Primary
To Electrostatic
Filters
Figure 2. Wazo Hill, Kiln 3, process lay-out
Energy and mass balances for kiln 3 are shown in Tables 2 and 3. Looking at the
specific energy use, it can be easily recognised that the burning and cooling section,
the cement kiln, has the highest energy use in cement production. Kiln 3 at Wazo
Hill consists of a 4-stage suspension pre-heater in which the hot kiln exhaust gasses
from the kiln (in which cement is burned at 1500°C) are mixed with the raw meal feed
using cyclones, and a clinker cooler in which the hot clinker is cooled using incoming
primary air (see Figure 2).
For this specific plant most data were measured by Samplonius
4
. Some data
however had to be calculated due to non available measuring equipment. Since the
dust precipitator (electrostatic filters) is not working properly at kiln 3, high amounts
of raw materials and product are lost via the exhaust.

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Table 2. Energy balance Kiln 3, Wazo Hill
(calculated per ton of Portland cement production) [Samplonius4
]
Process Energy Use
MJ, electrical energy in kWh
Total Energy use
MJ
Primary crushing 1.8 kWh 6.5
Drying and grinding 36 kWh 129.6
Weighing and blending 1.5 kWh 5.4
Burning and cooling 27 kWh and 4142 MJ 4239.2
Grinding and blending 42.0 kWh 151.2
Total process 108.3 kWh and 4142 MJ 4531.9
Table 3. Mass Balance of kiln 3, Wazo Hill. [Samplonius
4
]
In Out
Limestone 1.36 ton
Red soil 0.34 ton
(= Raw meal 1.7 ton) Dust 0.07 ton
Fuel 0.107 ton Exhaust air 2.71 ton
Combustion air 1.96 ton Clinker (raw cement) 1.00 ton
Total in: 3.8 ton Total out: 3.8 ton
The mass balance had to be calculated to some extent because plant mass measuring equipment was not available.
Exergy Analysis of kiln 3
For this system an exergy balance was calculated
5
using the black box method (see
Figure 3). The mass and energy streams in and out were calculated from the field
data. The exergy balance was calculated using energy and mass balances.
Figure 3. Black box model for kiln 3
Most data necessary for the exergy analysis like mass and heat balance,
temperature measurements for incoming and outgoing gases and products and
some data on the kiln burner was available
4
. Other data has been calculated: heat
loss, dust composition in exhaust gas, fuel composition, and exhaust gas
composition.
Preheater
Kiln
Cooler
Out
Clinker
Hot air
Dust
Heat losses
In
Raw meal
Air
Fuel

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Following exergetic aspects of the process were calculated:
- The physical exergy (heat) of streams in and out, see formula 6;
- Exergy loss due to heat loss (heat through kiln wall etc.), formula 7;
- The chemical exergy of material flows, in and out, formula 9;
- Mixing exergy (necessary due to calculation method), formula 10.
From this data the exergetic efficiency was determined, using formulas 12 and 13.
Results
The results for the calculations of the various exergetic quantities are given in
Tables 4 - 6. In Table 7 the calculated exergetic data are summarised.
Table 4. Chemical exergy values of raw meal and clinker products.
Raw meal IN Clinker OUT
Raw
meal
ni
mole/kg cl
exo
kJ/mole
Exch,I
kJ/kg cl
Clinker ni
mole/kg cl
exo
kJ/mole
Exch,I
kJ/kg cl
CaCO3 13.62 1.0 13.6 8-C2S * 0.44 301.3 131.8
SiO2 3.93 1.9 7.5 C3S 3.13 413.5 1293.8
Fe2O3 0.18 16.5 2.9 C3A 0.49 500.6 245.7
Al2O3 0.72 200.4 143.5 C4AF 0.16 "604 96.5
Total 167.5 Total 1767.8
*) 8-C2S: 2CaO+SiO2; CA:CaO+Al2O3; C2F: 2CaO+Fe2O3; C3S: 8-C2S+CaO; C3A: CA+2CaO; C4AF:CA+C2F+CaO
Table 5. Chemical exergy values of air, fuel and exhaust gases
Air IN Exhaust gases OUT
ni
mole/kg cl
exo
kJ/mole
Exch,I
kJ/kg cl
ni
mole/kg cl
exo
kJ/mole
Exch,I
kJ/kg cl
Air 69.3 0.126 8.7 N2 54.75 0.72 39.4
Fuel 0.63 7334.4 4620.7 O2 3.77 3.97 15.0
Water 0.28 3.12 0.9 CO2 21.16 19.87 520.4
H2O (g) 6.48 9.50 61.5
SO3 0.09 249.10 22.4
Total 4630.3 Total 558.8
Fuel (calculated via kg, molar weight of 170 estimated for Table 5. only)
The chemical exergy of the dust in the exhaust gas is calculated 123.7 kJ/kg clinker

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Table 6. Exergy of heat loss by kiln and cooler
Distance
m
qi
kJ/s
Temperature
K
d exi
kJ/kg cl
Cooler: 0-8 551 413-463 20.9
Kiln: 0-30 3268 550 175.2
30-60 3484 540 189.2
Total 7303 385,3
Table 7, Summary Table; exergy data kiln 3
Exergy IN
kJ/kg clinker
OUT
kJ/kg clinker
Chemical Exch:
Raw materials 167.5
Air 8.7
Fuel 4620.7
Water 0.9
Exhaust gas 558.8
Clinker 1767.8
Dust 123.7
Total Exch 4797.8 2450.3
Mixing Exmx gases -204.2
Thermal Exth 17.5 365.9
Heat loss Exrad 385.3
TOTAL 4815.3 2997.3
Total Eloss is 1433.7 kJ/kg clinker
The data from Table 7 are used to calculate both simple and rational efficiency
(Formulas 12 and 13):
η simple = 0.62 and η rational = 0.46
The irreversibility of the kiln process can be calculated, according to Formula 11.
The calculated irreversibility I = 1818 kJ/kg clinker (38%).
Besides these two efficiencies another efficiency, called the real efficiency can be
calculated. To calculate η real the used exergy is defined in another way, namely as
the sum of the exergetic input i.e. Ech,fuel + Ech,air + Etm,in. As in Formula 13 this
defines η real :
η
ch, clinker
ch, In tm, In
real
Ex
Ex Ex
=
+
(14)
The efficiency calculated in this way: η real = 0.37

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This real efficiency is a good tool for the definition of overall efficiency in clinker
production in kiln 3. When comparing kiln 3 with other cement kilns, this efficiency
definition should be used in stead of the ‘simple’ and ‘rational’ efficiency.
Discussion
Looking at the calculated efficiencies of kiln 3, Wazo Hill is not much better or worse
than efficiencies of several cement factories found in literature
6
. However, much of
those efficiencies were not well defined and data for the most advanced kiln systems
were not available in literature yet. Also the moisture content of the raw materials at
Wazo Hill is rather low compared to other factories, indicating a better starting
position on minimum energy requirements.
However, the irreversibility of the process (32%) is rather high and therefore its is
expected that there are possibilities to increase the overall efficiency.
As found in literature
7
improvements can be done by better insulation of the system
(heat loss accounts for up to 27% of the exergetic loss) especially in the kiln, pre-
heater and cooling sections.
More specific data on the losses in the sections is needed to give specific advises
on improvement opportunities. Estimates show the kiln section has the lowest
exergetic efficiency, chemical exergy of fuel is destroyed and heat is lost by exhaust
gases and heat transfer through the kiln wall. The efficiencies of pre-heater and
cooling section are found to be comparable. However there was not enough data to
calculate these efficiencies in an accurate manner.
Exergy loss due to the malfunctioning of the dust precipitators accounts for up to 8%
of the exergetic loss, better maintenance can improve the overall process
performance without large investments.
Process control is another way to increase efficiency. Not enough data was available
to calculate the losses through poor process control like temperature offset, fuel
efficiency, start-up procedures etc.
Other improvements of kiln 3 can be achieved by the installation of a so called pre-
calciner in the pre-heater section which has been shown by an exergy analysis. A
pre-calciner is an extra pre-heating device capable of burning fuel. The pre-calciner
provides an increase in the calcination reaction from 30 to 80%. Therefore the kiln
length can be shortened. In general the kiln capacity will be increased by 115%. In
the case of kiln 3 this means an increase from 800 to 1700 ton clinker/day. The
exergetic efficiency of this new system was calculated to be η real = 0.42. This
increase by 0.05% units, leads to a net fuel decrease of 14%.
An economical analysis of this retrofitted system showed a pay back time of 1.5
years.

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3b. Application of exergy analysis on heat exchangers in the sugar industry
Sugar-cane and beet sugar are the two plants which are used as raw materials in the
manufacture of sugar. The major steps involved in the processing of sugar-cane and
beet are essentially similar. These are extraction, purification, evaporation and
crystallisation, as shown in Figure 4.
In Zambia sugar is manufactured from sugar-cane. Zambia Sugar Plc (ZSPlc) is a
company that cultivates sugar cane and processes it into sugar and sugar by-
products. The company, which has an annual production capacity of over 150,000
tons is the only sugar processing company in Zambia. In Tanzania sugar is also
produced from sugar cane, by several companies such as the Kegera Sugar
Company Ltd., Kilombero Sugar Company Ltd., Mtibwa Sugar Company Ltd., TPC
Ltd., and Mahonda Sugar Company Ltd. This presentation focuses on the Zambia
Sugar Company.
Figure 4. Outline of sugar production
Sugar cane stalks are cut into small pieces by a series of rapidly rotating knives and
then shredded by hammer mill shredders. Juice is extracted by crushing the cane in
steam-powered mills. The average crushing capacity at Zambia Sugar is between
400 and 450 tonnes of cane per hour (t.c.h).
The extracted juice is acidic and turbid. It comprises of water, sucrose, glucose,
mineral matter and organic matter. Mineral matter consists of salts taken up by the
cane from the soil. These are mainly chlorides, sulphates, phosphates and silicates
of sodium, potassium, magnesium and calcium. Organic matter comprises of gums,
proteins, acids and colouring matter. Further the juice contains suspended matter in
the form of cane wax, fine sand particles and fibres.
To remove soluble and insoluble impurities the juice is subjected to a clarification
process. This is achieved by adding lime to the juice and then heating the mixture to
almost its boiling point The heating is done in series in two sets of heat exchangers;

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primary and secondary heaters. Phosphoric acid is added prior to the juice entering
the primary heaters. In the primary heaters the juice temperature is raised from 40
o
C
to about 65
o
C. From the primary heaters the juice is pumped to the liming tank
where milk of lime is added. Milk of lime is a mixture of calcium oxide and water.
Liming increases the pH of the juice from between 5 and 5.5 to between 7.5 and 8.
Limed juice is pumped to the secondary heaters where it is heated to about 104
0
C.
Heating the limed juice coagulates the proteins and some of the fats, waxes and
gums. The combined process of liming and heating leads to the formation of calcium
phosphate a precipitate which entraps suspended solids. This mixture is pumped to
the clarifier where the precipitate and the coagulated material are separated from the
juice.
The clarified juice is then evaporated to produce a thick syrup, containing about 60%
dissolved solids. The syrup is heated in heaters before being transferred to vacuum
pans for crystallisation.
Fouling Problems in cane sugar industries
One of the major problems encountered in the process of sugar extraction is fouling,
the unwanted deposition of material on heat transfer surfaces. Since the thermal
conductivity of the deposited layer is low, fouling leads to a reduction in thermal
efficiency of a heat transfer equipment. No effective method currently exists for
eliminating fouling. At the design stage, an allowance is made for fouling by
increasing the heat transfer surface area. During the operation stage periodic
cleaning is the only recourse.
Fouling of heat transfer equipment is a significant problem in cane sugar processing
plants. In Australia mills are stopped every 14 days for evaporator cleaning. The
average frequency of cleaning varies between 2 and 10 days for secondary heaters,
and between 2 and 6 weeks for primary heaters. At Zambia Sugar Plc juice heaters
and evaporators are cleaned every 7 days.
Crystallisation fouling, sometimes referred to as scaling, is the dominant fouling
mechanism in sugar cane factories. It is a mechanism by which salts of inverse
solubility nature, originally dissolved in the process fluid, crystallise on heat transfer
surfaces. The solubilities of such salts decrease with temperature as shown in
Figure 5 for calcium sulphate. Juice flowing in heaters and evaporators comprises
salts such as silicates, sulphates, phosphate and oxalates of calcium. The solubility
limits of these salts is normally exceeded at temperatures existing at the walls of the
heaters and evaporators. This situation leads to solid formation and deposition on
the heat transfer walls.
In the evaporators and syrup heaters, the deposits are hard and adhere strongly to
the surface. They are also thick, an average of 1 mm being the normal deposit
thickness in the syrup heaters.

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0,15
0,16
0,17
0,18
0,19
0,2
0,21
0,22
0 10 20 30 40 50 60 70 80 90 100
Temperature [°C]
Solubility[gofCaSO4in100gofwater]
Figure 5. Solubility of calcium sulphate (gypsum) in water
Influence of fouling on energy balance
The sugar industry is a high energy consumer. Energy in the form of heat is needed
for heating, evaporation, crystallisation and for driving machinery. Though most of
the energy needed is generated by burning bagasse obtained while crushing the
sugar cane, effective use of the generated steam is essential, considering the large
amounts of heat required. The effectiveness is achieved by interconnecting the
various energy users as depicted in Figure 6.
Figure 6. Cascade of heat exchangers in sugar production process
Steam generated by burning bagasse in the boilers is first used to drive steam
turbines. The exhaust heat from the turbines is used to preheat clear juice to the
temperature of the first effect evaporators, and to boil the juice in the first effect
evaporators. Part of the vapour generated in the first effect evaporators is used to
heat juice in the secondary heaters and for crystallisation. The remainder is used to
boil juice in the second effect evaporators. Similarly part of the vapour generated in
the second effect evaporators is used as heating medium for juice in the primary
heaters and the rest goes to boil juice in the third effect evaporators. From a point of
exergy this heat exchanger cascade is already very balanced; waste heat from one
heat exchanger is used in a second one.

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Exergy analysis can be used to assess how fouling affects the effectiveness of juice
heaters and evaporators. For a juice heater or evaporator, the driving force for heat
transfer is the temperature difference between the steam/vapour side and the juice
side (Figure 7).
Figure 7. Effect of fouling on the heat transfer
When the heat transfer surfaces are clean the driving force is ∆T T Tv j= − , where Tv
and Tj are vapour and juice temperatures respectively. After fouling has taken place
a layer of deposit develops on the heat transfer surface. Existence of a scale layer
on a heater or evaporator surface introduces additional resistance to thermal flow.
This means that required temperatures on the juice side will not be attained at the
same heat load. The heat load need to be increased in order to maintain the desired
temperature. This will lead to an increase in the vapour side temperature. The
temperature difference, in turn, will increase as depicted in figure 7.
An increase in the temperature difference will increase exergy losses. This is clear
from equation 3 which shows a direct relation between fouling and exergy loss. For
a heat exchanger then, higher exergy losses mean lower equipment effectiveness
i.e. higher amount of energy use. Though data is not yet available to quantify the
exergy loss resulting from fouling in juice heaters and evaporators, the above
discussion shows that fouling is undesirable as it leads to decrease in equipment
effectiveness. The significance of finding a solution to the problem of fouling in the
sugar industry cannot be overemphasised.
A project aimed at understanding the mechanism of scaling in juice heaters has
been started between the University of Zambia and Eindhoven University of
Technology in the Netherlands. Zambia Sugar plc is a co-operating partner.
An important parameter, besides dissolved salts, affecting the scaling mechanism is
the presence of particles in the system. Experimental results which showing that the
scaling mechanism is greatly influenced by particles suspended in the process fluid
have appeared in literature
8,9
. The reported results suggest that scaling is enhanced
or suppressed depending on the nature of particles. This observation is important
especially for industrial heat exchange systems since the likelihood of particles
being present in process fluids is high. The clarification methods used to remove
impurities from mixed juice in cane sugar industries, can not eliminate micro-
particles from the juice. Understanding how particles affect deposition and removal
mechanisms in the scaling process is therefore an important step towards
developing affective ways of minimising fouling.

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The first aim of the research is to investigate how particles affect the fouling and
contribute to understanding the physical phenomena involved. The second objective
is to develop models to describe fouling and use the models to set up design and
operational guidelines for industrial heat exchangers susceptible to fouling. The
focus is on the juice heaters used in sugar cane industries.
4. Conclusions
Part a, exergy analysis of the cement industry
It has been possible to perform an exergy analysis of kiln 3 from Wazo Hill Portland
cement factory. Only field data and some thermodynamic data were necessary to
perform a black box exergy analysis. However, if more detailed calculations on the
different subsystems like pre-heater or cooler are necessary, more detailed
measurements of these different sections are necessary.
Although the exergy efficiency in kiln 3 of Wazo Hill Portland cement company is
reasonable compared to other studies, there are still plenty of opportunities to
increase its energy efficiency, more than 15%, by:
• Decrease of dust loss by maintaining and renewing the electrostatic precipitators
• Better heat management in the kiln (better temperature set-up through process
control)
• Better insulation, especially in the high temperature areas of kiln, cooler and pre-
heaters,
• Preheating the fuel in the cooler section
• Enlargement and improved insulation of the preheater section
• Retrofitting by installing a pre-calciner, new dust filters and a larger preheating
system
Part b, Exergy analysis on heat-exchangers in the sugar industry
It can be concluded that an exergy analysis shows in a straightforward manner how
heat exchanger fouling leads to an increase in exergy losses.
General conclusions
It can be concluded that an exergy analysis is more accurate than a thermal analysis
because not only quantity but also quality of the energy used is calculated. Using
exergy analysis, a better view on the efficiency of processes can be obtained.
Exergetic efficiencies also make it more easy to compare the performance of
different processes. Therefore its is advisable to use exergy analysis when
considering process efficiency improvements.

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References
1] Wall, G., Exergy flows in industrial processes, Energy, vol 13(2), pp. 197-208, 1988.
2] Kotas, T.J., The Exergy Method of Thermal Plant Analysis, 2nd
edition, Krieger publishing
Company, USA, 1995.
3] Szargut, J. et al., Exergy Analysis of Thermal, Chemical, and Metallurgical Processes, 1st
edition,
Springer Verlag, Berlin, 1988.
4] Samplonius, C., Energy efficiency in the Tanzanian industry: The cement industry as a case study,
MSc. thesis, Eindhoven University of Technology, December 1994.
5] Boer, J.d., Exergy Analysis of Kiln-3 at Wazo Hill Portland Cement Company, MSc. Thesis,
Eindhoven University of Technology, 1998.
6] Vleuten, F.P. van der, Strategies and instruments to promote energy efficiency in developing
countries: project working paper 4 : energy and environment in the global cement industry. Petten,
Netherlands Energy Research Foundation ECN, 1995. (ECN-C--94-035)
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system, J. of the Chinese Ceramic Society, vol 23 (6), pp. 626-634, 1995.
8] Hasson D and Zahavi J., Mechanism of calcium sulphate scale deposition on heat transfer
surfaces, I&EC fundamentals, vol 9 (1), pp. 1-10, 1970.
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industrial heat exchangers, Engineering Foundation Conference, California, June, 1995.
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pp.113-121, 1990.
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the reduction of irreversibility, PhD thesis, Twente University of Technology, The Netherlands, 1997.
Appendix
When working with exergy a new definition for so-called reference states has to be
developed. The exergy value of materials which are found in the environment, for
instance air, equals to zero. Pure oxygen has an exergy value greater than 0. The
reference state temperature is chosen to be T0 = 298.15 K and the reference
pressure P0 = 100 kPa. However, other reference values also occur in literature
10
.
The Second Law states that: )Sirrev > 0.
This means that there is a production of entropy in every real, irreversible, process.
The loss is connected to the loss of work : Wlost = T0 )Sirrev (1)
Of course it is interesting to understand the reasons for entropy production. This can
be explained using so-called irreversible thermodynamics:
∆S Jirrev i
i
n
= ∑ iX (2)
where Ji stands for process streams, like mass and heat flow and reaction rate, and
X the associated driving forces such as temperature difference, thermodynamic
potential, pressure and Gibbs free energy of reactions. Thus, by combination of
formulas 1 and 2, formula 3 can be derived:
W T Jlost i
i
n
= ∑0 iX (3)
So the work loss, which is also called exergy loss or irreversibility of a given
process, is related to the driving forces. By decreasing these driving forces the
exergy losses of processes will also decrease; by this the efficiency will increase.
The exergy of an amount of mass is determined by the sum of its physical and
chemical exergy.

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15
Physical exergy is the work obtained by transferring a stream or substance via a
reversible process from the initial temperature T and pressure P to the reference
state T0 (298.15 K) and P0 (100 kPa):
E H H T S Sph = − − −0 0 0( ) (4)
Where H is the enthalpy and S the entropy. The physical exergy has several parts
namely a thermal and a pressure part. For a gas the physical exergy can be
calculated from:
e c T T T
T
T
RT
P
P
ph p= − − +{( ) ln } ln0 0
0
0
0
(5)
with cp the specific isobaric heat capacity and R the molar gas constant.
For liquids and solids the following formula is obtained:
e c T T T
T
T
v P Pph m= − − − −{( ) ln ( )} ( )0 0
0
0 (6)
where c is a specific heat and vm the specific volume (at T0).
When exergy is transferred due to heat transfer the formula from Carnot is used
This gives the following formula for exergy transfer:
E Q
T
T
ht A= −( )1
0
(7)
where QA is the heat transfer at temperature T, with uniform temperature distribution.
Chemical exergy is equal to the maximum amount of work available when a
substance is brought into equilibrium with the environment (T0 and P0) by processes
involving heat transfer and mass exchange only with the environment.
For example to calculate the chemical exergy of the reference gases (i.e. O2 and N2)
the work for getting the components at the standard pressure from the partial
pressure of the reference state has to be determined. The following formula can be
used: e RT P Pch = 0 0 00ln / (8)
where P00 is the partial pressure of the component in the reference state.
To calculate the chemical exergy of non-reference pure components the following
formula has to be used:
e G x e x ech i ch i
in
i
i ch i
out
i
= − − +∑ ∑∆ 0 , , (9)
where )G0 is the Gibbs free energy of formation, xi is the mole fraction of component
i and ech,I the chemical exergy value of reference state chemicals.
The chemical exergy of a mixture can be calculated with the following formula:
e x e RT x xch mix i ch i i
ii
i i, , ln= + ∑∑ 0 γ (10)
where (i is the activity coefficient. For ideal solutions this coefficient equals to one.
Efficiency calculations
To calculate the exergetic changes for a chemical process, a ‘black box’ of that
process can be used. In this case only the mass and heat flows into and out of the
process are taken into account by the exergy analysis, and not the actual processes
taking place inside. For such a calculation following exergetic balance can be used:

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16
E E Iin out= + (11)
With E the exergy flows in and out, respectively, and I the so called irreversibility.
In processes with a high irreversibility the I term is very high, this means the process
is far away from thermodynamic perfection and there is still ample room for efficiency
improvements. A solution is to decrease the driving forces in that process within the
economic range of the process.
An exergetic process efficiency can be calculated for different processes to rank
processes. However, there are many definitions of this exergetic efficiency. A
commonly used one is the so called ‘simple’ efficiency but a much better one is the
‘rational’ efficiency
11
:
Simple efficiency: η simple
out
in
E
E
= (12)
Rational efficiency: η rational
desired output
used
E
E
= (13)
In formula 13, E desired output is defined as Eproduct - Eraw materials. And Eused is defined as the
difference of ingoing and outgoing resource flows, i.e. air and exhaust gases.
The simple efficiency only says something about the irreversibility of a process.
Processes with high heat loss but low irreversibility can still have a high simple
efficiency. The rational efficiency only looks at the exergy of the useful products with
respect to the total exergetic input. Thus it gives a better overall view of the
efficiency of a process.