Exergy analysis - a tool for sustainable technology - in engineering education

P.P.A.J. van Schijndel, J.M.N. van Kasteren and F.J.J.G. JanssenEindhoven University of Technology (TUE), The NetherlandsFaculty of Chemistry and Chemical EngineeringCentre for Environmental Technology (CMT)

AbstractThe world is changing rapidly due to the increasing wealth and size of the population. There is agrowing need for more efficient and therefore sustainable production processes. Therefore, theeducational programme for engineers should contain the tools for the optimisation of processes intomore sustainable ones. Such tools are for instance process integration and exergy analysis.

For chemical and physical processes, exergy analysis is a powerful concept. Exergy is a measure forquality of mass and energy streams. By use of an exergy analysis, processes can be optimised intomore sustainable processes. Both environmental performance and economical aspects can becombined to improve its performance by the method of exergoeconomics, a new principle forcombined economical and environmental performance optimisation. In this paper, exergy analysesare presented, which have been carried out by chemical engineering students, focused on waste andbiomass gasification processes.

These experiences show how the environmental performance of existing processes can be improvedwithin economical constraints. Moreover, the students learn a method of analysing processes, whichis not yet incorporated in the engineering curriculum, although exergy is a well-known concept. Thefirst experiences have been encouraging so that a new course called ‘Exergy-route for a SustainableProcess Technology’, specifically on exergy, will start in the new academic year. The contents of thiscourse are included in this paper.

IntroductionAs our world is aiming at a more sustainable society, there are some major problems to overcome.The environmental burden of the society is related to population size, economical position andinfluence of GNP on environment (Welford, 1993). Since world population is expected to doublefrom 5 billion to 10 billion and GNP is going to raise by factor four within 30 years theenvironmental impact of industry and their products has to fall by about 90% to maintain the samelevel of environmental impact. This will be one of the greatest challenges of this time. So theenvironmental burden related to production, use and discarding of materials and products has to belowered. How is this to be achieved?First, production processes have to be reviewed followed by improving them into processes that aremore efficient. There are many tools for improving a process, e.g. environmental Life Cycleassessment, LCA, and exergy analysis. This study is focussed on exergy analysis.

Sustainable processesCleaner production of materials, goods and services is one of the tools for sustainable development.It means production in a way in which resources and energy are used in an efficient way and onlysmall amounts of waste and emissions are produced. Other important factors are the use ofrenewable resources and the increase in quality of the products. This does not mean that the cleanerproduction concept is contradictory to the economic approach of minimising costs and maximising

profits. It is the challenge to create win-win situations such as minimising the use of resources andcutting back on emissions, which can also decrease the costs of a given process.

An industrial process can be simply outlined, as a black box, see Figure 1. Resources and energy(work) are the inputs and products, wastes, emissions (air, soil, water), excess heat etc. are theoutputs of this process.

Figure 1. Schematical drawing of an industrial process

With the help of design tools like exergy analysis, LCA and others it is a goal for engineers tooptimise the process in a way it consumes fewer resources like raw materials and energy andproduces less emissions and waste.

Ordinary routes for achieving this used to be end-of-pipe treatment in the way of costly waste watertreatment plants, filters and scrubbers. These are both not real solutions, as they actually do notdecrease the environmental load, they only shift it from one phase, i.e. water or air to soil and water.In many cases, however, expensive end-of-pipe treatment solutions are unavoidable.

The tools, as mentioned before, are aiming at changing or optimising the given process so it turnsout more efficient and sustainable. In the next chapter, the exergy method of improving processeswill be described.

Exergy analysis as a tool for sustainable processesExergy analysis resembles the enthalpy or energy analysis. The difference is that in exergy analysisenthalpy and entropy are applied. An exergy balance can be performed for a whole plant or fordifferent unit operations. Information about exergy analysis can be found in literature (Szargut et al.,1988 and Kotas, 1995).

The following definition for exergy is used normally:‘Exergy is the maximum amount of work that can be obtained from a stream of matter, heat or workas it comes to equilibrium with a reference environment. It is a measure of the potential of a streamto cause change, as a consequence of not being completely stable relative to the referenceenvironment. Exergy is not subject to a conservation law, but it is destroyed due to irreversibility’sduring any process.’

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

Process

Useful Energy

Resources

Product(s)

Wastes

Materials

and

Waste Energy

Emissions

converted into work by 100% efficiency. Some examples of the difference between energy andexergy are shown in Table 1. From this table hot water and steam with the same enthalpy havedifferent exergy or quality values. Steam has a higher quality than hot water. Fuels like natural gasand gasoline have exergetic values comparable to their net combustion value. Work or electricity hasthe same exergy as enthalpy. Exergy can be calculated by product of energy and quality (Szargut etal., 1988 and Kotas, 1995).

Table 1. Examples of energy and exergy of different matter

Material Energy [J]

Exergy [J]

Quality [-]

Water 80°C 100 16 0.16Steam 1 bar and 120°C 100 24 0.24Natural Gas 100 99 0.99Electricity / work 100 100 1.00

Exergy values according to heat transfer (Carnot); Reference State is 298 K.

The calculations for the basis of table 1 are quite simple. For the calculations of exergy there areseveral components, which can be calculated separately like physical exergy (temperature andpressure), chemical exergy and mixing exergy. The exergy amount of a heat transfer streamaccording to the temperature difference of this stream and the environment is given by the Carnotfactor times the energy content:

Quality =

−

ST

T 01 (Carnot Quality factor)

Exergy = Energy (Transferred) ∗ Quality

Where T0 is the reference temperature (298 K) and Ts is the temperature of the stream.The Reference State is very important in exergy analysis because every compound or heat other thana reference substance or temperature is able to perform work.

The exergy analysis is more accurate and scientifically correct when compared to an ordinary energyanalysis because:• Exergy analysis provides a better view on the real efficiency of a process;• Exergy analysis is very useful to find the unit operation were efficiency improvements are the

most suitable or useful.

Each process designer or process engineer should perform an exergy analysis to make all exergylosses visible in the process under study. The method is very powerful when comparing two or moresolutions in an objective and quantitative manner. Of course the exergy analysis does not give directanswers on how to improve the process but it gives the best clues where to start, namely at the pointwhere the largest exergy losses appear.

Exergy analysis is especially useful in the design phase and during optimisation of new processes. Itis also a very useful tool when used for comparison of different production routes.

In using exergy analysis, it becomes clear that, for instance, a heat exchanger can be optimised byincreasing its heat-exchanging surface, because this decreases the temperature difference, ∆T, at thesame heat load conditions. At the same time costs will go up with increasing heat exchangingsurface. Therefore, there will be an economical/exergetical optimum as visualised in Figure 2.

Figure 2. Heat exchanger optimisation

Examples of processes reviewed with exergy analysisIn 1997 and 1998 several students have performed exergy analyses as a part of their study onprocess improvements. These studies involved cement and glass manufacturing and the processing ofwastewater sludge and PVC waste. Two cases, cement production and PVC waste gasification, arepresented in this paper.

Case 1: Cement production in TanzaniaThe production of cement is one of the most energy intensive production processes known. Thisprocess also emits a lot of CO2, due to the decomposition of CaCO3. Cement production accountsfor about 8% of total CO2 emissions from all human activities (Unanimous, 1993). It is beneficialfrom both an environmental as energetic point of view to optimise or redesign this process toimprove it’s efficiency. Therefore, a project was focussed on the possibility to perform an exergyanalysis on a real cement production plant. When focussing on the overall efficiency of a process it isbetter to perform an exergy analysis than to calculate only the energy use per ton of cement. This iscaused by the fact that the different resources have different exergetic values.

The plant chosen was the Tanzanian Portland Cement Company, TPCC, at Wazo Hill in Dar EsSalaam. Although there was not so much in depth process data available, several exergy analysescould be carried out successfully (Den Boer, 1998, Van Schijndel et al., 1998 and Hoenders, 1998).

Table 2. Exergetic efficiency at Wazo HillProcess unit Exergetic efficiency

(Fratzcher)Theoretical Efficiency*

Pre-heater 73 % 90 %Kiln 44 % 80 – 85 %Cooler 58 % 90 %

Overall 38 % 70 – 80 % *) Estimated values, zero heat loss

exergy consumption(operational costs)

Heat exchanger surface(capital costs)

minimum ∆T

Larger ∆T

Optimum ∆T

The analysis focussed on the pyroprocessing section, see Figure 3, where the raw grinded materialsare pre-heated, burned at 1450°C and cooled down to form clinker, the main product of Portlandcement.Results of the exergy studies in Table 2, showed that the overall efficiency of the pyroprocessingsection is about 38% (the so-called Fratzcher efficiency; see Sorin et al. 1998). This is low comparedto modern state of the art processes but average when compared to other old cement plants.

KilnCooler

Pre-Heater

Fuel

ExhaustGas

Raw meal

Hot meal

Air

HotClinker

Clinker

Primary

To ElectrostaticFilters

Figure 3. Cement production, process layout

According to this results the highest losses occur in the kiln (fuel burning, bad insulation) and cooler(bad heat transfer). Since all the equipment is coupled, optimisation has to be done by consideringthe whole process.

There are many opportunities to improve the process:• Only produce at an optimal throughput; decrease amounts of stops• Installation of high efficiency clinker cooler• Install new burner and automate clinker burning process• Better insulation in pre heater, kiln and clinker cooler• Improvement of pre heater• Improving dust system• Better training of process operators

When the plant is retrofitted to modern standards, using a precalciner, the efficiency will rise to 43%and higher. This optimisation is an economical and an environmental one since production capacitydoubles, the costs drop sharply and the fuel use decreases by over 20%. For TPCC, the pay backtime for the retrofitting has been estimated at 1.5 years. Several other exergetic optimisations, likepre-heater and cooler retrofitting, proved to be economically and environmentally feasible too.

Case 2: PVC waste gasification versus waste burningAt the TUE, a research project is running to develop a more environmental friendly route for thewaste processing of PVC. In stead of burning or recycling the process of gasification has beenchosen, see Figure 5. Since the experimental work has been successful (Slapak et al., 1996) a studentwas asked to perform an exergy study on both the new gasification process as the burning process of

PVC. In the gasification process, PVC waste is gasified in a fluidised bed reactor containing acatalyst, at 850° C together with steam. The gasses formed, HCl, CO, CO2 and H2 are quenched andHCl is stripped of. The gasses are then burned in a gasturbine, excess heat is used in the process in asteam turbine.

Figure 4. Schematical drawing of the PVC waste gasification process

Outcome of this analysis has been that the exergetic efficiency of PVC waste gasification is 60%higher than PVC burning (Table 3.). Main reason for the high efficiency of the gasification processare the use of a high temperature gas-turbine and the controlled gasification and burning processcompared to the chaotic PVC burning process. Some small optimisation calculations showed that thegasification process has potential for further optimisation. These studies are currently underinvestigation by post-graduate design course students.

Table 3. Burning versus gasification efficiencies.

Process Exergetic Efficiency (%)(10% heat loss)

Exergetic Efficiency (%)(zero heat loss)

PVC - Burning 29 32

PVC gasification 49 50

Both processes produce electricity and a 30% HCl-stream

Although the students were satisfied with the studies and the outcome of the research they felt thatthe time needed to understand and use the exergy analysis method took too long. One cause was theabsence of a graduate course in exergy analysis. Such course has been developed and started inSeptember 1998 for the first time.

PVC

Steam

WorkAir

Muratic acid

Work Work

Water

Gasifier

Stack gasses

Work

HCl-stripping Compressor Burner

Gas turbine

Steam cycle

New course in exergy analysis at TUESome years ago the TUE started a post graduate course for process and product design for chemicalprocess and product engineers. In this course extended thermodynamics including exergy wasintroduced. Since, as explained, second law thermodynamics are increasingly important in designinga process, combined by the growth of powerful simulation programmes which perform the extensivecalculations, there was the need to translate the course into a MSc. Course called ‘exergy routetowards sustainable development’.

Table 4. Overview of course ‘Exergy Route towards sustainable Process Technology’

1. Introduction: process efficiency and sustainable development.Economical welfare, sustainable development, efficiency of chemical processes, depletion of non renewableresources, environmental problems, ‘nature-oriented’ technology, thermodynamic analysis of industrial processes,social relevance of the second law of thermodynamics.

2. Thermodynamic background of exergy analysis.Entropy, the first and second law for an open system,, entropy balance and entropy production for irreversibleprocesses, dissipation of energy and materials, maximum of work potential (inclusive chemical reactions).

3. Fundamental aspects of energy.Energy sources, fossil fuels, nuclear energy, sustainable energy sources (solar energy, biomass), availability ofenergy, conversion technology of energy sources.

4. Exergy balance and irreversibility.Thermal exergy, exergy by work, exergy of material flows, physical and chemical exergy, conceptualsurroundings, exergy balance, irreversibility and the Gouy-Stodola relation, rational efficiency.

5. Exergy analysis of physical and chemical methods.Exergy analysis of processes: compression expansion, heat transfer, mixing and separation processes, distillation,chemical reactors, combustion processes.

6. Exergy analysis of energy systems.Exergy analysis of energy production and transfer, steam cycles, gas turbines, heat-power coupling, heat pumps,cooling installations.

7. Exergy analysis of chemical plants.Example: H2SO4 plant, Linde liquefaction of gasses.

8. Process integration.Improvement of the efficiency of separation processes, chemical reactors and plants by means of exergy analysis.

9. Thermodynamic design.Thermodynamic and economical utilisation of exergy, optimisation criteria for transport and separation processes,design of optimal system structure and equipment, common sense second law approaches for optimal design.

10. Environmental and ecological aspects of exergy.Depletion of non-renewable resources, cumulative exergy use, exergetic costs, ecological efficiency, life cycleassessment, recycling and ecological economy.

11. Guest lecture.Utilisation of exergy analysis in the process industry, recent examples from the industrial practice.

In the course there will be emphasis on the understanding the concept of exergy, the causes forexergy loss in any process and the possibilities to decrease exergy this loss by process optimisation.The following conceptions are lectured in the course: Process efficiency and sustainabledevelopment, fundamental aspects of energy, exergy balance and irreversibility, exergy analysis ofphysical and chemical processes and energy systems, analysis of whole plants and processintegration, environmental en ecological aspects of exergy. In Table 4 more elaborate contents of thecourse can be found.

Besides this special and non compulsory course for third-fourth years chemical and mechanicalengineering students, exergy analysis is also incorporated in other (environmental) courses at theEindhoven university of Technology. There will be a case study using exergy in the second yearcourse 'Sustainable development' and there are more examples of exergy calculation in the third yearscourse 'Environmental Technology'.

ConclusionsThe case studies have shown that there are many possibilities to increase the energy efficiency ofprocesses by using the method of exergy analysis.It is clear that the use of exergy analysis cannot be missed in the engineering curricula of chemicaland process engineers. Exergy analysis can add extra insight to the knowledge of the engineer. Thisknowledge is very essential (crucial) to design and optimise processes suitable for the next‘sustainable’ century

Literature-Den Boer J., 1998, Exergy Analysis of Kiln-3 at TPCC, MSc. report TUE.-Hoenders 1998, Exergy Analysis as Tool for Process Optimisation in Tanzania, research report,University of Dar Es Salaam Tanzania and Eindhoven University of Technology.-Kotas T.J. (1995), “The Exergy Method of Thermal Plant Analysis”, 2nd edition, Krieger publishingCompany, Malabar.-Ptasinski and Janssen, 1998, Contents of Course ‘Exergy route towards sustainable development’,Eindhoven University of Technology, internal memo.-Van Schijndel, P.P.A.J., Den Boer, J., Janssen, F.J.J.G., Mrema, G.D., Mwaba, M.G. (1998),“Exergy analysis as a tool for energy efficiency improvements in the Tanzanian and Zambianindustries”, ICESD Conference Engineering for sustainable development, July 27-29th 1998,University of Dar Es Salaam, Tanzania.-M.J.P. Slapak, J.M.N. van Kasteren and A.A.H. Drinkenburg, "Selection of a recycling route forheterogeneous PVC-waste", Proceedings First International working seminar on reuse , Eindhoven,nov. 11-13, 1996, ed. S.D. Flapper & A.J. de Ron, pag 267-275.-Sorin M., Lambert J., Paris J. (1998), Exergy flows analysis in chemical reactors, trans IchemE, vol76, Part A, pp. 389-395.-Szargut J., Morris, D.R., Stewart, F.R. (1988), “Exergy Analysis of Thermal, Chemical, andMetallurgical Processes”, 1st edition, Springer Verlag, Berlin.-Unanimous (1993), Environmental Building News.-Welford R., Gouldson ,A.(1994), Environmental management and business strategy, LondonPitman.

Personalia

The author;Patrick van Schijndel studied chemical engineering at the Eindhoven University of Technology andgraduated in 1994. He got his teaching degree in chemistry at Eindhoven University in 1995. Since1996 he is doing his PhD on cleaner production at CMT, and combines this with setting up a MSc.course in environmental technology for the University of Dar Es Salaam in Tanzania.

The co-authors;J.M.N. van Kasteren studied chemical engineering at the Eindhoven University of Technology and in1990 he received his PhD degree. In 1990 he worked at the Inter-University Environmental InstituteBrabant (IMB). From 1991 he works as appointed lecturer at the TUE, in the field of environmentaltechnology. In 1996 he was appointed director of PRI at the TUE. At PRI economic and technicalfeasibility studies of the recycling of wastes are carried out.

F.J.J.G. Janssen is head of the department responsible for gasification, combustion of fossil fuels andchemical processes at KEMA in Arnhem, The Netherlands. At KEMA he is working in the field ofresearch and development of gas cleanup systems for gasification of coal, heavy oils and biomass,pyrolysis of waste and biomass, energy saving technologies and water purification.At the TUE he is director of the Centre for Environmental Technology of the Faculty of ChemicalEngineering. CMT focuses on environmental education and environmental research.

Address:

Centre for Environmental technologyFaculty of Chemistry and Chemical EngineeringEindhoven University for TechnologyRoom STO 3.25P.O. Box 513, 5600 MB EindhovenThe Netherlands

Phone: +31 40 247 31 97Fax: + 31 40 245 37 62Email: p.p.a.j.v.schijndel@tue.nlhttp://www.chem.tue.nl/cmt

Published in proceedings of ENTRÉE ‘98 (Environmental Training in Engineering education), Innovationstrategies for Economy and Environment, edited by S. Poyry, J. Pringle and A. Hagstrom, 4-6 November1998, Deventer, The Netherlands.