T. P. Seager, T. L. Theis - 2002 - A Uniform Definition and Quantitative Basis for Industrial...

7/22/2019 T. P. Seager, T. L. Theis - 2002 - A Uniform Definition and Quantitative Basis for Industrial Ecology

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Journal of Cleaner Production 10 (2002) 225235

www.cleanerproduction.net

A uniform definition and quantitative basis for industrial ecology

T.P. Seager *, T.L. Theis

Center for Environmental Management, Environmental Manufacturing Management Program, Clarkson University, P.O. Box 5715, 104 Rowley

Laboratories, Potsdam, NY 13699, USA

Received 9 January 2001; accepted 15 August 2001

Abstract

Industrial ecology (IE) has been characterized by a fragmented approach encompassing a number of different perspectives and

analytical techniques. A uniform framework has yet to be established or proposed. This paper partially addresses this shortcomingby tracing some of the historical and intellectual antecedents of the field, providing a clear and concise lexicon of the biologicalanalogue, and contrasting the two most promising analytical methods by which IE research may be carried out: life cycle assessment(LCA) and systems analysis. Although a number of comparative environmental metrics may be employed in cost-minimization orthermodynamic efficiency studies, no single measure is sufficiently developed to prioritize among qualitatively disparate types ofenvironmental impacts. It is argued herein that the concept ofchemical exergy of mixingmay be the most promising basis for thedevelopment of a uniform, broad-based measure of chemical pollution, and that such a measure could significantly advance ascientific approach to IE. Some theoretical background is presented, although the reasoning herein is intended to be accessible toan interdisciplinary audience. 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Industrial ecology; Exergy; Environmental metrics

1. Intellectual and historical antecedents

As an emerging science, industrial ecology (IE) hasbeen accessible to researchers from a number of differentdisciplines [1], however, it has yet to establish a consist-ent definition or a uniform analytical framework inpart explaining why the concept of a natural analoguefor study of industrial systems is far from universallyembraced among environmental engineers, scientists ormanagers. IE has been variously described as a para-digm shift [2], a broad umbrella of concepts, ratherthan a unified theoretical construct [3] and an aggre-gation of trends (that) is still being defined by its pro-

ponents [4]. A broad review of the historical origins ofthe term shows that a myriad of definitions, descriptionsand new terms have appeared, disappeared or reappearedin the literature only to confuse experts and neophytesalike. A new terminology or vocabulary must eventually

* Corresponding author. Tel.: +1-315-268-3856; fax: +1-315-268-

4291.

E-mail addresses:[email protected] (T.P. Seager), theist@-

clarkson.edu (T.L. Theis).

0959-6526/02/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved.

PII: S 0 9 5 9 - 6 5 2 6 ( 0 1 ) 0 0 0 4 0 - 3

be established that reflects the evolution of scientificthought [5].

A comprehensive literature review shows that therehas been considerable uncertainty as to what IE is orshould be. In the seminal paper which popularized theneologism, Frosch and Gallapoulos [6] proposed thatindustrial systems would function more efficiently andwith fewer environmental impacts if they were modeledafter natural ecosystems wherein the consumption ofenergy and materials is optimized, waste generation min-imized, and the effluents of one process serve as theraw materials for another process.1 This supposition isan extension of what Ayres [7] called industrial metab-

olismand characterized as the energy-and-value-yield-

1 The paper mentions two additional terms that become important

to future discussions. Dematerialization is defined as the use of plas-

tics, composites, and high-strength alloys to reduce the mass of pro-

ducts, and cited as an important trend in the auto industry wherein

cars are becoming lighter, more fuel efficient, but also increasingly

difficult to recycle as the materials from which they are manufactured

become more diverse and complicated. Additionally, the paper refers

to the life cycle of various industrial materials (e.g. metals and

plastics), in which the origin, use and eventual fate of the materials

are studied together to identify opportunities for resource savings.


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226 T.P. Seager, T.L. Theis / Journal of Cleaner Production 10 (2002) 225235

ing process essential to economic development anal-ogous to the metabolic processes that are essential tolife.2 Industrial ecology may be considered a naturalbroadening of the metabolic analogy [10]. For a briefperiod, the two terms industrial metabolism and IEcoexisted and may have even been used interchangeably

[11]. However, the first textbooks in the field appearedto almost completely discard metabolism in favor ofecology [9,12,13]. Socolow [13] wrote that IE isintended to mean both the interaction of global indus-trial civilization with the natural environment and theaggregate of opportunities for individual industries totransform their relationships with the natural environ-ment, thereby subsuming the research that had beenconducted under a metabolism banner and expanding themeaning of IE to make a separate term superfluous. Inretrospect this seems unnecessary the two metaphorscreate different perspectives that need not be competi-tive, but complementary.

Subsequently, the terms industrial and ecologyhave also been subject to debate Socolow [13]claimed that industrial held different meaningsdepending upon whether a metabolism or ecology per-spective was adopted, while Commoner [14] has ques-tioned application of the term ecology. Together theyhave subsequently come to be interpreted broadly tomean practically any anthropogenic function, includingagriculture, information technology, etc. not justmanufacturing [15]. Extending the argument for a broaddefinition of industrial might lead to the logical con-clusion that the natural and industrial are not separatesystems, but inexorably linked and therefore must be

considered together. A merging of natural and IE maybe the inevitable consequence of a holistic applicationof the natural analogue, however, a theoretical basis formodeling the linkages has not yet been established.

While it is clear that an analogy has led to a new fieldof study and a new perspective from which to modelindustrial processes, the separate but overlapping defi-nitions offered to date are not adequately resolved byany author, although articulation of a vocabulary hasbeen identified as a prerequisite step [16]. Therefore, forthe purposes of this paper, IE shall be defined as a fieldof study (or branch of science) concerned with the inter-relationships of human industrial systems and their

environments. Similarly, industrial metabolism shall bedefined as the process by which mass and energy(exergy) flows are handled or transformed by the econ-omy. Lastly,an industrial ecosystem shall be understoodto mean a model of a community or system of firms that

2 See Thomas [8] for comments regarding Ayres [7] and Ayres and

Ayres [9].

is based upon a natural analogue.3 Alternative descrip-tions need not be entirely supplanted by these, as theymay offer explanatory power for a specialized agenda,but the definitions offered herein are broad enough fora wide range of applications and consistent with the bio-logical and etymological roots that inspired invention of

the terms. If it is not too late to define the nomenclatureof such a young and emergent field, these definitionswould serve to facilitate clarity, understanding andfurther research if they become more widely adopted.

2. Contrasting analytical perspectives

There are principally two perspectives which providean analytical basis for IE: life cycle assessment (LCA)and systems analysis.4 Each embodies the notion thatenvironmental problems should be examined with anincreasingly holistic, rather than reductionist, approach.

Although there are significant differences between thetwo (see Table 1), they are both extremely sensitive tohow the boundaries of study are defined. In fact, one ofthe principle motivating hypothesis of IE is the intuitivesense that as the boundaries expand, supraoptimal sol-utions may emerge. That is, the most favorable outcomemay be found by coordinating the activities of all systemcomponents, rather than by combining the individualbest option of each subsystem. Still, LCA and systemsanalysis approach this synthesis of separate subsystemsin different ways. Whereas the LCA approach reliesupon the metaphor of a product or process lifetime although this is difficult to define unambiguously sys-

Table 1

Summary comparison of LCA and system analysis

Characteristic Life cycle assessment Systems analysis

Purpose Descriptive Prescriptive

Boundaries Cradle to cradle/grave Scalable

Data requirements Broad Focused on

decision

Emphasis Materials cycling Any uniform metric

(e.g. dollars)

Applicability Industrial metabolism Industrial ecology

3 These definitions are not entirely original. One could obtain nearly

the same by looking up the words ecology, metabolism, and ecosystem

in the dictionary just substitute the word industry for organism in

the definitions provided therein. We happened to use Websters ninth

new collegiate dictionary, Merriam-Webster Inc., Springfield, MA

1988.4 ORourke et al. [3] have presented a critical review of IE litera-

ture, research strategies and tools which remains relevant. However,

they do not include a detailed discussion of LCA or systems analysis,

instead focusing on other methodological and theoretical aspects or

weaknesses (e.g. design for environment, ecofeedback, Pigouvian

taxes).


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227T.P. Seager, T.L. Theis / Journal of Cleaner Production 10 (2002) 225235

tems analysis is more flexible. The systems approach isamenable to any scale: from single product or processto an entire industry or geographic region. The objectiveof LCA is principally comparative, whereas in systemsanalysis the emphasis is explicitly placed upon optimiz-ation and decision-making. That is, LCA lends itself to

descriptive models, while systems analysis lends itselfto prescriptivemodels. For this reason, the data require-ments of LCA may be more extensive than systemsanalysis at least wherein systems analysis may neg-lect information that is beyond the boundaries of afocused study or irrelevant to the decision at hand. LCAis therefore more applicable to industrial metabolism,wherein the emphasis is on examining specific materialsflows and processes, whereas systems analysis is moreapplicable to IE, wherein the emphasis is on examin-ing interrelationships.

3. Life cycle assessment

LCA is the principal tool by which IE research is car-ried out. As Allenby [17] argues, virtually all modernapproaches to environmental issues begin with theassumption that the appropriate scale of the analysis isthe life cycle of the material, product or service at issue.Although a wide variety of approaches have been advo-cated, LCA researchers typically follow a four-stepmethodology consisting of scoping, inventory analysis,impact assessment and improvement assessment (seeGraedel [18], SETAC [19], or USEPA [20], etc. forfurther details). Scoping is a process of identifying the

goals that motivate the assessment and determining theproper boundaries of study. The inventory analysis is anaccounting of the resource requirements of a particularproduct, process or industry from virgin materialsextraction to final disposition. The impact assessment isconducted to relate the inventory data to specificenvironmental concerns. Finally, the improvementassessment (or interpretation phase) identifies thoseaspects of the materials life cycle that might be mostamenable to mitigation, or evaluates the potential forapplication of new strategies (e.g. design forenvironment) that offer the greatest leverage for environ-mental benefits. However, current LCA techniques have

been criticized as unreliable scientific tools subject toquantitative and qualitative errors [2124]. The limi-tations identified with current practices include a lackof adequate inventory data, difficulties in identifying theboundaries of the system, disparate underlying assump-tions, and impact assessments in terms that are notdirectly comparable. Nevertheless, LCA is perceived asa useful management tool that is growing in acceptanceamong industrial practitioners [25].

Graedel [18] suggests streamlining LCA to partiallyovercome the difficulties of completing an extensive

LCA. The methodology proposed is greatly simplified.A subjective score (14) is assigned to each componentof a matrix describing the type of impact (e.g. energyconsumption, solid waste generation) versus portion ofthe life cycle. The advantage of this method is thereduced time frame for completion a few days rather

than several months and the ease in communicatingthe results of the analysis to administrators and the pub-lic. However, no matter which methodology isemployed, widely disparate, albeit defensible, con-clusions can be drawn depending upon what informationis excluded from the study, or which underlying assump-tions are applied [26].

In many instances, a life cycle inventory requirescompilation of chemical information for intermediary orby-products for which manufacturers or suppliers main-tain little quantitative account. Even when such data doexist, it is often regarded as proprietary, and thereforeunavailable, unverifiable or unpublishable. Lave et al.

[24] have shown that the boundaries of LCA should beeven greater than suggested by current guidance docu-ments, especially when the alternatives comparedinclude significantly different raw materials. However, itis apparent that no logical beginning or end to a specificmaterials cycle may exist, and that establishing theboundaries of an inventory analysis may rely upon arbi-trary judgments. This presents a significant obstacle toproducing scientific research that is reproducible by dif-ferent investigators.

The principal advantage of LCA is that it provides abroad framework for asking questions it guides theinvestigative process beyond local boundaries, forcing

consideration of factors that may have previously beenignored which is informative as a management tooland as a vehicle for exploring an intuitive appreciationof a more holistic approach. The photographic industryprovides an illustrative example. In Fig. 1, a manage-ment perspective typical of current practices is rep-resented by the boxes drawn around manufacturingactivities. The emphasis is on reducing waste and pol-lution from the source of production. Raw materialrequirements and pollutant emissions are benchmarkedto production of finished product: for example, photo-graphic film, paper or chemicals. Herein, the principlesof waste minimization and pollution prevention may be

successfully applied (and are increasingly the focus oftraditional environmental engineers), but primarily as amethod of meeting stringent emissions goals morecost-effectively.

The portions of Fig. 1 outside the boxed boundariesillustrate a more comprehensive view consistent with theproduct life cycle analogy. Typically, photographerscapture extra exposures of critical scenes (i.e. the justin case shots), or take exposures that later prove to beunsatisfactory (e.g. I look fat in that photo), or takeadditional, unnecessary exposures just to use up the


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Fig. 1. Typical (Compliance) vs. life cycle perspective. The typical

compliance perspective (inside boundaries) measures finished product

and residuals at the factory, in terms of material production. The life

cycle perspective (complete figure) measures final product at the end

of the chain, rather than in the middle, in terms of value to the pho-

tographer excluding unwanted materials like unsatisfactory photo-

graphs.

roll. Current industry practice is to develop and printall of the exposures, even create double prints, prior toreturning the images to the photographer. That is,although not all photographs are equally valued by thephotographer they all consume equivalent thermodyn-amic and environmental resources. The fact that mostphotographic prints go into long-term storage (the shoebox) without ever being revisited is indicative that agreat portion of the film and paper which is perceivedby the manufacturer to be final product may be wasted.

An improvement assessment might focus on

employing digital technology to make photographic pro-cesses much more environmentally and economicallyefficient. For example, digital cameras typically allowphotographers to delete unsatisfactory exposures by dis-playing them on a small LCD screen at the back of thecamera. This obviates the need for developing and print-ing unwanted photographic prints. Other digital techno-logies operate on the same principle. For example, a filmdrive scans photos from a negative directly into com-puter memory without need for a paper print allowingthe photographer to preview and select which images or

what information is critical and may result in signifi-cant economic and environmental savings.

4. Systems analysis

Systems analysis typically requires a mathematicalmodel that characterizes the relationships and constraintsgoverning various systems components. The model isusually the result of a careful analysis of the system inquestion in which quantitative links among componentsare established. Boundaries may either be drawn nar-rowly (e.g. around a single manufacturing facility) ormore broadly (e.g. to include suppliers, partners, cus-tomers or to encompass wide geographic regions). Thefocus of systems analysis is the objective function,which must be expressed in uniform units of measure-ment (e.g. dollars). The goal of a systems analyst is tofind a solution which satisfies the mathematical model

for the maximum (or minimum) value of the objectivefunction. Therefore, systems analysis is a design toolwhich helps decision-makers focus all the elements of asystem towards a single objective.

Systems analysis is amenable to anymeasurableman-agement goal. Selection of a unifying objective metricis an analytical prerequisite. Current environmental man-agement approaches focus primarily on maximizing pro-fits (or minimizing manufacturing costs) while main-taining compliance with emissions regulations or self-imposed constraints. Revelle et al. [27], Haith [28] andOssenbruggen [29] exemplify current analyticalapproaches. However, not all business operations are

guided by a profit-maximizing principle. For example,public utilities may operate under constrained profit mar-gins and instead attempt to minimize costs while main-taining a high standard of service and low level of safetyrisks. By analogy, a hypothetical manufacturing facilitycould conceivably be optimized to generate fixed profitsat some minimal measure of environmental risk such astoxicity-weighted chemical releases [30]. This kind ofapproach has yet to be put into practice and may becounter productive to the extent that nontoxic environ-mental threats like global warming or eutrophicationwould be ignored it is not obvious that minimal toxicrelease is synonymous with minimal environmental

impact. Because it is difficult to simultaneously optimizefor multiple objective functions, the greatest obstacle tothe pursuit of a quantitative environmental systemsapproach for IE remains the fact that no single variableor metric exists which embodies a holistic measure ofenvironmental impact. This in part offers an explanationfor why the hypothesis originally articulated by Froschand Gallapoulos [6] is yet to be tested in any comprehen-sive, scientific way.

Fig. 2 illustrates a systems model of the silver halideand digital photographic imaging chains. Three steps are


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Fig. 2. Which is the optimum image pathway?

common to both: capture, processing, and output (orprinting), and the feasible or hypothetical links between

each step on each chain create a myriad of technologicalpossibilities that may efficiently serve a variety of pho-tographer preferences. Both LCA and systemsapproaches may be adapted to provide contrastinginterpretations of the photographic industry modeldepicted in Fig. 3. An LCA approach would entail separ-ate assessments of each imaging step or link to determinethe comparative environmental characteristics of eachpathway or identify any opportunities for improvementalong any particular pathway. The systems approachwould model the functional relationships at each box andlink to determine which pathway best meets the objec-tive criteria. Because the systems approach is more flex-

ible, it may have advantages as a decision or design tool.The data requirements can be less onerous and the sys-tem boundaries can be defined unambiguously.

The photographic industry again provides illustrativeexamples. Suppose the focus of one aspect of study isthe environmental impact of two methods of printing:ink jet and silver halide photographic. As far as each isprinted on identical paper base, the environmentalimpact of the base is irrelevant to comparison. Only thedifferent coatings, consumable chemicals and energyrequirements need be studied. Whereas an LCAapproach requires consideration of every aspect includ-ing the paper on which the image is printed, systems

analysis can exclude those factors which remain unin-fluenced by the decision without altering the con-clusions. In this way, systems analysis allows a morespecific definition of system boundries which partiallyovercomes one of the primary obstacles to LCA: theextensive data requirements called for by the life cycleinventory. This principle is applicable to other aspectsof the imaging chains as well. For example, both digitaland film cameras must contain similar elements: camerabody, a lens, etc. In some instances, these are inter-changeable. To effect a comparison for the purposes of

evaluating relative environmental impact of each tech-nology, a more focused approach is expedient. Conse-quently, it is not necessary to complete comprehensiveLCAs in order to draw effective comparisons; anabridged or marginal LCA may be adequate. However,this dilutes the principal benefit of the life cycle meta-

phor the notion of expanding the boundaries ofstudy and systems analysis is a more appropriate termfor such an approach. Table 1 summarizes the principaldifferences between the two perspectives.

5. Introduction to quantitative metrics for LCA

and systems analysis

Both LCA and systems approaches suffer from thesame shortcoming: the lack of a uniform basis for com-parison or expression of disparate material and energyrequirements, emissions or environmental impacts [31].

LCA is severely limited in its applicability to complexproblems (such as technology replacement) in which theresources and pollutants resulting from alternative analy-ses are likely to be incomparable, and environmental sys-tems analysis to date has primarily been a vehicle foridentifying cost-savings opportunities rather thanenvironmental improvements. Unfortunately, there iscurrently no scientific methodology for answering themost critical question of IE: which of the technologicalalternatives is preferable from an environmentalperspec-tive? There are three metrics that might be applied withinthe context of LCA or systems analysis to address theforegoing question: total financial cost, thermodynamic

(material and energy i.e. exergy) resource consump-tion, and environmental impact. The remainder of thispaper is primarily concerned with introducing the ther-modynamic concept of exergy of mixing as a promisingbasis for development of a scientific, objective measureof the latter quantity, but the first two must also beaddressed briefly to provide a basis for comparison.

6. Total cost assessment

Although accounting, finance and economics arehighly developed disciplines applicable within the con-

text of either LCA or systems analysis, it would be fal-lacious to take for granted that they may be applied todetermine environmental preferences. Gray [32] exemp-lifies the criticisms that have been levied against currentaccounting practices essentially arguing that these aretoo narrowly focused to provide accurate or objectiveinformation to decision-makers regarding the environ-mental implications of industrial activities. For example,in addition to the classic economic problem of externalcosts, there are several instances wherein it is commonpractice to neglect or misallocate internal financial costs


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such as: contingent liabilities and legal expenses, reme-diation, employee moral, brand image, and treatment ordisposal costs that are charged to overhead instead of tospecific production activities.Total cost assessmenthasbeen advocated as an alternative which is consistent withthe basic underlying principal of IE: that a more inclus-

ive, holistic treatment will lead to better decisions[33,34]. However, TCA approaches have rarely beenemployed within the context of LCA to determineenvironmental preferences. There may be circumstancesin which the most financially expedient strategy (evenby holistic measures) is not the environmentally prefer-able one [35], or the interests of future generations aredenigrated [36], or the most profitable strategy is to irre-versibly exploit natural resources to exhaustion (or in thecase of biological resources, extinction) [37].

7. Exergy as a measure of material and energetic

resource consumption

Hocking [38] suggests that net energy expendituremay be the most useful measure for judging environ-mental performance. However, Ayres et al. [39] pro-pose a unified basis for LCA based upon the thermodyn-amic concept ofexergy which is an improvement uponenergetic measures to create a more complete resourceand waste accounting. A term invented by Rant [40],exergy combines the first and second laws of thermodyn-amics in a manner analogous to Gibbs free energy,Helmholtz energy or availability. It is a thermodynamicproperty that expresses the capacity of a system to per-

form work under ideal conditions. Because all industrialprocesses and all material and energetic flows may bemodeled in terms of embodied exergy, thermodynamicscould theoretically provide a common scientific frame-work for both LCA and systems analysis, merging thetwo perspectives into complementary tools. A generalrelationship can be drawn between exergy and thematerial life cycle wherein high exergy (low entropy)resources are extracted from the environment, refined bythe economy, and returned to the environment as lowexergy (high entropy) wastes.5 Although exergy may befound in four basic forms, kinetic, potential, chemicaland physical (i.e. pressurevolume and heat exchange

type work), for the present purposes the most importantaspect of exergy analysis is chemical, which is anal-ogous to Gibbs free energy

BchemicalGHTS (1)

where Bchemical represents change in chemical exergy,

5 See Ayres [41] for a critical review of the role of thermodynamics

in economic theory and further treatment of exergy as a factor of pro-

duction.

Gchange in Gibbs free energy,Hchange in enthalpy,T absolute temperature and S change in entropybetween two thermodynamic states.

Computation of Gibbs free energies of formation hasbeen the subject of extensive research. Standard tablesare available as supplements or appendices to textbooks

that describe the general principles in application toenvironmental problems [42]. The standard referencestate is most commonly taken to be a pure elemental,zero valence form (e.g. O2 (g), Cl2 (g), Ag(metal)) and thefree energies of formation of all other compounds maybe computed by comparison to these. Unfortunately, thehighly reactive state of some standard forms (chlorine,hydrogen, etc.) makes them an inappropriate referencestate for exergetic analyses incorporating environmentalconsiderations. Moreover, pure reference conditions arenot representative of the state in which chemical com-pounds commonly occur in the environment.

The principal advantage of exergy compared to Gibbs

free energy is a system of environmental reference statesfirst proposed by Ahrendts [43] which identifies thechemical characteristics of three different referenceenvironments for computation of standard chemicalexergies: the atmosphere, the ocean, and the earthscrust. In many cases the most oxidized form of anelement serves as the appropriate reference state in eachenvironment, however consideration must also be givento the molar concentration of a compound in the speci-fied environmental sink. This is of utmost importance,as it is well recognized that natural systems are not atthermodynamic equilibrium, but rather must be approxi-mated as ongoing, quasi-steady-state reactions limited by

both kinetic and energetic considerations, and that puresubstances released into the environment will eventuallybecome dissipated to background concentration levels.

The principles for computation of the standard chemi-cal exergy of any compound are described by Szargut etal. [44]. Assuming standard pressures and temperatures,a simplified general formula may be employed

B0pG0

i

ninp

B0i (2)where G0 is the free energy of formation (kJ/mole) ofthe compound from the elements,nithe number of moles

and B

0

i the standard chemical exergy (kJ/mole) of the ithreactant required to form np moles of the product com-pound; ni and np are determined by the stoichiometricbalancing numbers of the appropriate chemical reaction.The formation of ammonia gas from nitrogen and hydro-gen provides a simple example. The chemical reactionis represented by

12N2(g)1

12H2(g)NH3(g) (3)

16.4812(0.72)11

2(236.1)338 kJ/mole. (4)


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The Gibbs free energy of formation of ammonia gasis given as 16.48 kJ/mol [42]. However, the standardchemical exergy is found to be 338 kJ/mol by substitut-ing thermodynamic data from Table 2 into Eq. (2).Whereas the Gibbs free energy is representative of theideal thermodynamic work required to sythesize pure

ammonia from pure elements, the standard chemicalexergy is representative of the maximum work that couldbe obtained under ideal conditions from pure ammoniagas. An extensive tabulation of standard chemical exerg-ies has been compiled by Ayres et al. [39], makingexergy the thermodynamic variable of choice for studiesrelated to the environment. Exergy analysis has sub-sequently been employed in LCA [45] and to advance ascientific basis for sustainability [46].

8. The problem of quantifying environmental

impact

As a common basis for resource accounting, exergyis an improvement upon mass or energy-based measures,but it is far from a panacea for the difficulties of con-ducting an environmental impact assessment or con-structing an environmental objective function. Althoughit has been argued that the concept of exergetic optimiz-ation of industrial processes can be justified on environ-mental [47,48] or moral [49] grounds, current appli-cations are for the most part limited to those industrieswherein thermodynamic and economic criteria areclosely aligned. Bejan [50] and Brodyansky et al. [51]exemplify current approaches in industries such as elec-

tric power generation, refrigeration, distillation, etc. Cur-rently, no methodology exists whereby exergy may bereliably related to environmental impact or ecosystemfunction. This is primarily because waste exergy thatwhich is released to the environment in the form ofchemical pollutants and waste heat comes in differentforms that may have different quantitative and qualitat-

Table 2

Thermodynamic data (R=8.314 J/mol/K, T=298.15 K, P=101.325 kPa)

G0 (kJ/mol)a B0 (kJ/mol)b yi0 RT ln(yi

0)

(mol/mol)b (kJ/mol)

N2 0 0.72 0.7479 0.720H2 0 236.1

NH3 16.48 337.9

CH4 50.79 831.65 1.7 ppm 32.9

O2 0 3.97 0.201 3.97

CO2 394.37 19.87 0.000331 19.87

H2O 228.57 9.49 0.0217 9.49

NO2 51.3 55.6 1 ppb (est.) 51.4

SO2 330.2 313.4 35 ppb (est.) 42.6

C(graphite) 0 410.26

a From [42].b From [44].

ive environmental effects. Therefore, a waste exergy (orexergy emissions) approach could provide only a firstapproximation of environmental impact (see Dincer [52]or Gunnewick and Rosen [53] for a summary of the sali-ent hypotheses).

To remedy this theoretical deficiency, many manufac-

turing companies have developed empirical metrics togauge improvements in environmental performance [5457]. Typically, these apply subjective value judgmentsregarding the importance of various characteristicimpacts allowing cross-comparison of different techno-logies, materials alternatives or material and energetictrade-offs. However, of the wide variety of approachescurrently employed, none are appropriate for develop-ment of a broad-based quantitative framework for IE.They may be too narrowly focused (e.g. on humantoxicity), rely on economic rather than environmentalmeasures (e.g. externalities), fail to capture the disparatequalitative characteristics of various materials (e.g.

materials intensity indices), or be applicable only to anarrow set of products, industries or single company.Consequently, the need for a universal, broad-basedenvironmental metric has increasingly been recognizedas a high priority for research. A better approach maybe to focus on the portion of the chemical exergy whichis due solely to material transfers or changes in compo-sition. This is referred to as the exergy of mixing, and itis a measure of the potential chemical change attribu-table to introduction of any pollutant into the environ-ment.

9. Calculation of exergy of mixing

There are two mechanisms by which chemical exergymay be converted into work or entropy: heat transferand mass transfer. The first of these is manifested in thechemical bond and released during reaction (e.g. com-bustion of fossil fuels to form carbon dioxide and water),and the second in dilution or dissipation of reaction end-products throughout the environment (e.g. theatmosphere). Whereas matter and energy are conservedthroughout the process, exergy is not. Assuming iso-thermal and isobaric conditions, the net chemical exergy

of heat transfer for a reaction is approximated by thechange in Gibbs free energy in accordance with Eq. (1),whereas the exergy of mixing (or composition-depen-dent component of chemical exergy [44]) is computedfor the ith chemical species of any system as

Bmi niRT0lnyiy0i (5)

whereBmi is exergy of mixing in joules,nithe total num-ber of moles of the species,yithe acitivity in the thermo-dynamic system under consideration, and y0i the refer-


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ence activity in the appropriate environmental sink. R isthe universal gas constant, taken to be 8.314 J/mol/K;T0 is standard temperature, 298.15 K. Because the workavailable in chemical reaction (i.e. heat transfer) is gen-erally much larger than that of mixing (i.e. masstransfer), researchers have to date nearly uniformly

ignored dissipative considerations ([58]; an exception isEdgerton [59]).For example, consider the chemical reaction rep-

resenting methane combustion

CH42O2CO22H2O (6)

G0[(50.79)2(0)][394.372( (7)

228.57)]800.72 kJ/mol.

The change in Gibbs free energy for the reaction isgiven in Eq. (7) and is computed from the thermodyn-amic data in Table 2 by subtracting the stoichiometric

equivalent standard free energies of formation of thereactants from those of the products. The standardchemical exergy is computed with Eq. (2) as831.7 kJ/mol from the formation reaction as in theammonia example. The difference of 31.0 kJ/molbetween the standard chemical exergy and the negativeGibbs free energy is entirely due to the net exergy avail-able from dissipation of the products (taken as pure car-bon dioxide and pure water vapor in accordance withthe Gibbs standard states) throughout the environment,minus the exergy required to concentrate pure oxygen atthe site of the reaction. From this example it is clearthat the standard chemical exergies B of the reference

compounds O2, CO2, and H2O may be determined solelyby their exergies of mixing computed from Eq. (5),assuming reference activities y0i shown in Table 2, andthat the results are consistent with those given by Eq. (2).From an energetic standpoint, the net exergy of mixing isnegligible, being in the order of 1/25th the net workavailable from combustion. Nevertheless, it embodiesthe thermodynamics of the final step wherein waste pro-ducts are introduced into and dispersed throughout theenvironment, and may provide a specific link betweenexergetic LCA and a holistic measure of the potentialfor unintended or adverse environmental consequences.

The case of coal, which is a mixture of hydrocarbons,

oxygen, sulfur, nitrogen, moisture, ash and traceelements, provides a basis for comparison. Mass frac-tions typical of moist coal are given in Table 3. Thetotal chemical exergy is estimated as 23,583 kJ/kg [44].Therefore, approximately 35.3 g of coal is required toprovide the same 831.7 kJ exergy as one mole of puremethane gas. A simplified chemical reaction (ignoringtrace elements and ash) is shown below. The stoichio-metric balancing numbers represent the number of molesof each substance involved in the reaction of 35.3 g coal(see Table 3):

Table 3

Properties typical of bituminous coal [44]

Mass m.w. (g/mol) Molal nifraction fraction (mol/35.3 g)

(kg/kg) (mol/kg)

C 0.577 12 48.1 1.70

H2 0.041 2 20.5 0.724H2O(l) 0.10 18 5.56 0.200

O2 0.112 32 3.50 0.124

S 0.013 32 0.406 0.015

N2 0.007 28 0.250 0.009

Ash 0.15

(1.70 C0.724 H20.200 H2O(l)0.124 O2

0.015 S0.009 N2)coal1.862 O2(g)1.70 CO2 (8)

0.924 H2O(g)0.009 NO2(g)0.015 SO2(g)

The exergy of mixing of the exhaust gases may be com-puted from Eq. (5). These total 43.65 kJ for coal, com-pared to 38.85 kJ for an exergetic equivalent amountof methane.

Although exergy of mixing of the exhaust gases isvery small (and uneconomical) compared to total exergy,from an environmental impact standpoint the wasteexergy of mixing is an objective measure of the chemicalchange engendered in the environment by release of thecombustion products to the atmosphere. Excluding oxy-gen (which is extracted from rather than wasted to theatmosphere) the sum of the exergies of mixing of the

coal exhaust gases is 12% higher than those of methane,indicating that coal combustion is likely to have a greateratmospheric environmental impact (due to increasedCO2emissions from carbon) than methane combustion.

6

Furthermore, it is possible to relate exergy of mixing toother measures of environmental impact. Fig. 3 (adaptedfrom [53]) compares the exergy of mixing of five atmos-pheric pollutants to estimates of environmental pol-lution costs.

In general, those chemical species which appear withthe greatest frequency in the environmental sink of inter-est are those with the lowest exergy of mixing and leastpotential for harm. Conversely, organisms and ecosys-

tems have little evolutionary experience with chemicalspecies which occur only rarely in nature and introduc-tion of these (having greater exergy of mixing) may beparticularly disturbing.

Chemical species which do not occur at all in nature(e.g. chlorinated hydrocarbons such as CFCs or PCBs)

6 Additional environmental considerations have not been included

in this analysis and may make coal even less attractive: e.g. the pres-

ence of mercury increases the loading of pollutants to the atmosphere

and ash disposal may present a problematic land impact.


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Fig. 3. Comparison of Environmental Pollutant Cost (EPC) with

exergy of mixing (adapted from [53]).

would have an infinite exergy of mixing in accordancewith Eq. (3) wherein the standard mole fraction in theenvironmental reference sink is taken as zero. However,this treatment contradicts common sense. (How caninfinite exergy emerge from a finite system?) The con-ventional thermodynamic assumption is to treat theenvironment as an infinite sink with immutable proper-ties and for many cases, the impact of mixing a smallamount of pollutant results in a negligible change in themole fraction of the reference environment. However,for exceedingly rare chemical species, or for massiveanthropogenic release (e.g. carbon dioxide), the refer-

ence condition may change over time. In these cases, theearth must be treated as a finite system. In the case ofchemicals with no natural sources, the reference con-dition (for the purposes of computing exergy of mixing)may be taken as some arbitrarily small figure such asthat which would be found in the environmental sink ofinterest after complete dissipation of the pollutant ofinterest.

Exergy of mixing computations are very sensitive tothe choice of an ideal reference environment, and somesubjectivity may be introduced in deciding what refer-ence mole fractions are to be employed. In someinstances, globally and/or annually averaged values

would be inappropriate, and reference conditions thatvary temporally or geographically are likely to becalled for.

When mixing is instantaneous, the exergy of mixingper mole of material is analogous to that portion ofGibbs chemical potential which is solely due to mixing(or the free energy of mixing[42,60]) albeit with refer-ence to environmental rather than standard states. How-ever, under environmental conditions, the actual exergyof mixing may be significantly less than the ideal forseveral reasons:

Degradation of the pollutant over time. Passing of the material beyond the geographic bound-

aries of interest. Sorption, deposition, or partitioning of the pollutant

into a different media of interest.

Therefore computation under actual, rather than ideal,conditions in the environment must necessarily includescalar and kinetic considerations, as well as thermodyn-amic. Actual exergies of mixing for any particular pol-lutant could conceivably be computed for any referenceenvironment provided the environmental fate and mixingkinetics (i.e. chemodynamics) within a particularlydefined geographic boundary can be approximated. Itmay be hypothesized that the results of a general meth-odology would show that those pollutants that occurmost rarely in nature, that mix quickly, and that are themost long-lived may intuitively be suspected ofembodying the greatest exergy of mixing and of being

the most dangerous to the environment. In theseinstances, a precautionary or preventative approach maybe justified to prevent future environmental surprises[61].

10. Conclusions

Either LCA or systems optimization approaches maybe feasible on the basis of any of the three criteriadescribed: TCA, exergetic efficiency, or chemical exergyof mixing. In the case of LCA, it is conceivable that lifecycle inventories could be condensed into common units

of total financial cost, total exergetic resources consumedor total waste exergy of mixing, and comparative assess-ments conducted on these bases. In the case of systemsanalysis, any of the three metrics may be cast in termsof an objective function, allowing maximization of theobjective criteria (or multi-objective criteria) under con-straints expressed in terms of the other two. Each meas-ure is applicable to all conceivable industrial processesand may synthesize widely disparate attributes. How-ever, the recommendations of otherwise identical studiesconducted under different criteria may vary. Except ininstances wherein financial, thermodynamic andenvironmental considerations are all aligned, there is no

basis within IE to select a strategy or design whichinvolves trade-offs among the three. The first measuremay be most amenable to cost/benefit or rate of returnstudies; the second may be central to studies related tosustainability; the last might be a comprehensive methodfor managing the disparate impacts of chemical contami-nation.

As a field of science, IE should uphold no other goalother than a further understanding of human industrialsystems and the environmental implications concomitantto them and not be called upon to make value judgments


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or advocate a principled agenda. Environmental man-agers, on the contrary, may have less philosophicalmotivations. They may be charged with the responsi-bility of meeting increasingly prohibitive environmentalconstraints or corporate goals, of reducing environmentaltreatment and disposal costs, or of managing environ-

mental risks or liabilities. Just as forest management maybe characterized as an application of forest ecology, somight the environmental management of industrial pro-cesses be characterized as an application of IE. The dis-tinction till date has been blurred, if it has existed atall. However, through the development of more reliableanalytical tools, IE will mature into a science of its ownthat furthers understanding of the ramifications of indus-trial strategies that at present are judged subjectively.

Acknowledgements

This research was supported by the National ScienceFoundation through the Lucent Technologies IndustrialEcology Research Fellowship Program (BES-9873589)and the Environmental Manufacturing Management Pro-gram (DGE-9870646) at Clarkson University. JeffreyMathews (Eastman Kodak Co. Inc.) and Randy Brown(Clarkson University) have provided helpful commentsregarding various sections of this paper.

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