INDICATORS FOR THE ENVIRONMENTAL IMPACT OF WASTE EMISSIONS:COMPARISON OF EXERGY AND OTHER INDICATORS

Marc A. RosenFaculty of Engineering and Applied Science, University of Ontario Institute of Technology

2000 Simcoe Street North, Oshawa, Ontario, LlH 7K4, CanadaEmail: marc.rosen@uoit.ca, Tel: 905/721-8668

Received January 2009, Accepted March 2009No. 09-CSME-20, E.r.e. Accession 3106

ABSTRACTExergy is compared with other indicators for the environmental impact of waste emissions in an effort

to better understand its potential as, or as the basis for, an effective indicator of the potential of an emittedsubstance to impact the environment. Relations between environmental impact and exergy in general andchemical exergy of waste emissions in particular are observed to support the use of exergy as such anindicator. The measure of disequilibrium with respect to a reference environment provided by exergy isconsidered, along with the consequence that the exergy of unrestricted waste emissions has the potential toimpact the environment. Exergy is observed to exhibit many characteristics of other indicators ofenvironmental impact, which are mainly empirical in nature. An exergy-based indicator could contribute tothe development of rational and objective procedures for assessing the harmful effects on the environmentof a substance and predicting its potential for environmental impact. The potential usefulness of exergy inaddressing environmental problems is concluded to be significant, but further research is needed to developobjective exergy-based indicators that are practical for the potential of a substance to impact on theenvironment.

INDICATEURS DE L'IMPACT DES EMISSIONS DE DECHETS SURL'ENVIRONNEMENT: COMPARAISON DE L'EXERGIE ET AUTRES INDICATEURS

RESUMEL'exergie est comparee avec d'autres indicateurs pour l'impact des emissions de Mchets sur l'environnement,

dans un effort pour mieux comprendre son usage potentiel, ou comme base, d'un indicateur potentiel efficace pourune substance emise et son impact sur l'environnement. Les relations entre l'impact environnemental et l'exergie engeneral, et en particulier de l'exergie chimique des emissions de dechets, est examinee pour appuyer I'utilisation del'exergie comme indicateur.

La mesure du desequilibre par rapport aun environnement de reference, demontree par l'exergie est examinee,en meme temps que les consequences de l'exergie des emissions illimitees de dechets et Ie potentiel d'impact surl'environnement. L'exergie est examinee dans Ie but d'exposer plusieurs caracteristiques des autres indicateursd'impacts environnementaux qui sont principalement empiriques par nature. Un indicateur base sur l'exergiepourrait contribuer au developpement de procedures rationnelles et objectives pour evaluer les effets dommageablessur l'environnement d'une substance donnee, et predire son potentiel d'impact sur l'environnement. L'utilitepotentielle de l'exergie comme moyen de s'attaquer aux problemes environnementaux est conc1uante, mais desrecherches plus poussees sont necessaires pour developper des indicateurs objectifs bases sur I'exergie qui seraientpratiques pour evaluer Ie potentiel d'impact d'une substance donnee sur I'environnement.

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1. INTRODUCTION

Exergy analysis is a thermodynamic technique for assessing and improving systems and processes,which is similar but advantageous to energy analysis, in large part because it is based on the second law ofthermodynamics. The exergy of an energy form or a substance is a measure of its usefulness.

Exergy can be applied beyond thermodynamics to understanding waste emissions and reducing theirenvironmental impact because exergy can also be viewed as a measure of the departure of a substance fromequilibrium with a specified reference environment, which is often modeled as the actual environment. Forclarity, a waste emission is defined here as a release to the environment of a material which is notconsidered usable and is therefore discarded. Furthermore, exergy is a measure of potential of a substance tocause change. The exergy of an emission to the environment, therefore, is a type of measure of the potentialof the substance to change or impact the environment. The greater the exergy of an emission, the greater isits departure from equilibrium with the environment, and the greater may be its potential to change orimpact the environment. The exergy of an emission is zero only when it is in equilibrium with theenvironment and thus benign. These points suggest that exergy may be, or provide the basis for, an effectiveindicator of the potential of an emitted substance to impact the environment.

Others have begun to recognize this potential application of exergy. For instance, the Consortium onGreen Design and Manufacturing at the University of California-Berkeley (http://cgdm.berkeley.edu)recently carried out a project entitled "Exergy as an Environmental Indicator" in an effort to increase thepractical application of exergy analysis for rectifying the problems associated with material and energyflows in industry. That work focussed on developing a generalizable technique to calculate exergy in anindustrial setting, exploring the significance of environmental ground states and how they might be utilizedto leverage results, and establishing the requisite databases for performing a wide range of highly diverseanalyses [1-3].

This article complements the work described above and focuses on the potential of exergy to be anindicator for the environmental impact of waste emissions. To support the arguments, the relations betweenexergy and the environment are described, and exergy is compared with other indicators for theenvironmental impact of waste emissions. It is demonstrated that an exergy-based indicator for theenvironmental impact of waste emissions can provide a better understanding of, and more rationalapproaches to mitigating, the environmental impact associated with waste emissions.

In the remainder of this article, brief background on exergy and exergy analysis are given. Then, theproblems with using energy as an indicator for environmental impact of waste emissions are discussed.Relations between exergy and the environment, particularly those that support the use of exergy as anindicator for environmental impact of waste emissions, are discussed. Finally, exergy's potential to be anindicator of environmental impact of waste emissions is described, and exergy is compared with otherindicators for environmental impact of emissions.

2. EXERGY

2.1. The Quantity Exergy

Exergy is a quantity that is similar to energy in some aspects, but different in others. Extensiveinformation on exergy can be found in elsewhere [4-17], but some key features of exergy relevant to thisarticle follow:

• Exergy (also known as available energy, availability, essergy) is generally defined as the maximum

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amount of work which can be produced by a flow of matter or energy as it comes to equilibrium with areference environment.

• Exergy is a measure of the potential of a flow to cause change, as a consequence of being indisequilibrium with the reference environment.

• The state of the reference environment, or the reference state, must be specified completely forevaluations of exergy, normally by specifying its temperature, pressure and chemical composition. Thereference environment is normally an idealization of the natural environment which is characterizedby a perfect state of equilibrium, i.e., the absence of any gradients or differences involving pressure,temperature, chemical potential, kinetic energy and potential energy. The reference environmentconstitutes a natural reference medium with respect to which the exergy of different systems isevaluated.

• Exergy is not subject to a conservation law, but instead is consumed or destroyed due to theirreversibilities in any process. The exergy consumed or destroyed during a process due toirreversibilities within the system boundaries is referred to as exergy consumption (or dissipation,irreversibility, lost work).

• Chemical exergy, which is of particular concern when assessing potential for environmental impact,is the maximum work obtainable from a substance when it is brought from the environmental state tothe dead state by means of processes involving interaction only with the environment.

2.2. Exergy Analysis

Exergy analysis uses the conservation of mass and conservation of energy principles together with thesecond law of thermodynamics for the design and analysis of energy systems. In exergy analysis, processperformance is assessed by examining exergy balances, losses and efficiencies. The exergy method isparticularly useful for attaining more efficient energy-resource use for two key reasons. First, it identifiesefficiencies that are true measures of the approach to ideality. Second, it enables the locations, types, andmagnitudes of wastes and losses to be determined. As a consequence, exergy analysis can reveal whether ornot, and by how much, it is possible to design more efficient energy systems by reducing the inefficienciesin existing systems.

2.3. Applications of Exergy Analysis

Numerous applications of exergy analysis to improve the efficiencies of processes and systems havebeen reported, ranging in size from complex processes such as electricity generation [17-20], cogeneration[17,21] and chemical processing [17,22], to simple processes such as thermal storage [17,23,24] andprocess heating [17,25].

Exergy concepts can also be applied to disciplines outside of thermodynamics. Two importantexamples follow:

• Applications of exergy are increasing in the area of economics, under such names asexergoeconomics and thermoeconomics [17,19,26-28].

• Uses of exergy are increasing in fields related to environmental impact [17,29-40]. That work isaimed at understanding environmental impact better, and developing better predictors and indicatorsof environmental impact, especially for environmental emissions.

The latter topic forms the focus of the present article.

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3. ENERGY AND ENVIRONMENTAL IMPACT

Energy production, transformation, transport and end-use generally impact the environment.Environmental costs are usually associated with thermal, chemical, nuclear and other emissions. Increasingefficiency can reduce the environmental impact of emissions, although increasing efficiency generallyentails greater use of materials and energy resources, increasing the environmental burdens associated withthese and somewhat offsetting the environmental gains of improved efficiency.

3.1 Reducing the Environmental Impact Based on Energy and Other Factors

One of the major challenges of designing technologies so as to reduce their environmental impactinvolves determining an environmentally optimal configuration or selecting the most appropriate fromcompeting options. This selection is made difficult by the complex relationship between the technologyconsidered and the characteristics of the residuals produced. Process or technology changes generally affectthe characteristics (e.g., flow rate, composition) of effluent streams. Evaluating alternatives, therefore, oftenrequires comparisons of the relative environmental merits of different residual streams. Existing methods forperforming such analyses focus primarily upon subjective ranking techniques, and often are based onenergy. Increasing attention has been devoted over the last few decades to better understanding therelationship between energy use and the environment.

3.2 Energy as an Indicator for Environmental Impact of Emissions

Although data on the geographic distribution of energy consumption indicate a close correlationbetween a country's energy consumption and economic development, a correlation between a country'senergy consumption and the decay of its environment does not appear evident. That is, energy itself doesnot seem to provide a good indicator of the environmental impact of waste emissions.

4. EXERGY AND ENVIRONMENTAL IMPACT

The exergy contents of waste emissions are more meaningful than the corresponding energy contents asmeasures of potential for environmental impact. By definition, material and energy flows only possessexergy when in disequilibrium with a reference environment. The exergy associated with waste emissionstreams has the potential to cause environmental damage, particularly when it is released in an unrestrictedmanner into the environment. Some believe that by considering the exergy content of a waste emission,rational and meaningful assessments can be made of the environmental impact potential of the emission.

The present author has carried out several studies on this topic [30,32,34-36]. To facilitate anexamination of such relations, a discussion is provided of the reference environment used in exergyanalysis.

4.1. Reference Environments for Exergy

The exergy Ex contained in a system may be written as

Ex= S( T - To)- V( P - Po )+ Nk( f.1 k - f.1 ko ) (1)

where the intensive properties are temperature T, pressure p, and chemical potential of substance k, 14.; and

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the extensive properties are entropy S, volume V, and number of moles of substance k, Nk• The subscript "0"

denotes conditions of the reference environment. Clearly, the exergy of a system is zero when it is inequilibrium with the reference environment because T =To, P =Po and J4. =J4.0 for all k.

Exergy is evaluated with respect to the reference-environment model, and the exergy of a flow orsystem is dependent on the intensive properties of the reference environment [17,41]. The exergy of thereference environment is zero, and the exergy of a stream or system is zero when it is in equilibrium withthe reference environment. The reference environment is considered to be in stable equilibrium, have allparts at rest relative to one another, behave like an infinite system, act a sink and source for heat andmaterials, and experience only internally reversible processes in which its intensive state remains unaltered.No chemical reactions can occur between the reference environment components.

In developing reference-environment models for exergy analysis, a compromise is often made betweenthe theoretical requirements of the reference environment and the actual behavior of the natural environmentbecause the theoretical characteristics of a reference environment are not observed in the naturalenvironment, which is not in equilibrium and which has intensive properties that vary spatially andtemporally. Also, many chemical reactions in the natural environment are blocked because the transportmechanisms necessary to reach equilibrium are too slow at ambient conditions. Thus, the exergy of thenatural environment is not zero, as work could be obtained if it were to come to equilibrium.

Several reference-environment models have been proposed, the most significant classes of which aredescribed in the following subsections, based on discussions in [34]. For clarity, three important statesrelated to the reference environment are defined here:

• The dead state is the state of a system when it is in thermal, mechanical and chemical equilibriumwith a conceptual reference environment (having intensive properties pressure Po> temperature T'l>and chemical potential flko for each of the reference substances in their respective dead states).

• The environmental state is the state of a system when it is in thermal and mechanical equilibriumwith the reference environment, i.e., at pressure Po and temperature To of the reference environment.

• The reference state is a state with respect to which values of exergy are evaluated. Several referencestates are used, including environmental state, dead state, standard environmental state and standarddead state.

Natural-environment-subsystem models. These models attempt to simulate realistically subsystemsof the natural environment, and are thus the most useful in evaluating the potential of exergy as an indicatorof environmental impact.

One such model consists of saturated moist air and liquid water in phase equilibrium.An extension of that model allows sulphur-containing materials to be analyzed. For this reference

environment (Table 1), the temperature is To = 25°C, the pressure is Po = 1 atm, and the chemicalcomposition consists of air saturated with water vapor, and the following condensed phases at To and Po:water (H20), gypsum (CaS04·2H20), and limestone (CaC03). The stable configurations of C, 0 and Nrespectively are taken to be those of CO2, O2and N2as they exist in air saturated with liquid water at To andPo; of hydrogen is taken to be in the liquid phase of water saturated with air at To and Po; and of Sand Carespectively are taken to be those of CaS04·2H20 and CaC03 at To and Po.

Other reference-environment models are used to facilitate the performance of exergy analysis forthermodynamic efficiency and loss evaluation, and some of these are described below. But these are lessuseful in evaluating the potential for environmental impact.

Process-dependent models. Such models contain only components that participate in the processconsidered in a stable equilibrium composition at the temperature and pressure of the natural environment.

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Bosnjakovic proposed such a model, which is not general but dependent on the process examined. Exergiesevaluated for a specific process-dependent model are relevant only to the process, and can not rationally becompared with exergies evaluated for other process-dependent models or used in environmentalassessments.

Table 1. A reference-environment modelTemperature, To 25°CPressure, Po 1 atmComposition Atmospheric air saturated with

HzO at To and PoAir constituents Mole fraction

0.75670.20350.03030.00910.00030.0001

Condensed phases at To and Po Water (HzO)Limestone (CaC03)

Gypsum (CaS04·2HzO)

Equilibrium and constrained-equilibrium models. Ahrendts proposed a model in which all thematerials present in the atmosphere, oceans and a layer of the crust of the earth are pooled together and anequilibrium composition is calculated for a given temperature. The selection of the thickness of crustconsidered is subjective and is intended to include all materials accessible to technical processes. For allthicknesses considered (1-1000 m) and a temperature of 25°C, the model differed significantly from thenatural environment. Exergy values obtained using these environments are significantly dependent on thethickness of crust considered, and represent the absolute maximum amount of work obtainable from amaterial. Since there is no technical process available which can obtain this work from materials, theequilibrium model does not give meaningful exergy values when applied to the analysis of real processes.

Ahrendts modified his equilibrium environment by excluding the possibility of the formation of nitricacid (HN03) and its compounds in calculating an equilibrium composition. That is, all chemical reactions inwhich these substances are formed are in constrained equilibrium, and all other reactions are inunconstrained equilibrium. When a thickness of crust of 1 m and temperature of 25°C are used, thisconstrained-equilibrium model is similar to the natural environment.

Reference-substance models. With these models, a "reference substance" is selected for everychemical element and assigned zero exergy. Szargut proposed a model in which the reference substances areselected as the most valueless substances found in abundance in the natural environment. The criterion forselecting such reference substances is consistent with the notion of simulating the natural environment, butis primarily economic in nature, and is vague and arbitrary with respect to the selection of referencesubstances. Part of this environment is the composition of moist air, including Nz, Oz, COz, HzO and thenoble gases; gypsum (for sulphur) and limestone (for calcium).

A related model, in which reference substances are selected arbitrarily, has also been proposed. This

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model is not similar to the natural environment. Consequently absolute exergies evaluated with this modeldo not relate to the natural environment, and can not be used rationally to evaluate efficiencies orenvironmental impact.

4.2 Exergy as an Indicator for Environmental Impact of Emissions

Many researchers agree that exergy is an objective indicator capable of providing significant insightinto potential environmental impact. Exergy analysis provides information about the quality of energy andmaterial that is physically meaningful. Because of its origins within the thermodynamic community,however, to date few applications and investigations have been undertaken of the potential of exergymethods for better understanding and reducing environmental impact, even though to appropriately reduceenergy consumption and the related environmental impact, such knowledge is beneficial.

Some work is nevertheless being carried out, such as the project "Exergy as an EnvironmentalIndicator" by University of California-Berkeley's Consortium on Green Design and Manufacturing(CGDM) noted earlier. That research was motivated by needs related to Design for the Environment (DFE)methods. A major challenge when applying DFE is the selection of an environmentally optimal processconfiguration from among competing process designs. Existing methods for such analyses focus primarilyon subjective scoring techniques, but CGDM researchers felt that exergy could be a less-subjective metricfor DFE assessments and performed research focused on the practical application and adaptation of exergyanalysis to the specific problems associated with the material and energy flows through industry [1-3].

4.3. Assessing and Mitigating Environmental Impact with Exergy

An understanding of the relations between exergy and the environment reveals some of the underlyingfundamental pattems and forces affecting environment changes, and can help researchers deal better withenvironmental damage. On a simple level, efforts to increase efficiency can reduce environmental impact byreducing exergy losses (including emissions). Increased efficiency also reduces the requirement for newfacilities for the production, transportation, transformation and distribution of the various energy forms, allof which impact the environment.

On a deeper level, several researchers have suggested that a potentially useful way to link the secondlaw and environmental impact is through exergy because it is a measure of the departure of the state of asystem from that of the environment [29-40]. The magnitude of the exergy of a system depends on the statesof both the system and the environment. This departure is zero only when the system is in equilibrium withits environment. Tribus suggested that performing exergy analyses of the natural processes occurring on theearth could form a foundation for ecologically sound planning because it would indicate the disturbancecaused by large-scale changes [42].

One recent study [36] of thermodynamics and sustainable development suggested that exergy is likelyan important tool for obtaining sustainable development. That study showed l) although energy can neverbe destroyed, exergy can be destroyed and this exergy destruction (irreversibility) should be appropriatelyminimized to make development more sustainable, and 2) environmental effects associated with emissionsand resource depletion can be expressed in terms of one indicator, which is based on physical principles.

4.4 Extending Exergy as an Environmental Impact Indicator to Economics and Policy

A methodology known as environomics has been developed for analyzing and improving energy-relatedsystems by simultaneously taking into account energy, exergy, economic and environmental factors [33].

Also, several projects have been carried out at the Delft University of Technology and University of

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Twente to determine whether exergy analysis can be used as an instrument for environmental policydevelopment, especially for the comparison of alternative production chains. These studies investigated ifthe linkages of exergy with phenomena such as pollution and dispersion can be converted into a reliable toolon which policy decisions can be based, and explored how the environmental effects of processes can belinked to or expressed in terms of exergy changes.

4.5. Types of Environmental Impact Indicated by Exergy

Three relationships are explained below between exergy and environmental impact that explain thetypes of environmental impact that can be predicted or indicated using exergy.

Emissions of waste exergy. The exergy associated with process wastes emitted to the environmentrepresents in some ways a potential for environmental damage. Typical process wastes have exergy, apotential to cause change, as a consequence of not being in stable equilibrium with the environment.When emitted to the environment, this exergy represents a potential to change the environment. Usually,emitted exergy causes a change which is damaging to the environment (e.g., deaths of fish and plants insome lakes due to the release of specific substances in stack gases as they react and come to equilibriumwith the environment), although in some instances the emitted exergy may cause a change perceived tobe beneficial (e.g., increased rate of growth of fish and plants near the cooling-water outlets from thermalpower plants).

Emissions of exergy to the environment can also interfere with the net input of exergy via solarradiation to the earth. The carbon dioxide emitted in stack gases from many processes changes theatmospheric CO2 content, affecting the receiving and re-radiating of solar radiation by the earth.

The relation between waste exergy emissions and environmental damage has been recognized byseveral researchers. By considering the economic value of exergy in fuels, Reistad proposed an air­pollution rating in which the air-pollution cost for a fuel was estimated as either the cost to remove thepollutant or the cost to society of the pollution (i.e., the appropriate tax if pollutants are not removedfrom emissions), which he claimed was preferable to the mainly empirical ratings then in use [43].

Degradation of resources and order. The degradation of resources found in nature is a form ofenvironmental damage.

One definition of a resource is a material, found in nature or created artificially, which is in a state ofdisequilibrium with the environment and thus have exergy. For some resources (e.g., metal ores), it istheir composition that is valued. Processes exist to increase the value of such resources by purifying them(i.e., by increasing their exergy), at the expense of consuming at least an equivalent amount of exergyelsewhere (e.g., burning coal to produce process heat for metal ore refining). For other resources (e.g.,fuels), it is normally their reactivity that is valued.

By preserving exergy through increased efficiency, i.e., degrading as little exergy as necessary for aprocess, environmental damage is reduced. Increased efficiency also has the effect of reducing exergyemissions which, as discussed in the previous subsection, also playa role in environmental damage.

More generally, the degradation or order is a type of environmental damage, whether it is the order inthe form of a fossil fuel or a pure substance. The combustion of a fuel reduces order as does the releaseof a pure substance like carbon dioxide into the atmosphere and allowing it to mix and dilute.

The earth is an open system subject to a net influx of exergy from the sun. It is the exergy (or orderstates) delivered with solar radiation that is valued; all the energy received from the sun is ultimatelyradiated out to the universe. Environmental damage can be reduced by taking advantage of the opennessof the earth and utilizing solar radiation, instead of degrading resources found in nature to supply exergy

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demands. This would not be possible if the earth was a closed system, for it would eventually becomemore and more degraded, or "entropic."

Creation of chaos. The creation of chaos or disorder is a form of environmental damage.Fundamentally, entropy is a measure of chaos. A system of high entropy is more chaotic or disorderedthan one of low entropy. For example, system of the atmosphere and industrial carbon dioxide emissions,where the carbon dioxide is stored in canisters, is more ordered than the same system when the carbondioxide is emitted to the atmosphere.

Since exergy is a measure of order, the exergy of an ordered system is greater than that of a chaoticone, relative to the same environment.

The difference between the exergy values of the two systems containing papers described above is ameasure of the minimum work required to convert the chaotic system to the ordered one. The differencebetween the exergy of the systems with the atmosphere and carbon dioxide emissions is a measure of theminimum work required to extract the emitted carbon dioxide from the atmosphere. In reality, more thanthis minimum work, which only applies if a reversible clean-up process is employed, is required. Theexergy destroyed when effluents are released to the atmosphere is a measure of the order destroyedduring each process.

Chaos creation

Resource/orderdegradation

Waste exergyemissions

Process exergy efficiency (%)

Figure 1. The relation between the exergy efficiency of a process and the associated environmentalimpact in terms of creation of chaos, degradation of resources and order, and emissions of waste exergy,qualitatively depicted.

Discussion. The decrease in the environmental impact of a process, in terms of the three measuresdiscussed in this section, as the process exergy efficiency increases is depicted qualitatively andapproximately in Fig. 1. Exergy methods also correlate with sustainability, considering sustainability to bethe degree to which human activity can continue without facing a lack of resources or negatively changingthe environment. To illustrate better the relation between exergy and sustainability and environmentalimpact, the diagram in Fig. 1 can be modified to Fig. 2, where sustainability is seen to increase andenvironmental impact to decrease as the process exergy efficiency increases. The two limiting efficiencycases in Fig. 2 are significant. As exergy efficiency approaches 0%, sustainability approaches zero

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because exergy-containing resources are used but nothing is accomplished. Also, environmental impactbecomes increasingly great because, to provide a fixed service, an ever-increasing quantity of resourcesmust be used and a correspondingly increasing amount of exergy-containing wastes are emitted. Asexergy efficiency approaches 100%, environmental impact approaches zero, since exergy is convertedfrom one form to another without loss (either through internal consumptions or waste emissions). Alsosustainability becomes increasingly great because the process approaches reversibility (i.e., no lossesoccur so the process can go forwards and backwards indefinitely).

Environmentalimpact

Sustainability

o 100Exergy efficiency (%)

Figure 2. Qualitative depiction of the dependence on the exergy efficiency of a process and itsenvironmental impact and sustainability.

The ideas discussed in this section are illustrated for a unit of a typical 500-MWe coal-fired electricalgenerating station. Overall balances of exergy and energy for the station are illustrated in Fig. 3. For thisstation, which has been examined by the present author elsewhere [20], each of the three types ofenvironmental impact indicated by exergy and described in the previous subsections is demonstrated:

• Waste exergy emissions from the plant occur with the stack gases, the solid combustor wastes, andwaste heat released to the lake, which supplies condenser cooling water, and the atmosphere. Each ofthese emissions has a potential to impact the environment. The societal consensus regardingemissions of harmful chemical constituents in stack gases and "thermal pollution" in local bodies ofwater indicates that the potential for impact of these emissions is already recognized, but not from anexergy perspective.

• During the electricity generation process, a finite resource (i.e., coal) is degraded. Although a degreeof degradation is necessary for any real process, increased process efficiency reduces thisdegradation for the same services or products. If the process considered were made ideal, the exergyefficiency would increase from 37% to 100%, and coal use as well as related emissions woulddecrease by over 60%. These insights are provided by the exergy, but not energy, considerations.

• The creation of chaos is observed in 1) the degradation of coal to stack gases and solid wastes, since

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less exergy is present in the products of the process, and 2) the emission of material and heatproducts of combustion to the environment without constraint.

(a)

Air Ash

oStackGas

62

(b)

Figure 3. Balances of a) exergy quantities and b) energy quantities, for a unit of a typical500-MWe coal­fired electrical generating station, represented by the rectangle in the center of each diagram. Widths offlow lines are proportional to the relative magnitudes of the represented quantities, and the hatchedregion denotes the exergy consumption rate in the station. Exergy flow rates (positive values) and exergyconsumption rate (negative value) are given in part a, while energy flow rates are given in part b, with allvalues in MW. CW denotes cooling water.

It may seem contradictory to pointed out that exergy in the environment is of value when in the formof resources and problematic when in the form of emissions. The key to rationalizing these statements isto include the word restricted (see Fig. 4). In that figure, a comparison of unrestricted and restrictedexergy is presented which illustrates how exergy restricted in a system represents a resource, whileexergy emitted to the environment becomes unrestricted and represents a driving potential forenvironmental damage. Restricted sources of exergy in the environment are of value. Most resourcesfound in nature are restricted. Unrestricted emissions of exergy to the environment can cause problems.Two potential benefits result if emissions to the environment are restricted (e.g., separating out sulfurfrom stack gases): the potential for environmental damage is blocked from entering the environment, andthe now-restricted emission potentially becomes a valued commodity or source of exergy.

Unrestrictedexergy

(potential tocause change in

theenvironment)

Emissions of exergy to theenvironment

Restrictedexergy

(potential tocause change)

Figure 4. Simple illustration of an emission of exergy and the comparison it provides betweenunrestricted and restricted exergy.

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5. ANALYSIS AND COMPARISON OF SELECTED INDICATORS FORENVIRONMENTAL IMPACT OF EMISSIONS

5.1. Environmental Impact of Exergy Losses

As noted earlier, exergy losses, which are made up of exergy destruction and waste exergy emissions,have a significant effect on environmental impact. Exergy destruction, in particular, can be used as one ofthe most significant criteria for assessing the depletion of natural resources. Exergy analysis can assistefforts to minimize the use of natural resources, by indicating where the work potential of natural resourcesis lost relative to the surroundings, i.e., where exergy is destroyed.

The exergy in a flow can only be entirely converted to products in a reversible process, where no exergyis destroyed in the system and no waste exergy is emitted to the environment. A reversible process is atheoretical ideal which we can strive towards but never actually realize. In real processes, which areirreversible, both exergy destruction and waste exergy emissions occur. Much effort is being expended onreducing resource exergy destructions and eliminating waste exergy emissions, often by converting theminto useful products.

A waste emission possesses exergy as a result of its being in a state of mechanical, thermal and/orchemical disequilibrium with the reference environment. A material generally has two components ofexergy: physical and chemical. The exergy of an emission due to chemical disequilibrium (i.e., the chemicalexergy) is often significant and not localized. The exergy of an emission attributable to mechanical andthermal disequilibrium is not usually significant as the potential impact on the environment of physicalexergy is limited. That is, pressure differences between an emission and the environment normally dissipateshortly after the emission reaches the environment, and temperature differences are normally localized nearthe emission source (e.g., thermal pollution in the region of a lake near the cooling water discharge of athermal power plant) and can be controlled.

5.2. Exergy and Other Indicators

Based on a previous analysis [32], the exergy of waste emissions is compared here to other selectedmeasures to assess or control the potential environmental impact of emissions, including

• air emission limits established by the government of Ontario, and• two proposed quantifications of "environmental costs" for emissions from fossil fuel combustion.

These comparisons are carried out to detect trends and patterns that may permit the exergy of asubstance to be a useful indicator of potential environmental impact and consequently as a tool inestablishing emission limits that are rationally based rather than formulated by "trial and error."

Air pollution limits in Ontario, Canada are covered by that province's legislation on environmentalquality, the Environmental Protection Act. The overall goal of the legislation is to ensure environmentalconditions such that human health and the ecosystem of the earth are not endangered. In Ontario, theMinistry of the Environment oversees the development and implementation of environmental legislation forindustry. Allowable air emission limits (i.e., allowable mass of pollutant per volume of air averaged over aspecified time period), which must be achieved prior to discharge (e.g., pre-stack), are listed for numeroussubstances. The potential of a substance to impact on the environment is evaluated by ten parameters:transport, persistence, bioaccumulation, acute lethality, sub-lethal effects on mammals, sub-lethal effects onplants, sub-lethal effects on non-mammalian animals, teratogenicity, mutagenicity/genotoxictiy and

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carcinogemcity. With this information, Point of Impingement (POI) air emission limits are determinedthrough a method known as the best available pollution control technology.

Two methods for developing environmental costs for emissions to air are considered here:

• For air emissions from fossil fuel combustion, the cost is considered of removing the pollutants from thewaste stream prior to discharge into the environment. This cost can be related to the exergy of thepollution, and is referred to as the Removal Pollution Cost (RPC). The removal costs for wasteemissions are evaluated as the total fuel cost per unit fuel exergy multiplied by the chemical exergy perunit fuel exergy, and divided by the exergy efficiency of the pollution removal process. The exergyefficiencies for the removal of pollutants from waste streams vary. Some sources indicate that exergyefficiencies are less than 5% when the removal process involves mechanical separation. For simplicity,exergy efficiencies of 1% for all pollutants are used here.

• Estimate environmental costs of pollutant, which are referred to here as Environmental Pollution Costs(EPCs). Such work is most advanced for emissions released to the atmosphere resulting from thecombustion of fossil fuels, e.g., carbon dioxide, sulphur dioxide, etc. Environmental costs for some ofthese emissions have been estimated for Canada (see Table 2). Values for EPCs are based onquantitative and qualitative evaluations of the cost to society for the correction or compensation ofenvironmental damage, and/or the cost incurred to prevent a harmful emission from escaping into theenvironment.

Table 2. Environmental pollution cost (EPe) for selected pollutantsPollutant EPC (2006 Can. $/kg pollutant)*

Particulates (including heavy metals, e.g., lead, 4.95cadmium, nickel, chromium, copper, manganese,vanadium)CO 4.46NOx 3.50S02 3.19CH4 1.17Volatile organic compounds (VOCs) 0.54CO2 0.036* EPCs are based on values from 1990, with an adjustment applied to the dollar values toaccount for inflation in Canada between 1990 and 2006, as measured by the ConsumerPrice Index for all products. Statistics Canada reports the adjustment factor as 1.401, whichrepresents a 40.06% increase over the 16 year period or an average annual inflation rate of2.13%.

5.3. Comparison

POI air emission limits, standard chemical exergies, RPCs and EPCs for emissions have been studiedand some preliminary relations discerned. Some evidence suggests that EPC increases with increasingstandard chemical. Also, EPCs generally appear to increase at a decreasing rate with increasing percentagesof pollution emission exergy. These general trends are expected and logical.

Overall, the two measures considered here for the environmental cost of pollutants (RPCs which arebased on the cost to remove a pollutant from the environment, and EPCs which are based on the costs

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attributable to the negative impact of the pollutant on the environment), although based on differentprinciples and evaluated differently, are of the same order of magnitude for a given pollutant. The RPCmethodology is based on a simple, theoretical concept, while the EPC methodology relies on subjectiveinterpretations of environmental impact data. Thus, exergy-based measures for environmental impact mayprovide a foundation for rational environmental indicators and tools.

For clarity, it is reiterated that environmental pollution cost and removal pollution cost are simply twodifferent types of indicators, among the many existing and possible ones. They provide good examples forcomparisons with exergy as indicators of environmental impact, since different rationales underlie them.EPC is the environmental cost of a pollutant, based on such factors as the cost to society for the correctionor compensation of environmental damage, and the cost incurred to prevent a harmful emission fromescaping into the environment. RPC is the cost of removing a pollutant from a waste stream prior todischarge into the environment.

6. CONCLUSIONS

The relations between environmental impact and exergy in general and the chemical exergy of wasteemissions in particular, suggest that exergy could be a meaningful and useful indicator of the potential forenvironmental impact. An exergy-based indicator could contribute to the development of simple, rationaland objective procedures for assessing the harmful effects on the environment and predicting the potentialfor environmental impact for a substance. This view is primarily based on the general premise that asubstance has exergy due to its disequilibrium with respect to a reference environment, and therefore theexergy of unrestricted waste emissions has the potential to impact on the environment. The potentialusefulness of exergy analysis in addressing and solving environmental problems is significant, but furtherwork is needed if utilizable, objective exergy-based indicators of the potential of a substance to impact onthe environment are to be developed.

ACKNOWLEDGMENTS

Financial support for this work was provided by the Natural Sciences and Engineering ResearchCouncil of Canada and is greatly appreciated.

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