Exergy and industrial ecology: an application to an integrated energy ...

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Int. J. Exergy, Vol. 5, No. 1, 2008

Exergy and industrial ecology: an application to an integrated energy system Mikhail Granovskii, Ibrahim Dincer* and Marc A. Rosen Faculty of Engineering and Applied Science, University of Ontario Institute of Technology, 2000 Simcoe Street North, Oshawa, Ontario L1H 7K4, Canada E-mail: [email protected] E-mail: [email protected] *Corresponding author Abstract: Exergy analysis can help integrate separate technologies following the principles of industrial ecology. An application of exergy analysis to calculate depletion numbers, which relate exergy destruction and total exergy use, is demonstrated for a gas turbine cycle combined with a hydrogen generation unit. The design includes a Solid Oxide Fuel Cell (SOFC) with internal natural gas reforming and a Membrane Reactor (MR) in place of a combustion chamber. The depletion number for the separate technologies is found to be more than two times greater than for the combined system, implying the latter is more environmentally benign and like an ecosystem. Keywords: exergy; industrial ecology; hydrogen; fuel cell. Reference to this paper should be made as follows: Granovskii, M., Dincer, I. and Rosen, M.A. (2008) ‘Exergy and industrial ecology: an application to an integrated energy system’, Int. J. Exergy, Vol. 5, No. 1, pp.52–63. Biographical notes: Mikhail Granovskii received his BS and MS Degrees in Chemical Engineering from the Russian University for Chemical Technology (Mendeleev Institute, Moscow) in 1981. In 1984, he started working in the Chemistry Department at the Moscow State University. In 1993, he received his PhD Degree from the same university. He currently works as Senior Research Associate at UOIT. His major interests are exergy analysis of chemical and power generation technologies, mathematical modelling and heat transfer optimisation in industrial systems. Ibrahim Dincer is a Professor of Mechanical Engineering in the Faculty of Engineering and Applied Science at UOIT. He has authored/co-authored several books and book chapters, many journal and conference papers and numerous technical reports. He has chaired many conferences, symposia, workshops and technical meetings. He has delivered many keynote and invited lectures. He is an active member of various international scientific organisations and societies and serves as Editor-In-Chief, Associate Editor, Regional Editor and Editorial Board Member on various prestigious international journals. He is a recipient of several research, teaching and service awards, including the Premier’s research excellence award in Ontario, Canada in 2004. Marc A. Rosen is founding Dean of the Faculty of Engineering and Applied Science at UOIT and President-Elect of the Engineering Institute of Canada. He served as President of the Canadian Society for Mechanical Engineering and is a fellow of that society as well as the Engineering Institute of Canada, Copyright © 2008 Inderscience Enterprises Ltd.

Exergy and industrial ecology

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the American Society of Mechanical Engineers and the International Energy Foundation. He received an Award of Excellence in Research and Technology Development from the Ontario Ministry of Environment and Energy and a Mid-career Award from the University of Toronto. He previously was a Professor and Chair of the Department of Mechanical, Aerospace and Industrial Engineering at Ryerson University in Toronto, from which he received a university distinguished scholar award.

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Introduction

Industrial ecology is an approach to designing industrial systems that promotes systems that are less damaging to the environment. The approach seeks a reasonable balance between industrial profit and environmental stewardship and thereby can contribute to sustainable development. Industrial ecology methods can beneficially incorporate exergy to provide more powerful tools. Exergy analysis is a technique that uses the conservation of mass and conservation of energy principles together with the second law of thermodynamics for the analysis, design and improvement of energy and other systems (Dincer and Rosen, 2004). Exergy analysis pinpoints processes and devices where the most significant exergy losses, including exergy destruction (or entropy generation or irreversibility) occur. These exergy losses lead to non-recoverable losses of fuel exergy. It is generally accepted that an increase in the efficiency of fossil fuel utilisation makes industrial technologies more ecologically benign and often more safe. Therefore, exergy methods can help in rationally modifying contemporary technologies. Szargut (2005) cites the following example. In a combined power plant equipped with a coal boiler and gas turbine, the heat transfer exergy losses in the heat recovery boiler of the gas turbine can be reduced by shifting the steam superheater from the coal boiler to the heat recovery boiler of the gas turbine. In another example, from the chemical industry (Husain et al., 2003), energy and exergy analyses of a traditional one-stage crude oil distillation unit and a newly proposed two-stage crude oil distillation unit are conducted to investigate their energy and exergy efficiencies and exergy losses. The results are compared for both one- and two-stage distillation units. The proposed two-stage distillation unit exhibits a 43.8% decrease in the overall exergy losses and a 125% increase in the overall exergy efficiency, leading to the recommendation to perform distillation in two stages rather than one to reduce the heat duty of the heating furnace and thus reduce irreversible losses. The main objective of this paper is to show how exergy analysis helps in the integration of separate technologies according to the principles of industrial ecology and in the evaluation of such designs. An evaluation measure that reflects the efficiency of energy and fuel consumption is the depletion number, which represents the relationship between the exergy destruction rate and total exergy use rate for a system. An application of exergy analysis to help evaluate depletion numbers is demonstrated for a gas turbine cycle combined with a hydrogen generation unit. The example illustrates how depletion numbers for separate and combined technologies can be compared so as to assess the effectiveness of the integration.

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M. Granovskii et al.

Industrial ecology

Industrial ecology is concerned with shifting industrial processes from linear (open loop) systems, in which resource and capital investments move through the system to become wastes, to closed loop systems where wastes become inputs for other processes (Graedel and Allenby, 2003). Industrial ecology was popularised by Frosch and Gallopoulos (1989) who asked why industrial systems do not behave like an ecosystem, where wastes of one species are a resource to another species. Why should not the outputs of one industry be the inputs of another, thereby reducing the use of raw materials and pollution and saving on waste treatment? Lowe and Evans (1995) note that industrial ecology suggests using the design of ecosystems to guide the redesign of industrial systems to achieve a better balance between industrial performance and ecological constraints and consequently to determine a path to sustainable development. According to this conception, modern industrial technologies should be designed like ecosystems where •

input mass and energy flows are minimised

•

energy supply is provided by renewable energy sources.

Minimisation of the fossil fuel energy consumption in industrial processes implies eliminating output waste energy flows or the emission of wastes that are in equilibrium with the conditions (pressure, temperature, composition) of the environment. Applying these principles to industrial processes, like power generation and transportation, leads to several interesting observations. The technical ability to transform renewable energy to electricity for industrial and other needs is developed, but the relevant technologies involve significant consumptions of resources such as construction materials per unit of output generated and are often less beneficial economically and sometimes less attractive environmentally than traditional fossil fuel plants.

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Linkage between exergy and industrial ecology

Graedel (1996) writes, “The term industrial ecology was conceived to suggest that industrial activity can be thought of and approached in much the same way as a biological ecosystem and that in its ideal form it would strive toward integration of activities and cyclisation of resources, as do natural ecosystems.”

He goes on to note that little had been done at that time to explore the usefulness of the analogy. The use of exergy in conjunction with industrial ecology can provide a useful tool that permits practical applications (Connelly and Koshland, 2001; Dewulf and van Langenhove, 2002; Kay, 2002). Waste exergy emissions and exergy destructions, unlike energy losses, can account for the environmental impacts of energy utilisation (Dincer and Rosen, 2005). Szargut et al. (2002) suggest that the cumulative consumption of non-renewable exergy provides a measure of the depletion of non-renewable natural resources.


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Reducing entropy generation leads to a decline in exergy destruction (losses) Ex D due to reducing the irreversibility of the processes constituting an industrial system. According to the Gouy-Stodola formula, = T ⋅ S Ex D 0 gen

(1)

where T0 is the reference environment temperature (often fixed at 298 K or the local temperature) and Sgen is the entropy generation rate in a process or device.

3.1 Depletion number Connelly and Koshland (2001) suggest that the efficiency of fossil fuel consumption be characterised by a depletion number Dp: Dp =

Ex D Ex

(2)

in

and total exergy which represents the relation between the exergy destruction rate Ex D input rate Exin (in this paper only direct exergies are considered). In line with the definition of exergy efficiency, if there are no waste exergy emissions the exergy efficiency ψ is expressible as follows:

ψ = 1 − Dp

(3)

The exergy efficiency is always a measure of how nearly a process approaches the ideal.

3.2 Integrated systems The efficiency of integrated or combined technologies (e.g., cogeneration) can be evaluated and compared by examining the depletion numbers Dp for the separate and combined technologies (see Figure 1). The consumption of non-renewable energy resources corresponds to lower depletion numbers (see equation (2)). Consequently, the depletion number for an advanced combined technology D p(comb) should be lower than the weighted sum of the depletion numbers D p(sep) for the separate technologies. For the system in Figure 1, D p(sep) is expressible as follows: D p(sep) =

comb comb Ex Ex p1 p2 (1) D D (2) + p comb comb comb comb p Ex p1 + Ex p 2 Ex p1 + Ex p2

(4)

comb where D p(1) and D p(2) are depletion numbers for two separate technologies and Ex p1 comb and Ex are the rates of output exergy flows for products 1 and 2, respectively. p2

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Figure 1

4

Input and output exergy rates for separate and combined technologies to produce two products

Illustrative example

The principles discussed in this article are demonstrated for a gas turbine cycle combined with a hydrogen generation unit (Granovskii et al., 2006). This design includes two important technologies: a SOFC with internal natural gas reforming and a MR and their combination with a hydrogen generation unit. A common feature of SOFCs and MRs is their utilisation of high-temperature oxygen ion-conductive membranes. Such membranes are conductive to negatively charged ions of oxygen and permit the separation of oxygen from air. This property accounts for their application as an electrolyte in SOFCs, where the chemical exergy of methane, through an intermediate stage involving its conversion to hydrogen and carbon monoxide and electrochemical oxidation with oxygen, is transformed into electrical work. In a MR, the membrane conducts both oxygen ions and electrons in opposite directions; such membranes are consequently often called mixed conducting membranes. In the present case, electrical work is not generated, but oxygen is separated from air and fuel combustion proceeds in an atmosphere of oxygen. Oxygen ion conductive membranes are made of ceramic materials (usually zirconia oxides) and have good performance characteristics at temperatures higher than 700°C. An SOFC-stack is often introduced into traditional power generation cycles, where it operates at temperatures of 800–1100°C (e.g., Kuchonthara et al., 2003; Chan et al., 2002). A MR is being developed for operation up to 1250°C, as a substitute for combustion chambers in advanced zero-emission power plants (AZEPs) (e.g., Sundkvist et al., 2001). New materials for the anodes of SOFCs contain a catalyst for the methane reforming process, allowing methane conversion into a mixture of hydrogen and carbon


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monoxide directly on the surface of the anode (Weber et al., 2002; Eguchi et al., 2002). SOFCs thereby become more flexible, compact and effective and avoid the need for preliminary reforming of methane.

4.1 The considered gas-turbine combined cycle with hydrogen generation A combined gas-turbine cycle with a hydrogen generation unit is presented in Figure 2. The initial stream of natural gas, after heating in device 14 (in order to achieve after compression the temperature of combustion products) and compression in device 15, is divided into two flows. The first is mixed with combustion products (carbon dioxide and steam) and directed to the anodes of the SOFC stack (device 4), where two processes occur simultaneously: conversion of methane into a mixture of carbon monoxide and hydrogen on the surface of the anodes and electrochemical oxidation of the resultant mixture with oxygen. The oxygen reduction is accompanied by electricity generation in the SOFCs. The gaseous mixture from the anodes (conversion and combustion products) is cooled in a heat exchanger (device 10), compressed in device 11 and directed to the MR (device 1), where the remainder of the conversion products combust in oxygen and then expand in a turbine (device 2). Figure 2

An application of a SOFC and MR in a combined gas turbine cycle with a hydrogen generation unit

Numbers indicate devices according to the following legend: 1 – MR; 2,3,6,8 – turbines; 11,13,15 – compressors; 4 – SOFC stack; 5 – methane converter; 7,9,10,12,14 – heat exchangers; a – oxygen ion-conductive membranes; b, c – anode and cathode of SOFC stack, respectively.

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The combustion products are then divided into two flows. The first is mixed with the initial flow of methane and directed to the SOFC stack, while the other is mixed with the second flow of methane and enters the catalytic methane converter (device 5). After methane conversion to hydrogen and carbon monoxide in device 5, the gaseous mixture is expanded in a turbine (device 8), cooled in a heat exchanger (device 9) and directed to the shift reactor, where the remainder of the carbon monoxide and steam is converted to hydrogen. Air is heated in device 12, compressed in device 13, directed to the MR (device 1), where some quantity of oxygen is transferred through the oxygen ion-conductive membrane and combusted with fuel. The air heating in device 12 is required in order to achieve after compression the temperature of the fuel flow which is directed, like air, to the MR. The temperature of air reaches its maximum at the MR (device 1) outlet, at which point it is expanded in the turbine (device 3) and directed to the cathodes of the SOFCs (device 4). In the SOFCs, the oxygen concentration in the air decreases and the air is heated and enters the space between pipes in the catalytic converter (device 5). In device 5, heat is transferred from the air to the reaction mixture in the pipes. The mixture is then expanded in the turbine (device 6) and cooled in the heat exchanger (device 7). The power generation design combines a traditional gas turbine cycle – which consists of compressors (devices 11 and 13), a combustion chamber (represented by the MR, device 1) and turbines (devices 2 and 3) – with the SOFC stack (device 4) and the methane converter (device 5). Heat exchangers are conditionally divided into heat releasing (devices 7, 9 and 10) and heat receiving (devices 12 and 14) types. Mechanical work is produced in the turbines and consumed in the compressors. The work is transformed into electrical energy, which is also directly generated in the SOFC stack. The endothermic process of methane conversion to hydrogen (via a synthesis gas) in device 5 is implemented in the power generation cycle.

4.2 Exergy analysis of the system The general assumptions applied in the exergy analysis of the considered design follow: •

gases are modelled as ideal

•

energy losses due to mechanical friction are negligible

•

thermodynamic and chemical equilibria are achieved at the outlet of the SOFC stack and methane converter

•

all combustible components are combusted completely in the MR.

The general parameters used in the combined power generation cycle are listed in Table 1. Values for the parameters ηt, ηcmp, Pmax, Pmin and Tmax are often cited (e.g., Kirillin et al., 1979). An exergy balance of a system permits evaluation of the efficiency with which input energy flows are utilised. For the power generation scheme in Figure 2 the exergy balance can be expressed as = Ex − Ex ∆Ex in out = ΣWi + ∆ExT + ∑ ExDi

(5)


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is the rate of exergy change in the system, Ex where ∆Ex in is the sum of the exergy rates of the input flows of methane and air, Ex is the sum of the exergy rates of the out output flows of conversion products (synthesis gas) directed to the shift converter and exhaust gases, ΣWi is the sum of powers generated in the turbines and in the SOFCs and is the sum of thermal exergy consumed in the compressors (with a negative sign), ∆Ex T rates released in heat exchangers 7, 9 and 10 and consumed in 4 and 12 (with a negative sign) and ∑ Ex Di is the sum of the exergy desrtuction rates in the devices of the system. The analysis results are presented in Tables 2–4. Table 1

General parameter values for the combined power generation cycle (Figure 2)

Parameter

Value

Isentropic efficiency of turbines, ηt

0.93

Isentropic efficiency of compressors, ηcmp

0.85

Operational circuit voltage of the SOFC stack, V

0.85

Maximum pressure in the gas turbine cycle pmax, atm

10

Minimum pressure in the gas turbine cycle pmin, atm

1

Maximum temperature in the cycle (at the MR outlet) Tmax, K

1573

Temperature of fuel at the inlet of the SOFC stack Ts, K

1273

Temperature of fuel and air at the outlet of the SOFC stack Ts, K

1273

Ratio of methane combusted in the power generation cycle to the methane converted

1.0 : 0.7

Molar ratio of combustion products after the MR to methane combusted in the power generation cycle

6

Ratio of amounts of combustion products directed to SOFC and methane converter

1:1

Standard temperature T0, K

298

Standard pressure p0, atm

1 21% O2, 79% N2

Air composition, volume percentage Table 2

Generated work and exergy destruction for the processes in the combined gas-turbine cycle (Figure 2)*

Device number in Figure 2

W (kJ/mol)

ExD (kJ/mol)

2

89.7

1.6

3

207.1

4.1

4

497.4

29.4

6

85.0

2.3

8

35.6

0.2

11

–89.8

4.2

13

–324.4

22.3

15

–18.8

0.7

Total

481.8

64.8

*Data are given per mole of methane combusted in the power generation cycle.

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Table 3 presents the mechanical and electrical work generated in the turbines and SOFC stack, the mechanical work consumed in the compressors (with a negative sign) and the exergy destruction accompanying these processes. Table 4 presents the exergy destruction in the MR and methane converter. Table 4 also lists the exergy destruction ExDtr accompanying the heat transfer from hot to cool flows and the excess of thermal exergy ∆ExT which can be converted to mechanical work in a bottoming steam-water (Rankine) cycle (not shown in Figure 2) with an exergy efficiency ψR of about 60% (Cengel and Turner, 2005), so that

WR = ηR ∆ExT and ExDR = ∆ExT − WR . Table 3

(6)

Exergy destruction in the MR and methane converter*

Device number in Figure 2

ExD (kJ/mol)

1

27.6

5

15.9

Methane mixing

10.0

Total

53.5

*Data are given per mole of methane combusted in the power generation cycle, which corresponds to 0.7 mole of methane converted in methane converter 5. Table 4

Released thermal exergy ∆ExT and its utilisation in the Rankine bottoming cycle*

∆ExT (kJ/mol) 58.4

ExDtr (kJ/mol)

WR (kJ/mol)

ExDR (kJ/mol)

26.3

35.0

23.4

*Data are given per mole of methane combusted in the power generation cycle.

After substituting WR and ExDR into equation (5) instead of ∆ET, the exergy change ∆Ex = 684.8 kJ/mol in the system is distributed only between work W = 516.8 kJ/mol and the exergy destruction ExD = 168.0 kJ/mol. Since data are calculated per mole of methane combusted to generate electricity and 0.7 mol of methane converted to hydrogen and the value of standard exergy of methane 0 ExCH = 831.7 kJ/mol (Szargut et al., 1988), the depletion number of the combined 4 system D p(comb) becomes D p(comb) =

∑ ExDi 0 1.7 ExCH 4

=

168.0 = 0.12. 1414.0

(7)

The combined system yields two products: electricity and synthesis gas (a mixture of carbon monoxide and hydrogen). The exergy of electrical work is equal to its energy and the standard exergies of carbon monoxide and hydrogen are ExH0 2 = 236.1 kJ/mol and 0 ExCO = 275.1 kJ/mol (Szargut et al., 1988). Then the exergy of the synthesis gas directed 0 to the shift reactor to produce hydrogen (see Figure 1) is ExSG == 656.1 kJ/mol (for one mole of methane combusted and 0.7 mol of methane converted). The exergy efficiency of a combined gas turbine steam power cycle where only electrical work is generated is taken to be Ψ(1) = 0.54 (e.g., Cleveland, 2004) and the exergy efficiency of


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methane conversion to synthesis gas is Ψ(2) = 0.84 (e.g., Rosen and Scott, 1998). With equation (3), the depletion numbers are calculated as D p(1) = 0.46 and D p(2) = 0.16. Substitution of these values into the expression for D p(sep) (equation (4)) yields the following: D p(sep) =

0 ExSG W D p(1) + D p(2) = 0.29. 0 0 W + ExSG W + ExSG

(8)

The depletion number for the separate technologies D p(sep) is seen to be more than two times greater than that for the combined system D p(comb) . The implication is that the combined technology is more environmentally benign (and behaves more like an ecosystem) than the separate devices and requires combustion of less natural gas. The limiting value of D p(1) for the separate electricity generation process can be obtained by equalising D p(comb) = D p(sep) with the given value of D p(2) . In this case, the limiting value is found to be D p(1) = 0.068, which corresponds to an exergy efficiency of electricity generation Ψ(1) = 0.93 (equation (3)). This value is unrealistic, as it exceeds even the highest SOFC efficiency obtained in laboratory experiments (e.g., Larminie and Dicks, 2003). Thus, this magnitude of efficiency can be attained only through an integrated process like cogeneration. The conducted analysis confirms that integrated energy systems, developed via an appropriate combination of technologies, represent an important opportunity for increasing the utilisation efficiency of natural resources and thereby achieving the aims of industrial ecology.

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Concluding remarks

Industrial ecology is an approach that suggests designing industrial systems like ecosystems, where the wastes of one species are often the resource of another. Important ways of implementing industrial ecology include the appropriate combination of separate technologies in order to match the waste outputs of one with the inputs of the other and the introduction of processes that reduce non-renewable energy consumption. Exergy analysis can help in designing industrial systems that follow the principles of industrial ecology and in the evaluation of the efficiencies and losses for such activities. One such evaluation measure is the depletion number, which relates the exergy destruction and exergy input for a system. An example has been used to illustrate how to compare depletion numbers for separate and combined technologies, so as to assess the effectiveness of their integration. The analysis suggests that an exergy-based approach to industrial ecology can be advantageous in the creation and modification of industrial systems, through integrating separate technologies and other measures.

Acknowledgements The financial support of an Ontario Premier’s Research Excellence Award, the AUTO 21 Network of Centres of Excellence (NCE) and the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.

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Nomenclature Dp

Depletion number

ExD , ExD

Exergy destruction rate and specific exergy destruction (kJ/s, kJ/mol)

. .

Exin , Exin

Exergy input rate and specific input exergy (kJ/s, kJ/mol)

Exout , Exout

Exergy output rate and specific output exergy (kJ/s, kJ/mol)

.

.

Ex p , Ex p

Product exergy rate and specific product exergy (kJ/s, kJ/mol)

ExT , ExT

Thermal exergy rate and specific thermal exergy (kJ/s, kJ/mol)

MR

Membrane Reactor

p

Pressure, atm

.

SOFC

Solid Oxide Fuel Cell

T W ,W

Temperature (K) Power and specific work (kJ/s, kJ/mol)

Greek letters ψ

Exergy efficiency

ψR

Exergy efficiency of steam conversion to work

ηt, ηcmp

Isentropic efficiencies of turbines and compressors

Subscripts cmp

Compressor

max

Maximum

min

Minimum

R

Rankine cycle

s

Solid oxide fuel cell

SG

Synthesis Gas

t

Turbine

0

Reference environment

Superscripts comb

Combined

sep

Separate

0

Reference environment