Materials Transactions, Vol. 44, No. 7 (2003) pp. 1244 to 1250 Special Issue on Growth of Ecomaterials as a Key to Eco-Society #2003 The Japan Institute of Metals
Exergy Analysis to Evaluate Integrated Environmental Impacts Yoshihiko Soeno1; * , Hiromitsu Ino1 , Kiiti Siratori1 and Kohmei Halada2 1 2
College of Engineering, Hosei University, Koganei 184-8584, Japan National Institute for Materials Science, Tsukuba 305-0047, Japan
It is essential to evaluate the environmental impacts of materials or products properly for improving the global environment. However the typical method called Life Cycle Assessment (LCA) has many problems. Especially, it is crucial that various kinds of environmental impacts cannot be integrated. We present an attempt to use the exergy concept to integrate them. We classified the different types of exergy and analyzed the impact of the life cycle of beverage containers (aluminum can, PET bottle) and generation of electricity as examples, focusing on chemical and nuclear materials. (Received March 3, 2003; Accepted May 7, 2003) Keywords: exergy, material wastes, environmental impacts, aluminum can, PET bottle, electric power plant, radioactive waste
1.
Introduction
The technological progress depends on three elements, materials, energy and information. Above all, it is materials that embody environmental impacts. Thus, it is essential to evaluate the environmental impacts of materials or products properly for improving the global environments. However, the typical evaluation method called Life Cycle Assessment (LCA) has many problems. Especially, it is crucial that the various kinds of environmental impacts cannot be integrated on the reliable scientific basis. Here, we present an attempt of using the exergy concept to integrate them. Exergy is a gap of free energy between a given state and that of environment, which gives the maximum work. It has been used to analyze thermal efficiency of heat engines, smelting processes of metal and so on. In recent years, Ayres suggested to measure environmental loads by exergy.1) It describes the potential ability of attack on the environment when wasted. To apply the exergy to environmental problems, exergy of materials is important since chemical reactions of wastes, such as SOx , NOx , heavy metal, are main causes of environmental harm or damage. We focus on exergy of materials in this paper. 2.
very important for environment. Using them, we estimated the amount of hydrogen fluoride emission and revised the LCI data. Consumption of aluminum fluoride in LCI data is explained by this reaction. 2.2 Calculation of exergy Exergy flow in a process can be expressed as follows: x þ EHx þ Ex ERx ¼ EPx þ EM
ð1Þ
x , EHx and Ex denote the exergy of where ERx , EPx , EM resources, products, material wastes, exhaust heat and loss, respectively. The last one is corresponding to the production of entropy in the process. From the standpoint of improvement of energy efficiency, the reduction of total exergy consumption is a main concern. However, the exergy of x material wastes, EM , is important from environmental point of view, since actions of material wastes cause environmental x problems. Hence, we classify wasted exergy into EM and EHx . Exergy of substance can be calculated by a product of the amount of the substance and the exergy embodied in unit mass. The value of standard exergy for each substance was quoted from JIS Z 9204, Ref. 8). Using the method described above, we can verify mass flow and calculate exergy of processed materials.
Methods
2.1 Accounting of mass balance Exergy analysis in our study is based on our previous Life Cycle Inventory (LCI) data,2) in which original data were corrected the following the references.3–7) However, mass balance was not considered fully, in that study, as usual in the LCI investigation. Therefore, we should add several materials to each process using knowledge of chemical reactions. For example, to analyze the Hall-Heroult process in electrolytic smelting of aluminum, we assumed following two chemical reactions: ‹
2Al2 O3 þ 3C ! 4Al þ 3CO2
›
2AlF3 þ 3H2 O ! Al2 O3 þ 6HF
The second reaction is spurious for aluminum production, but *Graduate
Student, Hosei University.
2.3 Diagrammatic expression of exergy analysis Figure 1 shows the result of exergy analysis of the HallHeroult process, as an example. This diagram shows a new expression that we propose to depict both exergy and mass flow in a process. Three sides of an equilateral triangle represent resources, products and wastes of the process. The length of an arrow expresses the magnitude of exergy and the width the mass of materials. Using this representation, two essential quantities, exergy and mass, can be represented just in one figure. The length of a side of the triangle represents total mass of materials processed or mobilized in the process. 3.
Results of Analysis
3.1 Beverage containers Figure 2 shows the results of exergy analyses of 1000 aluminum cans in each section (material production, manu-
Exergy Analysis to Evaluate Integrated Environmental Impacts
Fig. 1
Exergy analysis of a Hall-Heroult process.
facturing, discarding and recycling) of a life cycle, with and without recycling, on the basis of LCI data.2) The numerical data are listed in Table 1. When recycling is not done, the exergy of total material wastes of the life cycle is 513 MJ,
1245
which can be calculated by summing up the wasted exergy of aluminum and the material wastes in each section (material production, manufacturing and discarding). By full ‘‘can to can’’ recycling, the exergy is drastically reduced to 11 MJ, because of eliminating the aluminum production and discarding section. By recycling, the reduction of total exergy input into the life cycle is from 2513 to 674 MJ. These values in the present work are changed from our previous estimation,9) since in the previous report, the aluminum scrap produced in the manufacturing process was out from the system boundary, treated as byproduct. It is much realistic that scrap in factory is recycled in the system as treated in the present paper. There is a risk of creating dioxin in the remelting process, since polyvinyl chloride is used for coating in many cases. However, this result does not take into account the evaluation. Figure 3 and Table 2 show the result of 1000 PET bottles based on the reference10) data. Without recycling, the exergy of material wastes from the life cycle is 991 MJ. By cascade recycling, the wasted exergy is reduced to 316 MJ, whereas, input exergy shows a 147 MJ increase. 675 MJ of exergy is
Fig. 2 Integrated Exergy and mass flows in the processes of life cycle of one thousand aluminum cans in cases of (a) recycling: 0% and (b) recycling: 100%.
1246
Y. Soeno, H. Ino, K. Siratori and K. Halada Table 1
Mass and exergy balances in each section in life cycle of 1000 aluminum cans. Mass of 1000 aluminum cans is 16 kg.
Material produnction of 16 kg of aluminum M/kg
Ex /MJ
OUTPUT
Electricity
—
1100
Aluminum
16
469
Bauxite
76.9
0
HF
0.343
2.62
Carbon anodes
7.5
180
Red mud
54.3
(0)
INPUT
M/kg
Ex /MJ
Synthetic cryolite
0.73
2.79
NaF
0.288
0.670
Aluninum flouride
0.47
1.57
Cl2
6.34
4.21
Limestone
2.51
0
H2
0.178
21.1
H2 O
10.6
0
CO2
66.6
30.6
Rock salt
10.7
0
SO2
0.296
1.42
Fuel
15.5
646
NO2
0.284
0.345
O2
56.4
6.96
H2 O
25.9
0
N2
0.086
0.00207
Others
10.0
0
Total
181
1937
Exegry loss
—
and exhaust heat
1407
Total
181
1937
OUTPUT
M/kg
Ex /MJ
Manufacturing of 16 kg aluminum cans INPUT
M/kg
Ex /MJ
Aluminum
27
791
Aluminum cans
16
469
Electricity
—
248
Aluminum scrap
11
322
Fuel
6.2
324
CO2
17.5
8.05
O2
24.3
3.0
SO2
0.0257
0.124
N2
0.0125
0.0003
NO2
0.0412
0.050
Total
58
1367
H2 O
12.99
0
Exegry loss and exhaust heat
—
Total
58
1367
568
Discarding of 16 kg of aluminum cans INPUT
M/kg
Ex /MJ
OUTPUT
M/kg
Ex /MJ
Electricity
—
0.01
Wasted aluminum
16
469
Used aluminum cans
16
469
CO2
0.0559
0.0257
Fuel
0.02
0.82
SO2
0.0000
0.00023
O2
0.0609
0.01
NO2
0.0001
0.00011
N2
2.88E-05
6.90E-07
H2 O
0.0226
0
Total
16.1
470
Exegry loss
—
and exhaust heat
1
Total
16.1
470
M/kg
Ex /MJ
Recycling of 16 kg of aluminum cans INPUT
M/kg
Ex /MJ
OUTPUT
Used aluminum cans
16
469
Recycled aluminum
16
469
Electricity
—
9
CO2
5.68
2.61
Fuel
2
88
SO2
0.0286
0.137
O2
6.89
0.85
NO2
0.0320
0.039
N2
0.0097
0.00
H2 O
3.065
0
Total
25
567
Exegry loss and exhaust heat
—
Total
25
95 567
Exergy Analysis to Evaluate Integrated Environmental Impacts Table 2
1247
Mass and exergy balances in each section in life cycle of 1000 PET bottles. Mass of 1000 PET bottles is 36 kilogram.
Material produnction of 36 kilogram of plastics INPUT
M/kg
Ex /MJ
OUTPUT
M/kg
Ex /MJ 758
Electricity
—
135
PET
33
Fuel
129
427
PP
2.9
133
Petroleum
77.9
2382
PS
1.3
53.6
Permanganate
10.8
8.32
LDPE
0.29
13.4
H2 O
140
0
Pyrochroite
6.08
5.86
O2
65.5
8.08
Potassium hydroxide
3.84
8.86
N2
0.137
0.003
CO2
28.4
13.1
Total
424
2960
SO2
0.293
1.41
NO2
0.450
0.546
H2 O
297
0
Byprpducts
51
(1206)
—
766
Total
424
2960
Exegry loss and exhaust heat
Manufacturing of 36 kilogram of PET bottles INPUT
M/kg
Ex /MJ
OUTPUT
M/kg
Ex /MJ
PET
32.8
758
PET bottles
36.32
936
PP
2.87
133
PET Scrap
0.96
22
PS
1.29
54
CO2
2.82
1.293
LDPE
0.29
13
SO2
0.0337
0.162
Electricity
—
0.63
NO2
0.0114
0.014
Fuel
1.32
33
H2 O
4.79
0
O2
6.33
0.781
Exegry loss
—
33
N2
0.003
0.0001
and exhaust heat
Total
45
992
Total
45
992
INPUT
M/kg
Ex /MJ
OUTPUT
M/kg
Ex /MJ
Used PET bottles
36.32
936
Wasted PET bottles
36.32
936
Fuel
1.62
50
CO2
3.46
1.59
O2
7.88
0.972
SO2
0.009
0.042
N2
0.00
0.0001
NO2
0.014
0.017
Total
46
987
H2 O
6.02
0
—
50
Total
46
987
Discarding of 36 kilogram of PET bottles
Exegry loss and exhaust heat
Cascade recycling of 36 kilogram of PET bottles INPUT
M/kg
Ex /MJ
OUTPUT
M/kg
Ex /MJ
Used PET bottles
36.32
936
Recycled PET
25
675
Electricity
—
145
PET Scrap
11.32
261
Fuel
1.7
52
CO2
3.56
1.64
O2
8.35
1.03
SO2
0.009
0.0431
N2
0.005
0.0001
NO2
0.014
0.0175
Total
46
1134
H2 O
6.21
0
—
196
46
1134
Exegry loss and exhaust heat Total
1248
Y. Soeno, H. Ino, K. Siratori and K. Halada
impacts due to construction and discarding of these plants. Exergy of the radioactive waste was calculated by decay heat integrated from 101 to 2 108 s by using an approximation formula proposed by the American Nuclear Society (ANS).12) This formula was given to evaluate the decay heat released after the shut down of a reactor and has been widely accepted. When 1000 MJ of electricity is produced, exergy of material wastes from the processes of oil fired and nuclear power plant are 62 MJ and 8.9 MJ, respectively. The main part, 97% quantitatively, from the oil-fired power plant is caused by CO2 . This exergy is due to concentration difference from 300 ppm in atmosphere. In the case of the nuclear power plant, the main part of wasted material exergy is radioactivity, which makes up 96%. This value (8.5 MJ) is much higher than that of SOx and NOx (2.1 MJ) from the oilfired power plant. 4.
Fig. 3 Integrated Exergy and mass flows in the processes of life cycle of one thousand of PET bottles in cases of (a) recycling: 0% and (b) cascade recycling: 100%.
produced as recycled PET (cascaded materials), accompanied with 261 MJ of exergy of scrap. Figure 4 shows the results of the cases of full cascade recycling and incineration, adding fuel and electricity production process, whose results are shown in Table 4. In the next section, the analysis of the electricity production is discussed. By incineration, the exergy of material wastes are reduced from 999 to 126 MJ. However, the toxicity of substances contained in PET, for example antimony, which is used for polymerization catalyst, is not take into account in this analysis. 3.2 Generation of electricity In this work, we analyzed oil fired and nuclear power plant based on the data of reference.11) The result is shown in Table 3. These results show the potential impacts of operation only. In other words, we did not analyze the
Discussion
4.1 Standard exergy In this paper, standard exergy is quoted from reference.3) The state of environment cannot be determined uniquely, because it changes from place to place. In the case of Ayres’s analysis, crust, ocean or atmosphere is defined as standard or zero exergy point. In the Hall-Heroult process, according to Ayres, 56 MJ of electricity, 11 MJ of heat and 22 MJ of feedstock are put into the process, and 33 MJ of aluminum, 9 MJ of material wastes and 48 MJ of waste heat are output.1) One of the reasons of the discrepancy between Ayres and our analysis is the difference of standard exergy of alumina. Ayres set the exergy of alumina 1.96 MJ/kg, whereas that is zero in our study. The reason why we used JIS standard is to make the chemical exergy clear. It should be careful to compare different exergy analyses. Standard exergy depends on the premise of the state of environment. 4.2 Types of exergy In this study, we considered three types of exergy: thermal exergy, chemical and nuclear exergy, and exergy caused by concentration difference. Each exergy has different action style. The classification of them is needed.
Fig. 4 Exergy analyses of life cycle of one thousand of PET bottles.
Exergy Analysis to Evaluate Integrated Environmental Impacts Table 3
1249
Exergy analysis in production processes of electricity and fuel.
(a) Oil fired power plant (1000 MJ) INPUT
OUTPUT M/kg
Ex /MJ
Fuel
57.2
2728
Electricity
O2
266
32.8
CO2
N2
0.149
0.004
SO2
0.306
1.47
323
2761
NO2
0.491
0.60
H2 O
191
0
Exergy loss and exhaust heat
—
1698
Total
323
2761
M/kg
Ex /MJ
—
1000
0.00126
8.5
4.42
0
Total
M/kg
Ex /MJ
—
1000
131.3
60.3
(b) Nuclear power plant (1000 MJ) INPUT
OUTPUT M/kg
Ex /MJ
Uranic ores
4.42
3138
Electricity
Fuel
0.349
11.8
Radioactive waste
O2
1.587
0.196
Residual dross
N2
0.00029
0.00001
CO2
0.820
0.376
6.35
3150
SO2
0.00350
0.017
NO2
0.00096
0.001
H2 O
1.11
0
—
2141
6.35
3150
M/kg
Ex /MJ
Total
Exergy loss and exhaust heat Total (C) Production process of fuel (1000 MJ) INPUT
OUTPUT M/kg
Resource Fuel
Ex /MJ
20
1000
Produced fuel
20
1000
0.740
35.87
CO2
1.60
0.737
—
52.47
SO2
0.00143
0.00685
O2
1.17
0.144
NO2
0.00328
0.00398
N2
0.001
0.000024
H2 O
0.301
0
Total
21.9
1088
—
87.74
21.9
1088
Electricitty
Exergy loss and exhaust heat Total
Table 4 shows the exergy balance in the Hall-Heroult process in detail. According to this table, the main substances of environmental potential are HF and SO2 . This is consistent with the toxic point of view. Chemical and nuclear exergy has driving force to attack environments. Hence, material wastes should be discarded without chemical or nuclear exergy.
nuclei, dioxin and so on, are examples of type 2. They damage ecosystems through the disturbance information of living systems. Damage of gene due to radioactively is the typical example. Exergy analysis is a useful method in the case of type1, but it is difficult to evaluate the effect of information disturbances because they are magnified extremely by living system itself.
4.3 On the evaluation of toxic substances using exergy Considering the toxicity of radioactivity, the present result for nuclear power plant is not sufficient. Following two types of effect should be distinguished. Type 1: the case that exergy attacks structure of ecosystems. Type 2: the case that exergy attacks information of living systems. SOx , NOx and so on, are examples of type 1. Their chemical exergy attack structure of ecosystem such as trees, human bodies etc. Toxic substances such as radioactive
4.4
Evaluation of beverage containers and the effect of recycling We analyzed two kinds of beverage containers and showed the effect of recycling, focusing on the exergy of material wastes. Environmental impacts due to the discarding processes were evaluated by the amount of discarded exergy. Thus, we could evaluate the recycling effect by quantifying the exergy reduction discarded into environment. According to the results of analysis of life cycle of aluminum cans, ‘‘can to can’’ recycling is effective, if the
1250
Y. Soeno, H. Ino, K. Siratori and K. Halada Table 4
INPUT Alumina
Exergy analysis in Hall-Heroult process. (1)
(2)
Total
Ex /MJ
Ex /MJ
Ex /MJ
0
0
0
Synthetic cryolite
0.471
0
0.471
Aluninum flouride
0.063
0
0.063
Carbon anodes
11.34
0.11
11.45
Water
0
Fuel
0 11
0 11
O2
0
0.065
0.065
N2
0
0.00001
0.00001
Electricity
—
—
58
(1)
(2)
Total
Ex /MJ
Ex /MJ
Ex /MJ
Total OUTPUT
81
Al
29.3
0
29.3
Alumina
0
0
0
HF
0.164
0
0.164
NaF
0.042
0
0.042
Waste solids
0
0
0
Acknowledgements The authors would like to express their thanks to Dr. H. Koide of the Research Reactor Institute, Kyoto University, for giving them information of the approximation formula on the decay energy of a Uranium fueled reactor, provided by the American Nuclear Society.
CO2
0
0.833
0.833
NO2
0.0017
0.0001
0.0018
SO2
0.0398
0.0005
0.0403
Water
0
0
0
REFERENCES
—
—
51
1) Robert U. Ayres and Leslie W. Ayres: Accounting for Resources, 2, (Edward Elgar Pub., 1999) pp. 1–61. 2) K. Nakajima, H. Ino and K. Halada: J. Japan Inst. Metals 64 (2000) 591–596 (in Japanese). 3) NEDO, International Investigation on establishment of energy consumption evaluation procedure, (1995) pp. 76–131. 4) A.-M. Tillman, H. Baumann, E. Eriksson and T. Rydberg: Packaging and the Environment, Chalmers Industriteknik, (1991) pp. 1–53. 5) The society of Non-Traditional Technology: Fundamental investigation on establishment of environmental road evaluation procedure, (1995) pp. 5–109. 6) The Japan industrial furnace manufactures association, Industrial Furnace Handbook, (1997) pp. 93–193. 7) NEDO, International Investigation on establishment of energy consumption evaluation procedure, (1996) pp. 34–75. 8) Japanese Industrial standards Committee: JIS Z 9204 (1991) pp. 1–33. 9) Y. Soeno, Y. Akashi, H. Ino, K. Siratori, K. Nakajima and K. Halada: J. Japan Inst. Metals 66 (2002) 885–888 (in Japanese). 10) Project on the comparison of environmental loads of beverage containers: Report on the comparison of environmental loads of beverage containers by LCA (revised version), (2001) pp. 1–94, (in Japanese). 11) H. Hondo, Y. Uchiyama and Yue Moriizumi: Socio-economic Research Center, Reo. No, Y99009, pp. 11–31, (in Japanese). 12) ‘‘Decay Energy Release Rate Following Shutdown of Uranium Fueled Reactor’’, American Nuclear Society STANDARD, pp. 287–300.
Exergy loss and exhaust heat Total
81
(1) Chemical exergy (2) Exergy caused by concentration difference
creation of dioxin can be avoided in remelting process. It is needed that no-PVC-coating is established. From the results of PET bottles, cascade recycling creates no less than 261 MJ of scrap and incineration generates only 84 MJ of electricity, when 1000 bottles are processed. The cascade recycling and incineration process of PET leave much room for improvement for reducing the environmental impact. 5.
(1) We showed the importance of classification of each type of exergy. Especially, it is important to distinguish between thermal exergy, chemical and nuclear exergy, and exergy caused by concentration difference, for discussing the environmental impacts. (2) Standard exergy from the environmental point of view has not defined uniquely. It is needed to additional discussion about it for establishing useful exergy analysis as world standard. (3) In the case that exergy attacks information of living systems, exergy analysis is not sufficient. Therefore, we should consider other factors for the integrated evaluation of all kinds of environmental impacts of a given system or life cycle. (4) Our study provided new view of the environmental impacts of discarding processes and made the effect of recycling clear.
Conclusion
(0) We would like to insist the importance of the exergy of discarded materials to evaluate environmental impacts of life cycle of products.
