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Carbon Exergy Footprint to Evaluate the Greenhouse Impact of Operating Units
Available online at www.sciencedirect.com
ScienceDirect Energy Procedia 105 (2017) 3179 – 3184
The 8th International Conference on Applied Energy – ICAE2016
Carbon exergy footprint to evaluate the greenhouse impact of operating units Zhen Qina,b, Xiaomei Wua,b, Zaoxiao Zhanga,b,* a
School of Chemical Engineering and Technology, Xi’an Jiaotong University, No.28 Xianning West Road, Xi’an 710049, P.R. China b State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, No.28 Xianning West Road, Xi’an 710049, P.R. China
Abstract Increasingly serious global warming caused by greenhouse gases has urged more and more focus on the sustainability. Exergy and carbon footprint are generally considered as two separate indicators to assess the greenhouse effect and sustainability of industrial production. In this paper we propose carbon exergy footprint (CEF) for quantitatively associating carbon footprint with exergy to evaluate the greenhouse effect of operating units. The indicator synthesizes the advantages of exergy and carbon footprint. Specific physical meaning for this novel indicator is illustrated on the basis of the second law of thermodynamics and life cycle assessment. The process of water-gas shift is chosen as the case for elaborating the behavior of CEF. Results show that CEF is competent for the objective evaluation for the greenhouse effect of the operating units. Additionally, CEF also maintains consistency with the conventional exergy analysis in terms of sustainability. © 2017 Published by Elsevier Ltd. This © 2016The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE
Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy.
Keywords: Carbon exergy footprint; operating unit; greenhouse impact; indicator
1. Introduction In recent years more and more attention has been focused on sustainability, especially ecological sustainability, due to the evident contradiction between increasing economic development and its damage to the environment. 81.4% of the world’s primary energy supply in 2013 was originated from the fossil fuels (coal, oil and natural gas) according to IEA energy statistics [1]. It has been proved that as long as the fossil fuels are utilized for energy demand, inevitable greenhouse gas (GHG) associated with energy
* Corresponding author. Tel.: +86-29-82660689; fax: +86-29-82660689. E-mail address: [email protected] (Z. Zhang)
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.693
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Zhen Qin et al. / Energy Procedia 105 (2017) 3179 – 3184
conversion would finally release into the atmosphere in the perspective of the second law of thermodynamics and life cycle assessment (LCA) [2]. In chemical industry, energy conversion is always accompanied with the element metabolism, and operating units plays an indispensable role for the energy conversion. Thus the operating unit is certainly regarded as an individual unit of the industrial process and its sustainability deserves further discussion to guide the countermeasures for reducing the energy degradation and GHG emission. Nowadays, some indexes have been proposed to assess the sustainability and can be divided into two categories: “implicit indexes” and “explicit indexes”. The formers, just as energy and exergy, are deduced by the state parameters of the defined objective. Higher exergy efficiency makes a better sustainability. Advanced indicators like avoidable and unavoidable exergy destructions, cumulative exergy and carbon exergy tax have been presented to link different parameters together with the specific physical meanings [3-5]. The latter, just as GHG emissions and footprints, is defined as the amount of wastes poured into the environment by the unit of mass flow or volume flow. The less emissions and footprints human discharge, the better sustainability would be kept. GHG emission is the most intuitive metric for assessing the damage to ecological environment in perspective of greenhouse effect, and it is frequently used by governments to declare the specific GHG reduction target in some reports. In the recent years, footprint families have also become popular indicators to monitor the impact on sustainability [6, 7]. A portion of fuel consumption is due to the exergy destruction and loss in operating units, leading to superfluous carbon emissions beyond the theoretical horizon. Namely, inefficient energy use contributes to the undeserved part of carbon footprint. Thus, it is essential to establish the correlation between exergy and carbon footprint to discover the deeper potentiality of mitigation technologies. Numerous researches have studied the performance of retrofitted system in the perspective of energy, economy and environmental impact with a global viewpoint. However, few focused on the correlation between exergy and carbon footprint in terms of operating units. In this study, we consider exergy and carbon footprint as the artistically joint indicators for sustainability. We propose an indicator, carbon exergy footprint (CEF), to integrate these two indexes together to evaluate the greenhouse effect of operating units. The established indicator is based on the second law of thermodynamics and life cycle assessment. It compromises the merits of exergy analysis and carbon footprint analysis to intuitively evaluate the performance of operating units and process in a local or holistic perspective. In Section 2, we elaborate the relation between exergy and carbon footprint and then propose the definition of CEF. The process of water-gas shift (WGS) is chosen as the case for illustrating the behavior of CEF in Section 3 and the results will be discussed in Section 4. 2. Carbon exergy footprint definition Exergy is the combination of the quantity and quality of energy, practically representing the maximum useful work during a process when the system is brought into the reference-environment state or dead state [8]. In a specific system, exergy destruction occurring in the operating units is constitutionally derived from the original inlet exergy. In industrial application, the difference of temperature, pressure and chemical potential considered as the driving forces for the actual process is unavoidable in operating units. Carbon footprint is a member of footprint families [6] and is generally defined as the total carbon emissions including direct and indirect GHG caused by product, person, organization or activity over a full life cycle to evaluate the carbon intensity. It is an indicator for environmental impact and sustainability while exergy efficiency represents the design level of operating units or technological process.
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Zhen Qin et al. / Energy Procedia 105 (2017) 3179 – 3184
Fig. 1. Qualitative description of the influence of exergy efficiency on sustainability and environmental impact [8]
It is observed in Fig. 1 that as the exergy efficiency increases, the sustainability increases and environmental impact decreases. Therefore, the qualitative description also reveals the feasibility for linking carbon footprint with exergy in a quantitative expression. Based on the analogy with the definition of carbon footprint, carbon exergy footprint (CEF) is defined as the total carbon emission related with the operating unit, including the direct GHG generated by chemical reaction and indirect GHG caused by exergy destruction. It is an indicator for assessing the environmental impact of operating units and correlating carbon footprint with exergy. Thus the expression of CEF is defined as:
CEF ᷉
mi i
m j j᷊ ᷉ f Exd᷊
i
(1)
j
where m is the mass flow of the stream (kg/h); the subscripts, i and j, represent the outlet and inlet stream, respectively; is the mass fraction of GHG in the stream. The bracketed subtraction of the first two items represents a part of CEF caused by the composition change in chemical reaction; f (Exd) stands for another part of CEF caused by exergy destruction, including physical exergy destruction only. Wherever the exergy supplement is originated from, complete combustion of coal in boiler can be considered as the ultimate thermal resource for all heat demand. In terms of exothermal operating units, such as the unit of WGS, if the released heat is utilized in a higher efficient way, the consumption of feedstock coal would be kept in a comparatively lower level. Besides, the exergy destruction varies with different operating conditions. Therefore, in accordance with the terminal fuel consumption for exergy destruction, f (Exd) is expressed as
᷉ f Exd᷊
Exd mCO2 LHVcoal
(2)
where Exd is the exergy destruction (kJ/h); LHVcoal is the low heat value of the reference coal (kJ/kg coal); mCO2 is the amount of CO2 when 1kg coal is burned completely (kg/kg coal). In this study, kinetic and potential exergy are normally neglected and the reference environment is set at 25 ć and 1atm. 3. Case study To analyze the behavior of the proposed CEF, a part of the water-gas shift section is used as the example because it covers typical operating units, including reactor, electrical equipment and heat exchanger. The flow sheet is shown in Fig. 2. Water-gas shift reaction, CO+H2OėCO2+H2, is used for adjusting the H/C ratio in many chemical synthesis processes. Crude syngas with saturated steam from gasification section is first preheated by hot shifted gas to the temperature of about 280 ć and then enters high temperature shift reactor (R101),
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where the concentration of carbon monoxide (CO) is reduced to approximately 3.7% (vol.). For heat recovery, the outlet stream passes through the steam superheater (about 4.1 MPa) and then preheats the crude syngas. After that, syngas continues to be cooled down to 270 ć in E103 and then enters the low temperature shift reactor (R102) for further conversion. The temperature and CO concentration of the outlet syngas are 295 ć and 1.1%, respectively. The parameters of this process are extracted from a commercial chemical plant, as shown in Table 1.
Fig. 2. Flow sheet of WGS section Table 1. Basic parameters for case study Item
Description
Reference coal
Illinois 6# [9]
Mass flow rate of Stream 101
351714 kg/h
Mass flow rate of Stream 121
35865 kg/h
4. Results and discussion Exergy is calculated based on the properties of the streams, and the results of exergy destruction of the six operating units are shown in Table 2. In this section, we will discuss the behaviors of CEF for the three types of operating units, i.e., heat exchanger, reactor and pump. Table 2. Results of carbon exergy footprint calculation Operating unit
Exergy destruction (kJ/h)
CEF by exergy destruction (kg CO2,e/h)
CEF by the change of components (kg CO2,e/h)
Total CEF (kg CO2,e/h)
Specific CEF (kg CO2,e/MJ)
E101
3913000
336.04
0
336.04
11.87
E102
2336600
200.66
0
200.66
13.56
E103
7613230
653.81
0
653.81
8.11
R101
-38672000
-3321.09
138742.10
135421.01
1.045
R102
-4010000
-344.37
20736.00
20391.63
1.053
Pump
24446
2.10
0
2.10
36.76
Zhen Qin et al. / Energy Procedia 105 (2017) 3179 – 3184
4.1. Heat exchanger The CEF of heat exchanger is merely decided by exergy destruction because the components do not change in the heat exchange. It can be inferred from Equation (2) that the total CEF increases with the increase of exergy destruction. Though the exergy destruction and CEF of E102 are smaller than those of E101 and E103, it is seen that the specific CEF of E102 is the largest one. Table 2 shows that the specific CEFs of the three exchangers follow the order of E102˚E101˚E103, which fits in with the order of the log-mean temperature difference (LMTD). The reason is that temperature difference is conducted as the driving force for the irreversible process of heat transfer but also the causation for exergy destruction. On the basis of the same heat duty, the more exergy destruction occurred in the heat exchanger, the more exergy from feedstock coal is needed to make up this gap. Therefore, the specific CEF of heat exchanger can be characterized by the temperature difference of heat transfer. 4.2. Reactor The negative exergy destruction in reactors is due to the exclusion of chemical exergy when the exergy is calculated. Actually, exergy is considered to consist of four major parts: potential, kinetic, physical and chemical. Kinetic and potential exergy are normally negligible in many industrial cases. Physical exergy is characterized by the change of temperature and pressure from system state to reference-environment state while chemical exergy is expressed by the change of composition from reference-environment state to dead state. In the operating unit with chemical reaction, both physical and chemical exergy change due to the change of temperature and compositions, where the existing form of carbon element commonly varies with the demand of chemical production. By contrast, in the operating unit without chemical reaction, the destruction includes only physical exergy destruction. Therefore, in the perspective of greenhouse effect, the CEF caused by the change of composition is considered to correspond to the partial chemical exergy destruction. The conversion ratios of carbon monoxide in R101 and R102 are 82.8% and 70.4%, respectively. The amount of heat released from R101 is about 6.69 times larger than that of R102. Thus the absolute value of exergy destruction in R101, that is, the increase of physical exergy contributed by exothermal reaction, is larger than that of R102. On the other hand, the higher conversion ratio of CO in R101 leads to the higher CEF caused by the change of compositions. Consequently the total CEF of R101 is much larger than that of R102. Therefore, the total CEF of operating unit with chemical reaction is a balance of the reaction heat and the conversion of compositions. 4.3. Pump The total input exergy of pump consists of both electricity and inlet stream. The pump efficiency is set as 0.67 and the electricity consumption is calculated as 90015.52 kJ/h. If the pump efficiency is increased to 80%, it can be evaluated that the specific CEF decreases by 8.9% due to the higher exergy efficiency of electricity. Therefore, it is suggested that improving the exergy efficiency is a feasible way to deduce the CEF of electrical equipment. Additionally, compared with conventional life cycle assessment, the result of CEF in this study is obviously different from carbon footprint (CF) calculation, i.e. carbon footprint = emission factor × energy consumption. If the emission factor is set at 0.9746 kg CO 2, e/kWh [10], the carbon footprint is obtained as 24.37 kg CO2, e/h, about 11.6 times than the CEF. The concept of carbon footprint is based on the environmental impact of energy consumption in life cycle scale, considering all the process of energy
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conversion. Differently, CEF indicates the GHG impact in terms of the operating unit. Thus CEF could be considered as a part of CF in a sense. 5. Conclusions In this study, a novel indicator, carbon exergy footprint (CEF), is proposed to quantitatively associate carbon footprint with exergy to evaluate the greenhouse effect of operating units. On the basic case study of water-gas shift process, the behavior of CEF is illustrated by discussing three types of operating units at different operating conditions. Results show that the temperature difference, both quantity of heat and element conversion and electrical efficiency are the key factors that influence the CEFs of heat exchanger, reactor and pump, respectively. The results of CEF analysis show good accordance with the conventional exergy analysis. Thus it is possible to use the concept of carbon exergy footprint to compare the performance of operating units. Acknowledgements Financial supports of the National Natural Science Foundation of China (No. 51276141) is gratefully acknowledged. References [1] http://www.iea.org/stats/WebGraphs/WORLD4.pdf. [2] Messagie M, Mertens J, Oliveira L, Rangaraju S, Sanfelix J, Coosemans T, et al. The hourly life cycle carbon footprint of electricity generation in Belgium, bringing a temporal resolution in life cycle assessment. Appl Energ. 2014;134:469-76. [3] Callak M, Balkan F, Hepbasli A. Avoidable and unavoidable exergy destructions of a fluidized bed coal combustor and a heat recovery steam generator. Energ Convers Manage. 2015;98:54-8. [4] Li H, Chen J, Sheng D, Li W. The improved distribution method of negentropy and performance evaluation of CCPPs based on the structure theory of thermoeconomics. Appl Therm Eng. 2016;96:64-75. [5] Massardo AF, Santarelli M, Borchiellini R. Carbon exergy tax (CET): its impact on conventional energy system design and its contribution to advanced systems utilisation. Energy. 2003;28:607-25. [6] Čuček L, Varbanov PS, Klemeš JJ, Kravanja Z. Total footprints-based multi-criteria optimisation of regional biomass energy supply chains. Energy. 2012;44:135-45. [7] Qin Z, Zhai G, Wu X, Yu Y, Zhang Z. Carbon footprint evaluation of coal-to-methanol chain with the hierarchical attribution management and life cycle assessment. Energ Convers Manage. 2016;124:168-79. [8] Dincer I, Rosen MA. Exergy: energy, environment and sustainable development: Newnes; 2012. [9] Li S, Jin H, Gao L. Cogeneration of substitute natural gas and power from coal by moderate recycle of the chemical unconverted gas. Energy. 2013;55:658-67. [10] Shen W, Cao L, Li Q, Zhang W, Wang G, Li C. Quantifying CO2 emissions from China’s cement industry. Renew Sust Energ Rev. 2015;50:1004-12.
Biography Professor Zaoxiao Zhang works in the School of Chemical Engineering and Technology, Xi’an Jiaotong University. The research areas are focused on CO 2 capture and sequestration technologies, energy storage and utilization, and new type of chemical machinery.
ScienceDirect Energy Procedia 105 (2017) 3179 – 3184
The 8th International Conference on Applied Energy – ICAE2016
Carbon exergy footprint to evaluate the greenhouse impact of operating units Zhen Qina,b, Xiaomei Wua,b, Zaoxiao Zhanga,b,* a
School of Chemical Engineering and Technology, Xi’an Jiaotong University, No.28 Xianning West Road, Xi’an 710049, P.R. China b State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, No.28 Xianning West Road, Xi’an 710049, P.R. China
Abstract Increasingly serious global warming caused by greenhouse gases has urged more and more focus on the sustainability. Exergy and carbon footprint are generally considered as two separate indicators to assess the greenhouse effect and sustainability of industrial production. In this paper we propose carbon exergy footprint (CEF) for quantitatively associating carbon footprint with exergy to evaluate the greenhouse effect of operating units. The indicator synthesizes the advantages of exergy and carbon footprint. Specific physical meaning for this novel indicator is illustrated on the basis of the second law of thermodynamics and life cycle assessment. The process of water-gas shift is chosen as the case for elaborating the behavior of CEF. Results show that CEF is competent for the objective evaluation for the greenhouse effect of the operating units. Additionally, CEF also maintains consistency with the conventional exergy analysis in terms of sustainability. © 2017 Published by Elsevier Ltd. This © 2016The TheAuthors. Authors. Published by Elsevier Ltd.is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE
Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy.
Keywords: Carbon exergy footprint; operating unit; greenhouse impact; indicator
1. Introduction In recent years more and more attention has been focused on sustainability, especially ecological sustainability, due to the evident contradiction between increasing economic development and its damage to the environment. 81.4% of the world’s primary energy supply in 2013 was originated from the fossil fuels (coal, oil and natural gas) according to IEA energy statistics [1]. It has been proved that as long as the fossil fuels are utilized for energy demand, inevitable greenhouse gas (GHG) associated with energy
* Corresponding author. Tel.: +86-29-82660689; fax: +86-29-82660689. E-mail address: [email protected] (Z. Zhang)
1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.693
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Zhen Qin et al. / Energy Procedia 105 (2017) 3179 – 3184
conversion would finally release into the atmosphere in the perspective of the second law of thermodynamics and life cycle assessment (LCA) [2]. In chemical industry, energy conversion is always accompanied with the element metabolism, and operating units plays an indispensable role for the energy conversion. Thus the operating unit is certainly regarded as an individual unit of the industrial process and its sustainability deserves further discussion to guide the countermeasures for reducing the energy degradation and GHG emission. Nowadays, some indexes have been proposed to assess the sustainability and can be divided into two categories: “implicit indexes” and “explicit indexes”. The formers, just as energy and exergy, are deduced by the state parameters of the defined objective. Higher exergy efficiency makes a better sustainability. Advanced indicators like avoidable and unavoidable exergy destructions, cumulative exergy and carbon exergy tax have been presented to link different parameters together with the specific physical meanings [3-5]. The latter, just as GHG emissions and footprints, is defined as the amount of wastes poured into the environment by the unit of mass flow or volume flow. The less emissions and footprints human discharge, the better sustainability would be kept. GHG emission is the most intuitive metric for assessing the damage to ecological environment in perspective of greenhouse effect, and it is frequently used by governments to declare the specific GHG reduction target in some reports. In the recent years, footprint families have also become popular indicators to monitor the impact on sustainability [6, 7]. A portion of fuel consumption is due to the exergy destruction and loss in operating units, leading to superfluous carbon emissions beyond the theoretical horizon. Namely, inefficient energy use contributes to the undeserved part of carbon footprint. Thus, it is essential to establish the correlation between exergy and carbon footprint to discover the deeper potentiality of mitigation technologies. Numerous researches have studied the performance of retrofitted system in the perspective of energy, economy and environmental impact with a global viewpoint. However, few focused on the correlation between exergy and carbon footprint in terms of operating units. In this study, we consider exergy and carbon footprint as the artistically joint indicators for sustainability. We propose an indicator, carbon exergy footprint (CEF), to integrate these two indexes together to evaluate the greenhouse effect of operating units. The established indicator is based on the second law of thermodynamics and life cycle assessment. It compromises the merits of exergy analysis and carbon footprint analysis to intuitively evaluate the performance of operating units and process in a local or holistic perspective. In Section 2, we elaborate the relation between exergy and carbon footprint and then propose the definition of CEF. The process of water-gas shift (WGS) is chosen as the case for illustrating the behavior of CEF in Section 3 and the results will be discussed in Section 4. 2. Carbon exergy footprint definition Exergy is the combination of the quantity and quality of energy, practically representing the maximum useful work during a process when the system is brought into the reference-environment state or dead state [8]. In a specific system, exergy destruction occurring in the operating units is constitutionally derived from the original inlet exergy. In industrial application, the difference of temperature, pressure and chemical potential considered as the driving forces for the actual process is unavoidable in operating units. Carbon footprint is a member of footprint families [6] and is generally defined as the total carbon emissions including direct and indirect GHG caused by product, person, organization or activity over a full life cycle to evaluate the carbon intensity. It is an indicator for environmental impact and sustainability while exergy efficiency represents the design level of operating units or technological process.
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Zhen Qin et al. / Energy Procedia 105 (2017) 3179 – 3184
Fig. 1. Qualitative description of the influence of exergy efficiency on sustainability and environmental impact [8]
It is observed in Fig. 1 that as the exergy efficiency increases, the sustainability increases and environmental impact decreases. Therefore, the qualitative description also reveals the feasibility for linking carbon footprint with exergy in a quantitative expression. Based on the analogy with the definition of carbon footprint, carbon exergy footprint (CEF) is defined as the total carbon emission related with the operating unit, including the direct GHG generated by chemical reaction and indirect GHG caused by exergy destruction. It is an indicator for assessing the environmental impact of operating units and correlating carbon footprint with exergy. Thus the expression of CEF is defined as:
CEF ᷉
mi i
m j j᷊ ᷉ f Exd᷊
i
(1)
j
where m is the mass flow of the stream (kg/h); the subscripts, i and j, represent the outlet and inlet stream, respectively; is the mass fraction of GHG in the stream. The bracketed subtraction of the first two items represents a part of CEF caused by the composition change in chemical reaction; f (Exd) stands for another part of CEF caused by exergy destruction, including physical exergy destruction only. Wherever the exergy supplement is originated from, complete combustion of coal in boiler can be considered as the ultimate thermal resource for all heat demand. In terms of exothermal operating units, such as the unit of WGS, if the released heat is utilized in a higher efficient way, the consumption of feedstock coal would be kept in a comparatively lower level. Besides, the exergy destruction varies with different operating conditions. Therefore, in accordance with the terminal fuel consumption for exergy destruction, f (Exd) is expressed as
᷉ f Exd᷊
Exd mCO2 LHVcoal
(2)
where Exd is the exergy destruction (kJ/h); LHVcoal is the low heat value of the reference coal (kJ/kg coal); mCO2 is the amount of CO2 when 1kg coal is burned completely (kg/kg coal). In this study, kinetic and potential exergy are normally neglected and the reference environment is set at 25 ć and 1atm. 3. Case study To analyze the behavior of the proposed CEF, a part of the water-gas shift section is used as the example because it covers typical operating units, including reactor, electrical equipment and heat exchanger. The flow sheet is shown in Fig. 2. Water-gas shift reaction, CO+H2OėCO2+H2, is used for adjusting the H/C ratio in many chemical synthesis processes. Crude syngas with saturated steam from gasification section is first preheated by hot shifted gas to the temperature of about 280 ć and then enters high temperature shift reactor (R101),
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where the concentration of carbon monoxide (CO) is reduced to approximately 3.7% (vol.). For heat recovery, the outlet stream passes through the steam superheater (about 4.1 MPa) and then preheats the crude syngas. After that, syngas continues to be cooled down to 270 ć in E103 and then enters the low temperature shift reactor (R102) for further conversion. The temperature and CO concentration of the outlet syngas are 295 ć and 1.1%, respectively. The parameters of this process are extracted from a commercial chemical plant, as shown in Table 1.
Fig. 2. Flow sheet of WGS section Table 1. Basic parameters for case study Item
Description
Reference coal
Illinois 6# [9]
Mass flow rate of Stream 101
351714 kg/h
Mass flow rate of Stream 121
35865 kg/h
4. Results and discussion Exergy is calculated based on the properties of the streams, and the results of exergy destruction of the six operating units are shown in Table 2. In this section, we will discuss the behaviors of CEF for the three types of operating units, i.e., heat exchanger, reactor and pump. Table 2. Results of carbon exergy footprint calculation Operating unit
Exergy destruction (kJ/h)
CEF by exergy destruction (kg CO2,e/h)
CEF by the change of components (kg CO2,e/h)
Total CEF (kg CO2,e/h)
Specific CEF (kg CO2,e/MJ)
E101
3913000
336.04
0
336.04
11.87
E102
2336600
200.66
0
200.66
13.56
E103
7613230
653.81
0
653.81
8.11
R101
-38672000
-3321.09
138742.10
135421.01
1.045
R102
-4010000
-344.37
20736.00
20391.63
1.053
Pump
24446
2.10
0
2.10
36.76
Zhen Qin et al. / Energy Procedia 105 (2017) 3179 – 3184
4.1. Heat exchanger The CEF of heat exchanger is merely decided by exergy destruction because the components do not change in the heat exchange. It can be inferred from Equation (2) that the total CEF increases with the increase of exergy destruction. Though the exergy destruction and CEF of E102 are smaller than those of E101 and E103, it is seen that the specific CEF of E102 is the largest one. Table 2 shows that the specific CEFs of the three exchangers follow the order of E102˚E101˚E103, which fits in with the order of the log-mean temperature difference (LMTD). The reason is that temperature difference is conducted as the driving force for the irreversible process of heat transfer but also the causation for exergy destruction. On the basis of the same heat duty, the more exergy destruction occurred in the heat exchanger, the more exergy from feedstock coal is needed to make up this gap. Therefore, the specific CEF of heat exchanger can be characterized by the temperature difference of heat transfer. 4.2. Reactor The negative exergy destruction in reactors is due to the exclusion of chemical exergy when the exergy is calculated. Actually, exergy is considered to consist of four major parts: potential, kinetic, physical and chemical. Kinetic and potential exergy are normally negligible in many industrial cases. Physical exergy is characterized by the change of temperature and pressure from system state to reference-environment state while chemical exergy is expressed by the change of composition from reference-environment state to dead state. In the operating unit with chemical reaction, both physical and chemical exergy change due to the change of temperature and compositions, where the existing form of carbon element commonly varies with the demand of chemical production. By contrast, in the operating unit without chemical reaction, the destruction includes only physical exergy destruction. Therefore, in the perspective of greenhouse effect, the CEF caused by the change of composition is considered to correspond to the partial chemical exergy destruction. The conversion ratios of carbon monoxide in R101 and R102 are 82.8% and 70.4%, respectively. The amount of heat released from R101 is about 6.69 times larger than that of R102. Thus the absolute value of exergy destruction in R101, that is, the increase of physical exergy contributed by exothermal reaction, is larger than that of R102. On the other hand, the higher conversion ratio of CO in R101 leads to the higher CEF caused by the change of compositions. Consequently the total CEF of R101 is much larger than that of R102. Therefore, the total CEF of operating unit with chemical reaction is a balance of the reaction heat and the conversion of compositions. 4.3. Pump The total input exergy of pump consists of both electricity and inlet stream. The pump efficiency is set as 0.67 and the electricity consumption is calculated as 90015.52 kJ/h. If the pump efficiency is increased to 80%, it can be evaluated that the specific CEF decreases by 8.9% due to the higher exergy efficiency of electricity. Therefore, it is suggested that improving the exergy efficiency is a feasible way to deduce the CEF of electrical equipment. Additionally, compared with conventional life cycle assessment, the result of CEF in this study is obviously different from carbon footprint (CF) calculation, i.e. carbon footprint = emission factor × energy consumption. If the emission factor is set at 0.9746 kg CO 2, e/kWh [10], the carbon footprint is obtained as 24.37 kg CO2, e/h, about 11.6 times than the CEF. The concept of carbon footprint is based on the environmental impact of energy consumption in life cycle scale, considering all the process of energy
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conversion. Differently, CEF indicates the GHG impact in terms of the operating unit. Thus CEF could be considered as a part of CF in a sense. 5. Conclusions In this study, a novel indicator, carbon exergy footprint (CEF), is proposed to quantitatively associate carbon footprint with exergy to evaluate the greenhouse effect of operating units. On the basic case study of water-gas shift process, the behavior of CEF is illustrated by discussing three types of operating units at different operating conditions. Results show that the temperature difference, both quantity of heat and element conversion and electrical efficiency are the key factors that influence the CEFs of heat exchanger, reactor and pump, respectively. The results of CEF analysis show good accordance with the conventional exergy analysis. Thus it is possible to use the concept of carbon exergy footprint to compare the performance of operating units. Acknowledgements Financial supports of the National Natural Science Foundation of China (No. 51276141) is gratefully acknowledged. References [1] http://www.iea.org/stats/WebGraphs/WORLD4.pdf. [2] Messagie M, Mertens J, Oliveira L, Rangaraju S, Sanfelix J, Coosemans T, et al. The hourly life cycle carbon footprint of electricity generation in Belgium, bringing a temporal resolution in life cycle assessment. Appl Energ. 2014;134:469-76. [3] Callak M, Balkan F, Hepbasli A. Avoidable and unavoidable exergy destructions of a fluidized bed coal combustor and a heat recovery steam generator. Energ Convers Manage. 2015;98:54-8. [4] Li H, Chen J, Sheng D, Li W. The improved distribution method of negentropy and performance evaluation of CCPPs based on the structure theory of thermoeconomics. Appl Therm Eng. 2016;96:64-75. [5] Massardo AF, Santarelli M, Borchiellini R. Carbon exergy tax (CET): its impact on conventional energy system design and its contribution to advanced systems utilisation. Energy. 2003;28:607-25. [6] Čuček L, Varbanov PS, Klemeš JJ, Kravanja Z. Total footprints-based multi-criteria optimisation of regional biomass energy supply chains. Energy. 2012;44:135-45. [7] Qin Z, Zhai G, Wu X, Yu Y, Zhang Z. Carbon footprint evaluation of coal-to-methanol chain with the hierarchical attribution management and life cycle assessment. Energ Convers Manage. 2016;124:168-79. [8] Dincer I, Rosen MA. Exergy: energy, environment and sustainable development: Newnes; 2012. [9] Li S, Jin H, Gao L. Cogeneration of substitute natural gas and power from coal by moderate recycle of the chemical unconverted gas. Energy. 2013;55:658-67. [10] Shen W, Cao L, Li Q, Zhang W, Wang G, Li C. Quantifying CO2 emissions from China’s cement industry. Renew Sust Energ Rev. 2015;50:1004-12.
Biography Professor Zaoxiao Zhang works in the School of Chemical Engineering and Technology, Xi’an Jiaotong University. The research areas are focused on CO 2 capture and sequestration technologies, energy storage and utilization, and new type of chemical machinery.