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Exergy Analysis of Biomass-Based Hydrogen Production Processes
C H A P T E R
15 Exergy Analysis of Biomass-Based Hydrogen Production Processes Piyush Parkhey1, Bidyut Mazumdar2, S. Venkata Mohan3, 4 1
Amity Institute of Biotechnology, Amity University-Chhattisgarh, Raipur, India; 2Department of Chemical Engineering, National Institute of Technology-Raipur, Raipur, India; 3 Bioengineering and Environmental Sciences Lab, CEEFF, CSIR-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad, India; 4Academy for Scientific and Industrial Research (AcSIR), Hyderabad, India
1. INTRODUCTION Biohydrogen, to put simply, is the hydrogen produced biologically (biological routes or processes). As is well-known, hydrogen is a front-runner in the quest for finding alternative energy sources to fossil fuels. The primary reason being its clean combustion profile and highest energy density per unit mass (142 MJ/kg), which is more than two and a half times to that of gasoline and natural gas. The most common industrial process for industrial hydrogen production is steam reformation of methane, which involves reacting steam at high temperature with methane. However, this process, albeit highly efficient with respect to hydrogen production, also releases a considerable amount of carbon monoxide as a by-product (Eq. [15.1]). Therefore there has been an upsurge in the interests of hydrogen production through biological routes, which are environmentally friendly and sustainable. CH4 þ H2 O # CO þ 3 H2
(15.1)
For biological production to be viable for practical usage, energy efficiency and total yields of the process must be comparable to industrial routes. For this reason, it becomes extremely important to determine the energy content within the biological hydrogen production systems. Exergy analysis aids in calculating the net energy available to be used. Thermodynamically, exergy corresponds to the maximum useful work that can be withdrawn from a system when it comes into equilibrium with a heat reservoir. In simpler words, exergy is the total energy available for use. The present chapter deals with the exergy analysis of the various biohydrogen production processes and their comparison with the established industrial routes. There is only a little Biohydrogen, Second Edition https://doi.org/10.1016/B978-0-444-64203-5.00015-0
383
Copyright © 2019 Elsevier B.V. All rights reserved.
384
15. EXERGY ANALYSIS OF BIOMASS-BASED HYDROGEN PRODUCTION PROCESSES
information available regarding exergy analysis, specifically of biohydrogen production in comparison to that of steam reformation, gasification, and electrolysis. Therefore the present chapter is an attempt to compare the available data that could assist in designing more energy efficient and economical biohydrogen production processes. Prior to that, an introduction about the concept of exergy is discussed for better understanding of the idea.
2. CONCEPT OF EXERGY As mentioned above, exergy is the maximum useful work available from a system. The term was first put forward by Gibbs in 1873 [1] and later explained and discussed by Zoran Rant in 1956. It is a combination property of both system and its environment as it depends on both [2]. The term exergy is synonymous with available energy, utilizable energy, and maximum work content or available useful work of a system. In any reversible process, the maximum amount of work is obtainable when the system is brought at a thermodynamic equilibrium with the environment. Therefore exergy measures the deviation of a system from a state of equilibrium with its environment [3]. The first law of thermodynamics states that energy can neither be created nor be destroyed. But, exergy is always destroyed in an irreversible process, which is directly proportional to the increase in the entropy of the system and the surroundings. The exergy analysis is done with an aim of determining the losses and quantitatively estimating the thermodynamic imperfections of the process under consideration. The analysis, therefore, can assist in the overall improvement of any given thermodynamic process. For this very reason, exergy and not energy is often used as a measure of design and analysis of energy systems. The exergy-based economic-analysis methodologies put-up a clearer picture of the feasibility and commercial viability of such energy systems. The relationship between energy and exergy, and how they regulate the economics have been well-discussed by Dincer and Rosen [4] and therefore will not be detailed here.
3. EXERGY ANALYSIS The primary advantage associated with exergy analysis of a system is that it provides a relationship equation between the ideal output of the system and the real one. It generates a comparison point from where further process optimization can be carried out to maximize the output to reach the theoretical maximum. The exergy efficiency of any process can be defined as the ratio of total exergy output (Exout) to that of total exergy input (Exin) [5]. Mathematically, this can be represented as shown in Eq. (15.2). ε ¼
Exout Exin
(15.2)
This is the standard description of exergy analysis and variations as per the different considerations of hydrogen production processes may exist. The exergetic efficiency, as can be seen from Eq. (15.2), includes all the input exergy as used exergy and the output exergy as the useful exergy. Therefore the aim of any production process is to minimize the exergy loss or ExinExout. In this regards, the exergy analysis can be used for quantitative estimation of thermodynamic imperfections of a process and specify the possible improvements [3].
4. EXERGY ANALYSIS OF HYDROGEN PRODUCTION PROCESSES
385
4. EXERGY ANALYSIS OF HYDROGEN PRODUCTION PROCESSES 4.1 Biomass Gasification Biohydrogen production, of all the available renewable sources, has been most extensively studied using biomass as substrate. Biomass not only is a renewable energy source but is also virtually a nonexhaustible carbon source in bioprocess industry. Biomass gasification is one of the most viable methods for sustainable hydrogen production [6] and therefore the exergical analysis of hydrogen production from biomass through gasification has been comprehensively studied in different systems and different scales. Exergical analysis plays a major role in deciding the process efficiency of hydrogen production and identifying the bottlenecks that might hamper the production efficacy. However, one important issue in calculating exergy efficiency with biomass-based systems is difficulty in determining the chemical exergy contained within biomass. Therefore statistical correlation of Szargut and Styrylska [7] is described by Kalinci et al. [5] as shown in Eq. (15.3): eo;biomass ¼ b LHV biomass where LHV corresponds to lower heating value and b is defined as: ZH ZO ZH ZN 1:0412 þ 0:2160 0:2499 1 þ 0:7884 þ 0:0450 ZC ZC ZC ZC b ¼ ZO 1 0:3035 ZC
(15.3)
(15.4)
where ZO, ZC, ZH, and ZN are the weight fractions of oxygen, carbon, hydrogen, and nitrogen, respectively, in the biomass. The general exergy balance equation is given by Eq. (15.5) as: X X _ out þ I_ (15.5) Ex_ in ¼ Ex in
out
where I is the rate of loss in the internal exergy due to irreversibility. Using these equations, the exergy efficiency and the exergy destruction rate in an oxygen-rich air gasification of biomass process were determined. The exergy efficiency of the gasifier was calculated to be 56.8% with an exergy destruction rate of 670.43 kW. The exergy destructions occur in the gasifier while the loss occurs with the products. The exergy destructions, which reduce the efficiency of the process, may be minimized by optimization of parameters, such as temperature, pressure, steam biomass ratio, and equivalence ratio. To determine the exergical efficiency at pilot-scale level, Toonssen et al. [6] compared five different gasification systems in two different plant systems. The maximum exergy efficiency of about 66% was obtained. The thermodynamic calculations of the 10 processes suggested that the presence of nitrogen gas in producer gas reduced the process efficiency. Also, air separation unit causes significant energy consumption and losses. This can be avoided by using oxygen instead of air. A lower moisture content of about 10% has been also reported to increase the exergy efficiency of the process. The fourth major factor deciding the net
386
15. EXERGY ANALYSIS OF BIOMASS-BASED HYDROGEN PRODUCTION PROCESSES
efficiency of biohydrogen production is the gas cleaning temperature. While the hightemperature gas cleaning leads to lower exergy losses, the associated hydrogen yields could also be low, resulting in net lower exergy efficiency. Therefore it is recommended that hydrogen must be supplied at low temperatures to transport systems to improve the exergy efficiency of the overall process. Lu et al. [8] studied the energy and exergy analysis of hydrogen production from biomass gasification in supercritical water. The study revealed that exergy efficiency of the whole system was more than 40% and increased with increasing heat transfer efficiencies. In the system, the exergy loss was caused due to the irreversibility of chemical reaction and heat transfer. Improvement in process design and optimization of operational conditions to ensure maximum heat transfer efficiency may improve the system performance. The comparison of different renewable sources for hydrogen production was done by Christopher and Dimitrios [3]. Of the four sources, i.e., wind, solar, hydroelectric, and biomass studied, the second highest exergetic efficiency was obtained from biomass gasification. Liquefaction of produced hydrogen was an energy-intensive process and consumed a considerable exergy. Therefore liquefaction process and an efficient gas collection system should be redesigned to minimize the exergy loss and maximize the process efficiency. A number of other studies have been undertaken that relate the process efficiency of hydrogen production from biomass gasification with the related process exergy analysis. All these studies concluded that the results drawn from the exergy analysis can be well thought of in optimizing and designing a more efficient hydrogen production process. A summary of such reports is given in Table 15.1.
4.2 Biological Hydrogen Production So far, the exergy analysis of biomass gasification as a tool to determine the sustainability of hydrogen production process has been discussed. In this section, the exergy analysis of biological hydrogen processes would be discussed. Such reports are very less and therefore an urgent need to undertake such analysis is advocated. One of the earlier reports regarding exergy analysis of biohydrogen production was done in the anaerobic photosynthetic biohydrogen production using photosynthetic bacteria Rhodospirillum rubrum and Rhodospirillum palustris PT [13,14]. The determination of exergy efficiency of the production process included the total exergy input from microorganism, culture media, as well as syngas. The exergy balance equation for the photobioreactor was as shown in Eq. (15.6) ExMO;t þ ExSG;t þ ExCM;t þ ExDL;t þ ExOS;t ¼ ExCM;tþDT þ ExSG;tþDT þ ExMO;tþDT þ Exdes;ex
(15.6)
where ExMO, ExSG, and ExCM are the exergetic values of microorganisms, syngas, and culture media, respectively, at time t. Also, t þ Dt, ExDL, and ExOS denote the exergy delivered to the culture media from the tungsten light and the orbital shaker between the two-time intervals while Exdes,ex denotes the destroyed exergy based on conventional exergy concept.
387
4. EXERGY ANALYSIS OF HYDROGEN PRODUCTION PROCESSES
TABLE 15.1
The Comparative Exergy Efficiencies of Hydrogen Production Processes From Biomass Gasification
Biomass
Process
Efficiency
Remark
References
Oil palm shell
Gasification followed by steam methane reforming and shift reactions
Energye27% Exergye25%
A process simulation tool was used to assess the gasifier. The gasifier with a decomposition reactor was the most critical component.
Cohce et al. [9]
Poplar (lignocellulosic biomass)
Gasification
Exergetic efficiencye48% Total exergy losse4.6% Exergy destruction e47%
Life-cycle assessment from poplar cultivation to hydrogen purification suggests a promising energy performance.
Iribarren et al. [10]
Oil palm shell
Biomass gasification, steam methane reforming, and shift reaction
Energy efficiencye22% Exergy efficiencye19% Gasifier cold gas efficiencye18%
Gibbs free energy minimization used for modeling of gasifier.
Cohce et al. [11]
Sawdust wood
Biomass gasification
Exergy efficiencye 40%e60%
Decrease in hydrogen production with increase in biomass as the efficiency of using available energy decreases with biomass increase.
Abuadala et al. [12]
Similar to Eq. (15.6), the exergy of the culture media, ExCM, organic materials used to prepare the media, ExOM, chemical exergy of the syngas, ExSG, and mechanical work delivered to the culture media from the orbital shaker, ExOS, were evaluated as given in Eqs. (15.7e15.10), respectively. ! X X X i εi þ RT o X i lnðX i Þ (15.7) ExCM ¼ n i
i
where n denotes the mole number of the culture media, X denotes the molar fraction of each component, εi (kJ/mol) represents the standard chemical exergy of ith component, R denotes the universal gas constant (8.31446 J/mol K), and To denotes the dead state temperature (303.15 K). exOM;i ¼ 363:439C þ 1075:633H 86:308O þ 4:14N þ 190:798S 21:1A
(15.8)
where exOM gives the specific chemical exergy and C, H, O, N, S, and A represent the percent concentration of carbon, hydrogen, oxygen, nitrogen, sulfur, and ash, respectively. ! X X P X i εi þ RT o X i lnðX i Þ þ RT o ln ExSG ¼ (15.9) P O i i where P and PO are the absolute pressure (kPa) of syngas and dead state, respectively.
388
15. EXERGY ANALYSIS OF BIOMASS-BASED HYDROGEN PRODUCTION PROCESSES
ExOS ¼ mCM adDt
(15.10)
where mCM denotes the mass of culture media, a and d denote the acceleration of orbital shaker and amplitude of shaking, respectively. An additional component in biological hydrogen production is the microorganism carrying out the production process. The presence of a microorganism in the system impacts the overall exergy of the system and to accurately calculate the exergical efficiency of biological hydrogen production, the exergy content of the microorganism needs to be included. The exergy contained within the microorganism is called as ecoexergy. This concept was first put forward by Jørgensen et al. [15]. They proposed that the work energy carried by the living organisms in such processes is much more than the nonliving elements of the system that only carry the chemical energy, and this is due to the genetic information carried by the living moiety. The ecoexergy includes the work energy of the organism along with the information encoded within the genes. To calculate the ecoexergy of the microorganism, first, the chemical exergy associated with it needs to be calculated. This is given as given in Eq. (15.11). ExMO ¼ 18:7 mMO
(15.11)
here, mMO is the mass of microorganism and 18.7 kJ/g is the chemical exergy of the organic compounds produced by the microorganism. Using this, the mathematical expression of the ecoexergy (J) was calculated as shown in Eqs. (15.12)e(15.15). J ¼ 7:34 105 ExMO þ ExMO ln20number of nucleotidesð1no.of RT O b ¼ 1þ
repeating genesÞ=3
ln 20ðnumber of nucleotides ð1 no:of repeating genesÞ 3 7:34 105
(15.12)
(15.13)
now, as ln20 z 3, Eq. (15.13) can be rewritten as: b ¼ 1þ
number of nucleotides ð1 no: of repeating genesÞ 7:34 105
(15.14)
b, in Eqs. (15.13) and (15.14), is the ratio of the ecoexergy to the chemical exergy and is indicative of the information contained in the microorganism. Approximately, this value was calculated to be 8.5 for bacteria. Thus ecoexergy of the microorganism can be rearranged to J ¼ 18:7 8:5 mMO
(15.15)
Now, including the ecoexergy of the microorganism, the net exergy balance equation of the system (Eq. 15.6) can be modified as: JMO;t þ ExSG;t þ ExCM;t þ ExDL;t þ ExOS;t ¼ ExCM;tþDT þ ExSG;tþDT þ ExMO;tþDT þ Exdes;ex (15.16)
REFERENCES
389
The inclusion of ecoexergy makes the total exergical analysis of the system more purposeful. The application of this analysis can be extended towards designing more exergy and energy efficient photobiohydrogen production systems. A probable extension could be carrying out exergoeconomical and exergoenvironmental analyses to design large-scale continuous photobioreactors for biohydrogen production. Exergy-based fuzzy techniques have also been investigated in order to optimize the continuous photobioreactor for biohydrogen production from syngas. Fuzzy optimization tools can be helpful in designing cost-effective and environmentally safe biohydrogen production systems.
5. CONCLUSIONS AND PERSPECTIVES Hydrogen is the most ideal future clean energy carrier since it produces only water vapor as by-product and has the highest energy density per unit mass. Considering the advantages of hydrogen as an energy source, a new concept of “hydrogen energy economy” has been introduced to replace the existing energy supplies. However, for their practical applications in commercial market, it is utmost necessary that the overall efficiency (economic and energy) of processes associated with hydrogen, from production to packaging, must be competitive with the efficiency of conventional fuels. Exergy analysis, based on the second law of thermodynamics, is an effective method that uses conservation of mass and energy principles to identify the thermodynamic imperfections of the system. It gives a clear description of how better efficient hydrogen production systems can be designed by reducing the inefficiencies of the existing systems. Careful and rational determination of the exergetic losses associated with every step of hydrogen production, liquefaction, storage, and transport would further lead to the development of a more efficient, sustainable, and cost-effective hydrogen production system that would ideally be able to replace fossil-based fuels in near future.
References [1] J.W. Gibbs, A method of geometrical representation of the thermodynamic properties of substances by means of surfaces, Trans. Connect. Acad. Arts Sci. 2 (1873) 382e404. [2] Y.A. Cengel, M.A. Boles, Thermodynamics an Engineering Approach, fifth ed., McGraw Hill Education, 2004. https://cruncheez.files.wordpress.com/2015/06/thermodynamics-an-engineering-approach-5th-edition.pdf. [3] K. Christopher, R. Dimitrios, A review on exergy comparison of hydrogen production methods from renewable energy sources, Energy Environ. Sci. 5 (2012) 6640, https://doi.org/10.1039/c2ee01098d. [4] I. Dinç er, M. Rosen, EXERGY : Energy, Environment and Sustainable Development, Elsevier Science, 2012. https://books.google.co.in/books?id¼FrUolHxhGVYC&pg¼PA393&lpg¼PA393&dq¼exergyþanalysisþbioh ydrogen&source¼bl&ots¼9Jfki-6h36&sig¼9Fiow9NpXfbWcBSeobQpjwkXo1w&hl¼en&sa¼X&ved¼0ahUK EwjjnpDmk_XYAhXBKo8KHV4AAxM4FBDoAQgzMAI#v¼onepage&q¼exergy analysis biohydrogen. [5] Y. Kalinci, A. Hepbasli, I. Dincer, Biomass-based hydrogen production: a review and analysis, Int. J. Hydrogen Energy 34 (2009) 8799e8817, https://doi.org/10.1016/j.ijhydene.2009.08.078. [6] R. Toonssen, N. Woudstra, A.H.M. Verkooijen, Exergy analysis of hydrogen production plants based on biomass gasification, Int. J. Hydrogen Energy 33 (2008) 4074e4082, https://doi.org/10.1016/j.ijhydene. 2008.05.059. [7] J. Szargut, T. Styrylska, Approximate evaluation of the exergy of fuels, Brennstoff-Waerme-Kraft 16 (1964) 589e596.
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15. EXERGY ANALYSIS OF BIOMASS-BASED HYDROGEN PRODUCTION PROCESSES
[8] Y. Lu, L. Guo, X. Zhang, Q. Yan, Thermodynamic modeling and analysis of biomass gasification for hydrogen production in supercritical water, Chem. Eng. J. 131 (2007) 233e244, https://doi.org/10.1016/j.cej.2006.11.016. [9] M.K. Cohce, I. Dincer, M.A. Rosen, Thermodynamic analysis of hydrogen production from biomass gasification, Int. J. Hydrogen Energy 35 (2010) 4970e4980, https://doi.org/10.1016/j.ijhydene.2009.08.066. [10] D. Iribarren, A. Susmozas, F. Petrakopoulou, J. Dufour, Environmental and exergetic evaluation of hydrogen production via lignocellulosic biomass gasification, J. Clean. Prod. 69 (2014) 165e175, https://doi.org/ 10.1016/j.jclepro.2014.01.068. [11] M.K. Cohce, I. Dincer, M.A. Rosen, Energy and exergy analyses of a biomass-based hydrogen production system, Bioresour. Technol. 102 (2011) 8466e8474, https://doi.org/10.1016/j.biortech.2011.06.020. [12] A. Abuadala, I. Dincer, G.F. Naterer, Exergy analysis of hydrogen production from biomass gasification, Int. J. Hydrogen Energy 35 (2010) 4981e4990, https://doi.org/10.1016/j.ijhydene.2009.08.025. [13] S.S. Hosseini, M. Aghbashlo, M. Tabatabaei, G. Najafpour, H. Younesi, Thermodynamic evaluation of a photobioreactor for hydrogen production from syngas via a locally isolated Rhodopseudomonas palustris PT, Int. J. Hydrogen Energy 40 (2015) 14246e14256, https://doi.org/10.1016/j.ijhydene.2015.08.092. [14] S.S. Hosseini, M. Aghbashlo, M. Tabatabaei, H. Younesi, G. Najafpour, Exergy analysis of biohydrogen production from various carbon sources via anaerobic photosynthetic bacteria (Rhodospirillum rubrum), Energy 93 (2015) 730e739, https://doi.org/10.1016/j.energy.2015.09.060. [15] S.E. Jørgensen, N. Ladegaard, M. Debeljak, J.C. Marques, Calculations of exergy for organisms, Ecol. Modell 185 (2005) 165e175, https://doi.org/10.1016/J.ECOLMODEL.2004.11.020.
15 Exergy Analysis of Biomass-Based Hydrogen Production Processes Piyush Parkhey1, Bidyut Mazumdar2, S. Venkata Mohan3, 4 1
Amity Institute of Biotechnology, Amity University-Chhattisgarh, Raipur, India; 2Department of Chemical Engineering, National Institute of Technology-Raipur, Raipur, India; 3 Bioengineering and Environmental Sciences Lab, CEEFF, CSIR-Indian Institute of Chemical Technology (CSIR-IICT), Hyderabad, India; 4Academy for Scientific and Industrial Research (AcSIR), Hyderabad, India
1. INTRODUCTION Biohydrogen, to put simply, is the hydrogen produced biologically (biological routes or processes). As is well-known, hydrogen is a front-runner in the quest for finding alternative energy sources to fossil fuels. The primary reason being its clean combustion profile and highest energy density per unit mass (142 MJ/kg), which is more than two and a half times to that of gasoline and natural gas. The most common industrial process for industrial hydrogen production is steam reformation of methane, which involves reacting steam at high temperature with methane. However, this process, albeit highly efficient with respect to hydrogen production, also releases a considerable amount of carbon monoxide as a by-product (Eq. [15.1]). Therefore there has been an upsurge in the interests of hydrogen production through biological routes, which are environmentally friendly and sustainable. CH4 þ H2 O # CO þ 3 H2
(15.1)
For biological production to be viable for practical usage, energy efficiency and total yields of the process must be comparable to industrial routes. For this reason, it becomes extremely important to determine the energy content within the biological hydrogen production systems. Exergy analysis aids in calculating the net energy available to be used. Thermodynamically, exergy corresponds to the maximum useful work that can be withdrawn from a system when it comes into equilibrium with a heat reservoir. In simpler words, exergy is the total energy available for use. The present chapter deals with the exergy analysis of the various biohydrogen production processes and their comparison with the established industrial routes. There is only a little Biohydrogen, Second Edition https://doi.org/10.1016/B978-0-444-64203-5.00015-0
383
Copyright © 2019 Elsevier B.V. All rights reserved.
384
15. EXERGY ANALYSIS OF BIOMASS-BASED HYDROGEN PRODUCTION PROCESSES
information available regarding exergy analysis, specifically of biohydrogen production in comparison to that of steam reformation, gasification, and electrolysis. Therefore the present chapter is an attempt to compare the available data that could assist in designing more energy efficient and economical biohydrogen production processes. Prior to that, an introduction about the concept of exergy is discussed for better understanding of the idea.
2. CONCEPT OF EXERGY As mentioned above, exergy is the maximum useful work available from a system. The term was first put forward by Gibbs in 1873 [1] and later explained and discussed by Zoran Rant in 1956. It is a combination property of both system and its environment as it depends on both [2]. The term exergy is synonymous with available energy, utilizable energy, and maximum work content or available useful work of a system. In any reversible process, the maximum amount of work is obtainable when the system is brought at a thermodynamic equilibrium with the environment. Therefore exergy measures the deviation of a system from a state of equilibrium with its environment [3]. The first law of thermodynamics states that energy can neither be created nor be destroyed. But, exergy is always destroyed in an irreversible process, which is directly proportional to the increase in the entropy of the system and the surroundings. The exergy analysis is done with an aim of determining the losses and quantitatively estimating the thermodynamic imperfections of the process under consideration. The analysis, therefore, can assist in the overall improvement of any given thermodynamic process. For this very reason, exergy and not energy is often used as a measure of design and analysis of energy systems. The exergy-based economic-analysis methodologies put-up a clearer picture of the feasibility and commercial viability of such energy systems. The relationship between energy and exergy, and how they regulate the economics have been well-discussed by Dincer and Rosen [4] and therefore will not be detailed here.
3. EXERGY ANALYSIS The primary advantage associated with exergy analysis of a system is that it provides a relationship equation between the ideal output of the system and the real one. It generates a comparison point from where further process optimization can be carried out to maximize the output to reach the theoretical maximum. The exergy efficiency of any process can be defined as the ratio of total exergy output (Exout) to that of total exergy input (Exin) [5]. Mathematically, this can be represented as shown in Eq. (15.2). ε ¼
Exout Exin
(15.2)
This is the standard description of exergy analysis and variations as per the different considerations of hydrogen production processes may exist. The exergetic efficiency, as can be seen from Eq. (15.2), includes all the input exergy as used exergy and the output exergy as the useful exergy. Therefore the aim of any production process is to minimize the exergy loss or ExinExout. In this regards, the exergy analysis can be used for quantitative estimation of thermodynamic imperfections of a process and specify the possible improvements [3].
4. EXERGY ANALYSIS OF HYDROGEN PRODUCTION PROCESSES
385
4. EXERGY ANALYSIS OF HYDROGEN PRODUCTION PROCESSES 4.1 Biomass Gasification Biohydrogen production, of all the available renewable sources, has been most extensively studied using biomass as substrate. Biomass not only is a renewable energy source but is also virtually a nonexhaustible carbon source in bioprocess industry. Biomass gasification is one of the most viable methods for sustainable hydrogen production [6] and therefore the exergical analysis of hydrogen production from biomass through gasification has been comprehensively studied in different systems and different scales. Exergical analysis plays a major role in deciding the process efficiency of hydrogen production and identifying the bottlenecks that might hamper the production efficacy. However, one important issue in calculating exergy efficiency with biomass-based systems is difficulty in determining the chemical exergy contained within biomass. Therefore statistical correlation of Szargut and Styrylska [7] is described by Kalinci et al. [5] as shown in Eq. (15.3): eo;biomass ¼ b LHV biomass where LHV corresponds to lower heating value and b is defined as: ZH ZO ZH ZN 1:0412 þ 0:2160 0:2499 1 þ 0:7884 þ 0:0450 ZC ZC ZC ZC b ¼ ZO 1 0:3035 ZC
(15.3)
(15.4)
where ZO, ZC, ZH, and ZN are the weight fractions of oxygen, carbon, hydrogen, and nitrogen, respectively, in the biomass. The general exergy balance equation is given by Eq. (15.5) as: X X _ out þ I_ (15.5) Ex_ in ¼ Ex in
out
where I is the rate of loss in the internal exergy due to irreversibility. Using these equations, the exergy efficiency and the exergy destruction rate in an oxygen-rich air gasification of biomass process were determined. The exergy efficiency of the gasifier was calculated to be 56.8% with an exergy destruction rate of 670.43 kW. The exergy destructions occur in the gasifier while the loss occurs with the products. The exergy destructions, which reduce the efficiency of the process, may be minimized by optimization of parameters, such as temperature, pressure, steam biomass ratio, and equivalence ratio. To determine the exergical efficiency at pilot-scale level, Toonssen et al. [6] compared five different gasification systems in two different plant systems. The maximum exergy efficiency of about 66% was obtained. The thermodynamic calculations of the 10 processes suggested that the presence of nitrogen gas in producer gas reduced the process efficiency. Also, air separation unit causes significant energy consumption and losses. This can be avoided by using oxygen instead of air. A lower moisture content of about 10% has been also reported to increase the exergy efficiency of the process. The fourth major factor deciding the net
386
15. EXERGY ANALYSIS OF BIOMASS-BASED HYDROGEN PRODUCTION PROCESSES
efficiency of biohydrogen production is the gas cleaning temperature. While the hightemperature gas cleaning leads to lower exergy losses, the associated hydrogen yields could also be low, resulting in net lower exergy efficiency. Therefore it is recommended that hydrogen must be supplied at low temperatures to transport systems to improve the exergy efficiency of the overall process. Lu et al. [8] studied the energy and exergy analysis of hydrogen production from biomass gasification in supercritical water. The study revealed that exergy efficiency of the whole system was more than 40% and increased with increasing heat transfer efficiencies. In the system, the exergy loss was caused due to the irreversibility of chemical reaction and heat transfer. Improvement in process design and optimization of operational conditions to ensure maximum heat transfer efficiency may improve the system performance. The comparison of different renewable sources for hydrogen production was done by Christopher and Dimitrios [3]. Of the four sources, i.e., wind, solar, hydroelectric, and biomass studied, the second highest exergetic efficiency was obtained from biomass gasification. Liquefaction of produced hydrogen was an energy-intensive process and consumed a considerable exergy. Therefore liquefaction process and an efficient gas collection system should be redesigned to minimize the exergy loss and maximize the process efficiency. A number of other studies have been undertaken that relate the process efficiency of hydrogen production from biomass gasification with the related process exergy analysis. All these studies concluded that the results drawn from the exergy analysis can be well thought of in optimizing and designing a more efficient hydrogen production process. A summary of such reports is given in Table 15.1.
4.2 Biological Hydrogen Production So far, the exergy analysis of biomass gasification as a tool to determine the sustainability of hydrogen production process has been discussed. In this section, the exergy analysis of biological hydrogen processes would be discussed. Such reports are very less and therefore an urgent need to undertake such analysis is advocated. One of the earlier reports regarding exergy analysis of biohydrogen production was done in the anaerobic photosynthetic biohydrogen production using photosynthetic bacteria Rhodospirillum rubrum and Rhodospirillum palustris PT [13,14]. The determination of exergy efficiency of the production process included the total exergy input from microorganism, culture media, as well as syngas. The exergy balance equation for the photobioreactor was as shown in Eq. (15.6) ExMO;t þ ExSG;t þ ExCM;t þ ExDL;t þ ExOS;t ¼ ExCM;tþDT þ ExSG;tþDT þ ExMO;tþDT þ Exdes;ex
(15.6)
where ExMO, ExSG, and ExCM are the exergetic values of microorganisms, syngas, and culture media, respectively, at time t. Also, t þ Dt, ExDL, and ExOS denote the exergy delivered to the culture media from the tungsten light and the orbital shaker between the two-time intervals while Exdes,ex denotes the destroyed exergy based on conventional exergy concept.
387
4. EXERGY ANALYSIS OF HYDROGEN PRODUCTION PROCESSES
TABLE 15.1
The Comparative Exergy Efficiencies of Hydrogen Production Processes From Biomass Gasification
Biomass
Process
Efficiency
Remark
References
Oil palm shell
Gasification followed by steam methane reforming and shift reactions
Energye27% Exergye25%
A process simulation tool was used to assess the gasifier. The gasifier with a decomposition reactor was the most critical component.
Cohce et al. [9]
Poplar (lignocellulosic biomass)
Gasification
Exergetic efficiencye48% Total exergy losse4.6% Exergy destruction e47%
Life-cycle assessment from poplar cultivation to hydrogen purification suggests a promising energy performance.
Iribarren et al. [10]
Oil palm shell
Biomass gasification, steam methane reforming, and shift reaction
Energy efficiencye22% Exergy efficiencye19% Gasifier cold gas efficiencye18%
Gibbs free energy minimization used for modeling of gasifier.
Cohce et al. [11]
Sawdust wood
Biomass gasification
Exergy efficiencye 40%e60%
Decrease in hydrogen production with increase in biomass as the efficiency of using available energy decreases with biomass increase.
Abuadala et al. [12]
Similar to Eq. (15.6), the exergy of the culture media, ExCM, organic materials used to prepare the media, ExOM, chemical exergy of the syngas, ExSG, and mechanical work delivered to the culture media from the orbital shaker, ExOS, were evaluated as given in Eqs. (15.7e15.10), respectively. ! X X X i εi þ RT o X i lnðX i Þ (15.7) ExCM ¼ n i
i
where n denotes the mole number of the culture media, X denotes the molar fraction of each component, εi (kJ/mol) represents the standard chemical exergy of ith component, R denotes the universal gas constant (8.31446 J/mol K), and To denotes the dead state temperature (303.15 K). exOM;i ¼ 363:439C þ 1075:633H 86:308O þ 4:14N þ 190:798S 21:1A
(15.8)
where exOM gives the specific chemical exergy and C, H, O, N, S, and A represent the percent concentration of carbon, hydrogen, oxygen, nitrogen, sulfur, and ash, respectively. ! X X P X i εi þ RT o X i lnðX i Þ þ RT o ln ExSG ¼ (15.9) P O i i where P and PO are the absolute pressure (kPa) of syngas and dead state, respectively.
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15. EXERGY ANALYSIS OF BIOMASS-BASED HYDROGEN PRODUCTION PROCESSES
ExOS ¼ mCM adDt
(15.10)
where mCM denotes the mass of culture media, a and d denote the acceleration of orbital shaker and amplitude of shaking, respectively. An additional component in biological hydrogen production is the microorganism carrying out the production process. The presence of a microorganism in the system impacts the overall exergy of the system and to accurately calculate the exergical efficiency of biological hydrogen production, the exergy content of the microorganism needs to be included. The exergy contained within the microorganism is called as ecoexergy. This concept was first put forward by Jørgensen et al. [15]. They proposed that the work energy carried by the living organisms in such processes is much more than the nonliving elements of the system that only carry the chemical energy, and this is due to the genetic information carried by the living moiety. The ecoexergy includes the work energy of the organism along with the information encoded within the genes. To calculate the ecoexergy of the microorganism, first, the chemical exergy associated with it needs to be calculated. This is given as given in Eq. (15.11). ExMO ¼ 18:7 mMO
(15.11)
here, mMO is the mass of microorganism and 18.7 kJ/g is the chemical exergy of the organic compounds produced by the microorganism. Using this, the mathematical expression of the ecoexergy (J) was calculated as shown in Eqs. (15.12)e(15.15). J ¼ 7:34 105 ExMO þ ExMO ln20number of nucleotidesð1no.of RT O b ¼ 1þ
repeating genesÞ=3
ln 20ðnumber of nucleotides ð1 no:of repeating genesÞ 3 7:34 105
(15.12)
(15.13)
now, as ln20 z 3, Eq. (15.13) can be rewritten as: b ¼ 1þ
number of nucleotides ð1 no: of repeating genesÞ 7:34 105
(15.14)
b, in Eqs. (15.13) and (15.14), is the ratio of the ecoexergy to the chemical exergy and is indicative of the information contained in the microorganism. Approximately, this value was calculated to be 8.5 for bacteria. Thus ecoexergy of the microorganism can be rearranged to J ¼ 18:7 8:5 mMO
(15.15)
Now, including the ecoexergy of the microorganism, the net exergy balance equation of the system (Eq. 15.6) can be modified as: JMO;t þ ExSG;t þ ExCM;t þ ExDL;t þ ExOS;t ¼ ExCM;tþDT þ ExSG;tþDT þ ExMO;tþDT þ Exdes;ex (15.16)
REFERENCES
389
The inclusion of ecoexergy makes the total exergical analysis of the system more purposeful. The application of this analysis can be extended towards designing more exergy and energy efficient photobiohydrogen production systems. A probable extension could be carrying out exergoeconomical and exergoenvironmental analyses to design large-scale continuous photobioreactors for biohydrogen production. Exergy-based fuzzy techniques have also been investigated in order to optimize the continuous photobioreactor for biohydrogen production from syngas. Fuzzy optimization tools can be helpful in designing cost-effective and environmentally safe biohydrogen production systems.
5. CONCLUSIONS AND PERSPECTIVES Hydrogen is the most ideal future clean energy carrier since it produces only water vapor as by-product and has the highest energy density per unit mass. Considering the advantages of hydrogen as an energy source, a new concept of “hydrogen energy economy” has been introduced to replace the existing energy supplies. However, for their practical applications in commercial market, it is utmost necessary that the overall efficiency (economic and energy) of processes associated with hydrogen, from production to packaging, must be competitive with the efficiency of conventional fuels. Exergy analysis, based on the second law of thermodynamics, is an effective method that uses conservation of mass and energy principles to identify the thermodynamic imperfections of the system. It gives a clear description of how better efficient hydrogen production systems can be designed by reducing the inefficiencies of the existing systems. Careful and rational determination of the exergetic losses associated with every step of hydrogen production, liquefaction, storage, and transport would further lead to the development of a more efficient, sustainable, and cost-effective hydrogen production system that would ideally be able to replace fossil-based fuels in near future.
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