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Bioprocess engineering for biohythane production from low-grade waste biomass: technical challenges towards scale up
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ScienceDirect Bioprocess engineering for biohythane production from low-grade waste biomass: technical challenges towards scale up Zhidan Liu1, Buchun Si1, Jiaming Li1, Jianwei He2, Chong Zhang2,3, Yuan Lu2,3, Yuanhui Zhang1,4 and Xin-Hui Xing2,3 A concept of biohythane production by combining biohydrogen and biomethane together via two-stage anaerobic fermentation (TSAF) has been recently proposed and considered as a promising approach for sustainable hythane generation from waste biomass. The advantage of biohythane over traditional biogas are more environmentally benign, higher energy recovery and shorter fermentation time. However, many of current efforts to convert waste biomass into biohythane are still at the bench scale. The system bioprocess study and scale up for industrial application are indispensable. This paper outlines the general approach of biohythane by comparing with other biological processes. The technical challenges are highlighted towards scale up of biohythane system, including functionalization of biohydrogen-producing reactor, energy efficiency, and bioprocess engineering of TSAF. Addresses 1 Laboratory of Environment-Enhancing Energy (E2E), and Key Laboratory of Agricultural Engineering in Structure and Environment, Ministry of Agriculture; College of Water, Resources and Civil Engineering, China Agricultural University, Beijing 100083, China 2 MOE Key Lab of Industrial Biocatalysis, Institute of Biochemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China 3 Center for Synthetic and Systems Biology, Tsinghua University, Beijing 100084, China 4 Department of Agricultural and Biological Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA Corresponding authors: Zhang, Chong ([email protected]), Xing, Xin-Hui ([email protected])
Current Opinion in Biotechnology 2018, 50:25–31 This review comes from a themed issue on Energy biotechnology Edited by Akihiko Kondo and Hal Alper
http://dx.doi.org/10.1016/j.copbio.2017.08.014
wastes, agricultural residues and algal blooms, is a worldwide environmental concern. On the other hand, it contains renewable carbon and energy resources [2,3]. Value-added valorization of waste biomass is of great importance to a sustainable society [4]. Hythane, as a mixture of hydrogen and methane, also known as hydrogen enriched compressed natural gas (HCNG), has received extensive attention as a vehicle fuel [5]. Hythane displays remarkable advantages over compressed natural gas [6], such as reduced greenhouse gas (GHG) emissions, and improved fuel efficiency. One green alternative to provision of sustainable hythane instead of fossil base is two-stage anaerobic fermentation (TSAF) of waste biomass [7,8,9]. Note that much of previous knowledge about conventional TSAF is mainly focused on acidification (first stage) and methanogenesis (second), while not aiming at hydrogen production but enhanced methanogenesis [10]. In contrast, biohythane consists of hydrogen, methane and carbon dioxide harvested from the TSAF with the first stage for biohydrogen generation, which may be upgraded to biobased hythane by removing the carbon dioxide in one step [8]. Advantages of biohythane over traditional biogas are improved energy recovery, shortened fermentation time, flexible H2/CH4 ratio, and more environmentally benign and process robustness for handling waste biomass [7,8]. We have reviewed the research advances of TSAF for the coproduction of hydrogen and methane and proposed a new concept of biohythane in 2013 [8]. Since then, the term of biohythane has been gradually accepted in the field of gaseous biofuel [7,9,11–13,14,15–17]. However, scale up of biohythane system has yet to be developed, although a process demonstration for hydrogen and methane production from sugar-based kitchen waste via TASF was performed [18]. Process engineering for biohythane production is still in its infant stage.
0958-1669/ã 2017 Elsevier Ltd. All rights reserved.
Introduction Huge amount of waste biomass [1] generated from various social activities such as animal wastes, food processing www.sciencedirect.com
In this paper, we outline the biochemical reactions and thermodynamics of biohythane production by comparing with other biological processes for energy production from waste biomass. The technical challenges are highlighted towards scale up of biohythane production process, including functionalization of biohydrogen reactor, energy efficiency of biohythane system, and system engineering of TSAF. Current Opinion in Biotechnology 2018, 50:25–31
26 Energy biotechnology
Biochemical reactions and thermodynamics of biohythane Two-stage biohydrogen and biomethane (biohythane) production was compared with other typical biofuel processes using glucose as the model substrate, focusing on thermodynamic and technical evaluation (Table 1). Most of the single-stage bioprocesses (hydrogen [19], methane [20], or ethanol [21]) face the challenges on how to deal with the remaining residuals. Volatile fatty acids (VFAs), as the main components of the fermentation residuals in anaerobic fermentation, can be further converted into energy carriers such as methane [7], hydrogen [22], electricity [20] or other biochemicals [23] by establishing a second anaerobic stage. Among all the listed pathways, the maximum bioconversion of glucose to hydrogen (12 mol/mol glucose) results in the highest theoretical energy recovery. An energy recovery higher than 100% is the result of the absorption and conversion of external heat into biohydrogen. However, these pathways are limited by the strict requirements of the substrate [24], low process efficiency, or expense of the reactors [25]. A major difference between bioethanol and biohydrogen/ biomethane is that a pure strain and a narrow range of substrates are mostly needed for the former [21]. Harvesting electricity or value-added chemicals through microbial technologies is an emerging approach for waste valorization [26,27]. However, the scale up of MFC for practical application still suffers from the cost-intensive materials, and long-term operation stability [28]. Given that a hydrogen yield of 4 mol/mol glucose can be achieved through dark fermentation, theoretical energy recovery for hydrogen production (41%) is the lowest value among the biofuel processes (Table 1). In fact, the current hydrogen yield is normally lower than 2 mol/ mol glucose due to the limited metabolic fluxes [29]. Single-stage hydrogen production through dark fermentation is thus not energy and cost effective [19], and should be combined with other value-added processes. Methane fermentation has been well developed, which is, however, time-consuming and challengeable for treating high-solid organic waste. Instead, the biohythane system via TSAF resulted in enhanced energy recovery and reduced fermentation time [7,8]. In addition, in a biohythane system dealing with lignocellulosic biomass, saccharification and biohydrogen production could be simultaneously implemented in the first stage via microbial consortium engineering [30].
Technical challenges for scale up Microbial consortium and engineering control of biohydrogen and biomethane processes
The traditional anaerobic methane fermentation normally incorporated microorganisms with different functions to establish a synergic microbial consortium [31]. In particular, two kinds of bacteria take part in the methanogenesis process: one responsible for the conversion of Current Opinion in Biotechnology 2018, 50:25–31
acetic acid to methane, and the other for the reaction of carbon dioxide and hydrogen into methane. In order to harvest hydrogen from the overall process and generate biohythane, the hydrogen-to-methane pathway has to be inhibited [16]. Three aspects are most influential: (1) microbial physiological characteristics. Most hydrogen-generating microbes other than methanogens can produce spores in stress. Different pretreatment methods could be adopted to screen hydrogen producers [32]. In general, the most common pretreatment is heat treatment and pH shock. However, some studies reported the invalidity of such pretreatment [33], because not all hydrogen-producing bacteria are directly associated with the ability to form endospores. In addition, there are also many hydrogen-consuming bacteria that can form spores, such as acetogens, certain propionate and lactate producers [34]; (2) pH control. pH control is an important strategy for continuous operation of biohythane system, where pH varies depending on microbial species and activities, feedstock characteristics, organic loading, reactor structure, temperature, etc. The difference of pH is due to various microbial reactions involved, whereas the pH influences the distribution of respective metabolic products [35]. Low pH is one of the most critical strategies to inhibit the activity of methanogenesis. The suggested optimal pH for biohydrogen production ranges from 5.0 to 6.5, whereas the neutral pH is beneficial for methanogenesis; (3) growth rates of microbes. From the perspective of thermodynamics, changes of Gibbs free energy during hydrogen production were much larger than those of methanogenesis (Table S1). This means faster rates for microbial growth in biohydrogen fermentation. On the basis of this characteristic, a number of bioprocess parameters, such as hydraulic retention time (HRT) [34], temperature [36], oxidation-reduction potential (ORP) can be manipulated to enable microbial hydrogen process to be feasible in continuous operation. For instance, shortening HRT has been frequently used to wash out the methane producers in biohydrogen stage, further contributing to the two-stage separation [34]. Functionalization of biohydrogen reactor
From the perspective of microbial metabolisms, biohydrogen is an intermediate of biomethanation and can only be harvested through inhibiting or inactivating hydrogenotrophic methanogenesis. The performance of the biohydrogen-fermentation stage directly impacts the production of biomethane and formation of fermentation residues. The feedstock type and microorganism species contribute most to the functionalization of biohydrogen reactor [37]. For instance, sugar-rich substrates are ideal for hydrogen production considering the metabolic pathway of biohydrogen [8,38]. In comparison, proteinrich biowastes, such as animal manure is less desirable due to the limited hydrogen donor [38]. Cellulosic feedstock is also difficult because of its recalcitrance for microbial transformation [37]. Codigestion strategy could www.sciencedirect.com
www.sciencedirect.com Table 1 Thermodynamics and property of biohythane production compared with other biofuel processes. Energy recovery was defined as the combustion values of products divided by the combustion value of substrate. Specifically, theoretical energy recovery was defined as the combustion values of products divided by the combustion value of substrate, based on the theoretical conversion of substrate as illustrated in the biochemical reactions. In terms of theoretical energy recovery over 100% in some cases, this is because external heating for anaerobic fermentation was not considered. If the heating supply is included in the denominator, the total energy recovery will be less than 100% based on the law of energy conservation Bioprocess and energy carrier One stage H2
Biochemical reaction
DG (kJ/mol)
DH (kJ/mol)
Theoretical energy recovery
Technical stage of respective bioprocesses
206 27
89 628
41% 122%
Bench scale Bench scale
CH4
C6H12O6 = 3CH4 + 3CO2
419
137
90%
Industry scale
Ethanol
C6H12O6 = 2CH3CH2OH + 2CO2
236
68
97%
Industry scale
Butanol Electricity Two stages H2 + CH4
C6H12O6 = 2C4H10O + 2CO2 C6H12O6 + 6O2 = 6H2O + 6CO2 + electricity
273 2873
124 2803
95% 100%
Industry scale Bench/pilot scale
C6H12O6 + 2H2O = 4H2 + 2CH4 + 4CO2
242
117
104%
Bench/pilot scale
H2
C6H12O6 + 7H2O = 12H2 + 6CO2
27
628
122%
Bench scale
H2 + electricity
C6H12O6 + 4O2 = 4H2 + 2H2O + 6CO2 + electricity
2554
1663.8
100%
Bench scale
CH4 + electricity
C6H12O6 + 4O2 = CH4 + 4H2O + 6CO2 + electricity
1533
2306
100%
Bench scale
Dark fermentation: Clostridium, Enterobacter [34,37,50] Cell-free synthetic bioprocess [24], or electrohydrogenesis [25] Anaerobic digestion: anaerobic acetogens and methanogens [51] Anaerobic fermentation: Saccharomyces cerevisiae, Scenedesmus obliquus [21] Anaerobic fermentation: Clostridium acetobutylicum [52] Microbial fuel cell: Geobacter [26] Anaerobic fermentation: Clostridium, acetoclastic methanogens [7,8] Dark fermentation and Photo fermentation: Clostridium butyricum and Rhodopseudomonas palustris [22] Dark fermentation and Microbial fuel cell: Clostridium, Geobater [19] Anaerobic digestion and Microbial fuel cell: methanogens and Geobacter [20]
Technical challenges for scale up biohythane technology Liu et al. 27
Current Opinion in Biotechnology 2018, 50:25–31
C6H12O6 + 2H2O = 2CH3COOH + 4H2 + 2CO2 C6H12O6 + 7H2O = 12H2 + 6CO2
Bioprocesses and Examples of involved microorganisms
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Figure 1
Biohythane process (Vehicle fuel scenario)
Biohythane process (CHP scenario)
Biogas process (Vehicle fuel scenario)
Biogas process (CHP scenario)
Direct combustion
Compression
-0.25
Utilization stage Production stage Pretreatment stage Transportation stage Net value -0.20
-0.15
-0.10
-0.05
0.00
0.05
Global warming (Pt) Current Opinion in Biotechnology
Normalized results in four scenarios of global warming. Please refer to the supporting material for the detailed setup, operation and experimental data of TSAF pilot system.
be promising but needs further field verification. Simultaneous saccharification and biohydrogen generation is a new strategy and has been reported for biohydrogen production from cornstalk via microbial consortium engineering [30]. A large number of bench study were conducted employing pure culture for biohydrogen production. For example, metabolic engineering of known hydrogen-producer Enterobacter aerogenes has been extensively investigated via the control of NADH or formate pathways [29,39,40]. It remains a great challenge for longterm stable operations treating low-grade complex feedstocks, such as lignocellulosic biowaste [37,41]. Using microbial consortium enriched from the nature may be more competitive for biohythane production from real biowastes [32,42]. Energy recovery of biohythane system
It is known that biohythane production has higher theoretical energy recovery based on the chemical energy of the feedstock than biogas production (Table 1). However, compared to biogas production using only one reactor (in most cases), biohythane normally needs two reactors for biohydrogen and subsequent biomethane production. Additional energy loading are needed for heating up biohydrogen reactor [43]. Energy losses take place during anaerobic fermentation [43], via the stage shift [36], and release of products. Energy balance on large-scale biohythane system has yet to be fully evaluated. Current Opinion in Biotechnology 2018, 50:25–31
Utilization of products and the sustainability of biohythane system Utilization of biohythane and coproducts
Biohythane could be developed as a green and efficient vehicle fuel after upgrading in one purification step. The removal of carbon dioxide from biohythane is mature technology and hythane as transport fuel has been verified. Biohythane could be also used as the C1 feedstock for chemical production via metabolic engineering [4]. The challenges for biohythane utilization are undeveloped downstream network for hythane filling station and the lack of standard for implementation. Meanwhile, the use of coproducts during the biohythane process must be considered. The liquid digestate is the main coproduct and the valorization of liquid is a challenge [44]. In principle, the fermented water could be used as liquid fertilizer or recycled as nutrients for the TSAF [45]. The challenge for nutrient reuse is the limitation of land [44]. The challenge for recycling into TASF is the negative influence of methanogenesis on biohydrogen for longterm operation [45]. The solid residue released which contains lignin or cellulose could be developed for plant cultivation [46], or as biochemicals [47]. Environmental credit and challenges of biohythane system
In order to make TSAF practically feasible, biohydrogen is suggested to contribute to the increased efficiency and www.sciencedirect.com
Technical challenges for scale up biohythane technology Liu et al. 29
Figure 2
Energy supply for local community
Biohythane Local community
Waste biomass
Closed EcoEnergy Circle
Biohythane upgrading
Hythane vechile
Liquid digestate
Pretreatment Plant growth Solid residue
BioCH4 BioH2 Feedstock tank Waste biomass
Biohythane system via TASF
Biorefinery Current Opinion in Biotechnology
A biorefinery mode of waste biomass via biohythane process.
decreased GHG emissions [8,48]. TSAF of organic wastewater will extend the production value chain by supplying hydrogen and methane for heating and revenue from treating wastewater [49]. The costs of biomass feedstock will largely impact the profitability of two-stage systems for biohythane production [11], suggesting that low-grade biomass waste will be more suitable substrates for practical applications. We recently carried out life cycle assessment (LCA) of biohythane production from cornstalk based on a pilotscale TSAF demonstration in comparison with other model processes, including biogas, direct combustion, and compression (unpublished result, Supporting material). Biohythane in vehicle fuel scenario exhibited the most favorable GHG impact among all the model processes (Figure 1), indicating biohythane process is the most suitable approach for the energy production from cornstalk in terms of climate change effect.
Concluding remarks and future perspective Biohythane via TSAF using waste biomass could be a promising technology for higher energy recovery and cleaner transport biofuel than biogas. However, a number of technical challenges need to be addressed before largescale production of biohythane, including the stability of www.sciencedirect.com
biohydrogen reactor, control and integration of TSAF, and whole energy efficiency of biohythane system. At present, production of biohythane alone by TSAF is not economically and ecological feasible. To make TSAF viable, the feedstock should be waste biomass or wastewater in order to reduce the feedstock cost. Furthermore, nutrient-rich fermented broth and solid residue should be utilized as byproduct. A biorefinery mode is specifically important for biohythane system (Figure 2). Effective utilization of all products of waste biomass in an implementable biorefinery mode is the key issue of system engineering for the scale up of the biohythane process.
Conflict of interest The authors declare no conflict of interest.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51561145013), the National Key Research and Development Program of China (2016YFD0501402), Beijing Youth Top-notch Talent Support Project (2015000026833ZK10), and NSFC-JST Cooperative Research Project (21161140328).
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10. 1016/j.copbio.2017.08.014. Current Opinion in Biotechnology 2018, 50:25–31
30 Energy biotechnology
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ScienceDirect Bioprocess engineering for biohythane production from low-grade waste biomass: technical challenges towards scale up Zhidan Liu1, Buchun Si1, Jiaming Li1, Jianwei He2, Chong Zhang2,3, Yuan Lu2,3, Yuanhui Zhang1,4 and Xin-Hui Xing2,3 A concept of biohythane production by combining biohydrogen and biomethane together via two-stage anaerobic fermentation (TSAF) has been recently proposed and considered as a promising approach for sustainable hythane generation from waste biomass. The advantage of biohythane over traditional biogas are more environmentally benign, higher energy recovery and shorter fermentation time. However, many of current efforts to convert waste biomass into biohythane are still at the bench scale. The system bioprocess study and scale up for industrial application are indispensable. This paper outlines the general approach of biohythane by comparing with other biological processes. The technical challenges are highlighted towards scale up of biohythane system, including functionalization of biohydrogen-producing reactor, energy efficiency, and bioprocess engineering of TSAF. Addresses 1 Laboratory of Environment-Enhancing Energy (E2E), and Key Laboratory of Agricultural Engineering in Structure and Environment, Ministry of Agriculture; College of Water, Resources and Civil Engineering, China Agricultural University, Beijing 100083, China 2 MOE Key Lab of Industrial Biocatalysis, Institute of Biochemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China 3 Center for Synthetic and Systems Biology, Tsinghua University, Beijing 100084, China 4 Department of Agricultural and Biological Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA Corresponding authors: Zhang, Chong ([email protected]), Xing, Xin-Hui ([email protected])
Current Opinion in Biotechnology 2018, 50:25–31 This review comes from a themed issue on Energy biotechnology Edited by Akihiko Kondo and Hal Alper
http://dx.doi.org/10.1016/j.copbio.2017.08.014
wastes, agricultural residues and algal blooms, is a worldwide environmental concern. On the other hand, it contains renewable carbon and energy resources [2,3]. Value-added valorization of waste biomass is of great importance to a sustainable society [4]. Hythane, as a mixture of hydrogen and methane, also known as hydrogen enriched compressed natural gas (HCNG), has received extensive attention as a vehicle fuel [5]. Hythane displays remarkable advantages over compressed natural gas [6], such as reduced greenhouse gas (GHG) emissions, and improved fuel efficiency. One green alternative to provision of sustainable hythane instead of fossil base is two-stage anaerobic fermentation (TSAF) of waste biomass [7,8,9]. Note that much of previous knowledge about conventional TSAF is mainly focused on acidification (first stage) and methanogenesis (second), while not aiming at hydrogen production but enhanced methanogenesis [10]. In contrast, biohythane consists of hydrogen, methane and carbon dioxide harvested from the TSAF with the first stage for biohydrogen generation, which may be upgraded to biobased hythane by removing the carbon dioxide in one step [8]. Advantages of biohythane over traditional biogas are improved energy recovery, shortened fermentation time, flexible H2/CH4 ratio, and more environmentally benign and process robustness for handling waste biomass [7,8]. We have reviewed the research advances of TSAF for the coproduction of hydrogen and methane and proposed a new concept of biohythane in 2013 [8]. Since then, the term of biohythane has been gradually accepted in the field of gaseous biofuel [7,9,11–13,14,15–17]. However, scale up of biohythane system has yet to be developed, although a process demonstration for hydrogen and methane production from sugar-based kitchen waste via TASF was performed [18]. Process engineering for biohythane production is still in its infant stage.
0958-1669/ã 2017 Elsevier Ltd. All rights reserved.
Introduction Huge amount of waste biomass [1] generated from various social activities such as animal wastes, food processing www.sciencedirect.com
In this paper, we outline the biochemical reactions and thermodynamics of biohythane production by comparing with other biological processes for energy production from waste biomass. The technical challenges are highlighted towards scale up of biohythane production process, including functionalization of biohydrogen reactor, energy efficiency of biohythane system, and system engineering of TSAF. Current Opinion in Biotechnology 2018, 50:25–31
26 Energy biotechnology
Biochemical reactions and thermodynamics of biohythane Two-stage biohydrogen and biomethane (biohythane) production was compared with other typical biofuel processes using glucose as the model substrate, focusing on thermodynamic and technical evaluation (Table 1). Most of the single-stage bioprocesses (hydrogen [19], methane [20], or ethanol [21]) face the challenges on how to deal with the remaining residuals. Volatile fatty acids (VFAs), as the main components of the fermentation residuals in anaerobic fermentation, can be further converted into energy carriers such as methane [7], hydrogen [22], electricity [20] or other biochemicals [23] by establishing a second anaerobic stage. Among all the listed pathways, the maximum bioconversion of glucose to hydrogen (12 mol/mol glucose) results in the highest theoretical energy recovery. An energy recovery higher than 100% is the result of the absorption and conversion of external heat into biohydrogen. However, these pathways are limited by the strict requirements of the substrate [24], low process efficiency, or expense of the reactors [25]. A major difference between bioethanol and biohydrogen/ biomethane is that a pure strain and a narrow range of substrates are mostly needed for the former [21]. Harvesting electricity or value-added chemicals through microbial technologies is an emerging approach for waste valorization [26,27]. However, the scale up of MFC for practical application still suffers from the cost-intensive materials, and long-term operation stability [28]. Given that a hydrogen yield of 4 mol/mol glucose can be achieved through dark fermentation, theoretical energy recovery for hydrogen production (41%) is the lowest value among the biofuel processes (Table 1). In fact, the current hydrogen yield is normally lower than 2 mol/ mol glucose due to the limited metabolic fluxes [29]. Single-stage hydrogen production through dark fermentation is thus not energy and cost effective [19], and should be combined with other value-added processes. Methane fermentation has been well developed, which is, however, time-consuming and challengeable for treating high-solid organic waste. Instead, the biohythane system via TSAF resulted in enhanced energy recovery and reduced fermentation time [7,8]. In addition, in a biohythane system dealing with lignocellulosic biomass, saccharification and biohydrogen production could be simultaneously implemented in the first stage via microbial consortium engineering [30].
Technical challenges for scale up Microbial consortium and engineering control of biohydrogen and biomethane processes
The traditional anaerobic methane fermentation normally incorporated microorganisms with different functions to establish a synergic microbial consortium [31]. In particular, two kinds of bacteria take part in the methanogenesis process: one responsible for the conversion of Current Opinion in Biotechnology 2018, 50:25–31
acetic acid to methane, and the other for the reaction of carbon dioxide and hydrogen into methane. In order to harvest hydrogen from the overall process and generate biohythane, the hydrogen-to-methane pathway has to be inhibited [16]. Three aspects are most influential: (1) microbial physiological characteristics. Most hydrogen-generating microbes other than methanogens can produce spores in stress. Different pretreatment methods could be adopted to screen hydrogen producers [32]. In general, the most common pretreatment is heat treatment and pH shock. However, some studies reported the invalidity of such pretreatment [33], because not all hydrogen-producing bacteria are directly associated with the ability to form endospores. In addition, there are also many hydrogen-consuming bacteria that can form spores, such as acetogens, certain propionate and lactate producers [34]; (2) pH control. pH control is an important strategy for continuous operation of biohythane system, where pH varies depending on microbial species and activities, feedstock characteristics, organic loading, reactor structure, temperature, etc. The difference of pH is due to various microbial reactions involved, whereas the pH influences the distribution of respective metabolic products [35]. Low pH is one of the most critical strategies to inhibit the activity of methanogenesis. The suggested optimal pH for biohydrogen production ranges from 5.0 to 6.5, whereas the neutral pH is beneficial for methanogenesis; (3) growth rates of microbes. From the perspective of thermodynamics, changes of Gibbs free energy during hydrogen production were much larger than those of methanogenesis (Table S1). This means faster rates for microbial growth in biohydrogen fermentation. On the basis of this characteristic, a number of bioprocess parameters, such as hydraulic retention time (HRT) [34], temperature [36], oxidation-reduction potential (ORP) can be manipulated to enable microbial hydrogen process to be feasible in continuous operation. For instance, shortening HRT has been frequently used to wash out the methane producers in biohydrogen stage, further contributing to the two-stage separation [34]. Functionalization of biohydrogen reactor
From the perspective of microbial metabolisms, biohydrogen is an intermediate of biomethanation and can only be harvested through inhibiting or inactivating hydrogenotrophic methanogenesis. The performance of the biohydrogen-fermentation stage directly impacts the production of biomethane and formation of fermentation residues. The feedstock type and microorganism species contribute most to the functionalization of biohydrogen reactor [37]. For instance, sugar-rich substrates are ideal for hydrogen production considering the metabolic pathway of biohydrogen [8,38]. In comparison, proteinrich biowastes, such as animal manure is less desirable due to the limited hydrogen donor [38]. Cellulosic feedstock is also difficult because of its recalcitrance for microbial transformation [37]. Codigestion strategy could www.sciencedirect.com
www.sciencedirect.com Table 1 Thermodynamics and property of biohythane production compared with other biofuel processes. Energy recovery was defined as the combustion values of products divided by the combustion value of substrate. Specifically, theoretical energy recovery was defined as the combustion values of products divided by the combustion value of substrate, based on the theoretical conversion of substrate as illustrated in the biochemical reactions. In terms of theoretical energy recovery over 100% in some cases, this is because external heating for anaerobic fermentation was not considered. If the heating supply is included in the denominator, the total energy recovery will be less than 100% based on the law of energy conservation Bioprocess and energy carrier One stage H2
Biochemical reaction
DG (kJ/mol)
DH (kJ/mol)
Theoretical energy recovery
Technical stage of respective bioprocesses
206 27
89 628
41% 122%
Bench scale Bench scale
CH4
C6H12O6 = 3CH4 + 3CO2
419
137
90%
Industry scale
Ethanol
C6H12O6 = 2CH3CH2OH + 2CO2
236
68
97%
Industry scale
Butanol Electricity Two stages H2 + CH4
C6H12O6 = 2C4H10O + 2CO2 C6H12O6 + 6O2 = 6H2O + 6CO2 + electricity
273 2873
124 2803
95% 100%
Industry scale Bench/pilot scale
C6H12O6 + 2H2O = 4H2 + 2CH4 + 4CO2
242
117
104%
Bench/pilot scale
H2
C6H12O6 + 7H2O = 12H2 + 6CO2
27
628
122%
Bench scale
H2 + electricity
C6H12O6 + 4O2 = 4H2 + 2H2O + 6CO2 + electricity
2554
1663.8
100%
Bench scale
CH4 + electricity
C6H12O6 + 4O2 = CH4 + 4H2O + 6CO2 + electricity
1533
2306
100%
Bench scale
Dark fermentation: Clostridium, Enterobacter [34,37,50] Cell-free synthetic bioprocess [24], or electrohydrogenesis [25] Anaerobic digestion: anaerobic acetogens and methanogens [51] Anaerobic fermentation: Saccharomyces cerevisiae, Scenedesmus obliquus [21] Anaerobic fermentation: Clostridium acetobutylicum [52] Microbial fuel cell: Geobacter [26] Anaerobic fermentation: Clostridium, acetoclastic methanogens [7,8] Dark fermentation and Photo fermentation: Clostridium butyricum and Rhodopseudomonas palustris [22] Dark fermentation and Microbial fuel cell: Clostridium, Geobater [19] Anaerobic digestion and Microbial fuel cell: methanogens and Geobacter [20]
Technical challenges for scale up biohythane technology Liu et al. 27
Current Opinion in Biotechnology 2018, 50:25–31
C6H12O6 + 2H2O = 2CH3COOH + 4H2 + 2CO2 C6H12O6 + 7H2O = 12H2 + 6CO2
Bioprocesses and Examples of involved microorganisms
28 Energy biotechnology
Figure 1
Biohythane process (Vehicle fuel scenario)
Biohythane process (CHP scenario)
Biogas process (Vehicle fuel scenario)
Biogas process (CHP scenario)
Direct combustion
Compression
-0.25
Utilization stage Production stage Pretreatment stage Transportation stage Net value -0.20
-0.15
-0.10
-0.05
0.00
0.05
Global warming (Pt) Current Opinion in Biotechnology
Normalized results in four scenarios of global warming. Please refer to the supporting material for the detailed setup, operation and experimental data of TSAF pilot system.
be promising but needs further field verification. Simultaneous saccharification and biohydrogen generation is a new strategy and has been reported for biohydrogen production from cornstalk via microbial consortium engineering [30]. A large number of bench study were conducted employing pure culture for biohydrogen production. For example, metabolic engineering of known hydrogen-producer Enterobacter aerogenes has been extensively investigated via the control of NADH or formate pathways [29,39,40]. It remains a great challenge for longterm stable operations treating low-grade complex feedstocks, such as lignocellulosic biowaste [37,41]. Using microbial consortium enriched from the nature may be more competitive for biohythane production from real biowastes [32,42]. Energy recovery of biohythane system
It is known that biohythane production has higher theoretical energy recovery based on the chemical energy of the feedstock than biogas production (Table 1). However, compared to biogas production using only one reactor (in most cases), biohythane normally needs two reactors for biohydrogen and subsequent biomethane production. Additional energy loading are needed for heating up biohydrogen reactor [43]. Energy losses take place during anaerobic fermentation [43], via the stage shift [36], and release of products. Energy balance on large-scale biohythane system has yet to be fully evaluated. Current Opinion in Biotechnology 2018, 50:25–31
Utilization of products and the sustainability of biohythane system Utilization of biohythane and coproducts
Biohythane could be developed as a green and efficient vehicle fuel after upgrading in one purification step. The removal of carbon dioxide from biohythane is mature technology and hythane as transport fuel has been verified. Biohythane could be also used as the C1 feedstock for chemical production via metabolic engineering [4]. The challenges for biohythane utilization are undeveloped downstream network for hythane filling station and the lack of standard for implementation. Meanwhile, the use of coproducts during the biohythane process must be considered. The liquid digestate is the main coproduct and the valorization of liquid is a challenge [44]. In principle, the fermented water could be used as liquid fertilizer or recycled as nutrients for the TSAF [45]. The challenge for nutrient reuse is the limitation of land [44]. The challenge for recycling into TASF is the negative influence of methanogenesis on biohydrogen for longterm operation [45]. The solid residue released which contains lignin or cellulose could be developed for plant cultivation [46], or as biochemicals [47]. Environmental credit and challenges of biohythane system
In order to make TSAF practically feasible, biohydrogen is suggested to contribute to the increased efficiency and www.sciencedirect.com
Technical challenges for scale up biohythane technology Liu et al. 29
Figure 2
Energy supply for local community
Biohythane Local community
Waste biomass
Closed EcoEnergy Circle
Biohythane upgrading
Hythane vechile
Liquid digestate
Pretreatment Plant growth Solid residue
BioCH4 BioH2 Feedstock tank Waste biomass
Biohythane system via TASF
Biorefinery Current Opinion in Biotechnology
A biorefinery mode of waste biomass via biohythane process.
decreased GHG emissions [8,48]. TSAF of organic wastewater will extend the production value chain by supplying hydrogen and methane for heating and revenue from treating wastewater [49]. The costs of biomass feedstock will largely impact the profitability of two-stage systems for biohythane production [11], suggesting that low-grade biomass waste will be more suitable substrates for practical applications. We recently carried out life cycle assessment (LCA) of biohythane production from cornstalk based on a pilotscale TSAF demonstration in comparison with other model processes, including biogas, direct combustion, and compression (unpublished result, Supporting material). Biohythane in vehicle fuel scenario exhibited the most favorable GHG impact among all the model processes (Figure 1), indicating biohythane process is the most suitable approach for the energy production from cornstalk in terms of climate change effect.
Concluding remarks and future perspective Biohythane via TSAF using waste biomass could be a promising technology for higher energy recovery and cleaner transport biofuel than biogas. However, a number of technical challenges need to be addressed before largescale production of biohythane, including the stability of www.sciencedirect.com
biohydrogen reactor, control and integration of TSAF, and whole energy efficiency of biohythane system. At present, production of biohythane alone by TSAF is not economically and ecological feasible. To make TSAF viable, the feedstock should be waste biomass or wastewater in order to reduce the feedstock cost. Furthermore, nutrient-rich fermented broth and solid residue should be utilized as byproduct. A biorefinery mode is specifically important for biohythane system (Figure 2). Effective utilization of all products of waste biomass in an implementable biorefinery mode is the key issue of system engineering for the scale up of the biohythane process.
Conflict of interest The authors declare no conflict of interest.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (51561145013), the National Key Research and Development Program of China (2016YFD0501402), Beijing Youth Top-notch Talent Support Project (2015000026833ZK10), and NSFC-JST Cooperative Research Project (21161140328).
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at http://dx.doi.org/10. 1016/j.copbio.2017.08.014. Current Opinion in Biotechnology 2018, 50:25–31
30 Energy biotechnology
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