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Fossil fuel use drives greenhouse gas emissions and climate change, highlighting the need for alternatives like biomass-derived syngas. Syngas, mainly H 2 and CO, is produced via biomass gasification and offers a solution to environmental challenges. Syngas fermentation through the Wood-Ljungdahl pathway yields valuable chemicals under mild conditions. However, challenges in scaling up persist due to issues like unpredictable syngas composition and microbial fermentation contamination. This review covers advancements in genetic tools and metabolic engineering to expand product range, highlighting crucial enabling technologies that expedite strain development for acetogens and other non-model organisms. This review paper provides an in-depth exploration of syngas fermentation, covering microorganisms, gas composition effects, separation techniques, techno economic analysis, and commercialization efforts., Competing Interests: Declaration of interests The authors declare no conflicts of interest., (Copyright © 2024 Elsevier Ltd. All rights reserved.)

Electrocatalytic CO 2 reduction to CO and formate can be coupled to gas fermentation with anaerobic microorganisms. In combination with a competing hydrogen evolution reaction in the cathode in aqueous medium, the in situ, electrocatalytic produced syngas components can be converted by an acetogenic bacterium, such as Clostridium ragsdalei , into acetate, ethanol, and 2,3-butanediol. In order to study the simultaneous conversion of CO, CO 2 , and formate together with H 2 with C. ragsdalei , fed-batch processes were conducted with continuous gassing using a fully controlled stirred tank bioreactor. Formate was added continuously, and various initial CO partial pressures (pCO 0 ) were applied. C. ragsdalei utilized CO as the favored substrate for growth and product formation, but below a partial pressure of 30 mbar CO in the bioreactor, a simultaneous CO 2 /H 2 conversion was observed. Formate supplementation enabled 20-50% higher growth rates independent of the partial pressure of CO and improved the acetate and 2,3-butanediol production. Finally, the reaction conditions were identified, allowing the parallel CO, CO 2 , formate, and H 2 consumption with C. ragsdalei at a limiting CO partial pressure below 30 mbar, pH 5.5, n = 1200 min -1 , and T = 32 °C. Thus, improved carbon and electron conversion is possible to establish efficient and sustainable processes with acetogenic bacteria, as shown in the example of C. ragsdalei.

The tandem application of CO 2 electrolysis with syngas fermentation holds promise for achieving heightened production rates and improved product quality. However, the significant impact of syngas composition on mixed culture-based microbial chain elongation remains unclear. Additionally, effective methods for generating syngas with an adjustable composition from acidic CO 2 electrolysis are currently lacking. This study successfully demonstrated the production of medium-chain fatty acids from CO 2 through tandem acidic electrolysis with syngas fermentation. CO could serve as the sole energy source or as the electron donor (when cofed with acetate) for caproate generation. Furthermore, the results of gas diffusion electrode structure engineering highlighted that the use of carbon black, either alone or in combination with graphite, enabled consistent syngas generation with an adjustable composition from acidic CO 2 electrolysis (pH 1). The carbon black layer significantly improved the CO selectivity, increasing from 0% to 43.5% (0.05 M K + ) and further to 92.4% (0.5 M K + ). This enhancement in performance was attributed to the promotion of K + accumulation, stabilizing catalytically active sites, rather than creating a localized alkaline environment for CO 2 -to-CO conversion. This research contributes to the advancement of hybrid technology for sustainable CO 2 reduction and chemical production.

Syngas fermentation, in which microorganisms convert H 2 , CO, and CO 2 to acids and alcohols, is a promising alternative for carbon cycling and valorization. The intellectual landscape of the topic was characterized through a bibliometric analysis using a search query (SQ) that included all relevant documents on syngas fermentation available through the Web of Science database up to December 31st, 2021. The SQ was validated with a preliminary analysis in bibliometrix and a review of titles and abstracts of all sources. Although syngas fermentation began in the early 1980s, it grew rapidly beginning in 2008, with 92.5% of total publications and 87.3% of total citations from 2008 to 2021. The field has been steadily moving from fundamentals towards applications, suggesting that the field is maturing scientifically. The greatest number of publications and citations are from the USA, and researchers in China, Germany, and Spain also are highly active. Although collaborations have increased in the past few years, author-cluster analysis shows specialized research domains with little collaboration between groups. Based on topic trends, the main challenges to be address are related to mass-transfer limitations, and researchers are starting to explore mixed cultures, genetic engineering, microbial chain elongation, and biorefineries., (Copyright © 2022 Elsevier Ltd. All rights reserved.)

Syngas fermentation is a potential player for future emission reduction. The first demonstration and commercial plants have been successfully established. However, due to its novelty, development of syngas fermentation processes is still in its infancy, and the need to systematically unravel and understand further phenomena, such as substrate toxicity as well as gas transfer and uptake rates, still persists. This study describes a new online monitoring device based on the respiration activity monitoring system for cultivation of syngas fermenting microorganisms with gaseous substrates. The new device is designed to online monitor the carbon dioxide transfer rate (CO 2 TR) and the gross gas transfer rate during cultivation. Online measured data are used for the calculation of the carbon monoxide transfer rate (COTR) and hydrogen transfer rate (H 2 TR). In cultivation on pure CO and CO + H 2 , CO was continuously limiting, whereas hydrogen, when present, was sufficiently available. The maximum COTR measured was approximately 5 mmol/L/h for pure CO cultivation, and approximately 6 mmol/L/h for cultivation with additional H 2 in the gas supply. Additionally, calculation of the ratio of evolved carbon dioxide to consumed monoxide, similar to the respiratory quotient for aerobic fermentation, allows the prediction of whether acetate or ethanol is predominantly produced. Clostridium ljungdahlii, a model acetogen for syngas fermentation, was cultivated using only CO, and CO in combination with H 2 . Online monitoring of the mentioned parameters revealed a metabolic shift in fermentation with sole CO, depending on COTR. The device presented herein allows fast process development, because crucial parameters for scale-up can be measured online in small-scale gas fermentation., (© 2021 The Authors. Biotechnology and Bioengineering published by Wiley Periodicals LLC.)

An effective method for the production of value-added chemicals from food waste and lignocellulosic materials is a hybrid thermal-biological process, which involves gasification of the solid materials to syngas (primarily CO and H 2 ) followed by fermentation. This paper reviews the recent advances in this process. The special focus is on the cultivation methods that involve the use of single strains, defined mixed cultures and undefined mixed cultures for production of carboxylic acids and higher alcohols. A rate limiting step in these processes is the low mass transfer between the gas and the liquid phases. Therefore, novel techniques that can enhance the gas-liquid mass transfer including membrane- and trickle-bed bioreactors were discussed. Such bioreactors have shown promising results in increasing the volumetric mass transfer coefficient (k L a). High gas pressure also influences the mass transfer in certain batch processes, although the presence of impurities in the gas would impede the process., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

The hybrid gasification-syngas fermentation platform can produce more bioethanol utilizing all biomass components compared to the biochemical conversion technology. Syngas fermentation operates at mild temperatures and pressures and avoids using expensive pretreatment processes and enzymes. This study presents a new process simulation model developed with Aspen Plus® of a biorefinery based on a hybrid conversion technology for the production of anhydrous ethanol using 1200tons per day (wb) of switchgrass. The simulation model consists of three modules: gasification, fermentation, and product recovery. The results revealed a potential production of about 36.5million gallons of anhydrous ethanol per year. Sensitivity analyses were also performed to investigate the effects of gasification and fermentation parameters that are keys for the development of an efficient process in terms of energy conservation and ethanol production., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Clostridium carboxidivorans P7 is a strict anaerobic bacterium capable of converting syngas to biofuels. However, its fermentation profiles is poorly understood. Here, various end-products, including acetic acid, butyric acid, hexanoic acid, ethanol and butanol were supplemented to evaluate their effects on fermentation profiles in C. carboxidivorans at two temperatures. At 37°C, fatty acids addition likely led to more corresponding alcohols production. At 25°C, C2 and C4 fatty acids supplementation resulted in more corresponding higher fatty acids, while supplemented hexanoic acid increased yields of C2 and C4 fatty acids and hexanol. Supplementation of ethanol or butanol caused increased production of C2 and C4 acids at both temperatures; however, long-chain alcohols were still more likely produced at lower temperature. In conclusion, fermentation profiles of C. carboxidivorans can be changed in respond to pre-added end-products and carbon flow may be redirected to desired products by controlling culture conditions., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Technological solutions to reduce greenhouse gas (GHG) emissions from anthropogenic sources are required. Heavy industrial processes, such as steel making, contribute considerably to GHG emissions. Fermentation of carbon monoxide (CO)-rich off gases with wild-type acetogenic bacteria can be used to produce ethanol, acetate, and 2,3-butanediol, thereby, reducing the carbon footprint of heavy industries. Here, the processes for the production of ethanol from CO-rich off gases are discussed and a perspective on further routes towards an integrated biorefinery at a steel mill is given. Recent achievements in genetic engineering as well as integration of other biotechnology platforms to increase the product portfolio are summarized. Already, yields have been increased and the portfolio of products broadened. To develop a commercially viable process, however, the extraction from dilute product streams is a critical step and alternatives to distillation are discussed. Finally, another critical step is waste(water) treatment with the possibility to recover resources., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

Thermochemical-biological hybrid processing uses thermochemical decomposition of lignocellulosic biomass to produce a variety of intermediate compounds that can be converted into fuels and chemicals through microbial fermentation. It represents a unique opportunity for biomass conversion as it mitigates some of the deficiencies of conventional biochemical (pretreatment-hydrolysis-fermentation) and thermochemical (pyrolysis or gasification) processing. Thermochemical-biological hybrid processing includes two pathways: (i) pyrolysis/pyrolytic substrate fermentation, and (ii) gasification/syngas fermentation. This paper provides a comprehensive review of these two hybrid processing pathways, including the characteristics of fermentative substrates produced in the thermochemical stage and microbial utilization of these compounds in the fermentation stage. The current challenges of these two biomass conversion pathways include toxicity of the crude pyrolytic substrates, the inhibition of raw syngas contaminants, and the mass-transfer limitations in syngas fermentation. Possible approaches for mitigating substrate toxicities are discussed. The review also provides a summary of the current efforts to commercialize hybrid processing., (Copyright © 2015 Elsevier Inc. All rights reserved.)

This study proposed a submerged hollow fibre membrane bioreactor (HFMBR) system capable of achieving high carbon monoxide (CO) mass transfer for applications in microbial synthesis gas conversion systems. Hydrophobic polyvinylidene fluoride (PVDF) membrane fibres were used to fabricate a membrane module, which was used for pressurising CO in water phase. Pressure through the hollow fibre lumen (P) and membrane surface area per unit working volume of the liquid (A(S)/V(L)) were used as controllable parameters to determine gas-liquid volumetric mass transfer coefficient (k(L)a) values. We found a k(L)a of 135.72 h(-1) when P was 93.76 kPa and AS/VL was fixed at 27.5m(-1). A higher k(L)a of 155.16 h(-1) was achieved by increasing AS/VL to 62.5m(-1) at a lower P of 37.23 kPa. Practicality of HFMBR to support microbial growth and organic product formation was assessed by CO/CO2 fermentation using Eubacterium limosum KIST612., (Copyright © 2014 Elsevier Ltd. All rights reserved.)

Gas fermentation is gaining attention as a crucial technology for converting gaseous feedstocks into value-added chemicals. Despite numerous efforts over the past decade to investigate these innovative processes at a lab scale, to date, the evaluation of the technologies in relevant industrial environments is scarce. This study examines the fermentative production of acetate from biomass-derived syngas using Moorella thermoacetica. A mobile gas fermentation pilot plant was coupled to a bubbling fluidized-bed gasifier with syngas purification to convert crushed bark-derived syngas. The syngas purification steps included hot filtration, catalytic reforming, and final syngas cleaning. Different latter configurations were evaluated to enable a simplified syngas cleaning configuration for microbial syngas conversion compared to conventional catalytic synthesis. Fermentation tests using ultra-cleaned syngas showed comparable microbial growth (1.3 g/L) and acetate production (22.3 g/L) to the benchmark fermentation of synthetic gases (1.2 g/L of biomass and 25.2 g/L of acetate). Additional fermentation trials on partially purified syngas streams identified H2S and HCN as the primary inhibitory compounds. They also indicated that caustic scrubbing is an adequate and simplified final gas cleaning step to facilitate extended microbial fermentation. Overall, this study shows the potential of gas fermentation to valorize crude gaseous feedstocks, such as industrial off-gases, into platform chemicals. [ABSTRACT FROM AUTHOR]

BACKGROUND: Isopropanol and acetone production by syngas fermentation is a promising alternative to conventional fossil carbon‐dependent production. However, this alternative technology has not yet been scaled up to an industrial level owing to the relatively low product concentrations (about 5 wt% in total). This original research aims to develop cost‐effective and energy‐efficient processes for the recovery of isopropanol and acetone from highly dilute fermentation broth (>94 wt% water) for large‐scale production (about 100 ktIPA+AC y−1). RESULTS: Vacuum distillation and pass‐through distillation enhanced with heat pumps or multi‐effect distillation were efficiently coupled with regular atmospheric distillation and extractive distillation in several innovative intensified downstream processes. Over 99.2% of isopropanol and 100% of acetone were recovered as high‐purity end‐products (>99.8 wt%). Advanced heat pumping (mechanical vapor recompression) and heat integration techniques were implemented to decrease total annual costs (0.109–0.137 USD kgIPA+AC−1), reduce energy requirements (1.348–2.043 kWth h kgIPA+AC−1) and lower CO2 emissions (0.067–0.191 kgCO2 kgIPA+AC−1), resulting in highly competitive recovery processes. CONCLUSION: The proposed three novel isopropanol and acetone recovery processes from dilute broth significantly contribute to the expansion of sustainable industrial fermentation. Furthermore, this original research is the first one to develop novel pass‐through distillation technology for the complex isopropanol–acetone–water system. All the designed processes are highly economically competitive and environmentally viable. In addition to recovering efficiently both isopropanol and acetone, the designed downstream processes offer the possibility to enhance the fermentation process by recycling all the present microorganisms and reducing fresh‐water requirements. © 2023 The Authors. Journal of Chemical Technology and Biotechnology published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry (SCI). [ABSTRACT FROM AUTHOR]

Gasifiers produce a gaseous mixture of CO/CO2/H2, also known as synthesis gas (syngas), containing varying compositions and ratios depending on the lignocellulose material types, gasifier design, and gasification conditions. Different physicochemical and thermodynamic properties of each gas type in the various syngas blends can influence syngas fermentation performance for the production of chemicals such as acetate. This study examined the effect of syngas composition (CO, CO/H2, CO/CO2/H2, and CO/H2) and its corresponding ratio on acetate production using Moorella thermoacetica, a thermophilic homoacetogen as the biocatalyst. We also investigated the effect of yeast extract addition for enhancing acetate production. A syngas fermentation study performed at a total pressure of 19 psig (2.29 atm) demonstrated that syngas fermentation in the absence of CO (30%CO2/70%H2) or at low CO proportions (21%CO/24%CO2/55%H2) resulted in the highest volumetric productivity of acetate (0.046 ± 0.001 and 0.037 ± 0.001 g/L/h, respectively). Interestingly, syngas fermentation without CO reached the highest YP/X of 22.461 ± 0.574 g-acetate/g-biomass, indicating that more acetate was produced compared to cell biomass. Higher biomass production was obtained when the CO proportion was increased up to 75% in CO/H2 fermentation. However, the cell growth and acetate production dramatically decreased with increasing CO proportion up to 99.5% CO as the sole constituent of the syngas. Even so, acetate production using 99.5% CO could be improved by adding 2 g/L yeast extract. [ABSTRACT FROM AUTHOR]

Definition: Gas–liquid mass transfer is a major issue during various bioprocesses, particularly in processes such as syngas fermentation (SNF). Since SNF involves the movement of gases into the fermentation broth, there is always a rate-limiting step that reduces process efficiency. Improving this process could lead to increased efficiency, higher production of ethanol, and reduced energy consumption. One way to improve fluid transfer between gas and liquid is by incorporating nanoparticles (NPs) into the liquid phase. This entry describes recent advances in using NPs to improve gas–liquid mass transfer during SNF. The entry also describes the basics of SNF and the impact of NPs on the process and suggests areas for future research. For example, carbon nanotubes have been found to elevate the available surface area needed for gas–liquid transfer, thus improving the process efficiency. Another area is the use of NPs as carriers for enzymes involved in syngas fermentation. [ABSTRACT FROM AUTHOR]

Biotransformation of lignocellulose-derived synthetic gas (syngas) into acetic acid is a promising way of creating biochemicals from lignocellulosic waste materials. Acetic acid has a growing market with applications within food, plastics and for upgrading into a wide range of biofuels and bio-products. In this paper, we will review the microbial conversion of syngas to acetic acid. This will include the presentation of acetate-producing bacterial strains and their optimal fermentation conditions, such as pH, temperature, media composition, and syngas composition, to enhance acetate production. The influence of syngas impurities generated from lignocellulose gasification will further be covered along with the means to alleviate impurity problems through gas purification. The problem with mass transfer limitation of gaseous fermentation will further be discussed as well as ways to improve gas uptake during the fermentation. [ABSTRACT FROM AUTHOR]

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The aim of this study is to investigate the effect of sulfate on conversion of CO2 into organics using syngas fermentation with mixed culture. Fermentation tests were operated under H2 concentrations of 2.14 and 21.4 mmol/d, respectively, using substrates contained different initial concentrations of sulfate (i.e. 0, 2, 4, 6, 8, 10, and 12 mM). The addition of sulfate improved the acetate and formate production, and the enhancement was positively correlated with the sulfate concentrations from 2 to 8 mM. With 8 mM sulfate, the maximum acetate concentrations reached 75.4 ± 4.51 and 76.1 ± 7.77 mM under H2 concentrations of 2.14 and 21.4 mmol/d, respectively, which were 1.47 and 2.58 times higher than those of the treatment without sulfate. The biomass achieved with the sulfate addition was 52%-97% higher than that without sulfate. High-throughput pyrosequencing showed that with the presence of sulfate, Acetobacterium and Desulfovibrio were dominant in the microbial community with high relative abundance of 43% and 38%, respectively. This study suggested that the performance of syngas fermentation could be improved with co-metabolism between homoacetogen and sulfatereducing bacteria. [ABSTRACT FROM AUTHOR]

This work presents a strategy for optimizing the production process of ethanol via integrated gasification and syngas fermentation, a conversion platform of growing interest for its contribution to carbon recycling. The objective functions (minimum ethanol selling price (MESP), energy efficiency, and carbon footprint) were evaluated for the combinations of different input variables in models of biomass gasification, energy production from syngas, fermentation, and ethanol distillation, and a multi-objective genetic algorithm was employed for the optimization of the integrated process. Two types of waste feedstocks were considered, wood residues and sugarcane bagasse, with the former leading to lower MESP and a carbon footprint of 0.93 USD/L and 3 g CO2eq/MJ compared to 1.00 USD/L and 10 g CO2eq/MJ for sugarcane bagasse. The energy efficiency was found to be 32% in both cases. An uncertainty analysis was conducted to determine critical decision variables, which were found to be the gasification zone temperature, the split fraction of the unreformed syngas sent to the combustion chamber, the dilution rate, and the gas residence time in the bioreactor. Apart from the abovementioned objectives, other aspects such as water footprint, ethanol yield, and energy self-sufficiency were also discussed. [ABSTRACT FROM AUTHOR]

In this paper, a simulation model based on the non-stoichiometric equilibrium method via ASPEN Plus was established to analyze the gasification performance of 20 herbaceous and agricultural biomasses (H&ABs) linked with syngas fermentation and product purification units for ethanol production. The established simulation model does not consider the gasification system as a black box; it focuses the important processes in gasification such as drying, pyrolysis, gasification, and connection with bioethanol production plants. The results for the 20 H&AB options suggest that the specific mass flow rate of bioethanol from 1 kg of biomass input to the unit is in the range of 99–250 g/kg, and between them, the system fed by hazelnut shell biomass remarkably outranked other alternatives by 241 g/kg production due to the high beneficial results gained from the performance analysis. Additionally, a sensitivity analysis was performed by changing operating conditions such as gasification temperature and air-to-fuel ratio. The modeling results are given and discussed. The established model could be a useful approach to evaluate the impacts of a huge numbers of biomasses and operating parameters on bioethanol output. [ABSTRACT FROM AUTHOR]

In this study, the effects of organic nitrogen supplements, such as tryptone and peptone, on microbial growth and ethanol and acetic acid production were investigated to improve ethanol productivity in a Clostridium autoethanogenum culture using syngas as the sole carbon source. The concentration of yeast extract, which is an organic nitrogen source in a basal medium, was optimized to 0.5 g/L before adding the supplements. In the C. autoethanogenum culture using tryptone and peptone as supplements, microbial growth and acetic acid formation were proportionate to the supplement concentration till up to 5.0 g/L of tryptone or peptone. However, ethanol concentration reached a maximum level of 4.57 g/L and 4.23 g/L in the cultures supplemented with 0.5 g/L of tryptone and 0.5 g/L of peptone, respectively. Ethanol production did not increase at a supplement concentration higher than 0.5 g/L. Thus, the addition of tryptone and peptone at a concentration of 0.5 g/L showed the best volumetric ethanol productivities (0.286 and 0.264 g/L·d, respectively), which were enhanced by 45.9% and 44.3% compared to the results of the control experiment. [ABSTRACT FROM AUTHOR]

Syngas fermentation processes with acetogenic bacteria like Clostridium carboxidivorans have been proven to be a promising approach for the conversion of CO-rich waste gases into short- and medium-chain alcohols. The challenge of synthesis gas impurities, on the other hand, has always been a major concern for establishing an industrial-scale process, since some of the trace components in waste gases, such as NH3, H2S, and NO x , can have inhibiting or even toxic effects on microbial growth and product formation. Thus, this study aims to identify the effects of the main trace impurities in syngas from gasification of biogenic residues by the supply of defined concentrations of trace impurities to the cultivation medium. Autotrophic gas fermentation studies were performed with C. carboxidivorans in batch-operated fully-controlled stirred-tank bioreactors with continuous gas supply (80% CO and 20% CO2). The syngas components NH3 and H2S had a positive effect on both growth and alcohol formation (ethanol, 1-butanol, and 1-hexanol). The maximum biomass concentration was increased by more than 50%, and the maximum ethanol concentration was more than doubled with 5.0 g L−1 NH4Cl or 1.0 g L−1 H2S provided by the addition of 2.2 g L−1 thioacetamide. The addition of the nitrogen oxide species nitrate and nitrite, on the other hand, reduced biomass growth as well as alcohol concentrations. Already, the supply of 0.1 g L−1 NaNO3 resulted in reduced growth and 25% reduction of the maximum ethanol concentration. The production of the longer chain alcohols 1-butanol and 1-hexanol was reduced as well. All NaNO2 concentrations tested showed a strong toxic effect on the metabolism of C. carboxidivorans , and neither CO consumption nor product formation was observed after addition. As a consequence, NO x components in syngas from the gasification of biogenic residues should be reduced by the gasification process and/or selectively removed from the syngas after gasification. [ABSTRACT FROM AUTHOR]

Higher alcohols such as butanol (C4 alcohol) and hexanol (C6 alcohol) are superior biofuels compared to ethanol. Clostridium carboxidivorans P7 is a typical acetogen capable of producing C4 and C6 alcohols natively. In this study, the composition of trace metals in culture medium was adjusted, and the effects of these adjustments on artificial syngas fermentation by C. carboxidivorans P7 were investigated. Nickel and ferrous ions were essential for growth and metabolite synthesis during syngas fermentation by P7. However, a decreased dose of molybdate improved alcohol fermentation performance by stimulating carbon fixation and solventogenesis. In response to the modified trace metal composition, cells grew to a maximum OD600nm of 1.6 and accumulated ethanol and butanol to maximum concentrations of 2.0 and 1.0 g/L, respectively, in serum bottles. These yields were ten-fold higher than the yields generated using the original composition of trace metals. Furthermore, 0.5 g/L of hexanol was detected at the end of fermentation. The results from gene expression experiments examining genes related to carbon fixation and organic acid and solvent synthesis pathways revealed a dramatic up-regulation of the Wood–Ljungdahl pathway (WLP) gene cluster, the bcs gene cluster, and a putative CoA transferase and butanol dehydrogenase, thereby indicating that both de novo synthesis and acid re-assimilation contributed to the significantly elevated accumulation of higher alcohols. The bdh35 gene was speculated to be the key target for butanol synthesis during solventogenesis. [ABSTRACT FROM AUTHOR]

Biomass-generated syngas conversion through fermentation is a promising technique for bioethanol production due to increasing demand for clean and sustainable energy. However, lignocellulosic biomass is difficult to degrade wholly, and traditional pretreatment process has several drawbacks. The present study emphasizes on bioethanol production from lignocellulosic biomass-based syngas including the main composition of N 2 (45.58%), CO (22.92%), CO 2 (7.9%), H 2 (13.05%), and CH 4 (1.13%). Field emission electronic microscopic analysis was used to characterize freshly cultured Clostridium butyricum for syngas fermentation and experiment was run in a bioreactor (TFB). The obtained yield of bioethanol was analyzed by nuclear magnetic resonance and gas chromatography-mass spectrometry analyses. For this syngas fermentation, treated syngas was preferred, as most of the Clostridium butyricum grown on best fermentation conditions. The results show that except 0.03% of CO 2 , other gases were dissolved entirely. It is also found that extracted bioethanol was identified by corresponding NMR (1H) spectra of methyle group (CH 3 -), methylene group (–CH 2 –) and hydroxyl group (OH). The yield of bioethanol was 29.94 mmol from 1 L of syngas. Hence, this biomass-generated syngas is the appropriate renewable energy source for the meetup of future energy needs. Image 1 • Syngas fermentation was performed in a tar free bioreactor (TFB). • Syngas and charcoal were used as a substrate of Clostridium butyricum. • Clostridium butyricum is one of the suited microorganisms for bioethanol production. • 500 times higher CFU was found when treated syngas was used instead of untreated syngas. • Yield of bioethanol was 29.94 mmol from 1 L of syngas. [ABSTRACT FROM AUTHOR]

Syngas, the product of biomass gasification, can play an important role in moving towards the production of renewable chemical commodities, by using acetogenic bacteria to ferment those gaseous mixtures. Due to the complex and changing nature of biomass, the composition and the impurities present in the final biomass-derived syngas will vary. Because of this, it is important to assess the impact of these factors on the fermentation outcome, in terms of yields, productivity, and product formation and ratio. In this study, Clostridium ljungdahlii was used in a fed-batch fermentation system to analyze the effect of three different biomass-derived syngases, and to compare them to equivalent, clean syngas mixtures. Additionally, four other clean syngas mixtures were used, and the effects on product ratio, productivity, yield, and growth were documented. All biomass-derived syngases were suitable to be used as substrates, without experiencing any complete inhibitory effects. From the obtained results, it is clear that the type of syngas, biomass-derived or clean, had the greatest impact on product formation ratios, with all biomass-derived syngases producing more ethanol, albeit with lesser total productivity. [ABSTRACT FROM AUTHOR]

This study presents a new simulation model developed with ASPEN Plus of waste biomass gasification integrated with syngas fermentation and product recovery units for bioethanol production from garden waste as a lignocellulosic biomass. The simulation model includes three modules: gasification, fermentation, and ethanol recovery. A parametric analysis is carried out to investigate the effect of gasification temperature (500-1500 °C) and equivalence ratio (0.2-0.6) on the gasification performance and bioethanol production yield. The results reveal that, for efficient gasification and high ethanol production, the operating temperature range should be 700-1000 °C, as well as an equivalence ratio between 0.2 and 0.4. At optimal operating conditions, the bioethanol production yield is 0.114 kg/h per 1 kg/h input garden waste with 50% moisture content. It is worth mentioning that this parameter increases to 0.217 kgbioethanol/kggarden waste under dry-based conditions. [ABSTRACT FROM AUTHOR]

Bioethanol is an eco-friendly biofuel due to its merit that makes it a top-tier fuel. The present study emphasized on bioethanol production from hydrogen-rich syngas through fermentation using Sacharomyces cerevisiea. Syngas fermentation was performed in a tar free fermenter using a syngas mixture of 13.05% H 2 , 22.92% CO, 7.9% CO 2 , and 1.13% CH 4 , by volume. In the fermentation process, effects of various parameters including syngas impurity, temperature, pH, colony forming unit, total organic carbon and syngas composition were investigated. The yield of bioethanol was identified by Gas chromatography-Mass spectrometry analysis and further, it was confirmed by Nuclear magnetic resonance (1H) analysis. From GC-MS results, it is revealed that the concentration of bioethanol using Saccharomyces cerevisiae was 30.56 mmol from 1 L of syngas. Thus, hydrogen-rich syngas is suited for bioethanol production through syngas fermentation using Saccharomyces cerevisiae. This research may contribute to affordable and environment-friendly bioethanol-based energy to decrease the dependency on fossil fuels. Image 1 • Hydrogen-rich syngas was converted into bioethanol using a tar free fermenter (TFF) • Saccharomyces cerevisiae was suited microorganism for syngas fermentation • Produced bioethanol was detected using (1H) NMR and GC-MS analysis [ABSTRACT FROM AUTHOR]

The cost of culture media represents a considerable portion of the overall cost in syngas fermentation. Yeast extract is an expensive ingredient among the medium components required to prepare the culture medium. In this study, we evaluated the effectiveness of replacing yeast extract with relatively low-cost nutrient supplements such as corn steep liquor, malt extract, and vegetable extract in improving the feasibility of the syngas fermentation process in Clostridium autoethanogenum culture. The use of corn steep liquor, malt extract, and vegetable extract as nutrient supplements at a concentration of 0.5 g/L resulted in optical densities of 1.44, 1.37, and 1.71, and produced ethanol concentrations of 2.24, 3.37, and 3.76 g/L, respectively. The microbial growth and ethanol production in syngas fermentation with yeast extract alternatives was comparable with the results of fermentation with yeast extract (optical density of 1.48 and ethanol concentration of 2.16). Moreover, the use of malt and vegetable extracts showed considerable improvement (69% and 51%, respectively) in the specific ethanol productivity compared to yeast extract. [ABSTRACT FROM AUTHOR]

The fermentation of synthesis gas, or syngas, by acetogenic bacteria can help in transitioning from a fossil-fuel-based to a renewable bioeconomy. The main fermentation products of Clostridium ljungdahlii, one of such microorganisms, are acetate and ethanol. A sensitive, robust and reproducible system was established for C. ljungdahlii syngas fermentation, and several process parameters and medium components (pH, gas flow, cysteine and yeast extract) were investigated to assess its impact on the fermentation outcomes, as well as real time gas consumption. Moreover, a closed carbon balance could be achieved with the data obtained. This system is a valuable tool to detect changes in the behavior of the culture. It can be applied for the screening of strains, gas compositions or media components, for a better understanding of the physiology and metabolic regulation of acetogenic bacteria. Here, it was shown that neither yeast extract nor cysteine was a limiting factor for cell growth since their supplementation did not have a noticeable impact on product formation or overall gas consumption. By combining the lowering of both the pH and the gas flow after 24 h, the highest ethanol to acetate ratio was achieved, but with the caveat of lower productivity [ABSTRACT FROM AUTHOR]

Background: Ethanol production through fermentation of gas mixtures containing CO, CO2 and H2 has just started operating at commercial scale. However, quantitative schemes for understanding and predicting productivities, yields, mass transfer rates, gas flow profiles and detailed energy requirements have been lacking in literature; such are invaluable tools for process improvements and better systems design. The present study describes the construction of a hybrid model for simulating ethanol production inside a 700 m3 bubble column bioreactor fed with gas of two possible compositions, i.e., pure CO and a 3:1 mixture of H2 and CO2. Results: Estimations made using the thermodynamics-based black-box model of microbial reactions on substrate threshold concentrations, biomass yields, as well as CO and H2 maximum specific uptake rates agreed reasonably well with data and observations reported in literature. According to the bioreactor simulation, there is a strong dependency of process performance on mass transfer rates. When mass transfer coefficients were estimated using a model developed from oxygen transfer to water, ethanol productivity reached 5.1 g L−1 h−1; when the H2/CO2 mixture is fed to the bioreactor, productivity of CO fermentation was 19% lower. Gas utilization reached 23 and 17% for H2/CO2 and CO fermentations, respectively. If mass transfer coefficients were 100% higher than those estimated, ethanol productivity and gas utilization may reach 9.4 g L−1 h−1 and 38% when feeding the H2/CO2 mixture at the same process conditions. The largest energetic requirements for a complete manufacturing plant were identified for gas compression and ethanol distillation, being higher for CO fermentation due to the production of CO2. Conclusions: The thermodynamics-based black-box model of microbial reactions may be used to quantitatively assess and consolidate the diversity of reported data on CO, CO2 and H2 threshold concentrations, biomass yields, maximum substrate uptake rates, and half-saturation constants for CO and H2 for syngas fermentations by acetogenic bacteria. The maximization of ethanol productivity in the bioreactor may come with a cost: low gas utilization. Exploiting the model flexibility, multi-objective optimizations of bioreactor performance might reveal how process conditions and configurations could be adjusted to guide further process development. [ABSTRACT FROM AUTHOR]

Fermentation of gases provides a promising opportunity for the production of biochemicals from renewable resources, which has resulted in a growing interest in acetogenic bacteria. Thermophilic organisms provide potential advantages for the fermentation of, e.g., syngas into for example volatile compounds, and the thermophiles Moorella thermoacetica and Moorella thermoautotrophica have become model organisms of acetogenic metabolism. The justification for the recognition of the closely related species M. thermoautotrophica has, however, recently been disputed. In order to expand knowledge on the genus, we have here genome sequenced a total of 12 different M. thermoacetica and M. thermoautotrophica strains. From the sequencing results, it became clear that M. thermoautotrophica DSM 1974T consists of at least two different strains. Two different strains were isolated in Lyngby and Ulm from a DSM 1974T culture obtained from the DSMZ (Leibniz-Institut DSMZ-Deutsche Sammlung von Mikroorganismen und Zellkulturen GmbH, Brunswick, Germany). Phylogenetic analysis revealed a close relationship between all the sequenced genomes, suggesting that the two strains detected in the type strain of the species M. thermoautotrophica could not be distinguished at the species level from M. thermoacetica. Despite genetic similarities, differences in genomic features were observed between the strains. Differences in compounds that can serve as carbon and energy sources for selected strains were also identified. On the contrary, strain DSM 21394, currently still named M. thermoacetica , obviously represents a new Moorella species. In addition, based on genome analysis and comparison M. glycerini NMP, M. stamsii DSM 26217T, and M. perchloratireducens An10 cannot be distinguished at the species level. Thus, this comprehensive analysis provides a significantly increased knowledge of the genetic diversity of Moorella strains. [ABSTRACT FROM AUTHOR]

Syngas fermentation has been successfully implemented in commercial-scale plants and can enable the biochemical conversion of the driest fractions of biomass through synthesis gas (H2, CO2, and CO). The process relies on optimized acetogenic strains able to reach and maintain high productivity of ethanol and acetate. In parallel, microbial communities have shown to be the best choice for the production of valuable medium-chain carboxylates through anaerobic fermentation of biomass, demanding low technical complexity and being able to realize simultaneous hydrolysis of the substrate. Each of the two technologies benefits from different strong points and has different challenges to overcome. This review discusses the rationales for merging these two seemingly disparate technologies by analyzing previous studies and drawing opinions based on the lessons learned from such studies. For keeping the technical demands of the resulting process low, a case is built for using microbial communities instead of pure strains. For that to occur, a shift from conventional syngas-based to "syngas-aided" anaerobic fermentation is suggested. Strategies for tackling the intricacies of working simultaneously with communities and syngas, such as competing pathways, and thermodynamic aspects are discussed as well as the stoichiometry and economic feasibility of the concept. Overall, syngas-aided anaerobic fermentation seems to be a promising concept for the biorefinery of the future. However, the effects of process parameters on microbial interactions have to be understood in greater detail, in order to achieve and sustain feasible medium-chain carboxylate and alcohol productivity. [ABSTRACT FROM AUTHOR]

This paper reports the use of lignin-rich residues from second generation bioethanol production, to produce syngas that can be applied in the gas fermentation process. Three gasification technologies at a different scale were considered in this study. Fixed bed updraft gasification of about 30 kg/h solid feed, bubbling fluidized bed gasification of about 0.3 kg/h solid feed and indirect gasification of about 3 kg/h solid feed. Two lignin-rich residues with different properties were tested and the results were evaluated in terms of feedstock pretreatment (grinding, drying and pelleting) and syngas quality requirements for the fermentation process. The molar H 2 /CO ratio (ranging from 0.6 to 1.0) and the tar yield (18–108 g/Nm3) obtained from the three gasification technologies was quite different. For the syngas fermentation process, low H 2 to CO ratio is preferred, as most of the organisms grow better on CO than H 2. Furthermore, different contents of impurities that can reduce the fermentability of the gas (such as hydrocarbons, HCN, HCl, NH 3 , COS and other organic S- compounds) were detected in the product gas. The concentration of these compounds in the syngas is related to the content of the corresponding compounds in the original feedstock. The different characteristics of the lignin-rich feedstocks are related to the specific pre-treatment technologies for the (hemi)cellulose extraction. By tuning the pre-treatment technology, the properties of the feedstock can be improved, making it a suitable for gasification. Tar and unsaturated hydrocarbon compounds need to be removed to very low levels prior to the fermentation process. As a next step, the combination of the gasification and the appropriate product gas cleaning, with the syngas fermentation process for the production of bio-alcohols will be evaluated and the overall efficiency of the gasification-fermentation process will be assessed. [ABSTRACT FROM AUTHOR]

Syngas (mixture of CO, H2 and CO2) fermentation suffers from mass transfer limitation due to low solubility of CO and H2 in the liquid medium. Therefore, it is critical to characterize the mass transfer in syngas fermentation reactors to guide in delivery of syngas to the microorganisms. The objective of this study is to measure and predict the overall volumetric mass transfer coefficient, kLa for O2 at various operating conditions in a 7-L sparged and non-sparged continuous stirred-tank reactor (CSTR). Measurements indicated that the kLa for O2 increased with an increase in air flow rate and agitation speed. However, kLa for O2 decreased with the increase in the headspace pressure. The highest kLa for O2 with air sparged in the CSTR was 116 h-1 at 600 sccm, 900 rpm, 101 kPa, and 3 L working volume. Backmixing of the headspace N2 in the sparged CSTR reduced the observed kLa. The mass transfer model predicted the kLa for O2 within 10% of the experimental values. The model was extended to predict the kLa for syngas components CO, CO2 and H2, which will guide in selecting operating conditions that minimize power input to the bioreactor and maximize the syngas conversion efficiency. [ABSTRACT FROM AUTHOR]

• A bulk-gas-to-atomized-liquid (BGAL) concept was developed for syngas fermentation. • Mass transfer rate of the BGAL system was 20 times of that in stirred tank reactors. • The BGAL system reduced 4-fold energy consumption compared to stir tank reactors. • BGAL system enhanced ethanol productivity by 2-fold compared to stir tank reactors. Syngas fermentation for fuels and chemicals is limited by the low rate of gas-to-liquid mass transfer. In this work, a unique bulk-gas-to-atomized-liquid (BGAL) contactor was developed to enhance mass transfer. In the BGAL system, liquid is atomized into discrete droplets, which significantly increases the interface between the liquid and bulk gas. Using oxygen as a model gas, the BGAL contactor achieved an oxygen transfer rate (OTR) of 569 mg·L−1·min−1 and a mass transfer coefficient (K L a) of 2.28 sec−1, which are values as much as 100-fold greater than achieved in other kinds of reactors. The BGAL contactor was then combined with a packed bed to implement syngas fermentation, with packing material supporting a biofilm upon which gas saturated liquid is dispersed. This combination avoids dispersing these gas-saturated droplets into the bulk liquid, which would significantly dilute the dissolved gas concentration. Although this combination reduced overall K L a to 0.45–1.0 sec−1, it is still nearly 20 times higher than achieved in a stirred tank reactor. The BGAL contactor/packed bed bioreactor was also more energy efficient in transferring gas to the liquid phase, requiring 8.63–26.32 J mg−1 O 2 dissolved, which is as much as four-fold reduction in energy requirement compared to a stirred tank reactor. Fermentation of syngas to ethanol was evaluated in the BGAL contactor/packed bed bioreactor using Clostridium carboxidivorans P7. Ethanol productivity reached 746 mg·L−1·h−1 with an ethanol/acetic acid molar ratio of 7.6. The ethanol productivity was two-fold high than the highest level previously reported. The exceptional capability of BGAL contactor to enhance mass transfer in these experiments suggests its utility in syngas fermentation as well as other gas-liquid contacting processes. [ABSTRACT FROM AUTHOR]

The production of ethanol via syngas fermentation obtained from lignocellulose gasification provides a method for completely utilizing all of the carbon content from lignocellulosic feedstock. The low mass transfer rate of less soluble CO and H2 gas to liquid has been considered a major bottleneck in the overall process; however, microporous membrane has been proposed as a gas diffuser to improve gas-to-liquid mass transfer. In this study, a liquid batch of syngas fermentation employing Clostridium ljungdahlii with continuous gas supply was obtained using the configuration of a bioreactor connected to microporous hydrophobic polypropylene hollow fiber membrane (HFM) as a gas diffuser. Liquid recirculation between the fermentation vessel and membrane module was applied to enhance the gas-liquid contact as well as cell-recycle. The fermentation performance with and without HFM was compared and evaluated by cell growth, CO utilization, ethanol yield, and productivity. A higher ethanol yield, 0.22 mol/mol, was achieved using the system of an HFM-supported bioreactor with a higher ethanol titer of 1.09 g/L and an ethanol-acetate molar ratio of 1.43 mol/mol. The obtained result demonstrates that an HFM-supported bioreactor is the best fermentation system compared to stirred tank reactor (STR) without a membrane. [ABSTRACT FROM AUTHOR]

The production of ethanol via syngas fermentation obtained from lignocellulose gasification provides a method for completely utilizing all of the carbon content from lignocellulosic feedstock. The low mass transfer rate of less soluble CO and H2 gas to liquid has been considered a major bottleneck in the overall process; however, microporous membrane has been proposed as a gas diffuser to improve gas-to-liquid mass transfer. In this study, a liquid batch of syngas fermentation employing Clostridium ljungdahlii with continuous gas supply was obtained using the configuration of a bioreactor connected to microporous hydrophobic polypropylene hollow fiber membrane (HFM) as a gas diffuser. Liquid recirculation between the fermentation vessel and membrane module was applied to enhance the gas–liquid contact as well as cell-recycle. The fermentation performance with and without HFM was compared and evaluated by cell growth, CO utilization, ethanol yield, and productivity. A higher ethanol yield, 0.22 mol/mol, was achieved using the system of an HFM-supported bioreactor with a higher ethanol titer of 1.09 g/L and an ethanol-acetate molar ratio of 1.43 mol/mol. The obtained result demonstrates that an HFM-supported bioreactor is the best fermentation system compared to stirred tank reactor (STR) without a membrane. [ABSTRACT FROM AUTHOR]

[Display omitted] • Bioreactor operated at 28 °C/20 °C with pH 5.5 to 4.5 using mixed culture. • Lower temperature and pH led to increased electron yields to acetate. • The highest H 2 and CO consumption was at 20 °C and pH 4.5. • Methanogens were not completely suppressed under any conditions. • Different microbial genera dominated distinct experimental periods. Syngas fermentation to acetate offers a promising solution for its valorisation, particularly when syngas contains a high N 2 concentration, which otherwise impedes the utilisation of syngas biomethanation gaseous product in cogeneration or upgrading units. In this study, continuous lab-scale syngas fermentation assessing the effects of acidic pH and psychrophilic conditions (28 °C and 20 °C) on bioconversion efficiency and anaerobic consortium diversity was studied. The results showed that as temperature and pH decrease, acetate yield increases. The highest H 2 and CO consumption rates were observed at 20 °C and pH 4.5, reaching 48.4 mmol/(L·d) and 31.5 mmol/(L·d), respectively, and methanogenic activity was not completely suppressed. The microbial community composition indicated an enhanced abundance of acetate-producing bacteria and hydrogenotrophic methanogens at 28 °C. The PICRUSt2 prediction of metabolic potential indicated that temperature and pH changes appear to have a more pronounced impact on acetotrophic methanogenesis genes than carbon dioxide-based methanogenesis genes. [ABSTRACT FROM AUTHOR]

Biochar has functional groups, pH buffering capacity and cation exchange capacity (CEC) that can be beneficial in syngas fermentation. This study examined the properties of biochar made from switchgrass (SGBC), forage sorghum (FSBC), red cedar (RCBC) and poultry litter (PLBC), and their effects on ethanol and butanol production from syngas using Clostridium carboxidivorans . Experiments were performed in 250 mL bottle reactors with a 50 mL working volume at 37 °C fed syngas containing CO:H 2 :CO 2 (40:30:30 by volume). Results showed that PLBC and SGBC enhanced ethanol production by 90% and 73%, respectively, and butanol production by fourfold compared to standard yeast extract medium without biochar (control). CO and H 2 utilization in PLBC and SGBC media increased compared to control. PLBC had the highest pH buffering capacity, CEC and total amount of cations compared with SGBC, FSBC and RCBC, which could have contributed to its highest enhancement of ethanol and butanol production. [ABSTRACT FROM AUTHOR]

Syngas fermentation into acetate by mixed cultures is considered as a promising biotechnology. The effects of pH and temperature on the conversion efficiency, product composition, selectivity and microbial community composition during syngas fermentation are investigated in this work. The maximum total volatile fatty acids and ethanol concentrations can reach 3400 and 3100 mg/L both at pH 9 under mesophilic and psychrophilic conditions, respectively. The highest ratio of acetate to total volatile fatty acids and ethanol are also obtained at pH 9 and reach 91 and 90% under mesophilic and psychrophilic conditions, respectively. Although the production and selectivity are higher under mesophilic condition than psychrophilic condition, it’s also found that the activity of methanogens can be better inhibited under psychrophilic condition than mesophilic condition. The high-throughput sequencing analysis of 16S rRNA genes shows the abundance of genus Clostridium and Acetobacterium , containing several homoacetogens, are enriched in initial pH of 9 under both mesophilic condition and psychrophilic conditions, which may be related with the conversion of CO and H 2 into acetate. [ABSTRACT FROM AUTHOR]

A two-stage continuous system with two stirred tank reactors in series was utilized to perform syngas fermentation using Clostridium carboxidivorans . The first bioreactor (bioreactor 1) was maintained at pH 6 to promote acidogenesis and the second one (bioreactor 2) at pH 5 to stimulate solventogenesis. Both reactors were operated in continuous mode by feeding syngas (CO:CO 2 :H 2 :N 2 ; 30:10:20:40; vol%) at a constant flow rate while supplying a nutrient medium at different flow rates of 8.1, 15, 22 and 30 ml/h. A cell recycling unit was added to bioreactor 2 in order to recycle the cells back to the reactor, maintaining the OD 600 around 1 in bioreactor 2 throughout the experimental run. When comparing the flow rates, the best results in terms of solvent production were obtained with a flow rate of 22 ml/h, reaching the highest average outlet concentration for alcohols (1.51 g/L) and the most favorable alcohol/acid ratio of 0.32. [ABSTRACT FROM AUTHOR]

• Enhancing syngas fermentation plants by improving downstream processing. • Eco-efficient bioethanol recovery with full recycle of biomass, acetate and water. • Improved fluid separations with effective heat pumping and heat integration. • About 54% reduction in TAC, 66% in energy savings and 83% lower CO 2 emissions. Syngas fermentation is used industrially to produce diluted bioethanol (about 1–6 wt%). This research study proposes a novel downstream process that recovers bioethanol in an energy-efficient and cost-effective manner, improves fermentation yield by recycling all fermentation broth components (microbes, acetate and water), and is designed for full-scale industrial-level application. Therefore, vacuum distillation at fermentation temperature was conceptually studied as an initial ethanol recovery step, leading to a bottom stream that may be recycled. Advanced separation and purification techniques were designed to recover 99.5% of initially present ethanol as high-purity product (99.8 wt%). Mechanical vapor recompression and heat integration methods were used to maximize sustainability and eco-efficiency of the proposed recovery process. Implementation of these techniques on a process using 6 wt% ethanol feed stream decreased the total annual costs by 54.2% (from 0.175 to 0.080 $/kg EtOH), reduced the primary energy requirement by 66.1% (from 2.82 to 0.96 kW th h/kg EtOH), lowered the CO 2 emission by up to 82.6% (from 0.414 to 0.072 kg CO2 /kg EtOH), and reduced the fresh water usage by 62.6% (from 0.242 to 0.091 m3 W /kg EtOH). Sensitivity analysis for ethanol concentrations ranging from 6 to 1 wt% showed that the recovery costs and energy use increased to 0.336 $/kg EtOH and 1.78 kW th h/kg EtOH respectively. Since ethanol recovery performs better but fermentation will perform worse at higher ethanol concentration in fermentation broth, there is a trade-off concentration for the overall process. The current analysis is an important step toward determining this trade-off. [ABSTRACT FROM AUTHOR]

Biochar contains minerals and metals that can serve as nutrients for acetogens to produce ethanol via syngas fermentation. In this study, four fermentation media containing biochar from switchgrass (SGBC), forage sorghum (FSBC), red cedar (RCBC) and poultry litter (PLBC) were compared with standard yeast extract (YE) medium for syngas fermentation using Clostridium ragsdalei . Fermentations were performed in 250 mL bottle reactors at 150 rpm and 37 °C with syngas containing CO:H 2 :CO 2 (40:30:30) by volume. Results showed that media containing RCBC and PLBC improved ethanol production by 16.3% and 58.9%, respectively, compared to YE medium. C. ragsdalei consumed 69% more H 2 and 40% more CO in PLBC medium compared to YE medium. However, no enhancement of ethanol production was observed in SGBC and FSBC media. The highest release of Na, K, Ca, Mg, S and P was from PLBC, which was considered to contribute in enhancement of ethanol production. [ABSTRACT FROM AUTHOR]

Production of second generation ethanol can be accomplished with biomass gasification followed by syngas fermentation using acetogenic bacteria in a hybrid process. Using process simulation and financial analysis, this study evaluated the feasibility of producing hydrous ethanol from sugarcane bagasse in a conceptual hybrid plant designed to be energetically self-sufficient. The process was found to be competitive with other second generation routes, achieving an ethanol yield of 330 L per metric ton of dry biomass and an overall carbon conversion of 30%. The minimum ethanol selling price to achieve Net Present Value break-even with 10% Internal Rate of Return was estimated to be 706 US$/m 3 after taxes in the base model. When accounting for uncertainties in the fixed capital investment and the cost of raw materials, the Net Present Value was found to be non-negative in 80% of the cases for a selling price of 783 US$/m 3 . [ABSTRACT FROM AUTHOR]

Fuel ethanol has been recognized as a kind of potential alternative fuel as well as an additive to gasoline because of its oxygenated, and high octane characteristics. Syngas fermentation, a novel route for ethanol production, is getting more and more attention. Syngas can be generated from a lot of organic materials including biomass. Gasification technology is used to convert the biomass into a mixture of gases (consisting mainly of CO, CO2 and H2), which is subsequently fermented to ethanol by means of anaerobic microbial catalysts known as homoacetogens. As a homoacetogen, Clostridium autoethanogenum is able to metabolize syngas/CO and synthesize ethanol, but limited work has been accomplished with it and ethanol concentration achieved was low. In order to improve its ethanol production, fermentation process and some factors affecting cell growth or product formation were studied. Although xylose is an easily-used carbon source for C. autoethanogenum, results showed that the primary end product in xylose metabolism was acetate, while ethanol remained in a low level even with high level of xylose (8-10 g/L). C. autoethanogenum grew rapidly in the medium containing xylose and organic nitrogen source, and high cell density was achieved. When nitrogen source was switched to NH4Cl, C. autoethanogenum grew much slowly and the overall cell density diminished. However, nitrogen source didn’t have much influence on ethanol production if xylose was used as the carbon source. Two-step fermentation, i.e., growing on xylose (first stage) and then fermenting with CO (second stage), was performed in a 3 L bioreactor to study the batch fermentation of C. autoethanogenum. Results indicated that cell growth and acetate production occurred in the first stage, whereas ethanol was primarily produced in the second stage when xylose was exhausted and CO became the sole carbon source. Fermentation curves showed that pH value and oxidation-reduction potential (ORP) dropped with cell growth and acetate production, while ethanol production was accompanied by the decrease of acetate concentration and the rise of pH value and ORP. CO2 evolution was observed in both growth and fermentation stages, and a small amount of H2 was detected in the outlet gas during the CO fermentation stage. But due to the limitations of bioreactor and operating conditions, gas-liquid mass transfer in the bioreactor was poor, which resulted in the low efficiency of CO utilization, and only 1.71 g/L ethanol was obtained. To eliminate the limitations, the bioreactor was modified and equipped with a specific device which could keep a constant pressure in the headspace. One-step fermentation was carried out in the modified bioreactor using CO as the sole carbon and energy source. In spite of reduced growth rate and cell density, C. autoethanogenum produced more ethanol than it did in two-step fermentation. The maximum ethanol concentration obtained was 7.36 g/L, much greater than that reported in the previous studies. Moreover, during the whole fermentation process, acetate concentration remained lower than 1.1 g/L. Summarily, the results suggest that C. autoethanogenum is a promising strain in ethanol production from CO, and this study presents a reference for the scale-up of CO fermentation to ethanol with C. autoethanogenum. [ABSTRACT FROM AUTHOR]

The use of gas fermentation for production of chemicals and fuels with lower environmental impact is a technology that is gaining increasing attention. Over 38 Gt of CO 2 is annually being emitted from industrial processes, thereby contributing significantly to the concentration of greenhouse gases in the atmosphere. Together with the gasification of biomass and different waste streams, these gases have the potential for being utilized for production of chemicals through fermentation processes. Acetogens are among the most studied organisms capable of utilizing waste gases. Although engineering of heterologous production of higher value compounds has been successful for a number of acetogens, the processes are challenging due to the redox balance and the lack of efficient engineering tools. In this review, we address the availability of different gaseous feedstock and gasification processes, and we focus on the advantages of alternative fermentation scenarios, including thermophilic production strains, multi-stage fermentations, mixed cultures, as well as mixotrophy. Such processes have the potential to significantly broaden the product portfolio, increase the product concentrations and yields, while enabling the exploitation of alternative and mixed feedstocks. The reviewed processes also have the potential to address challenges associated with product inhibition and may contribute to reducing the costs of downstream processing. Given the widespread availability of gases, such processes will likely significantly impact the transition towards a more sustainable society. [ABSTRACT FROM AUTHOR]

In bioindustry, syngas fermentation is a promising technology for biofuel production without the use of plant biomass as sugar-based feedstock. The aim of this study was to identify optimal conditions for high efficient ethanol and volatile fatty acids (VFA) production from synthetic gas fermentation. Therefore, the effect of different gases (pure CO, H 2 , and a synthetic syngas mixture), media (acetate medium and acetate-free medium), and biocatalyst (pure and mixed culture) were studied. Acetate was the most dominant product independent on inoculum type. The maximum concentration of volatile fatty acids and ethanol was achieved by the pure culture ( Clostridium ragsdalei ). Depending on the headspace gas composition, VFA concentrations were up to 300% higher after fermentation with Clostridium ragsdalei compared to fermentation with mixed culture. The preferred gas composition with respect to highest VFA concentration was pure CO (100%) regardless of microbial composition of the inoculum and media composition. The addition of acetate had a negative impact on the VFA formation which was depending on the initial gas composition in head space. [ABSTRACT FROM AUTHOR]

Background: The product of current syngas fermentation systems is an ethanol/acetic acid mixture and the goal is to maximize ethanol recovery. However, ethanol currently has a relatively low market value and its separation from the fermentation broth is energy intensive. We can circumvent these disadvantages of ethanol production by converting the dilute ethanol/acetic acid mixture into products with longer carbon backbones, which are of higher value and are more easily extracted than ethanol. Chain elongation, which is the bioprocess in which ethanol is used to elongate short-chain carboxylic acids to medium-chain carboxylic acids (MCCAs), has been studied with pure cultures and open cultures of microbial consortia (microbiomes) with several different substrates. While upgrading syngas fermentation effluent has been studied with open cultures, to our knowledge, no study exists that has performed this with pure cultures. Results: Here, pure cultures of Clostridium kluyveri were used in continuous bioreactors to convert ethanol/acetic acid mixtures into MCCAs. Besides changing the operating conditions in regards to substrate loading rates and composition, the effect of in-line product extraction, pH, and the use of real syngas fermentation effluent on production rates were tested. Increasing the organic loading rates resulted in proportionally higher production rates of n-caproic acid, which were up to 40 mM day-1 (4.64 g L-1 day-1) at carbon conversion efficiencies of 90% or higher. The production rates were similar for bioreactors with and without in-line product extraction. Furthermore, a lower ethanol/acetic acid ratio (3:1 instead of 10:1) enabled faster and more efficient n-caproic acid production. In addition, n-caprylic acid production was observed for the first time with C. kluyveri (up to 2.19 ± 0.34 mM in batch). Finally, the use of real effluent from syngas fermentation, without added yeast extract, but with added defined growth factors, did maintain similar production rates. Throughout the operating period, we observed that the metabolism of C. kluyveri was inhibited at a mildly acidic pH value of 5.5 compared to a pH value of 7.0, while reactor microbiomes perform successfully at mildly acidic conditions. Conclusions: Clostridium kluyveri can be used as a biocatalyst to upgrade syngas fermentation effluent into MCCAs at pH values above 5.5. [ABSTRACT FROM AUTHOR]