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The use of mixed cultures in gas fermentations could reduce operating costs in the production of liquid chemicals such as alcohols or carboxylic acids. However, directing reducing equivalents towards the desired products presents the challenge of co-existing competing pathways. In this study, two trickle bed reactors were operated at acetogenic and chain elongating conditions to explore the fate of electron equivalents (ethanol, H2, and CO) and test pH oscillations as a strategy to target chain-elongated products. Hereby, the use of a H2-rich syngas increased gas conversion rates and the specificity towards acetic acid (86% of C-mol production, 9.0 g LEBV−1 day−1, with EBV referring to empty bed volume), while preliminary experiments with CO-rich syngas show promising results in increasing the ethanol production necessary to target chain-elongated products. On the other hand, ethanol supplementation hindered the endogenous ethanol production of the acetogenic culture but promoted butanol production (1.0 g LEBV−1 day−1) at high ethanol concentrations (9.6 g L−1) in the fresh media. Finally, pH oscillations improved chain elongation yields but negatively affected acetogenic growth, reducing production rates. [ABSTRACT FROM AUTHOR]

Clostridium ragsdalei was found to produce (S)-2-hydroxyisovalerate (2-HIV) as a novel product in addition to acetate, ethanol, and d-2,3-butanediol in heterotrophic (d-xylose), autotrophic (CO), and mixotrophic (d-xylose + CO) conditions. Mixotrophic batch processes in stirred-tank bioreactors with continuous gassing resulted in improved production of this alpha-hydroxy acid compared to batch processes solely with either d-xylose or CO. The maximal CO uptake rate was considerably reduced in mixotrophic compared to autotrophic processes, resulting in a concomitant decreased total CO2 production. Simultaneous conversion of 9.5 g L−1 d-xylose and 320 mmol CO enabled the production of 1.8 g L−1 2-HIV in addition to 1.1 g L−1 d-2,3-butanediol, 2.0 g L−1 ethanol, and 1.8 g L−1 acetate. With reduced initial d-xylose (3.1 g L−1), l-valine production started when d-xylose was depleted, reaching a maximum of 0.4 g L−1 l-valine. Using l-arabinose or d-glucose instead of d-xylose in mixotrophic batch processes reduced the 2-HIV production by C. ragsdalei. Considerable amounts of meso-2,3-butanediol (0.9–1.3 g L−1) were produced instead, which was not observed with d-xylose. The monomer 2-HIV can form polyesters that make the molecule attractive for application as bioplastic (polyhydroxyalkanoates) or new composite material. [ABSTRACT FROM AUTHOR]

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]

Electrocatalytic CO2 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, CO2, and formate together with H2 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 (pCO0) 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 CO2/H2 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, CO2, formate, and H2 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. [ABSTRACT FROM AUTHOR]

In a circular economy approach, syngas biomethanation is a promising technology for waste to energy conversion. However, syngas can contain impurities, notably tar, that can limit the processes upgrading syngas downstream gasification. The effect of tar on syngas biomethanation is unknown. Therefore, for the first time, common tar components were tested on a consortium adapted for syngas biomethanation to assess the resistance of the microorganisms. Four light tar components (benzene, toluene, styrene and phenol) commonly found in syngas were selected and tested at different concentrations in batch tests. Adding pollutant up to inhibitory concentrations affected both the lag phase of microbial growth and the rates of the bioreactions. Hydrogenotrophic methanogens were found to be more sensitive than carboxydotrophs. Amongst the four tested pollutants, phenol appears to be the most problematic, due not only to its high inhibitory effect but also to its high solubility, allowing phenol in the syngas to reach high inhibitory concentrations. This study paves the way for further research on the resistance of syngas biomethanation to impurities contained in the syngas. [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]

Providing simultaneously autotrophic and heterotrophic carbon sources is a promising strategy to overcome the limits of autotrophic syngas fermentations. D-xylose and L-arabinose are particularly interesting as they can be obtained by the hydrolysis of lignocellulosic biomass. The individual conversion of varying initial concentrations of these pentoses and D-fructose as reference was studied with C. autoethanogenum in fully controlled stirred-tank reactors with a continuous syngas supply. All mixotrophic batch processes showed increased biomass and product formation compared to an autotrophic reference process. Simultaneous CO and D-xylose or L-arabinose conversion was observed in contrast to D-fructose. In the mixotrophic batch processes with L-arabinose or D-xylose, the simultaneous CO and sugar conversion resulted in high final alcohol-to-acid ratios of up to 58 g g−1. L-arabinose was superior as a mixotrophic carbon source because biomass and alcohol concentrations (ethanol and 2,3-butanediol) were highest, and significant amounts of meso-2,3-butanediol (>1 g L−1) in addition to D-2,3-butanediol (>2 g L−1) were solely produced with L-arabinose. Furthermore, C. autoethanogenum could not produce meso-2,3 butanediol under purely heterotrophic conditions. The mixotrophic production of meso-2,3-butanediol from L-arabinose and syngas, both available from residual lignocellulosic biomass, is very promising for use as a monomer for bio-based polyurethanes or as an antiseptic agent. [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]

In large-scale syngas fermentation, strong gradients in dissolved gas (CO, H2) concentrations are very likely to occur due to locally varying mass transfer and convection rates. Using Euler-Lagrangian CFD simulations, we analyzed these gradients in an industrial-scale external-loop gas-lift reactor (EL-GLR) for a wide range of biomass concentrations, considering CO inhibition for both CO and H2 uptake. Lifeline analyses showed that micro-organisms are likely to experience frequent (5 to 30 s) oscillations in dissolved gas concentrations with one order of magnitude. From the lifeline analyses, we developed a conceptual scale-down simulator (stirred-tank reactor with varying stirrer speed) to replicate industrial-scale environmental fluctuations at bench scale. The configuration of the scale-down simulator can be adjusted to match a broad range of environmental fluctuations. Our results suggest a preference for industrial operation at high biomass concentrations, as this would strongly reduce inhibitory effects, provide operational flexibility and enhance the product yield. The peaks in dissolved gas concentration were hypothesized to increase the syngas-to-ethanol yield due to the fast uptake mechanisms in C. autoethanogenum. The proposed scale-down simulator can be used to validate such results and to obtain data for parametrizing lumped kinetic metabolic models that describe such short-term responses. [ABSTRACT FROM AUTHOR]

Syngas fermentation with clostridial co-cultures is promising for the conversion of CO to alcohols. A CO sensitivity study with Clostridium kluyveri monocultures in batch operated stirred-tank bioreactors revealed total growth inhibition of C. kluyveri already at 100 mbar CO, but stable biomass concentrations and ongoing chain elongation at 800 mbar CO. On/off-gassing with CO indicated a reversible inhibition of C. kluyveri. A continuous supply of sulfide led to increased autotrophic growth and ethanol formation by Clostridium carboxidivorans even at unfavorable low CO concentrations. Based on these results, a continuously operated cascade of two stirred-tank reactors was established with a synthetic co-culture of both Clostridia. An amount of 100 mbar CO and additional sulfide supply enabled growth and chain elongation in the first bioreactor, whereas 800 mbar CO resulted in an efficient reduction of organic acids and de-novo synthesis of C2-C6 alcohols in the second reactor. High alcohol/acid ratios of 4.5–9.1 (w/w) were achieved in the steady state of the cascade process, and the space-time yields of the alcohols produced were improved by factors of 1.9–5.3 compared to a batch process. Further improvement of continuous production of medium chain alcohols from CO may be possible by applying less CO-sensitive chain-elongating bacteria in co-cultures. [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]

Synthesis gas (syngas) fermentation is a biological carbon fixation process through which carboxydotrophic acetogenic bacteria convert CO, CO2, and H2 into platform chemicals. To obtain an accurate overview of the syngas fermentation research and innovation from 2012 to 2022, a systematic search was performed on Web of Science and The Lens, focusing on academic publications and patents that were published or granted during this period. Overall, the research focus was centered on process optimization, the genetic manipulation of microorganisms, and bioreactor design, in order to increase the plethora of fermentation products and expand their possible applications. Most of the published research was initially funded and developed in the United States of America. However, over the years, European countries have become the major contributors to syngas fermentation research, followed by China. Syngas fermentation seems to be developing at "two-speeds", with a small number of companies controlling the technology that is needed for large-scale applications, while academia still focuses on low technology readiness level (TRL) research. This systematic review also showed that the fermentation of raw syngas, the effects of syngas impurities on acetogen viability and product distribution, and the process integration of gasification and fermentation are currently underdeveloped research topics, in which an investment is needed to achieve technological breakthroughs. [ABSTRACT FROM AUTHOR]

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In order to utilize a wider range of low-grade syngas, the syngas biomethanation was studied in this work with respect to the gas–liquid mass transfer and the reactor start-up strategy. Two reactors, a continuous stirred tank (CSTR) and a bubble column with gas recirculation (BCR-C), were used in the experiment by feeding an artificial syngas of 20% H2, 50% CO, and 30% CO2 into the reactors at 55 °C. The results showed that the CH4 productivity was slightly increased by reducing the gas retention time (GRT), but was significantly improved by increasing the stirring speed in the CSTR and the gas circulation rate in the BCR-C. The best syngas biomethanation performance of the CSTR with a CH4 productivity of 22.20 mmol·Lr−1·day−1 and a yield of 49.01% was achieved at a GRT of 0.833 h and a stirring speed of 300 rpm, while for the BCR-C, the best performance with a CH4 productivity of 61.96 mmol·Lr−1·day−1 and a yield of 87.57% was achieved at a GRT of 0.625 h and a gas circulation rate of 40 L·Lr−1·h−1. The gas–liquid mass transfer capability provided by gas circulation is far superior to mechanical stirring, leading to a much better performance of low-grade syngas biomethanation in the BCR-C. Feeding H2/CO2 during the startup stage of the reactor can effectively stimulate the growth and metabolism of microorganisms, and create a better metabolic environment for subsequent low-grade syngas biomethanation. In addition, during the thermophilic biomethanation of syngas, Methanothermobacter is the dominant genus. [ABSTRACT FROM AUTHOR]

Background: Ethyl acetate is a bulk chemical traditionally produced via energy intensive chemical esterification. Microbial production of this compound offers promise as a more sustainable alternative process. So far, efforts have focused on using sugar-based feedstocks for microbial ester production, but extension to one-carbon substrates, such as CO and CO2/H2, is desirable. Acetogens present a promising microbial platform for the production of ethyl esters from these one-carbon substrates. Results: We engineered the acetogen C. autoethanogenum to produce ethyl acetate from CO by heterologous expression of an alcohol acetyltransferase (AAT), which catalyzes the formation of ethyl acetate from acetyl-CoA and ethanol. Two AATs, Eat1 from Kluyveromyces marxianus and Atf1 from Saccharomyces cerevisiae, were expressed in C. autoethanogenum. Strains expressing Atf1 produced up to 0.2 mM ethyl acetate. Ethyl acetate production was barely detectable (< 0.01 mM) for strains expressing Eat1. Supplementation of ethanol was investigated as potential boost for ethyl acetate production but resulted only in a 1.5-fold increase (0.3 mM ethyl acetate). Besides ethyl acetate, C. autoethanogenum expressing Atf1 could produce 4.5 mM of butyl acetate when 20 mM butanol was supplemented to the growth medium. Conclusions: This work offers for the first time a proof-of-principle that autotrophic short chain ester production from C1-carbon feedstocks is possible and offers leads on how this approach can be optimized in the future. [ABSTRACT FROM AUTHOR]

Summary: Fossil fuel depletion and environmental contamination coupled with food vs fuel conflict motivated sustainable bioethanol production through syngas fermentation utilizing lignocellulosic biomass as feedstock. Herein, Enterobacter sp. RF2 strain was selected for the investigation out of the nine anaerobes that were screened for bioethanol production using the pyridinium chlorochromate assay. Under optimal conditions, Enterobacter sp. RF2 and the reference strain Clostridium ljungdahlii ATCC‐55383 produced 21.8 and 37.7 g/L of bioethanol in an anaerobic culture bottle and 25.3 and 39.6 g/L of bioethanol in a stirred tank reactor via syngas fermentation. Furthermore, infra‐red spectra and proton nuclear magnetic resonance illustrated chemical shift patterns of bioethanol. The high bioethanol producer was identified as Enterobacter hormaechei RF2 by in silico analysis. Here, it was reported for the first time that E. hormaechei RF2 is high ethanol tolerant and genetically stable bioethanol producer that can be exploited for pilot‐scale production to meet the rising demand for fossil fuels and lessen eco‐pollution. Thus, this research will pave the way for the commercialization of green chemicals which is the need of the hour in the current scenario. However, the cost of biomass conversion to bioethanol must be less than the existing price of gasoline to be competitive and get economic acceptance. Although this method is now at the pre‐commercialization stage due to lack because of techno‐economic feasibility, but usage of suitable substrates and concurrent production of different valuable products can make the entire process cost affordable. Therefore, by providing employment to the rural population through the exploitation of lignocellulosic biomass, eco‐friendly fuel to the customer, and lucrative fuel business to industries, it might thus hypothesize an intriguing future for our global economy. [ABSTRACT FROM AUTHOR]

Synthesis gas fermentation is a microbial process, which uses anaerobic bacteria to convert CO-rich gases to organic acids and alcohols and thus presents a promising technology for the sustainable production of fuels and platform chemicals from renewable sources. Clostridium carboxidivorans and Clostridium autoethanogenum are two acetogenic bacteria, which have shown their high potential for these processes by their high tolerance toward CO and in the production of industrially relevant products such as ethanol, 1-butanol, 1-hexanol, and 2,3-butanediol. A promising approach is the coupling of gasification of biogenic residues with a syngas fermentation process. This study investigated batch processes with C. carboxidivorans and C. autoethanogenum in fully controlled stirred-tank bioreactors and continuous gassing with biogenic syngas produced by an autothermal entrained flow gasifier on a pilot scale >1200 °C. They were then compared to the results of artificial gas mixtures of pure gases. Because the biogenic syngas contained 2459 ppm O2 from the bottling process after gasification of torrefied wood and subsequent syngas cleaning for reducing CH4, NH3, H2S, NOX, and HCN concentrations, the oxygen in the syngas was reduced to 259 ppm O2 with a Pd catalyst before entering the bioreactor. The batch process performance of C. carboxidivorans in a stirred-tank bioreactor with continuous gassing of purified biogenic syngas was identical to an artificial syngas mixture of the pure gases CO, CO2, H2, and N2 within the estimation error. The alcohol production by C. autoethanogenum was even improved with the purified biogenic syngas compared to reference batch processes with the corresponding artificial syngas mixture. Both acetogens have proven their potential for successful fermentation processes with biogenic syngas, but full carbon conversion to ethanol is challenging with the investigated biogenic syngas. [ABSTRACT FROM AUTHOR]

Background: The replacement of fossil fuels and petrochemicals with sustainable alternatives is necessary to mitigate the effects of climate change and also to counteract diminishing fossil resources. Acetogenic microorganisms such as Clostridium spp. are promising sources of fuels and basic chemical precursors because they efficiently utilize CO and CO2 as carbon source. However the conversion into high titers of butanol and hexanol is challenging. Results: Using a metabolic engineering approach we transferred a 17.9-kb gene cluster via conjugation, containing 13 genes from C. kluyveri and C. acetobutylicum for butanol and hexanol biosynthesis, into C. ljungdahlii. Plasmid-based expression resulted in 1075 mg L−1 butanol and 133 mg L−1 hexanol from fructose in complex medium, and 174 mg L−1 butanol and 15 mg L−1 hexanol from gaseous substrate (20% CO2 and 80% H2) in minimal medium. Product formation was increased by the genomic integration of the heterologous gene cluster. We confirmed the expression of all 13 enzymes by targeted proteomics and identified potential rate-limiting steps. Then, we removed the first-round selection marker using CRISPR/Cas9 and integrated an additional 7.8 kb gene cluster comprising 6 genes from C. carboxidivorans. This led to a significant increase in the hexanol titer (251 mg L−1) at the expense of butanol (158 mg L−1), when grown on CO2 and H2 in serum bottles. Fermentation of this strain at 2-L scale produced 109 mg L−1 butanol and 393 mg L−1 hexanol. Conclusions: We thus confirmed the function of the butanol/hexanol biosynthesis genes and achieved hexanol biosynthesis in the syngas-fermenting species C. ljungdahlii for the first time, reaching the levels produced naturally by C. carboxidivorans. The genomic integration strain produced hexanol without selection and is therefore suitable for continuous fermentation processes. [ABSTRACT FROM AUTHOR]

Autotrophic syngas fermentation with clostridia enables the conversion of CO, CO2, and H2 into organic acids and alcohols. The batch process performance of Clostridium ragsdalei was studied in fully controlled and continuously gassed (600 mbar CO, 200 mbar H2, 200 mbar CO2) stirred-tank bioreactors. The final ethanol concentration varied as function of the reaction conditions. Decreasing the pH from pH 6.0–5.5 at a temperature of 37 °C increased the ethanol concentration from 2.33 g L−1 to 3.95 g L−1, whereas lowering the temperature from 37 to 32 °C at constant pH 6.0 resulted in a final ethanol concentration of 5.34 g L−1 after 5 days of batch operation. The sulphur availability was monitored by measuring the cysteine concentration in the medium and the H2S fraction in the exhaust gas. It was found that most of the initially added sulphur was stripped out within the first day of the batch process (first half of the exponential growth phase). A continuous sodium sulfide feed allowed ethanol concentrations to increase more than threefold to 7.67 g L−1 and the alcohol-to-acetate ratio to increase 43-fold to 17.71 g g−1. [ABSTRACT FROM AUTHOR]

Acetogen bacteria have been spotlighted to produce fuels and chemicals from CO, H2, and CO2, main components of waste gas from the steel industry and syngas derived from lignocellulose and plastics. The production of C2∼C4 acids/alcohols from syngas has been widely studied; however, there are much less studies on the production of C6 or higher acids/alcohols due to the limited number of acetogen producing C6 or higher carbon compounds. In this study, we report a newly isolated strain Clostridium sp. JS66 that has the ability to produce C2, C4, and C6 acids/alcohols from syngas and glucose. This isolate exhibited 99.9% 16S rRNA similarity and 64.5% digital DNA-DNA hybridization value to Clostridium carboxidivorans P7T, a known C6 acid/alcohol-producing acetogen. Characterization of gas fermentation using CO:CO2:H2 [30:30:40] was carried out at different temperature (25∼37°C), initial pH (5.5∼7.0), agitation speed (50∼150 rpm), and total pressure (100∼150 kPa). When the agitation speed decreased from 150 rpm to 100 rpm at 150 kPa, hexanoic acid production significantly increased from 0.20 g/L to 0.47 g/L. Notably, the production of hexanoic acid was slightly higher at 100 kPa and 125 kPa than at 150 kPa, despite the lower supply of syngas. Moreover, the chain elongation tendency to form C6 acid/ alcohol was enhanced at lower pressures. These results demonstrate that Clostridium sp. JS66 has the potential to produce C6 compounds from syngas, extending a spectrum of C6-producing acetogen. [ABSTRACT FROM AUTHOR]

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]

Energy consumption places growing demands on modern lifestyles, which have direct impacts on the world's natural environment. To attain the levels of sustainability required to avoid further consequences of changes in the climate, alternatives for sustainable production not only of energy but also materials and chemicals must be pursued. In this respect, syngas fermentation has recently attracted much attention, particularly from industries responsible for high levels of greenhouse gas emissions. Syngas can be obtained by thermochemical conversion of biomass, animal waste, coal, municipal solid wastes and other carbonaceous materials, and its composition depends on biomass properties and gasification conditions. It is defined as a gaseous mixture of CO and H2 but, depending on those parameters, it can also contain CO2, CH4 and secondary components, such as tar, oxygen and nitrogenous compounds. Even so, raw syngas can be used by anaerobic bacteria to produce biofuels (ethanol, butanol, etc.) and biochemicals (acetic acid, butyric acid, etc.). This review updates recent work on the influence of biomass properties and gasification parameters on syngas composition and details the influence of these secondary components and CO/H2 molar ratio on microbial metabolism and product formation. Moreover, the main challenges, opportunities and current developments in syngas fermentation are highlighted in this review. [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]

Background: Clostridium carboxidivorans P7 is capable of producing ethanol and butanol from inexpensive and non-food feedstock, such as syngas. Achieving improved ethanol and butanol production in the strain for industrial application depends on the energetics and biomass, especially ATP availability. Results: This study found that exogenous addition of citrulline promoted accumulation of ATP, increased specific growth rate, and reduced the doubling time of C. carboxidivorans P7. In heterotrophic fermentation experiments, the addition of citrulline increased intracellular ATP by 3.39-fold, significantly enhancing the production of total alcohol (ethanol + butanol) by 20%. Moreover, in the syngas fermentation experiments, the addition of citrulline improved the level of intracellular ATP and the biomass by 80.5% and 31.6%, respectively, resulting in an 18.6% and 60.3% increase in ethanol and the alcohol/acid production ratio, respectively. Conclusions: This is the first report that citrulline could promote the growth of C. carboxidivorans P7 and increase the level of intracellular ATP, which is of great significance for the use of C. carboxidivorans P7 to synthesize biofuels. [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 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 (CO2TR) 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 (H2TR). In cultivation on pure CO and CO + H2, 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 H2 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 H2. 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. [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]

A syngas-degrading enrichment culture, culture T-Syn, was dominated by a bacterium closely related to Desulfofundulus australicus strain AB33T (98% 16S rRNA gene sequence identity). Culture T-Syn could convert high CO concentrations (from pCO ≈ 34 kPa to pCO ≈ 170 kPa), both in the absence and in the presence of sulfate as external electron acceptor. The products formed from CO conversion were H2 and acetate. With sulfate, a lower H2/acetate ratio was observed in the product profile, but CO conversion rates were similar to those in the absence of sulfate. The ability of D. australicus strain AB33T to use CO was also investigated. D. australicus strain AB33T uses up to 40% CO (pCO ≈ 68 kPa) with sulfate and up to 20% CO (pCO ≈ 34 kPa) without sulfate. Comparison of the metagenome-assembled genome (MAG) of the Desulfofundulus sp. from T-Syn culture with the genome of D. australicus strain AB33T revealed high similarity, with an ANI value of 99% and only 32 unique genes in the genome of the Desulfofundulus sp. T-Syn. So far, only Desulfotomaculum nigrificans strain CO-1-SRB had been described to grow with CO with and without sulfate. This work further shows the carboxydotrophic potential of Desulfofundulus genus for CO conversion, both in sulfate-rich and low-sulfate environments. [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]

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]

The present study comparatively investigates the potential of waste plastic utilization as a feedstock for the production of liquid fuels to satisfy the rising liquid fuel demands of the transportation industry while simultaneously resolving the global plastic waste pollution challenge. For the first time, therefore, conceptual models simulating the production of transportation fuels of ethanol and gasoline from waste plastics via the technologies of thermo-syngas fermentation and hydrothermal liquefaction were assessed using classic technoeconomic assessment methods. The conceptual models were developed based on existing experimental data as obtained from the literature and simulated using ASPEN Plus as the preferred process simulation tool. This study demonstrated the technical viability of both conversion pathways with the hydrothermal liquefaction (HTL) of waste plastics for gasoline production shown to constitute a more economically preferable pathway. This was because the HTL of waste plastics presented a higher internal rate of return (IRR) value and a lower unit processing cost of 51.3% and USD 0.38 per kg compared to the thermo-syngas fermentation pathway that presented an IRR value and a unit processing cost value of 22.2% and USD 0.42 per kg, respectively. Payback periods of 5 years and 2 years were also determined as vital to recoup initial capital invested in the thermo-syngas fermentation project and the HTL project, respectively. Therefore, this study provides a basis for further work regarding waste plastic management strategies while offering a useful guide for policy makers in determining the most cost-effective way to utilize waste plastic and thus promote favorable environmental outcomes. [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]

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]

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]

In this study, novel and conventional techniques for the production of bioethanol from fruit and vegetable wastes (FVWs) by yeast and bacterial fermentation were investigated experimentally. Different pretreatment techniques (acid, heat, acid/heat, and microwave) for yeast fermentation were compared. Maximum ethanol concentrations of 11.7 and 11.8 g L-1 were observed from acid/heat and microwave pretreatment, respectively, by using Saccharomyces cerevisiae. On the other hand, biochar production from FVWs and syngas fermentation from the waste gas of this process were integrated. From waste gas with 12 % CO content, 5.5 g L-1 and 2.5 g L-1 ethanol production was observed by using anaerobic mixed culture and Clostridium ljungdahlii, respectively. The overall results emphasize the potential of bioethanol production from FVWs by economically feasible and environmentally friendly methods. [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]

Syngas fermentation is one of the bets for the future sustainable biobased economies due to its potential as an intermediate step in the conversion of waste carbon to ethanol fuel and other chemicals. Integrated with gasification and suitable downstream processing, it may constitute an efficient and competitive route for the valorization of various waste materials, especially if systems engineering principles are employed targeting process optimization. In this study, a dynamic multi‐response model is presented for syngas fermentation with acetogenic bacteria in a continuous stirred‐tank reactor, accounting for gas–liquid mass transfer, substrate (CO, H2) uptake, biomass growth and death, acetic acid reassimilation, and product selectivity. The unknown parameters were estimated from literature data using the maximum likelihood principle with a multi‐response nonlinear modeling framework and metaheuristic optimization, and model adequacy was verified with statistical analysis via generation of confidence intervals as well as parameter significance tests. The model was then used to study the effects of process conditions (gas composition, dilution rate, gas flow rates, and cell recycle) as well as the sensitivity of kinetic parameters, and multiobjective genetic algorithm was used to maximize ethanol productivity and CO conversion. It was observed that these two objectives were clearly conflicting when CO‐rich gas was used, but increasing the content of H2 favored higher productivities while maintaining 100% CO conversion. The maximum productivity predicted with full conversion was 2 g·L−1·hr−1 with a feed gas composition of 54% CO and 46% H2 and a dilution rate of 0.06 hr−1 with roughly 90% of cell recycle. [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]

Herein, an enzyme‐electrode based on the oxygen‐insensitive carbon monoxide dehydrogenase (CODH) containing molybdenum (Mo) and copper (Cu), flavin adenine dinucleotide (FAD) and two different [2Fe‐2 S] clusters as cofactors from the aerobic bacterium Hydrogenophaga pseudoflava, is proposed as a platform for dissolved CO concentration monitoring. The immobilized CODHs on Au electrode retain their catalytic activity and demonstrates changes to cyclic voltammetry and amperometry signals upon interactions with various dissolved CO concentrations. Cyclic voltammetry shows that CODHs are capable of direct electron transfer without any mediator use as oxidative current which starts around −0.268 V (vs Ag/AgCl) is observed in the presence of CO. When CO‐saturated standard solution was spiked sequentially into the gas‐tight reactor, amperometry analysis shows current increased accordingly with response time within 5 s. Our study demonstrates that this enzyme‐electrode is promising to serve as platform for developing an on‐line dissolved CO concentration monitoring tool which is essential to fill in the gap for conventional technologies which are limited to off‐line measurement. [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]

Ethanol production from non-edible feedstock has received significant attention over the past two decades. The utilisation of agricultural residues within the biorefinery concept can positively contribute to the renewable production of fuels. To this end, this study proposes the utilisation of bagasse in a hybrid conversion route for ethanol production. The main steps of the process are the gasification of the raw material followed by syngas fermentation to ethanol. Aspen plus was utilised to rigorously design the biorefinery coupled with Matlab to perform process optimisation. Based on the simulations, ethanol can be produced at a rate of 283 L per dry tonne of bagasse, achieving energy efficiency of 43% and according to the environmental analysis, is associated with low CO2 emissions. The conduction of a typical discounted cash flow analysis resulted in a minimum ethanol selling price of 0.69 $ L−1. The study concludes with multiobjective optimisation setting as objective functions the conflictive concepts of total investment costs and exergy efficiency. The total cost rate of the system is minimised whereas the exergy efficiency is maximised by using a genetic algorithm. This way, various process configurations and trade-offs between the investigated criteria were analysed for the proposed biorefinery system. [ABSTRACT FROM AUTHOR]

Cyanide is a minor constituent of crude syngas whose content depends on the feedstock and gasification procedure. It is a known poison to metal catalysts and inhibits iron-containing enzymes like carbon monoxide dehydrogenase of acetogenic organisms. Therefore, it is considered a component that has to be removed from the gas stream prior to use in chemical synthesis or syngas fermentation. We show that the growth rate and maximum biomass concentration of Clostridium ljungdahlii are unaffected by cyanide at concentrations of up to 1.0 mM with fructose as a carbon source and up to 0.1 mM with syngas as a carbon source. After the culture is adapted to cyanide it shows no growth inhibition. While the difference in growth is an increasing lagphase with increasing cyanide concentrations, the product spectrum shifts from 97% acetic acid and 3% ethanol at 0 mM cyanide to 20% acetic acid and 80% ethanol at 1.0 mM cyanide for cultures growing on (fructose) and 80% acetic acid and 20% ethanol at 0.1 mM cyanide (syngas). [ABSTRACT FROM AUTHOR]

Background: This study aimed at producing C6-C8 medium-chain carboxylates (MCCAs) directly from gaseous CO using mixed culture. The yield and C2-C8 product composition were investigated when CO was continuously fed with gradually increasing partial pressure. Results: The maximal concentrations of n-caproate, n-heptylate, and n-caprylate were 1.892, 1.635, and 1.033 mmol L-1, which were achieved at the maximal production rates of 0.276, 0.442, and 0.112 mmol L-1 day-1, respectively. Microbial analysis revealed that long-term acclimation and high CO partial pressure were important to establish a CO-tolerant and CO-utilizing chain-elongating microbiome, rich in Acinetobacter, Alcaligenes, and Rhodobacteraceae and capable of forming MCCAs solely from CO. Conclusions: These results demonstrated that carboxylate and syngas platform could be integrated in a shared growth vessel, and could be a promising one-step technique to convert gaseous syngas to preferable liquid biochemicals, thereby avoiding the necessity to coordinate syngas fermentation to short-chain carboxylates and short-tomedium-chain elongation. Thus, this method could provide an alternative solution for the utilization of waste-derived syngas and expand the resource of promising biofuels. [ABSTRACT FROM AUTHOR]

Renewable energy, including biofuels such as ethanol and butanol from syngas bioconversed by Clostridium carboxidivorans P7, has been drawing extensive attention due to the fossil energy depletion and global eco-environmental issues. Effects of zinc on the growth and metabolites of C. carboxidivorans P7 were investigated with model syngas as the carbon source. The cell concentration was doubled, the ethanol content increased 3.02-fold and the butanol content increased 7.60-fold, the hexanol content increased 44.00-fold in the medium with 280 μM Zn, when comparing with those in the control medium [Zn, (7 μM)]. Studies of the genes expression involved in the carbon fixation as well as acid and alcohol production in the medium with 280 μM Zn indicated that fdhII was up-regulated on the second day, acs A, fdhII, bdh35 and bdh50 were up-regulated on the third day and bdh35, acsB, fdhI, fdhIII, fdhIV, buk, bdh10, bdh35, bdh40 and bdh50 were up-regulated on the fourth day. The results indicated that the increased Zn content increased the alcohol production through increase in the gene expression of the carbon fixation and alcohol dehydrogenase. [ABSTRACT FROM AUTHOR]

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