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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.

Synthesis gas (syngas) fermentation is a biological carbon fixation process through which carboxydotrophic acetogenic bacteria convert CO, CO2, and H-2 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. info:eu-repo/semantics/publishedVersion

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 $/kgEtOH), reduced the primary energy requirement by 66.1% (from 2.82 to 0.96 kWthh/kgEtOH), lowered the CO2 emission by up to 82.6% (from 0.414 to 0.072 kgCO2/kgEtOH), and reduced the fresh water usage by 62.6% (from 0.242 to 0.091 m3W/kgEtOH). Sensitivity analysis for ethanol concentrations ranging from 6 to 1 wt% showed that the recovery costs and energy use increased to 0.336 $/kgEtOH and 1.78 kWthh/kgEtOH 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.

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.

Global energy demand has been escalating creating ever increasing pressure on climate crisis caused by fossil-based fuels. Humankind is now desperately in need of alternative and sustainable energy sources. Therefore, biofuels provide promising solution. Amongst the various biofuels, bioethanol from syngas, which is a mixture of, mostly, CO, CO2, N2, H2, and CH4 gases has been drawing increasing attention recently. Regarding this, the conversion of syngas to bioethanol, an alternative biofuel to fossil fuels, is considered a promising approach to reduce the negative effects of global warming by reducing greenhouse gas emissions. In this study, a novel immobilized cell bioreactor, where Clostridium ragsdalei was grown, was designed and used to achieve an efficient production of ethanol regarding volumetric production. For this purpose, a 300 mL immobilized reactor filled with ceramic balls as immobilization material was set and operated at 30oC throughout the study where CO gas as the main substrate was fed at rate of 6 ml/min continuously. Results showed ethanol and acetic acid concentrations reaching up to 1.4 g/L and 0.2 g/L, respectively, after 600h with a volumetric production rate of 0,0023g ethanol/L/h. We believe, the ceramic ball was used for bioethanol production for syngas for the first time.

The electrochemical conversion of CO2-H2O into CO-H2 using renewable energy is a promising technique for clean syngas production. Low-cost electrocatalysts to produce tunable syngas with a potential-independent CO/H2 ratio are highly desired. Herein, a series of N-doped carbon nanotubes encapsulating binary alloy nanoparticles (MxNi-NCNT, M= Fe, Co) were successfully fabricated through the co-pyrolysis of mela-mine and metal precursors. The MxNi-NCNT samples exhibited bamboo-like nanotubular structures with a large specific surface area and high degree of graphitization. Their electrocatalytic performance for syngas production can be tuned by changing the alloy compositions and modifying the electronic structure of the carbon nanotube through the encapsulated metal nanoparticles. Consequently, syngas with a wide range of CO/H2 ratios, from 0.5:1 to 3.4:1, can be produced on MxNi-NCNT. More importantly, stable CO/H2 ratios of 2:1 and 1.5:1, corresponding to the ratio to produce biofuels by syngas fermentation, could be realized on Co1Ni-NCNT and Co2Ni-NCNT, respectively, over a potential window of −0.8 to −1.2 V versus the reversible hydrogen electrode. Our work provides an approach to develop low-cost and potential-independent electrocatalysts to effectively produce syngas with an adjustable CO/H2 ratio from electrochemical CO2 reduction.

Norway is implementing several CO2 emissions control measures to become a low-emission society by 2050. As a part of this great vision, it strives to be the first nation to end the sales of fossil-fuelled vehicles by 2025. However, heavy transport still relies on fossil fuels that can be substantially replaced by liquified biogas. The demand-driven biogas market volume is more extensive than the current supply volume. The feedstock availability and lower methane yield are the potential bottlenecks that hinder the complete substitution of biomethane as a clean fuel. This PhD project studies syngas fermentation and microbial electrosynthesis (MES) as two sustainable technologies to advance methane yield. Fermenting syngas into a biogas reactor is an arduous process. Inefficient gas-liquid (GL) mass transfer of H2, the slow growth rate of the microorganisms that account for the fermentation, and lack of reducing equivalents are the foremost challenges. The original contribution of this PhD project to the knowledge is published as five open-access journal articles and a conference paper. Article 1 examines the effect of H2 partial pressure on syngas fermentation to overcome the GL mass transfer limitation. Article 2 explores the possibility of using moving bed biofilm to overcome the kinetic growth limitation of the slow-growing microorganisms. Articles 3, 4, and 5 investigate the impact of MES integration as a source of reducing equivalents for syngas fermentation; herein, the lowest benchmark potential for acetate synthesis and GL mass transfer are scrutinised. In addition, preliminary modelling work was performed in Modelica as an interface between the power electronic side and the biochemical side of the MES reactor, presented in a conference (Article 6). Digested sludge from Knarrdalstrand WWTP-AD, Porsgrunn, was pre-treated to enriched homoacetogens and utilised as the inoculum for all the experiments. In articles 1 to 4, the syngas was mimicked as H2 in the headspace and CO2 in the liquid medium as bicarbonate salt, while an industrially relevant syngas mixture (15 % CO, 15 % H2, 20 % N2 in 50 % CO2) was used in article 5. Initially, the impact of 1 to 25 bar H2 headspace pressure on the homoacetogenic medium with 0.41 volatile and total solid ratio (VS: TS) was studied. The 15 bar was identified as the optimum headspace pressure. There 47.24 mmol of H2 was consumed at an optimum rate of 6.22 mol h-1 L-1, and 3.0 g L-1 of acetic acid was synthesised. In study two, moving bed biofilm was incorporated into a 15 bar H2 pressurised reactor. The biofilm integrated batch depicted a 33 % improvement in the H2 uptake rate (200 mmol L-1 d-1) and 48 % in acetate synthesis rate (37.4 mmol L-1 d-1). The stereo microscopic images affirmed the biofilm accumulation on the carriers at an average thickness of 160 μm. In order to study the impact of MES on syngas fermentation (article 3), the experiment was performed in three phases. The fermentation was done with the suspended medium alone in the first phase. Then electrodes were integrated into the planktonic medium, referred to as a single-chamber MES reactor; therein, electrodes were not poised with any voltage in phase two (open circuit mode, OCM), while cathode was poised with negative potential in phase three (closed-circuit mode, CCM). In contrast to the planktonic fermentation, the biofilm assimilated on the electrodes in the OCM tripled the H2 (1021 mmol) consumption. Also, the gas uptake rate (13.5 mmol L-1 d-1) and acetate synthesis (57 mmol L-1) improved by 125 % and 63 %. An optimum potential (– 0.8 V vs. SHE) for acetate synthesis was adapted from previous USN studies to perform phase three. The study hypothesised that potential could trigger the H2 GL mass transfer and acetate synthesis. However, it failed because the applied potential (- 0.8 V) could have been deprived of its biotic nature; thus, the anodic potential exceeded the acetic acid oxidation potential, resulting in a rapid drop in concentration. Therefore article 4 attempted to determine the lowest benchmark potential for acetate synthesis and improved H2 GL mass transfer. Study 4 was performed in two phases, OCM and CCM. The CCM examined the impact of reducing power from – 25 to – 175 mV vs. Ag/AgCl (3.0 NaCl) on the acetic acid synthesis and H2 GL mass transfer. Compared to the OCM, the -175 mV enhanced the acetate synthesis rate (0.225 mmol L-1 h-1) by 26 % and was identified as the lowest benchmark potential, while there was no improvement in H2 consumption. The direct electron transfer from cathode to microorganism avoided the H2 consumption from headspace. Article 5 is quite similar to article 4. However, the distinct advancement is that instead of pure H2 in the headspace, industrial relevant syngas has been fermented to move the technology readiness level forward. The experiments were performed in both open and closed-circuit modes. During the CCM, the impact of reducing power from – 50 to – 400 mV vs. Ag/AgCl (3.0 NaCl) was examined on acetic acid synthesis, syngas utilisation and CO inhibition. The – 150 mV was figured out as the lowest benchmark potential for the optimum acetic acid synthesis rate (0.263 mmol L-1 h-1), which is 15 times higher than the OCM rate. Sixty per cent of the CO from the headspace was consumed without any noticeable inhibition. The overall PhD study concludes that elevated syngas pressure, MBB, and MES incorporation significantly enhanced acetate synthesis that can further be converted into methane into a biogas reactor, thus advancing biogas production.

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.

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 (CO2 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 (H2 TR). 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.

The increasing world population, as well as demands for energy and industrial commodities, have led to an overall rise in anthropogenic greenhouse gas emissions. Moreover, demands for fossil fuels and petrochemicals are also gradually escalating in the developing and underdeveloped countries to sustain the transportation sector, industrial power generation and commodity chemical production. To address these challenges and to meet the Paris Agreement on climate change, there has been a growing interest in the development of renewable energy resources. Hydrogen is considered a strong candidate for the next-generation energy infrastructures as an energy carrier and vector, as well as a precursor or auxiliary component for several industrial processes. This review outlines current researches and applications of hydrogen in the processing and upgrading of hydrocarbon fuels, fuel cells, platform chemicals synthesis, pharmaceuticals, aerospace and maritime uses, metallurgy and electronics. Besides, applications of hydrogen as a direct fuel, synthesis of liquid hydrocarbons via Fischer-Tropsch process and syngas fermentation to alcohols are also reviewed. The specific utilization of hydrogen in the metallurgical industry is highlighted for metal reduction and in the oxyhydrogen flame for cutting and welding of metals. Besides, the article also describes the use of hydrogen in pharmaceutical industries for drug discovery. Finally, a variety of socio-economic aspects of hydrogen as a renewable fuel is also outlined in this article.

The biomass is one of the best renewable energy sources for extracting syngas which is used for bio ethanol production. As compared to other renewable sources such as solar, wind and hydro, biomass gives CO2 and H2 which is mainly used for bio ethanol production. The thermo chemical and biological methods are two different conversion technology available for bio ethanol production. The major steps involved in entire process are syngas extraction from biomass, syngas cleaning and fermentation of syngas using microorganism and various type of reactor under specified condition of temperature, pressure and pH. The optimum temperature range for mesophilic microorganisms are 37 to 40 °C while for thermophilic microorganism it ranges 55 to 80 °C. The microorganisms of clostridium family are giving good yield of bio ethanol. The hybrid platform for gasification syngas fermentation can produce more bio-ethanol by using biomass components as compared to the biochemical conversion routes. The use of hybrid system for syngas fermentation process will takes place at mild temperature and pressures. It also avoids the use of expensive pre-treatment process and enzymes. The entire study concludes the possible routes for bio ethanol production from syngas using fermentation technology.

The massive consumption of fossil energy forces people to find new sources of energy. Syngas fermentation has become a hot research field as its high potential in renewable energy production and sustainable development. In this study, trophic anaerobic acetogen Morella thermoacetica was successfully immobilized by calcium alginate embedding method. The ability of the immobilized cells on production of acetic acid through syngas fermentation was compared in both airlift and bubble column bioreactors. The bubble column bioreactor was selected as the better type of bioreactor. The production of acetic acid reached 32.3 g·L−1 in bubble column bioreactor with a space–time yield of 2.13 g·L−1·d−1. The immobilized acetogen could be efficiently reused without significant lag period, even if exposed to air for a short time. A semi-continuous syngas fermentation was performed using immobilized cells, with an average space–time acetic acid yield of 3.20 g·L−1·d−1. After 30 days of fermentation, no significant decrease of the acetic acid production rate was observed.

Medium-chain carboxylic acids (MCCAs) are valuable platform chemicals and can be produced from waste biomass sources or syngas fermentation effluent through microbial chain elongation. We have previously demonstrated successful approaches to separate >90% purity oil with different MCCAs (MCCA oil) by integrating the anaerobic bioprocess with membrane-based liquid-liquid extraction (pertraction) and membrane electrolysis. However, two-compartment membrane electrolysis unit without pertraction was not able to separate MCCA oil. Therefore, we developed a five-compartment electrodialysis/phase separation cell (ED/PS). First, we tested an ED/PS cell in series with pertraction and achieved a maximum MCCA-oil flux of 1.7 × 103 g d-1 per projected area (m2) (19 mL oil d-1) and MCCA-oil transfer efficiency [100% × moles MCCA-oil moles electrons-1] of 74% at 15 A m-2. This extraction system at 15 A m-2 demonstrated a ∼10 times lower electric-power consumption (1.1 kWh kg-1 MCCA oil) than membrane electrolysis in series with pertraction (9.9 kWh kg-1 MCCA oil). Second, we evaluated our ED/PS as a stand-alone unit when integrated with the anaerobic bioprocess and demonstrated that we can selectively extract and separate MCCA oil directly from chain-elongating bioreactor broth with just an abiotic electrochemical cell. However, the electric-power consumption increased considerably due to the lower MCCA concentrations in the bioreactor broth compared to the pertraction broth.

Hexanol–butanol–ethanol fermentation from syngas by Clostridium carboxidivorans P7 is a promising route for biofuel production. However, bacterial agglomeration in the culture of 37 °C severely hampers the accumulation of biomass and products. To investigate the effect of culture temperature on biomass growth and higher-alcohol production, C. carboxidivorans P7 was cultivated at both constant and two-step temperatures in the range from 25 to 37 °C. Meanwhile, Tween-80 and saponin were screened out from eight surfactants to alleviate agglomeration at 37 °C. The results showed that enhanced higher-alcohol production was contributed mainly by the application of two-step temperature culture rather than the addition of anti-agglomeration surfactants. Furthermore, comparative transcriptome revealed that although 37 °C promoted high expression of genes involved in the Wood–Ljungdahl pathway, genes encoding enzymes catalyzing acyl-condensation reactions associated with higher-alcohol production were highly expressed at 25 °C. This study gained greater insight into temperature-effect mechanism on syngas fermentation by C. carboxidivorans P7.

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 N2 (45.58%), CO (22.92%), CO2 (7.9%), H2 (13.05%), and CH4 (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 CO2, other gases were dissolved entirely. It is also found that extracted bioethanol was identified by corresponding NMR (1H) spectra of methyle group (CH3-), methylene group (–CH2–) 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.

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% H2, 22.92% CO, 7.9% CO2, and 1.13% CH4, 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.

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.

Mass transfer limitations in syngas fermentation processes are mostly attributed to poor solubility of CO and H2 in water. Despite these assumed limitations, a syngas fermentation process has recently been commercialized. Using large-sale external-loop gas-lift reactors (EL-GLR), CO-rich off-gases are converted into ethanol, with high mass transfer performance (7–8.5 g.L-1.h−1). However, when applying established mass transfer correlations, a much poorer performance is predicted (0.3–2.7 g.L-1.h−1). We developed a CFD model, validated on pilot-scale data, to provide detailed insights on hydrodynamics and mass transfer in a large-scale EL-GLR. As produced ethanol could increase gas hold-up (+30%) and decrease the bubble diameter (≤2 mm) compared to air–water mixtures, we found with our model that a high volumetric mass transfer coefficient (650–750 h−1) and mass transfer capacity (7.5–8 g.L-1.h−1) for CO are feasible. Thus, the typical mass transfer limitations encountered in air–water systems can be alleviated in the syngas-to-ethanol fermentation process.

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 (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.

Dissertation, RWTH Aachen University, 2022; Aachen : RWTH Aachen University 1 Online-Ressource : Illustrationen, Diagramme (2022). = Dissertation, RWTH Aachen University, 2022, The goal of this thesis is the optimization of hexanol production from syngas. Hexanol can be used i.e.for the production of plastics, fuels and additives. Two potential biocatalysts, C. carboxidivorans and C. ljungdahlii, were characterized and evaluated for this application.These are the most important results obtained in this study: Bioenergetic calculations revealed that hexanol could be produced as the sole fermentation product from syngas. Hexanol toxicity was identified as a crucial limiting factor in hexanol production and could be circumvented by product extraction, increasing yields ~2.5 fold to 24 mM hexanol. The overall toxicity of hexanol towards both tested species was similar with acute inhibition at ≤ 20 mM. The hexanol tolerance of C. carboxidivorans was higher at 30°C (IC50 17.5 mM) than at 37°C (IC50 11.8 mM) and extraction led to detoxification only at the lower temperature. During scale-up from bottles to CSTR, gas delivery, measured as dissolved CO tension, and different nutrient requirements were identified as important bottlenecks. The highest hexanol titer produced in the CSTR was 37 mM. This is ~3 times the currently highest titer reported in other studies. To improve heterologous hexanol production with C.ljungdahlii, different enzymes were analyzed in transgenic C. ljungdahlii strains but the produced hexanol titers still remained lower than the titers obtained with C. carboxidivorans. In conjugation experiments with C. carboxidivorans, colonies could only be obtained either after heat shock of the recipient or after use of a donor strain expressing the C. carboxidivorans methyl transferases. These colonies displayed resistance towards the selection markers, but no verifiable liquid cultures could beobtained. The methylation pattern of C. carboxidivorans was investigated and the discovered motifs were attributed to respective methyl transferases. Converter gas (CG) was compared to synthetic CG with the same composition to investigate toxic effects of minor gas components. Without dilution of the gas and low cell densities, no growth was observed. High cell densities could partially circumvent inhibition and final product- and growth yields were similar with real and synthetic CG. If the gas phase was renewed, growth ceased even at high cell densities. No adaptation towards CG was observed. Compared to synthetic CG, hexanol production and growth yields were reduced with only H2+CO2 as substrate. Screening of different media compositions revealed improvement through addition of 2 g L-1 carbonate and a lower pH. In conclusion, C. carboxidivorans is a promising candidate for hexanol production from syngas. The hexanol titers obtained in this study of 37 mM (3.8 g L-1) is higher than titers reported for transgenic biocatalysts as well as co-cultivations. The major drawback is the lack of reliable methods for genetic manipulation. By addressing the crucial points identified in this work, hexanol production from syngas could be developed into a commercial process that can decrease net carbon emissions while simultaneously reducing dependence on fossil carbon sources. This is a step towards a greener andmore sustainable economy., Published by RWTH Aachen University, Aachen

The main aim of the present study was to evaluate the effects of the presence or absence of trace elements and yeast extract on growth and ethanol production by Clostridium ragsdalei grown on C1 gas dominated syngas (60% CO, 10% CO2, 10% CH4, 10% H-2, 10% N-2). Comparison of whey powder and glucose syrup as low-cost substitutes helped develop a more cost-effective medium formulation to produce ethanol from syngas with significantly higher yields, achieving up to a 3.8-fold improvement with co-utilisation of syngas and glucose syrup. In addition, the results provided further insights into medium manipulation and possible modifications to decrease total costs for increased ethanol production by solventogenic acetogens via syngas fermentation by achieving a 15.6% reduction in the cost of fermentation medium., TUBITAK-CAYDAG [118Y305], The authors wish to thank TUBITAK-CAYDAG under project number 118Y305.

In gas fermentations (using O2, CO, H2, CH4 or CO2), gas-to-liquid mass transfer is often regarded as one of the limiting processes. However, it is widely known that components in fermentation broths (e.g., salts, biomass, proteins, antifoam, and organic products such as alcohols and acids) have tremendous impact on the volumetric mass transfer coefficient kLa. We studied the influence of ethanol on mass transfer in three fermentation broths derived from syngas fermentation. In demineralized water, we observed that the addition of ethanol, the expected product, increased kLa two-fold in the 0–5 g L−1 range, after which near-constant kLa values were obtained. In the fermentation broths, kLa was increased significantly (2–4 fold compared to water) by ethanol supplementation, and to be highly influenced by broth salinity. Our results indicate that kLa is a dynamic parameter in gas fermentation experiments and can be significantly increased due to broth components.

Tese de doutoramento em Chemical and Biological Engineering, One of the major drawbacks of syngas fermentation is the limited gas-liquid mass transfer that generally limits productivities. Most research has been focusing in increasing the kLa (volumetric gas-liquid mass transfer coefficient), investigating different reactor typologies and gas dispersion devices. However, the driving force for mass transfer can also be increased, for instance, by operating at increased pressure. This thesis aims to explore the effect of increased pressure in syngas fermentation to improve the gas solubility, evaluating the effect on the biocatalysts and on the process itself. The starting point of this work was to use an adapted culture (from a syngas converting reactor) as inoculum. This culture was able to mainly produce methane and acetate from syngas. Methane was not produced directly from CO, but via the conversion of acetate and H2 by the bacteria present in the mixed culture (Acetobacterium and Sporomusa species). Later on, this culture was subsequentially transferred with syngas, originating an enriched culture mainly composed by Acetobacterium and Methanospirillum species that, besides acetate and methane, was able to produce propionate from syngas. From that highly enriched culture a new strain of Acetobacterium wieringae, strain JM, was isolated and characterized. This highly enriched culture was tested in an axial agitation reactor (AAR) with different initial syngas (CO, H2 and CO2 (60:30:10 %, v/v)) pressures, from 100 kPa to 600 kPa. No substrate inhibition was observed, even at the highest pressure and an increase of 45 % in titres (of acetate and propionate) were obtained. Moreover, the increase of pressure resulted in a shift in the metabolic pathways from acetate towards propionate, which is an uncommon product of syngas fermentation. The effect of pressure on the conversion of syngas by anaerobic mixed sludge was then studied using two different reactor typologies, AAR and GLR (gas-lift reactor). Initial syngas pressures of 100 kPa, 300 kPa and 500 kPa were tested. Overall, the GLR showed better performance in terms of CO consumption rates and product titres. The main product obtained was methane and the results showed, for the first time, that methanogenic activity was not inhibited with initial syngas pressures up to 500 kPa, achieving methane yields of 75 % for the GLR, and 92 % for the AAR. At 300 kPa and 500 kPa, volatile fatty acids were also produced, namely acetate, proprionate and n-butyrate. Propionate was the most abundant acid produced, reaching 4.4 mM at 300 kPa and 4.8 mM at 500 kPa in the AAR. Overall, the use of moderate pressures in syngas fermentation was shown beneficial, as it resulted in better productivities and titres without having significant detrimental effects on cell growth. The increase of pressure also did not inhibit methanogenesis. Moreover, higher pressures seem to induce the production of different chemicals, broadening the product spectrum of syngas fermentation. In this way, these findings could be the basis of new developments in the industrialization of syngas fermentation to produce platform chemicals and/or for biomethanation processes., Um dos principais desafios da fermentação do gás de síntese é a limitação na transferência de massa gás-líquido que geralmente limita a produtividade. A maioria das pesquisas têm-se concentrado no aumento do kLa (coeficiente de transferência de massa gás-líquido volumétrico), investigando diferentes tipologias de reatores e dispositivos de dispersão de gás. No entanto, a força motriz para a transferência de massa também pode ser aumentada, por exemplo, operando sistemas pressurizados. Esta tese tem como objetivo explorar o efeito do aumento da pressão na fermentação de gás de síntese para melhorar a solubilidade do gás, avaliando o efeito nos biocatalisadores e no próprio processo. O ponto de partida deste trabalho foi a utilização de uma cultura adaptada (de um reator de conversão de gás de síntese) como inóculo. Essa cultura foi capaz de produzir principalmente metano e acetato a partir do gás de síntese. O metano não foi produzido diretamente do CO, mas através da conversão de acetato e H2 pelas bactérias presentes na cultura mista (espécies dos géneros Acetobacterium e Sporomusa). Essa cultura foi consecutivamente transferida usando gás de síntese como substrato, originando uma cultura enriquecida composta principalmente por espécies de Acetobacterium e de Methanospirillum que, além de acetato e metano, foi capaz de produzir propionato a partir de gás de síntese. A partir dessa cultura altamente enriquecida, uma nova strain de Acetobacterium wieringae, strain JM, foi isolada e caracterizada. Esta cultura altamente enriquecida foi testada num reator de agitação axial (AAR) com diferentes pressões de gás de síntese (CO, H2 e CO2 (60:30:10%, v/v)), de 100 kPa a 600 kPa. Não foi observada qualquer inibição pelo substrato, mesmo na pressão mais elevada e obteve-se um aumento de 45 % na produção final de acetato e propionato. Além disso, o aumento da pressão originou um desvio metabólico de acetato para propionato, que é um produto incomum da fermentação do gás de síntese. O efeito da pressão na conversão de gás de síntese utilizando biomassa anaeróbia foi também estudado usando duas tipologias de reatores diferentes, AAR e GLR (reator gas-lift). Foram testadas as seguintes pressões iniciais de gás de síntese: 100 kPa, 300 kPa e 500 kPa. O GLR apresentou melhor desempenho em termos de taxas de consumo de CO e de concentração final de produtos. O principal produto obtido foi o metano e os resultados mostraram, pela primeira vez, que a atividade metalogénica não foi inibida com pressões iniciais de syngas de até 500 kPa, atingindo rendimentos de metano de 75 % para o GLR e 92 % para o AAR. A 300 kPa e 500 kPa, também foram produzidos ácidos gordos voláteis, nomeadamente, acetato, propionato e n-butirato. O propionato foi o ácido mais abundante produzido, atingindo 4,4 mM a 300 kPa e 4,8 mM a 500 kPa no AAR. No geral, o uso de pressões moderadas na fermentação do gás de síntese mostrou-se benéfico, pois resultou em melhores produtividades e concentrações finais de produto sem ter efeitos prejudiciais significativos no crescimento celular. O aumento da pressão também não inibiu a metanogénese. Além disso, pressões mais altas parecem induzir a produção de diferentes produtos químicos, ampliando o espectro de produtos da fermentação do gás de síntese. Desta forma, estas descobertas podem ser a base de novos desenvolvimentos na industrialização da fermentação do gás de síntese para a produção de produtos químicos e/ou para processos de biometanação.

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.

Microbial C1 gas conversion technologies have developed into a potentially promising technology for converting waste gases (CO2, CO) into chemicals, fuels, and other materials. However, the mass transfer constraint of these poorly soluble substrates to microorganisms is an important challenge to maximize the efficiencies of the processes. These technologies have attracted significant scientific interest in recent years, and many reactor designs have been explored. Syngas fermentation and hydrogenotrophic methanation use molecular hydrogen as an electron donor. Furthermore, the sequestration of CO2 and the generation of valuable chemicals through the application of a biocathode in bioelectrochemical cells have been evaluated for their great potential to contribute to sustainability. Through a process termed microbial chain elongation, the product portfolio from C1 gas conversion may be expanded further by carefully driving microorganisms to perform acetogenesis, solventogenesis, and reverse -oxidation. The purpose of this review is to provide an overview of the various kinds of bioreactors that are employed in these microbial C1 conversion processes., This study was conducted in collaboration with researchers from four different institu tions (Dokuz Eylul University, Turkey; University of Minho, Portugal; Izmir Democracy University, Turkey, and University of A Coruña, Spain), who were supported by the following funding bodies: A.A. [Dokuz Eylul University, Scientific Research Foundation (DEU-BAP) (#2011.KB.FEN.046) and TUBİTAK (#119R029)]; L.P. [Portuguese Foundation for Science and Technology (FCT) (UIDB/04469/2020), and FCT and European Social Fund (POPH-QREN) (POCI-01-0145-FEDER 031377)]; T.K. [TUBİTAK-CAYDAG (118Y305)]; and H.N.A. [Xunta de Galicia (ED431C 2021/55)]., A.A. acknowledges the support by Dokuz Eylul University, Scientific Research Foundation (DEU-BAP), Turkey, for the award on (#2011.KB.FEN.046) “Direct Electricity Generation from Treatment Plant Sludges by using MFCs” research project. A.A. acknowledges TUBİTAK for the support on #119R029 “Sustainable Energy Recovery from Treatment plant sludge, green waste and olive pomace via gasification process: Investigation of beneficial usage alternatives of gasification by-products”. L.P. acknowledges the Portuguese Foundation for Science and Technol ogy (FCT) under the scope of the strategic funding of UIDB/04469/2020 unit. Also, the financial sup port from Portuguese Foundation for Science and Technology (FCT) and European Social Fund (POPH-QREN) through the project INNOVsyn - Innovative strategies for syngas fermentation (POCI-01-0145-FEDER-031377) are gratefully acknowledged. T.K. acknowledges the support from The Scientific and Technological Research Council of Turkey (TUBITAK-CAYDAG) (project no: 118Y305). H.N.A. thanks the Xunta de Galicia (Spain) for his postdoctoral fellowship (ED481D 2019/033). H.N.A., belonging to the BIOENGIN group of the UDC, also acknowledges Xunta de Galicia for recognizing the group as a competitive Reference Research Group (GRC) (ED431C 2021/55)., info:eu-repo/semantics/publishedVersion

Sertkaya, Simge (Balikesir Author), Syngas fermentation via the Wood-Ljungdahl (WL) pathway is a promising approach for converting gaseous pollutants (CO and CO2) into high-value commodities. Because the WL involves several enzymes with trace metal components, it requires an adequate supply of micronutrients in the fermentation medium for targeted bioprocessing such as bioethanol production. Plackett-Burman statistical analysis was performed to examine the most efficient trace elements (Ni, Mg, Ca, Mn, Co, Cu, B, W, Zn, Fe, and Mo) and their concentrations for Clostridium ljungdahlii on ethanol production. Overall, 1.5 to 2.5 fold improvement in ethanol production could be achieved with designed trace element concentrations. The effects of tungsten and copper on ethanol and biomass production were determined to be the most significant, respectively. The model developed was statistically significant and has the potential to significantly decrease the cost of trace element solutions by 18-22%. This research demonstrates the critical importance of optimizing the medium for syngas fermentation in terms of product distribution and economic feasibility., Ege University FDK-2020-22039 ED481D 2019/033

The strict anaerobe Clostridium ljungdahlii can ferment CO or H2/CO2 via the Wood-Ljungdahl pathway to acetate, ethanol, and 2,3‐butanediol. This ability has attracted considerable interest, since it can be used for syngas fermentation to produce biofuels and biochemicals. However, the key enzyme methylenetetrahydrofolate reductase (MTHFR) in the Wood-Ljungdahl pathway of the strain has not been characterized, and its physiological electron donor is unclear. In this study, we purified the enzyme 46-fold with a benzyl viologen reduction activity of 41.2 U/mg from C. ljungdahlii cells grown on CO. It is composed of two subunits, MetF (31.5 kDa) and MetV (23.5 kDa), and has an apparent molecular mass of 62.2 kDa. The brownish yellow protein contains 0.73 flavin mononucleotide (FMN) and 7.4 Fe, in agreement with the prediction that MetF binds one flavin and MetV binds two [4Fe4S] clusters. It cannot use NAD(P)H as its electron donor or catalyze an electron-bifurcating reaction in combination with ferredoxin as an electron acceptor. The reduced recombinant ferredoxin, flavodoxin, and thioredoxin of C. ljungdahlii can serve as electron donors with specific activities of 91.2, 22.1, and 7.4 U/mg, respectively. The apparent Km values for reduced ferredoxin and flavodoxin were around 1.46 μM and 0.73 μM, respectively. Subunit composition and phylogenetic analysis showed that the enzyme from C. ljungdahlii belongs to MetFV-type MTHFR, which is a heterodimer, and uses reduced ferredoxin as its electron donor. Based on these results, we discuss the energy metabolism of C. ljungdahlii when it grows on CO or H2 plus CO2. IMPORTANCE Syngas, a mixture of CO, CO2, and H2, is the main component of steel mill waste gas and also can be generated by the gasification of biomass and urban domestic waste. Its fermentation to biofuels and biocommodities has attracted attention due to the economic and environmental benefits of this process. Clostridium ljungdahlii is one of the superior acetogens used in the technology. However, the biochemical mechanism of its gas fermentation via the Wood-Ljungdahl pathway is not completely clear. In this study, the key enzyme, methylenetetrahydrofolate reductase (MTHFR), was characterized and found to be a non-electron-bifurcating heterodimer with reduced ferredoxin as its electron donor, representing another example of MetFV-type MTHFR. The findings will form the basis for a deeper understanding of the energy metabolism of syngas fermentation by C. ljungdahlii, which is valuable for developing metabolic engineering strains and efficient syngas fermentation technologies.

Recycling residual industrial gases and residual biomass as substrates to biofuel production by fermentation is an important alternative to reduce organic wastes and greenhouse gases emission. Clostridium carboxidivorans can metabolize gaseous substrates as CO and CO2 to produce ethanol and higher alcohols through the Wood-Ljungdahl pathway. However, the syngas fermentation is limited by low mass transfer rates. In this work, a syngas fermentation was carried out in serum glass bottles adding different concentrations of Tween® 80 in ATCC® 2713 culture medium to improve gas-liquid mass transfer. We observed a 200% increase in ethanol production by adding 0.15% (v/v) of the surfactant in the culture medium and a 15% increase in biomass production by adding 0.3% (v/v) of the surfactant in the culture medium. The process was reproduced in stirred tank bioreactor with continuous syngas low flow, and a maximum ethanol productivity of 0.050 g/L.h was achieved.

Syngas fermentation to biofuels and chemicals is an emerging technology in the biobased economy. Mass transfer is usually limiting the syngas fermentation rate, due to the low aqueous solubilities of the gaseous substrates. Membrane bioreactors, as efficient gas–liquid contactors, are a promising configuration for overcoming this gas-to-liquid mass transfer limitation, so that sufficient productivity can be achieved. We summarize the published performances of these reactors. Moreover, we highlight numerous parameters settings that need to be used for the enhancement of membrane bioreactor performance. To facilitate this enhancement, we relate mass transfer and other performance indicators to the type of membrane material, module, and flow configuration. Hollow fiber modules with dense or asymmetric membranes on which biofilm might form seem suitable. A model-based approach is advocated to optimize their performance.

WOS: 000508234000005, 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., TUBITAK-CAYDAGTurkiye Bilimsel ve Teknolojik Arastirma Kurumu (TUBITAK) [118Y305], This work was supported by TUBITAK-CAYDAG [118Y305] Dr. Tugba Keskin-Gundogdu especially thanks Dr. Gozde Duman for providing pyrolysis gas from FWVs, and Prof. Dr. Nuri Azbar, and Prof. Dr. Jale Yanik for their support in providing laboratories for experiments. Special thanks to Dr. Haris Nalakth Abubackar and Dr. Hilal Betul Kaya Akkale for advisory support.

Acetyl-CoA is the central metabolic node connecting glycolysis, Krebs cycle and fatty acids synthase. Plant-derived polyketides, are assembled from acetyl-CoA and malonyl-CoA, represent a large family of biological compounds with diversified bioactivity. Harnessing microbial bioconversion is considered as a feasible approach to large-scale production of polyketides from renewable feedstocks. Most of the current polyketide production platform relied on the lengthy glycolytic steps to provide acetyl-CoA, which inherently suffers from complex regulation with metabolically-costly cofactor/ATP requirements. Using the simplest polyketide triacetic acid lactone (TAL) as a target molecule, we demonstrate that acetate uptake pathway in oleaginous yeast (Yarrowia lipolytica) could function as an acetyl-CoA shortcut to achieve metabolic optimality in producing polyketides. We identified the metabolic bottlenecks to rewire acetate utilization for efficient TAL production inY. lipolytica, including generation of the driving force for acetyl-CoA, malonyl-CoA and NADPH. The engineered strain, with the overexpression of endogenous acetyl-CoA carboxylase (ACC1), malic enzyme (MAE1) and a bacteria-derived cytosolic pyruvate dehydrogenase (PDH), affords robust TAL production with titer up to 4.76 g/L from industrial glacier acetic acid in shake flasks, representing 8.5-times improvement over the parental strain. The acetate-to-TAL conversion ratio (0.149 g/g) reaches 31.9% of the theoretical maximum yield. The carbon flux through this acetyl-CoA metabolic shortcut exceeds the carbon flux afforded by the native acetyl-CoA pathways. Potentially, acetic acid could be manufactured in large-quantity at low-cost from Syngas fermentation or heterogenous catalysis (methanol carbonylation). This alternative carbon sources present a metabolic advantage over glucose to unleash intrinsic pathway limitations and achieve high carbon conversion efficiency and cost-efficiency. This work also highlights that low-cost acetic acid could be sustainably upgraded to high-value polyketides by oleaginous yeast species in an eco-friendly and cost-efficient manner.

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 (KLa) 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 KLa 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 O2 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.

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.

Ethanol production via syngas fermentation obtained from lignocellulose gasification provides a method to completely utilize all the carbon content from lignocellulosic feedstock. A low mass transfer rate of less soluble gas CO and H2 to liquid has been considered as the major bottleneck of the overall process; thus, microporous membrane was 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 carried out with the configuration of a bioreactor connected to the 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. 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 by the system of HFM-supported bioreactor with higher ethanol titter of 1.09 g/L and ethanol to acetate molar ratio of 1.43 mol/mol. The obtained result indicates HMF-supported bioreactor is the best fermentation system compared to STR without the membrane.

Bioethanol has garnered a great interest as a potential energy source, mainly due to its sustainable and green nature. Generally, bioethanol is produced through the microbial conversion of biomass and biomass derived syngas. However, the dehydration and purification steps for achieving fuel-grade ethanol from the microbial production process consume tremendous amounts of energy. This high energy consumption limits the feasibility of microbial ethanol production on the commercial scale. In this context, selection of an optimal technology for product separation is essential for successful commercialization of microbially produced bioethanol. This article presents the recent developments in dehydration and purification technologies for bioethanol production using distillation and membrane based separation. Distillation and pervaporation are analyzed on the basis of the overall energy requirement, consumption, and economics. Pervaporation-assisted distillation approaches are also examined from the perspective of process systems engineering, including factors affecting the system performance. Furthermore, the role of simulation in technological development along with available mathematical models is discussed, and commercial status of pervaporation based separation is presented. Finally, the current status of the existing technology, challenges, and future research directions are identified from the perspective of achieving process sustainability on the industrial scale. Economic comparison between distillation and different hybrid schemes revealed that integrating distillation with membrane based separation techniques reduce the bioethanol production cost. Moreover, hybrid schemes that combine distillation with pervaporation, and steam stripping with vapor permeation are proved to be the best combinations for the cheapest bioethanol production.

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.

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.

Financiado para publicación en acceso aberto: Universidade da Coruña/CISUG [Abstract] In recent years, the possibility of merging technologies for waste recovery such as those based on syngas fermentation and chain elongation has been studied for the production of medium chain fatty acids (MCFAs) and bioalcohols, in an attempt to integrate the concept of circular economy in the industry. Nevertheless, one of the main issues of this approach is the pH mismatch between acetogens and chain elongating microorganisms. This work reports, for the first time, the suitability of a co-culture of C. aceticum and C. kluyveri metabolizing syngas at near neutral pH in stirred tank bioreactors. For this purpose, bioreactor studies were carried out with continuous syngas supply. In the first experiment, maximum concentrations of 𝑛-butyrate and 𝑛-caproate of 7.0 and 8.2 g/L, respectively, were obtained. In the second experiment, considerable amounts of 𝑛-butanol were produced as a result of the reduction, by C. aceticum, of the carboxylates already formed in the broth. In both experiments, ethanol was used as an exogenous electron agent at some point. Finally, batch bottle assays were performed with a pure culture of C. aceticum grown on CO in presence of n-butyrate to assess and confirm its ability to produce 𝑛-butanol, reaching concentrations up to 951 mg/L, with a 𝑛-butyrate conversion efficiency of 96%, which had never been reported before in this species. Therefore, this work contributes to the state of the art, presenting a novel system for the bioproduction of MCFAs by combining syngas fermentation and chain elongation at near neutral pH, as opposed to the acidic pH range used in all previously reported literature. This research was funded through the Spanish Ministry of Science and Innovation and European FEDER funds (PID2020-117805RB-I00). CFB thanks Xunta de Galicia for her doctoral contract (ED481A-2020/028). The authors, belonging to the BIOENGIN group, thank Xunta de Galicia for financial support to Competitive Reference Research Groups (ED431C 2021/55). Funding for open access charge provided by Universidade da Coruña/CISUG Xunta de Galicia; ED481A-2020/028 Xunta de Galicia; ED431C 2021/55

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 NOx, 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, NOx 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.

Electron donor scarcity is seen as one of the major issues limiting economic production of medium-chain carboxylates from waste streams. Previous studies suggest that co-fermentation of hydrogen in microbial communities that realize chain elongation relieves this limitation. To better understand how hydrogen co-feeding can support chain elongation, we enriched three different microbial communities from anaerobic reactors (A, B, and C with ascending levels of diversity) for their ability to produce medium-chain carboxylates from conventional electron donors (lactate or ethanol) or from hydrogen. In the presence of abundant acetate and CO2, the effects of different abiotic parameters (pH values in acidic to neutral range, initial acetate concentration, and presence of chemical methanogenesis inhibitors) were tested along with the enrichment. The presence of hydrogen facilitated production of butyrate by all communities and improved production of i-butyrate and caproate by the two most diverse communities (B and C), accompanied by consumption of acetate, hydrogen, and lactate/ethanol (when available). Under optimal conditions, hydrogen increased the selectivity of conventional electron donors to caproate from 0.23 ± 0.01 mol e–/mol e– to 0.67 ± 0.15 mol e–/mol e– with a peak caproate concentration of 4.0 g L–1. As a trade-off, the best-performing communities also showed hydrogenotrophic methanogenesis activity by Methanobacterium even at high concentrations of undissociated acetic acid of 2.9 g L–1 and at low pH of 4.8. According to 16S rRNA amplicon sequencing, the suspected caproate producers were assigned to the family Anaerovoracaceae (Peptostreptococcales) and the genera Megasphaera (99.8% similarity to M. elsdenii), Caproiciproducens, and Clostridium sensu stricto 12 (97–100% similarity to C. luticellarii). Non-methanogenic hydrogen consumption correlated to the abundance of Clostridium sensu stricto 12 taxa (p < 0.01). If a robust methanogenesis inhibition strategy can be found, hydrogen co-feeding along with conventional electron donors can greatly improve selectivity to caproate in complex communities. The lessons learned can help design continuous hydrogen-aided chain elongation bioprocesses.

This paper explores the construction and validation of an artificial neural network (ANN) model to accurately and efficiently predict the performance of a downdraft biomass gasification integrated with syngas fermentation plant for ethanol production. The study aims to predict the specific mass flow rate of bioethanol product from the systems derived by various kinds of biomass feedstocks under atmospheric pressure and various operating conditions. The input parameters used in the models are elemental analysis compositions (C, O, H, N and S), proximate analysis compositions (moisture, ash, volatile material and fixed carbon) and operating parameters (gasifier temperature and air to fuel ratio). The architecture of the model consisted of one input, one hidden and one output layer. 1008 simulated data from 84 different types of biomasses in various operating conditions were used to train the ANN. The developed ANN shows agreement with simulated data with Root Mean Square Error (RMSE) less than 0.05 in the case of product bioethanol. Moreover, the relative influence of biomass characteristics and some specific operating parameters on output are determined. Finally, to have a more detailed assessment, the variations of all input variables with respect to carbon content are compared and analyzed together. The suggested integrated ANN based model can be applied as a very useful tool for optimization and control of the process through the downdraft biomass gasification integrated with bioethanol production unit.

Industrially applied bioprocesses for the reduction of C1 gases (CO2 and/or CO) are based in particular on (syn)gas fermentation with acetogenic bacteria and on photobioprocesses with microalgae. In each case, process engineering characteristics of the autotrophic microorganisms are specified and process engineering aspects for improving gas and electron supply are summarized before suitable bioreactor configurations are discussed for the production of organic products under given economic constraints. Additionally, requirements for the purity of C1 gases are summarized briefly. Finally, similarities and differences in microbial CO2 valorization are depicted comparing gas fermentations with acetogenic bacteria and photobioprocesses with microalgae.

Financiado para publicación en acceso aberto: Universidade da Coruña/CISUG [Abstract] Syngas fermentation is a promising technology for bioalcohols production even though some produced volatile fatty acids (VFA) may remain unconsumed in the effluent. The present research explored the production of polyhydroxyalkanoates (PHA) from the remaining VFA of the syngas fermentation effluents in fed-batch bioreactors so that bioalcohols and biopolymers could be obtained in a combined way. In order to perform these PHA accumulation tests, two different syngas fermentation effluents, composed mainly of a mixture of VFA and alcohols, were used as substrate. Those effluents were characterized by different nitrogen availabilities, one being N-rich and the other N-limited. A mixed microbial culture (MMC) with a maximum PHA storage ability of 53.6% was used as inoculum. The microorganisms of the MMC mainly consumed the VFA rather than the alcohols, allowing the latter to accumulate as end product, besides PHA. However, when the N-rich effluent was used as substrate, the consumption rate of alcohols was 20 times higher (0.040 Cmmol-Alcohol Cmmol-X-1 h-1) compared to the N-limited effluent (0.002 Cmmol-Alcohol Cmmol-X-1 h-1). Despite not observing large differences in the maximum amount of accumulated PHA (40.5 – 41.5%), there was a decrease in the PHA content after reaching its maximum peak when the N-rich effluent was used as substrate. This trend was not observed with the N-limited effluent, in which the maximum PHA peak exactly matched with the end of the experiment. The present work has been financially supported by the Spanish Ministry of Science and Innovation and European FEDER funds (project PID2020–117805RB-I00). The authors, belonging to the BIOENGIN group, thank Xunta de Galicia for financial support to Competitive Reference Research Groups (GRC) (ED431C 2021/55). Besides, the authors would also like to acknowledge the support of INNOLACT S. L. Funding for open access charge provide by Universidade da Coruña/CISUG Xunta de Galicia; ED431C 2021/55

The integration of gasification with syngas fermentation is still in an early stage of development, where many questions exist concerning the syngas quality required. The challenge is to define the gasification conditions that will provide a H2:CO:CO2 ratio suitable for syngas fermentation, as well as to identify and remove the compounds that can inhibit the performance of the microorganisms. The right balance between the gasification conditions and a line up with minimal gas cleaning needs to be assessed to improve the economic feasibility of the process. In this work a first attempt to combine the two processes is presented, with the aim to provide an insight of the indirect gasification, the gas cleaning applied, the effect of the main impurities and the performance of syngas fermentation. A lignin rich feedstock was gasified with steam in an indirect gasifier, at TNO (Netherlands Organization for Applied Scientific Research). The gas, after removal of some components that could hinder the performance of the fermentation (tar compounds, BTX, unsaturated hydrocarbons and sulphur compounds), consisted of CO, H2, CO2, CH4, C2H6 and traces of other hydrocarbons. The influence of the obtained syngas quality and composition was evaluated in the fermentation process using Clostridium ljungdahlii, at KIT (Karlsruhe Institute of Technology). For comparison, product gas from beech wood gasification was also evaluated in the fermentation process under the same conditions.

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.

Carbon one industry flux gas generated from fossil fuels, various industrial and domestic waste, as well as lignocellulosic biomass provides an innovative raw material to lead the sustainable development. Through the chemical and biological processing, the gas mixture composed of CO, CO2, and H2, also termed as syngas, is converted to biofuels and high-value chemicals. Here, the syngas fermentation process is elaborated to provide an overview. Sources of syngas are summarized and the influences of impurities on biological fermentation are exhibited. Acetogens and carboxydotrophs are the two main clusters of syngas utilizing microorganisms, their essential characters are presented, especially the energy metabolic scheme with CO, CO2, and H2. Synthetic biology techniques and microcompartment regulation are further discussed and proposed to create a high-efficiency cell factory. Moreover, the influencing factors in fermentation and products in carboxylic acids, alcohols, and others such like polyhydroxyalkanoate and poly-3-hydroxybutyrate are addressed. Biological fermentation from carbon one industry flux gas is a promising alternative, the latest scientific advances are expatiated hoping to inspire more creative transformation.

This work investigates the effects of carbon allocation on the emission intensities of low-carbon products cogenerated in facilities that co‐process biogenic and fossil feedstocks and apply the carbon capture utilization and storage technology. Thus, these plants simultaneously sequester CO2 and synthesize fuels or chemicals. We consider an integrated steel mill that injects biomass into the blast furnace, captures CO2 for storage, and ferments CO into ethanol from the blast furnace gas. We examine two schemes to allocate the CO2 emissions avoided [due to the renewable feedstock share (biomass) and CO2 capture and storage (CCS)] to the products of steel, ethanol, and electricity (generated through the combustion of steel mill waste gases): 1) allocation by (carbon) mass, which represents actual carbon flows, and 2) a free-choice attribution that maximizes the renewable content allocated to electricity and ethanol. With respect to the chosen assumptions on process performance and heat integration, we find that allocation by mass favors steel and is unlikely to yield an ethanol product that fulfills the Renewable Energy Directive (RED) biofuel criterion (65% emission reduction relative to a fossil comparator), even when using renewable electricity and applying CCS to the blast furnace gas prior to CO conversion into ethanol and electricity. In contrast, attribution fulfills the criterion and yields bioethanol for electricity grid intensities 2/kWhel without CCS and yields bioethanol for grid intensities up to 800 gCO2/kWhel with CCS. The overall emissions savings are up to 27 and 47% in the near-term and long-term future, respectively. The choice of the allocation scheme greatly affects the emissions intensities of cogenerated products. Thus, the set of valid allocation schemes determines the extent of flexibility that manufacturers have in producing low-carbon products, which is relevant for industries whose product target sectors that value emissions differently. We recommend that policymakers consider the emerging relevance of co‐processing in nonrefining facilities. Provided there is no double-accounting of emissions, policies should contain a reasonable degree of freedom in the allocation of emissions savings to low-carbon products, so as to promote the sale of these savings, thereby making investments in mitigation technologies more attractive to stakeholders.

In the current scenario, the world is facing a shortage of fuels, and in the future, the existing reservoirs of fuels will be exhausted; this challenging situation could be solved by the method of syngas fermentation. Syngas is a mixed composition of gases including carbon monoxide, hydrogen and a lesser amount of carbon dioxide. Mostly, syngas synthesis is performed by acetogenic bacteria. These bacteria utilizes the Wood–Ljungdahl pathway for fermentation to take place. Syngas fermentation of waste biomass has led to the development of biofuels rich in energy and valuable chemicals. Biomass used as a substrate includes municipal waste, crops, chemical wastes, coal, lignin, natural gas and wood. Firstly, biomass gets transformed into carbon dioxide and hydrogen via gasification, these products formed act as substrate for syngas fermentation which is utilized by acetogenic bacteria to produce hydrocarbon-rich compounds. Bacteria used are genetically modified and the reactors are optimized for the scale-up studies for maximizing the yield. This chapter covered the important biochemical aspects of syngas fermentation and advantages, biochemical pathway and the microorganisms involved in syngas fermentation. The type of bioreactors used for syngas production, challenges faced during syngas fermentation and future perspectives have been included.