Bioconversion of syngas/waste gas components to produce ethanol appears to be a promising alternative compared to the existing chemical techniques. Recently, several laboratory-scale studies have demonstrated the use of acetogens that have the ability to convert various syngas components (CO, CO 2 , and H 2 ) to multicarbon compounds, such as acetate, butyrate, butanol, lactate, and ethanol, in which ethanol is often produced as a minor endproduct. This bioconversion process has several advantages, such as its high specifi city, the fact that it does not require a highly specifi c H 2 /CO ratio, and that biocatalysts are less susceptible to metal poisoning. Furthermore, this process occurs under mild temperature and pressure and does not require any costly pre-treatment of the feed gas or costly metal catalysts, making the process superior over the conventional chemical catalytic conversion process.The main challenge faced for commercializing this technology is the poor aqueous solubility of the gaseous substrates (mainly CO and H 2 ). In this paper, a critical review of CO-rich gas fermentation to produce ethanol has been analyzed systematically and published results have been compared. Special emphasis has been given to understand the microbial aspects of the conversion process, by highlighting the role of different micro-organisms used, pathways, and parameters affecting the bioconversion. An analysis of the process fundamentals of various bioreactors used for the biological conversion of CO-rich gases, mainly syngas to ethanol, has been made and reported in this paper. Various challenges faced by the syngas fermentation process for commercialization and future research requirements are also discussed.

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Fermentation of CO or syngas offers an attractive route to produce bioethanol. However, during the bioconversion, one of the challenges to overcome is to reduce the production of acetic acid in order to minimize recovery costs. Different experiments were done with Clostridium autoethanogenum. With the addition of 0.75 μM tungsten, ethanol production from carbon monoxide increased by about 128% compared to the control, without such addition, in batch mode. In bioreactors with continuous carbon monoxide supply, the maximum biomass concentration reached at pH 6.0 was 109% higher than the maximum achieved at pH 4.75 but, interestingly, at pH 4.75, no acetic acid was produced and the ethanol titer reached a maximum of 867 mg/L with minor amounts of 2,3-butanediol (46 mg/L). At the higher pH studied (pH 6.0) in the continuous gas-fed bioreactor, almost equal amounts of ethanol and acetic acid were formed, reaching 907.72 mg/L and 910.69 mg/L respectively.

The fermentation of waste gases rich in carbon monoxide using acetogens is an efficient way to obtain valuable biofuels like ethanol and butanol. Different experiments were carried out with the bacterial species Clostridium carboxidivorans as biocatalyst. In batch assays with no pH regulation, after complete substrate exhaustion, acetic acid, butyric acid, and ethanol were detected while only negligible butanol production was observed. On the other side, in bioreactors, with continuous carbon monoxide supply and pH regulation, both C2 and C4 fatty acids were initially formed as well as ethanol and butanol at concentrations never reported before for this type of anaerobic bioconversion of gaseous C1 compounds, showing that the operating conditions significantly affect the metabolic fermentation profile and butanol accumulation. Maximum ethanol and butanol concentrations in the bioreactors were obtained at pH 5.75, reaching values of 5.55 and 2.66 g/L, respectively. The alcohols were produced both from CO fermentation as well as from the bioconversion of previously accumulated acetic and butyric acids, resulting in low residual concentrations of such acids at the end of the bioreactor experiments. CO consumption was often around 50% and reached up to more than 80%. Maximum specific rates of ethanol and butanol production were reached at pH 4.75, with values of 0.16 g/h*g of biomass and 0.07 g/h*g of biomass, respectively, demonstrating that a low pH was more favorable to solventogenesis in this process, although it negatively affects biomass growth which does also play a role in the final alcohol titer.

Environmental problems associated with the use of fossil fuels as well as their expected scarcity in the near future requires a search for new alternative fuels produced from renewable sources. Bioethanol is a biofuel that can be obtained from biomass and waste as feedstocks through fermentation. Two major routes allow conversion of the feedstocks to fermentable substrates, i.e. the hydrolytic route and the thermochemical route. In the hydrolytic route, the feedstock undergoes a pretreatment stage first, aimed at facilitating the subsequent hydrolytic treatment. Chemical, physical or biological pretreatments can be applied. Lignocellulosic feedstocks are mainly composed of cellulose, hemicellulose and lignin. The pretreatment attacks the lignin and hemicellulose polymers and makes cellulose more accessible in the next, hydrolytic, stage. The hydrolytic treatment uses enzymes to convert the cellulose polymer to simple, fermentable, sugars, mainly glucose. Simple sugars obtained from hemicellulose and cellulose are then fermented by yeasts to bioethanol. In the thermochemical alternative, the feedstock is gasified, yielding syngas – a mixture largely composed of CO, CO2 and H2 – which can be fermented anaerobically, usually by clostridia, to ethanol or other products. In both cases, downstream processes are then applied to recover and purify the biofuel. The different stages involved in both alternatives are described, and both processes are compared in terms of their main characteristics and development stage. © 2015 Society of Chemical Industry

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Bioprocesses in conventional second generation biorefineries are mainly based on the fermentation of sugars obtained from lignocellulosic biomass or agro-industrial wastes. An alternative to this process consists in gasifying those same feedstocks or even other carbon-containing materials to obtain syngas which can also be fermented by some anaerobic bacteria to produce chemicals or fuels. Carbon monoxide, carbon dioxide and hydrogen, which are the main components of syngas, are also found in some industrial waste gases, among others in steel industries. Clostridium carboxidivorans is able to metabolise such gases to produce ethanol and higher alcohols, i.e. butanol and hexanol, following the Wood-Ljungdahl pathway. This does simultaneously allow the removal of volatile pollutants involved in climate change. The bioconversion is a two step process in which organic acids (acetate, butyrate, hexanoate) are produced first, followed by the accumulation of alcohols; although partial overlap in time of acids and alcohols production may sometimes take place as well. Several parameters, among others pH, temperature, or gas-feed flow rates in bioreactors, affect the bioconversion process. Besides, the accumulation of high concentrations of alcohols in the fermentation broth inhibits the growth and metabolic activity of C. carboxidivorans.

Butanol production from carbon monoxide-rich waste gases or syngas is an attractive novel alternative to the conventional acetone-butanol-ethanol (ABE) fermentation. Solvent toxicity is a key factor reported in ABE fermentation with carbohydrates as substrates. However, in the gas-fermentation process, kinetic aspects and the inhibition effect of solvents have not thoroughly been studied. Therefore, different batch bottle experiments were carried out with the bacterial species Clostridium carboxidivorans using CO as carbon source for butanol-ethanol fermentation. A maximum specific growth rate of 0.086 ± 0.004 h(-1) and a biomass yield of 0.011 gbiomass/gCO were found, which is significantly lower than in other clostridia grown on sugars. Besides, three assays were carried out to check the inhibitory effect of butanol, ethanol, and their mixtures. Butanol had a higher inhibitory effect on the cells than ethanol and showed a lower IC50, reduced growth rate, and slower CO consumption with increasing alcohol concentrations. A concentration of 14-14.50 g/L butanol caused 50 % growth inhibition in C. carboxidivorans, and 20 g/L butanol resulted in complete inhibition, with a growth rate of 0 h(-1). Conversely, 35 g/L ethanol decreased by 50 % the final biomass concentration respect to the control and yielded the lowest growth rate of 0.024 h(-1). The inhibitory effect of mixtures of both alcohols was also checked adding similar, near identical, concentrations of each one. Growth decreased by 50 % in the presence of a total concentration of alcohols of 16.22 g/L, consisting of similar amounts of each alcohol. Occasional differences in initially added concentrations of alcohols were minimal. The lowest growth rate (0.014 h(-1)) was observed at the highest concentration assayed (25 g/L).