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Bioelectrically engineered fuel produced by yeasts

Yeasts such as Saccharomyces cerevisiae are, as the name suggests, used for large scale production of beer and other alcoholic beverages. Their high salt and ethanol tolerance not only makes them useful for the production of beverages, but also suitable for the production of combustion fuels at high alcohol concentrations. Besides ethanol, long-chain fusel alcohols are of high interest for biofuel production as well. Bioethanol is already mixed with gasoline and thus improves the CO2 balance of internal combustion engines. This liquid biofuel is made from either starch or lignocellulose. The production and use of bioethanol supports local economies, reduces CO2 emissions and promotes self-sufficiency. The latter is especially important for resource-depleted landlocked countries.

In order to efficiently produce ethanol and other alcohols from lignocellulose hydrolysates, yeasts must use both glucose and pentoses such as xylose and arabinose. This is because biomass is rich in both lignocellulose and thus glucose and xylose. However, this is also the main disadvantage of using Saccharomyces cerevisiae because it does not ferment xylose. Consequently, the identification of another yeast strains capable of fermenting both these sugars could solve the problem. Highly efficient yeasts can be grown in co-cultures with other yeasts capable of lignocellulose fermentation for ethanol production. Such a yeast is, for example, Wickerhamomyces anomalous.

To further improve ethanol production, bioelectric fermentation technology supporting traditional fermentation can be used. The microbial metabolism can thus be controlled electrochemically. There are many benefits of this technology. The fermentation process becomes more selective due to the application of an electrochemical potential. This, in turn, increases the efficiency of sugar utilization. In addition, the use of additives to control the redox equilibrium and the pH is minimized. Ultimately cell growth can be stimulated, further increasing alcohol production.

Such bioelectric reactors are galvanic cells. The electrodes used in such a bioelectric reactor may act as electron acceptors (anodes) or source (cathodes). Such electrochemical changes affect the metabolism and cell regulation as well as the interactions between the yeasts used. Now, a research group from Nepal (a resource-depleted landlocked country) has used new yeast strains of Saccharomyces cerevisiae and Wickerhamomyces anomalous in a bioelectric fermenter to improve ethanol production from biomass. The results were published in the journal Frontiers in Energy Research.

For their study, the researchers chose Saccharomyces cerevisiae and Wickerhamomyces anomalus as both are good ethanol producers. The latter is to be able to convert xylose to ethanol. After the researchers applied a voltage to the bioelectrical system, ethanol production doubled. Both yeasts formed a biofilm on the electrodes, making the system ideal for use as a flow-through system because the microorganisms are not washed out.

Saccharomyces cerevisiae cells in a brightfield microscopic image of 600-fold magnification (Foto: Amanda Luraschi)

The researchers speculated that the increased ethanol production was due to the better conversion of pyruvate to ethanol − the yeast’s central metabolic mechanism. The researchers attributed this to accelerated redox reactions at the anode and cathode. The applied external voltage polarized the ions present in the cytosol, thus facilitating the electron transfer from the cathode. This and the accelerated glucose oxidation probably led to increased ethanol production.

Normally, pyruvate is converted into ethanol in fermentation yeast. External voltage input can control the kinetics of glucose metabolism in Saccharomyces cerevisiae under both aerobic and anaerobic conditions. Intracellular and transplasmembrane electron transfer systems play an important role in electron transport across the cell membrane. The electron transfer system consists of cytochromes and various redox enzymes, which confer redox activity to the membrane at certain sites.

The authors also found that an increased salt concentration improved conductivity and therefore ethanol production. The increased ethanol production from lignocellulosic biomass may have been also be due to the presence of various natural compounds that promoted yeast growth. When the cellulose acetate membrane was replaced by a Nafion™ membrane, ethanol production also increased. This was perhaps due to improved transport of xylose through the Nafion™ membrane as well as the decrease of the internal resistance. A further increase of ethanol production was observed when the bioelectrical reactor was operated with fine platinum particles coated on the platinum anode and neutral red deposited on the graphite cathode.

Several yeast cultures from left to right: Saccharomyces cerevisiae, Candida utilis, Aureobasidium pullulans, Trichosporum cutaneum, Saccharomycopsis capsularis, Saccharomycopsis lipolytica, Hanseniaspora guilliermondii, Hansenula capsulata, Saccharomyces carlsbergensis, Saccharomyces rouxii, Rhodotorula rubra, Phaffia rhodozyba, Cryptococcus laurentii, Metschnikowia pulcherrima, Rhodotorula pallida

At Frontis Energy, we think that the present study is promising. However, long-chain fusel alcohols should be considered in the future as they are less volatile and better compatible with current internal combustion engines. These can also be easily converted into the corresponding long-chain hydrocarbons.

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Cheap, high-octane biofuel discovered

Researchers from the National Renewable Energy Laboratory (NREL) have developed a cheap method for producing high-octane gasoline from methanol. They recently published their method in the journal Nature Catalysis. Methanol can be synthesized from CO2 via various routes, as we reported last year. Biomass, such as wood, is one possibility.

The production of biofuels from wood, however, is too expensive to compete with fossil fuels. To find a solution to this problem, the researchers combined their basic research with an economic analysis. The researchers initially aimed at the most expensive part of the process. Thereafter, the researchers found methods to reduce these costs with methanol as an intermediate.

So far, the cost of converting methanol to gasoline or diesel was about $1 per gallon. The researchers have now reached a price of about $0.70 per gallon.

The catalytic conversion of methanol into gasoline is an important research area in the field of CO2 recovery. The traditional method is based on multi-stage processes and high temperatures. It is expensive, producing low quality fuel in small quantities. Thus, it is not competitive with petroleum-based fuels.

Hydrogen deficiency was the initially problem the researcher had to overcome. Hydrogen is the key energy containing element in hydrocarbons. The researchers hypothesized that using the transition metal copper would solve this problem, which it did. They estimated that the copper-infused catalyst resulted in 38% more yield at lower cost.

By facilitating the reintegration of C4 byproducts during the homologation of dimethyl ether, the copper zeolite catalyst enabled this 38% increase in product yield and a 35% reduction in conversion cost compared to conventional zeolite catalysts. Alternatively, C4 by-products were passed to a synthetic kerosene meeting five specifications for a typical jet fuel. Then, the fuel synthesis costs increased slightly. Even though the cost savings are minimal, the resulting product has a higher value.

Apart from the costs, the new process offers users further competitive advantages. For example, companies can compete with ethanol producers for credits for renewable fuels (if the carbon used comes from biogas or household waste). The process is also compatible with existing methanol plants that use natural gas or solid waste to produce syngas.

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Fresh CO2 − Now Even Cheaper!

Hurry up while stocks last, you may want to add. Carbon dioxide (CO2) is a waste product from the combustion of fossil fuels such as oil, gas and coal. It is almost worthless because it finds little use. However, technologies such as power-to-gas or electrosynthesis of methanol are able to convert CO2 directly into a valuable, albeit cheap, product. This increases the commercial interest in CO2 and ultimately the filtering from the air becomes economically interesting. That is, filtering CO2 from the air is now more than just an expensive strategy to fight global warming. Recently, a detailed economic analysis has been published in the journal Joule, which suggests that this filter technology could soon become a viable reality.

The study was published by the engineers of the Canadian company Carbon Engineering in Calgary, Canada. Since 2015, the company has been operating a pilot plant for CO2 extraction in British Columbia. This plant − based on a concept called Direct Air Capture (DAC) − formed the foundation for the presented economic analysis. It includes the costs from suppliers of all major components. According to the study, the cost of extracting a ton of CO2 from the air ranges from $94 to $232, depending on a variety of design options. The latest comprehensive analysis of DAC estimated $600 per tonne and was published by the American Physical Society in 2011.

In addition to Carbon Engineering, the Swiss company Climeworks also works on DAC in Zurich. There, the company has launched a commercial pilot that can absorb 900 tonnes of CO2 from the atmosphere every year for use in greenhouses. Climeworks has also opened a second plant in Iceland that can capture 50 tonnes of CO2 per year and bury it in subterranean basalt formations. According to Daniel Egger of Climeworks, capturing a ton of CO2 at their Swiss site costs about $600. He expect the number to fall below $100 per ton over the next five to ten years.

Technically, CO2 is dissolved in an alkaline solution of potassium hydroxide which reacts with CO2 to form potassium carbonate. After further processing, this becomes a solid residue of calcium carbonate, which releases the CO2 when heated. The CO2 could then be disposed of underground or used to make synthetic, CO2-neutral fuels. To accomplish this, Carbon Engineering has reduced the cost of its filtration plant to $94 per ton of CO2.

CO2-neutral fuel, from carbon dioxide captured from the air and electrolytic hydrogen.

Assuming, however, that CO2 is sequestered in rock, a price of $100 per ton would translate into 0.2 cent per liter gasoline. Ultimately, the economics of CO2 extraction depend on factors that vary by location, including the price of energy and whether or not a company can access government subsidies or a carbon trading market. But the cost per ton of DAC-CO2 is likely to remain above the real market price of CO2 in the near future. For example, emission certificates in the European Union’s trading system are around €16 per tonne of CO2. If CO2 extraction technology were to gain a foothold in markets where carbon can be sold at DAC price, then DAC would of course become economical. Conversion into useful products product such as plastic or fuel could help to include the DAC premium. Alberta seems a great location because its oil is of low quality and comes at high production costs. Moreover, the size of the DAC plant suggests this is done best in Canada, given the size of the country. Albertans may want to reconsider their business model.

At Frontis Energy, we are excited about this prospect. CO2 is accessible everywhere and DAC is helping us convert it into methane gas. Power-to-gas is perfect for this. However, there would still have something to happen. $100 per ton is already good (compared to $600), but to be able to economically place a product like methane on the market it should be more like $10 per tonne:

CO2 economy of power-to-gas with electrolytic hydrogen. Cal, California, EOR, enhanced oil recovery.

Sure, we always complain, but we still cannot wait to see how the price of DAC continues to fall and wish Carbon Engineering to Climeworks all the best. Keep it up!

(Photos: Carbon Engineering)