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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 ferments neither glucose nor 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 60-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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Bioenergy

Bioenergy is renewable energy derived from biomass. Biomass is organic material that was produced by living organisms. Each type of biomass was once converted into chemical energy using sunlight and then stored.

Since biomass is stored chemical energy, it can be burned directly. Biofuels can be produced from biomass in solid, liquid or gaseous form. Bio-electricity is both the direct use of biomass and the conversion of biomass into oils, biogas or other fuels for power generation.

Wood that is burned to make fire is another example of biomass. Wood is the world’s most widely used biofuel. Ethanol is also a popular biofuel. It is produced by fermentation of sugars. The process is the same as in alcoholic fermentation for the production of beer or wine. Usually, yeasts carry out fermentation, but other microorganisms, such as clostridia are capable of producing alcohols and other volatile organics as well.

While combustion of biomass produces about the same amount of CO2 as fossil fuels, biofuels are produced in the present day and their combustion does not release additional CO2 into the atmosphere. Biofuels can also be used as fuel additives to reduce carbon emissions from gasoline prices. But there are also vehicles that are powered mainly by biofuels. Bioethanol is widespread in the United States and Brazil, while more biodiesel is produced in the European Union.

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Nanomaterials in bio-electrical systems could improve performance

Since Professor Potter’s discovery of the ability of microbes to turn organic molecules into electricity using microbial fuel cells (MFC) more than a century ago (Potter MC, 1911, Proc Roy Soc Lond Ser B 84:260–276), much research was done to improve the performance. Unfortunately, this did not not produce an economically viable technology. MFCs never made it out of the professors’ class rooms. This may change now that we have advanced nanomaterials available.

The testing of nanomaterials in bio-electrical systems has experienced a Cambrian explosion. The focus usually was on electrodes, membranes, and in the electrolyte with infinite possibilities to find high performing composites. The benefits of such materials include a large surface area, cost savings, and scalability. All are required to successfully commercialize bio-electrical systems. The large-scale commercial application could be wastewater treatment. In our recently published literature survey we discovered that there is no common benchmark for performance, as it is usual in photovoltaics or for batteries. To normalize our findings, we used dollar per peak power capacity as ($/Wp) as it is standard in photovoltaics. The median cost for air cathodes of MFCs is $4,700 /Wp ($2,800 /m²). Platinum on carbon (Pt/C) and carbon nanofibers are the best performing materials with $500 /Wp (Pt/C $2,800 /m²; nanofibers $2,000 /m²).

We found that carbon-based nanomaterials often deliver performance comparable to Pt/C. While MFCs are still far away from being profitable, microbial electrolysis cells already are. With these new carbon-based nanomaterials, MFCs however, are moving closer to become an economic reality. Graphene and carbon nanotubes are promising materials when they are combined with minerals such as manganese or iron oxides. However, the price of graphene is still too expensive to let MFCs become an economic reality in wastewater treatment. The costs of microbial electrolysis, however, are already so low that it is a serious alternative to traditional wastewater treatment as we show in the featured image above. For high strength wastewater, a treatment plant can in fact turn into a power plant with excess power being offered to surrounding neighborhoods. Reducing the costs of microbial electrolysis is accomplished by using a combination of cheap steel and graphite.

Relationship between MEC reactor capacity and total electrode cost including anode and cathode. Errors are standard deviations of four different tubular reactor designs. Anodes are graphite granules and cathodes are steel pipes

 

Graphite, in turn, is the precursor of graphene, a promising material for MFC electrodes. When graphite flakes are reduced to a few graphene layers, some of the most technologically important properties of the material are greatly improved. These include the overall surface and the elasticity. Graphene is therefore a very thin graphite. Many manufacturers of graphene use this to sell a material that is really just cheap graphite. In the journal Advanced Materials Kauling and colleagues published a systematic study of graphene from sixty manufacturers and find that many high-priced graphene products consist mainly of graphite powder. The study found that less than 10% of the material in most products was graphene. None of the tested products contained more than 50% graphene. Many were heavily contaminated, most likely with chemicals used in the production process. This can often lead to a material having catalytic properties which would not have been observed without contamination, as reported by Wang and Pumera.

There are many methods of producing graphene. One of the simplest is the deposition on a metallic surface, as we describe it in our latest publication:

Single-layer graphene (SLG) and multilayer graphene (MLG) are synthesized by chemical vapor deposition (CVD) from a carbon precursor on catalytic metal surfaces. In a surface-mediated CVD process, the carbon precursor, e.g. Isopropyl alcohol (IPA) is decomposed on the metal surface, e.g. Cu or Ni. In order to control the number of graphene layers formed, the solubility of the carbon precursor on the metal catalyst surface must be taken into account. Due to the low solubility of the precursor in Cu, SLG can be formed. It is difficult to grow SLG on the surface of a metal with a high affinity for the precursor.

Protocol:
The protocol is a cheap, safe and simple method for the synthesis of MLG films by CVD in 30-45 minutes in a chemistry lab. A nickel foil is submersed in acetic acid for etching and then transferred to an airtight quartz tube. The same protects the system from ambient oxygen and water vapor. Nitrogen gas is bubbled through the IPA and the resulting IPA saturated gas is passed through the closed system. The exhaust gases are washed in a beaker with a water or a gas wash bottle. The stream is purged for 5 minutes at a rate of about 50 cc/min. As soon as the flame reaches a Meker burner 575-625 °C, it is positioned under the nickel foil, so that sufficient energy for the formation of graphene is available. The flame is extinguished after 5-10 minutes to stop the reaction and to cool the system for 5 minutes. The graphene-coated Ni foil is obtained.

But how thin must graphite flakes be to behave as graphene? A common idea supported by the International Organization for Standardization (ISO) is that flakes with more than ten graphene layers consist essentially of graphite. Thermodynamics say that each atomic layer in a flake with ten or fewer layers at room temperature behaves as a single graphene crystal. In addition, the stiffness of the graphite flakes increases with the layer thickness, which means that thin graphene flakes are orders of magnitude more elastic than thicker graphite flakes.

Unfortunately, to actually use graphene in bioelectric reactors, you still have to make it yourself. The ingredients can be found in our DIY Shop.

 
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A Short Introduction to Bioenergy

Bioenergy is renewable energy derived from biomass. Biomass is organic material that was produced by living organisms. Each type of biomass was once converted into chemical energy using sunlight and then stored.

Since biomass is stored chemical energy, it can be burned directly. Biofuels can be produced from biomass in solid, liquid or gaseous form. Bio-electricity is both the direct use of biomass and the conversion of biomass into oils, biogas or other fuels for power generation.

Wood that is burned to make fire is another example of biomass. Wood is the world’s most widely used biofuel. Ethanol is also a popular biofuel. It is produced by fermentation of sugars. The process is the same as in alcoholic fermentation for the production of beer or wine. Usually, yeasts carry out fermentation, but other microorganisms, such as clostridia are capable of producing alcohols and other volatile organics as well.

While combustion of biomass produces about the same amount of CO2 as fossil fuels, biofuels are produced in the present day and their combustion does not release additional CO2 into the atmosphere. Biofuels can also be used as fuel additives to reduce carbon emissions from gasoline prices. But there are also vehicles that are powered mainly by biofuels. Bioethanol is widespread in the United States and Brazil, while more biodiesel is produced in the European Union.

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Mapping Waste-to-Energy

Most readers of our blog know that waste can be easily converted into energy, such as in biogas plants. Biogas, biohydrogen, and biodiesel are biofuels because they are biologically produced by microorganisms or plants. Biofuel facilities are found worldwide. However, nobody knows exactly where these biofuel plants are located and where they can be operated most economically. This knowledge gap hampers market access for biofuel producers.

At least for the United States − the largest market for biofuels − there is now a map. A research team from the Pacific Northwest National Laboratory (PNNL) and the National Renewable Energy Laboratory (NREL) published a detailed analysis of the potential for waste-to-energy in the US in the journal Renewable and Sustainable Energy Reviews.

The group focused on liquid biofuels that can be recovered from sewage sludge using the Fischer-Tropsch process. The industrial process was originally developed in Nazi Germany for coal liquefaction, but can also be used to liquefy other organic materials, such as biomass. The resulting oil is similar to petroleum, but contains small amounts of oxygen and water. A side effect is that nutrients, such as phosphate can be recovered.

The research group coupled the best available information on these organic wastes from their database with computer models to estimate the quantities and the best geographical distribution of the potential production of liquid biofuel. The results suggest that the United States could produce more than 20 billion liters of liquid biofuel per year.

The group also found that the potential for liquid biofuel from sewage sludge from public wastewater treatment plants is 4 billion liters per year. This resource was found to be widespread throughout the country − with a high density of sites on the east cost, as well as in the largest cities. Animal manure has a potential for 10 billion liters of liquid biofuel per year. Especially in the Midwest are the largest untapped resources.

The potential for liquid biofuel from food waste also follows the population density. For metropolitan areas such as Los Angeles, Seattle, Las Vegas, New York, etc., the researchers estimate that such waste could produce more than 3 billion liters per year. However, food leftovers also had the lowest conversion efficiency. This is also the biggest criticism of the Fischer-Tropsch process. Plants producing significant quantities of liquid fuel are significantly larger than conventional refineries, consume a lot of energy and produce more CO2 than they save.

Better processes for biomass liquefaction and more efficient use of biomass still remain a challenge for industry and science.

(Photo: Wikipedia)