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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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Better heat exchangers for concentrated solar power

Solar thermal systems are a good example of the particle-wave dualism expressed in Planck’s constant h: E = hf. Where h is the Planck constant, f is the frequency of the light and E is the resulting energy. Thus, the higher the frequency of the light, the higher the amount of energy. Solar thermal metal collectors transform the energy of high-frequency light by generating them to an abundance of low-frequencies through Compton shifts. Glass or ceramic coatings with high visible and UV transmittance absorb the low frequency light generated by the metal because they effectively absorb infrared light (so-called heat blockers). The efficiency of the solar thermal system improves significantly with increasing size, which is also the biggest advantage of such systems compared to photovoltaic generators. One disadvantage, however, is the downstream transformation of heat into electricity with the help of heat exchangers and turbines − a problem not only in solar thermal systems.

To provide the hot gas (supercritical CO2) to the turbines, heat exchangers are necessary. These heat exchangers transfer the heat energy generated by a power plant to the working fluid in a heat engine (usually a steam turbine) that converts the heat into mechanical energy. Then, the mechanical energy is used to generate electricity. These heat exchangers are operated at ~800 Kelvin and could be more efficient if the temperature were at >1,000 Kelvin. The entire process of converting heat into electricity is called a power cycle and is a critical process in power generation by solar thermal plants. Obviously, heat exchangers are pivotal elements in this process.

Ceramics are a great material material for heat exchanger because they can withstand extreme temperature fluctuations. However, unlike metals, ceramics are not easy to shape. Relatively coarse shapes, in turn, are made quickly and easily. In contrast, metals can be easily formed and have a high mechanical strength. Metals and ceramics have been valued for centuries for their distinctive properties. For example, bronze and iron have good impact resistance and are so malleable that they have been made into complex shapes such as weapons and locks. Ceramics, like those used to make pottery, have been formed into simpler shapes. Their resistance to heat and corrosion made ceramics a valued material. A new composite of metal and ceramic (a so-called cermet) combines these properties in amazing ways. A research group led by Mario Caccia reported now in the prestigious journal Nature about a cermet with properties that makes it usable for heat exchangers in solar thermal systems.

The history of such composites goes back to the middle of the 20th century. The advent of jet engines has created a need for materials with high resistance to heat and oxidation. On top of that, they had to deal with rapid temperature changes. Their excellent mechanical strength, which often surpassed that of existing metals, was highly appreciated by the newly created aerospace industry. Not surprisingly, the US Air Force funded more research into the production of cermets. Cermets have since been developed for multiple applications, but in most cases have been used for small parts or surfaces. The newly released composite withstands extreme temperatures, high pressures and rapid temperature changes. It could increase the efficiency of heat exchangers in solar thermal systems by 20%.

To produce the composite, the authors first produced a precursor, which was subject to further processing, comparable to potting the unfired version of a clay pot. The authors compacted tungsten carbide powder into the approximate shape of the desired article (the heat exchanger) and heated it at 1,400 °C for 2 minutes to bond the parts together. They then further processed this porous preform to produce the desired final shape.

Next, the authors heated the preform in a chemically reducing atmosphere (a mixture of 4% hydrogen in argon) at 1,100 °C. At the same temperature, they immersed the preform in a tank of liquid zirconium and copper (Zr2Cu). Finally, the preform was removed by heating to 1,350 °C. In this process, the zirconium displaces the tungsten from the tungsten carbide, producing zirconium carbide (ZrC) as well as tungsten and copper. The liquid copper is displaced from the ZrC matrix as the material solidifies. The final object consists of ~58% ZrC ceramic and ~36% tungsten metal with small amounts of tungsten carbide and copper. The beauty of the method is that the porous preform is converted into a non-porous ZrC / tungsten composite of the same dimensions. The total volume change is about 1-2%.

The elegant manufacturing process is complemented by the robustness of the final product. At 800 °C, the ZrC / tungsten cermet conducts heat 2 to 3 times better than nickel based iron alloys. Such alloys are currently used in high-temperature heat exchangers. In addition to the improved thermal conductivity, the mechanical strength of the ZrC / tungsten composite is also higher than that of nickel alloys. The mechanical properties are not affected by temperatures of up to 800 ° C, even if the material has previously been subjected to heating, e.g. for cooling cycles between room temperature and 800 °C. In contrast, iron alloys, e.g. stainless steels, and nickel alloys loose at least 80% of their strength.

(Photo: Wikipedia)

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A Durable Aluminum-Air-Battery

Non-rechargeable batteries, which depend on a reaction between aluminum and oxygen, can store significantly more energy than conventional lithium-ion batteries. The biggest limitation of such aluminum-air batteries is their short shelf life. An improved battery design could help eliminate this limitation. Aluminum and air batteries are based on the property of aluminum to corrode, which is also their weak spot:

4 Al + 3 O2 + 6H2O → 4 Al (OH)3

While an aluminum-air battery is not used, its electrodes corrode causing unwanted discharge. This self-discharge drastically shortens the shelf life of the battery. Brandon Hopkins, of the Massachusetts Institute of Technology in Cambridge, and his colleagues developed an aluminum-air battery that uses a conventional electrolyte during operation. When stored, however, the electrolyte is replaced by oil. Their article was recently published in the journal Science.

The new battery reaches a storage capacity of almost 900 Wh / kg. This makes the prototype comparable to other aluminum-air batteries. In contrast, the new corrosion protection extends the storage time 10,000-fold. The authors suggest that such a battery could be used in long-range drones and grid-independent power generation. At Frontis Energy, we believe that batteries with high storage capacity and durability can be used almost anywhere, for example for sensors and other applications.

(Photo: George Hodan)

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White Christmas, going … gone

In Germany, we seem to remember White Christmas from fairy tales only. Now there is also scientific evidence that winter snow cover in Europe is thinning. Thanks to global warming, the snow cover decrease accelerated

The research group behind Dr. Fontrodona Bach of the Royal Netherlands Meteorological Institute in De Bilt analyzed snow cover and climate data from six decades from thousands of weather stations across Europe. The researchers found that the mean snow depth, with the exception of some local extremely cold spots, has been decreasing since 1951 at 12% per decade. The researchers recently published their research results in the journal Geophysical Research Letters. The amount of “extreme” snow cover affecting local infrastructure has declined more slowly.

The observed decline, which accelerated after the 80s, is the result of a combination of rising temperatures and the impact of climate change on precipitation. The decreasing snow cover can reduce the availability of fresh water during the spring melt, the authors noted.

(Photo: Doris Wulf)

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An inexpensive scalable multi-channel potentiostat

As our preferred reader, you know already what we work on Power-to-Gas to combat Global Warming. We think that giving CO2 a value will incentivize its recycling and recycling it into fuel turns it into a commodity that everyone needs. While the price of CO2 from air is still too high to convert it into combustion fuel, working on the other end (the CO2 conversion) will help to accommodate such high prices. We have now published an research paper that shows how how to reduce the costs of electronic equipment needed for CO2 conversion. For Power-to-Gas as well es for electrosynthesis of liquid fuels, it is necessary to poise an electrochemical potential. So far, only electronic potentiostats could do that. We have developed a software solution that can control cheap off-the-shelf hardware to accomplish the same goal. Since the software controls µA as well as MA, it is freely scalable. By stacking cheap power supplies, it can also run unlimited channels.

Frontcell© potentiostat setup with two channels. From left to right: digital multimeter (in the back), relay board (in front), two H-type electrolysis cells, power supply, control computer.

We tested the software at a typical experimental Power-to-Gas setup at −800 mV and found that the recorded potential was stable over 10 days. The small electrochemical cells could also be replaced by a larger 7 liter reactor treating real wastewater. The potential was stable as well.

The potential of −800 mV controlled by the Frontcell© potentiostat was stable for 200 ml electrolysis cells (left) as well as for a larger 7 l reactor (right).

As instrument control of mass products also makes the chemical processes involved cheap, microbial electrolysis of wastewater becomes economically feasible. Removal of wastewater organics usually occurs at positive electrochemical potentials. Indeed, the software also stabilizes such potentials at +300 mV.

The Frontcell© potentiostat stabilized a 200 ml electrolysis cells at +300 mV for ten days.

The potentiostat is currently available as command line version. We are currently accepting pre-orders at a 50% discount for the commercial version that comes with a graphical user interface and remote control using an internet browser.

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Ammonia energy storage #1

The ancient, arid landscapes of Australia are not only fertile soil for huge forests and arable land. The sun shines more than in any other country. Strong winds hit the south and west coast. All in all, Australia has a renewable energy capacity of 25 terawatts, one of the highest in the world and about four times higher than the world’s installed power generation capacity. The low population density allows only little energy storage and electricity export is difficult due to the isolated location.

So far, we thought the cheapest way to store large amounts of energy was power-to-gas. But there is another way to produce carbon-free fuel: ammonia. Nitrogen gas and water are enough to make the gas. The conversion of renewable electricity into the high-energy gas, which can also be easily cooled and converted into a liquid fuel, produces a formidable carrier for hydrogen. Either ammonia or hydrogen can be used in fuel cells.

The volumetric energy density of ammonia is almost twice as high than that of liquid hydrogen. At the same time ammonia can be transported and stored easier and faster. Researchers around the world are pursuing the same vision of an “ammonia economy.” In Australia, which has long been exporting coal and natural gas, this is particularly important. This year, Australia’s Renewable Energy Agency is providing 20 million Australian dollars in funding.

Last year, an international consortium announced plans to build a $10 billion combined wind and solar plant. Although most of the 9 terawatts in the project would go through a submarine cable, part of this energy could be used to produce ammonia for long-haul transport. The process could replace the Haber-Bosch process.

Such an ammonia factories are cities of pipes and tanks and are usually situated where natural gas is available. In the Western Australian Pilbara Desert, where ferruginous rocks and the ocean meet, there is such an ammonia city. It is one of the largest and most modern ammonia plants in the world. But at the core, it’s still the same steel reactors that work after the 100 years-old ammonia recipe.

By 1909, nitrogen-fixing bacteria produced most of the ammonia on Earth. In the same year, the German scientist Fritz Haber discovered a reaction that could split the strong chemical bond of the nitrogen, (N2) with the aid of iron catalysts (magnetite) and subsequently bond the atoms with hydrogen to form ammonia. In the large, narrow steel reactors, the reaction produces 250 times the atmospheric pressure. The process was first industrialized by the German chemist Carl Bosch at BASF. It has become more efficient over time. About 60% of the introduced energy is stored in the ammonia bonds. Today, a single plant produces and delivers up to 1 million tons of ammonia per year.

Most of it is used as fertilizer. Plants use nitrogen, which is used to build up proteins and DNA, and ammonia delivers it in a bioavailable form. It is estimated that at least half of the nitrogen in the human body is synthetic ammonia.

Haber-Bosch led to a green revolution, but the process is anything but green. It requires hydrogen gas (H2), which is obtained from pressurized, heated steam from natural gas or coal. Carbon dioxide (CO2) remains behind and accounts for about half of the emissions. The second source material, N2, is recovered from the air. But the pressure needed to fuse hydrogen and nitrogen in the reactors is energy intensive, which in turn means more CO2. The emissions add up: global ammonia production consumes about 2% of energy and produces 1% of our CO2 emissions.

Our microbial electrolysis reactors convert the ammonia directly into methane gas − without the detour via hydrogen. The patent pending process is particularly suitable for removing ammonia from wastewater. Microbes living in wastewater directly oxidize the ammonia dissolved in ammonia and feed the released electrons into an electric circuit. The electricity can be collected directly, but it is more economical to produce methane gas from CO2. Using our technology, part of the CO2 is returned to the carbon cycle and contaminated wastewater is purified:

NH3 + CO2 → N2 + CH4

 

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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)

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A Graphene Membrane Becomes a Nano-Scale Water Gate

Biological systems can control water flow using channels in their membranes. This has many advantages, for example when cells need to regulate their osmotic pressure. Also artificial systems, e.g. in water treatment or in electrochemical cells, could benefit from it. Now, a group of materials researchers behind Dr. Zhou at the University of Manchester in the United Kingdom have developed a membrane that can electrically switch the flow of water.

As the researchers reported in the journal Nature, a sandwiched membrane of silver, graphene, and gold was fabricated. At a voltage of more than 2 V channels it opens its pores and water is immediately channeled through the membrane. The effect is reversible. To do this, the researchers used the property of graphene to form a tunable filter or even a perfect barrier to liquids and gases. New ‘smart’ membranes, developed using a low-cost form of graphene called graphene oxide, allow precise control of water flow by using an electrical current. The membranes can even be used to completely block water when needed.

To produce the membrane, the research group has embedded conductive filaments in the electrically insulating graphene oxide membrane. An electric current passed through these nanofilaments created a large electric field that ionizes the water molecules and thus controls the water transport through the graphene capillaries in the membrane.

At Frontis Energy we are excited about this new technology and can imagine numerous applications. This research makes it possible to precisely control water permeation from ultrafast flow-through to complete shut-off. The development of such smart membranes controlled by external stimuli would be of great interest to many areas of business and research alike. These membranes could, for instance, find application in electrolysis cells or in medicine. For medical applications, artificial biological systems, such as tissue grafts, enable a plenty of medical applications.

However, the delicate material consisting of graphene, gold, and silver nano-layers is still too expensive and not as resistant as our Nafion™ membranes. But unlike Nafion™ you can tune them. We stay tuned to see what is coming next.

(Illustration: University of Manchester)

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You Can Have the Pie and Eat It

In Paris, humanity has set itself the goal of limiting global warming to 1.5 °C. Most people believe that this will be accompanied by significant sacrifice of quality of life. That is one reason why climate protection is simply rejected by many people, even to the point of outright denial. At Frontis Energy, we think we can protect the climate and live better. The latest study published in Nature Energy by a research group around Arnulf Grubler of the International Institute for Applied Systems Analysis in Laxenburg, Austria, has now shown that we have good reasons.

The team used computer models to explore the potential of technological trends to reduce energy consumption. Among other things, the researchers said that the use of shared car services will increase and that fossil fuels will give way to solar energy and other forms of renewable energy. Their results show that global energy consumption would decrease by about 40% regardless of population, income, and economic growth. Air pollution and demand for biofuels would also decrease, which would improve health and food supplies.

In contrast to many previous assessments, the group’s findings suggest that humans can limit the temperature rise to 1.5 °C above preindustrial levels without resorting to drastic strategies to extract CO2 from the atmosphere later in the century.

Now, one can argue whether shared car services do not cut quality of life. Nevertheless, we think that individual mobility can be maintained while protecting our climate. CO2 recovery for the production of fuels (CO2 recycling that is) is such a possibility. The Power-to-Gas technology is the most advanced version of CO2 recycling and should certainly be considered in future studies. An example of such an assessment of the power-to-gas technology was published by a Swiss research group headed by Frédéric Meylan, who found that the carbon footprint can be neutralized with conventional technology after just a few cycles.

(Picture: Pieter Bruegel the Elder, The Land of Cockaigne, Wikipedia)

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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)