“Liquid Sunshine”—that is the promise of biofuel. It’s the promise of turning solar energy into an easy-to-use liquid fuel. But direct use of solar energy presents several problems. These include:
- the need to convert the direct current generated by photovoltaic cells to the alternate current used by homes and appliances;
- the fact that sun only shines only half a day, on average;
- the effects of climate;
- the latitude of the solar energy site, which can severely reduce available power;
- the fact that the cost of large collector arrays, and the land to put them on, can be prohibitively expensive; and
- the need to store energy for later use.
That last point is critical. Solar energy is produced when and where it is available, which is not necessarily when and where it is needed. Not only does solar energy need to be storable, it needs to be transportable. Solar energy can be stored electrically in batteries or thermally in heat sinks. But heat sinks are fixed and permanent structures. And while batteries can be used to run transportation vehicles, they, themselves, are no more transportable than any other bulk-shipping item. This is where biodiesel makes a big difference.
Biofuel, being liquid, is easily transportable via an existing network of pipelines and fleets of tanker trucks-just like gasoline, diesel, and natural gas. In short, we already have in place an infrastructure for the transport and delivery of biodiesel. Furthermore it has the kind of energy density that makes traditional fossil fuels so useful. Gasoline and diesel, for example, have an energy density of approximately 46 megajoules per kilogram (MJ/kg). By comparison, a rechargeable lithium-ion battery has an energy density of only 0.4 to 0.9 MJ/kg. Advanced lithium-air batteries under development for electrical vehicles have a potential energy density of 9.0 MJ/kg. Clearly, chemical energy storage is significantly more efficient. The trick, then, is to turn sunlight into a liquid fuel as convenient and useful as petroleum derived fuels. And this is accomplished using biological methods.
|Photo: Sandia National Laboratories
The journal Angewandte Chemie devoted its May 10 cover to a paper co-authored by Sandia’s Nils Hansen and Lawrence Livermore’s Charles Westbrook, which examines the essential elements of biofuel combustion.
Strictly speaking, every kind of energy (except nuclear) is a form of solar energy. Coal, oil, and natural gas are the solid, liquid, and gaseous forms of decayed plants that died millions of years ago. These plants, nourished by the sun so long ago transformed the sunlight, water and soil into a vegetate mass that initially formed peat bogs and eventually became the vast deposits of fossil fuels that make modern industrial civilization possible. Biofuels currently being marketed, and newer version under development, take this process and accelerate it so that the solar energy used by today’s plants can be used immediately without having to wait for millions of years. The kinds of biofuels are as varied as their organic source materials, each one having its strengths and weaknesses.
Depending on the source, biofuel can be as simple as vegetable oil and as rare as biobutanol. Vegetable oil, often taken from grease traps and old fryers, can be used to create biodiesel or even used directly in certain types of diesel engines that utilize indirect fuel injectors. Direct use of vegetable oil as fuel also requires cars that are equipped with extensive filtering systems to screen out impurities from oil that had been used to fry chicken the night before. Given its high viscosity and the difficulty an engine would have in atomizing the oil for combustion, some cars use either a separate diesel fuel source for starting and stopping (both heating up the engine to receive vegetable oil and then flushing out the residue before the engine gets turned off), or special glow plugs that are designed to handle vegetable oil. However, since vegetable oil is typically available only in small quantities from the diverse locations of multiple used vegetable oil sources, it won’t scale up to mass-market applications. So this kind of biofuel remains in the craft stage and represents a small (but enthusiastic) niche market. Vegetable oil can also be used in the production of biodiesel.
Biodiesel can be derived either from vegetable oils or animal fats. Since its composition is just like petro-diesel, it can be used by diesel engines directly or mixed with petro-diesel without the need for extensive engine modifications. Biodiesel is usually available in the retail market as a type of blend. A “B” factor is used to designate the amount of biodiesel in any blend. Standard blends are designated as such: B100 is 100% biodiesel, B20 is 20% biodiesel and 80% petro-diesel, and B5 is 5% biodiesel and 95% petro-diesel. This last mixture, B5, is what is commonly available at gas stations. Blends below B6 can be used in unmodified diesel engines. Pure biodiesel (B100) has different solvent properties than petro-diesel and can degrade rubber hoses and gaskets over time. Therefore, engines utilizing B100 often have these flexible connections replaced with components made from more resilient materials in order to avoid long-term maintenance issues.
Biodiesel is made from a variety or organic materials, including: virgin oil feedstock such as rapeseed and soybean (which accounts for half of the US production of biodiesel), other directly produced vegetable oils made from a variety of secondary crops (jatropha, mustard, flax, sunflower, coconut, palm, and hemp), reused vegetable oil, animal fats, algae, halophytes, and sewage sludge. Biodiesel is made via transesterification, a chemical process that strips the glycerin from the fat or vegetable oil. This process converts oils and fats into long-chain molecules called “mono alkyl esters,” also known as biodiesel. In simple terms, during transesterification 100 pounds of oil or fat are reacted with 10 pounds of methanol and a catalyst such as sodium hydroxide to form 10 pounds of glycerin and 100 pounds of biodiesel. The glycerin byproduct itself is a valuable chemical that can be extracted and used in the making of cosmetics and soap.
A secondary process can further refine the glycerin to produced a pure glycerin and extract any residual methanol. The methanol along with the catalyst is fed back into the transesterification stage to accelerate the process. The crude biodiesel is further refined until final market grade biodiesel is ready. The biodiesel can then mixed with petro-diesel at several stages in the delivery process. It can be mixed at the start in tanks at the production facility prior to pumping into a tanker truck, or it can be combined immediately prior to use via metered pump mixing. In this second example, petroleum diesel and biodiesel meters are set to a preferred percentage of the total volume and are pumped simultaneously from two storage tanks and mixed during pumping.
Ethanol and Other Alcohols
The other biofuel currently in large-scale use across the globe is ethanol. Second only to biodiesel in Europe, it is the number one biofuel in America and Brazil. The first makes ethanol from standard grain crops while the second mostly uses sugar cane as a feedstock. Like biodiesel, ethanol is mixed with gasoline. Standard automobile engines can work with fuel mixtures with as much as 85% ethanol (which is the standard mixture in the US). A mixture of ethanol and gasoline has a higher octane content than straight gasoline, producing a fuel that burns hotter and at greater efficiency. However, ethanol has a somewhat lower energy density than gasoline (only 27 MJ/kg compared to 46 MJ/kg for diesel and gasoline). Roughly speaking, ethanol has a gasoline gallon equivalency (GGE) of approximately 1.5 gallons. So roughly 1.5 gallons of ethanol are required to produce the same amount of energy as 1 gallon of gasoline. This means that an automobile will travel only two-thirds of the distance on the same amount of ethanol. And like biodiesel, ethanol can have a long-term corrosive effect on rubber parts as well as other parts of the fuel system.
The thing to remember about ethanol is that it is an alcohol. Also called ethyl alcohol or grain alcohol, it is created by traditional fermentation processes. When certain species of yeast metabolize sugar in reduced-oxygen conditions, they produce ethanol and carbon dioxide. It is the oldest know chemical reaction, dating back to the dawn of civilization and the making of the first beer and wine. To make ethanol from starchy vegetation such as cereal grains, their starch content has to be converted into sugar prior to fermentation. Some ethanol production methods use a different process. Instead of allowing the grain to malt (as in traditional beer making), the sugars needed for fuel ethanol can be converted faster and more economically into glucose by means of hydrolysis utilizing dilute sulfuric acid.
The industrial scale production of ethanol for use as fuel differs somewhat from traditional fermentation processes. Most of the ethanol produced in the world today is derived from the starch or sugar in a wide variety of common crops, or feedstocks. In the United States, corn is the most popular feedstock for ethanol production. One bushel of corn yields about 2.8 gallons of fuel. On the average American form this translates into 330 to 420 gallons of ethanol per acre. The residual by products of the distillation process can be used as livestock feed.
However, current production processes use only a fraction of the corn plant. Only the kernels are used and of them, only the starch (50% of the kernels dry mass) becomes ethanol. The use of the cellulosic parts of the plant is being explored by new techniques that utilize enzymes to help convert the cellulose to ethanol, or pyrolysis to convert the entire plant into syngas. But with current technology, only sugar from sugar cane and starch from corn can be economically fermented.
Once kernels have been separated from the rest of the plant, they go through a milling process where a hammer mill grinds the kernels into flour (cornmeal). The meal is combined with water and enzymes in a mixing tank to produce mash. The mash then gets sent to a jet cooker which heats the mash to 210°F and liquefies its starch content while removing bacteria. The cooked mash is than sent to “saccharification” tanks where additional enzymes turn the starch content of the mash into dextrose sugar. Passing through heat exchangers then cools the dextrose laden mash to 90°F. The cooled mash is pumped into fermentation tanks where yeast is added. The yeast eats the dextrose, producing excess heat, ethanol, and carbon dioxide. The result is a 15% alcohol, 85% non-fermentable solids mixture, which is sent to distillation tanks where the alcohol is separated out (the solids are processed in livestock feed). Since the fuel performance of ethanol is severally degraded by water, the extracted alcohol is through dehydration columns rendering pure, 200-proof ethanol. Lastly, the ethanol is made unfit for human consumption by means of denaturing. This is usually achieved by the addition of a small quantity of gasoline.
In addition to the mass marketed ethanol, other bio-alcohols include the less commonly used bio-propanol and biobutanol. Biobutanol can be used directly without admixtures in standard gasoline engines that require no modifications, and has a less corrosive effect than ethanol. This is due to the fact that biobutanol’s molecular configuration is more similar to gasoline than to ethanol. It can be produced from biomass via the ABE-process (acetone-butanol-ethanol fermentation). However, current fermentation processed are inefficient since the bacteria used to make butanol has a relatively low yield per amount of glucose converted to fuel. One very promising development is the recent discovery of a strain of bacteria that can efficiently convert cellulose into butanol, greatly expanding potential feedstocks.
Biogas consists of approximately 65% methane with the remainder being carbon dioxide and gaseous byproducts. It is produced from organic material and anaerobic bacteria in oxygen free conditions. This can occur either in an enclosed anaerobic digester or in the heart of a municipal solid waste landfill whose organic waste component is digested to produce landfill gas (approximately half methane and half carbon dioxide). Non-gaseous byproducts include digestate, which can be used as a biofuel itself or as a fertilizer.
Industrial scale production of biogas involves several steps that duplicate and accelerate this natural process. Organic waste is delivered to the facility and run through a mixer to create a more homogenous and uniform waste mass consisting of smaller organic particles. The waste is pumped to tanks where it is preheated to over 150°F via steam reformatting. The resultant sludge is than pumped to enclosed, oxygen-free tanks, where it is kept at a temperature of 100°F and digested by anaerobic microorganisms. This continues for about 30 days. During this time, biogas is siphoned off the top of the tank and digestate is extracted from the bottom. A further purification step increases the methane content to about 97%, creating biomethane, which has the same energy density and methane content as natural gas.
New Sources of Biofuel
Driven by a need to develop a cost effective biological substitute for petroleum based fuels, and to create fuel that is “carbon neutral”-won’t increase the amount of greenhouse gases in the atmosphere- researchers are exploring new methods and modifications to existing processes that utilize new and novel feedstocks for biofuel production. As an added bonus, these processes often rely on waste materials, which are normally disposed of instead of reused.
One such novel feedstock is raw sewage. Sewage sludge has a high lipid content, making it a potential (and cheap) source of fossil fuel. Most of the sludge’s lipids are produced by the bacteria living in the sludge. South Korean researchers have found a method that converts lipids to biodiesel at high yield and low cost. As a result, sewage can yield more lipids than soybeans while costing less to produce. The process is a non-catalytic method and involves heating with methanol. In this process, heat drives the reaction at 380°C (716°F), rather than a catalyst. To increase contact time between the lipids the reaction occurs in a porous material such as activated alumina that traps the reactants together. Adding carbon dioxide improves the yield, converting 98% of the lipids into biodiesel.
Algae has been investigated as a source of biofuel since the 1970s and the first oil crisis. It has potentially higher yields than any other bio-oil source and burns cleaner than petroleum. It can be grown almost anywhere from factory tanks, to off shore “farms” floating in the oceans, to ponds of brackish water, to raw sewage. But while growing algae is easy and cheap, extracting the oils for the manufacture of biodiesel is not. An hydraulic screw press (a technology identical to that of an ancient olive press) combined with the application of a hexane solvent is used to squeeze out three-fourths of the available oil. The process is energy intensive, expensive, and time consuming. It represents one of the biggest technological roadblocks to widespread use of algae oil and its ability to compete with less-expensive fossil fuels, which is unfortunate since algae has the potential to provide a large portion of our energy needs. Just a single 100-acre algae pond could produce 10 million gallons of biodiesel annually.
However, a newly developing process, hydrothermal liquefaction, can transform 2/3 of the algae into bio-oil that does not need special mixing or handling, and do so in less than an hour. Initial estimates predict that this process would allow the sale of algae derived biodiesel for as little as $2 a gallon (currently it goes for about $10 a gallon). The technology utilizes what is essentially a large pressure cooker. This new type of pressure cooker is designed to replace costly steps like drying the algae before processing with a more streamlined approach. As a result, all phases of production become much cheaper. The system can also extract oil efficiently from a wide variety of algae grown in various locales. The process starts with a mixture of 20% algae and 80% water by weight. The mixture is fed continuously though a long tube at 660°F and 3,000 psi pressure for 30 minutes while being stirred. This breaks down the algae completely into oil at a rate of 100 pounds of algae yielding 53 pounds of a bio-oil, which is similar chemically to light, sweet crude. Efficient heat recovery systems improve process efficiency. The process also yields valuable byproducts, which can be used to form natural gas and make fertilizer.
Some Paths Being Explored
As mentioned earlier, starch is relatively easy to convert to biofuel. Cellulose…not so much. But being able to use lignocellulosic biomass (the inedible cellulose from plant materials) as feedstock would greatly increase the efficiency and lower the cost of ethanol production. The resultant fuel is identical to ethanol, but its production involves a much more complicated process, cellulose hydrolysis. The first stage is pretreatment which breaks down the physical bonds and structure containing the cellulose. The second takes the exposed cellulose and uses enzymes to break it down into useable sugars. Both stages are expensive and have proven stumbling blocks to the economic competitiveness of cellulosic ethanol.
But research continues for new biological methods than can replace these mechanical and chemical processes. Scientists at Tulane University have discovered the first known strain of bacteria, TU-103, which produces biobutanol (which can be used as a replacement for gasoline) directly from cellulose. TU-103 (discovered in zebra feces) is the only known butanol-producing strain that can grow and produce butanol in an aerobic environment. Oxygen kills other butanol-producing bacteria, and not having to produce butanol in an isolated anaerobic tank will greatly reduce production costs. It will also allow for the mass conversion of the bulk of our solid waste (like old newsprint) into fuel that does not increase levels of greenhouse gases in the atmosphere.
Lastly, scientists at the University of Georgia are working on a near magical process that can create biofuel directly from thin air. One of the great benefits of using biofuel is that fact that it is largely carbon neutral. The biological processes and bacterial metabolisms used to create ethanol, biodiesel, algae oil, etc. extract carbon dioxide from the air while creating fuel. Later, burning the fuel releases approximately the same amount of carbon dioxide, making the use of biofuel potentially “carbon neutral.” So how is this new process different than photosynthesis, when plants use sunlight to transform water and carbon dioxide into sugar that are later extracted to ferment fuels like ethanol? This new discovery allows for the direct production of oils from a microorganism (pyrococcus furiosus) discovered in the super heated waters of a geothermal vent on the bottom of the ocean. A genetically modified version of this organism can feed directly off carbon dioxide at much lower temperatures while directly producing fuel oil and other useful chemicals.
New Applications for Biofuel
Its discoveries like these that point to the potentially revolutionary development of a new generation of biofuels, those that are derived from genetically modified organisms. These new discoveries, sources and methods have been matched by new large-scale facilities and small-scale applications that are bringing biofuels into the mainstream.
At the large scale, once such facility is being developed in Bridgeport, CT. Anaergia Inc. (located in Burlington, Ontario, Canada) has developed for the City of Bridgeport an anaerobic digestion facility that will convert organic waste, wastewater sludge, food wastes, and vegetation into a renewable source of electricity. In a strategic 20-year partnership with the City of Bridgeport Water Pollution Control Authority (WPCA), Anaergia (through its wholly owned subsidiary, Anaergia Services LLC.) will design, build, own, finance, and operate an anaerobic digester facility that will generate 10 million kWh of electricity (enough for 1,000 homes). The solid waste materials feeding the process will be diverted from the local landfill and waste incinerator, reducing both disposal needs and greenhouse gas emissions. A holistic economic analysis of the project argues in is favor since it utilizes waste and eliminates other external costs.
“We are proud to partner on this renewable energy project with the City of Bridgeport and the Water Pollution Control Authority,” says Steve Watzeck, CEO of Anaergia. “Our solution platform enables the city to transform what was once waste into a valuable resource. This project will serve as a showcase for other municipalities looking to implement sustainability programs that drive energy independence and reduce operating costs.”
The mayor of Bridgeport is equally enthusiastic about the project’s economic value and sustainability. “This project with Anaergia is another step toward realizing our vision outlined in the City of Bridgeport’s BGreen 2020 sustainability plan to help create jobs, save taxpayers money, and fight climate change,” says Bridgeport Mayor Bill Finch. “Generating clean energy from organic waste creates a “˜win-win’ scenario for us, enabling our city to tackle tough waste management challenges, cut costs and create a renewable energy source for our city.”
The biogas generated by the anaerobic digester facility will be used to run a generator providing electricity to West Side Wastewater Treatment Facility which has a treatment capacity of 30 million gallons per day for operational requirements and to provide energy supply resiliency in the event of a power grid failure.
Occupying the small-scale application are fuels cells that can utilize biogas for power. In standard fuel cells two fuels are fed into the fuel cell at high pressure to force their interaction across a membrane. In a standard hydrogen fuel cell, these fuels are hydrogen and oxygen. The result is an electricity producing reaction. Similar in principle but different in operation are fuel cells that utilize biofuels. Biofuel cells use biocatalysts to create electrical energy. They come in two types, biofuel cells that utilize the energy of whole organisms (microbial fuel cells) and those that use the enzymes produced by microbes (enzymatic fuel cells).
A third type of fuel cell that indirectly utilizes biofuels are fuel cells that use biogas/methane or natural gas as the primary fuel. A leading developer of biofuel cells is ClearEdge Power Inc. located in Hillsboro, OR. Their Purecell Model 5 and Purecell Model 400 systems utilize natural gas or high quality biogas to produce 5 kW (21,000 BTU per hour) and 440 kW (up to 0.78 MMBTU per hour), respectively. An efficient design, it utilizes what would normally be waste heat from the fuel cell reaction to provide “low grade” heat for boilers, pools, hot tubs, radiant flooring, and space heating, further offsetting a customer’s energy costs. Overall, their systems provide 90% peak system efficiency, making them one of the most efficient applications of biogas fuel. For increased energy output, individual systems can be arranged in a minimal site footprint. Flexibility is as important as efficiency to the Purecell, and its design allows for installation outdoors or indoors, from basements to rooftops and requires a fraction of the area and volume required by equivalent solar/wind systems.
And that sums up the promise of biofuel. It remains the only truly convenient form of solar energy—one that does not require a large physical footprint, can be efficiently stored in terms of energy per volume/mass, is easily transportable, is competitively priced with other liquid fuels, and is compatible with existing energy infrastructure. It has a bright future.