SQUEEZING POWER FROM POND SCUM
The merits of renewable fuels abound, but so do limiting factors. We can already produce biofuels to run our cars, but if it costs $10 per gallon and requires petroleum products for production, why bother? We must see green—economic and environmental—if we seek to reduce our dependence on fossil fuels.
The United States currently imports at least half of its petroleum, 60 percent of which is used for transportation fuels, and resources are dwindling. With the world's population now surpassing seven billion people, the nation seeks a competitive alternative to crude oil.
Biofuel is a popular venture today, and hundreds of companies in the United States are now pursuing algae as a source of fuel and other economically valuable products. The discovery that many species of algae can produce large amounts of combustible fats and oils dates back more than sixty years, and algal biofuels research gained traction during the energy price surges of the 1970s. But then researchers turned their attention to conversion of agricultural products such as corn and cellulosic material (stalks and fibrous plants) into ethanol. Unfortunately, this approach is not without its downsides, as demand for farm-grown fuels would cause crop prices to rise, potentially threatening to leave mouths empty. Additionally, it suffers from other common problems of big agriculture, including the clearing of rainforests and the deadening of marine zones due to fertilizer runoff. And so far, crop-based ethanol has proven to be a low-performance fuel compared to petroleum.
Recently, algae have garnered renewed interest. Because algal biofuel must compete with petroleum in price as well as performance, it's fortunate that not only is algae a good source for fuel, but some of its byproducts are far more valuable per gallon than crude oil. But it still costs more than petroleum to produce, so algae's economy-energy-environment equation must be carefully balanced.
A photobioreactor (PBR) is a device that can contain and grow algae without the need for sunlight or sugars (required by ponds or fermenters, respectively). Los Alamos chemical engineer Munehiro Teshima designed and built this unique PBR to achieve exquisite control over gas flows and other factors such as temperature, illumination, alkalinity, agitation, and cell density. The reactor uses sophisticated sampling robots to take measurements and make adjustments in order to maintain ideal growth conditions.
Algal Fuel for the Fire
Algae are aquatic microorganisms that use carbon dioxide (CO2), water, and energy from light to make sugars, oxygen, and combustible fuel sources including lipids (fatty, energy-rich molecules) and hydrocarbons. The unicellular variety most biofuel researchers focus on, microalgae, is found in freshwater and marine systems. Some microalgae species can double their mass in one day and may contain up to fifty percent of their body weight as lipids.
Algae are valuable sources for fertilizer, proteins, essential fatty acids for nutritional supplements and pharmaceuticals, animal feed, and industrial products such as polymers in biodegradable plastics and rubber. Algal biomass can produce more burnable fuel on less land than traditional bioenergy crops—at least 32 times more oil than corn per acre annually—and its use in fuel does not compete with the world's food demands. It can be cultivated on nonarable land that is also undesirable for urban development and can be grown in nonpotable water containing salt, industrial waste, or sewage.
Algae also have another advantage over other biofuels. Algal products produce enough energy to be used in high-energy density liquid fuels for aviation and long-haul trucks, something that cannot be said for corn-based ethanol, which only provides about half the energy per volume of jet fuel. In fact, algal-powered commercial planes took flight last year.
Biofuel production from algae, on the other hand, sounds simple and ideal: Add algae to wastewater tanks located in sunny locations with no commercial or agricultural value, and let the plant grow independently. Overfeed the algae to make them produce more fat (just like humans). And since algae require carbon dioxide for growth, they absorb dangerous greenhouse gases from the environment while simultaneously producing lots of valuable oil—a boon for our planet and our economy. It certainly sounds like a compelling plan. In reality, however, it's not so easy.
The production of biofuels from algae faces significant challenges. Large-scale algae-to-fuel production utilizing today's technologies is estimated to supply fuel that costs at least $8 per gallon (compared to about $4 per gallon for soybean oil). So production efficiency and output of valuable byproducts must be increased for algal biofuels to be a viable option. The organism must be improved for it to thrive on a diet, to survive in less-than-ideal environments, to surrender its fats without a fight, and to provide sufficient power when consumers put the pedal to the metal.
Scientists are improving methods to make the algae-to-fuel process more economical and environmentally friendly. This illustration reveals how algae can thrive in wastewater, remove carbon dioxide from the atmosphere, and surrender their entire biomass to be used. Algae's products are diverse—biogasoline, cosmetics, plastics, and food supplements, to name a few—and Los Alamos researchers have already made growth, harvest, and production methods more efficient.
The Perfect Strain
Countless species of microalgae exist, and scientists are looking for ideal algal strains that grow rapidly and accumulate large amounts of oil with minimal inputs that survive despite a large variation in temperature and water quality. Their target is to maximize production of both hydrocarbons and lipids through understanding and improving the biosynthetic pathways involved.
Renowned crop researcher and molecular biologist Richard Sayre recently brought a large team to Los Alamos to address algal problems. Sayre is currently working on multiple methods to increase algal performance. With one method, Sayre genetically engineers algae to absorb light more efficiently to increase production and decrease resource use. The Lab is a partner in two biofuels consortia—the National Alliance for Advanced Biofuels and Bioproducts (NAABB) and the National Advanced Biofuels Consortium—to develop "green" biofuels. Sayre is the chief scientist for NAABB, which is directed by his Los Alamos colleague José Olivares.
Algae evolved to grow in low light, so their photosynthetic antennae only use about one quarter of the energy they absorb from the sun. But even though they don't use all the solar energy, they hog it and prevent algae located deeper in the water from seeing any sunlight. Sayre created mutants with much smaller antennae designed to absorb only as much light as they need, thereby allowing other algae to soak up more light. This improvement allows biofuel producers to use deeper ponds with layers of algae, which decreases the amount of both land and water needed for cultivation. Thus, a given pond can produce more algal biofuel.
The Laboratory's Genome Science Group is sequencing and assembling five algal genomes, with three more in the near future. The group examined several algal gene sequences to determine what genes are important for lipid production. Because most algal production systems are open ponds, it's important to understand the entire pond community in order to maintain stable cultures. Therefore, the team is sequencing DNA from the mixed environmental samples.
Los Alamos biochemist David Fox and his team are pursuing algal compounds that produce hydrocarbons having a structure similar to gasoline—flammable liquid hydrocarbons. These hydrocarbons can simply be dropped into an existing oil refinery's infrastructure and "cracked" to provide auto fuel similar to gasoline. Fox's team is collaborating with researchers at Texas A&M University to identify the optimal process for conversion of CO2 and light to hydrocarbons by using genetic engineering to manipulate the network of metabolic processes performed by algae.
Other teams at Los Alamos are working on different angles. One led by Pat Unkefer, for example, increases the photosynthesis rates by raising the amount of carbon dioxide the organism takes from its environment and converts to carbohydrates, while also increasing the efficient use of nitrogen.
Los Alamos bioscientist Cliff Unkefer monitors algae's growth. Unkefer and his colleague Pete Silks recently demonstrated a high-yielding chemical process to convert triglycerides obtained from algal lipids into high-energy hydrocarbon biofuels for aviation. The process removes oxygen atoms from the algal triglycerides and reforms the resulting high-molecular-weight hydrocarbons into lower-molecular-weight hydrocarbons that can vaporize and ignite quickly.
The Sweet Spot
Scientists know how to speed up algal reproduction, but can they simultaneously accumulate lipids at a matching rate? To be cost competitive, algal biofuels production must be swift, but when the organism diverts all its energy towards proliferation, it consumes its fat reserves. An optimal combination of nutrients, light, and temperature must be created that balances high growth rates and lipid yields
"People underappreciate how difficult it is to improve upon nature," says Fox. "Gene function discovery is difficult, but bioengineering algae for much improved production of biofuels may border on an intractable problem. We want a fast growing organism that also has a high energy density. It's a major barrier to overcome."
Sayre agrees and thinks improved results may materialize by blending different algal growth strategies. Some species of algae can grow autotrophically—outdoors using sunlight as the energy source—but the process is slow. Some species can be grown heterotrophically—fed glucose or fructose inside a fermenter (sugar instead of sunlight)—but this is expensive, produces substantial CO2 as a byproduct, and risks contamination since bacteria thrive in the presence of sugar. Some species are mixotrophic, deriving energy from both sunlight and sugar. Sayre believes a hybrid model where the algae are grown outside with captured CO2 and wastewater is sustainable. No oil is produced until the next phase, when the mass is briefly fermented inside a tank with small amounts of sugar. There, the lipid content is increased from miniscule amounts to 70 percent with a 24-hour turnaround, according to Sayre.
Quenching a Mighty Thirst
Algae has a major advantage over traditional agriculture in that it can thrive in contaminated water, but it still requires a substantial amount of water. Experts predict mass cultivation of algae will require at least 350 gallons of water per gallon of oil produced. Water consumption needs to be reduced if algal biofuels are going to be economical and environmentally friendly.
Pilot plants are trying to use wastewater or water from a saline aquifer. According to Sayre, the saline water model is not ideal because the water evaporates, leaving behind salts or toxins that require expensive mitigation to prevent environmental contamination. However, it may be possible to drain briny water to evaporation ponds in order to recover the salts for use by the chemical industry.
Municipal wastewater as a growth medium may present a win-win situation for algal producers and communities because the algae remove contaminants and utilize nutrients that otherwise pollute the water, such as raw sewage or fertilizer runoff. Algal use of wastewater—rich in carbon, nitrogen, and phosphorous nutrients—could reduce the expense and the environmental cost of producing fertilizers, while removing environmentally damaging chemicals from the water. The residue could then be used as fertilizer.
Produced water, a byproduct pumped to the surface during fossil-fuel extraction that contains bicarbonate and other nutrients, is also becoming an attractive growth medium for algal biofuels, although it may require pretreatment to remove certain toxins. Currently, oil and gas production brings about 800 billion gallons of brackish produced water to the surface. Los Alamos scientist Enid Sullivan and colleagues at Eldorado Biofuels (a New Mexico company) recently joined industry and NAABB consortium partners to conduct the first pilot-scale test of algae growth using water from an oil-production well in New Mexico. The researchers grew salt-tolerant, oil-producing algae in 80-gallon reservoirs of city water with varying amounts of produced water mixed in. When the concentration of produced water was at low levels, algal growth was comparable to that in the control group containing unadulterated city water. Growth became limited as more produced water was added, and researchers are now testing bicarbonate and salinity levels to find the ideal mix.
Land, climate, and algal growth are interlinked, and not all regions in the country provide enough sun, water, and carbon dioxide for growing algae. Roughly 800–2600 acres are needed to produce 10 million gallons of algal oil, according to the Department of Energy. A 2010 study by Pacific Northwest National Laboratory (PNNL) reports ponds should not be placed in urban areas or on conservation and agricultural land if algal biofuels are to be profitable. Sunny regions with average temperatures above 55° are ideal, and analysis revealed the Gulf Coast states and portions of the Southwest are optimal. PNNL's study reveals that algal biofuels produced from 21 billion gallons of American algal oil could replace 17 percent of the United States' imported transportation fuels, and it could be grown on parcels of land that, taken together, are roughly the size of South Carolina.
"Production rates from algae are at least tenfold greater than regular agriculture," says Cliff Unkefer. "And it doesn't need to be grown in rich river-bottom land. If we can get productivity high enough we could lower the land requirement by a factor of ten compared to agricultural crops such as corn."
Nitrogen is essential for the growth of algae's DNA, RNA, and protein—building blocks of life. Earth's atmosphere is composed of 80 percent nitrogen, but not in a form plants can use, due to the nitrogen molecule's strong triple bonds. Therefore nitrogen and other mineral nutrients must be supplied to crops via fertilizers, the production of which is extremely energy intensive. Fertilizers are usually synthesized using atmospheric nitrogen and natural gas, a fossil fuel. Agriculture loses 75 percent of the fertilizer input to soil seepage, which is harmful to the environment. Fortunately, algal ponds are protected from this leaching into the ground, but the fertilizer is still expensive.
The ammonia (NH3) used to produce fertilizers is commonly synthesized using a process that requires high temperatures and very high pressures. As a result, 3–5 percent of the natural gas produced in the world is consumed in the process. Los Alamos chemist John Gordon and his colleagues are attempting to develop low-cost, low-energy (ambient temperature and pressure) synthetic approaches to generate NH3 using abundant metals such as iron.
An alternative to making fertilizers cheaper is to develop a way to use less of them. Stressing algae by removing or decreasing the availability of a key nutrient, including those found in common fertilizers, can result in high lipid production. Therefore, Pat Unkefer and her team are working on understanding this metabolic mechanism and exploiting it to generate high productivity. However, just removing the nutrients does not mean that the algae will become fat. In fact, they are more likely to die if the nutritional manipulations are not carefully executed. When algae are stressed from lack of nutrients (or other factors such as drought or temperature fluctuations), they absorb more sunlight in their chloroplasts than they can use during photosynthesis and CO2 fixation, and molecular oxygen is activated. The plants experience photo-oxidative damage, which can lead to cell death. However, as long as the plants are not overstressed, they produce lipids as well as gummy residues called isoprenoids (another biofuel source) to protect themselves by storing light energy as chemical energy.
Los Alamos researchers Richard Sayre (left) and José Olivares founded a new international journal, Algal Research, launching early this year. These Los Alamos scientists also organized the first International Conference on Algal Biomass, Biofuels and Bioproducts last year.
Capturing Carbon Dioxide
Algae consume CO2 during photosynthesis. They can get this CO2 from the atmosphere, but the uptake rate is limited by its slow diffusion through the surface resistance of the water in the cultivation system. Accelerating algae's growth rate requires boosting both the CO2 supply and its uptake speed.
To address the supply problem, Los Alamos researchers have designed and constructed a system for extracting CO2 from the exhaust streams of power plants and concentrating it for algal consumption. This method kills two birds with one stone, since the exhaust CO2 is an unwanted greenhouse gas emission anyway. Researchers are determining what algal species can survive the exhaust gases' blistering temperatures and optimizing the delivery system.
Two teams at the Laboratory (led by Sayre and Zoë Fisher) are boosting algae's metabolism by a genetic modification that uses an efficient human enzyme responsible for CO2 regulation in red blood cells to increase algal growth. The enzyme catalyzes the inter-conversion of CO2 and bicarbonate, which the algae readily take up. Fisher uses data from a joint neutron and x-ray study—the first of its kind—to better understand the enzyme properties and reaction rate. Sayre reports this genetic work increased algal photosynthesis rates 30–136 percent, depending on test conditions.
Los Alamos scientists are researching algal growth in open-air ponds in New Mexico and Texas. Open ponds that do not require much mechanical mixing (to incorporate nutrients) need 20–100 days for algal growth. High rate ponds like this one, which mix the solution with paddle wheels, allow cultivation in 4–10 days, but they are more expensive to run. CREDIT: NEW MEXICO STATE UNIVERSITY
Making Pondscum Profitable
So once researchers have mastered plant growth, how do they squeeze the precious oil out? Harvesting the algae from its growth medium and then extracting the oil can be quite costly, accounting for almost 30 percent of the total cost of current algal biofuel production systems. But Los Alamos researchers discovered a way to use sound waves to harvest algae. The Ultrasonic Algal Biofuel Harvester uses ultrasonic waves to concentrate algae in a solution, rupture the algae to release the lipids, and then collect the lipids and other useful byproducts. The researchers, led by Babetta Marrone, are optimizing this technology into a portable device.
A closed photobioreactor system is essentially a series of plastic or glass containers for water and algae. Proponents of such bioreactors (compared to open ponds) say the growth environment can be more easily controlled; they prevent evaporation; and light penetrates through all sides of the container, which increases cell density. However, bioreactors suffer high materials and energy costs as well as mixing and gas-exchange inefficiencies; therefore scalability remains problematic.
Conversion of algae to biofuels requires dewatering before extracting usable products. This can be daunting since the mass of water in a growth pond exceeds that of the algae by 999 to one. Recently, scientists Pulak Nath and Scott Twary, from the Los Alamos physics and bioscience divisions respectively, genetically engineered magnetic algae to investigate a novel harvesting method: pulling the algae from the water with a magnet. The team took a gene that is known to form magnetic nanoparticles in certain bacteria and expressed it in green algae. This project is in the early stages, and Nath is working to optimize it. Extraction—getting the oil out—is also being optimized. One option Twary is examining is to use rotating magnetic fields to heat up the algae, causing their cells to rupture and release lipids. The team is investigating how the metals in the magnetic nanoparticles could interfere with the algal chemistry or downstream byproducts (e.g., high levels of iron in biomass waste could be toxic to cattle therefore undesirable as an animal feed) and how to mitigate such potential problems. Once the algae is magnetically separated, some of it can be used for biofuels or commodity byproducts, and the magnetic nanoparticles can be recovered for use in biomedical imaging and cancer treatments.
Apart from the new magnetic approaches, oil extraction is usually done by one of two methods: mechanical or chemical. The mechanical method requires drying the algae, then pressing or crushing the oil out of the remaining biomass. It is energy intensive because of the dehydration process. The chemical method usually requires the use of toxic chemicals (such as benzene or hexane) as solvents, liquids that extract the oil from the plant. The oil is then distilled from the resulting solution. Los Alamos materials chemist Rico del Sesto and Fox developed a potentially new method for biofuel extraction using a benign class of solvents known as ionic liquids. Early results are promising, with the dual effects of extraction of hydrocarbons with minimal toxicity to the algae.
Each of these harvesting and extraction methods discovered at Los Alamos may help level the playing field against petroleum. But it's not easy being green. Energy links the environment, the economy, and our society together; all three are vulnerable to changes in the energy sector.
Since its inception, Los Alamos has been tasked to make pioneering discoveries to protect the nation, and today, energy security is an important component of that protection. In turn, algal fuels may become an important component of energy security if researchers can find ways to further cut biofuels' costs while increasing their production. Such progress would build upon the Laboratory's recent successes that helped rejuvenate decades of biofuels research and development. With promising leads on every aspect of biofuel production, Los Alamos has every reason to be optimistic.
Los Alamos student Calla Glavin (left) and Taraka Dale, a researcher in the Bioscience Division on the Ultrasonic Algal Biofuel Harvester team, measure the lipids harvested from algal cells.