The hydrothermal liquefaction process is flexible enough to use many species of algae feedstock because it converts not only lipids, but also protein and carbohydrate. Courtesy of PNNL
A new process extracts a greater amount of energy from algae than previously possible and show promise in producing an economically feasible alternative to fossil fuels.
January 21, 2014—What if it were possible to grow a plentiful, renewable source of gasoline? That intriguing question has entered and exited the public energy debate many times over the past four decades, with interest in the topic spiking along with crude oil prices. In recent years engineers have turned their attention to algae as a source of biofuel.
Researchers at the U.S. Department of Energy’s Pacific Northwest National Laboratory (PNNL) in Richland, Washington, have now developed a process for creating a form of biocrude oil from algae that shows promise in addressing many of the complications that have made the procedure economically unfeasible on a large scale before.
Early efforts to extract energy from algae concentrated on the lipid content of dried algae. This meant that some species of algae were better suited to the process than others and prompted researchers to focus their attention on efforts to develop algae with higher lipid content and often meant growing the algae at less-than-optimum rates. Early efforts also required an energy-intensive, time-consuming drying process. The process developed by PNNL, called hydrothermal liquefaction, eliminates both of these challenges.
“We convert not just the lipid, but the protein and the carbohydrate as well,” says Douglas Elliott, a laboratory fellow working on the project at PNNL. “They are all broken down in the hydrothermal processing and made into what we call a biocrude.” This means the process yields a much greater amount of potential energy.
“We are less particular about what kind of algae grows,” Elliott says. “We have shown that we can convert a wide range of the algae into biocrude. It may vary a little bit with different species, but we are not limited. If it gets contaminated with different algae that grow very little lipids, are you out of business? In our case, no.”
The algae is dewatered to create a thick green slurry that is fed into a continuously stirred tank reactor in which it is heated to 662° F under 3,000 psi. The slurry is then filtered to remove solid precipitated minerals, which enables the water and oil to better separate. With the oil removed, the by-product water stream is then subjected to hydrothermal gasification, which further yields a medium-quality methane gas and cleans the water of toxic organics. The water is then returned to the algae pond where the remaining nutrients can feed future algae growth. A tube-in-tube system exchanges heat from the process’s effluent, warming the algae slurry that is about to enter the reactor system.
“The heat recovery is a key part of this processing,” Elliott says. “We’ve done this kind of processing work for a number of years. We’ve shown that we can do the preheating of the feedstock by the effluent from the reactor [via] a tube in tube heat exchangers. That is important to do. We convert as much of the algae into a liquid fuel as will separate by gravity. Then the part that is still dissolved in the water, we get it out as a fuel gas,” Elliott says. “It’s why we can get such high overall energy yields from this work.”
The process works so well in the laboratory that efforts are under way to create a pilot-scale project to further test the process. Genifuel, a company headquartered in Salt Lake City, Utah, has an exclusive license to further develop the technologies advanced by PNNL. The company is constructing a pilot project now, Elliott says.
“What we are talking about is showing these things can be run at larger scale,” Elliott says. “These are scale-ups of 10 to 100 times [the laboratory] size. We think that just showing that you can do the pumping, you can do the heat exchange, you can make the oil and water separation work at larger scale—those are all the key points.”
The pilot project will also create enough oil that it can be hydrotreated, fractionated into fuel, and tested, he explains. Currently, what is known of the fuel properties from the systems are projections from detailed chemical composition analysis. “We’ll be able to actually make the fuel, burn it, and test it in engines,” Elliott notes.
Significant engineering challenges remain to bring algae oil to the public in large enough quantities to have an economic impact. The facilities would require large open ponds to grow sufficient algae, and because the ponds would be open, the algae would be subject to contamination from dust and other wind-blown matter. And algae growth is a natural process that can’t be completely controlled.
“People are trying to understand if it is even possible,” Elliott says. “I think that’s why there is so much interest in algae, you can draw it up on paper and see these very high production rates of algae—much higher than just growing plants, much more effective at capturing the sunlight—but can you really do it in the real world in an open system? What kinds of algae do you get? How well do they grow?”
The stakes are high. Preliminary economic estimates have a wide range, the projected cost of a gallon of biocrude ranging anywhere from $2 to $12 a gallon.
“The biggest driver on that was how much does it cost to get algae to our system,” Elliott says. “The processing has some variation, too. But the biggest factor was the algae feedstock cost.” If the optimistic projections about growing algae at a large scale prove to be correct, the costs will be at the lower end of the range, and “our fuel costs come out quite competitive,” he says.