The term "biofuels" actually encompasses a host of largely unrelated technologies. So far, it has largely meant ethanol, generated by fermenting the sugars found in crops, while research is attempting to make it from the cellulose that gives all plants their structure. But you can also convert biological materials to things like biodiesel or other replacements for the fuels we already use.

Even in that area, however, there are several different approaches. One is to engineer organisms to make molecules that we can drop directly into our fuel tanks. Another is to get those organisms to produce an excess of the lipids they normally require and then subject them to a mild bit of processing in order to convert the lipids to fuel. This week, researchers announced progress with a third approach: subjecting the microbes to a chemical process that converts them to a complex mixture that's a bit like crude oil.

The authors of the new paper point out the trade-offs between approaches in their introductions. It's possible to select algae that are naturally rich in lipids and then grow them under conditions that induce them to produce even more. But the lipid-rich strains don't grow as fast as many others, and the conditions that get the most fuel out of them slow down growth even more. If you just take a typical species of algae and grow them in optimum conditions, you get a lot more raw material much more quickly.

Unfortunately, very little of that raw material is in a form that can easily be converted to fuel. But it is possible to convert some of it to a mixture of hydrocarbons of various sizes that's a bit like light crude oil—termed biocrude. That biocrude can then be mixed in with normal oil and processed for use as fuel.

The challenge is doing that conversion efficiently. It takes heat, pressure, and a constant flow of hydrogen, so you need to use that energy efficiently. And you also need to be able to run the conversion at scale. That was the idea behind a demonstration project at the Pacific Northwest National Lab. The lab created a continuous-flow reactor that took algae and hydrogen at one end and released biocrude (as well as some other useful products) at the other.

The process started with a near-solid slurry of single-celled algae that grow rapidly in tanks. This raw material was sent through a number of tanks where it was pressurized and warmed using waste heat from the main reaction tank. Solids (mostly calcium phosphate) and sulfur were removed before the material entered the reaction chamber, where it met the hydrogen and a catalyst (ruthenium). The ensuing reactions used the hydrogen to remove oxygen from sugars, break down double bonds, and convert any nitrogen into ammonia.

The ammonia, along with some carbon dioxide, ended up in the gas phase and could be bled out of the system. Once the rest of the material leaves the reaction chamber and is chilled, it spontaneously separates into water and hydrocarbon phases. The water contains some carbon compounds like methanol and ethanol, which can then be extracted and used as fuel. It also contains much of the remaining phosphate and nitrogen, and the authors are optimistic that these can be isolated and used as fertilizer for the algae themselves.

But most of the material in the cells (53 percent by weight) ends up the biocrude. The authors show that this material is a mix of long-chain hydrocarbons (with anywhere from six to over 30 carbon atoms). Based on its properties, they estimate that over 80 percent of the mix could be blended into diesel stock.

They key thing about the process is that it is continuous-flow: you can keep stuffing algae in the front even as biocrude flows out the far end, which means that it can be operated continuously, keeping the heat in the system and raising the efficiency (although, in practice, the test setup was never run for more than a day). The authors indicate that all of the hardware of their system can also be scaled up to get things to work at an industrial level.

Will it make sense to do so? Right now, the uncertainties are pretty large. Some of the same researchers have done an economic analysis of the system, and they have found that it could make sense now, or it could only be worth it if fuel costs rose to $7 a gallon—the variability depended on the cost of raising the algae and the efficiency of the process at scale. Still, without basic work like this, it would be difficult to make any estimate of the cost at all.

Algal Research, 2013. DOI: 10.1016/j.algal.2013.08.005 (About DOIs).