Plants take energy from the Sun, and use it to make sugar. We humans extract the sugar, and ferment it to make fuel. That’s the process for ethanol and cellulosic biofuels in a nutshell.
If we were to replace every drop of the liquid fuels we use with biofuel made with commercially available technologies, the volume needed would demand unsustainable supplies of water, land, fertilizer, and energy. Even with the small fraction of the total we currently use—about 9% of US gasoline is actually plant-derived ethanol, for example—a single bad corn harvest can rock commodity markets with a food-versus-fuel scare.
Of course, it isn’t the corn that biofuel makers really want: It’s the sugar. And the way we get to sugar is problematic, says Kef Kasdin. Kasdin is chief executive at Proterro, a company that has taken a different approach to biofuels. Proterro cuts out plants entirely, instead using cyanobacteria to produce sugar directly.
The basic chemistry of stripping sugars from plant cellulose has been known for more than a century. Chemist Henri Braconnot published a paper describing the sulfuric acid process in 1819. Still fermentation was a popular method of producing ethanol right on the farm for lamp fuel. And in the early days of the automobile, ethanol fueled plenty of combustion engines. But by the early 20th century, the cost of farming, harvesting, and processing biomass for ethanol, together with US government-imposed alcohol taxes, made ethanol uncompetitive with gasoline.
That situation changed a century later, when Brazil’s massive sugarcane industry successfully produced ethanol from sugar. The Renewable Fuel Standard passed in the US in 2005 helped as well. At that time, Kasdin, a Battelle Ventures general partner, and cofounder John Aikens, founder of biotech development company Lybradyn, were evaluating cellulosic ethanol projects as potential investments. Kasdin and Aiken thought there ought to be a better, more efficient way to get it.
Aiken had previous experience in biotechnology, and both he and Kasdin were drawn to the idea of engineering cyanobacteria to make sugar. Cyanobacteria get all the energy they need from the Sun, and live on water and carbon dioxide. They pick up trace minerals from their aqueous environment, which does not have to be especially clean. Irrigation-grade or even waste water can be used. And depending on how they are grown, they can produce considerably more sugar per acre of land than sugarcane can.
Other groups in the biofuel space are working with cyanobacteria, but most of those focus on producing hydrocarbons such as alkanes and terpenes. Such hydrocarbons already make up a portion of conventional gasoline and jet fuel, and can be easily plugged into existing infrastructure. They are also higher in energy density than alcohols. The mixture of hydrocarbons in gasoline delivers one-and-a-half times more energy than an equal volume of ethanol.
Sugar serves a different part of the pipeline. The sucrose that Proterro’s cyanobacteria make doesn’t feed cars directly. But it can feed yeast to produce ethanol, or algae to produce biodiesel, or even amino acids for livestock feed. That flexibility means than Proterro could choose which markets to sell to, depending on price and demand.
The bioreactor sits in a field in Florida, a 5-meter-long cylinder suspended in black netting. Inside of it hang sheets of fabric housing cyanobacteria. Water slowly percolates through the fabric, and the gas inflating the reactor is rich in CO2. Proterro hopes this outdoor bioreactor replicates the success of its greenhouse demonstration, which produced the equivalent of 145 tons/acre of sucrose, thirty times the output of a field of sugarcane. If this bioreactor does as well, they will replicate it 100-fold. And if that works—if they can reliably get water and CO2 to that many bioreactors—the next step would be a commercial plant.
Proterro's bioreactor. CREDIT: Proterro
Ideally, such a plant would be hooked up directly to an ethanol fermenter, Kasdin says. Proterro would take the CO2 exhaled by the fermentation yeast in the ethanol vats and pump it through the bioreactors. The cyanobacteria in the bioreactors would take the carbon dioxide and make sucrose. The sugar water from the cyanobacteria’s bioreactors could then be pumped right back to the ethanol refinery and fed to the yeast.
“That sounds pretty easy, a pretty tight little loop there. I can imagine that being a very attractive industrial partnership,” says Nathanael Greene, director of renewable energy policy at NRDC. Getting biomass to the point of sustainability is the big challenge facing the entire industry. If Proterro’s bioreactors can sustain their high productivity at scale, they might make ethanol fermentation a sustainable system.
The "tight little loop" admired by Greene would supply a portion of the sugar needed by the yeast while eliminating the traditional costs of transport, harvest and storage for biomass. Once implemented, the process would sharply increase the energy return on energy investment (EROEI) compared to conventional ethanol production.
And the EROEI is a big deal: It tracks the amount of energy input into a system and the amount of energy extracted. Conventional petroleum, for example, has a huge EROEI. The energy equivalent of a single barrel of oil extracts the equivalent of tens or even hundreds of barrels of oil from a conventional well. Even unconventional oil fracked from tight shale formations has an EROEI of 10 or so, meaning that one barrel of oil invested in a fracked well extracts 10 barrels. But traditional cellulosic biomass technologies produce single-digit, and sometimes even negative, EROEIs.
Kasdin wouldn’t give the exact EROEI of Proterro’s process, but she says it’s looking pretty good. They’ve made the first principles calculations on energy potential from available solar photons, and it aligns closely with the productivity of their greenhouse project.
“Net-net, we really think we have the energy advantage,” over conventional biofuels, she says.
Much depends on the results of their new outdoor bioreactor in Florida. As a startup with limited funds, Proterro has stayed lean. Kasdin says when she worked for a large corporation she managed budgets for single projects that were larger than Proterro’s budget entire. The company outsources a lot, for the scientists who modify the cyanobacteria, the engineers who design them, and the company that fabricates them.
But if Proterro’s Florida demonstration is successful, there’s plenty of incentive to expand, and fast. The renewable fuel standard in the US is currently at 15 billion gallons by 2015. It will jump to 36 billion gallons by 2022. If US gasoline demand remains stable, nearly a quarter of the nation’s fuel will be biofuel. And that biofuel is going to need feedstock.
For Proterro, the future could be sweet.
Kim Krieger is an independent science writer. She has reported on science policy from Capitol Hill, energy from the floor of the New York Mercantile Exchange, and physics innovation everywhere.