91av

“If you’re working at a power plant, you just saw your carbon dioxide turned into something you can drive home with.” So says Isaac Berzin of GreenFuel Technologies in Cambridge, Massachusetts, which is developing a way of producing biofuel from the noxious emissions of power plants.

Two of the world’s greatest energy users are electricity generation and transport. Both are responsible for huge quantities of greenhouse gas emissions, as most power plants and vehicles still rely on fossil fuels. Now GreenFuel and others are hoping to marry the two together with an emerging technology that uses a by-product of one to supply fuel to the other. Doing so could dramatically reduce their overall carbon dioxide emissions.

At the heart of the technology is a plastic cylinder full of algae, which literally sucks the CO₂ out of a power plant’s exhaust. The algae can in turn be converted into biofuel, creating a cycle that takes the carbon from the smokestack to the gas tank before it enters the atmosphere.

If successful, the technology could capture all of a power plant’s CO₂ emissions. “Right now, when you say CO₂, people want to hide under the table. Carbon dioxide is not something you want to pump underground, it’s something you want to reuse,” says Berzin.

To produce fuel from CO₂, the flue gases are fed into a series of transparent “bioreactors”, which are 2 metres high and filled with green microalgae suspended in nutrient-rich water. The algae use the CO₂, along with sunlight and water, to produce sugars by photosynthesis, which are then metabolised into fatty oils and protein. As the algae grow and multiply, portions of the soup are continually withdrawn from each reactor and dried into cakes of concentrated algae. These are repeatedly washed with solvents to extract the oil.

The algal oil can then be converted into biodiesel through a routine process called transesterification, in which it is processed using ethanol and a catalyst. Enzymes are then used to convert starches from the remaining biomass into sugars, which are fermented by yeasts to produce ethanol.

GreenFuel is testing a pilot facility at the Redhawk power station in the Arizona desert. The size of a couple of trailers, it treats a only a tiny fraction of the plant’s exhaust, but it works, and has so far produced several gallons of algal oil, which the company is planning to convert into biodiesel for the first time this week. A second, larger prototype of around 1300 square metres is now under construction.

This new facility will also capture the heat produced by the plant and use it to help dry the algae before the oil is extracted and converted to biodiesel. This excess heat could also make it easier to recover the solvent from the oil after extraction. “The main energy requirement is recovering the solvent from the oil once it is extracted,” says Berzin. “Seventy per cent of a coal-burning plant’s energy is lost as heat. That’s a lot of waste heat to use.”

GreenFuel has so far received more than $18 million in venture capital funding, and hopes to install a full-scale algal farm at least 1 kilometre square near the Redhawk plant by 2009. Berzin calculates that if the farm has enough algae to absorb all the CO₂ produced by the 1000-megawatt plant, GreenFuel could ultimately produce more than 150 million litres of biodiesel and 190 million litres of ethanol a year. To do this, it would need a farm of between 8 and 16 square kilometres.

The idea of producing biofuel from algae is not new. The US Department of Energy began investigating algae in the 1970s during the global oil shortage. Researchers scoured the US, collecting more than 3000 different strains of “extremophile” algae that could withstand the high temperatures, salinity and pH required to absorb the exhaust from power plants.

The Aquatic Species Program, as it was known, grew the algae in open pond test sites in Hawaii, California and New Mexico, but was mothballed in 1996 when lower crude oil prices made it difficult for alternative fuels to compete. “It’s an entirely different world now,” says John Sheehan, an analyst with the National Renewable Energy Laboratory in Golden, Colorado, who worked on the project. “I’ve had a call or email a week enquiring about it.”

Although ahead of the competition in terms of developing prototype bioreactors, GreenFuel is not the first to use algae to produce samples of biofuel from power plant exhaust. In March Laurenz Thomsen and his team at the Greenhouse Gas Mitigation Project at the International University Bremen in Germany used microalgae to produce 10 millilitres of biodiesel. Thomsen is now working on a possible joint venture with GreenFuel to develop algae farms at CO₂-belching coal-fired plants in eastern Europe.

“Using technology based mainly on GreenFuel, we can mitigate 50,000 tonnes of CO₂ per square kilometre per year,” he says. Building a 1-square-kilometre facility would cost approximately $20 million, he estimates, but the payoffs would be equally large. “I think we are close to the point where we can gain $5 to $10 million a year by selling the fuel.”

Another company building a pilot algae reactor is New York-based Greenshift. The company plans to begin testing its reactor at a bioethanol plant in Iowa in early 2007, where waste CO₂ is emitted when corn is converted into ethanol. “Roughly one-third of the corn that goes into a facility comes out as ethanol,” says Kevin Kreisler of Greenshift. “With algae and other technologies we can increase that to two-thirds.” Like GreenFuel, the company eventually plans to use the technology at power plants.

Instead of exposing the algae directly to sunlight, Greenshift uses an array of mirrored troughs and fibre optics to carry sunlight to the plants. Algae don’t need strong sunlight for photosynthesis, so the bioreactors could feasibly be housed in buildings or underground. “It’s all about efficiency,” says Kreisler. “By diffusing the light we can take one square metre of sunlight and spread it out over 10 square metres of growth plates, thus reducing the amount of land we need by a factor of 10.”

Indeed, one key advantage of algae farms over other sources of biofuel such as corn and soybeans is that they need much less space (91av, 23 September, p 36). In Germany, where rapeseed is the primary crop used for biodiesel, it would take up to 33 times as much land as is needed by the algae bioreactors to produce the same amount of fuel, Thomsen says. What’s more, unlike other biofuel crops, algae do not require precious commodities like fresh water or fertile land. That makes the technology suitable for use in the deserts of the American south-west and China. “If you really want to make an impact on CO₂, you have to look at the US and China,” Berzin says.

If the technology is to be successful, though, the energy industry will need to be convinced. Barry Worthington of the US Energy Association in Washington DC, which represents the electricity generators, says the economics of algal biofuel still have to be borne out. But he is optimistic about its potential. All the conventional ways of reducing CO₂ emissions are considered a cost, he says. “This changes the dynamics dramatically.”