A New Biofuel: Propane

MIT researchers say they have developed an efficient chemical process for making propane from corn or sugarcane. They are incorporating a startup this week to commercialize the biopropane process, which they hope will find a place in the existing $21 billion U.S. market for the fuel.

While much of the attention on biofuels has focused on ethanol, the process developed by the MIT researchers produces propane, says Andrew Peterson, one of the graduate students who demonstrated the reactions. Propane is used in the United States for residential heating and some industrial processes, and to a limited extent as a liquid transportation fuel. “We’re making a demonstrated fuel” for which a market and an infrastructure already exist, says Peterson, who works in the lab of chemical-engineering professor Jefferson Tester and has founded the startup C₃ BioEnergy, based in Cambridge, MA, to commercialize the technology.

Propane, which is currently made from petroleum, has a higher energy density than ethanol, and although it is often used in its gaseous form, it’s the cleanest-burning liquid fuel.

The C₃ BioEnergy process depends on supercritical water–water at a very high temperature and pressure–which facilitates the reactions that turn a biological compound into propane. Peterson wouldn’t reveal the starting compound, but he says that it is a product of the fermentation of the sugars found in corn or sugarcane. The reaction is driven by heat, requiring no catalysts. At supercritical temperature and pressure, Peterson says, “water does bizarre things. It becomes like a nonpolar solvent” and mixes with the organic compounds. Once the reaction has taken place, the solution is kept under high pressure and cooled to room temperature so that the propane comes out of the solution and floats to the top. “We’ve demonstrated that we can make propane,” says Peterson. “Now we’re trying to optimize the reaction rate and get it to a scalable stage.”

Peterson says the biopropane conversion has a good energy balance: not much fossil fuel needs to be burned during production. The reaction does not require the input of a large amount of energy because the heat that’s key to the biopropane conversion is recoverable using a heat exchanger, a device that transfers heat in and out of a fluid.

“All biofuel reactions involve removing oxygen from the starting compound,” says George Huber, assistant professor of chemical engineering at the University of Massachusetts, in Amherst. There are a number of strategies for doing this, including reactions that rely on biological catalysts. But, says Huber, “supercritical fluids are a very promising way to make biofuels. You can do it in a very small reactor in a very short time, so you can do it very economically.”

Other academic labs are developing processes that use high-temperature, high-pressure fluids to make biofuels. Douglas Elliott, at the Pacific Northwest National Laboratory, in Richland, WA, is using near-supercritical conditions in combination with a catalyst to treat wastewater and unprocessed biomass. Under these conditions, organic compounds can be made into a mixture of methane (the main component in natural gas) and carbon dioxide. “We’ve gone all the way from small-batch reactors to treating a few gallons of wastewater per hour,” says Elliott, who is working with a company on commercializing the technology for water treatment. “We’re still in the lab with biomass.”

Huber and Elliott say the MIT biopropane process is novel. “I’ve never seen anyone make propane with supercritical fluids,” says Huber.

In some countries, including Australia, propane is more widely used as a transportation fuel. In the United States, “you would have to modify engines to use it,” says Huber. “Biopropane could be used where we already use propane.”