Flash volatilization: a new biomass-to-liquids process

Chemical engineer Lanny Schmidt and his research team have found a new carbon-neutral process to make syngas from vegetable oil and sugar, a first step toward creating usable biofuels from plant wastes like sawdust or cornstalks. Schmidt, a Regents Professor of Chemical Engineering and Materials Science, and graduate students James Salge, Brady Dreyer and Paul Dauenhauer describe their work on this innovative biomass-to-liquids (BTL) path in the Nov. 3, 2006, issue of Science.

The process, called reactive flash volatilization, consists of pyrolising biomass, yielding a mixture of hydrogen and carbon monoxide gases called synthesis gas, which is now used to make synthetic diesel fuel (dimethyl ether, also a substitute for propane gas) and ammonia, a constituent of fertilizer. Hydrogen is also the energy source for fuel cells and may someday be burned in car engines instead of fossil-based gasoline. The syngas can be turned into a clean burning liquid biofuel.

If scaled up, their process could slash the cost of producing renewable fuels and chemicals from biomass while eliminating the fossil fuel input now needed for turning vegetable oil into usable liquid biofuels. The new process works 10 to 100 times faster than current technologies and could be done in facilities about 10 times smaller than today. Facilities could be placed on farms to produce fertilizer or energy for local consumption, or in centralized locations to produce fuels for transportation.

Schmidt's 'reacitve flash volatilization' process:

1. The researchers start with either pure soy oil or a thick sugar syrup.
2. The reactor [see picture] consists of an automotive fuel injector, used to spray the oil or syrup as fine droplets through a tube. Sitting like a plug in the tube is a porous ceramic disk made of a rhodium-cerium catalyst material.
3. As the droplets hit the disk-whose surface temperature is 1,000 degrees C-the heat and oxygen break apart the molecules of oil or sugar.
4. The catalyst guides the breakdown toward the production of synthesis gas rather than toward water vapor and carbon.
5. The synthesis gas passes through the porous disk and is collected downstream in the tube.
6. No external heating is needed because the chemical reactions release enough heat to break up molecules of oil or sugar following in their wake.

While the Schmidt team used fresh soybean oil and a sugar-glucose-in their experiments, those were just practice materials. In particular, glucose was a stand-in for related starchy compounds like cellulose, a major building block of plant cell walls. The real targets of the research are underutilized plant oils and fibers:

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"It's a way to take cheap, worthless biomass and turn it into useful fuels and chemicals," says Schmidt. "Potentially, the biomass could be used cooking oil or even products from cow manure, yard clippings, cornstalks or trees. It's better than bringing oil from Saudi Arabia to fuel your gas station."

The breakthrough came with the perfection of a technique that heats fuel to a temperature so hot that the smoking reaction is bypassed, said Bradon Dreyer, a chemical engineering and materials science graduate student at the University of Michigan who co-authored the article.

Dreyer and his colleagues built a reactor capable of producing hydrogen from soybean oil, biodiesel or sugar water without any of the buildup that would have resulted from a conventional process. To get the reactor warmed up, the researchers ignited a mixture of methane and oxygen in order to bring the catalyst to a searing 1,000 degrees Fahrenheit.

Addressing concerns about keeping the process carbon-neutral, Paul Dauenhauer, another graduate student working on the project, notes that while methane is a fossil fuel, there are other ways to heat the catalyst that don't involve burning petrochemicals. What's more, once the reaction is running, it's self-sustaining, and methane and oxygen are no longer required.

A fuel injector like those used in a car atomized the biofuels into tiny droplets that landed on a hot rhodium-cerium catalyst, which converted the fuel to syngas. This reaction released energy and heated the catalyst. The heat and ratio of carbon and oxygen in the reaction kept the buildup from sticking to the catalyst. For each type of biofuel, nearly all the fuel was converted and about 70 percent of the hydrogen bound up in the fuel molecules was given off as gas, the researchers report in this week's Science. "We find we reach the theoretical maximum," says Dauenhauer.

The whole reaction takes less than 50 milliseconds. "Faster means smaller," says Dreyer, who predicts that because of its speed, their reactor can be scaled down and remain efficient. Dreyer also notes that their reactor could work on other fuels, including used cooking oil. Best of all, no more carbon comes out of their system than went into it.

Currently, soy oil can be modified to make a fuel called biodiesel, but the process requires the addition of methanol, a fossil fuel derived from natural gas. And while cellulose can be digested into simple sugars-which can be fermented into ethanol or turned into other fuels-these processes require special enzymes and lots of time.

What makes vegetable oil, sugars and starches so hard to turn into fuels is the fact that they don't evaporate when heated. As a drop of oil sits on a hot surface, its bottom layer is exposed to heat but not oxygen. In the absence of oxygen, the heat will break down the molecules of oil into water vapor and carbon "gunk" rather than into synthesis gas. A similar situation applies to crystals of sugar.

The new process quickly vaporizes the oil and sugar and exposes them to extreme heat. There's no time for carbon gunk to form because oxygen in the air snatches the carbon atoms and transforms them into carbon monoxide. It's over in one-hundredth of a second, potentially 100 times faster than current means of making synthesis gas and hydrogen.

"What Lanny does is sorcery," says Frank Bates, head of the chemical engineering and materials science department. "This is classic Minnesota chemical engineering in the tradition of understanding how to steer chemical reactions to get more of the products you want and less of those you don't."

"We need radically new technologies on the road to renewable fuels. This is a possibility," says Schmidt. "We need a lot of research like this to make renewable technologies work."


More information:

J. R. Salge, B. J. Dreyer, P. J. Dauenhauer, L. D. Schmidt, Renewable Hydrogen from Nonvolatile Fuels by Reactive Flash Volatilization, Science, Nov. 3, 2006

Scientific American: Biofuels Discovery Promises to End Dependence on Natural Gas - November 03, 2006

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