Washington DC (SPX) Feb 17, 2011
Chemists, chemical engineers, and synthetic biologists have largely met the technical challenge of developing biofuels to supplement and then replace petroleum-derived transportation fuels in the coming decades. For biofuels to reach the U.S. market, however, these technologies have to fit into the existing transportation fuel infrastructure. Every major chemical and petrochemical firm has claimed a stake in the race to biofuel commercialization, as have dozens of start-up companies.
Biofuels have multiple starting points, including sugars, starches, vegetable oil, recycled paper and cardboard, and raw biomass, which can be processed by biological or chemical methods, or both. Whichever ones win, the competing technologies' versatility ensures that companies will make money, and the country will gain energy security by eliminating dependence on imported oil, as well as climate security by reducing greenhouse gas emissions.
"We all know how to get from the beginning to the end and make biofuels-we've all done it," says James A. Dumesic, a chemical engineer at the University of Wisconsin, Madison. "What you would like to do is put raw biomass in one end and get a ready-to-use fuel out the other end, using as few steps and engineering unit operations as possible. Now, we are to try to get the costs down so it can be affordable. The winning processes, whatever they will be, will need to be as light as possible on the capital investment in order to be practical. Everyone is looking to develop processes that can compete without subsidies."
"Because the energy industry is so large, there is room for everybody to play, as long as you can meet the economics," says Jay D. Keasling, a synthetic biologist at the University of California, Berkeley. "That is the great thing about this problem. Chemical technologies can be engineered to happen more quickly. It does take a long time to engineer the biology. But the beauty of biology is that it can work under dirtier conditions, and you can get the specific molecule you want under a range of conditions."
Synthetic biology seemed to have the early edge in the race to the pump. But despite success in ethanol production, synthetic biology's limitations-the primary products are alcohols, not alkanes typical of transportation fuels, and fermentation processes are slow-have stalled progress. That has created an opening for chemical technologies.
"Chemical approaches offer plenty of advantages," says Mark Mascal, a chemistry professor at UC Davis whose group is working on several biofuel projects. "Generally, if you have an inexpensive catalyst and a fast method, a chemical approach can be more cost-effective and doesn't take a few days or a week the way most fermentation processes do," he notes. "A consistent feedstock isn't needed as is the case with microbes in sugar fermentation-you can use anything as long as it has sugar or cellulose in it."
Chemical methods offer a broader platform from which to operate, Mascal says, "because chemically manipulating carbohydrates, as opposed to fermenting sugars, allows you to make alcohols, esters, and furans from a single starting point that can be used to make different types of transportation fuels."
One of the primary pathways to bio-fuels is aqueous-phase chemistry. Mascal's group has developed a biphasic acid/solvent reactor to make substituted furans from a cellulosic feedstock in a single step, which eliminates the need to first pretreat or break down the biomass, a step normally required for solution-phase chemistry. The researchers use a hydrochloric acid solution to digest the cellulosic starting material, continuously extracting the reaction mixture with dichloroethane to obtain the substituted furan 5-(chloromethyl)furfural, a biofuel intermediate.
The team has evolved the process to convert biomass crops such as grasses or waste biomass such as corn stover, wood, straw, and recycled paper into either 5-(chloromethyl)furfural or another biofuel intermediate, levulinic acid, depending on reaction conditions, in yields up to 95% (Green Chem., DOI: 10.1039/b918922j). "To our knowledge, this level of conversion of carbohydrate feedstocks into simple organic molecules is unrivaled," Mascal says.
As an added benefit, the single-reactor processing doesn't give off any carbon dioxide like most biofuel technologies do, he notes. A key scale-up issue for Mascal is the low efficiency and poor carbon economy of most biofuel processes, which spells poor economics and contradicts the carbon-neutral goal of biofuels.
Microorganisms readily convert glucose into ethanol, but inefficiently because one-third of the available carbon ends up as CO2, Mascal notes. Plus, there's a variety of five- and six-carbon sugars in the cellulose and hemicellulose polysaccharides that make up biomass, but yeasts typically used in fermentation consume only the six-carbon sugars. The microbes also work more slowly than an industrial chemical process and can't tolerate the high levels of ethanol they produce, which limits batch processing.
For producers of biofuels other than ethanol, a significant portion of the carbon is also lost as CO2, compromising the yield of hydrocarbons, Mascal says. For these reasons, he thinks his single-reactor route to furans and others like it have an edge.
One drawback, however, is the halogenated solvent, which might have to be replaced in an industrial-scale process.
When derivatized, the furfural or levulinic acid leads to other furans or levulinate esters that can be used as stand-alone fuels-which would require regulatory approval-or more likely as blend stocks to make traditional gasoline, diesel fuel, or jet fuel. Mascal is exploring opportunities with potential commercial partners-there's no shortage of interested capital investors knocking on his door-and he has been collaborating with Nevada-based Bently Biofuels to test some biodiesel candidates.
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