strains of ethanol-excreting bacteria that Ralph Tanner, a microbiologist at the University of
Oklahoma, had discovered years before in the oxygen-free sediments of a swamp. These
anaerobic bugs make ethanol by voraciously consuming syngas.
F
The “heart and soul of the Coskata process,” as Tobey puts it, is the bioreactor in which the
bacteria live. “Rather than searching for food in the fermentation mash in a large tank, our
bacteria wait for the gas to be delivered to them,” he explains. The firm relies on plastic tubes,
the filter-fabric straws as thin as human hair. The syngas flows through the straws, and water
is pumped across their exteriors. The gases diffuse across the selective membrane to the
bacteria embedded in the outer surface of the tubes, which permits no water inside. “We get an
efficient mass transfer with the tubes, which is not easy,” Tobey says. “Our data suggest that
in an optimal setting we could get 90 percent of the energy value of the gases into our fuel.”
After the bugs eat the gases, they release ethanol into the surrounding water. Standard
distillation or filtration techniques could extract the alcohol from the water.
G
Coskata researchers estimate that their commercialized process could deliver ethanol at under
$1 per gallon-less than half of today’s $2-per-gallon wholesale price, Tobey claims. Outside
evaluators of Argonne National Laboratory measured the input-output “energy balance” of the
Coskata process and found that, optimally, it can produce 7.7 times as much energy in the end
product as it takes to make it.
H
The company plans to construct a 40,000-gallon-a-year pilot plant near the GM test track in
Milford, Mich., by the end of this year and hopes to build a full-scale, 100-million-gallon-a-year
plant by 2011. Coskata may have some company by then; Bioengineering Resources in
Fayetteville, Ark., is already developing what seems to be a similar three-step pathway in
which syngas is consumed by bacteria isolated by James Gaddy, a retired chemical engineer at
the University of Arkansas. Considering the advances in these and other methods, plant
cellulose could provide the greener ethanol everyone wants.
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