Science News - August 1, 2009

This information, along with yields and quality of plant material, is all being entered into a database to help predict which plants will grow best where.

Biomass breakdown

Bioenergy is not just about growing crops up, though. It’s even more about tearing them down. Biomass must be harvested from the field or forest, perhaps stored, and then shipped to a refinery for processing. Harvesting equipment, travel distances and processing methods must all be considered to determine whether biofuels make economic and energy sense.

“What is becoming a bigger and bigger issue to people is the logistics of it all—that’s becoming a barrier to the whole thing,” says J. Richard Hess, the technology manager of the Idaho National Lab program.

An essential part of biofuel logistics is the preprocessing of plants — cutting, baling and hauling the bales somewhere for storage before transporting them to a refinery. Those preprocessing steps pose problems with a material that isn’t very dense or evenly shaped. “It’s like moving air or feathers,” Hess says.

Ideally, preprocessing would provide an end product that is uniform and easy to handle, like grain — the biomass equivalent of crude oil. “We’re not aiming for a certain size, but a certain density that’s easy to ship, is flowable,” says INL’s Christopher Wright.

Wright and Neal Yancey, also of INL, are trying to achieve the optimal density by finding the right balance of shredding and compacting, ultimately producing something like the alfalfa pellets fed to pet rabbits, or perhaps Matchbox car– sized blocks. This crude can then be shipped to a refinery to be heated into an oil-like liquid or broken down by enzymes into the desired fuel.

Breaking biomass down into fuel is no small task. The dominant method is known as biochemical conversion: processes that use heat, chemicals or enzymes to turn the biomass into sugars that can be fermented by microbes such as yeast into ethanol. This ethanol

Running on algae

Pond scum gets a bad rap. but microalgae —tiny, single-celled aquatic organ-
isms — are rising stars in the renewable energy sector. they can provide
oil that can be turned into liquid fuels such as biodiesel and jet fuel.
algal oil is mostly triacylglycerides —long fatty acid chains with glycerol back-
bones —that can be converted to diesel and other fuels in relatively few steps.
algae’s potential lies in their speedy growth rate, efficient photosynthesis and flex-
ible habitat preferences. Many strains can grow in saltwater or wastewater from
treatment plants. in open ponds or closed bioreactors, the microorganisms can
potentially make more than 50 times as much oil as land plants on the same area.
this potential fuel has a long history. in 1978 the Department of energy
launched the aquatic Species Program to develop fuels from algae, but the
program was shut down in 1996. in the intervening years, more than 3,000
strains were investigated, included species from yellowstone national Park’s
hot springs and the Caribbean Sea.
now algae research is surging once again in
both the private and public sectors. Problems still
loom, including how to best extract the oil, scale
up algae farms and control contamination by
unwanted strains or tiny critters like rotifers that
graze on the algal crop. but in June the algae-
to-ethanol company algenol biofuels announced
plans for a pilot plant with Dow Chemical Co.
in freeport, texas. and in January, Continental
airlines conducted a 90-minute test flight of a
boeing 737 fueled in part by a blend derived from
algae and Jatropha plants. Prospects for fuel
from pond scum are starting to look up. — R.E.

is the same whether its origins are corn or other biomass. But it is currently a lot easier to get the fermentable sugars out of a starchy corn kernel than from something like wood chips or a weedy grass.

Plant cell walls are about 75 percent complex sugars, but getting at these sugars is a bit like trying to get the mortar and minerals out of a castle’s rampart. Cell walls, one of the defining features of plants as a life-form, were made to resist degradation. Even termites and cows need special microbes in their guts to get the job done.

That’s because those sugars are embedded in a complex architectural structure called lignocellulose — cellulose (long, unbranched chains of glucose) embedded in a matrix of more sugars ( hemicellulose) embedded in the tough, gluelike lignin. (Biofuels researchers refer to the “recalcitrance” of the cell wall, as if it were an obstinate child.) Not only did cell walls evolve for strength, they also

are a primary defense against microbial attack, and critters that are up to the task aren’t common.

“Lignin is a highly problematic polymer from the point of view of processing, but an exemplary evolutionary achievement,” researchers at the University of York in England commented in May 2008 in New Phytologist.

To prep for the cell wall attack, plant matter is usually pretreated: the shredded, chopped or pelletized biomass is typically mixed with dilute acids or ammonia. At a biofuels symposium held in May in San Francisco, scientists presented work describing pretreatment with proton beam irradiation, steam explosion and microwave reactors. Ionic liquids — basically liquid salts — are also under investigation.

“Cellulose doesn’t liquefy in minutes to hours — it’s hours to days,” says Jim McMillan of the national lab in Golden. This step is the main bottleneck in cel-