The Biofuel Future

Scientists seek ways to make green energy pay off

Biofuels are liquid energy Version 2.0. Unlike their fossil fuel counterparts — the cadaverous remains of plants that died hundreds of millions of years ago — biofuels come from vegetation grown in the here and now. So they should offer a carbon-neutral energy source: Plants that become biofuels ideally consume more carbon dioxide during photosynthesis than they emit when processed and burned for power. Biofuels make fossil fuels seem so last century, so quaintly carboniferous.

And these new liquid fuels promise more than just carbon correctness. They offer a renewable, home-grown energy source, reducing the need for foreign oil. They present ways to heal an agricultural landscape hobbled by intensive fertilizer use. Biofuels could even help clean waterways, reduce air pollution, enhance wildlife habitats and increase biodiversity.

Yet in many respects, biofuels are in their beta version. For any of a number of promising feedstocks — the raw materials from which biofuels are made — there are logistics to be worked out, such as how to best shred the original material and ship the finished product. There is also lab work — for example, refining the processes for busting apart plant cell walls to release the useful sugars inside. And there is math. A lot of math.

The only way that biofuels will add up is if they produce more energy than it takes to make them. Yet, depending on the crops and the logistics of production, some analyses suggest that it may take more energy to make these fuels than they will provide. And if growing biofuels creates the same environmental problems that plague much of large-scale agriculture, then air and water quality might not really improve. Prized ecosystems such as rain forests, wetlands and savannas could be destroyed to grow crops. Biofuels done badly, scientists say, could go very, very wrong.

“Business as usual writ larger is not an environmentally welcome outcome,” states a biofuels policy paper authored by more than 20 scientists and published in Science last October.

Many scientists have expressed concern that political support for the biofuels industry has outpaced rigorous analyses of the fuels’ potential impacts. Others see this notion as manure. Research needed to resolve that disagreement is now underway, as scientists in industry, national labs and universities across the country are assessing every aspect of these fuels, from field to tailpipe. Researchers are growing crops, evaluating yields and comparing harvesting techniques. Computer models are providing stats on each crop’s effect on environmental factors such as soil nutrients and erosion. The plant cell wall is under attack from several angles. And chemists and microbiologists are cajoling an expanding menagerie of microorganisms into producing higher fuel yields.

THE BIOFUEL FUTURE Perennial plants, like this Miscanthus giganteus in a University of Illinois test plot, could replace corn as a source of plant-based liquid fuel. S. Long/Univ. of Illinois; USDA-NRCS PLANTS Database

BIOMASS BENEFITS | Greenhouse gas emissions drop, and air and water quality improve, when switchgrass and forest residues from logging replace corn as a raw material for fuel, suggests a recent life cycle analysis. The chart shows the improvement relative to corn for these two next-generation biofuel hopefuls. Williams et al./Environmental Science & Technology 2009

POWER FROM PLANTS | Scientists are studying biofuels from the field to the pump to make each step more efficient and environmentally friendly. Here’s a typical blueprint for ethanol production. Newhouse Design

Green goals

Ideally, high biofuel yields come with minimal environmental baggage and maximum efficiency at every step. The raw materials for these fuels run the gamut from corn to municipal waste to algae, and each has its own benefits and headaches. To make fuels, researchers must first process the raw material to create fermentable sugars or a crude oil-like liquid. Further refinement yields fuels such as ethanol, butanol, jet fuel or biodiesel.

In some cases, such as algae-based biodiesel, the technologies are far from mature. Squeezing ethanol from crops such as corn, on the other hand, uses a technology as old as whiskey. An infrastructure already exists for growing and moving grain, and distillation and fermentation techniques work at large scales.

But grain-based fuels raise several environmental issues, such as emissions of the potent greenhouse gas nitrous oxide from heavy fertilizer use. So, many scientists see corn ethanol as a bridging technology for use until the next-generation feedstocks fulfill biofuels’ real promise. Nonfood plants rich in cellulose or even residual waste diverted from landfills may define the biofuel future.

Several studies attest to the benefits of fuels made from such feedstocks, although the degree of benefit varies depending on what factors are included in the analysis. Overall, dedicated energy crops such as switchgrass and waste residues from sources like commercial logging fare better than corn-based ethanol, concludes a recent modeling analysis and literature review citing more than 100 papers. Published online May 27 in Environmental Science & Technology, the analysis reports that municipal waste-based ethanol production emits an estimated 60 to 80 percent less greenhouse gas than corn-based ethanol production. Dedicated energy crops, especially when grown on marginal land, also fare better than corn in terms of greenhouse gas emissions, and require less water and generate less air pollution, report researchers from the National Renewable Energy Laboratory in Golden, Colo., and E Risk Sciences in Boulder, Colo.

Research also suggests that these new fuels will be priced competitively with gasoline from petroleum. A new assessment coauthored by Lee Lynd, head scientist and cofounder of the Boston-based ethanol start-up Mascoma Corp., found that the production costs of cellulose-based ethanol, when made on a commercial scale, could be competitive with gasoline at oil prices of $30 or more per barrel.

Both of these recent big-picture studies, while optimistic, call for continued research to improve existing production processes and better define each fuel’s associated trade-offs.

Such research is in progress at the Idaho National Laboratory in Idaho Falls, where scientists David Muth Jr. and Thomas Ulrich take part in a coordinated, national effort to watch grass grow. In partnership with scientists at Oak Ridge National Laboratory in Tennessee and at several universities, Muth and Ulrich are keeping track of more than 50 field trials of various feedstocks across the country. The researchers are growing switchgrass and Miscanthus, an 11-foot tall perennial grass. Energy cane, an über-biomass relative of sugar cane, is also under study.

The research suggests that there is not one silver bullet source for biofuels. While there are some generally desirable plant characteristics — such as needing few nutrients and flourishing on degraded land — the future biofuels landscape will likely be a patchwork of different sources that work best in different regions.

“What’s emerging pretty quickly is how site-specific both the production systems and problems are,” says Muth.

Energy cane, for example, has “huge yields, but it is a water sink,” he says. So it may be best for water-rich Gulf Coast states. Miscanthus, which has been tested in Europe for several years, produces very high yields and has the genes to withstand cold climates.

Part of biofuels’ allure lies in the variety of ingredients from which the fuels may be spun. The Idaho National Lab is also investigating strains of algae that pump out oils as a raw material for biodiesel. At other sites agricultural and municipal waste, such as straw stalks, corn cobs and tree cuttings, are under investigation. Some researchers are focused on crops dedicated to energy, such as prairie grasses, and fast-growing softwoods, such as willow, poplar and eucalyptus. A pilot-scale system for growing the diminutive pond plant duckweed on wastewater is underway at North Carolina State University.

In Idaho, Muth is also using several computer models to calculate the effect that growing and removing the feedstocks has on factors such as soil’s nutrients, carbon and water content. 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 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 (hemi-cellulose) 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 cellulosic fuel production, Lynd and several other researchers conclude in a February 2008 commentary in Nature Biotechnology.

Lignin is typically removed after pretreatment and then burned in the refinery’s boiler, replacing some fossil fuel use. The remaining plant matter is then broken into simple sugars, typically by a cocktail of microbial enzymes known as cellulases. Other microbes are then called in to ferment the sugars into ethanol.

Breaking down cellulose with enzymes is usually a separate step from fermentation — and a very costly one. But recent attempts to combine the conversion of cellulose to sugars with the conversion of sugars to fuel — called consolidated bioprocessing — have been successful. A strain of the soil-dwelling bacterium Clostridium phytofermentans will happily munch biomass such as wood pulp waste and will ferment it into ethanol. That discovery, by microbiologist Susan Leschine of the University of Massachusetts Amherst, led to the development of Qteros, a cellulosic-ethanol start-up in Marlborough, Mass. And in May, Mascoma researchers reported the engineering of a yeast and the bacterium Clostridium thermocellum to produce cellulases and ethanol in a single step.

At the San Francisco conference, posters reported on investigations of even more enzymes from various sources: bacteria that live in the deep sea, penicillin, diseased sea squirts, the bread mold Neurospora, a yeast that grows on wood-boring beetles and soil microbes from a Puerto Rican rainforest. Scientists are also fighting recalcitrance from the inside out by breeding lines of low-lignin plants.

Of course, getting a lot of ethanol in a benchtop flask is one thing. Scaling up to a silo-sized bioreactor is another. Industrial models exist — such as wringing pulp from trees for the paper industry or mass-producing cornstarch. “But we haven’t done it with cellulose yet,” says McMillan.

More than a dozen pilot plants for producing cellulosic ethanol are under construction and a handful are operating, with 2011 seen as the year for cellulosic technologies to walk the walk. The group at Idaho National Lab hopes to be able to demonstrate a system from field to refinery by autumn of 2010.

Environmental cost

Yet concerns remain that the environmental side of the biofuels equation is still not worked out. Some argue that the numbers are too fuzzy to proceed with confidence that environmental burdens and benefits have been fully considered.

“There are people who say we don’t have enough knowledge to move forward — to some extent that is true,” says Michigan State University’s Philip Robertson, coauthor of the Science policy paper. “But we do know a lot about sustainability — enough to implement logical science-based standards.” This includes things like the strategic use of cover crops, fertilizer and tilling.

There is also the consideration of land-use changes — if forests are cleared for biofuels production, far more carbon will be released than is saved by the nonpetroleum fuels, several studies suggest. Such findings have led to scrutiny that has stung many in the industry who argue that biofuels are being held to a much higher standard than fossil fuels. If the petroleum isn’t “charged” for the greenhouse gas emissions of the U.S. military keeping supply lanes open in the Persian Gulf, why should emissions from cleared forests be included in the biofuels ledger? asks Bruce Dale of Michigan State University in a recent editorial in the journal Biofuels, Bioproducts & Biorefining.

Congress is now considering legislation that may determine whether indirect land use can or cannot be a mark on the ruler used by the U.S. Environmental Protection Agency to measure biofuels’ impacts. Eventually, many researchers hope, a more detailed picture will emerge of the benefits and costs across all stages of the life cycles of fossil and next-generation fuels.

“Some really interesting services are going to emerge from these crops,” says Muth, of the Idaho National Lab. Some biofuel plants help sequester carbon in the soil, for example. A 2002 analysis reported that by the second or third planting year, switchgrass plots experience far less soil erosion than annual crops such as corn. Species that do well near wetlands can act as filters, preventing nitrates and phosphates from getting into the water, Muth says. “If there is a value on carbon sequestration … a value on clean water, there may be economic benefits for a lot of these crops.”

Robertson adds, “If certain practices were being promoted with incentives, it would ensure that we have a biofuels industry that is sustainable with a net benefit, not a cost. We don’t have that yet — I say ‘yet’ hopefully.”

With appropriate carrots and sticks, biofuels could play a big role in the energy portfolio of the future. There may even be a day when, Back to the Future style, garbage can be thrown into a personal-sized bioreactor that yields fuel. (Trash biomass in the form of sugar beet pulp, tomato pomace, cashew apple, grape pomace, sweet gum and coffee pulp are all being investigated.) Several lines of research are investigating biofuel “coproducts,” high-value molecules that can be extracted during processing, such as proteins for animal feed or aromatics for perfumes and drugs. These products will also bring the net costs of these fuels down, one of several variables that can help the biofuels math add up to success as a fossil fuel substitute.

“It’s difficult to compare the costs of not changing with the costs of changing,” Lynd said at the May meeting in San Francisco. “Asking is this or that realistic is well-intentioned, but all solutions involve changes — we don’t have an option. Business as usual? Well, we think of it as a baseline, but it is a fantasy — even if you don’t care about carbon — just as a supply issue. Fossil fuels will all be gone. They’ll all be gone.”

A researcher examining a sample of algal oil.
A researcher examining a sample of algal oil.
Running on algae

Pond scum gets a bad rap. But microalgae — tiny, single-celled aquatic organisms — 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 backbones — 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 flexible 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.

From the Nature Index

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