Biofuels: The Sequel — The Science Behind Second Generation Biofuels

July 15, 2009


Making fuel from cellulose? Will it be the fuel of the future and how is the U.S. system promoting or haring it?

New political carrots and sticks are leading the biofuels industry into a second generation phase, and many critics of the biofuels industry think it's long overdue. The question for the industry, though, is whether the science is ready to scale up.

Among the carrots is a USDA budget request for $1.1 billion in funding to support development of advanced bio-refineries and other stimulus funding for renewable energy, along with about $384 million previously allocated in 2007.

The sticks include the low carbon fuel standards passed by the California Air Resources Board in April (See C02 sidebar) and possible new EPA limits on carbon emissions.

At least eight major cellulosic biofuels plants are in production or under construction in the U.S. and Canada. (See industry sidebar)

So, it's now or never for cellulosic biofuels — the "fuel of the future" for almost a century, and long seen as the only way to replace petroleum in a liquid fuel system.

"We think, ultimately, cellulosic materials are the only materials where you can produce enough under environmentally sustainable conditions," said Chris Somerville, director of the Energy Biosciences Institute at the University of California at Berkeley at the 2008 Society of Environmental Journalists conference.

But which cellulosic materials, how are they to be harvested and processed, and what fuels will come out the other side of the pipeline?


Materials (feedstocks)
Energy crops – trees (eg, hybrid poplars); perennial field crops (miscanthus and switchgrass). Advantages: Higher yield per acre than corn, lower carbon footprint, lower collection costs, more predictable components better suited to enzymatic processes.

Waste — paper from garbage, leftover crop residues (corn husks, rice hulls) and timbering waste. Lower unit costs but higher collection costs, sometimes better suited to acid or pyrolysis processes.

Aquatic and marine biomass — Algae and kelp — Higher photosynthetic efficiency, less energy needed to break down cellulose, unlimited resource availability; disadvantage is purity, dewatering and collection costs. Potential for third generation biofuels and oils that would need less processing.


Pretreatment — Often a combination of pressure, acid and agitation separates cellulose from other components of biomass, such as lignin (glue) and hemicellulose (five-carbon sugars) and makes it more accessible.

Acid — Cheap, well-known and used for centuries to break wood down into materials for paper. Disadvantages include environmental impacts and lower process efficiencies. For instance, newsprint is cellulose with a high lignin content, which is why it is cheap and ages quickly. Higher purity cellulose paper is used by artists and by book publishers for longevity. The product is then fermented to ethanol or other biofuels (eg butanol).

Enzyme — Better process efficiencies, lower environmental impacts, but higher engineering standards needed. Some pilot plants have lost one batch in three due to contamination in the process. Fermentation to ethanol also required.

Gasification (pyrolysis) — Applying heat to biomass in a closed chamber results in a release of gas. The gas can then be processed and fermented using varieties of clostridium bacteria or it can be reformed in the presence of a catalyst. One advantage of reformed gas is the increased variety of fuels that can be made.

History and background

The idea that cellulose would be the foundation for replacing petroleum was championed by Henry Ford, Isaac Asimov, and even, 90 years ago, by the scientist who founded the Cellulose Chemistry division of the American Chemical Society — Harold Hibbert.

"It looks as if in the rather near future, this country will be under the necessity of paying out vast sums yearly in order to obtain supplies of crude oil from Mexico, Russia and Persia," Hibbert said in a 1921 journal article. "It is believed, however, that the chemist is capable of solving this difficult problem ... (and) it would seem that cellulose in one form or another is capable of filling that role."

In 1925, Henry Ford told reporters: "The fuel of the future is going to come from fruit like that sumac out by the road, or from apples, weeds, sawdust — almost anything." Ford's optimism about cellulosic biofuels was unusual for the auto, oil and chemical industries, which had all placed their bets on leaded gasoline and foreign oil.

Of course, cellulose processes were (and still are) important for paper and chemicals such as celluloid and rayon. During the early 20th century, the acid process was improved to allow a greater variety of woody feedstock such as southern pine. Paper mills of this era were well known for billowing clouds of foulsmelling pollutants, although steam and pressure pulping eventually reduced costs to the environment.

The idea of turning cellulose into renewable fuels remained attractive, and science writers followed it over the years. In 1940, for example, New York Times science writer William L. Laurence wrote about Ernst Berl, a Jewish scientist who left Germany to work at Carnegie Institute. Berl developed a pressurizing process for reducing cellulose from all kinds of plant materials to either liquid or solid biofuels.

Berl's work "assures mankind of an illimitable supply of the prime movers of the wheels of civilization for all time, after natural deposits have been exhausted," Laurence said.

The idea was compelling, especially in light of the possible exhaustion of coal and oil reserves which, even in the 1940s, had long been a concern for scientists and policy makers.

Another WWII era development was the discovery of a voracious cellulose-eating fungus in the remote jungles of the Pacific. Soldiers called it "jungle rot," because the fungus was turning their cotton clothing into sugar. Polyester clothing solved the problem, but Elwyn T. Reese and other Army chemists recognized a key to one of the great longstanding problems of science: How to efficiently split the strong bond that holds molecules of glucose together to form cellulose. Although it was possible to produce fuel as a side-stream at paper mills, an enzymatic process could make fuel cheaper, many believed.

In the 1970s, Reese and others told congressional committees that they could produce fuel from cellulose at low cost, and without affecting food supplies, but they were unable to attract much research support as grain-state and oil-state politicians fought for control of energy markets.

Reese's optimism notwithstanding, cellulosic biofuels are an enormously complex area of biochemical engineering. Researchers in hundreds of university and government labs have taken decades to create an industry that is nearly commercial — isolating, characterizing and testing the complex chemical structures of plants, and working on cascading systems of enzyme reactions. One of the scientists intrigued with Reese and his discoveries was Patrick Foody, who founded Iogen Corp. in 1974. The company now has a commercial scale enzyme biorefinery under construction in Saskatchewan.

Science fiction writer Isaac Asimov found all this fascinating. "Cellulose can be broken down into glucose molecules," Asimov said in a 1986 article, "and the glucose solution can be fermented into alcohol ... (and) used as a liquid fuel." The advantage? "Cellulose is self-renewing if we are careful to conserve our forests, so the fuel we get from it could last indefinitely, whereas oil from the ground must be completely used up eventually." Yet Asimov found it hard to resist the science fiction notion that we need to beware of mutant microbes that might get outside their tanks and dissolve the forests.

Low oil prices in the 1980s dissolved political support for second-generation biofuels research, but higher energy costs and the need for non-toxic octane-boosting gasoline additives in the 1990s launched the grain ethanol industry. Questions about the energy efficiency and carbon footprint of grain ethanol kept high interest in second-generation biofuels.

One milestone was the 2005 "billion-ton" biomass study at Oak Ridge National Labs. Waste wood, switchgrass and other cellulose sources amounted to 1.3 billion tons, which could replace at least 30 percent of U.S. petroleum, the study said. The billion-ton study changed the federal government's approach to energy, but there are concerns about the use of Conservation Reserve Program land, about increased forestry, and other impacts from intensified biomass harvesting.

Research today on switchgrass and miscanthus shows high potential — more than 1,000 gallons of biofuel per acre, as op posed to hundreds of gallons of ethanol per acre with corn. Among researchers working on energy crops are Ken Vogel at the University of Nebraska, David Bransby at Auburn, Stephen Long at the University of Illinois, John Sheehan of the National Renewable Energy Laboratory (NREL), and Chris Somerville of the University of California at Berkeley.

One flaw of cellulose crops according to Somerville, is that they require high energy processing to break down cellulose into glucose, ferment the glucose, and then distill the ethanol. Envisioning a third generation of biofuels, Somerville says more research is needed on plants that produce oils and fuel-like substances that would be very close to gasoline and diesel, and consequently need less energy to refine. Another issue involving cellulosic biomass came up with research by Timothy Searchinger, published in 2008, that indicated CO2 releases from converting forests or pastures to cropland are significant. The carbon intensity of various crops was considered in recent carbon standards issued by the California Air Resources Board.

Several interesting efforts to dramatically broaden the resource base using aquatic and marine organisms are under way. One company (Algenol) is hoping to make ethanol efficiently from algae in fresh water situations where lots of carbon dioxide gas is available.

Two marine research efforts involve cellulose from kelp at the University of Costa Rica and the Scottish Association for Marine Sciences.

As it turns out, this too is nothing new. As early as 1918, the Pasteur Institute was reporting in Scientific American that it had been able to distill about 10 gallons of fuel ethanol per ton of seaweed.

Bill Kovarik, an SEJ board member, is working on The Summer Spirit, a book about the history of renewable energy.

** From SEJ's quarterly newsletter SEJournal Summer 2009 issue



Enzyme process — Combinations of mild acid and pressure pre-treat the plant material, then enzymes break cellulose down into glucose, and then ferment the glucose into ethanol or other chemicals.

  • POET – 20,000 gal/yr— Scotland, S.D. Enzyme process. Operating, will lead to 25 million gallons per year commercial facility in Emmetsburg, Iowa, making ethanol from corn cobs and stalks in tandem with a standard grain ethanol plant.
  • ABENGOA Bioenergy — Hugoton, Kan. Enzyme process. Wheat straw. Starting construction in 2010, in production by 2011.
  • IOGEN — Ottawa, Canada — Enzyme process. One of the earliest firms to work on the cellulosic enzyme process, Iogen declined a partial US-funded deal and is working with a start-up plant in Canada.
  • DUPONT DANISCO — Vonore, Tenn. — Under construction, plant will use switchgrass and enzyme processing.
  • VERENIUM - Jennings, La.— 1.4 million gallon demonstration- scale plant / waste biomass sugarcane. Technology Review. 

    Advanced enzyme process — Along with enzyme breakdown of cellulose into glucose, a chain of enzymes can produce a variety of products, not just ethanol.

  • MASCOMA - Rome, N.Y. — Began in February 2009 with capacity of 200,000 gallons of cellulose ethanol, gasoline or other chemicals from wood chips, grasses, corn and sugar cane residues. An affiliate is developing a commercial-scale facility in Kinross, Mich.

    Concentrated acid process— Strong sulfuric acid is added to dried biomass, heated and then separated under pressure. This is very similar to the way cellulose is separated for paper.

  • BLUEFIRE Ethanol — Irvine, Calif. Acid process. Garbage, wood waste, ag residues — Still hung up on siting. 

    Synthesis gas — Heat and pressure are applied and biomass is turned into biogas — hydrogen and carbon dioxide streams — that are then re-combined in the presence of catalysts to create different kinds of fuels or chemicals.

  • RANGE BIOFUELS — Soperton, Ga. Pyrolysis to synthesis gas (syngas) using heat, pressure and steam, and catalytic treatments. Under construction. First 20 million gallon phase by March 2010.