Editor's Note: George W. Huber and Bruce Dale are chemical engineers at the University of Massachusetts Amherst and Michigan State University, respectively.
In this article, a rough draft of which appears below, Huber and Dale point out that biofuels remain one of the most technically promising alternatives to oil. The key will be learning to convert cellulosic biomass (like stalks and stems, and unlike edible cereal corn, which is noncellulosic) into fuel. Please help us edit the following piece by suggesting factors the researchers may have overlooked or refinements to their argument.
Here are some questions to get started:
What is your reaction to their assertion that "huge amounts of cellulosic biomass can be sustainably harvested to produce fuel"?
What do you think are the most promising avenues of exploration in figuring out how to deconstruct cellulosic material?
What other viable biofuel manufacturing processes might Huber and Dale want to consider?
Do Huber and Dale present a persuasive case that cellulosic biofuels are the most technically promising alternative to oil?
What is your reaction to their assertion that the "raw feedstocks that go into making the biofuel are far less expensive than raw crude?"
How do you think a "move toward 'grassoline'" would "fundamentally change the world"?
Your feedback will be considered by the writers and editors as they complete the final draft of this article, which will appear in an upcoming edition of Scientific American magazine.
By now it ought to be clear that we must get off oil. We can no longer afford the dangers that our overwhelming dependence on petroleum poses for our national security, our economic security or our environmental security. Yet civilization is not about to not stop moving, and so we must develop a new way to power the world’s transportation fleet. Biofuels, or liquid fuels made from plant material, remain the most technically promising alternative.
Biofuels can be made from anything that is, or ever was, a plant. First-generation biofuels are made from edible biomass such as corn or sugarcane. Although we already possess the technology to convert these feedstocks into fuels (as evidenced by the nearly 200 refineries currently processing corn into ethanol in the U.S.), there is simply not enough corn, sugar cane or vegetable oil to provide more than about 10 percent of the liquid fuel needs of developed countries such as ours. These first generation biofuels also compete for farmland with crops used for human food and animal feed, which complicates the calculations of the environmental costs and benefits associated with them. We need biofuel raw materials that are cheap, abundant and that do not interfere with food production.
The winner in all three categories is cellulosic biomass—woods, grasses and inedible stalks of plants. Fuel made out of this biomass—what we’ll call “grassoline”—could come from dozens, if not hundreds, of potential sources, from wood residues such as sawdust and construction debris, to agricultural wastes such as corn stalks, to “energy crops”—fast-growing grasses and woody materials that are grown expressly for their energy content.
Huge amounts of cellulosic biomass can be sustainably harvested to produce fuel. According to an upcoming study by the U.S. Department of Agriculture and Department of Energy, the U.S. can produce at least 1.3 billion dry tons of cellulosic biomass every year, and all without decreasing the amount of biomass available for our food, animal feed or exports. This much biomass could produce more than 100 billion gallons per year of grassoline, or about half the current annual consumption of gasoline and diesel in the U.S. Similar projections estimate that the global supply of cellulosic biomass has an energy content equivalent to between 34 billion to 160 billion barrels of oil per year, numbers that exceed the world’s current annual consumption of 30 billion barrels of oil. And unlike biofuel made from corn, cellulosic biomass can be converted to any type of fuel—ethanol, ordinary gasoline, diesel or even jet fuel.
Yet growing the cellulosic biomass isn’t the problem (or, at least, isn’t yet the problem—as the industry grows, agricultural techniques will become increasingly important). Because cellulose is so strong, it can be very difficult to extract the energy stored within. Many approaches have been attempted, but only now are scientists and engineers beginning to understand how we might scale grassoline production up to industrial levels. Successfully doing so may be the most important technical and environmental challenge our civilization now faces.
The Energy Lock
Cellulosic biomass has been designed by evolution to give structure to a plant. It features rigid scaffolds of interlocking molecules that provide support for vertical growth. It also stubbornly resists biological breakdown. Scientists must find a way to defeat nature’s highly effective design.
On the molecular level, biomass consists largely of carbon, hydrogen and oxygen stored in the plant cell wall. Liquid fuels are made of carbon and hydrogen. Thus, from a chemical engineering perspective, refining biomass requires a simple removal of most of the oxygen molecules from the biomass feedstock. The remaining carbon and hydrogen (with some remaining oxygen) become the various fuel products. One of the huge advantages of grassoline—as opposed to first generation biofuels—is that grassoline isn’t synonymous with ethanol. You can make anything that’s found in crude oil from cellulosic biomass.
The general strategy that scientists use to create fuels from biomass involves first deconstructing the solid biomass into smaller molecules, then refining these products into a fuel. Low-temperature deconstruction (50 degrees C to 200 degrees C) produces sugars can be fermented into ethanol in much the same way that grain or sugar crops are. Deconstruction at higher temperatures (400 degrees C to 600 degrees C) produces a bio-crude or bio-oil, which can be converted into gasoline or diesel. Extremely high temperature deconstruction (700 degrees C to 1,000 degrees C) produces a gas called syn-gas, which can be converted into fuel by 80-year-old syn-gas conversion technologies.
The “best” pathway is one that converts the maximum amount of the biomass energy into a liquid biofuel at the lowest costs. We don’t know yet which pathway(s) will be the most economical. It may be that different pathways will be used on different cellulosic biomass materials. High-temperature processing might be best for woods, whereas grasses may work better at low temperatures.
The most technically developed pathway to biofuels is the route that runs through the high-temperature gasification process to produce syn-gas. Syn-gas is a mixture of carbon monoxide and hydrogen that can be made from any carbon containing material. Syn-gas can be transformed into diesel fuel, gasoline or ethanol using a process called Fischer-Tropsch Synthesis (FTS) that was developed by German scientists in the 1920s. The Third Reich used FTS during World War II to create liquid fuel out of Germany’s coal reserves. Most of the major oil companies still have a syn-gas conversion technology that they may introduce under the right economic conditions.
The first step to produce syn-gas is called gasification. In this process biomass is fed into a reactor and heated to temperatures between 800 degrees C to 1,200 degrees C. It is then mixed with steam or oxygen to produce a gas containing carbon monoxide (CO), hydrogen gas, and tars. The tars must first be cleaned out and the gas compressed to 20 to 70 atmospheres. Both of these steps can be expensive. The compressed syn-gas then flows over a specially designed catalyst—a solid material that’s designed to hold the individual reactant molecules and preferentially encourage particular chemical reactions. Syn-gas conversion catalysts have been developed by the petroleum chemistry primarily for converting natural gas and coal-derived syn-gas into fuels.
Though the technology is well understood, the reactors are expensive. An FTS plant in Qatar to convert natural gas into 34,000 barrels per day of liquid fuels was projected to cost $1 billion to 1.5 billion. To pay off these high capital costs, a biomass gasification plant would have to consume around 5,000 tons of biomass per day, every day, for a period of 15 to 30 years. There are significant logistic and economic challenges with getting this amount of biomass to a single location, and so research in this area focuses on ways to reduce the capital costs.
Another way to convert biomass into fuel is to first transform it into an oil, then refine this bio-oil in much the same way you would refine raw crude. A refinery heats up the biomass to 300 degrees C to 600 degrees C in an oxygen-free environment. The heat breaks the biomass down into a charcoal-like solid and the bio-oil, giving off some gas in the process. The bio-oil produced by this method is the cheapest liquid biofuel on the market today, perhaps $0.50 per gallon (in addition to the cost of the raw biomass). The process can also be carried out in relatively small factories that are close to where biomass is harvested. Yet like a crude, this bio-oil is very poor quality. It is highly acidic, not soluble with petroleum-based fuels and has half the energy content of gasoline. Although you can burn bio-oil directly in a diesel engine, you should only attempt it if you no longer have a need for your diesel engine.
Oil refineries could convert this bio-crude into a usable fuel, however, and many companies are studying how they could convert their existing refineries into plants capable of doing so. Some are already producing green diesel fuel by co-feeding vegetable oils and animal fats with petroleum oil directly into their refinery. Conoco-Phillips currently produces around 300 barrels of biodiesel per day in its Borger, Texas, plant by refining the waste fat from cows at a nearby slaughterhouse.
Researchers are also figuring out ways to process biomass using the chemical engineering equivalent of one-pot cooking—converting the solid biomass to oil then the oil into fuel inside a single reactor. The research group of one of us (Huber) is developing an approach called catalytic fast pyrolysis that would do just that. The “fast” in fast pyrolysis comes from the initial heating—once biomass enters the reactor, it is heated to 500 degrees C in a second. This heating breaks down the large biomass molecules into smaller molecules. Like eggs and an egg carton, these small molecules are now the perfect size and shape to fit into the surface of a catalyst. Once ensconced inside the catalyst’s pores, the molecules go through a series of reactions that change them into gasoline—specifically, the high-value aromatic components of gasoline that increase the octane. The entire process takes just two to 10 seconds. Already the startup company Anellotech is attempting to scale this process up to the commercial scale. Its first test plant is expected by 2014.
The route that has attracted most of the public and private investment thus far relies on a more traditional mechanism—unlock the sugars in the plant, then ferment these sugars into ethanol. Yet nature designed plant materials to resist biological breakdown; thus, the job of the engineer is to defeat those barriers that nature put up. Perhaps not surprisingly, this task has proven to be task.
Scientists have studied literally dozens of possible ways to break down cellulose. You can do it mechanically, using heat or gamma rays. You can grind the material into a fine slurry or subject it to high-temperature steam. You can douse it with concentrated acids or bases, or bathe it in an appropriate solvent. You can even genetically engineer biological organisms that will eat and digest the cellulose.
While it’s impossible to say which methods will end up being the most successful (and different methods will probably tailored for the particular feedstock), many techniques that may be successful in the lab have no chance of succeeding in commercial practice. The pretreatments must generate easily fermentable sugars at high yields and concentrations, conserve nutrients in the biomass and be implemented with modest capital costs. They shouldn’t use toxic materials or require too much energy input to work. Most important, they must be cheap.
The most promising approaches right now involve subjecting the biomass to extremes of pH and temperature. One approach that uses ammonia—a strong base—is being developed at a laboratory run by one of us (Dale). In the ammonia fiber expansion (AFEX) process, biomass plant material is cooked with hot concentrated ammonia under pressure. When the pressure is released, the ammonia evaporates and is recycled. The treated biomass gives high sugar yields of 90 percent or more following a final conversion by enzymes. This approach minimizes the side effect of sugar degradation that often occurs in acid or high temperature environments. The AFEX process also is “dry to dry”: biomass starts as a mostly dry solid and is left dry after treatment, undiluted with water. It thus provides high ethanol at high concentrations.
It also has the potential to be very cheap: a recent economic analysis showed that, assuming biomass can be delivered to the plant for around $80 a ton, AFEX pretreatment can produce cellulosic ethanol for around $1.40 per gallon. If we project to a future where a streamlined agricultural infrastructure exists, and assume a “mature” process in which the processing costs are about 30 percent of the overall grassoline production costs (as it is in the oil refining business today), grassoline will be delivered to the pump for around $2 per gallon.
The Cost of Change
Cost, of course, will be the major determinant of how fast grassoline will grow. Its main competitor is petroleum, and the petroleum industry has been reaping the technological benefits of dedicated research programs for over a century. In addition, most petroleum refineries in use today have already paid off their initial capital costs; a lignocellulosic plant built using today’s technology will cost between $300 million and $500 million, a price that will have to be integrated into the cost of the fuel it produces through the years.
Grassoline, on the other hand, enjoys two major advantages over fuels from petroleum. First, the raw feedstocks that go into making the fuel are far less expensive than raw crude. This should help keep costs down once the industry gets up and running.
Second, new analytical tools and computer modeling techniques will let us build better, more efficient biorefinery operations at a rate that would have been unattainable to petroleum engineers just a decade ago. We’re gaining a deeper understanding of our raw feedstocks and the processes we can us to convert them into fuel at an ever-increasing pace. The government’s support for research into alternative forms of energy should help this process to accelerate even further.
Indeed, if we maintain our current national commitment to move beyond oil, we will see an explosive growth in cellulosic biofuels over the next five to 15 years as biomass conversion technologies move from the laboratory to commercial scale. This move towards grassoline will fundamentally change the world. It is a move that is now long overdue.