Scientists are turning agricultural leftovers,
wood and fast-growing grasses into a huge variety
of biofuels- even jet fuel. But before these
next-generation biofuels go mainstream, they have
to compete with oil at $60 a barrel
By George W. Huber and Bruce E. Dale
Scientific American
July 2009
| Key Concepts
| Second-generation biofuels made from the inedible
| parts of plants are the most environmentally
| friendly and technologically promising near-term
| alternatives to oil.
| Most of this "grassoline" will come from
| agricultural residues such as cornstalks, weedlike
| energy crops and wood waste.
| The U.S. can grow enough of these feedstocks to
| replace about half the country's total consumption
| of oil without affecting food supplies. - Scientific
| American Editors
By now it ought to be clear that the U.S. must get off
oil. We can no longer afford the dangers that our
dependence on petroleum poses for our national
security, our economic security or our environmental
security. Yet civilization is not about to stop moving,
and so we must invent a new way to power the world's
transportation fleet. Cellulosic biofuels- liquid fuels
made from inedible parts of plants-offer the most
environmentally attractive and technologically feasible
near-term alternative to oil.
Biofuels can be made from anything that is, or ever
was, a plant. First-generation biofuels derive from
edible biomass, primarily corn and soybeans (in the
U.S.) and sugarcane (in Brazil). They are the low-
hanging fruits in a forest of possible biofuels, given
that the technology to convert these feedstocks into
fuels already exists (180 refineries currently process
corn into ethanol in the U.S.). Yet first-generation
biofuels are not a long- term solution. There is simply
not enough available farmland to provide more than
about 10 percent of developed countries' liquid-fuel
needs with first-generation biofuels. The additional
crop demand raises the price of animal feed and thus
makes some food items more expensive- though not nearly
as much as the media hysteria last year would indicate.
And once the total emissions of growing, harvesting and
processing corn are factored into the ledger, it
becomes clear that first-generation biofuels are not as
environmentally friendly as we would like them to be.
Second-generation biofuels made from cellulosic
material-colloquially, "grassoline"-can avoid these
pitfalls. Grassoline can be made from dozens, if not
hundreds, of sources: from wood residues such as
sawdust and construction debris, to agricultural
residues such as cornstalks and wheat straw, to "energy
crops"-fast- growing grasses and woody materials that
are grown expressly to serve as feedstocks for
grassoline [see box on page 57]. The feedstocks are
cheap (about $10 to $40 per barrel of oil energy
equivalent), abundant and do not interfere with food
production. Most energy crops can grow on marginal
lands that would not otherwise be used as farmland.
Some, such as the short- rotation willow coppice, will
decontaminate soil that has been polluted with
wastewater or heavy metals as it grows.
Huge amounts of cellulosic biomass can be sustainably
harvested to produce fuel. According to a study by the
U.S. Department of Agriculture and the Department of
Energy, the U.S. can produce at least 1.3 billion dry
tons of cellulosic biomass every year without
decreasing the amount of biomass available for our
food, animal feed or exports. This much biomass could
produce more than 100 billion gallons of grassoline a
year- about half the current annual consumption of
gasoline and diesel in the U.S. [see bottom left graph
on page 57]. 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 a year, numbers that exceed the world's
current annual consumption of 30 billion barrels of
oil. Cellulosic biomass can also be converted to any
type of fuel- ethanol, ordinary gasoline, diesel, even
jet fuel.
Scientists are still much better at fermenting corn
kernels than they are at breaking down tough stalks of
cellulose, but they have recently enjoyed an explosion
of progress. Powerful tools such as quantum-chemical
computational models allow chemical engineers to build
structures that can control reactions at the atomic
level. Research is done with an eye toward quickly
scaling conversion technologies up to refinery scales.
And although the field is still young, a number of
demonstration plants are already online, and the first
commercial refineries are scheduled for completion in
2011. The age of grassoline may soon be at hand.
The Energy Lock
Blame evolution. Nature designed cellulose to give
structure to a plant. The material is made out of rigid
scaffolds of interlocking molecules that provide
support for vertical growth [see box on opposite page]
and stubbornly resist biological breakdown. To release
the energy inside it, scientists must first untangle
the molecular knot that evolution has created.
In general, this process involves first deconstructing
the solid biomass into smaller molecules, then refining
these products into fuels. Engineers generally classify
deconstruction methods by temperature. The low-
temperature method (50 to 200 degrees Celsius) produces
sugars that can be fermented into ethanol and other
fuels in much the same way that corn or sugar crops are
now processed. Deconstruction at higher temperatures
(300 to 600 degrees C) produces a biocrude, or bio-oil,
that can be refined into gasoline or diesel. Extremely
high temperature deconstruction (above 700 degrees C)
produces gas that can be converted into liquid fuel.
So far no one knows which approach will convert the
maximum amount of the stored energy into liquid
biofuels at the lowest costs. Perhaps different
pathways will be needed for different cellulosic
biomass materials. High- temperature processing might
be best for wood, say, whereas low temperatures might
work better for grasses.
Hot Fuel
The high-temperature syngas approach is the most
technically developed way to generate biofuels. Syngas-
a mixture of carbon monoxide and hydrogen-can be made
from any carbon- containing material. It is typically
transformed into diesel fuel, gasoline or ethanol
through a process called Fischer-Tropsch synthesis
(FTS), developed by German scientists in the 1920s.
During World War II the Third Reich used FTS to create
liquid fuel out of Germany's coal reserves. Most of the
major oil companies still have a syngas conversion
technology that they may introduce if gasoline becomes
prohibitively expensive.
The first step in creating a syngas is called
gasification. Biomass is fed into a reactor and heated
to temperatures above 700 degrees C. It is then mixed
with steam or oxygen to produce a gas containing carbon
monoxide, hydrogen gas and tars. The tars must be
cleaned out and the gas compressed to 20 to 70
atmospheres of pressure. The compressed syngas then
flows over a specially designed catalyst-a solid
material that holds the individual reactant molecules
and preferentially encourages particular chemical
reactions. Syngas conversion catalysts have been
developed by the petroleum chemistry primarily for
converting natural gas and coal- derived syngas into
fuels, but they work just as well for biomass.
Although the technology is well understood, the
reactors are expensive. An FTS plant built in Qatar in
2006 to convert natural gas into 34,000 barrels a day
of liquid fuels cost $1.6 billion. If a biomass plant
were to cost this much, it would have to consume around
5,000 tons of biomass a day, every day, for a period of
15 to 30 years to produce enough fuel to repay the
investment.
Because significant logistic and economic challenges
exist with getting this amount of biomass to a single
location, research in syngas technology focuses on ways
to reduce the capital costs.
Bio-Oil
Eons of subterranean pressure and heat transformed
Cambrian zooplankton and algae into present-day
petroleum fields. A similar trick- on a much reduced
timescale-could convert cellulosic biomass into a
biocrude. In this scenario, a refinery heats up biomass
to anywhere from 300 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 that is produced by
this method is the cheapest liquid biofuel on the
market today, perhaps $0.50 per gallon of gasoline
energy equivalent (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,
thus limiting the expense of biomass transport.
Unfortunately, this crude is highly acidic, is
insoluble with petroleum- based fuels and contains only
half the energy content of gasoline. Although you can
burn biocrude directly in a diesel engine, you should
attempt it only if you no longer have a need for the
engine.
Oil refineries could convert this biocrude into a
usable fuel, however, and many companies are studying
how they could adapt their existing hardware to the
task. Some are already producing a different form of
green diesel fuel, suggesting that refineries could
handle cellulosic biocrude as well. At the moment, the
facilities co-feed vegetable oils and animal fats with
petroleum oil directly into their refinery.
ConocoPhillips recently demonstrated this approach at a
refinery in Borger, Tex., creating more than 12,000
gallons of biodiesel a day out of beef fat shipped from
a nearby Tyson Foods slaughterhouse [see box on page
59].
Researchers are also figuring out ways to carry out the
two-stage process using the chemical engineering
equivalent of one-pot cooking- converting the solid
biomass to oil and then the oil into fuel inside a
single reactor. One of us (Huber) and his colleagues
are developing an approach called catalytic fast
pyrolysis. The "fast" in the name comes from the
initial heating- once biomass enters the reactor, it is
cooked to 500 degrees C in a second, which breaks down
the large molecules into smaller ones. Like eggs in 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. (High-octane fuels allow engines to run at
higher internal pressures, which increases efficiency.)
The entire process takes just two to 10 seconds.
Already the start-up company Anellotech is attempting
to scale up this process from the laboratory to the
commercial level. It expects to have a commercial
facility in operation by 2014.
Sugar Solution
The route that has attracted most of the public and
private investment thus far relies on a more
traditional mechanism-unlock the sugars in plants, then
ferment these sugars into ethanol or other biofuels.
Scientists have studied literally dozens of possible
ways to break down the digestion-resistant cellulose
and hemicellulose- the fibers that bind cellulose
together inside the cells [see box on page 54]-to their
constituent sugars. You can heat the biomass, irradiate
it with gamma rays, grind it into a fine slurry, or
subject it to high-temperature steam. You can douse it
with concentrated acids or bases or bathe it in
solvents. You can even genetically engineer microbes
that will eat and degrade the cellulose.
Unfortunately, many techniques that work in the lab
have no chance of succeeding in commercial practice. To
be commercially viable, the pretreatments must generate
easily fermentable sugars at high yields and
concentrations and be implemented with modest capital
costs. They should not use toxic materials or require
too much energy input to work. They must also be able
to produce grassoline at a price that can compete with
gasoline.
The most promising approaches involve subjecting the
biomass to extremes of pH and temperature. We are
developing a strategy that uses ammonia-a strong base-
in one of our laboratories (Dale's). In this ammonia
fiber expansion (AFEX) process, cellulosic biomass is
cooked at 100 degrees C with concentrated ammonia under
pressure. When the pressure is released, the ammonia
evaporates and is recycled. Subsequently, enzymes
convert 90 percent or more of the treated cellulose and
hemicellulose to sugars. The yield is so high in part
because the approach minimizes the sugar degradation
that often occurs in acidic or high-temperature
environments.
The AFEX process is "dry to dry": biomass starts as a
mostly dry solid and is left dry after treatment,
undiluted with water. It thus can provide large amounts
of highly concentrated, high-proof ethanol.
AFEX also has the potential to be very inexpensive: a
recent economic analysis showed that, assuming biomass
can be delivered to the plant for around $50 a ton,
AFEX pretreatment, combined with an advanced
fermentation process called consolidated bioprocessing,
can produce cellulosic ethanol for approximately $1 per
gallon of equivalent gasoline energy content, probably
selling for less than $2 at the pump.
The Cost of Change
Cost, of course, will be the primary determinant of how
fast the use of grassoline will grow. Its main
competitor is petroleum, and the petroleum industry has
been reaping the technological benefits of dedicated
research programs for more than a century. Moreover,
most petroleum refineries now in use have already paid
off their initial capital costs; grassoline refineries
will require investments of hundreds of millions of
dollars, a cost that will have to be integrated into
the price of the fuel it produces through the years.
Grassoline, on the other hand, enjoys several major
advantages over fuels from petroleum and other
petroleum alternatives such as oil sands and liquefied
coal. First, the raw feedstocks are far less expensive
than raw crude, which should help keep costs down once
the industry gets up and running. Grassoline will be
domestically produced, with the national security
benefits that confers. And it is far better for the
environment than any fossil fuel-based alternative.
In addition, new analytical tools and computer-
modeling techniques will let researchers build better,
more efficient biorefinery operations at a rate that
would have been unattainable to petroleum engineers
just a decade ago. We are gaining a deeper
understanding of the properties of our raw feedstocks
and the processes we can use to convert them into fuel
at an ever increasing pace. The U.S. government's
support for research into alternative forms of energy
should help this process to accelerate even further.
The stimulus bill signed into law by President Barack
Obama earlier this year contained $800 million in
funding for the Department of Energy's Biomass Program,
which will accelerate advanced biofuels research and
development and provide funding for commercial- scale
biorefinery projects. In addition, the bill contained
$6 billion in loan guarantees for "leading edge biofuel
projects" that will commence construction by October
2011.
Indeed, if the U.S. maintains its current commitment to
biofuels, the logistical and conversion challenges the
industry now faces should be readily overcome. Over the
next five to 15 years, biomass conversion technologies
will move from the laboratory to the market, and the
number of vehicles powered by cellulosic biofuels will
grow dramatically. This move toward grassoline can
fundamentally change the world. It is a move that is
now long overdue.
| The Fat of the Matter
| There is a new drive to make fuel off the fat of the
| land. In April, High Plains Bioenergy opened a
| biorefinery next to a pork-processing plant in
| Guymon, Okla. The refinery takes pork fat-an
| abundant, low- value by-product of the industrial
| butchering process- and converts it, along with
| vegetable oil, into biodiesel. The plant is expected
| to turn 30 million pounds of lard into 30 million
| gallons of biodiesel a year. In 2010 the High Plains
| facility will be joined by a plant in Geismar, La.,
| that will be run by Dynamic Fuels, a joint venture
| between Tyson Foods and energy company Syntroleum.
| That plant will use the fat from Tyson's beef,
| chicken and pork operations to create 75 million
| gallons of biodiesel and jet fuel annually.
| Yet the biodiesel industry has been battered
| recently, with many plants sitting idle for lack of
| demand. Low oil prices have made petroleum-based
| diesel fuel less expensive than biodiesel, which in
| the U.S. is typically made from soy and vegetable
| oils. A $1 per gallon federal tax credit for
| biodiesel has helped soften the blow, but that
| credit is set to expire at the end of the year. Some
| manufacturers worry that if the credit disappears,
| so will their business. Tyson had earlier partnered
| with ConocoPhillips to produce biodiesel at an
| existing ConocoPhillips refinery in Borger, Tex. But
| insecurity about the status of the tax break has put
| the project on hold. - Scientific American Editors
More To Explore
Breaking the Chemical and Engineering Barriers to
Lignocellulosic Biofuels. A research road map from the
Biomass to Biofuels Workshop:
www.ecs.umass.edu/biofuels
Development of Cellulosic Biofuels. Video lecture given
by Chris Somerville, director of the Energy Biosciences
Institute at the University of California, Berkeley:
http://tinyurl.com/grassolineU.S. Department of Energy Biomass Program Web site:
http://eere.energy.gov/biomass_____________________________________________
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