Cellulosic ethanol is a biofuel produced
from wood, grasses, or the inedible parts of plants.
It is a type of biofuel produced
from lignocellulose, a structural material that
comprises much of the mass of plants. Lignocellulose is composed mainly of cellulose, hemicellulose
and lignin. Corn stover,
Panicum
virgatum (switchgrass), Miscanthus
grass species, wood chips and the byproducts of lawn and tree
maintenance are some of the more popular cellulosic materials for ethanol
production. Production of ethanol from lignocellulose has the advantage of
abundant and diverse raw material compared to sources such as corn and cane
sugars, but requires a greater amount of processing to make the sugar monomers
available to the microorganisms typically used to produce ethanol by
fermentation.
Switchgrass and Miscanthus are the major biomass
materials being studied today, due to their high productivity per acre.
Cellulose, however, is contained in nearly every natural, free-growing plant,
tree, and bush, in meadows, forests, and fields all over the world without
agricultural effort or cost needed to make it grow.
According to Michael Wang of Argonne National Laboratory, one of the
benefits of cellulosic ethanol is it reduces greenhouse
gas emissions (GHG) by 85% over reformulated gasoline. By contrast, starch
ethanol (e.g., from corn), which most frequently uses natural gas
to provide energy for the process, may not reduce GHG emissions at all
depending on how the starch-based feedstock is produced.According to the National Academy of Sciences, there is
no commercially viable bio-refinery in existence to convert lignocellulosic
biomass to fuel. Absence of production of cellulosic ethanol in the quantities
required by the regulation was the basis of a United
States Court of Appeals for the District of Columbia decision announced
January 25, 2013 voiding a requirement imposed on car and truck fuel producers
in the United States by the Environmental Protection Agency requiring addition
of cellulosic biofuels to their products. These issues, along with many other
difficult production challenges, led George Washington University policy
researchers to state that "in the short term, [cellulosic] ethanol cannot
meet the energy security and environmental goals of a gasoline
alternative."
History
The French chemist, Henri Braconnot, was the first to
discover that cellulose could be hydrolyzed into sugars by treatment with
sulfuric acid in 1819. The hydrolyzed sugar could then be processed to form
ethanol through fermentation. The first commercialized ethanol production began
in Germany in 1898, where acid was used to hydrolyze cellulose. In the United
States, the Standard Alcohol Company opened the first cellulosic ethanol
production plant in South Carolina in 1910 during WWI. Later a second plant was
opened in Louisiana. However, both plants were closed after WWI due to economic
reasons.
The first attempt at commercializing a process for ethanol
from wood was done in Germany in 1898. It involved the use of dilute acid to
hydrolyze the cellulose to glucose, and was able to produce 7.6 liters of
ethanol per 100 kg of wood waste (18 US gal (68 L) per
ton). The Germans soon developed an industrial process optimized for yields of
around 50 US gallons (190 L) per ton of biomass. This process soon found
its way to the US, culminating in two commercial plants operating in the
southeast during WWI. These plants used what was called "the American
Process" — a one-stage dilute sulfuric acid hydrolysis. Though the yields
were half that of the original German process (25 US gallons (95 L) of
ethanol per ton versus 50), the throughput of the American process was much
higher. A drop in lumber production forced the plants to close shortly after
the end of WWI. In the meantime, a small but steady amount of research on
dilute acid hydrolysis continued at the USFS's Forest Products Laboratory.[8][9][10]
During World War II, the US again turned to cellulosic ethanol, this time for
conversion to butadiene
to produce synthetic rubber. The Vulcan Copper and Supply Company was
contracted to construct and operate a plant to convert sawdust into ethanol.
The plant was based on modifications to the original German Scholler process as
developed by the Forest Products Laboratory. This plant achieved an ethanol
yield of 50 US gal (190 L) per dry ton, but was still not
profitable and was closed after the war.
With the rapid development of enzyme technologies in the
last two decades, the acid hydrolysis process has gradually been replaced by
enzymatic hydrolysis. Chemical pretreatment of the feedstock is required to
prehydrolyze (separate) hemicellulose, so it can be more effectively converted
into sugars. The dilute acid pretreatment is developed based on the early work
on acid hydrolysis of wood at the USFS's Forest Products Laboratory. Recently,
the Forest Products Laboratory together with the University of Wisconsin–Madison
developed a sulfite pretreatment to overcome the recalcitrance of
lignocellulose[12]
for robust enzymatic hydrolysis of wood cellulose.
US President George
W. Bush, in his State of the Union address delivered January 31,
2006, proposed to expand the use of cellulosic ethanol. In his State of the Union Address on January
23, 2007, President Bush announced a proposed mandate for 35 billion US
gallons (130,000,000 m3) of ethanol by 2017. It is widely
recognized that the maximum production of ethanol from corn starch is
15 billion US gallons (57,000,000 m3) per year, implying a
proposed mandate for production of some 20 billion US gallons (76,000,000 m3)
more per year of cellulosic ethanol by 2017. Bush's proposed plan includes $2
billion funding (from 2007 to 2017?) for cellulosic ethanol plants, with an
additional $1.6 billion (from 2007 to 2017?) announced by the USDA on January
27, 2007.
In March 2007, the US government awarded $385 million in
grants aimed at jump-starting ethanol production from nontraditional sources
like wood chips, switchgrass and citrus peels. Half of the six projects chosen
will use thermochemical methods and half will use cellulosic ethanol methods.
The American company Range Fuels
announced in July 2007 that it was awarded a construction permit from the state
of Georgia to build the first commercial-scale 100-million-US-gallon
(380,000 m3)-per-year cellulosic ethanol plant in the US.
Construction began in November, 2007. The Range Fuels plant was built in
Soperton, GA, but was shut down in January 2011, without ever having produced
any ethanol. It had received a $76 million grant from the US Dept of Energy,
plus $6 million from the State of Georgia, plus an $80 million loan guaranteed
by the U.S. Biorefinery Assistance Program.
Production methods
Bioreactor for cellulosic ethanol research.
The two ways of producing ethanol from cellulose
are:
- Cellulolysis processes which consist of hydrolysis on pretreated lignocellulosic materials, using enzymes to break complex cellulose into simple sugars such as glucose, followed by fermentation and distillation.
- Gasification that transforms the lignocellulosic raw material into gaseous carbon monoxide and hydrogen. These gases can be converted to ethanol by fermentation or chemical catalysis.
As is normal for pure ethanol production, these methods
include distillation.
Cellulolysis (biological approach)
The stages to produce ethanol using a biological approach
are:
- A "pretreatment" phase, to make the lignocellulosic material such as wood or straw amenable to hydrolysis
- Cellulose hydrolysis (cellulolysis), to break down the molecules into sugars
- Separation of the sugar solution from the residual materials, notably lignin
- Microbial fermentation of the sugar solution
- Distillation to produce roughly 95% pure alcohol
- Dehydration by molecular sieves to bring the ethanol concentration to over 99.5%
In 2010, a genetically engineered yeast strain was developed
to produce its own cellulose-digesting enzymes. Assuming this
technology can be scaled to industrial levels, it would eliminate one or more
steps of cellulolysis, reducing both the time required and costs of production.
Pretreatment
Although lignocellulose is the most abundant plant material
resource, its usability is curtailed by its rigid structure. As the result, an
effective pretreatment is needed to liberate the cellulose from the lignin seal
and its crystalline structure so as to render it accessible for a subsequent
hydrolysis step. By far, most pretreatments are done through physical or
chemical means. To achieve higher efficiency, both physical and chemical
pretreatments are required. Physical pretreatment is often called size
reduction to reduce biomass physical size. Chemical pretreatment is to remove
chemical barriers so the enzymes can have access to cellulose for microbial
destruction.
To date, the available pretreatment techniques include acid
hydrolysis, steam explosion, ammonia fiber expansion,
organosolve, sulfite pretreatment, alkaline wet oxidation and
ozone pretreatment. Besides effective cellulose liberation, an ideal
pretreatment has to minimize the formation of degradation products because of
their inhibitory effects on subsequent hydrolysis and fermentation
processes.The presence of inhibitors will not only further complicate the
ethanol production but also increase the cost of production due to entailed
detoxification steps. Even though pretreatment by acid hydrolysis is probably
the oldest and most studied pretreatment technique, it produces several potent
inhibitors including furfural and hydroxymethyl furfural (HMF) which are by far
regarded as the most toxic inhibitors present in lignocellulosic hydrolysate.
Ammonia Fiber Expansion (AFEX) is a promising pretreatment with no inhibitory
effect in resulting hydrolysate.
Most pretreatment processes are not effective when applied
to feedstocks with high lignin content, such as forest biomass. Organosolv
and SPORL ('sulfite pretreatment to overcome recalcitrance of lignocellulose')
are the only two processes that can achieve over 90% cellulose conversion for
forest biomass, especially those of softwood species. SPORL is the most energy
efficient (sugar production per unit energy consumption in pretreatment) and
robust process for pretreatment of forest biomass with very low production of
fermentation inhibitors. Organosolv pulping is particularly effective for
hardwoods and offers easy recovery of a hydrophobic lignin product by dilution
and precipitation.
Cellulolytic processes
The cellulose molecules are composed of long chains of sugar
molecules. In the hydrolysis process, these chains are broken down to free
the sugar before it is fermented for alcohol production.
There are two major cellulose hydrolysis (cellulolysis)
processes: a chemical reaction using acids, or an enzymatic reaction.
Chemical hydrolysis
In the traditional methods developed in the 19th century and
at the beginning of the 20th century, hydrolysis is performed by attacking the
cellulose with an acid. Dilute acid may be used under high heat and high
pressure, or more concentrated acid can be used at lower temperatures and
atmospheric pressure. A decrystalized cellulosic mixture of acid and sugars
reacts in the presence of water to complete individual sugar molecules
(hydrolysis). The product from this hydrolysis is then neutralized and yeast
fermentation is used to produce ethanol. As mentioned, a significant obstacle
to the dilute acid process is that the hydrolysis is so harsh that toxic
degradation products are produced that can interfere with fermentation. BlueFire Renewables uses concentrated acid
because it does not produce nearly as many fermentation inhibitors, but must be
separated from the sugar stream for recycle [simulated moving bed (SMB)
chromatographic separation, for example] to be commercially attractive.
Agricultural Research Service scientists
found they can access and ferment almost all of the remaining sugars in wheat straw. The
sugars are located in the plant’s cell walls, which are notoriously difficult
to break down. To access these sugars, scientists pretreated the wheat straw
with alkaline peroxide, and then used specialized enzymes to break down the
cell walls. This method produced 93 US gallons (350 L) of ethanol per ton
of wheat straw.
Enzymatic hydrolysis
This reaction occurs at body temperature in the stomachs of ruminants such
as cattle and sheep, where the enzymes are produced by microbes. This process
uses several enzymes at various stages of this conversion. Using a similar
enzymatic system, lignocellulosic materials can be enzymatically hydrolyzed at
a relatively mild condition (50°C and pH 5), thus enabling effective cellulose
breakdown without the formation of byproducts that would otherwise inhibit
enzyme activity. All major pretreatment methods, including dilute acid, require
an enzymatic hydrolysis step to achieve high sugar yield for ethanol
fermentation. Currently, most pretreatment studies have been laboratory-based,
but companies are exploring means to transition from the laboratory to pilot,
or production scale.
Various enzyme companies have also contributed significant
technological breakthroughs in cellulosic ethanol through the mass production
of enzymes for hydrolysis at competitive prices.
The fungus Trichoderma reesei is used by Iogen
Corporation to secrete "specially engineered enzymes" for an
enzymatic hydrolysis
process.Their raw material (wood or straw) has to be pre-treated to make it
amenable to hydrolysis.
Another Canadian company, SunOpta, uses steam
explosion pretreatment, providing its technology to Verenium (formerly Celunol
Corporation)'s facility in Jennings, Louisiana, Abengoa's facility in Salamanca,
Spain, and a China Resources Alcohol Corporation in Zhaodong. The
CRAC production facility uses corn stover as raw material.
Genencor and Novozymes have received United States Department of Energy
funding for research into reducing the cost of cellulases, key enzymes in the
production of cellulosic ethanol by enzymatic hydrolysis.
Other enzyme companies, such as Dyadic International,
are developing genetically engineered fungi which would
produce large volumes of cellulase, xylanase and hemicellulase
enzymes, which can be used to convert agricultural residues such as corn
stover, distiller grains, wheat straw and sugarcane bagasse and energy
crops such as switchgrass into fermentable sugars which may be used to
produce cellulosic ethanol.
In 2010, BP Biofuels bought out the cellulosic ethanol
venture share of Verenium, which had itself been formed by the
merger of Diversa
and Celunol, and with which it jointly owned and operated a
1.4-million-US-gallon (5,300 m3) per year demonstration plant
in Jennings, LA, and the laboratory facilities and staff in San Diego, CA. BP
Biofuels continues to operate these facilities, and has begun first phases to
construct commercial facilities. Ethanol produced in the Jennings facility was
shipped to London and blended with gasoline to provide fuel for the Olympics.
KL Energy Corporation,
formerly KL Process Design Group, began commercial operation of a
1.5-million-US-gallon (5,700 m3) per year cellulosic ethanol
facility in Upton, WY in the last quarter of 2007. The Western Biomass Energy
facility is currently achieving yields of 40–45 US gallons (150–170 L) per
dry ton. It is the first operating commercial cellulosic ethanol facility in
the nation. The KL Energy process uses a thermomechanical breakdown and
enzymatic conversion process. The primary feedstock is soft wood, but lab tests
have already proven the KL Energy process on wine pomace, sugarcane bagasse,
municipal solid waste, and switchgrass.
Microbial fermentation
Traditionally, baker’s yeast (Saccharomyces cerevisiae), has long
been used in the brewery industry to produce ethanol from hexoses (six-carbon
sugars). Due to the complex nature of the carbohydrates
present in lignocellulosic biomass, a significant
amount of xylose
and arabinose
(five-carbon sugars derived from the hemicellulose portion of the
lignocellulose) is also present in the hydrolysate. For example, in the
hydrolysate of corn stover, approximately 30% of the total fermentable
sugars is xylose. As a result, the ability of the fermenting microorganisms to
use the whole range of sugars available from the hydrolysate is vital to
increase the economic competitiveness of cellulosic ethanol and potentially
biobased proteins.
In recent years, metabolic engineering for microorganisms
used in fuel ethanol production has shown significant progress. Besides Saccharomyces
cerevisiae, microorganisms such as Zymomonas mobilis and Escherichia
coli have been targeted through metabolic engineering for cellulosic
ethanol production.
Recently, engineered yeasts have been described efficiently
fermenting xylose, and arabinose, and even both together. Yeast cells are
especially attractive for cellulosic ethanol processes because they have been
used in biotechnology for hundreds of years, are tolerant to high ethanol and
inhibitor concentrations and can grow at low pH values to reduce bacterial
contamination.
Combined hydrolysis and fermentation
Some species of bacteria have been found capable of direct
conversion of a cellulose substrate into ethanol. One example is Clostridium thermocellum, which
uses a complex cellulosome to break down cellulose and synthesize
ethanol. However, C. thermocellum also produces other products
during cellulose metabolism, including acetate and lactate,
in addition to ethanol, lowering the efficiency of the process. Some research
efforts are directed to optimizing ethanol production by genetically engineering bacteria that focus on
the ethanol-producing pathway.
Gasification process (thermochemical approach)
The gasification process does not rely on chemical
decomposition of the cellulose chain (cellulolysis). Instead of breaking the
cellulose into sugar molecules, the carbon in the raw material is converted
into synthesis
gas, using what amounts to partial combustion. The carbon monoxide, carbon
dioxide and hydrogen may then be fed into a special kind of fermenter. Instead of sugar fermentation with
yeast, this process uses Clostridium ljungdahlii bacteria.
This microorganism will ingest carbon monoxide, carbon dioxide and hydrogen and
produce ethanol and water. The process can thus be broken into three steps:
- Gasification — Complex carbon-based molecules are broken apart to access the carbon as carbon monoxide, carbon dioxide and hydrogen
- Fermentation — Convert the carbon monoxide, carbon dioxide and hydrogen into ethanol using the Clostridium ljungdahlii organism
- Distillation — Ethanol is separated from water
A recent study has found another Clostridium
bacterium that seems to be twice as efficient in making ethanol from carbon
monoxide as the one mentioned above.
Alternatively, the synthesis gas from gasification may be
fed to a catalytic reactor where it is used to produce ethanol and other higher
alcohols through a thermochemical process. This process can also generate other
types of liquid fuels, an alternative concept successfully demonstrated by the
Montreal-based company Enerkem at their facility in Westbury, Quebec.
Hemicellulose to ethanol
Studies are intensively conducted to develop economic
methods to convert both cellulose and hemicellulose
to ethanol. Fermentation of glucose, the main product of cellulose hydrolyzate,
to ethanol is an already established and efficient technique. However,
conversion of xylose, the pentose sugar of hemicellulose hydrolyzate, is a
limiting factor, especially in the presence of glucose. Moreover, it cannot be
disregarded as hemicellulose will increase the efficiency and cost-effectiveness
of cellulosic ethanol production.
Sakamoto (2012) et al. show the potential of genetic
engineering microbes to express hemicellulase enzymes. The researchers created
a recombinant Saccharomyces cerevisiae strain that was able to:
- hydrolyze hemicellulase through codisplaying endoxylanase on its cell surface,
- assimilate xylose by expression of xylose reductase and xylitol dehydrogenase.
The strain was able to convert rice straw hydrolyzate to
ethanol, which contains hemicellulosic components. Moreover, it was able to
produce 2.5x more ethanol than the control strain, showing the highly
effectiveness process of cell surface-engineering to produce ethanol.
Economics
The shift to a renewable fuel resource has been a target for
many years now. However, most of its production is with the use of corn
ethanol. In the year 2000, there was only 6.2 billion liters produced in the
United States and it has expanded over 800% to 50 billion litres in just a
decade (2010). Government pressures to shift to renewable fuel resources has
been apparent since the U.S Environmental Protection Agency has implemented the
2007 Renewable Fuel Standard (RFS) to use a percentage of renewable fuel in
products or face penalties. The shift to cellulosic ethanol production instead
of corn has been strongly promoted by the US government. Even with these
policies in place and the government attempting to create a market for
cellulose ethanol, there was no commercial production of this fuel in 2010 and
2011. The Energy Independence and Security Act originally set goals of 100
million, 250 million and 500 million gallons for the years 2010, 2011 and 2012
respectively. However, as of 2012 it was projected that the production of
cellulosic ethanol would be approximately 10.5 million far from its target. In
2007 alone, the US government provided 1 billion US dollars for cellulosic
ethanol projects, while China invested 500 million US dollars into cellulosic
ethanol research.
Due to the lack of existing commercialized plant data it is
difficult to determine the exact method of production that will be most
commonly employed. Model systems try to compare different technologies costs,
however these models cannot be applied to commercial-plant costs. Currently
there are many pilot and demonstration facilities open that show cellulosic
production on a smaller scale. These main facilities are summarized in the table below.
Start-up costs for pilot scale lignocellulosic ethanol
plants are high. On 28 February 2007, the U.S. Dept. of Energy announced $385 million in
grant funding to six cellulosic ethanol plants. This grant funding accounts for
40% of the investment costs. The remaining 60% comes from the promoters of
those facilities. Hence, a total of $1 billion will be invested for
approximately 140-million-US-gallon (530,000 m3) capacity. This
translates into $7/annual gallon production capacity in capital investment
costs for pilot plants; future capital costs are expected to be lower. Corn-to-ethanol
plants cost roughly $1–3/annual gallon capacity, though the cost of the corn
itself is considerably greater than for switchgrass
or waste biomass.
As of 2007, ethanol is produced mostly from sugars or
starches, obtained from fruits and grains. In contrast, cellulosic ethanol is
obtained from cellulose, the main component of wood, straw, and much of the
structure of plants. Since cellulose cannot be digested by humans, the production
of cellulose does not compete with the production of food, other than
conversion of land from food production to cellulose production (which has
recently started to become an issue, due to rising wheat prices.) The price per
ton of the raw material is thus much cheaper than that of grains or fruits.
Moreover, since cellulose is the main component of plants, the whole plant can
be harvested. This results in much better yields—up to 10 short tons per acre
(22 t/ha), instead of 4-5 short tons/acre (9–11 t/ha) for the best
crops of grain.
The raw material is plentiful. An estimated 323 million tons
of cellulose-containing raw materials which could be used to create ethanol are
thrown away each year in US alone. This includes 36.8 million dry tons of urban
wood wastes, 90.5 million dry tons of primary mill residues, 45 million dry
tons of forest residues, and 150.7 million dry tons of corn stover and
wheat straw. Transforming them into ethanol using efficient and cost-effective
hemi(cellulase) enzymes or other processes might provide as much as 30% of the
current fuel consumption in the United States. Moreover, even land marginal for
agriculture could be planted with cellulose-producing crops such as
switchgrass, resulting in enough production to substitute for all the current
oil imports into the United States.
Paper, cardboard, and packaging comprise a substantial part
of the solid waste sent to landfills in the United States each day, 41.26% of all
organic municipal solid waste (MSW) according to California Integrated
Waste Management Board's city profiles. These city profiles
account for accumulation of 612.3 short tons (555.5 t) daily per landfill
where an average population density of 2,413 per square mile persists. Organic
waste consists of 0.4% manure, 1.6% gypsum board, 4.2% glossy
paper, 4.2% paper ledger, 9.2% wood, 10.5% envelopes,
11.9% newsprint, 12.3% grass and leaves, 30.0% food scrap,
34.0% office paper, 35.2% corrugated cardboard, and
46.4% agricultural composites, makes up 71.51% of landfill. All these,
except gypsum board, contain cellulose, which is transformable into cellulosic
ethanol. This may have additional environmental benefits because decomposition
of these products produces methane, a potent greenhouse gas.
Reduction of the disposal of solid waste through cellulosic
ethanol conversion would reduce solid waste disposal costs by local and state
governments. It is estimated that each person in the US throws away 4.4 lb
(2.0 kg) of trash each day, of which 37% contains waste paper, which is
largely cellulose. That computes to 244 thousand tons per day of discarded
waste paper that contains cellulose. The raw material to produce cellulosic
ethanol is not only free, it has a negative cost—i.e., ethanol producers can
get paid to take it away.
In June 2006, a U.S. Senate hearing was told the current
cost of producing cellulosic ethanol is US $2.25 per US gallon (US
$0.59/litre), primarily due to the current poor conversion efficiency .
At that price, it would cost about $120 to substitute a barrel of oil (42 US
gallons (160 L)), taking into account the lower energy content of ethanol. However,
the Department of Energy is optimistic and has requested a doubling of research
funding. The same Senate hearing was told the research target was to reduce the
cost of production to US $1.07 per US gallon (US $0.28/litre) by 2012.
"The production of cellulosic ethanol represents not only a step toward
true energy diversity for the country, but a very cost-effective alternative to
fossil fuels. It is advanced weaponry in the war on oil," said Vinod
Khosla, managing partner of Khosla
Ventures, who recently told a Reuters Global Biofuels Summit that he could
see cellulosic fuel prices sinking to $1 per gallon within ten years.
In September 2010, a report by Bloomberg
analyzed the European
biomass infrastructure and future refinery development. Estimated prices for a
litre of ethanol in August 2010 are EUR 0.51 for 1g and 0.71 for 2g. The report
suggested Europe should copy the current US subsidies of up to $50 per dry
tonne.
Recently on October 25, 2012, BP, one of the leaders in fuel
products, announced the cancellation of their proposed $350 million
commercial-scale plant. It was estimated that the plant would be producing 36
million gallons a year at its location in Highlands County of Florida. BP has
still provided 500 million US dollars for biofuel research at the Energy
Biosciences Institute. General Motors (GM) has also invested into cellulosic
companies more specifically Mascoma and Coskata. There are many other companies
in construction or heading towards it. Abengoa
is building a 25 million-gallon per year plant in \ technology platform
based on the fungus Myceliopthora Thermophila to convert lignocellulose into
fermentable sugars. Poet is also in midst of producing a 200 million dollar, 25-million-gallon
per year in Emmetsburg, Iowa. Mascoma now partnered with Valero has declared
their intention to build a 20 million gallon per year in Kinross, Michigan.
China Alcohol Resource Corporation has developed a 6.4 million liter cellulosic
ethanol plant under continuous operation.
Also, since 2013 the Brazilian company GranBio is working to
become a producer of biofuels and biochemicals. The family-held company is
commissioning a 82 million liters per year (22 MMgy) cellulosic ethanol plant
(2G ethanol) in the state of Alagoas, Brazil, which will be the first
industrial facility of the group. GranBio's second generation ethanol facility
is integrated to a first generation ethanol plant operated by Grupo Carlos
Lyra, uses process technology from Beta Renewables, enzymes from Novozymes and
yeast from DSM. Breaking ground in January 2013, the plant is in final
commissioning. According to GranBio Annual Financial Records, the total
investment was 208 million US Dollars.
Enzyme-cost barrier
Cellulases and hemicellulases used in the production of
cellulosic ethanol are more expensive compared to their first generation
counterparts. Enzymes required for maize grain ethanol production cost
2.64-5.28 US dollars per cubic meter of ethanol produced. Enzymes for
cellulosic ethanol production are projected to cost 79.25 US dollars, meaning
they are 20-40 times more expensive. The cost differences are attributed to
quantity required. The cellulase family of enzymes have a one to two order
smaller magnitude of efficiency. Therefore, it requires 40 to 100 times more of
the enzyme to be present in its production. For each ton of biomass it requires
15-25 kilograms of enzyme. There is also relatively high capital costs
associated with the long incubation times for the vessel that perform enzymatic
hydrolysis. Altogether, enzymes comprise a significant portion of 20-40% for
cellulosic ethanol production.
Feedstocks
In general there are two types of feedstocks: forest
(woody) Biomass and agricultural biomass. In the US, about 1.4
billion dry tons of biomass can be sustainably produced annually. About 370
million tons or 30% are forest biomass. Forest biomass has higher cellulose and
lignin content and lower hemicellulose and ash content than agricultural
biomass. Because of the difficulties and low ethanol yield in fermenting
pretreatment hydrolysate, especially those with very high 5 carbon
hemicellulsoe sugars such as xylose, forest biomass has significant advantages
over agricultural biomass. Forest biomass also has high density which
significantly reduces transportation cost. It can be harvested year around
which eliminates long term storage. The close to zero ash content of forest
biomass significantly reduces dead load in transportation and processing. To
meet the needs for biodiversity, forest biomass will be an important biomass
feedstock supply mix in the future biobased economy. However, forest biomass is
much more recalcitrant than agricultural biomass. Recently, the USDA Forest Products Laboratory together
with the University of Wisconsin–Madison
developed efficient technologies[12][58]
that can overcome the strong recalcitrance of forest (woody) biomass including
those of softwood species that have low xylan content. Short-rotation intensive
culture or tree farming can offer an almost unlimited opportunity for forest
biomass production.
Woodchips from slashes and tree tops and saw dust from
saw mills, and waste paper pulp are common forest biomass feedstocks for
cellulosic ethanol production.
The following are a few examples of agricultural biomass:
Switchgrass (Panicum
virgatum) is a native tallgrass
prairie grass. Known for its hardiness and rapid growth, this perennial
grows during the warm months to heights of 2–6 feet. Switchgrass can be
grown in most parts of the United States, including swamplands, plains,
streams, and along the shores & interstate highways. It is self-seeding
(no tractor for sowing, only for mowing), resistant to many diseases and pests,
& can produce high yields with low applications of fertilizer and other
chemicals. It is also tolerant to poor soils, flooding, & drought; improves
soil quality and prevents erosion due its type of root system.
Switchgrass is an approved cover crop for land protected
under the federal Conservation Reserve Program (CRP).
CRP is a government program that pays producers a fee for not growing crops on
land on which crops recently grew. This program reduces soil erosion, enhances
water quality, and increases wildlife habitat. CRP land serves as a habitat for
upland game, such as pheasants and ducks, and a number of insects. Switchgrass
for biofuel production has been considered for use on Conservation Reserve
Program (CRP) land, which could increase ecological sustainability and lower
the cost of the CRP program. However, CRP rules would have to be modified to
allow this economic use of the CRP land.
Miscanthus × giganteus is another viable feedstock
for cellulosic ethanol production. This species of grass is native to Asia and
is the sterile triploid hybrid of Miscanthus sinensis and Miscanthus sacchariflorus. It can
grow up to 12 feet (3.7 m) tall with little water or fertilizer input.
Miscanthus is similar to switchgrass with respect to cold and drought tolerance
and water use efficiency. Miscanthus is commercially grown in the European
Union as a combustible energy source.
Corn cobs and corn stover are the most popular
agricultural biomass.
It has been suggested that Kudzu may become a
valuable source of biomass.
Environmental effects
The environmental impact from the production of fuels is an
important factor in determining its feasibility as an alternative to fossil
fuels. Over the long run, small differences in production cost, environmental
ramifications, and energy output may have large effects. It has been found that
cellulosic ethanol can produce a positive net energy output. The reduction in
green house gas (GHG) emissions from corn ethanol and cellulosic ethanol
compared with fossil fuels is drastic. Corn ethanol may reduce overall GHG
emissions by about 13%, while that figure is around 88% or greater for
cellulosic ethanol.As well, cellulosic ethanol can reduce carbon dioxide
emissions to nearly zero.
Croplands
A major concern for the viability of current alternative
fuels is the cropland needed to produce the required materials. For example,
the production of corn for corn ethanol fuel competes with cropland that may be
used for food growth and other feedstocks.The difference between this and
cellulosic ethanol production is that cellulosic material is widely available
and is derived from a large resource of things. Some crops used for cellulosic
ethanol production include switchgrass, corn stover, and hybrid poplar. These
crops are fast-growing and can be grown on many types of land which makes them
more versatile. Cellulosic ethanol can also be made from wood residues (chips
and sawdust), municipal solid waste such as trash or garbage, paper and sewage
sludge, cereal straws and grasses. It is particularly the non-edible portions
of plant material which are used to make cellulosic ethanol, which also
minimizes the potential cost of using food products in production.
The effectiveness of growing crops for the purpose of
biomass can vary tremendously depending on the geographical location of the
plot. For example, factors such as precipitation and sunlight exposure may
greatly effect the energy input required to maintain the crops, and therefore
effect the overall energy output. A study done over five years showed that
growing and managing switchgrass exclusively as a biomass energy crop can
produce 500% or more renewable energy than is consumed during production. The
levels of GHG emissions and carbon dioxide were also drastically decreased from
using cellulosic ethanol compared with traditional gasoline.
Corn-based vs. grass-based
See also: Environmental and social impacts of
ethanol fuel in the U.S., Indirect land use change
impacts of biofuels and Low-carbon fuel standard
|
Summary of Searchinger et al.
comparison of corn ethanol and gasoline GHG emissions with and without land use change (Grams of CO2released per megajoule of energy in fuel)[70][71] |
||||
|
Fuel type
(U.S.) |
Carbon
intensity |
Reduction
GHG |
Carbon
intensity + ILUC |
Reduction
GHG |
|
Gasoline
|
92
|
-
|
92
|
-
|
|
Corn ethanol
|
74
|
-20%
|
177
|
+93%
|
|
Cellulosic ethanol
|
28
|
-70%
|
138
|
+50%
|
|
Notes: Calculated using default assumptions for 2015
scenario for ethanol in E85.
Gasoline is a combination of conventional and reformulated gasoline. |
||||
In 2008, there was only a small amount of switchgrass
dedicated for ethanol production. In order for it to be grown on a large-scale
production it must compete with existing uses of agricultural land, mainly for
the production of crop commodities. Of the United States' 2.26 billion
acres (9.1 million km2) of unsubmerged land, 33% are
forestland, 26% pastureland and grassland, and 20% crop land. A study done by
the U.S. Departments of Energy and Agriculture in 2005 determined whether there
were enough available land resources to sustain production of over
1 billion dry tons of biomass annually to replace 30% or more of the
nation’s current use of liquid transportation fuels. The study found that there
could be 1.3 billion dry tons of biomass available for ethanol use, by
making little changes in agricultural and forestry practices and meeting the
demands for forestry products, food, and fiber. A recent study done by the
University of Tennessee reported that as many as 100 million acres
(400,000 km2, or 154,000 sq mi) of cropland and
pasture will need to be allocated to switchgrass production in order to offset
petroleum use by 25 percent.
Currently, corn is easier and less expensive to process into
ethanol in comparison to cellulosic ethanol. The Department of Energy estimates
that it costs about $2.20 per gallon to produce cellulosic ethanol, which is
twice as much as ethanol from corn. Enzymes that destroy plant cell wall tissue
cost 30 to 50 cents per gallon of ethanol compared to 3 cents per
gallon for corn. The Department of Energy hopes to reduce production cost to
$1.07 per gallon by 2012 to be effective. However, cellulosic biomass is
cheaper to produce than corn, because it requires fewer inputs, such as energy,
fertilizer, herbicide, and is accompanied by less soil erosion and improved
soil fertility. Additionally, nonfermentable and unconverted solids left after
making ethanol can be burned to provide the fuel needed to operate the
conversion plant and produce electricity. Energy used to run corn-based ethanol
plants is derived from coal and natural gas. The Institute for Local Self-Reliance
estimates the cost of cellulosic ethanol from the first generation of
commercial plants will be in the $1.90–$2.25 per gallon range, excluding
incentives. This compares to the current cost of $1.20–$1.50 per gallon for
ethanol from corn and the current retail price of over $4.00 per gallon for
regular gasoline (which is subsidized and taxed).
One of the major reasons for increasing the use of biofuels
is to reduce greenhouse gas emissions. In comparison to gasoline, ethanol burns
cleaner, thus putting less carbon dioxide and overall pollution in the air.
Additionally, only low levels of smog are produced from combustion. According
to the U.S. Department of Energy, ethanol from cellulose reduces greenhouse gas
emission by 86 percent when compared to gasoline and to corn-based
ethanol, which decreases emissions by 52 percent. Carbon dioxide gas
emissions are shown to be 85% lower than those from gasoline. Cellulosic
ethanol contributes little to the greenhouse effect and has a five times better
net energy balance than corn-based ethanol. When used as a fuel, cellulosic
ethanol releases less sulfur, carbon monoxide, particulates, and greenhouse
gases. Cellulosic ethanol should earn producers carbon reduction credits,
higher than those given to producers who grow corn for ethanol, which is about
3 to 20 cents per gallon.
It takes 0.76 J of energy from fossil fuels to produce 1 J
worth of ethanol from corn. This total includes the use of fossil fuels used
for fertilizer, tractor fuel, ethanol plant operation, etc. Research has shown
that fossil fuel can produce over five times the volume of ethanol from prairie
grasses, according to Terry Riley, President of Policy at the Theodore
Roosevelt Conservation Partnership. The United States Department of Energy
concludes that corn-based ethanol provides 26 percent more energy than it
requires for production, while cellulosic ethanol provides 80 percent more
energy. Cellulosic ethanol yields 80 percent more energy than is required to
grow and convert it. The process of turning corn into ethanol requires about
1700 times (by volume) as much water as ethanol produced. Additionally,
it leaves 12 times its volume in waste. Grain ethanol uses only the edible
portion of the plant.
Cellulose is not used for food and can be grown in all parts
of the world. The entire plant can be used when producing cellulosic ethanol.
Switchgrass yields twice as much ethanol per acre than corn.Therefore, less
land is needed for production and thus less habitat fragmentation. Biomass
materials require fewer inputs, such as fertilizer, herbicides, and other
chemicals that can pose risks to wildlife. Their extensive roots improve soil
quality, reduce erosion, and increase nutrient capture. Herbaceous energy crops
reduce soil erosion by greater than 90%, when compared to conventional
commodity crop production. This can translate into improved water quality for
rural communities. Additionally, herbaceous energy crops add organic material
to depleted soils and can increase soil carbon, which can have a direct effect
on climate change, as soil carbon can absorb carbon dioxide in the air. As
compared to commodity crop production, biomass reduces surface runoff and
nitrogen transport. Switchgrass provides an environment for diverse wildlife
habitation, mainly insects and ground birds. Conservation Reserve Program (CRP)
land is composed of perennial grasses, which are used for cellulosic ethanol,
and may be available for use.
For years American farmers have practiced row cropping, with
crops such as sorghum and corn. Because of this, much is known about the effect
of these practices on wildlife. The most significant effect of increased corn
ethanol would be the additional land that would have to be converted to
agricultural use and the increased erosion and fertilizer use that goes along
with agricultural production. Increasing our ethanol production through the use
of corn could produce negative effects on wildlife, the magnitude of which will
depend on the scale of production and whether the land used for this increased
production was formerly idle, in a natural state, or planted with other row
crops. Another consideration is whether to plant a switchgrass monoculture or
use a variety of grasses and other vegetation. While a mixture of vegetation
types likely would provide better wildlife habitat, the technology has not yet
developed to allow the processing of a mixture of different grass species or
vegetation types into bioethanol. Of course, cellulosic ethanol production is
still in its infancy, and the possibility of using diverse vegetation stands
instead of monocultures deserves further exploration as research continues.
A study by Nobel Prize winner Paul
Crutzen found ethanol produced from corn had a "net climate
warming" effect when compared to oil when the full life cycle assessment properly considers the nitrous
oxide (N20) emissions that occur during corn
ethanol production. Crutzen found that crops with less nitrogen
demand, such as grasses and woody coppice species, have more favourable climate
impacts.
Cellulosic ethanol commercialization
Cellulosic ethanol commercialization is the process of
building an industry out of methods of turning cellulose-containing organic
matter into fuel. Companies such as Iogen, POET, and Abengoa are
building refineries that can process biomass and turn it into ethanol, while
companies such as DuPont,
Diversa, Novozymes,
and Dyadic are producing
enzymes which could enable a cellulosic ethanol future. The shift from food
crop feedstocks to waste residues and native grasses offers significant
opportunities for a range of players, from farmers to biotechnology firms, and
from project developers to investors.
The cellulosic ethanol industry developed some new
commercial-scale plants in 2008. In the United States, plants totaling 12
million liters (3.17 million gal) per year were operational, and an additional
80 million liters (21.1 million gal.) per year of capacity - in 26 new plants -
was under construction. In Canada, capacity of 6 million liters per year was
operational. In Europe, several plants were operational in Germany, Spain, and
Sweden, and capacity of 10 million liters per year was under construction.
Italy-based Mossi & Ghisolfi
Group broke ground for its 13 MMgy cellulosic ethanol facility in
northwestern Italy on April 12, 2011. The project will be the largest
cellulosic ethanol project in the world, 10 times larger than any of the
currently operating demonstration-scale facilities.
|
Commercial Cellulosic Ethanol Plants in the U.S.
(Operational or under construction) |
||
|
Company
|
Location
|
Feedstock
|
|
Abengoa Bioenergy
|
Hugoton, KS
|
Wheat straw
|
|
Irvine, CA
|
Multiple sources
|
|
|
Sacramento, CA
|
Waste rice straw
|
|
|
Warrenville, IL
|
Biomass, Agricultural and Municipal wastes
|
|
|
Vonore, TN
|
Corn cobs, switchgrass
|
|
|
Nevada, IA
|
||
|
Reno, NV
|
Municipal solid waste
|
|
|
Mossy Head, FL
|
Wood waste
|
|
|
Upton, WY
|
Wood
|
|
|
Lansing, MI
|
Wood
|
|
|
Emmetsburg, IA
|
Corn cobs, husks, and stover
|
|
|
Treutlen County, GA
|
Wood waste
|
|
|
Little Falls, MN
|
Wood chips
|
|
|
Rochester, NY
|
Multiple Sources
|
|
|
Highlands County, FL
|
Sweet sorghum
|
|
|
Auburndale, FL
|
Citrus peels
|
|
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