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3
Biochemical Conversion of Biomass
INTRODUCTION
This chapter focuses on the biochemical conversion of biomass to liquid
transportation fuels. It addresses the questions raised in the statement of task related to
the application of biochemical conversion to the production of alternative liquid
transportation fuels from biomass by discussing the following:
• The technology alternatives for converting biomass to liquid transportation fuels.
• The status of development of biochemical conversion of lignocellulosic biomass
to ethanol.
• The projected costs, performance, environmental impact, and barriers to
deployment of biochemical conversion of lignocellulosic biomass to ethanol.
• Challenges and needs in research and development (R&D), including basic-
research needs for the long term.
• Other technologies for converting biomass to liquid fuels that are not likely to be
ready for commercial deployment before 2020.
TECHNOLOGY ALTERNATIVES
Liquid fuels can be derived from biomass through biochemical processing,
chemical catalysis, or thermochemical conversion. Biochemical conversion and chemical
conversion typically transform the biomass into sugars as intermediates. In contrast,
thermochemical conversion uses heat to convert the biomass into building blocks, such as
carbon monoxide (CO) and hydrogen (H2), which can be used for the synthesis of fuels
(Figure 3-1). Other thermochemical conversion processes include pyrolysis and
liquefaction.
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Biochemical Conversion to Fuels
Biochemical conversion uses enzymes to break down structural carbohydrates
(for example, the cellulose1 and hemicellulose2 found in plant cell walls) into sugars,
which are transformed into alcohols, organic acids, or hydrocarbons by microorganisms
in fermentation. The conversions typically take place at atmospheric pressure and
temperatures ranging from ambient to 70°C.
Early ethanol production technology based on biochemical conversion of sugar
and starch has been deployed commercially. In that technology, ethanol is produced when
wild-type yeast ferments six-carbon sugars. Sugar can be obtained directly from
sugarcane (Brazil) and sugar beets (Europe) or indirectly from the hydrolysis of starch-
based grains, such as corn (United States) and wheat (Canada and Europe). In the latter
case, the starch feedstock needs to be ground to a meal that is hydrolyzed to glucose by
enzymes. The resulting mash is fermented by natural yeast and bacteria. Finally, the
fermented mash is separated into ethanol and residues (for feed production) via
distillation and dehydration (Figure 3-2).
Corn grain is the major source of ethanol in the United States, and its potential for
growth is defined by production efficiencies, food-vs-fuel debates, and the question of
sustainability and carbon footprint. Developments aimed at future processes are targeting
cellulose conversions that could address those issues by providing a growth potential, a
low carbon footprint, and sustainability. The infrastructure that was established by the
corn grain ethanol industry will benefit the future cellulosic-ethanol industry because the
use of ethanol as a transportation fuel has been proved to be feasible, a distribution
system exists, and automobiles with internal-combustion engines that use ethanol
efficiently are on the road.
Recent analyses of the full life cycle of corn grain ethanol have indicated that it
provides society with small net energy gains over the fossil energy needed to produce it
(Farrell et al., 2006; Hill et al., 2006) and might lead to only small net greenhouse-gas
advantages (Farrell et al., 2006; Hill et al., 2006; Fargione et al., 2008; Searchinger et al.,
2008) or might release more greenhouse gas than do production and combustion of an
energetically equivalent amount of gasoline once direct and indirect land-use changes are
taken into account (Fargione et al., 2008; Searchinger et al., 2008). Issues with corn grain
ethanol have led to increased interest in second-generation biofuel feedstocks—including
switchgrass, Miscanthus, hybrid poplar, and the other lignocellulosic feedstocks—and in
conversion methods that potentially can make biofuels that, relative to corn ethanol, offer
larger energy gains and greenhouse-gas benefits and reduced competition with food
crops.
The development of biofuels needs to move toward conversion of lignocellulosic
materials (so-called second-generation biofuels) that are unused agricultural or forestry
residues, agricultural cover crops, dedicated perennial crops grown on marginal lands that
are not suitable for commodity-crop production even with high commodity prices, or
municipal solid wastes. The need to move away from corn grain ethanol is highlighted by
the renewable fuel standard (RFS) as amended in the 2007 Energy Independence and
Security Act. The RFS mandates that production of ethanol from corn grain level off
1
A complex carbohydrate, (C6H10O5)n, that forms cell walls of most plants.
2
A matrix of polysaccharides present in almost all plant cell walls with cellulose.
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from 2008 to 2015 and that production of cellulosic and other advanced biofuels to
increase from 2008 to 2020. The key differences in production between grain ethanol and
cellulosic ethanol are the pretreatment of the biomass and the use of byproducts (Figure
3-2). This chapter focuses on the conceptual design, conversion technologies, and
economics of the biochemical conversion of cellulosic biomass to ethanol. It will also
discuss other technologies to produce advanced biofuels that use nonfood renewable
feedstocks. The other technologies could produce fuels more desirable than ethanol—for
example, lipids, higher alcohols, hydrocarbons, and other products that can be separated
by low-energy distillation. New routes of biochemical conversion of biomass to liquid
fuels will probably encounter complications as they are being developed and scaled up;
these issues will have to be addressed in a continuous R&D program.
Chemical Conversion to Fuels
In contrast with biochemical conversion, chemical conversion uses inorganic
catalysts in a series of aqueous-phase reactions to convert sugars to hydrocarbons that can
be used as fuels. It is a developing technology that will not be ready for commercial
deployment by 2020, but it will be discussed later in this chapter.
Thermochemical Conversion to Fuels
In what is currently the most developed thermochemical route, biomass is initially
converted into CO and H2 via gasification. The gas stream can be cleaned of impurities
and shifted to the needed H2:CO ratio, and CO2 can be removed to produce a gas stream
that can be catalytically converted to liquid fuels by several routes, including Fischer-
Tropsch (FT) and methanol synthesis followed by methanol-to-gasoline (MTG)
conversion. Thermochemical conversion is discussed in Chapter 4. Other thermochemical
conversion routes involve production of bio-oil by pyrolysis or liquefaction and
refinement of the bio-oil (Huber et al., 2006); this technology is not as well developed as
FT or MTG.
BIOCHEMICAL CONVERSION OF CELLULOSIC BIOMASS
This section discusses the biochemical processes for converting cellulosic
biomass to ethanol in a biorefinery. The processes discussed here occur at the end of the
supply chain, when the biomass has been delivered to the biorefinery (Figure 3-3). The
process economies are those within the biorefinery.
Process Overview
The biochemical conversion of cellulosic biomass involves six major steps:
feedstock preparation, pretreatment to release cellulose from the lignin shield,
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saccharification (breaking down of the cellulose and hemicellulose by hydrolysis to
sugars, such as glucose and xylose), fermentation of sugar to ethanol, and distillation to
separate the ethanol from the dilute aqueous solution (Figure 3-4). The residues,
primarily lignin, can be combusted to provide energy (Figure 3-4). The integration of
those steps with each other and with the living microorganisms and enzymes that carry
out the catalytic conversions in a biorefinery is essential to the development of cost-
effective processes.
Feedstock Preparation
Some feedstocks have to be washed to remove inorganic and other undesirable
materials before pretreatment. Whether washing is needed depends on the source and the
manner of storage before the feedstock is delivered to the conversion facility. The
biomass is then chopped or ground to the desirable size range to feed into the
pretreatment stage. The extent of grinding and size reduction will depend on the type of
biomass and the pretreatment technology being used. Cellulosic feedstock can be
chopped or ground with existing forestry or agricultural techniques.
Pretreatment
Producing fuel ethanol from lignocellulosic feedstocks has been challenging
because of the recalcitrant nature of the cellulose that is embedded in the plant cell-wall
structure. Therefore, pretreatment is a key step in production of cellulosic ethanol.
Pretreatment greatly increases the rates and extents of enzyme action in breaking down
cellulose to fermentable sugars (Ladisch et al., 1978) by improving the accessibility of
the structural carbohydrates in the cell wall (Figure 3-5). Yields of fermentable sugars
from untreated native lignocellulosic materials are low because of the highly packed
cellulose structure and the presence of hemicellulose and lignin, which shield cellulose
from acid or enzymatic hydrolysis.
Maximizing the use of all lignocellulosic material that is capable of yielding
simple (six- and five-carbon) sugars is essential for improving ethanol yield and lowering
the cost of ethanol production. Hence, pretreatment of lignocellulosic material is required
to improve the hydrolytic efficiency of cellulose by removing and hydrolyzing
hemicellulose, by separating the cellulose from the lignin, and by loosening the structure
of cellulose and thereby increasing its porosity. The pretreatment of lignocellulosics is
particularly important for enzymatic hydrolysis to reduce the amount of enzyme and the
time required to convert cellulose to glucose.
Among the various pretreatment methods, hydrothermolysis with steam or water
has been shown to be effective in removing and solubilizing hemicellulose and thus in
improving hydrolytic efficiency (Mosier et al., 2005; Wyman et al., 2005a; Wyman et al.,
2005b). Hot-water pretreatment of lignocellulosic biomass at a controlled pH effectively
dissolves hemicellulose and some of the lignin and minimizes the formation of
monosaccharides and other coproducts that could interfere with biological processes
downstream (Yang and Wyman, 2008). For example, monosaccharides inhibit cellulase
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in the hydrolysis of cellulose downstream. The sugars could degrade further to form such
toxic substances as furfural during the pretreatment step (Ladisch et al., 1998; Kim and
Ladisch, 2008; Hendriks and Zeeman, 2009). Other pretreatments are similarly effective,
and they use acid, bases, ammonia, or other materials (Mosier et al., 2005; Jorgensen et
al., 2007; Murnen et al., 2007; Sendich et al., 2008; Yang and Wyman, 2008; Hendriks
and Zeeman, 2009). Several of the promising pretreatment methods have been
demonstrated on a pilot scale, but the lowest-cost approach is yet to be determined.
Saccharification
In the saccharification step, the cellulose polymers (long chains of sugar) are
broken down by hydrolysis into five-carbon and six-carbon sugars (xylose and glucose)
for fermentation into alcohol (Figure 3-6). The enzymes used for hydrolysis are referred
to as cellulolytic enzymes, and they are classified into three main groups:
cellobiohydrolases, endoglucanases, and beta-glucosidases. The cellobiohydrolases and
endoglucanases are modular proteins with two distinct independent domains; the first
domain is responsible for the hydrolysis of the cellulose chain, and the second is a
cellulose-binding domain (CBD) that has the dual activity of increasing adsorption of
cellulolytic enzymes onto insoluble cellulose and affecting cellulose structure. A
schematic of the action is shown in Figure 3-7. By intercalating between fibrils and
surface irregularities of the cellulose surface, CBDs help to reduce particle size and
increase specific surface area. Microscopy of cellulose treated with isolated CBDs
generated from recombinant organisms has shown the release of small particles from
insoluble cellulose with no detectable hydrolytic activity and an increase in the roughness
of highly crystalline fibers.
The cellobiohydrolases are the most important cellulolytic enzyme group in that
cellobiohydrolase I makes up 60 percent of the protein mass of the cellulolytic system of
Trichoderma reesei, and its removal by gene deletion reduces overall cellulase system
activity on crystalline cellulose by 70 percent. The concerted effects of pretreatment and
enzymatic hydrolysis affect the plant at the cellular level as illustrated in Figure 3-8.
According to the prevailing understanding, cellobiohydrolases attack the chain ends of
cellulose polymers to release cellobiose, the repeat unit of cellulose. Endoglucanases
decrease the degree of polymerization of cellulose by attacking amorphous regions of
cellulose through random scission of the cellulose chains. Beta-glucosidase completes the
process by hydrolyzing cellobiose to glucose. Cellulolytic systems, such as those in
filamentaous T. reesei, have enzymes in all three groups: two cellobiohydrolases, four
endoglucanases, and one beta-glucosidase (Mosier et al., 1999). The mechanism by
which cellulolytic enzymes act in hydrolyzing cellulose is complex and requires a system
of different enzymes to achieve deploymerization of the oligosaccharides to
monosaccharides, such as glucose and xylose. Most studies have been done with
cellulases, which are produced industrially from T. reesei.
The development of the cellulases has resulted in effective systems that are
capable of hydrolyzing cellulose to glucose almost completely. Similar studies are being
done on hemicellulases, enzymes that are responsible for breaking down hemicellulose to
xylose. Hemicellulases are not as well developed as cellulases.
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Despite the complexities, much progress has occurred in the development of
enzymes for the hydrolysis of pretreated cellulose. Costs are being reduced, with the
ultimate goal of combining cellulases with glucose- and xylose-fermenting
microorganisms in a concept referred to as consolidated bioprocessing (Lynd et al.,
2005). Hydrolysis and fermentation (combined bioprocessing) are being demonstrated on
a pilot scale with the goal of reducing costs.
Fermentation
Pretreatment and enzymatic hydrolysis of plant matter—such as wood, corn
stover, or grasses—result in a mixture of five-carbon and six-carbon sugars. Many
microorganisms, particularly yeasts, will ferment glucose to ethanol. Typically, however,
25-30 percent of the sugar derived from likely candidates as cellulosic feedstocks for
bioprocessing (for example, hardwoods, agricultural residues, and some types of grasses)
are pentoses—sugars that have five carbon atoms rather than six carbon atoms. Other
potential sources of biomass, such as softwoods, have a lower proportion of
hemicelluloses and hence less pentoses. Pentoses are not readily fermented to ethanol, so
yeasts or bacteria that have been genetically modified to ferment both hexoses (six-
carbon sugars) and pentoses are needed to maximize the yield of ethanol from cellulosic
materials. Some researchers have been successful in engineering microorganism that are
able to use pentose efficiently but cannot do so naturally to produce ethanol. An
alternative would be to supply ethanol-producing microorganisms with pentose-using
pathways (Nevoigt, 2008). Development of such microorganisms presents a number of
challenges. They have to be capable of fermenting the sugars to ethanol, and they have to
be sufficiently robust to withstand antimicrobial agents released by the pretreatment and
hydrolysis of the lignocellulose and to withstand relatively high alcohol concentrations.
The glucose and xylose that result from the saccharification step are fermented
into ethanol by microorganisms. Traditionally, fermentation is a separate step from
saccharification. As noted above, the ideal development would be organisms that could
do both simultaneously. Although Saccharomyces cerevisiae (wild-type yeast) has been
used for fermentation in ethanol production from corn grain, it cannot ferment xylose
sugars obtained from lignocellulose unless it is genetically modified (Sonderegger et al.,
2004). Organisms that ferment xylose and glucose have been developed through
metabolic engineering (Aristidou and Penttila, 2000).
The composition of the biomass feedstock determines the amount of ethanol that
can be produced per ton of biomass (Table 3-1). The ultimate yield is determined by the
maximum yield of sugars that can be obtained from a given biomass type, and the yield
of sugars is determined by the combined starch, cellulose, and hemicelluloses content of
the biomass. The ethanol yield can range from 105 to 135 gal/ton (on a dry-weight basis)
if all bioprocessing steps occur at 100 percent efficiency, that is, all the structural
carbohydrates—starch, cellulose, and hemicelluloses—are used to produce ethanol
(Table 3-1). Because the efficiency is typically less than 100 percent—ranging from
about 95 percent overall for a corn-grain-to-ethanol plant to 50 percent with some current
cellulose-conversion technologies—the actual yields are substantially lower. Ethanol
yields can be improved with a combination of advanced pretreatments and enzymes to
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improve cellulose-conversion efficiency and with the application of fermenting
microorganisms that are able to convert glucose, xylose, and other pentoses to ethanol
(Wyman et al., 2005a).
Cellulolytic enzymes are subject to product inhibition in which the rate of an
enzymatic reaction is inhibited by the end products of the reaction (Gong et al., 1977;
Gong et al., 1979; Ladisch et al., 1981). Some work has been done on process strategies
and modified enzymes to reduce that inhibition, but more is needed. Simultaneous
saccharification, in which the enzyme and the microorganism are used in the same tank,
could enable reduced enzyme use because the reaction is not inhibited by the product of
enzymatic action, glucose or cellobiose (Gauss et al., 1976; Takagi et al., 1977; Wright et
al., 1977; Saddler et al., 1982a; Saddler et al., 1982b; Wyman et al., 1986; Spindler et al.,
1987; Lynd et al., 2002). The ethanol product that is formed also inhibits enzymatic
activity but to a smaller extent than glucose and cellobiose. The reduction in inhibition
allows the reaction to proceed to the final desired product—ethanol in this case—more
rapidly.
TABLE 3-1 Compositions of Corn Grain, Corn Cob, Corn Stover, and Poplar
Graina Cobb Stoverc Poplarc
Type of Material
Starch 071.7% n/m n/m 000n/m
Cellulose 002.4% 042.0% 36.0% 040.3%
Hemicellulose 005.5% 033.0% 26.0% 22.0%
Protein 010.3% n/m 005.0% n/m
Oil 00.43% n/m n/m n/m
Lignin 000.2% 18.0% 19.0% 23.7%
Ash 001.4% 1.5% 12.0% 0.6%
Other 004.2% 5.5% 2.0% 13.4%
Total 100.0% 100.0% 100.0% 100.0%
Maximum yield of
monosaccharides (lb/ton 0.1,778 0.1,684 001,392 0.1,396
with 100% efficiency)
Calculated best-case ethanol
yield (gal/ton with 100% 000135 000128 105 000106
efficiency)
Dry weight d 00.52% 10% 0.48% .0052%
a
Gulati et al., 1996.
b
Corn-cob composition measured at Laboratory of Renewable Resources Engineering,
Purdue University.
c
DOE, 2007a.
d
Pordesimo et al., 2005. Absolute weight of corn grain is based on corn grain data
provided by the U.S. Department of Agriculture National Agricultural Statistics Service
(NASS, 2005), which were used for calculation of ethanol yields. Absolute weights of
corn cob and corn stover are derived from the given weight percentages based on the
absolute weight of corn grain.
Note: n/m, not measured.
SOURCE: Adapted from Schwietzke et al. (2008b).
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Distillation
The ethanol solution from the fermentation step is distilled to produce 95 percent
ethanol. Ethanol is further dried with molecular sieves to produce the required purity. The
distillation requires large quantities of energy (Katzen et al., 1981; Shapouri et al., 2002).
Distillation of ethanol is a well-established technology and is used in commercial corn
grain ethanol plants. However, recovery of solids will require improved engineering
design.
Combustion of Residual Solids
The bottoms from distillation are centrifuged to concentrate them. Solids from
other portions of the process might also be added. The residual solids are rich in lignin
and can be burned in a boiler to generate steam and electricity for the biorefinery.
Pathways of using lignin as a fuel with low carbon emission are well defined.
Current Status
A substantial amount of ethanol has been produced in the United States since the
1970s, and the feedstock of choice has been corn grain. The process of producing ethanol
from corn grain has been steadily improved in efficiency, and costs have been greatly
reduced. The process can be considered fully commercial, well understood, and
optimized. There were over 130 corn grain ethanol plants in the United States in 2008. In
contrast, there are no large-scale commercial cellulosic-ethanol plants, although many of
the components are in pilot demonstration. In February 2008, the Department of Energy
announced that it would invest up to $385 million for six biorefinery projects over 4
years to bring cellulosic ethanol to market (DOE, 2007b), and a number of private
companies are actively pursuing pilot demonstration of integrated cellulosic plants, which
could lead to commercial-scale demonstrations and eventual deployment of cellulosic-
ethanol plants. If the current effort in development and demonstration of cellulosic
ethanol is sustained or accelerated, cellulosic-ethanol plants could be ready for
commercial demonstration in the next 3-5 years, and the first plant could be built by
2015.
Technical Challenges
As discussed above, the key issues associated with cellulosic ethanol are related
to the ability to develop enzymes and microorganisms that can break cellulose bundles
and depolymerize cellulose and hemicellulose to produce soluble sugars and the ability to
produce the enzymes and microorganisms at a reasonable cost. Microorganisms capable
of fermenting five-carbon and six-carbon sugars in a separate steps are available, and
their performance is rapidly being proved in the laboratory. The challenge will be to
demonstrate their robustness under industrial conditions. A longer-term challenge
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involves development of microorganisms and enzymes that can break down cellulose
structures, depolymerize them to sugars, and ferment the sugars in the same vessel. If
hydrolysis and fermentation are to be carried out separately, the development of
microorganisms that can withstand and ferment sugars at higher temperatures would
provide opportunities to increase the rate of fermentation and reduce the need for cooling
the fermentation itself. The amount of energy that would be required to heat the broth to
distillation temperature would also be reduced.
Research, Development, and Demonstration
One research gap is in understanding of the molecular biology of the plants that
provide the feedstock and of the microorganisms that provide the biocatalysts and
enzymes for transforming the feedstocks into ethanol and other biofuels. Fundamental
studies on the structure and function of the enzymes that catalyze the breakdown of
cellulosic components to fermentable sugars are key to improving the rates at which the
enzymes operate and to reducing product inhibition. The nature and chemical structure of
inhibitors and how they interact with the active catalytic sites of the enzymes need to be
defined so that strategies to mitigate the inhibition can be developed at the catalytic level
or through engineering approaches to remove the inhibitors by bioseparation.
Another research gap is in the fundamental understanding of how the plant cell-
wall structure resists enzyme attack. Genomic studies could provide insights into how
lignin (the key agent in resistance of the cell wall to enzymatic conversion) might be
modified at the molecular level to enable facile transformation but retain the properties
that provide resistance to pathogens. The molecular biology and metabolic pathways
involved in directing the flow of carbon into ethanol or other advanced biofuels and the
manner in which modified bacteria and other microorganisms are able to break down
cellulose are also important topics. Research will help to reduce enzyme costs, a major
component of cellulose-conversion costs. Research on pretreatments that exploit the
knowledge gained and biochemical engineering that will define design principles of low-
cost manufacturing through enzyme-catalyzed reactions, thermal processing,
fermentation, and advanced bioseparation needs to be pursued. The fundamental research
would also address how temperature, pressure, and pretreatment media interact at the
microscopic and macroscopic levels so that plant cells are readily attacked by biological
or chemical catalysts.
COST AND PERFORMANCE
Prior Work
The corn grain ethanol plants have created an industry, and they provide a basis
for estimating some of the capital and operating costs of cellulosic-ethanol plants. Several
key studies also provide information on and analysis of the design and performance of
cellulosic-ethanol plants (Aden et al., 2002; Johnson, 2006; Kwiatkowski et al., 2006;
Kim et al., 2008).
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Modeling
On the basis of experience gained from corn-ethanol plants, published studies,
and its own expertise, the panel used SuperPro Designer to estimate the capital and
operating costs of ethanol-production plants (Intelligen, 2009). SuperPro Designer is a
chemical-process simulation software that contains a set of unit operations that can be
customized to the specific modeling needs of the corn grain-to-ethanol and cellulosic
biomass-to-ethanol processes. It has a well-developed economic-evaluation package,
which includes an extensive database of chemical components and mixtures, extensive
equipment and resource databases, equipment sizing and costing information, and
economic-evaluation parameters—such as financing, depreciation, running royalty
expenses, inflation rate, and taxes—for cost estimates.
A grain-ethanol plant was used as the baseline to validate the model and input
parameter values. Once validated, the model was applied to estimate capital and
operating costs for cellulosic-ethanol production. Most of the unit operations for
cellulosic-ethanol production are similar to those for grain-ethanol production.
The panel decided to develop its own model for cost estimates. The process-flow
diagram used for the panel’s modeling is shown in Figure 3-9. The panel conducted three
sets of analyses with the cellulosic-ethanol plant model. First, it assessed the sensitivity
of the capital and manufacturing cost estimates with respect to such process parameters
as enzyme cost, types of feedstock, plant size, solids loading, and yields in the
pretreatment, hydrolysis, and fermentation steps. The results are presented in the form of
process cost estimates for three scenarios representing the most pessimistic with little
advancement in technologies or process efficiencies from where they are today (2008) to
reduce costs (low), reasonable advancement (medium), and the most optimistic (high) in
terms of technology and process improvement. Second, the panel assessed the sensitivity
of operating costs to different types of feedstock. As mentioned earlier, process efficiency
depends on feedstock composition. Assuming reasonable advancement in process
technologies (that is, using the medium case), the panel compared the costs of ethanol
produced from different biomass alternatives. Third, the panel assessed whether and how
the capital and operating costs of a cellulosic-ethanol plant vary with size; in this
analysis, the panel used the medium case to estimate costs and assumed that plants of
different size used poplar chips as a feedstock.
Design criteria
The panel used a biorefinery with a capacity of 40 million gallons of ethanol per
year as the basis of its calculations and assessed the effect of biorefinery size later. The
capacity was used because it is comparable with that of grain-ethanol plants, matches
well previous studies and current cellulosic-ethanol projects in the planning stage, and
could serve well geographic areas that have a lower concentration of biomass. The
distribution of biomass could pose a limit on the volumes that could be economically
supplied to a biorefinery in some areas, and transportation of large volumes of biomass
over a long distance could be expensive and consume more fossil fuel (see Chapter 2).
Producing 40 million gallons of ethanol per year with a fermentor effluent
concentration of 4-5 percent ethanol by weight requires nine fermentors, each of which
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has a capacity of 800,000 gal if a fermentation time of 72 h is assumed. The 800,000-gal
capacity of the fermentors was selected on the basis of typical industry practice. The
runtime was assumed to be 80 h/batch. The current analysis is based on an output of 40
million gallons per year. Further economic benefits as a result of scale will be realized as
the technologies develop and larger plants are constructed.
In the model, each fermentor has a train of pretreatment and saccharification tanks
that work in batch mode. Each train supplies its own fermentor, thereby avoiding the
need for holding tanks for intermediate storage and minimizing the potential for fouling
the saccharified mash. The pretreatment unit, saccharification tank, and fermentor,
therefore, scale linearly with the size of the plant. Other units—such as distillation,
centrifugation, shredding, drum drying, and boiler and steam generator—operate
continuously. The continuously operating units were sized so that they can supply and
process the contents of a single fermentor over a period equal to t/N, where t is the
duration of each batch, and N is the number of processing units. Because the
fermentations are staggered for efficient loading and emptying, overall batch time is set at
80 h, which is the time required for cycling the full set of eight to 10 fermentors.
Although fermentations operate in batch mode, using multiple vessels enables a staged
operation so that the upstream processing, including pretreatment, can be carried out
continuously.
The major process determinants that affect the size and operation of a plant are
• Yield in saccharification,
• Yield in pretreatment,
• Fermentation time and yield and
• Solids loading.
With the exception of fermentation time, which was set at 72 h for all simulations, the
parameters listed above were varied with the ranges indicated in Table 3-2 representing
low, medium, and high performance scenarios. Once those parameters were defined, the
design of the various units, the sizing, and cost calculations were automatically handled
by SuperPro Designer. Details of the modeling and analyses are presented in Appendix I.
TABLE 3-2 Assumptions for Low, Medium, and High Performance Cases
Variable Low Medium High
Size of reactors 800,000 gal 800,000 gal 800,000 gal
Solids loading for fermentation 18% 21% 25%
and hydrolysis
Pretreatment yield 80% 85% 95%
Saccharification extent 90% 95% 95%
Fermentation glucose 85% 90% 90%
Fermentation xylose 75% 81% 81%
Cellulose cost $0.40/gal $0.25/gal $0.10/gal
Cost of biomass (wet weight) $44/ton $35/ton $25/ton
Cost of biomassa (dry weight) $88/ton $70/ton $50/ton
a
Biomass costs in this table are used as illustrations and do not represent the range of costs
estimated by the panel.
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potential and are receiving renewed interest (Savage, 2007). The approaches being taken
include using well-established recombinant-DNA techniques to insert genes into
microorganisms to make specific fuel precursors or even direct synthesis of hydrocarbon
fuel components. Another approach involves redesigning genes with computer assistance
to perform specific reactions and then synthesizing the desired genes for insertion into
microorganisms. Yeasts can also be engineered to produce larger amounts of lipids,
which with additional metabolic engineering can be converted to useful products,
potentially fuels. That work has not progressed as far as the work on bacteria.
Those techniques might make it possible to modify bacteria to produce and
excrete specific hydrocarbon molecules that have desired fuel or other chemical
properties. Microorgaisms that produce and excrete specific hydrocarbons minimize the
costs of energy-consuming separation, although developing organisms that excrete the
fuel products is a major challenge in that most synthesis products, including
hydrocarbons, accumulate in the cell. No specific processes can be considered to be
approaching commercial production at this point, but the magnitude of activity and the
current rate of progress could change that in the not-too-distant-future. Several companies
are using synthetic biology to produce bacteria that make increased amounts of fatty
acids or other lipids that are then converted to hydrocarbons and excreted. The bacteria
make and excrete hydrocarbons of any desired length and structure. The phase-separation
of hydrocarbons from the growth medium, markedly reducing separation costs. The
feedstock for the bacteria is renewable sugars, which can be obtained from sugar cane or
grain or from cellulosic biomass (LS9, 2008). It is difficult to project future
developments. Some companies are producing fuels, but projected costs of fuels have not
been reported (Service, 2008).
Technologies to Improve Biochemical Conversion
Important advances are being made in genomics, molecular breeding, synthetic
biology, and metabolic and bioprocess engineering that will probably enable
discontinuous innovation and advancement in alternative transportation fuels. Those
advances and related technologies have the potential to accelerate the creation of
dedicated or dual-purpose energy crops and microorganisms that can be used for both
biofuel production and feedstock conversion.
Genomics
The sequencing of full genomes continues to become faster and less expensive,
and this is enabling the sequencing of energy crops, such as trees, perennial grasses, and
such nonedible oilseeds as castor and jatropha. Their sequence data are extremely
important for improving overall yields, for enabling improved nutrient and water use, and
for understanding and manipulating biochemical pathways to enhance the production of
desired materials. Sequence data can also be used to target specific genes for
downregulation by classical methods, such as antisense and RNA interference, and via
complete inactivation with new and evolving procedures for homologous recombination-
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based gene disruption. Rapid sequencing of breeding populations of energy crops will
enable marker-assisted selection to accelerate breeding programs in ways previously not
possible. Furthermore, rapid and inexpensive sequencing of fermentative and
photosynthetic microorganisms is redefining and shortening the timelines associated with
strain-development programs for converting sugars, lignocellulosic materials, and CO2 to
alternative liquid fuels. Strains generated through classical mutagenesis that have
improved biocatalytic properties can now be analyzed at the molecular level to determine
the specific genetic changes that result in the improved phenotype, and this allows the
changes to be implemented in additional strains. In addition, “metagenome” sequence
data obtained by randomly sequencing DNA isolated from environmental samples is
providing huge numbers of new gene sequences that can be used in genetic engineering
to improve crops and microorganisms.
Synthetic Biology and Synthetic Genomics
Improved technologies for synthesizing megabase DNA molecules are being
developed to allow the introduction of entire biochemical pathways into energy crops and
biofuel-producing microorganisms. The technologies could have a great effect on
scientists’ ability to generate plants and microorganisms with specific desirable traits. For
example, it is becoming conceivable to replace large portions of, or even complete,
chromosomes of microorganisms (including photosynthetic microorganisms) in ways that
will focus the vast majority of their cells’ biochemical machinery toward production of
next-generation biofuel molecules and thus provide cost and product advantages.
Maintaining the purity of such cultures and finding ways to disadvantage mutants that
gain competitive ability by producing less of the desired secondary chemicals could be
serious hurdles.
Metabolic and Bioprocess Engineering
In addition to genetic manipulation, new bioengineering technologies that will
lower the cost of biofuel formation and recovery are coming on line. Synthetic biology
can now provide synthetic DNA for transferring heterologous genes into suitable host
cells, but metabolic engineering is the enabling technology for constructing functional
and optimal pathways for microbial fuel synthesis. This field has matured in only a few
years and has an impressive record of accomplishments, many already being applied in
industry (for example, in the production of biopolymers, alcohols, 1,3-propanediol, oils,
and hydrocarbons). Microbial strains that secrete hydrophobic fuels that are similar to
constituents of diesel fuel and gasoline into the culture medium have been developed.
The fuels can be separated from the aqueous phase in a manner that simplifies distillation
and thereby reduces energy inputs and facilitates continuous production. By taking a
systems view of metabolism, metabolic engineering developed tools for overall
biosystems optimization that are now facilitating the optimal construction of biosynthetic
pathways and elicitation of novel multigenic cellular properties of critical importance for
biofuels production, such as tolerance of fuel toxicity. In the bioprocessing sphere, the
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successful development of membrane-based alcohol separation would greatly reduce
energy costs relative to the typically used distillation process (Vane, 2008). Gas-
stripping, liquid-liquid extractions of secreted fuel molecules or new adsorbent materials
that will allow continuous production modes for fermentation-based products are also
being developed (Vane, 2008). For photosynthetic production of biofuels, the
development of low-cost photobioreactors and associated recovery systems for algal
biofuel production is of great interest and could have substantial beneficial effects on
overall process economics.
FINDINGS AND RECOMMENDATIONS
Grain-based ethanol is a bridge to advanced biofuels that has important potential
for greenhouse-gas displacement. Advanced biofuels do not directly compete with food
and feed supply, and they minimize indirect land-use change if appropriate feedstocks are
selected and sustainable practices are used in their production. Grain ethanol has initiated
public awareness of the use of ethanol in the current and future transportation fleet and of
the pitfalls of feedstock supply for a new industry. Grain ethanol has helped to establish
an industrial infrastructure for advanced biofuels and for distribution and use of fuel
ethanol.
Lignocellulosic feedstocks for production of advanced biofuels could be
agricultural or forestry residues, agricultural cover crops, dedicated perennial crops
grown on marginal lands that are not suitable for commodity-crop production even with
high commodity prices, or municipal solid wastes. Biochemical conversion of cellulose
to liquid fuels emulates commercial corn grain-to-ethanol technology but might require
additional processing steps and could result in other types of alcohols and hydrocarbon-
rich fuels.
The technologies for biochemical conversion of cellulosic biomass to ethanol are
in the early stages of demonstration and commercial development. Several demonstration
plants are expected to be operational by 2012. The panel judges that cellulosic bioethanol
will be commercially deployable before 2020, and other advanced biofuels are likely to
emerge after 2020.
Finding 3-1
Engineering and operational knowledge can be gained only from designing and
building commercial-scale, integrated cellulosic-ethanol facilities and then operating
them for a reasonable period. The first few commercial plants will be more expensive
than commercial facilities that follow because of the learning that occurs with a first-of-
its-kind facility. The initial learning that occurs with first-of-a-kind plants will lead to
further cost-reducing improvements in commercial facilities deployed thereafter. The
pace of learning is expected to be similar to that in the chemical industry, in which costs
have historically decreased by 30-40 percent over several cycles of deployment and
concurrent process improvement.
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Recommendation 3-1
The federal government and industry should aggressively pursue technology
demonstration or small-scale commercial plants, which will lead to full-scale
commercial production of cellulosic ethanol to define its potential and to provide
data on engineering and cost performance to help in preparation for full
commercial deployment.
In the immediate term, pretreatment and enzymatic hydrolysis, fermentation, or
combined enzymatic hydrolysis and fermentation need to be substantially improved to
allow efficient deconstruction of carbohydrate polymers to simple sugars and
fermentation of the sugars to ethanol. Research in and improvement of pretreatment, with
engineering of appropriate microorganisms for optimal use of the resulting simple sugars
in an adverse fermentation environment, will have a direct impact on reducing the cost of
transforming cellulosic feedstocks to ethanol. The cost of producing sugars directly
affects the cost of ethanol. In addition, the sugars have to be converted to ethanol
efficiently to minimize feedstock and operational costs.
Feedstock, pretreatment, and enzymes are key components of a cellulose-to-
ethanol process, and they are all related to the goal of preparing lignocellulosic
feedstocks (through agronomics, plant molecular genetics, and pretreatment) so that they
are readily transformed to sugars and ethanol at low cost. Other targets for improvement
include increasing solids loading and developing engineered microorganisms and
enzymes that have increased tolerance of toxic compounds in biomass hydrolysates and
of the biofuel products themselves. Incremental improvements in biochemical conversion
technologies and the learning and experience gained from R&D and demonstration can
be expected to reduce nonfeedstock processing costs by 25 percent by 2020 and 40
percent by 2035 (see Table 3-3).
Finding 3-2:
Process improvements in cellulosic-ethanol technology are expected to be able to
reduce the plant-related costs associated with ethanol production by up to 40
percent over the next 25 years. Over the next decade, process improvements and cost
reductions are expected to come from evolutionary developments in technology, from
learning gained through commercial experience and increases in scale of operation, and
from research and engineering in advanced chemical and biochemical catalysts that will
enable their deployment on a large scale.
Recommendation 3-2
The federal government should continue to support research and development to
advance cellulosic-ethanol technologies. R&D programs should be pursued to resolve
the major technical challenges facing ethanol production from cellulosic biomasss:
pretreatment, enzymes, tolerance to toxic compounds and products, solids loading,
engineering microorganisms, and novel separations for ethanol and other biofuels. A
long-term perspective on the design of the programs and allocation of limited resources is
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needed; high priority should be placed on programs that address current problems at a
fundamental level but with visible industrial goals.
Recommendation 3-3:
The pilot and commercial-scale demonstrations of cellulosic-ethanol plants should
be complemented by a closely coupled research and development program. R&D is
necessary to resolve issues that are identified during demonstration and to reduce costs of
sustainable feedstock acquisition. Industrial experience shows that such reductions
typically occur as processes go through multiple phases of implementation and
expansion.
Finding 3-3:
Future improvements in cellulosic technology that entail invention of biocatalysts
and biological processes could produce fuels that supplement ethanol production in
the next 15 years. In addition to ethanol, advanced biofuels (such as lipids, higher
alcohols, hydrocarbons, and other products that are easier to separate than ethanol) should
be investigated because they could have higher energy content and would be less
hydroscopic than ethanol and therefore could fit more smoothly into the current
petroleum infrastructure than ethanol.
Recommendation 3-4:
The federal government should ensure that there is adequate research support to
focus advances in bioengineering and the expanding biotechnologies on developing
advanced biofuels. The research should focus on advanced biosciences—genomics,
molecular biology, and genetics—and biotechnologies that could convert biomass
directly to produce lipids, higher alcohols, and hydrocarbons fuels that can be directly
integrated into the existing transportation infrastructure. The translation of those
technologies into large-scale commercial practice poses many challenges that need to be
resolved by R&D and demonstration if major effects on production of alternative liquid
fuels from renewable resources are to be realized.
Finding 3-4:
Biochemical conversion processes, as configured in cellulosic-ethanol plants,
produce a stream of relatively pure CO2 from the fermentor that can be dried,
compressed, and made ready for geologic storage or used in enhanced oil recovery
with little additional cost. Geologic storage of the CO2 from biochemical conversion of
plant matter (such as cellulosic biomass) further reduces greenhouse-gas life-cycle
emissions from advanced biofuels, so their greenhouse-gas life-cycle emissions would
become highly negative.
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Recommendations 3-5:
Because geologic storage of CO2 from biochemical conversion of biomass to fuels
could be important in reducing greenhouse-gas emissions in the transportation
sector, it should be evaluated and demonstrated in parallel with the program of
geologic storage of CO2 from coal-based fuels.
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