MEMORANDUM

Date: November 17, 2010

Subject: Assessment of Advanced Biofuel Technologies

From: Lester Wyborny

Chemical Engineer

To: EPA Air Docket Number EPA-HQ-OAR-2010-0133

1. Background Information of Cellulosic Biofuel Production

The biochemical conversion of cellulosic biomass to fuels requires a detailed 

understanding of the chemistry of the cellulosic biomass. Unlike grain feedstocks where 

the major carbohydrate is starch, lignocellulosic biomass is composed mainly of cellulose 

(40-60 %) and hemicellulose (20-40 %). The remainder consists of lignin, a complex 

polymer which serves as a stiffening and hydrophobic (water-repelling) agent in cell 

walls.1 Cellulose and hemicellulose are made up of sugars linked together in long chains 

called polysaccharides. Once hydrolyzed, they can be fermented into ethanol or other 

biofuels. Currently, lignin cannot be fermented into ethanol, but could be burned as a byproduct to generate electricity or converted by the other technologies into biofuels as 

discussed in this document. 

Both starch (e.g. corn grain) and cellulosic feedstocks must be hydrolyzed prior to 

fermentation. Structural differences at the molecular level make it far more difficult, and 

therefore more costly, to hydrolyze cellulosic biomass than it is to hydrolyze starch. 

Glucose, C6H12O6, the repeating monomer in both starch and cellulose, is a six-sided 

ring, similar in conformation to the classic ‘chair’ conformation of cyclohexane or 

benzene, except one carbon atom in the ring is replaced by an oxygen atom. For 

uniformity (and ease) of discussion, it is generally assumed that the first carbon atom 

next to the oxygen, is carbon #1; the numbering, 2-5, continues around the ring with 

oxygen in the 6th position; one of the four bonds of the fifth carbon atom is attached to 

the oxygen atom to complete the ring, one is attached to hydrogen atom and the fourth to 

a -CH2OH group. Thus, a glucose molecule/monomer is a six-sided molecule, but not a 

six-carbon ring (although there are six-carbon molecules present, one of which is in the –

methylhydroxy group). 

The main difference between starch and cellulosic plant matter is that starch 

polysaccharides are made up of -glucose monomers, uniformly strung together by -

linked 1,4-glucosidic bonds whereas cellulosic polysaccharides are made up of β-glucose 

monomers, strung together through β-linked 1,4-glucosidic bonds. In starch with the -

conformation, the hydroxyl group on carbon #1 is in the axial or -position, which causes 

the -OH’s on each successive glucose monomer to end up on the same side of the 

polymer. There are also 1,6-linked glucose branches that occur irregularly on 

approximately one in twenty-five glucose units.2 The -OH groups on the same side of 
the polymer, along with the randomly attached 1,6-glucose branches, leaves starch 

polymers relatively weak, flexible, and able to easily wrap and twist together to form tiny

granules ( e.g., common, everyday corn starch).

Cellulosic polysaccharides are in the β-conformation with the hydroxyl group on 

carbon #1 is positioned away from the ring, in the equatorial or β-position, which causes 

the -OH’s on each successive glucose monomer, added to the chain, to end up on 

opposite sides of the polymer. The hydroxyl groups lined up evenly and uniformly along 

opposite sides of each polymer strand allow intra-molecular hydrogen bonds to develop 

within each monomer. They also allow inter-molecular hydrogen bonds to develop 

between adjacent polymers to form tight, rigid, strong, mostly straight polymer bundles 

called microfibrils that act as the core constituent in the formation of plant cell walls that 

are also insoluble in water and resistant to chemical attack. The β-conformation and the 

resulting hydrogen bonds stabilize the glucose chair structure to help minimize the 

polymer’s flexibility (which hinders hydrolysis) and to add to its strength. 

The second cellulosic component is called hemicellulose. It consists mainly of a 

random mixture of highly branched and heavily substituted five- and six-carbon rings. 

The five-carbon residues are usually D-xylose and L-arabinose; the six-carbon residues 

are usually D-galactose, D-glucose, and D-mannose, and uronic and acetic acid. 

Hemicellulose is not as rigid or strong as cellulose, but does contribute additional 

strength and helps protect the plant cell wall against attack by microbes or water. 

Hemicellulose is relatively easy to hydrolyze, due to its highly branched, somewhat 

random or non-uniform structure. 

Lignin, the third principle component, is a complex, cross-linked polymeric, high 

molecular weight substance derived principally from coniferyl alcohol by extensive 

condensation polymerization. Covalently bonded to the hemicellulose, it is essentially a 

glue-like polymer that covers the cellulose and hemicellulose polymer cell walls and 

helps hold them together, provides additional strength, helps resist microbial decay, and 

perhaps most importantly, for this discussion, inhibits hydrolysis. Its molecular weight is 

around 10,000.3 While both cellulose and hemicellulose contribute to the amount of 

fermentable sugars for ethanol production, lignin does not, but can be combusted to 

provide process energy in a biochemical plant or used as feedstock to a thermochemical 

process.4 

A significant part of the reason it is more difficult and more costly to produce 

ethanol from cellulosic feedstocks, has to do with the differences in the molecular 

structures of simple starches and oils and those of cellulosic plant matter. That is, as a 

plant grows, glucose monomers are added to the polysaccharide chains of the plant cell 

walls through condensation reactions. In general, condensation is a chemical process by

which two molecules are joined together to make a larger, more complex molecule, and a 

molecule of water is a byproduct of the reaction. In the formation of polysaccharides, 

and enzyme catalyzes the reaction wherein the -OH group on carbon #1 of one monomer, 

or glucose residue, reacts with the -OH on carbon #4 or #6 of another residue. An H-OH 

(H2O or water) molecule is removed leaving an -O- that links the monomers together to 
form the polysaccharide chain. Again, depending on the direction of the –OH group at 

carbon 1, it may be called an alpha (as in starch) or a beta (as in cellulose) linkage.5 

Hydrolysis is the reverse reaction. The -H from an H-OH (water) molecule is 

added to one monomer and the remaining -OH is added to its pair, e.g., to the next 

monomer on the chain, to regenerate separate glucose monomers. During starch 

hydrolysis, water and water borne hydrolyzing enzymes can easily penetrate the 

randomly formed polymers (the tiny granular particles or bundles) in order to break the 

bonds to release glucose monomers. However, the cellulosic or glucan polymers formed 

in tightly packed, dense, rigid microfibrils are especially resistant to water and 

hydrolyzing enzymes. Xylan, the main constituent of hemicellulose, is more easily 

hydrolyzed than cellulose, but not easily fermented. Cellulose is not easily hydrolyzed, 

but readily ferments into alcohol. These are two of the major problems that must be 

satisfactorily resolved for biochemical conversion of cellulosic feedstocks.

While the biochemical process primarily uses enzymes or acids to disassemble the 

cellulose and hemicellulose followed by fermentation, the thermochemical process 

operates substantially differently. In the thermochemical process, biomass is gasified at 

temperatures that are above 850°C. The gasification process partially oxidizes all 

biomass carbonaceous feed stock into gas, which is high in energy and consists of CO, 

CO2, CH4, H2 and H2O, along with other minor constituents. The CO and hydrogen are 

the main building block components for making liquid fuels via conversion through 

downstream synthesis reactions. 

Because of the thermal oxidation that occurs, all of the cellulosic biomass 

feedstock, including the lignin component, can be used to make renewable fuels. Due to 

its ability to convert the lignin into fuels, the thermochemical process may be more 

feedstock flexible than the biochemical process. In general, the lignin fraction content in 

woody feedstocks is higher than fractions found in herbaceous biomass (e.g., grasses, 

straw, corn stalks). Therefore, thermochemical technologies may be more compatible 

than biochemical cellulosic biofuel technologies for processing wood-based feedstocks. 

Wood products generally have higher heating values than other biomass stocks, such as 

stover, grasses etc which also makes them better feedstocks for a thermochemical unit. 

Higher energy cellulosic feeds yield more CO and hydrogen synthesis gas, resulting in 

higher liquid yields per ton of feedstock.

2. Emerging Technologies

When evaluating the array of biofuel technologies which could produce one or 

more fuels that could qualify under RFS2, we found that it is helpful to organize them 

into fuel technology categories. Organizing them into categories eases the task of 

understanding the costs and life cycle impacts of these technologies because like 

technologies likely have similar cost and life cycle impacts. The simplest organization is 

by the fuel produced. However, we frequently found that additional subdivisions were 

also helpful. Table 2.1 provides a list of technologies, the cellulosic fuels produced and a 
list of many of the companies which we learned are pursuing the technology (or 

something very similar to the technology listed in the category).

Table 2.1

List of Biofuel Categories, the Fuels Produced and the Companies Pursuing Cellulosic 

Biofuel Technologies

Technology 

Category

Technology Fuels 

Produced 

Companies

Biochemical Enzymatic Hydrolysis Ethanol Abengoa, AE Fuels, 

DuPont/Danisco, Florida 

Crystals, Gevo, Poet, ICM, 

Iogen, BPI, Energy, Fiberight, 

KL Energy.

Acid Hydrolysis Ethanol Agresti, Arkenol, Blue Fire, 

Pencor, Pangen, Raven 

Biofuels

Dilute Acid, Steam Explosion of 

Cellulose

Ethanol Verenium, BP, Central 

Minnesota Ethanol Coop.

Consolidated Bioprocessing (one 

step hydrolysis and fermentation) 

of Cellulose

Ethanol Mascoma, Qteros

Conversion of Cellulose via 

carboxylic acid

Ethanol, 

Gasoline, Jet 

Fuel, Diesel 

Fuel

Terrabon, Swift Fuels

One step Conversion of Cellulose 

to distillate

Diesel, Jet 

Fuel or 

Naphtha

Bell Bioenergy, LS9

Thermochemical Thermochemical/Fischer Tropsch Diesel Fuel 

and Naphtha

Choren, Flambeau River 

Biofuels, Baard, Clearfuels, 

Gulf Coast Energy, Rentech, 

TRI.

Thermochemical/Fischer Tropsch DME Chemrec, New Page.

Thermochemical/Catalytic 

conversion of syngas to alcohols

Ethanol Range Fuels, Pearson 

Technologies, Fulcrum 

Bioenergy, Enerkem, and Gulf 

Coast Energy.

Hybrid Thermochemical w/ Biochemical 

catalyst

Ethanol Coskata, INEOS Bio

Acid Hydrolysis of cellulose to 

intermediate; hydrogenation 

using Thermochemical syngas 

from non-cellulose fraction

Ethanol, 

Other 

alcohols

Zeachem

Depolymerization Catalytic Depolymerization of 

Cellulose

Diesel, Jet 

Fuel or 

Naphtha

Cello Energy, Covanta, Kior, 

University of Massachusetts

Pyrolysis of Cellulose Diesel, Jet 

Fuel, or 

Gasoline

Envergent (UOP/Ensyn), 

Dynamotive, Petrobras

Other Catalytic Reforming of Sugars 

from Cellulose

Gasoline Virent
Of the technologies listed above, many of them are considered to be “second 

generation” biofuels or new biofuel technologies capable of meeting either the advanced 

biofuel or cellulosic biofuel RFS standard. The following sections describe specific 

companies and the new biofuel technologies which the companies have developed or are 

developing. This summary is not meant to be an unabridged list of new biofuel 

technologies, but rather a description of some of the more prominent or interesting of the 

new biofuel technologies that serve to provide a sense of the technology categories listed 

above. The process technology summaries are based on information provided by the 

respective companies. EPA has not been able to confirm all of the information, 

statements, process conditions, and the process flow steps necessary for any of these 

processes and companies. 

3. Biochemical Process for the Production of Cellulosic Biofuels

Biochemical conversion refers to a broad grouping of processes that use 

biological organisms or enzymes from such organisms to convert cellulosic feedstocks 

into biofuels. While no two processes are identical, many of these processes follow a 

similar basic pathway to convert cellulosic materials to biofuel. The general process of 

most biochemical cellulosic biofuel processes consists of five main steps, feedstock 

handling, pretreatment, hydrolysis, fermentation/fuel conversion, and 

distillation/separation. The feedstock handling step reduces the particle size of the 

incoming feedstock and removes any contaminants that may negatively impact the rest of 

the process. In the pretreatment step the structure of the lignin and hemicellulose is 

disrupted, usually using some combination of heat, pressure, acid, or base, to allow for a 

more effective hydrolysis of the cellulosic material to simple sugars. In the hydrolysis 

stage the cellulose and any remaining hemicellulose is converted into simple sugars, 

usually using an enzyme or strong acid. In the fermentation or fuel conversion step, the 

simple sugars are converted to the desired fuel by a biological organism. In the final step 

the fuel that is produced is separated from the water and other byproducts by distillation 

or some other means. A basic diagram of the biochemical conversion process can be 

found in Figure 3.1.

Figure 3.11

 

1

Image From: http://www.afdc.energy.gov/afdc/ethanol/production_cellulosic.html
While this diagram shows the production of ethanol from cellulosic biomass, it is 

possible to use the same process to produce other fuels or specialty chemicals using 

different biological organisms. The following sections will discuss each of these steps in 

greater detail, discuss some of the variations to this general process, and discuss some of 

the advantages and disadvantages of the biochemical process of producing biofuel from 

cellulosic materials as compared to other fuel production processes.

Three of the five companies that EPA believes may produce cellulosic biofuel in 

2011 plan to use a biochemical process to produce biofuels. All three of these 

companies, Dupont Danisco Cellulosic Ethanol, Fiberight, and KL energy, plan to use an 

enzymatic hydrolysis, while a variety of other companies are exploring other production 

methods. The main reason for the dominance of biochemical technologies in 2011 is the 

relatively low capital costs of these projects compared to other cellulosic biofuel 

facilities. Biochemical projects also benefit less from economies of scale, making 

smaller and less capital intensive commercial facilities more feasible. The following 

sections provide more information on the biochemical processes being pursued by the 

majority of the companies we expect to produce cellulosic biofuels and make them 

commercially available in 2011, as well as many other companies planning to begin 

production in later years.

3.1 Feedstock Handling

The first step of the biochemical conversion process is to insure that the biomass 

stream can be utilized by the rest of the conversion process. This most often takes the 

form of size reduction, either by grinding or chipping as appropriate for the type of 

biomass. While this is a relatively simple process it is essential to allow the following 

steps of the process to function as designed. It is also a potentially energy intensive 

process. It may be possible for biofuel producers to purchase cellulosic material that is 

already of the appropriate size, however we believe that in the near term this is unlikely

and most biofuel producers will have to invest in equipment to reduce the size of the 

material they receive as needed for their process. In coming years, as the market for 

cellulosic materials expands, purchasing feedstock that has already been ground or 
chipped may be possible and cost effective, as these processes increase the density of this 

material and may reduce transportation costs.

In addition to size reduction, steps must also be taken to remove any material 

from the feedstock that might be detrimental to the fuel production process. At best, 

contaminants in the feedstock would travel through the fuel production process 

unchanged, perhaps resulting in a slight reduction in fuel production volume. Depending 

on the type of contaminant they may also be converted to undesired byproducts that must 

be separated from the fuel. At worst, these contaminants could be toxic to the biological 

organisms being used to convert the sugars to fuel, necessitating a shut down and restart 

of the plant. Either of these scenarios would result in a significant cost to the fuel 

producer. Feedstocks such as agricultural residues, wood chips, or herbaceous or woody 

energy crops are likely to contain far fewer contaminants than more heterogeneous 

feedstocks such as municipal solid waste (MSW).

3.2 Biomass Pretreatment

The purpose of the biomass pretreatment stage is to disrupt the structure of the 

cellulosic biomass to allow for the hydrolysis of the cellulose and hemicellulose into 

simple sugars. The ideal pretreatment stage would allow for a high conversion of the 

cellulose and hemicellulose to simple sugars, minimize the degradation of these sugars to 

undesired forms that reduce fuel yields and inhibit fermentation, not require especially

large or expensive reaction vessels, and be a relatively robust and simple process. No 

single biomass pretreatment method currently exists that meets all of these goals, but 

rather a variety of options are being used by various cellulosic fuel producers, each with 

their own strengths and weaknesses. Dilute acid pretreatment and alkaline pretreatment 

are two methods currently being used that attack the hemicellulose and lignin portions of 

the cellulosic biomass respectively. Other methods, such as steam explosion and 

ammonia fiber expansion, seek to use high temperature and pressure, followed by rapid 

decompression to disrupt the structure of the cellulosic biomass and allow for a more 

efficient hydrolysis of the cellulose and hemicellulose to simple sugars. The cost and 

characteristics of the cellulosic feedstock being processed is likely to have a significant 

impact on the pretreatment process that is used. 

3.2.1 Dilute Acid Pretreatment

The dilute acid pretreatment uses a dilute acid solution, generally ~1%, sulfuric, 

hydrochloric, or nitric acid, at a high temperature, usually greater than 160° C to 

hydrolyze the hemicellulose to simple sugars. The dilute acid pretreatment produces very

high yields of xylose, the most important sugar in the hemicellulose, and requires very

short reaction times, often as low as several minutes. It also allows for high conversion 

rates of the cellulose to glucose in the later stages of the process. Using a dilute acid 

pretreatment, however, is a relatively expensive method. Corrosive resistant materials 

must be used for the reaction vessels due to the presence of the acid. The acid must also 

be neutralized prior to the cellulose hydrolysis stage if enzymes are to be used, and before 

the fuel production stage as it is harmful to the biological organisms used for the fuel 
production. Lime can be used to neutralize the acid, however this produces gypsum that 

must be removed from the process. Other compounds, such as ammonia, can be used that 

do not produce solids, but the higher costs for these compounds generally negates any 

savings from the lack of equipment required to separate the gypsum.

3.2.2 Alkaline Pretreatment

The alkaline pretreatment process is designed to dissolve and disrupt the structure 

of the lignin portion of the lingocellulosic materials rather than the hemicellulose. This 

method exposes the feedstock to an alkaline solution, usually a solution of 1-2% sodium 

hydroxide, calcium hydroxide, or ammonia. The reaction time is approximately one 

hour, however this can vary depending on the feedstock being used. High temperatures 

are not required for this process, however they are sometimes used to decrease the 

reaction time or increase the sugar conversion of the cellulose and hemicellulose in later 

stages of the process. If an alkaline pretreatment is used a different combination of 

enzymes are required for the subsequent hydrolysis step, as now the hemicellulose must 

be hydrolyzed in addition to the cellulose. Alkaline pretreatment has lower capital costs 

than dilute acid pretreatment as cheaper materials are able to be used for the reaction 

vessels. The alkaline solution can also be recovered and recycled rather than neutralized 

which decreases the cost of chemical usage and waste water treatment. The main 

disadvantage of the alkaline pretreatment process is that yields of the sugars from the 

cellulose and hemicellulose tend to be lower than those for other processes. This must be 

weighed against the lower capital costs when considering using an alkaline pretreatment 

process. DuPont Danisco Cellulosic Ethanol is one company planning on using an 

alkaline pretreatment in the production of cellulosic biofuels.

3.2.3 Steam Explosion

In the steam explosion pretreatment process the lingocellulosic feedstock is 

treated with high temperature and pressure saturated steam. The feedstock is held at high 

temperature and pressure, up to 50 bar and 290° C for a period of time ranging from 

several seconds to ten minutes, and then undergoes a sudden decompression to 

atmospheric pressure. This decompression causes a disruption of the lignin and 

hemicellulose structure allowing for hydrolysis of the remaining hemicellulose and 

cellulose in later steps. This process is advantageous in that it uses no acids or bases for 

the pretreatment resulting in decreased costs and environmental impacts. In the rapid 

decompression, however, compounds can be formed that can inhibit the later hydrolysis 

and fuel production processes. The pretreated biomass will likely have to be washed to 

remove these compounds. This washing can also result in the loss of soluble 

hemicellulose and ultimately simple sugars and fuel produced. Some steam explosion 

processes add small amounts of acid with the saturated steam to increase the hydrolysis 

of hemicellulose and cellulose in later steps and to decrease the production of undesired 

compounds that inhibit simple sugar and fuel production. The addition of acid, however, 

increases the capital costs and adds acid neutralization costs as discussed in section 3.2.1 

above. Companies using steam explosion to produce cellulosic biofuels include IOGEN 

and Verinium.
3.2.4 Ammonia Fiber Explosion

Ammonia Fiber Explosion (AFEX) operates much like steam explosion. 

Lignocellulosic feedstock is treated with liquid ammonia at a high temperature (~90° C) 

and pressure. These conditions are held for much longer than with steam explosion, 

approximately 30 minutes, before being rapidly decompressed to atmospheric pressure. 

As with the steam explosion process where acid has been added, compounds are not 

produced that would interfere with later hydrolysis and fuel production steps , so a wash 

step to remove these compounds is not necessary. In order for the AFEX pretreatment 

process to be economically feasible the ammonia must be recovered and reused. This 

process produces a treated feedstock with high rates of simple sugar production in the 

hydrolysis step as long as the lignocellulosic feedstocks have relatively low lignin 

content. The AFEX process is effective at treating herbaceous energy crops and 

agricultural residues such as corn stover and wheat and barley straw. AFEX is less

effective as a pretreatment process for feedstocks with high lignin content such as wood 

chips or newspaper. If the AFEX pretreatment is used for feedstocks with high lignin 

content the result will be low sugar production in the hydrolysis stage.

3.3 Hydrolysis

In the hydrolysis step the cellulose and any remaining hemicellulose are 

converted to simple sugars. There are two main methods of hydrolysis, acid hydrolysis 

and enzymatic hydrolysis. Acid hydrolysis is the oldest technology for the conversion of 

cellulosic feedstock to ethanol and can only be used following an acid pretreatment 

process. An alternative method is to use a mixture of different enzymes to perform the 

hydrolysis after the biomass has been pretreated. This process is potentially more 

effective at hydrolyzing pretreated biomass but in the past has not been economically

feasible due to the prohibitively high cost of the enzymes. The falling cost of these 

enzymes in recent years has made the production of cellulosic biofuels using enzymatic 

hydrolysis possible. The lignin is largely unaffected by the hydrolysis and fuel 

production steps but is carried through these processes until it is separated out in the fuel 

separation step and burned for process energy or sold as a co-product.

3.3.1 Acid Hydrolysis

Acid hydrolysis is a technique that has been used for over 100 years to convert 

cellulosic feedstocks into fuels. In the acid hydrolysis process the lignin and cellulose 

portions of the feedstock that remain after the hemicellulose has been dissolved, 

hydrolyzed, and separated during the dilute acid pretreatment process is treated with a 

second acid stream. This second acid treatment uses a less concentrated acid than the 

pretreatment stage but at a higher temperature, as high as 215° C. This treatment 

hydrolyzes the cellulose into glucose and other 6 carbon sugars that are then fed to 

biological organisms to produce the desired fuel. It is necessary to hydrolyze the 

hemicellulose and cellulose in two separate steps to prevent the conversion of the pentose 

sugars that result from the hydrolysis of the hemicellulose from being further converted 
into furfural and other chemicals. This would not only reduce the total production of 

sugars from the cellulosic feedstock, but also inhibit the production of fuel from the 

sugars in later stages of the process. 

The acidic solution containing the sugars produced as a result of the hydrolysis 

reaction must also be treated so that this stream can be fed to the biological organisms 

that will convert these sugars into fuel. In order to operate an acid hydrolysis process 

cost effectively the acid must be recovered, not simply neutralized. Methods currently 

being used to recover this acid include membrane separation and continuous ion 

exchange. The advantages of using an acid hydrolysis are that this process is well 

understood and capable of producing high sugar yields from a wide variety of feedstocks. 

Capital costs are high however, as materials compatible with the acidic streams must be 

extensively utilized. The high temperatures necessary for acid hydrolysis also result in 

considerable energy costs, and profitability is highly dependent on the ability to 

effectively recover and reuse the acid.

3.3.2 Enzymatic Hydrolysis

The enzymatic hydrolysis process uses enzymes, rather than acids, to hydrolyze 

the cellulose and any remaining hemicellulose from the pretreatment process. This 

process is much more versatile than the acid hydrolysis and can be used in combination 

with any of the pretreatment processes described above, provided that the structure of the 

lignocellulosic feedstock has been disrupted enough to allow the enzymes to easily access 

the hemicellulose and cellulose. After the feedstock has gone through pretreatment a 

cocktail of cellulose enzymes is added. These enzymes can be produced by the cellulosic 

biofuel producer or purchased from enzyme producers such as Novozymes, Genencor, 

and others. The exact mixture of enzymes used in the enzymatic hydrolysis stage can 

vary greatly depending on which of the pretreatment stages is used as well as the 

composition of the feedstock.

The main advantages of the enzymatic hydrolysis process are a result of the mild 

operating conditions. Because acid is not used, special materials are not required for the 

reaction vessels. Enzymatic hydrolysis is carried out at relatively low temperatures, 

usually around 50° C, and atmospheric pressure, and therefore has low energy

requirements and incurs lower capital costs. These conditions also result in less 

undesired reactions that would reduce the production of sugars and potentially inhibit fuel 

production. Enzymatic hydrolysis works best with a uniform feedstock, such as 

agricultural residues or energy crops, where the concentration and combination of 

enzymes can be optimized for maximum sugar production. If the composition of the 

feedstock varies daily, as can be the case with fuel producers utilizing MSW or other 

waste streams, or even seasonally, it would make it more difficult to ensure that the 

correct enzyme cocktail is being used to carry out the hydrolysis as efficiently as 

possible. The main hurdle to using an enzymatic hydrolysis has been, and continues to 

be, the costs of the enzymes. Recent advances by companies that produce enzymes for 

the hydrolysis of cellulosic materials have resulted in a much lower purchase price for 

these enzymes. If, as many researchers and cellulosic biofuel producers expect, the cost 
of these enzymes continues to fall, it is likely that enzymatic hydrolysis will be a lower 

cost option than acid hydrolysis, especially for cellulosic biofuel producers utilizing 

uniform feedstocks.

3.4 Fuel Production

After the cellulosic biomass has been hydrolyzed to simple sugars this sugar 

solution is converted to fuel by biological organisms. In some biochemical fuel 

production processes the sugars (principally five carbon) produced from the fermentation 

of the hemicelluloses are converted to fuel in a separate reactor and with a different set of 

organisms than the sugars (principally six carbon) produced from the hydrolysis of 

cellulose. Others processes, however, produce fuel from the five and six carbon sugars in 

the same reaction vessel.

A wide range of biological organisms can be used to convert the simple sugars 

into fuel. These include yeasts, bacteria, and other microbes, some of which are naturally 

occurring and others that have been genetically modified. The ideal biological organism 

converts both five and six carbon sugars to fuel with a high efficiency, is able to tolerate a 

range of conditions, and is adaptable to process sugar streams of varying compositions 

that may result from variations in feedstock. Many cellulosic biofuel producers have 

their own proprietary organism or organisms optimized to produce the desired fuel from 

their unique combination of feedstock, pretreatment and hydrolysis processes, and fuel 

conversion conditions. Other cellulosic fuel producers license these organisms from 

biotechnology companies who specialize in their discovery and production. 

The many different biological organisms being considered for cellulosic biofuel 

production are capable of producing many different types of fuels. Many cellulosic 

biofuel producers are working with organisms that produce ethanol. In many ways this is 

the most simple fuel to produce from lignocellulosic biomass as the production of ethanol 

from simple sugars is a well understood process. Others intend to produce butanol or 

other alcohols that have higher energy content. Butanol may be able to be blended into 

gasoline in greater proportion to ethanol and therefore has a potentially greater market as 

well as higher value due to its higher energy content. Yields for butanol, however, are 

currently significantly lower per ton of feedstock than ethanol. Some of the fuel 

producers who plan to produce alcohols are considering purchasing and modifying 

already existing grain ethanol plants. This would potentially have significant capital cost 

savings as many of the units used in a grain ethanol process are very similar to those 

required by the biochemical fuel production process and could be used with minimal 

modification.

Other cellulosic biofuel producers intend to produce hydrocarbon fuels very 

similar to gasoline, diesel, and jet fuel. These fuels command a higher price than 

alcohols, have a greater energy density, and are potentially drop-in fuels that could be 

used in conventional vehicles without strict blending limits. They could also be 

transported by existing pipelines and utilize the same infrastructure as the petroleum 

industry. Some of the processes being researched by fuel producers produce a single 
compound, such as iso-octane, that would need to be blended into petroleum gasoline in 

order to be used while others produce a range of hydrocarbons very similar to those 

found in gasoline or diesel fuel refined from petroleum and could potentially be used in 

conventional vehicles without blending. While the prospect of producing hydrocarbon 

fuels from cellulosic feedstock is promising, the current yields of fuel produced by these 

organisms are significantly lower than those that are producing ethanol and other 

alcohols. Improvement in the yields of these organisms will have to be realized in order 

for cellulosic hydrocarbon fuels produced via a biochemical process to compete with 

cellulosic ethanol, and ultimately petroleum based fuels.

3.5 Fuel Separation

In the fuel separation stage the fuel produced is separated from the water, lignin, 

any un-reacted hemicellulose and cellulose, and any other compounds remaining after the 

fuel production stage. The complexity of this stage is highly dependent on the type of 

fuel produced. For processes producing hydrocarbon fuels this stage can be as simple as 

a settling tank, where the hydrocarbons are allowed to float to the top and removed. 

Recovering the ethanol is a much more difficult task. To recover the ethanol a distillation 

process, nearly identical to that used in the grain ethanol industry, is used. The ethanol 

solution is first separated from the solids before being sent to a distillation column called 

a beer column. The overheads of the beer column are fed to a second distillation column, 

called a rectifier for further separation. The rectifier produces a stream with an ethanol 

concentration of approximately 96%. A molecular sieve unit is then used to dehydrate 

this stream to produce fuel grade ethanol with purity greater than 99.5%. The distillation 

of ethanol is a very energy intensive process and new technologies, such as membrane 

separation, are being developed that could potentially reduce the energy intensity, and 

thus the cost, of the ethanol dehydration process. The neat ethanol which is produced is 

blended with natural gasoline at a ratio of 2 % natural gasoline to 98 % ethanol to 

denature the ethanol before it is stored. The solids which were separated from the 

ethanol solution, which contains the unused lignin, are dried and either burned on site to 

provide process heat and electricity or sold as a byproduct of the fuel production process. 

The waste water is either recycled or sent to a water treatment facility.

3.6 Process Variations

While the process described above outlines the general biochemical process used 

by many cellulosic biofuel producers, there are several prominent variations being 

pursued by prospective biofuel producers. These variations usually seek to simplify the 

biochemical fuel production process by combining several steps into a single step or 

using other means to reduce the capital or operating costs of the process. Simultaneous 

Saccharification and Fermentation (SSF), Simultaneous Saccharification and CoFermentation (SSCF), Consolidated Bio-Processing (CBP), and Single Step Fuel 

Production are all production methods being developed by various biofuel production 

companies to combine two or more of the steps outlined above. These modifications are 

usually enabled by a proprietary technology or biological organism that makes these 

changes possible.
3.6.1 Simultaneous Saccharification and Fermentation (SSF)

In simultaneous saccharification and fermentation (SSF), the cellulose hydrolysis 

and fuel conversion steps are combined. After the lingocellulosic biomass has been 

pretreated cellulose enzymes and fuel conversion organisms, usually yeast, are added 

simultaneously. As the cellulase enzymes convert the cellulose to simple sugars the yeast 

immediately converts the sugars to ethanol. There are several benefits of the SSF

process. The first is the potential lowering of capital costs because only a single reaction 

vessel is required for both the hydrolysis of the cellulose and the fuel production from the 

resulting sugars. In addition, the products of the cellulose hydrolysis, the glucose and 

other simple sugars, are inhibitors to the enzymes performing the hydrolysis. By

converting them to ethanol as they are produced the sugar concentration is kept low and 

the conversion rate of the cellulose into simple sugars is maximized. There are several 

drawbacks to the SSF process. The rate of ethanol production is now limited by the 

enzyme activity. The ideal conditions for cellulose hydrolysis and the fermentation of the 

resulting sugars are not the same. This can result in slower overall reaction rates, and the 

fuel yields using this technology are not currently as high as when these two steps are 

performed separately.

3.6.2 Simultaneous Saccharification and Co-Fermentation (SSCF)

Simultaneous saccharification and co-fermentation (SSCF) is very similar to SSF. 

This process, however, combines the hydrolysis of the cellulose and the fermentation of

the resulting six carbon sugars with the fermentation of the five carbon sugars produced 

form the hydrolysis of the hemicellulose. The advantages and disadvantages of this 

process are also very similar to SSF, with further reductions in capital cost possible. 

Conditions also move further away from the optimal conditions for each of the three 

reactions and slower reaction rates and lower yields can result.

3.6.3 Consolidated Bio-Processing (CBP)

Consolidated Bio-Processing (CBP) further increases the simplicity of the process 

from SSCF. CBP uses genetically engineered organisms that are capable of producing 

cellulase enzymes and fermenting the simple sugars to ethanol. By combining all these 

steps into one process the fuel production process is greatly simplified and the cost of 

fuel production is potentially reduced. The simple process and mild conditions could 

result in both capital cost reductions as well as operating cost reductions. While CBP is a 

promising technology, there are currently no organisms that can efficiently produce 

cellulase enzymes and ferment sugars to ethanol. Organisms have been found in nature 

that possesses one or the other of these qualities, but not both together. Significant 

research is being done to produce such an organism, usually by starting with an organism 

that naturally expresses one of the desired traits and inserting genetic material to allow 

for the expression of the other desired trait. The commercial viability of CBP is 

dependent on the discovery of an organism that can perform both these tasks efficiently.
3.6.4 Single Step Fuel Production

Further simplification of the biochemical cellulosic biofuel process would include 

the elimination of the pretreatment stage. Bell Bio-Energy claims to have developed such 

a process. In this process the lingocellulosic feedstock is first ground to reduce the size 

and then the feedstock is immersed into water containing genetically engineered bacteria 

. The bacteria begin to digest the cellulose after only several hours, but require 30 to 60 

days to fully digest the cellulose. The produced fuel is constantly removed from the 

reaction vessel, and a significant amount of organic material is also produced which will 

be marketed as potting soil. The process is expected to produce 30 to 40 gallons of 

renewable fuel per ton of feedstock and the simplicity of the process results in low capital 

costs per volume produced.

While this process may appear very similar to those discussed above (SSF, SSCF, 

and CBP) it differs in several important ways. The fuel produced by the process is a 

diesel-like fuel that would need to be upgraded before being used as a transportation fuel, 

rather than ethanol. Bell Bio-Energy is currently working with a University to develop a 

process for upgrading the fuel they produce. The amount of time required for the 

organisms to produce fuel from the lingocellulosic feedstock is also much longer, 30 – 60 

days rather than hours. The yields of fuel per ton of biomass are much lower in this 

process, 30 – 40 gallons per ton versus 80 – 100 gallons per ton with most biochemical 

processes. Finally, the byproduct of this process is a potting soil, which Bell Bio-Energy

has found avenues to sell, rather than lignin. Despite the disadvantages listed above, the 

extremely low capital and operating costs of this process and high value byproduct may

allow for commercial success. As this technology has not been studied as extensively as 

many of the alternative methods at a small scale, there is greater technical uncertainty 

associated with this process.

3.6.5 Gravity Pressure Vessel

Another variation of the general biochemical cellulosic fuel production process is 

the use of a gravity pressure vessel for the pretreatment and hydrolysis of the 

lingocellulosic biomass. Using this process the feedstock material is pumped deep 

underground, to a depth of approximately 2000 feet. At this depth the pressure is near 60 

bar and the solution is heated to approximately 250° C. Under these conditions the lignin 

is dissolved and oxygen is added to combust the lignin to provide heat to maintain this 

high temperature. Next a weak acid is added to hydrolyze the remaining hemicellulose 

and cellulose into simple sugars. Once the hydrolysis is complete the acid is neutralized, 

the solution is returned to the surface, and the sugars are fed to the fuel producing 

organisms. 

The gravity pressure vessel allows the extreme conditions necessary for the break 

down and hydrolysis of the lingocellulosic material to be achieved with relatively little 

energy inputs, taking advantage of the naturally high pressure deep underground and the 

combustion of the lignin to provide the required heat. The two main downsides of using 

the gravity pressure vessel are high capital costs and no byproduct recovery. The gravity 
pressure vessel is an expensive piece of equipment, and because the lignin is combusted 

to provide process heat for the hydrolysis it cannot be recovered and used to provide 

process heat for fuel separation or sold as a potentially valuable byproduct. Agresti 

Biofuels is planning to use a gravity pressure vessel to produce ethanol from MSW at a 

facility in Pike County, Kentucky.

3.7 Current Status of Biochemical Conversion Technology

The biochemical cellulosic fuel production industry is currently transitioning from 

an industry consisting mostly of small scale research and optimization focused facilities 

to one capable of producing fuel at commercial scale facilities. Companies such as 

Iogen, DuPont Danisco Cellulosic Ethanol, Fiberight, and KL Energy are just beginning 

to market the fuel they are producing at their first small scale commercial fuel production 

facilities. By 2011 we expect several other cellulosic fuel production facilities using 

biochemical processes to come online. Many other facilities, including some large scale 

facilities capable of producing tens of millions of gallons of fuel, are planned to come 

online starting in 2012.

There are many factors that are likely to continue to drive the expansion of the 

cellulosic biofuel industry. The high price of petroleum fuels and the mandates put into 

place by the RFS2 program have created a large demand for cellulosic biofuels. The 

biochemical production process also has several advantages including relatively low 

capital costs, highly selective fuel production, flexibility in the type of fuel produced, and 

the promise of future production cost reductions.

While the poor worldwide economy and tight credit markets has had a negative 

impact on the biofuel industry as a whole the cellulosic biofuel producers utilizing 

biochemical processes have not been as hard hit as many others in the industry. This is 

partially due to the relatively low capital costs of biochemical production plants as a 

result of the relative simplicity and mild operating conditions of these plants. Several 

companies have been able to purchase distressed grain ethanol plants and are in the 

process of modifying them to produce cellulosic ethanol, further reducing the capital 

costs of their initial facilities. Once biochemical fuel production facilities have been 

constructed another advantage they have over other fuel production processes is that their 

high selectivity in the fuels they produce. Unlike chemical catalysts, which often 

produce a range of products and byproducts, biological organisms often produce a single 

type of fuel, which leads to very high fuel production rates per unit sugar. Finally, there 

is a large potential to further decrease the production costs of cellulosic biofuels using the 

biochemical processes. Unlike other production methods such as gasification which are 

relatively mature technologies, biochemical production of fuels is a young technology. 

One of the major costs of the biochemical fuel production processes currently are the 

enzymes. Great strides have been made recently in reducing the cost of these enzymes, 

and as the price of enzymes continues to fall so will the operating costs of biochemical 

fuel production processes.

3.8 Path to Commercialization
While there are many promising qualities of the biochemical fuel production 

process we have identified several different aspects of the process which can be further 

improved. The pretreatment processes can be improved to speed the conversion of 

cellulose and hemicellulose to simple sugars and to minimize the production of other 

undesired compounds, especially those that may inhibit the fuel production process. The 

ability of the biological fuel production organisms to process a wide range of both five 

and six carbon sugars can also be improved. Both these improvements will increase the 

fuel yield per ton of cellulosic feedstock, reducing the operating costs of the process. 

Finally, the enzyme production processes can be further optimized which would lower 

the price for enzymes and improve the economics of hydrolyzing cellulose to sugars.

Another opportunity for improvement would be the profitable utilization of the 

lignin portion of the cellulosic feedstock. Unlike some of the other cellulosic biofuel 

production processes, the biochemical process does not convert the lignin to fuel. 

Cellulosic feedstock can contain up to 40% lignin, depending on the type of feedstock 

used, so the effective utilization of this lignin is an important piece of the profitability of 

the biochemical process. One option for the use of the lignin is to burn it to provide 

process heat and electricity, as well as excess electricity that could be sold to the grid. 

While this would provide good value for the lignin, it would require fairly expensive 

boilers and turbines that increases the capital cost of the facility. If the lignin cannot be 

used as part of the fuel production process it may be able to be marketed as a solid fuel 

with high energy density and low carbon intensity. These various improvements to the 

cellulosic biofuel plants would make biochemical processes more cost-competitive with 

petroleum and other cellulosic biofuels.

While decreases in production costs are not a given there is a wide consensus 

within the cellulosic biofuels industry that future technological developments and 

economy of scale benefits from commercial scale production will result in significant 

production cost reductions. An analysis conducted by the National Renewable Energy

Laboratory in 2008 projected the cost of cellulosic ethanol production via the 

biochemical pathway decreasing from $2.43 per gallon in 2010 to $1.21 per gallon in 

20226. A table from this report, found below, shows how the operating costs of different 

parts of the cellulosic ethanol production process are expected to change over time.

Table 3.17

Changes in Operating Cost of Cellulosic Ethanol via the Biochemical Process Over Time
In considering NREL’s analysis of changes in the cost of production of cellulosic 

biofuels via the biochemical pathway over time, as well as studies conducted by other 

parties, there are three main areas from which the majority of the cost improvements are 

expected; feedstock cost, the cost of enzymes, and yields per ton of feedstock. The cost 

of feedstock is a large contributor to the cost of ethanol, responsible for approximately

40% of the cost of a gallon of ethanol in all three cases above. Any reductions in the cost 

of cellulosic feedstocks to the biofuel producers will have a direct and potentially

significant impact on the overall cost of cellulosic biofuel production. Efforts to decrease 

the cost of cellulosic feedstocks are taking place on a wide variety of fronts. Work in the 

area of plant breeding and genetics is underway to increase the efficiency with which 

plants utilize water and nutrients in order to decrease the cost of growing plants used by

cellulosic ethanol facilities. Another area of much interest is increasing the annual 

growth rate of crops that can potentially be use for biofuel production. This would 

decrease the number of acres needed to supply a cellulosic biofuel production facility as 

well as decrease the transportation cost for the feedstock by decreasing the area from 

which a production facility would have to import feedstock. New harvesters are being 

developed to increase the efficiency of harvesting cellulosic biomass. These are just 

some of the many different ways in which the cost of feedstocks for cellulosic biofuel 

producers is likely to decrease in the future.

A second area of significant potential cost savings for cellulosic biofuel producers 

is the cost of enzymes. Enzyme cost has long been a major hurdle to the economic 

production of cellulosic biofuel via the biochemical process, and it continues to be a 

major factor in the overall production cost. The recent trends in the cost of enzymes 

necessary to produce cellulosic biofuel, however, are promising. In February 2010 

Novozymes announced that it had reduced the cost of enzymes require for cellulosic 

ethanol production to less than $0.50 per gallon of ethanol produced, an cost reduction of 

80% in the past two years alone8. It is likely that the costs of the enzymes required for 

cellulosic biofuel production, and thus the overall cost of cellulosic biofuel production 

via the biochemical process, will continue to decrease.

A third way in which the cost of cellulosic biofuel production is expected to 

decrease is by increasing the yield of fuel per dry ton of biomass. As with the feedstock 

costs, there are many different ways in which cellulosic biofuel yields may be increased. 

Feedstocks are being developed with lower lignin content and higher cellulose and 

hemicellulose content to enable a larger portion of the biomass to be converted into 

biofuels by the biochemical process. More effective pretreatment methods and enzymes 

will likely convert more of the cellulose and hemicellulose into simple sugars. More 

versatile biological organisms may be able to convert more of the simple sugars into fuel. 

The cost impacts of increasing the yield of biofuel per ton of feedstock are similarly

diverse. Greater yields mean a lower feedstock cost on a per-gallon of fuel basis. They

also mean lower capital costs due to relatively smaller production facilities to produce the 

same amount of biofuel. Relatively smaller biofuel production facilities mean lower 

costs for other raw materials and energy inputs. The combination of these many effects 

would have a significant impact on the cost of cellulosic biofuel production. While it is 
impossible to precisely determine the extent of the progress in each of these areas over 

time, it highly likely that progress in some or all of these areas will contribute to future 

decreases in cellulosic biofuel production costs via the biochemical process .

4. Thermochemical Process for the Production of Cellulosic Biofuels

Thermochemical conversion involves biomass being broken down into 

intermediates using heat and upgraded to fuels using a combination of heat and pressure

in the presence of catalysts.9For the sake of the discussion here, we are defining 

Thermochemical to mean gasification processes (partial oxidation in the presence of a 

gasifying agent, usually air, oxygen, and/or steam) that convert the biomass to a syn-gas, 

but not including other processes such as pyrolysis (discussed below) which occurs in the 

absence of oxygen and at lower temperatures. The thermochemical process is also 

applicable to other feedstocks (e.g., coal or natural gas); the main difference is that a 

renewable feedstock is used (i.e. biomass) to produce cellulosic biofuel. A 

thermochemical unit can also complement a biochemical processing plant to enhance the 

economics of an integrated biorefinery by converting lignin-rich, non-fermentable 

material left over from high-starch or cellulosic feedstocks conversion.10 

Thermochemical conversion processes using gasification can be designed to 

either produce primarily ethanol (alcohols) or other petroleum like fuels such as diesel 

and naphtha, which is commonly referred to as a biomass-to-liquids (BTL) 

thermochemical process. The processing steps, however, used to produce either ethanol 

or BTL fuels in a thermochemical unit are similar until the fuel synthesis and product 

separation stages, as discussed below. 

Compared to corn ethanol or biochemical cellulosic ethanol plants, the use of 

biomass gasification may allow for greater flexibility to utilize different biomass 

feedstocks at a specific plant. Mixed biomass feedstocks may be used, based on 

availability of long-term suppliers, seasonal availability, harvest cycle, and costs. 

Agricultural residue, energy crops, wood residues, and municipal solid waste are all 

being considered as potential feedstocks. Geographic location, availability of biomass, 

the existence of biomass suppliers, and costs would all likely influence the mix of 

biomass feedstocks utilized. The general steps of the gasification thermochemical 

process include: feedstock handling, gasification, gas cleanup and conditioning, fuel 

synthesis, and separation. Refer to Figure 4.1 for a schematic of the thermochemical

cellulosic ethanol production process through gasification. For greater detail on the 

thermochemical mixed-alcohols route refer to NREL technical documentation.11 Refer to 

Figure 4.2 for a schematic of a block diagram of a BTL Thermochemical process. 
Figure 4.1

 Cellulosic Ethanol Thermochemical Gasification Process

Figure 4.2

 Biomass to Liquids (BTL) Thermochemical Gasification Process
4.1 Feedstock Handling

The particle size requirement for a thermochemical process is around 10-mm to 

100-mm in diameter.12 Once the feedstock is ground to the proper size, flue gases from 

the char combustor and tar reformer catalyst regenerator dry the feed from the asreceived moisture level of around 30% to 50% moisture to the level required by the 

gasifier. 

4.2 Gasification 

Ideally the gasification process would completely convert biomass to CO and H2, 

although the process is not completely efficient as some CO2, water, methane and coke/ 

tars are also produced. The gasification of biomass feedstock is carried out in the 

presence of a low concentration of oxygen, with oxygen levels that are only about 30% of 

that required for complete combustion. Tars and coke are also formed as biomass is 

gasified, which is primarily influenced by the gasifier operating conditions, gasifier 

residence time, gasifier design and operating temperature, with less influence based on 

the cellulosic feedstock type. The tar yield can vary by a wide range based on these 

parameters. There are many types of gasifiers, many designs resulting in different 

performance and conversion of biomass. For most gasifiers, the overall efficiency for 

converting biomass into synthesis gas of CO and hydrogen, is only about 70 to 80% 

efficient, as some of the biomass energy is converted into CO2, H2O and tars. 

Additionally, there is heat loss and other energy losses in the gasifier, that also lower 

efficiency. Downstream of the gasifier, additional energy is also lost when converting 

the synthesis gas of CO and H2 into ethanol and liquids, which lowers the overall process 

efficiency to about 42-57% (amount of biomass energy converted to liquid fuels) for 

currently explored thermochemical designs13.

The amount of carbon, hydrogen and oxygen present within the cellulosic 

biomass feedstock can vary from feedstock to feedstock or even within a feedstock. As 

the relative amount of carbon and hydrogen in the feedstock varies, the amount of CO 

and hydrogen produced during gasification would vary as well which ultimately affects 

the amount of fuel which can be produced. If the oxygen content of the feedstock varies, 

the amount of air or oxygen which is added to the gasifier to help oxidize the biomass

and combust any char being formed, can also be varied to balance the overall quantity

oxygen needed for gasification. An elemental chemical analysis is conducted on the 

feedstock to the thermochemical unit to identify the amounts of carbon, oxygen, 

hydrogen, sulfur, nitrogen and other inorganic elements present to indicate the potential 

volume of liquid fuels that can be produced. However, not all of the available energy in 

biomass feedstock is converted to fuels, due to inefficiencies in the thermochemical 

process.

The presence of moisture in feed stocks, though, has a detrimental impact on the 

process yields and process efficiency of a gasification unit. This is because any moisture 

in the feedstocks must be vaporized in the gasifier, which diverts energy away from the 

production of liquid fuels. So, a feedstock with a higher moisture content will have lower 

overall process efficiency than that same feedstock with lower moisture content. Wood 
has higher moisture content, with values around 50%, than other cellulosic stocks that 

have dried in the field (around 15%)vi. Wood’s high heating value, though, outweighs the 

negative effects of its higher moisture content versus other cellulosic feedstocks.

There are two general classes of gasifiers. First, partial oxidation (POx) gasifiers 

(directly-heated gasifiers) use the exothermic reaction between oxygen and organics to 

provide the heat necessary to devolatilize biomass and to convert residual carbon-rich 

chars. In POx gasifiers, the heat to drive the process is generated internally within the 

gasifier. 

The second general class, called indirect gasification, uses steam gasifiers to 

accomplish gasification through heat transfer from a hot solid or through a heat transfer 

surface. Either the byproduct char and/or a portion of the product gas can be combusted 

with air (external to the gasifier itself) to provide the energy required for gasification. 

Most steam gasifiers operate at low pressure and therefore require product gas 

compression for downstream purification and synthesis unit operations. 14,15

For both the direct or indirect types of gasifiers, it advantageous to use pure 

oxygen instead of air to the gasifier, as pure oxygen does not add nitrogen into the 

synthesis gas which results in cost savings from smaller equipment sizes for downstream 

equipment. A disadvantage of using pure oxygen as a feedstock is that oxygen 

production is expensive and typically requires large plant sizes to improve economics. 

There are different subcategories of gasifiers which are either directly or 

indirectly heated. One subcategory is termed a bubbling fluidized bed gasifier and it 

employes a bubbling fluidized bed of inert material and the reactant (biomass) is also 

bubbled through the fluidized bed. A second variant is the circulating fluidized bed 

gasifier which is similar to the bubbling fluidized bed reactor except that a high feedstock 

and air flow rate circulates the fluidized bed out of and back into the reactor. For the 

fluidized bed, the bed material may either be inert alumina or sand which helps the heat 

transfer. There are also fixed bed reactors which either feed the reacting gas (oxygen or 

air) upward or downward through a fixed bed of the reactant (biomass). Because of the 

tar formed when using biomass as a feedstock, a second reactor is sometimes added 

which solely targets converting the tar to syn-gas. If the biomas feedstock is ground to a 

sufficiently small particle size, or liquefied, the biomass is considered to be “entrained” 

in the reactor, and the reactor is defined as an entrained flow reactor. 

The indirect gasification process begins as the biomass is fed to the reactor 

containing a heat transfer media, such as sand, and is partially reacted with air (or 

oxygen) which is introduced to the bottom of the reactor. The air serves as the carriergas and as the oxidant for partially oxidizing the biomass to syn-gas, carbon monoxide 

and hydrogen. In addition to the syngas produced, char and coke are also formed. The 

heat for the endothermic gasification reactions is supplied by circulating heat transfer 

media (e.g. sythetic sand) between the gasifier and the char combustor. The heat 

generated by the combustion of the char and coke heats the heat transfer media to over 

1800°F. The syngas is separated from the sand and ash and sent to gas cleanup. 
4.3 Gas Cleanup & Conditioning 

Once the biomass is gasified and converted to syngas, the syngas must be cleaned 

and conditioned. Inorganic materials and minerals, such as sulfur, sodium, silica and 

potassium are contained within the cellulosic material and are also accumulated from dirt 

and the environment as the biomass is collected from the field and transported to the 

plants. The minor entrained contaminants such as tars, sulfur, nitrogen oxides, alkali 

metals, and particulates have the potential to negatively affect the syngas conversion 

steps. Most of this material is deposited in the gasifier as a molten slag and is removed as 

a waste byproduct. Additionally, ash content in agriculture residues (from fertilzers, dirt) 

and woods place a burden on the process, as ash must be scrubbed out of flue gas that 

exits the process, though woody feedstocks have less ash than agriculture resides. The 

scrubbing of the ash from the syn-gas consumes power and requires expensive capital for 

a flue gas scrubber system. Sulfur in biomass is converted into gaseous H2S in the 

gasifier which is poisonous to the Fischer Tropsch process as it deactivates the 

downstream reformer and Fischer Tropsch synthesis catalyst. H2S is therefore removed 

from the gasifier synthesis gas via an acid gas scrubbing section prior to entry into the 

reformer catalyst. Wood has low sulfur levels with a typical values estimated around 10 

ppm, while energy crops, such as corn stover and switch grass have much higher values 

with estimates around 500 ppm16. The gasifier also converts some of the nitrogen in 

biomass to NH3 and HCN, which is also removed in the downstream acid gas scrubbing 

section, as these are poisonous to the Fischer Tropsch catalyst. If scrubbers are being 

used anyways, thermochemical technologies may be better suited to process municipal 

solid waste which can contain a significant amount of contaminants. The benefit to using 

MSW is that it is priced lower than other feedstocks and the lower feedstock price would 

help to offset the thermochemical plant’s much higher capital costs. The gasification 

reactors and the downstream scrubbers typically used in thermochemical processes 

reduces the chance that contaminants contained in the feedstock make it into the final 

fuel. Gas conditioning steps include sulfur polishing to remove trace levels of H2S and 

water-gas shift to adjust the final H2/CO ratio for optimized fuel synthesis. 

4.4 Fuel Synthesis and Product Separation

After cleanup and conditioning, the “clean” syngas is comprised of essentially CO 

and H2. The syngas is then converted into a liquid fuel by either a catalytic process or 

through the use of a microorganism. The fuel producer has the choice of producing 

diesel fuel or alcohols from syngas by optimizing the type of catalyst used and the H2/CO 

ratio. Diesel fuel has historically been the primary focus of such processes, as it produces 

a high quality distillate product, however, with the 45 cent tax subsidy currently available 

for ethanol production, it may be economically advantageous for fuel producers to 

convert syngas to ethanol instead of to diesel fuel. Below, we discuss the processing 

steps for a plant configured to make either ethanol or BTL fuels.

4.5 Heat & Power 
For either an ethanol or BTL plant configuration, a carefully integrated 

conventional steam cycle produces process heat and electricity (excess electricity is 

exported). Pre-heaters, steam generators, and super-heaters generate steam that drives 

turbines on compressors and electrical generators. The heat balance around a 

thermochemical unit or thermochemical/biochemical combined unit must be carefully 

designed and tuned in order to avoid unnecessary heat losses.17 Any unreacted carbon 

monoxide and hydrogen and any gaseous hydrocarbon material from the process are 

burned to produce electricity in a turbine. The waste heat from the gas turbine along with 

the steam created to cool the syn-gas, may be sent to steam turbines to produce additional

electricity. Most of the electricity would be used within the thermochemical plant, 

however, some could be sold to raise additional revenues.

4.6a Ethanol Production

Conceptual designs and techno-economic models have been developed for 

ethanol production via mixed alcohol synthesis using catalytic processes. The proposed 

mixed alcohol process produces a mixture of ethanol along with higher normal alcohols 

(e.g., n-propanol, n-butanol, and n-pentanol). The by-product higher normal alcohols 

have value as commodity chemicals and fuel additives. Typically the mixed alcohol 

products are high in methanol, but contain a wide distribution of several different 

alcohols. One concept proposed in literature is to completely recycle this methanol in 

order to increase the production of ethanol and higher alcohols which are generally more 

valuable. This concept was modeled by NREL for the thermochemical production of 

ethanol for the year 2012, using a Molydenum Sulfide type catalyst. Total mixed alcohol 

yield was 94.1 gallons per dry ton, in which 85% of the total alcohol product was ethanol. 

This was made possible through the addition of an almost complete recycle of methanol 

within the process.18 For the Renewable Fuels rulemaking, we worked with NREL to 

develop the thermochemical mixed-alcohols model for the 2015 and 2022 timeframe19.

The liquid rundown from the low-pressure separator is dehydrated in vapor-phase 

molecular sieves, producing the dehydrated mixed alcohol feed into a methanol/ethanol 

overhead stream and a mixed, higher molecular weight alcohol bottom stream. The 

overhead stream is further separated into a methanol stream and an ethanol stream.

Range Fuels produces cellulosic biofuel via a two step thermochemical process. 

Their technology converts biomass to syngas followed by catalytic conversion of the 

syngas to alcohols. Range claims that their technology is capable of producing more 

ethanol than other cellulosic technologies based on yields per energy input. They utilize

a two step process which can use many forms of non food biomass, such as agriculture 

waste, wood, and corn stocks. Additionally, the technology can process feed stocks with 

variable water content.

Range has operated a pilot plant for over 7 years using over 20 different nonfood 

feedstocks. Range built its first commercial demonstration plant which started late in 

2010. This plant is located in Soperton, Georgia and is partially funded from proceeds of 

DOE and USDA grants. The plant uses wood, grasses, and corn stover as feedstocks. In 
its initial phase, the Range plant will be producing 4 million gallons per year of 

methanol. Once the company is confident in its operations, Range intends to add 

additional reaction capacity to convert much of the methanol to ethanol and then expand 

the plant. 

Enerkem is another company pursuing cellulosic ethanol production via the 

thermochemical route. The Canadian-based company was recently announced as a 

recipient of a $50 million grant from DOE to build a 10 MGY woody biomass-to-ethanol 

plant in Pontotoc, MS. The U.S. plant is not scheduled to come online until 2012, but 

Enerkem is currently building a 1.3 MGY demonstration plant in Westbury, Quebec. 

According to the company, plant construction in Westbury started in October 2007 and 

was completed in December 2009. Since then, the plant has been in the commissioning 

and start-up phase with methanol production projected to occur in the fall of 2010. Later, 

the plant will switch from methanol production to ethanol. While this fuel would likely

be eligible to generate RINs under the RFS program if imported, Enerkem has indicated 

that they plan to use all the fuel locally and do not plan to export any fuel to the United 

States. 

4.6b BTL Production 

In this configuration, the syngas is fed to a Fischer-Tropsch (FT) reactor to 

primarily produce diesel fuel and some naphtha.

20 Usually the ratio of carbon monoxide 

to hydrogen is too high not allowing maximum fuel production volumes with the syngas, 

If that is the case, the syngas must first be sent to a water shift reactor where some of the 

carbon monoxide is used to free up hydrogen from water which is also present in the 

reactor. The cleaned and water-shifted syngas is sent to the FT reactor where the carbon 

monoxide and hydrogen are reacted over a FT catalyst. The FT catalyst is either ironbased or cobalt-based. The cobalt catalyst is more expensive, although it does not require 

a recycle, while the less expensive iron catalyst does require a recycle. The FT reactor 

creates a syncrude, which is a variety of hydrocarbons that boil over a wide distillation 

range (a mix of heavy and light hydrocarbons). The syncrude from the FT reactor is sent 

to a distillation column where it is separated into various components based on their 

vapor pressure, mainly liquid petroleum gas (LPG), naphtha, distillate and wax fractions. 

The heavier compounds are hydrocracked to maximize the production of diesel fuel. The 

distillate boiling compounds have high cetane and thus are of high quality for blending 

into diesel fuel. Conversely, the naphtha material is very low in octane thus, it would 

either have to be upgraded, or blended down with high octane blendstocks (i.e., ethanol), 

or be upgraded to a higher octane blendstock to have much value for use in gasoline. The 

naphtha could also be sold as feedstock for the petrochemical market for manufacturing 

chemical products such as ethylene and benzene.

Choren has a technology called Carbo-V, which is a Fischer-Tropsch process that 

can be used to make diesel fuel. The process can process a wide variety biomass and 

recycled material materials as feedstocks. The process converts agriculture biomass, 

forestry biomass, biogenic waste and recycling substances into a synthesis gas which can 

be further converted to a diesel fuel using a Fischer-Tropsch reactor. The Carbo-V 
process can also be configured without the Fischer-Tropsch hydrocracking technology, so 

as to produce electricity, heat and power, methanol, and other chemical feedstocks.

The principal aspect of the Carbo-V Process is a three-stage gasification process 

consisting of low temperature gasification, high temperature gasification and endothermic 

entrained bed gasification. In the first stage, biomass is partially oxidized with air or 

oxygen at temperatures between 400 and 500 °C. This breaks down the feedstock into a 

gas containing tar and solid carbon. In the second stage, the tar is oxidized at 

temperatures higher than the tar’s melting point, converting the tar into a synthesis gas. 

In the third stage, solid carbon is mechanically pulverized and blown into the hot 

gasification stream. The fluidized carbon endothermicly reacts with the gasification 

stream and is converted into a synthesis gas. In the next Fischer-Tropsch stage of the 

process, the synthesis gas (CO and H2) reacts with the aid of a catalyst to form 

hydrocarbons. The resulting heavy hydrocarbons produced from the three stages can 

then be sent to a hydrocracking process to primarily produce diesel fuel. 

Choren will be building a commercial Plant in Freiberg/ Saxony Germany that is 

expected to be operational in 2011 or 2012. Initially, the plant will use biomass from 

nearby forests, wood waste from the wood-processing industry and straw from farmland.

4.7 Pathways to Commercialization 

Although there are examples of thermochemical gasification processes which

process coal and natural gas, a thermochemical plant which must gather, pretreat and 

gasify cellulosic biomass has not yet been demonstrated on a commercial scale. The 

capital costs for thermochemical plants are much higher per volume of fuel produced 

than other cellulosic biofuel technologies, due to the complexity of their design which 

requires many process stages, expensive equipment pieces and high catalyst costs. The 

first investors in these thernochemical plants that process cellulosic biofuels are expected 

to be those which are willing to tolerate the somewhat higher risk of investing in a new 

application of an existing technology. Once the first several thermochemical plants 

which produce cellulosic biofuels are built, more investors are expected to be confident 

that these plants will have long term reliable performance and that the product yields will 

be satisfactory for economic payoff and invest in this technology. Additionally, 

optimization and improvements to the gasification stage, catalyst synthesis stages and 

overall configuration of the plant will help to lower the capital and operating costs to 

further improve the competitiveness of this technology.

5. Hybrid Thermochemical/Biochemical Processes

Hybrid technologies include process elements involving both the gasification 

stage of a typical thermochemical process, as well as the fermentation stage of a typical 

biochemical process and therefore cannot be placed easily into either category. For more 

specific information regarding either biochemical processes or thermochemical, please 

see Sections 3 and 4 of this technical memo respectively. Currently, there are several 

strategies for the production of ethanol through hybrid processes; these strategies are 
differentiated by the order in which the thermochemical and biochemical steps take place 

within the process, as well as how the intermediate products from each step are used.

The following subsections describe these different processes; specifically how they differ 

from processes that may use only one technology type (either thermochemical or 

biochemical).

5.1 “Thermochemical First” Process

The ‘thermochemical first’ strategy involves the gasification of all feedstock 

material to syngas before being processed into ethanol using a fermenter. Hence, the first 

steps in this process most closely resemble a typical thermochemical process, while the 

following steps most closely resemble that of a typical biochemical process.

Stage 1 – 3: Feedstock Handling, Gasification and Cleanup

The first stages do not differ in any significant way from the respective stages 

found in a traditional thermochemical process. Gasification converts carbonaceous 

material under high heat and pressure to a mixture of mainly carbon monoxide and 

hydrogen gasses. This mixture is commonly referred to as syngas. This gas may also 

contain other compounds such as metallics or sulfur; these compounds can be detrimental 

or poisonous to steps farther downstream in the process and therefore must be removed 

before further treatment of the syngas. More information regarding these steps can be 

found in Section 4 of this technical memo.

Stage 4: Fuel Synthesis

After gasification, the syngas stream is cooled and bubbled into a fermenter 

containing genetically modified microorganisms. Unlike traditional fermentation, where 

C5 and C6 sugars are digested to ethanol, these microorganisms are engineered to convert 

the carbon monoxide and hydrogen contained in the syngas stream directly into ethanol.

Using a fermenter in this hybrid process replaces the catalyst needed in a thermochemical 

process. Traditional thermochemical catalysts require a very specific ratio of carbon 

monoxide to hydrogen in the syngas stream in order to reach efficient conversions. This 

requirement necessitates the inclusion of an additional water-shift step for 

thermochemical fuel production. A water-shift reaction is used to balance the ratio of 

carbon dioxide and hydrogen gas to that required by the traditional thermochemical 

catalyst. The microorganisms used in a hybrid process allow for more variation of the 

incoming syngas stream ratio, therefore avoiding the water-shift reaction preceding 

typical thermochemical conversion using chemical-based catalysts. While 

thermochemical catalysts require high pressures and temperature, the microorganisms 

are able to operate at significantly lower temperatures and pressures than those required 

for a traditional catalytic conversion to ethanol. Microorganisms, unlike a catalyst, are 

also self-sustaining and do not require periodic replacement. They are; however, 

susceptible to bacterial and viral infections which requires periodic cleaning of the 

fermentation reactors.
Stage 5: Product Separation

After fermentation, the effluent water/ethanol stream from the fermenter is 

separated similarly to a biochemical process; usually using a combination of distillation 

and molecular sieves. The separated water can then be recycled back into the 

fermentation stage of the process. Typical yields of ethanol are predicted in the 100-130 

gallon per ton range, similar to that of a traditional thermochemical ethanol process. For 

more information regarding product separation, refer to Section 3.5 biochemical product 

separation.

Coskata has a gasification-based technology which produces ethanol from 

biomass and other forms of carbon through a biofermentation route. A wide variety of 

feedstocks can be used including municipal solid waste, agriculture waste and other 

carbonaceous containing material. Since this process uses combustion and 

biofermentation, we classified it as a hybrid technology. This process requires that the 

biomass or carbonaceous material be processed to a small particle size and then it is 

injected into a gasifier. 

In the Coskata process, the gasifier combusts any dry carboneous feed stocks into 

syngas. The syngas produced is fermented in a reactor by micro-organisms, which 

convert the carbon monoxide and hydrogen directly into ethanol. The micro-organisms 

are low cost and can process a wide range of carbon monoxide and hydrogen molar ratios 

in the syngas, providing feedstock processing flexibility. No other enzymes are required 

by this process for producing ethanol, providing significant cost savings over current 

cellulosic and corn based fermentation production methods. The Coskata microorganism reactor is operated at low temperatures and pressures, which offers savings on 

capital and energy costs. Additional energy savings can be realized by employing 

membrane technology to separate ethanol from the reactor decant liquid. This 

technology uses gravity and filtration to recover ethanol, resulting in significant savings 

on distillation capital and energy costs used in other cellulosic and corn based ethanol 

production methods. According to Coskata, initial ethanol production cost estimates are 

lower than the biochemical and thermochemical cellulosic technologies.

For woody biomass, Coskata estimates that each ton of this feedstock would 

generate about 100 gallons of ethanol and small amounts of ash which would be burned 

to supply energy needs for the process. Corn stover is expected to provide similar 

ethanol yields as woody biomass feed stocks, though details about yields from the various 

feed supply stocks are not yet public.

 

Coskata has a bench scale pilot plant in Warrenville, IL, and its larger 40,000 

gallon per year pilot plant became operational in 2009 in Madison, Pennsylvania. 

Coskata is targeting to design and build a 55 million gallon per year commercial 

demonstration plant that it expects to be operational in 2012.

INEOS Bio also has been developing a thermochemical first hybrid biofuel 

production technology. INEOS Bio (along with partner New Planet Energy) has recently 
been selected for a $50MM DOE grant for the construction of an 8 MGPY plant in River 

County, Florida; predicted to finish construction in late 2011.

5.2 “Biochemical First” Process

A ‘biochemical first’ process ferments the cellulose and hemicellulose like a 

typical biochemical process, but the typically unfermentable feedstock fraction (lignin) is 

gasified concurrently with a fermentation step. The syngas created from the lignin is then 

reacted with the product of the fermented stream. 

Stage 1 – 2: Feedstock pretreatment and hydrolysis

Feedstocks are prepared for this process in the same fashion as a traditional 

biochemical plant. Incoming feedstock is pretreated and hydrolyzed to separate 

fermentable sugars from non-fermentable compounds such as lignin. More information 

on these steps can be found in Sections 3.2 and 3.3 of the biochemical technology Section

of this technical memo.

Stage 3a: Fermentation

Stage 3a and 3b in this process take place concurrently; before fermentation, the 

unfermentable portion of feedstock (lignin, ash and other residue) is fractioned and sent 

to a gasifier, see stage 3b. The remaining fraction of feedstock is fermented using an 

acetogen microorganism, in a manner similar to that of a traditional biochemical process. 

The acetogen converts the sugars and mixed carbohydrates of the feedstock to acetic acid. 

This reaction creates no carbon dioxide, unlike a typical fermentation using yeast, 

preserving a higher amount of carbon in the finished fuel. Acetogens are also naturally

occurring and therefore more robust against infection or contamination versus 

microorganisms that would be genetically engineered. The acetic acid stream is then 

stabilized to ethyl acetate. Ethyl acetate is both easier to store, as well as forming a 

precursor to many other chemical compounds that can be created at a later stage in the 

process.

Stage 3b: Gasification of unfermentable feedstock

Meanwhile, the unfermentable fraction separated in stage 3a is fed to a 

gasification reactor similar to that found in a traditional thermochemical process. Due to 

the reduced volume of feedstock going to this reactor, the size of the gasifier is greatly 

reduced compared to that of a thermochemical-only process. The syngas stream 

produced from the gasification of lignin and other residue is then separated into its carbon 

monoxide and hydrogen components. The hydrogen is fed to the next stage of the 

process, while excess carbon monoxide is combusted, producing enough energy to meet 

the needs of the other process stages.

Stage 4: Product Production
The hydrogen stream from stage 3b is reacted with the ethyl acetate stream from 

stage 3a to form ethanol. It is also possible, by changing reaction parameters, to form 

other chemical compounds from the ethyl acetate in order to meet the needs of changing 

market conditions; acetic acid and ethyl acetate form the precursors to many other 

chemical compounds and therefore may also be sold in addition to ethanol. It is predicted 

that this process can achieve conversions of 130 - 150 gallons of ethanol per ton of 

feedstock.

A company currently pursuing a biochemical first process technology is Zeachem 

Inc. Zeachem is currently constructing a 250 thousand gallon per year demonstration 

plant in Boardman, Oregon. They have received a $25MM DOE grant and expect to 

have a full commercial production facility operational in 2013. 

5.3 Path to Commercialization

Since these hybrid process technologies include steps from both biochemical and 

thermochemical processes, they also have some of the same commercialization issues as 

both of these technology types. ‘Thermochemical first’ processes carry the same high 

capital costs as traditional thermochemical operations, as well as the complexity of heat 

transfer between stages required for these processes. However, because of the use of a 

biochemical stage instead of a catalyst, initial catalyst cost and tuning can be eliminated. 

This is offset by initial microorganism cost and first batch startup of the microorganisms. 

The addition of a biological step also increases the need for sterile procedures in that part 

of the plant, adding costs that would not be present in a typical thermochemical 

operation. ‘Biochemical first’ processes also share many of the commercialization issues 

of a typical biochemical process. Feedstock pretreatment and hydrolysis steps need to be 

improved on a large scale to achieve efficient production. The addition of a 

thermochemical processing step for lignin and unfermentable residue solves the problem 

of wasted feedstock; however, it adds both complexity and capital cost with the addition 

of a thermochemical step. Both hybrid process pathways are able to achieve higher 

conversion rates than either of the sole technology processes, with the tradeoff of added 

complexity. This added complexity adds to startup time and capital costs, which may

lengthen construction times.

6. Pyrolysis and Catalytic Depolymerization

Pyrolysis and catalytic depolymerization encompass a group of technologies 

which are capable of creating biofuels from cellulose by either only relying on heat, or by

relying on a combination of heat with catalysis. Pyrolysis technologies are usually

thought of being solely or primarily thermal technologies, however, pyrolysis 

technologies are being developed integrate some catalysts into the technology. These are 

all unique processes, typically with single companies developing the technologies, so 

they are discussed separately. 
6.1 Pyrolysis 

Pyrolysis oils, or bio-oils, are produced by decomposing cellulosic biomass at 

lower temperatures (400 to 800 degree Celsius) than the gasification process, thus 

producing a liquid bio-oil instead of a synthesis gas.

21 22 23 24

 The reaction can occur 

either with or without the use of catalysts, but it occurs without any additional oxygen 

being present. The bio-oil produced varies in oxygen content or viscosity according to 

the feedstock used and the reactor conditions. Typically, to maximize the production of 

liquid fuels, the reactor temperature is controlled at 400 to 500 degrees Celsius and the 

residence time is only 2 seconds – hence the name “Fast pyrolysis.” The fast pyrolysis 

process uses sand (not catalyst) in a fluidized bed to transform bio-fuels into a product 

named bio-oil. The sand is used to enable the quick heat transfer needed to keep the 

reaction time to 2 seconds. Three products are produced by the fast pyrolysis process, 

bio oil (60-75% by weight), char (15-20% wt.) and non-condensable gases (10-20% wt.). 

The char produced is similar to coke and can be used as fuel by other industries while the 

gases yielded from the process can be used to supply about 75% of the energy

requirements of the pyrolysis process. Once the reaction is complete, the oil must have 

particulates and ash removed in filtration to create a homogenous product. One common 

way to conduct the separation of particulate matter and ash from the bio-oil is through the 

use of a cyclone. 

The bio-oil is typically high in oxygen content (i.e., 20% – 30%) which makes 

this product immisible with petroleum, but enables it to be miscible with water.25 Bio-oil 

is also very low in cetane and has about half the energy content per gallon of petroleumbased diesel fuel. Because of the high oxygen content and the immersed water, the biooil has a density more like that of water. Also the acid compounds in the bio-oil cause 

the bio-oil to be acidic which requires the metals in contact with the bio-oil to be acid 

resistant stainless steels. This makes bio-oil very challenging to use in internal 

combustion engines. If a use can be found for the bio-oil, the production cost is very low 

due to the very low capital and operating costs for these plants.26 

If pyrolysis is to be widely used to generate as transportation fuels or for other 

uses, the bio-oil will have to be upgraded to hydrocarbon fuels.

27 28 The upgrading 

process would likely involve one or more reactor vessels which likely would contain a 

catalyst that reduces the acidity, reduces the viscosity and reduces the density of the biooil. One likely technology for this upgrading step would be hydrotreating.29 30 31 32

 The 

Pacific Northwest National Laboratory (PNNL) has modeled hydrotreating as an 

upgrading strategy for bio-oils sourced from pyrolysis.33 PNNL projected that two 

hydrotreating reactors would be needed. The first hydrotreater is projected to operate at a 

higher space velocity and lower temperature than conventional hydrotreaters - the lower 

temperature is used to avoid coke formation. The purpose of the first reactor would be to 

deoxygenate the more reactive acid and aldehyde oxygen-containing hydrocarbon 

species, and saturate the more reactive olefins. After the first reactor, the bio-oil would 

likely no longer be miscible with water and be blendable with hydrocarbons, which 

would allow any entrained water to be separated from the bio-oil. 
The second hydrotreater would operate at a temperature and space velocity more 

like a conventional refinery hydrotreater and its primary purpose would be to drastically 

remove the oxygen contained within the hydrocarbon molecules. The resulting fuel 

would still contain a small fraction of the oxygen originally contained within the 

cellulose, but it would be much more like hydrocarbon compounds sourced from crude 

oil. The downside for the hydrotreating route is that the hydrogen consumption from the 

two upgrading hydrotreating reactors would be a very high – on the order of 4600 

standard cubic feet per barrel of feed. This level of hydrogen consumption is an order of 

magnitude higher than that typically experienced with distillate hydrotreating. 

Instead of solely relying on hydrotreating as the upgrading technology, the first 

reactor after the pyrolysis reactor could contain a catalyst which upgrades the bio-oil 

without the use of hydrogen. The somewhat upgraded bio-oil would still contain a 

significant amount of oxygen, but it may be sufficiently stable to store for a second 

upgrading step that could involve hydrotreating. Because the bio-oil is stable, it perhaps 

could be transported to a refinery with extra hydrotreating capacity to hydrotreat the biooil. An important advantage of employing a nonhydrotreating catalyst as a first step 

would be to significantly reduce the amount of hydrogen required. The second advantage 

of this approach is the two upgrading duties can be optimized separately depending on 

the cellulosic biomass location and best location for fuels upgrading. For example, the 

pyrolysis reactors and upgrading reactor (first step) could be located close to where the 

cellulosic biomass is located avoiding the long distance shipments of the biomass 

(biomass is a solid which is more costly to handle, and it contains about 50% oxygen, 

which is deadweight because oxygen does not add any energy value). Once the biomass 

has been “liquefied” and at least partially deoxygenated, transportation is made easier, 

perhaps by barges, ships and pipelines. The second upgrading step can be located further 

away from the biomass harvesting locations where fewer but larger, more cost-efficient 

upgrading equipment could be utilized. The upgrading step can even occur using the 

existing capital at today’s refineries.34

There is another means other than using catalysts to stabilize bio-oil. The 

National Renewable Energy Laboratory (NREL) has a proprietary technology named 

“hot filtration” that apparently was able to stabilize bio-oil samples for a very long period 

of time (years). NREL’s pyrolysis reactor uses sand, but the resulting bio-oil after their 

hot filtration will contain a much lower amount of contaminant metals and is stable. So, 

it will be easier to solve the second problem, the bio-oil upgrading. 

Dynamotive Energy Systems Corporation employs a fast pyrolysis technology

that uses medium temperatures and oxygen free reactions to convert dry waste biomass 

and energy crops into specialty products.35 The process is flexible on the types of 

biomass feedstock’s that can be processed. The bio-oil produced by their process 

contains significant amounts of water, thus it is not directly useable as fuel in 

conventional vehicles and would have to be converted via another catalytic conversion 

processing step. The additional catalytic step Dynamotive envisions for upgrading their 

bio-oil for this would combust the material into a synthesis gas which would then be 
converted into diesel fuel (the BTL process). If upgraded, the diesel fuel produced is 

expected to be compatible with existing petroleum diesel fuels. 

Dynamotive has two small demonstration plants. One demonstration plant is 

located in Guelph, Ontario, Canada and its capacity is 66,000 dry tons of biomass a year

with an energy output equivalent to 130,000 barrels of oil per year. Dynamotive also has 

a demonstration plant in West Lorne, Ontario Canada. The West Lorne plant has a 

capacity to convert 130 tonnes of biomass into bio-oil per day which, if proportional to 

the Guelph plant, translates to an energy-equivalent of 84,500 barrels of oil per year. 

This plant started operation in early 2005 using waste sawdust as a feedstock. 

Dynamotive does not have any contracts in place to supply U.S. based clients with 

transportation fuel in year 2010 or 2011. Additionally, Dynamotive has not announced 

plans for building a bio-oil upgrading facility.

Envergent is a company formed through a joint venture between Honeywell’s 

UOP and the Ensyn Corporation. Although Ensyn has been using fast pyrolysis for more 

than a decade to produce specialty chemicals, UOP is relying on its decades of 

experience developing refining technologies to convert the pyrolysis oils into 

transportation fuels.36 Envergent is also working with Federal laboratories to further their 

technology. Based on their current technology and depending on the feedstock 

processed, about 70 % of the feedstock is converted into liquid products. The gasoline 

range products produced are high in octane, while the diesel fuel products are low in 

cetane. Envergent is licensing this technology as well as working with a U.S. oil 

company to test out this technology in a commercial setting here in the U.S. 

Petrobras is working on developing a pyrolysis technology.

37

 A United States and 

Brazil Memorandum of Understanding to advance the cooperation on biofuels was signed 

by United States’ Secretary of State and Brazil’s External Relations Minister on March 9, 

2007. A second Memorandum of Understanding was signed by Petrobras and the 

National Renewable Energy Laboratory (NREL) on September 2008 aiming at 

collaborating to maximize the benefit of their respective institutional interests in second 

generation biofuels. Petrobras is working with NREL to develop a pyrolysis technology 

to produce biofuels from agricultural wastes such as sugar cane bagasse, wood chips or

corn stover. NREL and Petrobras intend to use their combined expertise to develop a 

two-step pyrolysis route to biofuels production. 

Petrobras is also working to develop a one-step pyrolysis technology, although the 

company is somewhat skeptical that even if an effective one-step pyrolysis catalyst can 

be developed, it may not be economically viable for use in rural areas. The Cooperation 

Agreement with NREL covers the Life Cycle Assessment and Technoeconomic Analysis 

of both the two-step and one-step routes.

6.2 One Step Catalytic Pyrolysis and Catalytic Depolymerization

It is possible to use a sophisticated catalyst (instead of sand) to heat and 

depolymerize cellulose and at the same time produce a stable bio-oil or even produce 
transportation quality fuels.38

 Some papers in literature address these problems by using 

ZSM-5 zeolite.39 The resulting bio-oil is very aromatic, but theoretically it could be used 

directly as transportation fuel and the posterior upgrading would be no longer necessary, 

since fuel properties can be improved by decreasing the amount of oxygen in the bio-oil 

through deoxygenation and cracking to form H2O, CO and CO2. The challenge of this 

route is that many of the available catalysts which are candidates are relatively expensive 

and they degrade quickly due to the metals and water which are present.40

There are at least two ways to address the cost impacts of using expensive 

catalysts which degrade over time from the contaminants. One solution would be to find 

a catalyst formulation which is less expensive, but also robust enough to handle the high 

level of water and metal contaminants present in biomass, namely sodium, potassium, 

magnesium, iron. Otherwise, the process will not be economically viable. Zeolytic 

catalysts are likely not candidates because they deactivate from being exposed to most of 

these metals. A second approach would be to develop a new catalyst which may still be 

expensive, but which is resistant to the contaminants. Alternatively, one could use a 

susceptible catalyst, but somehow protect the catalyst from the contaminants, such as a 

guard bed. A guard bed would be placed above the rest of the catalyst and its role would 

be to adsorb the contaminants preventing the more expensive one-step pyrolysis catalysts 

located lower down in the reactor from being exposed to the contaminants. 

There seem to be several companies which have found catalysts which can 

function in the environments present when converting cellulosic biomass to biofuels. 

Since their processes are proprietary, some of the companies will not share what catalysts 

they are using, nor do we know the cycle length of the catalysts they are using. 

Covanta is currently operating 44 energy-from-waste facilities which annually

convert 20 million tons of municipal solid waste materials into 9 million megawatt hours 

of electricity and 10 billion pounds of steam, which is sold to a variety of industries. 

Covanta has secured license rights to a catalytic depolymerization technology developed 

by AlphaKat GmbH. Covanta constructed an AlphaKat demonstration plant in West 

Wareham, Massachusetts designed to process 45 tons of waste per day into renewable 

diesel fuel. If successful, the total liquid fuel production capacity of this demonstration 

plant will be 1 million gallons per year. This plant started up in mid-2010 and after 

experimenting with the technology to further understand its capabilities, Covanta expects 

to use the liquid distillate fuel produced from this demonstration plant within its own 

plant as heating oil and nonroad diesel fuel. 

The AlphaKat process licensed by Covanta catalytically depolymerizes cellulosic 

feedstocks at moderate temperatures and low pressure to produce diesel fuel. The 

proposed feedstock is municipal solid waste (MSW) or other waste material such as 

animal waste, non-recyclable plastics, agriculture residue, woody biomass and sewage 

waste. The feedstock must be prepared by shredding it to a size smaller than 2 inches, 

and removing both ferrous and non-ferrous metals. The prepared feedstock is fed to the 

process along with a powdered zeolite catalyst and some lime which serves as a 

neutralizing agent, where they are mixed with a non-reactive carrier oil, which provides a 
liquid medium for the reactions to take place. This carrier oil slurry is then circulated 

through a friction turbine reactor, which heats the slurry to about 280 degrees Celsius and 

turbulently mixes the feedstock and catalyst. In the reactor the waste feedstock is 

catalytically converted to diesel fuel, which is recovered in a distillation column. 

Depending on its use, the diesel product may require further processing to remove sulfur. 

The process is capable of producing approximately 60 gallons of diesel product per ton 

of waste feedstockI Inorganic material that cannot be converted will be removed from the 

process via a purge stream, which may be further processed to recover catalyst and 

carrier oil to be returned to the process. Carbon dioxide and water are also formed in the 

catalytic depolymerization reactions and are released from the process. 

The Cello-Energy process is also a catalytic depolymerization technology. At 

moderate pressure and temperature, the Cello-Energy process catalytically removes the 

oxygen and minerals from the hydrocarbons that comprise finely ground cellulose. This 

results in a mixture of short chain (3, 6 and 9 carbon) hydrocarbon compounds. These 

short chain hydrocarbon compounds are polymerized to form compounds that boil in the 

diesel boiling range, though the process can also be adjusted to produce gasoline or jet 

fuel. The resulting diesel fuel meets the ASTM standards, is in the range of 50 to 55 

cetane and typically contains a very low concentration of sulfur. 

The Cello process is reported to be on the order of 82 % efficient at converting the 

feedstock energy content into the energy content of the product, which is very high 

compared to most of today's biochemical and thermochemical processes which are on the 

order of 50 % efficient or less. Because of the simplicity of the process, the capital costs 

are very low. A 50 million gallon per year plant is claimed to only incur a total cost of 

$45 million. Because of its high efficiency in converting feedstocks into liquid fuel, the 

production and operating costs are also estimated to be very low. 

In December 2008, Cello completed construction of a 20 million gallon per year 

commercial demonstration plant. However, they are still working to resolve process 

issues that have arisen upon scaleup from their pilot plant. However, we are doubtful that 

Cello will be able to produce any volume of cellulosic biofuel in 2011.

Greenpower has a process similar to the Alphakat process licensed by Covanta. 

Greenpower completed construction of a demonstration plant located in Fife, Washington 

in March of 2008. Greenpower is working on obtaining additional funding and an air 

permit through the State of Washington Environmental Office. While we do not expect 

that Greenpower will have its plant operational in 2011, it is possible that outstanding 

issues could be resolved to allow this company to produce renewable fuel that could help 

refiners comply with the cellulosic biofuel volume standard for 2011. 

6.3 Pathways to Commercialization 
Pyrolysis technologies produce very low quality products which cannot be used in 

today’s vehicles and engines. Therefore, the bio-oil must be upgraded to improve the 

stability and the usability of the fuel and continued effort will likely be invested in 

improving the efficiency and reducing the overall cost of upgrading the bio-oil. For those 

working with catalysts for combining with a pyrolysis process, or for a catalytic 

depolymerization process, the companies will likely continue to improve the 

effectiveness and lower the costs of their catalyst.

7. Catalytic Reforming of Sugars to Gasoline

Virent is pursuing a process called “Bioforming” which functions similarly as the 

gasoline reforming process used in the refining industry. While refinery-based reforming 

raises natural gasoline’s octane value and produces organic chemicals, benzene, xylene 

and toluene as a byproduct, Bioforming reforms biomass-derived sugars into 

hydrocarbons for blending into gasoline and diesel fuel. The process operates at 

moderate temperatures and pressures. The Bioforming process which initially was 

conceived at the University of Wisconsin, is being developed through a partnership 

between Virent, Shell, Cargill and Honda. Virent currently has 16 benchscale pilot plants 

in operation, each of which is capable of producing about one gallon of product per day. 

In March of 2010, Virent announced that they had begun operating a larger pilot plant 

capable of about 30 gallons per day. Commercialization of the Virent process will 

happen sometime after 2011. 

Biomass feedstocks for the Bioforming process are sugar feeds, such a corn syrup, 

sucrose, glycerol, sorbitol, xylose, glucose, cellulose and hemi cellulose. These are 

primarily converted into gasoline and diesel fuel, though other hydrocarbons such as jet 

fuel, LPG, benzene, toluene, xylene, hydrogen and natural gas can also be produced.

Water is also produced, as the reforming process removes oxygen from the sugar feeds. 

The resulting properties and energy content of gasoline and diesel produced, though, are 

physically comparable to those yielded from refining industry. The variable operating 

costs of the Virent process are low because no distillation equipment is needed to 

separate the produced gasoline, diesel and other hydrocarbons, as these separate naturally 

from the aqueous solutions generated in the reforming process. It appears that 

Bioforming is a promising technology, as production cost estimates are low in 

comparison to many other renewable and biomass production processes while the 

products are compatible with traditional petroleum stocks so they don’t need special 

handling. 

For this technology to become a cellulosic biofuel technology, it will be necessary

to link this reforming technology with a technology which breaks cellulose down into 

starch or sugars. In parallel with its Bioreforming work, Virent is working on a 

technology to break down cellulose into sugars upstream of its technology which reforms 

sugars to gasoline. 
 

1 DOE. “Biomass Program: ABC’s of Biofuels”. Accessed at: 

http://www1.eere.energy.gov/biomass/abcs_biofuels.html#content

2

Starch Hydrolysis; http://www.bio-link.org/pdf/starch.pdf

3 Glossary of Biomass Terms, National Renewable Energy Laboratory, Golden, CO.

http://www.nrel.gov/biomass/glossary.html

4 Glossary of Biomass Terms, National Renewable Energy Laboratory, Golden, CO.

http://www.nrel.gov/biomass/glossary.html

5

 Condensation and Hydrolysis; 

http://www.biotopics.co.uk/as/disaccharideformation.html 

6 Aden, Andy and Ling Tao. “Technoeconomic Modeling to Support the EPA Notice of 

Proposed Rulemaking (NOPR)”. November 3, 2008.

7

Ibid

8 Novozymes Press Release. “New Enzymes Turn Waste into Fuel.” February 16, 2010. 

Accessed at: 

http://www.novozymes.com/en/MainStructure/PressAndPublications/PressRelease/2010/

New+enzymes+turn+waste+into+fuel.htm

9 US. DOE. Technologies: Processing and Conversion. Accessed at:

http://www1.eere.energy.gov/biomass/processing_conversion.html on October 28, 2008

10 EERE, DOE, Thermochemical Conversion, & Biochemical Conversion, Biomass 

Program Thermochemical R&D . 

http://www1.eere.energy.gov/biomass/thermochemical_conversion.html 

http://www1.eere.energy.gov/biomass/biochemical_conversion.html

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vi

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16 A. Aden, National Renewable Energy Laboratory, Golden, Colorado 80401-3393, 

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18 S. Phillips, A. Aden, J. Jechura, and D. Dayton, National Renewable Energy

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Thermochemical Ethanol via Indirect Gasification and Mixed Alcohol Synthesis of 

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19 Aden, Andy, Mixed Alcohols from Woody Biomass – 2010, 2015, 2022, National 

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20

 Davis, Ryan; Techno-economic analysis of current technology for Fischer-Tropsch 

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24 Saffron, Christofer M.; Biomass Fast Pyrolysis; Presentated to the Michigan AgriEnergy Conference, March 30, 2009.

25 Bridgwater, AV: Thermal Conversion of Biomass and Waste: The Status.

26 Stewart, Gerald W.; Bio-Oil Commercialization Plan; Prepared for the New Hampshire 

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27 Chiaramonti, David; Power Generation using Fast Pyrolysis Liquids from Biomass; 

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28 Bridgwater, Anthony V.; Biomass Fast Pyrolysis.

29 Brown, Robert C., Holmgren, Jennifer; Fast Pyrolysis and Bio-Oil Upgrading.

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31 Venderbosch, R.H.; Stabilization of Biomass-Derived Pyrolysis Oils; J ChemTechnol 

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34 Marinangeli, Richard; Opportunities for Biorenewables in Oil Refineries; Final 

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35 Dynamotive Brochure.

36 Goodfellow, Randal; Biomass to Green Hydrocarbons; November 6, 2008.

37 E-mail from Andrea Pinho, Senior Researcher FCC Catalysts, Petrobras.
 

38 Huber, George W.; Synergies between Bio- and Oil Refineries for the Production of 

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39 Carlson, Torren C., Green Gasoline by Catalytic Fast Pyrolysis of Solid Biomass 

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40 Huber, George H.; Breaking the Chemical and Engineering Barriers to Lignocellulosic 

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