Feedstock Considerations and Impacts on Biorefining

Andy Aden, PE

NREL

Dec. 10, 2009

Introduction

Biomass is ubiquitous. It can be found in nearly every part of the globe except from the harshest of 

climates. This makes it an important renewable resource of interest to a large number of stakeholders 

internationally. However, not all biomass is created equal. There are a wide variety of lignocellulosic 

materials that could potentially be utilized for biorefining into liquid fuels and other products. 

Biomass can be grouped and classified in many ways. Near-term biomass feedstocks can be byproducts 

from existing processes, such as the stalks, leaves and other plant matter left over after harvesting (i.e. 

corn stover, wheat straw, etc). Forest residues from thinning operations or from existing lumber 

processing are also a potential feedstock, as is bagasse, the plant fiber left after extracting sugar from 

sugar cane. Dedicated biomass crops have also been envisioned for biorefining purposes. These would 

include fast-growing tree plantations (poplar), energy cane, and grasses (switchgrass, miscanthus). 

Significant variability exists for all biomass feedstocks of interest to refiners. Even within a single type of 

biomass (i.e. corn stover), significant compositional variation has been seen1. The reasons for this 

variation are due to a wide variety of genetic and environmental factors, including soil conditions, 

precipitation, harvesting time, harvesting method, and plant genetics. This variation in composition can 

have a very large impact in processability and in the overall economics of biorefining. Therefore, the 

objective of this report is to outline the key biomass feedstock considerations and the potential impact 

they may have on overall yield and processability within the biorefinery. Much of this information 

comes from decades of study of the structure and composition of biomass resources, as well as handson pilot plant experience with conversion of these materials to liquid fuels and other products at NREL.

Background

Biomass feedstocks considered here represent a mixture of those identified as being of potentially highvolume in the United States2

. These include wheat straw, corn stover, forest residues and thinnings, 

switchgrass, and plantation trees such as poplar. Other feedstocks will be discussed but will not be 

primary foci, including bagasse, sorghum, energy cane, rice straw, and municipal solid waste (MSW). 

While there are many sources of biomass, there are also many envisioned methods of converting that 

biomass to fuels. To focus the discussion, three representative conversion technologies will be 

considered here: one biochemical (using enzymes and fermentative organisms) and two 

thermochemical (gasification coupled with mixed alcohol and/or Fischer-Tropsch synthesis). These are 

well documented in two NREL design reports3.4. The Fischer-Tropsch (FT) design was documented in an 

earlier report to EPA from NREL. Other processes will not be discussed here, including Coskata’s syngas 

fermentation, Bluefire Ethanol’s concentrated acid process, and certain pyrolysis processes.
Many methods of feedstock compositional analysis have been developed over the years. Quantifying 

the components of biomass and biomass intermediates remains an active area of research, aimed at 

producing more accurate and faster methods of analysis. When considering a specific biomass resource 

for refining, it is always best to conduct this analysis on representative materials because of the large 

variability described above. However, several collections of existing compositional data exist5,6.

MSW can be considered a potential feedstock for biomass conversion processes, both biochemical and 

thermochemical. In either process, the organic fraction of MSW is targeted. However, there are many 

challenges and considerations associated with this particular feedstock. These largely relate to the 

potential quantity and quality of biomass from MSW. Maintaining a consistent quality MSW feedstock is 

extremely difficult. MSW is also not easily defined. In many parts of the country, “trash” is not 

segregated in any fashion and can be extremely variable in contents. It would have to be pre-sorted to 

better isolate a biomass fraction and this would add cost. However, in certain parts of the country, this 

pre-sorting infrastructure is in place, such as California. Sometimes called a MRF or MERF (Materials 

Recovery Facility), this facility separates co-mingled trash into separate fractions of aluminum, paper, 

cardboard, etc. Each of these can then be sold to recycle markets overseas and domestic. Sometimes, 

curbside segregation can result in materials such as “urban greenwaste” (food/wood/yard trimmings) 

that could more easily be utilized as a biomass feedsource. However this is not widely practiced in the 

US which makes it difficult to visual urban greenwaste as a high-impact biomass feedstock. 

It’s also important to keep in mind that current markets do exist for these MSW materials. Cardboard 

and paper can be sold overseas for over $100/ton7, which makes using such materials for biofuels cost 

prohibitive. Urban greenwaste is utilized for commercial mulch and organic composting. In the past, 

several biomass companies have developed business plans that require negative cost feedstocks. In 

other words, they count on tipping fees remaining in place for companies to dispose of waste. In 

essence the biomass company gets paid to take the feedstock instead of paying for feedstock. However, 

the problem with this strategy is that the value of these materials could quickly rise with significant 

demand present, which could cause feedstock costs to rise. 

Discussion

The general topics of comparison for the various feedstocks include:

 Type of analysis conducted (carbohydrate vs. ultimate)

 Energy content (heating value of biomass, HHV, LHV)

 Moisture, water content

 Physical structure of biomass

 Cellulose

 Hemicellulose

 Lignin

 Acetyl groups

 Other organics

 Ash and other inorganics
Type of analysis conducted. The type of analysis conducted is largely determined by the process 

under consideration. For a biochemical process, a carbohydrate analysis is more informative 

because of the known stoichiometric reactions within a biochemical process for carbohydrate 

conversion. This is typically accomplished using published laboratory analytical procedures and 

equipment such as high performance liquid chromatography (HPLC). In a biochemical process, only 

the carbohydrate fractions of the biomass are converted to ethanol. The remaining fractions (lignin, 

ash, etc) are either converted to heat and power, or remain relatively unconverted. For a 

thermochemical process, an elemental (also known as “ultimate”) analysis is more informative. This 

type of analysis has been used to analyze other solid resources such as coal for many years. It 

simply quantifies how much of the organic elements are present within a sample (Carbon, 

Hydrogen, Oxygen, Nitrogen, Sulfur, Chlorine, etc). Other inorganic compounds are found in the Ash 

fraction, which can further be broken down into fractions of potassium, sodium, etc. The C,H, and O 

in particular represent the potential available materials for synthesizing mixed alcohols 

downstream.

Energy content. The energy content of biomass is often expressed in terms of a higher or lower 

heating value. In thermodynamic literature, these are often referred to as a heat of combustion; the 

heat released from the biomass when it’s combusted and the chemical bonds are broken. The 

higher heating value (HHV) assumes that all the water is in the liquid phase at the end of 

combustion, while the LHV assumes that the water is in the vapor phase. In this fashion, the HHV 

includes the heat that can be obtained by condensing the water vapor produced by combustion. 

Biomass feedstocks with higher energy contents have more energy available from which to create 

liquid fuels. Woods, for example, often have higher heating values compared to grasses like stover. 

It is important to calculate heating values on a moisture-free or dry basis because the presence of 

water in a biomass feedstock will reduce the gross heating value.

Moisture, water content. The presence of moisture in a feedstock can have a significant impact on 

the biorefining process, especially if it is a thermochemical process. Processing feedstocks with 

higher moisture content in a thermochemical process will produce lower overall efficiencies than 

drier feedstocks. This is due to the energy lost in vaporizing this moisture. Wetter feedstocks will 

also have lower efficiencies in the gasification step as well. In a biochemical process, the moisture 

content of the biomass has less of an impact on the overall efficiency. Because these processes are 

mostly aqueous-phase systems to begin with, water is often added in order to slurry the biomass 

and make it more “flowable”. In the design report process as designed, water is added until the 

biomass slurry entering pretreatment is approximately 30% total solids, or approximately 70% 

moisture. Therefore, starting with a feedstock that is 15% moisture will require the addition of 

more water than a feedstock that is 50% moisture. Technoeconomic analysis has shown that 

maximizing the solids concentrations through the biochemical processes helps to reduce cost 

because of economies of scale. However, this is limited by a pretreatment’s ability to disrupt the 

biomass matrix sufficiently. Certain pretreatment reactors and feeding systems can also have 

limitations on the solids concentration they can utilize. Woods are expected to arrive at a 

biorefinery with moisture concentrations near 50%, while feedstocks that have been field dried are 
expected to be closer to 15% moisture. The moisture content can be affected by time of biomass 

collection, and other processing steps such as pelletization, drying, torrefaction, etc.

Physical structure of biomass. Biomass can be analyzed and classified at many different levels. For 

example, biomass often contains different anatomical fractions that can be visualized without the 

aid of any microscopic tools. Fractions of a corn plant, for example, include stalks, husks, cobs, 

nodes, etc. As one dives deeper into the biomass, different classification systems come in to play. 

Within the anatomical fractions, different cell types exist. Ultimately, the biomass polymers of most 

interest (cellulose, hemicelluloses, and lignin) are found within the plant’s cell walls. 

Nature has made these biomass materials resistant to degradation so as to guard against 

environmental elements and biological attack. The three most common compounds are cellulose, 

hemicellulose, and lignin. These compounds are interwoven physically with each other and also 

chemically bonded to one another. Cellulose within plant cell walls comes in the form of 

microfibrils. Hemicellulose is a general group of other polysaccharides and organic acids that form a 

sheath around the cellulose. They also form covalently bonds and ester linkages with the lignin 

present. Different biomass resources will contain different versions of how these compounds are 

physically arranged. These differences can lead to varying degrees of “convertability”. For example, 

wheat straw has been shown to be more amenable to biochemical conversion (at lower severity 

conditions) than corn stover. This is part of the reason why it’s a targeted feedstock. Corn stover 

and switchgrass are less amenable to conversion and require more severe conditions to effectively 

pretreat. Feedstocks that have already been pre-processed, such as bagasse or corn fiber, can also 

be more amenable to conversion because part of the matrix has already been disrupted through the 

initial processing steps. 

Cellulose. Cellulose is a long polymer chain of glucose monomer units. It is very similar in this 

fashion to starch. The difference between starch and cellulose is how the glucose units are bonded. 

Starch is bonded through alpha linkages, which are much easier to break than the beta linkages 

found in cellulose. In this fashion, nature uses these compounds differently, using starch as an 

energy storage system, and cellulose as a structural function. Together, cellulose and hemicellulose 

make up the total potential fraction of the biomass that can be directly converted to ethanol and 

other biofuels in a biochemical process. Large variability in cellulose concentration is seen in 

biomass feedstocks. Some niche biomass feedstocks have been seen to have almost 75% cellulose1

on a dry-weight basis, however this is not typical. In general, cellulose concentrations for most 

feedstocks range between 30% and 45%. Woods (both hardwood and softwood) tend to have 

higher cellulose concentrations than grasses and agricultural residues and therefore, have higher 

theoretical yield potentials of biofuel. 

Hemicellulose. Hemicellulose is a general definition that captures additional biomass carbohydrates 

(xylan, galactan, arabinan, mannan) and organic acids (uronic acid, ferulic acid). Hemicellulose also 

contains the acetyl functional groups that become acetic acid during dilute acid pretreatment. 

 

1

Some urban green waste and pulp wastes are of this sort.
Hemicellulose is generally thought of as “easier to convert” than cellulose, which is why processes 

are often designed to use cheaper chemicals to hydrolyze the hemicellulose during pretreatment as 

opposed to expensive enzymes. Hemicellulase enzyme systems are under development but are not 

as developed as the cellulase systems. Commercial hemicellulase preps are currently used in 

pulping and other industries. However, they will contain much different functional activities than 

cellulase preps, such as xylanase, ferulic-acid-esterase, etc. In a biochemical process, if the 

hemicellulose is not hydrolyzed chemically, the burden becomes placed on the enzymatic system. 

This is seen with some of the alternative pretreatments such as AFEX, for example. Under this 

pretreatment, the ammonia helps to swell fibers and disrupt the biomass matrix, however, does 

very little to hydrolyze or otherwise chemically convert the hemicellulose. Concentrations of 

hemicellulose can range dramatically between biomass feedstocks (typically 15-30% dry weight of 

biomass). The relative concentrations of hemicellulosic sugars can also vary. For example, 

softwoods typically contain higher concentrations of mannan than hardwoods or grasses. 

Lignin. Lignin is the substance within a cell wall that gives a plant structural rigidity. It is a large 

molecular weight compound that contains aromatic rings and several functional groups (methoxy, 

etc.). The empirical formula for lignin will vary between biomass feedstocks. In other words, the 

lignin in hardwoods is different than lignin found in softwoods, which is different from lignin found 

in grasses and straws. Lignin can be depolymerized through base-catalyzed reactions. It can also be 

extracted from biomass using various organic solvents. Several methods of quantifying lignin within 

biomass exist, including Klason lignin, and other such determinations. In a thermochemical process, 

the lignin, hemicellulose, and cellulose are broken down to produce a mixture of gases (CO, H2, CO2, 

CH4, etc), tars (organics with molecular weight greater than or equal to benzene), solid char, and 

ash. In a dilute acid biochemical process, the lignin carries through the process relatively 

unchanged. Some of the lignin is solubilized if temperatures in pretreatment exceed the glass 

transition temperature of lignin (~135C). However, the lignin will recondense into solid form once 

the temperature is reduced. Lignin is known to inhibit enzymatic hydrolysis reactions however this 

interaction between cellulose, lignin, and enzymes is complex and the subject of much research. 

Lignin is known to play a role in “non-specific binding” which means that the cellulase enzymes will 

bind to the lignin instead of the cellulose. This causes enzyme to try to act on a substrate that it 

wasn’t meant for, which equates ultimately to the need for higher enzyme loadings. However, 

digestibility experiments have also shown that completely extracting the lignin does not necessarily 

create more digestible cellulose. The theory here is that when lignin and hemicellulose are 

completely extracted, the cellulose microfibrils may hydrogen bond to one another and become 

even more recalcitrant than before. But aside from that, feedstocks with less lignin are generally 

more amenable to enzymatic conversion, and feedstocks with higher lignin are more recalcitrant, 

requiring higher enzyme doses or have yields that are much lower. In general, woods have higher 

lignin fractions than grasses and straws. Lignin composition of some woody biomass has been seen 

in the high 20’s percent on a dry weight basis.

Acetyl groups. The amount of acetylation of biomass depends on the specific feedstock. The acetyl 

groups are part of the hemicelluloses and are easily cleaved during dilute acid hydrolysis. In this low 
pH environment, they become acetic acid, which is a well-known fermentation inhibitor. For some 

ammonia-based alternative pretreatments the acetyl groups convert to acetimide, which helps to 

reduce the amount of acetic acid a fermenting organism will see. During gasification, the specific 

fate of these compounds are unknown, but they in some fashion become part of the syngas, char, 

and tars. In general higher acetylation is seen with hardwoods than grasses or straws. For biomass 

that is more highly acetylated (>4% dry wt), additional processing is required to reduce the acetic 

acid levels to a more manageable level (<10 g/L). This can include membrane separation, ion 

exchange, etc. This can add cost to a process.

Other organics. Other organic compounds that can be present in biomass include acids (uronic 

acids), proteins, “extractives”, other sugars (fructose, sucrose), pectins, and other unidentified

“soluble solids”. Methods for quantifying these compounds continue to undergo development. The 

fate of many of these compounds within a biochemical process are largely unknown. In process 

models these are largely assumed to be combusted with lignin residue to provide heat and power, 

or can end up in wastewater treatment. Helping to identify the additional organic compounds 

present within these biomass fractions can help to close material balances to a greater degree.

Ash and other inorganics. Ash is comprised of the inorganic materials and minerals entering the 

process. They can either be “structural inorganics” which are metals such as potassium, sodium, 

sulfur, etc. found in the biomass itself, or they can be non-structural inorganics. The structural 

inorganics help provide certain functionalities within the living plant just as they do for human 

bodies. Non-structural inorganics are sources of material that come from the environment when 

the biomass is collected. For example, when corn stover is collected off of the field, a certain 

amount of soil and dirt will come with the stover. When the biomass is analyzed, this will show up 

in the analysis as ash. This is part of why grasses and straws typically have higher ash contents than 

woods. Ash is effectively inert material. Therefore, it is desirable to have low-ash biomass for 

biorefining because you don’t want to pay a lot of money for biomass fractions that can’t be 

converted into anything useful. 

In a biochemical process, the ash is largely inert and will end up as dry ash coming from the boiler. 

However, if ash fractions (potassium, etc) that go to wastewater treatment are too high, they can 

become problematic. They can impact process equipment by eroding it if present in high enough 

concentration. Rice straw is a primary example. Rice straw is notorious for having high 

concentrations of silica8. This has been shown to erode process equipment at those concentrations. 

This can increase processing costs and is unfavorable. In a thermochemical process, the ash will 

come out of the gasifier as a solid dry ash for lower temperature systems. In high temperature 

gasification systems, the ash will leave as a molten form called “slag”. This will cool into a glassy 

looking substance that has a number of potential uses, including being added to road base, cement 

and other structural foundation material. 

Sulfur is one inorganic that is of particular concern to thermochemical processes. The 

thermochemical mixed alcohol process as designed uses a nickel-based tar reforming catalyst to 

reform tars into additional syngas yield. Sulfur is a known poison to nickel-based catalysts. As such, 
experiments have shown catalyst to deactivate more quickly in the presence of sulfur and lose full 

reforming potential when regenerated. This means that certain active sites have been poisoned 

irreversibly. Woods typically have low sulfur levels, around 10 parts per million (ppm). However 

agricultural residues and energy crops (corn stover, switchgrass) can have sulfur levels as high as 500 

ppm, which can greatly impact the process. In a gasifier’s reducing atmosphere, sulfur will typically 

come in the form of H2S. Sulfur can also impact fuel synthesis catalysts. In this particular mixed 

alcohol process, a Cobalt-moly-sulfide based catalyst is used to convert syngas into mixed alcohols. 

This particular catalyst actually requires the presence of small amounts of sulfur to remain sulfided. 

However, alternative catalysts may run the risk of being poisoned by sulfur, which would require 

additional cost due to more stringent acid gas scrubbing.

Summary

Given all of the information described thus far, a qualitative comparison of process yield, efficiency, 

etc. for the chosen feedstocks was completed. This is summarized in Table 1. There are many 

considerations that can impact these parameters. Many experimental lessons have been learned 

throughout the years for comparing feedstocks to one another. Those learnings were incorporated 

into this comparison as much as possible.

In a biochemical process, corn stover and switchgrass have shown to behave similarly. They have 

similar composition, are anatomically similar, and therefore at this juncture would be expected to 

produce similar yields in such a process. While wheat straw also maintains a similar composition to 

corn stover and switchgrass, experiments have shown it to be more amenable to conversion at 

lower severity. Poplar and other hardwoods, however, are expected to perform differently in a 

biochemical process. The total carbohydrate potential, and therefore the theoretical ethanol yield is 

larger due to these woods having more cellulose. The heating value of the woods is also greater. 

The difference in moisture in a biochemical process (15% vs. 50%) has little impact on the efficiency 

or yield. Because of the much greater lignin content, the electric coproduct credit for a biochemical 

process is also anticipated to be greater for woods than for stover or switchgrass. However, this 

increased lignin can potentially have negative impacts on certain sections of the process, particularly 

the enzymatic hydrolysis. For the woods, the yields would likely be achieved through certain 

process improvements that help to overcome these negative impacts. These improvements may 

cost more money, but cost was not a factor in this particular analysis. Another area for potentially 

increased cost comes from the higher levels of acetylation in hardwoods compared to stover and 

grasses. Additional unit operations might be needed to reduce the acetic acid levels prior to 

fermentation to avoid inhibition of the cells.

For a thermochemical process, woods make an excellent feedstock. They have high heating values 

compared to stover and switchgrass, and they are low in sulfur (which can poison the tar reforming 

catalyst). One of their drawbacks is their higher moisture content. This tends to lower the overall 

efficiency of these processes, however, higher yields from woods are still observed compared to 
stover and switchgrass. Forest residues have a very large compositional variability depending on 

what forests they come from (hardwood, softwood), how they are harvested, etc. For the purposes 

of this study, they were assumed to be similar to poplar, however, detailed compositional analysis 

would be required to verify this for a specific feedstock or biorefinery project. The yields for the FT 

process lower than the mixed alcohols process, but the overall energy efficiencies of both processes 

are approximately the same.

For the data summarized in Table 1, a few cells were noted with “?”. These are the overall 

efficiencies for biochemical conversion of poplar for 2010, 2015, and 2022. Researchers at NREL 

don’t currently have a good understanding of how the overall efficiency might differ between woods 

and grasses (or residues) in a biochemical process. As such, NREL’s current recommendation is to 

assume that the efficiencies are similar for both feedstock classes until this is better understood 

through future research. 
Table 1 – Biomass feedstock comparison between biochemical and thermochemical processes

Year Pathway Feedstock

(dry matter) 

Heating Value

(Btu/ton) LHV Moisture

Ethanol Yield 

(gal/ton)

Diesel

Equivalent/

dry ton

Electricity 

Produced 

(kwh/gal)

% overall plant

efficiency [fuel 

products] (LHV)

2010 Biochem Fermentation Corn Stover 14,036,000 15 71.9 2.61 49.00

2010 Biochem Fermentation Switchgrass (Herbaceous) 14,797,555 15 71.9 2.61 49.00

2010 Biochem Fermentation Hybrid Poplar (Farm Trees) 16,120,000 50 73.7 3.92 ?

2010 Thermochem Mixed Alcohols Hybrid Poplar 16,120,000 50 63.2 0.514 38.60

2010 Thermochem Mixed Alcohols Corn Stover 14,036,000 15 59.1 0 41.30

2010 Thermochem Mixed Alcohols Switchgrass (Herbaceous) 14,797,555 15 59.1 0 41.30

2010 Thermochem Mixed Alcohols Forest Residues 16,120,000 50 63.2 0.514 38.60

2015 Biochem Fermentation Corn Stover 14,036,000 15 89.8 2.36 52.00

2015 Biochem Fermentation Switchgrass (Herbaceous) 14,797,555 15 89.8 2.36 52.00

2015 Biochem Fermentation Hybrid Poplar (Farm Trees) 16,120,000 50 92.0 3.54 ?

2015 Thermochem Mixed Alcohols Hybrid Poplar (Farm Trees) 16,120,000 50 73.4 0 42.00

2015 Thermochem Mixed Alcohols Corn Stover 14,036,000 15 68.6 0 45.00

2015 Thermochem Mixed Alcohols Switchgrass (Herbaceous) 14,797,555 15 68.6 0 45.00

2015 Thermochem Mixed Alcohols Forest Residues 16,120,000 50 73.4 0 42.00

2020 Biochem Fermentation Corn Stover 14,036,000 15 92.3 3.59 55.00

2020 Biochem Fermentation Switchgrass (Herbaceous) 14,797,555 15 92.3 3.59 55.00

2020 Biochem Fermentation Hybrid Poplar (Farm Trees) 16,120,000 50 94.6 5.39 ?

2020 Thermochem Mixed Alcohols Hybrid Poplar (Farm Trees) 16,120,000 50 80.1 0 45.90

2020 Thermochem Mixed Alcohols Corn Stover 14,036,000 15 74.9 0 49.11

2020 Thermochem Mixed Alcohols Switchgrass (Herbaceous) 14,797,555 15 74.9 0 49.11

2020 Thermochem Mixed Alcohols Forest Residues 16,120,000 50 80.1 0 45.90

2020 Thermochem F-T Diesel Hybrid Poplar (Farm Trees) 16,120,000 50 62.7 0 46.70

2020 Thermochem F-T Diesel Corn Stover 14,036,000 15 58.6 0 49.97

2020 Thermochem F-T Diesel Switchgrass (Herbaceous) 14,797,555 15 58.6 0 49.97

2020 Thermochem F-T Diesel Forest Residues 16,120,000 50 62.7 0 46.70
 

1

Ruth, M.F.; Thomas, S.; “The effect of corn stover composition on ethanol process economics”, Presentation for 

25th Symposium for Biotechnology for Fuels and Chemicals, Breckenridge, CO, 2003.

2

USDA/DOE report “Biomass as a Feedstock for a Bioenergy and Bioproducts Industry: The Technical Feasibility of 

a Billion-ton Annual Supply”, April 2005.

3

Aden et al “Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-current Dilute Acid 

Prehydrolysis and Enzymatic Hydrolysis for Corn Stover”, NREL/TP-510-32438, June 2002.

4

Phillips et al “Thermochemical Ethanol Via Indirect Gasification and Mixed Alcohol Synthesis of Lignocellulosic 

Biomass”, NREL/TP-510-41168, April 2007.

5

Phyllis, database for biomass and waste, http://www.ecn.nl/phyllis Energy research Centre of the Netherlands 

6

Department of Energy, Biomass Program, Feedstock composition and Property Database, 

http://www1.eere.energy.gov/biomass/feedstock_databases.html

7

Personal conversations with MRF managers in California, 2007.

8

Summers, M.D., et.al. “Yields and Property Variations for Rice Straw in California”, UCDavis, submitted abstract to 

Biomass conference of the Americas (BCOTA).