Document ID: EPA-HQ-OAR-2012-0401-0240
Agency: epa
Document Type: Supporting & Related Material
Title: 
Posted Date: 2014-07-18T04:00Z

MEMORANDUM
TO: 		Air and Radiation Docket EPA-HQ-OAR-2012-0401
FROM: 	EPA Office of Transportation and Air Quality
DATE:		July 1, 2014
SUBJECT: 	Cellulosic Content of Various Feedstocks  -  2014 Update

This memorandum provides information on EPA's evaluation of components generally found in feedstocks that would be used to produce cellulosic biofuels.  

A. BACKGROUND
For purposes of this memorandum, the terms "cellulosic" and "cellulosic content" generally mean cellulose, hemicellulose, and lignin.  Cellulose, hemicellulose, and lignin are the main components of plant cell walls that provide strength, mechanical support and protection to the cell.  These components can be defined as follows:  

   * Cellulose  --  A structural carbohydrate that is the principal constituent of wood and other biomass. It is a polymer of glucose with a repeating unit of C6H10O5 strung together by ß-glycosidic linkages. 

   * Hemicellulose  --  A structural carbohydrate that consists of short, highly branched chains of sugars. In contrast to cellulose, which is a polymer of only glucose, a hemicellulose is a polymer of multiple different sugars, including five-carbon sugars (usually D-xylose and L-arabinose), six-carbon sugars (D-galactose, D-glucose, and D-mannose) and uronic acid. The sugars are highly substituted with acetic acid. 

   * Lignin  --  The major noncarbohydrate structural constituent of plant biomass, lignin is an irregular polymer that gives plant cells much of their mechanical strength.  It has a large, complex, cross-linked, and highly aromatic structure derived principally from coniferyl alcohol (C10H12O3).  Because of its extensive cross-linking and complex chemical structure, lignin is very difficult to break down biochemically.

Biomass also includes other organic compounds that perform essential functions in the cells, including energy storage, enzymatic catalysis of biochemical reactions and selective transport of molecules into or out of the cell.  These other compounds include but are not restricted to:  

   * Extractives  --  Any number of different compounds in biomass that are not an integral part of the cellular structure. The compounds can be extracted from biomass by means of polar and non-polar solvents including hot or cold water, ether, benzene, methanol and other solvents. The types of extractives found in biomass samples may include free sugars, pigments, membrane lipids, oils, and many other compounds.

   * Protein  --  A protein molecule is a chain of up to several hundred amino acids that is folded into a more or less compact structure. Because about 20 different amino acids are used by living matter in making proteins, the variety of protein types is enormous. In their biologically active states, proteins function as catalysts in metabolism and to some extent as structural elements of cells and tissues.

   * Starch  --  A molecule composed of long chains of -glucose molecules linked together. These linkages occur in chains of -1,4 linkages with branches formed as a result of -1,6 linkages. This polysaccharide is widely distributed in the vegetable kingdom and is stored in all grains and tubers.  Starch is highly amorphous, making it readily susceptible to digestion by human and animal enzyme systems that break it down into glucose.

In addition to these organic components, plants also contain ash, which is the residue remaining after ignition of a sample, determined by a prescribed procedure.  Ash is inorganic material, which includes soil that is present in the biomass ("soluble" or "extractable" ash) as well as minerals that are part of the structure of the biomass ("insoluble" or "structural" ash).  

There is significant variation in the data reported on feedstock component compositions, due in part to the natural heterogeneity of plants, as well as:
   * Differences between different varieties or species within a generic feedstock type,
   * Variability in the environmental conditions where the feedstock was grown,[2]
   * Sampling of different parts of the plant, which may have different composition (e.g., whole plant versus stalks and leaves),[2]
   * Harvesting of the feedstock at different stages of maturity,
   * Variability in feedstock composition in different years,[2]
   * Differences in the agronomic practices used to grow the crop,[2] and 
   * Degradation or alteration of the biomass during storage.
Additional variability may appear if comparing results determined using different analytical methods or even from variability between analytical batches in the same laboratory.

The purpose of this document is to present EPA's understanding of the cellulosic content of different biofuel feedstocks.  Given the variability in the data, we report ranges whenever available.  We also take a consistent approach to looking at the data (described in the next section).  The data itself were grouped into broad categories including cellulosic components (cellulose, hemicellulose and lignin), other organics (extractives, protein), and inorganics (soil, ash).  Each of these main components has sub-components as described in the following section.  More detailed information about each analysis is available in a spreadsheet available in the docket.

B.  EPA'S APPROACH

In order to determine the cellulosic content of feedstocks typically considered "cellulosic," such as corn stover and switchgrass, EPA surveyed the scientific literature and assembled a range of estimates of the composition of different feedstocks.  Our analysis is based on data from over 25 journal articles, the DOE Biomass online database, and other technical reports.  Many of the studies used some version of the procedure outlined by the National Renewable Energy Laboratory's (NREL) Standard Biomass Analytical Procedure.  These procedures involve separation of extractives followed by determination of structural components via a two-stage sulfuric acid hydrolysis, followed by analysis of the individual sugars via High Performance Liquid Chromatography (HPLC).  Lignin is measured in the acid soluble and acid insoluble fractions.  Proteins are determined based on the nitrogen content of the material, and ash content is determined after heating of the sample.  Some studies instead used a detergent fiber analysis, in which non-cellulosic material is dissolved so that the remaining material is the total fiber (i.e. cellulosic material), which can be measured gravimetrically.[,]  In a detergent fiber analysis, subsequent steps can be performed to dissolve hemicellulose or both hemicellulose and cellulose, so that the content of cellulose, hemicellulose, and lignin can be determined.  Some of the studies looking at energy cane only measured the total fiber content, and the Brix content (total soluble solids).  Two studies used near-infrared spectroscopy (NIRS) / partial least squares modeling, calibrated with the NREL methods or detergent fiber analysis.  An accompanying spreadsheet lists the methods used for each study, along with the compounds reported by the authors and how EPA made its calculations based on this reported data.

Table 1 shows an example of how EPA combined the data reported in one paper into the values used for our analysis.  Some studies reported a concentration for "cellulose" or "hemicellulose," and some reported "Neutral Detergent Fiber" (the sum of cellulose, hemicellulose, and lignin), but many studies reported concentrations for the individual sugars that comprise these biomolecules.  For example, some studies report a concentration for glucan or glucose, almost all of which is contained in cellulose.  We assumed that the concentration of cellulose was equivalent to the reported concentration of glucan or glucose, as shown in Table 1.  This may be a slight overestimation since hemicellulose also contains some glucose; however, concentrations of glucose in cellulose are likely to dwarf that in hemicellulose, so this approximation should be reasonably accurate.  For hemicellulose, studies reported some combination of concentrations for xylan/xylose, galactan/galactose, arabinan/arabinose, mannan/mannose, rhamnose and fucose, as well as acetyl/acetate and uronic acids.  To estimate total hemicellulose concentration, we summed the concentrations of these individual components.  Note that this value may slightly underestimate the true concentration of hemicellulose since it excludes any glucose contained in this biomolecule.  Lignin was reported as either "lignin", "Klason lignin" or both "acid soluble lignin" and "acid insoluble lignin."  For our estimates, we combined the concentrations of all types of lignin reported.  One source[4] reported only acid detergent lignin (ADL) for a number of different feedstocks.  This study also stated that acid insoluble lignin tends to be 2-4 times more concentrated than ADL for grasses, whereas acid insoluble lignin is only about 30% higher than ADL for legumes.  Accordingly, for these samples, we multiplied the reported ADL value by 3 for grasses and by 1.3 for legumes to determine the acid insoluble lignin content.  However, this estimate introduces considerable uncertainty into the total cellulosic content values, so these numbers should be considered less certain than the other values.  Studies reported a number of different types of "extractives," including compounds extracted with ethanol, ethanol/toluene, water, and hot water, as well as sucrose, organic acids, individual sugars (Glc, Fru, Raf, Sta) and fats/lipids.  We added all of these components together to estimate the concentration of total extractives.  Studies generally determined the concentration of protein (if calculated) by multiplying the nitrogen content of the sample (as mass %) by 6.25 to scale up to total protein content.  When reporting the total inorganic, or "ash," content of a sample, studies mostly reported one value.  However, some studies reported a value for extractable inorganics as well as one for structural inorganics (ash; Table 1).  Three studies reported a value for "volatile solids," which is essentially everything but the ash,[,][,] and two other studies reported values for "organic matter."[,]  For these studies, we calculated the ash content by subtracting the volatile solids or organic matter concentration from 100%.  We added together the cellulose, hemicellulose and lignin concentrations to determine the total cellulosic content, and added protein, extractives, and starch together to determine the concentration of "other organics."

Once we had calculated the concentrations (as mass %) for cellulose, hemicellulose, lignin, extractives, protein, and ash, we added these components together to determine the total percent recovery for each analysis (Table 1).  The total percent recovery is expected to be 100% if all of the components listed in Table 1 are reported, because we assume that these components represent all of the dry matter in the plant.  A total percent recovery of 100% means that all of the dry matter in the plant has been accounted for.  In the studies we looked at, total percent recoveries ranged from 88% to 106%.  Recoveries may be lower than 100% because of losses during sample processing or because portions of the material were not analyzed.  Recoveries greater than 100% indicate that one or more analytical method used over-reports the amount of material recovered.  For cases where all components were analyzed, but the total percent recovery did not equal 100%, we adjusted the percent concentration of each component so that the total percent recovery equaled 100%.  To do this, the percent concentration of each component was divided by the total percent recovery. For cases where not all of the components were reported (e.g. some studies did not report extractives), this adjustment for recovery was not made, because the total percent recovery is not expected to equal 100% when not all of the plant material is measured.  We also adjusted the cellulosic content to account for the ash content of the feedstock.  Inorganic (ash) materials are not likely to end up in the final fuel product made from the feedstock, and if they did, they would not contribute to the fuel heating content (which is how RINs are determined).  The "adjusted cellulosic content" is the percent of organic (non-ash) material that is cellulose, hemicellulose, or lignin.  Table 1 shows the results of these calculations for corn stover from one study.  Appendix A shows the equations used to calculate the adjusted cellulosic content.

Table 1.  Average biochemical composition of corn stover based on 508 samples from 47 sites in 8 states over 3 harvest years, including the standard deviation and range of the estimates.[2]  Values labeled "Reported" are as reported in Templeton et al. (2009).  Values labeled "Used" are the values used in our analysis, which are calculated as described in the text based on the reported values. 
 
 
 
Average
Std. Dev.
Range
Cellulose
 
 
 
 
 

Reported:
Glucan
                                     31.9
                                      2.0
                                   26.5-37.6
 
Used:
Total Cellulose
                                     31.9
                                      2.0
                                       
Hemicellulose

                                       
                                       
                                       

Reported:
Xylan
                                     18.9
                                      1.3
                                   14.8-22.7

Galactan
                                      1.5
                                      0.2
                                    0.8-1.9

Arabinan
                                      2.8
                                      0.3
                                    1.6-3.6

Mannan
                                      0.3
                                      0.1
                                    0.0-0.7

 
Acetyl
                                      2.2
                                      0.3
                                    0.9-2.9
 
Used:
Total Hemicellulose
                                     25.7
                                      1.4*
 
Lignin

                                       
                                       

Reported:
Lignin
                                     13.3
                                      1.1
                                   11.2-17.8
 
Used:
Total Lignin
                                     13.3
                                      1.1
                                       
Extractives

                                       
                                       
                                       

Reported:
Ethanol solubles
                                      3.3
                                      0.4
                                    1.7-4.1

Sucrose
                                      3.6
                                      2.1
                                   -1.0-10.0

 
Water-extractible (other)
                                      8.6
                                      2.3
                                   1.4-15.7
 
Used:
Total extractives (organic)
                                     15.5
                                       3.1*
                                       
Protein

                                       
                                       
                                       

Reported:
Protein
                                      3.7
                                      0.8
                                    1.1-5.4
 
Used:
Protein
                                      3.7
                                      0.8
                                       
Inorganics/Ash

                                       
                                       
                                       

Extractible inorganics (soil)
                                      2.5
                                      0.5
                                    0.0-4.8

Structural inorganics (ash)
                                      3.9
                                      0.9
                                    0.8-6.6

Used:
Total inorganics
                                      6.4
                                      1.0*
                                       
Total Percent Recovery

                                       
                                       
                                       

Reported:
Component closure
                                     96.4
                                      1.6

 
Used:
% Recovery
                                     96.4
                                      1.6
 
Cellulosic Content
                                       
                                       

Cellulosic Content
                                     70.9
                                      2.7*

Other Organics
                                     19.2
                                      3.2*

Inorganics (Ash)
                                      6.4
                                      1.0*

 
 
Adjusted Cellulosic Content
                                     78.6
                                      3.3*
 
*Standard deviation calculated by propagation of error.

C.  VARIABILITY IN CELLULOSIC CONTENT

Examination of one research study provides an example of both the variability observed within one feedstock and of how different factors may contribute to this variability (Table 1).  This study analyzed the composition of 508 commercial hybrid corn stover samples collected from 47 sites in 8 states over 3 harvest years.[2]  Glucan content (a proxy for cellulose content) in these samples varied by more than 10% of the total mass, ranging from 26.5% to 37.6% of total mass.  Lignin content was between 11.2% and 17.8% of total mass.  Standard deviations (as percentages) for the three major components were 5% for hemicellulose, 6% for cellulose and 8% for lignin, indicating that most of the measurements fell within more narrow ranges.  Standard deviations for the minor components (extractives, protein and inorganics/ash) are relatively larger, ranging from 16% for ash to 22% for protein.  The authors found that harvest year was the largest source of variability, probably reflecting the important influence of weather on composition.  Within a given sample year, environmental factors such as soil type and weather appeared to account for more variability than did the corn hybrid variety, although the hybrid variety also affected the variability in composition.

It is likely that variability when comparing different studies would be greater than that observed for this one study because of differences in the analytical methods used, as well as due to other factors that this study controlled for such as the part of the plant used and how the material was stored.  Accordingly, this study likely underestimates the amount of variability we could expect to see in our comparison.

D. CELLULOSIC CONTENT IN BIOMASS
   
EPA considered the cellulosic content of several different types of feedstocks, including several types of crop residue, grasses including miscanthus and switchgrass, and a variety of woods and branches, based on the categories of "cellulosic" feedstocks included in Table 1 to CFR § 80.1426.  Data were derived from journal articles, technical reports, and the Department of Energy's Biomass Feedstock Composition and Property Database, which contains compositional data for over 150 feedstock samples.  Because of the high degree of natural variability in biomass (discussed above), the average adjusted cellulosic contents are likely more meaningful than any single value.  This is because no single value can reflect the compositional range and variability present in that type of biomass, particularly if this value derives from a small number of samples.  However, examination of the range of values observed and the average values calculated should provide insight into the degree of variability present, as well as the most common composition, and thus be more informative.  The data are summarized in Tables 2-4, and a more detailed presentation of the data reported in each paper and EPA's use of this data can be found in an accompanying spreadsheet.

         a. Crop Residue

EPA found information about the cellulosic content of a number of types of crop residue, including corn stover, corn kernel fiber, sugarcane bagasse, and straw from wheat, rice, oats, soybeans, rye and barley (Table 2).  Of these, corn stover and wheat straw are the most commonly analyzed crop residues for biomass production.  As shown in Table 2, EPA examined data on the composition of corn stover from six different sources.  Based on the reported data, the cellulosic content was between 71 and 86% of the total mass of corn stover (Table 2).  Other organic compounds made up 10 to 20% of the total mass and inorganics were 5-12% of the total mass.  When the data have been adjusted for total percent recovery and inorganic content, the adjusted cellulosic content of corn stover was 78-92%, with an average value of 85%.  

Data for wheat straw came from seven sources, and agreed very well with data for corn stover, with the adjusted cellulosic content ranging between 73% and 92%, with an average of 85%.  EPA also considered the cellulosic content of sugarcane bagasse, corn kernel fiber, and straw from rice, soybeans, oats, rye, and barley, with data derived from two to four sources for each biomass type.  Bagasse had an extremely high adjusted cellulosic content of 94-98%.  Corn kernel fiber had an adjusted cellulosic content of 87%.  The cellulosic content of rice straw, barley straw, rye straw, soybean straw, and oat straw were similar to that of both corn stover and wheat straw, with average adjusted cellulosic contents between 78 and 83% (Table 2).  Both soybean straw and rye straw had one source that reported cellulosic contents that were several percent lower than the cellulosic content reported by other sources (69% for both).  However, for these samples, total lignin was not reported so we adjusted the reported acid detergent lignin content using commonly observed relationships.[4]  These numbers are accordingly only estimates and may in fact be much higher.  Similarly, using data from the same study, the adjusted cellulosic content of barley straw was 102%, which is physically impossible.  We performed a similar adjustment of the acid detergent lignin content of this sample, so it is likely that we overestimated the lignin content of this sample.  The average adjusted cellulosic content of all the crop residues listed in Table 2 (excluding those with estimated lignin content) is 84%, with a range of 78-96% (Table 2).  These data indicate that on average a high percentage of the organic mass of crop residues is composed of cellulose, hemicellulose and lignin.  Since all crop residues for which we have collected relevant data contain more than 75% adjusted cellulosic content, and since we believe the types of crop residue for which we have data are representative of the type of material that meets the regulatory definition of "crop residue," we conclude that crop residues as a class are predominantly cellulosic.  
Table 2.  Composition of crop residues, including percentages as reported (raw and unadjusted) for cellulosic content, other organic materials and inorganics (ash). "Adjusted cellulosic content" was adjusted for total percent recovery and inorganics.
 
 
                             Reported Composition
                                       
 
 
 
                              Cellulosic Content
                                Other Organics
                               Inorganics (Ash)
                          Adjusted Cellulosic Content
Source

Crop Residue
                                       
                                       
                                       
                                       
 

Corn Stover
                                      80%
                                      10%
                                      12%
                                      89%
DOE Database[15]

                                      71%
                                      19%
                                      6%
                                      79%
Templeton et al. (2009)2

                                      72%
                                      20%
                                      5%
                                      78%
Templeton et al. (2010)5 

                                      83%
                                      11%
                                      6%
                                      88%
Lee et al. (2007)4

                                      86%
                                     n.d.
                                      7%
                                      92%
López et al. (2005)[13]

                                      78%
                                     n.d.
                                      6%
                                      83%
Yalchi and Hajieghrari (2011)

                                                            Corn Stover Average
                                      78%
                                      15%
                                      7%
                                      85%
 

Wheat Straw
                                      74%
                                      17%
                                      10%
                                      81%
DOE Database

                                      83%
                                      7%
                                      9%
                                      92%
NREL data

                                      82%
                                      12%
                                      6%
                                      87%
Lee et al. (2007)

                                      85%
                                     n.d.
                                      7%
                                      91%
Stanton and LeValley (2010)

                                      75%
                                     n.d.
                                      5%
                                      79%
López et al. (2005)

                                      72%
                                     n.d.
                                      2%
                                      73%
Adapa et al. (2007)

                                      78%
                                     n.d.
                                      13%
                                      90%
Yalchi and Hajieghrari (2011)

                                                            Wheat Straw Average
                                      79%
                                      12%
                                      8%
                                      85%
 
Table 2, continued. Composition of crop residues.
                                                                               

                             Reported Composition
                                       

                                                                               

                              Cellulosic Content
                                Other Organics
                               Inorganics (Ash)
                          Adjusted Cellulosic Content
Source
                                                                               
Barley Straw
                                     91%*
                                     -2%*
                                      11%
                                     102%*
Lee et al. (2007)
                                                                               
                                                                               
                                      82%
                                     n.d.
                                      7%
                                      88%
Stanton and LeValley (2010)
                                                                               
                                                                               
                                      72%
                                     n.d.
                                      6%
                                      76%
López et al. (2005)
                                                                               
                                                                               
                                      71%
                                     n.d.
                                      2%
                                      72%
Adapa et al. (2007)

                                                           Barley Straw Average
                                      75%
                                     n.d.
                                      7%
                                      79%
 

Rye Straw
                                     65%*
                                     29%*
                                      6%
                                     69%*
Lee et al. (2007)

                                                                               
                                      71%
                                     n.d.
                                      6%
                                      76%
Stanton and LeValley (2010)

                                                                               
                                      84%
                                     n.d.
                                      4%
                                      88%
López et al. (2005)

                                                                               
                                      75%
                                      19%
                                      6%
                                      80%
Sun and Cheng  (2005)

                                                              Rye Straw Average
                                      77%
                                      19%
                                      6%
                                      81%
 

Soybean Straw
                                     65%*
                                     29%*
                                      6%
                                     69%*
Lee et al. (2007)

                                                                               
                                      70%
                                     n.d.
                                      6%
                                      74%
Stanton and LeValley (2010)

                                                                               
                                      81%
                                     n.d.
                                      3%
                                      83%
Maheri-Sis et al. (2011)[14]

                                                                               
                                      73%
                                     n.d.
                                      9%
                                      80%
Yalchi and Hajieghrari (2011)

                                                          Soybean Straw Average
                                      75%
                                     n.d.
                                      6%
                                      79%
 

Oat Straw
                                      70%
                                     n.d.
                                      8%
                                      76%
Stanton and LeValley (2010)

                                                                               
                                      80%
                                     n.d.
                                      5%
                                      84%
López et al. (2005)

                                                                               
                                      74%
                                     n.d.
                                      2%
                                      75%
Adapa et al. (2007)

                                                              Oat Straw Average
                                      75%
                                     n.d.
                                      5%
                                      78%
 

Rice Straw
                                      74%
                                     n.d.
                                      15%
                                      86%
Guo et al. (2009) 

                                                                               
                                      76%
                                     n.d.
                                      11%
                                      85%
Agbagla et al. (2001)

                                                                               
                                      69%
                                     n.d.
                                      11%
                                      77%
Jin and Chen (2007)

                                                             Rice Straw Average
                                      73%
                                     n.d.
                                      12%
                                      83%
 

Corn Kernel Fiber
                                      87%
                                      13%
                                      1%
                                      87%
Yoshida et al. (2012)

                                                      Corn Kernel Fiber Average
                                      87%
                                      13%
                                      1%
                                      87%

Sugarcane Bagasse
                                      90%
                                     n.d.
                                      8%
                                      98%
Guo et al. (2009)

                                                                               
                                      92%
                                      6%
                                      4%
                                      94%
Templeton et al. (2010)

                                                                Bagasse Average
                                      91%
                                      6%
                                      6%
                                      96%
 
 
                                                           Crop Residue Average
                                      78%
                                      15%
                                      7%
                                      84%
 
*For these samples, lignin was reported as acid detergent lignin (ADL), which can be considerably lower than total lignin, which was reported for the other samples.  Total lignin was estimated for these samples by multiplying ADL by 1.3 for soybeans (a legume) or by 3 for rye and barley (types of grass) as discussed in the text.  These estimated values were not used in calculating the average.
         
         
         
         b. Grasses 

For our analysis of the cellulosic content of grasses, EPA gathered information about switchgrass, miscanthus and several other types of grasses (Table 3).  Of these grasses, switchgrass and miscanthus have been intensively studied for use as biofuels feedstocks.  The cellulosic content of these two types of grasses were similar to each other, with studies reporting 71-85% cellulosic content for switchgrass and 77-86% cellulosic content for miscanthus.  The average adjusted cellulosic content of both switchgrass and miscanthus are 85%.  EPA considers both of these feedstock types to be predominantly cellulosic.  EPA has added energy cane, giant reed (Arundo donax), and napier grass (Pennisetum purpureum) as allowable feedstocks for cellulosic fuels under RFS in recent rulemakings.[,]  Napier grass and giant reed have average adjusted cellulosic contents of 78% and 83%, respectively, so also are predominantly cellulosic.    As explained in more detail in Section E, different cultivars of cane that some would consider to be energy cane can have a range of cellulosic content.  The cultivars listed in Table 3 have an average adjusted cellulosic content of 67%.  EPA is defining the term "energy cane" to only include cultivars with an average adjusted cellulosic content of at least 75%.  Such cultivars are predominantly cellulosic.  For example, cane cultivar L 79-1002 has an average adjusted cellulosic content of 75%, so is consistent with the definition of  "energy cane" being adopted in the final rule today.  The adjusted cellulosic content of the remaining grasses ranged from 71% to 98%, although most of the data on other grasses is from a study that only reported acid detergent lignin, requiring us to estimate total lignin content based on this number as discussed above, leading to greater uncertainty in the cellulosic content (Table 3).  The data on other grasses are shown for comparison purposes and may be used in the future if EPA is petitioned to include these feedstocks in the RFS program. 

Table 3.  Composition of grasses, including percentages as reported (raw and unadjusted) for cellulosic content, other organic materials and inorganics (ash). "Adjusted cellulosic content" was adjusted for total percent recovery and inorganics.

 
                                       
                                       
                                       
                                       
 
 
 
                             Reported Composition
                                       
 
 
 
                              Cellulosic Content
                                Other Organics
                               Inorganics (Ash)
                          Adjusted Cellulosic Content
Source
Grasses
 
                                       
                                       
                                       
                                       
 

Switchgrass
                                       
                                       
                                       
                                       
 

                                      79%
                                      17%
                                      5%
                                      82%
DOE Database[15]

                                      75%
                                      15%
                                      6%
                                      83%
NREL data[17]

                                      71%
                                      13%
                                      7%
                                      85%
Dien et al. (2006)[3]

                                      85%
                                      9%
                                      6%
                                      90%
Lee et al. (2007)[4]

                                      70%
                                      12%
                                      7%
                                      86%
Vogel et al. (2011)

                                      71%
                                      13%
                                      9%
                                      84%
Vogel et al. (2013)
                                                            Switchgrass Average
                                      75%
                                      13%
                                      7%
                                      85%
 

Miscanthus
                                       
                                       
                                       
                                       

                                      85%
                                     n.d.
                                      4%
                                      89%
Guo et al. (2009)[21] 

                                      86%
                                      12%
                                      2%
                                      88%
Lee et al. (2007)

                                      77%
                                     n.d.
                                      1%
                                      78%
Villaverde et al. (2010) 

                                      79%
                                      14%
                                     n.d.
                                      85%
Fedenko et al. (2013)
                                                             Miscanthus Average
                                      83%
                                      13%
                                      2%
                                      85%
 

Energy cane
                                       
                                       
                                       
                                       

                                                           unspecified cultivar
                                      75%
                                      20%
                                      5%
                                      79%
BP data

                                                                      L 79-1002
                                      73%
                                      23%
                                     n.d.
                                      76%
Fedenko et al. (2013)

                                                                      L 79-1002
                                    (26%)^
                                    (10%)^
                                     n.d.
                                      73%
Bischoff et al. (2008)

                                                                      L 79-1002
                                    (27%)^
                                     (9%)^
                                     n.d.
                                      75%
Hale (2010)

                                                                      Ho 02-113
                                    (26%)^
                                    (10%)^
                                     n.d.
                                      71%
Hale (2010)

                                                                      Ho 00-961
                                    (16%)^
                                    (18%)^
                                     n.d.
                                      47%
Tew et al. (2007a)

                                                                    HoCP 91-552
                                    (15%)^
                                    (17%)^
                                     n.d.
                                      48%
Tew et al. (2007b)
                                                            Energy Cane Average
                                       
                                       
                                       
                                       
 
                                                                    (L 79-1002)
                                      73%
                                      23%
                                     n.d.
                                      75%

                                                      Energy Cane Average (all)
                                      74%
                                      21%
                                      5%
                                      67%
 

Napier grass
                                       
                                       
                                       
                                       

                                      83%
                                      13%
                                      4%
                                      86%
BP data

                                      74%
                                     n.d.
                                      6%
                                      79%
Ansah et al. (2010)

                                      59%
                                     n.d.
                                      14%
                                      68%
Grant et al. (1975)

                                      76%
                                      19%
                                     n.d.
                                      80%
Fedenko et al. (2013)
                                                           Napier Grass Average
                                      73%
                                      13%
                                      8%
                                      78%
 

^Fiber and Brix were reported as a percentage of whole cane, including water.  This sample was not used in calculating the "cellulosic content" or "other organics" averages.

Table 3, continued.  Composition of grasses.

                             Reported Composition
                                       

                              Cellulosic Content
                                Other Organics
                               Inorganics (Ash)
                          Adjusted Cellulosic Content
Source

Giant reed
                                       
                                       
                                       
                                       

                                      84%
                                     n.d.
                                      4%
                                      87%
Scordia et al. (2011)

                                      78%
                                     n.d.
                                      6%
                                      83%
Scordia et al. (2012)

                                      73%
                                      21%
                                     n.d.
                                      78%
Fedenko et al. (2013)
                                                             Giant Reed Average
                                      78%
                                      21%
                                      5%
                                      83%
 

Other grasses
                                       
                                       
                                       
                                       
 

                                                               Reed Canarygrass
                                      58%
                                      21%
                                      11%
                                      73%
Dien et al. (2006)

                                                               Reed Canarygrass
                                     66%*
                                     26%*
                                      8%
                                     72%*
Lee et al. (2007)

                                                                   Big Bluestem
                                      83%
                                      11%
                                      6%
                                      88%
Lee et al. (2007)

                                                                    Tall Fescue
                                      64%
                                      25%
                                      11%
                                      72%
Lee et al. (2007)

                                                                    Indiangrass
                                     86%*
                                      6%*
                                      8%
                                     93%*
Lee et al. (2007)

                                                                Little Bluestem
                                   (66%)†
                                   (27%)†
                                      7%
                                   (71%)†
Lee et al. (2007)

                                                              Prairie Cordgrass
                                     92%*
                                      2%*
                                      6%
                                     98%*
Lee et al. (2007)
                                                        Intermediate Wheatgrass
                                     82%*
                                     12%*
                                      6%
                                     87%*
Lee et al. (2007)
                                                              Smooth Bromegrass
                                     86%*
                                      6%*
                                      8%
                                     93%*
Lee et al. (2007)

                                                                        Timothy
                                     73%*
                                     21%*
                                      6%
                                     78%*
Lee et al. (2007)
                                                          Other Grasses Average
                                      68%
                                      19%
                                      8%
                                      78%
 

*For these samples, lignin was reported as acid detergent lignin (ADL), which can be considerably lower than total lignin, which was reported for the other samples.  Total lignin was estimated for these samples by multiplying ADL by 3 as discussed in the text.  These estimated values were not used in calculating the average.
†Lignin was not measured for this sample, so these values are much lower than the true value.  This sample was not used in calculating the average for grasses.

         c. Wood and Leaves

EPA gathered compositional data for wood (from whole trees, debarked wood, or lumber yard wood) from 10 different species of tree (Table 4), including seven hardwoods and three softwoods, as well as from two different samples of branches from a mixture of trees.  The studies reported between 83 and 94% cellulosic content for all woods and adjusted cellulosic contents of 83 to 97%.  The adjusted cellulosic content of the mixed branch samples was very similar to that of the samples from individual species, with 86% and 89% adjusted cellulosic content, although one of the branch samples contained large amounts (18%) of inorganic materials, probably due to incorporation of non-branch materials such as soil.  Two studies determined the adjusted cellulosic content of leaves from a mixture of trees to be 74% and 77%.  The very high (37% by mass) proportion of inorganic materials in one of the leaf samples is almost certainly due to the inclusion of soil in the sample, as mentioned for the branch sample from this same study.  The average adjusted cellulosic content from wood was 92%, whereas the value for leaves was 75%.

Table 4.  Composition of wood and leaves, including percentages as reported (raw and unadjusted) for total cellulosic content, other organic materials and inorganics (ash). "Adjusted cellulosic content" was adjusted for total percent recovery and inorganics. 

 
 
                             Reported Composition
                                       
 
 
 
                              Cellulosic Content
                                Other Organics
                               Inorgan-ics (Ash)
                          Adjusted Cellulosic Content
Source
Wood & Leaves
                                       
                                       
                                       
 

Black Locust
                                      88%
                                      8%
                                      3%
                                      92%
DOE Database[15]

Eucalyptus
                                      94%
                                      3%
                                      1%
                                      97%
DOE Database

Eucalyptus
                                      83%
                                     n.d.
                                      0%
                                      83%
Wang et al. (2011)[12]

Hybrid Poplar
                                      89%
                                      4%
                                      1%
                                      96%
DOE Database

American Sycamore
                                      87%
                                      4%
                                      1%
                                      96%
DOE Database

Eastern Cottonwood
                                      88%
                                      3%
                                      1%
                                      97%
DOE Database

Yellow Poplar
                                      89%
                                      3%
                                      2%
                                      97%
DOE Database

Red Oak
                                      84%
                                     n.d.
                                      0%
                                      84%
Wang et al. (2011)

Monterey Pine
                                      91%
                                      3%
                                      0%
                                      97%
DOE Database

Radiata Pine
                                      88%
                                     n.d.
                                      1%
                                      89%
Wang et al. (2011)

Spruce
                                      90%
                                     n.d.
                                      2%
                                      91%
Wang et al. (2011)

Mixed Branches
                                      86%
                                     n.d.
                                      3%
                                      89%
Eleazer et al. (1997)[10]

Mixed Branches
                                      70%
                                     n.d.
                                      18%
                                      86%
Komilis and Ham (2003)[11]
                                                                   Wood Average
                                      87%
                                      4%
                                      3%
                                      92%
 

Mixed Leaves
                                      70%
                                     n.d.
                                      10%
                                      77%
Eleazer et al. (1997)

Mixed Leaves
                                      47%
                                     n.d.
                                      37%
                                      74%
Komilis and Ham (2003)
 
                                                                 Leaves Average
                                      58%
                                     n.d.
                                      23%
                                      75%
 

E.  IMPLICATIONS FOR CELLULOSIC BIOFUELS

In the March 2010 final RFS rule, EPA identified a number of feedstocks that may be used in approved pathways for the production of cellulosic biofuels.  In subsequent rulemakings,[,] we added energy cane, Arundo donax, and Pennisetum purpureum to the list of feedstocks that may be used to make cellulosic biofuels.  The  feedstocks currently identified in Table 1 to 80.1426 as available for use in cellulosic biofuel production pathways are:

      Crop residue, slash, pre-commercial thinning and tree residue, switchgrass, miscanthus, energy cane, Arundo donax, Pennisetum purpureum, separated yard waste, separated MSW, cellulosic components of separated food waste; and cellulosic components of annual cover crops.
       
In the current Final Rulemaking (FRM), "Regulation of Fuels and Fuel Additives: RFS Pathways II, and Technical Amendments to the RFS Standards and E15 Misfueling Mitigation Requirements," we are establishing requirements for determining the number of cellulosic biofuel Renewable Identification Numbers (RINs) that will be generated for fuel made from a range of cellulosic feedstocks.  We have determined that fuel produced according to the cellulosic biofuel pathways listed in Table 1 to § 80.1426, including use of specified feedstock types, is eligible to generate cellulosic RINs for 100% of the fuel, with special considerations for fuels made from cane and annual cover crops, as explained below.  In the future, EPA will calculate the average adjusted cellulosic content of feedstocks at the same time that we evaluate the lifecycle GHG emissions associated with use of these feedstocks to make fuel.  Fuel made with feedstocks with an average adjusted cellulosic content of at least 75% will also be eligible to generate cellulosic RINs for all of the finished fuel volume.  In this rulemaking, we have also determined that certain fuels produced from biogas from landfills, municipal wastewater treatment facility digesters, agricultural digesters, and separated MSW digesters qualify as cellulosic biofuels.  In this section, we discuss the cellulosic content of feedstocks that are already identified in Table 1 to 80.1426 as part of a pathway eligible to generate cellulosic RINs.  To help this discussion, the data from Tables 2-4 above are summarized in Table 5.

Table 5.  Summary of the average adjusted cellulosic content of different types of feedstocks.  Data are derived from Tables 2-4 above.

 Feedstock Type
                      Average Adjusted Cellulosic Content
Crop Residue
                                      84%
Switchgrass
                                      85%
Miscanthus
                                      85%
Energy cane cultivar L 79-1002
                                      75%
Giant reed
                                      83%
Napier grass
                                      78%
Wood and Branches
                                      92%
Leaves
                                      75%

         a. Crop Residues

As discussed above, the crop residues for which we have data have an average adjusted cellulosic content of 84% (Tables 2 & 5).  The average adjusted cellulosic content of each type of crop residue for which we have data was at least 78%.  Based on this analysis, it is clear that a large majority of the biomass of different types of crop residues is derived from cellulose, hemicellulose and lignin, and that these crop residues meet the 75% threshold.  EPA concludes that crop residues are predominantly cellulosic in content.

The types of residues analyzed here are considered to be representative of the crop residue category as listed in Table 1 to § 80.1426.  In the current FRM we are finalizing the definition of crop residue as: 

      Crop residue means biomass left over from the harvesting or processing of planted crops from existing agricultural land and any biomass removed from existing agricultural land that facilitates crop management (including biomass removed from such lands in relation to invasive species control or fire management), whether or not the biomass includes any portion of a crop or crop plant.  Biomass is considered crop residue only if the use of that biomass for the production of renewable fuel has no significant impact on demand for the feedstock crop, products produced from that feedstock crop, and all substitutes for the crop and its products, nor any other impact that would result in a significant increase in direct or indirect GHG emissions.

Furthermore, Table IV.D.3-1 to the FRM also lists the feedstocks that would be considered under the crop residue category including: 

   * Sugarcane and Sweet Sorghum Bagasse
   * Kernel Fiber from Barley, Corn, Oats, Rice, Rye, Grain Sorghum, and Wheat
   * Stover from Corn and Grain Sorghum
   * Straw from Barley, Oats, Rice, Rye, Soybeans, and Wheat

We presented data specifically for corn stover, wheat straw, rice straw, barley straw, rye straw, soybean straw, oat straw, corn kernel fiber, and bagasse in the previous section.    
         
         b. Slash, Pre-Commercial Thinnings, and Tree Residue

The March 2010 final RFS rule established the following definitions for slash, pre-commercial thinnings and tree residue:

      Slash is the residue, including treetops, branches, and bark, left on the ground after logging or accumulating as a result of a storm, fire, delimbing, or other similar disturbance.
      
      Pre-commercial thinnings are trees, including unhealthy or diseased trees, removed to reduce stocking to concentrate growth on more desirable, healthy trees, or other vegetative material that is removed to promote tree growth.
      
      Tree residue is slash and any woody residue generated during the processing of planted trees from tree plantations for use in lumber, paper, furniture or other applications, provided that such woody residue is not mixed with similar residue from trees that do not originate in tree plantations.

Based on these definitions, it is clear that woody material, ranging in size from tree trunks to branches to twigs, would comprise the majority of these feedstocks, along with some bark and likely some residual leaves.  Based on the data in Tables 4 and 5, the average adjusted cellulosic content of wood and branches was 92%, and the average adjusted cellulosic content of leaves was 75%.  Accordingly, EPA concludes that slash, pre-commercial thinnings and tree residue are predominantly cellulosic in content.

         c. Annual cover crops

A January 2012 supplemental rulemaking defined annual cover crops as follows:

      Annual cover crop means an annual crop, planted as a rotation between primary planted crops, or between trees and vines in orchards and vineyards, typically to protect soil from erosion and to improve the soil between periods of regular crops. An annual cover crop has no existing market to which it can be sold except for its use as feedstock for the production of renewable fuel.

Common cover crops include a wide variety of types of plants.  Because cover crops are so diverse, it is likely that their cellulosic content also varies widely.  We do not have enough data about annual cover crops to be confident that they will always meet the 75% threshold set for cellulosic feedstocks.  Therefore, we are adding a new pathway for "non-cellulosic components of annual cover crops," which will be eligible for advanced biofuel RINs.  In the future, as more information becomes available, we may revisit this determination.  In addition, we may  consider the cellulosic content of cover crops on a case-by-case basis in a future rulemaking or in response to an individual pathway petition.  

         d. Switchgrass

As shown in Tables 3 and 5, six different studies of switchgrass found adjusted cellulosic contents of 82-90%, with an average of 85%. EPA concludes that switchgrass is predominantly cellulosic in content.

         e. Miscanthus

Similar to switchgrass, of the average adjusted cellulosic content of miscanthus is 85% (Tables 3 and 5). EPA concludes that miscanthus is predominantly cellulosic in content.

         f. Giant reed
            
Giant reed has an average adjusted cellulosic content of 83% (Tables 3 and 5).  EPA concludes that giant reed is predominantly cellulosic in content.

         g. Napier grass
Napier grass has an average adjusted cellulosic content of 78% (Tables 3 and 5).  EPA concludes that napier grass is predominantly cellulosic in content.

         h. Sugar/Energy Cane

Sugarcane can be and is bred for a variety of uses and a wide range of sugar and fiber contents.  As shown in Table 3, some cultivars of cane are well below the 75% adjusted cellulosic content threshold.  EPA has decided to amend the definition of energy cane to distinguish between cane cultivars that have 75% or greater cellulosic content and those that do not.  The revised definition of energy cane is: 

      Energy cane means a complex hybrid in the Saccharum genus that has been bred to maximize cellulosic rather than sugar content. For the purposes of this section:
      (1) Energy cane excludes the species Saccharum spontaneum, but may include hybrids derived from S. spontaneum that have been developed and publicly released by USDA; and 
      (2) Energy cane only includes cultivars that have, on average, at least 75% adjusted cellulosic content on a dry mass basis. 

Based on this definition, some cultivars that are commonly referred to as energy cane would not meet EPA's definition of energy cane, such as Ho 00-961 and HoCP 91-552 (see Table 3).  Cultivars that do not meet the 75% adjusted cellulosic content threshold will be considered sugarcane.  

         i. Separated yard waste; Separated MSW; and Cellulosic components of separated food waste

For these feedstocks we specifically outlined how they would be considered as part of the final rule published on March 26, 2010 (75 FR 14670).  We determined that:  

      Yard waste kept separate since generation from other waste materials are expected to be composed almost entirely of woody material or leaves, and therefore is  deemed to be composed entirely of cellulosic materials.  See 40 CFR 80.1426(f)(5)(A).
      
      Food waste kept separate since generation from other waste materials may contain both cellulosic and non-cellulosic materials.  We deem such waste streams to be composed entirely of non-cellulosic materials unless a party demonstrates that a portion of the feedstock is cellulosic through approval of their facility registration.  See 40 CFR 80.1426(f)(5)(B).
      
      For separated MSW, EPA is not aware of a test method that is able to distinguish between cellulosic- and starch-derived renewable fuel.  Under these circumstances, EPA believes that it is appropriate for producers to base RIN assignment on the predominant component and, therefore, to assume that the biogenic portion of their fuel is entirely of cellulosic origin.  See 75 FR 14706.

This FRM does not change those determinations.  Therefore, biofuel produced from separated yard waste is considered to be 100% cellulosic, biofuel produced from the biogenic portion of separated MSW is assumed to be 100% cellulosic, and biofuel produced from separated food waste is assumed to be non-cellulosic unless the producer demonstrates that some portion of the food waste is cellulosic. 

         j. Biogas from landfills, municipal wastewater treatment facility digesters, agricultural digesters, and separated MSW digesters 
         
A separate memo to the docket discusses in depth the cellulosic content of biogas derived from MSW landfills, municipal wastewater treatment facility digesters, agricultural digesters, and separated municipal solid waste (MSW) digesters.  The cellulosic content data from this memo are summarized here.  Landfill MSW has an adjusted cellulosic content of 90%.  This material is also used in separated MSW digesters.  Agricultural digesters use a mixture of animal manure, crop residue and separated yard waste.  In aggregate, animal manure has an adjusted cellulosic content of 80%, and as noted elsewhere, we have determined that crop residues as a class are predominantly cellulosic.  Wastewater treatment facility digesters process biosolids from municipal wastewater.  In aggregate, the biosolids entering the digester have an adjusted cellulosic content of 76%.  As with the other feedstocks examined in this memo, biogas produced from MSW landfills, municipal wastewater treatment facility digesters, agricultural digesters, and separated MSW digesters is derived from materials that are  predominantly cellulosic. 

APPENDIX A. EQUATIONS USED TO CALCULATE THE ADJUSTED CELLULOSIC CONTENT

These equations assume that all data are in units of % feedstock, on a dry matter basis.

Step 1: Calculate the total amount of cellulosic material, other organic material, and inorganic material.
C = cellulose + hemicellulose + lignin
where C is the total cellulosic content.

O = extractives + starch + protein
where O is the total "other organic" content.

I  = soluble ash + insoluble ash
where I is the total inorganic content.

Step 2: Determine the total percent recovery 
R = C + O + I
where R is the total percent recovery.

Step 3: Adjust C, O and I to account for the total percent recovery.
If R does not equal 100%, and all of the feedstock components have been reported, adjust the cellulosic content, other organic content, and inorganic content so that the total percent recovery equals 100%.
Cr  = C/R
Or = O/R
Ir = I/R
where Cr, Or, and Ir are the cellulosic content, other organic content, and inorganic content adjusted for the total percent recovery.

After this adjustment is made, by definition, 
Cr + Or + Ir = 100%

Step 3 is only taken if all of the feedstock components have been reported because if some of the components are not reported (e.g. extractives), the total percent recovery is not expected to be 100%.  

Step 4: Adjust for the inorganic material.
If all feedstock components were reported, and cellulosic content, other organic content, and inorganic content have been adjusted for total percent recovery, there are two equivalent equations for calculating the adjusted cellulosic content (ACC):
ACC = Cr / (100% - Ir)
Or
ACC = Cr / (Cr + Or)

If some of the components were not reported (such as extractives), Step 3 should be skipped. If the inorganic content was reported, the first equation of Step 4 should be used to calculate the ACC.  If the inorganic content was not reported, and instead, all of the "other organic" components were reported, the second equation of Step 4 should be used.  

For examples of how EPA has used these equations, see the accompanying spreadsheet posted to the docket.