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MEMORANDUM

DATE: May 30, 2014

TO: Air and Radiation Docket EPA-HQ-OAR-2014-0401

SUBJECT: Additional Detail on the Calculation of the Cellulosic Converted Fraction

This memorandum provides additional information about the implications for cellulosic fuel producers 

processing feedstock materials with an adjusted cellulosic content below EPA’s cellulosic content 

threshold of 75%. All cellulosic biofuel processes using biomass feedstocks must employ a process with 

the demonstrated capability of converting cellulosic material to biofuel. However, while processes using

a feedstock with an adjusted cellulosic content greater than 75% will be allowed to generate cellulosic 

renewable identification numbers (RINs) for the entire fuel volume, processes using a feedstock with an 

adjusted cellulosic content below 75% will have to apportion different D-codes to the RINs produced 

based on the amount of cellulosic fuel produced. In practice, such producers will treat the feedstock 

material as two separate feedstocks – one cellulosic feedstock and one non-cellulosic feedstock – and 

allocate RINs using existing regulations for generating fuels using multiple feedstocks. For most 

producers this apportionment will be straight-forward, but for producers simultaneously processing 

cellulosic and non-cellulosic feedstocks in a single reactor using a biochemical process, the 

apportionment will be more complicated – requiring additional record keeping and reporting 

requirements. The regulatory framework EPA established to verify that cellulosic RINs generated by such 

processes correspond to cellulosic biofuel involves verifying that the calculated converted fractions (CF) 

applied to each portion of the feedstock material (cellulosic or non-cellulosic) is accurate. Establishing 

the accuracy of these determined CF values requires obtaining reasonably accurate feedstock and 

process data to perform a mass balance accounting of the various feedstock materials allowing portions 

of the final fuel to be attributed either to the cellulosic or non-cellulosic feedstock. This memorandum 

will explain: 1) why EPA has established separate requirements for various types of processes, 2) what 

types of data can be considered reasonably accurate to form the basis for calculating cellulosic and noncellulosic CF values for feedstocks, and 3) how accurate feedstock CF values and existing regulations can 

be used to apportion RINs for a single type of fuel produced from multiple feedstocks.
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I. Background

a. The cellulosic content threshold, and the implications for feedstocks with lower cellulosic 

contents

EPA is finalizing a cellulosic content threshold for feedstocks to qualify for cellulosic RINs for the total 

volume of fuel produced.1 If a feedstock has an average adjusted cellulosic content of at least 75%, all of 

the fuel produced from the feedstock is eligible to generate cellulosic RINs, provided that the producer 

is using a method that can produce fuel from the cellulosic portion of the feedstock. “Adjusted cellulosic 

content” is the percent of organic matter that is cellulosic material (cellulose, hemicellulose, and lignin). 

In other words, the percent cellulosic content is adjusted for the inorganic (ash) content of the 

feedstock.

In cases where two or more feedstocks are processed simultaneously, producers are eligible to generate 

cellulosic RINs for a percentage of the fuel produced, based on the cellulosic converted fraction and the 

equations Table 4 to §80.1426. The purpose of this Memorandum is to provide details on how the 

cellulosic converted fraction can be calculated, and examples of methods that could be used to generate 

the data needed to calculate the cellulosic converted fraction.

b. Registration, reporting and record keeping requirements for thermochemical and 

biochemical processes

EPA is requiring all producers of cellulosic RINs converting cellulosic biomass to complete additional 

registration requirements. These additional requirements are designed to demonstrate that the process 

being registered is capable of performing cellulosic biomass conversion, and that the relevant unit 

process equipment to perform the described conversion to biofuel is both in place and operational.

Process technology plays a key role in how much of the final fuel product is actually produced from 

cellulose, hemicellulose, or lignin. There are two basic processes for converting cellulosic feedstocks 

into fuel: thermochemical and biochemical. Thermochemical processes mainly consist of pyrolysis – in 

which cellulosic biomass is decomposed with temperature to bio-oils that can be further processed to 

produce a finished fuel – and gasification – in which cellulosic biomass is decomposed to synthesis gas 

(“syngas”) that with further catalytic processing can produce a finished fuel product. Thermochemical 

processes typically convert all of the organic components of the feedstock into finished fuel, and the 

portion of the finished fuel derived from cellulosic material is proportional to the cellulosic content of 

the organic fraction of the feedstock. Therefore, EPA’s additional registration requirements for 

thermochemical producers are intended to demonstrate how much of the feedstock is cellulosic, and 

that the conversion equipment to be used in the process is both in place and functional at the time of 

registration. 

By contrast, the biochemical process requires the release of sugars from biomass and the use of 

microorganisms to convert sugars into fuels. The amount of cellulosic material converted to fuel will 

depend on the details of the process, such as the types and combinations of enzymes used. Variations 

 

1

See Notice of Final Rulemaking placed in docket EPA-HQ-OAR-2012-0401.
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from process to process will establish different hydrolysis efficiencies for cellulosic and non-cellulosic 

carbohydrates meaning that the cellulosic portion of the fuel will not necessarily be proportional to the 

cellulosic composition of the feedstock.

For in situ processes, where cellulosic and non-cellulosic material are simultaneously hydrolyzed to 

intermediate sugars or converted to fuel, the registration, reporting, and recordkeeping requirements, 

as well as the method for determining the cellulosic converted fraction, are different depending on 

whether the cellulosic biomass is converted by biochemical hydrolysis or by thermochemical treatment. 

EPA’s additional registration and record keeping requirements for in situ biochemical producers of 

cellulosic fuel are intended to establish and monitor the relative conversion efficiencies for both the 

cellulosic and non-cellulosic portions of the feedstock in order to ensure that RIN assignments have 

been made appropriately. 

II. Definition of the cellulosic Converted Fraction

The EPA defines the converted fraction (CF) to be the average mass percent (typically a yearly average) 

representing that portion of the feedstock that is converted into renewable fuel. The CF is calculated by 

dividing the mass of the feedstock that is converted to renewable fuel by the mass of the feedstock that 

is capable of being converted to fuel. The cellulosic converted fraction, therefore, is the average mass 

percent representing that portion of the cellulosic feedstock that is converted into renewable fuel. This 

calculation is straight forward for producers using a single feedstock that meets the 75 percent adjusted 

cellulosic content threshold, or mechanically separating the cellulosic and non-cellulosic portions of a 

mixed feedstock and processing those portions separately. However it is more complicated to calculate 

separate CF and cellulosic CF values among in situ biochemical processes because the mass of the 

resulting fuel corresponding to the non-cellulosic and cellulosic portions cannot be directly measured. 

However, existing EPA regulations detail how RINs can be allocated where multiple feedstocks are being 

simultaneously processed. This memo provides more detail on how the formulas in 80.1426(f)(3)(vi) 

should be used to calculate RINs. 

III. Description of various process configurations for “in situ” processes

Biochemical processes typically employ engineered yeast strains to ferment aqueous sugars into 

alcohols that can be separated and used as biofuels. Release of sugars from complex carbohydrates is 

facilitated by enzymatic treatment in a process called hydrolysis. The polymeric linkages in these 

carbohydrates can be cellulosic or non-cellulosic, and this distinction determines which enzymatic 

treatment will result in hydrolyzing the chemical bonds linking the sugars together. In this case, enzymes 

can be viewed as keys that un-lock the sugars, and only the proper key will fit a particular chemical 

bond’s lock. Released sugars will be converted to biofuel in the same manner regardless of whether 

they were derived from cellulosic or non-cellulosic carbohydrates. Therefore, hydrolysis is the critical 

step controlling the fraction of the finished fuel that can be considered to be derived from cellulosic 

material. Determining the extent to which the cellulosic portion of a feedstock was hydrolyzed can be 

quantified in a number of ways, but this validation is more difficult for processes that perform in situ
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hydrolysis of both the cellulosic and non-cellulosic feedstock portions. This is due to the fact that the 

fraction of sugars or fuel derived from either type of carbohydrate cannot be measured directly, but 

rather requires a mass balance accounting to identify the source of the sugars or fuel. Figure 1 shows a 

simplified example of a biochemical process performing in situ biochemical hydrolysis, and Figure 2 

shows a separated hydrolysis scenario. 

Figure 1 – Simplified schematic of a biochemical biofuel process with in situ hydrolysis

Figure 2 – Simplified schematic of a biochemical biofuel process with separated hydrolysis 
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To determine the relative amounts of cellulosic and non-cellulosic feedstock hydrolyzed, a mass balance 

accounting of the converted feedstock and resulting products and co-products is required. There are a 

number of ways to perform a mass balance accounting for the converted portion of the cellulosic 

feedstock. It will be easier for producers using separated hydrolysis because the cellulosic conversion

can be quantified directly from the cellulosic hydrolysis unit as indicated by the dashed boundary line in 

Figure 2. In this case, complex analysis of the inputs and outputs is not required because the aqueous 

sugars in the hydrolysis unit’s effluent are assumed to be derived from cellulosic carbohydrates. 

Similarly, if the cellulosic feedstock and resulting intermediates were kept separate from the 

downstream fermentation, separations, and refining, then all of the resulting fuel would be assumed 

cellulosic as well. A direct accounting for the converted cellulosic fraction will allow RINs to be easily 

attributed.

By contrast, in situ processes require a more complex mass balance accounting to confirm the 

portions of the biofuel resulting from cellulosic hydrolysis relative to non-cellulosic hydrolysis. 

Assessment methods can be designed in a number of ways, but would typically account for cellulosic 

conversion by either quantifying overall biofuel increases relative to non-cellulosic treatment, or by 

directly analyzing the structural make-up of the process’ feedstock and residual materials, as well as the 

overall fuel yield, to determine relative feedstock conversion. EPA is allowing producers to design 

analytical protocols specific to their process’ configuration as long as the mass balance accounting 

calculation method is verified by an independent engineer, these calculations can independently identify 

both a cellulosic and non-cellulosic CF, the data used to perform the calculations is regularly reported 

along with the calculated CF values, and the data itself is collected and reported in scientifically 

reasonable manner.

IV. Expectations for data collection and analysis

a. General requirements regarding data used to calculate CF

EPA is providing clarifying information on the type of methods that can be used to generate data to 

calculate the cellulosic converted fraction. Data used to calculate the cellulosic CF must be obtained 

using an analytical method certified by a voluntary consensus standards body (VCSB), or using a method 

that would produce reasonably accurate results. If a VCSB-approved method is not used to generate the 

data required to calculate the cellulosic CF for a given process, then the producer will need to show that 

the method used is an adequate means of generating the data by providing peer reviewed references to 

the third party engineer performing the engineering review at registration. An example of an 

appropriate Voluntary Consensus Standards Body test method certified by a voluntary consensus 

standards body is ASTM E1758, “Determination of Carbohydrates in Biomass by High Performance 

Liquid Chromatography.” Since this ASTM test method has been adjudicated through the voluntary 

consensus standards body process we are confident in its use in practice. An example of another type of 

method would be a non-voluntary consensus standards body test method such as the National 

Renewable Energy Laboratory (NREL) Laboratory Analytical Procedure to measure structural 
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carbohydrates2and is an example of such a methodology that is commonly presented in peer-reviewed 

literature.3 Since this NREL method has not been adjudicated through the voluntary consensus 

standards body process, the user of these types of non-VCSB test methods should have the independent 

third party engineering review certify that the non-VCSB test method would provide reasonably 

accurate results. These examples of test method analyses quantify the amount of cellulose, 

hemicellulose, and lignin in biomass. Data from these analyses can be used in a mass balance equation 

to determine the cellulosic CF. In the next sections we describe some of the types of methods that can 

be used to determine the cellulosic fraction of a feedstock, which can be used to calculate the cellulosic 

CF of the finished fuel.

b. Structural Carbohydrate Analysis

One method of determining the cellulose and hemicellulose content of biomass is using a High 

Performance Liquid Chromatography (HPLC) structural carbohydrate analysis, also known as a dietary 

fiber analysis. This type of structural carbohydrate analysis has been commonly used by researchers 

who are interested in the composition of a feedstock that will be used for biofuel.

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 For this type of test,

acid hydrolysis is used to break down both cellulose and hemicellulose into monomeric sugars (i.e., 

cellulose is converted to glucose; hemicellulose is converted to xylose, galactose, arabinose, and 

mannose). The concentrations of these sugars are measured using HPLC. The glucose concentration 

can be used as a measure of the cellulose concentration in the plant, and the sum of the xylose, 

galactose, arabinose, and mannose concentrations can be used as a measure of the hemicellulose 

concentration. The sample can also be analyzed for acetyl content, which is part of hemicellulose. 

Examples of this type of procedure are the National Renewable Energy Lab (NREL) Laboratory Analytical 

Procedure, “Determination of Structural Carbohydrates and Lignin in Biomass” and ASTM E1758, 

“Determination of Carbohydrates in Biomass by High Performance Liquid Chromatography.” Although 

this type of method has been successfully used to measure cellulose and hemicellulose for many 

feedstocks, an important caveat is that the results obtained are highly dependent on the exact methods 

used, and small changes to the procedures can have a large effect on the results.5In addition, the NREL 

 

2

“Determination of Structural Carbohydrates and Lignin in Biomass,” National Renewable Energy Laboratory. 

Available at: http://www.nrel.gov/biomass/analytical_procedures.html

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See e.g. Sluiter, J.B. et al. (2010) Compositional Analysis of Lignocellulosic Feedstocks. 1. Review and Description 

of Methods. Journal of Agricultural and Food Chemistry, 58, 9043-9053. Templeton, D.W. et al. (2010) 

Compositional Analysis of Lignocellulosic Feedstocks. 2. Method Uncertainties. Journal of Agricultural and Food 

Chemistry, 58, 9054-9062. 

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E.g. Guo, G. et al., 2009. “Characterization of enzymatic saccharification for acid-pretreated lignocellulosic 

materials with different lignin composition.” Enzyme and Microbial Technology, 45:80-87; Dahlberg, J. et al., 2011. 

“Compositional and agronomic evaluation of sorghum biomass as a potential feedstock for renewable fuels.” 

Journal of Biobased Materials and Bioenergy, 5:1-7; Fedenko, J.R. et al. 2013. “Biomass production and 

composition of perennial grasses grown for bioenergy in a subtropical climate across Florida, USA.” Bioenergy 

Research, 6:1082-1093.

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For example, using a sample particle size that is smaller than recommended can cause a low bias in measured 

carbohydrate content. For more details about how procedure changes can affect the results, see the Laboratory 

Analytical Procedures, “Summative Mass Closure” and “Determination of Structural Carbohydrates and Lignin in 

Biomass,” National Renewable Energy Laboratory. Available at: 

http://www.nrel.gov/biomass/analytical_procedures.html
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and ASTM procedures warn that biases may be introduced if the sample has a high protein content, or if 

extractives have not been removed. 

Because the HPLC structural carbohydrate analysis only measures cellulose and hemicellulose, a 

separate method is needed to measure the lignin concentration. A procedure for measuring the lignin 

concentration is detailed in the NREL method cited above, as well as in ASTM E1721 “Determination of 

Acid-Insoluble Residue in Biomass.” Briefly, the acid hydrolysis step that breaks down the cellulose and 

hemicellulose also separates lignin into an “acid soluble” portion and an “acid insoluble” portion. The 

concentration of acid soluble lignin can be measured using a UV-Visible spectrophotometer. The 

concentration of acid insoluble lignin can be measured gravimetrically. 

In addition to the HPLC structural carbohydrate method, near infrared spectroscopy (NIRS) can be used 

to determine the structural carbohydrate composition, as well as the presence of other components.

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NIRS measures the chemical bonds in a biomass sample, which can be used to determine its

composition. This method is rapid and non-destructive. In order for measurements to be made using 

NIRS, the spectra must be calibrated using other methods, such as an HPLC compositional analysis. 

c. Detergent Fiber Analysis

For a determination of the total fiber content of biomass, a neutral detergent fiber (NDF) test can be 

used. The NDF method dissolves non-fiber components such as proteins, lipids, and sugars. The 

leftover material is the total fiber, which can be measured gravimetrically. This method is detailed in 

AOAC 2002.04 Amylase-Treated Neutral Detergent Fiber in Feeds.

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 This method is commonly used for 

determining the forage potential of biomass.

The NDF method can be used along with an acid detergent fiber (ADF) test and an acid detergent lignin 

(ADL) test to determine the amount of cellulose, hemicellulose and lignin, rather than just the total fiber 

content. The ADF method breaks down hemicellulose, so that cellulose and lignin remain. The 

measurement of ADF can be subtracted from the measurement of NDF to determine the concentration 

of hemicellulose in the sample. The ADL test measures the lignin concentration by breaking down both

cellulose and hemicellulose using sulfuric acid and ignition. The ADL measurement can be subtracted 

from the ADF measurement to determine cellulose concentration. The detergent fiber analyses are

much simpler than the HPLC structural carbohydrate analysis. However, these two types of methods do 

not always give the same results. For example, one study showed that the detergent fiber methods 

overestimate cellulose and hemicellulose and underestimate lignin relative to the structural 

 

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E.g. Sanderson, M.A. et al., 1996. “Compositional analysis of biomass feedstocks by near infrared reflectance 

spectroscopy.” Biomass and Bioenergy, 11(5):365-370; Vogel, K.P. et al., 2011. “Quantifying actual and theoretical 

ethanol yields for switchgrass strains using NIRS analyses.” Bioenergy Research, 4:96-110.

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See also, Mertens, D.R., 2002. “Gravimetric determination of amylase-treated neutral detergent fiber in feeds 

with refluxing in beakers or crucibles: Collaborative study.” Journal of AOAC International, 85(6):1217-1240.
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carbohydrate analysis.8 Another study has shown that NDF and ADF cannot be used to accurately 

determine glucan and xylan content on an extractives-free basis.9

d. Quantification of Starch Content 

Measurement of the starch content can be used to indirectly determine the cellulosic converted 

fraction. 10 To measure the starch content, enzymes are used to convert the starch to maltodextrin and 

then to glucose. A subsequent reaction forms a dye in proportion to the amount of glucose present. The 

absorbance of this dye is measured with a spectrophotometer. Standards are used to calibrate the 

spectrophotometer measurements and determine the starch content of the initial sample.

V. Allocating RINs using the 1426 equations

a. Description of how EPA views cellulosic and non-cellulosic components of a feedstock as 2 

separate feedstocks for RIN allocation

For simplicity, EPA selected to specify that if a single plant’s average adjusted cellulosic content is less 

than 75%, then the feedstock is considered to be two feedstocks – specifically a cellulosic and noncellulosic feedstock. This designation allows the equations in Table 4 to 80.1426 to be used to allocate 

both cellulosic and non-cellulosic RINs for the resulting mixed batch of fuel. This will require a separate 

converted fraction (CF) to be calculated for both the cellulosic and non-cellulosic fractions. 

b. Calculation of the cellulosic CF and identification of the volume of D3-RINs

The converted fraction (CF) is a unitless average mass percent representing that portion of the feedstock 

that is converted into renewable fuel. This is calculated by dividing the mass of the feedstock that is 

converted to renewable fuel by the mass of the feedstock that is capable of being converted to fuel. For 

in situ biofuel producers, this calculation will require some type of material balance accounting because 

the fuel will be a mixture of both cellulosic and non-cellulosic fuel.

There are several possible combinations of system boundaries and analytical methods that could be

used to perform a mass balance accounting of cellulosic conversion, which would offer the data needed 

to calculate the corresponding cellulosic biofuel production via an in situ process. Thus, there are

different ways to obtain the data to be used to calculate the CF values need to perform RIN allocation. 

We will present an example of how to use data characterizing feedstock composition, cellulosic 

conversion and certain process parameters to allocate RINs. In this example we assume that an in situ

process is simultaneously processing corn starch and cellulosic polysaccharides associated with corn 

kernel fiber to produce ethanol.

 

8

Dien, B.S., et al., 2006. “Chemical composition and response to dilute-acid pretreatment and enzymatic 

saccharification of alfalfa, reed canarygrass, and switchgrass.” Biomass and Bioenergy, 30:880-891.

9 Wolfrum, E.J., et al., 2009. “Correlating detergent fiber analysis and dietary fiber analysis data for corn stover 

collected by NIRS.” Cellulose, 16:577-585.

10 e.g. Megazyme Total Starch Assay (Amylglucosidase/alpha-Amylase Method); AOAC Official Method 996.11, 

“Starch (Total) in Cereal Products, Amylglucosidase-alpha-Amylase Method”; AACC Method 76.13, “Total Starch 

Assay Procedure (Megazyme Amyloglucosidase/alpha-Amylase Method)”
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Data characterizing the feedstock corn composition (Table 1) and process specific data to elucidate the 

relative starch and cellulosic converted fraction (CF) will be required to appropriately allocate RINs. For 

this example, the validation protocol for obtaining the cellulosic CF requires performing an analytic 

process that involves 1) removing starch, 2) quantifying an initial level of cellulosic polysaccharides prior 

to hydrolysis, 3) quantifying a final level of cellulosic polysaccharides following hydrolysis, and 4) 

identifying the cellulosic ethanol by calculating the difference between the initial and final levels of 

cellulosic polysaccharides. In this example, the CF is determined to be the identified cellulosic ethanol 

(Step 4) divided by the theoretical ethanol yield for the total initial amount of cellulosic polysaccharides. 

Once the amount of ethanol resulting from the cellulosic polysaccharides has been identified, data 

characterizing the combined feedstock composition can be used to identify the cellulosic portion of the 

feedstock. An analysis of a representative sample of the feedstock would be used to characterize the 

corn composition (Table 1). The data from these various analytical methods can be used to allocate RINs 

for the example batch (Table 2).

Table 1 – Relevant feedstock characteristics and composition

Corn Composition

Kernel Mass (M), lb wet 61.2 lb/bu

Corn Moisture (m), % wet 15%

Kernel Mass, lb dry 52.0 lb/bu

Starch, % of kernel dry 75.0%

Cellulosic polysaccharides, % of kernel dry 4.0%

 

RIN allocation for the cellulosic and non-cellulosic portions of the biofuel can be performed by applying 

the RIN volume (VRIN ) equation.

 =  ∙  ∙



 + 

Where EV is the equivalence value for the renewable fuel (in this case it is 1 since the fuel is ethanol), VS

is the standardized volume of the renewable fuel produced, and FE is the feedstock energy value. Note 

that this equation will be applied separately to calculate the RIN volume for each D-code. The feedstock 

energy (FE) is characterized by the FE equation.

 =  ∙ (1 − ) ∙  ∙ 

Where M is the mass of the feedstock, m is the feedstock moisture content, CF is the converted fraction

and E is the energy content of the components of the feedstock converted to fuel (Table 2). 
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Table 2 – Example: assumed inputs, intermediate values, and calculations

Example in situ RIN Allocation Calculation

Assumed feedstock for example (M), lb wet

Total standardized volume of fuel (VS), gal

1,000,000

64,158 Note: 77,000 Btu/gal ethanol

Starch Cellulose Comments

Feedstock, thousand lb dry / batch 637.5 34

Account for moisture content 

and composition (Table 1).

Energy Content (E), Btu/lb dry 8,000 8,000

Theoretical feedstock energy 

assuming complete conversion11.

Converted Fraction (CF) 0.95 0.35

Calculated using method

approved at registration12.

Feedstock Energy (FE), thousand Btu 4,845,000 95,200

Obtained by multiplying the dry 

feedstock, E, and CF.

RIN Volume (VRIN), gallons 62,922 1,236

Number of D6 and D3 RINs that 

would be assigned respectively 

for this example batch.

For the example presented, 1,000,000 pounds of wet corn feedstock was processed to produce 64,158 

gallons of ethanol. Based on established feedstock characteristics, the wet feedstock was composed of 

637,500 pounds of starch and 34,000 pounds of cellulose on a dry mass basis. By establishing and using 

the dry mass of these components for subsequent calculation, the moisture content term in the FE

equation simplifies to 1. Multiplying the dry mass of each feedstock by its corresponding converted 

fraction (CF) and energy content (E) yields the feedstock energy (FE). Using the resulting FE values for 

starch and cellulose respectively, and applying the VRIN equation for allocating RIN volumes to batches 

with these two feedstocks yields the RIN volume for each type of D-code. Note that since the fuel

produced is ethanol, the energy equivalence value (EV) is 1.

This example shows how information about the wet feedstock mass, total fuel output, feedstock 

characteristics, and calculated process parameters can be used to allocate separate RINs for an in situ

process converting both starch and cellulose to ethanol. 

 

11 This theoretical value will depend on the process’ fermentation and relevant reaction chemistry as well as the 

processes ability to convert C5 as well as C6 carbohydrates. 

12 CF values are typically an annual mass percent average, but for in situ processes generating D3 RINs, a new CF 

will have to be calculated for every 500,000 cellulosic RINs generated or every year – whichever comes first. The 

same CF is used for all RIN allocation between each required update.