A national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Biochemical Production of 

Ethanol from Corn Stover: 2007 

State of Technology Model 

Andy Aden 

Technical Report 

NREL/TP-510-43205

May 2008 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
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Contract No. DE-AC36-99-GO10337

Technical Report 

NREL/TP-510-43205

May 2008 

Biochemical Production of 

Ethanol from Corn Stover: 2007 

State of Technology Model 

Andy Aden 

Prepared under Task No. BB07.2410 
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Contents 

List of Figures ............................................................................................................................ iii

List of Tables ............................................................................................................................. iii

1 Introduction......................................................................................................................... 1

2 State of Technology for FY 2007......................................................................................... 1

2.1 Updated Economic Indices .......................................................................................... 2

2.2 Updated Feedstock Cost.............................................................................................. 4

2.3 Alternative Hydrolysate Condition Technology............................................................. 5

2.4 Improved Xylose Yields in Pretreatment ...................................................................... 6

3 Results and Conclusions..................................................................................................... 8

Appendix...................................................................................................................................10

List of Figures 

Figure 1. State of technology progress toward the 2012 goal (in 2002 dollars) .......................... 2

Figure 2. Chemical engineering plant cost index, July 2007 (see www.che.com)....................... 3

Figure 3. Progression from $1.07/gal cost target to $1.33/gal cost target................................... 4

Figure 4. Xylan component closure and yields for corn stover pretreatment in ZipperClave and 

steam gun reactors .................................................................................................................... 7

Figure 5. State of technology progress toward the 2012 goal (estimated 2007 dollars).............. 9

List of Tables 

Table 1. Conditioning Data Used in State of Technology Models ............................................... 6

Table 2. Pretreatment Data and Processing Conditions Used in State of Technology Models ... 8

Table 3. State of Technology Summary ..................................................................................... 9

 iii
 1

1 Introduction 

In 2002, the National Renewable Energy Laboratory (NREL) published a detailed report1

documenting a process design and economic analysis for the biochemical conversion of 

lignocellulosic biomass (corn stover) to ethanol. The design makes use of dilute acid 

pretreatment followed by enzymatic saccharification and cofermentation with recombinant 

Zymomonas mobilis. This design is not optimized; rather, it represents one technology package. 

Although experimentally verified data are contained in the report, it serves a more important 

function—to set the technological targets necessary for attaining U.S. Department of Energy 

(DOE) cost goals. This report still serves as the basis for the 2012 DOE goals. 

The material and energy balances contained in the report were generated using Aspen Plus 

models created by NREL. The data were then placed in a spreadsheet framework for economic 

calculations. Using a discounted cash flow rate of return analysis, we calculated the minimum 

ethanol selling price (MESP) needed to meet a 10% internal rate of return for a biorefinery 

capable of producing 2,000 dry metric tonnes per day. An “Nth” plant assumption was used to 

eliminate any first-of-a-kind costs that might be incurred while building the pioneer facilities. In 

year-2000 dollars, this equated to $1.07 per gallon ($1.07/gal) ethanol. When inflation is factored 

into capital, chemicals, and labor costs, this MESP equates to $1.33/gal ethanol, in estimated 

2007 dollars. 

2 State of Technology for FY 2007 

Since 2001, NREL has kept track of technical research progress in the biochemical process 

through what are known as “State of Technology” (SOT) assessments. Once termed 

“experimentally verified” cases, these annual updates are an essential activity at NREL. Data 

generated at either the bench or pilot scale in the research and development (R&D) areas of 

pretreatment, conditioning, enzymatic hydrolysis, and fermentation are input to technoeconomic 

models in order to calculate what the commercial-scale economics might be under specific 

conditions. This allows researchers to quantify progress achieved in the laboratory in terms of 

economic improvements in the overall cellulosic ethanol process. As such, progress toward the 

2012 goal of cost-competitive ethanol can easily be seen. Researchers are also able to determine 

where process improvements are needed and where research efforts and funds should be focused. 

The purpose of this report is to update the FY 2005 SOT model with the latest research results 

from the past two years. Because of budget limitations and resource constraints, an FY 2006 

assessment was never completed. A technical memo describing the FY 2005 state of technology 

results (Doc. #9382)2

 is not yet available to the public. The MESP resulting from that assessment 

was $2.26/gal, in year-2002 dollars. In addition, all prior assessments were described in the same 

2002 dollars, as was the design case cost target of $1.07/gal. These are shown in Figure 1. 

 

1 A. Aden et al., Lignocellulosic Biomass to Ethanol Process Design and Economics Utilizing Co-Current Dilute 

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

www.nrel.gov/docs/fy02osti/32438.pdf 2

 J. Jechura, “Sugar Platform Post-Enzyme Subcontract Case,” NREL Technical Memorandum, Doc. #9382, 

October 11, 2005. 
$0.00

$1.00

$2.00

$3.00

$4.00

$5.00

$6.00

2000 2005 2010

Minimum Ethanol Selling Price ($/gal

Conversion

Feedstock

Pre-initiative Targets

President's Biofuels Initiative

Integrated with

Thermochemical

Processing

$0.00

$1.00

$2.00

$3.00

$4.00

$5.00

$6.00

2000 2005 2010

Minimum Ethanol Selling Price ($/gal

Conversion

Feedstock

Pre-initiative Targets

President's Biofuels Initiative

Integrated with

Thermochemical

Processing

Figure 1. State of technology progress toward the 2012 goal (in 2002 dollars) 

Four changes were made to update the FY 2005 SOT model to an FY 2007 SOT model: 

1. Updated economic indices from 2002 dollars to estimated 2007 dollars 

2. Updated feedstock cost 

3. Investigated alternative hydrolysate conditioning (ammonium hydroxide) 

4. Improved monomeric xylose yields from xylan during pretreatment 

2.1 Updated Economic Indices 

Several cost indices are used within the model to update capital, raw material, and labor costs to 

the desired costing year. These cost indices have risen dramatically since 2002 as a result of a 

variety of international pressures affecting the availability of steel and skilled labor. As an 

example, the chemical plant cost indices, which are used to index capital costs in the model, are 

shown in Figure 2. 

In addition, a newly developed feedstock cost index was used to index the delivered corn stover 

cost to the proper year-dollars. The index was developed by researchers at Idaho National 

Laboratory (INL). This specifically impacted the 2012 design case by increasing the feedstock 

 2
cost from $35/dry ton in 2002 dollars to $46/dry ton in estimated 2007 dollars. The impact of this 

feedstock cost increase is shown in Figure 3, along with other updated indices. 

As a result, the process economics and cost targets rise significantly when updated to estimated 

year-2007 dollars. The 2012 cost target rose from $1.07/gal in 2002 dollars to $1.33/gal in 

estimated 2007 dollars (see the progression in Figure 3). Actual indices were reported only 

through 2006; actual 2007 indices will be available some time in 2008. These cost numbers are 

estimated from the first few months’ index values. 

Chemical Engineering Plant Cost Index

350.0

370.0

390.0

410.0

430.0

450.0

470.0

490.0

510.0

1990 1992 1994 1996 1998 2000 2002 2004 2006

Year

Index

Figure 2. Chemical engineering plant cost index, July 2007 (see www.che.com) 

 3
Figure 3. Progression from $1.07/gal cost target to $1.33/gal cost target 

2.2 Updated Feedstock Cost 

The feedstock cost used in the FY 2005 SOT model was $53/dry ton delivered to the plant gate. 

Since then. researchers at INL have made further developments to the feed-handling logistics 

needed to bring the stover from the field to the biorefinery. The feed-handling logistics designs 

and targets are described in more detail in the DOE Biomass Program’s Multi-Year Program 

Plan (MYPP)3. The important distinction now is that the new 2007 feedstock cost represents a 

cost of feedstock delivered (in the proper form and quality) to the throat of the pretreatment 

reactor. Therefore, any on-site feed handling (bale breaking, washing, etc.) is now included as 

part of the overall delivered cost of $60/dry ton. 

A large amount of research has been conducted and reported on under the biochemical platform 

since the last SOT case was developed in FY 2005. This includes research conducted on biomass 

characterization and sample analysis, new surface characterization imaging techniques and 

results, and pretreatment and enzymatic conversion experiments at both the bench and pilot 

scales. Some experimental data (e.g., testing of advanced enzyme preparations by Genencor or 

Novozymes) remains protected by a Cooperative Research and Development Agreement, or 

CRADA, and therefore cannot be used as part of this assessment. “Knowledge” enhancements 

include improved sample analysis methods or updated knowledge of the relationship between 

enzyme structure and function on cellulosic substrate. These help to enhance the quality of 

research being done at NREL, but at present are difficult to capture as specific and applied 

process improvements within the model. They will undoubtedly lead to cost reductions in later 

 

3 U.S. Department of Energy, Biomass Multi-Year Program Plan, November 2007, Biomass Program, Office of 

Energy Efficiency and Renewable Energy, Washington, D.C. 

$0.12 $1.21

$1.07

$1.33 $?.??

Actual indices 

only through 

2006. $0.14 2002 to

2007

2007 index is Inflation

extrapolated. Impact:

2000 to Adjusted Cost of steel 2007 For:

and other • Feedstock Inflation manufacturing Impact: items are Adjusted increasing For: rapidly – could • Capital be under- • Labor predicted. • Energy 

President’s AEI

Goal:

2000 real 

dollars

$35 feedstock

(corn stover)

President’s

AEI Goal:

2007 real 

dollars

$35 feedstock

(corn stover) 

President’s

AEI Goal:

2007 real 

dollars

$46 feedstock

(corn stover) 

 4
years as the knowledge is applied. Process improvements that are being captured as part of the 

FY 2007 SOT are alternative conditioning and improved xylose yield. 

2.3 Alternative Hydrolysate Condition Technology 

The 2005 state of technology documented significant sugar degradation (13% xylose, 12% 

glucose) as a result of using a process known as “overliming” to condition the dilute acid 

hydrolysate. Consequently, alternative conditioning technologies were investigated by 

researchers. A milestone report prepared by Jennings et al.4 in December 2006 evaluated the 

effectiveness of lime and ammonium hydroxide as conditioning agents for dilute acid pretreated 

corn stover. Pretreated stover was produced in a pilot-scale reactor, and liquor extracted from the 

pretreated slurry was treated with an “overliming” process or with ammonium hydroxide at 

various concentrations and treatment temperatures. Glucose (spiked) and xylose were fermented 

to ethanol using recombinant Zymomonas mobilis 8b. A shake flask screening study identified 

conditions in which the treatment with ammonium hydroxide did not result in the sugar losses 

normally seen during overliming treatments. Improved fermentation yields were also observed 

when researchers used ammonium hydroxide rather than overliming. 

Using these data, researchers conducted an economic evaluation5 to determine the potential 

economic impact of alternative conditioning agents on the overall process. At the base-case 

conditions chosen, the use of ammonium hydroxide appeared to have minimal economic benefit. 

In general, the benefits of lower sugar losses (increasing the sugar available for fermentation) 

were counterbalanced by the increased cost of the detoxification agent (ammonia vs. lime). In the 

end, a slight improvement in MESP was found to result from the improved fermentation yields. 

This model showed that the economics were strongly dependent on both the fermentation yields 

and the price of ammonia. Although laboratory experiments did show a fermentation yield 

increase as a result of using ammonium hydroxide, this was not captured as part of the FY 2007 

SOT assessment because relatively high ethanol yields (85% glucose conversion, 76% xylose 

conversion) were already modeled in the FY 2005 SOT. As new integrated testing data become 

available, fermentation yield improvements may in fact be seen. 

Using ammonium hydroxide during conditioning may also negate the need for solid-liquid 

separations of the hydrolysate slurry before conditioning. When lime is used as a conditioning 

agent, it is applied to the hydrolysate liquor fraction only. An insoluble agent such as this does 

not disperse well in a high-solids slurry. However, ammonium hydroxide is a liquid, which has a 

much easier time dispersing throughout the entire hydrolysate slurry. As a result, costly solidliquids separation equipment could potentially be eliminated from the overall design. A more 

detailed look at this issue, and the potential ramifications downstream, will be done before this 

design change is included in a state of technology assessment. Water recycle effects and the 

destination of ammoniated compounds are two specific items to be investigated. 

 

4

 E. Jennings and D. Schell, “Evaluate Alternative Conditioning Technology With the Potential to Eliminate Sugar 

Losses,” NREL Milestone completion report, Dec. 30, 2006, Doc. #9777. 

5

 A. Aden, “Economic Evaluation of Alternative Conditioning Technology,” NREL Technical Memo, April 5, 2007, 

Doc. #9865. 

 5
Sugar losses during conditioning with ammonium hydroxide were significantly lower (0.6% 

glucose, 1.8% xylose) than they were with overliming. This brought the state of technology 

much closer to the goals represented in the 2012 case for conditioning (1% xylose loss, 0% 

glucose loss). The conditioning data input to the model are shown in Table 1 and compared with 

the data used in the FY 2005 SOT. 

Table 1. Conditioning Data Used in State of Technology Models 

Conditions FY 2005 SOT FY 2007 SOT 

 Temperature (°C) 50 30 

 Conditioning agent Lime Ammonium Hydroxide 

Conversions

 Glucose sugar loss 12.0% 0.6% 

 Xylose sugar loss 13.0% 1.8% 

 Arabinose sugar loss 20.0% 1.8% 

 Mannose sugar loss 0% 0.6% 

 Galactose sugar loss 28.0% 0.6% 

2.4 Improved Xylose Yields in Pretreatment 

The 2005 state of technology documented a xylan-to-monomeric xylose conversion of 63% 

using dilute acid pretreatment. In 2007, a milestone report by Weiss et al.6 documented the 

progress thus far in improving xylan-to-xylose conversion in the dilute acid pretreatment 

process. Two bench-scale pretreatment reactor systems were operated at high-solids loadings for 

1⁄4-inch and 3⁄4-inch particles of whole corn stover. The particles were impregnated with 1.1% 

(wt/wt) sulfuric acid and pretreated in both the ZipperClave (2-liter) and steam gun (4-liter) 

reactors at temperatures ranging from 180° to 200°C and reaction times between 1 and 2 

minutes. Results from the liquor and solids analysis indicated that the yield target of 75% 

conversion to xylose was exceeded in several instances. These results, shown in Figure 4, were 

achieved in both reactors for both particle sizes. As expected, higher levels of degradation 

products (furfural) were obtained at the high-severity conditions. Depending on the reactor 

system and conditions, approximately 3%-10% of the xylan was recovered as oligomeric xylose. 

Building on this work, the 2008 target for the platform is to demonstrate this same 75% target at 

a larger continuous scale. 

 

6 N. Weiss et al., “Achieve 75% Xylose Yield in Laboratory Scale High Solids Pretreatment Reactor,” NREL 

Milestone Completion Report, June 30, 2007, Doc. #9926. 

 6
0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

2.5 2.6 2.8 2.9 3.1 3.2 2.5 2.8 2.9 3.1 2.6 2.6 2.9 2.9 3.2 3.2

Severity Ro

% Initial xylan

Calculated xylan remaining in

solids

Furfural

Oligomeric xylose

Monomeric xylose

1/4" Steam gun 3/4" Steam gun 1/4" ZipperClave 

Figure 4. Xylan component closure and yields for corn stover pretreatment in ZipperClave and 

steam gun reactors 

The focus of this work was on xylan conversion; however, some cellulose hydrolysis, with 

resulting glucose formation, did occur as well. Total glucose yields generally ranged between 

10% and 20%, with very little degradation to hydroxymethylfurfural (HMF). 

The pretreatment data were used in the FY 2007 SOT process model. Specifically, an average of 

the results was used from the ZipperClave at severity (Ro) of 2.9 using a 1⁄4-inch stover particle 

size. The reactor temperature was 190°C, residence time was 105 seconds, reactor acid 

concentration was 1.64% (wt/wt), and total solids loading was 46%. The data input to the model 

are shown in Table 2 in comparison to the data used in the FY 2005 SOT. In the model, the 

solids loading to the reactor remained at 30% total solids. Xylan conversion to “tar” is used in 

the model to capture differences between the initial and final insoluble components not identified 

as “xylan.” In this fashion, the 5.7% remaining xylan shown in Figure 5 is equivalent to 5.7% 

unconverted xylan in the model. Because other hemicellulosic sugars (arabinose, galactose, 

mannose) were not measured during pretreatment but behave similarly, they were given identical 

conversions to xylan. 

 7
Table 2. Pretreatment Data and Processing Conditions Used in State of Technology Models 

Conditions FY 2005 SOT FY 2007 SOT 

 Temperature (°C) 190 190 

 Acid concentration (wt%) 1.90% 1.64% 

 Solids loading (wt%) 30% 30% 

Conversions

 Cellulose Æ glucose olig 2.7% 0.3% 

 Cellulose Æ glucose 18.5% 9.9% 

 Cellulose Æ HMF 0.0% 0.3% 

 Xylan Æ xylose olig 10.2% 6.0% 

 Xylan Æ xylose 63.2% 75.0% 

 Xylan Æ furfural 10.4% 4.8% 

 Xylan Æ “tar” 2.4% 8.5% 

 Other hemis to oligs 10.2% 6.0% 

 Other hemis to monomers 63.2% 75.0% 

 Other hemis to degrad. prod. 10.4% 4.8% 

3 Results and Conclusions 

As a result of these model changes, the minimum ethanol selling price decreased from $2.52/gal 

to $2.43/gal. The new SOT cost progression curve with the updated FY 2007 SOT case is shown 

in Figure 5. The economic summary sheet for the FY 2007 SOT case is shown in the appendix. 

All of the FY 2007 research targets were met and are shown in Table 3. Several basic scientific 

milestones were achieved that will provide a basis for other out-year cost-reduction milestones. 

In FY 2008, significant progress will be made toward the $1.33/gal target. In particular, benchscale pretreatment results will be validated at larger continuous scale. The relationship between 

corn stover hydrolysate conditioning and fermentation will be better defined and understood. 

Commercial cellulase preparations will be analyzed for baseline performance (specific activity), 

and cellulase function will be better understood. Integrated testing of whole slurry and recycle 

options will be conducted. Last, the efficacy of advanced enzyme preparations (including 

oligomerases and/or hemicellulases) will be tested in conjunction with alternative pretreatment 

technologies. The goal is to reduce the cost of pretreatment, reduce the hydrolysis time, and 

increase the sugar (xylose and glucose) yields. 

 8
$1.33

$2.43 $2.52

$5.95

$0.00

$1.00

$2.00

$3.00

$4.00

$5.00

$6.00

$7.00

2000 2003 2006 2009 2012

Minimum Ethanol Selling Price ($ per gal)

Feedstock Conversion

Figure 5. State of technology progress toward the 2012 goal (estimated 2007 dollars) 

Table 3. State of Technology Summary 

2007 

Targets

2007 

Results 2008 2010

2012 

Market 

Target

Minimum Ethanol Selling Price $2.43

$4.17

72

75%

13%

2%

1%

$1.33

TPI/Annual Gallon $3.17

Yield (Gallon/dry ton) 68 90

Feedstock

Feedstock Cost ($/dry ton) $60 $60 $46

Pretreatment

 Solids Loading (wt%) 30% 30% 30% 30% 30%

 Xylan to Xylose 75% 75% 85% 90%

 Xylan to Degradation Products 13% 8% 6% 5%

Conditioning

 Xylose Sugar Loss 11% 2% 2% 1%

 Glucose Sugar Loss 12% 1% 1% 0%

Enzymes

Enzyme Contribution ($/gal EtOH) $0.32 $0.32 $0.32 $0.16 $0.10

Saccharification & Fermentation 

 Total Solids Loading (wt%) 20% 20% 20% 20% 20%

 Combined Saccharification & Fermentation Time (d) 7 7 7 5 3

 Overall Cellulose to Ethanol 85% 85% 85% 85% 85%

 Xylose to Ethanol 76% 76% 80% 80% 85%

 Minor Sugars to Ethanol 0% 0% 40% 80% 85%

 9
Appendix 

FY 2007 Biochemical SOT Economic Summary 

Reference Model: A0802A 

Minimum Ethanol Selling Price $2.43

Ethanol Production (MM Gal. / Year) 55.5 Ethanol at 68°F

Ethanol Yield (Gal / Dry US Ton Feedstock) 71.9

Feedstock Cost $/Dry US Ton $60

Internal Rate of Return (After-Tax) 10%

Equity Percent of Total Investment 100%

Capital Costs Operating Costs (cents/gal ethanol)

 Feed Handling $0 Feedstock 83.6

 Pretreatment $23,000,000 Biomass to Boiler 0.0

 Neutralization/Conditioning $8,400,000 CSL 13.8

 Saccharification & Fermentation $20,500,000 Cellulase 32.2

 Distillation and Solids Recovery $23,400,000 Other Raw Materials 31.1

 Wastewater Treatment $3,400,000 Waste Disposal 1.9

 Storage $3,800,000 Electricity -10.4

 Boiler/Turbogenerator $45,500,000 Fixed Costs 16.0

 Utilities $5,600,000 Capital Depreciation 20.9

Total Installed Equipment Cost $133,500,000 Average Income Tax 15.7

Average Return on Investment 38.3

Added Costs $98,200,000

 (% of TPI) 42% Operating Costs ($/yr)

Feedstock $46,400,000

Total Project Investment $231,700,000 Biomass to Boiler $0

CSL $7,700,000

Installed Equipment Cost/Annual Gallon $2.41 Cellulase $17,900,000

Total Project Investment/Annual Gallon $4.17 Other Raw Matl. Costs $17,200,000

Waste Disposal $1,000,000

Loan Rate N/A Electricity -$5,800,000

Term (years) N/A Fixed Costs $8,900,000

Capital Charge Factor 0.180 Capital Depreciation $11,600,000

Average Income Tax $8,700,000

Denatured Fuel Prod. (MMgal / yr) 58.1 Average Return on Investment $21,300,000

Denatured Fuel Min. Sales Price $2.35

Denaturant Cost ($/gal denaturant) $0.641 Excess Electricity (KWH/gal) 2.61

Plant Electricity Use (KWH/gal) 2.05

Maximum Yields (100% of Theoretical)

 Ethanol Production (MM Gal/yr) 87.0 Plant Steam Use (kg steam/gal) 18.1

 Theoretical Yield (Gal/ton) 112.7 Boiler Feed -- LHV (Btu/lb) 1,594

Current Yield (Actual/Theoretical) 64% Boiler Feed -- Water Fraction 0.538

All Values in 2007$

All Values in 2007$

Corn Stover Design Case: 2007 State of Technology Case

Ethanol Production Process Engineering Analysis

 10
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An update to the FY 2005 assessment of the state of technical research progress toward biochemical process goals. 

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