Overall Sugar Yields from Corn
Stover via Thermochemical
Hemicellulose Hydrolysis Followed
by Enzymatic Hydrolysis
Todd A. Lloyd and Charles E. Wyman
Thayer School of Engineering
Dartmouth College
Hanover, New Hampshire 03755
AIChE Annual Meeting
San Francisco, CA
November 20, 2003
Biomass Refining CAFI
Objectives
• Determine pretreatment conditions that lead to
highest overall sugar yields
• Develop comparitive information on a consistent
basis
• Identify opportunities to lower production costs
while maintaining high yields
Biomass Refining CAFI
Corn Stover Composition
• NREL supplied corn stover to all project participants
(source: BioMass AgriProducts, Harlan IA)
• Stover washed and dried in small commercial
operation, knife milled to pass ¼ inch screen
Glucan
36.1 %
Xylan
21.4 %
Arabinan
3.5 %
Mannan
1.8 %
Galactan
2.5 %
Lignin
17.2 %
Protein
4.0 %
Acetyl
3.2 %
Ash
7.1 %
Uronic Acid
3.6 %
Non-structural Sugars
1.2 %
Biomass Refining CAFI
Overall Flow Diagram for Dilute
Acid Pretreatment
H2O or H2SO4(aq)
Enzyme
Corn Stover
Batch
Pretreatment
Stage 1
Digestion Sugars
Hydrolysis
Stage 2
Residue
Pretreatment Sugars
Fermentation
Ethanol
Biomass Refining CAFI
Stage 1 – Summary of Pretreatment
Conditions
• Steam Gun-no added
acid
•
– 150-220
– ~25% solids
oC
• Dilute Acid
–
–
–
–
140-200 oC
0.22-1.0 % H2SO4
5-25% solids
Heated in sand bath
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Batch Reaction Systems
1. NREL steam gun
2. ½”o.d. batch tubes
3. 1l stirred autoclave
NREL Steam Gun
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4”
Batch Tube Experimental Apparatus
for Dilute Sulfuric Acid
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Parr Reactor Experimental
Apparatus for Dilute Sulfuric Acid
Biomass Refining CAFI
Stage 2 – Enzymatic Digestion
• Spezyme from Genencor - used by all
investigators
• NREL LAP-009 used to evaluate
digestibility
– 60 FPU/g original glucan used to determine
ultimate digestibility
– 15 FPU/g tests done on selected samples
Biomass Refining CAFI
Severity Parameter
• For no acid addition applied approach used by Overend
and Chornet
Ro = t*exp[(T-100)/14.75]
• Modified equation including acid addition has the form:
n
CS = t*A exp[(T-100)/14.75]
CS = Combined Severity
A = Added Acid, %
n = Arbitrary exponent
T = Reaction Temperature, oC
• Provides useful tools to compare results from a broad
range of conditions
Biomass Refining CAFI
Combined Stage 1 and 2 Steam Gun
Results - No Acid
1
0.9
Yield, (X/X o + G/Go)
0.8
Combined Xylose
Combined Glucose
0.7
0.6
60 FPU/g original glucan
Ro=t*exp((T-100)/14.75)
Overall Combined
o
210 C
o
190 C
0.5
0.4
0.3
0.2
0.1
0
3.2
3.4
3.6
3.8
4
Log (Ro)
Biomass Refining CAFI
4.2
4.4
4.6
Stage 1 Pretreatment Yield
for 0.49% H2SO4 Addition in Batch Tubes
1
0.9
CS=t*[aH+]0.5*exp((T-100)/14.75)
Batch Tube
Yield, X/Xo or G/Go
0.8
0.7
0.6
180 C Xylose Yield
0.5
180 C Glucose Yield
0.4
160 C Xylose Yield
0.3
160 C Glucose Yield
140 C Xylose Yield
0.2
140 C Glucose Yield
0.1
0
1.00
1.50
2.00
2.50
Log CS
Biomass Refining CAFI
3.00
3.50
4.00
Stage 2 Digestion Yield
for 0.49% H2SO4 Addition in Batch Tubes
1
0.9
Batch Tube
Yield, X/Xo or G/Go
0.8
0.7
0.6
180 C Glucose Yield
0.5
0.4
180 C Xylose Yield
60 FPU/g Original Glucan
160 C Glucose Yield
0.5
0.3
160 Xylose Yield
CS=t*[aH+] *exp((T-100)/14.75)
140 Glucose Yield
140 Xylose Yield
0.2
0.1
0
1.00
1.50
2.00
2.50
Log CS
Biomass Refining CAFI
3.00
3.50
4.00
Combined Stage 1 and Stage 2 Yield
for 0.49% H2SO4 Addition in Batch Tubes
1.00
Batch Tube
CS=t*[aH+]0.5*exp((T-100)/14.75)
Yield, (X+G)/(Xo+ Go)
0.95
0.90
60 FPU/g glucan
enzyme loading
0.85
0.80
180 C Combined Yield
160 C Combined Yield
140 C Combined Yield
15 FPU/g glucan loading
0.75
0.70
0.65
0.60
1.00
1.50
2.00
2.50
Log CS
Biomass Refining CAFI
3.00
3.50
4.00
Combined Stage 1 and Stage 2 Yield
for 0.98% H2SO4 Addition in Batch Tubes
Biomass Refining CAFI
Mass Balance for Dilute Acid Pretreatment with
Digestion (15 FPU/ g of glucan) in Parr Reactor
1 wt% H2SO4
140 oC
Corn Stover Parr Reactor
Enzyme
@ 15 FPU/g of glucan, 48h
Hydrolyzate
Liquid
Hydrolysis
100 lb Pretreatment 64.0 lb
Treated
(dry basis)
Solids
21.4 lb xylan
2.1 lb xylan
36.1 lb glucan
1.3 lb xylose (X)
30.9 lb glucose (G)
32.4 lb
Residual Solids
33 lb glucan
2.29 gal
ethanol
21.8lb xylose (X)
Fermentation
0.1 lb xylo-oligomer (XO) 1.79 gal
ethanol
3.0 lb glucose (G)
Ethanol 4.08 gal
0.2 lb gluco-oligomer (GO)
89.7% mass balance closure ( all solids + G + GO + X + XO)
92% theoretical ethanol yield from glucose + xylose
84.2% glucan to glucose + 54.5% xylan to xylose conversion at 15FPU/g glucan
Biomass Refining CAFI
Summary of Batch Tube Dilute Acid
Performance at 15 FPU/g Glucan
T, oC
A, %
t, min
140
0.98
40
160
0.49
20
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X Yield, %
G Yield, %
Stage 1 - 85 Stage 1 - 8
Stage 2 - 8 Stage 2 - 82
Total - 93 Total 90
Stage 1 - 85 Stage 1 - 6
Stage 2 - 9 Stage 2 - 85
Total - 94 Total 91
Understanding Role of Dilute Acid
and Oligomer Release
• Why does acid enhance performance?
• What is the role of oligomers in sugar release?
Biomass Refining CAFI
Understanding Role of Dilute Acid
and Oligomer Release
• Why does acid enhance performance?
• What is the role of oligomers in sugar release?
Biomass Refining CAFI
Predicting pH at Elevated
Temperature
K1
H 2 SO4  H   HSO4
(1)
K2
HSO4  H   SO4
(2)
Assume reaction (1) goes to completion
M  [ H 2 SO4 ]
N  [ SO4 ]
[ H  ][ SO4 ]
K2 
[ HSO4 ]
(Amount of Added Acid)
(Amount of Acid Neutralized)
[H  ] 
Biomass Refining CAFI
 K 2  M  N  
K 2  M  N 2  8K 2 M
2
(3)
(4)
Predicting pH at Elevated
Temperature
[ H  ][ SO4 ]  H   SO4
K 
[ HSO4 ]  HSO
* log K 2th  (56.9  19.9 log T  2307.9 / T  .0065T )
th
2
4
K2  K
th
2
 HSO

4
 H  SO

K
th
2

4
*Marshall and Jones
Davies' Empirical Expression
2

 log  i  Az 
 0.2 I 
1 I

I
Biomass Refining CAFI
1
 SO
 (10
( 56.9 19.9 logT  2307.9 / T .0065T )

4
)
1
 SO

4
1
mi zi2

2
 i  ionic activity coefficient
I
A  1.825 x106 T 1.5
  132.88  .208T
z  ionic charge
m  ion molarity
Predicting pH at Elevated
Temperature
pH   log aH     log [ H  ] H  
   K  M  N  
2
pH   log  



 H 




K2  M  N 2  8K2 M 
2
• Similar to approach used by Bienkowski et al. to predict
the sulfuric acid degradation of glucose at elevated
temperature and of Springer and Harris for the hydrolysis
of wood
Biomass Refining CAFI
Predicted pH vs. Temperature
5% solids
No Neutralization
4.0
3.5
3.0
pH
2.5
2.0
0.22% H 2 SO 4
1.5
0.45% H 2 SO 4
0.98% H 2 SO 4
1.0
0.5
0.0
20
40
60
80
100
120
o
T em perature, C
Biomass Refining CAFI
140
160
180
Predicted pH vs. Temperature
5% solids
10 mg/g Biomass Neutralization
4.0
3.5
3.0
pH
2.5
2.0
0.22% H2SO 4
0.45% H2SO4
0.98% H2SO 4
1.5
1.0
0.5
0.0
20
40
60
80
100
120
o
Temperature, C
Biomass Refining CAFI
140
160
180
Predicted pH vs. Temperature
5% solids
20 mg/g Biomass Neutralization
4.0
3.5
3.0
pH
2.5
2.0
0.22% H2SO4
0.45% H2SO4
0.98% H2SO4
1.5
1.0
0.5
0.0
20
40
60
80
100
120
o
Temperature, C
Biomass Refining CAFI
140
160
180
Predicted pH vs. Temperature
25% solids
20 mg/g Biomass Neutralization
4.0
3.5
3.0
pH
2.5
2.0
0.22% H2SO4
1.5
0.45% H2SO4
0.98% H2SO4
1.0
0.5
0.0
20
40
60
80
100
120
o
Biomass Refining CAFI
Temperature, C
140
160
180
Understanding Role of Dilute Acid
and Oligomer Release
• Why does acid enhance performance?
• What is the role of oligomers in sugar release?
Biomass Refining CAFI
Typical Stage 1 Yield vs Time Data
100
Batch Tube
Reactors
o
140 C
0.49% Sulfuric Acid
% Maximum Potential Xylose
80
60
Residual Xylan
Soluble Monomers and Oligomers
40
Soluble Xylooligomers
20
0
0
20
40
60
80
100
120
Reaction Time, min
Biomass Refining CAFI
140
160
180
200
Models to Predict
Hemicellulose Hydrolysis
• Abundance of literature data
• Most kinetic studies neglect oligomers
• Saeman model:k
k
H
h
M
d
D
• Have also added consideration of two fractions of
hemicellulose in biphasic reaction (Kobayashi et al.)
Hf
Hs
Biomass Refining CAFI
kf
ks
M
kd
D
Hemicellulose Hydrolysis
Kinetics
• A few models in literature include oligomers
Hf
Hs
kf
ks
O
kh
M
kd
D
• These treat oligomers as only one or possibly two
species
• In fact, we would expect a number of different species of
oligomers with varying chain lengths
• Some inconsistencies among models
Biomass Refining CAFI
Hemicellulose Hydrolysis
Kinetics
• A few models in literature include more than one species
of oligomer and suggest that oligomers may react
directly to degradation products without first forming
xylose monomer
Hs
ks
XOH
kf
XOL
Garrote et. al, 2000, Process Biochemistry, 36/571-78
Biomass Refining CAFI
kh
M
kOd
kd
D
Structure of Hemicellulose
Xylose
Glucuronic Acid
+ H2O
Acetyl
Arabinose
Ferulic Acid
p-Coumaric Acid
Biomass Refining CAFI
Fragmentation
Products
Polymer Degradation1
N-mer
dCN/dt = -kh(N-1)CN
CN = CN0exp[-kh (N-1) t]
Assumes random bond scission and uniform rate constant
1
Simha, R., (1941), Journal of Applied Physics, 12:569-578
Biomass Refining CAFI
Generalized Form for j-mer
Distribution
N
dCj/dt = 2kh S Ci - kh(j-1)Cj
i=j+1
Cj = CN0 [1 – a ]
@ t=0, CN=CN0; Cj=0
a[ 2 + (N-j-1)a ]
(j-1)
1< j < N-1
Where a = 1 - exp[ -kht ]
For reactive monomer:




2kh  N  1 e kht   e kd t  N  2 e2 kht   e kd t  
C1 



kd  kh 
kd  2kh 
N 

Biomass Refining CAFI
o
Depolymerization Model 140 C,
0.49% Added Acid
1
0.9
0.8
Yield, X/X0
0.7
0.6
kh=0.126
kd=0.0016
0.5
0.4
Residual Xylan
Assumptions
Original DP = 100
Cutoff DP = 8
0.3
Total Xylose and Xylooligomers
Xylooligomers
0.2
0.1
0
0
20
40
60
80
100
120
Pretreatment Time, min
Biomass Refining CAFI
140
160
180
200
Modified Depolymerization Model
•
Assumes a change in activation
energy with conversion
dC j
dt
n
 2k h a  Ci  k h a(j  1)C j
i  j 1
ae
k a X
Biomass Refining CAFI
n
;
Ci
wher e X  1  
i m Ci 0
for j  m : n
Modified Depolymerization Model
o
140 C, 0.49% Added Acid
1
dC j
Assumptions:
0.9
 2k h a  Ci  k h a(j  1)C j
dt  ka X i  j 1
ae
Cutoff DP = 8
Original DP = 100
0.8
Yield, X/X0
0.7
Kh=.516 min-1
0.6
Ka=7.9
Kf=.0016 min-1
Ko=.094 min-1
0.5
0.4
n
for j  m : n
n
m-1
dC1
 2kha Ci  2ko  Ci  ko (1 - j)C j
dt
i m
i 2
n
m-1
dC1
 2khaCi  2ko Ci  kdC1
dt
i m
i 2
for 1  j  m
for j  1
Residual Xylan
0.3
Soluble Xylooligomers
Soluble Xylose + Xylooligomers
0.2
0.1
0
0
20
Biomass Refining CAFI
40
60
80
100
120
Pretreatment Time, min
140
160
180
200
Conclusions
• Water only hydrolysis with a steam gun produced a
maximum yield of about 60% X+G with 60 FPU/g
glucan.
• For dilute acid pretreatment conditions for >95% yield
of glucan and xylan ranged from 140oC with 0.98%
added acid for 40 minutes to 180oC with 0.49% added
acid for 5 minutes
• Xylanase activity in stage 2 enhances xylose yields
• Neutralization of added acid can have a significant
effect on pH
• Hydrolysis can be viewed as a depolymerization
process
Biomass Refining CAFI
Acknowledgements





The United States Department of Agriculture Initiative
for Future Agricultural and Food Systems Program
through Contract 00-52104-9663 for funding our
research
The United States Department of Energy Office of the
Biomass Program and the National Renewable Energy
Laboratory
Our partners from Auburn University, Michigan State,
Purdue, and Texas A&M Universities and the National
Renewable Energy Laboratory
The National Institute of Standards and Technology for
funds to purchase some of the equipment used in this
research
The Thayer School of Engineering at Dartmouth
College
Biomass Refining CAFI
Questions?
Biomass Refining CAFI