Biomass Reforming Processes in Hydrothermal Media
by
Andrew A. Peterson
B.Ch.E., University of Minnesota (1999)
B.S., University of Minnesota (1999)
M.S.C.E.P., Massachusetts Institute of Technology (2006)
Submitted to the Department of Chemical Engineering
in partial fulfillment of the requirements for the degree of
Doctor of Philosophy in Chemical Engineering
at the
MASSACHUSETTS INSTITUTE OF TECHNOLOGY
May 2009
© Massachusetts Institute of Technology 2009. All rights reserved.
Author . . . .
Department of Chemical Engineering
May 22, 2009
Certified by . . . .
Jefferson W. Tester
H. P. Meissner Professor of Chemical Engineering
Thesis Supervisor
Accepted by . . . .
William M. Deen
Biomass Reforming Processes in Hydrothermal Media
by
Andrew A. Peterson
Submitted to the Department of Chemical Engineering on May 22, 2009, in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy in Chemical Engineering
Abstract
While hydrothermal technologies offer distinct advantages in being able to process a wide variety of biomass feedstocks, the composition of the feedstock will have a large effect on the processing employed. This thesis characterizes the role of two major components of biomass, salts and proteins, that if dealt with intelligently can create valuable byproducts, but if dealt with inefficiently can cause processes to fail or run sub-optimally.
In supercritical-water, most ionic species exhibit only sparing solubility. Thus, in the supercritical-water gasification of biomass, a supercritical-water reverse-flow vessel (RFV) may be employed to precipitate the salts from the biomass before the biomass is brought into contact with catalysts. However, these vessels are subject to blockage, and flow charac-terization within the vessels has largely been unknown.
Therefore, the technique of neutron radiography is introduced to the supercritical-water processing field in order to characterize the behavior within these RFVs. This allows for non-invasive, in situ, real-time measurements of precipitation phenomena within thick-walled vessels. Two methods that provide complimentary information were developed to observe the precipitation behavior of salts from supercritical water. In the first method (“normal-phase” radiography), the salts have a lower attenuation coefficient than the continuous1H
2O
phase. This allowed for the visualization of major blockages, buildups of salts at the walls,
and flow pattern changes. By using the strongly attenuating 1H
2O form of water, fluid
den-sity changes resulting from flow pattern changes were also apparent. In the second method (“reverse-phase” radiography), strongly attenuating salts, such as those containing boron,
were employed in a continuous phase of D2O, which has a weak neutron attenuation
coeffi-cient. In this manner, the onset of precipitation events could be seen with finer resolution.
The neutron radiographical method was also employed to perform an elegant 1H2
O-in-D2O tracer study of the fluid mechanics within the RFV, in which a stream of water
that spans its critical temperature undergoes a reversing flow pattern. The flow was found to penetrate deep into the vessel and to mix thoroughly with the contents of the vessel, presumably because of the strong buoyant forces generated by heat at the walls. In the absence of wall heating, the flow was observed to reverse at a point much nearer to the
top of the vessel. Also, as compared to comparable conditions at subcritical pressures, the supercritical jet was found to be more diffuse and to penetrate the vessel at a lower velocity. A jet entrainment model was derived to describe the flow within the vessel. Due to the near-critical conditions in the vessel, the model was developed without the Boussinesq ap-proximation or the density deficiency formulation commonly applied to such models. The model was solved numerically and was found to provide excellent agreement with the neu-tron radiographical data for the two cases studied: an isothermal case and a case in which the temperatures spanned the critical point of water. In the isothermal limit, the model accurately predicted the jet reversal point. In the case with temperatures that spanned the critical point, the model accurately predicted that the jet would not fully reverse before reaching the bottom of the vessel. The model also predicted the characteristic decay time of a tracer within the vessel with near-perfect accuracy.
Proteins are a second constituent of biomass that require special treatment in the devel-opment of hydrothermal processes. Industrial and research laboratory reports have indicated that the presence of proteins may create processing conditions that lead to equipment foul-ing and reduced gasification yields in hydrothermal processfoul-ing. In studies on the interaction
between glycine and glucose as model compounds at 250◦C and 10 MPa, strong kinetic and
qualitative evidence is presented that a Maillard-type reaction occurs in hydrothermal pro-cessing. This reaction, which is very common at the lower temperatures encountered in food and medicinal chemistry, is known to lead to the formation of polymeric material that may be desirable for color and flavor generation in food processing, but is generally undesirable in hydrothermal processing, since these polymers will act to foul process equipment.
Glucose and glycine were found to strongly influence the reaction pathway of the other compound, and the resulting reactor effluent had UV absorbances typical of Maillard reaction products. Compounds with the same functional groups, a primary amine and a carbonyl group, were substituted for glucose and glycine and were found to have similar effects. Also, the degradation pathway of glucose was found to be altered, with the significant product hydroxymethylfurfural being suppressed with the addition of glycine.
The reactive form of glucose is the acyclic, aldehydoglucose form. Glucose can also exist as an unreactive, cyclic form, which dominates at room temperature, making up over 99.99% of the aqueous equilibrium composition. However, this equilibrium is unknown at elevated temperatures and pressures. A preliminary computational chemistry study was undertaken to predict this equilibrium under hydrothermal conditions. The acyclic form was predicted to rise to be of comparable prevalence to the cyclic forms. The study indicates that this was caused by two main effects, a change in the relative stability imparted by the decreased dielectric constant of water, and by an entropic effect in which the much larger number of conformers that are possible in the acyclic state become more energetically populated at higher temperatures.
Thesis Supervisor: Jefferson W. Tester
Acknowledgments
Thanks to my advisor, Jeff Tester, for the opportunities, the advice, the support, the criti-cism, and for dragging me out on several long bike trips. And to the real boss, Gwen: thanks for everything. Nothing would get done without you.
Conducting experiments in supercritical systems is a tricky business, and it is thanks to the support of our fellow lab members that any of us are able to get anything done. Thanks in particular to Russ Lachance for all his early mentorship and support. Thanks to the rest of the crew: Chad, Scott, Jason, Rocco, Heather, Sam, Drew, Matt, Kurt, Russell, Michael, Justin for hours of fun with no windows except the sapphire type.
And to my second lab in Switzerland: Thanks in particular to Fr´ed´eric Vogel at the Paul Scherrer Institut for all the mentorship and support as we decided to combine the complex-ities of supercritical water with the complexcomplex-ities of particle physics. The Swiss system is very pleasant to work in, and I have two incredible technicians to thank for the construction and maintenance of my equipment: Erich DeBoni and Peter Hottinger. Thanks to Peter Vontobel and Eberhard Lehmann for the advice, assistance, and beamtime at NEUTRA (the neutron radiography facility). Running the neutron beam is a round-the-clock affair, and I thank a small army of other PSI researchers for assisting me with shifts at NEUTRA: Martin Schubert, Hans Regler, Fredi Vogel, Ashaki Rooufs, Erich DeBoni, Martin Branden-berger, Thanh-Binh Truong, Stefan Rabe. Thanks also to Maurice Waldner, Marcello Bosco, Dimitris Bachelin, and Martin Seemann for all their advice, orientation, and fun lunch chats. Writing a review article and learning about topics as varied as jet entrainment and the Maillard reaction led to a lot of interesting conversations. I would like to thank a number of people who made intellectual contributions to my thesis. In particular, Michael Antal (U. Hawaii), Phil Marrone (SAIC), Glenn Hong (General Atomics), KC Swallow (Merrimac), Jeff
Resch (General Mills), Doug Elliott (PNNL), Ahmed Ghoneim, Ken Smith, Samuel St¨ucki
(PSI), Terry Adams (CWT), Brian Appel (CWT) and my committee members Charles Cooney, Bill Green, Greg Stephanopoulos, Russ Lachance, Fredi Vogel, and Terry Adams all contributed significant insights in my quest for understanding of the field.
The MIT energy community is famously strong, and gave me the opportunity to travel to Saudi Arabia, to meet with a bunch of energy rockstars like Sam Bodman and Lee Raymond, to get wonky with congressional staffers in DC, to help in running a few conferences, and to found and then unfound an energy company. Thanks to a ton of people on this, but particularly Curt Fischer, Tracy Mathews, Dave Danielson, and Daniel Enderton.
And finally, to my father, who didn’t get to see me enter graduate school, but would have loved to hear about my research and to discuss all the politics of renewable energy with me: thanks for all the support growing up and for being a wonderful and supportive role model. To Mom, Brenda, Pete, Emma, and Hannah: sorry to complicate the family get-togethers by being so far away, and thanks for all of your loving support over the years. And finally,
to my fiance´e Alissa, I don’t know what I was thinking in suggesting we schedule my defense
and our wedding this close together, but your patience in putting up with me as I assembled this thesis speaks volumes about our future prospects.
Contents
1 Background 14
2 Thesis objectives and approach 21
2.1 Motivation . . . 21
2.2 Objectives . . . 22
3 Review of hydrothermal biomass conversion research 26 3.1 Chemical reactions of biological molecules in hydrothermal systems . . . 26
3.1.1 Reactions of carbohydrates . . . 31
3.1.2 Reactions of lignocellulose . . . 38
3.1.3 Reactions of lipids . . . 43
3.1.4 Reactions of proteins and amino acids . . . 46
3.1.5 Formation of hydrocarbons and lipids from small organic materials . 48 3.2 Liquefaction in subcritical water . . . 49
3.2.1 “Bio-oil” and “bio-crude” production . . . 49
3.2.2 Refinement of bio-oils into fuels . . . 54
3.2.3 Hydrothermal liquefaction of food processing waste . . . 55
4 Neutron radiography applied to supercritical-water systems 58 4.1 Background and motivation . . . 58
4.1.1 Salt behavior in supercritical water . . . 59
4.1.2 Fertilizer recovery in biomass gasification . . . 60
4.1.3 Existing salt separation techniques . . . 61
4.2 Materials and methods . . . 62
4.2.1 Imaging with neutron radiography . . . 62
4.2.2 Salt separation vessel and test rig . . . 64
4.2.3 Model systems employed . . . 67
4.3 Data analysis methodology . . . 70
4.3.1 Raw data and median filtering . . . 70
4.3.2 Detector background intensity . . . 70
4.3.3 Flatfield neutron intensity . . . 71
4.3.4 Image division to highlight changes . . . 73
4.3.6 De-skew . . . 77
4.3.7 Edge location . . . 77
4.3.8 Matlab code . . . 79
5 Neutron radiography analysis of salt precipitation phenomena 80 5.1 Results of reverse-phase tests . . . 80
5.1.1 Operational characteristics of salt separator . . . 81
5.1.2 Effects of salt type . . . 82
5.1.3 Precipitation and condensation . . . 84
5.1.4 Effect of flow rate . . . 84
5.1.5 Length of entrance dip tube . . . 86
5.1.6 Blockage causing a shorter effective residence time . . . 86
5.2 Results of normal-phase tests . . . 88
5.3 Conclusions . . . 98
6 Supercritical water fluid dynamics measured in situ 100 6.1 Background . . . 100
6.2 Characterization of the flow . . . 105
6.2.1 Dimensionless numbers . . . 105
6.2.2 Related flows . . . 111
6.2.3 Jet flow entrainment . . . 114
6.3 Materials and methods . . . 116
6.4 Radiographic results . . . 120
6.5 Conclusions . . . 130
7 Jet entrainment modeling of supercritical-water fluid dynamics 138 7.1 Modeling with the entrainment approach . . . 139
7.2 Numerical solution procedure. . . 150
7.2.1 Steady-state ODE solutions . . . 150
7.2.2 Unsteady-state tracer ODEs . . . 152
7.3 Model results and comparison to data . . . 152
7.4 Conclusions and recommendations . . . 163
8 Sugar and protein interactions in hydrothermal media 165 8.1 Introduction . . . 165
8.2 Materials and methods . . . 175
8.2.1 Reactor system overview . . . 175
8.2.2 Reactor design . . . 176
8.2.3 Chemical reagents and solution preparation . . . 188
8.2.4 Analysis of reactor effluent . . . 188
8.3 Results and discussion . . . 190
9 Prediction of the relative stability of the dominant cyclic and acyclic
glu-cose isomers in hydrothermal media 207
9.1 Motivation and background . . . 207
9.2 Literature search . . . 208
9.2.1 Other ab initio studies . . . 208
9.2.2 Experimental data . . . 210
9.3 Methods . . . 211
9.3.1 Ab initio calculations . . . 211
9.3.2 Solvent effects . . . 211
9.3.3 Extrapolation of free energies to 250◦C . . . 213
9.3.4 Multiple conformers . . . 215
9.4 Results . . . 218
9.4.1 Conformer searches . . . 218
9.4.2 Free energies of conformers and predicted product distribution . . . . 222
9.4.3 Ab initio calculations simulating room-temperature conditions . . . . 223
9.5 Conclusions . . . 224
9.6 Recommendations . . . 225
10 Conclusions and recommendations for further research 228 10.1 Overall conclusions and recommendations . . . 228
10.2 Recommendations for further research in decarboxylation of fermentative in-termediates . . . 237
Bibliography 240 Appendices A Matlab code developed to process neutron radiography data 259 B Numerical solution of jet entrainment model 271 B.1 Solution of mass, momentum, and enthalpy ODEs. . . 271
B.2 Tracer transient differential equations . . . 276
List of Figures
1-1 Enthalpy of water versus temperature, at 1 and 300 atmospheres of pressure. 18
1-2 Zones of hydrothermal biomass processing, referenced to the phase diagram
of pure water. . . 19
3-1 Conceptual schematic of hydrothermal processes . . . 29
3-2 Pathways for the degradation of D-glucose and D-fructose . . . 32
3-3 Assumed first-order Arrhenius plot of glucose degradation data . . . 33
3-4 Products from the degradation of starch at 220◦C . . . 37
3-5 Arrhenius plot of the natural logarithm of pseudo-first-order reaction rate versus inverse temperature for cellulose decomposition. . . 40
3-6 Cross-polarized light microscopy of cellulose being heated in high-pressure water 41 3-7 Arrhenius plot overlay of first-order decomposition rate constants for the degradation of glucose and cellulose . . . 42
3-8 (Left.) The solubility of saturated fatty acids in water at 15 MPa. (Right.) The solubility of water in fatty acids at the vapor pressure of the system . . 45
3-9 Wood liquefying in water at 340◦C . . . 53
3-10 Process flow diagram with mass flow rates in tons per day, as reported by CWT 57 4-1 Density, dielectric constant, and ion dissociation constant of water at 30 MPa. 60 4-2 Neutron attenuation coefficients and effective neutron spectrum of the NEU-TRA source. . . 63
4-3 Neutron transmission image of the reverse-flow vessel. . . 66
4-4 Simplified schematic of the experimental apparatus. . . 68
4-5 Raw neutron transmission data before (left) and after (right) application of a median filter. . . 71
4-6 Correction of neutron intensity for the background detector intensity . . . . 72
4-7 Correction of neutron intensity for the flatfield intensity . . . 72
4-8 Radiograph of the empty vessel and the vessel full of supercritical D2O . . . 74
4-9 Fourier transform filter method applied to a trace of the vessel . . . 76
4-10 The location of the center of the vessel versus vertical position, as found with the Fourier transform filter method. . . 77
4-11 Determined vessel boundary condition vs. vertical position . . . 78
5-2 Thermowell temperature versus vertical position in the vessel for salt-free
1H
2O runs. . . 82
5-3 Examples of the type of precipitation and condensation that occur in the supercritical-water salt separator . . . 85
5-4 The effect of flow rate on the location of buildup in the vessel . . . 87
5-5 Observed conductivity and exit temperature from Run Condition X. . . 88
5-6 Illustration of the phase reversal employed . . . 90
5-7 Neutron radiographs superimposed on a chart of exit temperature and exit conductivity versus time for Run A . . . 92
5-8 Solubility of Na2SO4 in 1H2O at 30 MPa . . . 94
5-9 Neutron radiographs superimposed on chart of exit temperature and exit con-ductivity versus time for Run B. . . 95
5-10 Neutron radiographs superimposed on chart of exit temperature and exit con-ductivity versus time for Run C . . . 97
6-1 Physical and thermal properties of water at 30 MPa . . . 101
6-2 Computational fluid dynamics validations from Oh et al. . . 103
6-3 Computational fluid dynamics validations from Hunt . . . 104
6-4 Illustration of the vessel geometry highlighting key length scales. . . 107
6-5 Calculated Reynolds number at the jet exit . . . 108
6-6 Calculated Reynolds number versus effective diameter . . . 108
6-7 Calculated Grashof number versus wall temperature. . . 110
6-8 Calculated Grashof number versus bulk temperature. . . 110
6-9 Calculated Froude number at the jet exit . . . 111
6-10 Calculated Froude number versus effective diameter . . . 112
6-11 Negatively-buoyant (fountain) jet flow. . . 113
6-12 Craya-Curtet flow . . . 115
6-13 Experimental apparatus for the 1H2O in D2O tracer experiments. . . 118
6-14 Neutron radiographs of the injection time period of Run A. . . 121
6-15 Neutron radiographs of the decay time period of Run A. . . 122
6-16 Mean absorbance vs time of the top 9 cm for Run A. . . 123
6-17 Limiting dispersion models for flow patterns within the RFV. . . 125
6-18 Mean absorbance vs time of the top 9 cm for Runs A, B, and C. . . 126
6-19 Mean absorbance vs time of the top 9 cm for Runs A, K, G, and H. . . 126
6-20 Mean absorbance vs time of the top 9 cm for Runs G, J, and I. . . 127
6-21 Neutron radiographs of the injection time period of Run I. . . 128
6-22 Mean absorbance vs time of the lower zones for Runs G, J, and I. . . 129
6-23 Neutron radiographs of Zone C of Run I . . . 131
6-24 Neutron radiographs of Zone C of Run J . . . 132
6-25 Neutron radiographs of Zone A of Run L . . . 133
6-26 Bulk absorbance and decay time fit for each zone of Runs A, B, C and D. . . 135
6-28 Bulk absorbance and decay time fit for each zone of Runs I, J, K and L. . . . 137
7-1 Assumed geometry of the flow within the vessel used in entrainment coefficient analysis . . . 140
7-2 Control volumes and body forces . . . 143
7-3 Calculated Rayleigh number versus bulk temperature. . . 149
7-4 Results of the jet entrainment model in the isothermal limit . . . 154
7-5 Results of the jet entrainment model for supercritical water conditions com-parable to Run ID G . . . 158
7-6 The simulated movement of tracer through the vessel at various times for the supercritical simulation . . . 160
7-7 Comparison of the entrainment model results to the neutron radiographic measurements for the top zone of the vessel . . . 161
7-8 Bulk neutron absorbance of the upper zone of the model for both the neutron radiographical results and the model prediction . . . 162
8-1 The initial steps of the Maillard reaction leading to the Amadori rearrange-ment product . . . 167
8-2 Overview of the Maillard reaction network. . . 169
8-3 Undesirable product produced under certain processing conditions by Chang-ing World Technologies, Inc. . . 171
8-4 Mean molecular weight of melanoidin degradation products versus final reac-tion temperature, as reported by Inoue et al. . . 173
8-5 Product fractions reported by Inoue et al. for the degradation of melanoidins in hydrothermal media . . . 173
8-6 Plug-flow reactor schematic. . . 177
8-7 Temperatures and reactant concentrations needed in the component streams 180 8-8 Schematic of the jacketed injection port. . . 180
8-9 Mixing geometry assumed by Forney and Gray . . . 182
8-10 Actual velocity ratio, and optimal velocity ratio for mixing, as a function of injection diameter . . . 183
8-11 Reynolds number as a function of tube diameter for the three relevant flow conditions. Points A, B, C, and D indicate relevant diameters to the mixing tee geometry and are described in the text. . . 184
8-12 Mixing tee detail. . . 185
8-13 Characteristic mixing time versus vessel diameter . . . 186
8-14 Calculations of the expected temperature, rate constant, and conversion versus time . . . 187
8-15 Calculations comparing the expected effluent concentration exiting the cooling bath for a cooling bath with O.D. of 1/8” and 1/16” . . . 188
8-16 Destruction of glycine as a function of initial glucose concentration . . . 191
8-17 Destruction of glucose in the presence of 5 mmolal glycine as a function of initial glucose concentration . . . 191
8-18 Absorbance as a function of initial glucose concentration . . . 192
8-19 Destruction of glucose as a function of initial glycine concentration . . . 193
8-20 Destruction of glycine as a function of initial glycine concentration . . . 194
8-21 Absorbance at 450 nm as a function of initial glycine concentration . . . 194
8-22 Reactor effluent samples from reactions of glycine and glucose. . . 195
8-23 UV/vis absorbance spectra of reactor effluent from reactions of 100 mmolal glycine with varying amounts of glucose . . . 195
8-24 Destruction of 100 mmolal glycine as a function of initial glucose concentra-tion, for experimental conditions of 250◦C, 10 MPa, and 5.0 s residence time. 196 8-25 Destruction of 1.25 mmolal of glucose as a function of initial glycine concen-tration . . . 197
8-26 Absorbance at 450 nm as a function of initial glycine concentration . . . 197
8-27 Glucose destruction for three different initial glycine loadings . . . 198
8-28 Glucose destruction in the absence of glycine . . . 199
8-29 Reaction rate order for glucose in hydrothermal media as reported by Mat-sumura et al. . . 200
8-30 Reactor effluent concentration of glycine anhydride versus residence time . . 200
8-31 Yield of 5-hydroxymethylfurfural from 5 mmolal of glucose with varying amounts of glycine . . . 201
8-32 Chemical structure of pyruvic aldehyde. . . 202
8-33 Chemical structure of ethanolamine. . . 202
8-34 Destruction of 5 mmolal glycine with 1.25 mmolal and 10 mmolal of pyruvic aldehyde . . . 203
8-35 Destruction of 10 mmolal of glucose by glycine and ethanolamine . . . 204
8-36 Destruction of 1.25 mmolal of glucose by glycine and ethanolamine . . . 205
9-1 (a) Aldehydo-d-glucose and (b) β-d-glucopyranose, the dominant acyclic and cyclic forms of glucose present at room temperature . . . 209
9-2 Literature values of d-glucose isomers equilibrium concentrations . . . 210
9-3 Density and dielectric constant of water at 100 bar as a function of temperature212 9-4 An example of a dihedral scan (blue line) showing three wells, or stable con-formers . . . 216
B-1 Flow chart of the iterative solution method employed to solve the system of ordinary differential equations in the jet entrainment problem. . . 272
B-2 Assumed geometry in tracer movement analysis. . . 277
B-3 Geometry for simulated neutron radiography images, for a horizontal slice at any position z . . . 280
List of Tables
3.1 Representative biomass feedstock chemical compounds and reaction
interme-diates encountered in hydrothermal processing. . . 28
3.2 Glucose/fructose degradation products observed in various studies . . . 34
3.3 Comparison of sample bio-oils produced from hydrothermal liquefaction and from fast-pyrolysis . . . 50
3.4 Partitioning of heating content, oxygen, and mass amongst the products for the hydrothermal liquefaction of sugar beet pulp at 300-360◦C, 100-180 bar and 5-20 minutes . . . 51
3.5 Typical operating conditions for hydrodeoxygenation tests with bio-oils . . . 56
4.1 Ionic constituents of Swiss swine manure solids as measured with ion chro-matography after Soxhlet extraction . . . 61
4.2 Critical parameters and normal boiling point of H2O and D2O . . . 67
5.1 Run conditions and conductivity reductions . . . 83
5.2 Neutron attenuation coefficients at 1.54 ˚A . . . 89
5.3 Comparison of reverse-phase and normal-phase methods employed. . . 90
5.4 Run conditions for the blocked and the clear condition. . . 91
6.1 Assumed run conditions for dimensionless number calculations. . . 106
6.2 Classification of buoyant jets. . . 112
6.3 Neutron attenuation coefficients at 1.54 ˚A. . . 117
6.4 Run conditions for 1H 2O in D2O tracer tests . . . 119
6.5 Decay times for each zone of the vessel for each run . . . 124
7.1 Assumed conditions for the two model cases studied. . . 153
9.1 Solvent properties used for ab initio free energy calculations in hydrothermal water . . . 213
9.2 Summary of scans undertaken in searching for low-energy conformers of acyclic d-glucose . . . 220
9.3 Minimum energy conformers of acyclic glucose that were submitted for full
optimization and energy calculations at the rb3lyp/cbsb7/iefpcm level of theory.221
9.5 The calculated free energies and Boltzman sums for the lowest energy con-formers of cyclic and acyclic forms of d-glucose . . . 223
Chapter 1
Background
The twin problems of climate change and supplying the world’s growing demand for energy are surely among the largest our society has faced since the industrial revolution. In the modern era, we have relied primarily on the large amount of energy stored in the chemical bonds of fossil fuels to supply our energy needs, which has resulted in a continuously in-creasing concentration of carbon dioxide in our atmosphere and political instability related to energy supplies and price fluctuations.
In the search for new energy supplies, carbon-neutrality and distributed sources will be prized. A plethora of these carbon-neutral energy supplies exist that can give us electricity, including nuclear, hydroelectric, geothermal, wind, solar, and fossil-fuel burning with carbon capture. Clearly, all of these technologies have considerable hurdles and drawbacks, or they would represent a much larger share of our current energy supply; nonetheless, there are multiple options for electricity from carbon-neutral sources.
Even if all of the above technologies are quite successful, our society will still need chemi-cal fuels, particularly for transportation, heating and industrial uses. Unfortunately, mother nature has only given us two sources of reduced carbon: fossil fuels and biomass. Although some promising research is on the horizon that focuses on fixing carbon dioxide via artificial photosynthetic methods, in the near term, to substitute a neutral fuel for a carbon-intensive fuel, our only option is to use biomass. Since we depend on our biomass resource for
food, as well as for many other services including maintaining bio-diversity, it is imperative that we use this unique and precious resource in the most efficient manner possible.
Current inefficiencies in biofuel production. Our first massive attempt to use the
biomass resource as a substitute for fossil-based conventional fuels has been to convert it into ethanol. The reasons our society has chosen ethanol as our first biofuel are obvious: we learned how to convert biomass into ethanol nine millennia ago. There are essentially no technological risks and no intellectual property barriers, so it is easy to roll out on a large scale. However, as easy as ethanol is to make, it has a large number of very well documented problems. These can be summarized into three main issues:
1. Poor energy efficiency in the conversion. Debate exists about which quantity is greater: the energy contained in a gallon of ethanol, or the fossil fuels required to produce that gallon of ethanol, but all agree that the two numbers are of the same order of magnitude. It should be noted, though, that even if the ethanol contains only 90% of the energy that is used to produce it, this is not necessarily a reason to not pursue the production of ethanol, since the net effect is to turn solid (coal) and gaseous (natural gas) fuels into a liquid fuel (ethanol). The best chemical processes to perform coal-to-liquids (CTL) and gas-to-liquids (GTL) are only around 60% efficient. However, if the efficiency remains at this low level, ethanol may best be thought of as an energy conversion strategy to produce liquid fuels from coal and gas, and not as a biomass-based energy source.
2. Competition with food and land-use issues. Current ethanol production in the United States relies almost exclusively on corn. U.S. corn production has been in surplus for decades and for much of that time the price of corn has actually been below the cost of production of the corn (with the difference being made up by government subsidies). In this situation, diverting part of the corn surplus for the production of ethanol makes sound policy sense. However, in recent years a number of factors, including increasing global demand for meat, droughts, and the use of biomass for
energy, have acted to push up demand for all food crops, which has acted to push up the price of corn and other food crops to levels unseen in recent times; the additional drain of biofuels on the system exacerbates the situation.
Clearly the amount of corn that can be diverted to biofuel production is limited at
best. Simple calculations show this; at 2.8 gallons of ethanol per bushel of corn,
and 120 bushels of corn per acre, it would take 320 of our nation’s 450 million acres of agricultural land to replace just 1/3 of our transportation fuels with ethanol. By contrast, a DOE/USDA study [1] showed that if processes could be developed that were feedstock agnostic, plenty of agricultural and forestry supplies would be available to sustainably supply 1/3 of our transportation fuels, at an assumed conversion efficiency of 50%.
3. Low energy density and fuel incompatibility with existing infrastructure. As a liquid chemical fuel, the energy density of ethanol is much higher than many other energy storage schemes, including batteries, pumped hydro, or compressed air. However, relative to other chemical fuels, the energy density of ethanol is quite low – ethanol has only about 2/3 of the volumetric energy density of the gasoline that it replaces. Additionally, ethanol presents handling issues as a fuel: it is hygroscopic and has only limited compatibility with existing infrastructure such as pipelines and engines. However, it does have the sizeable advantage of being less toxic than most conventional fuels in the case of environmental spills.
Because of these drawbacks associated with the current methods of producing ethanol from corn, society would be well-served by the development of more efficient means of us-ing the biomass resource. To develop more efficient biomass processes, it is important to understand the sources of the inefficiencies in conventional biofuel production. Although the farmer and the fertilizer production receive much of the popular criticism for the ineffi-ciencies of ethanol production, in reality, as shown by Johnson [2], the growing of the corn, including fertilizer, irrigation, planting, harvesting, and transporting of the product to the
ethanol production facility, only accounts for about a third of the energy inputs in ethanol processing. The majority of the energy inputs are consumed in the processing of the biomass into the biofuel: a natural fit for chemical engineers to improve.
As further shown by Johnson, approximately 50% of the total energy consumed in pro-ducing ethanol is taken up by just two processing steps: distillation and drying. Distillation is the separation of the ethanol from the water in which it is produced. Drying is associated with removal of water from the distiller’s grain by-product, which is typically converted to animal feed. From a engineer’s standpoint, these two large energy sinks are caused by the same thermodynamic quantity: the large enthalpy of vaporization of water.
Hydrothermal processing. If processes can be devised that avoid the need to vaporize
water, much more efficient means of converting biomass into fuels may be feasible. Hy-drothermal technologies look to address this point of avoiding water’s enthalpy of vapor-ization by processing under high pressures, above either the vapor pressure or the critical pressure of water. By so doing, water is always kept in a condensed, liquid or supercritical state, and never goes through the phase change associated with the enthalpy of vaporization. Figure 1-1 shows the enthalpy curves of water at atmospheric pressure and at 300 atm (30.4 MPa). For example, a process that requires heating a wet biomass stream to process
conditions of 250◦Ctakes about 2,900 kJ to heat each kilogram of water in an atmospheric
pressure process; while if the same process was at elevated pressure only about 950 kJ would be required. This represents an enormous energy savings by avoiding the enthalpy of vaporization of water, which is apparent as the large step change on the 1 atm curve.
As temperatures rise to and above the critical point of water (374◦C) the two heating
curves in Figure 1-1 begin to approach each other, although the high-pressure curve always remains below the low-pressure curve. However, perhaps more importantly than the differ-ence in the value of the curves is the shape of the curves: the high-pressure heating curve is a smooth heating curve, without discontinuities. This enables heat recovery in conventional heat exchange processes without the great difficulties and inefficiencies that are associated
0 100 200 300 400 500 Temperature (oC) 0 1000 2000 3000 4000 E nt ha lp y (k J/ kg ) 300 atm 1 atm
Figure 1-1: Enthalpy of water versus temperature, at 1 and 300 atmospheres of pressure.
with a singularity in the heat capacity.
Therefore, hydrothermal processes allow the high throughputs of thermochemical pro-cessing without the large enthalpic penalties associated with the vaporization of water. The field of hydrothermal biomass processing encompasses a range of different processes, includ-ing liquefaction and gasification, on a range of different mixed and pure feedstocks, includinclud-ing waste products and lignocellulosics, in both the presence and absence of catalysts. We have recently published a thorough review on hydrothermal biofuel technologies, including pro-cessing technologies, hydrothermal chemistry, and research needs. The major hydrothermal technologies will be introduced here; further detail is contained in Chapter 3 of this thesis and in Peterson et al. [3].
Most hydrothermal processing of interest to this thesis can be summarized into two categories: liquefaction and gasification technologies. These zones are shown, referenced to the phase diagram of water, in Figure 1-2. Hydrothermal liquefaction technologies have been shown to take a wide variety of biomass feedstocks, from turkey offal to onion peels, and to convert them into a crude oil that is suitable for processing into conventional liquid fuels.
Figure 1-2: Zones of hydrothermal biomass processing, referenced to the phase diagram of pure water.
Processing conditions are generally at temperatures below the critical temperature of water
(374◦C), and at high enough pressures to keep the water in a liquid state. Temperatures
employed vary from fairly mild, at about 250◦C, for fatty feedstocks that are already high
in oil content, to higher temperature processes, around 330◦C, for more lignocellulosic-rich feedstocks. Hydrothermal liquefaction technologies generally produce a product in which the energy density of the feedstock has been significantly increased, the oxygen content has been decreased, and the feedstock has been changed into a liquid form that is processable by more conventional techniques.
Gasification technologies have also been shown to work on a variety of feedstocks, but product gas yields are generally higher on lignocellulosic and starchy feedstocks than on protein-containing feedstocks. Gasification technologies, often referred to as supercritical-water gasification (SCWG), take place at temperatures from near supercritical-water’s critical point to
temperatures up to about 700◦C. Very high gas yields have been reported with very little
char and tar formation. Gasification in the lower temperature end of this range generally takes place with the aid of a heterogeneous catalyst.
Although hydrothermal technologies offer a number of unique advantages, including high energy efficiencies and the ability to use mixed feedstocks, most technologies are still in the research stage of development. This thesis deals with processing complications associated with variations in biomass feedstocks, specifically on the influence of salts and proteins. The specific thesis objectives and approach are discussed in Chapter 2.
Chapter 2
Thesis objectives and approach
2.1
Motivation
Hydrothermal technologies may offer tremendous advantages in the processing of biomass into fuels. One of the key advantages is that they are “feedstock agnostic”; that is, they can handle a variety of mixed streams and do not require specific chemical feedstocks. This is in contrast to current biofuels processes: ethanol requires a glucose feedstock, and biodiesel requires a triacylglyceride feedstock.
While hydrothermal technologies have been shown to handle a range of mixed feedstocks, in actuality, the processes are susceptible to the individual make-up of the feedstocks and must be tailored to the individual chemistry taking place as a result of the feedstock compo-sition. These hydrothermal processing methods have demonstrated high conversion of a wide range of mixed feedstocks into fuels rich in hydrocarbons; however, the non-hydrocarbon-forming portions of the biomass will change the chemistry and physical characteristics of the processing, and processes will need to be adapted in order to accommodate them.
The two most germane examples to this thesis of these modifications deal with processes at the Paul Scherrer Institut, in Switzerland, and at Changing World Technologies’ processing plant in Carthage, Missouri.
biomass into a methane-rich gas stream. At the batch scale, the process has been shown to gasify as much as 99% (on a carbon basis) of feedstocks as difficult to handle as ground wood and swine manure. However, while the process works well on the batch scale, when run continuously, any salts present in the feedstock have been observed to create processing issues. As water is heated to and above its critical point, the solubility of most salts drops rapidly: most salts are only sparingly soluble in supercritical water. Thus, when salt-containing biomass feedstocks are heated in the PSI process, ionic constituents begin to precipitate. This leads to heat exchanger fouling, plugging of pipes, and, most importantly, catalyst deactivation. Thus, the salt portion of the feedstock must be dealt with specially.
As a second example, the company Changing World Technologies has developed a process to hydrothermally convert the offal associated with a turkey processing plant into fuel oils. In their process, the turkey waste, which includes skin, bones, feathers, organs and other
tissues, is hydrothermally processed at around 250◦C to produce an oil phase and an aqueous
phase. The oil phase is further refined to produce heating oil or diesel fuel, the aqueous phase is treated to produce fertilizers and other by-products. Under certain conditions encountered in starting up their plant, instead of producing distinct oil and aqueous phases, CWT re-ports producing an inseparable emulsion, with a brown polymeric material formed. Their feedstock, made of meat waste, is much higher in proteins than most biomass feedstocks, and a chemical reaction between proteins and carbohydrates is suspected of producing this occurrence.
2.2
Objectives
This thesis deals with two main issues involved with impurities in hydrothermal processing of biomass: the physical aspects of salt precipitation from supercritical-water streams as it relates to biomass processing, and the chemical interactions between carbohydrate and protein fractions of biomass streams. Specific thesis objectives and approach are:
hy-drothermal media, including discussions of liquefaction and gasification, chemical phenom-ena, catalyst development, and research needs.
Although the process of supercritical water gasification of biomass was invented at MIT in the late 1970s, scant research on biomass processing has occurred in the MIT super-critical fluids laboratory in recent decades. The first objective was to perform a thorough review of the state of the art in hydrothermal biomass processing, to both familiarize ourselves with this changing field, and to fill a gap within the literature. The paper that resulted from this survey was published as Peterson et al. in 2008 [3], much of which is covered in Chapter 3 of this thesis.
Objective 2: To develop a method to apply neutron radiography to understand phenomena in a supercritical water vessel, including construction of a new supercritical-water vessel.
The separation of salts from supercritical water, as well as the operation of reverse-flow vessels with supercritical water, has been very difficult to predict and observe. As part of this thesis, we for the first time apply the powerful tool of neutron radiography to the field of supercritical water processing. Neutron radiography allows for non-invasive, in situ, real-time measurement of phenomena within the thick-walled vessels which are necessary to contain the high-pressure, high-temperature and corrosive environment as-sociated with supercritical water processing. The neutron radiographic method that was developed along with the data analysis methodology developed with it is described in detail in Chapter 4 and was published as part of Peterson et al. [4].
Objective 3: To apply neutron radiographic methodology to the study of model systems of
D2O and boron-containing salts under supercritical water conditions.
To achieve high contrast and good visibility of salt precipitation from supercritical water,
a model system was developed that consisted of supercritical D2O as a surrogate for
supercritical H2O with a tracer of boron-containing salts. The boron strongly attenuated
neutrons, while the D2O only weakly attenuated neutrons, resulting in clear visibility of
in Section 5.1 of this thesis as well as in Peterson et al. [4].
Objective 4: To apply neutron radiographic methodology to study general salt precipitation in supercritical water.
The neutron radiographic method was applied to systems using real salts of interest to biomass gasification with supercritical water. This method provided complimentary in-formation to that provided by the model system: although precipitation events couldn’t be seen with as high of resolution as in the model system, this method provided the additional information of a signal that was proportional to the density of the water phase, providing additional information on fluid density changes when blockages or flow pattern changes occurred. These results are presented in Section 5.2 of this thesis as well as in Peterson et al. [5].
Objective 5: To perform an quantitative water-in-water tracer study to elucidate fluid me-chanics behavior in a reverse-flow supercritical water vessel.
The fluid mechanics in supercritical-water reverse-flow vessels has proven notoriously dif-ficult to model with computational fluid mechanics, and until now, meaningful measure-ments of flows within these reactors have proven unfeasible. The neutron radiographic
method provided the means to conduct an elegant tracer study by using a tracer of1H2O
in D2O, tracking the movement of the 1H2O through the system by means of the
neu-tron radiographic measurements. The results of these studies are presented in Chapter 6 of this thesis. A jet entrainment model was developed to describe the results of these experiments; the derivation and results of this model are presented in Chapter 7.
Objective 6: To make kinetic determinations of the occurrence of protein-carbohydrate inter-actions in hydrothermal media.
Industrial experience suggests that Maillard-type reactions may be occurring in hy-drothermal media, which can cause processing complications. Glucose and glycine were studied as model compounds of carbohydrates and proteins, respectively, with
empha-sis on kinetic measurements of an interaction between the two compounds, as well as measurements in the absorbance range typical of Maillard-type reaction products. Ad-ditionally, other model compounds with similar functional groups were used as further surrogates to demonstrate the interaction, and limited measurements were made on changes in the reaction pathway that occurred as the result of the interactions. The results of this investigation are detailed in Chapter 8 of this thesis.
Objective 7: To predict trends in the equilibrium between the major isomeric forms of glucose under hydrothermal conditions with the tools of computational chemistry.
To react with glycine in a Maillard-type reaction, glucose must have a carbonyl group in its chemical structure. Glucose can exist in a number of ring forms, all of which do not have a carbonyl group, and in a chain form, which does has a reactive aldehyde group. At room temperature equilibrium, the reactive aldehydoglucose makes up less than 0.01% of the glucose present when dissolved in water; however, the relative pro-portion of glucose present under hydrothermal conditions is unknown. Computational chemistry studies were undertaken in order to make predictions of how the equilibrium between the major isomers of glucose shifts with an increase in temperature. The results were used to provide physical insight on the thermodynamic reasons for predicted shifts in equilibrium. These results are summarized in Chapter 9 of this thesis.
Chapter 3
Review of hydrothermal biomass conversion
research
This chapter has appeared in modified form as an invited article in the premier issue of the
journal Energy and Environmental Science [3]. Michael Antal, Jefferson Tester, Fr´ed´eric
Vogel, Morgan Fr¨oling, and Russ Lachance all contributed to this work. For more
infor-mation on hydrothermal processing, including discussions of supercritical-water gasification, catalysts, and research needs, see the published article.
3.1
Chemical reactions of biological molecules in hydrothermal
systems
The chemistry behind reactions of individual biochemicals under hydrothermal conditions is well studied for a number of common materials, such as glucose and triacylglycerides. See Table 3.1 and Figure 3-1 for representative biomass feedstocks. However, the chemical pathways of, kinetics of, and interactions between most other components of biomass at these conditions are largely uncharacterized. This section reviews hydrothermal reactions of biological materials, as well as some condensation reactions that may contribute to the formation of oils in hydrothermal systems under reducing conditions. The main focus of
this section is on chemistry in the subcritical water zone, leading largely to extraction, depolymerization, fragmentation, and liquefaction, as supercritical-water chemistry tends to favor gas formation which is discussed more thoroughly in Section 3 of Peterson et al. [3].
Generalities of hydrothermal reactions. In the early 1980s, many researchers expected
altered or enhanced rates of chemical reactions occurring near the critical point of solvents such as carbon dioxide or water. However, it is now commonly accepted that no such en-hancement takes place for water, as shown, for example, by Antal et al. [6] for the dehydration chemistry of 1-propanol. However, observed rates can be significantly enhanced by the loss of mass-transfer limitations (as most organic species become miscible with supercritical water) as well as the ability of supercritical water to sustain ionic as well as free-radical reactions [7]. Generally speaking, in producing fuels from biomass, one overall objective is to remove oxygen; biomass feedstocks often contain 40-60 wt-% oxygen and conventional fuels and oils typically have only trace amounts, under 1%. Oxygen heteroatom removal occurs most readily by dehydration, which removes oxygen in the form of water, and by decarboxylation, which removes oxygen in the form of carbon dioxide. Thermodynamically, since both water and carbon dioxide are fully oxidized and have no residual heating value, they can make ideal compounds in which to remove oxygen without losing heating value to the oxygen-containing chemicals removed.
Although an excess of water is present, dehydration reactions commonly occur in hy-drothermal media at elevated temperatures and pressures. In fact, thermodynamics
calcula-tions for the alcohol/alkene equilibrium [8] show that, for a 1M solution at 400◦C and 34.6
MPa, we would expect ethanol to equilibrate to a mixture of about 74 mol-% ethylene / 26 mol-% ethanol, and n-propanol would equilibrate to a mixture of 97 mol-% propylene / 3 mol-% propanol. The chemistry of cellulose and hemicellulose is dominated by their polyol structure (see Table 3.1 and Figure 3-1), and degradation occurs by a mixture of dehydration and hydrolysis (fragmentation) reactions, as discussed further in Section 3.1.2.
Table 3.1: Representative biomass feedstock chemical compounds and reaction intermediates en-countered in hydrothermal processing.
Substance Chemical formula Structural information
Feedstocks
cellulose [C6H10O5]n n ≈ 500-10,000; β(1→4)
linkages between glucose residues
hemicellulose typical monomers:
[C5H8O4], [C6H10O5]
branched with variable monosaccharide residues; degree of polymerization ∼500-3,000
lignin typical monomers: polymer of aromatic
subunits in random structure (see Fig. 3-1); molecular weight: >10,000 u
triacylglycerides (fats) R-CH2CH(R0)CH2-R00 R,R0,R00 are fatty acids with
ester linkages to the glycerol backbone
proteins [NHCH(R)C(O)]n monomer is amino acid
residue with various side (R) groups; n ≈ 50-2000 Intermediates
glucose C6H12O6 exists as 6-membered ring,
5-membered ring, and open chain (see Fig. 3-2)
xylose C5H10O5 exists as 6-membered ring,
5-membered ring, and open chain
amino acid H2NCH(R)COOH R is the side group, varies
from H to heterocyclic group
fatty acid RCOOH R is an alkyl group,
typically of 12-20 carbons with 0-4 double bonds 5-Hydroxymethylfurfural
Figure 3-1: Conceptual schematic of hydrothermal processes. Biomass feedstocks (top), including cellulose, triacylglycerides, and lignin, are processed in the aqueous phase. The lack of a phase change (bottom-left) allows for increased heat recovery.
(involving the alkene product), as shown for ethanol [8, 9] and n-propanol [6, 8]. Tertiary
alcohols’ dehydration chemistry is dominated by E1 and AdE2 (also involving the alkene
product) mechanisms [10, 11]. For secondary alcohols, the mechanism is less clear – for isopropanol, both E2 and E1 mechanisms give a good fit to the data [12]. In all three cases, ethers (which may be formed from alcohols via substitution reactions) play an important role in the dehydration chemistry. Generally, dehydration reactions are accelerated by the catalytic effect of a small amount of an Arrhenius acid such as H2SO4.
Decarboxylation reactions provide a second means of removing oxygen from biomass compounds; unfortunately, compared to dehydration reactions, fewer fundamental studies have been initiated. (Decarboxylation reactions are discussed for amino acids and fatty acids later in Section 3.1.4 and 3.1.3, respectively.) Decarboxylation reactions are attractive because they not only decrease the oxygen content of the feedstock, but because they also increase the H:C ratio, which typically leads to more attractive fuels. For example, typical
wood may have an empirical chemical formula of C6H9O6, or an H:C ratio of about 1.5.
Gasoline, if taken as the model compound iso-octane, has an H:C ratio of about 2.25; natural gas (CH4) has an H:C ratio of 4.
Relative to water-free conditions, decarboxylation reactions in hydrothermal media can be suppressed [13] or enhanced [14], and some suppressed reactions can return to similar levels (as water-free) with the addition of a catalyst such as KOH [15]. Goudriaan and Peferoen [14], as well as Boocock and Sherman [16] have shown that under liquefaction
conditions of 300 to 350◦Cin liquid water, a large portion of the oxygen is removed from
lignocellulose as carbon dioxide. However, the mechanism of this is unclear: deoxyhexonic acids, which are formed via the dehydration of many sugars, have not been found to undergo
selective decarboxylation under hydrothermal conditions of 340◦C [17].
Of course, in mixtures containing multiple functional groups, reactions and interactions (both inter- and intramolecular) between these groups can change chemical pathways. In acids such as lactic acid [18] and citric acid [19], which contain both hydroxyl and carboxylic acid groups, a decarbonylation pathway (involving the loss of CO) is opened and can occur
instead of dehydration or decarboxylation reactions.
3.1.1
Reactions of carbohydrates
Monosaccharides. All carbohydrates, including sugars, starches, cellulose, hemicellulose,
and chitin, are fundamentally polymers of monosaccharides. (See Table 3.1.) As discussed in Section 3.1.2, cellulose breaks down to form glucose (and other products) under hydrother-mal conditions, and hemicellulose breaks down to form a number of monosaccharides, the most prevalent being the 5-carbon sugar xylose. An understanding of the subsequent reac-tions of monosaccharides is important in hydrothermal reacreac-tions involving any of these large molecules, including cellulose pretreatment to produce glucose.
Glucose and fructose. When D-glucose dissolves in water, it exists in three forms: as
an open chain, a pyranose ring, and a furanose ring. Similarly, when D-fructose dissolves in water, it can also exist as an open chain, a pyranose ring, and a furanose ring. Glucose reversibly isomerizes into fructose via the LBAE (Lobry de Bruyn, Alberda van Ekenstein) transformation. Hence, when glucose or fructose is present in water, at least six forms of monosaccharide are present, and glucose and fructose will follow the same general reaction pathways in hydrothermal systems, as shown in Figure 3-2.
However, the rate of inter-isomerization is slow relative to the rates of degradation of both glucose and fructose. Antal et al. [22] saw that, when starting with glucose (or fructose), the amount of fructose (or glucose) formed was quite small compared to the amounts of other degradation products formed. Fructose is reportedly more reactive than glucose; for instance, Kabyemela et al. [27] observed that the rate of glucose isomerization to fructose was important in hydrothermal media; however, the reverse reaction of fructose to glucose was not important. His observations are based on experiments in which glucose or fructose were the starting material at temperatures of 300 to 400◦C and pressures of 25 to 40 MPa. In agreement with Kabyemela et al., Salak Asghari and Yoshida [28] have seen that despite the isomerization, fructose reacts much faster than glucose, at least in the presence of phosphoric
Figure 3-2: Pathways for the degradation of D-glucose and D-fructose. References for individual reactions are given in brackets. [B1985] = Bonn et al., 1985 [20]; [K1986] = Krishna et al., 1986 [21]; [A1990] = Antal et al., 1990 [22]; [A1990a] = Antal et al., 1990 [23]; [L1993] = Luijkx et al., 1993 [24]; [K1999] = Kabyemela et al., 1999 [25]; [J2004] = Jin et al., 2004 [26].
Figure 3-3: Assumed first-order Arrhenius plot of glucose degradation data. Data from Bobleter and Pape, 1968 [30], Amin et al., 1975 [31], Meyer, 1993 [32], Kabyemela et al., 1997 [27] (40 MPa data), and Matsumura et al., 2006 [29] (25 MPa data).
acid: after two minutes at 340◦C fructose was 98% destroyed, but glucose was only 52%
destroyed. They noted that at room temperature the more-reactive acyclic form of glucose is in much lower relative abundance than the acyclic form of fructose, and speculated the same principle may be driving the lower reactivity at hydrothermal conditions.
The hydrolysis of glucose and fructose has been studied for well over a century, and all confirm rapid degradation at hydrothermal conditions. Figure 3-3 is an Arrhenius figure showing the glucose degradation rate as a function of temperature from a number of stud-ies. Glucose destruction is drastic under hydrothermal conditions; for instance, Kabyemela
et al. [25] saw 55% conversion of glucose after 2 seconds at 300◦C and 90% conversion
af-ter 1 second at 350◦C. Most researchers have assumed that glucose undergoes 1st-order
degradation kinetics; however, Matsumura et al. [29] observed a reaction order of ∼0.8 for
temperatures above about 250◦C.
Table 3.2 shows common degradation products observed by in six different studies. An overall degradation network for glucose under hydrothermal conditions is presented in Fig-ure 3-2, which has been compiled from a number of sources.
Table 3.2: Glucose/fructose degradation products observed in various studies. Conditions given at end of table. Compound Study acetaldehyde [33, 34] acetic acid [22, 27, 33–35] acetol (hydroxyacetone) [22] acetone [33] acetonylacetone (2,5-hexanedione) [34] 2-acetylfuran [34] acrylic acid (propenoic acid) [33, 34]
arabinose [22] 1,2,4-benzenetriol [33] cellobiose [35] dihydroxyacetone [22, 27, 33, 36] erythrose [27] formic acid [22, 27, 33–35] fructose [22, 27, 35, 36] 2-furaldehyde (furfural) [22, 33, 34, 36] glyceraldehyde [22, 27, 33, 36] glycolaldehyde [22, 27, 33, 36] glycolic acid [33] 5-hydroxymethylfurfural (5-HMF) [22, 27, 33–36] lactic acid [22, 33, 34] levulinic acid [22, 35] levoglucosan (1,6-anhydroglucose) [22, 27, 35] mannose [22] 5-methylfurfural [34] pyruvaldehyde [22, 27, 33, 36] solid precipitate (“humic solid”) [35]
gaseous products [35] Sources:
[36] (Bonn and Bobleter): glucose/fructose at 220-270◦C, 5 sec - 15 min. [22] (Antal et al.): fructose at 250◦C, 34.5 MPa, 1-95 sec.
[27] (Kabyemela et al.): glucose at 300-400◦C, 25-40 MPa, 0.02-2 sec. [35] (Xiang et al.): glucose at 200-230◦C, unreported pressure (sealed in
ampoules), 0.5-30 min, with weak H2SO4.
[34] (Holgate et al.): glucose at 425-600◦C, 24.6 MPa, 6 sec. [33] (Srokol et al.): glucose at 340◦C, 27.5 MPa, 120 sec.
Because the rate of isomerization between glucose and fructose is slow relative to their degradation rates, different major products are observed when starting with glucose or fruc-tose. While different reaction conditions and analytical techniques cause the products re-ported to differ, most publications agree that glucose degrades mostly to fragmentation prod-ucts (glycolaldehyde, pyruvaldehyde, glyceraldehyde, etc.) [33,36] while fructose will react to a higher amount of the dehydration product 5-hydroxymethylfurfural (5-HMF) [22, 33, 36]. (5-HMF has been proposed as an industrial building block chemical for bio-based products by Kunz [37] and was the starting material for biomass-derived dimethylfuran, a bio-based gasoline replacement proposed by Dumesic and co-workers [38].)
Interestingly, Luijkx et al. [24] reported that the aromatic compound 1,2,4-benzenetriol could be formed in significant yields from fructose. They determined that this compound was being formed in yields of up to 46% from 5-HMF (as shown in Figure 3-2). This is noteworthy because in lignocellulosic pretreatments, aromatic compounds are often assumed to originate from the lignin portion. Thus Luijkx and co-workers’ results show that aromatics can also be formed from the cellulosic sugars. Indeed, Nelson et al. [39] reported, in 1984, the formation
of aromatic compounds from hydrothermal reactions of pure cellulose at 250 to 400◦C.
Temperature can have a profound impact on the reaction pathway. The first studies of glucose hydrolysis over a range of temperatures including supercritical water, conducted in the 1970s [31], reported that product spectra changed from char and liquid organics below
the critical temperature of water (374◦C) to gases, with little char, and liquid furans and
furfurals above water’s critical temperature [31, 40, 41].
Various reactions within the pathways are sensitive to pH. Xiang et al. [35] studied the
kinetics of glucose decomposition in dilute-acid mixtures at 180 to 230◦C in sealed glass
ampoule reactors at unspecified pressures, and found, at 200◦C, that lower ambient pH
solutions increased glucose destruction with the highest conversion being approximately 68% after 30 minutes at an (ambient) pH of 1.5. In experiments with fructose as the starting
material at 250◦C, Antal et al. [22] noted that adding 2-mM H2SO4 significantly affected
decreased yields of pyruvaldehyde and lactic acid, but had no measurable effect on the isomerization of fructose to glucose. Salak Asghari and Yoshida [28] worked to optimize yields of 5-HMF from fructose, and found phosphoric acid to be the best acid catalyst they tried, giving an optimal yield of 65% 5-HMF at 240◦C after 120 seconds using 0.05-M fructose in a phosphoric acid solution with an initial pH of 2. Rates were observed to decrease with increasing fructose concentration, concurrent with the build up of solid humin.
Xylose. Xylose is a five-carbon sugar that is one of the most common monosaccharide
residues contained in hemicellulose. Industrially, most of the global production of furfural (2-furaldehyde) is produced from hemicellulose-derived xylose.
Xylose can exist in water as a pyranose ring, a furanose ring, or as an open-chain structure. Antal et al. [42] have proposed a mechanism for the conversion of xylose into furfural. Perhaps counterintuitively, furfural (itself a five-membered ring) was found to be formed from the pyranose ring form of xylose; the furanose ring was relatively stable to further chemical transformations under their test conditions. The open-chain form was found to produce glyceraldehyde, pyruvaldehyde, lactic acid, glycolaldehyde, formic acid, and acetol, which are fragmentation by-products in furfural production. The stability of the furanose ring, coupled with relatively slow rates of isomerization between the three forms of xylose, explained the presence of a small and enduring concentration of xylose in the products even after relatively
long residence times at 250◦C. This mechanism was recently confirmed by ab initio molecular
dynamics simulations by Qian et al. [43] at the National Renewable Energy Laboratory. While Antal et al. showed how furfural is formed directly from xylose, Jing et al. [44] showed that furfural also degrades under hydrothermal conditions, but at a much lower rate than the xylose transforms into it. Sasaki et al. [45] saw, as the temperature rose higher into
the 360-420◦C range, that the measured quantity of fragmentation products (glycolaldehyde,
glyceraldehyde, pyruvaldehyde, and dihydroxyacetone) dominated the measured quantity of furfural after reactions with residence times of 0.1-0.25 sec and pressures of 25-40 MPa.
time, min yi e ld ( % o n c a rb o n b a si s) 0 5 10 15 20 0 10 20 30 40 50 60 70 glucose fructose maltose 5-HMF
Figure 3-4: Products from the degradation of starch at 220◦C. Adapted from Nagamori and Funazukuri [47].
observed by Nelson et al. [46] for acidic mixtures at 300◦C.
Starch. Starch is a polysaccharide consisting of glucose monomers bound with α-(1→4)
and α-(1→6) bonds. Starches are easily hydrolyzed in hydrothermal conditions. However, while starches can be broken down rapidly without the addition of acids or enzymes, the reported yields of glucose are lower than those achievable with conventional enzymatic meth-ods, presumably due to further decomposition of glucose or degradation of the starch into oligomers, like the ones produced in cellulose degradation, that cannot be further hydrolyzed to glucose.
Nagamori and Funazukuri [47] studied starch (from sweet potato) decomposition and quantified the yields of glucose, fructose, maltose, and 5-hydroxymethylfurfural (5-HMF)
versus time at 180 to 240◦C in a batch reactor at unspecified pressures. They found their
highest yield of glucose to be 63% (carbon basis) at 200◦C and 30 minutes. Figure 3-4 shows
product spectra vs. time for reactions at 220◦C; a large amount of glucose is produced in
the early period, but it is significantly degraded at longer residence times, primarily into 5-hydroxymethylfurfural (5-HMF).
Miyazawa and Funazukuri [48] report significantly lower yields of glucose from starch at
similar conditions: 3.7% glucose yield after 15 minutes at 200◦C and unspecified pressures.
However, they had a key finding that the glucose production increased drastically to 53% with CO2 addition at the ratio of 0.1 g CO2 per g H2O. The CO2 was likely acting as an acid
in hydrothermal media. The amount of glucose extracted increased approximately linearly with increasing CO2 concentration, in the range of 0 to 0.1 g CO2 / g H2O.
3.1.2
Reactions of lignocellulose
Lignocellulosic materials constitute the bulk of the dry weight of woody and grassy plant materials, and as such are amongst the most abundant biochemicals on earth. Lignocellulose is expected to be available at higher industrial yields than starch, by utilizing “energy crops” such as switchgrass, willow and poplar and from agricultural and forest-product residuals such as corn stover, wheat and rice straw, and wood waste.
Lignocellulose is composed of three primary components: cellulose, hemicellulose, and lignin. Garrote et al. [49] give typical cellulose, hemicellulose, and lignin fractions of vari-ous hardwoods, softwoods, and agricultural residues. These three chemical components of lignocellulose behave quite differently under hydrothermal conditions. For instance, in
hy-drothermal experiments with woody and herbaceous biomass at 200 to 230◦C without added
acid or base, Mok and Antal [50] found that 100% of the hemicellulose was extracted over the span of just a few minutes, as compared to just 4-22% of the cellulose and 35-60% of the lignin over the same time period.
Hydrothermal media, often with the addition of acids and bases, have long been studied for the decomposition of lignocellulose into monomers. See, for example, Bobleter’s excellent review article [51] and more recent updates by Mosier et al. [52] and Yu et al. [53]. The monosaccharides produced can make suitable sugars for fermentative processes, such as the production of (cellulosic) ethanol and other biofuels and materials. (However, it is suggested that some aromatic compounds formed in hydrothermolysis may inhibit some fermentation products [46].) Hydrothermal technologies can also liquefy and gasify lignocellulose. This
section focuses on sugar extraction; liquefaction is discussed in Section 3.2 and gasification is discussed in Part 4 of Peterson et al. [3].
Cellulose. Cellulose, like starch, is a polysaccharide composed of units of glucose. See
Table 3.1 and Figure 3-1. However, unlike starch, the glucose monomers are connected via β-(1→4)-glycosidic bonds, which allows strong intra- and inter-molecular hydrogen bonds to form, and makes them crystalline, resistant to swelling in water, and resistant to attack by enzymes. Water at elevated temperatures and pressures can both break up the hydrogen-bound crystalline structure and hydrolyze the β-(1→4)-glycosidic bond, resulting in the production of glucose monomers. However, competing reactions hinder high glucose yields: glucose itself is subject to hydrothermal degradation (as discussed in Section 3.1.1) and cellulose has been found to also break down into oligomers, some of which can hydrolyze into glucose and some of which cannot [54].
Cellulose from different biological sources has different properties, and both its physical (crystalline) and chemical structure can effect its behavior. Perhaps as expected, there is considerable variation in reported degradation rates for cellulose. Schwald and Bobleter [55] show classic first-order Arrhenius kinetics for cotton cellulose degradation with an activation
energy of 129.1 kJ/mol in the temperature range of 215 to 274◦C. However, in a semi-batch
system, Adschiri et al. [56] showed a significantly higher activation energy of ∼165 kJ/mol on powdered cellulose of unspecified plant origin. In an experiment involving a hydrothermal thermogravimetric apparatus measuring loss-in-weight of a cellulose sample at isothermal conditions, Mochidzuki et al. [57] found an activation energy of 220 kJ/mol. Meanwhile, Sasaki et al. [58, 59] report a drastic acceleration of the reaction kinetics as water becomes near-critical, associated with a change in activation energy from 146 to 548 kJ/mol as the system is heated past 370◦C! Sasaki et al.’s data [58] is plotted with Schwald and Bobleter’s, Adschiri et al.’s and Mochidzuki et al.’s in Figure 3-5, which makes the apparent change in activation energy observed by Sasaki et al. appear less dramatic. If a straight line is fit to all of the data in Figure 3-5, an apparent activation energy of 215 kJ/mol is obtained. For
1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 × 10-3
Schwald & Bobleter, 1989 Adschiri et al., 1993 Mochidzuki et al., 2000 Sasaki et al, 2000 441 394 352 315 282 253 227 203 181 1/T (T in K) ln k (k in s ) T, C
Figure 3-5: Arrhenius plot of the natural logarithm of pseudo-first-order reaction rate versus inverse temperature for cellulose decomposition. From Schwald and Bobleter [55], Adschiri et al. [56], Mochidzuki et al. [57], and Sasaki et al. [58].
comparison, the activation energy for cellulose pyrolysis in the absence of condensed water is about 228 to 238 kJ/mol [60].
Other researchers have undertaken temperature scanning techniques to determine when the breakdown of cellulose becomes significant. Deguchi et al. [61] have used polarized light microscopy to observe the loss of crystallinity in cellulose fibers using similar techniques to those conventionally used to monitor starch gelatinization, namely a loss of birefringence
which corresponds to a loss of crystallinity. When scanning at 11 to 14◦C/min at 25 MPa,
they observed a loss of birefringence at around 320◦C, indicating the cellulose crystallinity disappeared at these conditions. As illustrated in Figure 3-6, they observed breakup of the cellulose fibers very shortly after the loss of crystallinity, suggesting that the crystallinity was preventing breakdown of the cellulose.
