Energy & Fuels 1993, 7, 514-577
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Hydrogen Production by Steam Reforming Glucose in Supercritical Water? Dehui Yu, Masahiko Aihara, and Michael Jerry Antal, Jr.' Hawaii Natural Energy Institute and the Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, Hawaii 96822 Received January 13,1993. Revised Manuscript Received May 10, 1993
Gasification of glucose in supercritical water at 600 "C, 34.5 MPa, and 30 s residence time results in the formation of hydrogen, carbon dioxide, carbon monoxide, and methane. The yields of these gases are strongly influenced by the reactor wall and are far from values predicted by a thermochemical equilibrium program. For glucose concentrations ranging from 0.1 to 0.8 MI the gasification carbon efficiency equals or exceeds 85 % in a Hastelloy C276 capillary tube reactor. Virtually no tar or char products are detected in the effluent of the reactor.
Introduction In 1974 biomass was proposed112as a potential feedstock for hydrogen production via steam reforming reactions of the type: C6H1206+ 6H20
-
6C02 + 12H2
(1)
In eq 1glucose serves as a model compound which mimics the reaction chemistry of the many carbohydrates that (together with lignin) compose biomass. Subsequent research3-6revealed that the pyrolysis of biomass in steam at atmospheric pressure results in the formation of a hydrocarbon-rich synthesis gas, a refractory tar, and a char. Hydrogen yields were not high. Consequently, interest in biomass as a feedstock for hydrogen production declined, although more conventional gasification work c0ntinued.I More recently, seemingly unrelated findings concerning glucose decomposition in supercritical water renewed our interest in this area. Amin et almsobserved no char formation during glucose decomposition in water at 218 atm and 374 "C. As much as 20% of the feed carbon was detected in the gaseous decomposition products. Subsequent studies involvingcellulose and various wet biomass feeds confirmed the fact that lignocellulosicmaterials can be liquefied by rapid hydropyrolysis.g-12 Furthermore, ground-breaking work by Elliott and his colleagues at the t This paper is dedicated to Dr. Johndale C. Solem,Los Ala" National Laboratow. ___..-... (1)An&, M. J. In Hydrogen Energy, Part A; Veziroglu, T. N., Ed.; Plenum: New York, 1975. (2) Antal, M. J. In Toward a Solar Ciuilization; Williams, R. H., Ed.; MIT Press: Cambridge, MA, 1978. (3) Antal, M. J. Ind. Eng. Chem. Prod. Res. Deu. 1983,22, 366-375. (4) Antal, M. J. In Research in Thermochemical Biomass Conuersion; Bridgwater, A. V., Kuester, J. L.,Eds.; Elsevier Applied Science: London,
_""". 1 RFIA
(5)A n a ,M. J. In Aduances in Solar Energy; Boer, K. W., Duffie, J. A., Eds.; American Solar Energy Society: New York, 1984; Vol. 1. (6) Antal, M. J. In Advances in Solar Energy; Boer, K. W., Duffie, J. A., Eds.; Plenum Press: New York, 1985; Vol. 2. (7) See: Research in Thermochemical Biomass Conuersion; Bridgwater, A. V., Kuester, J. L., Eds.; Elsevier Applied Science: London, 1988. (8)Amin, S.; Reid, R. C.; Modell, M. Presented at the Intersociety Conference on Environmental Systems, San Francisco, CA, 21-24 July 1975; ASME Paper No. 75-ENh-21. (9) Modell, M.; Reid, R. C.; Amin,S. US.Patent 4,113,446, 1978. (10).Modell,M.; Gaudet, G. G.; Simmon, M.; Hong, G. T.; Biemann, K. Solrd Waste Management, August 1982. (11)Beckman, D.; Boocock, D. G. Can. J. Chem. Eng. 1983,61,80-86.
Battelle Pacific Northwest Laboratory demonstrated the ability of various alkali carbonate and nickel catalysts to convert wet biomass into a methane-rich gas at temperatures between 400 and 450 "C and pressures as high as 34.5MPa.13-15 These results caused us to wonder if higher temperatures might result in superior yields of a hydrogenrich synthesis gas. We found that glucose at low concentrations (ca. 0.1 M) can be completely gasified in SCW after about 20 s at 600 "C and 34.5 MPa.16 The major products were hydrogen and carbon dioxide. No char or tar products were detected. Consequently, we adopted 600 "C, 34.5 MPa, and 0.5 min residence time as baseline reaction conditions for the studies described here. The goals of this work were to identify the effects of the reactor wall and increasing glucose reactant concentration on gasification results. High biomass concentrations (comparable to about 1 M glucose in water) are necessary for this work to enjoy commercial promise. Also, we wanted to compare experimental results with those predicted by a thermochemical equilibrium computer program. Closely related work concerning SCW oxidation as a means for destroying hazardous organic wastes has attracted much attention. The current status of this field is summarized in an authoritative review by Tester et al.17 A key finding from Tester's laboratory is the important role of water in oxidizing carbon monoxide via the water gas shift reaction. Above 500 "C over 20%of the carbon dioxide formed from carbon monoxide in the presence of oxygen resulted from the water gas shift reaction.18Jg As detailed below, the water gag shift reaction also plays an (12) Boocock, D. G.; Sherman, K.M. Can. J. Chem. Eng. 1985, 63, 627-633. (13)Butner,R. S.; Sealock, L. J.; Elliott,D. C. InEnergyfrom Biomass and Wastes IX,Klass, D. L., Ed.; I G T Chicago, 1985. (14) Sealock, L. J.; Elliott, D. C.; Butner, R. S.; Neuenschwander, G. G. Low Temperature Conversion of High-Moisture Biomass. Topical Rept. Jan. 1984-Jan. 1988, PNL-6726; Oct. 1988. (15) Elliott, D. C.; Sealock, L. J.; Butner, R. S.; Baker, E. G.; Neuenschwander, G. G. Low-Temperature Conversion of High-Moisture Biomass. PNL-7126; Oct. 1989. (16) Manarungson, S.; Mok, W. S. L.; Antal, M. J. in Progress in Thermochemical Biomass Conuersion, in press. (17) Tester,J. W.;Holgate,H.R.;Armellini,F. J.; Webley,P.A.;Killilea, W. R.; Hong, G. T.; Barner, H. E. Supercritical Water Oxidation Technology: Process Development and Fundamental Research, In Emerging Technologies for Hazardous Waste Management III, ACS Symposium Series 518; Tedder, D. W.; Pohland, F. G., Eds.; American Chemical Society: Washington, DC, 1993. (18)Helling, R. K.; Tester, J. W. Energy Fuels 1987, 1,417.
0 1993 American Chemical Society Q~~l-O624/93/25Ql-Q514~Q4.QQlQ
Energy & Fuels, Vol. 7, No. 5, 1993 575
Glucose in Supercritical Water Table I. Composition of Ni Cr Mo Ha~telloy bal 14.5-16.5 15-17 (2-276 Inconel 58b 2G23 &lo 625 a Maximum. b Minimum.
Alloys Used in This Work Fe W Si Co Mn 1.0b 4-7 3-4.5 0.08' 2.5' 5'
0.50 3.15-4.15
important role in the production of hydrogen (and carbon dioxide) from glucose in SCW. Unfortunately, research concerninghazardous waste destruction has not shed much light on the pyrolytic reactions of carbohydrates in water. At temperatures above 200 OC carbohydrates rapidly undergo isomerization, dehydration, and fragmentation reactions,mt21resulting in the formation of a plethora of reactive species, each having its own pyrolysis chemistry. Whole biomass materials, containing lignin, extractives, ash, proteins, and other species, in addition to cellulose and hemicellulose carbohydrates may be expected to evidence an even more complex reaction chemistry. Nevertheless, it is well-known that glucose is difficult to gasify;22 consequently we view glucose to be a good model compound for exploratory work. Recent studies with water hyacinth, banana tree stem, and microalga (tobe published in the near future) have evidenced higher gas yields than the yields obtained from glucose under identical conditions. Clearly, much work will be required before we understand the mechanisms which underlie the conversion of biomass and its components into light, permanent gases in SCW. Apparatus and Experimental Procedures Four tubular flow reactors have been employed in the course of this work. Early experiments involved a supercritical annular flow reactor (SCAF'R) that was described elsewhere.l6*" The other three reactors are 6.1 m long, 3.15 mm 0.d. and 1.44 mm i.d. coiled Hastelloy C276 and Inconel 625 capillary tubes. The composition of each of these two alloys is given in Table I. These supercritical coil flow reactors (SCCFR) and associated equip ment, including the gas-sampling systems, are described in the Results obtained MSE theses of S. Manarungson" and D. Y U . ~ from the SCAFR have been published;lBconsequently this paper focuses on recent data obtained from the three SCCFR. We note that one of these reactors (the "corroded Hastelloy" SCCFR) was exposed on numerous occasions to fluid salt (KC1 and/or NaC1) and water mixtures at 650 OC and 34.5 MPa. These salts are soluble in fluid water at these conditions and pass through the reactor without noticeable precipitation at concentrations as high as 0.01 M. Because the salts attacked the walls of the Hastelloy tube, we replaced it with a new Hastelloy tube which was employed in most of the work reported here. The Inconel SCCFR was briefly exposed to 0.05 M KC1-water fluid at 650 OC, but it suffered no evident corrosion. Results presented here reflect recent significant improvements in our sampling procedures. Gas samples in our earlier work% were occasionally compromised by hydrogen gas retention in the sample loop and incomplete mixing of gases after the carbon gas phase change, which occurs when the dioxide liquid pressure of the sample drops from 34.5 MPa to subatmospheric.
-
~
~
(19)Holgate, H. R.; Webley, P. A.; Tester, J. W. Energy Fuels 1992, 6,586. (20)Antal, M. J.; Mok, W. S. L.; Richards,G. N. Carbohydr.Re8.1990, 199,91-109. (21)Antal, M. J.; Leesomboon, T. C.; Mok, W. S. L.; Richards, G. N. Carbohydr. Rea. 1991,217, 71-85. (22)Orsi, F.J. Therm. Anal. 1973,5, 329-335. (23)Manarungaon, 5.MSE Thesis, Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, HI, 1991. (24) Yu, D. MSE Thesis, Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, HI, 1993.
Table 11. Formic Acid and Acetic Acid Decomposition in Water in Various Reactors at 600 "C,34.6 MPa, and 34 s Residence Time 0.6 M 0.6 M acetic acid formic acid new corroded Inconel Inconel Hastellov Hastellov Hz yielda 0.90 0.065 0.039 0.20 COz yield 0.88 0.14 0.086 0.39 CO yield 0.031 0.012 0.013 0.079 CI& yield 0.018 0.12 0.096 0.58 0.002 0.002 0.003 Cz& yield 0.0 CzHr yield 0.0 0.0 0.001 0.0 carbon balancebl % 93 14 10 53 Yield = mol &mol reactant. b Carbon balance = mol carbon in gas/mol carbon in reactant. ~~
Our current procedure captures the total effluent of the reactor at reaction pressure and room temperature during a measured period of time (typically 15 s to a few minutes) in a capillary tube. The flow of products into the tube displaces water at reaction pressure out of the tube during the sampling period. All the captured products are released from the capillary tube into a large evacuated test tube through a 10port valve/needle/septum arrangement, and the capillary tube is back-flushed into the test tube to ensure collection of all products. Results indicate that this sampling procedure does not retain hydrogen and ensures adequate mixing of the gases. The current work also employs a new Hewlett-Packard Model 5890 SeriesI1Gas Chromatograph equippedwith flame ionization and thermal conductivity detectors. Gas analyses are accomplished using a sO/lOOmesh Carbosphere molecular sieve packed column, heated at 35 "C for 4.2 min, followed by a 15 "C/min ramp to 227 "C and a 70 "C/min ramp to 350 "C with a plateau at 350 "C of 5.3 min. Gas calibration curves spanning the range of concentrations observed in this work are built up by analyses of syringe injections of five different certified gas standards and air. Qualitative analysis of liquid products is accomplishedusing a Hewlett-Packard Model 5790A gas chromatograph coupled to a Hewlett-Packard Model 5970A mass selective detector employing a J & W Scientific 30 m X 0.25 mm DB-1701 column.
Results To gain confidence in our new sampling and analysis procedures we executed an experiment with 0.6 M formic acid as the reactant (see Table 11). It is well-known that formic acid immediately decomposes in SCW to a gas mixture composed primarily of hydrogen and carbon dioxide.2s Thus formic acid serves as a good reactant to test sampling and analysis procedures for products rich in these two gases. We regard the carbon balance (carbon balance = (total carbon in gas)/(carbon in feed)) and measured gas yields (yield = (mol of gas species produced)/ (mol of glucose in feed)) given in Table I1to be indicative of the experimental accuracy which can be achieved using our current equipment. Table I1 also displays the results of experiments using 0.6 M acetic acid as the reactant in each of the three reactors. Acetic acid is much more stable than formic acid, undergoing only 10-14 % conversion in the Inconel and new Hastelloy reactors. On the other hand, the conversion is more than 3 times higher in the "corroded Hastelloy" reactor. Moreover, we observed initial conversions as high as 61 5% in this reactor, which gradually fell to 47 7% during the course of the run. This dramatic finding points to the role of wall-catalyzed reactions in the degradation chemistry of acetic acid in SCW. (25) Antal, M. J.; Brittain,A.; DeAlmeida, C.; Ramayya,S.; Roy, J. C. In Supercritical Fluids; Squires, T. G., Paulaitis, M. E., Eds.; American Chemical Society: Washington, DC, 1987.
576 Energy & Fuels, Vol. 7,No. 5, 1993
Yu et al. Table 111: Gas Yields from 0.2 M Glucose Gasification in Water in Various Reactors at 600 "C, 34.5 MPa, and 34 s Residence Time
\p
H2 yield C02 yield CO yield CHr yield C2H6 yield C2H4 yield
.e.
6
F 4
A
i,
2
0
0.0
0.2
carbon balance/ % 0.4
0.6
0.6
Concentration of Glucose [ M ] Figure 1. Effect of reactant concentration on gasification. Lines, equilibrium data; Symbols, experimental data at 600 "C, 34.5 MPa, 34 s using the Inconel SCCFR. 1oo2l - [
10
- a p
6
F 2
0
0.0
0.2
0.4
0.6
0.6
Concentration of Glucose [ M ]
Figure 2. Effect of reactant concentration on gasification. Lines, equilibrium data; Symbols, experimental data at 600 "C, 34.5 MPa, 34 s using the new Hastelloy SCCFR. To offer baseline theoretical results against which subsequent experimental observations could be compared, we used the STANJAN26 thermochemical equilibrium computer program to predict the equilibrium composition of products formed from the gasification of glucose in water at 600 "C and 34.5 MPa. Results displayed in Figures 1 and 2 indicate that hydrogen, carbon dioxide, methane, and traces of carbon monoxide are the only expected products in equilibrium. The hydrogen yield drops by more than a factor of 3 and the methane yield increases by a factor of 20 as glucose concentration increases from 0.1 to 0.8 M. A 33 % decrease in the carbon dioxide yield and a small increase in the carbon monoxide yield are also predicted. Glucose gasification results are also displayed in Figures 1and 2. At low (0.1M) glucose concentrations, the carbon balance (which is a measure of gasification efficiency) is above 90% in both the Inconel (Figure 1) and new Hastelloy (Figure 2) reactors. A typical sample standard deviation for the carbon balance is 3%. In the new Hastelloy reactor the carbon balance falls to a value of about 85% at 0.4 M and remains at this value to 0.8 M. In the Inconel reactor the carbon balance declines steadily to a value of about 68% as the reactant concentration increases. In both cases the yields of hydrogen and carbon dioxide decrease by a factor of 2 or more as the reactant concentration increases. In the Inconel reactor the carbon monoxide yield increases and the methane yield holds steady with increasing glucose concentration; whereas in the new Hastelloy reactor both the carbon monoxide and methane yields are steady. (26) Reynolds, W. C. STANJAN chemical equilibrium solver, V 3.90 IBM-PC (c), Stanford University, 1987.
Inconel corroded Hastelloy new Hastelloy 6.3 7.7 1.8 3.1 3.7 0.90 0.57 0.16 3.0 0.90 1.2 0.96 0.17 0.036 0.21 0.002 0.03 82 86 89
At low concentrations the water effluent of both the Inconel and new Hastelloy reactors is clear with no measurable solids content; whereas with a glucose reactant concentration of 0.8 M the water effluent of these reactors has a very slight yellow hue and a barely detectable burnt smell. We dried a 4-mL undiluted sample of this effluent from the new Hastelloy reactor and a 4-mL sample of HPLC grade water in weighing vials in an oven at 105 "C and measured the difference in weight gain several times over a three-day period. The dissolved solids content of the reactor effluent was 0.33 mg/mL with a sample standard deviation of 0.06 mg/mL. This is about 0.2 % by weight of the original feed solids content. These results caused us to obtain a TOC analysis of a sample of the water effluent from each reactor taken during high (0.8 M) glucose concentration experiments. The analysis was completed by an independent laboratory employing wet persulfate oxidation techniques with an 01 Analytical Model 700 TOC Analyzer. The TOC content of the sample from the Inconel reactor was 14% of the original feed, and the TOC content of the sample from the new Hastelloy reactor was 17% of the feed. Only one significant peak was present in a GC-MSD analysis of these two liquid samples. This peak exhibited the retention time and mass spectrum of acetic acid. Evidently, acetic acid is a major refractory product of the steam reforming reactions of glucose in supercritical water. The fact that acetic acid was found to be relatively stable at these conditions (see Table 11) is consistent with this finding. The trends in the yields of hydrogen, carbon dioxide, and carbon monoxide displayed in Figure 1for the Inconel reactor parallel those predicted by the STANJAN code; however, the experimental yields of hydrogen and carbon dioxide are lower and those of carbon monoxide much higher than expected. Experimental values of the methane yield are steady and do not increase as predicted by STANJAN. In the new Hastelloyreactor, observed trends for hydrogen and carbon dioxide also followthe equilibrium predictions, but the actual values of the yields are much lower than expected. On the other hand, the yield of carbon monoxide is much higher than expected, and the methane trend does not follow the STANJAN prediction. Because the "corroded Hastelloy" reactor evidenced strong catalytic activity in the destruction of acetic acid, we were curious to learn if it would also be effective in the gasificationof glucose. Table I11compares results obtained from the three reactors with a glucose concentration of 0.2 M. To our surprise, gas products from the "corroded Hastelloy" reactor were similar to those from the Inconel reactor in this test. Results displayed in Tables I1 and I11 and Figures 1and 2 point to the catalytic role of the reactor wall in determining the gas composition. The carbon balance obtained from the new Hastelloy reactor is consistently higher than that of the Inconel reactor, indicating the
Glucose in Supercritical Water
effectiveness of its wall in catalyzing the steam reforming (gasification) reactions. The wall of the Inconel reactor seems to catalyze the water gas shift reaction, producing a gas rich in hydrogen and carbon dioxide; whereas the gas exiting the new Hastelloy reactor is rich in carbon monoxide. Conclusions
At low concentrations (ca. 0.1 M) glucose can be completely gasified in SCW after about 30 s at 600OC and 34.5 MPa. The main products are hydrogen, carbon dioxide, carbon monoxide, and methane. Trace amounts of ethane and ethene are also detected. The gasification efficiency falls to about 85% in the new Hastelloy reactor and 68% in the Inconel reactor, as the reactant concentration increases to 0.8 M. Inconel strongly catalyzes the water gas shift reaction; whereas Hastelloy does not. A “corroded Hastelloy”wall catalyzes the decomposition of aceticacid in supercriticalwater; whereas Inconel and new Hastelloy walls are leea effective. The “corroded Hastelloy” wall behaves similarly to the Inconel wall with glucose as
Energy & Fuels, Vol. 7, No. 5,1993 677
a reactant. These results point to opportunities to identify effectivecatalytic agents for the production of a hydrogenrich synthesis gas from wet biomass. They also suggest novel possibilities for the catalytic nonoxidative destruction of hazardous organic wastes in supercritical water. Tubular packed bed catalytic flow reactors are now being fabricated in our laboratory to enable us to undertake further studies of the catalytic gasification of both model compounds (such as glucose and acetic acid) and whole biomass materials in supercritical water. Acknowledgment. This work was supported by the NREL/DOE under Contract XN-0-19164-1.The GCMSD analysis reported in this paper was accomplished by Ms. Astrid Berg (HNEI). The authors thank Dr. Ralph Overend (NREL),Dr. Robert Williamsand Dr. Eric Larson (Princeton), and Dr. Patrick Takahashi, Dr. Charles Kinoshita, Dr. Kelton McKinley, and Dr. Richard Rocheleau (HNEI)for their interest in this work. We also thank three anonymous reviewers for their constructive comments.
