Energy & Fuels 2008, 22, 871–877
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Supercritical Water Gasification of Phenol and Glycine as Models for Plant and Protein Biomass Gregory J. DiLeo, Matthew E. Neff, Soo Kim, and Phillip E. Savage* Chemical Engineering Department, UniVersity of Michigan, Ann Arbor, Michigan 48109-2136 ReceiVed August 16, 2007. ReVised Manuscript ReceiVed NoVember 1, 2007
We examined the gasification of phenol and glycine in supercritical water (SCW). For phenol SCW gasification, the water density and phenol loading were varied to determine their effects. Increasing the water density at a constant phenol loading lowered the phenol conversion for Ni-catalyzed reactions. Increasing the phenol loading at a fixed water density increased the conversion. The H2 yields at equilibrium increased with decreasing water density, decreasing phenol loading, and increasing temperature. Glycine was much more resistant to gasification than phenol. Large amounts (20%–90%) of the initial carbon remained in the aqueous phase even after 1 h for both homogeneous and Ni-catalyzed reactions. Solid material was also produced. Of the gases that were formed, CO was most abundant from the homogeneous reactions, while hydrogen was the most abundant in the presence of a nickel catalyst. The Ni catalyst assisted in glycine gasification, as less carbon was found in the aqueous phase, and gas yields were increased.
Introduction As the demand for energy grows and atmospheric CO2 levels rise, renewable and sustainable energy sources become increasingly important. Converting biomass into useful fuels that retain its chemical energy is one path toward sustainable energy systems. One way to achieve this conversion is through gasification—chemically converting the biomass into gaseous products. Conventional gasification technology requires a drying step. This step becomes energy and cost intensive for biomass with a high moisture content. Using water as the gasification medium, as is done in aqueous phase gasification1 and gasification in supercritical water,2 would eliminate the need for drying. Supercritical water gasification (SCWG) is the process investigated here. Its origins date to the 1970s.3 The reaction medium is water above its thermodynamic critical point (Tc ) 374 °C, Pc ) 22.1 MPa). SCWG is more attractive than conventional gasification for biomass with >30 wt % water.4 A well-designed SCWG process can produce a gas with >90% of the chemical energy in the original biomass and, more importantly, with 4.5 J of energy for every 1.0 J of unrecovered heat or work put into the process.5 In this article, we report on the gasification of both phenol and glycine in supercritical water. Phenol serves as a model for woody plant materials. The macromolecular chemical structure of woody biomass contains many phenolic monomers. This article expands on our earlier work on phenol SCWG6 by * Corresponding author. E-mail:
[email protected]. (1) Davda, R. R.; Shabaker, J. W.; Huber, G. W.; Cortright, R. D.; Dumesic, J. A. Appl. Catal., B 2005, 56, 171–186. (2) Matsumura, Y.; Minowa, T.; Potic, B.; Kersten, S. R. A.; Prins, W.; van Swaaij, W. P. M.; van de Beld, B.; Elliott, D. C.; Neuenschwander, G. G.; Kruse, A.; Antal, M. J., Jr. Biomass Bioenergy 2005, 29, 269–292. (3) Modell, M.; Reid, R. C.; Amin, S. I. Gasification process. US Patent 4,113,446, 1978. (4) Yoshida, Y.; Dowaki, K.; Matsumura, Y.; Matsuhashi, R.; Li, D. Y.; Ishitani, H.; Komiyama, H. Biomass Bioenergy 2003, 25, 257–272. (5) Calzavara, Y.; Joussot-Dubien, C.; Boissonnet, G.; Sarrade, S. Energy ConVers. Manage. 2005, 46, 615–631.
determining the effects of water density and phenol loading on the kinetics and gas yields. The encouraging results from the phenol experiments led us to investigate SCWG reactions for glycine, which serves as a model for animal material. The behavior of amino acids in hydrothermal systems has been studied, but with little attention on SCW systems.7–12 One feature that makes the present work unique is that the reactions were conducted in quartz capillary tubes, at times with a nickel wire inserted. These reactors allowed us to separate clearly the effects of homogeneous and metalcatalyzed reactions. Experimental Section All chemicals were purchased commercially in high purity and used as received. Water was deionized and purified via an in-house treatment system prior to use. We prepared aqueous stock solutions of phenol (2 and 5 wt %) and glycine (10 wt %) and loaded aliquots of these into batch reactors fashioned from a 2 mm i.d. × 5 mm o.d. quartz tube that was flame-sealed at one end. The amount of solution added was chosen such that a desired density was reached at reaction conditions. Nickel wire (0.5 mm diameter) was added to some of the reactors to observe its catalytic effect. The wire was cut to the desired length and scoured with emery paper to roughen and remove impurities from the surface. After adding the reactant and water, and in some cases Ni (12 cm), the open end of the quartz tube was flame-sealed at a length of 13.7 cm. The reactors were next placed horizontally in an isothermal Techne fluidized sand bath preheated to the desired reaction (6) DiLeo, G. J.; Neff, M. E.; Savage, P. E. Energy Fuels 2007, 21, 2340–2345. (7) Li, J.; Brill, T. B. J. Phys. Chem. A 2003, 107, 5987–5992. (8) Li, J.; Wang, X.; Klein, M. T.; Brill, T. B. Int. J. Chem. Kinet. 2002, 34, 271–277. (9) Islam, M. N.; Kaneko, T.; Kobayashi, K. Bull. Chem. Soc. Jpn. 2003, 76, 1171–1178. (10) Li, J.; Brill, T. B. Int. J. Chem. Kinet. 2003, 35, 602–610. (11) Rogalinski, T.; Herrmann, S.; Brunner, G. J. Supercrit. Fluids 2005, 36, 49–58. (12) Abdelmoez, W.; Nakahasi, T.; Yoshida, H. Ind. Eng. Chem. Res. 2007, 46, 5286–5294.
10.1021/ef700497d CCC: $40.75 2008 American Chemical Society Published on Web 01/12/2008
872 Energy & Fuels, Vol. 22, No. 2, 2008 temperature. Our previous article6 displays a complete temperature profile for a reactor during this heat-up stage. About 30 s was required for the reactor to reach the sand bath temperature. After the desired reaction time had elapsed, the reactors were immediately removed from the sand bath and allowed to cool to ambient conditions. The reaction times were selected to ensure that gasification would be incomplete in most instances. Our interest is in the SCWG reaction pathways and kinetics, so it is essential that data be obtained over a wide range of conversions. Several reactors were used for each experimental condition, and the results reported are mean values from at least three independent reactor trials. The liquid-phase products from the phenol experiments were recovered by breaking open the quartz tube and rinsing it several times with acetone. The acetone with the dissolved liquid effluent was transferred to a sample vial and analyzed via gas chromatography, as described previously.6 The liquid-phase products of the glycine experiments were analyzed for total organic carbon (TOC) on a Shimadzu TOC 500 instrument. Deionized water was used to recover the products. The reactors were rinsed several times, and the recovered solution was diluted to a total volume of 2 mL. These samples were divided into 0.5 and 1.5 mL portions. The 0.5 mL portion was further diluted with 5 mL of deionized water to bring it within the detection range of the TOC analyzer. The 1.5 mL portion was stored for repeat analysis if necessary. Standard solutions of organic carbon were prepared by dissolving glycine in deionized water in concentrations of 125, 62.5, and 31.25 mg/L. The standard solution of inorganic carbon was prepared by dissolving both 23.2 mg of sodium carbonate and 17.52 mg of sodium bicarbonate in 500 mL of water. The TOC analyzer used 5-7 injections of 10 µL to analyze for total carbon content and the same to analyze for inorganic carbon content for each sample. Samples of the deionized water were also injected to determine the background carbon amounts. Calibration curves for both total carbon and inorganic carbon were made from the standard solutions. The concentrations of total carbon, inorganic carbon, and organic carbon (difference between total and inorganic carbon) were calculated from the analysis of each sample. The carbon content of the reactors was then calculated from the concentrations of the analyzed samples, taking into account the dilutions used. The gaseous products from both phenol and glycine were recovered by placing a quartz reactor tube in a sealed metal tube. This metal tube was purged with helium and then pressurized to 10 psig. Next the total pressure was taken to 50 psig by adding argon. The helium acted as an internal standard for gas species analysis. The quartz tube was next shattered to release the gas products into the metal tube. The gaseous products were then released from the metal tube into a gas sample loop on a gas chromatograph (GC). The gas sample was then injected into the GC for analysis. This technique is discussed in further detail in an earlier publication.6 Gas mixtures of known concentrations were also analyzed to create calibration curves for the gaseous species. To validate the gas analysis method, gases produced from the decomposition of formic acid in supercritical water were measured. This system was chosen because formic acid decomposes completely and quickly in supercritical water to form gases.13 A stock solution was made by dissolving 200 µL of formic acid in 1 mL of water. Several quartz reactors were charged with 40 µL of this solution. A 12 cm length of Ni wire was added, and the reactor was sealed at a length of 13.7 cm. To ensure complete conversion, the reactors were heated in the sand bath for a long period of time. Three reactors were run at 600 °C for 5 h. Six reactors were run on a different day at 500 °C for 3 h. Every reactor in the first batch and four of the six in the second batch were analyzed for gas products. The other two reactors were used for TOC analysis to determine whether any ungasified carbon in the liquid phase. The mole fraction of each gas was calculated from calibration curves, and the total moles of each gas were calculated using the added He as an internal standard. (13) Yu, J.; Savage, P. E. Ind. Eng. Chem. Res. 1998, 37, 2–10.
DiLeo et al. Table 1. Equilibrium and Experimental Gas Compositions from Formic Acid SCWG (500 °C, Gw ) 0.09 g/mL) H2 CO2 CH4 CO
equilibrium mole fraction
measured mole fraction
0.16 0.67 0.17 0.002
0.18 ( 0.03 0.66 ( 0.01 0.15 ( 0.02 0.004 ( 0.010
The total amount of carbon detected in the gas phase was averaged for all seven reactors. The mean value for the percent of carbon recovered in the gas phase was 98% with a standard deviation of 45%. The mean carbon recovery in the liquid phase was 1.6% with a standard deviation of 0.5%. These results show that accurate carbon balances can be obtained when multiple experiments are run and the mean value is taken. Equilibrium calculations were done using ASPEN Plus to determine the relative amounts of each product one should expect from SCWG of formic acid at the long times used in our experiments. Table 1 shows both the equilibrium and experimental mole fraction of each gaseous product at 500 °C. It is clear that the experimental values are in very good agreement with those expected at equilibrium. This agreement demonstrates that out experimental methods provide accurate results for the gas composition. The results from experiments at 600 °C show the same degree of accuracy.
Phenol Gasification The gasification of phenol (5 wt %) in supercritical water (Fw ) 0.079 g/mL) with and without a catalyst and at different temperatures was presented in an earlier publication.6 The results of that study were that both the addition of a Ni catalyst and increasing the reaction temperature accelerated the reaction. The pseudo-first-order rate constant at 600 °C for homogeneous gasification of phenol is (3.0 ( 0.4) × 10-4 s-1, and the rate constant for Ni-catalyzed gasification is (1.1 ( 0.1) × 10-3 cm/s. In this article we report on additional experiments to determine the effects of water density and phenol loading. The reaction conditions used in the earlier study (Fw ) 0.079 g/mL and 5 wt % initial phenol loading) are used as the base case here. Effect of Water Density. An important parameter in many supercritical water reactions is the density of the water (Fw) at reaction conditions. In this section we report the effect of the water density on the phenol conversion and yields of gaseous products at 600 °C and a 5 wt % phenol loading. The water density was changed by varying the reactor size and volume of stock solution loaded. Shortening the reactor length to 12.5 cm (from 13.7 cm) and decreasing the amount of phenol solution loaded to 25 µL (from 35 µL) led to a water density of Fw ) 0.064 g/mL, which is lower than the base case density. This density at 600 °C leads to a pressure just slightly above the critical pressure of water. Experiments at a density higher than the base case were also conducted at 600 °C. This water density, Fw ) 0.159 g/mL, was obtained by changing the reactor length to 14 cm and the stock solution loading to 70 µL. Figure 1 shows the effect of water density on phenol conversion at 600 °C. For both homogeneous and Ni-catalyzed SCWG the water density clearly has an effect on phenol conversion. We also observe that at each water density Nicatalyzed SCWG is always faster than homogeneous SCWG. The difference is most pronounced at the lowest water density. In Ni-catalyzed SCWG, as Fw is increased, the conversion of phenol is decreased. For homogeneous SCWG of phenol, gasification at the intermediate density is much faster than at both the higher and
Supercritical Water Gasification of Phenol and Glycine
Energy & Fuels, Vol. 22, No. 2, 2008 873
Figure 1. Effect of water density (Fw) on phenol conversion at 600 °C and 5 wt % loading. Table 2. Pseudo-First-Order Rate Constants for SCWG of Phenol at 600 °C phenol Fw [phenol]0 [water]0 loading (wt %) (g/mL) (mol/L) (mol/L) 2 5 5 5
0.079 0.064 0.079 0.159
0.0142 0.0555 0.0628 0.1230
4.522 3.537 4.522 8.840
khomogeneous (10–4 s–1) not 0.9 3.5 1.3
analyzed ((0.07) ((0.30) ((0.10)
kNi-catalyzed (10–3 cm/s) 0.76 ((0.15) 1.30 ((0.40) 1.10 ((0.10) 0.14 ((0.03)
Figure 2. Gas yields from Ni-catalyzed SCWG of phenol at 600 °C and 5 wt % loading: (a) Fw ) 0.064 g/mL; (b) Fw ) 0.159 g/mL. Solid lines connect experimental values, and dashed lines show equilibrium values.
lower density. This result indicates that an optimal water density may exist for homogeneous SCWG of phenol. From the phenol conversion data we calculated pseudo-first-order rate constants for the disappearance of phenol as the slope from a linear regression of ln (1 – X) vs time, where X is the phenol conversion. The rate constants appear in Table 2. Figure 2 shows the yields (moles of gas formed per mole of phenol loaded) of gaseous products from the low- and highdensity experiments with a nickel catalyst. There was no solid material formed; thus for phenol SCWG, the total gas yields were calculated from the amount of aqueous-phase carbon
Figure 3. Gas yields from homogeneous SCWG of phenol at 600 °C and 5 wt % loading: (a) Fw ) 0.064 g/mL; (b) Fw ) 0.159 g/mL.
converted. The experimental results are shown along with dashed lines that indicate the equilibrium yields calculated by ASPEN Plus. The gas yields are much higher for the low-density experiments; therefore, lower water density promotes both phenol conversion and gas formation. At the low water density (Figure 2a) hydrogen has the highest yield at the shorter reaction times, but it is seemingly consumed and carbon dioxide becomes the major product as the reaction time approaches 60 min. Methane is present in moderate amounts while carbon monoxide is present at very low yields. The hydrogen yield behaves similarly at high water density—it is high at short reaction times and decreases as the reaction time approaches 60 min. This outcome is consistent with the equilibrium yields at high density (Figure 2b). The measured methane yield is lower than what is expected at equilibrium with high water density. The highdensity experiments did not reach complete phenol conversion (Figure 1) so these reactions may not yet be at equilibrium. Figure 3 shows the yields of gaseous products from the lowand high-density homogeneous SCWG experiments. Recall that in both cases the phenol conversion is far from complete. The gas yields are low, and they generally increase with time. The only exception is the carbon monoxide yield at Fw ) 0.159 g/mL, which approaches zero after 60 min. The disappearance of CO is likely due to the water gas shift reaction. These results suggest that in a homogeneous reaction environment the water gas shift reaction is faster with higher water densities. Both Rice et al.14 and Araki et al.15 observed an increase in water gas shift reaction rate with increasing water density. The dependence of the reaction rate was shown to be greater than first order. Sato et al.,16 however, did not see an increase in the calculated rate constant, but an increase in water density did selectively promote the water gas shift reaction toward CO2 and H2. (14) Rice, S. F.; Steeper, R. R.; Aiken, J. D. J. Phys. Chem. A 1998, 102, 2673–2678. (15) Araki, K.; Fujiwara, H.; Sugimoto, K.; Oshima, Y.; Koda, S. J. Chem. Eng. Jpn. 2004, 37, 443–448. (16) Sato, T.; Kurosawa, S.; Smith, R. L., Jr.; Adschiri, T.; Arai, K. J. Supercrit. Fluids 2004, 29, 113–119.
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Figure 4. Effect of phenol loading (wt %) on phenol conversion at 600 °C (Ni-catalyzed, Fw ) 0.079 g/mL).
Figure 5. Gas yields from 2 wt % loading SCWG of phenol (Fw ) 0.079 g/mL, Ni-catalyzed at 600 °C). Solid lines connect experimental values, and dashed lines show equilibrium values.
Effect of Phenol Loading. The initial concentration of reactant is an important parameter in many reactions so we examined its influence on the SCWG of phenol. The base case experiments reported previously used a phenol loading of 5 wt %. This was the maximum loading that could be achieved in a stock solution based on the solubility of phenol in water at room temperature. We performed new experiments with a phenol loading of 2 wt %. The results of these experiments are compared to those with a 5 wt % loading to determine the effect of the initial phenol loading. Figure 4 shows the effect of phenol loading on the phenol conversion in Ni-catalyzed SCWG. The reaction with 5 wt % loading is slightly faster. The pseudo-first-order rate constant at a 2 wt % loading was calculated as k2% ) (0.76 ( 0.15) × 10-3 cm/s. This value compares to the rate constant for the 5 wt % loading at k5% ) (1.1 ( 0.1) × 10-3 cm/s. If the reaction were truly first order in phenol, the rate constant would be independent of the phenol loading in these experiments. The results in this section, though, show that decreasing the phenol loading may result in a modest decrease in the pseudo-firstorder rate constant for Ni-catalyzed SCWG. Figure 5 shows the gas yields for nickel-catalyzed phenol SCWG with a phenol loading of 2 wt %. The yields measured here are very close to those expected at equilibrium. The hydrogen yield of about 9 mol per mole of phenol is the highest of any seen in this work or in our previous article.6 The carbon dioxide and methane yields are comparable to those obtained in other experiments. Once more, the carbon monoxide yield is extremely low, most likely due to the water gas shift reaction. Hydrogen from Water. In our previous article6 we showed that about two-thirds of the hydrogen atoms in the gaseous products came from water molecules at the base case conditions. The present work allows us to determine the influence of phenol loading and water density on the conversion of hydrogen atoms in water into H2 and CH4. From the phenol conversions and
DiLeo et al.
Figure 6. Yield of hydrogen atoms in the gaseous products at equilibrium from phenol SCWG.
equilibrium calculations shown for all experiments, one can see that nickel-catalyzed SCWG of phenol generally reaches equilibrium in less than 1 h. Therefore, we will use the equilibrium yields for this analysis. Figure 6 shows the number of hydrogen atoms in the gaseous product at equilibrium per mole of phenol for all of the conditions studied in this work and in our previous article.6 These hydrogen atoms are in the form of H2 or CH4. If hydrogen were only being supplied by phenol, the maximum hydrogen atom yield would be six. As can be seen in Figure 6, however, these reactions can produce up to 22 hydrogen atoms in the gas phase per mole of phenol. Thus, more than 70% of the hydrogen atoms in the product gases come from the supercritical water and not the lignin model compound. The results indicate that the low reactant loading, low water density, and high temperature liberate the most hydrogen atoms from water during phenol SCWG. SCWG of Glycine The results from the phenol SCWG experiments were encouraging in that gas yields were high, and we were able to determine the influences of important process variables. To continue this research, we next investigated how a model for animal biomass behaves in SCW. One of the major constituents of animal matter and wastes is proteins. Proteins are made up of amino acids and decompose into amino acids in hydrothermal conditions.17 The goal of this portion of the study was to determine the effectiveness of SCW as a reaction medium for glycine gasification. Glycine was chosen because it is the simplest amino acid. Using an amino acid is important for two reasons. The first is that it can be used to model proteins. An understanding of the behavior of glycine in SCW can be used to gain knowledge on the behavior of proteins and other animal matter in SCW. The other important reason to study the gasification of glycine is that it is, along with the other amino acids, a nitrogen-containing compound. The study of glycine SCWG can help show any effect nitrogen could have on SCWG. Experiments were conducted to determine how effective SCWG is for converting glycine into gaseous products. A solution of 10 wt % glycine in water was heated to 500 and 600 °C for up to 60 min. The water density at reaction conditions was 0.079 g/mL. These reactions were run both homogeneously and with a nickel wire acting as a catalyst. The liquid effluent was analyzed for its carbon content to determine the extent of gasification. (17) Qian, Y.; Engel, M. H.; Macko, S. A.; Carpenter, S.; Deming, J. W. Geochim. Cosmochim. Acta 1993, 57, 3281–3293.
Supercritical Water Gasification of Phenol and Glycine
Figure 7. Fraction of initial carbon in aqueous phase from glycine SCWG at 500 °C.
Aqueous Phase Analysis. Figure 7 shows the fraction of carbon remaining in the aqueous phase from glycine SCWG at 500 °C. This quantity is the fraction of the carbon atoms initially loaded into the reactor that were detected by TOC analysis in the liquid effluent. For both the homogeneous and catalyzed reactions the aqueous phase contains a significant amount of the initial carbon. In the homogeneous reaction, about 60% of the initial carbon remains in the liquid phase, and the amount remaining is insensitive to the reaction time. For the Ni-catalyzed reaction, the aqueous-phase carbon is in the range of 20%–40% of that originally loaded into the reactor. The results from reactions at 600 °C were very similar to those at 500 °C. The higher temperature did not have much effect on the amount of aqueous-phase carbon in the reactor effluent. Also, the fraction of carbon remaining in the aqueous phase did not change much with reaction time, and the nickel catalyst left less aqueous-phase carbon than did the homogeneous reaction. The hydrothermal stability of amino acids has been studied previously.7–12 Glycine is stable at low temperatures (
