Energy Fuels 2009, 23, 6213–6221 Published on Web 10/08/2009
: DOI:10.1021/ef9007278
Expanded and Updated Results for Supercritical Water Gasification of Cellulose and Lignin in Metal-Free Reactors Fernando L. P. Resende and Phillip E. Savage* University of Michigan, Department of Chemical Engineering, 2300 Hayward, Ann Arbor, Michigan 48109-2136 Received July 13, 2009. Revised Manuscript Received September 22, 2009
We report updated and expanded results from a systematic study of the effects of process variables on the gasification of cellulose and lignin in supercritical water in quartz reactors. Quartz reactors eliminate potential catalytic effects from metallic walls. The gases produced were far from their equilibrium composition. High temperatures, high water densities, and long reaction times provide the highest H2 yields. The total gas yields and energy content of the gas were largely insensitive to the water density and biomass loading, with lignin gasification at 1.0 wt % loading being the exception. The yields of CO2, CH4, and H2 displayed an Arrhenius-like dependence on temperature. The apparent activation energies for CO2 and H2 production were about 15 and about 60 kJ/mol, respectively, and this difference suggests that these products did not primarily form together in a common reaction pathway. The activation energies for each of the product gases, however, were about the same for both cellulose and lignin. Comparing new results for gasification in quartz with literature data for gasification at identical conditions in stainless steel indicates that the significance of catalysis by metal reactor walls depends upon the experimental conditions used. Supercritical water (SCW) has been proposed as a gasification medium that can address these issues. When water is at temperatures and pressures that exceed its thermodynamic critical point, its hydrogen bonds are weaker than at room temperature conditions. As a result, its dielectric constant decreases from about 78 (at 298 K) to the range of 2-20 around the critical point, which is similar to that of polar organic solvents at room temperature.5,6 This low dielectric constant gives SCW the ability to dissolve many organic compounds completely, resulting in a single homogeneous phase,4,7-9 and making rapid reactions of organic compounds possible.10,11 These properties support the concept of supercritical water gasification (SCWG), also called hydrothermal gasification, in which the organic waste is hydrolyzed and decomposed to fuel gases in SCW. Hydrothermal gasification emerged in the early 1980s from the pioneering work of Modell,12 who processed maple sawdust in SCW. The biomass quickly decomposed to tars and some gas without the formation of char. Since then, many researchers have investigated SCW as a medium for gasification systems. Four recent literature reviews1,13-15 summarize
Introduction Producing energy and fuels from renewable resources could lessen many nations’ dependence on imported oil, as well as ameliorate environmental issues associated with energy production from fossil fuels. The use of waste biomass for this purpose may also help to reduce waste accumulation. The abundance and reasonably even distribution of biomass throughout the world make it an interesting alternative to fossil fuels. In addition, all of the CO2 released during the decomposition of biomass material originates from the current atmosphere. Gasification is a technology commonly used to produce fuel gases (H2, CH4, and CO) from biomass feedstocks. Technical difficulties prevent large-scale biomass gasification systems from being used more widely. One of these difficulties is the low thermal efficiency of gasifying high-moisture biomass feedstocks. Some waste biomass resources, such as sewage sludge, cattle manure, and food industry waste may contain more than 85% moisture.1 When the moisture content of the biomass is higher than 40%, the thermal efficiency of conventional gasification decreases substantially.2 Another difficulty with conventional biomass gasification systems is the generation of tars along with the desired light gases.3 Any production of tar represents an effective loss of gas that could have been formed otherwise. When tars are formed, they are difficult to gasify and to separate from the main gas stream,4 compromising its use in downstream applications.
(5) Savage, P. E. Chem. Rev. 1999, 99, 603–621. (6) Akiya, N.; Savage, P. E. Chem. Rev. 2002, 102, 2725–2750. (7) Lee, I.; Kim, M.; Ihm, S. Ind. Eng. Chem. Res. 2002, 41, 1182–1188. (8) McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction: Principles and Practice; Butterworths: Boston, MA, 1986; p 512. (9) Martino, C. J.; Savage, P. E.; Kasiborski, J. Ind. Eng. Chem. Res. 1995, 34, 1941–1951. (10) Yoshida, Y.; Dowaki, K.; Matsumura, Y.; Matsuhashi, R.; Li, D.; Ishitani, H.; Komiyama, H. Biomass Bioenergy 2003, 25, 257–272. (11) Matsumura, Y. Energy Convers. Manage. 2002, 43, 1301–1310. (12) Modell, M. In Fundamentals of Thermochemical Biomass Conversion; Overend, R. P., Milne, T. A., Mudge, L. K. Eds.; Elsevier Applied Science Publishers: London, 1985; pp 95-119. (13) Peterson, A. A.; Vogel, F.; Lachance, R. P.; Froling, M.; Antal, M. J.; Tester, J. W. Energy Environ. Sci. 2008, 1, 32–65. (14) Loppinet-Serani, A.; Aymonier, C.; Cansell, F. ChemSusChem 2008, 1, 486–503. (15) Kruse, A. Biofuels, Bioprod. Biorefin. 2008, 2, 415–437.
*To whom correspondence should be addressed. E-mail: psavage@ umich.edu. (1) Elliott, D. C. Biofuels, Bioprod. Biorefin. 2008, 2, 254–265. (2) Schmieder, H.; Abeln, J.; Boukis, N.; Dinjus, E.; Kruse, A.; Kluth, M.; Petrich, G.; Sadri, E.; Schacht, M. J. Supercrit. Fluids 2000, 17, 145– 153. (3) Antal, M. J.; Allen, S. G.; Schulman, D.; Xu, X.; Divilio, R. J. Ind. Eng. Chem. Res. 2000, 39, 4040–4053. (4) Osada, M.; Sato, O.; Watanabe, M.; Arai, K; Shirai, M. Energy Fuels 2006, 20, 930–935. r 2009 American Chemical Society
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current knowledge. One of the issues these reports mention as an important gap in the literature concerns the evaluation of catalysts. When catalysts are added to SCWG reactors, their effect on gas yields can be confounded with catalytic effects from metal reactors walls.13,15 The lack of knowledge on catalytic effects originating from metallic walls may lead to problems at scale-up.13 To address this issue, some researchers have performed SCWG experiments using quartz reactors, thus eliminating the possibility of catalytic effects from metallic walls.16-20 Previous studies with cellulose and lignin (in both metal and quartz reactors) have been reported.4,19-29 Our previous articles19,20 were the first and only studies of SCWG for cellulose and lignin in the absence of metals, and they were the first to report a systematic study of the influence of different process variables. In the present paper, we again report on cellulose and lignin SCWG in quartz reactors, but we report results from entirely new experiments that used a substantially improved method to measure gas yields with better reproducibility, accuracy, and precision. Details are provided in the Experimental Section. Additionally, we have expanded the range of experimental conditions beyond those used in our earlier work.19,20 We added a very low biomass loading (1.0 wt %) for both cellulose and lignin and a subcritical temperature (365 °C) and pyrolysis (0.00 g/mL water density) to the experimental conditions investigated for cellulose. Finally, the present contribution provides experimentally determined uncertainties for the reported values, the energy content for the gas produced at all conditions, experimental activation energies for gas formation, the equilibrium compositions for a set of base-case gasification conditions, and the first direct comparison of gas yields from lignin and cellulose SCWG in quartz and stainless steel reactors at identical reaction conditions. None of these items appeared in our earlier articles on this topic. Therefore, this article represents a significant advancement beyond our earlier articles on this topic. To keep this article as brief as possible, we generally present the new results in tabular form, and we limit the discussion of topics already covered in the previous articles to those where the new data show different trends or have different implications for the SCWG pathways.
Experimental Section Our previous articles19,20 provide experimental details, so this section will give only an overview and focus on the improvements we made to the experimental methods. We purchased microcrystalline cellulose powder and organosolv lignin from SigmaAldrich and used them as received. Quartz capillary tubes (2 mm I.D., 6 mm O.D.) served as mini-batch reactors. The amounts of biomass and water loaded into each reactor varied with the reaction conditions. The water loadings ranged from none (water density 0.00 g/ cm3) to 103 μL (0.18 g/cm3), and the biomass loadings ranged from 0.5 mg (1.0 wt % loading) to 26 mg (33.3 wt.% loading). After being loaded, the reactors were flamesealed, placed in a fluidized sand bath or tube furnace at the desired isothermal temperature for the desired batch holding time, and then cooled to room temperature before recovering the gaseous products. To collect the gases formed during SCWG, we first inserted the cooled quartz reactor into a steel tube, which was then pressurized to 10 psi with helium to facilitate filling later the gas sample loop on the gas chromatograph (GC). Next, the sealed metal tube was struck sharply several times to shatter the quartz reactor within and release the product gases. We then waited at least 45 min before introducing a gas sample into the GC for analysis. There was no minimum waiting time in our earlier work. Subsequent study,30 however, revealed that if the waiting time were too short, the calculated yields were sensitive to this variable, which led to more run-to-run variability in the previous results. We verified experimentally that there was no further change in the calculated gas yields beyond a 45 min waiting time. This amount of time appeared to be adequate for the added helium, residual air, and gaseous reactor contents to mix and reach a uniform composition throughout the metal tube. The subsequent GC analysis, which provides the gas composition, followed the procedure described previously.19 The absolute amount of each component present in the gas phase was calculated using N2 (from air initially in the reactor, the metal tube, and the cylinder connections) as an internal standard. Accounting for the air in the cylinder connections is one of the changes we made to improve accuracy in the present study. Another improvement we made was to purge the metal tube with flowing air for about five minutes between uses with different reactors. We verified experimentally that this purging prevented carryover from one reactor analysis to the next.
SCWG of Cellulose We performed experiments at several batch holding times (2.5-30 min) and temperatures (365 °C (to evaluate yields at subcritical conditions), 400, 500, and 600 °C). We also varied the cellulose loading (1.0, 5.0, 9.0, and 33 wt %) and the water density (0.00 (no water), 0.05, 0.08, and 0.18 g/cm3). All results reported herein for both cellulose and lignin represent mean values from multiple independent experiments at the same nominal reaction conditions. The uncertainties reported are standard deviations. The line segments in the graphs that connect experimental data points are provided solely to help the reader see the trends in the data. Effect of Batch Holding Time. We first present results for the cellulose base case conditions (500 °C, 0.08 g/cm3, 9.0 wt % cellulose) as a function of time. Figure 1 shows the temporal variation of the molar composition (dry basis) of the gases. CO2 and CO are the major products initially. CO2 remains the major product throughout and its mole % remains relatively stable at all times examined. The mole % of CO decreases with time whereas the mole % of H2 and
(16) Potic, B.; Kersten, S. R. A.; Prins, W.; van Swaaij, W. P. M. Ind. Eng. Chem. Res. 2004, 43, 4580–4584. (17) DiLeo, G. J.; Savage, P. E. J. Supercrit. Fluids 2006, 39, 228–232. (18) DiLeo, G. J.; Neff, M. E.; Kim, S.; Savage, P. E. Energy Fuels 2008, 22, 871–877. (19) Resende, F. L. P.; Neff, M. E.; Savage, P. E. Energy Fuels 2007, 21, 3637–3643. (20) Resende, F. L. P.; Fraley, S. A.; Berger, M. J.; Savage, P. E. Energy Fuels 2008, 22, 1328–1334. (21) Yoshida, T.; Matsumura, Y. Ind. Eng. Chem. Res. 2001, 40, 5469–5474. (22) Watanabe, M.; Inomata, H.; Arai, K. Biomass Bioenergy 2002, 22, 405–410. (23) Williams, P. T.; Onwudili, J. Energy Fuels 2006, 20, 1259–1265. (24) Osada, M.; Sato, T.; Watanabe, M.; Adschiri, T.; Arai, K. Energy Fuels 2004, 18, 327–333. (25) Minowa, T.; Ogi, T.; Dote, Y.; Yokoyama, S. Renewable Energy 1994, 5, 813–815. (26) Hao, X.; Guo, L.; Zhang, X.; Guan, Y. Chem. Eng. J. 2005, 110, 57–65. (27) Lu, Y. J.; Guo, L. J.; Ji, C. M.; Zhang, X. M.; Hao, X. H.; Yan, Q. H.; Int, J Hydrogen Energy 2006, 31, 822–831. (28) Sato, T.; Furusawa, T.; Ishiyama, Y.; Sugito, H.; Miura, Y.; Sato, M.; Suzuki, N.; Itoh, N. Ind. Eng. Chem. Res. 2006, 45, 615–622. (29) Watanabe, M; Inomata, H.; Osada, M.; Sato, T.; Adschiri, T.; Arai, K. Fuel 2003, 82, 545–552.
(30) Resende, F. L. P. Supercritical Water Gasification of Biomass, Ph.D. Thesis, University of Michigan: Ann Arbor, 2009; p 196.
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Figure 1. Effect of batch holding time on gas composition for cellulose SCWG (500 °C, 0.08 g/cm3, 9.0 wt % loading). Dashed lines display calculated equilibrium composition.
Figure 2. Effect of batch holding time on gas yields for cellulose SCWG (500 °C, 0.08 g/cm3, 9.0 wt % loading).
CH4 increase with time. These trends in the present data are much clearer than those discussed in our previous article19 on cellulose SCWG. Figure 1 also shows equilibrium compositions, calculated by ASPEN Plus. We used the RGIBBS reactor block in ASPEN, which calculates equilibrium concentrations by minimizing the Gibbs free energy of the mixture. The feed to the equilibrium reactor in ASPEN contained cellulose monomer (C6H12O6) and water in the same initial compositions as used experimentally in the base case. The reactor effluent was permitted to contain monomer, water, CO, H2, CO2, and CH4. The equilibrium mole % of H2 (20.5%) and CH4 (33.3%) are higher than the experimental values at 30 min. The CO2 molar % in equilibrium (46%) is smaller than the experimental value, and the equilibrium CO molar % is much smaller than the experimental one. It is clear that the system is far from equilibrium under these reaction conditions. Figure 2 shows the yields of CO, CO2, CH4, and H2 as a function of time for cellulose SCWG. The yield of CO2 increases during the first five minutes, and then is relatively stable at about 6 mmol/g for about 10 min before increasing again to 8.4 mmol/g at 30 min. The yield of CO remains stable at about 2.5 mmol/g after 5 min. Small amounts of H2 and CH4 are detected at 2.5 min. In our previous article,19 we posited steam reforming as the main pathway for CO2 formation at short times. The new data show that this hypothesis is not likely since the yield of H2, a steam reforming coproduct, is low. Instead, pyrolysis of cellulose intermediates could be taking place, forming both CO2 and CO. Figure 3 shows the effect of batch holding time on the total gas yield; the energetic content; and the H, C, and O yields for the base case conditions. We define the energy content of the gas (%) as the lower heating value (LHV) of the product gas (i.e., the mole fraction weighted averages of the LHVs for CH4, H2, and CO) relative to the LHV of the original feedstock (cellulose in the present case). The energy content of the gas is largely determined by the CH4 content (LHV = 0.80 MJ/mol). H2 (LHV = 0.24 MJ/mol) and CO (LHV = 0.28 MJ/mol) influence the energy content to a much lesser extent. The conversion of cellulose into gases increases in the first 5 min to about 35%, and then increases again at 30 min to 49%. Since the most abundant product gas is CO2, the C yield follows a trend similar to that species. The O and H yields also increase with time. The O yield is 63% after 30 min, and the H yield is 24%. The energy content of the gas remains steady at about 11% until 30 min, when it
Figure 3. Effect of batch holding time on C, H, O, and total gas yields and energy content for cellulose SCWG (500 °C, 0.08 g/cm3, 9.0 wt % loading).
increases to 19%. For comparison, if cellulose completely undergoes steam reforming to produce H2 and CO2, the energy content would be 103%. To conclude this discussion of the base case results we note that the gas yields reported here are far from equilibrium and are usually higher than those reported for the base case in our previous article.19 Also, the trends are much clearer with the present data. Our earlier article suggested that the total gas yield and the CO2 yield decreased at longer times. The present data, which are both more precise and accurate, show that no such decrease actually occurs. Effect of Temperature. Table 1 shows the effect of temperature on the composition and yields of the product gas at 5 min at the base case water density and cellulose loading. This report extends the range of temperatures considered in our previous work by including a subcritical temperature. As temperature increases, the mole fractions of H2 and CH4 increase, the mole fraction of CO goes through a maximum, and the mole fraction of CO2 decreases. The H2 mole % at 600 °C is about 7 times larger than it is at 365 °C. CH4 is only produced above 400 °C. These trends are similar to those reported previously, but the data set now includes uncertainties for all data points, which increases confidence in the trends reported. In general, the gas yields increase with temperature and they are lowest at 365 °C. The results in Table 1 clearly show that temperature can be used to increase gas yields from SCWG of cellulose. The effect of temperature is similar for all yields except that of CO. The major effect of temperature appears to be accelerating the rates of steam reforming and decomposition of intermediates into gases. A large increase in H2 and CH4 yields occurs when going from 500 to 600 °C. After 5 min, the yield of H2 is more than 4 times higher 6215
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: DOI:10.1021/ef9007278
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Table 1. Effect of Temperature on Gas Yields and Composition from Cellulose SCWGa 365 °C H2 mol % CO mol % CH4 mol % CO2 mol % H2 yield (mmol/g) CO yield (mmol/g) CH4 yield (mmol/g) CO2 yield (mmol/g) C yield (%) H yield (%) O yield (%) total yield (%) energy content (%) a
2.55 ( 0.05 21.6 ( 0.3 n.d. 75.9 ( 0.4 0.13 ( 0.01 1.1 ( 0.1 n.d. 3.8 ( 0.3 13.1 ( 0.9 0.43 ( 0.05 27.9 ( 2.1 19.6 ( 1.5 1.9 ( 0.2
400 °C
500 °C
600 °C
4.1 ( 0.5 6.1 ( 0.0 19.2 ( 1.3 26.5 ( 7.4 29.8 ( 4.9 13.0 ( 5.1 n.d. 10.0 ( 2.0 18.6 ( 2.5 69.3 ( 7.9 54.1 ( 2.9 49.3 ( 1.4 0.2 ( 0.1 0.65 ( 0.01 2.9 ( 0.2 1.4 ( 0.7 3.2 ( 0.5 1.9 ( 0.7 n.d. 1.1 ( 0.2 2.8 ( 0.4 3.6 ( 0.3 5.8 ( 0.4 7.4 ( 0.3 13.8 ( 1.9 27.2 ( 1.8 32.6 ( 2.4 0.7 ( 0.2 9.1 ( 1.5 27.3 ( 2.7 28.3 ( 2.8 47.9 ( 2.9 53.9 ( 3.1 20.1 ( 2.2 36.3 ( 2.2 42.8 ( 2.6 2.7 ( 1.1 11.1 ( 1.3 20.0 ( 2.3
Figure 4. Effect of cellulose loading on gas composition at 10 min (500 °C, 0.08 g/cm3 water density).
5 min, 0.08 g/cm3, 9.0 wt % loading. n.d.: not detected
at 600 °C than at 500 °C, reaching 2.9 mmol/g. CH4, which is not detected at 400 °C, reaches 2.8 mmol/g at 600 °C. The data in Table 1 also suggest an exponential increase in the yields of H2, CH4, and CO2 with temperature, which suggests an Arrhenius-type variation of the yields with temperature. If we take the yields to be proportional to the average rate of gas formation, we can calculate apparent activation energies for the formation of H2 and CO2. Similar activation energies for different gas species would suggest common reaction paths for their formation. We calculated the activation energies to be 59 ( 6 kJ/mol for H2, and 15 ( 2 kJ/mol for CO2. These values suggest that the reactions that increase the yields of H2 and CO2 with temperature are not the same. The implication is that neither water gas shift nor steam reforming is the sole dominant pathway for these two products. The data in Table 1 also show that the activation energy for the total gas yield is 17 ( 3 kJ/mol, and for the energy content it is 43 ( 7 kJ/mol. Effect of Cellulose Loading. We next varied the loading of cellulose to determine its effect on gas production. We used 1.0, 5.0, 9.0, and 33.3 wt % cellulose mixtures. The data at 1.0 wt % expands the range considered in our previous work.19 Water was always present in excess of the stoichiometric amount needed for steam reforming. The temperature (500 °C) and water density (0.08 g/cm3) remained at their base case values. Figure 4 shows how the composition of the gas is affected by the cellulose loading. Increasing the loading to 33.3 wt % significantly reduces the mole fraction of CO. The H2 mole fraction increases modestly with loading, which is fortuitous for H2 production from biomass because high loadings are desirable for commercial implementations of SCWG. Our previous work19 indicated that the CH4 and CO2 mole fractions were sensitive to the loading, but the uncertainties were high. The new data show that these gas compositions are largely insensitive to the loading. Figure 5 shows the effect of cellulose loading on the yields of C, H, O, and total gas and the energy content at 10 min. At all loadings, the C yield is about 25-30%, the H yield is about 10%, the O yield is about 45-50%, and the total gas yield is about 35-40%. The energy content of the gas remains at about 10%. Our previous report19 on cellulose SCWG suggested that most of the yields increased with cellulose loading, but the data were more limited and less certain than these reported here in Figure 5. The new data show that the cellulose loading has little effect on the yields, which is an important insight. It means that one can gasify high loadings of cellulose without taking a penalty in lower gas yields.
Figure 5. Effect of cellulose loading on C, H, O, and total gas yields and energy content at 10 min (500 °C, 0.08 g/cm3 water density).
Figure 6. Effect of cellulose loading on gas yields at 10 min (500 °C, 0.08 g/cm3 water density).
Figure 6 shows the molar yields of each gas as a function of cellulose loading. While the yields of H2, CH4, and CO2 remain nearly constant (given the experimental uncertainties) within the range of cellulose loadings studied, the CO yield decreases from about 3 mmol/g at 1.0 wt % to about 1 mmol/g at 33.3 wt %. These new data clearly show that cellulose SCWG is largely insensitive to the biomass loading. This result differs from that in our earlier study. Effect of Water Density. This section expands upon the results we reported earlier for cellulose SCWG19 by including experiments with no water. Also, interpolated data were used in the previous article, whereas all data reported here are direct measurements. Figure 7 shows the effect of water density on the gas product composition at 7.5 min. The H2 mole fraction, which was only 1.0% without water added, increases with water density to 26% at 0.18 g/cm3. The CO content decreases with density: it is nearly absent at 0.18 g/cm3. The CO2 and CH4 mole fractions are not affected much by water density or even by the absence of water. 6216
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Figure 7. Effect of water density on gas composition at 7.5 min from cellulose SCWG (500 °C, 9.0 wt % loading).
Figure 8. Effect of water density on C, H, O, and total gas yields and energy content from cellulose SCWG at 7.5 min (500 °C, 9.0 wt % loading).
Figure 8 shows the effect of water density on the C, H, O, and total gas yields and the energy content of the gas. The yields were lowest when no water was present. As expected, the major effect of adding water is the increase in H and O yields. The H yield steadily increases with water density from 6% with no water to 18% at 0.18 g/cm3. The O yield increases from 32% with no water to 47% when water is added. This experimental evidence suggests the participation of water as a reactant in the system. The C yield increases from 19 to 27% when water is added, and remains nearly constant for all water densities. The total gas yield increases from 25% without water to 36% in the presence of water. The energy content of the gas is nearly constant at about 10% regardless of the water density. This outcome is due to the high LHV of CH4 and to its yield being largely insensitive to the water density (see Figure 9). Figure 9 shows how the water density affects gas yields. The major effect of increasing the water density is the increase of the H2 and CO2 yields and the decrease in CO at the highest density (0.18 g/cm3). The CO2 yield at 0.18 g/ cm3 is more than twice as much as the yield with no water. When the density increases from 0.08 g/cm3 to 0.18 g/cm3, the H2 and CO2 yields increase by about 2.5 mmol/g, which is the same amount that the CO yield decreases. This balance in the yields of these species is consistent with the water-gas shift reaction being responsible for the effects of water density from 0.08 g/cm3 to 0.18 g/cm3. The yield of CH4 remains largely unchanged for all water densities, including the situation where no water is present. The trends in Figures 8 and 9 differ from those we reported previously. The previous data suggested that the gas yields experienced a minimum as density increased. Interpolated data were used, however, and uncertainties were large. The new data show that there is no minimum. Effect of Reactor Surface. In this section, we report additional new experimental results in quartz reactors obtained at SCWG conditions identical to those used in published work on SCWG of cellulose in metal reactors with no added catalyst. Performing new experiments at the precise conditions that matched those used in the literature allows us to make a much better comparison between SCWG in quartz and in stainless steel reactors. In our previous article,19 the comparisons were less definitive because they used interpolated data from quartz reactors and they relied on experiments done at different water densities and cellulose loadings. This present comparison provides clearer insight into the contribution of catalytic reactions from the metal reactor wall during nominally uncatalyzed SCWG. Table 2 compares results from the new experiments in our lab with those of Hao et al.26 From information provided in
Figure 9. Effect of water density on gas yields from cellulose SCWG at 7.5 min (500 °C, 9.0 wt % loading). Table 2. Gas Yields (mmol/g) from SCWG of Cellulose with No Added Catalysta reactor material CO2 CO H2 CH4 sum a
This Work
Hao et al.26
quartz 5.1 ( 0.5 2.4 ( 0.3 1.2 ( 0.2 1.3 ( 0.2 10.0 ( 0.6
316 stainless 6 5.5 4 3 18.5
500 °C, 20 min, 0.07 g/mL, 9.1 wt % cellulose.
the article, we estimate the metal reactor surface area/ biomass weight to be about 18 mm2/mg. The yields of H2, CH4, CO, and CO2 from SCWG in a stainless steel autoclave with no added catalyst are all higher than the yields we obtained from SCWG in quartz with no added catalyst, but at otherwise identical conditions. Although the difference in CO2 yields is relatively small, the CO and CH4 yields are more than twice as high in stainless steel than they were in quartz, and the H2 yield is more than three times as high. Thus it appears that a significant portion of the gas yields observed by Hao et al. may be attributable to heterogeneous reactions catalyzed by the reactor wall. Table 3 compares results from new experiments in our lab in quartz with a different set of previous results obtained in stainless steel reactors with no added catalyst. In this case, the experimental conditions used by Watanabe22 and Osada24 include very high water densities (0.33 g/cm3) and low cellulose loading (about 5.0 wt %). From information provided in the articles, we estimate the metal surface area/ biomass ratio in both cases to be about 19 mm2/mg. This comparison leads to a different outcome, with the H2 and CO2 yields higher in quartz than in stainless steel, and CO yields in quartz being lower. CH4 is nearly absent in all cases. 6217
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Effect of Temperature. Table 5 shows the effect of temperature on the composition and yields of the product gas at 45 min. The trends in the data do not differ appreciably from those reported previously.20 The table also shows new information about the energy content of the gas. The percentage of the chemical energy in the lignin that appears in the product gas increases from 3% at 365 °C to 38% at 725 °C. Higher temperatures are therefore extremely important to obtain higher gas yields for noncatalyzed lignin SCWG. If we assume the molar yield of a gas species at some fixed time to be proportional to its average rate of formation over that time interval, we can estimate activation energies for gas formation from the data in Table 5. The apparent activation energies are 63 ( 3 kJ/mol for H2, 34 ( 5 kJ/mol for CH4, and 15 ( 6 kJ/mol for CO2. This result suggests that different chemical reaction paths are responsible for the increase in the yields of H2, CH4, and CO2 with temperature. Also, the apparent activation energies are 17 ( 3 kJ/mol for total gas yield (similar to that of CO2 formation, the most abundant gas), and 36 ( 4 kJ/mol for the energy content (similar to that of CH4, the gas with the highest LHV). Several of the activation energies reported here for lignin are very similar to those obtained for cellulose. This outcome intimates that the reaction paths forming some of the different gaseous species may be similar for the two components of woody biomass that we have investigated. The effect of temperature on the gas yields is very similar for cellulose and lignin. The yields of CO2, CH4, and H2 increase with temperature whereas the yield of CO decreases at the higher temperatures. The total gas yield and energy content of the product gas both increase with temperature. Effect of Lignin Loading. The lignin loading is an important process variable because one typically desires to use as high a biomass loading as possible to reduce the capital and operating costs for a SCWG process. We used 1.0, 5.0, 9.0, and 33.3 wt % lignin mixtures, and water was always present in excess. The temperature (600 °C) and water density (0.08 g/cm3) remained at their base case values. The data at 1.0 wt % are new, and the trends in the updated data sometimes differ from those in the earlier study.20 Therefore, we present some of these results graphically and discuss the new trends observed with these expanded and updated data. Table 6 shows how the composition of the gas is affected by the lignin loading. The results and trends are very similar to those reported in our earlier article. The chief differences are the slightly lower H2 mole fraction and the slightly higher CO mole fraction in the new data. Figure 11 shows the molar yields of the individual gases at the different lignin loadings. In general, changing the lignin loading does little to change the gas yields in the range from 5.0 to 33.3 wt %. The exception is the 1.0 wt % loading, where substantial increases in yields are observed for all gases. When the lignin loading decreases from 5.0 to 1.0 wt %, the H2 yield increases from 1.3 to 3.3 mmol/g, the CO yield increases from 3.6 to 6.9 mmol/g, the CH4 yield increases from 4.1 to 8.9 mmol/g, and the CO2 yield increases from 4.4 to 16.7 mmol/g. These trends differ from the earlier data, which indicated that gas yields were significantly suppressed at the 9.0 wt % loading. Figure 12 shows the effect of lignin loading on the C, H, O, and total gas yields, and the energy content at 15 min. All the yields are largely independent of the lignin loading in the range of 5.0-33.3 wt %. The C yield stays around 25%, the
Table 3. Gas Yields (mmol/g) from SCWG of Cellulose with No Added Catalysta 3
Water dens. (g/cm ) wt % cellulose reactor material CO2 CO H2 CH4 sum
This Work
Watanabe et al.22
Osada et al.24
0.33 5.0 quartz 5.9 ( 0.4 0.4 ( 0.1 1.4 ( 0.2 0.12 ( 0.01 7.3 ( 0.4
0.35 4.8 316 stainless 4.1 1.8 0.6 n.d. 6.5
0.33 5 316 stainless 2.6 1.4 0.8 0.1 4.9
a 400 °C, 15 min. Yields in mmol/g were calculated from data provided in the article.
Figure 10. Effect of batch holding time on gas composition from lignin SCWG (600 °C, 0.08 g/cm3, 9.0 wt % loading).
The reason why the yields in quartz are higher than in stainless steel is unclear. It seems that the relative importance of catalysis by metal walls may depend upon the water density, the biomass loading, and perhaps temperature. SCWG of Lignin We investigated temperatures of 365, 500, 600, and 725 °C; lignin loadings of 1.0, 5.0, 9.0, and 33.3 wt %; and water densities of 0.00, 0.05, 0.08, and 0.18 g/cm3. Reactions were performed for batch holding times of 2.5, 5, 10, 15, 30, 45, 60, and 75 min. Effect of Batch Holding Time. This section presents results for the influence of the reaction time on gas yields at the base case conditions (600 °C, 0.08 g/cm3, 9.0 wt % loading) for lignin SCWG. Figure 10 shows the temporal variation of the molar composition (dry basis) of the gases formed along with the equilibrium compositions as calculated by ASPEN Plus. We again used the RGIBBS reactor block. The feed contained a hypothetical lignin monomer (C10H12O4) and water in the same initial compositions as used in the experiments. The product could contain the monomer, water, CO, H2, CO2, and CH4. The H2 equilibrium molar % (36%) is always much higher than the experimental value. The CH4 and CO equilibrium molar % (26 and 0.9%, respectively) are both lower than the experimental values. It seems, again, therefore, that the system is far from equilibrium at these conditions. Table 4 provides the updated results for the individual gas yields; the total gas yield; and the H, C, and O atom yields. In addition, it provides new information about the energy content of the product gas. The trends in the data do not differ appreciably from those reported in our earlier article.20 Moreover, the trends in the temporal variations for lignin are very similar to those for cellulose. For both feedstocks, the yields of CO2, CH4, and H2 increase with time, whereas the yield of CO does not. Likewise, the total gas yield and energy content of the product gas increase with time. 6218
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Table 4. Effect of Batch Holding Time on Yields from Lignin SCWGa 2.5 min
5.0 min
10.0 min
15.0 min
30.0 min
45.0 min
60.0 min
75.0 min
1.4 2.8 4.1 4.4 20.2 34.0 68.5 33.9 18.0
1.4 3.0 4.8 4.7 22.3 39.3 73.8 37.1 20.6
1.8 ( 0.4 2.8 ( 0.6 5.1 ( 0.6 5.7 ( 1.0 24.2 ( 2.3 42.3 ( 4.6 84.4 ( 12.2 41.4 ( 4.8 21.6 ( 2.2
1.8 ( 0.5 3.0 ( 0.4 5.2 ( 0.3 5.2 ( 0.4 23.8 ( 1.2 43.3 ( 2.7 79.7 ( 5.1 40.0 ( 2.1 22.2 ( 1.2
2.4 1.2 5.6 6.2 23.4 48.6 81.5 40.5 22.3
2.0 ( 0.1 1.5 ( 0.2 5.0 ( 0.3 5.1 ( 0.4 20.7 ( 1.0 42.7 ( 2.3 69.7 ( 5.3 35.2 ( 2.1 20.2 ( 1.1
2.6 ( 0.3 1.95 ( 0.01 5.9 ( 0.7 6.0 ( 1.0 24.2 ( 2.2 51.0 ( 5.1 82.5 ( 12.5 41.7 ( 4.8 24.1 ( 2.3
3.2 ( 0.9 1.6 ( 0.6 6.3 ( 1.0 7.6 ( 1.9 27.7 ( 4.0 56.5 ( 8.0 99.9 ( 23.3 48.8 ( 8.8 25.8 ( 3.6
H2 yield (mmol/g) CO yield (mmol/g) CH4 yield (mmol/g) CO2 yield (mmol/g) C yield (%) H yield (%) O yield (%) total yield (%) energy content (%) a
600 °C, 0.08 g/cm3, 9.0 wt % loading.
Table 5. Effect of Temperature on Gas Yields and Composition from Lignin SCWGa 365 °C
500 °C
600 °C
725 °C
1.9 ( 0.1 6.2 ( 2.2 14.6 ( 0.6 27.0 ( 0.1 H2 mol % CO mol % 16.2 ( 6.2 20.4 ( 5.8 11.3 ( 1.8 1.6 ( 0.2 12.0 ( 1.7 33.1 ( 0.5 36.2 ( 2.2 32.6 ( 1.7 CH4 mol % 69.8 ( 4.4 40.3 ( 3.7 37.4 ( 3.3 38.8 ( 1.8 CO2 mol % 7.5 ( 0.5 H2 yield (mmol/g) 0.10 ( 0.01 0.6 ( 0.3 2.0 ( 0.1 CO yield (mmol/g) 0.9 ( 0.5 2.0 ( 0.5 1.5 ( 0.2 0.45 ( 0.01 9.0 ( 0.1 CH4 yield (mmol/g) 0.60 ( 0.03 3.3 ( 0.2 5.0 ( 0.3 3.5 ( 0.5 4.1 ( 0.7 5.1 ( 0.4 10.7 ( 1.2 CO2 yield (mmol/g) C yield (%) 8.8 ( 1.2 16.9 ( 1.6 20.7 ( 1.0 35.9 ( 2.1 H yield (%) 4.6 ( 0.2 26.1 ( 1.9 42.7 ( 2.3 90.5 ( 1.8 O yield (%) 46.8 ( 6.2 60.6 ( 8.5 69.7 ( 5.3 129.8 ( 13.7 total yield (%) 18.8 ( 3.4 29.2 ( 3.3 35.2 ( 2.1 64.3 ( 5.1 energy content (%) 3.1 ( 0.6 14.0 ( 1.0 20.5 ( 1.3 37.4 ( 0.5 a
Figure 11. Effect of lignin loading on gas yields from SCWG (15 min, 600 °C, 0.08 g/cm3).
45 min, 9.0 wt % loading, 0.08 g/mL.
Table 6. Effect of Lignin Loading on Gas Composition from SCWGa H2 mol % CO mol % CH4 mol % CO2 mol % a
1.0 wt %
5.0 wt %
9.0 wt %
33.3 wt %
9.3 ( 3.9 19.1 ( 3.2 25.0 ( 6.0 46.5 ( 6.7
9.9 ( 0.6 26.2 ( 6.3 30.9 ( 2.3 33.0 ( 3.4
11.6 ( 3.1 19.5 ( 3.0 34.3 ( 2.2 34.6 ( 2.2
7.3 14.7 42.4 35.6
15 min, 600 °C, 0.08 g/cm3.
H yield around 40%, the O yield around 80%, the total gas yield about 40%, and the energy content about 25%. This outcome contrasts with the previous report20 where a minimum yield was observed at the 9 wt % loading. In light of the updated data, it appears that the yields reported previously for the 5 and 33.3% loadings were too high. At the 1.0 wt % loading the C yield increases to 58%, the H yield increases to 74%, the O yield increases to 239%, and the energy content increases to 40%. Based on these results, the 1.0 wt % loading is attractive, but one needs to keep in mind that such a low lignin loading will require a large amount of water and will negatively affect the reactor size and capital cost. With the exception of the high yields from lignin at 1% loading, the effect of loading on gas yields was very similar for both cellulose and lignin. This variable did not have much influence on the yields of any of the gases. Only the CO yield appeared to be sensitive. This relative insensitivity of the yields to high biomass loadings differs from our previous results. Effect of Water Density. Table 7 shows how water density affects the gas composition and yields at 60 min. For pyrolysis, the yields of H2, CH4, and CO2 are lower than for SCWG. For SCWG, the H2, CH4, and CO2 yields increase only modestly with increasing density. In contrast, the CO yield decreases as the water density increases. Almost all the CO formed at 0.18 g/cm3 had reacted away at 60 min. Thus, the main effect of higher densities is to reduce
Figure 12. Effect of lignin loading on C, H, O, and total gas yields and energy content from SCWG (15 min, 600 °C, 0.08 g/cm3).
(or eliminate) the CO yields and increase the CO2 yields. The previous data20 suggested a minimum gas yield existed at 0.08 g/cm3, but the new data show no such minimum. Table 7 also shows the effect of water density on the C, H, O, total gas yields and energetic content. For pyrolysis, the yields are lower than for SCWG at any water density. With water present, the H and O yields substantially increase (as expected). The influence of water density is small as long as water is present. The C yield remains at about 20-25%, the H yield remains at about 50-55%, and the O yield stays at about 90%. The results suggest the use of low water densities would be preferred if the total gas yield or energy content is the most important outcome. The use of higher water densities requires higher pressures, and it does not seem to generate any advantage for lignin SCWG in terms of increasing yields. This conclusion differs from that in our earlier study,20 which relied on data with less accuracy and with more uncertainty. The influence of water density on the yields is similar for both lignin and cellulose. The CO yield decreases, the H2 and CO2 yields increase, and the CH4 yield is largely unaffected as density increases. The total gas yield and energy content are not strongly affected as long as water is present. 6219
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: DOI:10.1021/ef9007278
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quartz reactors. It is interesting to note that Osada’s experiments24 were performed at high water densities and low biomass loadings. These conditions provided a similar outcome for cellulose as well, with yields in quartz being higher than the ones in stainless steel. The results suggest that the water density and biomass loading may affect the relative importance of catalytic effects from reactors walls for SCWG of both cellulose and lignin.
Table 7. Effect of Water Density on Gas Yields and Composition from Lignin SCWGa 0.00 g/cm3 0.05 g/cm3 0.08 g/cm3 0.18 g/cm3 H2 mole % 9.4 ( 1.7 13.5 ( 1.3 15.7 ( 0.1 15.9 ( 3.0 CO mole % 28.4 ( 1.3 15.9 ( 0.5 12.0 ( 1.4 1.0 ( 0.4 35.3 ( 2.0 35.3 ( 4.2 35.9 ( 0.3 39.3 ( 1.0 CH4 mole % 26.9 ( 5.1 35.3 ( 3.3 36.4 ( 1.8 43.9 ( 3.6 CO2 mole % 0.8 ( 0.6 2.3 ( 0.4 2.6 ( 0.3 2.71 ( 0.02 H2 yield (mmol/g) CO yield (mmol/g) 2.4 ( 1.5 2.7 ( 0.1 1.95 ( 0.01 0.2 ( 0.1 3.0 ( 1.9 6.1 ( 0.3 5.9 ( 0.7 6.8 ( 1.1 CH4 yield (mmol/g) 2.1 ( 0.9 6.1 ( 1.0 6.0 ( 1.0 7.7 ( 2.0 CO2 yield (mmol/g) C yield (%) 13.4 ( 4.5 26.5 ( 1.9 24.6 ( 2.2 26.1 ( 4.4 H yield (%) 24.2 ( 13.4 51.4 ( 2.5 51.0 ( 5.1 58.1 ( 7.6 O yield (%) 39.5 ( 13.5 88.7 ( 12.0 82.5 ( 12.5 92.2 ( 23.9 total yield (%) 21.0 ( 6.3 44.7 ( 4.5 41.7 ( 4.8 45.7 ( 9.0 energy content (%) 13.4 ( 6.4 25.3 ( 1.0 24.1 ( 2.3 25.2 ( 3.5 a
Conclusions This section summarizes the key findings from these new experiments and their implications. We will not repeat any of the conclusions that remain unchanged from the earlier articles. Rather, we provide new conclusions and highlight any differences from the previous sets of conclusions. Although the highest total gas yield obtained in this study of noncatalytic SCWG is 108%, the highest energy content of the gases (40%) is lower than one would desire for a large-scale process. The gases produced in these experiments, however, are far from their equilibrium composition. Therefore, tremendous opportunities exist for catalysis to increase reaction rates and gas yields in SCWG. The yields of CO2 and H2 consistently increased with increasing temperature and water density, whereas the yield of CO decreased. The reason CO failed to follow this trend is most likely its consumption via the water-gas shift reaction becoming faster at the higher temperatures and higher water densities. The activation energies for production of H2 and CO2 are very different for both lignin and cellulose gasification, which suggests that these two products are largely formed by different reaction pathways. The implication of this result is that neither steam reforming nor water gas shift alone is primarily responsible for the formation of both of these products at the conditions investigated. The activation energies for the production of H2 and CO2, though different from one another, are very similar for SCWG of both cellulose and lignin. This similarity intimates similar reaction paths for the production of the individual gases from the two different starting materials. There is no minimum in gas yields from lignin as either the biomass loading or water density is varied. Moreover, the CH4/H2 ratio is not nearly as sensitive to loading and density as indicated by our earlier data, which were less accurate and less precise. SCWG can be used to produce H2, fuel gas, or syngas. Different reaction conditions can be used to obtain the desired products. High temperatures, high water densities, and long reaction times lead to high H2 yields. The energy content of the product gas is increased by the use of higher temperatures (to increase H2 and CH4 yields) and low biomass loadings (to increase CO yield). One can produce syngas with CO/H2 ratios around 1:1 or 1:2 by using short reaction times, low biomass loadings, and low water densities. These conditions favor the survival of the CO that is produced during the early stages of gasification. Of course, overall gas yields are low at these conditions. The syngas produced has the relative amounts of CO and H2 needed for synthesis of liquid fuels such as methanol or Fischer-Tropsch liquids. Finally, this article provides the first direct comparison of biomass SCWG in quartz and stainless steel at nominally identical reaction conditions. At moderate water densities (∼0.07 g/cm3), the total gas yield and the H2 mole fraction
60 minutes, 600°C, 9.0 wt % loading.
Table 8. Gas Yields (mmol/g) from SCWG of Lignin with No Added Catalysta reactor material CO2 CO H2 CH4 sum a
This Work
Watanabe et al.29
quartz 3.3 0.3 0.5 1.4 5.6
316 stainless steel 3.33 0.56 0.76 2.78 7.42
400°C, 60 min, 0.35 g/cm3, 12.6 wt %.
Table 9. Gas Compositions and Yields from Lignin SCWG with No Catalyst Addeda reactor material CO2 (%) CO (%) H2 (%) CH4 (%) C yield (%) H yield (%) O yield (%) a
This Work
Osada et al.24
quartz 53.2 ( 0.9 17.8 ( 1.2 4.5 ( 0.6 24.5 ( 0.2 6.8 ( 0.3 7.6 ( 0.5 29.4 ( 1.4
316 stainless steel 42 16 7 33 3.7 5.8 13.5
400°C, 15 min, 0.33 g/cm3, 5.0 wt %.
Effect of Reactor Surface. The literature contains reports of lignin SCWG in metal reactors with no added catalyst. We could not compare our earlier results in quartz reactors with these published data because no common temperatures were investigated. Therefore, we have now performed new SCWG experiments at precisely the same conditions as those used in the published studies. Table 8 compares these new results in quartz with results obtained by Watanabe et al.29 in stainless steel. From information in the article, we estimate the reactor surface area/biomass weight ratio to be about 6 mm2/mg. The CO2 yield is the same in quartz and stainless steel. All the other yields are higher in stainless steel than in quartz, and the total gas yield is 33% higher in stainless steel. These results suggest that catalysis from the stainless steel walls can increase gas yields in lignin SCWG at these conditions. Of course, this conclusion must remain tentative since neither data set provides estimates for the experimental uncertainty. Table 9 provides another comparison of SCWG results in quartz and stainless steel reactors. We estimate the stainless steel surface area/biomass ratio to be about 19 mm2/mg. The molar % reported for CO and H2 are close to the ones we obtained in quartz reactors. It seems, though, that CO2 is present in higher % in quartz, and CH4 is present in higher % in stainless steel. The C, H, and O yields are all higher in 6220
Energy Fuels 2009, 23, 6213–6221
: DOI:10.1021/ef9007278
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from SCWG of cellulose are lower in quartz than in stainless steel reactors. This comparison indicates that the reactor surface can influence both the rate of gas formation and the composition of the gas. At high water densities (∼0.33 g/cm3) and low biomass loadings (∼5 wt %), however, H2 and CO2 are modestly higher in quartz then they were in stainless steel reactors. These comparisons indicate that the relative influence of metal surfaces is likely a function of the precise reaction conditions employed. It seems that water density
and biomass loading affect the relative importance of catalysis from reactor walls in SCWG. Acknowledgment. We thank Harald Eberhardt, Master Glassblower at The University of Michigan, for experimental assistance and access to the glass lab. F. L. P. R. gratefully acknowledges the Ph.D. fellowship awarded from the CNPq, an institution of the Brazilian Government committed to promote research and higher education. This research was also supported by the National Science Foundation (CBET-0755617).
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