Ind. Eng. Chem. Res. 1997, 36, 1507-1518
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Biomass to Hydrogen via Fast Pyrolysis and Catalytic Steam Reforming of the Pyrolysis Oil or Its Fractions D. Wang, S. Czernik, D. Montane´ ,† M. Mann, and E. Chornet*,‡ National Renewable Energy Laboratory, 1617 Cole Boulevard, Golden, Colorado 80401-3393
Pyrolysis of lignocellulosic biomass and reforming of the pyroligneous oils are being studied as a strategy for producing hydrogen. A process of this nature has the potential to be cost competitive with conventional means of producing hydrogen. We propose a regionalized system of hydrogen production, where small- and medium-sized pyrolysis units (500 °C) to be effectively steam reformed. The following mechanism has been proposed (Ross et al., 1978): (1) methane, or any other hydrocarbon, is dissociatively adsorbed on the metal sites; (2) H2O is also dissociatively adsorbed on the Al sites, hydroxylating the surfaces; (3) metal-catalyzed dehydrogenation takes place, creating adsorbed hydrocarbon-derived fragments; (4) the OH surface groups migrate to the metal sites, activated by the temperature, and they eventually form intermediates leading to carbon oxides. Methanol can be steam reformed at much lower temperatures (99.95 >99.95 >99.95 >99.95 >99.95 99.7 >99.95 >99.95 >99.95 >99.95 >99.95
95 86 96 99 100 69 100 102 100 41 62
93 90 103 98 100 47 87 98 93 35 60
8 5 5 6 6 12 3 3 5 3 3
a Averaged results from triplicates. Reaction conditions were 600 °C, t ) 0.1 s, S/C ) 10-13. Catalyst used: UCI G-90C. b Limited by the detection capability of the MBMS instrument. Also see discussion in text. c S/C ) 4.5. d Samples pyrolyzed in batches of 5-10 mg at 600 °C; the residence time before reaching the catalyst bed was about 0.5 s.
decomposition. However, because other molecules present in the steam reforming reactions interfere very little with the molecular ion of hydrogen at m/z 2, hydrogen can be effectively quantified. Hydrogen was calibrated at several known levels of flow rates to cover the predicted range of hydrogen production. Pure hydrogen gas was metered by a mass flow controller (Tylan, Model FC280, 0-100 mLSTP‚min-1) into the outer flow of the reactor under operating conditions. The calibration was repeated as often as necessary throughout the day; at least one level of calibration was repeated when the reactor temperature was changed. A constant flow (10 mLSTP‚min-1) of argon (m/z 40) was used as an internal standard to correct for small changes in the total flow rate of gases exiting the reactor. The linear regression line fit of intensity at m/z 2 versus flow rate was used to predict the flow rate of hydrogen produced by steam reforming of the model oxygenates. The data were fitted with straight lines through the origin with correlation coefficients better than 0.9999. For steady-state experiments using continuous feeding, the calculated hydrogen flow rate was first multiplied by the correction factor obtained from the internal argon standard, and the resulting value was used to calculate hydrogen yield by dividing it with the stoichiometric amount. Therefore, all yield results for hydrogen are reported here as the percentage of theoretical maximum according to the stoichiometry of the reaction. They are estimated to be accurate to within (3%. Hydrogen yields measured using methanol as the standard feed under complete conversion conditions with excessively high S/C ratios (>15) were consistent with this estimation. For batchfeeding experiments where a quartz boat containing a known amount of the solid sample was placed in a sample holder and inserted to the pyrolysis zone of the vertical reactor (Figure 2), the averaged signal during the whole evolution curve and the averaged sample feed rate were used to calculate the yield of hydrogen. Errors associated with the latter results are estimated to be (5%. Larger errors ((10%) are expected in cases involving incomplete conversions. We did not measure the yields of other major products of steam reforming reactions, CH4 (m/z 15 and 16), CO (m/z 28), and CO2 (m/z 44) at the microreactor scale. However, they were estimated by assuming a complete conversion of methanol to H2 and CO2, and a comparison on relative yields can still be made. It should be noted that CO•+ (m/z 28) is also a fragment ion in the EI mass spectrum of CO2, with an intensity of 3-6% relative to that of m/z 44, depending on the mass spectrometer tuning. For cases involving incomplete conversions, qualitative product mass spectra were obtained by
subtracting the mass spectrum of the reactant (assumed to be unconverted following vaporization conditions and rapid passage through the bed at low temperatures) from that of the reactant converted by pyrolysis or reforming reactions. Product assignments are based on a user library of mass spectral data collected for pure compounds in separate experiments. Screening of Feedstocks and Catalysts. We reformed a series of oxygen-containing model compounds, of in situ pyrolysis vapors of biomass and its major components (lignin, cellulose, and hemicellulose), and of vaporized pyrolysis oils of biomass produced in a large-scale pyrolysis reactor (Diebold et al., 1995). Methanol steam reforming was used as a standard test for checking the performance of the system and catalyst deactivation. Acids, ketones, alcohols, and aldehydes were represented by acetic acid, acetone, ethylenediol, glycerol, and hydroxyacetaldehyde. The phenolic series included phenol, anisole, cresols, resorcinol, 2,6-dimethylphenol, guaiacol, syringol, and 4-allyl-2,6-dimethoxyphenol (ADP). The furan family of model compounds consisted of furan, 2-methylfuran, 2,5-dimethylfuran, 2-furfuraldehyde, furfuryl alcohol, 5-methylfurfural, and 5-(hydroxymethyl)furfural. Parameters for the steam reforming operation included catalyst temperature, molar steam-to-carbon ratio (S/C), gas hourly space velocity (GC1HSV), and residence time (t, calculated from the void volume of the catalyst bed divided by the total flow rate of gases at the inlet of the reactor; void fraction ) 0.4). Table 6 summarizes quantitative results obtained experimentally for some of these oxygenates. These tests were performed with the same catalyst (UCI G-90C), as described below, under conditions of 600 °C, S/C ) 10-13, GC1HSV ) 180-1680 h-1, and t ) 0.1 s. Vapor residence time was similar in all experiments because the feed flow rate was small compared to that of the helium carrier gas and steam. Essentially complete conversion of all oxygenates was achieved, and, in most cases, the yields of hydrogen were close to stoichiometry. However, ADP, lignin, and aspen feedstocks produced hydrogen yields far below those predicted by stoichiometry and equilibrium calculations, because nonvolatile deposits (ash and char) formed during vaporization or pyrolysis of the feed prior to entering the catalyst bed. The other feedstocks formed little or no such non-volatile deposits during vaporization or pyrolysis. Better yields can be obtained if this side effect is eliminated. The abundance of m/z 28 relative to that of m/z 44 (Table 6) can be used to infer the amount of CO generated during steam reforming. Thus, ADP produced the greatest amount of CO; other feedstocks produced much less. No clear explanation
1514 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 Table 7. Conversions (%) of Model Compounds by Catalytic Steam Reforminga ADP/MeOH
catalyst ID
MeOH
HAc
HAA
ADP
MeOH
MeOH, repeat
A E G D, #1 D, #2 C, #1 C, #2 B, #1 B, #2 B, #3
>99.95 >99.95 99.91 >99.95 >99.95 >99.95 99.5 >99.95 >99.95 >99.95
99.85 >99.95 99.95 >99.95 >99.95 >99.95 99.9 >99.95 >99.95 >99.95
>99.95 99.8 >99.95 >99.95 99.95 >99.95 99.7 >99.95 99.90
>99.95 99.6 99.7 98.0 95.9 >99.95 99.3 >99.95 99.8
99.7 99.7 99.95 99.4 98.3 >99.95 99.5 >99.95 >99.95
>99.95 99.92 99.8 >99.95 99.88 >99.95 99.5 >99.95 >99.95
a Experimental conditions: 700 °C, S/C ) 5, G HSV ) 6725 C1 h-1, and t ) 0.01 s. Conversions >99.95% correspond to the instrument detection limit.
Figure 4. Yields of hydrogen for catalytic steam reforming of methanol (0/9), acetic acid (O/b), hydroxyacetaldehyde (4/2), and 4-allyl-2,6-dimethoxyphenol (41 wt % in methanol) ([) at varying temperatures. Open symbols denote conditions of GC1HSV ) 336 h-1, S/C ) 10, and t ) 0.1 s, and solid symbols, conditions of GC1HSV ) 1680 h-1, S/C ) 4.5, and t ) 0.08 s. The thick solid and dashed lines represent equilibrium yields for C2H4O2 (acetic acid and hydroxyacetaldehyde) from thermodynamic calculations, while the thin solid and dashed lines are only visual aids.
Figure 3. H2 yield for six catalysts and four model compounds under the same steam reforming conditions (700 °C, S/C ) 5, GC1HSV ) 6725 h-1, and t ) 0.01 s).
can be advanced given the limited data. However, the results indicate that reforming kinetics are competitive for the different species and that aromatic compounds (ADP) are harder to reform than other constituents found in bio-oil. We also screened various Ni-based catalysts using four model compounds under the following conditions: 700 °C, S/C ) 5, GC1HSV ) 6725 h-1, and t ) 0.01 s. The shift catalyst was tested at lower temperatures of 350-400 °C. Table 5 lists the physical and chemical properties of all catalysts screened in this study. The Ni-based catalysts included two research formulations and samples obtained from several commercial catalyst suppliers, with variations in loading, supports, and promoters. The four model compounds used for the catalyst screening were methanol (MeOH), acetic acid (HAc), hydroxyacetaldehyde (HAA), and 4-allyl-2,6dimethoxyphenol (ADP). Catalyst performance was measured by compound conversion (shown in Table 7), the yield of hydrogen (relative to stoichiometric yield, shown in Figure 3), and resistance to deactivation. It should be noted that these screening tests did not discriminate between the high activity catalysts. The low-temperature shift catalyst (UCI C18HC, containing CuO/ZnO/Al2O3/C) worked only for methanol. Methanol was completely converted at typical operating temperatures (99.95%) at 421 °C and above. ADP was more difficult to reform with steam, and higher temperatures (>600 °C) were required for complete conversion. At lower temperatures of 421, 500, and 600 °C, the conversions of ADP were 21%, 81%, and 99.7%, respectively. Catalyst deactivation was indicated by decreased yields of hydrogen obtained from repeated experiments with the used catalyst. This deactivation probably resulted from steam reforming operations at the low temperatures used in these experiments, temperatures that were much lower than the optimal values. More meaningful studies on catalyst deactivation, lifetimes, and regen-
Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1515
Figure 5. Effect of (a) residence time and (b) steam-to-carbon ratio on hydrogen yields from catalytic steam reforming of methanol (0), acetic acid (O), hydroxyacetaldehyde (4), and 4-allyl2,6-dimethoxyphenol (41 wt % in methanol) (]) at 600 °C, GC1HSV ) 1680 h-1, and (a) S/C ) 4.5 and (b) t ) 0.08 s. Lines are only visual aids.
eration are underway using both model compounds and real bio-oils in a larger bench-scale fixed-bed reactor. Within experimental error limits, varying residence time from 0.04 to 0.15 s and increasing S/C from 4.5 to 7.5 did not significantly affect (Figure 5) the yield of hydrogen under the conditions of 600 °C and GC1HSV ) 1680 h-1 (UCI G-90C catalyst). These conditions resulted in almost complete conversion of all four model compounds (methanol, acetic acid, HAA, and ADP). Interestingly, except for the case of HAA, methane formation did show some dependence on residence time and S/C (Figure 6). In the case of HAA, there was almost no change in the amount of methane formed upon varying residence time and S/C. However, as the residence time increased from 0.04 to 0.15 s, acetic acid produced less methane but both methanol and ADP/ MeOH produced significantly more (Figure 6a). As expected, less steam also favored the formation of methane, especially from methanol (Figure 6b). At 700 °C, there was no significant change in the yield of hydrogen. The microreactor-MBMS system continuously monitored in real time the intermediate products present in the gas phase and also allowed the study of the reaction mechanism in low-conversion experiments. More detailed studies on the various types of model compounds mentioned above are underway; we have recently published the results of thermal decomposition and catalytic steam reforming reactions of HAc and HAA (Wang et al., 1996). These results indicate that both thermal decomposition and catalytic reforming reactions take place during the steam reforming of oxygenates found in pyrolysis oils. The decomposition involves both thermal cracking reactions prior to entering the catalyst bed and the acid-catalyzed reactions at the acidic sites of the catalyst support. Only very few oxygen-containing compounds are stable enough to reach the catalyst bed without thermal cracking. These competing reactions may form carbonaceous materials (coke), which
Figure 6. Effect of (a) residence time and (b) steam-to-carbon ratio (S/C) on the relative amount of methane per moles of carbon in the feed (max. ) 100) formed from catalytic steam reforming of methanol (9), acetic acid (b), hydroxyacetaldehyde (2), and 4-allyl2,6-dimethoxyphenol (41 wt % in methanol) ([) at 600 °C, GC1HSV ) 1680 h-1, and (a) S/C ) 4.5 and (b) t ) 0.08 s. Lines are only visual aids.
could plug the reactor, significantly lower the yield of hydrogen, and even deactivate the catalyst. However, under suitable operating conditions (700-750 °C, GC1HSV ) 2000 h-1, S/C ) 5, and t ) 0.1 s) a highly active catalyst such as the UCI G-90C catalyst used here can effectively steam reform all compounds, including the secondary cracking products, into H2 and CO2. The key to the steam reforming process for pyrolysis oils is hence to properly inject the oil into the reactor in order to minimize coke buildup at the entrance of the catalyst bed. Process Design and Preliminary Economics Two ways to produce hydrogen from biomass pyrolysis oil can be envisioned: (1) A regionalized system of distributed small- and medium-sized pyrolysis units provides condensed biooil to a central reforming unit. At the reforming plant, the separation of the lignin-derived oligomers will generate a “clean” aqueous stream containing simple carbohydrate-derived soluble organics that will be fed to the reformer. (2) An integrated system in which a larger pyrolysis unit processes biomass transported to it. Uncondensed vapors will be fed directly to an integrated reforming unit located in the same plant. The heavier fraction of the pyrolysis vapors can be recovered by fractional condensation and the remaining uncondensed vapors fed to the reformer. The first way can use cheaper feedstocks, which include biomass residues. The integrated system, however, can avoid the costs of entirely condensing the vapors to produce bio-oil and transporting the bio-oil to the hydrogen facility. A conceptual process design to make hydrogen from pyrolysis oil in a regionalized system is shown in Figure 7. Because of the low sulfur content of biomass and bio-oil, a sulfur removal system is not likely to be required. Also, according to thermodynamic simulations and the screening results, a tem-
1516 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997
per day). Several parameters (for instance, a lower cost for biomass) can lower this price to $3-5/GJ (Mann et al., 1996). The process can also sustain large changes in coproduct selling price, capital cost, and hydrogen production capacity before the hydrogen becomes more expensive than current markets will allow. Conclusions
Figure 7. Process flow diagram for the fast pyrolysis and reforming of biomass.
perature reformer ramping up to 700-750 °C (which is lower than 825-900 °C required for reforming natural gas) will be needed. The ratio of steam to carbon will be determined by experimental results and economic optimization; it will be in the 5-7 range, based on the literature and experimental data already obtained. In the process being evaluated, bio-oil generated from fluid-bed pyrolysis of biomass will be refined through a separation step (using water and ethyl acetate) to recover an oxyaromatic coproduct which will be used as a phenolic substitute in resin formulations. The remaining aqueous fraction will be catalytically steam reformed to produce hydrogen, using a process based on that used for natural gas reforming. Laboratory experiments will provide the basis for the choice of the most suitable catalyst and reactor configuration; the base case will use a fixed-bed catalytic reactor. A pressure-swing adsorption unit will be used to purify the H2 produced. A feasibility analysis was performed on this process to determine if the process could have economic viability and to specify areas where research will help to lower the production cost. Both laboratory data and standard process data, where applicable, were used. Although this analysis is not of design quality, it does provide useful information on this research project before scaleup and commercialization. The capital investment of the pyrolysis plant was taken from Beckman and Graham (1993). Biomass was considered available at a cost of $25/dry tonne. A 15% internal rate of return was assumed for both the pyrolysis and reforming facilities. The phenolics substitute coproduct was assumed to be sold for $0.44/kg, a fraction of the selling price of phenol. Steam is produced through heat integration and is sold as a byproduct. The design assumes that a number of small pyrolysis operations, using biomass where it is available, will supply bio-oil to a central extraction and reforming facility. Because biomass pyrolysis oil is similar to petroleum crude oil in that many fuels and chemicals can be derived from it, further coproduct options can also be considered as a function of current economic and market conditions. The current selling price of H2 in industry is generally comprised between $5/GJ and $14/GJ, depending on the size of the production facility. This range is for hydrogen as produced by the plant: purified but not compressed or stored. For our conceptual process, the cost of hydrogen has been estimated to be $7.70/GJ for the base case (production capacity: 35.5 tonne of hydrogen
Fast pyrolysis of biomass is an advanced technology for producing a bio-oil in high yields (70-75 wt % of anhydrous biomass). This bio-oil is a complex aqueous mixture of simple aldehydes, alcohols, and acids together with more complex carbohydrate- and ligninderived oligomeric materials. Fractional condensation of the pyrolytic vapors could separate the simple monomeric materials, which compose about one-third to half of the biocrude, from the complex oligomeric fraction. Steam reforming of the simple monomeric materials is thermodynamically and chemically feasible. Steam reforming of the entire bio-oil is also thermodynamically feasible. Reforming the complex oxygenates seems chemically possible, but it may require high steam-tocarbon ratios because the oxygenates rapidly dehydroxylate and aromatics are formed on the surface of the catalyst. Carbon-carbon bonds are then ruptured and only a large supply of OH and H species will suppress (or at least minimize) the formation of coke. Our screening tests have shown that such catalysis is possible using commercially available Ni-based catalysts. Improvements in their formulation may be needed to optimize the activity, selectivity, and timeon-stream relationships when reforming the biomassderived oxygenates. The preferred implementation strategy consists of small- to medium-sized regional fast pyrolysis units which will produce bio-oil from either dedicated crops or plantations or from waste lignocellulosics. The biooil produced will be transported to a central reforming unit serving a given region where H2 will be produced. An alternative strategy consists of transporting the biomass to a central conversion site where fast pyrolysis and reforming operations can be conducted jointly. Fractional condensation of the vapors can be carried out in order to recover, for instance, the depolymerized lignin fraction which has value as a mixture of phenolics to be used in resin formulations for adhesives. The process concept is simple: a desulfurization unit is not needed, and the bio-oil can be atomized in a flow of steam which is then processed through a reforming unit. By a proper choice of conditions the near-equilibrium design can be driven to maximize H2 production. A pressure-swing adsorption unit will purify the gas stream. The preliminary economic analysis of the process indicates that the necessary selling price of hydrogen is well within current market values. This analysis is based on the coproduction of hydrogen and a phenolic substitute for resin formulation. The process studied considers pyrolysis oil production at distributed locations followed by extraction and steam reforming at a centralized facility. Acknowledgment The authors are indebted to the U.S. DOE Hydrogen Program (Mr. Neil Rosemeissel and Ms. Cathy Gre´goirePadro´, managers) for financial support of this research project under Contract DE AC 36-83CH10093.
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Received for review July 8, 1996 Revised manuscript received January 1, 1997 Accepted January 8, 1997X IE960396G
X Abstract published in Advance ACS Abstracts, February 15, 1997.
