Biomass, a product of photosynthesis, is a renewable resource that can be used for sustainable production of hydrogen. We propose an approach that combines production of hydrogen with valuable coproducts and shows promising economics. The concept is based on a two-stage process: fast pyrolysis of biomass to generate bio-oil, followed by catalytic steam reforming of the bio-oil, or a fraction thereof, to produce hydrogen. The preferred option is separation of the bio-oil into a lignin-derived fraction, which could be used for producing phenolic resins or fuelblending components, and a carbohydrate-derived material, which would be reformed to produce hydrogen. The coproduct strategy can also be applied to residual fractions derived from pulping operations or ethanol production and to effluents from other biomass conversion technologies such as transesterification of vegetable oils or food processing residues. In addition, all of the biomass-derived liquids can be coprocessed with natural gas to produce hydrogen from mixed fossil-biomass feedstocks, a strategy similar to cofiring biomass and coal for power generation. This work focuses on the second stage of the process: catalytic steam reforming of various biomass-derived liquids. We have used a commercial nickel-based naphtha reforming catalyst in a fluidized-bed reactor to produce hydrogen from the various biomass-derived liquids. Yields have approached or exceeded 80% of those theoretically possible for stoichiometric conversion.

A parametric study of the gasification of four feedstocks (corn stover, switchgrass, wheat straw, and wood) has been performed on an experimental, pilot-scale (0.5 ton/day) gasification facility. A comparison was made of the performance of the gasifier as a function of feedstock, in terms of the syngas production and composition. In these experiments, pelletized feedstock was used, so that the shapes and sizes of the materials did not influence the results. A total of 22 statistically designed experimental conditions were examined for each feedstock, including the effects of varying the temperature of the fluidized bed, the temperature of the secondary thermal cracker, and the steam-to-biomass ratio. For each experimental condition, the permanent-gas composition was measured continuously by gas chromatography (GC). Tars were measured continuously using a molecular-beam mass spectrometer (MBMS). Sulfur analysis by GC was also conducted for three of the feedstocks studied. The results from this study show that there were significant differences between the feedstocks studied in terms of light gases formed, but less apparent variation in tar formation. In general, the variations in products were smaller at higher temperatures. A preliminary analysis of gasifier efficiency was performed using an Aspen Plus process model for selected gasification conditions. Finally, a comparison was made between the results of this work and other similar biomass gasification studies.

The objective of this research project was to test the hypothesis that separation of char with its associated mineral matter from pyrolysis vapors before condensation will lead to improved bio-oil quality and stability with respect to storage and transportation. The metric prescribed by the U.S. Department of Energy (DOE) to evaluate stability in this case was a 10-fold reduction in the rate of increase of viscosity as determined by an accelerated aging test. The primary unit operation that was investigated for this purpose was hot gas filtration. A custom-built heated candle filter system was fabricated by the Pall Corporation and furnished to the National Renewable Energy Laboratory (NREL) for this test campaign. This system consisted of a candle filter element in a containment vessel surrounded by heating elements on the external surface of the vessel. The filter element and housing were interfaced to NREL’s existing 0.5 MTD pyrolysis process development unit (PDU). For these tests, the pyrolysis reactor of the PDU was operated in the entrained-flow mode. The hot gas filter (HGF) test stand was installed on a slipstream from the PDU, so that both hot gas filtered oil and bio-oil that was not hot gas filtered could be collected for purposes of comparison. Two filter elements from Pall Corporation were tested: (1) porous sintered stainless-steel (PSS) metal powder and (2) sintered ceramic powder. A sophisticated bio-oil condensation and collection system was designed and fabricated at NREL and interfaced to the slipstream filter unit. The test campaign on vapor-phase filtration of biomass-derived pyrolysis oil demonstrated that a bio-oil with substantially improved properties can be obtained by application of hot gas filtration. The ceramic filter element and test stand supplied by Pall Corporation and the vapor condensation and collection system designed and fabricated by NREL both demonstrated very good operability. Application of periodic blowback was shown to be effective in maintaining the filter element pressure drop within acceptable limits, and filter plugging was never experienced. A bio-oil with greatly reduced alkali and alkaline earth metals and very low solids content was produced. Bio-oil obtained by hot gas filtration with a PSS element had elevated iron content, suggesting that the material of construction is not suitable for this application. The PSS-filtered bio-oil also did not pass the viscosity metric of a 10-fold reduction in the rate of viscosity increase as determined by the accelerated aging test at 80 °C. Bio-oil obtained by hot gas filtration with a ceramic (Dia-Schumalith sintered ceramic powder) filter element was also low in alkali and alkaline earth metals and total solids and did not exhibit high iron content. The ceramic-filtered oil passed the viscosity metric, indicating that this oil should be much improved with respect to storage and transport stability. Total mass loss because of hot gas filtration was estimated to be in the range of 10–30% by weight.

The effects of feedstock type and biomass conversion conditions on the speciation of sulfur in biochars are not well-known. In this study, the sulfur content and speciation in biochars generated from pyrolysis and gasification of oak and corn stover were determined. We found the primary determinant of the total sulfur content of biomass to be the feedstock from which the biochar is generated, with oak and corn stover biochars containing 160 and 600–800 ppm sulfur, respectively. In contrast, for sulfur speciation, we found the primary determinant to be the temperature combined with the thermochemical conversion method. The speciation of sulfur in biochars was determined using X-ray absorption near-edge structure (XANES), ASTM method D2492, and scanning electron microscopy–energy-dispersive spectroscopy (SEM–EDS). Biochars produced under pyrolysis conditions at 500–600 °C contain sulfate, organosulfur, and sulfide. In some cases, the sulfate contents are up to 77–100%. Biochars produced in gasification conditions at 850 °C contain 73–100% organosulfur. The increase of the organosulfur content as the temperature of biochar production increases suggests a similar sulfur transformation mechanism as that in coal, where inorganic sulfur reacts with hydrocarbon and/or H2 to form organosulfur when the coal is heated. EDS mapping of a biochar produced from corn stover pyrolysis shows individual sulfur-containing mineral particles in addition to the sulfur that is distributed throughout the organic matrix.

Mitigation of tars produced during biomass gasification continues to be a technical barrier to developing systems. This effort combined the measurement of tar-reforming catalyst deactivation kinetics and the production of syngas in a pilot-scale biomass gasification system at a single steady-state condition with mixed woods, producing a gas with an H 2 -to-CO ratio of 2 and 13% methane. A slipstream from this process was introduced into a bench-scale 5.25 cm diameter fluidized-bed catalyst reactor charged with an alkali-promoted Ni-based/Al 2 O 3 catalyst. Catalyst conversion tests were performed at a constant space time and five temperatures from 775 to 875 °C. The initial catalyst-reforming activity for all measured components (benzene, toluene, naphthalene, and total tars) except light hydrocarbons was 100%. The residual steady-state conversion of tar ranged from 96.6% at 875 °C to 70.5% at 775 °C. Residual steady-state conversions at 875 °C for benzene and methane were 81% and 32%, respectively. Catalytic deactivation models with residual activity were developed and evaluated based on experimentally measured changes in conversion efficiencies as a function of time on stream for the catalytic reforming of tars, benzene, methane, and ethane. Both first-and second-order models were evaluated for the reforming reaction and for catalyst deactivation. Comparison of experimental and modeling results showed that the reforming reactions were adequately modeled by either first-order or second-order global kinetic expressions. However, second-order kinetics resulted in negative activation energies for deactivation. Activation energies were determined for firstorder reforming reactions and catalyst deactivation. For reforming, the representative activation energies were 32 kJ/g‚mol for ethane, 19 kJ/g‚mol for tars, 45 kJ/g‚mol for tars plus benzene, and 8-9 kJ/g‚mol for benzene and toluene. For catalyst deactivation, representative activation energies were 146 kJ/g‚mol for ethane, 121 kJ/g‚mol for tars plus benzene, 74 kJ/g‚mol for benzene, and 19 kJ/g‚mol for total tars. Methane was also modeled by a second-order reaction, with an activation energy of 18.6 kJ/g‚mol and a catalyst deactivation energy of 5.8 kJ/g‚mol.

Progress in the production of hot-gas filtered biocrude oils from a dry hybrid poplar feedstock in the NREL vortex ablative pyrolysis reactor is discussed. In particular, adjusting the pyrolysis severity in the vortex reactor and the cracking severity in the char baghouse resulted in increased oil yields of very low-ash and low-alkali biocrude oils. The viscosity of these oils meets the requirements for American Society for Testing and Materials (ASTM) #4 fuel oils. Increasing the water content to 3 0% decreased the viscosity by half, but not enough to meet the viscosity requirement for ASTM #2 fuel oil. Viscosity contours for water and methanol dilution are shown. The addition of water or methanol or both to make a more consistent product may be advantageous. Aging studies of this low-alkali oil showed a slower increase in viscosity with time equal to one-third the rate of a biocrude oil with higher alkali contents. It appears that removal of the char fines results in a more stable oil. In fact, after 24 hours at 90 o C, the viscosity of this low-ash biocrude oil was lower than that seen previously for the unaged sample of higher ash oil. It is concluded that the removal of char fines to produce a premium biocrude oil will be even more important than was previously supposed.

An NREL-designed vortex reactor fast pyrolysis process development unit (PDU) has been used to investigate hot gas filtration of biomass pyrolysis vapors. Most of the experimental work employed a conventional baghouse type of filter that used NEXTEL™ ceramic cloth filter bags as the filter medium.A series of experimental runs demonstrated that hot gas filtered biocrude oils having less than 10 ppm of total alkali could be reproducibly made. Removal of the char cake from the filter elements proved to be a difficult problem. The char appears to become progressively more sintered to itself and the filter as a function of the cumulative biomass processed. Controlled oxidation does remove this dense char from the filters, but leaves residual ash on the filter cloth fibers. This ash may in turn cause subsequent biomass pyrolysis vapors that pass through the filter to produce additional char (coke) in the interstices of the filter cloth. Data are presented that suggest this char formation may contribute to a more rapid rise in the rate of filter blinding as measured by the increase in recovered filter pressure drop.

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