(97e) Intermediate Temperature Catalytic Reforming of Bio-Oil for Distributed Hydrogen Production

With the world's energy demands rapidly increasing, it is necessary to look to sources other than fossil fuels, preferably those that minimize greenhouse emissions. One such renewable source of energy is biomass, which has the added advantage of being a near-term source of hydrogen. While there are several potential routes to produce hydrogen from biomass thermally, given the near-term technical barriers to hydrogen storage and delivery, distributed technologies such that hydrogen is produced at or near the point of use are attractive. One such route is to first produce bio-oil via fast pyrolysis of biomass close to its source to create a higher energy-density product, then ship this bio-oil to its point of use where it can be reformed to hydrogen and carbon dioxide. This route is especially well suited for smaller-scale reforming plants located at hydrogen distribution sites such as filling stations. There is also the potential for automated operation of the conversion system.²

A system has been developed for volatilizing bio-oil with manageable carbon deposits using ultrasonic atomization and by modifying bio-oil properties, such as viscosity, by blending or reacting bio-oil with methanol. Non-catalytic partial oxidation of bio-oil is then used to achieve significant conversion to CO with minimal aromatic hydrocarbon formation by keeping the temperature at 650°C or less and oxygen levels low. The non-catalytic reactions occur primarily in the gas phase. However, some nonvolatile components of bio-oil present as aerosols may react heterogeneously. The product gas is passed over a packed bed of precious metal catalyst where further reforming as well as water gas shift reactions are accomplished completing the conversion to hydrogen.⁴

The approach described above requires significantly lower catalyst loadings than conventional catalytic steam reforming due to the significant conversion in the non-catalytic step. The goal is to reform and selectively oxidize the bio-oil and catalyze the water gas shift reaction without catalyzing methanation or oxidation of CO and H₂, thus attaining equilibrium levels of H₂, CO, H₂O, and CO₂ at the exit of the catalyst bed. Experimental

Bio-oil (mixed with varied amounts of methanol to reduce the viscosity and homogenize the bio-oil) or selected bio-oil components (such as methanol or HAA) are introduced at a measured flow rate through the top of a vertical quartz reactor which is heated using a five zone furnace. The ultrasonic nozzle used to feed the reactants allows the bio-oil to flow down the center of the reactor at a low, steady flow rate. Additionally, the fine mist created by the nozzle allows for intimate mixing with oxygen and efficient heat transfer, providing optimal conditions to achieve high conversion at relatively low temperatures in the non-catalytic step thus reducing the required catalyst loading. Generation of the fine mist is especially important for providing good contact between non-volatile bio-oil components and oxygen. Oxygen and helium are also delivered at the top of the reactor via mass flow meters with the amount of oxygen being varied to maximize the yields of H₂ and CO and the amount of helium being adjusted such that the gas phase residence time in the hot zone is ~0.3 and ~0.45 s for bio-oil and methanol experiments, respectively.

A catalyst bed can be located at the bottom of the reactor tube. To date, catalyst screening experiments have used Engelhard noble metal catalysts. The catalysts used for these experiments were 0.5 % rhodium, ruthenium, platinum, and palladium (all supported on alumina).  Experiments were performed using pure alumina as well.  Both the catalyst type and the effect of oxygen and steam on the residual hydrocarbons and accumulated carbon containing particulates were investigated.  The residence time before the catalyst is varied to determine the importance of the non-catalytic step and its potential effect on the required catalyst loading. Non-catalytic experiments (primarily homogeneous cracking) use a bed of quartz placed to capture any deposits that are formed in the volatilization and cracking zones.

The inner reactor effluent is quenched by a flow of 10 SLPM He which serves to sweep the products quickly (~0.03 s) to a triple quadrupole molecular beam mass spectrometer (MBMS) for analysis.  The MBMS serves as a universal detector and allows for real time data collection.  The study of pyrolysis by MBMS has been described previously.⁵ The dilution of the reactor effluent reduces the potential problems caused by matrix effects associated with the MBMS analysis. Argon is used as an internal standard in the quantitative analysis of all the major products (CO, CO₂, H₂, H₂O, and benzene) as well as any residual carbon, which is determined by subsequent oxidation of carbon (monitored as CO₂) after shutting off the feed and maintaining the oxygen/helium flow.  Any carbon left on the walls of the reactor (measured by weighing) is also counted as residual carbon.

Results and Discussion

Non-Catalytic Partial Oxidation. To date, experiments have been performed using 50:50 and 70:30 bio-oil:methanol (wt. %) mixtures at 650°C. Earlier experiments showed that temperatures above 600°C yielded the most promising results. However, it should be noted that a significant amount of bio-oil conversion occurs at temperatures as low as 550°C.

The maximum CO yields from the bio-oil (expressed as percentage of carbon converted to CO after the CO attributed to methanol is subtracted out) for the 50:50 mixture (~50 %) and the 70:30 mixture (~55 %) were remarkably similar and occurred at nearly the same effective molar oxygen to carbon (O:C) ratio (~1.5).   The effective O:C ratio does not include the oxygen due to the water content of the bio-oil is subtracted out (since water is an inert under these conditions).

The maximum overall H₂ yields calculated as percent of the amount of hydrogen that could be produced by stoichiometric conversion of bio-oil to CO₂ and H₂ (the H₂ attributed to methanol is included) for the 50:50 (~23 %) and 70:30 (~24 %) mixtures are nearly identical and occur at an effective O:C ratio of ~1.5. It should be noted that the maximum H₂ and CO yields occurred at the same O:C ratios.

Catalytic Auto-thermal Reforming.Limited catalyst screening has identified rhodium supported on alumina as the most promising catalyst since it yields results (~84% yield for H₂ and ~61% yield for CO) that come closest to equilibrium predictions of 80% H₂ yield and 69% CO yield (see Table 1).  The other catalysts used fell well short of equilibrium (28 to 34% H₂ yield and CO yield ranging from 45 to 51%).  However, the use of any of the catalysts does result in a significant increase in the H₂ and CO (13% H₂ yield and 39% CO yield).  Methanol conversion also increases significantly from 49% in the non-catalytic experiments to 100% (the value predicted by equilibrium) for the rhodium catalyst.  The other catalysts yielded methanol conversions ranging from 75 to 88%.  Hence, the catalyst serves a dual purpose of increasing the conversion of the components of the bio-oil as well as catalyzing the water gas shift reaction.

It should be noted that the catalysts used were chosen because they were on hand and no efforts have been made to date to optimize the catalyst size or loading. Future experiments will involve exploring the optimal size and loading of the catalyst as well as determining how effective the non-catalytic partial oxidative step is at reducing the catalyst loading.  A suitable catalyst that will minimize or eliminate the use of expensive rhodium is highly desirable and efforts will be made to discover such a catalyst.

The non-catalytic results described above will be talked about briefly as these results will be discussed in great detail in a separate presentation.  The focus of this presentation will be on the catalytic results.

References

(1)   Milbrandt A., A Geographic Perspective on the Current Biomass Resource Availability in the United States; National Renewable Energy Laboratory: Golden, CO, TP-560-39181, 2005.

(2)   Czernik, S.; Elam, C.; Evans R.; and Milne, T., in Thermal and Chemical Biomass Conversion; Bridgwater, A.V. and Boocock D.G.B., eds.; CPL Press: Newbury, UK, 2006; pp.1752-1761.

(3)   U.S. Department of Energy: Hydrogen, Fuel Cells and Infrastructure Technologies Program; Multi-Year Research, Development and Demonstration Plan, Section 3.1 Hydrogen Production; U.S. Department of Energy: Office of Energy Efficiency and Renewable Energy: Washington, D.C, 2006.

(4)   Evans, R.J.; Czernik, S.; French, R.; and Marda, J.; Distributed Bio-Oil Reforming, DOE Hydrogen Program FY2006 Annual Progress Report, 2006.

(5)   Evans, R. J.; and Milne, T. A., Energy & Fuels, 1, 1987, 123-137.