PARTIAL HYDROGENATION OF BIOMASS PYROLYSIS OILS TO LIQUID FUEL INTERMEDIATES

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1 PARTIAL HYDROGENATION OF BIOMASS PYROLYSIS OILS TO LIQUID FUEL INTERMEDIATES Richard J. French National Renewable Energy Laboratory 1617 Cole Blvd, MS 3322 Golden, CO richard ABSTRACT Wood, crop residues, and other organic materials may be flash-pyrolyzed by rapid heating to C followed by rapid cooling to produce a high yield (70wt%) of a combustible liquid called pyrolysis oil. This liquid is a poor fuel because of its high concentration of oxygen, water, acid and reactive functional groups. It can be upgraded to a fuel-like hydrocarbon via hydrogen and a catalyst in a process similar to petroleum hydrotreatment. However, hydrogen and transportation costs are high. Costs may be reduced by performing a partial hydrogenation to produce a liquid with acceptable physical and chemical properties acid number < 2, volatility > 90%, and miscibility with petroleum (py-oil:petroleum ~1:9) and completing the upgrading in a petroleum refinery where the refinery s economies of scale and existing infrastructure can be used to advantage. Results will be presented showing that a material suitable for feeding to a refinery can be produced with about one-half the hydrogen consumption of a deep hydrogenation and that acidity can be reduced by washing. 1. INTRODUCTION Concerns over global climate change and economic and socio-political issues associated with energy security and wealth transfer have highlighted the need to develop renewable and sustainable technologies for the manufacture of liquid transportation fuels. It is estimated that the United States alone can sustainably produce a billion tons of biomass per year that could be used to make cellulosic biofuels. 3 Biomass-derived fuels can, in principle, be carbon-neutral where the carbon dioxide released from the process and final products is compensated for by the carbon captured in the next year s crop. Such renewable carbonneutral fuels are a goal of the US government. 1,4 If biomass is heated rapidly (ΔT/Δt 1000 K/s) in the absence of oxygen to temperatures in the range of C (a process known as fast pyrolysis), a combustible liquid (bio-oil) retaining ca. 75% of the energy content of the feed material is produced in high yield. Though superficially resembling a heavy fuel oil, this liquid contains about 50% oxygen, 15-30% water, and has many undesirable physical and chemical properties when compared to petroleum liquids. Bio-oil is corrosive, only partly volatile, and largely immiscible with hydrocarbons. 5,6,7,8 Bio-oil can be converted to a gasoline- or diesel-like liquid by catalytic hydroprocessing using catalysts and conditions that are very similar to those used in petroleum hydrodesulfurization, hydrotreating and hydrocracking processes. A recent review highlights the history and current status of this technique. 7 When this approach was applied to fast pyrolysis oil, the oil was found to coke severely in the original single-stage process, 6 hence a twostage process was developed. 9,10 In this improved process, the oil was stabilized at a lower temperature ( C) before it was fed to a high temperature reactor ( C) where the majority of the oxygen removal took place. Standard petroleum-industry hydrotreating catalysts were 1

2 used including both nickel-molybdenum (NiMo) and cobalt-molybdenum (CoMo) on γ-alumina support. A DOE design report for the production of finished fuels via fast pyrolysis followed by hydrotreating showed the potential for producing the fuels at cost-competitive prices. 11 The oil was hydrotreated in two stages at temperatures of C and psig pressure to produce hydrotreated oil containing 1.5% oxygen with a yield of 44%. Hydrogen consumption was assumed to be 5 wt% of the feed. This product oil was then hydrocracked as necessary and separated into gasoline and diesel streams. While these projections are promising, other studies have concluded that the process is too expensive to compete with low-cost oil because of the large amount of hydrogen consumed, low product yields, low quality products that would require further upgrading in a refinery, and the corrosiveness of the raw oil. 12,13 The Global Energy Management Institute at the University of Houston carried out an assessment of pyrolysis oil upgrading. They found the upgrading costs very large compared to petroleum refining costs. 14 Costs could be significantly reduced by mild hydrotreating of the bio-oil and then co-processing the partially deoxygenated products with petroleum-derived material in a refinery, thus taking advantage of the economies of scale and existing, highly efficient infrastructure of refineries. Therefore the authors recommended reducing the severity of hydrotreating to leave about 7% oxygen in the bio-oil, thus avoiding hydrogenating aromatics while reducing hydrogen consumption, catalyst costs, and hydrotreater capital costs. The residual acidity of the oil could then be accommodated by diluting with crude oil or an internal refinery stream (naphtha, gas oil, etc.). For this strategy to work, a number of important criteria must be met by the products from mild hydrotreating: 1) the acidity of the bio-oil must be reduced from the typical TAN (total acid number) value of over to about 15, assuming that hydrotreated bio-oil would be blended in a 1:8 ratio (acidity of blend less than 2 mg- KOH/g-oil) 15 2) the hydrotreated bio-oil must be miscible with hydrocarbons 3) the hydrotreated bio-oil must be highly volatile so that it is amenable to fractional distillation (some high-boiling residue is acceptable) The goal of NREL hydrotreating work is to reduce costs by optimizing conditions for producing oils that can be introduced into refineries for further processing. During We performed a parametric screening for hydrotreating to define the range of temperature, pressure and hydrogen flow under which oils with suitable properties can be produced. 17 The catalyst studied was NiMo/γ-Al 2 O 3. The oxygen content and volatility of the upgraded oil was in the range of interest at all conditions tested C stabilization temperature, C final temperature, and psig pressure (70-170Bar). The destruction of acid was satisfactory at the highest pressure, but at low pressure the acid was physically removed in the exit gas stream before it could react. While nickel catalyst performed satisfactorily in short-term experiments and the high rate of decarboxylation of organic acids, which may be advantageous in this work, the alumima support is not stable in this aqueous environment and they may be too selective for aromatic ring hydrogenation, which leads to excessive hydrogen consumption 22,23 and thus higher cost. Precious metal catalysts on carbon, silica-alumina, or zirconia support have been suggested as promising alternatives for the traditional catalysts as these supports are more stable at these conditions than alumina. 18,19,20,25 They may also be active for bio-oil deoxygenation at lower temperatures and pressures than standard hydrotreating catalysts. 21 Thus the objective of the series of experiments reported here was to evaluate the impact of catalyst type (precious metals versus traditional NiMo catalyst) on the hydrotreating process. Bio-oil was hydrotreated in the presence four precious metal catalysts at varying temperatures and pressures and the results were compared to those of a more standard NiMo catalyst. The aim was to produce oil suitable as refinery-ready intermediate at a carbon conversion of 55% or above. The oil quality criteria were good volatility (>90% volatile matter), low oxygen content (<10%), low TAN (<15, to give an acid number after blending < 2), and a miscibility of at least 1:10 in representative hydrocarbons. 2. EXPERIMENTAL A 1-L stirred autoclave (Autoclave Engineers) was used. It was configured to operate in a semibatch mode (see Figure 2

3 1) the catalyst and oil were a single charge but hydrogen gas was flowed through the reactor continuously. The oil used in the study was produced from white oak during 2008 by entrained flow pyrolysis at 550 C in the NREL Thermochemical Process Development Unit. The residence time in the entrained-flow reactor was ~0.5 s. Proximate and ultimate analyses of the oil (wet basis) gave a water content of 32.8%, carbon, hydrogen, nitrogen, and sulfur contents of 41.7, 3.9, 0.07, and 0.02% respectively. Organic oxygen was 19.9% by difference, volatile matter was 53% and fixed carbon 14%. The carboxylic acid number by the method of Nicolaides 24 was equivalent to 87 mg-koh)/g-oil. The catalysts were 5% Pd, Pt, or Ru on powdered char or carbon supports, provide by Johnson Matthey. The Ru catalyst was a paste containing 45% water. The catalysts were reduced by heating to the final operating temperature under hydrogen. NiMo catalyst on a high-surface alumina support (5% NiO, 25% MoO 3 )was provided by Grace Davison. The catalyst was milled to a fine powder then pre-sulfided in H 2 S in a bench-scale fixed-bed up-flow reactor. 16 The oil (300 g) and catalyst (30 g) were heated together under a pressure of flowing hydrogen first to a moderate stabilization temperature to stabilize reactive moieties that tend to cause severe polymerization and coking. Then it was heated further, at which point the bulk of the water was removed, and allowed to hydrogenate further at a higher final temperature. Three conditions of stabilization temperature, final temperature and pressure were used C, 340 C, 70 bar (low severity); 215 C, 370 C, 120 bar (medium severity); and 280 C, 400 C, 170 bar (high severity). During the experiments, the content of carbon monoxide and carbon dioxide were monitored via non-dispersive infrared spectrometers (California analytical instruments NDIR 100). Also, gas samples were collected and analyzed by GC (Varian 4600 micro GC with MS5A, CP-sil, and PBQ columns) for C1-C4 hydrocarbons. The aqueous and organic liquids collected in the high-pressure condensers (Fig. 1) and the liquid and solid residues left in the autoclave were analyzed for acid (Nicolaides 24 ); C, H, N, S, and direct oxygen (ultimate analysis) and volatiles (proximate analysis). Miscibility with hydrocarbons was assessed by mixing the organic phases in ratios of 9:1 and 1:1 in toluene or heptanes and looking for phase separation or cloudiness. 3. RESULTS The mass balances for the experiments range from %. The aqueous phase was the largest product (41-66wt%) while the gas was relatively minor ( %) except when platinum catalyst was used (16-22%). The total yields of organic liquids ranged from 23-43%. Coke yield (on the catalyst) was % and hydrogen consumption was % except for platinum where it was %. The miscibility tests, while only qualitative, indicated that for all catalysts at low severity the miscibility was poor, while at high severity it was satisfactory. Key results are shown below in Figure 2. Desired values are indicated by dashed lines. Oxygen concentrations were close to the desired value for most moderate and severe conditions while acid contents were rather high except for a few severe conditions. Carbon-to-liquid yields were close to the desired value for most experiments. Since the precious metal catalysts all produced oil with acidities that were higher than desired, we tested water washing as a means to reduce the oil acidity. The results for the organic condensates B and C (Fig. 1) from the Pd/char catalyst experiment performed at the severe condition are shown in Table 1. These condensates were collected as the temperature was increasing from 280 to 400C and the water and acetic acid were evaporating out of the reactor. These liquids had higher acid content than the other liquids collected during this experimental run. This run gave a good yield of organic liquid and good miscibility but the acidity was high. The results show there is a substantial improvement in the acidity with low loss of the organic phase. This is a promising improvement and refinement of this procedure may well produce oil with the desired level of acidity. 4. DISCUSSION AND CONCLUSIONS These results show that the volatility and miscibility criteria and the desired oxygen content can be achieved with several of the catalysts. The severe condition with the nickel catalyst also met the acidity target. Hydrogen consumption was considerably less ( % vs. 5%) than that used in the design case. Thus, it is reasonably expected 3

4 that a product satisfactory for blending into a refinery can be achieved at a yield (based on carbon) of 55%. The precious metal catalysts may also produce a satisfactory product by water washing of the product to remove acid. Thus two possibilities are suggested for an improved process the use of nickel on a stable support or the use of a precious-metal catalyst followed by water washing of the light product. Platinum has added promise because of its reduced hydrogen consumption. Pressure letdown & instruments Bottled H slm Bio-oil/catalyst slurry (batch) Autoclave (CSTR) A B C D GAS ORG AQ C Bar High P condenser manifold 5-15 C Figure 1. Diagram of Semibatch Autoclave Apparatus 4

5 Figure 2. Summary of Key Results Table 1. Effect of Water Washing on Acidity of Organic Condensates Condensates B C Original CAN Water Added/Organic Liq. (g/g) After washing Organic phase (% or org. liquid) Aqueous phase (% of water addition) CAN in the aqueous phase CAN in the organic phase

6 5. ACKNOWLEDGEMENTS This work would not have been possible without the dedicated help of James Stunkel, Stuart Black and Michele Myers as well as the support of Kristiina Iisa, Robert Baldwin and Adam Bratis, and financial support from the Department of Energy. REFERENCES (1) U.S. Department of Energy, Biomass Multi-Year Program Plan 2011, Office of the Biomass Program, Energy Efficiency and Renewable Energy. Available at pdf (2) Beaudry-Losique, J. Growing a Robust Biofuels Economy, Venture Capital Forum. August 21-22, Available at mass.pdf (3) U.S. Department of Energy U.S. Billion-Ton Update: Biomass Supply for a Bioenergy and Bioproducts Industry. R.D. Perlack and B.J. Stokes (Leads), ORNL/TM-2011/224. Oak Ridge National Laboratory, Oak Ridge, TN. 227p. Available at (4) U.S. Environmental Protection Agency, EPA Proposes New Regulations for the National Renewable Fuel Standard Program for 2010 and Beyond 2009, Available at (5) Czernik, S., Bridgwater, A.V. Overview of Applications of Biomass Fast Pyrolysis Oil. Energy & Fuels 2004, 18, (6) Elliott, D. C., Baker, E. G. Catalytic Hydrotreating of Biomass Liquefaction Products to Produce Hydrocarbon Fuels: Interim Report 1986, Pacific Northwest National Laboratory, Richland, WA (7) Elliott, D. C. Historical Developments in Hydroprocessing Bio-oils. Energy & Fuels 2007, 21, (8) Conti, L., Scano, G., Boufala, J., Mascia, S. Experiments of bio-oil hydrotreating in a continuous bench-scale plant. Bio-Oil Prod. Util., Proc. EU-Can. Workshop Therm. Biomass Process 1996, (9) Baker, E. G., Elliott, D. C. Catalytic upgrading of biomass pyrolysis oils. In A.V. Bridgwater, J.L. Kuester, (Eds.), Research in Thermochemical Biomass Conversion 1988, pp , Barking, England: Elsevier Science Publishers, LTD (10) Baker, E. G., Elliott, D. C. Method of Upgrading Oils Containing Hydroxyaromatic Hydrocarbon Compounds to Highly Aromatic Gasoline. US Patent 5,180,868, 1993 (11) Jones, S., Valkenburg, C., Walton, C., Elliott, D., Holladay, J., Stevens, D., Kinchin, C., Czernik, S. Production of Gasoline and Diesel from Biomass via Fast Pyrolysis, Hydrotreating and Hydrocracking: A Design Case, Report PNNL Rev , Pacific Northwest National Lab, Richland, WA (12) Baldauf, W., Balfanz, U. Upgrading of fast pyrolysis liquids at Veba Oel AG, Biomass Gasif. Pyrolysis 1997, (13) Marker, T. L. Opportunities for Biorenewables in Oil Refineries, Final Technical Report 2005, DOE Contract number DE-FG36-05GO15085 (14) Arbogast, S. V. Preferred Paths for Commercializing Pyrolysis Oil at Conventional Refineries, Deliverable 1, Quality Considerations for the Transport and Processing of Pyrolysis Oil in Existing Petroleum Refineries 2008, Houston, TX: Global Energy Management Institute (15) Fu, X., Dai, Z., Tian, S., Long, J., Hou, S., Wang, X. Catalytic Decarboxylation of Petroleum Acids from High Acid Crude Oils over Solid Acid Catalysts, Energy & Fuels 2008, 22, (16) French, R. J., Stunkel, J., Baldwin, R. M., Mild Hydrotreating of Bio-Oil: Effect of Reaction Severity and Fate of Oxygenated Species, Energy & Fuels 2011, 25, 3268 (17) French, R. J., Stunkel, J., Baldwin, R. M., Mild Hydrotreating of Bio-Oil: Effect of Reaction Severity and Fate of Oxygenated Species, Energy & Fuels 2011, 25, (18) Douglas C. Elliott, D., C., Hart, R. H., Catalytic Hydroprocessing of Chemical Models for Bio-oil, Energy & Fuels 2009, 23, (19) Wildschut, J., Farchad H. M.,, F. H., Venderbosch, R. H., Heeres, H., J., Hydrotreatment of Fast Pyrolysis Oil Using Heterogeneous Noble-Metal Catalysts, Ind. Eng. Chem. Res. 2009, 48, (20) Mercader, F. d M.,, Groeneveld, M.J., S.R.A. Kersten, S.R.A.,, N.W.J. Way, N.W.J., Schaverien, C.J., Hogendoorn,, J.A. Production of advanced biofuels: Coprocessing of upgraded pyrolysis oil in standard refinery units, Applied Catalysis B: Environmental, 2010, 96,

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