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January 2000 • NREL/SR-570-27613 A Review of the Chemical and Physical Mechanisms of the Storage Stability of Fast Pyrolysis Bio-Oils J.P. Diebold Thermalchemie, Inc. Lakewood, Colorado National Renewable Energy Laboratory 1617 Cole Boulevard Golden, Colorado 80401-3393 NREL is a U.S. Department of Energy Laboratory Operated by Midwest Research Institute • Battelle • Bechtel Contract No. DE-AC36-99-GO10337 January 2000 • NREL/SR-570-27613 A Review of the Chemical and Physical Mechanisms of the Storage Stability of Fast Pyrolysis Bio-Oils J.P. Diebold Thermalchemie, Inc. Lakewood, Colorado NREL Technical Monitor: Stefan Czernik Prepared under Purchase Order Number 165134 National Renewable Energy Laboratory 1617 Cole Boulevard Golden, Colorado 80401-3393 NREL is a U.S. Department of Energy Laboratory Operated by Midwest Research Institute • Battelle • Bechtel Contract No. DE-AC36-99-GO10337 NOTICE This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. 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Box 62 Oak Ridge, TN 37831-0062 phone: 865.576.8401 fax: 865.576.5728 email: reports@adonis.osti.gov Available for sale to the public, in paper, from: U.S. Department of Commerce National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 phone: 800.553.6847 fax: 703.605.6900 email: orders@ntis.fedworld.gov online ordering: http://www.ntis.gov/ordering.htm Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste PREFACE This literature review was suggested by the Pyrolysis Network (PyNe) and the National Renewable Energy Laboratory (NREL), as a necessary means to collect and compare the known chemistry and physical mechanisms of the storage instability of bio-oils. Because of the chemical similarities between bio-oils derived by fast pyrolysis with wood distillates and liquid smoke used for flavors, the author expanded the review to include these pyrolysis-derived condensates. ACKNOWLEDGMENTS The financial support to perform this review was provided equally by PyNe (managed by Prof. A.V. Bridgwater, Director of the Energy Research Group, Department of Chemical Engineering and Applied Chemistry, Aston University, Birmingham, UK) and by the Biomass Power Program (managed by Mr. Kevin Craig) at NREL, Golden, CO of the U.S. Department of Energy with purchase order 165134 of June 3, 1999. This support is gratefully acknowledged. The encouragement of Dr. Stefan Czernik of NREL, Mr. Jan Piskorz of Resource Transforms International, Dr. Dietrich Meier of the Institute of Wood Chemistry, and Ms. Anja Oasmaa of the Technical Research Centre (VTT) of Finland is also gratefully acknowledged. ii Contents Page Preface...........................................................................................................................ii Acknowledgments ..........................................................................................................ii Abstract......................................................................................................................... 1 1.0 Introduction .......................................................................................................................... 1 1.1 Storage Stability Problem.......................................................................... 1 1.2 Combustion Problems Caused by Aging or Excessive Heat...................... 5 2.0 2.1 2.2 Composition of Bio-Oils ........................................................................... 5 Organics in Bio-Oil .................................................................................... 6 Inorganics in Bio-Oil .................................................................................. 6 3.0 Probable Chemical Mechanisms of Storage Instability........................................ 11 3.1 Reactions of Organic Acids ..................................................................... 12 3.1.1 Esterification................................................................................ 12 3.1.2 Transesterification ....................................................................... 15 3.2 Reactions of Aldehydes........................................................................... 15 3.2.1 Homopolymerization .................................................................... 15 3.2.2 Hydration ..................................................................................... 16 3.2.3 Hemiacetal Formation.................................................................. 16 3.2.4 Acetalization ................................................................................ 17 3.2.5 Transacetalization ....................................................................... 20 3.2.6 Phenol/Aldehyde Reactions and Resin........................................ 20 3.2.7 Polymerization of Furan Derivatives ............................................ 21 3.2.8 Dimerization of Organic Nitrogen Compounds............................. 21 3.3 Sulfur-Containing Compounds ................................................................ 22 3.4 Unsaturated Organic Reactions .............................................................. 22 3.4.1 Alcohol Addition........................................................................... 22 3.4.2 Olefinic Condensatio ................................................................... 22 3.5 Oxidation................................................................................................. 22 3.6 Gas-Forming Reactions .......................................................................... 23 3.6.1 Carbon Dioxide............................................................................ 23 3.6.2 Hydrogen..................................................................................... 24 3.7 Insights to Be Gained from the Chemical Mechanisms of Aging.............. 24 4.0 Observed Chemical Reactions in Wood Distillates, Wood Smoke, and Bio-Oils............................................................................................................... 26 4.1 Wood Distillates ...................................................................................... 26 4.2 Wood Smoke .......................................................................................... 27 4.3 Bio-Oils ................................................................................................... 27 4.3.1 Aging ........................................................................................... 27 4.3.2 Esterification and Acetalization .................................................... 28 4.3.3 Hydrogenation ............................................................................. 29 4.3.4 Polymerization with Formaldehyde .............................................. 30 4.3.5 Air Oxidation................................................................................ 30 iii 4.3.6 4.3.7 Effect of Entrained Char .............................................................. 30 Off-Gassing during Storage ......................................................... 31 5.0 Methods to Slow Aging in Bio-Oils ...................................................................... 32 5.1 Solvent Addition to Reduce Viscosity and Aging Rates ........................... 32 5.2 Mild Hydrogenation ................................................................................. 34 5.3 Limiting Access to Air and Antioxidants................................................... 35 6.0 Physical Mechanisms of Phase Instability........................................................... 35 6.1 Co-Solvency of Bio-Oil Components ....................................................... 35 6.2 Changes in Mutual Solubility with Aging.................................................. 38 6.3 Micelles, Suspensions, and Emulsions.................................................... 38 6.4 Off-Gassing during Aging ........................................................................ 39 7.0 Comparisons of the Storage Instability Mechanisms of Bio-Oils and Petroleum Oils .................................................................................................... 40 8.0 Summary ............................................................................................................ 41 9.0 Conclusions and Recommendations................................................................... 42 10.0 References ......................................................................................................... 43 Figures Figure 1. Aging of Bio-Oils at 35ºC to 37ºC................................................................. 2 Figure 2. Effect of Measurement Temperature on Apparent Aging of Poplar Hot-Gas Filtered Bio-Oil ................................................................ 2 Figure 3. Rate of Viscosity Increase with Temperature during Storage of Bio-Oils...... 4 Figure 4. Viscosity and Molecular Weight after Aging of a Bio-Oil Made from Oak...... 4 Figure 5. Hydrolysis Rate of Ethyl Acetate and pH.................................................... 14 Figure 6. Calculated Equilibrium Composition of Pseudo Bio-Oil with Water Content at Start of Storage ........................................................................ 25 Figure 7. Calculated Equilibrium Composition of Pseudo Bio-Oil with Added Methanol .................................................................................................... 26 Figure 8. Noncatalytic Esterification in Whole Smoke Condensate at 25ºC............... 28 Tables Table 1. Compounds Identified in Bio-Oils and Similar Pyrolysis Products ............... 7-8 Table 2. Inorganic Compositions of the Chars and Bio-Oils Made from Various Biomass Feeds at NREL with Char Removal by Cyclones or Filtration........ 10 Table 3. Normal Boiling Points of Probable Alcohols, Acids, and Esters in Bio-Oils, Liquid Phase, and Vapor Phase Equilibrium Constants for Ester Formation from Alcohol and Organic Acids at 25ºC....................... 13 Table 4. Equilibrium Constants for Liquid Acetal Formation (at 25ºC) and Normal Boiling Point of Resulting Acetals ................................................................ 19 iv Table 5. Effect of Adding Solvents on Aging Rates ................................................... 33 Table 6. Hansen Solubility Parameters for Solvents in Bio-Oil of Potential Interest to Bio-Oil Producers........................................................................ 37 Table 7. Hansen Solubility Parameters for Polymers Possibly Relevant to Bio-Oils.................................................................................................... 38 v A REVIEW OF THE CHEMICAL AND PHYSICAL MECHANISMS OF THE STORAGE STABILITY OF FAST PYROLYSIS BIO-OILS ABSTRACT Understanding the fundamental chemical and physical aging mechanisms is necessary to learn how to produce a bio-oil that is more stable during shipping and storage. This review provides a basis for this understanding and identifies possible future research paths to produce bio-oils with better storage stability. Included are 108 references. The literature contains insights into the chemical and physical mechanisms that affect the relative storage stability of bio-oil. Many chemical reactions that are normally thought to require catalysis, proceed quite nicely without them (or with catalysts indigenous to the bio-oil) during the long reaction times available in storage. The literature was searched for information about the equilibrium constants and reaction rates of selected aging mechanisms, to determine whether they apply to storage times. The chemical reactions reported to occur in pyrolytic liquids made from biomass are presented. As the bio-oil composition changes during aging, the mutual solubility of the components changes to make phase separation more likely. With these insights into the aging mechanisms, the use of additives to improve storage stability is examined. Comparisons are then made to the storage stability of petroleum fuels. The review is summarized, conclusions are drawn, and recommendations are made for future research to improve the storage stability of bio-oils. 1.0 INTRODUCTION 1.1 Storage Stability Problem Pyrolysis of biomass under conditions of rapid heating and short reactor residence times can produce a low-viscosity, single-phase pyrolysis liquid (bio-oil) in yields reportedly higher than 70%. Most projected uses of bio-oil require that it retain these initial physical properties during storage, shipment, and use. Unfortunately, some bio-oils rapidly become more viscous during storage. Figure 1 shows this increase for three bio-oils made from three hardwoods using different pyrolysis conditions, after aging 3 months at 35ºC to 37ºC. These three bio-oils exhibit very different initial viscosities and rates of viscosity increase. Figure 2 shows the effect of temperature on viscosity for three samples of a bio-oil made from poplar that had been aged at 90ºC for 0, 8, and 20.5 hours. Aging effectively shifts the viscosity curve to the right on the temperature axis, resulting in higher viscosities. The effect of aging on viscosity is greater at lower measurement temperatures (Diebold and Czernik 1997). In this example, the change in viscosity appears to be about twice as high, if measured at 40ºC rather than 50ºC. At the higher measurement temperature of 70ºC, the effects of aging amount to an increase of only a few centipoise (mPas). The measurement temperature is usually chosen to compare to petroleum fuel oil specifications (e.g., 40ºC in the United States, 40ºC and 50ºC have been used in Finland). 1 300 Czernik et al. (1994) meas. @ 40°C Oasmaa and Sipilä (1996) meas. @ 50°C Diebold and Czernik (1997) meas. @ 40°C 200 150 100 50 0 0 20 40 60 80 100 Time, days Figure 1. Aging of Bio-Oils at 35°C to 37°C (cP = mPas) 350 300 250 Viscosity, cP Viscosity, cP 250 20.5 h at 90°C 200 8 h at 90°C 150 0 h at 90°C 100 50 0 20 30 40 50 60 70 Temperature of Viscosity Measurement, °C Figure 2. Effect of Measurement Temperature on Apparent Aging of Poplar Hot-Gas Filtered Bio-Oil (Diebold and Czernik 1997) 2 The aging effects occur much faster at higher temperatures. Figure 3 shows that the viscosity increase rate of the hardwood bio-oils (shown in Figure 1) and for a softwood bio-oil varied more than four orders of magnitude, from 0.009 cP/day at –20ºC to more than 300 cP/day at 90ºC. This is approximately a doubling of the viscosity increase rate for each 7.3ºC increase in storage temperature. The aging rate of softwood bio-oil is about the same as for hardwood bio-oils at 20ºC, with some possible differences at lower storage temperatures. However, the viscosity change during aging is very small (below 20ºC), making low-temperature aging rates subject to measurement errors. Because the viscosity change rates may be represented as Arrhenius exponential functions of the inverse of absolute temperature, chemical reactions appear to be involved. Figure 3 shows that the bio-oils must be cooled quickly after being produced and then stored at low temperatures to maintain their low viscosity. The pyrolysis oils referred to in Figure 3 initially contained 10 to 21 wt % water. A loss of volatiles will increase the viscosity of bio-oil, so the bio-oils shown in Figures 1-4 were carefully aged in sealed containers to prevent such losses. Using gel permeation chromatography with ultraviolet detection of the aromatic compounds, the weight-average molecular weights of the aromatic compounds in aged bio-oils made from oak were determined (Czernik et al. 1994). Figure 4 shows that molecular weight correlated very well with viscosity during aging, in this case with a linear-regression R2 value of 0.96, for all aging data at 37ºC, 60ºC, and 90ºC treated as one data set. (The regression R2 values are slightly improved if the data set is divided into three sets, one for each aging temperature.) Figure 4 strongly implies that if a pyrolysis process more thoroughly cracks the bio-oil to lower molecular weights, the initial viscosity is desirably lower. Thus, partially pyrolyzed particles and droplets must not be entrained prematurely from the reactor system, because if they are soluble in the bio-oil, they will cause the molecular weight and viscosity to increase. During aging, chemical reactions, which apparently increase the average molecular weight, take place in bio-oil. Based on the good correlation for the aging data treated as one data set, relatively similar chemical reactions appear to occur over this temperature range. This is the basis for conducting accelerated aging research at elevated temperatures and then applying the results to predict storage of biooils at lower temperatures. The advantage of accelerated aging tests is the short time required to demonstrate the aging properties of a particular bio-oil. Bio-oil is not a product of thermodynamic equilibrium during pyrolysis, but is produced with short reactor times and rapid cooling or quenching from the pyrolysis temperatures. This produces a condensate that is also not at thermodynamic equilibrium at storage temperatures. Bio-oil contains a large number of oxygenated organic compounds with a wide range of molecular weights, typically in small percentages. During storage, the chemical composition of the bio-oil changes toward thermodynamic equilibrium under storage conditions, resulting in changes in the viscosity, molecular weight, and co-solubility of its many compounds. In addition to simple viscosity increases, the single-phase bio-oil can separate into various tarry, sludgy, waxy, and thin aqueous phases during aging. Tarry sludges and waxes still in suspension have caused rapid plugging of fuel filters. They can form during storage in previously filtered bio-oils and in aqueous phases. Bio-oils seem to be more unstable during storage than are petroleum-derived fuel oils, although there appear to be many similarities in their mechanisms. 3