Pyrolysis Oils from Biomass - American Chemical Society

Chapter 1

Of Biomass, Pyrolysis, and Liquids Therefrom Ed

J.

Soltes

Downloaded by 117.253.160.136 on November 22, 2015 | http://pubs.acs.org Publication Date: September 30, 1988 | doi: 10.1021/bk-1988-0376.ch001

Department of Forest Science, Texas Agricultural Experiment Station, Texas A&M University System, College Station, TX 77843-2135 Renewable biomass (harvest and process residues in forestry and agricultural operations, specific terrestrial or aquatic crops grown for fuel, often animal wastes, refuse derived fiber, etc.) represents an important energy resource in the United States, with future potentials especially important as fossil fuels are depleted (1). Biomass is however generally poorly suited for direct energy use, with pretreatments necessary for altering physical and chemical form. Moisture content can be in excess of 50%, so costs of collection, transportation, and energy conversion become relatively inefficient. Further, biomass availability (e.g., from row crops) is sometimes seasonal, so storage is necessary. As biomass can spoil, it has to be covered and processed soon after receipt. Fortunately, thermal conversion processes are somewhat insensitive to type, form and shape, and can convert biomass into stable, storable and transportable energy forms, and in physical or chemical forms that can be used in higher efficiency energy conversion processes developed for liquid petroleum, coal and natural gas. Under pyrolysis process conditions, a liquid tar (pyrolysis oil) can be produced which can be upgraded to liquid engine fuels (2). Why liquid? It can be argued that the principal advantage of petroleum is that it is a liquid (3): liquid fuels, besides being energy dense, are especially easy to store, transport and meter, thus being really the only choice for transportation fuels. Biomass conversion processes (specifically pyrolysis) which have potential for producing liquid fuels, especially liquid fuels that can be direct replacements for gasoline or diesel engine fuels, are then of much interest. The Nature of Biomass Pyrolysis Biomass thermochemical processes have been studied for at least two reasons: (1) a better understanding of the combustion process to control biomass flammability; and, (2) research into improved processes for converting biomass into useful energy forms. The late Fred Shafizadeh (see, e.g., 4,5) laid the groundwork for all recent studies in both arenas. The work on combustion mechanisms continues at the Wood Chemistry Laboratory of the University of Montana in Missoula (see, e.g., 6). Antal has recently (7,8) reviewed all aspects of biomass pyrolysis, and the reader is directed to his reviews for detail study of the processes involved. Chatterjee has produced an excellent summary (9) of biomass pyrolysis 0097-6156/88/0376-0001$06.00/0 ° 1988 American Chemical Society

In Pyrolysis Oils from Biomass; Soltes, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.


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PYROLYSIS OILS FROM BIOMASS

rocesses relative to application in lesser developed countries. This author as also written several reviews on biomass thermochemical processes, and specifically pyrolysis (2,3,13). Several volumes have been published recently on thermal conversion (10,11), with another on the way for the 1988 Phoenix conference (12). The opening paragraph to this overview chapter relates the need for pyrolysis relative to liquid transportation fuels production. This is a rather specific energy need, and in fact, biomass can serve other energy needs as well. Charcoal from wood is the principal fuel of rural regions of most lesser developed countries. The gas from biomass gasification has found use in the retrofit of natural gas furnaces and engines, and highly efficient cogeneration plants. The energy content of residues from forestry and agricultural operations can often serve on-site process energy needs, but the biomass materials are generally poorly suited for direct use. Modern agricultural and process machinery seldom use solid fuels, therefore pretreatments are often required to change their chemical or physical nature to liquids or gases. Similar considerations apply to the conversion of biomass crops and to the use of biomass fuels off-farm. Thermochemical processes for biomass are basically pretreatment processes, processes that alter the chemical and physical nature of biomass to permit higher efficiency use. Thermochemical processes are often characterized as combustion, gasification, pyrolysis, carbonization or tarification processes. Except for pyrolysis, these names reflect types of products produced. Thermochemical processing is variable and flexible. Depending on conditions used, (primarily the temperature, the oxygen-to-fuel ratio, and residence time at temperature), biomass can be altered very slightly, or be completely changed. These three variables define conditions for pyrolysis, gasification and combustion but there is often little distinction between these processes. In fact, there is a continuum of process conditions. Selection of treatment conditions permits a variety of outcomes of importance to the production of bioenergy products. The mechanisms for the formation of these products are indeed complex, and are still unfolding (see e.g., 7,8,14). Knowledge of these mechanisms has permitted the identification of conditions under which it is possible to produce tars in good yield from large, moist wood particles (15), or novel reactor design for the production and capture of desired tar products (16). Still, gases, liquids and solids are always produced, with relative yields and chemical or elemental compositions dependent on process variables. Definition of Pyrolysis. The word pyrolysis has had some problems in definition, especially when applied to biomass. The older literature generally equates pyrolysis to carbonization, in which the principal p r o duct is a solid char. Today, the term pyrolysis is generally used to describe processes in which liquid oils are preferred products. This symposium was concerned with the latter pyrolysis - processes which offer enhanced yields of liquid oils, especially those with desirable chemical compositions and physical attributes for liquid fuels, fuel supplements and chemical feedstocks. Definition of Pyrolysis Oil vs. Tar. Throughout this volume, "pyrolysis oil", "tar" and "pyrolytic tar" are used almost interchangeably. Tar or pyrolytic tar is a more generic term, now becoming used in reference to its formation in a secondary sense, e.g., as undesired in gasification, or the "creosote tar" of incomplete combustion or the usually


1. SOLTES

Of Biomass, Pyrolysis, and Liquids Therefrom

3


wasted tar byproducts of charcoal manufacture. Oil or pyrolysis oil refers specifically to the liquid product of biomass pyrolysis when the oil is the principal product of interest* Thus, not only "pyrolysis" but also "tar" and "oil" definitions are being cast while research is being conducted in this area. To add to this confusion, this author more than once tried (in retrospect fortunately unsuccessful) to coin the term "tarification" as any pyrolytic process where the product of interest was the liquid tar product! Other Feedstocks and Processes. Although most of the papers in the symposium deal with thermochemical conversion of wood in what may be called the conventional pyrolysis process mode (heating in the absence of air, or under air-starve conditions), other papers were invited which cover the conversion of other biomass materials, such as from municipal solid waste (17) or black liquor (18), or conversion under more novel processing conditions. Mention is made of liquefaction relative to pyrolysis. Although there has not been an attempt to define liquefaction in the literature, to the author's knowledge, it usually refers to the one-step high liquid yield process incorporating heat in combination with reducing gases and/or catalysts (19), but may now also include water-based processes (20). Liquids are also produced in acid solution (21), alkaline solution (22) and in solvolysis (23). Characterization of Biomass Pyrolysis Oils The chemical compositions of pyrolysis oils are very complex. High temperature reactions in the absence of oxygen or under air-starve conditions are not specific, and the availability of sufficient energy for alternative pathways results in a series of complex concurrent and consecutive reactions which provide a wide spectrum of pyrolytic products, usually in small yields (3,14). Most of the literature in the past, including that by the author (13), reflected composition of tars produced as a byproduct of charcoal manufacture. These tars, from different biomass materials, exhibit like chemical compositions. The chars produced under similar carbonization conditions also exhibit similar physical and chemical properties (24). During the last decade or so, pyrolysis process research has confirmed that both the yield and chemical composition of pyrolysis oils are v e r y dependent on reaction conditions (see e.g., 14-16,25,26). All biomass oils are not the same: oil formation conditions determine oil composition. Similarly, it has been reported that different biomass feedstocks pyrolyzed under similar process conditions can give oil products with similarities in chemical composition (e.g., 27-30). Characterization Tools for Pyrolysis Oils. It wasn't too many years ago that the only tools available to the scientist interested in pyrolysis oil composition were gas chromatography and thermogravi-metric analysis. The complexity of the pyrolysis oils demands high performance equipment, and a list of such equipment mentioned during the symposium would include proton and carbon nuclear magnetic resonance spectroscopy, free-jet molecular beam/mass spectrometry (16,25), diffuse reflectance Fourier transform infrared spectrometry (31), photoelectron spectroscopy (31), as well as procedures such as computerized multivariate analysis methods (32) - truly a display of the some of the most sophisticated analytical tools known to man, and a reflection of the difficulty of the oil composition problem.



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PYROLYSIS OILS FROM BIOMASS

Not all scientists have access to this type of equipment, and gas chromatography (GC) is still used extensively, especially in capillary mode (see, e.g., 26,27). The higher resolution of capillary GC is preferred over column chromatography because of the many oxygenated species of similar polarity in pyrolysis oils. Much (usually most) of the pyrolysis oil is also non-volatile, and this causes additional problems for capillary columns: nonvolatiles can cause column deterioration of the stationary phase. It is usually with much trepidation that the average scientist injects for the first time a black, viscous, mostly non-volatile pyrolysis oil into his pristine capillary column (especially when coupled to someone else's mass spectrometer), but be assured that it works, and that the column can live a long and useful life after many such experiences! Column deterioration can be minimized by periodic cleaning or rejuvenation of the column, as well as by occasionally removing the first centimeter or two of the column at the injection end containing the nonvolatiles. Prior separations of complex oils into volatile or functional fractions such as with the use of High Performance Size Exclusion Chromatography (HPSEC) (27,33), or silica gel column chromatography (26) can help the researcher in obtaining more suitable fractions for capillary GC work. Although a number of phases are used in capillary GC for separating the volatiles of pyrolysis oils, the most popular appears to be a bonded silica of moderate polarity, such as the DB-5 column in 30 cm length. HPS EC by itself is becoming increasingly used as a tool for characterizing pyrolysis oils (see, e.g., 27,34). Post-Pyrolysis Processing The utility of biomass pyrolysis oils in any fuel or chemical sense must recognize this complexity in chemical composition, and it is commonly suggested that the tar either be fractionated into simpler mixtures, or be reprocessed or converted into more useful mixtures. Unfortunately, oils usually do not fractionate well (depends on storage conditions and type of oil), and will even undergo changes upon storage, resulting in higher concentrations of non-volatile high molecular weight materials. Extraction techniques work, but these processes are unwieldy. Further, functional fractions are generally poor feedstocks for conversion. Other than a limited amount of work conducted in using the phenolic fractions for adhesives (35-37), most product research has been concerned with reprocessing the oils into a more useful mixture of compounds, usually fuel hydrocarbons. As detailed above, composition of the oils will depend on pyrolysis process conditions. If the oils are primarily phenolic, then hydrotreating (oxygen removal) is necessary to produce hydrocarbon fuels. Single ring phenolics and cyclic ketones present in the biomass pyrolytic oils can be upgraded through deoxygenation to hydrocarbons (a low oxygen content mixture of all types) in the gasoline (19,29) and diesel (29) boiling point ranges. Heavier, higher molecular weight products like the polycyclic aromatics need also be hydrocracked, with at least partial ring saturation prior to cracking. A number of catalysts have been tried, initially at high pressures with typical petroleum hydrotreating or hydrocracking catalysts (19,29), but more recently at lower pressures with acidic zeolites (e.g., 38,39). Kinetics of the hydrotreating/hydrocracking reactions are being studied (33). Other post-pyrolysis process variations include the treating of primary pyrolysis vapors to gasoline hydrocarbons (40), and pyrolysis oil cofed over zeolite catalyst with methanol derived from char gasification


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(41). New characterization tools such as molecular beam mass spectrometry help in understanding the mechanisms by which wood vapor and related compounds react over catalysts.


Prospects for Biomass Pyrolysis It's easy to say that the key to commercial implementation of biomass pyrolysis for tar production will be the identification of economically competitive technology for the production of higher-valued products. As the primary virtues of pyrolysis oils are those attributable to petroleum (liquid fuels and, under some pyrolytic conditions, also olefins), it can be assumed that pyrolysis can become an avenue to petroleum-type products from renewable biomass. Is biomass pyrolysis, coupled with oil upgrading, the renewable route to petroleum? Pyrolysis, after all, allows for the production of biomass-derived fuels in efficient-to-use petroleum forms. Fuels are however not the highest values obtainable from biomass (nor are they from petroleum, coal or natural gas), nor oil the only product of pyrolysis - nor is pyrolysis the only, or even most efficient, biomass conversion process. Perhaps biomass pyrolysis can provide the economic base that would permit the further exploitation of the chemical values of biomass materials to enhance overall process profitability (cf. history of olefins usage in petroleum). This author is bold in suggesting that, more appropriately, pyrolysis should be considered as an upgrading process for residues from chemical/ materials options for biomass, that efficient fuel/energy processes such as pyrolysis be considered in tandem with, if not after, efficient biochemical or chemical processing of biomass for polymer and oxychemical production. Biomass is composed of complex structures and polymers that have merit in their complexity, and usage of biomass should first consider chemicals/ materials production based on this complexity, geared to market demand, with only higher entropy states of biomass, process residues, relegated to fuel use status. Literature Cited 1.

2.

3.

4.

5.

6.

Stevens, D.J. "An overview of biomass thermochemical liquefaction research sponsored by the U.S. Department of Energy." In Production, Analysis and Upgrading of Oils from Biomass, Vorres, K.S., Ed., American Chemical Society, Division of Fuel Chemistry Abstracts, 1987, 32(2), 223. Soltes, E.J. "Thermochemical processes for bioenergy production." In Biomass Energy Development, Smith, W.H., Ed., Plenum Press: New York, 1986, 321. Soltes, E.J. "Thermochemical routes to chemicals, fuels and energy from forestry and agricultural wastes." In Biomass Utilization, Cote, W.A., Ed., Plenum Press: New York, 1983, 537. Shafizadeh, F. "Introduction to pyrolysis of biomass." In Proc. Specialists' Workshop on Fast Pyrolysis of Biomass, Copper Mountain, Diebold, J . , Ed., SERI/CP-622-1096, Solar Energy Research Institute: Golden CO, 1980, 79. Shafizadeh, F. "The Chemistry of Pyrolysis and Combustion." In The Chemistry of Solid Wood, Rowell, R.M., Ed. Advances in Chemistry Series 207, American Chemical Society: Washington DC, 1984. DeGroot, W.F.; Pan, W-P.; Rahman, M.D.; Richards, G.N. "Early products of pyrolysis of wood." In Production, Analysis and Upgrading of Oils from Biomass, Vorres, K.S., Ed., American Chemical Society, Division of Fuel Chemistry Abstracts, 1987, 32(2), 36.


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12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

28.

PYROLYSIS OILS FROM BIOMASS Antal, M.J., Jr. "Biomass pyrolysis: a review of the literature, Part I– carbohydrate pyrolysis." In Adv. in Solar Energy, Boer, K.W.; Duffie, J.W.; Eds., American Solar Energy Society: Boulder CO, 1984, 1, 61. Antal, M.J., Jr. "Biomass pyrolysis: a review of the literature, Part II– lignocellulose pyrolysis." In Adv. in Solar Energy, Boer, K.W.; Duffie, J.W.; Eds., American Solar Energy Society: Boulder CO, 1985, 2, 175. Chatterjee, A.K. State-of-the-Art Review on Pyrolysis of Wood and Agricultural Biomass. Final Report, Contract No. 53-319-R-0-206, AC Project P0380, USDA Forest Service: Washington DC, 1981. Bridgwater, A.V. Thermochemical Processing of Biomass, Butterworths: London, 1984. Overend, R.P.; Milne, T.A.; Mudge, L.K.; Eds. Fundamentals of Thermochemical Biomass Conversion, Elsevier: New York, 1985. Proceedings of the International Thermochemical Biomass Conversion Conference, Phoenix, AR. Elsevier: New York, 1988, in press. Soltes, E.J.; Elder, T.J. "Pyrolysis." In Organic Chemicals from Biomass, Goldstein, I.S., Ed., CRC Press: Boca Raton FL, 1981, 63. Elliott, D.C. "Relation of reaction time/temperature to the chemical composition of pyrolysis oils." In This Volume. Lede, J.; Li, H.Z.; Villermaux, J. "Pyrolysis of biomass: evidences for a fusion-like phenomena." In This Volume. Diebold, J.P.; Scahill, J.W. "Production of primary pyrolysis oils in a vortex reactor." In This Volume. Helt, J.E.; Agrawal, R.K. "Liquids from municipal solid waste." In This Volume. McKeough, P.J.; Johansson, A.A. "Oil production by high-pressure thermal treatment of black liquors: aqueous-phase products." In This Volume. Baker, E.G.; Elliott, D.C. "Catalytic hydrotreating of biomass-derived oils." In This Volume. Boocock, D.G.B.; Allen, S.G.; Chowdhury, Α.; Fruchtl, R. "The production, evaluation and upgrading of oils from the steam liquefaction of poplar chips." In This Volume. Nelson, D.A.; Hallen, R.T.; Theander, O. "Formation of aromatic compounds from carbohydrates: X. Reaction of xylose, glucose and gluconic acid in acidic solution at 300°C." In This Volume. Krochta, J.M.; Hudson, J.S.; Tillin, S.J. "Kinetics of alkaline thermochemical degradation of polysaccharides to organic acids." In This Volume. Bouvier, J.M.; Gelus, M.; Maugendre. S. "Direct liquefaction of wood by solvolysis." In This Volume. Wenzl, H.F.J. The Chemical Technology of Wood. Academic Press: New York, 1970, 267. Piskorz, J.; Scott, J.S.; Radlein, D. "The composition of oils obtained by the fast pyrolysis of different woods." In This Volume. Pakdel, H.; Roy, C. "Chemical characterization of wood oils obtained in a vacuum pyrolysis multiple hearth reactor." In This Volume. Soltes, E.J.; Lin, S-C.K. "Chromatography of non-derivatized pyrolysis oils and upgraded products." In Production, Analysis and Upgrading of Oils from Biomass, Vorres, K.S., Ed., American Chemical Society, Division of Fuel Chemistry Abstracts, 1987, 32(2), 178. Soltes, E.J.; Lin, S-C.K.; Sheu, Y-H.E. "Catalyst specificities in high pressure hydroprocessing of pyrolysis and gasification tars." In Production, Analysis and Upgrading of Oils from Biomass, Vorres,


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29. 30. 31.


32. 33. 34. 35. 36.

37. 38. 39. 40. 41. 42.


K.S., Ed., American Chemical Society, Division of Fuel Chemistry Abstracts, 1987, 32(2), 229. Soltes, E.J.; Lin, S-C.K. "Hydroprocessing of biomass tars for liquid engine fuels." In Progress in Biomass Conversion, Tillman, D.A.; Jahn,E.C., Eds., Academic Press: New York, 1984, 5, 1. Elliott, D.C.; Sealock, J.J., Jr.; Butner, R.S. "Product analysis from direct liquefaction of several high-moisture biomass feedstocks." In This Volume. Grandmaison, J.L.; Ahmed, Α.; Kalaiguine, S. "Solid residues from supercritical extraction of wood: characterization of their constituents." In This Volume. Hoesterey, B.L.; Windig, W.; Meuzelaar, H.L.C.; Eyring, E.M.; Grant, D.M.; Pugmire, R.J. "An integrated spectroscopic approach to the chemical characterization of pyrolysis oils." In This Volume. Sheu, Y-H.E. Kinetic Studies of Upgrading Pine Pyrolytic Oil by Hydrotreatment, Ph.D. Dissertation, Texas A&M University, 1985. Johnson, D.K.; Chum, H.L. "Some aspects of pyrolysis oils characterization by high performance size exclusion chromatography (HPSEC)." In This Volume. Elder, T.J. The Characterization and Potential Utilization of the Phenolic Compounds Found in a Pyrolytic Oil, Ph.D. Dissertation, Texas A&M University, 1981. Chum, H.; Diebold, J.; Scahill.; Johnson, D.K.; Black, S.; Schroeder, H.A.; Kreibich, R.E. "Biomass pyrolysis oil feedstocks for phenolic adhesives". In Adhesives from Renewable Resources, Conner, Α.; Hemingway, R., Eds., American Chemical Society: Washington DC, 1988, in press. Soltes, E.J.; Lin, S-C.K. "Adhesives from natural resources." In Progress in Biomass Conversion, Tillman, D.A.; Jahn,E.C., Eds., Academic Press: New York, 1983, 4, 79. Renaud, M.; Grandmaison, J.L.; Roy, C.; Kaliaguine, S. "Low pressure upgrading vacuum pyrolysis oils from wood." In This Volume. Dao, L.H.; Haniff, M.; Houle, Α.; Lamothe, D. "Reactions of model compounds of biomass pyrolysis oils over ZSM-5 zeolite catalysts." In This Volume. Diebold, J.P.; Scahill, J.W. "Biomass to gasoline (BTG): upgrading pyrolysis vapors to aromatic gasoline with zeolite catalysts at atmospheric pressure." In This Volume. Chen, N.Y.; Walsh, D.E.; Koenig, L.R. "Fluidized bed upgrading of wood pyrolysis liquids and related compounds." In This Volume. Evans, R.J.; Milne, T.A. "Molecular beam mass spectrometric studies of wood vapor and model compounds over HZSM-5 catalyst." In This Volume.

RECEIVED May 26, 1988


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Pyrolysis Oils from Biomass - American Chemical Society

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