AN AMERICAN CHEMICAL SOCIETY JOURNAL VOLUME 1, NUMBER 4
JULYIAUGUST 1987
Q Copyright 1987 by the American Chemical Society
Articles Molecular Characterization of the Pyrolysis of Biomass. 2. Applications Robert J. Evans and Thomas A. Milne* Solar Energy Research Institute, Golden, Colorado 80401 Received September 29, 1986. Revised Manuscript Received March 9, 1987
The determination of the underlying chemistry in thermochemical conversion systems is generally difficult because of the reaction conditions and the complex nature of the products. Part 1of this series described the technique of free-jet molecular-beam mass spectrometric sampling (MBMS) for real-time studies of pyrolysis at high temperature and atmospheric pressure. This approach allowed the observation of the fundamental transformations that occur in solid-phase (primary) pyrolysis and the subsequent sequential (secondary and tertiary) gas-phase transformations as a function of temperature and residence time. In this paper, MBMS is used within that mechanistic perspective to investigate several aspects of pyrolysis that are pertinent to practical conversion processes. The chemical nature of the collected oil is characterized, and four major types of pyrolysis liquids are identified and discussed. The effect of primary pyrolysis parameters on oil composition is discussed, and results show that physical effects such as particle size, moisture content, and heating rate have only secondary effects on the composition of the oil although the effect on yield may be dramatic. The gas-phase chemistry of conversion practices has been investigated from two perspectives: (1) attempts to upgrade the primary products such as continued pyrolysis in reactive environments such as methane and direct, low-pressure catalytic upgrading to aromatic gasoline; (2) the nature of emissions from high-temperature conversion devices such as gasifiers and combustors. Gas-phase pyrolysis of wood-derived vapors in methane has shown that olefins are enhanced, in accordance with literature reports. The direct catalytic upgrading of the wood-derived vapors over zeolites has shown the posssibility of complete conversion of the primary products, although the yield of organic products has not yet been determined. The direct sampling of updraft and downdraft gasifier effluents has shown the nature of the condensible products, with the updraft producing primary oils and the downdraft producing tertiary, aromatic tars. A summary of the implications and future applications of this technique is given.
Introduction In a companion paper1 the technique of free-jet molecular-beam mass spectrometric sampling (MBMS) was described. Its use in elucidating the molecular pathways in the primary and secondary pyrolysis of biomass and its major components was presented. This unique apparatus allows the observation of changes in product composition as a function of reaction conditions. The analysis of pyrolysis products is difficult because of the reaction conditions, the distinction between solid-phase and gas-phase (1) Evans, R.J.; Milne, T . A. Energy Fuels 1987,1, 123-137.
processes, and the complexity of the products, which cover a range of molecular weight and polarity. The use of MBMS allows the observation, in real time, of a wide range of products including low molecular weight producta, such as methane; reactive species, such as coniferyl alcohol; thermally labile products, such as levoglucosan; and high molecular weight products, such as polynuclear aromatic hydrocarbons. This range of detection has allowed the development of a mechanistic perspective of both the solid-phase (primary) pyrolysis reactions and the subsequent gas-phase (secondary and tertiary) reactions that occur in most practical thermochemical conversion devices. For fast pyrolysis at
0887-0624/87/2501-0311$01.50/00 1987 American Chemical Society
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atmospheric pressure, a relatively simple sequence of primary, secondary, and tertiary reactions (defined in 1) explain much of the observed behavior, though the actual chemistry is complex, involving 50 or more significant primary species. In this paper, several applications of MBMS to the qualitative understanding of actual conversion systems are illustrated and are briefly described in this Introduction. The difficulty of pyrolysis oil analysis is a major problem in the development of pyrolysis techniques. Typically only 50% by weight of the product can be mobilized by chromatographic techniques, after condensation. As stated above, the products are complex and methods of analysis are time consuming. We have used the MBMS technique to characterize the products of several conversion processes, and although this analysis does not take the place of careful systematic conventional analysis, as performed by Elliott? Menard: and others, it allows rapid screening of the products with a data set that can be readily interpreted, especially when manipulated by statistical techniques such as multivariate a n a l y ~ i s . ~ Another important problem in producing liquid fuels by biomass pyrolysis is that the composition of primary oil is less than optimum. The physical properties of the oil are not good for use as a premium fuel, or even a boiler fuel, due to acidity, corrosiveness, and high viscosity, as well as reactivity, which may possibly lead to polymerization during storage. The primary oil is also highly polar and generally has almost as much oxygen as the biomass feedstock. One of the major goals of the work reported in part 1' is the understanding of how the different classes of products are formed so that the oil composition can be controlled by selection of feedstock pretreatment, by the primary pyrolysis operating conditions, or by the subsequent thermal or catalytic upgrading of the vapors. Because of this limited utility of the primary oil and the need for premium liquid fuels, the question of how the product can be upgraded is of prime importance. A process that can do so without using high pressure or hydrogen is particularly desirable. In the past, the use of thermal cracking to produce olefins and light aromatics has been e ~ p l o r e d . However, ~ one problem with this approach is the formation of CO by straight thermal cracking, and work in this area- has shown that the yield of CO remains high (approximately 50%) over the cracking temperature range of 500-800 OC while the amount of ethylene produced is around 10%. The loss of yield as CO has limited the application of this approach to produce premium fuels. Steinberg has studied the flash pyrolysis of wood in the presence of methane and proposed that the methane enhanced the formation of ethylene and light aromatics by interacting with intermediate products of biomass pyrolysis. The same effect has been reported for coal flash
pyrolysis. Calkins et al.'O have proposed that the increase in ethylene yield was due entirely to the pyrolysis of methane, perhaps catalyzed by the presence of the coal solids, the coal-derived pyrolysis products, or the reactor metal. A promising approach to primary product upgrading is the direct catalytic conversion in the vapor phase at low pressure and without hydrogen. This avoids the problem of revolatilizing the oil and allows pyrolysis and upgrading in one thermal cycle. The principal method for this has been the use of zeolites, and several workers have applied HZSM-5 to wood and other related products."-'5 A major reported problem in the use of zeolite is the rate of coke deposition, especially in comparison to the methanol to gasoline process.16 In comparison to the direct production of liquid fuels, the thermochemical conversion of biomass by gasification and combustion is a well-advanced technology. Gasification involves the partial oxidation of volatiles followed by char reaction to form CO and Ha, and the subject has been reviewed by Fteed.l7 Two major methods are updraft and downdraft gasification. In updraft gasification fuel flows countercurrent to upflowing air and steam. Flaming combustion of char occurs at the grate where temperatures reach 900-1200 "C to produce COP,H20, CO, and H2. The rising hot gas then pyrolyzes the incoming fuel to make more gas but also produces up to 30% oil by pyrolysis of the wood. In the downdraft gasification mode, oxygen or air and the wood enter the gasifier cocurrently from the top and the pyrolysis products produced above the char zone are burned in limited air. These hot combustion gases are rich in C 0 2and H20 and react with the remaining char to give the final fuel gas and only 0.1-1% condensible product. Other types of gasifiers, such as fluidized beds or entrained flow gasifiers, also produce small quantities of condensible products. In part 1of this series, we showed that the condensed aromatics are tertiary products. This distinction between primary, secondary, and tertiary products is relevant to gasification, and the tar is dependent on the thermal history of the process.1s The nature of the oils vs. tars in downdraft and updraft gasifiers will be discussed in this paper. Another important pollution problem arises from wood stove emissions, which produce significant amounts of CO, particulates, and condensable organic species. Although outside the scope of this work, the nature of the products surviving combustion can be discussed in relationshjip to the primary, secondary, and tertiary sequence of products. CO is a product of incomplete combustion or gasification, and the particulates are a physical problem. However, the formation of condensable organic species is due to flame front irregularities, where limited oxygen and transient
(2) Elliott, D. C. Final Report No. PNL-4931 on IEA Co-Operative Project DI, Biomass LiquefactionTest Facility Project, Pacific Northwest Laboratory: Richland, WA, 1983, Vol. 4. (3) Menard, H.; Belanger, D.; Chauvette, G.; Gaboury, A.; Khorami, J.; Grise, N.; Martel, A.; Potvin, E.; Roy, C.; Langlois, R. Fifth Canudian Bioenergy R&D Seminar; Hasnain, S., Ed.; Elsevier Applied Science: New York, 1985; pp 418-439. (4) Windig, W.; Meuzelaar, H. L. C., A d . Chem. 1984,56,2297-2303. (5) Diebold, J. P.; Scahill, J. W.; Evans, R. J. In Proceedings of the 1985 Biomass Thermochemical Conversion Contractors' Meeting, PNLSA-13571, CONF-8510167; NTIS: Springfield, VA, 1986; pp 31-56. (6)Antal, M. J. Jr. Adu. Solar Energy 1982, 1, 61-112. (7) Antal, M. J. Jr., in Adu. Solar Energy 1985,2, 175-255. ( 8 ) Diebold, J. P. Thesis T.-3007, Colorado School of Mines, Golden, co, 1985. (9) Steinberg, M.; Fallon, P. T.; Sundaram, M. S. In Proceedings of the 1985 Biomass Thermochemical Conversion Contractors Meeting, PNL-SA-13571, CONF-8510167;"IS: Springfield, VA, 1986;pp 15-30.
(IO) Calkins, W. H.; Bonifaz, C. Fuel 1984, 63, 1716. (11) Chantal, P. D.; Kaliaquine, S.;Grandmaisen, J. L. Appl. Catal. 1986,18, 133-145. (12) Eager, R. L.; Mathews, J. F.; Pepper, J. M. In Specialists Meetina. Biomass Liauefaction Processes: National Research Council of Canada: Ottawa, Canada, 1985. (13) Diebold, J. P.; Chum, H. L.; Evans, R. J.; Milne, T. A.; Reed, T. B.; Scahill, J. W. Energy Biomass Wastes 1986, 10th. (14) Frankiewicz, T. C. In Proceedings of Specialists' Workshop on Fast Pyrolysis of Biomass, SERI/CP-672-1096; Solar Energy Research Institute: Golden, CO, 1980; pp 123-136. (15) Chen, N. Y.; Koenig, L. R. U.S. Patent 4549031, 1985. (16)Chang, C. D. Hydrocarbons from Methanol; Marcel, Dekker: New York, 1983. (17) Reed, T. B. Adu. Solar Energy 1985,2, 125-174. (18) Elliott, D. C., 'Analysis and Comparison of Biomass Pyrolysis/ Gasification Condensates"; Final Report No. PNL-UC-61D; Pacific Northwest Laboratories; Richland, WA, 1986.
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Pyrolysis of Biomass Quadrupole MS
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high temperatures lead to the formation of tertiary products. These topics-pyrolysis oil characterization,the control of primary oil composition, the upgrading of oil by thermal cracking, by the subsequent pyrolysis in reactive environments, and by direct catalytic conversion, and the nature of gasifier condensible species and combustion products-have been studied to varying degrees by the MBMS technique and are discussed below in relationship to the mechanistic scheme presented in part 1of this series. Experimental Section The system used for these studies is shown in Figure 1,where the bottom reactor system varies depending on the application. This system is described in detail in part one and in references therein. Through MBMS, one extractively samples light gases and heavy vapors, while preserving reactive and condensable species through rapid quenching and wall-less conversion to molecular flow before line-of-sight introduction into the mass spectrometer ion source. Sampling is in real time, with millisecond time resolution for following the evolution of single ions or complete, repetitivescans of 300 amu/s. This allows the simultaneous observation of time-variant processes and the total product distribution by integrating the scans over the whole pyrolysis or samplingtime interval. Mass spectra to only 300 amu are shown here. As discussed in the first paper in this series, although higher-mass species exist, they are estimated to constitute a few percent or less of total vapor. Oil characterization was performed by introducing a quartz boat of the oil into hot flowing gas (typically 300-500 "C), using the lower section of the reactor system shown in Figure 1and repetively scanning the mass range as the liquid is volatilized. Greater than 90% of most samples is volatilized, and residence time in the gas phase was kept as short as possible (less than 75 ms) to minimize potential gas-phase secondary reactions. Direct studies of secondary cracking in real time (discussed in part 1)have shown only minimal changes in product distribution below 500 OC and residence times of 1s. Nevertheless, chemical changes in the oil are possible during the heating and volatilization process. Replicate samples have shown this technique to be reproducible. The sources of the oils used in this work are as follows: (1)the softwood *
primary pyrolysis oil was produced by Diebold and Scahil16at SERI by a vortex-entrained flow reactor using pine wood as a feedstock; (2) the hardwood primary pyrolysis oil was produced by Scott et al.19 by a fluidized bed reactor using aspen wood as a feedstock; (3) the partially cracked updraft gasifier oil was produced at Rome, GA," in an updraft gasifier from an unspecified feedstock; (4) the high-pressure liquefaction oil was produced at Albany, OR, with Douglas fir as the feedstock;" (5) the downdraft gasifier tar was produced by using the oxygen downdraft gasifier at SERI from an unspecified feedstock.2l Aging effects with these samples are not known, but the primary oils produced in the vortex-entrained flow reactor appear not to change qualitatively over several years. Elliott has performed extensive analyses on the first three oil samples.18 The pyrolysis systems used to study the effect of pyrolysis parameters on oil composition were of two types: (1)the typical reactor shown in Figure 1 and used for the work in part 1was used for most pyrolysis studies in part 2; (2) to demonstrate the effect of heating rate, a contact pyr0lyzer~~9~~ was coupled with the MBMS to study the effect of high heat-transfer rates and to investigate whether the possible direct formation of liquid products would have any effect on the composition of the pyrolysis products. The contact pyrolyzer consisted of a heated copper block with a hole where a spinning wood dowel could be inserted under pressure. Pyrolysis occurred at the surface and the vapors exited through a smaller concentric hole in the block and were directly sampled by the MBMS. (See ref 22 and 23 for details of the process.) The slow heating rate experiment was performed in the standard reactor with the sample inserted when the gas temperature was low, and the products were monitored as the gas temperature increased at 30 "C/min from 150 to 400 "C. The gasifier experimentswere performed in collaboration with Reed23and utilized an inverted-downdraft gasifier configuration to simulate downdraft gasification. An insulated 2.5-cm i.d. quartz tube was packed with wood chips, and with air flowing upward (4 L/min), the top of the bed was lit with a propane torch. The char bed and combustion zone then moved downward through the wood, and the effluent was sampled directly by the MBMS. Initially, the bed operated in a combustion mode, but as the char bed developed, the effluent gas became combustible and the bed operated for about 7 min in the gasification mode. After ignition a small, porous ceramic plug and some quartz wool were placed on top of the bed to compact the char and prevent fly ash from entering the sampling orifice. The same apparatus was operated in the updraft mode by establishing a bed of burning char at the bottom of the tube and rapidly pouring biomass on top of the charcoal. The hot gases produced by char gasification rise through the biomass, producing mostly primary pyrolysis oils. The study of biomass pyrolysis in methane was done in the quartz reactor shown in Figure 1. Cold methane flows of 2-5 L/min were used, which gave residence times up to 750 ms over the temperature range of 500-1000 "C. Pyrolysis of the methane alone was only observed above 975 OC under these conditions. In each methane experiment,a blank spectrum was automatically obtained for spectra taken before and after the insertion of the wood sample. These blank spectra are routinely subtracted before the data is printed out. For catalyst experiments, the secondary zone of the quartz reactor was packed with 0.6 g of pure 5-10-pm ZSM-5 zeolite catalyst that was suspended in 3 g of quartz wool. Microscopic examination of the catalysbloaded wool indicated that the catalyst appeared to have a mild attraction (possibly electrostatic)to the wool that could survive initial temperature cycling in hot helium. The pure ZSM-5 was made available by Mobil Research and (19) Scott, D. S.; Piskorz, J. Can. J. Chem. Eng. 1984, 62, 404. (20) Sample supplied by D. C. Elliott, Pacific Northwest Laboratories, . Richland, WA. (21) Sample supplied by T. B. Reed, Colorado School of Mines, Golden, CO. (22) Reed, T. B.; Diebold, J. P.; Chum, H. L.; Evans, R. J.; Milne, T. A.; Scahill, J. W. Overview of Biomass Fast Pyrolysis and Catalytic Upgrading to Liquid Fuels, ASES 86; American Solar Energy Society: Boulder, CO, 1986. (23)Reed, T. B.; Levie, B.; Scahill, J.; Evans, R.; Milne, T. In Proceedings of the 1985 Biomass Thermochemical Contractors' Meeting, PNL-SA-13571,CONF-8510167; NTIS: Springfield, VA, 1986.
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Development Corp. in the ammoniated form. The ammoniated ZSM-5 was converted to HZSM-5 by heating it at 550 "C for about 18 h with a low flow of inert gas to sweep out the released ammonia. The inertness of the quartz wool was confiied by a blank experiment. Batch pyrolysis was carried out in the usual fashion with He as the heating and carrier gas. During the pyrolysis wave, 30 mg of wood was pyrolyzed over 30 s, giving a vapor concentration of about 25 w t % (or 1%by volume assuming a vapor molecular weight of 100). This resulted in a weight hourly space velocity (WHSV) of 6 for the experiment shown in this paper.
Applications of MBMS to Applied Systems Rapid Characterization of Pyrolysis Oils. The revolatilization of oils, as described in the Experiment Section, allows the rapid screening of many oils with vaporization of more than 90% of the oil (as determined by residue after flash evaporation at 400-500 "C). This compares with the much lower values (often less than 50%) mobilized in typical programmed injection in GC's or through standard distillation cycles.24 From a survey of many oils, four distinguishabletypes can be defined25and these are illustrated by their mass spectra in Figure 2. The primary oils are those that have not been exposed to temperatures and residence times sufficient to cause gas-phase cracking reactions. From the results of the fundamental investigation described in part 1,this would be approximately a temperature of less than 600 "C and residence time of less than 1s. Two examples of primary oils are shown in Figure 2. The spectrum of a softwoodderived oil produced by Diebold and Scahil15 shows a distribution of products that closely resembles the directly sampled vapors discussed in part one (see Figure 8a for a primary pyrolysis vapor spectrum of pine). The major difference in the collected oil spectrum is that coniferyl alcohol (m/z 180) is missing, which is typical of all collected primary oils. The peak at m / z 137 is still present, which in the real-time spectra is most probably a fragment ion of coniferyl alcohol. However, for the collected oil the fragment ion is possibly due to higher molecular weight lignin-like condensation products. The absence of coniferyl alcohol is probably due to condensation reactions since the oil is acidic and coniferyl alcohol is known to polymerize in acidic environments. Isoeugenol is still predominant, a t m/z 164, which indicates that it does not undergo polymerization to the same extent as coniferyl alcohol. For the carbohydrate-derived products, the major peak is m/z 43, the fragment ion of carbonyl compounds, which is typical of primary pyrolysis oils. In part 1of this series,l the two major carbohydrate primary pyrolysis mechanisms were discussed. It was shown that by the addition of 0.1 wt % K as K2C03to cellulose, the pyrolysis spectrum could be connected to the same distribution of peaks as observed in whole biomass. The alkali-metal-catalyzed product spectrum had m/z 43 and 126 as major peaks while the pure cellulose had m/z 60 and 144. The relative amounts of products from the two major carbohydrate pyrolysis pathways can be judged by comparing m / z 43 and 126 from the alkali-metal-catalyzed route to m / z 60 and 144 from levoglucosan formation. In this oil the amount of m/z 144 is somewhat higher than in most pyrolysis oils. Water (m/z 18) is detectable although the sensitivity is low so it, along with methanol, are underrepresented by the relative peak heights. In most primary oils the water is 10-20% by weight.2 A hardwood-derived oil produced by Scott et al.19 shows the same basic trends as noted for the softwood with some ~~~~
(24) Beckman, D.; Elliott, D.
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notable exceptions. The dimethoxy series of lignin products is predominant (mlz 154,168,182,194,208, and 210). There is a smaller amount of m/z 272 in the hardwood oil than in the softwood oil, which is also the distribution observed in the direct pyrolysis spectra.' The xylan in hardwood has a significant amount of acetyl groups that form acetic acid during pyrolysis, and conventional analysis has shown levels of 7Ti2. The high abundance of m / z 61 is very probably due to the acetic acid although the reason for the protonated molecular ion is unknown. As with the softwood oil, the coniferyl and sinapyl alcohol peaks at m/z 180 and 210 (mlz 180 can also be due to vinyl syringol in hardwood pyrolysis) are low compared to the directly sampled pyrolysis vapors. Also, the dimethoxy products are more prevalent than the monomethoxy products. The methanol peaks (mlz 31 and 32) are not larger than for the softwood oil, which is not in agreement with the known fact that hardwoods are usually a better source of methanol, but this may be a fractionation effect of oil collection techniques. The updraft gasifier oil produced a t Rome, GA, is an example of a slightly cracked oil where the more thermally labile products have been cracked, but no new significant peaks have been formed. The lack of low molecular weight material could be due to incomplete collection of the volatiles or possibly because the stable lignin products are less susceptible to cracking and therefore are the predominant products. There is no significant peak at m/z 136 as has been observed in secondary products in the gas phase,' but there is a large peak at m/z 164, which indicates that there was probably only minimal secondary cracking that primarily affected the carbohydrate-derived products. This oil has a lower viscosity than those of the primary oils.18 The high-pressure liquefaction oil produced by the PERC process at Albany, OR, has a dramatically different product slate than the primary oils. This oil, produced in the presence of hydrogen and with alkali-metal catalysts, has a significantly reduced oxygen content compared to the primary oils. There is some m/z 136, but it is not likely the same product that is observed in secondary cracking studies as described in part 1. Elliott2 reported some methylethylphenol, which has a molecular weight of 136. There is a possible homologous series at m/z 136,150,164, and 178 that is separated by 14 amu. Other homologous series are also possible, giving the regular distribution observed in the spectrum. The tertiary products have been totally absent in the above oils, showing that in well-controlled situations, the primary and tertiary products are mutually exclusive. The downdraft gasifier tar in Figure 2 is dominated by condensed polynuclear aromatics: m / z 152, acenaphthene; m / z 166, fluorene or benzindene isomers; mlz 178, anthracene or phenanthrene; m / z 192, methylanthracene isomers; m / z 202, fluoranthene, pyrene, or benzacenaphthylene; m/z 216, methylpyrene isomers; m/z 228, benzanthracene isomers, chrysene, or triphenylene; m / z 252, benzopyrene isomers or perylene. The lack of low molecular weight aromatic species is due to incomplete collection. The nature of gasifier effluents will be addressed below. Fingerprinting provides a rapid test of the thermal history of collected oils and tars. As an example, a "tar" was analyzed that had been generated in the gas-processing train of a 10 ton/day air gasifier being tested by Syngas Systems Inc.2e The spectrum clearly revealed that the (26) Graboski, M. S. Syngas Systems, Inc., Golden, CO, unpublished results, 1985.
Energy &Fuels, Vol. 1, No. 4, 1987 315
product was largely primary oil, indicating it may have collected during startup. In another example, a set of collected creosotes were analyzed from wood stoves undergoing systematic tests in the Northea~t.~'These ran the gamut from slightly cracked products to highly aromatic tars. Such fingerprinting is also a rapid way of detecting solvent or other types of contamination in collected oils. Quantitative approaches to the same information have been reported by Elliott.2s It is interesting to note that revaporized collected oils seem to be of low molecular weight in contrast to results for the liquid from gel permeation chromatography, where molecular weights as high as 2000 are i n f e r d B It appears that liquid-phase bonding is strong enough to affect GPC separation but too weak to survive revaporization. Effect of Primary Pyrolysis Parameters on Oil Composition. Though we have not systematicallystudied the effects of temperature, heating rate, particle size, and moisture on primary oil composition, a few very qualitative observations are in order based on a first look at several major parameters. These are made in the context of the rather fast heating of relatively small particles, with the minor dimension not exceeding 6 mm in.). In the context used here, minor changes mean that peak ratios seldom vary over a factor of 2 and the chemical species present are largely the same (e.g., no secondary cracking to benzene etc.). For both wood and cellulosemthe primary product slate varies little with surrounding gas temperatures over the range 400-900 "C, provided the gases are sampled close to the surface to avoid secondary cracking. This is not surprising since the endothermic, ablative fast pyrolysis likely buffers the solid temperature to values below which extensive internal secondary cracking can occur. The effect of heating rate on product slate also is not dramatic, over the range 0.5-50 OC/s, although the yield of oil and char are affected. The effect of moisture and particle dimensions on primary vapors is illustrated for ponderosa pine in Figure 3. With the exception of m/z 137, 180, 226 and an unusual peak at mlz 239 the proportions of species are rather constant (e.g., compare the m/z 164, 124, and 60 ratios). One possibility is that coniferyl alcohol reacts with other volatile species or with the solid wood while escaping from the interior of the large particle. A series of pyrolyses for dry pine of dimensions from 1 mm X 1mm X 20 mm to 6 mm X 7 mm X 20 mm gave virtually identical spectra except for m / z 180, in spite of pyrolysis times varying from 15 to 90 s. For specified fuel dimensions and thermal history, our techniques could be used to quickly diagnose major perturbations in product slate from that for fast, primary pyrolysis due to product interactions and cracking while escaping from large particles in applied pyrolysis systems. Perhaps the best example that can be shown for the effect of heating rate on primary pyrolysis slate is the spectrum obtained from the direct sampling of a contact pyrolyzer. Reed23has conceived and constructed a pyrolysis device that forces a rotating wood dowel against a thermostated copper block. With pressure applied to the wood (typically 300 psi), rapid heat-transfer occurs and (27) Barnet, P. Omni Environmental Services, private communication, 1986. (28) Elliott, D. C. In Proceedings of the 1985 Biomass Thermochemical Conversion Contractors' Meeting, PNL-SA-13571, CONF-8510167; N T I S Springfield, VA, 1986. (29) Chum, H. L.; Johnson, D. K.; Ratcliff, M.; Posey, F., and Black, S. In SERI/PR-234-2665; Diebold, J. P., Scahill, J. W. Eds.; Solar Energy Research Institute: Golden, CO, 1985; Appendix B. (30) Milne, T. A.; Soltys, M. N. Fundamentals of Thermochemical Biomass Conversions; Overend, R. P., Milne, T. A., Mudge, L. K., Eds.; Elsevier Applied Science: New York, 1985; pp 361-386:
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an apparently charless pyrolysis is achieved. We have sampled the vapor directly from this device, with the results shown in Figure 4. It is seen that only minor changes in primary vapor slate occur under this ultrafast pyrolysis. Higher relative amounts of m/z 44 are observed as well as a different distribution of lignin peaks. By virtue of our real-time sampling capability, we are able to observe the degree of sequential evolution of products, under slow and rapid heating conditions. The upper plot in Figure 5 shows, for slow heating (0.5 "C/s) the early evolution of extractives ( m / z 226) and the primary lignin species, coniferyl alcohol (m/z 180), followed by the characteristic hemicellulose indicator species (m/z 114), with a primary cellulose indicator (m/z 43) and a secondary lignin peak ( m / z 138) evolving the latest.
and of vapor freshly &shed from a birch dowel in a copper-contact pyrolyzer (b).
Further elaboration of evolution behavior, under fast heating (estimated 20 "C/sec), is shown in Figure 5b,c. The series of ion profiles are for wet (Figure 5c) and dry (Figure 5b) large (-600 mg) specimens of ponderosa pine being heated in 500 "C He. Note the delay in pyrolysis of the wet specimen, as the unbound water is driven off over the first 75 s. The same sequential evolution is present in the fast heating of the dry wood (Figure 5b) as in the slow heating described above (Figure 5a), but with less resolution of the individual products. In the wet sample (Figure 5 4 , however, the sequence of evolution is more uniform. Interestingly, coniferyl alcohol (m/z 180) starts evolving before the moisture is driven off, but the extractive at m/z 226 is retarded until the major part of the pyrolysis wave. This differential evolution would not appear to provide practical separation of products in engineered pyrolysis systems designed for high liquid yields (fast heating) but is useful in understanding mechanisms of pyrolysis. Gas-Phase Processes in Gasification. In collaboration with Reed, we coupled our MBMS sampling system to a small insulated quartz fixed-bed gasifier that could be operated in either an updraft or a downdraft mode. Details of the 2.5-cm4.d. gasifier and its mode of operation are given in the Experimental Section and by Reed et al.= In the updraft mode, the spectrum shown in the middle of Figure 6b was obtained under operation with air flowing upward through small birch dowels. The spectrum is dominated by typical primary products as shown for the feedstock in Figure 6A. The observed high C 0 2concentration (m/z 44), reflecting the partial oxidation in gasification, is not shown in this mass range. In contrast, the lower spectrum, Figure 6c, was obtained by direct sampling from the gasifier operating in the inverted-down draft mode. In this case, sampled gases passed through the hot char zone before exit. Note the domination of the spectrum by aromatics (e.g., benzene at m / z 78, toluene at m/z 92, indene at m/z 116 and naphthalene at m/z 128) and the relatively low value of primaries such as m/z 60 and 73. Once or twice during
Energy & Fuels, Vol. 1, No. 4, 1987 317
Pyrolysis of Biomass
(a) Birch wood (Reed)
Time (min)
(b) Updraft gasifier (b) 13841
TIC
19898
m/z 10
m/z 114
65535
m/z 100
pi-.-_
m/z 226
51805
lk
50
150
2W
Time (s)
IW
13393
13189
1
L15482u5-
-
-
-~~ 58
-
m/z 226
m/z 124
,v.d...,
d
.
j!;; ..-,m/z2;0
+ ,*,
22883
158
58
I&
m/z 60 i.
158
2k
Time ( 0 )
Figure 5. Time-resolved evolution of selected primary pyrolysis products under different conditions: (a) the slow heating of pine wood at 30 C/min in Helium from 150 to 400 "C; (b and c) the
evolution of species from oven-dry (b)and wet (c)ponderosa pine (50% moisture), both under fast heating rate (about 30 "C/s).
the operation in this mode, shaking of the gasifier caused a sudden, increased contact of the hot char with the pyrolyzed wood, analogous to what would occur when batch feeding drops new material onto a hot char bed. When this happened, spikes of primary species appeared to pass through the bed, presumably by flooding and cooling the bed temporarily. Similarly, in direct sampling of a small quartz combustor, surges of organics like benzene could be seen when a fresh batch of wood was added to the simulated stove. Direct probing of operating gasifiers or combustors would appear to be most useful in identifying the condition that allow pollutants to form in higher than expected amounts due to addition of fuel, poor mixing, and other transient upsets. The larger commerical gasifiers may have on-line MS monitoring of gas and minor constituents products. The knowledge derived here could permit monitoring of operation through a few selected peaks that would show the relative amount of primary and secondary pyrolysis products and in turn would accurately pinpoint gasifier malfunctions. Gas-Phase Product Enhancement in Reactive Environments. A novel approach to the gas-phase pro-
cessing of flash pyrolysis vapors has been taken by Steinberg and co-workersgwho have pyrolyzed biomass in reactive environments, specifically hydrogen and methane. The effect of hydrogen under pressure was to increase the amount of methane that is formed, but in the presence of methane at temperatures above 900 "C, the yield of ethylene and benzene was significantly enhanced and doubled under some conditions. We attempted to study this reaction using the MBMS from the perspective of the reaction scheme that was outlined in part 1. However, because of certain limitations of our experimental system, the experiment of Steinberg et al. was somewhat different than ours. Biomass is entrained in the methane in their reactor whereas, in ours, captive samples are placed in the flowing methane. This allows us to separate potential primary, secondary, and tertiary sources of the observed product enhancement, our experiment was performed at atmospheric pressure while theirs was at 15-50 psig, and our temperature range did not go as high as theirs (975 vs. 1100 "C,respectively). Although the exact effects seen by Steinberg et al. were not observed in our work, probably due to one or more of the above reasons, real chemical interactions were detected.
318 Energy & Fuels, Vol. 1, No. 4, 1987
Evans and Milne
PINE PYROLYSIS
(a) Pine wood, no catalyst, 500" C
HELIUM
m /
500
600
m / z 42
700
800
TEMPERATURE,
OC
900
(b) Pine wood, active ZSM-5,500" C 191
Figure 7. Effect of temperature on the intensity of products from pine pyrolysis in helium and methane. The peak at m/z 43 is due to a fragment ion of carbonyl compounds and is representative of primary products, the peak at m/z 42 is due to propylene and is representative of the behavior of the C3 and C4 unsaturated hydrocarbons in these reaction conditions, and the peak at m/z 26 is due to acetylene. Ethylene could not be measured accurately, due to CO interference, but no obvious enhancementwas observed in methane. Gas-phase residence times are approximately 750 ms.
The results of a comparison of selected gas-phase reaction products in helium and methane environments is shown in Figure 7. The decrease in primary products is represented by m/z 43, which decreased steadily from 600 to 900 "C. No significant difference was observed between methane and helium. This indicates that olefin enhancement in methane does not involve the primary pyrolysis reactions or the primary reaction products. This was also the conclusion in another experiment where no significant changes in primary product distribution in helium and methane were observed with less than 75-ms residence time and temperature from 500 to lo00 "C. The secondary products are represented by propylene (m/z 42), and this shows a significant increase in relative abundance in methane as well as a maximum at a significantly higher temperature. There was no contribution to propylene from the methane alone, indicating that pyrolysis in methane involves unique chemistry. A comparison of acetylene (m/z 26) gives some insight as to the mechanism, with a significant decrease in acetylene occurring in methane. Steinberg and co-workersgreport the absence of acetylene under their experimental conditions. Although the exact mechanism is unknown, these results support the observatin that continued gas-phase pyrolysis of biomass-derived vapors in methane enhances the yields of olefins due to the interaction of methane and gas-phase species. Catalytic Upgrading of Biomass Primary Pyrolysis Products. The catalytic conversion of primary vapors is one approach to the conversion of biomass to products with the properties of gasoline or fuel oil. This approach has been summarized in a recent paper.13 Figure 8 shows the conversion of the primary vapors on a fully activated catalyst and on a catalyst after partial deactivation due to coking. It is surprising that all the primary products appear to be converted without regard to size since the zeolite channels are shape selective with about 5-A diameters. One possible explanation for this observation is that the larger molecules are initially cracked on the macro surface. The organic products from the active catalyst, however, do appear to be shape selective, with most products below m/z 150. In other experiments with more reaction time (WHSV = 1-2 h-' versus 6 h-l in Figure 8), larger products such as methylated naphthalenes (m/z 142, 156,170) become more prevalent and are also possibly due
(c) Pine wood, deactivated ZSM-5,500° C
,i'
I
m7.
Figure 8. Effect of HZSM-5zeolite catalyst on the primary pyrolysis products of pine: (a) the primary products spectrum; (b) the spectrum after directly passing the primary vapors over fresh catalyst; (c) the spectrum after partial deactivation of the catalyst due to coking. Conditions: quantity of catalyst, 0.6 g; WHSV = 6; He carrier gas.
to macrosurface catalysis. Water, CO, and C 0 2 are the products that contain the oxygen and represent a substantial portion of the yield. These light species are underrepresented by the intensities in the mass spectrum shown in Figure 8 because of instrumental discrimination (the mass spectrometer was tuned to favor the higher organics). The products from the partially deactivated catalyst show a mixture of the peaks in the primaryspectrum and the fully active catalyst spectrum. The lignin peaks are more prevalent, such as m / z 138,150, and 164, as well as the larger masses, such as m / z 272. The carbohydrates are still partially converted to furan derivatives: furan, m / z 68; methylfuran, m/z 82; benzofuran, m/z 118; methylbenzofuran, m / z 132. These furans are the only intermediate products identified in the zeolite conversion of biomass primary vapors. There may be sequences of reactions as observed in methanol conversion over ZSM-5 where dimethyl ether, alkenes, and aromatics and paraffins are observed sequentially.16 The furans disappear completely on active catalysts and are still formed on partially deactivated catalysts when the aromatics are no longer formed. This possibly indicates that the conversion of carbohydrates to furans (cyclic ethers) is analogous to the formation of dimethyl ether from methanol, which also forms as an intermediate and continues to form after
Pyrolysis of Biomass
further conversion to aromatics no longer occurs due to deactivation. The yields of hydrocarbons, degree and rate of coking, and the regenerability of the catalyst are major considerations in the application to biomass pyrolysis products. Since the biomass is relatively deficient in hydrogen, the propensity for coking is greater than with the hydrogenrich feedstocks such as methanol. Coking can be followed by monitoring the decrease in final products and the increase in starting material, as shown in Figure 8, as a function of time. Model compounds can be tested to determine the species that contribute to coke deposition. Coniferyl alcohol does not appear in the partially deactivated slate as the other higher molecular weight lignin products do. It is probable that the coniferyl alcohol, which polymerizes in the presence of acid, is being converted to coke and significantly contributing to the coking problem. The burn-off of coke can be followed by monitoring COP,CO, and H20 evolution as oxygen is introduced into the reactor. In this way, the coking propensity of model compounds can be monitored. This is a subject of current research activity as is the effect of primary pyrolysis vapor concentration on chemistry. (These early results are under quite dilute conditions.) Conclusions and Future Applications The screening approach allows a variety of processes to be quickly studied by using the mechanistic perspective developed with the MBMS experiments, since most conversion routes can be adequately described by a combination of the major solid-phase and gas-phase pathways: the two classes of carbohydrate primary pyrolysis pathways, transglycosylation and alkali-metal-catalyzed glycosidic fission; the two classes of lignin primary pyrolysis pathways, the prompt formation of alkenylmethoxyphenols such as coniferyl alcohol and the later formation of low molecular weight aromatics such as guaiacol; the four zones of gas-phase processes, primary product preservation below 500 "C, slight cracking of thermally labile species from 500 to 600 "C, secondary reactions from 600 to 750 "C and tertiary reactions from 750 to 1000 "C, as discussed in ref 1. In the majority of small samples and under most conditions likely to exist in pyrolysis reactors, the vapor composition does not vary greatly, with some notable exceptions: (1)the alkali-metal-catalyzedglycosidic rupture is the major effect in determining the composition of products from carbohydrates; (2) coniferyl alcohol is a major lignin primary product that can be affected by reaction conditions such as particle size, secondary gas phase residence time, the presence of acid, or reactivity after collection of the oil (which is acidic and can cause the coniferyl alcohol to polymerize); (3) temperatures below 600 "C and residence times of less than 1 s appear to be below the regime where significant loss of oil yield to CO
Energy &Fuels, Vol. 1, No. 4, 1987 319
occurs, or aromatics are formed. Research in this range is continuing to determine the kinetics of these gas-phase reactions and determine if mild gas-phase conversion may possibly be used to improve the nature of the liquid product by cracking the coniferyl alcohol to lower molecular weight methoxyphenols and benzodioxoles, cracking the levoglucosan to oxygenated fragments, decarboxylating the carboxylic acids, etc. The rapid sereening of oils allows comparisons that can be used for product characterization and process evaluation. This is the basis of current work in progress, which involves the development of a compound class characterization technique based on multivariate analysis4 of MBMS data. The multivariate analysis results can be compared with compound classes and be correlated with fuel-related properties. Analysis of effluents from gasification and combustion systems has shown the full range of products from the major classes of primary, secondary,and tertiary reactions. This is due to the variable-temperature and stoichiometry conditions that occur in systems where oxidation is occurring and solid fuel is being fed. Current research is being performed on the effect of partial oxidation on the survival of the different types of products from primaries to tertiaries. The preliminary investigation of the direct catalytic upgrading of the vapors shows promise, although the yield is as yet unknown. MBMS allows rapid analysis of products with observation of intermediates and catalyst deactivation. The current work in progress is the study of the extent of coking and the species responsible for oxygen removal, which can be monitored as a function of operating conditions and reactant composition. The possibility of thermal preprocessing, to crack potential coking species such as coniferyl alcohol, is also being pursued. Acknowledgment. The authors gratefully acknowledge the support of the DOE Biofuels and Municipal Waste program under Simon Friedrich and Gary Schiefelbein and the technical monitoring by Donald Stevens. Tom Reed collaborated on the experiments with the contact pyrolyzer and the small-scale gasifiers. The oil samples were provided by Doug Elliott and Jim Diebold. Crucial experimental and design help with the MBMS was provided by Michael Soltys. Mobil Research and Development Corp. provided the ZSM-5 catalyst, and Jim Diebold collaborated on the catalyst studies. Registry No. Methane, 74-82-8;isoeugenol, 97-54-1;levoglucosan, 49807-7;methanol, 67-56-1; acetic acid, 64-19-7;coniferyl acenaphthene, 83-32-9; alcohol, 458-35-5;sinapyl alcohol, 537-33-7; methylanthracene, 613-12-7;fluoranthene, 206-44-0;methyl benzanthracene, 56-55-3; benzene, 71-43-2; pyrene, 27577-90-8; toluene, 108-88-3; indene, 95-13-6; naphthalene, 91-20-3; propylene, 115-07-1; acetylene, 7486-2;methylfuran, 27137-41-3;benzofuran, 271-89-6; furan, 110-00-9; methylbenzofuran, 25586-38-3.
