single Droplet Combustion of Biomass Pyrolysis Oils - ACS Publications

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Single Droplet Combustion of Biomass Pyrolysis Oils Mary J. Wornat,* Bradley G. Porter, a n d Nancy Y. C. Yang Sandia National Laboratories, Combustion Research Facility and Center for Materials and Applied Mechanics, Livermore, California 94550 Received February 2, 1994. Revised Manuscript Received May 11, 1994@

In an investigation of the combustion behavior of biomass-derived liquids, we have performed single droplet experiments with two biomass oils, produced from the pyrolysis of oak and pine. The experiments are conducted a t 1600 K on 320 pm diameter droplets introduced into a laminar flow reactor, operating at 0 2 concentrations of 14-33 mol %. In-situ video imaging of burning droplets reveals that biomass oil droplets undergo several distinct stages of combustion. Initially biomass oil droplets burn quiescently in a blue flame. The broad range of component volatilities and inefficient mass transfer within the viscous biomass oils bring about an abrupt termination of the quiescent stage, however, causing rapid droplet swelling and distortion, followed by a microexplosion. Droplet coalescence follows, and subsequent burning occurs in a faint blue flame with occasional smaller scale bursts of fuel vapor. At the late stages of biomass oil combustion, droplets are accompanied by clouds of soot, produced from gas-phase pyrolysis. Liquid-phase polymerization or pyrolysis of the oxygenate-rich biomass oils leads to the formation of carbonaceous cenospheres, whose burnout signifies the final stage of biomass oil droplet combustion. Oak and pine oils behave similarly during combustion, though differences in their physical properties cause pine oil to show more susceptibility to fragmentation during the microexplosion. Changes in oxygen concentration alter the timing of the events during biomass oil combustion, but not their nature. Comparison of the biomass oils with No. 2 fuel oil reveals vast differences in combustion mechanisms, which are attributable to differences in the physical properties and chemical compositions of the fuels. Despite these differences, the biomass oils and No. 2 fuel oil exhibit surprisingly comparable burning times under the conditions of our experiments.

Introduction Because of its renewability and availability, one of the more attractive alternatives to conventional energy sources is biomass, which includes wood, agricultural residues, and fast-growing crops. Although many forms of biomass can be burned directly as combustor fuels,1,2 the pyrolysis and liquefaction of b i o m a ~ s lead ~ - ~to solid and liquid fuels of lower moisture content. For certain specialized applications, additionally upgraded fuels can * To whom correspondence should be addressed: Department of Mechanical & Aerospace Engineering, Engineering Quadrangle, Room D329B, Princeton University, Princeton, New Jersey 08544-5263. Telephone: (609) 258-5278. @Abstractpublished in Advance ACS Abstracts, July 1, 1994. (l)Tillman, D. A. The Combustion of Solid Fuels and Wastes; Academic Press: New York, 1991; pp 65-119. (2) Jaasma, D. R. Combust. Sci. Technol. 1986, 49, 213-225. (3) Diebold, J.; Scahill, J. In Pyrolysis Oils from Biomass: Producing, Analyzing, and Upgrading; Soltes, E. J., Milne, T. A., Eds.; ACS Symposium Series 376; American Chemical Society: Washington, DC, 1988; pp 31-40. (4) Pakdel, H.; Roy, C. In Pyrolysis Oils from Biomass: Producing, Analyzing, and Upgrading; Soltes, E. J., Milne, T. A,, Eds.; ACS Symposium Series 376; American Chemical Society: Washington, DC, 1988; pp 203-219. (5) Soltes, E. J. In Pyrolysis Oils from Biomass: Producing, Analyzing, and Upgrading; Soltes, E. J., Milne, T. A., Eds.; ACS Symposium Series 376; American Chemical Society: Washington, DC, 1988; pp 1-7. (6) Bridgewater, A. V.; Cottam, M.-L. Energy Fuels 1992, 6 , 115120. (7) Elliott, D. C.; Beckman, D.; Bridgewater, A. V.; Diebold, J. P.; Gevert, S. B.; Solantausta, Y. Energy Fuels 1991, 5, 399-410. (8) Kasper, J. M.; Jasas, G. B.; Trauth, R. L. Presented at the ASME Gas Turbine Conference and Exhibit, Phoenix, Arizona, March, 1983. American Society of Mechanical Engineers: New York ASME Publication 83-GT-96.

be produced from thermochemical treatment of biomass pyrolysis p r o d ~ c t s . ~ , ~ , ~ ~ - ~ ~ Our current work concerns the combustion of chars and oils produced from biomass pyrolysis. The combustion of biomass chars is treated elsewhere.14J5 This paper focuses on the combustion of oils produced from the pyrolysis of wood. Since wood comprises principally lignin, cellulose, and hemicel1ulose,l6-l8the oxygen-rich nature of these components is reflected in the products (9) van de Kamp, W. L.; Smart, J. P. Evaluation of the Combustion Characteristics of Pyrolytic Oils Derived from Biomass. Presented at the Sixth European Conference on Biomass for Industry and Energy, Greece, April 1991. (10)Scahill, J.; Diebold, J.; Power, A. Presented at the IEA International Conference on Research in Thermochemical Biomass Conversion, Phoenix, Arizona, May 1988. (11)Baker, E. G.; Elliott, D. C. In Pyrolysis Oils from Biomass: Producing,Analyzing, and Upgrading;Soltes, E. J.,Milne, T. A,, Eds.; ACS Symposium Series 376; American Chemical Society: Washington, DC, 1988; pp 228-240. (12) Dao, L. H.; Haniff, M.; Houle, A.; Lamothe, D. In Pyrolysis Oils from Biomass: Producing, Analyzing, and Upgrading; Soltes, E. J., Milne, T. A., Eds.; ACS Symposium Series 376; American Chemical Society: Washington, DC, 1988; pp 328-341. (13) Evans, R. J.; Milne, T. A. Energy Fuels 1987, I , 311-319. (14) Wornat, M. J.; Hurt, R. H.; Yang, N. Y. C.; Headley, T. J. Structural and Compositional Transformations of Biomass Chars During Combustion accepted for publication in Combust. Flame and for presentation at the Twenty-Fifth Symposium (International) on Combustion, Irvine, CA, August 1994. (15)Wornat, M. J.; Hurt, R.' H.; Yang, N. Y. C.; Frew, E. W., manuscript in preparation. (16) Haygreen, J . G.; Bowyer, J. L. Forest Products and Wood Science; Iowa State University Press: Ames, IA,1982. (17) Shafizadeh, F. In Fundamentals of Thermochemical Biomass Conuersion; Overend, R. P., Milne, T. A,, Mudge, L. K., Eds.; Elsevier: London, 1985; Chapter 11, pp 183-217. (18) Evans, R. J.; Milne, T. A. Energy Fuels 1987, 1 , 123-137.

0887-0624/94/2508-1131$04.5~IQ 0 1994 American Chemical Society

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of pyrolysis at low to moderate temperatures: organic acids, aldehydes, ketones, furans, alcohols, phenols, guiacols, syringols, and complex organic oxygenates.18-21 High levels of both oxygenated organics and water distinguish biomass oils from petroleum-based liquid fuels. These oxygen-rich constituents influence the physical properties of the fuel as well as their behavior during combustion. Another characteristic of the biomass oils, relevant to combustion, is their multicomponent nature. Several ~ o r k s address ~ ~ - the ~ ~issues of heat and mass transport relevant to the evaporation and combustion of multicomponent fuels. In many cases, droplet swelling andfor fragmentation occurs as a consequence of the presence of components with a wide range of volatilities.27,28The formation of carbonaceous residues, including cenospheres, has also been o b ~ e r v e d . ~ ~ , ~ ~ In our present study, we investigate the overall combustion behavior of oils produced from the rapid, moderate-temperature pyrolysis of two biomass feedstocks-a hard wood, oak; and a soft wood, Southern pine. In order to accentuate the features of combustion peculiar to biomass oils and t o facilitate comparison of biomass oils with more conventional fuels, we choose single droplet combustion in a laminar flow environment, where droplets are uniformly sized, moving in a well-characterized flow field, and widely spaced so as not t o incur interactions with other droplets. In the present paper, we report the time-resolved combustion history of oak and pine pyrolysis oils-presenting insitu images of the oils and the products of their partial reaction a t various stages throughout their combustion lifetime. We interpret the results in light of the characteristic features of these biomass-derived fuels and make comparisons t o liquid hydrocarbon fuels, as appropriate.

Experimental Materials, Equipment, and Procedures Combustion experiments a r e conducted on a sample of No. 2 fuel oil a n d on two biomass pyrolysis oils. The No. 2 fuel oil, supplied by Union Oil of California, has density, 0.86 g/cm3; viscosity, 2.2 CP( a t 40 "C); a n d a boiling point range of 182351 "C. ~~~

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(19) Elliott, D. C. In Pyrolysis Oils from Biomass: Producing, Analyzing, and Upgrading; Soltes, E. J., Milne, T. A,, Eds.; ACS Symposium Series 376; American Chemical Society: Washington, DC, 1988; pp 55-65. (20) Huffman, D. R.; Vogiatzis, A. J.; Graham, R. G.; Freel, B. A. Presented at the First European Forum on Electricity Production from Biomass and Solid Wastes by Advanced Technologies,Florence, Italy, November 1991. (21) Piskorz, J.; Scott, D. S.; Radlein, D. In Pyrolysis Oils from Biomass: Producing, Analyzing, and Upgrading; Soltes, E. J., and Milne, T. A,, Eds.; ACS Symposium Series 376; American Chemical Society: Washington, DC, 1988; pp 167-178. (22) Mawid, M.; Aggarwal, S. K. Combust. Flame 1991,84, 197209. (23) Sirignano, W. A,; Law, C. K. In Euaporation-Combustion of Fuels, Zung, J. T., Ed.; Advances in Chemistry Series 166; American Chemical Society: Washington, DC, 1981; Chapter 1, pp 3-25. (24) Makino, A.; Law, C. K. Combust. Flame 1988,73,331-336. (25) Law, C. K. Prog. Energy Combust. Sci. 1982,8 , 171-201. (26) Megaridis, C. M.; Sirignano, W. A. Combust. Sci. Technol. 1992, 87, 27-44. (27) Sangiovanni, J. 3. A n Investigation of the Combustion Characteristics of Fuel Droplet Arrays; United Technologies Research Center Report No. R80-954425, 1980. (28) Lefebvre, A. H. In Fossil Fuel Combustion: A Source Book; Bartok, W., Sarofim, A. F., Eds.; John Wiley & Sons: New York, 3991; Chapter 9, pp 529-652. (29) Williams, A. Prog. Energy Combust. Sci. 1976,2, 167-179.

Table 1. Characteristics of the Biomass Oils Oak Oil 154 Pine Oil 150 vapor cracker temperature ("C) proximate analysis ash (wt 5%) water (wt %) volatiles (diff., wt %) fixed carbon (wt 5%) elemental analysis (water-free basis) carbon (wt %) hydrogen (wt S ) oxygen (diff., wt 5%) nitrogen (wt 5%) sulfur (wt a) calcium (ppm) potassium (ppm) sodium (ppm) higher heating value ( M J k g ) PH density (g/mL) viscosity (cP) a t 21 "C at 30 "C a t 40 "C a t 50 "C

450

510

0.05 16.1 69.75 14.1

0.05 18.5 65.95 15.5

55.6 5.0 39.2 0.1 40.05 80 40 10 22.5 2.8 1.23

56.3 6.5 36.9 0.3 40.05 60 < 10 10 23.0 2.9 1.21

666 322 141 80

175 96 53 30

The biomass oils a r e produced by t h e National Renewable Energy Laboratory (Golden, CO) from t h e pyrolysis of oak or pine particles i n a vortex reactor operating at 625 "C and ~ oak and pine oils of this study coupled to a vapor ~ r a c k e r .The result from vapor cracker temperatures of 450 and 510 "C, respectively. Because some char particles a r e entrained into t h e vapor cracker, t h e biomass oils a r e filtered prior t o t h e combustion experiments. The characteristics of t h e oak a n d pine oils a r e listed i n Table 1. Relative to petroleum-based liquid fuels, t h e biomass pyrolysis oils contain large amounts of water a n d organic oxygenates and a r e quite acidic. The components of t h e biomass oils also span a wide range of volatilities. The viscosities of t h e two biomass oils differ substantially, as do their physical states. As Figure 1 portrays, t h e oak oil is a continuum ( t h e d a r k spherical inclusion is an a i r bubble), whereas t h e pine oil appears to be a suspension with solids t h a t a r e not fully solubilized at room temperature. Neither of t h e oils displays evidence of an emulsion character, which would be manifested a s a dispersion of multiple miniature spherical droplets within a continuous phase.30 The pyrolysis oil combustion experiments a r e conducted in Sandia's Biomass Fuels Combustion System (BFCS), which consists of a droplet generator, a laminar flow reactor, and a video imaging system. Illustrated in Figure 2, t h e droplet generator is a n adaptation of t h e design by Green et u L . , ~ ~ which features droplet production by t h e application of aerodynamic principles. Biomass oil is pumped slowly by syringe pump through t h e inner capillary tube and is disengaged a s a droplet when t h e surface tension holding t h e liquid at t h e capillary tip is overcome by t h e d r a g force of t h e coaxial gas flow. A cooling jacket surrounding t h e coaxial gas prevents in-line boiling of biomass oils with highly volatile components. The aerodynamic droplet generator produces uniformly sized droplets of 300-700 p m , spaced greater t h a n 60 diameters a p a r t t o ensure that t h e droplets b u r n independently of one an~ther.~~,~~ The biomass oil droplets a r e released downwardly into t h e centerline of t h e BFCS laminar flow reactor, operating at a total g a s flow of 49 L(STD)/min. A laminar flow burner a t t h e top of t h e reactor provides t h e high-temperature gas environment for t h e combustion experiments. A mixture of (30) Marrone, N. J.; Kennedy, I. M.; Dryer, F. L. Combust. S a . Technol. 1983,33, 299-307. (31) Green, G. J.; Takahashi, F.; Walsh, D. E.; Dryer, F. L. Rev. Sci. Instrum. 1989,60, 646-652. (32) Sangiovanni, J. J.; Dodge, L. G. Symp. (Int.1 Combust., [Proc.] 17th 1979,455-465.

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50 pm Figure 1. Photographs of filtered biomass pyrolysis oils. Magnification, 150x. (a)Oak oil; (b) pine oil. The dark sphere in (a) is a n air bubble.

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FLAT FLAME BURNER FACE CENTERING RING

Figure 2. BFCS aerodynamic droplet generator. hydrogen and methane is supplied through each of the more than 600 fuel tubes. Oxygednitrogen mixtures are fed through the honeycomb surrounding the fuel tubes-forming small diffusion flames at each fuel tube end. The stoichiometry of the reactor's HdCHdOfl2 flame is tailored to produce 1600 K postflame gases with oxygen concentrations of 14-33 mol %. The quartz walls of the BFCS reactor provide optical access for imaging of the burning droplets. In-situ imaging of burning biomass droplets is accomplished with a Panasonic WV-CL700 super-VHS stationary chargecoupled device camera, a Panasonic AG7350 super-VHS recorder, and a Sony PVM 13426 super-VHS monitor. Back lighting is provided by a strobe, which is laser-triggered to flash when a droplet passes through the focal point of the system. Adjustment of the vertical position of the reactor,

relative to the optical system, permits imaging at different heights along the reactor chimney so that droplet sizes, morphologies, and velocities can be monitored as functions of residence time. Droplet velocities are measured along the length of the reactor so that droplet residence times can be determined as a function of distance. To make the measurement, the laser trigger system is programmed to signal the back-lighting strobe with two pulses, separated by 500 ,us. The video camera records the two positions of the droplet, and the droplet velocity is calculated by dividing the distance of separation by the time of pulse spacing. The instantaneous values of the reciprocal of velocity are then integrated over the distance along the reactor in order to calculate droplet residence times. In selected biomass oil combustion experiments, solid residues are collected manually from the exhaust stream of the BFCS reactor for subsequent analysis. Particle morphology and surface composition are determined by secondary electron imaging on a JEOL Model 840 scanning electron microscope (SEM), operating at 15 kV, and by energydispersive X-ray spectrometry (EDS) with a Tracor Northern 5502 spectrometer. Combustion experiments on biomass oil solid residues from the BFCS reactor are conducted in Sandia's Captive Particle Imaging (CPI)system, described in detail by Hurt and Davisa3 In this reactor, particles are placed on a low-density support and inserted into the flow reactor through a n open test section in the quartz reactor wall. The support is surrounded by a small conical cooling coil that maintains the particle at 200300 "C, while it is brought into the focal volume of a modified long-focal-length microscope. The cooling coil is then retracted, the particle is rapidly heated by the surrounding gases, and its ignition, combustion, and burnout behavior are imaged with reflected visible light.

Results and Discussion BFCS Combustion Experiments with No. 2 Fuel Oil. When introduced into the BFCS reactor at 24 mol % 0 2 , 320 p m droplets of No. 2 fuel oil ignite at 7 ms and burn quiescently until full burnout is achieved at 139 ms. Except for the initial millisecond and final two milliseconds (which are accompanied by a blue flame), No. 2 fuel oil droplets burn in a yellow flame indicative (33) Hurt, R. H.;Davis, K. A. Near-Extinction and Final Burnout in Coal Combustion accepted to the Twenty-FifthSymposium (International) on Combustion, Irvine, California, August, 1994.

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Figure 3. Normalized droplet diameter squared as a function of residence time in the BFCS reactor for No. 2 fuel oil. Instantaneous droplet diameters normalized to the initial droplet diameter of 320 ,um. Gas composition, 24 mol % 0 2 .

of soot. Other than soot, no carbonaceous solids are produced during the combustion of No. 2 fuel oil in the BFCS reactor. The soot-forming and non-coking tendencies of No. 2 fuel oil have been previously noted.34 Figure 3 displays the monotonic decrease in droplet size as the No. 2 oil burns in the BFCS reactor. For a single-componentfuel burning at steady state, a plot of diameter squared versus time is linear, in accordance with the constant surface regression rate characteristic of heat and mass diffusion control.25 The slight curvature in the data of Figure 3 is consistent with literature results on the quiescent burning of nonviscous solutions of components with a fairly narrow range of volatilities.25928 BFCS Combustion Experiments with Oak Oil. Biomass oil burns very differently from No. 2 fuel oil. Figure 4 portrays oak oil droplets burning in an atmosphere of 24 mol % 0 2 in the BFCS reactor. Evident from the photograph are the different stages of biomass oil droplet combustion. After release from the droplet generator, the droplets ignite at a residence time of approximately 12 ms and burn quiescently in a spherical blue flame approximately two droplet diameters wide. Due to the long exposure time of the photograph (3 min), the burning ofthousands of droplets is recorded in Figure 4,and the succession of moving spherical blue flames appears as a continuous blue streak. After 34 ms, droplet burning is briefly accompanied by the onset of broadband luminosity, which appears as the bright oval-shaped region, about 5 mm in diameter, in Figure 4. Very small luminous fragments are occasionally observed to be released from this luminous region. The broadband luminous burning gives way to a fainter, smaller-diameter blue flame, occasionallypunctuated with additional luminous bursts that are much smaller scale than the initial broadband luminous event. Combustion of the biomass oil concludes in a yellow flame, indicative of the presence of soot. (34)Bonczyk, P.A.;Sangiovanni, J. J. Combust. Sci. Technol. 1984, 36,135-147.

Figure 4. Photograph of oak oil droplets burning in the BFCS reactor at 24 mol % 0 2 . Blue flame enhanced during photographic development.

Figure 5 presents in-situ video images of droplets of oak oil, initially 320 pm in diameter, burning in 24 mol % 0 2 for residence times of 21-121 ms, corresponding to the different stages of burning. Under these conditions, full burnout occurs a t 149 ms. Because the exposure time is only 1 p s (the duration of a pulse from the laser-triggered strobe), these images of moving droplets are frozen at each selected residence time. The intensity of the strobe, the source of the back-lighting, overwhelms any luminosity from the flame, so no details of the flame are evident from the images in Figure 5. Figure 5a depicts the quiescent phase of biomass oil droplet combustion, sustained from 12 to 34 ms, during which burning occurs in a blue flame and the spherical shape of the droplet is not perturbed. Similar behavior is observed throughout the combustion lifetime of

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500 pm Figure 6. Video images of droplets of oak oil, burning at 24 mol % 0 2 in the BFCS reactor. Residence times of (a)21.4 ms; (b) 34.8 ms; (c) 42.7 ms;(d) 121.2 ms.

droplets of pure methanol or acetone in the BFCS reactor. The quiescent combustion stage of the oak oil terminates, however, at 34 ms, the residence time corresponding to the beginning of the broadband luminous region in Figure 4. Here the droplet images change abruptly, as demonstrated in Figure 5b. As Figure 5b reveals, combustion at this stage is characterized by sudden buildup of vapor within the droplet, leading to expansion and distortion of the droplet. Rupture of the droplet surface is accompanied by the release of fuel vapor and small droplet fragments-most with diameters of tens of microns or less. Successive images of the same droplet, not shown here, reveal that droplet expansion occurs within 500 ps. As noted by Yap et al.35for the combustion of hexanehexadecane mixtures, the broadband luminosity observed during the microexplosion most probably results from radiating soot particles produced by gas-phase pyrolysis of fuel vapor, present at high concentration in the vicinity of the droplets at the time of the microexplosion. After the release of vapor during the microexplosion, oak oil droplets coalesce, due to surface tension, as depicted in Figure 5c. Burning at this stage occurs (35) Yap, L. T.; Kennedy, I. M.; Dryer, F. L. Combust.Sci. Technol. 1984,41,291-313.

again in a blue flame, fainter than the first. The nonspherical shape and small yet identifiable regions of transparency in the droplet of Figure 5c indicate that vapor is still trapped inside droplets at this stage, although droplet swelling is not as pronounced at this longer residence time. The final stage of burning of oak oil is depicted by the droplet in Figure 5d, taken at a residence time of 121 ms. This image reveals that droplets at this stage are indeed surrounded by a soot cloud, which accounts for the observed yellow flame. It is also interesting to note that even though this residence time corresponds to the last 30 ms of burning, droplets are still of an appreciable size. In the evaporation and combustion of multicomponent liquid fuel droplets, there are two extremes of beh a v i 0 r . ~ ~ - ~In, 3the ~ first, corresponding to low values of the liquid-phase Peclet number Pe or Lewis number Le,mass transfer within the liquid phase is rapid, and surface regression due to evaporation is balanced by the migration of volatilizing components to the droplet (36) Ramos-Arroyo, N. A; Berrekam, H.; Chauveau, C.; Odbide, A; Gtikalp, 1. uAnExperimental Study of Convective Bicomponent Droplet Vaporization,” presentation at the Western States Section Meeting of the Combustion Institute, Berkeley, California, Fall, 1992.

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surface. This “batch distillation”mechanism is favored in situations of rapid internal circulation, which may be induced by large relative velocities between droplet and gas. The other extreme, favored by highly viscous liquids with minimal internal circulation, corresponds t o high Pe and Le and to rate control by liquid-phase mass transfer. In this case, pockets of vapor build up within the droplet as a result of the nucleation of volatilizing componentsthat are deterred from reaching the droplet surface. Swelling of the vapor-filled droplet occurs until surface tension forces, holding the droplet together, can no longer balance the pressure differential across the liquid surface. At that point, the droplet surface ruptures, resulting in a microexplosion. The combustion of biomass oils adheres more closely to the latter of the above extreme cases for three reasons: the biomass oils are viscous; internal circulation within the droplets is low, due to the low droplet Reynolds number Re of our system (Re x 2); and the biomass oils comprise components of a wide range of volatilities. The physical properties of biomass oil also change during a droplet’s combustion lifetime, as volatile species evaporate and remaining species undergo thermally induced chemical transformations that could lead to higher viscosity. Quiescent burning of the biomass oil occurs until the temperature at some position within the droplet reaches the local homogeneous nucleation t e m p e r a t ~ r e .For ~ ~ oak oil, that point is reached a t a residence time of 34.8 msec, when the microexplosion occurs. Our observation of the time of droplet expansion of ~ 5 0 psec 0 is consistent with the observations of Lasheras et u Z . ~ * that homogeneous nucleation occurs in a time scale on the order of 100 psec. The sequence of quiescent burning followed by microexplosion has been observed for a variety of other multicomponent fuels: mixtures of alkanes;35alkane/ alcohol mixture^;^^^^^ alcohollether solutions;39residual heavy fuel ethanomo. 2 fuel oil mixtures;42 alkane/water emulsions;43and an alkane with dissolved pyrolysis products.44 As Lasheras et observe, the manner of the microexplosion depends on whether the multicomponent fuel is a solution or an emulsion. Differences in the two cases arise as consequences of differences in fluid “structure,”the conditions for internal boiling, and the nature and growth of the bubbles formed during n u c l e a t i ~ n . ~A~major , ~ ~ difference between fuel solutions and emulsions concerns swelling. In solutions, droplet diameters may increase by a factor of 337 prior to the microexplosion. In the case of ~

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(37) Lasheras, J. C.; Femandez-Pello, A. C.; Dryer, F. L. Symp. (Int.) Combust., [Proc.] 18th 1981, 293-305. (38) Lasheras, J. C.; Femandez-Pello, A. C.; Dryer, F. L. Combust. Sci. Technol. 1980, 22, 195-209. (39) Wood, B. J.; Wise, H.; Inami, S. H. Combust. Flame 1960,4 , 235 -242. (40)Marrone, N. J.; Kennedy, I. M.; Dryer, F. L. Combust. Sci. Technol. 1984,36, 149-170. (41) Urban, D. L.; Dryer, F. L. Symp. (Int.) Combust. [Proc.]23rd 1990, 1413-1421. (42) Lasheras, J . C.; Yap, L. T.; Dryer, F. L. Symp. (Int.) Combust., [P~oc.] 20th 1984, 1761-1772. (43) Lasheras, J. C.; Femandez-Pello, A. C.; Dryer, F. L. Combust. Sci. Technol. 1979,21, 1-14. (44) Shaw, B. D.; Dryer, F. L.; Williams, F. A.; Haggard, J. B., Jr. Sooting and Disruption in Spherically Symmetrical Combustion of Decane Droplets in Air. Presented at the 38th Congress of the International Astronautical Federation, Brighton, England, 1987. (45) Lasheras, J. C.; Kennedy, I. M.; Dryer, F. L. Combust. Sci. Technol. 1981,26, 161-169.

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emulsions, however, where droplets of 1-5 pm are dispersed throughout a continuous phase, no swelling of the overall emulsion droplets occurs prior to disruptive fragmentati0n.~5 Figure 6 portrays four oak oil droplets at the time of microexplosion. Despite the high water content (16.1%) of the oak oil, it appears to behave more like a multicomponent solution than an emulsion since the oak oil droplets swell by up t o a factor of 3 in diameter prior to rupturing. This behavior is consistent with the onephase nature evident in Figure 1. Compared to the in-situ images of swollen droplets of other multicomponent fuels during ~ o m b u s t i o n , ~ ~ , ~ ~ , ~ ~ , 4 5 those of the biomass oils appear to be unique. The oak oil droplets in Figure 6 exhibit a departure from sphericity and an irregular distribution of outer surface thickness during the microexplosion. Numerous bubbles, bordered by liquid membranes, appear to be present within the swollen biomass oil droplets. One possible explanation of the unique appearance of the swollen biomass oil droplets is the high concentration of oxygencontaining organic species, which make this fuel more prone to polymerization r e a c t i o n ~ . ~These ~ t ~ ~biomass oils are known t o polymerize over a period of days at a temperature of 37 “C and hours at 90 0C.47 At the higher temperatures incurred during combustion, it is very likely that polymerization of the oxygen-rich organic constituents would occur within the residence times of our experiment-producing very highly viscous material that may not permit rapid coalescence of individual bubbles within the droplets. Polymerization or liquid-phase pyrolysis to produce solids would also affect nucleation and bubble growth, as these processes have been shown t o be affected by the presence of ~ u r f a c e s - f i l a m e n t s ~and ~ ~carbon ~~~~~ particles.49 Comparison of Oak and Pine Oils. Under the same combustion conditions, pine oil displays the same stages of burning as oak oil: (a) quiescent burning, accompanied by a blue flame, in which the spherical shape of the droplet is not perturbed; (b) the microexplosion, accompanied by broadband luminosity, sudden expansion and distortion of the droplet, and release of fuel fragments; (c) coalescence of the droplet (due to surface tension) while burning in a faint blue flame, with the release of occasional small bright luminous bursts; and (d) the final stage of burning, which occurs in a yellow flame, indicating the presence of soot. The quiescent phase of burning lasts longer for the pine oil droplets than for the oak oil droplets-a result, perhaps, of the pine oil’s lower viscosity, which would lead to a higher liquid-phase diffusivity and more rapid liquidphase mass transfer. Although the pine and oak oils behave similarly during combustion, differences are apparent during the microexplosion. As shown in Figure 6, droplets of oak oil behave fairly uniformly, usually releasing only tiny fragments of tens of microns or smaller. Droplets of pine oil, on the other hand, exhibit more variability, as (46) Morrison, R. T.; Boyd, R. N. Organic Chemistry, 4th ed.; Allyn and Bacon: Boston, 1983. (47) Czemik, S.; Johnson, D. K.; Black, S. Biomass Bioenergy, in press. (48) Dryer, F. L. Symp. (Int.) Combust., [Proc.] 16th 1976, 279295. (49) Sangiovanni, J. J.; Kesten, A. S. A n Experimental Study of the Ignition and Combustion Characteristics of Fuel Droplets; United Technologies Research Center Report No. R76-952180 (1976).

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a

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500 pm Figure 6. Video images of droplets of oak oil, microexplodingwhile burning at 24 mol % 0 2 in the BFCS reactor. Residence time of 34.8 ma.

shown in Figure 7. Swollen pine oil droplets often exhibit high transparency, as in Figure 7a-a property only rarely displayed by oak oil. Many microexploding pine oil droplets contract to one primary droplet, as those of oak oil, but many break into two or more fkagments of appreciable size. Figure 8 displays droplet fragments observed at 64.9 ms, just 18 ms after the microexplosion. By this residence time, most of the fragments have assumed a spherical shape. The close proximity of these fragments ensures that they have all evolved from the same parent droplet since oil droplets are introduced into the reactor at >60 droplet diameters apart. Although we are not certain of the cause, the greater propensity of the pine oil droplets to fragment (compared to the oak oil droplets) appears to be due to differences in the viscosities of the two oils. The lower viscosity of the pine oil permits more rapid coalescence of gas bubbles within the oil droplet and more rapid transfer of the bubbles to the droplet’s outer surface, where surface rupture occurs, resulting in droplet fragmentation. Other factors may have influence, however, eg., surface tension and the nature and quantity of the vaporizable components generated by the two oils at the time of microexplosion. It should also be noted that despite the suspensionlike character of the pine oil at room temperature, this

oil displays solution-like behavior during combustion since it undergoes substantial swelling prior to the microexplosion, as reported for solutions.34 It is likely that the suspended solids are soluble at the droplet temperatures achieved during combustion. Droplet Size Distributions. More detail of the burning history of the biomass oil droplets is obtained from a n analysis of the sizes of droplets recorded by video-imaging over the range of droplet residence times in the reactor. Figure 9 portrays oak oil droplet size distributions as a function of residence time in the BFCS reactor for the experimental conditions corresponding to Figure 5. When droplets are spherical, the droplet’s diameter is used. For nonspherical droplets, an equivalent diameter is calculated from the square root of the product of the lengths measured along two axes of the droplet. Except for the data at 98.7 ms, each of the droplet size distributions in Figure 9 represents approximately 100 drops. The droplet size distribution in Figure 9a depicts the monodisperse nature of the droplets generated in the BFCS. Within the quiescent burning regime (up to approximately 34 ms), the droplets stay spherical but swell slightly, as indicated by Figure 9b. This slight expansion of the droplet occurs despite the loss of droplet mass by he1 evaporation.

1138 Energy & Fuels, Vol. 8, No. 5, 1994

(a)

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(b)

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500 pm Figure 7. Video images of droplets of pine oil, microexploding while burning at 24 mol % 0 2 in the BFCS reactor. Residence time of 47.2 ms.

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200 pm Figure 8. Video images of droplet fragments of pine oil, burning at 24 mol % 0 2 in the BFCS reactor. Residence time of 64.9 ms.

Figure 9b shows that at 32.3 ms, a small proportion of oak oil droplets have undergone the sudden expansion that accompanies the microexplosion. The greatest

number of droplets reach the microexplosion stage at 34.8 ms, however, as illustrated by Figure 9c. Figure 9, d and e, demonstrates the coalescence and gradual

Energy & Fuels, Vol. 8, No. 5,1994 1139

Combustion of Biomass Pyrolysis Oils

' d

2:.;::

.s

e 0.08

*s0.20 -E 0.15--

32.3 msec

I

2060-

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n

55.6 msec

.. I

0.06

; 0.05

Size (microns)

-

-

-

Figure 9. Size distributions of droplets of oak oil burning at 24 mol % 0 2 in the BFCS reactor for (a) 26.6 ms;(b) 32.3 ms;(c) 34.8 ms; (d) 40.2 ms;(e) 50.4 ms;(055.6 ms;(g) 65.3 ms;(h) 98.7 ms.

contraction of the droplets after they have undergone the microexplosion. The occurrence of larger droplets again at 55.6 ms (Figure 90 coincides with the visual observation, at this residence time, of a secondary microexplosion, much smaller than the first one. Because during the primary microexplosiondroplets do not expand or release vapor in exactly the same manner, postmicroexplosion droplets are not identical and the second microexplosion occurs over a broader range of residence times than the first. Following the secondary microexplosion, any subsequent bursts of fuel vapor release are of considerably smaller scale and are more erratic in timing. The fact that, at 98.7 ms (Figure 9h), droplets are still of an appreciable size indicates that even at the late stages of burning, oak oil droplets contain significant volumes of vapor. The droplet size distributions of pine oil show the same trends in behavior as those of oak oil, with three minor exceptions: Pine oil swells less during the quiescent phase. During the microexplosion and afterward, droplet size distributions are shifted to slightly smaller diameters, reflecting the greater propensity of pine droplets to fragment during the microexplosion. The greater variability in the manner of the primary microexplosion of the pine oil droplets causes the second microexplosion of the pine oil droplets to occur over a n even broader range of residence times than for the oak oil droplets. Analysis of Biomass Oil Residues. All of the results presented above for the combustion of the biomass oils come from experiments performed in an

environment of 24 mol % 0 2 , where the microexplosion and full burnout of the oak oil occur at 34.8 and 149 ms, respectively. An increase in 0 2 concentration to 29% speeds up the entire combustion process: The microexplosion occurs at 26.6 ms, and full burnout occurs at 113 ms. At 19 and 14 mol % 0 2 , full burnout of oak oil does not occur within the residence times of the BFCS reactor, and the microexplosion is delayed to 40.2 and 53.2 ms, respectively. Similar results are found for pine oil. Under these conditions, all phases of droplet combustion are retarded, and solid residues are found in the reactor exhaust. Representative solid residues from biomass oils are portrayed in Figure 10, SEM photographs of solids produced from pine oil at 14 mol % 0 2 and manually extracted from the exhaust stream of the BFCS reactor. As shown in Figure loa, the solid residues in these experiments are of two types: dense, glassy spheres and fragile, thin-walled cenospheres. As the SEM photograph in Figure 10b reveals, the glassy spheres contain cavities that indicate the existence of vapor pockets within these spheres at the time of their hardening. EDS analysis of the glassytype solids reveals that they are composed of C and 0, in a mass ratio of approximately 4 to 1. Most of the material in these solids dissolves in methanol, so they have retained the polar character of the parent fuel and have not yet undergone severe carbonization. There are small amounts of insoluble residue left behind, however, that signify the initial stages of carbonization. EDS analysis of the cenosphere-type residues reveals a C to 0 mass ratio of approximately 9 to 1 in these

1140 Energy & Fuels, Vol. 8,No.5, I994

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. 1T ’W -

--

b

Figure 10. Scanning electron micrographs of solids produced from pine oil during burning at 14 mol % 0 2 in the BFCS reactor: (a)cenosphere and glassy types; (b)interior of a glassy type.

materials. Consistent with their lower oxygen contents, these solids do not dissolve in methanol. Nor do they dissolve in dichloromethane, a solvent for aromatic materials. They therefore appear to result from a later stage of combustion than the dense glassy residues-a stage in which carbonization has taken effect. This conclusion is consistent with the fact that residues, collected at the same time, from combustion at 19 mol % oxygen contain a higher proportion of cenospheres than the residues from 14 mol % oxygen. The oak oil produces the same types of solid residues as the pine oil, as illustrated in Figure 11, SEM photographs of residues from oak oil combusted at 19% oxygen. Figure 11,a and b, depicts three glassy solids and one cenosphere before and after gold-coating, respectively. The oxygen-rich dense solids, if uncoated, are susceptible to charging and prove to be unstable under the electron beam of the scanning electron microscope. After gold coating, however, the images of the oxygen-rich dense solids are sharpened, due to the cessation of the charging phenomenon associated with these polar solids. The carbon-rich, oxygen-deficient cenospheres, on the other hand, exhibit stability under the electron beam and show no evidence of charging. The cenospheres are mechanically fragile, as evident from the collapse of the one in Figure lla,b, as a result of the evacuation step of the gold-coating procedure. The thin nature of the cenosphere membrane (-0.2 pm), portrayed in Figure l l c , partially accounts for this fragility.

Figure 11. Scanning electron micrographs of solids produced from oak oil during burning at 14 mol % 0 2 in the BFCS reactor: (a)three glassy type and one cenosphere type before gold coating; (b) three glassy type and one cenosphere type after gold coating, (c)close-up of a lighbcolored cell membrane of the cenosphere residue in (b).

Evident from Figure l l a , b is the cellular nature of the biomass oil cenosphere strudure, a structure similar to those displayed in the char of a softening bituminous coal5Oand in the partially oxidized residue of a bitumenin-water emulsion droplet.51 The cells resemble those associated with Marangoni convection, which results from local gradients in surface The mul~~~

~

(50)Fletcher, T. H.; Hardesty, D. R. Compilation of Sadiu Coal DevolatilizationData; SAND92-8209UC-362,May, 1992. (51) Hampartsoumian,E.; Hannud, B.; Williams, A. J . Inst. Energy

1993,66,13-16.

Combustion of Biomass Pyrolysis Oils

Energy & Fuels, Vol. 8,No. 5,1994 1141

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200 pm Figure 12. Video images of an oak oil cenosphere type residue burning in the CPI flow reactor at 6 mol % 0 2 . Residence times of (a) 0 s; (b) 0.52 s; (c) 0.58 s; (d) 0.93 s.

A "

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200 pm Figure 13. Video images of a pine oil glassy residue burning in the CPI flow reactor at 6 mol % 0 2 . Residence times of (a) 0 s; (b) 0.13 s; (c) 0.20s; (d) 0.30 s; (e) 0.90 s; (01.07 s; (g) 1.40 s; (h) 2.17 s.

ticomponent nature of the biomass oils, coupled with the carbonization transformations occurring in these materials at the latter stages of combustion, may be responsible for such local gradients. EDS analysis of the darker regions bordering the cells in Figure 11 shows them to be mostly carbon (C to 0 mass ratio of 19), whereas the lighter colored cell membranes are higher in oxygen (C to 0 mass ratio of 2). Figure l l c suggests that the material comprising these cell membranes is very smooth and glassy, similar to the glassy carbons produced by the polymerization of oxygen-rich furfuryl The thin, oxygen-rich nature of this membrane material would most probably make it more susceptible to oxidation than the more carbon-rich border regions. It is interesting to note that the biomass oil cenospheres bear no resemblance to the rough, porous, nonglassy cokes produced during the combustion of residual oils from p e t r ~ l e u m Whereas . ~ ~ ~ ~the ~ ~cokes ~ (52)Oertel, H., Jr. In Conuectiue Transport and Instability Phenomena;Zierep, J., Oertel,H., Jr., Eds.; G. Braunsche Hofbuchdruckerei und Verlag Karlsruhe, 1982; pp 3-24. (53)SBrensen, T. S. In Convective Tramport and Instability Phenomena;Zierep, J., Oertel, H., Jr., Eds.; G. Braunsche Hofbuchdruckerei und Verlag Karlsruhe, 1982; pp 339-369. (54) Kenning, D. B. R. Appl. Mech. Rev. 1968,21, 1101-1111. (55) Senior, C. L.; Flagan, R. C. Symp. (Znt.)Combust., [Pm.]20th 1984,921-929. (56) Urban, D. L.; Dryer, F. L. Heat Transfer Combust. System, HTD 1990,142,83-88. (57) Urban, D. L.; Huey, S. P. C.; Dryer, F. L. Symp. (Znt.)Combust., [Proc.]24th 1992,1357-1364.

from petroleum-based oils arise from the liquid-phase pyrolysis and polymerizationof hydrocarbon-rich,oxygendeficient petroleum c o n s t i t ~ e n t s , 4 ~ the * ~biomass ~ - ~ ~ oil residues arise from the thermally induced pyrolysis and/ or polymerization of oxygen-rich organic material. Combustion of Biomass Oil Residues. Figure 12 depicts the burning, within the CPI reactor, of a cenosphere-type residue produced from combustion of oak oil at 14 mol % 0 2 in the BFCS reactor. As predicted, the thin, oxygen-rich membranes comprising the cells are the first material to be depleted. The carbon-rich skeleton then slowly burns away, leaving no observable residue. Figure 13 reveals the different stages of combustion of a glassy sphere residue of pine oil, collected after burning in 14 mol % oxygen in the BFCS reactor. Upon heating, the glassy sphere rapidly expands to the cenosphere-type residue, which retains its skeletal structure throughout subsequent burning until no detectable residue exists. The absence of any noticeable ash, after combustion of the residues, is consistent with the fact that no inorganic elements are detected by EDS in the uncombusted oil residues. The transformation, during combustion, of the glassy sphere to the enlarged cenosphere is also consistent with the earlier observa(58) Be&, J. M.; Chigier, N. A. Combustion Aerodynamics; Meger: Melbourne, FL, 1983; Chapter 6. (59IHotte1, H. C.; Williams, G. C.; Simpson, H. C. Symp. (Znt.) Combust., [Pm.]5th 1955, 101-129.

1142 Energy & Fuels, Vol. 8, No. 5, 1994

tions, related to oxygen content and solubility, that indicate that the glassy spheres are precursors to the cenospheres and that the combustion of the cenospheres is the final step of biomass oil droplet combustion. This finding accounts for the observation that at the later stages of combustion in the BFCS reactor, the biomass oil “droplet” diameters, obtained by in-situ imaging, maintain an appreciable size until full burnout. The transformation of the glassy-type residue into the swollen cenosphere parallels the plasticizing behavior of softening bituminous coals and accounts, perhaps, for the similar cellular appearance of the biomass oil cenospheres and the bituminous coal pyrolysis product char of Fletcher.50

Conclusions Single droplet experiments with No. 2 fuel oil and oak and pine oils highlight several contrasts in the combustion behavior of these multicomponent fuels. For No. 2 fuel oil, efficient mass transfer within the droplet (related to the fuel’s low viscosity) and the fuel’s narrow range of component volatilities lead to quiescent burning throughout the droplet’s lifetime. The hydrocarbon-rich, oxygenate-deficient nature of the No. 2 fuel oil’s components facilitates soot production in the fuel vapor cloud surrounding the droplet. No coke or solid residues are found from the droplets themselves. In contrast, the biomass oils initially burn quiescently, free of soot, in a blue flame, indicative of volatile oxygenated components. The broad range of component volatilities and inefficient mass transfer within the viscous biomass oils bring about an abrupt termination of the quiescent stage, however, at about a quarter of the way into the droplet’s lifetime. Vapor suddenly builds up within the droplets, causing rapid swelling and distortion, followed by surface rupture and the release of fuel fragments. Droplet coalescence follows,

Wornat et al.

and subsequent burning occurs in a faint blue flame with occasional smaller scale releases of bursts of vapor. At the late stages of biomass oil combustion, droplets are accompanied by clouds of soot, produced from the gas-phase pyrolysis of fuel vapor. Liquid-phase polymerization and/or pyrolysis of the oxygenate-rich biomass oils leads to the formation of carbonaceouscenospheres, whose burnout signifies the final stage of biomass oil droplet combustion. Oak and pine oils behave very similarly during combustion, though differences in their physical properties cause pine oil to show more variability during the microexplosion and more susceptibility to fragmentation. Changes in oxygen concentration alter the timing of the events during biomass oil combustion, but not their nature. Despite large differences in fuel properties and combustion mechanisms, the burning times of the biomass oils and No. 2 fuel oil are fairly comparable-those of the biomass oils about 10% longer under our experimental conditions. This finding underlines the importance of the microexplosion in dispersing the biomassderived fuel into the oxidizing environment.

Acknowledgment. The authors gratefully acknowledge the National Renewable Energy Laboratory for financial support of this project. They thank Stefan Czernik of NREL for supplying the biomass oils and characterization data. The authors also express their appreciation to Donald Hardesty for management of the project; Joseph Sangiovanni (UTRC),Allen Salmi, Ken Hencken, Bill Kent, and Michel Bonin for help in the design of the BFCS; Eric Frew and Lynn Yang for their efforts in data analysis; James Ross, Alan Pomplun, and Lynda Hadley for their contributions in imaging and photography; and Robert Hurt, Adel Sarofim (MIT), Antonio D’Alessio (Universita Degli Studi di Napoli), and Alon Gany (Technion-Israel Institute of Technology) for helpful discussions.