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Real-Time and Post-reaction Microscopic Structural Analysis of Biomass Undergoing Pyrolysis Thomas J. Haas,† Mark R. Nimlos,‡ and Bryon S. Donohoe*,† Chemical and Biosciences Center, and National Bioenergy Center, National Renewable Energy Laboratory, 1617 Cole BouleVard, Golden, Colorado 80401 ReceiVed March 6, 2009. ReVised Manuscript ReceiVed May 15, 2009
The structural complexity of unprocessed plant tissues used for thermochemical conversion of biomass to fuels and energy impedes heat and mass transfer and may increase the occurrence of tar-forming secondary chemical reactions. At industrial scales, gas and liquid products trapped within large biomass particles may reduce net fuel yields and increase tars, impacting industrial operations and increasing overall costs. Real-time microscopic analysis of poplar (Populus sp.) wood samples undergoing anoxic, pyrolytic heat treatment has revealed a pattern of tissue and macropore expansion and collapse. Post-reaction structural analyses of biomass char (biochar) by light and transmission electron microscopy have provided direct structural evidence of pyrolysis product mass-transfer issues, including trapped pyrolysis products and cell wall compression, and have demonstrated the impact of heat-transfer problems on biomass particles. Finally, microscopic imaging has revealed that pyrolyzed/gasified biochars recovered from a fluidized bed reactor retain a similar pre-reaction basic plant tissue structure as the samples used in this study, suggesting that the phenomena observed here are representative of those that occur in larger scale reactors.
Introduction Gasification technologies have been used to produce fuels since the early 19th century;1 however, the ubiquity of natural gas distribution systems and increasing worldwide use of crude oil distillates had largely halted industrial gasification efforts by the mid-20th century. While the primary historical source of matter and energy for gasification has been of fossil origin, modern demands for environmentally friendly, politically secure, and non-fossil-based fuel and electricity has once again turned attention to biomass. Gasification and other biomass-based thermochemical energy production processes, such as pyrolysis and liquefaction, are important and necessary options to add to the growing portfolio of renewable energy production strategies. Additionally, biomass pyrolysis waste (“biochar”) may become an important fertilization, soil and water conditioning, and carbon sequestration material in many areas, especially those with nutrient-poor and degraded soils.2,3 One of the notable advantages of gasification technology is that it has been used and refined for 2 centuries, has proven its production capabilities at industrial scales, and continues to be an active area of academic and industrial research.1,4-7 The principal challenge for thermochemical energy production today * To whom correspondence should be addressed. E-mail:
[email protected]. † Chemical and Biosciences Center. ‡ National Bioenergy Center. (1) Everard, S. The History of the Gas Light and Coke Company 18121949; Ernest Benn: London, U.K., 1950. (2) Laird, D. A. The charcoal vision: A win-win-win scenario for simultaneously producing bioenergy, permanently sequestering carbon, while improving soil and water quality. Agron. J. 2008, 100 (1), 178–181. (3) Glaser, B.; Lehmann, J.; Zech, W. Ameliorating physical and chemical properties of highly weathered soils in the tropics with charcoalsA review. Biol. Fertil. Soils 2002, 35 (4), 219–230. (4) United States Department of Energy. The early days of coal research. http://www.fe.doe.gov/aboutus/history/syntheticfuels_history.html (accessed Jan 22, 2009).
is switching from a proven source with well-known physical and chemical properties (coal) to an unproven and structurally and chemically complex source (biomass and other plant-based waste materials), which are complex mixtures of cellulose, hemicelluloses, and lignin, as well as smaller amounts of proteins, lipids, and ash. These challenges are directly tied to crucial production goals, which include increasing energy yield and production scale and decreasing tar yields to improve activity and longevity of catalysts. To keep production costs as low as possible, biomass feedstocks must be minimally processed (i.e., chopped, ground, milled, and sieved). Energy used in feedstock processing and transportation can quickly turn an otherwise environmentally and economically friendly process unfriendly; however, as particle size increases, the added cellular- and tissue-scale barriers have an increasing impact on gasification and pyrolysis (Figure 1). Unfortunately, both size and complexity lead to problems. Herguido and colleagues report that gasified Maritime pine (Pinus pinaster) wood chips, compared to gasified Maritime pine sawdust, yield less gas, more tar, and more char, leading to significantly less overall carbon conversion in wood chips.8 Beaumont and Schwob state that, during fast pyrolysis, larger particle sizes yield more char and gas and less pyrolysis oil and that gas produced from larger particles has a different composition. They conclude that these effects are due to an increasingly non-isothermal process as the particle size in(5) Sasol. Technologies and processes. http://www.sasol.com/sasol_ internet/frontend/navigation.jsp?navid)1600033&rootid)2 (accessed Jan 22, 2009). (6) Abazajian, A. N.; Tomlinson, H. L.; Havlik, P. Z.; Clingan, M. D. Integrated Fischer-Tropsch process with improved alcohol processing capability. U.S. Patent 6,939,999, 2003/04/29, 2005. (7) Abbott, P. E. J.; Fernie, M. J. Process for the production of hydrocarbons. U.S. Patent 7,087,652, 2002/12/23, 2006. (8) Herguido, J.; Corella, J.; Gonzalezsaiz, J. Steam gasification of lignocellulosic residues in a fluidized-bed at a small pilot scalesEffect of the type of feedstock. Ind. Eng. Chem. Res. 1992, 31 (5), 1274–1282.
10.1021/ef900201b CCC: $40.75 2009 American Chemical Society Published on Web 06/23/2009
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Figure 1. Scale-down through biomass from the organismal to the molecular level. (A) Populus sp. in The Netherlands. (B) This study’s source of poplar wood. (C) Light micrograph of a cross-section of the unembedded poplar sample. Wood cell type examples: X, xylem element; F, wood fiber; R, ray parenchyma. (D) Transmission electron micrograph of a poplar xylem/fiber cell corner; SW, secondary cell wall; PW, primary cell wall; ML, middle lamella; L, cell lumen. (E) Artistic representation of the plant cell wall macromolecular structure. Red, cellulose microfibrils; yellow, hemicelluloses and pectins; green, lignin; blue, structural proteins. (F) Artistic representation of plant cell wall polymers (from top): cellulose, hemicellulose, lignin, and protein (not shown: pectins and ions). Scale bars: (B) 5 cm, (C) 100 µm, and (D) 2 µm.
creases.9 Haykiri-Acma determines that her study’s largest particles yield more char and require an over 70% increase in apparent activation energy compared to the smallest particles,10 and Rapagna` and Latif report decreases in gas yield and increases in char and heavy tar yields as biomass particle size is increased at all tested temperatures.11 While much work has been performed studying thermochemical processing of coal as well as deciphering the chemical mechanisms of the thermochemical breakdown of biomass and analysis of products, including microscopy and structural (9) Beaumont, O.; Schwob, Y. Influence of physical and chemical parameters on wood pyrolysis. Ind. Eng. Chem. Process Des. DeV. 1984, 23 (4), 637–641. (10) Haykiri-Acma, H. The role of particle size in the non-isothermal pyrolysis of hazelnut shell. J. Anal. Appl. Pyrolysis 2006, 75 (2), 211–216. (11) Rapagna, S.; Latif, A. Steam gasification of almond shells in a fluidised bed reactor: The influence of temperature and particle size on product yield and distribution. Biomass Bioenergy 1997, 12 (4), 281–288.
analysis of chars and other residues,12-18 to our knowledge, no known studies have microscopically imaged and analyzed the (12) Biagini, E.; Narducci, P.; Tognotti, L. Size and structural characterization of lignin-cellulosic fuels after the rapid devolatilization. Fuel 2008, 87 (2), 177–186. (13) Branca, C.; Iannace, A.; Di Blasi, C. Devolatilization and combustion kinetics of Quercus cerris bark. Energy Fuels 2007, 21 (2), 1078– 1084. (14) Cetin, E.; Moghtaderi, B.; Gupta, R.; Wall, T. F. Influence of pyrolysis conditions on the structure and gasification reactivity of biomass chars. Fuel 2004, 83 (16), 2139–2150. (15) Della Rocca, P. A.; Cerrella, E. G.; Bonelli, P. R.; Cukierman, A. L. Pyrolysis of hardwoods residues: On kinetics and chars characterization. Biomass Bioenergy 1999, 16 (1), 79–88. (16) Guerrero, M.; Ruiz, M. P.; Millera, A.; Alzueta, M. U.; Bilbao, R. Oxidation kinetics of eucalyptus chars produced at low and high heating rates. Energy Fuels 2008, 22 (3), 2084–2090. (17) Sharma, R. K.; Wooten, J. B.; Baliga, V. L.; Lin, X. H.; Chan, W. G.; Hajaligol, M. R. Characterization of chars from pyrolysis of lignin. Fuel 2004, 83 (11-12), 1469–1482. (18) Shen, D. K.; Gu, S.; Luo, K. H.; Bridgwater, A. V. Analysis of wood structural changes under thermal radiation. Energy Fuels 2009, 23 (2), 1081–1088.
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coal or biomass pyrolysis process in real time nor has any detailed microscopic structural study of pyrolyzing and postpyrolyzed biomass been carried out. The goals of this research are to structurally analyze the biomass and biomass-derived products during and after thermochemical treatment and to view the findings in light of the anatomy of plant tissues and any complications or restrictions that this structural complexity may impose on a thermochemical biomass-to-fuels process. Because of the complexity of the plant tissues used in this process, a greater understanding of the reactions of plant cellular and tissue structure to thermochemical processes in real time as well as any issues or implications that the use of relatively unprocessed biomass may introduce may help reduce tar formation and increase the energy yield of thermochemical production processes. Experimental Section Biomass Materials. Biomass used for hot-stage experiments was wood from an approximately 6-year-old hybrid poplar (Populus × canadensis), a fast-growing and high-yielding hardwood tree commonly proposed for use in biomass energy crop plantations,19 supplied by Idaho National Laboratory, Idaho Falls, ID (INL). Materials used in the generation of chars in the fluidized-bed reactor were corn stover pellets. Chars and control pellets were supplied by Calvin Feik, National Renewable Energy Laboratory (NREL).20 Sample Preparation for Microscopy. For real-time analysis of pyrolysis via light microscopy (LM) using the hot stage, large “matchsticks” of wood (approximately 10 × 10 mm in crosssection) were chiseled from the first- and second-year growth rings of a cross-section of poplar trunk. These pieces were cross-sectioned to 30 µm thick on a Leica RM2255 microtome with Leica-supplied razors (Leica, Wetzlar, Germany). For production of pyrolyzed material later fixed and embedded and used for LM and transmission electron microscopy (TEM) imaging, pieces of wood approximately 5 × 5 mm in cross-section and 1-2 cm in length were used (see below for information about hot-stage treatment). After treatment on the hot stage, some samples were embedded and used for light and electron microscopy with a microwave fixation/embedding protocol at 30 °C. Samples were fixed 2 × 4 min in 2.5% gluteraldehyde buffered in 0.1 M sodium cacodylate buffer at pH 7.2 (EMS, Hatfield, PA) under vacuum. The samples were dehydrated by treating with increasing concentrations of ethanol for 1 min at each dilution (30, 60, 90, and 3× 100% ethanol). After dehydration, the samples were infiltrated with LR White resin (Ted Pella, Inc., Redding, CA) at room temperature (RT) over 3 days in increasing concentrations of resin (30, 60, 90, 3× 100% resin diluted in ethanol). Infiltrated samples were transferred to flat-bottomed TAAB capsules (Ted Pella, Inc., Redding, CA) and polymerized in a nitrogen-purged vacuum oven at 60 °C for 48 h. Polymerized samples were sectioned to 2 µm with glass knives on a Leica EM UTC ultramicrotome (Leica, Wetzlar, Germany) for LM or sectioned to 80 nm with a Diatome diamond knife (EMS, Hatfield, PA) on a Leica EM UTC ultramicrotome for TEM. Light Microscopy and Hot-Stage Setup. LM images were captured on a Nikon Eclipse E800 microscope with an attached RT KE color SPOT camera and SPOT software (Diagnostic Instruments, Inc., Sterling Heights, MI). Adobe Photoshop (Adobe Systems, Inc., San Jose, CA) was used to crop, resize, and adjust contrast, brightness, and color levels of images and to assemble light microscopy figures. Helicon Focus (Helicon Soft Ltd., Kharkov, Ukraine) was used with some images to increase the depth of focus. (19) McKendry, P. Energy production from biomass (part 1): Overview of biomass. Bioresour. Technol. 2002, 83 (1), 37–46. (20) Carpenter, D. L.; Feik, C. J.; Gaston, K. X.; Jablonski, W.; Bain, R. L.; Phillips, S. D.; Nimlos, M. R., Pilot-scale gasification of corn stover, wood, switchgrass, and wheat straw. Energy Fuels, manuscript to be submitted.
Haas et al. The hot stage was an Instec HCS621G with a SCT200 temperature controller and WinTemp control software (Instec, Inc., Boulder, CO), manufacturer-modified for a temperature range of -190 °C (liquid N2 cooled) to approximately 700 °C, a light aperture of 3 mm, and gastight seals. Poplar sections were sandwiched between two 12 mm diameter glass coverslips and secured on the silver heating block with a thin, bored piece of aluminum and screws, which connected directly into the heating block. The reaction chamber was purged before and had a constant flow-through during heating of gaseous N2. For all hot-stage experiments, the heating block heating rate was approximately 150 °C/min. Instec-supplied, NREL-modified SPOT software camera control macros were used to capture image sequences with the RT KE color SPOT camera and record time and temperature from the hot-stage temperature controller via WinTemp software. Images were captured at a rate of approximately 1 frame/s depending upon hotstage and camera-controller response times. Image sequences were converted to.AVI video format and exported to ImageJ (National Institutes of Health, Bethesda, MD) for analysis. ImageJ macros were written to batch process and analyze videos. Processing of videos included the use of image binarization/threshold and measure functions of ImageJ. For area analysis, all white pixels were considered one unit of pore area and all black pixels were considered one unit of biomass area for area statistics and any interface between a black pixel and a white pixel was considered one unit of distance for perimeter statistics. Data from frames that were too out-of-focus or time points where the controller did not update heating block temperature before the image was captured were not included in the analysis. Full-frame resolution (1600 × 1200) with a 10× objective lens is approximately 3 µm/pixel. TEM. LR white-embedded ultrathin sections were collected on 0.35% Formvar-coated copper slot grids (SPI Supplies, West Chester, PA). Grids were post-stained for 6 min with 2% uranyl acetate (w/v) H2O and 6 min with 1% KMnO4 (w/v) H2O. Images were taken with a 4 megapixel Gatan UltraScan 1000 camera (Gatan, Pleasanton, CA) on a FEI Tecnai G2 20 Twin 200 kV LaB6 TEM (FEI, Hilsboro, OR). Adobe Photoshop was used to crop, resize, and adjust contrast and brightness and to assemble TEM figures. ImageJ was used to convert Digital Micrograph (Gatan, Pleasanton, CA).DM3 format images to 8-bit grayscale.TIF format.
Results Heat Treatment of Biomass Causes Material Expansion and Contraction. To study biomass undergoing pyrolysis in real time, a microscope hot stage was used. The biomass, sandwiched between two pieces of cover glass, is placed between a silver heating block and an aluminum cover plate in a gastight reaction chamber. The hot stage fits underneath the objective of a light microscope and the biomass heat treatment can be magnified with the microscope optics and recorded with a digital camera for later visualization and digital image analysis. A constant flow of gaseous nitrogen purges initial atmospheric gases and gases produced during biomass pyrolysis from the reaction chamber and prevents combustion of the biomass or its pyrolysis products. After visually analyzing videos of the biomass undergoing pyrolysis, a few things are noticeable. First, as the biomass is heated from room temperature (Figure 2A) to around 300 °C (Figure 2B), it expands in the x-y plane (i.e., plane perpendicular to the wood grain). z dimension (i.e., axis parallel to the wood grain) expansion is negligible (Supplemental Figure 3 in the Supporting Information). This moderate expansion could be due to heat-induced expansion of the gaseous contents of the biomass, expansion or phase change of specific molecular species in the cell wall, such as lignin, or escape of gases from chemical reactions taking place. The expansion also coincides with a darkening of cell walls, evidence of ongoing chemical
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Figure 3. Graph of image analysis data from 12 videos of poplar undergoing pyrolysis on the hot stage shows a trend of macropore behavior. Gross data for each type of analysis was smoothed using a 7-period centralized moving average. The relative pore area is shown in red, and the relative pore perimeter is shown in blue.
Figure 2. Still frames from a video of poplar undergoing pyrolysis on the microscope hot stage. Heating block temperatures: (A) 26 °C, (B) 299 °C, and (C) 501 °C. Red outlines of xylem cells track individual areas through each frame. Scale bars ) 1 mm.
reactions. As solid devolatilization begins, gases such as H2 and CH4 are being produced as well as new solid- and liquid-phase molecules, making the physical properties of the cell walls different from that of untreated biomass. These new products could be contributing to tissue expansion. It should also be noted that this temperature range is employed in biomass torrefaction, a type of thermal pretreatment used to decompose hemicelluloses and drive away water to increase gasification efficiency.21 As devolatilization continues and the biomass approaches a temperature of ∼350 °C and climbs to 500+ °C (Figure 2C), a flux of amber-colored liquid pyrolysis products along with a cell wall contraction is seen. Additionally, some parts of the cell wall appear to become molten. This liquid appears to leave the cell walls at approximately the same time as the remaining cell wall material contracts. This tissue collapse is possibly (21) Prins, M. J.; Ptasinski, K. J.; Janssen, F. J. J. G. More efficient biomass gasification via torrefaction. Energy 2006, 31 (15), 3458–3470.
caused by an outflux of liquid and gas pyrolysis products or a contraction of remaining reacted and unreacted cell wall material. As more material undergoes chemical reaction at increasing temperatures, the biomass continues to darken and the cell walls continue to weaken, contract, and carbonize. As the temperature exceeds 500 °C, cell wall contraction decelerates and the biomass continues to carbonize and darken (see Supplemental Figures 1-3 in the Supporting Information for videos of poplar undergoing reaction on the hot stage). Biomass Expansion and Contraction Increases Material Macroporosity and Induces Changes in Macropore Surface Area. During pyrolysis experiments, the biomass tissue structure appears to open up during the expansion phase (up to ∼350 °C), with the cell lumen inflating and the cell walls stretching until taut. Across the sample, this creates a 10-15% increase in total two-dimensional pore area per unit of biomass cross-sectional surface area (red data in Figure 3). This increase in cross-sectional pore area, when translated into three dimensions, is an increase in pore volume or porosity, which is the measure of the gaseous space of the material.22 As the material proceeds into the contraction phase (above 350 °C), the remaining solid cell wall material in wood fiber cells and ray parenchyma cells (Figure 1C) collapses the cell lumen as some of the liquid-phase material flows out of the cell wall. This collapse of cell lumen actually increases overall material porosity (red data in Figure 3), despite the collapse of many small cell lumina, for two main reasons. First, liquidand gas-phase products leave the biomass, taking up less volume. Second, although fewer cell lumina are visible at this level of magnification after material collapse, weakened cell walls and tears and cracks forming around contracting and increasingly carbonized and brittle tissues allow the largest cell lumina, those of the xylem elements, to grow to very large sizes as adjacent cell lumina with compromised cell walls combine to form a single lumen. The temperature range of high-porosity increase rates reported here (red data in Figure 3; approximately 350-450 °C) correlates well with temperature ranges reported for the maximum thermal decomposition rates of lignocellulosic materials under a nitrogenous atmosphere.23-25 These changes (22) Miura, M.; Kaga, H.; Sakurai, A.; Kakuchi, T.; Takahashi, K. Rapid pyrolysis of wood block by microwave heating. J. Anal. Appl. Pyrolysis 2004, 71 (1), 187–199. (23) Cagnon, B. T.; Py, X.; Guillot, A.; Stoeckli, F.; Chambat, G. R. Contributions of hemicellulose, cellulose and lignin to the mass and the porous properties of chars and steam activated carbons from various lignocellulosic precursors. Bioresour. Technol. 2009, 100 (1), 292–298.
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in porosity slow and eventually appear to end above 500 °C, as the biomass becomes increasingly carbonized and the contraction phase ends. While the changes in pore area are apparent from viewing a video of the reaction, changes in cell wall perimeter, which are translated into surface area in three dimensions, are not. Blue data in Figure 3 show the graph of the total biomass perimeter, measured as the sum of the lengths of the borders between cell walls (and other visible biomass material) and the cell lumina (and other void spaces created during pyrolysis). As the graph shows, the measured perimeter exhibits two increase-decrease phases. The first is due to the material expansion, similar to the first increase measured in pore area. As the lumina slightly inflate, the border between the lumen and cell wall is slightly stretched, leading to an increase in perimeter at this resolution. This is followed by the beginning of the biomass contraction and lumen collapse phase, again lowering the lumen/cell wall perimeter. The second increase-decrease is due to initial tearing and cracking of biomass cell walls, increasing perimeter. Further contraction and collapse again lowers macropore perimeter. Heat Treatment Leads to Changes in Biomass at the Ultrastructural Level. While real-time analysis of poplar undergoing pyrolysis on the hot stage has shown changes in biomass at the tissue level, issues of focal distance and aperture/ light levels with the hot-stage setup on the microscope have limited observation of real-time changes at the subcellular level. Gathering information about the pyrolysis process at this level is important because processes that take place at subcellular scales will have a significant impact on efficient heat and mass transfer. To study pyrolyzed biomass at this scale, hot-stage samples were embedded in a plastic resin and sectioned for higher magnification viewing by LM and TEM. In comparison to brightfield light microscope images of untreated poplar ray parenchyma (Figure 4A), which are semitransparent, pyrolyzed poplar ray parenchyma are dark ambercolored (Figure 4B). Some heat-treated cells have pits that, when intact, have increased in area more than 10 times compared to the control (white arrows in panels A and B of Figure 4), although the pit sizes of other ray parenchyma cells in heattreated poplar appear to have only doubled (or less) in area. This growth may in part be due to the collapse phenomenon at the cell wall level, but noting the number of pits of such large sizes in treated cells versus the number of normal-sized pits in untreated cells, the size in treated cells may be partially due to the combining of pit areas as the cell wall is stressed and tears in the biomass contraction/collapse phase described above. Additionally, as seen in Figure 2 and described above, light micrographs of embedded material show tissue and cell collapse. In panels A and B of Figure 5, the black and white arrows give examples of ray parenchyma and fiber cells, respectively, in untreated and heat-treated tissue. These images also illustrate thinned cell walls post-pyrolysis. Changes in cell wall size are also visible in transmission electron micrographs. While wood fiber cells of untreated control poplar have a regularly shaped lumen and full, clearly differentiated cell walls (Figure 6A) treated wood fibers appear compressed and gnarled (Figure 6B). Additionally, crosssectional widths of pyrolyzed secondary cell walls appear 5-10 times thinner than those of control fiber cell walls, depending upon cell type and treatment severity (panels A and B of Figure 5 and Figure 6). Furthermore, pyrolyzed cell walls in TEM (24) Raveendran, K.; Ganesh, A.; Khilar, K. C. Pyrolysis characteristics of biomass and biomass components. Fuel 1996, 75 (8), 987–998. (25) Raveendran, K.; Ganesh, A. Adsorption characteristics and poredevelopment of biomass-pyrolysis char. Fuel 1998, 77 (7), 769–781.
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Figure 4. Light micrographs of ray parenchyma in control and hotstage-pyrolyzed poplar. (A) Longitudinally sectioned control poplar showing cell wall faces of ray parenchyma, with white arrows pointing to examples of cell wall pores (plasmodesma). (B) Longitudinally sectioned hot-stage poplar shows cell wall faces of ray parenchyma with collected pyrolysis products (black arrows). Compare cell wall pores (white arrows) in this image to those in A. Scale bars ) 50 µm.
micrographs overall appear more uniform compared to the differentiation of cell wall layers visible in control images, likely because of increased carbonization post-pyrolysis. Pyrolysis Product Entrapment in Heat-Treated Poplar. In addition to wall darkening and thinning, light and transmission electron micrographs also reveal that some products of biomass pyrolysis remained contained within the biomass. Figure 5B shows a light micrograph of black char-like residue trapped within a fiber cell (black arrowhead), while Figure 4B shows amber-colored pyrolysis product droplets (black arrows) trapped within a ray parenchyma cell. The diameters of these amber-colored droplets range from 1 mm diameter) particles. Heat was applied to the lower surface (black arrowhead) for 3-4 min at an average rate of ∼150 °C/min to a temperature of approximately 500 °C. Notice the change in cell morphology and darkness of the cell walls as the distance from the heating surface increases. Areas farthest from the heating surface (white arrowhead) were relatively unaffected by pyrolysis. Scale bar ) 500 µm.
stocks is not ideal, it serves as a useful illustration of the reactions of plant biomass to pyrolysis and gasification processes. These results mean that tissue and cellular structure is intact as pyrolysis and gasification products are being released, emphasizing the importance of overcoming the complexities of plant tissue and cellular structure during thermochemical fuel and energy production. Because of this, we are confident that our small-scale reactions on the hot-stage reactor are representative of larger scale, faster pyrolysis and gasification reactions and provide valuable insight into the structural changes occurring in biomass particles during pyrolysis/gasification.
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Evidence presented in this study and others referenced previously suggests that increasing particle size, especially to “wood chip” size, in pyrolysis and gasification leads to degraded products, but there is an unexplored possibility that biomass particles of increased size and complexity may not harm the overall processes or at least do less harm than one might predict. This possibility is that, although a large particle may increase tar yields, it may also entrap them, as shown in this study. Tar sequestration could reduce catalyst fouling, although the increased tar yields in larger particles would still result in decreased fuel product yields. Additionally, some entrapped pyrolysis products may increase the biochar value if applied as a fertilizer and soil conditioner because of the presence of partially oxidized carbon compounds that can be used as carbon sources by soil microbes. Heat and Mass Transfer. While this study does not provide quantitative data for the heat- and mass-transfer rates of pyrolyzing poplar wood, our images illustrate that it is an issue and probably will be even in reactors with very high heating rates. Larger particles are more economical but also contribute to creating a non-isothermal reactor environment, and more even heating is likely to create a more uniform, more predictable product that can be adjusted depending upon production needs. Figure 8 very well illustrates the impacts of transferring heat from the energy-receiving surface to even fairly small “wood chip”-sized pieces of biomass. Although orientation of the biomass to the heating surface may impact heat-transfer rates up to 2-fold,26,27 this likely had little impact on our studies and even less so in an industrial process, because the biomass particles or pieces would be randomly oriented in a heating bed and heated from all sides. Of more importance is the material that did not escape the biomass volume over the course of the process, which in our case takes course over 3-4 min. Ideal product residence times are on the order of hundreds of milliseconds,16 while at least some of our products have resided (26) Bridgwater, A. V.; Meier, D.; Radlein, D. An overview of fast pyrolysis of biomass. Org. Geochem. 1999, 30 (12), 1479–1493. (27) Scott, D. S.; Piskorz, J. The continuous flash pyrolysis of biomass. Can. J. Chem. Eng. 1984, 62 (3), 404–412.
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in the reacting biomass (approximately 5 × 5 × 10 mm) at temperatures above 400 °C for at least 1 min. This residence has likely contributed to the formation of heavy tars and a loss in liquid and gas yield. Pyrolysis product residence times can be reduced using small particles or an ablative process; however, because industrial processes are likely to use large particle sizes for economical reasons, product escape from wood-chip sized particles within the ideal time frame is improbable. This extended residence time leads to secondary reactions. Besides being simply trapped, even if liquid or gas products escape an area undergoing reaction at a particular temperature, uneven heating of biomass particles because of poor heat transfer may cause these products to recondense in a cooler area and undergo another heating cycle, which could continue several times. This cyclical reactionmigration-recondensation may lead to products that are not typically seen with small particle sizes or in simple pyrolysis reactions of model compounds as well as an increase in tar formation. Because most research into thermochemically produced renewables is focused on chemistry and industrial scale-up, we hope that the approach of this study has shed light on issues that can be useful in modeling efforts and chemical and industrial engineering. Acknowledgment. The authors thank Calvin Feik (NREL) for providing recovered fluidized-bed chars. This work was supported by the Office of Biomass Program under the U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy at the National Renewable Energy Laboratory under contract DEAC36-08GO28308. Supporting Information Available: Video examples of poplar cross-sections pyrolyzing on the hot stage in real time (Supplemental Figures 1 and 2), video example of a poplar longitudinal section pyrolyzing on the hot stage in real time (Supplemental Figure 3), and light micrographs of pyrolysis products from nitrogen outflow trapped on paper filters (Supplemental Figure 4). This material is available free of charge via the Internet at http://pubs.acs.org. EF900201B
