Article pubs.acs.org/EF
Is Elevated Pressure Required to Achieve a High Fixed-Carbon Yield of Charcoal from Biomass? Part 2: The Importance of Particle Size Liang Wang,† Øyvind Skreiberg,† Morten Gronli,‡ Gregory Patrick Specht,§ and Michael Jerry Antal, Jr.*,§ †
SINTEF Energy Research, Sem Saelands vei 11, Trondheim, Norway Department of Energy and Process Engineering, Norwegian University of Science and Technology, Kolbjørn Hejes vei 1B, Trondheim, Norway § Hawaii Natural Energy Institute, School of Ocean and Earth Science and Technology, University of Hawaii at Manoa, Honolulu, Hawaii 96822, United States ‡
ABSTRACT: The prosperity of Silicon Valley is built upon a foundation of wood charcoal that is the preferred reductant for the manufacture of pure silicon from quartz. Because ordinary pyrolysis processes offer low yields of charcoal from wood, the production of silicon makes heavy demands on the forest resource. The goal of this paper is to identify process conditions that improve the yield of charcoal from wood. To realize this goal, we first calculate the theoretical fixed-carbon yield of charcoal by use of the elemental composition of the wood feedstock. Next, we examine the influence of particle size, sample size, and pressure on experimental values of the fixed-carbon yields of the charcoal products and compare these values with the calculated theoretical limiting values. The carbonization by thermogravimetric analysis of small samples of small particles of wood in open crucibles delivers the lowest fixed-carbon yields, closely followed by standard proximate analysis procedures that employ a closed crucible and realize somewhat improved yields. The fixed-carbon yields (as determined by thermogravimetry) improve as the sample size increases and as the particle size increases. Further gains are realized when pyrolysis occurs in a closed crucible that hinders the egress of volatiles. At atmospheric pressure, high fixed-carbon yields are obtained from 30 mm wood cubes heated in a closed retort under nitrogen within a muffle furnace. The highest fixed-carbon yields are realized at elevated pressure by the flash carbonization process. Even at elevated pressure, gains are realized when large particles are carbonized. These findings reveal the key role that secondary reactions, involving the interaction of vapor-phase pyrolysis species with the solid substrate, play in the formation of charcoal. Models of biomass pyrolysis, which do not account for the impacts of sample size, particle size, and pressure on the interactions of volatiles with the solid substrate, cannot predict the yield of charcoal from biomass. These findings also offer important practical guidance to industry. Size reduction of wood feedstocks is not only energy and capital intensive; size reduction also reduces the yield of charcoal and exacerbates demands made on the forest resource.
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INTRODUCTION Most residents of Silicon Valley would be surprised to learn that their prosperity, reflecting the worldwide demand for iPhones, E-books, and laptop computerswhose performance grows exponentially according to Moore’s lawis built upon a foundation of wood charcoal. Silicon is produced in electric arc furnaces where carbon serves to reduce quartz (SiO2) to molten silicon according to the idealized reaction: SiO2 + 2C = Si + 2CO. Because of its extraordinary reactivity and unusual purity, charcoal is the carbon reductant of choice for silicon production, although coal is used when charcoal is not available or is too expensive. Roughly 1.4 mt of charcoal and/or coal together with 0.39 mt of wood are consumed to produce 1 mt of Si with the emission of about 4.7 mt of CO2 to the atmosphere.1 Silicon alloys produced in the United States during 2010 were valued at about $770 million, and ferrosilicon consumption during that year increased by 38% relative to 2009.2 But the United States is responsible for only about 2.5% of world silicon and ferrosilicon production; increasingly the PRC and Brazil are principal suppliers of silicon to Silicon Valley and the world. Moore’s law governs the advance of technologies built of silicon, whereas the state-of-the-art of carbonization technologies in Brazil and the PRC is century-old stasis,3 delivering emissions of CO and other noxious gases that © 2013 American Chemical Society
pose serious health hazards together with very low yields of charcoal from wood.3 The contrast between the bright hightech future of Silicon Valley and its dreary foundation of charcoal production is inexplicable; handsome rewards (e.g., conservation of the wood resource, emissions remediation and improvement of human health, reduction of greenhouse gases that contribute to climate change, and improvements in the profitability of charcoal and Si production) await the industry that succeeds in improving the yield of charcoal from wood. Metallurgical grade charcoal is characterized by its high fixedcarbon content with concomitant low ash and volatile matter contents. These values are measured by the conventional proximate analysis procedure that was developed to rank coals. Briefly, (see ASTM D1762-84 (2007) for details), dry charcoal is heated in a closed crucible to 950 °C for 11 min. The measured fractional weight loss is the charcoal’s volatile matter content (%VM). Then, the charcoal is ashed in an open crucible at 750 °C for 6 h, and its fractional residual weight (relative to its dry weight) is the charcoal’s ash content (%ash). Received: January 7, 2013 Revised: March 14, 2013 Published: March 15, 2013 2146
dx.doi.org/10.1021/ef400041h | Energy Fuels 2013, 27, 2146−2156
Energy & Fuels
Article
Finally the charcoal’s fixed-carbon content (%fC) is given by % fC = 100 − %VM − %ash. Because of the importance of a charcoal’s fixed-carbon content to the metallurgical industry, throughout this paper, we emphasize the fixed-carbon yield (yfC) of charcoal realized by the carbonization process. The fixed-carbon yield is given by yfC = ychar*(%fC)/(100 − %feed ash), where ychar = Mchar/Mbio; Mchar and Mbio are the dry masses of the charcoal and biomass feedstock, respectively, and %feed ash is the ash content of the dry feedstock. Aside from its significance to the metallurgical industry, in previous papers,4−11 we reasoned that values of ychar are meaningless as a metric of charcoal production efficiency because they do not account for the quality of charcoal. Values of yfC capture the quality of the charcoal in their evaluation and thereby serve as a meaningful metric. Moreover, values of yfC can be compared to the theoretical yield of pure carbon that could be realized when thermochemical equilibrium is established in a closed vessel containing only biomass feedstock.4−6 Such theoretical values of yfC are easily calculated using a thermochemical equilibrium code (e.g., StanJan12) once the elemental composition of the feedstock is known. A comparison of the experimental value of yfC with its limiting, theoretical equilibrium value serves as a particularly stringent measure of the success of the carbonization procedure. In this paper, we examine the influences of particle size, confinement of the pyrolysis vapors, sample size, pressure, and gas flow rate on values of ychar and yfC for oak and sweet gum woods, whose carbonization behaviors are of interest to the silicon industry. Our results unveil the importance of secondary reactions in the formation of charcoal and suggest possibilities for substantially improving the fixed-carbon yield of charcoal from wood.
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Table 1. Temperature Program pyrolysis method step dynamic 1 2 3 4
isothermal
time (min)
−
30
−
jump −
−
heating rate (°C/min)
30 10
temp (°C) 25 25 → 105 105 105 → 950
measured at 950 °C by the dry mass measured at the end of the drying period at 105 °C. A temperature controllable muffle furnace (approximately 0.009 m3), a stainless-steel retort (approximately 0.004 m3), and two ceramic crucibles (approximately 200 mL) were employed by NTNU to determine the maximum charcoal and fixed-carbon yields that can be realized at atmospheric pressure from oven-dried material. Samples of 60−70 g were loaded into the crucibles. Afterward, the crucibles were placed in the retort, and its lid was sealed. Small crucibles with powder samples of 5 g were also placed in the retort. The retort was purged with nitrogen for 30 min before heating as well as during the run to ensure carbonization in an inert atmosphere. The muffle furnace was heated from room temperature to 950 °C at a rate of 5 °C/min. Thermocouples were placed within the crucibles and the retort to monitor the temperature history during the experiment. At the University of Hawaii (UH), all wood and corn cob samples were air-dried prior to carbonization. In most cases, 0.7 kg of corncob was loaded into the top of the flash carbonization (FC) canister above the wood. The corncob was sacrificial in the sense that its presence protected the wood charcoal from oxidation when the air delivery exceeded the bare minimum needed for full carbonization of the wood. The canister was subsequently loaded into the top of the flash carbonization reactor that was then pressurized with air. Electric power was delivered for 360 s to a heating coil at the bottom of the pressure vessel to ignite the wood at the bottom of the canister. Following ignition, compressed air was delivered to the top of the pressure vessel and flowed downward through the packed bed while the flash fire moved upward. The pressure within the reactor was continuously monitored and maintained at a specified level by a valve located downstream of the reactor. After sufficient air was delivered, the airflow was halted, and the reactor cooled overnight. The charcoal product was removed from the reactor and divided into separate corncob and wood charcoal products. The charcoals were allowed to equilibrate under a fume hood for 2 days before proximate analysis was performed. For moisture content determination, the charcoal samples were dried in a Fisher Scientific Isotemp model 282A vacuum oven evacuated below 0.015 MPa (4 in. Hg). The volatile matter and ash analyses were performed using a Thermolyne 1300 muffle furnace. At NTNU, all wood samples were subjected to proximate analysis according to American Society for Testing and Materials (ASTM) E871 and E872. NTNU employed ASTM D1102 for wood ash determination, whereas UH employed ASTM E1755-95. However, NTNU ashes a 1 g wood sample at 550 °C for a minimum of 12 h until no black carbon is visible, whereas UH ashes a 1 g wood sample at 575 °C for a minimum of 3 h and measures weight loss thereafter. This ashing procedure is repeatedly continued for a minimum of 1 h until the overall measured weight loss is constant within 0.3 mg. At NTNU, the wood samples were also subjected to proximate analysis according to ASTM E1131-08 using the coal interlaboratory compositional test parameters with the TGA/SDTA 851e. A flow of 200 mL/min nitrogen was used to purge the furnace. An additional flow of 53 mL/min oxygen (79% nitrogen and 21% oxygen by volume) was introduced into the furnace for ash determination. We note that the ASTM D1102 procedure involves ashing at 550 °C, whereas the ASTM standard E1131-08 procedure involves ashing at 950 °C that may incur additional oxidation of the minerals present in the ash. Elemental analyses of the wood samples with direct O measurement were obtained from Huffman Laboratories, whereas complementary elemental analyses with O by difference were
APPARATUS AND EXPERIMENTAL PROCEDURES
Moist, debarked strips of red (Quercus rubra L.) and white oak (Quercus alba L.) woods, mixed debarked oak planks and oak sawdust, laurel wood (a kind of oak wood) planks, and moist sweet gum (Liquidambar styraciflua) branches were supplied to us by the Dow Corning Corp. We also studied oak wood and its sawdust that we received from the Cowboy Charcoal Co. more than a decade ago. At the Norwegian University of Science and Technology (NTNU), two different sample preparations were employed: either the wood was ground in a cutting mill mounted with a 1 mm sieve, or a cubic sample was cut from the sapwood using a sharp knife. The cubic particles weighed 5, 10, 20, and 40 mg before drying, corresponding to cube sizes from 2 to 4.5 mm. Also, some large cubic particles measuring 7 mm were cut for manual insertion into the thermogravimetric analyzer (TGA). For the muffle furnace experiments, wood samples were cut in three different sizes: 14 mm cubes, 30 mm cubes, and 30 mm × 20 mm × 75 mm blocks. Subsequently, all samples were dried in an oven at 105 °C for 24 h prior to carbonization. A Mettler Toledo model TGA/SDTA 851e atmospheric pressure TGA was used by NTNU to study the carbonization behavior of small samples. All TGA runs employed nitrogen (99.999% pure) as purge gas for the furnace with a flow rate of 200 mL/min and a balance flow rate of 20 mL/min. Prior to each experiment, a measured amount of material (5, 10, 20, and (nominal) 30/40 mg in single particle or powder form) was loaded into the appropriate crucible (see below). Each experiment began with a 30 min purge at room temperature, followed by 30 min of drying at 105 °C. Afterward, the sample was heated from 105 to 950 °C at a heating rate of 10 °C/min and subsequently cooled to 50 °C at 100 °C/min (see Table 1 for a summary). Two types of TGA experiments were performed: an open (i.e., no lid) crucible loaded with sample and a loaded crucible covered by a lid with a small pinhole in the center (i.e., closed crucible). The charcoal yield ychar was calculated by dividing the sample mass as 2147
dx.doi.org/10.1021/ef400041h | Energy Fuels 2013, 27, 2146−2156
Energy & Fuels
Article
851e. There is good agreement between the two methods concerning the VM and fC content of the two woods but significant disparity in the measured ash contents. This disparity is a known limitation of TGA proximate analysis techniques and may be due to the short ashing time.25 If we think of proximate analysis as a kind of carbonization procedure, we can calculate its fixed-carbon yield yfC. For red oak wood, this value of yfC lies between 13.9 wt % and 14.2 wt %, and for sweet gum, the yfC value lies between 14.1 wt % and 14.4 wt % (see Table 2). In previous work, involving three different species of corn cob, the comparable values of yfC ranged from 17.7 to 18.2 wt %. Surprisingly, by this measure, corn cob is a more promising feedstock than wood for biocarbon production. Table 3 displays elemental analyses of the feedstocks as determined by Huffman Laboratories Inc. and Hazen Research Inc. together with previously published analyses of oak wood. In Table 3, we correct the 2003 elemental analyses of oak wood and oak wood sawdust given to us by the Huffman Laboratories. We understood their laboratory report to be on a dry basis,6 whereas it was in fact on a wet basis.11 The wide range in values of the elemental analyses and ash contents displayed in Table 3 for the same wood is typical of what we have observed in previous research and is indicative of the accuracy of the measurement. For example, for red oak Huffman reported a C content of 49.05%, whereas Hazen obtained 51.02%. Likewise, for Dow Corning oak wood blocks, Huffman measured an O content of 43.97%, whereas Hazen reported 40.55% (by difference). Similarly, the ash content of
determined by Hazen Research Inc. We note that Huffman ashes a 1 g ground wood sample at 600 °C for 3 h; whereas Hazen ashes a 1 g ground wood sample at 750 °C overnight (a minimum of 8 h). Both NTNU and UH employed the ASTM D1762-84 procedure for proximate analyses of the charcoal product. Small differences in our applications of the details of this procedure were discussed in a previous publication.11 The microstructure and surface topography of the charcoal particles were investigated using a Zeiss Supra-55 variable-pressure, fieldemission, scanning electron microscope (LV FE-SEM). Samples were mounted on carbon tape without further preparation and scanned by the SEM.
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RESULTS Table 2 offers a comparison of proximate analyses of the sweet gum and red oak wood according to ASTM standards E871, Table 2. Proximate Analyses and Fixed-Carbon Yields of Feed Materials proximate analysis (wt %)
a
feed
VM
fC
ash
yfC (wt %)
red oaka red oak (TGA)b sweet guma sweet gum (TGA)b
85.98 85.27 84.91 84.69
13.84 14.06 14.27 13.89
0.18 0.66 0.82 1.43
13.87 14.15 14.38 14.09
ASTM standards E872 and D1102. bASTM E1131-08.
E872, and D1102 using conventional methods in a muffle furnace and ASTM standard E1131-08 using the TGA/SDTA
Table 3. Ultimate Analyses of Oak and Sweet Gum Woods and the Calculated Theoretical Fixed-Carbon Yields yfC ultimate analysisb (wt %) no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21
feed c
sweet gum (Hazen) sweet gum (Huffman) white oak (Hazen)c white oak (Huffman) red oak (Hazen)c red oak (Huffman) Cowboy oak 2003 (Huffman) Cowboy oak sawdust 2003 (Huffman) Cowboy oak 2000 (Huffman) Dow Corning oak blocks (Hazen)c Dow Corning oak blocks (Huffman) Dow Corning oak coarse (Hazen)c Dow Corning oak coarse (Huffman) Dow Corning oak fine (Hazen)c Dow Corning oak fine (Huffman) Cowboy oak coarse (Hazen)c Cowboy oak coarse (Huffman) Cowboy oak fine (Hazen)c Cowboy oak fine (Huffman) laurel wood (Hazen)c laurel wood (Huffman) mean oak (Hazen)d standard deviation oak (Hazen)d mean oak (Huffman)e standard deviation oak (Huffman)e
MCa (wt %)
C
H
O
N
S
ash
total
yfC (wt %)
6.67 30.88 10.06 7.85 9.59 8.10 8.60 9.12 5.26 12.83 5.38 11.30 6.57 11.78 6.59 7.72 6.10 8.29 6.33 33.14 31.07
49.93 47.89 50.69 48.78 51.02 49.05 50.81 50.89 50.13 49.66 48.56 50.44 48.62 49.83 49.04 49.76 49.06 51.63 49.22 50.35 48.94 50.43 0.74 48.90 0.25
6.01 5.91 6.02 5.97 6.03 5.92 6.00 5.83 5.98 5.95 5.73 5.95 5.89 5.87 5.88 5.86 5.98 6.16 6.08 5.82 5.90 5.98 0.10 5.92 0.11
42.24 44.31 42.74 44.42 42.49 44.58 43.53 42.31 44.76 40.55 43.97 41.41 43.82 43.73 44.17 44.04 44.80 41.49 44.32 41.49 43.05 42.35 1.28 44.30 0.34
0.09 0.12 0.06 0.07 0.06 0.06 0.11 0.17 0.08 0.35 0.12 0.21 0.13 0.21 0.11 0.14 0.09 0.14 0.09 0.16 0.20 0.17 0.10 0.10 0.03
0.03 0.03
