Peculiarities of Rapid Pyrolysis of Biomass Covering Medium- and

Energy & Fuels 2006, 20, 2705-2712

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Peculiarities of Rapid Pyrolysis of Biomass Covering Medium- and High-Temperature Ranges Yan Zhang,*,† Shiro Kajitani,† Masami Ashizawa,† and Kouichi Miura‡ Central Research Institute of Electric Power Industry (CRIEPI), Energy Engineering Research Laboratory, 2-6-1 Nagasaka, Yokosuka-shi, Kanagawa-ken 240-0196, Japan, and Department of Chemical Engineering, Kyoto UniVersity, Katsura Campus, Kyoto 615-8510, Japan ReceiVed April 21, 2006. ReVised Manuscript ReceiVed August 24, 2006

Rapid pyrolysis of Hinoki cypress sawdust was conducted over a wide temperature range from 600 to 1400 °C in a lab-scale drop-tube furnace (DTF). Attention was paid to the influence of temperature on carbon conversion, gas yield, tar destruction, and coke deposition. A new observation was the decrease in carbon conversion from 900 to 1100 °C followed by a positive increase as the temperature was raised. Gravimetric analysis and gas chromatography were used as tools for determining heavy tar and light tar, respectively. It was found that temperature had a significant effect on tar destruction. The achievement of complete tar destruction in the product gas required an extremely high pyrolysis temperature of at least 1200 °C. An attractive result of this study was the satisfactory separation of the char and coke produced in pyrolysis, which described an unfamiliar behavior of the char formation and coke deposition. Char yield indicated a sharp drop between 600 and 800 °C then followed an almost plateau up to 1100 °C. Apparent coke deposition was observed from 900 °C accompanied by the secondary decomposition of tars and light hydrocarbon gases. It was concluded that the enhanced carbon conversion at the pyrolysis temperatures above 1100 °C was attributed to the enhancement of the gasification reactions between solid products (char and coke) and reactive gases (H2O and CO2).

Introduction Fossil fuel energy generated from coal, petroleum, and natural gas plays an important role in the development of the world economy but at the same time causes serious damage to the environment and human health. This is because all fossil fuels emit massive amounts of greenhouse gas (GHG) and other harmful gases (N2O, NOx, and SO2) during their utilization. The use of renewable energy for fossil fuel substitution is one of the most promising options for minimizing the emission of GHG and acid rain precursor gases. Among the different renewable energy sources, biomass appears to be the most important in terms of technical and economic feasibility during the next few decades.1 The United States Environmental Protection Agency (USEPA) predicts that the commercialized biomass energy will supply 25.8% and 31.7% of the world primary energy needs in 2025 and 2050, respectively.2 Biomass is traditionally consumed by direct combustion at the household level for cooking and heating. This primitive technology generally produces high levels of indoor and outdoor air pollutants and is low in efficiency. In recent years, extensive effort has been devoted to developing and improving some thermochemical conversion processes for biomass, such as gasification using various types of gasifers and combustion using industrial boilers. The two processes generally operate at medium and high temperatures from 600 to as high as 1200 °C,3 * Corresponding author. E-mail: [email protected]. † Energy Engineering Research Laboratory. ‡ Kyoto University. (1) Hall, D. O.; House, J. I. In Proc. International Conference: National Action to Mitigate Global Climate Change, Copenhagen, Denmark, 1994. (2) US EPA. Policy options for stabilizing global climate; Report to the Congress. Office of Policy, Planning and Evaluation: Washington, DC, 1990.

depending on the process types and purpose. It is considered that rapid pyrolysis is the first step in both gasification and combustion, which occurs at the same temperature of gasification or combustion in an industrial gasifier or boiler. The properties of the solid and volatile products under the pyrolysis determine their behavior or reactivity in the subsequent gasification and combustion. Although a number of studies have so far been reported on the rapid pyrolysis of biomass in freefall reactors similar to the drop-tube furnace (DTF) used in this study, most of the previous works were conducted in medium-temperature range (600-800 °C) or in rare cases up to 1000 °C.4-7 The pyrolytic characteristics of the biomass at higher temperatures above 1000 °C were not well-known. In this work, rapid pyrolysis tests for Hinoki cypress sawdust were conducted in a lab-scale drop-tube furnace (DTF) over a wide temperature range from 600 to 1400 °C. The general use of DTF is for assessing the influence of fuel characteristics and operating conditions on the gasification reactivity.8,9 Attention was paid to the impact of temperature on carbon conversion and product distribution. Total tar amount was determined by the combination of gravimetric analysis and gas chromatography. An attempt has been made to separate the char and coke (3) Ryu, C.; Yang, Y.; Nicola, E.; Yates, A. K.; Sharifi, V. N.; Swithenbank, J. Fuel 2006, 85, 1039-1046. (4) Zanzi, R.; Sjo¨stro¨m, K.; Bjo¨rnbom, E. In DeVelopment in Thermochemical Biomass ConVersion; Bridgwater, A. V., Boocock, D. G. B.; 1996; Vol. 1, pp 61-66. (5) Zanzi, R.; Sjo¨stro¨m, K.; Bjo¨rnbom, E. Fuel 1996, 75, 545-550. (6) Li, S.; Xu, S.; Liu, S.; Yang, C.; Lu, Q. Fuel Process. Technol. 2004, 85, 1201-1211. (7) Kinoshita, C. M.; Wang, Y.; Zhou, J. J. Anal. Appl. Pyrolysis 1994, 29, 169-181. (8) Kajitani, S.; Hara, S.; Matsuda, H. Fuel 2002, 81, 539-546. (9) Kajitani, S.; Nakagawa, S.; Miura, K.; Hara, S. In Proc. International Conference on Coal Science and Technology, Okinawa, Japan, 2005; p 3D10.

10.1021/ef060168r CCC: $33.50 © 2006 American Chemical Society Published on Web 10/17/2006

2706 Energy & Fuels, Vol. 20, No. 6, 2006

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Figure 1. Schematic diagram of drop-tube furnace. Table 1. Proximate and Ultimate Analyses of Hinoki Cypress Sawdust Proximate Analysis (wt %, db) ash volatile matter fixed carbon HHVa (kcal/kg, db)

0.2 89.3 10.5 4870

Ultimate Analysis (wt %, daf) C H N S Ob a

49.2 6.49 0.10 0.01 44.20

High heating value. b By difference.

produced in pyrolysis. The scientific knowledge obtained in this study will be helpful to the engineering and operation of the industrial biomass conversion processes. Experimental Section Feed Material and Chemicals. The feedstock used in pyrolysis tests was Hinoki cypress sawdust, which was pulverized below 0.5 mm and air-dried at 107 °C before use. Its main properties are given in Table 1. All chemicals were purchased from either Kanto Chem. Co., Inc., or Wako Pure Chem. Indus., Ltd., and used as received. Apparatus and Procedures. The schematic construction of the drop-tube furnace (DTF) system is shown in Figure 1. The reactor tube is constructed using a pure alumina tube (150 cm long and 5 cm internal diameter). It is inserted into a vertical furnace and heated by four independent electric heaters with a total heated length of 95.4 cm. The system also consists of a screw feeder, a tar-sampling train, a char hopper, a drain, a cartridge filter, a microgas chromatography, etc. A DTF can be operated in either active gas flow (air, steam, or the mixtures of them) for studying combustion and/or gasification or inert gas flow for testing pyrolysis of fuels. In the present study, nitrogen was used as carrier gas. For a certain pyrolysis run, the DTF was preheated to a setting temperature. Biomass was then continuously fed by a screw feeder and injected into the DTF through a slender pipe by a nitrogen

stream. The feeding rate of feedstock was controlled in the range of 60-70 g/h. Generally, the feeding rate becomes stable 3-5 min after starting. The gas residence times and linear velocities in the reactor tube were converted by varying nitrogen flow rate. They were 3-4.5 s and 200-300 mm/s, respectively. Gas Analysis and Determination. The product gas was online analyzed by microgas chromatography (Agilent Technologies, 3000 Micro GC), which equipped with three different columns and thermal conductivity detectors (TCDs) and took a sample every 5 min. CO2 and light hydrocarbon species with carbon numbers from two to three (C2-C3) were analyzed using a Pora Plot Q column connected to a backflush inlet. Other gas species were analyzed using two MS-5A columns. One was for H2 and the other was for N2, CO, O2, and CH4. The yield of each gas was determined by nitrogen balance and gas composition and was converted to mole number per kilogram of biomass in dry base (mol/kg, db). Carbon conversion is defined as the molar fraction (mol %) of carbon in the feedstock converted to gaseous products (including CO, CO2, CH4, and C2-C3 species). Tar Sampling. The setup for tar sampling used in this study is basically the same as that proposed by van Paasen et al.10 with some modification. The product gas containing tar and H2O was continuously taken from the tar-sampling chamber by using a sampling train. The tar-sampling chamber was generally kept at 250-260 °C by an electric heater. The train consists of a sampling probe, a quartz filter that is set at the tip of the probe, a methanol-CO2 ice bath, three impinger bottles, a sucking pump, and a flow meter. In each test run, tar and H2O were trapped with isopropanol in the three impinger bottles. Two were exposed to air, and the remaining one was placed in a methanol-CO2 ice bath. Product gas was drawn with a sucking pump at a flow rate of 0.7-1 NL/min for 30-40 min through this system to get one sample. Determinations of Heavy Tar and Light Tar. In the present study, heavy tar refers to the nonvolatile components at room temperature, the quantity of which was determined by gravimetric analysis as follows: A clean evaporating dish was weighed accurately three times on a balance. Parent tar (PT) solution (10 mL) was measured and poured into the evaporating dish. The solution in the dish was then evaporated at room temperature for 20-30 h to constant weight. The dish was then weighed and the final mass of the evaporation residue was taken as the mean of two parallel determinations. In addition to the evaporation residue obtained above, partial heavy tar condensed in the inner walls of the sampling line, which was separately collected by washing with tetrahydrofuran (THF). Consequently, the total content of heavy tar was determined according to eq 1,

CHT ) (m1Vt/Va + m2)(Ts + 273)/273Vs

(1)

where CHT is the content of heavy tar in product gas (g/Nm3), m1 is the mass of the evaporation residue in the evaporating dish (g), m2 is the mass of heavy tar collected from the inner walls of the sampling line by THF, Vt is the total volume of the sample solution (mL), Va is the volume of tar solution taken for gravimetric analysis (mL), Vs is the volume of sampled gas (m3), and Ts is the temperature of the gas in sampling (C). (10) van Paasen, S. V. B.; Kiel, J. H. A.; Neeft, J. P. A; Knoef, H. A. M.; Buffinga, G. J.; Zielke, U.; Sjo¨stro¨m, K.; Brage, C.; Hasler, P.; Simell, P. A.; Suomalainen, M.; Dorrington, M. A.; Thomas, L. Guideline for Sampling and Analysis of Tar and Particles in Biomass Producer Gases; ECN-C-02-090; ECN Biomassa: Petten, The Netherlands, 2002.

Peculiarities of Rapid Pyrolysis of Biomass

In contrast to heavy tar, light tar means the volatilized and/ or sublimated components during gravimetric analysis described above. The content of light tar was determined on the basis of gas chromatographic analyses. The detailed procedures were as follows: The PT solution was first analyzed using a HP6980 gas chromatography connected to a HP5973A mass spectroscopy (GCMS). The temperature of the mass detector was 230 °C, and electron-impact mass spectra (EI-MS) were recorded over the range 15-500 m/z. GC-MS analysis provided identifications of the organic compounds in tar solution. The individual content of the compounds in tar solution identified by GC-MS was then determined using the same model of the gas chromatography equipped with a flame-ionization detector (FID). The evaporation residue after gravimetric analysis was redissolved by adding 10 mL of isopropanol to the evaporating dish. This redissolved evaporation residue (ER) solution was subjected to the same analyses as those of PT solution. The total content of light tar in the product gas was consequently estimated according to eq 2,

CLT ) Vt Σ(Ci PT - Ci ER)(Ts + 273)/273Vs (i ) 1, 2, ..., n) (2) where CLT is the total content of light tar in the product gas (g/Nm3) and Ci PT and Ci ER are the contents of the species i in parent tar and evaporation residual solutions determined by GC (g/mL), respectively. The yields of heavy tar and light tar were finally calculated to give the weight percent to feedstock in dry base (wt %, db) according to their contents and nitrogen balance. H2O Determination. H2O adsorbed in isopropanol was then determined using a Karl Fischer moisture titrator (MKC-610, Kyoto Electronics Manufacturing Co., Ltd.). Definition and Collection of Char and Coke. In this study, char and coke (or soot) were separately determined. In principle, char means the solid particles that dropped from the gas stream to the char hopper set at the bottom of the reactor tube, while coke sums up two kinds of solid products collected from different locations. One is the fine solid particles that passed through the char hopper along with the gas stream and stopped in the drain and cartridge filter. The other is the membranous solids that deposited on the inner walls of the reactor tube positioned in the hot zone. Both char and coke were collected at the end of each run and given in a weight ratio (wt %, db) to feedstock throughput. Morphology Analysis. Scanning electron microscopy (SEM) was used to investigate the morphological changes of resulting solid products at different pyrolysis temperatures. The SEM analysis was performed using a HITACHI S-3500N system. A secondary electron mode was applied for surface observation at an accelerating voltage of 20 or 30 kv. The sample surfaces were previously subjected to coating with a gold layer. Size Distribution Analysis. Size distributions of the solid products were measured by using a laser diffraction-type particle-size analyzer (SEISHIN, LMS-30). The mixture of water and methanol was used as a dispersion medium. The detecting limit of the particle size is 1 µm. Thermogravimetry. The non-isothermal temperatureprogrammed reaction (TPR) was conducted using a thermogravimetric analyzer (TG-DTA2000S, Mac Science Co., Ltd.), for the purpose of estimating the content of char and coke in their mixtures. The sample size of 5 mg was heated at a constant

Energy & Fuels, Vol. 20, No. 6, 2006 2707

Figure 2. Carbon conversion as a function of pyrolysis temperature. Table 2. Product Distribution of Cypress Sawdust at Different Pyrolysis Temperatures yields (wt %, db) temp. (°C)

char

coke

heavy tara

light tarb

gas

H2O

total

600 800 900 1000 1100 1200 1400

13.5 4.9 4.3 4.0 3.0 2.4 1.9

0.0 0.0 0.6 6.6 15.4 14.7 11.8

12.5 3.3 2.2 1.0 0.10 0.0 0.0

15.4 8.2 5.6 3.0 1.0 0.1 0.0

30.9 65.3 19.7 65.4 64.8 68.5 77.6

27.0 20.5 69.0 17.8 15.3 13.0 7.8

99.3 102.2 101.4 97.7 99.6 98.7 99.1

a Residues after evaporating isopropanol at room temperature (determined by gravimetric analysis). b Vaporized and/or sublimated components at room temperature (determined by gas chromatography).

heating rate of 10 °C/min in a pure CO2 flow of 450 mL/min. A personal computer was used for system control and data acquisition. Results and Discussion Overall Mass Balance Analysis. The products from the pyrolysis of sawdust were gas, condensates (H2O and tar), and two kinds of solid products, char and coke. In this study, all of these products were determined as carefully as possible. Mass balance analysis was performed to assess the precision of the product collection and determination. A nitrogen balance was chosen as the basis for determining gaseous and liquid (H2O and tar) product yields. Nitrogen balance means that the outflowing rate of the nitrogen in the product gas is always the same as the in-flowing rate of the nitrogen injected into the DTF. According to this character, the flow rate of the total product gas could be calculated based on the in-flowing rate of the nitrogen and its concentration (determined by microgas chromatography) in the product gas. Then, the yields of the gaseous and liquid products can be obtained according to their concentrations (determined by microgas chromatography for gases, by Karl Fischer moisture titrator for H2O, and by gravimetric analysis and GC-MS for tars) and the flow rate of the total product gas. Table 2 shows the yields of all products determined on the basis of mass balance analysis. The overall mass balances ranged from 98 to 102 wt %, suggesting that the experimental error for collecting and determining products in this study was acceptable. This is a fundamental requirement to give a scientific interpretation of the experimental appearance. Carbon Conversion. Figure 2 shows the effect of temperature on the carbon conversion of Hinoki cypress sawdust in pyrolysis tests over the temperature range from 600 to 1400 °C. Carbon conversion increased linearly with increasing temperature from 600 to 900 °C. This fact is well-understood because,


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Figure 3. Product distribution and mass balance vs pyrolysis temperature.

when temperature is increased, the extent of the primary pyrolysis also increases. Somewhat more difficult to explain is why carbon conversion decreased from 900 to 1100 °C followed by a positive increase as the temperature was raised. This is a new observation for biomass pyrolysis. This type of phenomenon suggested a complex mechanism in the high-temperature pyrolysis of biomass. The discussion on the product profiles as indicated below would provide a reasonable and satisfactory explanation on these peculiarities. H2O Content in Product Gas. H2O in the product gas may both result from the moisture in the feedstock and be formed by dehydration during pyrolysis.11 In the present study, because feedstock was air-dried at 107 °C before use, the H2O determined in the product gas should mainly originate from the dehydration of sawdust under pyrolysis conditions. In a coexistence environment with CO gas, char (or coke), and hydrocarbons (including tar and hydrocarbon gases), H2O may act as a reactive agent to contribute to the following gasification reactions,

CO + H2O T CO2 + H2

(3)

C + H2O f CO + H2

(4)

CnHm + nH2O f nCO + (n + m/2)H2

(5)

where C denotes char (or coke) and CnHm signifies tar and hydrocarbon gases. Reaction 3 is exothermic, while reactions 4 and 5 are endothermic. High temperature will favor the progresses of reactions 4 and 5 rather than reaction 3. For a convenient view, data in Table 2 are graphically plotted in Figure 3. With the increase in temperature, H2O content in the product gas greatly decreased from 27 wt % at 600 °C to 7 wt % at 1400 °C. This fact provided evidence that gasification reactions occurred in the pyrolysis process. Gas Yield. Product gas included CO, CO2, H2, CH4, and light hydrocarbons with carbon numbers of two (C2H2 and C2H4) and three (C3H6 and C3H8). For convenience, the latter two components (C3H6 and C3H8) were joined together as C3+, because they were of very minor amounts in the product gas. It was of interest that the profile of the total gas yield was very similar to that of carbon conversion as indicated in Table 2 and Figure 3, i.e., gas yield increased in the temperature range from 600 to 900 °C, while it decreased from 900 to 1100 °C and then followed a positive increase as temperature was raised. In (11) Shafizadeh, F. Appl. Polym. Symp. 1975, 28, 153.

Figure 4. Yields of H2, CO, and CO2 at different pyrolysis temperatures.

order to give a satisfactory explanation of these special characteristics, there is a need to view the profile of each individual product gas. Figure 4 shows the profiles of H2, CO, and CO2. H2 monotonously increased over the temperature range from 600 to 1400 °C, which may be principally generated in the following ways: first, the dehydrogenation through the cleavage of a C-H or O-H bond in biomass or in its pyrolyzied products (chars, tars, and hydrocarbon gases); second, as indicated above, gasification reactions 3-5 can produce additional H2, depending on the H2O concentration and temperature. CO increased initially from 600 to 800 °C, then indicated a plateau in the temperature range from 800 to 1100 °C, and finally increased sharply with the increase in temperature. It is known that the generation of CO in the pyrolysis process is mainly attributed to the decomposition of oxygen-containing functional groups such as hydroxyl,12 ether,13 and carbonyl14 at relatively lower temperatures. At high temperature, however, in addition to reactions 4 and 5, the following reactions between carbon and CO2 would become dominant in the production of CO:

C + CO2 f 2CO

(6)

CnHm + nCO2 f 2nCO + (m/2)H2

(7)

Both reactions are endothermic. Therefore, high temperature will favor their progression. Different from the profiles of H2 and CO, CO2 increased with increasing temperature up to 1100 °C and then decreased sharply as temperature further increased. A reasonable explanation, regarding the positive increase in carbon conversion at temperatures above 1100 °C as indicated in Figure 4, may be drawn from the discrepancy between CO and CO2 profiles. That is, the decrease in CO2 yield accompanied with the increase in CO yield may principally be attributable to reaction 6. This explanation will be further confirmed by the profiles of char and coke in the same temperature region. Figure 5 indicates the profiles of light hydrocarbon gases. In the case of pyrolysis, the decompositions of methoxyl groups and aliphatic side chains are principal formation sources of CH4 and light hydrocarbons, respectively. CH4 is the most abundant species among the light hydrocarbon gases, which exhibited the (12) Antal, M. J., Jr. Biomass Pyrolysis: A Review of the Literature. Part I. Carbohydrate Pyrolysis. In AdVances in Solar Energy; Boer, K. W., Duffield, J. A., Eds.; Solar Energy Society: New York, 1983; pp 61-111. (13) Chatterjee, K.; Stock, M. L.; Zabransky, R. Fuel 1989, 68, 13491353. (14) Demirbas¸ , A. Energy ConVers. Manage. 2000, 41, 633-646.


Figure 5. Yields of light hydrocarbon gases at different pyrolysis temperatures.

maximum production at 900 °C and then decreased with increasing temperature. C2H4 and C2H2 indicated profiles similar to that of CH4. As shown in Figure 5, C2H4 reached the maximum production at 800 °C and completely disappeared at 1000 °C. The formation of C2H2, accompanied by the beginning of the reduction of C2H4 at the temperatures above 800 °C, suggested that C2H2 originated from the decomposition of C2H4. Different from the above hydrocarbon gases, C3+ gas species only showed a destruction profile, which seemed to have already decomposed to a small amount at 600 °C and entirely disappeared at 900 °C. It was realized that the decreases in yield and/or disappearances of these light hydrocarbon gases with increasing temperature were principally attributed to the occurrence of their secondary decomposition. This explains the fact that both carbon conversion (Figure 2) and gas yield (Figure 3) decreased in the temperature range from 900 to 1100 °C. Tar Yield. The so-called tars are generally summarized as the condensable hydrocarbons in the product gas, which condense at reduced temperature and lead to blocking and fouling of the process equipment such as engines and turbines. In spite of the importance of the tar problem, the terminology tar has no collective definition in the literature. For example, it can refer to a sum of components with boiling points higher than 150 °C,15 or it may include all organic condensables with a molecular weight larger than benzene.16 Uncertainties in tar definition also originate from different sampling and analysis methods.17 Discussion on the definition of tar is out of the scope of this study. Emphasis is given to the amounts of, as much as possible, all condensable components in the product gas. “Heavy tar” and “light tar” were used to express nonvolatile and volatile components at room temperature, respectively. Specific explanations about light tar should be taken here. As mentioned above, light tar was determined by gas chromatography, which counted the components that were detectable in the chromatogram of the parent tar sample (tar-containing isopropanol solution) but disappeared in the chromatogram of the evaporation residue. Figure 6 illustrates the chromatograms of the parent tar (Figure 6a) sample and the evaporation residue (Figure 6b). It was evident that the evaporation procedure led more than 20 species of compounds to complete vaporization or sublimation. Identification results indicated that these vapor(15) Brown, M. D. et. al. In Proc. Energy from Biomass & Wastes X. London-New York: Elsevier Sci. 1986, p. 655-676. (16) Neeft, J. P. A.; Knoef, H. A. M.; Onaji, P. Energy from Waste and Biomass; Novem Report No. 9919; EWAB: The Netherlands, 1999. (17) Knoef, H. A. M.; Koele, H. J. Biomass Bioenergy 2000, 18, 5559.


Figure 6. Chromatograms of (a) parent tar and (b) its evaporation residue obtained at 800 °C.

Figure 7. Production and destruction of benzene and light tar as a function of pyrolysis temperature.

ized or sublimated compounds were in the molecular weight range from 32 (methanol) to 154 (isomers of biphenyl). The forthcoming paper will show the details on light tar components with temperature and with gasification reagent variations. In the present paper, only benzene was individually taken for discussion, because it was the most abundant species in light tars. Figure 7 illustrates the profile of benzene with temperature variation. The yield of light tar was also replotted in the same figure for comparison. It can be seen that benzene occupied the majority of light tars at the temperatures above 900 °C and existed in trace amounts even at 1200 °C. It is reported that benzene shows a saturation concentration in pure nitrogen in the range of 100 g/m3 at 25 °C.18 Since its concentration in the product gas of the practical gasification processes is usually at least 1 order of magnitude below this value, it can be assumed that it will not cause any problems due to condensation.17 The yields of heavy and light tars are also shown in Table 2 and Figure 3, which indicated two important features. One is that the yields of both tars monotonically decrease with increasing temperature. Another is that the yield of light tars is always higher than that of heavy tars over the temperature range studied. The heavy tar and light tar yields declined to ca. 0.1 wt % and ca. 1 wt % at 1100 °C, and to almost 0 wt % and ca. 0.1 wt % at 1200 °C, respectively. The results suggested that the temper(18) Moersch, O.; Spliethoff, H.; Hein, K. R. G. Biomass Bioenergy 2000, 18, 79-86.


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Figure 8. SEM images of Hinoki cypress sawdust and its solid products after pyrolysis: (a) parent feeding powders, (b) char obtained at 600 °C (collected from char hopper), (c) char obtained at 1200 °C (collected from char hopper), (d) cakelike coke obtained at 1200 °C (collected from cartridge filter), (e) enlarged image of cakelike coke, and (f) membranous coke obtained at 1200 °C (collected from inner walls of reactor tube).

ature at 1200 °C or above seemed to be appropriate for achieving a product gas with less or no problems due to tar condensation. Characterization of Char and Coke. The solid residues that resulted from pyrolysis and/or gasification processes are frequently called char. However, the terminology char is not always a befitting expression for all solid products, especially in the case of the high-temperature biomass conversion processes. This is because coke (or soot) may form under severe conversion conditions, which generally indicates a significantly different physical property and chemical reactivity from char. Here, the origins of the formation distinguish between char and coke. Char formation is linked to the primary decomposition of biomass and is accompanied by the formation of gas and vapors, while coke is generated from the secondary decomposition and/or condensation reactions of vapor phases (including light hydrocarbon gases and tars). Figure 8 shows a comparison of SEM images of chars and cokes generated from pyrolysis, as well as the feeding powder. The feeding powder mostly showed a quadrilateral shape, which consists of long cell walls strongly bounded but with evident cavities (Figure 8a). The solid particles collected from the char hopper accounted for the majority of char. Parts b and c of Figure 8 show the SEM images of chars obtained in 600 and 1200 °C pyrolysis tests, respectively. Both char samples seemed to undergo melting and/or fusion. The char particles obtained at 600 °C almost held the shapes and the sizes of the feeding powder, while those obtained at 1200 °C took the shape of a sphere with a lot of voids and pores created by volatile matter. It should be noted that chars obtained at pyrolysis temperatures

Table 3. Ultimate Analysis and Bulk Density of Chars and Cokes 1200 °C charb C H N (S + O)a

1400 °C cokec

charb

cokec

Ultimate analysis (wt %, daf) 89.26 98.83 91.77 0.58 0.28 0.30 0.39 0.06 0.33 9.77 0.83 7.60

98.77 0.20 0.05 0.98

Bulk density (g/cm3) 0.24 0.13 a

b

0.23

0.17

c

By difference. Collected from char hopper. Collected from cartridge filter.

above 800 °C indicated almost the same morphological changes. Cokes were available from three different locations: the drain, the cartridge filter, and the inner walls of the reactor tube. Figure 8d shows a typical SEM image of coke collected from the cartridge filter at 1200 °C. The cokes collected from the drain exhibited apparently the same morphological image. These products looked like irregular cakes when viewed at a low magnification (Figure 8d) but were confirmed to be actual aggregates of innumerable fine particles near nanosize by high resolution (Figure 8e). The elemental analysis results in Table 3 indicate that the carbon content of these products was near 99%. In reality, both morphology and elemental composition are the characteristics worthy of the terminology coke. An SEM image of coke collected from the inner walls of the reactor tube is shown in Figure 8f. It indicated a membranous image. Three facts should be emphasized here. First, the formation of the


Figure 9. Particle distributions of chars collected from char hopper and cokes collected from cartridge filter: (a) char obtained at 1200 °C, (b) char obtained at 1400 °C, (c) coke obtained at 1200 °C, and (d) coke obtained at 1400 °C.

cakelike cokes greatly depended on the pyrolysis temperature. There were no collectable solids in the drain and cartridge filter at the pyrolysis temperatures below 800 °C. Second, char particles were scarcely observed in the products collected from the drain and the cartridge filter. Finally, membranous cokes were only obtained at the pyrolysis temperatures above 900 °C. The above findings indicated an efficient separation of coke from char. No exact mechanism could be presented in this paper to describe the separation phenomenon. However, from the viewpoint of the fluid dynamics, the separation phenomenon should have a close relation to the particle size and density of the char and coke. Figure 9 and Table 3 show the size distributions and bulk densities of chars and cokes obtained at 1200 and 1400 °C, respectively. It was found that the chars were more than 10 times the particle size and near twice the bulk density of the cokes. The character of the fine coke particles (size range from 1 to 20 µm) shown in Figure 9 suggested that the cakelike particles with size ∼100 µm as observed by SEM (Figure 8d) easily smashed in the dilute fluids. According to Stokes’ law, particles with larger size and higher density would be favorable for fast settlement from a gas flow, whereas the fine particles would preferably take their motion following the gas streamlines rather than settlement. This allows a reasonable assumption that the separation of coke from char may be attributed to the significant differences in particle sizes and densities between char and coke. Further study is needed to investigate their Stokes’ numbers to confirm this assumption. A few more notes should be added here regarding the collection and separation of the solid products. The sum of the char collected from the char hopper and coke collected from the drain and cartridge filter occupied 80-90 wt % of total solid products. The remaining 10-20 wt % of the solid products were generally deposited on the inner walls of the tar-sampling chamber and on the surface of the quartz filter during tar sampling. According to SEM observation, these deposits were confirmed to be chars in the cases of pyrolysis temperature e900 °C and the mixtures of char and coke g1000 °C. For those deposits containing char and coke, further investigation was pursued using a temperatureprogrammed reaction (TPR) technique, which has been employed to differentiate char and coke generated from coal pyrolysis.19 Figure 10 shows typical TPR traces of char, cakelike coke, and deposits collected from the tar-sampling chamber obtained in the pyrolysis test at 1200 °C. As expected, char and coke indicated a remarkable difference in gasification (19) Miura, K.; Nakagawa, H.; Nakai, S.; Kajitani, S. Chem. Eng. Sci. 2004, 59, 5261-5268.


Figure 10. TPR traces of solid products obtained in 1200 °C pyrolysis test (CO2 flow rate ) 450 cm3/min): (a) char collected from char hopper, (b) deposits collected from tar-sampling chamber, and (c) coke collected from cartridge filter.

reactivity with CO2. The weight loss of char (curve a) started from ca. 790 °C and ended at ca. 940 °C, while coke (curve c) lost its weight from ca. 910 °C and ended at ca. 1180 °C. The deposits (curve b) indicated a stepwise profile in weight loss. The first step (from 790 to 910 °C) corresponded to char, and the second step (from 910 to 1120 °C) corresponded to coke. Char content in the deposits could be estimated according to curve b, i.e., weight loss in the temperature range from 790 to 910 °C. The results of TPR indicated that the deposits obtained in the pyrolysis tests above 1000 °C were rich in coke and only contained 7-11 wt % of char. An emphasis is that the yields of char and coke in Table 2 and Figure 3 have actually summed up the results estimated by TPR. Char and Coke Yields. The char yields shown in Figure 3 indicated a sharp decrease from 600 to 800 °C followed by a gentle change or a plateau up to 1100 °C, above which it decreased further. The profile of char yield in Figure 3 suggested that the primary pyrolysis of sawdust in the present study was almost completed even at 800 °C. On the other hand, as indicated in the same figure, cokes were generated from 900 °C and increased with temperature to a maximum deposition at 1100 °C, and then decreased in the same way as that of char. The variation profiles of char and coke yields in Figure 3 could provide reasonable and satisfactory explanations on the behaviors of carbon conversion and gas yield as shown in Figures 2 and 3. In detail, the sharp decrease in char yield in the temperature range from 600 to 800 °C corresponded to the sharp increase in carbon conversion in the same region. The increase in coke yield from 900 to 1100 °C was mainly attributed to the secondary decomposition of light hydrocarbon gases (Figure 5) and tars (Figure 3) in the following way,

CnHm f nC + (m/2)H2

(8)

where C means coke in this case. The decreases in both char and coke yields above 1100 °C should be due to the contribution of reactions 4 and 5, which is reasonable because CO2 (Figure 4) and H2O (Figure 3) yields also decreased above 1100 °C. Conclusions The characteristics of the rapid pyrolysis of Hinoki cypress sawdust were studied over a wide temperature range from 600 to 1400 °C, using a lab-scale drop-tube furnace. The operating


temperature was found to have a strong influence on both carbon conversion and all product distributions in the pyrolysis process. The findings obtained herein are summarized as follows: Carbon conversion indicated a peculiarity in that it decreased from 900 to 1100 °C because of the secondary decomposition of heavy tars and light hydrocarbon gases, whereas it increased above 1100 °C because of the enhancement of gasification reactions of carbon (char and coke) with CO2 and H2O. The combination of gravimetric analysis and gas chromatography made it possible to obtain reasonable and reliable information on the destructions of heavy tar and light tar. Tar content in the product gas linearly decreased with increasing pyrolysis temperature. It was concluded that an extremely high

Zhang et al.

pyrolysis temperature of at least 1200 °C was required to achieve complete tar destruction. Char and coke were sufficiently separated. The profile of char formation suggested that the primary pyrolysis of sawdust used in this study seemed to complete at temperatures around 800 °C. Apparent coke deposition started from 900 °C and increased with temperature to a maximum at 1100 °C. The decreases in char and coke yields above 1100 °C were attributed to the gasification of solid products (char and coke) with reactive gases (H2O and CO2), which explained the increases in carbon conversion and gas yield in the same region. EF060168R

Peculiarities of Rapid Pyrolysis of Biomass Covering Medium- and

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