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Plasma Pyrolysis of Biomass for Production of Syngas and Carbon Adsorbent L. Tang†,‡ and H. Huang*,†,§ Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, No. 1 Nengyuan Road, Guangzhou 510640, People’s Republic of China, Department of Engineering Thermophysics, The University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China, and Department of Environmental Engineering, Guangdong University of Technology, No. 729 Dongfengdong Road, Guangzhou 510090, People’s Republic of China Received July 12, 2004. Revised Manuscript Received January 1, 2005
A radio-frequency (RF) plasma pyrolysis reactor was presented in this paper. Application of this reactor to the pyrolysis treatment of biomass at different operating pressures (3000-8000 Pa), with various input powers (1600-2000 W), was investigated. Interest was focused on the effect of the pyrolysis conditions on the yield of gas and char, the gas composition, and the quality of the char. On average, the gas yield can reach 66 wt % of the biomass feed at an input power of 1800 W and an operating pressure of 5000 Pa. The total content of CO and H2 in the gas product reached 76 vol % on a nitrogen-free basis, which can be used as syngas components. The obtained solid product has a large Brunauer-Emmett-Teller (BET) surface area and high pore volume, with a significant prevalence of micropores, and may have potential use as an activated carbon.
1. Introduction Thermal plasma pyrolysis offers some unique advantages for biomass conversion, in comparison to conventional pyrolysis at low temperatures and slow heating rates.1-3 The high energy density and temperature associated with thermal plasmas and the corresponding fast reaction times provide a potential solution for the problems occurred in conventional pyrolysis processes, such as low gas productivity and the generation of heavy tarry compounds.4-7 Nevertheless, until now, thermal plasmas have usually only been applied for the destruction of noxious materials,8-10 because of the high electrical power consumption, and thermal plasma pyrolysis of biomass for energy and chemical production * Author to whom correspondence should be addressed. Fax: 8620-87057761. E-mail address:
[email protected]. † Chinese Academy of Sciences. ‡ The University of Science and Technology of China. § Guangdong University of Technology (1) Zhao, Z.; Huang, H.; Wu, C.; Li, H.; Chen, Y. Biomass Pyrolysis in an Argon/Hydrogen Plasma Reactor. Chem. Eng. Technol. 2001, 24 (5), 197-199. (2) Fulcheri, L.; Schwob, Y. From Methane to Hydrogen, Carbon Black and Water. Int. J. Hydrogen Energy 1995, 20 (3), 197-202. (3) Baumann, H.; Bittner, D.; Beiers, H. G.; Klein, J.; Juntgen, H. Pyrolysis of Coal in Hydrogen and Helium Plasmas. Fuel 1998, 67 (8), 1120-1123. (4) Yaman, S. Pyrolysis of Biomass to Produce Fuels and Chemical Feedstocks. Energy Convers. Manage. 2004, 45 (5), 651-671. (5) Me´rida, W.; Maness, P.-C.; Brown, R. C.; Levin, D. B. Enhanced Hydrogen Production from Indirectly Heated, Gasified Biomass, and Removal of Carbon Gas Emissions Using a Novel Biological Gas Reformer. Int. J. Hydrogen Energy 2004, 29 (3), 283-290. (6) Bridgwater, A. V. Renewable Fuels and Chemicals by Thermal Processing of Biomass. Chem. Eng. J. 2003, 91 (2-3), 87-102. (7) Chen, G.; Andries, K.; Luo, Z.; Spliethoff, H. Biomass Pyrolysis/ Gasification for Product Gas Production: The Overall Investigation of Parametric Effects. Energy Convers. Manage. 2003, 44 (11), 18751884.
are seldom studied even in laboratory investigations for technical and economic reasons.1,11 In fact, the temperature initiated in thermal plasma (usually 3000-10000 K) is much too high for biomass pyrolysis; a great portion of the heat from the thermal plasma has been released to the surroundings by means of radiation and conduction. In this paper, we present a new process: a radiofrequency (RF) capacitively coupled plasma pyrolysis reactor that has been designed for organic waste treatment. It is different from previous RF cold plasma processes operating in a vacuum12,13 and RF thermal plasma processing at atmospheric pressure.14,15 Our RF (8) Katou, K.; Asou, T.; Kurauchi, Y.; Sameshima, R. Melting Municipal Solid Waste Incineration Residue by Plasma Melting Furnace with a Graphite Electrode. Thin Solid Films 2001, 386 (2), 183-188. (9) Kim, S.-W.; Park, H.-S.; Kim, H.-J. 100 kW Steam Plasma Process for Treatment of PCBs (Polychlorinated Biphenyls) Waste. Vacuum 2003, 70 (1), 59-66. (10) Nishikawa, H.; Ibe, M.; Tanaka, M.; Ushio, M.; Takemoto, T.; Tanaka, K.; Tanahashi, N.; Ito, T. A Treatment of Carbonaceous Wastes Using Thermal Plasma with Steam. Vacuum 2004, 73 (3-4), 589-593. (11) Brown, D. S. Plasma Arc Reduction of Biomass for Production of Synthetic Fuel Gas. Prepr.sAm. Chem. Soc., Div. Pet. Chem. 1979, 24 (2), 429-431. (12) Wang, Y.-F.; Lee, W.-J.; Chen, C.-Y. CFC-12 Decomposition in a RF Plasma System. J. Aerosol Sci. 1997, 28 (Supplement 1), S279S280. (13) Hsieh, L.-T.; Lee, W.-J.; Chen, C.-Y. Decomposition of CH3Cl/ CO2 Mixtures by Using an RF Plasma Reactor. J. Aerosol Sci. 1997, 28 (Supplement 1), S519-S520. (14) Guddeti, R. R.; Knight, R.; Grossmann, E. D. Depolymerization of Polypropylene in an Induction-Coupled Plasma (ICP) Reactor. Ind. Eng. Chem. Res. 2000, 39 (5), 1171-1176. (15) Guddeti, R. R.; Knight, R.; Grossmann, E. D. Depolymerization of Polyethylene Using Induction Coupled Plasma Technology. Plasma Chem. Plasma Process. 2000, 20 (1), 37-63.
10.1021/ef049835b CCC: $30.25 © 2005 American Chemical Society Published on Web 02/10/2005
Plasma Pyrolysis of Biomass in Syngas Production
Figure 1. Schematic of the RF plasma system.
plasma reactor system operates at low pressure (approximately 3000-8000 Pa, in terms of absolute pressure); by changing the operating pressure and the input power, a moderate energy density and gas temperature can be obtained, which could ensure biomass pyrolysis with high yields of gas, high-quality char, and little tar formation. 2. Experimental Section A schematic representation of the experimental setup is given in Figure 1. The device consists of a capacitively coupled, high-density, RF (13.56 MHz) plasma source with a power output of 0-2000 W and a matching network. The pyrolysis reactor is a cylindrical quartz tube with an inner diameter of 16 mm and a length of 500 mm. Surrounding this tube are two cylindrical copper electrodes that couple power from the RF power source to the gas as it flows through the tube; these copper electrodes were each 2.5 cm wide and were spaced 3 cm apart. An inert gas entrance was also incorporated, to purge oxygen and serve as a working gas to generate the plasma. A variable-speed screw feeder situated on the top of the quartz tube ensured that the particles were fed toward the center of the cylindrical quartz tube. The biomass used in our experiments is fir sawdust with the mean particle size of 200 µm. The proximate and ultimate analyses, as well as the heating value of the sample, are given in Table 1. The biomass particles were fed into the plasma reactor at a velocity of 0.3 g/min; the nitrogen carrier gas (0.5 L/min) was used here to maintain the stability of the plasma discharge under particle injection conditions. The pyrolysis product vapors were evacuated from the reactor by means of a variablespeed rotary vacuum pump. After the reaction, the residue in the char collector was assumed to be the solid yield, based on the feeder charge. Gaseous products then were analyzed, using a gas chromatography (GC) system (model N2010, manufactured by Kechuang GC instruments, Shanghai, PRC). The physical properties (porosity, surface area, structure) and chemical properties (elemental composition, heating value, surface functional groups) of the pyrolytic char were examined.
3. Results and Discussion 3.1. Plasma Characteristics. The input power and operating pressure of the RF plasma reactor have considerable influence on the plasma characteristics, such as energy density and gas temperature. A summary of the plasma characteristics under different operating conditions is shown in Table 2. The discharge
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length was measured from the discharge photographs taken in a dark room at night using the discharge as the primary light source. The power density was calculated by dividing the input power by the discharge volume. Table 2 shows that the discharge length was 9-14 cm, and the gas temperature was in the range of 1173-1773 K as the input power and operating pressure changed. The gas temperature should result from a balance of the RF input power and plasma heat loss to the surroundings by means of radiation and conduction. For a given operating pressure of 5000 Pa, when the input power was 1600 W, a lower energy density of 61.24 W/cm3 has led to a lower system temperature. Elevating the input power to 2000 W caused an increase in the energy density, to 82.93 W/cm3, and, hence, an increase in temperature to 1623 K. Table 2 also shows that the gas temperature for all discharges studied increased as the pressure increased for the same input power. The energy density is probably one of the main parameters accounting for this trend. Also note that the gas temperature achieved using an input power of 2000 W and operating pressure of 5000 Pa, with the energy density as high as 82.93 W/cm3 (1623 K), is lower than that initiated in 1800 W and 8000 Pa with a lower energy density of 81.43 W/cm3 (1773 K). This may be due to two reasons: (i) increasing the operating pressure decreases the discharge length and volume, as shown in Table 2, so the heat loss through radiation and conduction is reduced and the temperature is increased; and (ii) increasing the operating pressure results in an increase of the particle concentration in the plasma, so the electrons exchange their energy with ions and neutrals by a greatly increased rate of collision, and, as a result, the temperature of the system increases. In view of the heat released to the surrounding air by means of radiation and conduction, the measured temperature presented in Table 2 are somewhat lower than the maximum temperature and must be regarded as only an approximate estimate. Because of the limit of the maximum power of the RF generator, we did not get the information of the temperature and energy density for higher input powers and operating pressures; a plausible tread is that the temperature will increase with the input power and operating pressure. 3.2. Gas Product. High temperature, combined with a high heating rate, is beneficial to gas production and tar cracking. In contrast to previous conventional pyrolysis studies, we found that almost no tar was produced in the RF plasma pyrolysis process; this is consistent with the result of other thermal plasma pyrolysis studies, although the temperature under the current conditions is lower than that of thermal plasma.1 Probably, the energetic species (e.g., electrons, ions, atoms, and free radicals) initiated in the RF plasma can enhance tar cracking. Table 3 lists the gas yield, char yield, and conversion data of biomass pyrolysis under various experimental conditions. From Table 3, one can observe that the gas yield and the conversion efficiency are strongly influenced by the input power and operating pressure. A higher input power and operating pressure increased the temperature and reaction rate, resulting in increased gas yield and higher conversion efficiency.
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Table 1. Analysis of Wood and Char Samples material
Proximate Analysis (wt %) volatile fixed carbon ash
wood char a
78.82 naa
18.83 naa
2.35 14.62
C 47.10 83.17
Ultimate Analysis (wt % dried) H O N 6.40 2.21
46.50
