Ind. Eng. Chem. Res. 2003, 42, 3929-3936
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Effect of Heating Rate on Steam Gasification of Biomass. 2. Thermogravimetric-Mass Spectrometric (TG-MS) Analysis of Gas Evolution Chihiro Fushimi, Kenichi Araki, Yohsuke Yamaguchi, and Atsushi Tsutsumi* Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan
Evolution rates of low-molecular-weight gas products (H2, CH4, CO, and CO2) in pyrolysis and steam gasification of biomass (cellulose and lignin) were studied using thermogravimetric-mass spectrometric (TG-MS) analysis. Total gas yields were measured with a TCD-micro gas chromatograph (micro GC) at heating rates of 1, 10, and 100 K s-1. Steam gasification of biomass demonstrated heating rate effects on gas evolution: 81 wt % of cellulose was converted into tar in pyrolysis; also, slow heating and steam gasification of nascent char occurred above 700 K, evolving H2. Rapid heating significantly enhanced secondary pyrolysis of cellulose tar to yield H2, CO, and CH4. In contrast, char formation at 500-773 K was dominant in lignin pyrolysis. Evolution of H2 and CO2 were significantly increased by steam gasification of char. Insignificant influence of heating rate on carbon and hydrogen yield of lignin was observed. Over 70% of lignin’s chemical energy was converted into gas, especially hydrogen. Introduction Steam gasification of biomass is a promising technology for thermochemical hydrogen production from biomass. A large amount of tar is produced from biomass in the conventional biomass gasification process.1,2 This reduces thermal efficiency and causes tar troubles such as pipeline plugging and defluidization. Thus, it is important to convert tar into gaseous products such as H2 and CO during gasification for improvement in controllability and thermal efficiency of biomass gasifiers. It is known that very rapid-heating pyrolysis of biomass at moderate temperatures results in fragmentation of polymeric components in biomass to attain up to 70 wt % yields of volatiles composed of oxygenated monomers and polymer fragments.1,2 It can be considered that in the rapid-heating steam gasification of biomass, steam introduction promotes production of lowmolecular-weight gases due to steam-reforming reactions of volatiles and nascent char. However, few studies have addressed the effect of heating rates in steam gasification of biomass.3,4 The first paper in this series reported that higher heating rates decreased char production and increased char reactivity in steam gasification.5 The first objective of this study is to examine experimentally the effect of heating rates on production of low-molecular-weight gases such as H2, CH4, CO, and CO2 in steam gasification of biomass. In addition, evolution profiles of gaseous products during steam gasification were investigated to explore the reaction mechanism for steam gasification of biomass. So far, several studies have reported biomass pyrolysis.6-27 Va´rhegyi et al. studied gas evolution characteristics of biomass charcoal pyrolysis by thermogravimetric-mass spectrometric (TG-MS) analysis.6,7 Serio et al. analyzed gaseous products in pyrolysis of lignin using thermogravimetric, Fourier transform infrared (TG-FTIR) spectroscopy.8 Blasi et al. conducted biomass * To whom correspondence should be addressed. Tel: +813-5841-7336. Fax: +81-3-5841-7270. E-mail: tsutsumi@ chemsys.t.u-tokyo.ac.jp.
pyrolysis using a fixed-bed reactor; they measured gas evolution of H2, CO, CO2, and hydrocarbons with a TCD gas chromatograph.9,10 However, few studies have investigated gas evolution profiles in steam gasification, especially in the case of rapid heating. Herein, gas evolution profiles in steam gasification of biomass with heating rates of up to 100 K s-1 were measured using thermogravimetric-mass spectrometric (TG-MS) analysis. Experimental Section Apparatus. Experimental apparatus mainly comprises a 25-mm inner diameter quartz thermobalance reactor, an inner quartz tube with 13 mm in inner diameter, an infrared gold image furnace, and a balance sensor. The furnace’s isothermal zone length was ≈210 mm. An 8-mm-diameter, 10-mm-long ceramic basket was suspended in the thermobalance. Two platinum mesh sheets (150 mesh) were placed at the bottom of the ceramic basket. Temperature was measured by an R-type thermocouple placed near a sample. The quartz reactor’s lower portion was cooled by a water-cooling jacket to prevent secondary gas-phase reactions such as tar-cracking and steam-reforming. Consequently, the estimated residence time of volatiles was ≈1 s. Details of the apparatus were described in the first paper in this series.5 Procedure. A 10-20 mg biomass sample was put into the ceramic basket. Then, Ar gas of 2.72 cm3 s-1, which was 0.55 cm s-1 at the standard state, was fed into the thermobalance reactor; oxygen in the reactor tube was purged until its concentration declined below 80 ppm. Subsequently, the reactor was heated and kept at 383 K for 5 min to dry the sample. After the temperature was increased and kept at 473 K for 5 min to prevent steam from condensing in the reactor, steam was introduced into the reactor with carrier-gas Ar (50:50 vol %). When thermobalance output stabilized (≈5 min after steam introduction), the reactor was heated to a desired temperature to start the reaction.
10.1021/ie0300575 CCC: $25.00 © 2003 American Chemical Society Published on Web 07/18/2003
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Ind. Eng. Chem. Res., Vol. 42, No. 17, 2003
Figure 1. Profiles of temperature, relative mass of char, gases, and tar along with the gas evolution rate of cellulose with the heating rate of 1 K s-1: (a) pyrolysis; (b) steam gasification.
Figure 2. Profiles of temperature, relative mass of char, gases, and tar along with the gas evolution rate of lignin with the heating rate of 1 K s-1: (a) pyrolysis; (b) steam gasification.
The temperature was held at 973 K until the reaction was completed. The heating rate was varied up to 100 K s-1. Sample weight loss and temperature during reaction were recorded on a personal computer at the sampling rate of 2 or 5 Hz. After produced tar and water were eliminated in a CaCl2 column, gaseous products (H2, CH4, CO, and CO2) were analyzed by a quadrupole mass spectrometer
Table 1. Elemental Compositions of Biomass Samples elemental compositions [wt %, d.a.f. basis] sample
C
H
O
cellulose lignin
44.4 64.5
6.2 5.6
49.4 27.0
(Standom; ULVAC, Inc.) at time intervals of 1 s. Ions of m/z ) 2, 15, 28, and 44 were selected to measure the
Ind. Eng. Chem. Res., Vol. 42, No. 17, 2003 3931
Figure 3. Profiles of temperature, relative mass of char, gases, and tar along with the gas evolution rate of cellulose with the heating rate of 100 K s-1: (a) pyrolysis; (b) steam gasification.
Figure 4. Profiles of temperature, relative mass of char, gases, and tar along with the gas evolution rate of lignin with the heating rate of 100 K s-1: (a) pyrolysis; (b) steam gasification.
concentrations of H2, CH4, CO, and CO2, respectively. In conjunction with mass spectrometry, gaseous products were also measured by a TCD micro-gas chromatograph (model M-200H; Hewlett-Packard Co.) to verify accuracy of mass spectrometer data. We measured H2, O2, N2, CH4, and CO with an MS-5A column and Ar
carrier gas; CO2 was measured with a Pora Plot Q column and He carrier gas at time intervals of ≈90 s. Effluent gas from the reactor was collected in a gas bag to measure total gas production. Char was burned to measure the amount of char and to calculate biomass conversion: oxygen was introduced
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Figure 5. Cumulative gas yield vs temperature for cellulose: (a) H2; (b) CH4; (c) CO; (d) CO2.
into the reactor. Pyrolysis was also performed, introducing only Ar gas for comparison with steam gasification. After char combustion, a blank experiment was conducted with exactly the same procedure as steam gasification or pyrolysis to compensate output drift of the thermobalance reactor. All experiments were carried out at atmospheric pressure. Biomass Samples. Cellulose (Merck Co. Ltd.,
