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Effect of Heating Rate on Steam Gasification of Biomass. 1. Reactivity of Char 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
Steam gasification or pyrolysis of biomass was conducted at heating rates of 1, 10, and 100 K s-1 using a thermobalance reactor. Three kinds of biomass samples (cellulose, lignin, and bagasse) were used. Time profiles of weight change of biomass samples during steam gasification and pyrolysis were measured at 973 K. Char obtained in steam gasification of lignin was analyzed with a scanning electron microscope and a CHNS elemental analyzer. The effect of the heating rate on final conversion and reaction rate of char was investigated. It was found that a higher heating rate substantially increased the reaction rate of lignin char in steam gasification because porous char was produced during devolatilization due to rapid evolution of volatiles. A higher heating rate also increased final conversion of biomass. The heating rate showed no pronounced influence on elemental composition of char. Introduction The amount of carbon dioxide emission due to biomass combustion is proportionate to the amount of carbon dioxide absorption by photosynthesis; therefore, biomass does not increase the net amount of atmospheric carbon dioxide. Consequently, biomass is considered to be a clean energy resource that can reduce the environmental impact of carbon dioxide emission caused by fossil fuels combustion. Hydrogen is also considered to be a clean energy carrier because it emits no carbon dioxide during combustion. Conversion of biomass energy to hydrogen energy can reduce exergy loss engendered in combustion for power generation because hydrogen has the lowest exergy rate among conventional hydrocarbon fuels.1-4 Gasification is the most effective reaction for hydrogen production from biomass. However, cold gas efficiency (the ratio of energy of produced gases to that of biomass) of existing biomass gasification remains low. Cold gas efficiency of air gasification of biomass is in the range of 35-70%.5-7 Fluidized bed steam-oxygen gasification demonstrates efficiency of ≈70% without using a catalyst and 65-125% using a tar-cracking catalyst.8-10 Encinar et al. conducted steam gasification of Cynara cardunculus L. and reported that cold gas efficiency of Cynara was 64% at 973 K and 88% at 1073 K.11 In principle, cold gas efficiency of hydrocarbon gasification can be increased by decreasing oxygen consumption and enhancing steam utilization; this results in a lower reaction temperature, which is not generally preferred for rapid and complete conversion.12 It is known that rapid-heating pyrolysis yields a large amount of volatiles and a small amount of char.13,14 Chen et al. carried out pyrolysis of birch wood both in a free-fall tubular reactor (rapid heating) and a thermobalance reactor (slow heating); to measure the gasification rate of char, they then gasified that char with carbon dioxide and steam in a thermobalance.15 They reported that * To whom correspondence should be addressed. Tel: +813-5841-7336. Fax: +81-3-3841-7270. E-mail: tsutsumi@ chemsys.t.u-tokyo.ac.jp.
Figure 1. Experimental apparatus.
rapid-heating pyrolysis of birch wood char possessed higher reactivity in reactions with both carbon dioxide and steam than char which was formed with a slow heating rate. Therefore, it is inferred that rapid heating is effective for enhancement of steam gasification of biomass char and for rapid and complete conversion of volatiles into gases at low temperature by pyrolysis. Although many studies have been conducted with thermobalance reactors to study reactivity of biomass with various heating rates,16-25 those heating rates were 2-3 K s-1 at most. Few studies have been conducted to investigate reactivity of steam gasification of biomass char formed in situ in thermobalance reactor at high heating rates. In this study, reactivity of biomass in steam gasification at a low temperature (973 K) was examined with a new thermobalance reactor operated at the heating rates of 1, 10, and 100 K s-1. Time profiles of sample weight were measured to investigate the effect of heating rate in steam gasification of biomass. Experimental Section Apparatus. Figure 1 shows the schematic diagram of experimental apparatus. The system mainly consists of a quartz thermobalance reactor of 25 mm in inner
10.1021/ie030056c CCC: $25.00 © 2003 American Chemical Society Published on Web 07/18/2003
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diameter, an inner quartz tube of 13 mm in inner diameter, an infrared gold image furnace, and a balance sensor. The inner quartz tube was installed to avoid influence of buoyancy change due to gas convection during heating. Length of the furnace’s isothermal zone was ≈210 mm. A ceramic basket of 8 mm in diameter and 10 mm in length was suspended in the thermobalance. The ceramic basket was covered with a platinum sheet to absorb radiative heat from the furnace for rapid heating. Two platinum mesh sheets (150 mesh) were put at the bottom of the ceramic basket. Temperature was measured by an R-type thermocouple put near the sample. Steam was fed through a coil above the sample basket. The lower part of the quartz reactor 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. Procedure. A biomass sample of 10-20 mg 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 became less than 80 ppm. Subsequently, the reactor was heated and kept at 383 K for 5 min to dry the sample. Temperature was increased and was kept at 473 K for 5 min to prevent steam from condensing in the reactor. Then, steam was introduced into the reactor at 473 K with carrier gas Ar (50:50 vol %). When the thermobalance output became stable (≈5 min after introduction of steam), the reactor was heated to 973 K. The heating rate and the temperature were controlled by a programmable controller. The heating rate was variable up to 100 K s-1. Temperature and weight loss of the sample during reaction were recorded on a personal computer at sampling rates of 2 or 5 Hz. Pyrolysis was also carried out without introducing steam for comparison with steam gasification. After steam gasification or pyrolysis was completed, char and tar were burned by introducing oxygen into the reactor to measure the amount of char and to calculate biomass conversion. The blank experiment was conducted with exactly the same procedure as steam gasification or pyrolysis after the tar/char combustion to compensate for the output drift of the thermobalance. All experiments were carried out at atmospheric pressure. Characterization of char in steam gasification was performed by elemental analysis and scanning electron microscopy (SEM) observation. The sample was heated to a temperature ranging from 673 to 1073 K and then cooled to ambient temperature by introducing Ar gas. Elemental analysis of char was conducted by a CHNS elemental analyzer (Perkin-Elmer 2400II). Char surface morphology was examined with a scanning electron microscope (Hitachi S-900). Biomass Samples. Cellulose (Merck Co. Ltd.,
