A kinetic study of in situ CO2 removal gasification of woody biomass for hydrogen production
Introduction
There has been considerable focus on hydrogen as an energy carrier because only water is produced when it is used and an energy device with high efficiency, such as a fuel cell, can be developed. In Japan, hydrogen stations have been constructed in order to demonstrate its feasibility, and hydrogen fuel cell vehicles have been introduced. Many efforts are underway to establish the hydrogen society.
At present, hydrogen is produced by reforming of naphtha or natural gas or obtained as a byproduct from coke production. However, hydrogen production from renewable resources is expected to gain importance in the future. Only biomass is a renewable and organic resource, and it can be directly converted into hydrogen [1].
Steam gasification using a CO2 sorbent, termed “in situ CO2 removal gasification,” is a hydrogen production method from biomass. In situ CO2 removal gasification is carried out at high temperature (873–973 K) and pressure , and clean gas, mainly hydrogen, can be produced from carbonaceous materials since CO2 is absorbed in the CO2 sorbent, CaO, during the gasification. Based on the principle proposed by Lin et al. [2], [3], [4], [5], the theoretical overall reaction equation is as follows:The obtained CaCO can be regenerated by calcination:
Since the reaction heat absorbed during the calcination is generated during the carbonation, the gasification (1) is exothermic. Generally, in the case of the production of hydrogen via gasification, three processes are needed: steam gasification, water–gas shift reaction, and CO2 removal process. In the in situ CO2 removal gasification, these three reactions can occur in a single reactor, and therefore, high conversion efficiency is expected.
In order to develop the gasification process, the effects of the operating conditions on the gasification should be examined. In a previous study, we have clarified the effects of temperature, pressure, ratio of calcium to wood, etc., on the gasification yield and properties of the gas by using a batch reactor to obtain fundamental information [6], [7]. In this paper, we focus on a kinetic parameter for the design of the apparatus or process. A gasification model for wood pyrolysis, shown in Fig. 1, has been proposed by Shafizadih and Chin [8]. In this model, wood is first degraded into gas, tar, and char by primary degradation. The tar is then decomposed into gas and char by secondary degradation. Many kinetic studies for various pyrolysis and gasification processes have been carried out using this model [9], [10], [11], [12], [13], [14], [15], [16], [17], [18].
In a previous study, we gasified wood at different reaction temperatures up to 923 K and the effect of the temperature on the gasification was discussed [19]. In the present study, the result was reconsidered for a kinetic study and the kinetic constants of in situ CO2 removal gasification were examined from the product distribution with the reaction model proposed by Shafizadih and Chin.
Section snippets
Materials
Japanese oak (Quercus serrata, ) dried overnight in an oven was used as the woody biomass feedstock for the experiments. The proximate and ultimate analyses are shown in Table 1. Here, in the ultimate analysis, oxygen was determined by difference and includes trace elements that were relatively small such as nitrogen and sulfur. Commercial calcium hydroxide powder (Wako Pure Chemical Industries, Ltd., Japan) was employed as a CO2 sorbent without further preparation.
Reaction
The
Gas yield and its composition
Fig. 4 shows the gas conversion ratio (X). It is observed that sufficient gasification did not occur below 573 K. Two remarkable increases in X were observed above 573 K. The first increase appeared at approximately 573 K, while the second one, observed above 773 K, was more remarkable. X reached approximately 50% at 923 K. From these results, it is suggested that the first increase depended on the primary degradation and that the second one was affected by the secondary degradation.
The yield of
Conclusions
Woody biomass was gasified in steam at high temperature and pressure in the presence of a CO2 sorbent using an autoclave with capacity. The product distribution at different reaction temperatures ranging from 473 to 923 K was examined to obtain fundamental information on the biomass degradation during the in situ CO2 removal gasification. The following findings were obtained:
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The evolved CO2 was completely absorbed by the sorbent in all experiments.
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Below 773 K, primary
Acknowledgments
This work was carried out under the “Clean Gas Production from Biomass” project funded by the Ministry of Economy, Trade and Industry (METI). We greatly appreciate Mr. Kenji Kamei and Dr. Michiaki Harada of Center for Coal Utilization, Japan (CCUJ) for the overall support of the project. We are grateful to Drs. Hiroyuki Hatano, Yoshizo Suzuki, Koji Kuramoto, and Kiyohide Yoshida of AIST for their valuable comments. We also sincerely thank Ms. Kayo Kiribuchi, Yoshie Nakashima, Miyuki Nakata, and
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