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Ind. Eng. Chem. Res. 2005, 44, 1950-1953
RESEARCH NOTES Characteristics of Hydrogen Production from Coal Tar with Subcritical Steam Kazuhiro Kumabe,† Hiroshi Moritomi,*,† Kazunobu Yoshida,† Ryo Yoshiie,‡ and Shinji Kambara† Environmental and Renewable Energy Systems (ERES), Graduate School of Engineering, and Department of Human and Information Systems (HIS), Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu, Gifu 501-1193, Japan
A laboratory-scale batch reactor was used to perform a limited number of experiments on the steam gasification of coal tar formed by pyrolyzing Taiheiyo coal at 873 K for 2 h in N2. The study was intended to support the HyPr-RING process in which coal is gasified with steam in a fluidized bed containing Ca-based CO2 sorbent. H2 is the desired product, while substantial amounts of CH4 and small amounts of C2H6 were also produced. The amounts were decreased but the molar ratio of CH4 to H2 in the product gases was increased with a decrease of the temperature. The predominant production occurs in the initial stage including the heating period. The behavior for tar is similar to that for coal, suggesting that CH4 and H2 production from coal should be caused by tar produced from coal in the initial stage and especially that H2 production in the second stage should be by char from coal. 1. Introduction
Table 1. Proximate and Ultimate Analyses of Coal Tar Used
The use of H2 is the most favored option for minimizing the environmental impact of energy production from fossil fuels. The H2 production from coal is one of the clean coal technologies. HyPr-RING (hydrogen production by reaction integrated novel gasification) is a new gasification method to produce H2 from hydrocarbon resources such as fossil fuel, biomass, and waste with high-pressure steam and simultaneously to reduce CO2 by a calcium-based sorbent.1 Almost all of the organic materials such as coal can be reacted with steam and the sorbent to produce mainly H2 and CaCO3. CaCO3 can be regenerated to CaO by calcinations, while a highconcentration stream of CO2 is generated. In the initial stage of the HyPr-RING project, “the supercritical water gasification condition” was studied by using an autoclave reactor.1 Afterward, to develop a bench-scale continuous reactor, the gasification condition was shifted to “the high-pressure and -temperature steam gasification condition”. In the process of this development, we have focused on the kinetics of coal gasification in the high-pressure and -temperature steam condition by using a tubing-bomb (TB) reactor in order to seek the possibilities of the application to a fluidized-bed reactor.2,3 The objective of this study is to make the behavior of tar produced in the initial stage of coal gasification clear, * To whom correspondence should be addressed. Tel.: +81 58 293 2591. Fax: +81 58 293 2591. E-mail: moritomi@ cc.gifu-u.ac.jp. † Environmental and Renewable Energy Systems (ERES), Graduate School of Engineering. ‡ Department of Human and Information Systems (HIS), Faculty of Engineering.
proximate analysis [wt %, dry basis] volatile fixed matter carbon 98.1
1.46
ultimate analysis [wt %, dry ash-free basis]
ash
moisture
0.46
0.00
C
H
N
75.5 9.74 0.59
O, S (difference) 14.2
which would be devolatilized with pyrolysis reactions during the heating period of coal particles fed to a fluidized bed. The tar would produce CH4 as a byproduct, together with H2 as a target product. In the present study, we focused on CH4 and H2 production from the tar. 2. Experimental Section 2.1. Tar Sample. Tar was continuously produced from a fixed-bed reactor by thermal cracking of Taiheiyo coal (Japanese sub-bituminous coal) at 873 K for 2 h in a N2 downflow. The results of proximate and ultimate analyses of the tar are shown in Table 1. Reagent-grade Ca(OH)2 (Nacalai Tesque, Kyoto, Japan) was used as the CO2 sorbent. 2.2. Experimental Apparatus. The experimental apparatus used in the present study was the same as that used in our previous work.2,3 A small reactor (a TB reactor) was a seamless tube (100 mm long with an outer diameter of 1.3 cm and a volume of 7.2 cm3) made of stainless steel (SUS-316). The reactor was sealed at both ends with SUS-316 caps. It was externally heated to a given temperature with a fluidized sand-bath heater (inner diameter of 400 mm, bed height of 450 mm, and bed material of silica sand). 2.3. Experimental Procedure. Under ambient conditions and a nitrogen atmosphere, the TB reactor was
10.1021/ie049009q CCC: $30.25 © 2005 American Chemical Society Published on Web 02/18/2005
Ind. Eng. Chem. Res., Vol. 44, No. 6, 2005 1951 Table 2. Mixing Ratios of Coal Tar, Sorbent, and Water Used temp pressure [K] [MPa] 873 923 973
19.8 19.8 19.8
tar [g] 0.062 0.057 0.060
Ca/C Ca(OH)2 water H2O/C [g] [cm3] [mol/mol] [mol/mol] 0.407 0.376 0.350
0.407 0.376 0.350
5.9 5.9 5.2
1.4 1.4 1.3
loaded with a mixture of the tar, distilled water, and reagent-grade Ca(OH)2 prior to being sealed. The TB reactor was soaked in the fluidized sand bath at temperatures of about 873-973 K for each desired soaking time, i.e., 1-30 min, and was rapidly heated to the temperatures within about 3 min2 and then held at the temperatures. The mixing ratios of the tar, sorbent, and water are shown in Table 2. The target pressure (19.8 MPa) in the reactor was calculated based on the reactor volume (7.2 cm3), water quantity, and practical gasification temperature and confirmed by a separate measurement using a pressure transducer.2 After gasification, the reactor was rapidly removed from the sand bath and cooled to room temperature with water in order to quench the reaction. After gaseous products were collected through the water substitution method,2,3 the volume of collected gaseous products was measured with a graduated cylinder. Then, the concentrations of H2, O2, N2, CH4, CO, CO2, C2H4, and C2H6 were analyzed using thermal conductivity detector micro gas chromatographs [Aera, model M200; (1) with MS-5A and PPQ columns and argon carrier gas and (2) with a PPU column and helium carrier gas]. The instrument accuracy was within 0.01 vol %. 3. Results and Discussion 3.1. Change of Gases Produced from the Tar with the Soaking Time. First of all, the carbon balance at 973 K and 19.8 MPa is shown in Figure 1.
Figure 2. Changes of gases such as H2 (b), CH4 (9), and C2H6 (2) produced from raw coal, coal char, and coal tar with the soaking time.
Figure 1. Distribution of carbon to gases (0) and char (9) with the soaking time for raw coal.
The unknown loss of carbon increased at longer soaking times because of the fact that residual solids at longer soaking times were agglomerated by melting in the Ca(OH)2-CaCO3 system2 and some of the agglomerates adhered to the wall of the reactor could not be collected. The carbon conversions to gas were about 40% and 67% within 2 and 30 min, respectively. The yields of gas produced from the coal and steam are shown in Figure 2, together with those produced from the char and tar as reactants.
From these figures, it can be seen that in the condition of the HyPr-RING method by using the TB reactor, which enabled rapid heating, the composition of gas produced from the coal is strongly dependent on the initial stage of gasification or pyrolysis reaction up to 5 min. The carbon source for the production of H2 is generated from the reaction between the coal and steam and is generally divided roughly into two parts, i.e., the char and the tar. The char is mainly converted into H2 and the tar into CH4 and H2. The objective of the HyPrRING method is to produce H2, which means that CH4 is a byproduct. Because CH4 is produced from the tar, CH4 from the coal should originate from the tar produced in the initial stage of coal pyrolysis and gasification. The effect of the temperature on the product gas yields in the initial stage is shown in Figure 3. From Figure 3 and the result of the tar in Figure 2, it can be seen that the amount of CH4 was decreased with a decrease in the temperature but the ratio of the
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Ind. Eng. Chem. Res., Vol. 44, No. 6, 2005
Figure 3. Change of gases such as H2 (b), CH4 (9), and C2H6 (2) produced from coal tar at 873 K with the soaking time.
yields of CH4 to H2 was increased, i.e., 1.2 at 973 K and 2.7 at 873 K. Additionally, at 873 and 973 K, C2H6 and a minute amount of CO2 were also produced. In the heating period and the initial stage within 5 min, the yields of H2, CH4, and C2H6 produced from the tar were increased at higher temperature. The higher production should be caused by thermal cracking and gasification of the tar. After 5 min, the yields of H2, CH4, and C2H6 were not significantly increased with time. These results in the batch experiments show that the initial gas and tar production is rapid and the second stage reaction is very slow. It is suggested that if tar is polymerized to char, then more H2 might be seen from gasification of the char. Furthermore, a little of the increase of CH4 might be due to the production of the reactive char and the reaction of 2C + Ca(OH)2 + H2O f CH4 + CaCO3.4 Consequently, even if the tar was changed into char, the rate of steam gasification of the char would be very slow under the present experimental conditions because the H2 production rate was low after 5 min. On the other hand, C2H6 was slowly decreased after 5 min at 973 K. This suggests that a part of C2H6 might participate in H2 and CH4 production after 5 min. 3.2. Changes of H2 and CH4 Produced from the Tar with the Reaction Temperature. As previously described on the dependence of the temperature on the yields of the produced gases, the ratio of the yields of CH4 and H2 was increased with a decrease in the temperature. The yields of CH4 and H2 produced from the tar at 30 min are plotted as a function of the reaction temperature in Figure 4. Both yields of H2 and CH4 were linearly increased with the temperature. In addition, the production rate of H2 with increasing temperature was higher than that of CH4. This suggests that the increase of H2 production in the second stage may be due to char produced from the tar and steam in high-temperature conditions by the reaction of C + CaO + 2H2O f 2H2 + CaCO3. 3.3. Application to a Fluidized-Bed Reactor. The HyPr-RING test with a continuous bench-scale fluidized-bed reactor at lower pressure has been started. Generally, gas and tar production are dependent on the heating rate with thermal decomposition in the initial stage. Although the conditions were different between the continuous fluidized-bed reactor and the batch TB reactor, the results obtained by the batch reactor with much higher heating and cooling rates than those of the autoclave provided the available CH4 and tar data because they should be also produced in the initial stage
Figure 4. Change of gases of H2 (b), CH4 (9), and C2H6 (2) produced from coal tar at a soaking time of 30 min with the reaction temperature.
by the fluidized-bed reactor. Those issues could be solved by using a higher and/or catalytic bed material to decompose the tar and to reform CH4. Additionally, the fluidized-bed reactor adopted the convenient lower pressure condition and would rather increase the molar ratio of H2 to CH4. When coal particles were fed into the fluidized-bed reactor, tar may be produced during the heating period and the initial stage of gasification with steam, which is discharged into the freeboard above the bed and thus causes tar trouble or carbon loss. The tar production should be reduced as much as possible. In addition, if CH4 produced from tar is needless, tar in the bed should be reduced and selective H2 production by a catalytic reaction or polymerization of tar should be promoted in the bed. Because tar is easily produced at lower temperature, heating by passing rapidly through the lower temperature period and a coking reaction by deposition of tar on the bed material at higher temperature should be recommended. 4. Conclusions To understand the reaction mechanism and the role of tar to produce rich H2 by the HyPr-RING method, the tar gasification under the presence of subcritical steam by using a small batch reactor was studied and the following conclusions were obtained: (1) The products from tar gasification were mainly H2 and CH4, and the amounts increased with time. The yield of H2 is lower than that of CH4. (2) The yields of H2 and CH4 were increased with increased temperature, which are rapidly produced in the initial stage. The increasing rates in the second stage after 5 min were increased with increased temperature. (3) The behavior of CH4 production in the initial stage of coal gasification is similar to that of tar gasification. Acknowledgment This study was supported by Center for Coal Utilization, Japan (CCUJ). The authors are also grateful to National Institute of Advanced Industrial Science and Technology (AIST) for its technical support. Literature Cited (1) Lin, S.-Y.; Suzuki, Y.; Hatano, H.; Harada, M. Hydrogen Production from Hydrocarbon by Integration of Water-Carbon Reaction and Carbon Dioxide Removal (HyPr-RING Method). Energy Fuels 2001, 15, 339.
Ind. Eng. Chem. Res., Vol. 44, No. 6, 2005 1953 (2) Kuramoto, K.; Furuya, T.; Suzuki, Y.; Hatano, H.; Kumabe, K.; Yoshiie, R.; Moritomi, H.; Lin, S.-Y. Coal Gasification with a Subcritical Steam in the Presence of a CO2 Sorbent: Products and Conversion under Transient Heating. Fuel Process. Technol. 2003, 82, 61. (3) Kumabe, K.; Moritomi, H.; Yoshiie, R.; Kambara, S.; Kuramoto, K.; Suzuki, Y.; Hatano, H.; Lin, S.-Y.; Harada, M. Gasification of Organic Waste with Subcritical Steam under the Presence of a Calcium-Based Carbon Dioxide Sorbent. Ind. Eng. Chem. Res. 2003, 43, 6943.
(4) Wang, J.; Takarada, T. Role of Calcium Hydroxide in Supercritical Water Gasification of Low-Rank Coal. Energy Fuels 2001, 15, 356.
Received for review October 13, 2004 Revised manuscript received December 23, 2004 Accepted January 18, 2005 IE049009Q
