Energy Fuels 2010, 24, 68–75 Published on Web 09/10/2009
: DOI:10.1021/ef9004994
High-Temperature Gasification Reactivity with Steam of Coal Chars Derived under Various Pyrolysis Conditions in a Fluidized Bed† Hao Liu,*,‡ He Zhu,‡ Mashhiro Kaneko,§ Shigeru Kato,§ and Toshinori Kojima§ ‡
Department of Thermal Energy & Power Engineering, School of Physical Science and Technology, Soochow University, Suzhou, 215006, People’s Republic of China, and §Department of Materials and Life Science, Seikei University, Tokyo 180-8633, Japan Received May 21, 2009. Revised Manuscript Received August 4, 2009
In a unique fluidized-bed reactor, the gasification reactivity with steam of chars derived under various conditions was investigated at elevated temperatures. It was revealed that, similar to char-CO2 gasification, even at elevated temperatures, the gasification rates in steam were very different between different coals. The gasification rate of char with steam was very different from that of raw coal. A longer pyrolysis time led to lower reactivity of char during char gasification with steam. No significant influence of pyrolysis time on the temperature dependence of gasification reactivity (i.e., activation energy for a char) was identified. The reaction rate for coal had a tendency to level off at a high temperature for simultaneous pyrolysis/ gasification experiments, which was probably caused by the ash fusion in a reducing atmosphere. Generally, the activation energy of char gasification with steam, was lower than that with CO2 gasification. Moreover, the former activation energy in the low-temperature range was not significantly different from that in the high-temperature range. At elevated temperatures, the rate of char gasification with steam was not necessarily much higher than that of char-CO2 gasification.
not for the composition of the gas produced. Matsui et al.7 studied the fluidized-bed steam gasification of char by thermogravimetrically obtained kinetics in a temperature range of 1096-1311 K. Kasaoka et al.8 studied steam gasification of char with a thermogravimetric analyzer (TGA) and noted that the char reactivity with steam was dependent on the type of coal but was almost independent of the pyrolysis conditions such as the heating rate, gaseous atmosphere, and quenching at temperatures below ∼1273 K, while pyrolysis at temperatures of >1373 K reduced the char reactivity. Peng et al.9 also studied steam gasification of char with a TGA (thermogravimetric analyzer). A tubular reactor was also adopted by Ma et al.10 to examine the intrinsic kinetics of steam gasification of a coal char. Saffer et al.6 studied coal gasification with steam in a fluidized bed and concluded that the kinetic factors are more important than the hydrodynamic factors, and that the primary criteria of performance can be interpreted qualitatively in terms of reaction kinetics. However, their experiments were conducted at temperatures 1923
1588 >1873 >1923
1628 >1873 >1923
1643 >1873 1908
1703 >1873 >1923
1748 >1873 >1923
Figure 2. Gasification reaction rate versus time for SS017 chars pyrolyzed at various pyrolysis times (char gasification with steam).
lead to a lower reaction rate in the beginning and a longer time for the reaction to complete. At 1237 K, for example, the maximum reaction rates (dX/dt) are 0.080, 0.067, and 0.037 m/s for pyrolysis times of 0, 0.5, and 10 min, respectively. Moreover, at high temperatures, the gasification reaction rate is higher than that at low temperatures in the beginning, and decreases more rapidly with time, suggesting a shorter period for complete gasification at higher temperatures. This result (i.e., the influence of pyrolysis time on the rate of char gasification with steam) was similar to the case of char-CO2 gasification.18 The reaction rate results for SS033 char and SS035 char are shown in Figures 3 and 4, respectively. Similarly, the effect of pyrolysis time on gasification reaction rate was also demonstrated. In our previous research,13 it was found that the high gasification reactivity of rapid heating chars can be explained by the high surface area in
to derive char reactivity. The gasification rate of carbon was given as the sum of CO and CO2 production rates, based on the chemical stoichiometry of carbon atom, and the main gasification reactions of C þ H2O f CO þ H2 and C þ 2H2O f CO2 þ 2H2. The composition of the pyrolysis product, i.e., the volatile matter released from pyrolysis, was measured as a background in a nitrogen atmosphere through separate experiments. The production rates of CO and CO2 from pyrolysis were subtracted from the production rates of CO and CO2 when calculating the gasification reaction rates, so as to avoid the influence of pyrolysis product.
Results and Discussions Reaction Rate versus Time for Various Chars. The reaction rates versus time during gasification are illustrated in Figure 2 for SS017 chars derived from various pyrolysis times. Obviously, a longer pyrolysis time has a tendency to 70
Energy Fuels 2010, 24, 68–75
: DOI:10.1021/ef9004994
Liu et al.
Figure 3. Gasification reaction rate versus time for SS033 chars pyrolyzed at various pyrolysis times (char gasification with steam).
the mesopore range. This finding can be also recognized as the correlation between pyrolysis time and char structure considering that the pyrolysis time for rapid heating is shorter than the pyrolysis time for slow heating. Referring to these previous results, changes in structure and active site of a char were very likely the main mechanism accounting for the effect of pyrolysis time on the char reactivity. Temperature Dependence of Reaction Rate for Various Chars. Figure 5 shows the temperature dependence (i.e., an Arrhenius plot) of the initial reaction rates for SS017 chars derived from various pyrolysis times. The initial reaction rates were derived by fitting the experimental data with the random pore model,19 to minimize the error due to the possible uncertainty in the induction period. Thus, the initial reaction rates were determined, in fact, by all of the experimental data points, rather than directly by the initial data point. Because a random pore model can describe the change of reaction rate with conversion very well, fitting all the data with this model gives initial reaction data through the entire tendency of change in reaction rate during the reaction history and, therefore, minimizes the influence of the induction period. Figure 5 shows that a longer pyrolysis time leads to lower initial reactivity of SS017 char. Moreover, direct gasification (at a pyrolysis time of 0 min) had a much higher initial reaction rate than those chars derived from 0.5 and 10 min, which also suggested the very likely effect of atmosphere on gasification rate, considering the atmosphere of direct gasification is different, because of volatile matter.
Similar to the case of SS017 char, the influence of pyrolysis time on the reaction rate is also demonstrated for SS033 char and SS035 char in Figures 6 and 7, respectively. Figures 5-7 show that, at elevated temperatures, the gasification rates in steam were very different among different coals, despite the same particle size used for all the coals. From these results, it was determined that film diffusion was at least not the dominant factor of the overall reaction; otherwise, the reaction rates of chars with the same size would not be so different. Moreover, in Figures 5-7, no significant influence of pyrolysis time on the activation energy was identified for all three coals tested. Furthermore, Figures 6 and 7 demonstrate that the reaction rate levels off at a high temperature for the direct gasification of SS033 and SS035 coals. Probably the reducing atmosphere produced from devolatilization facilitated ash fusion and, accordingly, decreased the gasification rate in the case of direct gasification. Kasaoka et al.4 noted that coal ash melts at ∼1673 K during both char-steam and char-CO2 gasification. Radovic et al.20 found that the commonly observed deactivation of coal chars with increasing severity of pyrolysis conditions was correlated with a decrease in the active surface areas. Kasaoka et al.8 also found that pyrolysis at a temperature higher than 1373 K brought about a significant change in the pore structure of the char. Change of Reaction Rate with Carbon Conversion. Figure 8 shows the change of reaction rate with carbon conversion for SS017 char at various temperatures. The lines in Figure 8 (20) Radovic, L. R.; Walker, P. L., Jr.; Jenkins, R. G. Fuel 1983, 62, 849–856.
(19) Bhatia., S. K.; Perlmutter, D. D. AIChE J. 1980, 26 (3), 379–386.
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Energy Fuels 2010, 24, 68–75
: DOI:10.1021/ef9004994
Liu et al.
Figure 4. Gasification reaction rate versus time for SS035 chars pyrolyzed at various pyrolysis times (char gasification with steam).
Figure 6. Temperature dependence of initial reaction rate for SS033 char (char gasification with steam).
Figure 5. Temperature dependence of initial reaction rate for SS017 char (char gasification with steam).
tion period. It can be seen that the reaction rate increases in the beginning, reaches a maximum, and decreases thereafter. The curves at different temperatures all demonstrated almost the same tendency, which is presumed to be due to the formation of pores in the initial stage and coalescence of
were drawn to approximate the experimental data based on the random pore model,19 which best fits the experimental data. The data points at low conversions do not agree with the lines drawn, because these points belonged to the induc72
Energy Fuels 2010, 24, 68–75
: DOI:10.1021/ef9004994
Liu et al.
Figure 9. Complete gasification time for SS017 char derived at various pyrolysis times (char gasification with steam).
Figure 7. Temperature dependence of initial reaction rate for SS035 char (char gasification with steam).
Figure 10. Complete gasification time for SS033 char derived at various pyrolysis times (char gasification with steam). Figure 8. Reaction rate versus carbon conversion for SS017 char (char gasification with steam).
are shown in Figure 9. The pyrolysis time strongly influences the complete gasification time. Figures 10 and 11 present the results for SS033 char and SS035 char, respectively. Similarly, the strong effect of pyrolysis time on the complete gasification time for SS033 and SS035 chars was demonstrated. Comparison with Char Gasification with CO2. The reaction rate of char gasification with steam was compared with that of char gasification with CO2 for SS017, SS033, and SS035 chars in Figures 12-14, respectively. It was demonstrated that, at elevated temperatures, the rate of char gasification with steam was not necessarily much higher than that of char gasification with CO2, which was much different from the conventional impression for the gasification at relatively low temperatures. One interpretation of this fact is their different dependence on temperature, or activation energy, which makes the difference in reaction rate between
the pores in the latter stage. SS033 and SS035 chars demonstrated the similar tendency. This tendency is in agreement with the theory of random pore model.19 It can be seen that pyrolysis time affected the reactivity of a char not only in the beginning, but also in the latter period of gasification. Complete Gasification Time. To investigate the effect of pyrolysis time on the reactivity of a char in the entire gasification process, particularly the less-reactive period, the complete gasification time was estimated from experimental data. It was derived by extrapolating the straight line regressed from several of the last data points of the CO production rate;reaction time curve, drawn in a graph with normal coordinates. The reaction time corresponding to the cross-point on the horizontal coordinate was taken as the complete gasification time. The results for SS017 char 73
Energy Fuels 2010, 24, 68–75
: DOI:10.1021/ef9004994
Liu et al.
Figure 13. Comparison of reaction rates between char gasification with steam and char gasification with CO2 for SS033 char. Figure 11. Complete gasification time for SS035 char derived at various pyrolysis times (char gasification with steam).
Figure 14. Comparison of reaction rates between char gasification with steam and char gasification with CO2 for SS035 char. Table 3. Comparison of Activation Energy between Various Coals and Pyrolysis Times Activation Energy (kJ/mol) SS017 Char
Figure 12. Comparison of reaction rates between char gasification with steam and char gasification with CO2 for SS017 char.
gasification with CO2 and H2O pronounced at relatively low temperatures. At relatively lower temperatures, the rate of gasification with steam is much higher than gasification with CO2, whereas the latter increases stronger with temperature. That is why at elevated temperatures, the rate of char gasification with steam was not necessarily much higher than that of char gasification with CO2. Different reactions have different activation energy, because the energy of molecules needed for these reactions to occur is different. Gasification reactions are no exception. Carbon is the main composition of coal char. In fact, char gasification is the gasification of carbon. For carbon, the activation energy of gasification with steam and gasification with CO2 is different. Therefore, similarly, for coal char, the
SS033 Char
SS035 Char
pyrolysis time (min)
with H2O
with CO2
with H2O
with CO2
with H2O
0 0.5 10 30
57 135 67
188 208 205 211
121 116 108
91 128
97 126 119
activation energy differs between gasification with steam and gasification with CO2. This is mainly caused by chemical factors. The activation energy for various chars and various pyrolysis times are listed in Table 3. It can be seen that the influence of pyrolysis time on the activation energy was limited. However, the activation energy has a strong dependence on coal types and quite different between gasification with steam and gasification with CO2. 74
Energy Fuels 2010, 24, 68–75
: DOI:10.1021/ef9004994
Liu et al.
As explained previously, char gasification is practically carbon gasification, because carbon is the main composition of coal char. The activation energy is different between carbon gasification with steam and carbon gasification with CO2. Accordingly, the activation energy of char-H2O gasification differs from char-CO2 gasification. On the other hand, the difference between coal types is probably due to the different pore structure and chemical composition, considering the fact that mineral matter may have a catalytic effect on the gasification reaction. It is difficult to quantitatively compare our results with the results of other researchers, because the results of the same coals are not available. Nevertheless, qualitative comparison, or a comparison of tendency, is possible even if different coals are tested. The influence of pyrolysis conditions on the reactivity of the derived chars agreed with the results obtained by other researchers. For instance, Kasaoka et al.8 found that pyrolysis above 1373 K reduced the char reactivity. Kasaoka et al. studied steam gasification of char with TGA and noted that the char reactivity with steam was dependent on the type of coal but was almost independent of the pyrolysis conditions of heating rate, gaseous atmosphere, and quenching at temperatures below ∼1273 K. Generally speaking, the char from pyrolysis with a higher heating rate is different with regard to physical structure (specific surface area and pore volume, etc.) from that with a lower heating rate; accordingly, chars with different physical structures have different gasification reactivities. The results of Kasaoka et al. were likely attributed to the limited range of heating rates with TGA. Besides, the results of this work were similar to the lower char reactivity, because of the high-temperature treatment reported by many investigators concerning char gasification with CO2.8,13,20-22
Conclusions The effect of pyrolysis time on the gasification reactivity of three types of chars in steam was investigated through experiments at elevated temperatures and high heating rates, with a unique fluidized bed. The following conclusions were reached: (1) Similar to char-CO2 gasification, even at elevated temperatures, the gasification rates in steam were very different among different coals. The gasification rate of a char was very different from that of a parent coal (direct gasification without pyrolysis in advance). A longer pyrolysis time led to lower reactivity of a char during char gasification with steam, probably due to a structure change of the char during pyrolysis. Accordingly, this method was more adequate to measure the gasification kinetics of a coal, because this work revealed that pyrolysis time influences the gasification reactivity of a coal and this method can control the pyrolysis time exactly. (2) No significant influence of pyrolysis time on the activation energy for a char was identified for all three coals tested. (3) The reaction rate for coal had a tendency to level off at a high temperature for simultaneous pyrolysis/gasification experiments, which was probably caused by the ash fusion in a reducing atmosphere at a high temperature. (4) Generally, the activation energy of char gasification with steam was lower than that of char-CO2 gasification. Moreover, the former activation energy in a lower-temperature range was not significantly different from that in a hightemperature range. At elevated temperatures, the rate of char gasification with steam was not necessarily much higher than that of char-CO2 gasification, which was very different from that at low temperatures. Acknowledgment. This work is supported by the Foundation of State Key Laboratory of Coal Combustion (China). The authors also would like to thank NEDO/CCUJ for financial support of this work under BRAIN-C program (Japan).
(21) Kajitani, S.; Matsuda, H. Influence of pyrolysis conditions on coal gasification reactivity, Yokosuka Research Laboratory Report, Japan, 1998; No. W97020. (22) Miura, K.; Makino, M.; Silveston, P. L. Fuel 1990, 69, 580–589.
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