Characterization of Char Gasification in a Micro Fluidized Bed

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Characterization of Char Gasification in a Micro Fluidized Bed Reaction Analyzer Xi Zeng,† Fang Wang,‡,† Yonggang Wang,‡ Aoming Li,§ Jian Yu,*,† and Guangwen Xu*,† †

State Key Laboratory of Multi-phase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ School of Chemical and Environmental Engineering, China University of Mining and Technology, Beijing 100083, People’s Republic of China § Shanghai Amtek Industrial Equipment Limited Company, Shanghai 200131, People’s Republic of China ABSTRACT: This study is devoted to characterizing the isothermal reaction kinetics of char gasification with CO2 in a micro fluidized bed reaction analyzer (MFBRA) in comparison to the measurement in a thermogravimetric analyzer (TGA). Under minimized inhibition of heat and mass transfer, the reaction rate was found to be much higher in the MFBRA than in the TGA. The maximal rate appeared at a conversion of about 0.15 in the MFBRA but at 0.45 in the TGA. The shrinking core model described well the char−CO2 gasification reaction in both the MFBRA and TGA. In the temperature range of 760−1000 °C, the char−CO2 gasification reaction can be divided into two stages. At lower temperatures, the activation energy from the MFBRA and TGA is very close, validating the reliability of the MFBRA for analyzing gas−solid reaction kinetics. At higher temperatures, the estimated activation energy was obviously higher for the MFBRA than for the TGA, showing the lower diffusion limitation prevailing in the MFBRA. The frequency factor for the Arrhenius equation was found to be much higher for the MFBRA than for the TGA, complying with the higher reaction rate observed in the MFBRA. The variation in the reaction atmosphere composition during gas switching in the TGA was also investigated.

1. INTRODUCTION Gasification is the core technology for converting coal and other carbon-containing fuels cleanly and environmentally friendly.1,2 The reaction process of gasification can be divided into two steps: the initial rapid pyrolysis of raw fuel and the subsequent gasification of pyrolysis-formed char.3,4 Being the rate-limiting step, char gasification dominates the whole reaction process of gasification. Therefore, the precise understanding of the kinetic characteristics of the involved char gasification reactions, including char−CO2, char−steam, and char−O2 reactions, is important and essential to the design, optimization, and scale-up of a gasifier.5,6 Despite extensive studies on such a topic in the past, the kinetics of the char gasification reaction was mainly measured using a fixed-bed reactor, especially a thermogravimetric analyzer (TGA).7−10 As a useful, simple, and fast measurement tool, the TGA can continuously monitor the mass change of a char sample held in the crucible cell during a specified heating program. However, limited by its measurement principle and heating rate (usually 0.8), the values predicted by the VRM model exhibited larger deviations than those from the SCM model, which agreed well with the results reported by Zhang et al.20 Furthermore, from Table 2, one can see that the fitting coefficient for the SCM model was above 0.99 at all of the tested temperatures. The similar results can also be obtained by comparing the experimental data from TGA and the predicted values by the two models. Consequently, the shrinking core model was finally adopted to depict the char gasification reactions and also to estimate the kinetic parameters in both the MFBRA and TGA. The activation energy and pre-exponential factor can be obtained from the linear fitting of ln k versus 1/T for the measurements in both MFBRA and TGA, as shown in Figure 10. In the figure, the fitting lines were basically parallel for different conversions. According to the difference in slope, the curves can be divided into a low-temperature stage (MFBRA, 760−850 °C; TGA, 760−820 °C) and a high-temperature stage (MFBRA, 850−1000 °C; TGA, 820−1000 °C), indicating the different rate controls during the entire period of gasification. In the low-temperature stage, the effect of heat and mass transfer is minimized through adopting the optimized measurement conditions, so that the gasification rate is subject mainly to the kinetic control. With an increasing temperature, the diffusion effect becomes gradually significant to alter and, thus, the dominant factor for determining the reaction rate.30 Table 3 shows the activation energy (Ea) and frequency factors (A) determined from the Arrhenius plots in Figure 10. For the low-temperature region, the activation energies from the MFBRA and TGA are very close, which are 282.82 and 285.46 kJ/mol, respectively, agreeing very well with the results reported by Boyd et al. and Kajitani et al.31,32 Considering the minimized effects of heat and mass transfer under the testing conditions, such activation energy would be very close to its intrinsic value for char−CO2 gasification. Thus, the similarity in the determined kinetic parameters for the two analyzers verified

Figure 7. Char gasification reaction rate versus conversion in the MFBRA and TGA at different reaction temperatures.

Figure 7 shows that the reaction rate increased quickly with the raising temperature in 760−1100 °C, especially when the temperature was above 850 °C. Correlating the reaction rate R and conversion X in Figure 7 demonstrates further that the MFBRA and TGA have led to much different curve shapes and also the reaction rate R itself. In the MFBRA (panels a and b of Figure 7), the gasification rate reached its peak quickly after an initial rapid and short increasing stage at conversions below 0.15 and then the rate gradually decreased as the conversion became higher. Corresponding to this, the char gasification rate in the TGA changed slowly throughout the entire period of reaction, and its maximal rate appeared at conversion X of about 0.45 (panels c and d of Figure 7). The latter for the TGA in fact agreed well with the results reported in the literature.29,30 Taking the gasification rates and their corresponding conversions at 800, 850, and 900 °C in Figure 7 as examples, Figure 8 compares further the reaction rates for the MFBRA and TGA through correlating the rate ratio of RMFBRA/RTGA and the carbon conversion X. For each of the plotted curves in Figure 8, one can see that the rate ratio quickly decreases with the rise in the conversion. In the main stage of gasification reactions, that is to say, at conversions below 0.9, the ratio of RMFBRA/RTGA was all above 1, indicating the higher gasification rate in the MFBRA for a specified conversion below 0.9. At rather higher carbon conversion (>0.9), the rate difference is little between the MFBRA and TGA and also for different temperatures. This indicates essentially that, when the char gasification tends to end, the reaction is subject mainly to the diffusion of the gasification agent (CO2 here) into the residual reactant carbon. Figure 8 also shows that the rate ratio RMFBRA/ RTGA is lower for higher temperatures, clarifying that the difference in the reaction rate between the MFBRA and TGA decreased with increasing the reaction temperature. This is 1842

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Figure 9. Comparison between the experiment and prediction by models (a) VRM and (b) SCM.

Table 2. Fitting Coefficient for the VRM and SCM Models Based on the Measurement in the MFBRA temperature (°C)

760

780

800

820

850

900

950

1000

VRM SCM

0.96 0.99

0.97 0.99

0.97 0.99

0.99 0.99

0.97 0.99

0.99 0.99

0.99 0.99

0.99 0.99

effective impaction between the char particles and CO2 molecules in the MFBRA.33 3.4. Insight into the Isothermal Reaction in the TGA. For the TGA, the gas switching method is always adopted to implement the measurement of isothermal reaction characteristics. After switching the gas, the newly introduced agent has to diffuse gradually into the reaction atmosphere to replace its former gas species (usually inert). This would cause a continuous change in the concentration or partial pressure of the gasification agent in the TGA, thus affecting the char gasification reactions. To examine this, blank experiments without char load in the crucible were conducted by connecting MS to the outlet of the TGA. Figure 11 shows the change in the gas concentration after switching N2 to CO2 at temperatures of 800, 850, 900, 950, and 1000 °C. The gas

Figure 10. Arrhenius plots at different conversions for tests in (a) MFBRA and (b) TGA.

Table 3. Kinetic Data of Char−CO2 Gasification Measured in the MFBRA and TGA instrument MFBRA TGA

temperature (°C)

activation energy (kJ/mol)

760−850 850−1000 760−820 820−1000

282.82 175.06 285.46 138.60

frequency factors (s−1) 1.04 3.04 3.91 2.09

× × × ×

1012 106 1011 105

in fact the applicability of the MFBRA for measuring the isothermal reactions. For the high-temperature region with somehow evident limitations on the reaction rate from the heat and mass transfer, the estimated apparent activation energy is thus obviously higher for the MFBRA (175.06 kJ/mol) than for the TGA (138.60 kJ/mol). This further demonstrates that the limitation effect from diffusion is weaker in the MFBRA than in the TGA for coal char gasification. Table 3 also shows that the frequency factor for the Arrhenius equation is much higher for the MFBRA than for the TGA, complying with the higher reaction rate in the MFBRA shown in section 3.3. The larger frequency factor means possibly the higher frequency of

Figure 11. Diffusional effect existing in switching an inert gas to a gasification agent in the TGA. 1843

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concentration sharply changed in the initial stage, and it required 5−10 min to reach a steady state. During this period, the char gasification reaction must proceed partially or mostly (if with only a tiny sample mass), especially at temperatures above 850 °C, as shown in Figure 6. No doubt, this would affect the char gasification rate and contribute to the lower reaction rate measured in the TGA than in the MFBRA. Hence, it is reasonable to expect that the kinetic data from the MFBRA would be more precise to reflect the reaction nature.

4. CONCLUSION The isothermal differential reaction characteristics and kinetics of char−CO2 gasification were investigated in the indigenous MFBRA under minimized inhibition of external diffusion. A comparison was made with the measurement in the TGA. It was shown that the reaction rate was much higher in the MFBRA than in the TGA, and the maximal reaction rate appeared at a conversion of about 0.15 for the MFBRA but at about 0.45 for the TGA. At lower temperatures (MFBRA, 760−850 °C; TGA, 760−820 °C), the determined activation energy based on the data from the MFBRA and TGA was very close, indicating the reliability of the MFBRA for applying to gas−solid reactions, such as char gasification. At high temperatures (MFBRA, 850−1000 °C; TGA, 820−1000 °C), the determined activation energy was much higher for the MFBRA than for the TGA, showing the lower diffusion limitation ensured by the MFBRA. In the experimental temperature range, the determined frequency factor for the kinetic equation of the gasification reactions was much higher for the measurement in the MFBRA than in the TGA, agreeing with the observed higher reaction rate for the MFBRA. It was shown also that the gas switching method applied to the TGA has to cause an obvious transient variation in partial pressure or species concentration of the newly introduced gas, which would affect the reaction analysis result in the TGA.

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AUTHOR INFORMATION

Corresponding Authors

*Telephone/Fax: +86-10-8254-4886. E-mail: [email protected]. ac.cn. *Telephone/Fax: +86-10-8254-4886. E-mail: [email protected]. ac.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to the Natural Science Foundation of China (21306209) and the financial support from the National Basic Research Program of China (2011CB201304) and the Strategic Priority Research Program of Chinese Academy of Sciences on clean and high-efficiency utilization of low-rank coal (XDA07050400).

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