Distinctive Hydrodynamics of a Micro Fluidized Bed and Its Application

Dec 15, 2017 - Both the analysis method and testing performance of the adopted analyzer significantly affect the design, optimization, and scale up of...
2 downloads 13 Views 4MB Size
Download PDF
Review pubs.acs.org/EF

Cite This: Energy Fuels XXXX, XXX, XXX−XXX

Distinctive Hydrodynamics of a Micro Fluidized Bed and Its Application to Gas−Solid Reaction Analysis Fang Wang,† Xi Zeng,*,†,‡ Sulong Geng,† Junrong Yue,† Shibai Tang,† Yanbin Cui,† Jian Yu,† and Guangwen Xu†,§ †

State Key Laboratory of Multi-Phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ Sino-Danish College, School of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § Institute of Industrial Chemistry and Energy Technology, Shenyang Institute of Chemical Technology, Shenyang, Liaoning 110142, People’s Republic of China ABSTRACT: The measurement and analysis of reaction behavior, kinetics quantification, and mechanism clarification for gas−solid reactions are essential to the research and development of process technologies in fields of chemical engineering, energy production, mineral processing, etc. By far, different approaches and instruments have been applied for gas−solid reaction analysis and measurement, but a recent development appears distinctive because of its use of a micro fluidized bed (MFB) in implementing the so-called isothermal differential analysis of gas−solid reactions at the Institute of Process Engineering, Chinese Academy of Sciences (CAS). Systematic work has been reported on hydrodynamic studies in MFBs and the standardized development as well as its utilization of the so-called micro fluidized bed reaction analyzer (MFBRA) for investigating various particle-involved reactions, such as pyrolysis, combustion, gasification, reduction, catalysis, absorption, etc. The obtained results well demonstrated the distinctiveness of MFBs from the common large-size fluidized beds and the obvious advantages of the MFBRA for reaction analysis in aspects of rapid heating of the reactant sample, minimization of diffusion limitation to the reaction, real-time online capture of isothermal product formation characteristics, etc. It was shown that MFBRA is particularly capable of analyzing rapid complex reactions and providing some exceptional functions, such as steam atmosphere, online particle sampling, and decoupling of complex reactions in series. A series of applications well verified that the MFBRA provides actually a fundamental approach and the kind of instruments that are complementary to those of thermogravimetric analysis (TGA) for gas−solid reaction measurement and analysis.

1. INTRODUCTION 1.1. Approaches for Gas−Solid Reaction Analysis. Characteristics of the gas−solid reaction and its dynamics analysis are important research content in the field of process engineering, such as chemical engineering, metallurgy, energy conversion, materials science and engineering, etc. Both the analysis method and testing performance of the adopted analyzer significantly affect the design, optimization, and scale up of these processes.1,2 Thus, understanding the reaction behavior accurately and obtaining the reasonable kinetic data for a heterogeneous gas−solid reaction become essential and necessary. According to the change of the reaction temperature or not, the analysis method of the gas−solid reaction can be divided into the isothermal method or non-isothermal method.3 Up to now, the reactors and analyzers adopted in the gas−solid reaction analysis are diverse, including a self-made fixed bed reaction analyzer, thermogravimetric analyzer, differential thermal analyzer, drop-tube reactor, fluidized bed (FB) analyzer, Curie point reactor, etc.4,5 Figure 1 summarized the use of different kinds of analyzers for the research of carbon-containing fuel gasification published in the journal of Fuel in 2015. From it, one can see that thermogravimetric analysis (TGA) is widely adopted, showing good competitiveness. As we know, in TGA measurement, prior to the preset heating program, certain milligrams of sample are placed in a sample holder. When the variation of sample mass loaded in a © XXXX American Chemical Society

Figure 1. Use of different kinds of reaction analyzers.

crucible holder is continuously monitored, it is very easy and convenient for TGA to examine the reaction behavior. Special Issue: 6th Sino-Australian Symposium on Advanced Coal and Biomass Utilisation Technologies Received: October 6, 2017 Revised: November 23, 2017

A

DOI: 10.1021/acs.energyfuels.7b03003 Energy Fuels XXXX, XXX, XXX−XXX

Review

Energy & Fuels

result of the large reactor parameter and low measurement accuracy of the balance adopted (a weighing range of 150 kg with a reading precision of 0.1 g), serious gas back mixing and diffusion will be inevitable and the accuracy and repeatability of results are also difficult to ensure. For the latter, to test the burning rate of coal char in the FB reactor, Fennell et al.18 proposed that the reaction rate can be measured by measuring the content change of CO and CO2 by an infrared gas analyzer in 2007. Then, on a 51 mm diameter bench-scale FB reaction analyzer, Nilsson et al.19 systematically examined the physical effects on the char gasification reaction. The composition of the exit gas was measured by a Siemens analyzer using a nondispersed infrared method for CO, CO2, and CH4 and thermal conductivity and paramagnetic methods for H2 and O2. Via a FB reactor with the inner diameter of 22 mm and an overall length of 870 mm, Schwebela et al.20 conducted the chemical looping combustion experiments and finally obtained the apparent kinetic parameters by fitting to different model approaches. To measure the kinetic parameters of the particle reaction in gasification, a self-made FB reaction analyzer, with a diameter of 34 mm and a typical fluidized height fluctuating around 70 mm, was established by Haustein et al.21 The gasification behavior in this FB reactor was examined and further compared to the TGA method. After reviewing this literature, we can see that, as a result of the lack of systematical research in hydrodynamics, the structure and parameter of the FB reactor adopted were diverse, making an obvious difference in reaction behavior even for the same reaction. Moreover, limited by the measuring principle and the data acquisition frequency of analyzers, it is always not suitable for measuring the rapid reaction.22−24 Therefore, despite the obvious advantages mentioned above, accomplishing the investigations of reaction behavior and kinetics, especially the intrinsic kinetics in a FB reactor, for the existing FB reaction analyzer mentioned above is still difficult. 1.3. Micro Fluidized Bed Reaction Analysis (MFBRA). To overcome the bottlenecks of the gas−solid reaction by the FB reactor, many works has been conducted. Among them, miniaturization of the FB reactor is given more and more attention because a small-size bed has good operability and availability for some particularly required characteristics.25−27 The concept of measuring reaction behavior and kinetics by a micro fluidized bed (MFB) for liquid−solid reaction and gas−solid reaction has been proposed in 2005 and 2007, respectively.28,29 The previous studies show that, by controlling the effective size of the reactor to a certain degree, the gas back mixing in it and the operating stability will be much improved. This will make it true to realize real-time analysis for a complex chemical reaction. On this basis, a new analytical method adopting the MFB as the reactor has been proposed by the Institute of Process Engineering (IPE), Chinese Academy of Sciences (CAS), to test gas−solid reaction characteristics and determine the reaction kinetics.30 The studies about the reactor structure of the MFB, particle fluidization characteristics, gas−solid back-mixing feature, wall effect, and operating stability have been performed systematically. On this basis, a novel MFB reaction analyzer has been developed and standardized. By integration of the key technologies of pulse sample injection and online quick analysis of the formed gas product, the so-called isothermal differential MFBRA is able to measure the reaction behavior, calculate the kinetic parameters, and analyze the reaction mechanism, which fills the technology gap of the isothermal differential analyzer. 1.4. Intention of This Review. This paper is devoted to summarizing the theoretical research of the MFB reaction

However, limited by the measuring principle and heating rate of TGA (usually below 100 K/min), the experimental sample always needs to undergo a heat treatment period during the temperature-rising stage. When the gas stream flows across the mouth of the crucible holder, a stagnant region near the upper surface of the sample will form inevitably.6−8 Moreover, for the gas−solid isothermal reaction, during the switching period from inert gas to reactant gas, the influence of gas mixing and diffusion on the measured reaction will be very serious, only if the reaction rate is very high. Figure 2 shows the change of the gas content

Figure 2. Diffusion effect in the gas switching period for TGA:9 (1) cylinder, (2) needle valve, (3) gas mass flowmeter, (4) micro fluidized bed reactor, (5) differential pressure transducer, (6) sampling port, (7) mass spectrometer, and (8) data acquisition instrument.

after switching N2 to CO2 at temperatures of 800 and 1000 °C during the char isothermal gasification reaction. From it, one can see that the content of N2 and CO2 changed strongly in the initial stage, needing 5−10 min to reach a steady state. However, during this period, the char will be gasified partially or mostly, especially at a high temperature. For the violent chemical reaction, such as combustion, perhaps this effect will become more significant. Therefore, it is reasonable to believe that all of these mentioned above will affect the accuracy and rationality of the testing result, making the obtained data deviate from the industrial commercial apparatus.9 Many other reactors, including mesh reactor, drop-tube furnace, FB, etc., have also been used to measure the gas−solid reaction behavior and calculate kinetic parameters, but for these analyzers, there are no standardized analytical approaches and customized instruments, which still suffer from the limitations of an uneven temperature distribution in the reactor, serious gas back mixing, and diffusion.10−13 Consequently, the kinetic data reported in the literature were always much different, even for the same feedstock and under nearly approximate experimental conditions. 1.2. FB Reaction Analyzer. With more FB reactors adopted in the fundamental studies, the exploration of the FB reaction analyzer for gas−solid reaction analysis has become more and more interesting and attractive because of the obvious properties, such as excellent heat and mass transfer, easy scale up, good flexibility for feedstock, etc.14,15 According to the difference in the measurement method, the existing analyzers adopting the FB reactor can be divided into two categories, namely, gravimetrical analysis and concentration test. For the former, Reschmeier et al.16,17 designed a test rig by adopting the FB reactor, mainly consisting of a FB reactor with an inner diameter of 100 mm and a balance for testing the mass change of the system. At present, this analyzer has been used to examine the wood pyrolysis and char gasification. However, as a B

DOI: 10.1021/acs.energyfuels.7b03003 Energy Fuels XXXX, XXX, XXX−XXX

Review

Energy & Fuels analyzer, including the reactor structure, feeding style, hydrodynamic characteristics in the reactor, heat and mass transfer, gas testing method, etc. On this basis, a standard MFB reactor enabling the separation of the kinetic information from physical diffusion and/or fluid dynamics will be optimized. Then, the typical applications of this analyzer on catalysis, thermal chemical conversion, reduction of iron ore, etc. will be conducted to display the testing property in good heat and mass transfer, limited gas back mixing, online real-time analysis, etc. Meanwhile, a primary comparison between TGA and MFBRA will be conducted. All of these will be used to point out the new development direction and further improve the gas−solid analytical method.

2. HYDRODYNAMICS IN THE MFB REACTOR 2.1. Wall Effect. The fluidization characteristics of solid particles in the MFB reactor will be much different from the ordinary-size FB reactor because of the strong wall effect.31 As shown in Figure 3, Liu et al.32 investigated the influence of

Figure 4. Variations of the wall effect with the diameter of the FB reactor under different Hs and dp.32

bed heights from 20 to 50 mm to make the bed relatively stable and the particle homogeneous fluidization in the reactor. The MFB reactor with the size mentioned above is believed for suppressing gas mixing and external diffusion effectively, making it very suitable for reaction kinetics measurements. 2.2. Numerical Simulation. For the proper design of MFB devices, it is critical to gain a good understanding of the hydrodynamics in the reactor. Liu et al.33 adopted the Eulerian multiphase model via commercial computational fluid dynamics (CFD) software (FLUENT 6.3.26) to predict the fluidization behavior of Geldart A particles in the MFB reactor. The results revealed that, in comparison to the Gidaspow drag model, the modified Gibilaro drag model was more suitable for predicting the minimum bubbling velocity and bed porosity by setting the scaling factor of 0.2, which can enable the flow regime transition from bubbling to turbulent flow and a good fitting degree between the simulation value and experimental value. An analysis of the boundary wall effects indicated that, although the specularity coefficient played a minor role on the predicted minimum bubbling velocity and bed porosity, it had a substantial impact on the flow structure of the gas and solid. In comparison to the no-slip wall boundary condition, the free-slip boundary conditions presented smaller bubbles, a lower frequency of bubble formation, and more vigorous gas and solid circulations. This indicates that the wall boundary conditions need to be specified carefully when the gas−solid MFBs are modeled. 2.3. Characterization of Gas Back Mixing. As we know, gas backing in the FB reactor has a serious influence on realtime analysis of physical processes and chemical reactions, which is much related to the reactor structure and operating conditions, such as the particle size (dp), static bed height (Hs), gas velocity (Ug), and reactor diameter (Dt).34−36 At present, most of the FBs adopted in the studies about gas back mixing had the diameter above 50 mm. The authors’ group examined the effect of Dt (10−30 mm), Ug (0.009−0.048 m/s), and Hs (10−50 mm) on gas back mixing by the apparatus shown in Figure 5, which was quantified via the relationship between variance σt2 and peak height E(t)h of gas residence time distribution (RTD). The typical result in Figure 5 shows that, with the increase of Ug and the decrease of D, σt2 of E(t) can decrease until 0. For σt2 below 0.25, E(t)h increased exponentially. On the contrary, with the decrease of Ug and increase of D, σt2 will be infinite, while E(t)h tends to 0. For σt2 above 5.0, the declining rate of E(t)h reduces. The relationship

Figure 3. Schematic diagram of the experimental setup.32

reactor inner diameters (Dt = 12, 20, and 32 mm), static bed heights (Hs = 20, 35, and 50 mm), and particle size of the bed material (dp = 96, 242, and 464 μm) on the wall effect and characteristic fluidization velocity systematically. A newly defined parameter Pw,max/VB, as shown in eq 1, can be used to provide an alternative measure to the wall-effect-induced extra barrier, which needs to be overcome for full fluidization of solid particles ΔPw,max VB

=

(ΔPB/HB − ΔPErgun /HB)ug = uc πDt 2 /4

(1)

where ΔPw,max/VB represents the maximum extra pressure drop per unit volume of particle bed induced by the bed wall, ug and uc are the superficial gas velocity (m/s) and critical gas velocity (m/s), and ΔPB and ΔPErgun are the pressure drop across the particle bed (Pa) and across the fixed bed (Pa). For the examined reactors, the minimum fluidization velocity (Umf) can be determined by the classic Richardson method. However, for the minimum bubbling velocity (Umb), it can be difficult to measure by the traditional method, namely, the bubble observation method. As shown in Figure 4, in comparison to Dt above 20 mm, the parameters of Dt, dp, and Hs have a significant effect on Umf, Umb, and ΔPw,max/VB. With the increase of Dt, dp, and Hs, the values of Umf, Umb, and ΔPw,max/ VB decreased sharply. Finally, an inner diameter of about 20 mm was suggested to be suitable in the range of the static C

DOI: 10.1021/acs.energyfuels.7b03003 Energy Fuels XXXX, XXX, XXX−XXX

Review

Energy & Fuels

certain degree. It can be seen that the inhibition of the gas bubble was very obvious, especially for the structure A. The nearer the distance between the feeding pipe and reactor wall, the weaker the effect of trance gas on mixing. 2.5. Definition of the MFB. The study mentioned above showed that, by minimizing the reactor inner diameter of 21 mm, keeping the static bed height in the range of 20−50 mm, adopting the feeding style optimized, and using the Geldart B particles, such as silica sand with dp above 155 μm, as bed materials, the physical diffusion in the reactor will be inhibited effectively with the σt2 and E(t)h of RTD smaller than 0.25 and greater than 1.0, respectively. Under these conditions, the gas and solid phases in the reactor displayed nearly plug flow and perfect mixing flow, respectively, which will ensure the isothermal differential conditions for real-time analysis.

Figure 5. Correlation between σt2 and E(t)h.

3. PRINCIPLE AND PROPERTY OF MFBRA 3.1. Principle of MFBRA. On the basis of studies mentioned above, IPE, CAS, for the first time, proposed to use the MFB for testing gas−solid reaction characteristics and determining the reaction kinetics, as shown in Figure 8. A kind of MFB reactor with an inner diameter of 21 mm and a static bed height for a particle of 20 mm is adopted to take advantage of the quick heat and mass transfers in FB reactors and ensure the differential feature of the involved reaction. Moreover, this analyzer can also feed the sample online by pulse at an arbitrary experimental temperature and continuously monitor the change of gas composition by a fast process mass spectrometer. Thus, MFBRA can provide an isothermal differential reaction analysis tool to study the reaction characteristics and also measure the reaction rate and kinetic parameters.37 3.2. Data Analysis Approaches. In comparison to TGA, the measuring principle of MFBRA is much different. In TGA, the change of the sample mass can be continuously monitored by a high-precision balance, while for MFBRA, the variations of product gas composition are tested through online gas analyzers, such as mass spectrometry (MS) and gas chromatography (GC). By the fast mass spectrometer, it is very easy to determine the starting and ending point accurately and effectively, while micro GC adopted can be used to measure the average gas components.9 To be more clear, Figure 9 takes the gasification reaction of char with CO2 as an example, whose carbon conversion can be determined at the arbitrary reaction time by eqs 2 and 3, shown as follows:

between σt2 and E(t)h can be used as a criterion for judging the gas flow pattern. At the values of σt2 below 0.25 and E(t)h above 1.0, the gas flow in the MFB is very closed to the plug flow. Under this condition, very limited back mixing will be present in the FB reactor. On the contrary, at the values of σt2 above 5.0 or E(t) peak below 0.25, it will suffer from serious gas dispersion. 2.4. Choice of Online Feed Method. The feeding method and position of the feeding tube in the reactor will seriously affect the mixing performance between the sample and bed material and further hydromechanics in MFBs. Generally, the pulse method has the advantage of instantaneous charging operation simply and reliably and loading samples in the reactor at an arbitrary reaction temperature. Yang et al.36 examined the effect of different injector structures and locations in the reactor on the gas−solid mixing characterization. As shown in Figure 6,

Figure 6. Schematic diagram of injector structures.36

for the adopted structures of A and E, nozzles are close to the reactor wall, while for the structures of B, C, and D, nozzles were located in the reactor diameters of 1/4, 1/2, and 1/4, respectively. The nozzle shape of A and B was a straight tube, making the gas and sample inject into the reactor aslant, while the nozzle shape for C, D, and E was a bending pipe, enabling the gas and sample to inject into the reactor vertically to the gas distributor. Figure 7 shows the effect of injector structures and locations on the distribution of bed materials after trace gas injection by the pulse method. For the pulse gas injected vertically to the distributor, namely, the structures of C, D, and E, a big bubble generated quickly in the reactor. The nearer the distance between the feeding pipe and reactor wall, the larger the bubbles formed and, thus, the stronger effect on the existing flow state. As a result of the countercurrent flow and the formation of a bubble, accumulation of a solid particle near the reactor wall will be inevitable, causing the non-uniform mixing between the sample and bed materials. For the structures of A and B, the trace sample was injected aslant to the gas distributor with a

wi(CO) =

S0 → ti(CO) S0 → t f(CO)

t

wf(CO) =

ti t0 ∫0 i (IMS −CO − IMS−CO) dt

wf(C−CO) wf(C−CO) =

t

ti t0 ∫0 f (IMS −CO − IMS−CO) dt

(2)

28LC̅CO(tf − t0) 22.4

(3)

where wi and wf mean the carbon mass in generated CO and CO2 from the reaction starting point to the arbitrary time ti or the ending time tf, respectively, S0→ti and S0→tf represent the integrating areas of the spectrum curves for CO or CO2 (against the baseline) in MS between the reaction starting point t0 ti and ti or tf, respectively, IMS and IMS mean the intensity in the MS spectrum for CO or CO2 at the reaction starting point and ti, respectively, and L and C̅ refer to the gas flow rate at the reactor outlet and the average molar concentration of CO or D

DOI: 10.1021/acs.energyfuels.7b03003 Energy Fuels XXXX, XXX, XXX−XXX

Review

Energy & Fuels

Figure 7. Solid concentration for different injector structures.36

Figure 8. Schematic diagram and picture of adopted MFBRA.37

gasification reaction of char with CO2 in MFBRA, measured the kinetic parameters, and compared them to the results from commercial TGA. Under the minimized inhibition of heat and mass transfer, the curve shapes and also the reaction rate (R) measured by the two analyzers were much different, as shown in Figure 10. For MFBRA, the shape of R can be divided into two stages: an initial rapid increasing stage (X below 0.15) and then a gradual decreasing stage (X above 0.15), while for TGA, the char gasification rate changed slowly throughout the entire period of the reaction and its maximal rate appeared at a conversion of about 0.45. By adoption of the shrinking core model and fitting ln k and 1/T linearly, the value of activation energy (Ea) and preexponential factor tested by MFBRA and TGA at different temperature stages can be calculated, as shown in Figure 11. In comparison to TGA, the lower temperature stage controlled by the chemical reaction tested by MFBRA was wider (MFBRA, 760−850 °C; TGA, 760−820 °C), while the values of Ea were much similar, which verified the applicability of MFBRA for measuring the isothermal reactions. For the high-temperature region (MFBRA, 850−1000 °C; TGA, 820−1000 °C) with somehow evident limitations on the reaction rate from the heat and mass transfer, the estimated apparent Ea from MFBRA (175.06 kJ/mol) was obviously higher than that for TGA

Figure 9. Analysis approach adopted for the char−CO2 reaction in MFBRA.9

CO2 in the effluent gas (measured by GC) throughout all of the reaction period, respectively. Moreover, in eq 3, it is worthwhile to point out that, as a result of the small value of the CO concentration, L can be seen as a constant. 3.3. Characterization for the Isothermal Reaction. Isothermal methods have been commonly used in the research of the gas−solid reaction. Zeng et al.9 examined the isothermal E

DOI: 10.1021/acs.energyfuels.7b03003 Energy Fuels XXXX, XXX, XXX−XXX

Review

Energy & Fuels

are attributed to the lower limitation from heat and transfer including gas diffusion in MFBRA than in TGA. 3.5. Comparison of Reaction Analyzers. Because of the difference in the reactor structure and measuring principle, MFBRA, TGA, and other analyzers have significant differences in measuring performance and applicability. To be more clear, Table 1 shows the performance comparison of these analyzers and their respective applicable scope.

4. TYPICAL APPLICATIONS The newly developed MFB reaction analyzer can make up the limitations of existing TGA, which is a good complement and perfect for the gas−solid analysis method. At present, this analyzer has been standardized and used in many fields, such as the chemical industry, metallurgy, materials, thermal conversion, and other fields. Some of its typical applications were listed in Table 2. 4.1. Isothermal Differential Characteristics. As a result of the negligible internal diffusion and relative simplicity, the combustion reaction between graphite powder and air was chosen by Yu et al. on MFBRA for testing the isothermal differential reaction characteristics.43 With minimized inhibitions from physical diffusions, the reaction was further found to be subject to the nucleation and growth model expressed by G(a) = −ln(1 − a). The isothermal differential approach was used to calculate to the kinetic parameters of graphite combustion, obtaining Ea of 165 kJ/mol and a pre-exponential factor of 106 s−1. The nucleation and growth model was suitable to fit the experimental results. In comparison to the non-isothermal approach for TGA that involves complicated mathematical calculations, the isothermal differential approach for MFBRA allowed for the separation of the temperature effect (i.e., the reaction rate constant) and kinetic function model, thus providing a simple and reliable determination of the gas−solid reaction kinetics. On this basis, Liu et al.44 further examined the isothermal combustion characteristics of activated carbon with high specific surface area in MFBRA during the temperature range of 700−1000 °C. Under the minimized physical diffusion, Ea obtained using a random pore model to calculate the isothermal experimental data was about 178 kJ/mol with a structure parameter of about 0.17 m−3. This value was about twice of that from the thermal analytical iso-conversional method (95 kJ/mol), higher than the results of 137 kJ/mol from the non-isothermal TGA test, and closer to the intrinsic value. 4.2. Lower Diffusion Inhibition. Chen et al.45 examined the reduction characteristics of iron ore in a CO atmosphere by MFBRA and TGA. During the experimental temperature range of 700−850 °C, the reaction rate in TGA was very low, even in the high temperature, making it very difficult to react completely. However, in MFBRA, because the effects of external and internal diffusion were significantly inhibited, the reaction rate was much higher and increased sharply with the variation of the temperature from 700 to 850 °C, making it very easy to fully react. Under these conditions, the obtained kinetics was very close to the intrinsic kinetics. Lin et al. further examined the reduction reaction of iron ores in a continuous stream of 100% CO in MFBRA.46 With the minimized effects of physical diffusion, the kinetic data were obtained, with Ea of 26.28 kJ/mol and a pre-exponential factor of 0.03946 s−1. The experimental results complied with the gas internal diffusion model were shown with F(a) = 1 − 3(1 − a)2/3 + 2(1 − a). Moreover, experiments further show that the

Figure 10. Reaction rate varying with conversion for char−CO2 gasification in MFBRA and TGA.9

Figure 11. Linear fitting of ln R and 1/T for the char gasification reaction tested by MFBRA and TGA.9

(138.60 kJ/mol). This further indicated that the limitation effect from physical diffusion was weaker in MFBRA than in TGA for the char gasification reaction. 3.4. Non-isothermal Characterization of MFBRA. To examine the applicability of MFBRA for the non-isothermal gas−solid reaction, Wang et al.38 conducted a gasification experiment between char with CO2 at different heating rates. As shown in Figure 12, the results show that the heating rate obviously affected the coal char gasification reaction, especially the representative reaction temperature, such as the initiating reaction temperature (Ti), the temperature with a maximal reaction rate (Tm), and the finishing reaction temperature (Tf). Comparing the data tested by the two analyzers under the same heating rate clarified that the temperatures of Ti, Tm, and Tf were all relatively lower by MFBRA than those by TGA. The difference in characteristic reaction temperatures between TGA and MFBRA become larger with raising the heating rate. Moreover, Ea calculated from MFBRA was always larger than that from TGA, regardless of the single heating rate and the combination heating rate methods. Perhaps all of these results F

DOI: 10.1021/acs.energyfuels.7b03003 Energy Fuels XXXX, XXX, XXX−XXX

Review

Energy & Fuels

Figure 12. Variation of the reaction rate with the reaction temperature and heating rate for non-isothermal gasification of coal char in (a) TGA and (b) MFBRA.38

Table 1. Comparison of the Testing Property of MFBRA and TGA item

TGA4

MFBRA

drop tube39,40

wire mesh reactor41,42

sample mass feeding method heating rate testing principle external diffusion internal diffusion complex atmosphere reflection in actual apparatus applicability for rapid reaction