(MFB-TGA) for chemical looping combustion - ACS Publications

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

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CO2 Gasification of a Lignite Char in Microfluidized Bed Thermogravimetric Analysis for Chemical Looping Combustion and Chemical Looping with Oxygen Uncoupling Ye Li,† Hui Wang,† Weicheng Li,‡,§ Zhenshan Li,*,† and Ningsheng Cai†

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Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, People’s Republic of China ‡ Clean Combustion and Flue Gas Purification Key Laboratory of Sichuan Province, Chengdu, Sichuan 611731, People’s Republic of China § Dongfang Boiler Group Company, Limited, Zigong, Sichuan 643001, People’s Republic of China ABSTRACT: Chemical looping combustion (CLC) and chemical looping with oxygen uncoupling (CLOU) are novel combustion technologies with low energy penalty CO2 capture. In a CLC/CLOU unit, the gasification of char is the ratecontrolling step as a result of the slow gasification kinetics of char under a CO2 and H2O atmosphere. Reliable measurements of char gasification kinetics are absolutely necessary for CLC/CLOU reactor design, operation, and optimization and are the focus of this paper. To solve the problems of measuring gasification kinetics using thermogravimetric analysis (TGA) and fluidized bed (FB) reactors with gas measuring, a microfluidized bed thermogravimetric analysis (MFB−TGA) method was developed and reported in this paper to investigate the CO2 gasification of a lignite char. The feasibility of the novel MFB−TGA approach was tested, and its stability and accuracy were validated. MFB−TGA was then used to study the CO2 gasification of a lignite char. First, the CO2 gasification kinetics of the lignite char in MFB−TGA were compared to those obtained using a TGA-Q500 instrument. The char gasification kinetics in MFB−TGA were found to be much faster than those in the TGA-Q500. Second, the effect of the particle size on the gasification kinetics was investigated, and it was found that 900−1250 μm was the threshold size range, below which there was no obvious particle size effect on the kinetics. However, for particle sizes larger than 900− 1250 μm, the effect of the particle size on the kinetics was significant. Then, using 900−1250 μm particles, the influence of the temperature, CO2 concentration, and O2 concentration was studied, and the experimental results obtained from MFB−TGA showed that the lignite char could be fully converted within 300 s at 900 °C with 75 vol % CO2. The time for the full conversion of the char could be reduced to 190 s when the gasification temperature was increased to 950 °C or when 3 vol % O2 was present in the atmosphere. Finally, a predictive kinetic model was developed from the experimental results, which could predict both the conversion of carbon versus time and the reaction rate versus carbon conversion well.

1. INTRODUCTION Chemical looping combustion (CLC) has arisen as a promising combustion technology for power plants and industrial applications.1,2 In CLC, CO2 capture is inherent to the process and requires no extra energy input, which makes this technology competitive. Instead of air, it uses an oxygen carrier, usually in the form of a metal oxide, to provide oxygen for fuel conversion. Normally, the metal oxide reacts with CO, H2, and an additional reducing gas that comes from the pyrolysis and gasification of the fuel. For some types of oxides, gaseous oxygen is released and reacts with the fuel directly.3 This process is called chemical looping with oxygen uncoupling (CLOU). CLC of solid fuels has been a major topic in recent years, and its scale-up toward application in megawatt plants will be the future focus.4,5 Until now, nearly 3000 h of operational experience has been reported in approximately 20 pilot plants around the world. The thermal power of these pilot plants ranges from several kilowatts to several megawatts. However, the autothermal operation of CLC units has not yet been successful. Therefore, an improved CLC unit design is still necessary. In a CLC unit, the gasification of char is the rate-controlling step as a result of the © XXXX American Chemical Society

slow char gasification kinetics under the CO2 and H2O atmosphere.6−8 Therefore, accurate knowledge of the kinetics of fuel char gasification under CLC conditions is necessary to successfully design a large-scale unit. In addition to the kinetic data, the particle size is an important factor to be considered when designing a CLC unit. Fine particles can easily escape from the system, leading to considerable char loss.9,10 However, coarse particles may react more slowly and lead to difficulty during separation from the oxygen carrier. Thus, a proper choice of fuel particle size is important to optimize the unit design. In the field of CLC, thermogravimetric analysis (TGA) has been the most popular tool for the study of the kinetics of oxygen carriers. On the basis of the data from a review,2 approximately 70% of related kinetic data were established by TGA and the rest was derived from fluidized bed reactor, fixed bed reactor, and temperature-programmed reduction/oxidation analyses. Recent publications with contributions to the Received: August 22, 2018 Revised: December 16, 2018 Published: December 18, 2018 A

DOI: 10.1021/acs.energyfuels.8b02909 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels Table 1. Properties of Lignite proximate analysis (wt %)

ultimate analysis (wt %)

Mar

Var

FCar

Aar

Cdaf

Hdaf

Odaf

Ndaf

Sdaf

35.29

34.60

26.50

3.61

74.16

5.48

19.79

0.44

0.13

field of reaction kinetics of reduction/oxidation of oxygen carriers reinforce this trend. Different kinds of oxygen carriers were investigated, and the most popular are Ni-based,11−14 Cubased,15−17 Fe-based,18,19 and Mn-based20 oxides. In TGA, the real-time weight signal of the sample is recorded, and the kinetic information is derived from this signal. However, for a particle sample located on the crucible of the TGA apparatus in a packed state, poor mass or heat transfer in TGA could significantly affect the kinetic data.21,22 This means that a great deal of care must be taken to ensure that all of the particles are distributed separately. Furthermore, this means that a miniscule amount of sample must be used, which may cause poor reproducibility as a result of sample heterogeneity. The fluidized bed reactor is also used for the kinetic study of oxygen carriers in CLC.15,16 Furthermore, it experiences a great amount of use in the testing of oxygen carriers using gaseous fuels and solid fuels, for example, the in situ gasification of char using an oxygen carrier as the bed material.23,24 This is because the bubbling fluidized bed is still a mainstream choice for the fuel reactor in CLC. Therefore, it is popular to use a fluidized bed reactor to simulate the fuel reactor and conduct the related study. Char gasification kinetics have been researched extensively25 using various types of experimental equipment. According to a summary by Stoesser et al.,26 approximately 60% of experiments have been carried out in TGA systems,27−29 followed by fixed bed reactors,30 drop-tube furnaces,31,32 and fluidized bed reactors.6,7,33 Various influencing parameters were investigated with regard to the char gasification process, such as the coal rank, pressure, gas temperature, gas composition, particle size, etc. These parameters were varied throughout a wide range as a result of the demands of different applications. For example, high temperature and high pressure were favored for the entrained flow gasification.34,35 For fluidized bed gasification, relatively lower temperatures are preferred.36,37 The influence of the particle size is favored because it is important to determine the diffusive restrictions for industrial application. Additionally, the conclusions reached may differ among cases. For example, Kovacik et al.38 found that diffusion of CO2 within the particle affects the gasification rate for char particles larger than 105 mm. Meanwhile, other researchers did not find diffusion restrictions even with larger particles.39 Therefore, the particle size should always be noted when the char gasification kinetics are studied for a specific purpose. As described above, the fluidized bed reactor owns a natural advantage in the field of CLC because it can simulate the status of the fuel reactor better than other equipment. In addition, in a fluidized bed reactor, the particles are well-mixed in a fluidized bed, which can provide isothermal conditions and enhanced mass and heat transfers. The above advantages make the kinetics derived from the fluidized bed reactor reliable and suitable for CLC reactor design. For the current kinetic study in a fluidized bed reactor, the kinetic analysis is based on the concentration signals of the flue gas species. In fact, the concentrations of the gas species could be easily affected by imperfect plug flow inside the reactor, and the axial mixing of gases could also produce significant errors

in the gas species concentration signals. In other words, great effort must be taken to optimize the fluidized bed system and make the gas concentration signals reliable. The University of Cambridge, Imperial College London, and many other research institutions have performed a great deal of work to fix this problem.40,22,41 A well-designed gas sampling system and a fast gas analyzer were used, and a rapid system response was achieved. Still, the data processing remains complicated; the signal-to-noise ratio becomes too low for a slow reaction; or the effect of mixing in the bed and sampling system cannot be adequately solved for a fast reaction. Thus, it is interesting to combine a fluidized bed reactor and TGA for a kinetic study to best use the merits of both methods. It can be expected that a real-time weight measurement can avoid the problems of gas sampling and measurement in the traditional fluidized bed reactor. This novel idea has already been implemented by Samih and Chaouki42 for coal gasification. However, their experiment used ∼5 g of coal, which is too great a quantity for a kinetic study, and the mass transfer around the particle should be considered, especially for fast gas−solid reactions. To improve this method, the accuracy must be comparable to that of existing TGA or fluidized bed reactor apparatuses and must meet the requirements for a kinetic study; i.e., a relatively small amount of the solid sample should be used in the experiment, so that mass transfer around the particle will not be the limiting factor. In kinetic studies of fluidized bed reactors, the mass transfer of gas species from the bubble phase to the emulsion should be considered, especially for fast gas−solid reactions. If an excessive solid sample is used, the reactive gas species in the emulsion phase will be rapidly consumed by the particles in the emulsion phase. In this case, the mass transfer of the reactive gas species from the bubble phase to the emulsion phase is the limiting factor and the measured experimental data are not realistic kinetic data. Only when the mass transfer of the reactive gas species from the bubble to the emulsion is sufficient can the measured data reflect the kinetic behaviors. Therefore, in this paper, a microfluidized bed thermogravimetric analysis (MFB−TGA) method was reported and only milligram levels of the solid sample were added to the fluidized bed for kinetic experiments. The use of a micro amount of the solid sample assured that the gas−solid reactions were controlled by kinetics and that the mass transfer from the bubble phase to the emulsion phase was sufficient for particle reaction. First, the validity of this new method was verified using a designed test. Next, lignite char gasification experiments under typical CLC conditions were carried out, and the kinetic data were obtained. The results from MFB−TGA and a commercial TGA apparatus were compared, and a kinetic model was proposed on the basis of the results of MFB−TGA.

2. EXPERIMENTAL SECTION 2.1. Fuel. The fuel used in this experiment was a lignite, whose properties are listed in Table 1. 2.2. MFB−TGA. The MFB−TGA apparatus consisted of a quartz reactor with an internal diameter of 20 mm, a distributor thickness of 5 mm, an electric furnace with a maximum temperature of 1473 K, B


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data from the weight transducer at the start of the reaction, m(t) is the real-time data, and mc is the total mass of char. The conversion rate r (s−1) was defined in eq 2.

and a sensor system (see Figure 1). The sensor system consisted of a weighing transducer with a precision of 1 mg, a K-type thermocouple,

X(t ) =

r(t ) =

m0 − m(t ) mc

(1)

dX(t ) dt

(2)

3. RESULTS 3.1. Test of the MFB−TGA System. It is important to provide a more detailed illustration of the principle of accurately weighing the fluidization bed. As is well-known, there will be a fluctuant pressure difference across the bed when it is in the fluidization state. This pressure difference is due to the interactions between the fluidizing gas and bed material. The fluidizing gas “lifts” the bed material, and the bed material exerts resistance against the fluidizing gas, which results in the pressure difference. However, all of the interactions occur inside the reactor, and it is concluded that the total mass of the reactor system will not change according to Newton’s laws if all of the particles remain in the fluidizing bed. To demonstrate the above analysis, some tests were designed. The detailed results are displayed in the following section. First, the stability of the weight measurement under the fluidization conditions was tested; the results are shown in Figure 2. This test was conducted at room temperature using

Figure 1. Schematic diagram of MFB−TGA. and a pressure difference transducer. Gas was supplied through a line connected at the lower part of the system. The flow rates of the reactive gas and the inert gas were controlled with newly calibrated flow meters, and magnetic valves were used to switch the gas during the experiment. In the MFB−TGA system, the reactor rests entirely on the weighing transducer and does not contact any part of the furnace, to stabilize the mass signals. In a typical experiment, the reactor was filled with ∼10 g of silica sand with a particle diameter of 200−300 μm. The system was then heated to the desired temperature using pure argon as the fluidizing agent. When the temperature became stable, coal was then added to the reactor from the top of the fluidized bed, which produced rapid heating with drying and pyrolysis. The mass of the coal was usually 200 mg, with ∼50 mg of char for the gasification study. This amount of fuel was used to ensure that the variation in the CO2 concentration could be controlled at a low level and that problems caused by insufficient mass transfer between the bubble and emulsion phases could be minimized. Previously, 50 mg of sample was verified to be small enough for a gasification kinetic study.6 When the pyrolysis was complete, the weight measured by the weighing transducer became stable and the gasification was initiated after gas switching. During the gasification period, the typical mole fraction of CO2 was 75 vol % with nitrogen as balance to simulate CLC conditions. For the CLOU conditions, 3 vol % O2 was added to replace the same amount of CO2. The total volumetric flow rate of the reaction gas was set to affect a superficial velocity of 5−6Umf.43 Each experiment was repeated 3 times to ensure reproducibility. 2.3. TGA Experiment. The thermogravimetric analyzer Q500 used in this experiment was a commercial analyzer from TA Instruments. In the TGA experiment, the mass of coal was 10 mg and the gas flow rate was 100 mL min−1 (at 273.15 K and 1 bar). The temperature and gas composition were the same as in the MFB−TGA experiments. In a typical TGA experiment, the sample was heated under a N2 atmosphere at a heating rate of 30 K/min to the desired temperature. When the mass became stable, N2 was replaced by the reaction gas and isothermal char gasification began. 2.4. Data Evaluation. A real-time weight signal was recorded by the weight transducer during the experiment. Therefore, the conversion of char could be calculated from eq 1. In eq 1, m0 is the

Figure 2. Mass change with the variation of gas velocity.

N2 as the fluidizing agent and silica sand as the bed material. No reaction occurred during the test. As seen from Figure 2, the mass measured by the weight transducer remained fairly stable as the gas velocity was increased from 0 to 6Umf. The mass change remained within ∼2 mg (0 ± 1 mg), as shown in Figure 2; this mass change could be the result of some perturbation as a result of the fluidization. However, this result C


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Energy & Fuels clearly indicated that fluidization does not destabilize the weight signal within this gas velocity range. When the gas velocity becomes larger (i.e., 10Umf), the mass signal would be quite unstable, as shown in Figure 2. This can be explained by a more vigorous fluidization behavior in the bed. According to the fluidization theory, the particles are easily thrown into the free space of the reactor and leave the bed when the gas velocity is high. This causes the freefall of some particles, and these particles cannot be measured by the weight transducer in a period of time. Therefore, the weight signal becomes quite unstable. On the basis of the above results, it was concluded that the apparent mass change as a result of the gas−solid reaction could be measured accurately if the gas velocity was controlled at a proper value. This is the basis of MFB−TGA. In this paper, the gas velocity was controlled below 6Umf, which is safe for the conducted weight measurement. Second, the response time of MFB−TGA was verified by adding a known mass to the reactor; the results are displayed in Figure 3. Data points were recorded every 0.2 s. It can be

was calculated to estimate the bubble to particulate phase resistance of the fluidized bed.6 When Xf becomes larger than 3, the mass transfer between the bubble and particulate phases are fast enough that the difference in the gas concentration between the two phases becomes negligible. For group B particles, Xf can be decided by eq 3. Xf =

6.34Hmf Umf db,m(gdb,m)1/2

(3)

In eq 3, Hmf indicates the height of the bed at minimum fluidization, g is the gravitational acceleration, and db,m is the mean diameter of a bubble, determined from the correlation by Darton.44 db =

0.54(U − Umf )0.4 (h + 4 A 0 )0.8 g 0.2

(4)

In eq 4, h is the height of the bed above the distributor; here, the mean size of a bubble was assumed to be approximately 50% of the size at height H, which is the expanded height of the fluidized bed. A0 is the area of the distributor divided by the number of orifices on the distributor. A0 is thus nearly zero in this case. According to the fluidization theory, H can be decided by eq 5.45 H − Hmf U − Umf =Y H Ub

(5)

In eq 5, Ub is the velocity of the bubble and Y is a correction factor that is related to Archimedes number. Here, Ub can be decided by eq 6, and the value of Y is approximately 0.8.45 Ub = 0.95 gdb/2 + U − Umf

Figure 3. Mass change after a known mass increase.

(6)

On the basis of the above correlations, Xf was 3.68 and 3.13 at 1073 and 1173 K, respectively, for the experiments here in MFB−TGA. The high cross-flow factor in all experiments herein can suggest complete mixing throughout the entire bed. 3.2. Typical Data for the Char Gasification Experiment. For MFB−TGA developed in this study, real-time signals for the weight data and pressure difference data were measured. The former contained the conversion/kinetic information, and the latter indicated whether the fluidization conditions of the system were adequate. Figure 4 exhibits typical real-time weight and pressure difference signals. Figure 4a shows that the weight transducer could successfully record the mass change of the char sample during

seen from Figure 3 that there are approximately six points in the unstable period; MFB−TGA restabilized within 1.2 s when a mass of 450 mg was added. For a smaller mass of 50 mg, the response time was faster, with only 0.6 s being required to restore stability, as shown in Figure 3. In a typical gasification experiment, the total mass change was around 50 mg and the gasification process usually required several hundred seconds. Thus, the response time of the weight measurement was sufficiently rapid for measurement of the char gasification kinetics. The cross-flow factor Xf, defined as the number of times the bubble gas is replaced when the bubble passes through the bed,

Figure 4. (a) Mass data of a typical experiment (800 °C and 75% CO2) and (b) pressure difference data of a typical experiment. D


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Figure 5. Repeated test results (900 °C and 75% CO2) in (a) TGA-Q500 and (b) MFB−TGA.

Figure 6. Comparison of the conversion in TGA-Q500 and MFB−TGA over time.

during the entire experimental process. When the fluidization is good, the pressure fluctuates around a stable value. The amplitude of the fluctuation is around 30 Pa in these cases. This is the nature of the fluidized bed, and the fluctuation phenomenon has been extensively researched.46 3.3. Comparison of the Results of TGA-Q500 and MFB−TGA. TGA remains the most popular method for a kinetic study; therefore, it is necessary to compare the results obtained from TGA to those of MFB−TGA. This comparison will clearly illustrate how different systems influence the gasification kinetics. Before comparison, it is necessary to perform the repeated tests to obtain reliable data. The sample amount used within TGA experiments must be carefully controlled because it may greatly influence the kinetic results. Figure 5a shows the reproducibility results of TGA-Q500, and the influence of the sample amount was investigated. Lignite coal was used in these experiments, which means that the mass

the gasification process. The reaction started after the gas was switched. An obvious instantaneous instability was observed, which may have been due to the gas switching. However, this instantaneous instability was very short, lasting only a few seconds before the weight signal became stable again before the reaction. It should be noted that, in this MFB−TGA experimental setup, the original mass change curve was not as smooth as the TGA curve. Actually, the local fluctuation of mass signals was observable. Although the mass change curve was not perfect, the trend of the curve was still valid, as shown in Figure 4a, and this experimental curve was believed to be sufficient for analysis of the gasification kinetics. The start point and end point of the gasification reaction can be clearly identified from the curve, and the kinetic data could be derived from this curve. Figure 4b shows the pressure drop in the fluidized bed during the experiment, which confirmed that the reactor was operating under sufficient fluidization conditions E


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Figure 7. Comparison of conversion of a coal sample in MFB−TGA (blue data), a coal sample in TGA-Q500 (red data), and a char sample in TGA-Q500 (black data).

Figure 8. Conversion of fuel char with different particle sizes under (a) CLC and (b) CLOU conditions.

is above 1000 K/s.47 Therefore, it is necessary to clarify how different pyrolysis rates influence the gasification kinetics of char, as shown in Figure 7. In Figure 7, two of the three samples are lignite coal, and the remaining sample is a char sample that is prepared in the fluidized bed at 900 °C. This means that this char sample underwent a rapid pyrolysis process. The char sample was then used for TGA experiments. It can be seen that the conversion rate was markedly increased when using the prepared char sample. However, a significant difference remains between the TGA-Q500 results and those of MFB−TGA, as shown in Figure 7, even though the pyrolysis conditions were identical for both samples. Through the above analysis, it has been demonstrated that not only the pyrolysis step but also the fluidization conditions can affect the char gasification kinetics as a result of the complicated couplings of mass transfer, heat transfer, reaction, and particle motion. In conclusion, it is a better choice to obtain the kinetics under conditions that are as similar as possible to the actual fluidization process, such as in a CLC reactor. The char gasification kinetics will be used for a fluidized bed reactor design of CLC, and therefore, it is necessary and reasonable to test char gasification using MFB−TGA. 3.4. Effect of the Particle Size on Char CO2 Gasification in MFB−TGA. Understanding the effect of the particle size on the gasification kinetics is critical for the design of CLC reactors, and determining the optimal fuel particle size is necessary in actual CLC units. Small particles can easily be separated from the oxygen carrier; however, the feeding of small particles is difficult, and fine particles can also more easily escape from the cyclone. However, coarse particles may react

of char is less than 3 mg for 10 mg of lignite sample. It is believed that this microscale amount is small enough for a kinetic study. When a smaller amount of coal (∼3 mg of lignite) was used, as shown in Figure 5a, the repeatability worsened. This is because of the heterogeneity of lignite particles, which becomes a significant influencing factor when the sample amount is too small, such as 3 mg. Therefore, the results for 10 mg of lignite are used for the experiment in TGA-Q500. Figure 5b shows the reproducibility results of MFB−TGA. It can be seen that the reproducibility is sufficient because a relatively larger amount (200 mg) of lignite sample was used (∼50 mg lignite char). Figure 6 shows that the conversion rate in MFB−TGA was obviously much faster than that in TGA under the same conditions. It can be seen from Figure 6 that the time to reach a conversion of 0.8 in TGA-Q500 was almost double that required in MFB−TGA, which is a significant difference. Multiple factors were responsible for this difference, including different mass and heat transfer parameters, the rapid pyrolysis in MFB−TGA compared to the slow pyrolysis in TGA, etc. The reasons for these differences are discussed briefly in the following section. On the basis of the results reported here, it can be concluded that the kinetic data derived from MFB− TGA are more realistic and reliable when the kinetic data are used in the design of a fluidized bed reactor, as is the case for CLC. Considering the different heating rates of TGA and fluidized bed, the different pyrolysis rates may result in the differences shown in Figure 6. In TGA experiments, the heating rate is 30 K/min, while the heating rate in the microfluidized bed reactor F


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Figure 9. Conversion of char at (a) different temperatures and (b) gas concentrations.

words, the fuel reactor is designed to convert the fuel and char is an important part of the fuel, which illustrates the importance of the burnout time. In this experiment, the lignite char was fully converted within 300 s at 900 °C and 75 vol % CO2. Additionally, the increased temperature and the presence of more active reaction species, such as O2, can notably improve the kinetics, which means that the burnout time of this fuel char could be reduced to below 190 s. Considering the composition of this lignite, the high volatility matter content was easily converted by the oxygen carrier and the remaining fuel char could also be converted within an acceptable time with little ash remaining. Thus, it is believed that this type of lignite is a good choice for CLC. 3.6. Gasification Kinetics of Lignite Char under CLOU Conditions. CLOU is a method to combust solid fuels that avoids the slow char gasification step as a result of the release of gaseous oxygen from the oxygen carrier. To design a successful CLC unit, knowledge of the gasification kinetics of char under CLOU conditions is necessary; this was investigated by introducing 3 vol % O2 into MFB−TGA, and the results are shown in Figure 9b. The presence of 3 vol % O2 accelerated the gasification and reduced the burnout time to less than 190 s, even at 900 °C.

too slowly because they do not provide as much surface area as small particles. Thus, it is important to investigate the appropriate particle size for a CLC unit design. In this study, lignite particles were sieved to five sizes: 300−450, 600−800, 900−1250, 2000−2500, and 4000−5000 μm, and the effect of the particle size on the CO2 gasification kinetics was experimentally investigated; the results are shown in Figure 8. Figure 8 shows that the conversion rate increased as the particle size decreased from 5000 to 900 μm. This was mainly due to the increasing surface area and better mass transfer. In other words, mass transfer across the boundary layer around the particles still played an important role for particles larger than 900−1250 μm. When the particles were smaller than 900 μm, the impact of size on the kinetics was almost negligible; i.e., the intrinsic kinetics dominated. Therefore, 900−1250 μm was determined to be the optimal diameter for this kind of lignite coal, because particles of this size provide a similar conversion rate while exhibiting enhanced resistance to entrainment. In the kinetic investigation in the following section, the particle size was fixed as 900−1250 μm, while the temperature or CO2 concentration was modulated. From the results in Figure 8, it can be assumed that the gasification process is almost entirely kinetically controlled for a particle size of 900−1250 μm. 3.5. Impact of the Temperature and CO2 Concentration. On the basis of the above results, the particle size of 900−1250 μm was chosen and the following kinetic study is based on this particle size. The temperature in the study ranged from 800 to 950 °C, which is a reasonable temperature range for the fuel reactor in CLC. The concentration of CO2 was varied from 18 to 75 vol % to investigate the influence of the gas concentration on the gasification kinetics. Figure 9 shows the char conversion under different temperature and CO2 conditions. Figure 9a shows that the temperature greatly influenced the conversion rate from 800 to 950 °C. The “burnout time” (the gasification time for the full conversion of char) decreased from 1500 s at 800 °C to 300 s at 900 °C. When the temperature was further increased to 950 °C, the “burnout time” was reduced to 190 s. The influence of the gas concentration is displayed in Figure 9b. The CO2 concentration had an obvious effect on the gasification of this lignite char. The higher the CO2 concentration, the faster the char gasification occurred. For the design of the fuel reactor of the CLC unit, the burnout time of the fuel char is important, because it is used to set the residence time of the solid particles inside the fuel reactor. In turn, the particle residence time is closely connected to the size of the fuel reactor. In other

4. DISCUSSION The data in Figure 9 were used to investigate the kinetic expression. The particle size was fixed at 900−1250 μm. On the basis of the results presented above, it can be assumed that the gasification process is kinetically controlled for this particle size, and thus, the Arrhenius expression was used. To consider the influence of the CO2 concentration, a Langmuir− Hinshelwood (L−H) equation was also applied.7 In the L− H expression, the CO concentration, which can be estimated by the consumption of CO2, was ignored. This is reasonable because CO will be consumed by the oxygen carrier in the CLC or CLOU processes, and therefore, there will be a very small amount of CO around the char particle in the fluidized bed.6 In our MFB−TGA experiment, the mass of the char sample is small and the CO concentration is low. CO is produced inside the fluidized bed, and there will be a CO concentration profile along the reactor height; this effect is coupled with the vigorous mass transfer in the fluidized bed, making it more complicated. Thus, eq 7 was used for the kinetic study. In this equation, f(X) is a non-dimensional term that describes the variation of the reaction rate with conversion. In previous research, many expressions have been used for this purpose, including the random pore G


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Energy & Fuels model,48 the homogeneous model, and others.25 In this study, many well-known previously developed models were tested, but none of them fit the experimental data sufficiently. Therefore, an empirical equation was calculated on the basis of the dimensionless analysis of the data, and the results were then compared to the original data to prove their validity. dX = f (X )k 0 exp( −Ea /RT )[yCO /(yCO + β)] 2 2 dt

expression f(X) is obtained, as shown in eq 11, and the remaining part is shown in eq 12.

(7)

Figure 10 shows the variation in conversion as a function of dimensionless time, which is defined in eq 8. The term t0.4

dX 1 b = exp( −aX ) dt t0.4 a

(10)

f (X ) = exp( −aX )

(11)

1 b = k 0 exp( −Ea /RT )[yCO /(yCO + β)] 2 2 t0.4 a

(12)

In combination of eqs 8, 9, and 12, the carbon conversion as a function of time can be obtained, as shown in eq 13 X=

(13)

where A = k0 exp(−Ea/RT)[yCO2/(yCO2 + β)]. In this study, eq 12 was used to calculate the kinetic data; only experimental data with X = 0.4 were used to derive the kinetic parameters. For example, when the temperature was changed from 800 to 850, 900, and 950 °C, four t0.4 values corresponding to each of the four temperatures were obtained. Because the CO2 fraction was not changed and the plot of ln(1/t0.4) versus 1/T displayed in Figure 11a was linear, the kinetic parameter can be obtained from Figure 11a. To obtain the value of β, the temperature was fixed and the fraction of CO2 was changed. In this case, it can be seen from Equation 12 that the plot of t0.4 versus 1/yCO2 displayed in Figure 11b was linear, and therefore, the value of β can be obtained from it. On the basis of the above procedures, the kinetic parameters were calculated, and their values are shown in Table 2. The activation energy was similar to the values in some previous reports.7

Figure 10. Conversion of char versus dimensionless time at different temperatures.

refers to the time at which the conversion reached 0.4. The conversion was used to establish the model because it is more relevant for dimensionless purposes than the rate.49 τ = t /t0.4

1 ln(aAt + 1) a

(8)

Table 2. Estimated Values of the Kinetic Parameters for the CO2 Gasification of Lignite Char

Figure 10 shows highly coincident curves for the different temperatures. The fitting line was calculated using the least squares method. The expression is shown in eq 9, where a and b are constants whose values were 1.9 and 1.2, respectively, for the tested lignite char. 1 X = ln(1 + bτ ) (9) a

kinetic parameter

k0 (s−1)

Ea (kJ/mol)

β

a

b

5.68 × 105

171.38

0.345

1.9

1.2

It should be noted that the kinetic parameters were obtained using only the experimental data for X = 0.4. Therefore, it is necessary to determine the influence of the value of the characteristic conversion. As such, 0.5 and 0.6 are also used as the characteristic conversion values to analyze the model, and the activation energies obtained are 170.69 and 169.96 kJ/mol,

Equation 10 was derived by deriving eq 9 and then combining the result with eq 8. Equation 10 is in the form of a reaction rate, which is comparable to the original kinetic expression shown in eq 7. Therefore, by comparison of eqs 7 and 10, the

Figure 11. Arrhenius plots to obtain the kinetic parameters. H


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Energy & Fuels respectively. Furthermore, the slight difference shown in Figure 12 potentially indicates that the selection of characteristic conversion does not influence the accuracy of the model.

Figure 14. Comparison between the experimental and model results for the char reaction rate.

and that there will be some deviation regarding when the gasification was deemed complete.

Figure 12. Model results using different values of characteristic conversion.

i dX y i dX y R = jjj zzz/jjj zzz k dt { k dt { X = 0

The validity of the obtained kinetic parameters using 0.4 as the characteristic conversion was determined for other experimental data. Figure 13 shows the comparison between the experimental data and the modeled results of char conversion at different temperatures and CO2 concentrations. The model results were all calculated using eq 13. As seen, the proposed model and the obtained kinetic parameters were able to accurately predict the evolution of the degree of char conversion versus time. In addition to the char conversion curves shown in Figure 13, it is also important to validate the model with the conversion rate data. A dimensionless reaction rate R is defined in eq 14, using the initial reaction rate as the base point. The comparison between the model results and the experimental data is displayed in Figure 14. As seen, the model also provided a good estimate for the reaction rate. It is also worth noting that this dimensionless rate does not become zero when full conversion is achieved. That is because an apparent model is used to fit the conversion curve, which means that this model represents a comprehensive result of the entire process. Furthermore, it is the nature of the fluidized bed reactor that not all of the particles can be consumed inside the fluidized bed, especially when the particle size becomes too small. Considering that the model here is established on the basis of the conversion curve, it is believed that this model describes the reaction process of a char particle inside this fluidized bed

(14)

5. CONCLUSION In this work, MFB−TGA was newly developed for optimal design of a CLC reactor. The stability of the system was verified, and the mass change was ±1 mg when the gas velocity increased from 0 to 6Umf. The response time of the measurement was less than 1 s. The gasification of a lignite char was investigated using MFB−TGA and a conventional TGA-Q500 instrument. The results showed that the time to reach a conversion of 0.8 in TGA-Q500 was almost double that required in MFB−TGA. It was found that not only the pyrolysis step but also the fluidization conditions can affect the char gasification kinetics. On the basis of the experimental results, 900−1250 μm was chosen as the optimal particle size for reactor design and the kinetic study was carried out using the chosen particle size. When the temperature exceeds 900 °C, the burnout time of the char would be less than 300 s. Increasing the temperature or adding the more active gas O2 could reduce this time to 190 s. A predictive kinetic model was developed, which was able to accurately predict both the conversion and reaction rates for the experimental results. The activation energy was calculated to be 171.38 kJ/mol. On the basis of the gasification kinetic data and considering the composition of this lignite, it is believed that this kind of lignite is a good choice for CLC.

Figure 13. Comparison between the experimental and model results for char conversion at (a) different temperatures and (b) different CO2 concentrations. I


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AUTHOR INFORMATION

Corresponding Author

*Telephone: +86-10-62792478. E-mail: [email protected]. ORCID

Ye Li: 0000-0002-4568-5547 Hui Wang: 0000-0001-5910-9922 Zhenshan Li: 0000-0003-3407-8079 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Plan “Chinese−European Emission-Reducing Solutions Project (2017YFE0112500)”.

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