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Comparative kinetics of coal and oil shale pyrolysis in a micro fluidized bed reaction analyzer
Journal Pre-proofs Original Article Comparative Kinetics of Coal and Oil Shale Pyrolysis in a Micro Fluidized Bed Reaction Analyzer Yuming Zhang, Mengxuan Zhao, Rongxuan Linghu, Chengxiu Wang, Shu Zhang PII: DOI: Reference:
S2588-9133(19)30033-X https://doi.org/10.1016/j.crcon.2019.10.001 CRCON 56
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Carbon Resources Conversion
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22 June 2019 24 September 2019 3 October 2019
Please cite this article as: Y. Zhang, M. Zhao, R. Linghu, C. Wang, S. Zhang, Comparative Kinetics of Coal and Oil Shale Pyrolysis in a Micro Fluidized Bed Reaction Analyzer, Carbon Resources Conversion (2019), doi: https:// doi.org/10.1016/j.crcon.2019.10.001
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Title： Comparative Kinetics of Coal and Oil Shale Pyrolysis in a Micro Fluidized Bed Reaction Analyzer Author information: #1 Author Yuming Zhang, Associate Professor, State Key Laboratory of Heavy Oil Processing, China University of Petroleum-Beijing E-mail: [email protected]; Tel: +86-15001296129 #2 Author Mengxuan Zhao, Post-graduate, State Key Laboratory of Heavy Oil Processing, China University of Petroleum-Beijing E-mail: [email protected] #3 Author Rongxuan Linghu, Post-graduate, State Key Laboratory of Heavy Oil Processing, China University of Petroleum-Beijing E-mail: [email protected] #4 Author Chengxiu Wang, Associate Professor, State Key Laboratory of Heavy Oil Processing, China University of Petroleum-Beijing E-mail: [email protected] #5 Author Shu Zhang, Professor, College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, Jiangsu, P.R. China
E-mail: [email protected]
Comparative Kinetics of Coal and Oil Shale Pyrolysis in a Micro Fluidized Bed Reaction Analyzer Yuming Zhanga, Mengxuan Zhaoa, Rongxuan Linghua, Chengxiu Wanga*, Shu Zhangb* a State
Key Laboratory of Heavy Oil Processing, College of Mechanical and Transportation Engineering, China University of Petroleum-Beijing, Beijing 102249, P. R. China b College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, Jiangsu, P.R. China
ABSTRACT
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The characteristics and kinetics of coal and oil shale pyrolysis were comparatively studied by using a micro fluidized bed reaction analyzer (MFBRA). The isothermal differential model was first applied to calculate the kinetic parameters of activation energy and frequency factor according to the major gas components during pyrolysis. The results showed that the major gas components released from coal and oil shale under the isothermal condition had different initiating and ending time points, and the difference was more significant under the programmed heating conditions. The shrinking core model allowed better fitting relevance for the coal pyrolysis, while the three-dimension model was more suitable for oil shale pyrolysis, indicating that the gases from the pyrolysis process of coal and oil shale might go through different reaction paths. The activation energy of oil shale pyrolysis was 36.96 kJ·mol-1, larger than the value of pyrolysis of the two coals, which was 21.16 and 32.17 kJ·mol-1, respectively. The above results justified that the oil shale pyrolysis with high ash contents was somehow more difficult to take place in terms of higher activation energy and the MFBRA could be a useful tool to give some insight into the intrinsic kinetics and reaction mechanisms of coal and oil shale pyrolysis. KEYWORDS: Micro fluidized bed reaction analyzer; Coal; Oil shale; Pyrolysis; Kinetics;
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1. INTRODUCTION With the gradual depletion of petroleum sources, more and more attention has been paid to the development of alternative energy sources. Oil shale, a sedimentary rock containing organics, that is, soluble bitumen and insoluble kerogen, has being considered to be an important alternative fuel with abundant reserves [1, 2]. Pyrolysis, as the initial step of the thermal conversion process (i.e., gasification, combustion) of solid fuels, is essentially important to the utilization of the carbonaceous materials [3, 4]. Oil shale could be converted into light pyrolysis shale oil, non-condensable gas and char during lowtemperature pyrolysis (i.e., retorting or distillation). However, oil shale usually had high contents of inorganic substance or ash, which were mainly composed of carbonate, silicate, quartz and a small amount of copper, nickel, cobalt, molybdenum, titanium, vanadium and other compounds [5]. As a result, the pyrolysis behavior of oil shale was complicated and different from that of coal, biomass and other solid fuels with low ash contents due to the interaction between kerogens and inorganic matters [6]. The deep insights into the pyrolysis kinetics of carbonaceous materials played a key role in the development of pyrolysis technologies. In general, the thermogravimetric analyzer (TGA) has been widely used to study the carbonaceous materials pyrolysis process and deduce the corresponding kinetic parameters via monitoring the mass loss at different heating rates. Al-Harahsheh et al. [7] used a TGA to study the oil shale pyrolysis at different heating rates, finding that the pyrolysis reaction of oil shale belonged to the firstorder reaction and the pyrolysis activation energy (Ea) was about 75-95 kJ·mol-1. He et al. [8] calculated the average Ea of coal pyrolysis to be about 304.56 and 303.97 kJ·mol-1 by using the Kissinger-AkahiraSunose (KAS) and Flynn-Wall-Ozawa (FWO) method, respectively. The isothermal reaction characteristics of the different carbonaceous fuels had also been studied with the TGA via rapid heating or changing the reaction atmosphere. However, the structure of the pre-prepared fuel samples in TGA might have changed during the heating process, which resulted in misleading information of the pyrolysis kinetic parameters. Furthermore, it was difficult for the TGA to simulate the actual pyrolysis reaction process under fluidized operating conditions because the sample was put in the fixed crucible and suffered from serious gas diffusion effects. Researchers have attempted to use other apparatus to study the pyrolysis of solid fuels under the isothermal conditions. Han et al. [9] selected three types of lignite to study their fast pyrolysis in a moving furnace via on-line recording the mass change of the sample and also the emission of hydrocarbon components. The results showed that the apparent Ea under rapid pyrolysis conditions was lower than that of slow pyrolysis. Bar et al. [10] used fluidized bed to measure the Ea of coal pyrolysis to be 42.6 kJ·mol-1, while the Ea of four kinds of oil shale pyrolysis was 39.9, 44.9, 51.0, and 46.3 kJ·mol-1, respectively. Bradley et al. [11] applied the down-flow tube for the Ea of lignite and sub-bituminous coal, finding that the Ea was about 10.49-16.76 kJ·mol-1 in the range of 600-1600°C. Shapatina et al. [12] carried out the
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pyrolysis of lignite in a down-flow tube and the corresponding Ea to be about 4.18-15.88 kJ·mol-1. The above kinetic data were obtained via using different types of reactors and operating conditions. Some reactors might suffer from restriction by gas diffusion, which resulted in incomparability of the kinetics data in terms of different reactors. Recently, a micro-fluidized bed reaction analyzer (MFBRA) was used to study the kinetics of coal pyrolysis under isothermal conditions by analyzing gas-releasing characteristics [13-15]. Compared to the TGA, the MFBRA was developed to enhance heat exchange and mass transfer, also having rapid heating rate and minimizing the limitation of gas diffusion via fluidization, which in turn guaranteed the kinetics data close to the intrinsic reaction. The MFBRA had been employed for kinetics analysis of many gas-solid reactions, including biomass pyrolysis, gasification of coal char or petroleum coke [16, 17], etc. The MFBRA has proved to be a new approach to determine the thermal conversion of the different carbonaceous fuels. In this study, the MFBRA was employed to analyze the pyrolysis characteristics and kinetics of coal and oil shale under isothermal and non-isothermal conditions via monitoring gas-releasing behaviors. The kinetic data for the formation of four major gas products (CO2, CO, CH4, and H2) were deduced by the model-fitting approach. The different pyrolysis characteristics of coal and oil shale were investigated based on the releasing behaviors of each gas component and thus obtained the corresponding kinetic parameters. The results were expected to provide an in-depth understanding of the pyrolysis of carbonaceous fuels in the fluidized-bed under the isothermal conditions. 2. EXPERIMENTAL SECTION 2.1. Sample preparation In this experiment, Zhundong (ZD) sub-bituminous coal, Guobiao (GB) anthracite and Huadian (HD) oil shale were used as the samples, with their proximate and ultimate analyses given in Table 1. These three samples were dried to remove moisture in an oven at 105°C for 24 h before the experiment. The ZD coal had higher volatile content and lower fixed carbon content than that of the high-grade GB coal. The HD oil shale had similar volatile contents as ZD coal, while the ash content of oil shale was much higher than that of coals. Table 1. Properties of the tested samples. Proximate analysis (wt.%. db)
Ultimate analysis (wt.%. daf)
Sample
V
A
FC
C
H
O*
N
S
ZD coal
30.02
8.56
61.42
77.95
3.98
16.28
0.74
1.05
GB coal
8.73
13.72
77.55
92.57
2.14
4.12
0.68
0.37
HD oil shale
33.14
49.94
16.92
67.86
7.03
21.79
1.75
1.54
db: dry-base; daf: dry ash free base; V: volatile; A: ash; FC: fixed carbon. *By difference
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2.2. Apparatus and procedure The pyrolysis experiments were conducted in the MFBRA to obtain the releasing behaviors of major gas components as a function of time under the isothermal conditions. The schematic diagram of MFBRA was shown in Figure 1. It mainly consisted of four parts, that is, the micro fluidized bed, the sample feeding system, the pressure and temperature sensor, the gas cleaning and detecting system. The fluidized bed reactor was made of quartz with an inner diameter of 20 mm, to prevent surging and channel flow [18]. The feeding system was controlled by an electromagnetic valve and could instantly inject the feedstocks into MFBRA. The effluent gas components were measured by using an online mass spectrometer. In order to prevent particles escaping, the MFBRA was equipped with a double-layer dispersion plate and the main reaction zone was in-between the two plates. The main reaction zone of the reactor was about 40 mm in height, and 10-50 mg samples were injected for a single test. A computer was used to monitor the following parameters, including the temperatures of the resistance furnace and the exact reaction zone in the reactor, the pressure drop between the reactor inlet and outlet, the flow rate of carrier gas, the actions of sample feeding device and the output data from the mass spectrometer.
Figure 1. A schematic diagram of the MFBRA experimental apparatus. Before the experiment, about 3 g quartz sand with an average particle size of 70-100 mesh was loaded into the lower plate as the solid heat carriers and also the fluidized mediums. The quartz reactor was heated to a pre-set temperature (650-800°C) in a fluidized state, and then 10 mg sample was injected into the quartz reactor. The releasing characteristics of four major pyrolysis gases (H2, CO2, CH4, and CO2) were continuously measured by the mass spectrometry. Each experiment was repeated at least three times with
5
the relative error to be less than 5% and the average value was used for the kinetic analysis to ensure the reliability of the experimental results. 2.3. Isothermal differential kinetics The pyrolysis kinetics of solid fuels were determined via analyzing the reaction rate and the yield of reaction products, according to the generation process of major gas components under the isothermal condition. The kinetic study was carried out to describe the generation process of major gas and the mechanisms of fuel pyrolysis under the isothermal condition. The kinetic parameters from coal and oil shale pyrolysis were calculated based on the change of gas concentration under the isothermal conditions. Based on the above results, the integral data from the gas generation were measured via the online mass spectrometer. The conversion ratio X was shown in Eq. (1). via calculating the gas components from the pyrolysis reaction t
X
t iqvdt t t iqvdt
(1)
0
e
0
where t0, t and te (s) stands for the initiating time, lasting time and the ending time of pyrolysis reaction, respectively; φi representes the concentration of volatile components i (therein, i = CO2, CO, H2, and CH4), vol%; qv represents the volume of all gases releasing from the MFBRA, ml/min. The conversion ratio of solid fuels during pyrolysis with time was analyzed in terms of the production of gas mixture, t
（ X （ t0 te
t0
CO 2
CO CH 4 H 2 )qv dt
CO 2 CO CH 4 H 2 ) qv dt
(2)
where φCO2, φCO, φCH4 and φH2 represent respectively the volume fractions of CO2, CO, CH4 and H2 in the formed gas. According to the isothermal differential reaction kinetics, the kinetic equation of pyrolysis could be expressed as
dX k T f x dt
(3)
where k(T) was the reaction rate constant under the isothermal conditions; f(x) was the model mechanism function. Based on the variation of feedstocks properties and operating conditions, various reaction models had been used for the coal pyrolysis. Wang et al. [19] calculated the kinetic parameters of coal pyrolysis using the shrinking core model (SCM), suggesting that the reaction rate was controlled by the un-reacted surface area. The equation of the SCM was given in Eq. (4) f x 1 x
n
(4)
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where n represents the order of the reaction. The three-dimension diffusion model was found to have better fitting relevance for the oil shale pyrolysis process due to its high ash contents and its organic matters dispersed over the inorganic skeletons [20]. Eq. (5) showed the formula of the three-dimensional diffusion model, with the assumption that the mass diffusion of material went through the layered barrier [21]. f x 1 x
2/3
[1 1 x
2/3
]
(5)
Substitute f(x) in the Eq. (3) with the Eq.(4) and took the logarithm of the formula, we could obtain the Eq (6), where ln k(T) and n corresponds to the intercept and the slope of the line fitted by ln (dX/dt) and ln (1-X), respectively.
ln
dx ln k T n ln 1 x dt
(6)
Thus, the straight line of the rate constant k(T) and the temperature T was obtained. The apparent activation energy Ea and the frequency factor A could be obtained according to the slope and the intercept of the straight line according to Eq. (7)
ln k ln A Ea / RT
(7)
where A represents requency factor, s-1; Ea represents the activation energy of the reaction, kJ·mol-1, R represents the gas constant, 8.314 J·mol-1·K-1, and T represents the reaction temperature, K. 3. RESULTS AND DISCUSSION 3.1 Parametric investigation in MFBRA The pyrolysis process in MFBRA was defined via the volume change of the produced pyrolysis gases. In order to get the intrinsic kinetics, it was essential to minimize the restriction of gas diffusion. Yu [15] proved that the effect of particle size on internal diffusion was negligible when the particle size was less than 120 μm. When the gas flow rate was higher than 300 ml/min, the effect of external diffusion was negligible. Figure 2 showed the influence of fluidized gas flow rates and the particle size on the pyrolysis gas conversion. The reaction time decreased via increasing the gas flow rate and the conversion varied little when the flow rate was above 300 ml/min. This critical fluidized flow rate represented the velocity to minimize the limitation of the external diffusion on the reaction. Moreover, the reaction completion time decreased by reducing the particle size of the samples. In order to obtain the experimental results under the minimized internal and external diffusion conditions, the particle size of the sample was adopted in the range of 74-120 μm and the flow rate of argon gas was 500 ml/min in this study.
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Figure 2. Influence of gas flow rate (a) and particle size (b) on the pyrolysis gas conversion behavior 3.2 Pyrolysis characteristics in MFBRA The pyrolysis of coal and oil shale was conducted in the MFBRA at temperatures of 650°C, 700°C, 750°C and 800°C, respectively. The variation of four major gas components including CO2, CO, CH4 and H2 from the pyrolysis of ZD bituminous coal, GB anthracite and HD oil shale at 650°C and 800°C, were shown in Figure 3, respectively. The releasing sequence of the major gas components was CO2, CO, CH4 and H2 at 650°C. However, the difference of the gas releasing sequence had nearly vanished at about 800°C. All the pyrolysis reaction in the MFBRA finished within 10 s, which was much shorter than the experiment time conducting in the TGA [22, 23]. The fuel samples were instantly injected to the reactor in MFBRA, and rapidly heated under the isothermal conditions in a rate of about 1000-10000°C/s [13]. The intensity of each gas components increased and reached its peak quickly, mainly resulting from the high heating rate and the minimized gas diffusion effects. Coal with different ranks had different pyrolysis characteristics. In comparison with the GB anthracite, the releasing behavior of CO2 and CO was found to be more obvious for the ZD sub-bituminous coal at 650°C and 800°C. In addition, the pyrolysis characteristics of oil shale were different from coal under the isothermal conditions. The varied releasing behaviors of different fuels were related to their volatile contents and structure characteristics. The low-rank ZD sub-bituminous contained a large number of functional groups, while the high-rank GB anthracite with few side chains or functional groups tended to condense and had a weaker gas-releasing intensity [24, 25]. The releasing intensity of HD oil shale was
8
even lower than that of GB anthracite, suggesting that the pyrolysis of oil shale was harder to take place and generated less volatiles than that of coal.
3.0E-6
ZD
a ZD T=650℃ CO2 CO H2 CH4
b T=800℃
GB
c GB T=650℃
d T=800℃
e HD T=650℃
f T=800℃
2.0E-6 1.0E-6
Intensity
0.0 1.0E-6 5.0E-7 0.0 1.0E-6 HD 5.0E-7 0.0 0
5
0 Time/s
5
10
Figure 3. Gas-releasing behaviors of ZD, GB coals and HD oil shale pyrolysis at 650°C and 800°C in the MFBRA. Figure 4 showed that the pyrolysis of ZD coal and HD oil shale under the programmed heating conditions. The sample was placed in the MFBRA at normal atmospheric temperature and then heated to pre-set temperature at heat rating of 20°C/min. The different releasing characteristics of four major gas components were magnified for comparison. The releasing order of gas components was CO2, CO, CH4, and H2 during the ZD coal pyrolysis and the initiating temperatures for these gases were at about 280°C, 450°C, 500°C and 600°C, respectively. The different functional groups and reaction behaviors would lead to the evolution of each gas component [9, 24]. The CO2 was mainly derived from the dissociation of carboxyl functional groups in organic volatiles, which had poor thermal stability and could be easily decomposed at low temperature. The generation of CO at low temperature was mainly from phenols, ether bonds and oxygen-containing heterocyclic in coal due to its wide releasing temperature range. At high temperature, such as 800°C, lots of CO was further released because of the intense pyrolysis of the oxygencontaining heterocyclic. The CH4 was mainly formed by the shedding of aliphatic hydrocarbon side chains. Compared to the other gas components, H2 released at rather higher temperatures by the condensation of free radicals and the polymerization of aromatic rings [26]. Besides, Table 1 shows that the ZD coal and
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HD oil shale had similar volatile contents, while the HD oil shale had higher ash content than that of ZD coal. The variation of composition properties could in return determine the pyrolysis behavior of carbonaceous fuels.
Figure 4. Gas-releasing characteristics of ZD coal and HD oil shale pyrolysis under non-isothermal in the MFBRA. During the pyrolysis of HD oil shale, CO2 began to generate at about 250°C, CH4 releasing at about 380°C, then CO at about 400 °C. The CO2, CO and CH4 together reached to their peak at about 500°C, and this temperature was close to the temperature of maximum weight loss rate during oil shale pyrolysis in TG [27]. Initially, the reaction occurred on the surface of solid particles, resulting in only a small fraction of gas. The kerogen, as the main organic part in oil shale, dispersed over the inorganic minerals and might combine with ash. At higher temperature, the kerogen was cracked into small molecules (i.e., CO2, CO, and CH4), which was in accordance with the description of the three-dimensional diffusion model [28]. When the temperature was higher than 500°C, the intensity of CO decreased rapidly because the oxygencontaining functional groups had been substantially released. The higher intensity of the second CO2 peak at about 650°C might be attributed to the decomposition of the carbonate minerals [29]. The CO releasing peak at around 760°C was possibly caused by the interaction of CO2 and char.
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Figure 5. FTIR analysis of ZD coal and HD oil shale Figure 5 shows the FTIR analysis of HD oil shale and ZD coal, which were different in their functional groups. Both HD oil shale and coal had strong peaks at the range of 3000~2700 cm-1 (aliphatic chains) attributing to the methylene antisymmetric and symmetric stretching vibration. The weak absorption peak at 755 cm-1 could be from the vibration section of the long methylene chain. The absorption peak of oil shale was generally stronger than that of coal. Oil shale possessed more aliphatic chains and easily to be broken into CH4 [30]. Coal had an obvious absorption peak at 1589 cm-1 and there were some small peaks nearby, indicating that the increase of aromatization occurred due to the combining of C=O and C=C bonds. Besides, oil shale had an obvious absorption peak at 1033cm-1, which was connected to the C-O bond of organic matter alcohol and alkyl ether. 3.3 Pyrolysis kinetics analysis Figure 6 showed the relationship between gas conversion X of four major gases and time during ZD coal pyrolysis at different temperature. The reaction rate first increased with the increasing temperature and then reached to a plateau. However, the accelerating degree was different for each individual gas component. The slope of the H2 conversion curve had the most remarkable change via promoting temperature, indicating that the temperature had the utmost effect on the formation of H2. The whole gas products (i.e., the sum of all releasing gases) from pyrolysis were used to analyze the reaction kinetics, as
11
shown in Figure 6(e). The gas mixture released at around 30s due to the mutual compensation effect between all gas products. For example, CO2 and CO forming at about 15s, that is, the former part of the reaction, could compensate the late-releasing gas, such as, H2 forming at around 30 s. After 30 s, the gas mixture was completely released with the conversion ratio reaching 100%.
Figure 6. Gas conversion versus reaction time of ZD coal in MFBRA at different temperatures for the individual gas component. Figure 7 shows the profile by converting the data of Figure 6 into the correlation of ln(dX/dt) with ln(1-X). The reaction process of four individual gas could be divided into three stages. At the first stage, the injected sample was mixed with the high-temperature fluidized medium (i.e., quartz sand) and heated rapidly in the MFBRA. At the second stage, the sample reached pre-set temperature and began to pyrolysis. Initially, the reaction mainly occurred on the surface of the samples [31]. Because the gas diffusion effects on the reaction were negligible when the particle size was smaller than 75 μm, the reaction was mainly controlled by chemical reaction during the intrinsic pyrolysis process. The functional groups were exhausting with the proceeding of pyrolysis, and the gas generation rate was gradually reduced as well. The curves tended to divergence at the ending part of the reaction, as evidenced by the fact that the reaction rate
12
reduced to a minimum. At the final stage, the pyrolysis reaction changed from the surface to its inner side, and the reaction process changed from chemical reaction control to the internal diffusion control. Considering that the chemical reaction stage with the minimized internal diffusion was the main part we desired for the pyrolysis reaction in the MFBRA, the second stage was selected to analyze the intrinsic kinetic parameters.
Figure 7. Correlation of ln(dX/dt) and ln(1 - X) for individual components of ZD coal pyrolysis in the MFBRA. Table 2 shows the calculated reaction order n and ln k(T), with the reaction order of CO, CO2, CH4, H2, and gas mixture being 1.12-1.34, 1.12-1.24, 1.21-1.64, 0.83-1.16, 1.16-1.27 at different temperatures, respectively. Besides, the reaction order n of ZD pyrolysis was about 1.1-1.2 with the fitting correlation coefficient above 0.95. The pyrolysis process of coal was generally conformed to the first-order reaction, which was consistent with the previous research results of Klose et al [32]. The different reaction orders n of the four major gas components might be caused by the variation of formation paths or reaction mechanisms. Among them, the reaction order of H2 changed significantly, indicating that the reaction mechanism of H2 formation was different at varying temperatures.
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Table 2. Reaction order and rate constant of gas components at different temperatures Gas
CO
CO2
H2
CH4
Gas Mixture
T/°C
ln k(T)
n
R2
650
-0.57
1.28
0.99
700
-0.50
1.12
0.98
750 800 650
-0.41 -0.30 -0.87
1.34 1.17 1.24
0.98 0.99 0.98
700
-0.83
1.19
0.99
750
-0.74
1.12
0.99
800
-0.71
1.12
0.99
700
-2.18
1.64
0.98
750
-1.74
1.24
0.91
800
-1.53
1.21
0.96
650
-0.42
0.83
0.95
700
-0.23
1.1
0.97
750
-0.15
1.1
0.99
800
-0.03
1.16
0.98
650
-1.42
1.27
0.95
700
-1.32
1.21
0.96
750
-1.17
1.23
0.99
800
-1.04
1.16
0.99
3.4 Comparison of kinetics data The apparent activation energy (Ea) and frequency factor (A) of ZD coal, GB coal and HD oil shale were listed in Table 3. The Ea of the oil shale pyrolysis was 36.96 kJ·mol-1, higher than the Ea of two coals, i.e., 21.16 and 32.17 kJ·mol-1, respectively. With the increases of coal ranks, the pyrolysis required more energy to generate volatiles [24], thus leading to different Ea for pyrolysis of two coals. The process of oil shale pyrolysis at low temperature was similar to that of bituminous coal, while the Ea of oil shale pyrolysis was higher than high-rank coal at high temperature under the isothermal conditions [33]. The oil shale pyrolysis mainly involved with the decomposition of kerogen, while the reaction of inorganic matters required a large amount of energy. Thus, the pyrolysis Ea of oil shale pyrolysis was higher than that of coal. Table 3. Kinetic parameters of solid fuels pyrolysis in the MFBRA Reactor
Sample
Gas
T/°C
Ea /(kJ·mol-1)
A/s-1
14
ZD
MFBRA
GB
HD
CO2 CO CH4 H2 Gas Mixture CO2 CO CH4 H2 Gas Mixture CO2 CO CH4 H2 Gas Mixture
650-800
9.38 14.52 20.93 56.70 21.16
1.40 3.66 10.16 129.02 3.74
650-800
20.94 24.47 38.52 64.16 32.17
4.93 6.29 16.75 137.84 16.54
650-800
28.09 30.23 15.39 68.60 36.96
0.55 0.96 0.49 200.87 117.80
Table 4 compared the kinetics data from MFBRA with other reactors from the literature, such as, moving furnace and drop tube. The kinetic values of coal and oil shale pyrolysis obtained in MFBRA was close to the kinetics data obtained in the reactors with high heating rate. Han [9] and Tolvanen [34] respectively used a moving furnace (heating rate of 100-300 °C/s) and a drop-tube reactor (heating rate of 1000 °C/s) to conduct pyrolysis experiments under the isothermal condition. The resulting Ea and A was almost the same in these two reactors with Ea to be 28.14-38.23kJ·mol-1 and A of 420 s-1, respectively, which was a slightly higher Ea and the larger A than that of MFBRA. This deviation of kinetic parameters might be caused by the larger gas external diffusion in the drop-tube reactor and moving furnace. Table 4. Comparison of kinetic data of solid fuels pyrolysis in different reactors Reference This study
Reactor MFBRA
500-900
Ea/(kJ·mol-1) 21.16 32.17 36.96 11.77
A/s-1 3.74 16.54 117.80 1.45
Yu [15]
MFBRA
Fuel Sub-bituminous coal Anthracite coal Oil shale Beer lees
T/°C
Guo [37]
MFBRA
Herb residue
600-850
10.48-18.90
1.90-4.30
Han [9]
Moving furnace
Brown coal
300-800
28.14-36.90
0.73-1.53
Bar [10]
Fluidized bed
400-500
Tolvanen [34]
Drop-tube reactor
Oil shale Coal Coal
700-900
39.9-51.0 42.6 37.2-38.23
449.61 178.41 276.2-420
Geng [23]
TGA
Bituminous
200-600
293-473
Guo [24]
TGA
Coal
850-930
348.48-378.62
Al-Ayed [35]
TGA
Oil shale
350-550
98-120
Scaccia [36]
TGA
Coal
30-1000
189
2.26×10191.74×1027 1.03×10151.12×1016 9.51×1051.16×106 1.2×1012
650-800
15
The heating rate in TG was generally at 40°C /min, far less than that of MFBRA and other isothermal reactors [35, 36]. The reported Ea and A from TG were somehow high, with Ea to be 98-473 kJ·mol-1 and A of 9.51×105-1.74×1027 s-1. The kinetic data of the fast reaction like pyrolysis could be greatly affected by the heating process. The lower activation energy and frequency factor indicated that the fuels in MFBRA could receive more energy instantly to complete reactions [37]. Besides, the external and internal diffusion effects in the MFBRA could be minimized via the turbulent gas-solid reactions and tiny particle size. Consequently, the MFBRA would be a reliable reactor to analyze the gas-solid reaction under the isothermal condition, especially for the fuels with complex compositions like oil shale. Due to the mutual effects of minimized internal and external diffusion and rapid heating, the MFBRA could reflect the kinetics more accurately and get insight into the intrinsic reaction mechanism. 4. CONCLUSIONS The pyrolysis of two coals and oil shale was studied in the MFBRA to characterize the releasing behaviors of the individual gas component and their mixtures, thus to obtain the kinetic data under the isothermal conditions. The results showed that the major gas components released from coal and oil shale pyrolysis under the isothermal condition had different initiating and ending time points, and with the more distinct difference in the programmed heating conditions. The kinetic data i.e., activation energy (Ea) and frequency factor (A), were varied with each other for various gas components. The shrinking core model allowed better fitting relevance for the coal pyrolysis, while the threedimension model was more suitable for oil shale pyrolysis, indicating that the major gases released from the pyrolysis process of coal and oil shale might go through different reaction paths. The Ea of oil shale pyrolysis was 36.96 kJ·mol-1, larger than the Ea value during pyrolysis of two coals, which was 21.16 and 32.17 kJ·mol-1, respectively. The oil shale pyrolysis with high ash content was harder to take place in terms of its high activation energy. A comparison kinetic data of coal and oil shale pyrolysis in different reactors showed that the kinetic data was somehow subjected to the heating rates of fuels. The MFBRA could be a reliable tool to give an insight into the intrinsic kinetics and reaction mechanisms of gas-solid reactions, especially for the fast reaction, such as, pyrolysis. AUTHOR INFORMATION
Corresponding Authors E-mails: [email protected] (C. Wang); [email protected] (S. Zhang). ACKNOWLEDGMENTS The study was conducted with the research programs financed by the National Natural Science Foundation of China (U1862107, 21406264), and Science Foundation of China University of Petroleum Beijing (Grant No.2462018BJC003).
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Graphical Abstract:
19
Figure 1. A schematic diagram of the micro-fluidized bed reaction analyzer (MFBRA).
Highlights： The pyrolysis kinetics of oil shale and coals were comparatively studied by using a micro fluidized bed reaction analyzer (MFBRA). The releasing-gas behaviors from pyrolysis of coals and oil shale were different. Oil shale generally had higher value of pyrolysis activation energy than coals. The MFBRA could be a reliable tool for the intrinsic kinetics of gas-solid reactions.
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S2588-9133(19)30033-X https://doi.org/10.1016/j.crcon.2019.10.001 CRCON 56
To appear in:
Carbon Resources Conversion
Received Date: Revised Date: Accepted Date:
22 June 2019 24 September 2019 3 October 2019
Please cite this article as: Y. Zhang, M. Zhao, R. Linghu, C. Wang, S. Zhang, Comparative Kinetics of Coal and Oil Shale Pyrolysis in a Micro Fluidized Bed Reaction Analyzer, Carbon Resources Conversion (2019), doi: https:// doi.org/10.1016/j.crcon.2019.10.001
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Title： Comparative Kinetics of Coal and Oil Shale Pyrolysis in a Micro Fluidized Bed Reaction Analyzer Author information: #1 Author Yuming Zhang, Associate Professor, State Key Laboratory of Heavy Oil Processing, China University of Petroleum-Beijing E-mail: [email protected]; Tel: +86-15001296129 #2 Author Mengxuan Zhao, Post-graduate, State Key Laboratory of Heavy Oil Processing, China University of Petroleum-Beijing E-mail: [email protected] #3 Author Rongxuan Linghu, Post-graduate, State Key Laboratory of Heavy Oil Processing, China University of Petroleum-Beijing E-mail: [email protected] #4 Author Chengxiu Wang, Associate Professor, State Key Laboratory of Heavy Oil Processing, China University of Petroleum-Beijing E-mail: [email protected] #5 Author Shu Zhang, Professor, College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, Jiangsu, P.R. China
E-mail: [email protected]
Comparative Kinetics of Coal and Oil Shale Pyrolysis in a Micro Fluidized Bed Reaction Analyzer Yuming Zhanga, Mengxuan Zhaoa, Rongxuan Linghua, Chengxiu Wanga*, Shu Zhangb* a State
Key Laboratory of Heavy Oil Processing, College of Mechanical and Transportation Engineering, China University of Petroleum-Beijing, Beijing 102249, P. R. China b College of Materials Science and Engineering, Nanjing Forestry University, Nanjing 210037, Jiangsu, P.R. China
ABSTRACT
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The characteristics and kinetics of coal and oil shale pyrolysis were comparatively studied by using a micro fluidized bed reaction analyzer (MFBRA). The isothermal differential model was first applied to calculate the kinetic parameters of activation energy and frequency factor according to the major gas components during pyrolysis. The results showed that the major gas components released from coal and oil shale under the isothermal condition had different initiating and ending time points, and the difference was more significant under the programmed heating conditions. The shrinking core model allowed better fitting relevance for the coal pyrolysis, while the three-dimension model was more suitable for oil shale pyrolysis, indicating that the gases from the pyrolysis process of coal and oil shale might go through different reaction paths. The activation energy of oil shale pyrolysis was 36.96 kJ·mol-1, larger than the value of pyrolysis of the two coals, which was 21.16 and 32.17 kJ·mol-1, respectively. The above results justified that the oil shale pyrolysis with high ash contents was somehow more difficult to take place in terms of higher activation energy and the MFBRA could be a useful tool to give some insight into the intrinsic kinetics and reaction mechanisms of coal and oil shale pyrolysis. KEYWORDS: Micro fluidized bed reaction analyzer; Coal; Oil shale; Pyrolysis; Kinetics;
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1. INTRODUCTION With the gradual depletion of petroleum sources, more and more attention has been paid to the development of alternative energy sources. Oil shale, a sedimentary rock containing organics, that is, soluble bitumen and insoluble kerogen, has being considered to be an important alternative fuel with abundant reserves [1, 2]. Pyrolysis, as the initial step of the thermal conversion process (i.e., gasification, combustion) of solid fuels, is essentially important to the utilization of the carbonaceous materials [3, 4]. Oil shale could be converted into light pyrolysis shale oil, non-condensable gas and char during lowtemperature pyrolysis (i.e., retorting or distillation). However, oil shale usually had high contents of inorganic substance or ash, which were mainly composed of carbonate, silicate, quartz and a small amount of copper, nickel, cobalt, molybdenum, titanium, vanadium and other compounds [5]. As a result, the pyrolysis behavior of oil shale was complicated and different from that of coal, biomass and other solid fuels with low ash contents due to the interaction between kerogens and inorganic matters [6]. The deep insights into the pyrolysis kinetics of carbonaceous materials played a key role in the development of pyrolysis technologies. In general, the thermogravimetric analyzer (TGA) has been widely used to study the carbonaceous materials pyrolysis process and deduce the corresponding kinetic parameters via monitoring the mass loss at different heating rates. Al-Harahsheh et al. [7] used a TGA to study the oil shale pyrolysis at different heating rates, finding that the pyrolysis reaction of oil shale belonged to the firstorder reaction and the pyrolysis activation energy (Ea) was about 75-95 kJ·mol-1. He et al. [8] calculated the average Ea of coal pyrolysis to be about 304.56 and 303.97 kJ·mol-1 by using the Kissinger-AkahiraSunose (KAS) and Flynn-Wall-Ozawa (FWO) method, respectively. The isothermal reaction characteristics of the different carbonaceous fuels had also been studied with the TGA via rapid heating or changing the reaction atmosphere. However, the structure of the pre-prepared fuel samples in TGA might have changed during the heating process, which resulted in misleading information of the pyrolysis kinetic parameters. Furthermore, it was difficult for the TGA to simulate the actual pyrolysis reaction process under fluidized operating conditions because the sample was put in the fixed crucible and suffered from serious gas diffusion effects. Researchers have attempted to use other apparatus to study the pyrolysis of solid fuels under the isothermal conditions. Han et al. [9] selected three types of lignite to study their fast pyrolysis in a moving furnace via on-line recording the mass change of the sample and also the emission of hydrocarbon components. The results showed that the apparent Ea under rapid pyrolysis conditions was lower than that of slow pyrolysis. Bar et al. [10] used fluidized bed to measure the Ea of coal pyrolysis to be 42.6 kJ·mol-1, while the Ea of four kinds of oil shale pyrolysis was 39.9, 44.9, 51.0, and 46.3 kJ·mol-1, respectively. Bradley et al. [11] applied the down-flow tube for the Ea of lignite and sub-bituminous coal, finding that the Ea was about 10.49-16.76 kJ·mol-1 in the range of 600-1600°C. Shapatina et al. [12] carried out the
3
pyrolysis of lignite in a down-flow tube and the corresponding Ea to be about 4.18-15.88 kJ·mol-1. The above kinetic data were obtained via using different types of reactors and operating conditions. Some reactors might suffer from restriction by gas diffusion, which resulted in incomparability of the kinetics data in terms of different reactors. Recently, a micro-fluidized bed reaction analyzer (MFBRA) was used to study the kinetics of coal pyrolysis under isothermal conditions by analyzing gas-releasing characteristics [13-15]. Compared to the TGA, the MFBRA was developed to enhance heat exchange and mass transfer, also having rapid heating rate and minimizing the limitation of gas diffusion via fluidization, which in turn guaranteed the kinetics data close to the intrinsic reaction. The MFBRA had been employed for kinetics analysis of many gas-solid reactions, including biomass pyrolysis, gasification of coal char or petroleum coke [16, 17], etc. The MFBRA has proved to be a new approach to determine the thermal conversion of the different carbonaceous fuels. In this study, the MFBRA was employed to analyze the pyrolysis characteristics and kinetics of coal and oil shale under isothermal and non-isothermal conditions via monitoring gas-releasing behaviors. The kinetic data for the formation of four major gas products (CO2, CO, CH4, and H2) were deduced by the model-fitting approach. The different pyrolysis characteristics of coal and oil shale were investigated based on the releasing behaviors of each gas component and thus obtained the corresponding kinetic parameters. The results were expected to provide an in-depth understanding of the pyrolysis of carbonaceous fuels in the fluidized-bed under the isothermal conditions. 2. EXPERIMENTAL SECTION 2.1. Sample preparation In this experiment, Zhundong (ZD) sub-bituminous coal, Guobiao (GB) anthracite and Huadian (HD) oil shale were used as the samples, with their proximate and ultimate analyses given in Table 1. These three samples were dried to remove moisture in an oven at 105°C for 24 h before the experiment. The ZD coal had higher volatile content and lower fixed carbon content than that of the high-grade GB coal. The HD oil shale had similar volatile contents as ZD coal, while the ash content of oil shale was much higher than that of coals. Table 1. Properties of the tested samples. Proximate analysis (wt.%. db)
Ultimate analysis (wt.%. daf)
Sample
V
A
FC
C
H
O*
N
S
ZD coal
30.02
8.56
61.42
77.95
3.98
16.28
0.74
1.05
GB coal
8.73
13.72
77.55
92.57
2.14
4.12
0.68
0.37
HD oil shale
33.14
49.94
16.92
67.86
7.03
21.79
1.75
1.54
db: dry-base; daf: dry ash free base; V: volatile; A: ash; FC: fixed carbon. *By difference
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2.2. Apparatus and procedure The pyrolysis experiments were conducted in the MFBRA to obtain the releasing behaviors of major gas components as a function of time under the isothermal conditions. The schematic diagram of MFBRA was shown in Figure 1. It mainly consisted of four parts, that is, the micro fluidized bed, the sample feeding system, the pressure and temperature sensor, the gas cleaning and detecting system. The fluidized bed reactor was made of quartz with an inner diameter of 20 mm, to prevent surging and channel flow [18]. The feeding system was controlled by an electromagnetic valve and could instantly inject the feedstocks into MFBRA. The effluent gas components were measured by using an online mass spectrometer. In order to prevent particles escaping, the MFBRA was equipped with a double-layer dispersion plate and the main reaction zone was in-between the two plates. The main reaction zone of the reactor was about 40 mm in height, and 10-50 mg samples were injected for a single test. A computer was used to monitor the following parameters, including the temperatures of the resistance furnace and the exact reaction zone in the reactor, the pressure drop between the reactor inlet and outlet, the flow rate of carrier gas, the actions of sample feeding device and the output data from the mass spectrometer.
Figure 1. A schematic diagram of the MFBRA experimental apparatus. Before the experiment, about 3 g quartz sand with an average particle size of 70-100 mesh was loaded into the lower plate as the solid heat carriers and also the fluidized mediums. The quartz reactor was heated to a pre-set temperature (650-800°C) in a fluidized state, and then 10 mg sample was injected into the quartz reactor. The releasing characteristics of four major pyrolysis gases (H2, CO2, CH4, and CO2) were continuously measured by the mass spectrometry. Each experiment was repeated at least three times with
5
the relative error to be less than 5% and the average value was used for the kinetic analysis to ensure the reliability of the experimental results. 2.3. Isothermal differential kinetics The pyrolysis kinetics of solid fuels were determined via analyzing the reaction rate and the yield of reaction products, according to the generation process of major gas components under the isothermal condition. The kinetic study was carried out to describe the generation process of major gas and the mechanisms of fuel pyrolysis under the isothermal condition. The kinetic parameters from coal and oil shale pyrolysis were calculated based on the change of gas concentration under the isothermal conditions. Based on the above results, the integral data from the gas generation were measured via the online mass spectrometer. The conversion ratio X was shown in Eq. (1). via calculating the gas components from the pyrolysis reaction t
X
t iqvdt t t iqvdt
(1)
0
e
0
where t0, t and te (s) stands for the initiating time, lasting time and the ending time of pyrolysis reaction, respectively; φi representes the concentration of volatile components i (therein, i = CO2, CO, H2, and CH4), vol%; qv represents the volume of all gases releasing from the MFBRA, ml/min. The conversion ratio of solid fuels during pyrolysis with time was analyzed in terms of the production of gas mixture, t
（ X （ t0 te
t0
CO 2
CO CH 4 H 2 )qv dt
CO 2 CO CH 4 H 2 ) qv dt
(2)
where φCO2, φCO, φCH4 and φH2 represent respectively the volume fractions of CO2, CO, CH4 and H2 in the formed gas. According to the isothermal differential reaction kinetics, the kinetic equation of pyrolysis could be expressed as
dX k T f x dt
(3)
where k(T) was the reaction rate constant under the isothermal conditions; f(x) was the model mechanism function. Based on the variation of feedstocks properties and operating conditions, various reaction models had been used for the coal pyrolysis. Wang et al. [19] calculated the kinetic parameters of coal pyrolysis using the shrinking core model (SCM), suggesting that the reaction rate was controlled by the un-reacted surface area. The equation of the SCM was given in Eq. (4) f x 1 x
n
(4)
6
where n represents the order of the reaction. The three-dimension diffusion model was found to have better fitting relevance for the oil shale pyrolysis process due to its high ash contents and its organic matters dispersed over the inorganic skeletons [20]. Eq. (5) showed the formula of the three-dimensional diffusion model, with the assumption that the mass diffusion of material went through the layered barrier [21]. f x 1 x
2/3
[1 1 x
2/3
]
(5)
Substitute f(x) in the Eq. (3) with the Eq.(4) and took the logarithm of the formula, we could obtain the Eq (6), where ln k(T) and n corresponds to the intercept and the slope of the line fitted by ln (dX/dt) and ln (1-X), respectively.
ln
dx ln k T n ln 1 x dt
(6)
Thus, the straight line of the rate constant k(T) and the temperature T was obtained. The apparent activation energy Ea and the frequency factor A could be obtained according to the slope and the intercept of the straight line according to Eq. (7)
ln k ln A Ea / RT
(7)
where A represents requency factor, s-1; Ea represents the activation energy of the reaction, kJ·mol-1, R represents the gas constant, 8.314 J·mol-1·K-1, and T represents the reaction temperature, K. 3. RESULTS AND DISCUSSION 3.1 Parametric investigation in MFBRA The pyrolysis process in MFBRA was defined via the volume change of the produced pyrolysis gases. In order to get the intrinsic kinetics, it was essential to minimize the restriction of gas diffusion. Yu [15] proved that the effect of particle size on internal diffusion was negligible when the particle size was less than 120 μm. When the gas flow rate was higher than 300 ml/min, the effect of external diffusion was negligible. Figure 2 showed the influence of fluidized gas flow rates and the particle size on the pyrolysis gas conversion. The reaction time decreased via increasing the gas flow rate and the conversion varied little when the flow rate was above 300 ml/min. This critical fluidized flow rate represented the velocity to minimize the limitation of the external diffusion on the reaction. Moreover, the reaction completion time decreased by reducing the particle size of the samples. In order to obtain the experimental results under the minimized internal and external diffusion conditions, the particle size of the sample was adopted in the range of 74-120 μm and the flow rate of argon gas was 500 ml/min in this study.
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Figure 2. Influence of gas flow rate (a) and particle size (b) on the pyrolysis gas conversion behavior 3.2 Pyrolysis characteristics in MFBRA The pyrolysis of coal and oil shale was conducted in the MFBRA at temperatures of 650°C, 700°C, 750°C and 800°C, respectively. The variation of four major gas components including CO2, CO, CH4 and H2 from the pyrolysis of ZD bituminous coal, GB anthracite and HD oil shale at 650°C and 800°C, were shown in Figure 3, respectively. The releasing sequence of the major gas components was CO2, CO, CH4 and H2 at 650°C. However, the difference of the gas releasing sequence had nearly vanished at about 800°C. All the pyrolysis reaction in the MFBRA finished within 10 s, which was much shorter than the experiment time conducting in the TGA [22, 23]. The fuel samples were instantly injected to the reactor in MFBRA, and rapidly heated under the isothermal conditions in a rate of about 1000-10000°C/s [13]. The intensity of each gas components increased and reached its peak quickly, mainly resulting from the high heating rate and the minimized gas diffusion effects. Coal with different ranks had different pyrolysis characteristics. In comparison with the GB anthracite, the releasing behavior of CO2 and CO was found to be more obvious for the ZD sub-bituminous coal at 650°C and 800°C. In addition, the pyrolysis characteristics of oil shale were different from coal under the isothermal conditions. The varied releasing behaviors of different fuels were related to their volatile contents and structure characteristics. The low-rank ZD sub-bituminous contained a large number of functional groups, while the high-rank GB anthracite with few side chains or functional groups tended to condense and had a weaker gas-releasing intensity [24, 25]. The releasing intensity of HD oil shale was
8
even lower than that of GB anthracite, suggesting that the pyrolysis of oil shale was harder to take place and generated less volatiles than that of coal.
3.0E-6
ZD
a ZD T=650℃ CO2 CO H2 CH4
b T=800℃
GB
c GB T=650℃
d T=800℃
e HD T=650℃
f T=800℃
2.0E-6 1.0E-6
Intensity
0.0 1.0E-6 5.0E-7 0.0 1.0E-6 HD 5.0E-7 0.0 0
5
0 Time/s
5
10
Figure 3. Gas-releasing behaviors of ZD, GB coals and HD oil shale pyrolysis at 650°C and 800°C in the MFBRA. Figure 4 showed that the pyrolysis of ZD coal and HD oil shale under the programmed heating conditions. The sample was placed in the MFBRA at normal atmospheric temperature and then heated to pre-set temperature at heat rating of 20°C/min. The different releasing characteristics of four major gas components were magnified for comparison. The releasing order of gas components was CO2, CO, CH4, and H2 during the ZD coal pyrolysis and the initiating temperatures for these gases were at about 280°C, 450°C, 500°C and 600°C, respectively. The different functional groups and reaction behaviors would lead to the evolution of each gas component [9, 24]. The CO2 was mainly derived from the dissociation of carboxyl functional groups in organic volatiles, which had poor thermal stability and could be easily decomposed at low temperature. The generation of CO at low temperature was mainly from phenols, ether bonds and oxygen-containing heterocyclic in coal due to its wide releasing temperature range. At high temperature, such as 800°C, lots of CO was further released because of the intense pyrolysis of the oxygencontaining heterocyclic. The CH4 was mainly formed by the shedding of aliphatic hydrocarbon side chains. Compared to the other gas components, H2 released at rather higher temperatures by the condensation of free radicals and the polymerization of aromatic rings [26]. Besides, Table 1 shows that the ZD coal and
9
HD oil shale had similar volatile contents, while the HD oil shale had higher ash content than that of ZD coal. The variation of composition properties could in return determine the pyrolysis behavior of carbonaceous fuels.
Figure 4. Gas-releasing characteristics of ZD coal and HD oil shale pyrolysis under non-isothermal in the MFBRA. During the pyrolysis of HD oil shale, CO2 began to generate at about 250°C, CH4 releasing at about 380°C, then CO at about 400 °C. The CO2, CO and CH4 together reached to their peak at about 500°C, and this temperature was close to the temperature of maximum weight loss rate during oil shale pyrolysis in TG [27]. Initially, the reaction occurred on the surface of solid particles, resulting in only a small fraction of gas. The kerogen, as the main organic part in oil shale, dispersed over the inorganic minerals and might combine with ash. At higher temperature, the kerogen was cracked into small molecules (i.e., CO2, CO, and CH4), which was in accordance with the description of the three-dimensional diffusion model [28]. When the temperature was higher than 500°C, the intensity of CO decreased rapidly because the oxygencontaining functional groups had been substantially released. The higher intensity of the second CO2 peak at about 650°C might be attributed to the decomposition of the carbonate minerals [29]. The CO releasing peak at around 760°C was possibly caused by the interaction of CO2 and char.
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Figure 5. FTIR analysis of ZD coal and HD oil shale Figure 5 shows the FTIR analysis of HD oil shale and ZD coal, which were different in their functional groups. Both HD oil shale and coal had strong peaks at the range of 3000~2700 cm-1 (aliphatic chains) attributing to the methylene antisymmetric and symmetric stretching vibration. The weak absorption peak at 755 cm-1 could be from the vibration section of the long methylene chain. The absorption peak of oil shale was generally stronger than that of coal. Oil shale possessed more aliphatic chains and easily to be broken into CH4 [30]. Coal had an obvious absorption peak at 1589 cm-1 and there were some small peaks nearby, indicating that the increase of aromatization occurred due to the combining of C=O and C=C bonds. Besides, oil shale had an obvious absorption peak at 1033cm-1, which was connected to the C-O bond of organic matter alcohol and alkyl ether. 3.3 Pyrolysis kinetics analysis Figure 6 showed the relationship between gas conversion X of four major gases and time during ZD coal pyrolysis at different temperature. The reaction rate first increased with the increasing temperature and then reached to a plateau. However, the accelerating degree was different for each individual gas component. The slope of the H2 conversion curve had the most remarkable change via promoting temperature, indicating that the temperature had the utmost effect on the formation of H2. The whole gas products (i.e., the sum of all releasing gases) from pyrolysis were used to analyze the reaction kinetics, as
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shown in Figure 6(e). The gas mixture released at around 30s due to the mutual compensation effect between all gas products. For example, CO2 and CO forming at about 15s, that is, the former part of the reaction, could compensate the late-releasing gas, such as, H2 forming at around 30 s. After 30 s, the gas mixture was completely released with the conversion ratio reaching 100%.
Figure 6. Gas conversion versus reaction time of ZD coal in MFBRA at different temperatures for the individual gas component. Figure 7 shows the profile by converting the data of Figure 6 into the correlation of ln(dX/dt) with ln(1-X). The reaction process of four individual gas could be divided into three stages. At the first stage, the injected sample was mixed with the high-temperature fluidized medium (i.e., quartz sand) and heated rapidly in the MFBRA. At the second stage, the sample reached pre-set temperature and began to pyrolysis. Initially, the reaction mainly occurred on the surface of the samples [31]. Because the gas diffusion effects on the reaction were negligible when the particle size was smaller than 75 μm, the reaction was mainly controlled by chemical reaction during the intrinsic pyrolysis process. The functional groups were exhausting with the proceeding of pyrolysis, and the gas generation rate was gradually reduced as well. The curves tended to divergence at the ending part of the reaction, as evidenced by the fact that the reaction rate
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reduced to a minimum. At the final stage, the pyrolysis reaction changed from the surface to its inner side, and the reaction process changed from chemical reaction control to the internal diffusion control. Considering that the chemical reaction stage with the minimized internal diffusion was the main part we desired for the pyrolysis reaction in the MFBRA, the second stage was selected to analyze the intrinsic kinetic parameters.
Figure 7. Correlation of ln(dX/dt) and ln(1 - X) for individual components of ZD coal pyrolysis in the MFBRA. Table 2 shows the calculated reaction order n and ln k(T), with the reaction order of CO, CO2, CH4, H2, and gas mixture being 1.12-1.34, 1.12-1.24, 1.21-1.64, 0.83-1.16, 1.16-1.27 at different temperatures, respectively. Besides, the reaction order n of ZD pyrolysis was about 1.1-1.2 with the fitting correlation coefficient above 0.95. The pyrolysis process of coal was generally conformed to the first-order reaction, which was consistent with the previous research results of Klose et al [32]. The different reaction orders n of the four major gas components might be caused by the variation of formation paths or reaction mechanisms. Among them, the reaction order of H2 changed significantly, indicating that the reaction mechanism of H2 formation was different at varying temperatures.
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Table 2. Reaction order and rate constant of gas components at different temperatures Gas
CO
CO2
H2
CH4
Gas Mixture
T/°C
ln k(T)
n
R2
650
-0.57
1.28
0.99
700
-0.50
1.12
0.98
750 800 650
-0.41 -0.30 -0.87
1.34 1.17 1.24
0.98 0.99 0.98
700
-0.83
1.19
0.99
750
-0.74
1.12
0.99
800
-0.71
1.12
0.99
700
-2.18
1.64
0.98
750
-1.74
1.24
0.91
800
-1.53
1.21
0.96
650
-0.42
0.83
0.95
700
-0.23
1.1
0.97
750
-0.15
1.1
0.99
800
-0.03
1.16
0.98
650
-1.42
1.27
0.95
700
-1.32
1.21
0.96
750
-1.17
1.23
0.99
800
-1.04
1.16
0.99
3.4 Comparison of kinetics data The apparent activation energy (Ea) and frequency factor (A) of ZD coal, GB coal and HD oil shale were listed in Table 3. The Ea of the oil shale pyrolysis was 36.96 kJ·mol-1, higher than the Ea of two coals, i.e., 21.16 and 32.17 kJ·mol-1, respectively. With the increases of coal ranks, the pyrolysis required more energy to generate volatiles [24], thus leading to different Ea for pyrolysis of two coals. The process of oil shale pyrolysis at low temperature was similar to that of bituminous coal, while the Ea of oil shale pyrolysis was higher than high-rank coal at high temperature under the isothermal conditions [33]. The oil shale pyrolysis mainly involved with the decomposition of kerogen, while the reaction of inorganic matters required a large amount of energy. Thus, the pyrolysis Ea of oil shale pyrolysis was higher than that of coal. Table 3. Kinetic parameters of solid fuels pyrolysis in the MFBRA Reactor
Sample
Gas
T/°C
Ea /(kJ·mol-1)
A/s-1
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ZD
MFBRA
GB
HD
CO2 CO CH4 H2 Gas Mixture CO2 CO CH4 H2 Gas Mixture CO2 CO CH4 H2 Gas Mixture
650-800
9.38 14.52 20.93 56.70 21.16
1.40 3.66 10.16 129.02 3.74
650-800
20.94 24.47 38.52 64.16 32.17
4.93 6.29 16.75 137.84 16.54
650-800
28.09 30.23 15.39 68.60 36.96
0.55 0.96 0.49 200.87 117.80
Table 4 compared the kinetics data from MFBRA with other reactors from the literature, such as, moving furnace and drop tube. The kinetic values of coal and oil shale pyrolysis obtained in MFBRA was close to the kinetics data obtained in the reactors with high heating rate. Han [9] and Tolvanen [34] respectively used a moving furnace (heating rate of 100-300 °C/s) and a drop-tube reactor (heating rate of 1000 °C/s) to conduct pyrolysis experiments under the isothermal condition. The resulting Ea and A was almost the same in these two reactors with Ea to be 28.14-38.23kJ·mol-1 and A of 420 s-1, respectively, which was a slightly higher Ea and the larger A than that of MFBRA. This deviation of kinetic parameters might be caused by the larger gas external diffusion in the drop-tube reactor and moving furnace. Table 4. Comparison of kinetic data of solid fuels pyrolysis in different reactors Reference This study
Reactor MFBRA
500-900
Ea/(kJ·mol-1) 21.16 32.17 36.96 11.77
A/s-1 3.74 16.54 117.80 1.45
Yu [15]
MFBRA
Fuel Sub-bituminous coal Anthracite coal Oil shale Beer lees
T/°C
Guo [37]
MFBRA
Herb residue
600-850
10.48-18.90
1.90-4.30
Han [9]
Moving furnace
Brown coal
300-800
28.14-36.90
0.73-1.53
Bar [10]
Fluidized bed
400-500
Tolvanen [34]
Drop-tube reactor
Oil shale Coal Coal
700-900
39.9-51.0 42.6 37.2-38.23
449.61 178.41 276.2-420
Geng [23]
TGA
Bituminous
200-600
293-473
Guo [24]
TGA
Coal
850-930
348.48-378.62
Al-Ayed [35]
TGA
Oil shale
350-550
98-120
Scaccia [36]
TGA
Coal
30-1000
189
2.26×10191.74×1027 1.03×10151.12×1016 9.51×1051.16×106 1.2×1012
650-800
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The heating rate in TG was generally at 40°C /min, far less than that of MFBRA and other isothermal reactors [35, 36]. The reported Ea and A from TG were somehow high, with Ea to be 98-473 kJ·mol-1 and A of 9.51×105-1.74×1027 s-1. The kinetic data of the fast reaction like pyrolysis could be greatly affected by the heating process. The lower activation energy and frequency factor indicated that the fuels in MFBRA could receive more energy instantly to complete reactions [37]. Besides, the external and internal diffusion effects in the MFBRA could be minimized via the turbulent gas-solid reactions and tiny particle size. Consequently, the MFBRA would be a reliable reactor to analyze the gas-solid reaction under the isothermal condition, especially for the fuels with complex compositions like oil shale. Due to the mutual effects of minimized internal and external diffusion and rapid heating, the MFBRA could reflect the kinetics more accurately and get insight into the intrinsic reaction mechanism. 4. CONCLUSIONS The pyrolysis of two coals and oil shale was studied in the MFBRA to characterize the releasing behaviors of the individual gas component and their mixtures, thus to obtain the kinetic data under the isothermal conditions. The results showed that the major gas components released from coal and oil shale pyrolysis under the isothermal condition had different initiating and ending time points, and with the more distinct difference in the programmed heating conditions. The kinetic data i.e., activation energy (Ea) and frequency factor (A), were varied with each other for various gas components. The shrinking core model allowed better fitting relevance for the coal pyrolysis, while the threedimension model was more suitable for oil shale pyrolysis, indicating that the major gases released from the pyrolysis process of coal and oil shale might go through different reaction paths. The Ea of oil shale pyrolysis was 36.96 kJ·mol-1, larger than the Ea value during pyrolysis of two coals, which was 21.16 and 32.17 kJ·mol-1, respectively. The oil shale pyrolysis with high ash content was harder to take place in terms of its high activation energy. A comparison kinetic data of coal and oil shale pyrolysis in different reactors showed that the kinetic data was somehow subjected to the heating rates of fuels. The MFBRA could be a reliable tool to give an insight into the intrinsic kinetics and reaction mechanisms of gas-solid reactions, especially for the fast reaction, such as, pyrolysis. AUTHOR INFORMATION
Corresponding Authors E-mails: [email protected] (C. Wang); [email protected] (S. Zhang). ACKNOWLEDGMENTS The study was conducted with the research programs financed by the National Natural Science Foundation of China (U1862107, 21406264), and Science Foundation of China University of Petroleum Beijing (Grant No.2462018BJC003).
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Graphical Abstract:
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Figure 1. A schematic diagram of the micro-fluidized bed reaction analyzer (MFBRA).
Highlights： The pyrolysis kinetics of oil shale and coals were comparatively studied by using a micro fluidized bed reaction analyzer (MFBRA). The releasing-gas behaviors from pyrolysis of coals and oil shale were different. Oil shale generally had higher value of pyrolysis activation energy than coals. The MFBRA could be a reliable tool for the intrinsic kinetics of gas-solid reactions.
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