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Thermogravimetric study on the pressurized hydropyrolysis kinetics of a lignite coal
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Thermogravimetric study on the pressurized hydropyrolysis kinetics of a lignite coal Linbo Yan a, Boshu He a,*, Tianyi Hao b, Xiaohui Pei a, Xusheng Li a, Chaojun Wang a, Zhipeng Duan a a
Institute of Combustion and Thermal System, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China b Department of Thermal Engineering, Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China
article info
abstract
Article history:
Hydropyrolysis of coal is considered to be a third coal conversion technology between the
Received 30 December 2013
coal liquification and gasification technologies. It is also the primary process for coal
Received in revised form
hydrogasification (CHG). However, the detailed kinetic characteristics of coal hydro-
4 March 2014
pyrolysis (CHP) are still rarely studied, which is adverse to the further development of the
Accepted 10 March 2014
CHP and CHG technologies. In this work, the hydropyrolysis kinetics of a lignite coal is
Available online 13 April 2014
studied in a pressurized thermogravimetric analyzer (P-TGA). The non-isothermal thermogravimetric method is used and the effect of pressure on the pyrolysis kinetics of the
Keywords:
lignite coal is detected. Finally, some useful results are found from the analyses for the
Hydropyrolysis
lignite hydropyrolysis under P-TGA. With the increment of the pyrolysis pressure, the
Thermogravimetric
initial pyrolysis temperature increases when the pressure is higher than 1 MPa; the tem-
Kinetic triplet
perature span of the pyrolysis process shrinks; the weight loss peak value position of the
Lignite coal
derivative thermogravimetric (DTG) curve shifts rightwards when the pressure is lower than 1 MPa, while it shifts leftwards when the pyrolysis pressure is higher than 1 MPa; the reaction process will be restrained when the pressure is lower than 2 MPa. In addition, the kinetic triplets including the pre-exponential factor, the activation energy and the kinetic mechanism function are defined for the hydropyrolysis process under different pressures. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction It is an undisputed fact that the coal deposit on the earth is abundant and widely distributed compared with crude oil and natural gas [1]. Unfortunately, coal is also the dirtiest fuel in the world and traditional coal utilization method has emitted large amount of pollutants including NOx, SOx, CO2, fine particles, heavy metal and other trace elements into the
atmosphere [2]. Recently, with the advent of fears of climate change and environment pollution, the significance of developing clean and efficient coal utilization technologies has become self-evident. Coal hydrogasification (CHG) is such a promising technology and has attracted more and more attentions due to its obvious advantages [3e5]. As the primary process of CHG, coal hydropyrolysis (CHP) plays an important role during the CHG process and can affect the gasification products significantly if the reaction time is not long enough.
* Corresponding author. Tel.: þ86 10 5168 8542; fax: þ86 10 5168 8404. E-mail address: [email protected] (B. He). http://dx.doi.org/10.1016/j.ijhydene.2014.03.060 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 7 8 2 6 e7 8 3 3
Moreover, CHP is also considered to be a third way for coal conversion between coal liquification and gasification [6]. Up to now, a lot of work on CHP has been done by researchers from all over the world, which promotes the CHP technologies greatly. Sulimma et al. [6] once studied the catalytic hydropyrolysis of coal using a thermogravimetric analyzer and compared the effects of different catalysts on the CHP properties. Tang et al. [7] once studied the effect of coal rank on the yields of flash hydropyrolysis of Chinese coal in an entrainment reactor. Xu et al. once studied the nitrogen evolution [8] and sulfur evolution [9] during rapid hydropyrolysis of coal using a continuous free fall pyrolyzer. Recently, Zhou et al. [10] studied the hydropyrolysis characteristics and kinetics of potassium-impregnated pine wood using a thermogravimetric analyzer (TGA) in an atmosphere of 90% N2 and 10% H2 mixture. Researchers [11e13] from the State Key Lab of Coal Conversion of the Institute of Coal Chemistry in Chinese Academy of Science had also done a lot of work on the yields and kinetic characteristics of hydropyrolysis [11e13]. Besides the research mentioned above, many other studies on hydropyrolysis had also been reported and they are not listed here. However, most of the studies only focused on the yields of the hydropyrolysis process and those on the kinetics of CHP were rarely reported. Thus, further research on the kinetics of CHP is still needed to better reflect the CHG and CHP processes. The present work mainly focuses on the kinetic characteristics of the CHP process which are rarely studied up to now. A pressurized thermogravimetric analyzer (P-TGA) is used and the CHP kinetic characteristic of a lignite coal is studied. To visually exhibit the effects of reaction pressure on the CHP process, the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves at different reaction pressures are illustrated and compared. To reflect the difficulty of the reaction process, a pyrolysis difficulty index (PDI) correlation is developed based the original form and the PDI at different pressures are calculated. To find out the most probable kinetic mechanism function, a conventional method proposed to do the selection is developed and the application of the developed method is performed. When the most probable kinetic mechanism is detected, the apparent activation energies and the pre-exponent factors of the CHP processes at different pressures are then calculated and analyzed.
research institute and the parameters are obtained based on the testing standards in China. The total moisture in coal was tested based on GB/T211, the proximate analysis was implemented based on GB/T212, the heating value of coal was tested based on GB/T213, the total sulfur in coal was tested based on GB/T214, the carbon and hydrogen in coal was tested based on GB/T476 and the nitrogen in coal was tested based on GB/T19227. The coal particles are grinded and sieved to be smaller than 120 mm. It has been found that the particle size has slight effect on the weight loss and the yields of the CHP products when the coal particles are smaller than 160 mm [14]. The coal particles are dried to remove the moisture before CHP so that the moisture volatilization process will not occur during the CHP process.
Apparatus and procedure A pressurized thermogravimetric analyzer (P-TGA), TherMax 500 produced by the Thermo Fisher Scientific Corporation, is used in this work. The whole system includes a suspension pressurized balance, a furnace, an elevator, a control panel, a holder, several gas sources and a supervisory control computer running the Cahn Thermal Analysis software. The crucible with coal sample is hung in the furnace where the hydropyrolysis takes place. Three gas streams including the purge gas (He), the furnace gas (He) and the reaction gas (H2) are injected into the P-TGA as shown in Fig. 1. The purge gas is used to keep the pressure of the balance chamber and prevent the high temperature reaction gas from entering the balance chamber, the furnace gas is used to neutralize the pressure in
Experiments Coal sample Lignite coal is used in this study and its analysis results are listed in Table 1. In the table, Mad, Vad, Cad and Aad denote the mass fractions of moisture, volatile, fixed carbon and ash in the air-dried coal, respectively. The coal was tested in a
Table 1 e Analysis of coal. Approximate analysis
Ultimate analysis
Mad
Vad
Cad
Aad
C
H
O
N
S
9.41
40.19
47.15
12.67
65.11
4.48
16.64
0.77
0.33
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Fig. 1 e The schematic diagram of the P-TGA.
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the reactor and the reaction gas is used to react with the coal sample [15]. About 14.5 mg pre-dried coal sample is loaded in the crucible for each experiment. The temperature is controlled ranging from the ambient value to 1000 C with a ramping rate of 20 C/min and the pyrolysis pressure is set as 0.1 MPa, 1 MPa, 2 MPa, 3 MPa and 4 MPa, respectively. For each CHP experiment, a blank sample experiment in which the quartz sand is substituted for coal particles is also implemented under the same CHP condition to offset the temperature drift caused by the buoyancy.
Results Thermogravimetric analysis The TG and DTG curves of the lignite coal hydropyrolysis are shown in Fig. 2(a) and (b). From these figures, the initial pyrolysis temperature Ts, the maximum weight loss rate rmax and the corresponding temperature Tmax, and the mean weight loss rate rmean can be detected. Thereinto, rmean is calculated as the ratio of total weight loss to the pyrolysis time during the experiments. Because all the reactive material will be completely converted when the reaction time is long enough in the pure hydrogen atmosphere, the final weight loss VN for all the cases is then considered to be the sum of mass fractions of volatile and fixed carbon in the dry-base coal sample. The PDI, I, defined by Eq. (1) is then used to reflect the degree of difficulty of the CHP process under different pressures. Those values can be found in Table 2. I¼
rmax $rmean $VN Ts $Tmax $Dt1=2
(1)
where, Dt1/2 is the time span when r/rmax equates 0.5, namely Dt1/2 is the half-peak width. In some articles [16,17], temperature instead of reaction time is used to reflect the half-peak width. But, when the temperature reaches 1273 K, it will maintain constant. Thus, the half-peak width denoted by temperature cannot exactly reflect this width. To avoid this deficiency, the reaction time is used in this work. In order to make the following analyses for the effects of pressure on the CHP and CHG processes more precisely, it is necessary to analyze the CHP and CHG mechanisms firstly in detail. The coal structure can be imagined as the one shown in Fig. 3 as reported in literatures [18,19]. During the pyrolysis process, the labile bridges in the coal lattice break or
Table 2 e The pressurized hydropyrolysis characteristic parameters of the lignite coal. Pressure (MPa)
Ts ( C)
Tmax ( C)
rmax (%/min)
rmean (%/min)
0.1 1 2 3 4
165.3 143.2 212.9 305.6 322.3
414.8 841.6 765.3 710.4 642.8
3.16 2.55 2.66 2.83 3.08
0.60 0.83 0.77 0.82 0.99
I (min3 C2) 3.45E-06 5.48E-07 4.07E-07 4.45E-07 5.14E-07
transform and the small free radicals detach and combine with hydrogen from coal itself and the gasification agent to generate light gases. A certain fractions of coal detach from the coal lattice and finally become heavy-molecular-weight tar. The left materials in the coal lattice are bonded with stable char bridge and finally become char. Then, the pyrolysis products react with hydrogen and the hydrogasification occurs. The pyrolysis and gasification processes are shown in Fig. 4 as reported in our previous work [19]. It should be noted that although many gaseous species such as O2, H2O and CO2 can react with char and tar, only the reactions between the pyrolysis products and hydrogen will dominate in the hydrogen atmosphere. As is known, the pyrolysis process will generate gaseous products and tend to increase the reaction pressure while the char hydrogasification process will consume the gaseous species and tent to decrease the reaction pressure. Thus, based on the Le Chatelier’s principle and the mass action law [20], increasing pressure will restrain the hydropyrolysis process and promote the hydrogasification process on the whole. From Fig. 2(a) it can be seen that the mass of coal sample decreases with the reaction time increment and the TG curves have a reversed S-shape. This is due to the extraction of the volatile in coal and the hydrogasification of char. From Fig. 2(b) it can be seen that the peak position where the minimum value of the DTG curve is detected shifts rightwards obviously when the pyrolysis pressure increases from 0.1 MPa to 1 MPa. Then, with the increment of pressure when it is higher than 1 MPa, the peak position moves leftwards gradually. The peak value, namely the minimum value in the DTG curve, decreases greatly when the pyrolysis pressure increases from 0.1 MPa to 1 MPa. Then, it begins to increase gradually with the pressure increment when the pressure is higher than 1 MPa. It can also be seen that when the reaction pressure is lower than 1 MPa especially when it is close to 0.1 MPa, the hydropyrolysis
Fig. 2 e The thermogravimetric results.
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Fig. 3 e Chemical structures of coal.
will dominate and the whole pyrolysis process will take place in a narrow temperature range quickly. When the reaction pressure is higher than 1 MPa, the volatile extraction will be restrained while the hydrogasification will be promoted. This is because only the extraction of gaseous matter from coal can contribute to the weight loss. As the pressure increases, the boiling point of the heavy oil will be increased and the pyrolysis process will be deferred. Moreover, due to the Le Chatelier’s principle and the mass action law, the decomposition of the large molecular to generate light gases can also be restrained by increasing the pyrolysis pressure. Inversely, increasing pyrolysis pressure is beneficial to hydrogasification. Thus, when the pressure is higher than 1 MPa, the weight loss rate can be promoted and the peak position will move leftwards. It can also be seen from Fig. 2(b) that a second peak will appear at high temperature. This is mainly due to the hydrogasification process [21]. From Table 2 it can be seen that when the pyrolysis pressure increases from 0.1 MPa to 1 MPa, Ts and rmax will decrease with the pressure increment while Tmax will increase. When the pyrolysis pressure increases from 1 MPa to 4 MPa, those three
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parameters will change inversely. This is because when the pyrolysis pressure is not high, increasing pressure can promote the combination of the heteroatoms (O, N, S) and small free radicals with H2 [22]. Thus, more gaseous species will be generated at the beginning and Ts will be lowered. But, when the pyrolysis pressure is high, H2 will not obviously promote the extraction of the gaseous volatile since almost all the heteroatoms and small free radicals have already been saturated. On the contrary, the coal lattice [23] which is like the solvent of the volatile can hold more volatile at high pressure. Thus, Ts will increase. The explanation for the change of Tmax with the pyrolysis pressure is the same with that for the change of the peak position as depicted in Fig. 2(b). The explanation for the change of rmax with the pyrolysis pressure is the same with that for the change of the peak value. It can also be seen from Table 2 that I decreases with the pyrolysis pressure when it is within 2 MPa. Then, when the pressure is higher, I will increase with the pressure increment. This is mainly due to the restricting effect of pressure on the hydropyrolysis process and the promotion effect of pressure on the hydrogasification. From Fig. 2 and Table 2, it can be seen that 1 MPa seems to be a critical pressure for the hydropyrolysis process.
Kinetic analysis General theory for the integral method The mechanism function, the activation energy and the preexponent factor are the so called kinetic triplet which can exactly reflect the kinetic process. To quantitatively investigate the kinetic characteristics of the CHP process, the kinetic mechanism function for the CHP processes is firstly defined. Then, the activation energies and the pre-exponent factors for the CHP processes at different pressures are calculated. The kinetic equation for the CHP can be written as Eq. (2) [24,25]. da ¼ Kf ðaÞ ¼ AeE=RT f ðaÞ dt
(2)
where, a denotes the conversion rate and can be calculated with Eq. (3); t is the pyrolysis time; K is the reaction rate constant; A is the pre-exponent factor; E is the activation energy; R is the universal gas constant; T is the pyrolysis temperature; f(a) is the kinetic mechanism function. a¼
w0 w w0 wN
(3)
where, w0 is the initial mass; w is the mass of coal sample at any time during the pyrolysis process; wN is the mass left after the pyrolysis. Because only ash will be left if the hydropyrolysis time is long enough, wN is considered to be equal to the ash mass. The non-isothermal thermogravimetric method is used and the temperature ramping rate, b, can be expressed with Eq. (4). b¼
dT dt
(4)
The correlation of the integral form of f(a) can be written as Eq. (5). Za Fig. 4 e The general coal pyrolysis and gasification processes.
GðaÞ ¼ 0
da f ðaÞ
(5)
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Based on Eqs. (2), (4) and (5), G(a) can be calculated with Eq. (6). GðaÞ ¼
AE pðuÞ bR
(6)
where, u denotes E/RT and p(u) can be calculated using the integration by parts integral method. Zu pðuÞ ¼ N
eu eu 2! 3! 4! þ du ¼ þ / 1 u u2 u3 u2 u2
(7)
In the common range of the reaction temperature, u is much higher than unit. Thus, only the first two terms in the brackets on the right side of Eq. (7) are kept to do the simplification. Then, Eq. (6) can be cast into Eq. (8). ART2 2RT E=RT e 1 GðaÞ ¼ E bE
(8)
(9)
Determination of the kinetic mechanism function From Eq. (9) it can be seen that for the proper kinetic mechanism function f(a), its integral form G(a) will perform a linear relationship with the reciprocal of temperature. But, in fact it is found that many mechanism functions can meet the linear relationship due to the compensatory effect of E and A [26]. For different mechanism functions, different Es and As can be obtained. But which pair of E and A is the right one still needs determination. Many methods have been developed by researchers to help to determine the most probable mechanism function up to now [26]. In this work, the integral-differential method (IDM), namely the Bagchi method [27], is chosen and some improvement for this method is also implemented [28]. Using the variable separation method for Eq. (2) and taking log for both sides, Eq. (10) can then be obtained and this is the so called AchareBrindleyeSharpeWendworth method [29,30]. da E A ¼ þ ln ln f ðaÞdT RT b
(10)
The concept of IDM is that both E and A calculated from the integral method and the differential method, respectively, should be very close when the mechanism function is properly selected and determined. Some researchers do calculated E and A using the two method and then do the comparison [28]. But this way is cumbersome, especially when the curve obtained from Eq. (9) or Eq. (10) is piecewise linearity. In this work, a relatively simple and effective method is developed based on the IDM. From Eqs. (9) and (10), it can be seen that both the equations have the same slope form, E/RT. Thus, if E values calculated from the two equations are the same, the curves generated from the two equations should be parallel to each other. In addition, using h1 to represent ln½GðaÞ=T2 and h2 to represent ln½da=f ðaÞdT, if A calculated from the two equations are the same, make difference of them and Eq. (11) can then be obtained.
(11)
Eq. (11) indicates that the difference between the two curves obtained by both the integral method and differential method should be ln(R/E) and E can be calculated only using the integral method. In this work, 41 common mechanism functions reported in literature [26] are investigated and finally the random nucleation and later growth mechanism function (RNLGM, n ¼ 4) developed by Avrami [31e33] is found to have the best performance. Thus, RNLGM with n ¼ 4 is chosen as the kinetic mechanism function for the CHP process. The mechanism function and the corresponding integral form are written as Eqs. (12) and (13). f ðaÞ ¼
Because u is much higher than unit, the value in the bracket on the right side of Eq. (8) is about 1. Taking log of both sides and Eq. (8) can be transformed into Eq. (9). GðaÞ E AR þ ln ¼ ln 2 T RT bE
AR A R h1 h2 ¼ ln ln ¼ ln bE b E
1 ð1nÞ ð1 aÞ½ lnð1 aÞ n
GðaÞ ¼ ½ lnð1 aÞ
n
(12) (13)
Calculation results When the pyrolysis process occurs not in the inert atmosphere, both the pyrolysis and gasification processes will take place. Some researchers used the temperature to distinguish the pyrolysis process and the gasification process. But for different pyrolysis conditions, the dividing temperature should be different and the temperature is not precisely known [34]. In this work, 5%e95% weight loss of the volatile contained in the coal sample is considered to be the major hydropyrolysis process, and the kinetic parameters in this range are calculated. This is because during the whole reaction process, CHP rate is usually much higher than that of CHG and it is usually considered that the char gasification process follows the pyrolysis process. The plots of h1 ðln½GðaÞ=T2 Þ vs. 1/ T and the corresponding fitted curves are shown in Fig. 5(a1)e(e1). The corresponding fitted equations are also listed. The comparisons of the curves obtained from the integral method and the differential method are shown in Fig. 5(a2)e(e2). The calculated kinetic results are listed in Table 3. Dave in Table 3 is the average of h1 h2 determined by Eq. (11) in the corresponding temperature range. From Fig. 5(a2)e(e2), it can be seen that the curve obtained from the integral method basically parallels with that obtained from the differential method, which indicates that the activation energy calculated from the two methods are almost the same. By comparison of ln(R/E) and Dave listed in Table 3, very good agreement is found for these two values, indicating that the pre-exponent factor calculated from the two methods should be close to each other. In addition, all the correlation coefficient, Ra shown in Table 3, are all greater than 0.98, indicating that the data generated with RNLGM has good linearity. Thus, the kinetic mechanism function chosen in this work is believed justified and the kinetic parameters calculated based on this mechanism are reliable. It can also be seen from Table 3 that the temperature range of the hydropyrolysis process shrinks with the increase of the pyrolysis pressure. This is because increasing pressure can promote the combination of H2 and the free radicals generated during the heating process. When the pressure is higher than 1 MPa, the initial extraction
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Fig. 5 e Linear fitting and methods comparison.
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Table 3 e Kinetic parameters for the CHP process under different pressures. Pressure (MPa) 0.1
1 2 3 4
Temperature range ( C)
E (kJ/mol)
165e314 314e476 476e902 143e781 213e765 306e772 322e690
44.291 148.75 34.59 66.30 90.96 139.90 152.23
temperature increases and the hydropyrolysis temperature range becomes compact.
Conclusion To investigate the hydropyrolysis kinetic characteristics of a lignite coal under different pyrolysis pressures, a series of experiments are implemented in the P-TGA. The TG and DTG curves are drawn and compared. In order to find out the proper kinetic mechanism function, the IDM is used and an advanced method based on this method is proposed. Finally, using the RNLGM kinetic mechanism function when n ¼ 4, the kinetic parameters of the CHP are calculated. The meaningful conclusions obtained from this work can be drawn as follows: 1) When the pyrolysis pressure is lower than 1 MPa, Ts will decrease with the increment of the pressure. When the pressure is higher than 1 MPa, Ts will then increases with the increment of the pyrolysis pressure. When the pyrolysis pressure is lower than 1 MPa, Tmax will shift rightwards with the increment of the pressure. When the pyrolysis pressure is higher than 1 MPa, Tmax will then decrease gradually with the pressure increment. 2) Although increasing pressure is beneficial to CHG, it is not for CHP. When the reaction pressure is increased from 0.1 MPa to 2 MPa, PDI decreases with the pyrolysis pressure and the whole reaction process of the lignite coal will be restrained. Then, when the reaction pressure is higher than 2 MPa, PDI will increase and the reaction process will be promoted with the pressure increment. 3) With the increment of pyrolysis pressure, the hydropyrolysis temperature ranges shrinks gradually. 4) The RNLGM with n ¼ 4 can reflect the hydropyrolysis process very well. The kinetic process for the CHP under 0.1 MPa includes three steps and those for 1 MPae4 MPa are all one step.
Acknowledgments The authors gratefully acknowledge financial supports from the National Natural Science Foundation of China (NSFC, 50876008, 51176009) for this work.
A (min1)
Ra
ln(R/E)
Dave
102 108 101 101 102 105 107
0.993 0.996 0.988 0.996 0.987 0.990 0.999
8.58 9.79 8.33 8.98 9.30 9.73 9.82
8.71 9.76 8.69 9.24 9.46 9.77 9.90
3.02 1.66 4.6 2.97 6.56 3.52 2.03
references
[1] International energy outlook 2010. Washington, DC: U.S. Energy Information Administration; 2011 Sep.. Contract no.: DOE/EIA-0484(2011), http://me.queensu.ca/Courses/430/ InternationalEnergyOutlook-summaryonly.pdf. [2] Franco A, Diaz AR. The future challenges for “clean coal technologies”: joining efficiency increase and pollutant emission control. Energy 2009;34:348e54. [3] Ziock HJ, Lackner KS, Harrison DP. Zero emission coal power, a new concept. Los Alamos, New Mexico: Los Alamos National Laboratory. Contract no.: LA-UR-01e2214 http:// www.netl.doe.gov/publications/proceedings/01/carbon_seq/ 2b2.pdf. [4] Yan LB, He BS, Ma LL, Pei XH, Wang CJ, Li XS. Integrated characteristics and performance of zero emission coal system. Int J Hydrogen Energy 2012;37:9669e76. [5] Yan LB, He BS, Pei XH, Li XS, Wang CJ. Energy and exergy analyses of a zero emission coal system. Energy 2013;55:1094e103. [6] Sulimma A, Leonhardt P, Van Heek KH, Ju¨ntgen H. Thermogravimetric study on catalytic hydropyrolysis of coal. Fuel 1986;65:1457e61. [7] Tang LH, Zhu ZB, Gu HX, Zhang CF. The effect of coal rank on flash hydropyrolysis of Chinese coal. Fuel Process Technol 1999;60:195e202. [8] Xu WC, Kumagai M. Nitrogen evolution during rapid hydropyrolysis of coal. Fuel 2002;81:2325e34. [9] Xu WC, Kumagai M. Sulfur transformation during rapid hydropyrolysis of coal under high pressure by using a continuous free fall pyrolyzer. Fuel 2003;82:245e54. [10] Zhou LM, Jia YY, Nguyen TH, Adesina AA, Liu ZR. Hydropyrolysis characteristics and kinetics of potassiumimpregnated pine wood. Fuel Process Technol 2013;116:149e57. [11] Li BQ, Danheux CB, Cypre`s R. Catalytic hydropyrolysis by impregnated sulphided Mo catalyst. Fuel 1991;70:254e8. [12] Li W, Wang N, Li BQ. Product analysis of catalytic multi-stage hydropyrolysis of coal. J China Univ Min Technol 2002;31:246e51 [in Chinese]. [13] Sun QL, Li W, Chen HK, Li BQ. Compositions of coal tar from pyrolysis and hydropyrolysis of Shenmu coal macerals. J Fuel Chem Technol 2005;3:412e5 [in Chinese]. [14] Xu WC, Matsuoka K, Akiho H, Kumagai M, Tomita A. High pressure hydropyrolysis of coals by using a continuous freefall reactor. Fuel 2003;82:677e85. [15] Zhu L. Hydrogasification characteristics of coals in TGA [Master thesis]. Beijing Jiaotong University; 2010 [in Chinese]. [16] Xiao J, Shen LH, Wang ZM, Zhong XL. Pressurized thermogravimetric analysis of pyrolysis of biomass. J Combust Sci Technol 2005;11:415e20 [in Chinese]. [17] Wang XH, Ju FD, Yang HP, Xu J, Zhang SH, Chen HP. Kinetics and properties analysis of Shenfu coal pressurized pyrolysis. Proc CSEE 2011;31:40e4 [in Chinese].
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 7 8 2 6 e7 8 3 3
[18] Fletcher TH, Kerstein AR, Pugmire RJ, Grant DM. Chemical percolation model for devolatilization: 2. Temperature and heating rate effects on product yields. Energy Fuels 1990;4(1):54e60. [19] Yan LB, He BS, Pei XH, Li XS, Wang CJ, Liang HX. Kinetic model and prediction for coal hydrogasification. Int J Hydrogen Energy 2013;38:4513e23. [20] George WR. Chemical reactions and chemical reactors. Hoboken, NJ: John Wiley and Sons, Inc.; 2009. [21] Sun QL, Li W, Chen HK, Li BQ. Investigation of hydropyrolysis behavior of Shenmu macerals by TG-MS. J Fuel Chem Technol 2004;32:647e51 [in Chinese]. [22] Ye CP. Characterization and modeling of coal high molecular weight components [Ph.D. dissertation]. Taiyuan University of Technology; 2008 [in Chinese]. [23] He XM. Coal chemistry. Beijing: Metallurgical Industry Press; 2010 [in Chinese]. [24] Sbirrazzuoli N. Determination of pre-exponential factors and of the mathematical functions f(a) or G(a) that describe the reaction mechanism in a model-free way. Thermochim Acta 2013;564:59e69. ˛ zko ska M, Plis A, Scia _ [25] Łabojko G, Kotyczka-Moran M. Kinetic study of polish hard coal and its char gasification using carbon dioxide. Thermochim Acta 2012;549:158e65.
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[26] Hu RZ, Shi QZ. Thermal analysis kinetics. Beijing: Sciences Press; 2001 [in Chinese]. [27] Bagchi TP, Sen PK. Combined differential and integral method for analysis of non-isothermal kinetic data. Thermochim Acta 1981;51:175e89. [28] Zhang CQ, Wei LH, Ren GP, Jiang XM. Research on pyrolysis characteristics and mechanism of micro-pulverized and common-pulverized coal. J Harbin Inst Technol 2006;38:1948e51 [in Chinese]. [29] Achar BNN, Bridley GW, Sharp JH. Thermal decomposition kinetics of some new unsaturated polyesters. In: Proc Int Clay Conf, 1; 1966. p. 67. [30] Sharp JH, Wentworth SA. Kinetic analysis of thermogravimetric data. Anal Chem 1969;41:2060e2. [31] Avrami M. Kinetics of phase change. I General theory. J Chem Phys 1939;7:1103e12. [32] Avrami M. Kinetics of phase change. II Transformation-time relations for random distribution of nuclei. J Chem Phys 1940;8:212e24. [33] Avrami M. Granulation, phase change, and microstructure kinetics of phase change. III. J Chem Phys 1941;9:177e84. [34] Xiao J, Shen LH, Deng X, Wang ZM, Zhong X. Thermogravimetric study on catalytic pyrolysis gasification of biomass. Acta Energy Solaris Sin 2009;30:1252e7 [in Chinese].
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Thermogravimetric study on the pressurized hydropyrolysis kinetics of a lignite coal Linbo Yan a, Boshu He a,*, Tianyi Hao b, Xiaohui Pei a, Xusheng Li a, Chaojun Wang a, Zhipeng Duan a a
Institute of Combustion and Thermal System, School of Mechanical, Electronic and Control Engineering, Beijing Jiaotong University, Beijing 100044, China b Department of Thermal Engineering, Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China
article info
abstract
Article history:
Hydropyrolysis of coal is considered to be a third coal conversion technology between the
Received 30 December 2013
coal liquification and gasification technologies. It is also the primary process for coal
Received in revised form
hydrogasification (CHG). However, the detailed kinetic characteristics of coal hydro-
4 March 2014
pyrolysis (CHP) are still rarely studied, which is adverse to the further development of the
Accepted 10 March 2014
CHP and CHG technologies. In this work, the hydropyrolysis kinetics of a lignite coal is
Available online 13 April 2014
studied in a pressurized thermogravimetric analyzer (P-TGA). The non-isothermal thermogravimetric method is used and the effect of pressure on the pyrolysis kinetics of the
Keywords:
lignite coal is detected. Finally, some useful results are found from the analyses for the
Hydropyrolysis
lignite hydropyrolysis under P-TGA. With the increment of the pyrolysis pressure, the
Thermogravimetric
initial pyrolysis temperature increases when the pressure is higher than 1 MPa; the tem-
Kinetic triplet
perature span of the pyrolysis process shrinks; the weight loss peak value position of the
Lignite coal
derivative thermogravimetric (DTG) curve shifts rightwards when the pressure is lower than 1 MPa, while it shifts leftwards when the pyrolysis pressure is higher than 1 MPa; the reaction process will be restrained when the pressure is lower than 2 MPa. In addition, the kinetic triplets including the pre-exponential factor, the activation energy and the kinetic mechanism function are defined for the hydropyrolysis process under different pressures. Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction It is an undisputed fact that the coal deposit on the earth is abundant and widely distributed compared with crude oil and natural gas [1]. Unfortunately, coal is also the dirtiest fuel in the world and traditional coal utilization method has emitted large amount of pollutants including NOx, SOx, CO2, fine particles, heavy metal and other trace elements into the
atmosphere [2]. Recently, with the advent of fears of climate change and environment pollution, the significance of developing clean and efficient coal utilization technologies has become self-evident. Coal hydrogasification (CHG) is such a promising technology and has attracted more and more attentions due to its obvious advantages [3e5]. As the primary process of CHG, coal hydropyrolysis (CHP) plays an important role during the CHG process and can affect the gasification products significantly if the reaction time is not long enough.
* Corresponding author. Tel.: þ86 10 5168 8542; fax: þ86 10 5168 8404. E-mail address: [email protected] (B. He). http://dx.doi.org/10.1016/j.ijhydene.2014.03.060 0360-3199/Copyright ª 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 7 8 2 6 e7 8 3 3
Moreover, CHP is also considered to be a third way for coal conversion between coal liquification and gasification [6]. Up to now, a lot of work on CHP has been done by researchers from all over the world, which promotes the CHP technologies greatly. Sulimma et al. [6] once studied the catalytic hydropyrolysis of coal using a thermogravimetric analyzer and compared the effects of different catalysts on the CHP properties. Tang et al. [7] once studied the effect of coal rank on the yields of flash hydropyrolysis of Chinese coal in an entrainment reactor. Xu et al. once studied the nitrogen evolution [8] and sulfur evolution [9] during rapid hydropyrolysis of coal using a continuous free fall pyrolyzer. Recently, Zhou et al. [10] studied the hydropyrolysis characteristics and kinetics of potassium-impregnated pine wood using a thermogravimetric analyzer (TGA) in an atmosphere of 90% N2 and 10% H2 mixture. Researchers [11e13] from the State Key Lab of Coal Conversion of the Institute of Coal Chemistry in Chinese Academy of Science had also done a lot of work on the yields and kinetic characteristics of hydropyrolysis [11e13]. Besides the research mentioned above, many other studies on hydropyrolysis had also been reported and they are not listed here. However, most of the studies only focused on the yields of the hydropyrolysis process and those on the kinetics of CHP were rarely reported. Thus, further research on the kinetics of CHP is still needed to better reflect the CHG and CHP processes. The present work mainly focuses on the kinetic characteristics of the CHP process which are rarely studied up to now. A pressurized thermogravimetric analyzer (P-TGA) is used and the CHP kinetic characteristic of a lignite coal is studied. To visually exhibit the effects of reaction pressure on the CHP process, the thermogravimetric (TG) and derivative thermogravimetric (DTG) curves at different reaction pressures are illustrated and compared. To reflect the difficulty of the reaction process, a pyrolysis difficulty index (PDI) correlation is developed based the original form and the PDI at different pressures are calculated. To find out the most probable kinetic mechanism function, a conventional method proposed to do the selection is developed and the application of the developed method is performed. When the most probable kinetic mechanism is detected, the apparent activation energies and the pre-exponent factors of the CHP processes at different pressures are then calculated and analyzed.
research institute and the parameters are obtained based on the testing standards in China. The total moisture in coal was tested based on GB/T211, the proximate analysis was implemented based on GB/T212, the heating value of coal was tested based on GB/T213, the total sulfur in coal was tested based on GB/T214, the carbon and hydrogen in coal was tested based on GB/T476 and the nitrogen in coal was tested based on GB/T19227. The coal particles are grinded and sieved to be smaller than 120 mm. It has been found that the particle size has slight effect on the weight loss and the yields of the CHP products when the coal particles are smaller than 160 mm [14]. The coal particles are dried to remove the moisture before CHP so that the moisture volatilization process will not occur during the CHP process.
Apparatus and procedure A pressurized thermogravimetric analyzer (P-TGA), TherMax 500 produced by the Thermo Fisher Scientific Corporation, is used in this work. The whole system includes a suspension pressurized balance, a furnace, an elevator, a control panel, a holder, several gas sources and a supervisory control computer running the Cahn Thermal Analysis software. The crucible with coal sample is hung in the furnace where the hydropyrolysis takes place. Three gas streams including the purge gas (He), the furnace gas (He) and the reaction gas (H2) are injected into the P-TGA as shown in Fig. 1. The purge gas is used to keep the pressure of the balance chamber and prevent the high temperature reaction gas from entering the balance chamber, the furnace gas is used to neutralize the pressure in
Experiments Coal sample Lignite coal is used in this study and its analysis results are listed in Table 1. In the table, Mad, Vad, Cad and Aad denote the mass fractions of moisture, volatile, fixed carbon and ash in the air-dried coal, respectively. The coal was tested in a
Table 1 e Analysis of coal. Approximate analysis
Ultimate analysis
Mad
Vad
Cad
Aad
C
H
O
N
S
9.41
40.19
47.15
12.67
65.11
4.48
16.64
0.77
0.33
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Fig. 1 e The schematic diagram of the P-TGA.
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the reactor and the reaction gas is used to react with the coal sample [15]. About 14.5 mg pre-dried coal sample is loaded in the crucible for each experiment. The temperature is controlled ranging from the ambient value to 1000 C with a ramping rate of 20 C/min and the pyrolysis pressure is set as 0.1 MPa, 1 MPa, 2 MPa, 3 MPa and 4 MPa, respectively. For each CHP experiment, a blank sample experiment in which the quartz sand is substituted for coal particles is also implemented under the same CHP condition to offset the temperature drift caused by the buoyancy.
Results Thermogravimetric analysis The TG and DTG curves of the lignite coal hydropyrolysis are shown in Fig. 2(a) and (b). From these figures, the initial pyrolysis temperature Ts, the maximum weight loss rate rmax and the corresponding temperature Tmax, and the mean weight loss rate rmean can be detected. Thereinto, rmean is calculated as the ratio of total weight loss to the pyrolysis time during the experiments. Because all the reactive material will be completely converted when the reaction time is long enough in the pure hydrogen atmosphere, the final weight loss VN for all the cases is then considered to be the sum of mass fractions of volatile and fixed carbon in the dry-base coal sample. The PDI, I, defined by Eq. (1) is then used to reflect the degree of difficulty of the CHP process under different pressures. Those values can be found in Table 2. I¼
rmax $rmean $VN Ts $Tmax $Dt1=2
(1)
where, Dt1/2 is the time span when r/rmax equates 0.5, namely Dt1/2 is the half-peak width. In some articles [16,17], temperature instead of reaction time is used to reflect the half-peak width. But, when the temperature reaches 1273 K, it will maintain constant. Thus, the half-peak width denoted by temperature cannot exactly reflect this width. To avoid this deficiency, the reaction time is used in this work. In order to make the following analyses for the effects of pressure on the CHP and CHG processes more precisely, it is necessary to analyze the CHP and CHG mechanisms firstly in detail. The coal structure can be imagined as the one shown in Fig. 3 as reported in literatures [18,19]. During the pyrolysis process, the labile bridges in the coal lattice break or
Table 2 e The pressurized hydropyrolysis characteristic parameters of the lignite coal. Pressure (MPa)
Ts ( C)
Tmax ( C)
rmax (%/min)
rmean (%/min)
0.1 1 2 3 4
165.3 143.2 212.9 305.6 322.3
414.8 841.6 765.3 710.4 642.8
3.16 2.55 2.66 2.83 3.08
0.60 0.83 0.77 0.82 0.99
I (min3 C2) 3.45E-06 5.48E-07 4.07E-07 4.45E-07 5.14E-07
transform and the small free radicals detach and combine with hydrogen from coal itself and the gasification agent to generate light gases. A certain fractions of coal detach from the coal lattice and finally become heavy-molecular-weight tar. The left materials in the coal lattice are bonded with stable char bridge and finally become char. Then, the pyrolysis products react with hydrogen and the hydrogasification occurs. The pyrolysis and gasification processes are shown in Fig. 4 as reported in our previous work [19]. It should be noted that although many gaseous species such as O2, H2O and CO2 can react with char and tar, only the reactions between the pyrolysis products and hydrogen will dominate in the hydrogen atmosphere. As is known, the pyrolysis process will generate gaseous products and tend to increase the reaction pressure while the char hydrogasification process will consume the gaseous species and tent to decrease the reaction pressure. Thus, based on the Le Chatelier’s principle and the mass action law [20], increasing pressure will restrain the hydropyrolysis process and promote the hydrogasification process on the whole. From Fig. 2(a) it can be seen that the mass of coal sample decreases with the reaction time increment and the TG curves have a reversed S-shape. This is due to the extraction of the volatile in coal and the hydrogasification of char. From Fig. 2(b) it can be seen that the peak position where the minimum value of the DTG curve is detected shifts rightwards obviously when the pyrolysis pressure increases from 0.1 MPa to 1 MPa. Then, with the increment of pressure when it is higher than 1 MPa, the peak position moves leftwards gradually. The peak value, namely the minimum value in the DTG curve, decreases greatly when the pyrolysis pressure increases from 0.1 MPa to 1 MPa. Then, it begins to increase gradually with the pressure increment when the pressure is higher than 1 MPa. It can also be seen that when the reaction pressure is lower than 1 MPa especially when it is close to 0.1 MPa, the hydropyrolysis
Fig. 2 e The thermogravimetric results.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 7 8 2 6 e7 8 3 3
Fig. 3 e Chemical structures of coal.
will dominate and the whole pyrolysis process will take place in a narrow temperature range quickly. When the reaction pressure is higher than 1 MPa, the volatile extraction will be restrained while the hydrogasification will be promoted. This is because only the extraction of gaseous matter from coal can contribute to the weight loss. As the pressure increases, the boiling point of the heavy oil will be increased and the pyrolysis process will be deferred. Moreover, due to the Le Chatelier’s principle and the mass action law, the decomposition of the large molecular to generate light gases can also be restrained by increasing the pyrolysis pressure. Inversely, increasing pyrolysis pressure is beneficial to hydrogasification. Thus, when the pressure is higher than 1 MPa, the weight loss rate can be promoted and the peak position will move leftwards. It can also be seen from Fig. 2(b) that a second peak will appear at high temperature. This is mainly due to the hydrogasification process [21]. From Table 2 it can be seen that when the pyrolysis pressure increases from 0.1 MPa to 1 MPa, Ts and rmax will decrease with the pressure increment while Tmax will increase. When the pyrolysis pressure increases from 1 MPa to 4 MPa, those three
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parameters will change inversely. This is because when the pyrolysis pressure is not high, increasing pressure can promote the combination of the heteroatoms (O, N, S) and small free radicals with H2 [22]. Thus, more gaseous species will be generated at the beginning and Ts will be lowered. But, when the pyrolysis pressure is high, H2 will not obviously promote the extraction of the gaseous volatile since almost all the heteroatoms and small free radicals have already been saturated. On the contrary, the coal lattice [23] which is like the solvent of the volatile can hold more volatile at high pressure. Thus, Ts will increase. The explanation for the change of Tmax with the pyrolysis pressure is the same with that for the change of the peak position as depicted in Fig. 2(b). The explanation for the change of rmax with the pyrolysis pressure is the same with that for the change of the peak value. It can also be seen from Table 2 that I decreases with the pyrolysis pressure when it is within 2 MPa. Then, when the pressure is higher, I will increase with the pressure increment. This is mainly due to the restricting effect of pressure on the hydropyrolysis process and the promotion effect of pressure on the hydrogasification. From Fig. 2 and Table 2, it can be seen that 1 MPa seems to be a critical pressure for the hydropyrolysis process.
Kinetic analysis General theory for the integral method The mechanism function, the activation energy and the preexponent factor are the so called kinetic triplet which can exactly reflect the kinetic process. To quantitatively investigate the kinetic characteristics of the CHP process, the kinetic mechanism function for the CHP processes is firstly defined. Then, the activation energies and the pre-exponent factors for the CHP processes at different pressures are calculated. The kinetic equation for the CHP can be written as Eq. (2) [24,25]. da ¼ Kf ðaÞ ¼ AeE=RT f ðaÞ dt
(2)
where, a denotes the conversion rate and can be calculated with Eq. (3); t is the pyrolysis time; K is the reaction rate constant; A is the pre-exponent factor; E is the activation energy; R is the universal gas constant; T is the pyrolysis temperature; f(a) is the kinetic mechanism function. a¼
w0 w w0 wN
(3)
where, w0 is the initial mass; w is the mass of coal sample at any time during the pyrolysis process; wN is the mass left after the pyrolysis. Because only ash will be left if the hydropyrolysis time is long enough, wN is considered to be equal to the ash mass. The non-isothermal thermogravimetric method is used and the temperature ramping rate, b, can be expressed with Eq. (4). b¼
dT dt
(4)
The correlation of the integral form of f(a) can be written as Eq. (5). Za Fig. 4 e The general coal pyrolysis and gasification processes.
GðaÞ ¼ 0
da f ðaÞ
(5)
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Based on Eqs. (2), (4) and (5), G(a) can be calculated with Eq. (6). GðaÞ ¼
AE pðuÞ bR
(6)
where, u denotes E/RT and p(u) can be calculated using the integration by parts integral method. Zu pðuÞ ¼ N
eu eu 2! 3! 4! þ du ¼ þ / 1 u u2 u3 u2 u2
(7)
In the common range of the reaction temperature, u is much higher than unit. Thus, only the first two terms in the brackets on the right side of Eq. (7) are kept to do the simplification. Then, Eq. (6) can be cast into Eq. (8). ART2 2RT E=RT e 1 GðaÞ ¼ E bE
(8)
(9)
Determination of the kinetic mechanism function From Eq. (9) it can be seen that for the proper kinetic mechanism function f(a), its integral form G(a) will perform a linear relationship with the reciprocal of temperature. But, in fact it is found that many mechanism functions can meet the linear relationship due to the compensatory effect of E and A [26]. For different mechanism functions, different Es and As can be obtained. But which pair of E and A is the right one still needs determination. Many methods have been developed by researchers to help to determine the most probable mechanism function up to now [26]. In this work, the integral-differential method (IDM), namely the Bagchi method [27], is chosen and some improvement for this method is also implemented [28]. Using the variable separation method for Eq. (2) and taking log for both sides, Eq. (10) can then be obtained and this is the so called AchareBrindleyeSharpeWendworth method [29,30]. da E A ¼ þ ln ln f ðaÞdT RT b
(10)
The concept of IDM is that both E and A calculated from the integral method and the differential method, respectively, should be very close when the mechanism function is properly selected and determined. Some researchers do calculated E and A using the two method and then do the comparison [28]. But this way is cumbersome, especially when the curve obtained from Eq. (9) or Eq. (10) is piecewise linearity. In this work, a relatively simple and effective method is developed based on the IDM. From Eqs. (9) and (10), it can be seen that both the equations have the same slope form, E/RT. Thus, if E values calculated from the two equations are the same, the curves generated from the two equations should be parallel to each other. In addition, using h1 to represent ln½GðaÞ=T2 and h2 to represent ln½da=f ðaÞdT, if A calculated from the two equations are the same, make difference of them and Eq. (11) can then be obtained.
(11)
Eq. (11) indicates that the difference between the two curves obtained by both the integral method and differential method should be ln(R/E) and E can be calculated only using the integral method. In this work, 41 common mechanism functions reported in literature [26] are investigated and finally the random nucleation and later growth mechanism function (RNLGM, n ¼ 4) developed by Avrami [31e33] is found to have the best performance. Thus, RNLGM with n ¼ 4 is chosen as the kinetic mechanism function for the CHP process. The mechanism function and the corresponding integral form are written as Eqs. (12) and (13). f ðaÞ ¼
Because u is much higher than unit, the value in the bracket on the right side of Eq. (8) is about 1. Taking log of both sides and Eq. (8) can be transformed into Eq. (9). GðaÞ E AR þ ln ¼ ln 2 T RT bE
AR A R h1 h2 ¼ ln ln ¼ ln bE b E
1 ð1nÞ ð1 aÞ½ lnð1 aÞ n
GðaÞ ¼ ½ lnð1 aÞ
n
(12) (13)
Calculation results When the pyrolysis process occurs not in the inert atmosphere, both the pyrolysis and gasification processes will take place. Some researchers used the temperature to distinguish the pyrolysis process and the gasification process. But for different pyrolysis conditions, the dividing temperature should be different and the temperature is not precisely known [34]. In this work, 5%e95% weight loss of the volatile contained in the coal sample is considered to be the major hydropyrolysis process, and the kinetic parameters in this range are calculated. This is because during the whole reaction process, CHP rate is usually much higher than that of CHG and it is usually considered that the char gasification process follows the pyrolysis process. The plots of h1 ðln½GðaÞ=T2 Þ vs. 1/ T and the corresponding fitted curves are shown in Fig. 5(a1)e(e1). The corresponding fitted equations are also listed. The comparisons of the curves obtained from the integral method and the differential method are shown in Fig. 5(a2)e(e2). The calculated kinetic results are listed in Table 3. Dave in Table 3 is the average of h1 h2 determined by Eq. (11) in the corresponding temperature range. From Fig. 5(a2)e(e2), it can be seen that the curve obtained from the integral method basically parallels with that obtained from the differential method, which indicates that the activation energy calculated from the two methods are almost the same. By comparison of ln(R/E) and Dave listed in Table 3, very good agreement is found for these two values, indicating that the pre-exponent factor calculated from the two methods should be close to each other. In addition, all the correlation coefficient, Ra shown in Table 3, are all greater than 0.98, indicating that the data generated with RNLGM has good linearity. Thus, the kinetic mechanism function chosen in this work is believed justified and the kinetic parameters calculated based on this mechanism are reliable. It can also be seen from Table 3 that the temperature range of the hydropyrolysis process shrinks with the increase of the pyrolysis pressure. This is because increasing pressure can promote the combination of H2 and the free radicals generated during the heating process. When the pressure is higher than 1 MPa, the initial extraction
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Fig. 5 e Linear fitting and methods comparison.
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Table 3 e Kinetic parameters for the CHP process under different pressures. Pressure (MPa) 0.1
1 2 3 4
Temperature range ( C)
E (kJ/mol)
165e314 314e476 476e902 143e781 213e765 306e772 322e690
44.291 148.75 34.59 66.30 90.96 139.90 152.23
temperature increases and the hydropyrolysis temperature range becomes compact.
Conclusion To investigate the hydropyrolysis kinetic characteristics of a lignite coal under different pyrolysis pressures, a series of experiments are implemented in the P-TGA. The TG and DTG curves are drawn and compared. In order to find out the proper kinetic mechanism function, the IDM is used and an advanced method based on this method is proposed. Finally, using the RNLGM kinetic mechanism function when n ¼ 4, the kinetic parameters of the CHP are calculated. The meaningful conclusions obtained from this work can be drawn as follows: 1) When the pyrolysis pressure is lower than 1 MPa, Ts will decrease with the increment of the pressure. When the pressure is higher than 1 MPa, Ts will then increases with the increment of the pyrolysis pressure. When the pyrolysis pressure is lower than 1 MPa, Tmax will shift rightwards with the increment of the pressure. When the pyrolysis pressure is higher than 1 MPa, Tmax will then decrease gradually with the pressure increment. 2) Although increasing pressure is beneficial to CHG, it is not for CHP. When the reaction pressure is increased from 0.1 MPa to 2 MPa, PDI decreases with the pyrolysis pressure and the whole reaction process of the lignite coal will be restrained. Then, when the reaction pressure is higher than 2 MPa, PDI will increase and the reaction process will be promoted with the pressure increment. 3) With the increment of pyrolysis pressure, the hydropyrolysis temperature ranges shrinks gradually. 4) The RNLGM with n ¼ 4 can reflect the hydropyrolysis process very well. The kinetic process for the CHP under 0.1 MPa includes three steps and those for 1 MPae4 MPa are all one step.
Acknowledgments The authors gratefully acknowledge financial supports from the National Natural Science Foundation of China (NSFC, 50876008, 51176009) for this work.
A (min1)
Ra
ln(R/E)
Dave
102 108 101 101 102 105 107
0.993 0.996 0.988 0.996 0.987 0.990 0.999
8.58 9.79 8.33 8.98 9.30 9.73 9.82
8.71 9.76 8.69 9.24 9.46 9.77 9.90
3.02 1.66 4.6 2.97 6.56 3.52 2.03
references
[1] International energy outlook 2010. Washington, DC: U.S. Energy Information Administration; 2011 Sep.. Contract no.: DOE/EIA-0484(2011), http://me.queensu.ca/Courses/430/ InternationalEnergyOutlook-summaryonly.pdf. [2] Franco A, Diaz AR. The future challenges for “clean coal technologies”: joining efficiency increase and pollutant emission control. Energy 2009;34:348e54. [3] Ziock HJ, Lackner KS, Harrison DP. Zero emission coal power, a new concept. Los Alamos, New Mexico: Los Alamos National Laboratory. Contract no.: LA-UR-01e2214 http:// www.netl.doe.gov/publications/proceedings/01/carbon_seq/ 2b2.pdf. [4] Yan LB, He BS, Ma LL, Pei XH, Wang CJ, Li XS. Integrated characteristics and performance of zero emission coal system. Int J Hydrogen Energy 2012;37:9669e76. [5] Yan LB, He BS, Pei XH, Li XS, Wang CJ. Energy and exergy analyses of a zero emission coal system. Energy 2013;55:1094e103. [6] Sulimma A, Leonhardt P, Van Heek KH, Ju¨ntgen H. Thermogravimetric study on catalytic hydropyrolysis of coal. Fuel 1986;65:1457e61. [7] Tang LH, Zhu ZB, Gu HX, Zhang CF. The effect of coal rank on flash hydropyrolysis of Chinese coal. Fuel Process Technol 1999;60:195e202. [8] Xu WC, Kumagai M. Nitrogen evolution during rapid hydropyrolysis of coal. Fuel 2002;81:2325e34. [9] Xu WC, Kumagai M. Sulfur transformation during rapid hydropyrolysis of coal under high pressure by using a continuous free fall pyrolyzer. Fuel 2003;82:245e54. [10] Zhou LM, Jia YY, Nguyen TH, Adesina AA, Liu ZR. Hydropyrolysis characteristics and kinetics of potassiumimpregnated pine wood. Fuel Process Technol 2013;116:149e57. [11] Li BQ, Danheux CB, Cypre`s R. Catalytic hydropyrolysis by impregnated sulphided Mo catalyst. Fuel 1991;70:254e8. [12] Li W, Wang N, Li BQ. Product analysis of catalytic multi-stage hydropyrolysis of coal. J China Univ Min Technol 2002;31:246e51 [in Chinese]. [13] Sun QL, Li W, Chen HK, Li BQ. Compositions of coal tar from pyrolysis and hydropyrolysis of Shenmu coal macerals. J Fuel Chem Technol 2005;3:412e5 [in Chinese]. [14] Xu WC, Matsuoka K, Akiho H, Kumagai M, Tomita A. High pressure hydropyrolysis of coals by using a continuous freefall reactor. Fuel 2003;82:677e85. [15] Zhu L. Hydrogasification characteristics of coals in TGA [Master thesis]. Beijing Jiaotong University; 2010 [in Chinese]. [16] Xiao J, Shen LH, Wang ZM, Zhong XL. Pressurized thermogravimetric analysis of pyrolysis of biomass. J Combust Sci Technol 2005;11:415e20 [in Chinese]. [17] Wang XH, Ju FD, Yang HP, Xu J, Zhang SH, Chen HP. Kinetics and properties analysis of Shenfu coal pressurized pyrolysis. Proc CSEE 2011;31:40e4 [in Chinese].
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 9 ( 2 0 1 4 ) 7 8 2 6 e7 8 3 3
[18] Fletcher TH, Kerstein AR, Pugmire RJ, Grant DM. Chemical percolation model for devolatilization: 2. Temperature and heating rate effects on product yields. Energy Fuels 1990;4(1):54e60. [19] Yan LB, He BS, Pei XH, Li XS, Wang CJ, Liang HX. Kinetic model and prediction for coal hydrogasification. Int J Hydrogen Energy 2013;38:4513e23. [20] George WR. Chemical reactions and chemical reactors. Hoboken, NJ: John Wiley and Sons, Inc.; 2009. [21] Sun QL, Li W, Chen HK, Li BQ. Investigation of hydropyrolysis behavior of Shenmu macerals by TG-MS. J Fuel Chem Technol 2004;32:647e51 [in Chinese]. [22] Ye CP. Characterization and modeling of coal high molecular weight components [Ph.D. dissertation]. Taiyuan University of Technology; 2008 [in Chinese]. [23] He XM. Coal chemistry. Beijing: Metallurgical Industry Press; 2010 [in Chinese]. [24] Sbirrazzuoli N. Determination of pre-exponential factors and of the mathematical functions f(a) or G(a) that describe the reaction mechanism in a model-free way. Thermochim Acta 2013;564:59e69. ˛ zko ska M, Plis A, Scia _ [25] Łabojko G, Kotyczka-Moran M. Kinetic study of polish hard coal and its char gasification using carbon dioxide. Thermochim Acta 2012;549:158e65.
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[26] Hu RZ, Shi QZ. Thermal analysis kinetics. Beijing: Sciences Press; 2001 [in Chinese]. [27] Bagchi TP, Sen PK. Combined differential and integral method for analysis of non-isothermal kinetic data. Thermochim Acta 1981;51:175e89. [28] Zhang CQ, Wei LH, Ren GP, Jiang XM. Research on pyrolysis characteristics and mechanism of micro-pulverized and common-pulverized coal. J Harbin Inst Technol 2006;38:1948e51 [in Chinese]. [29] Achar BNN, Bridley GW, Sharp JH. Thermal decomposition kinetics of some new unsaturated polyesters. In: Proc Int Clay Conf, 1; 1966. p. 67. [30] Sharp JH, Wentworth SA. Kinetic analysis of thermogravimetric data. Anal Chem 1969;41:2060e2. [31] Avrami M. Kinetics of phase change. I General theory. J Chem Phys 1939;7:1103e12. [32] Avrami M. Kinetics of phase change. II Transformation-time relations for random distribution of nuclei. J Chem Phys 1940;8:212e24. [33] Avrami M. Granulation, phase change, and microstructure kinetics of phase change. III. J Chem Phys 1941;9:177e84. [34] Xiao J, Shen LH, Deng X, Wang ZM, Zhong X. Thermogravimetric study on catalytic pyrolysis gasification of biomass. Acta Energy Solaris Sin 2009;30:1252e7 [in Chinese].