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Fast pyrolysis comparison of coal–water slurry with its parent coal in Curie-point pyrolyser
Energy Conversion and Management 50 (2009) 1976–1980
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Fast pyrolysis comparison of coal–water slurry with its parent coal in Curie-point pyrolyser Hui Wang a, Xiumin Jiang a,*, Hui Liu b, Shaohua Wu b a b
Institute of Thermal Energy Eng., School of Mechanical Engineering, Shanghai Jiao Tong Univ., Shanghai 200240, China Institute of Combustion Eng., School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 10 December 2007 Received in revised form 29 August 2008 Accepted 6 April 2009 Available online 12 May 2009 Key words: Coal–water slurry Curie-point pyrolyser Pyrolysis
a b s t r a c t Curie-point pyrolyser is an instrument that can be used to analyze powder or slurry samples at medium heating rates. It can keep a constant heating rate to heat up the samples until the wire temperature reaches Curie-point temperature and remains the same temperature. In this paper, coal–water slurry (CWS) and its parent coal were studied by using a Curie-point pyrolyser. Kinetic parameters of pyrolysis were calculated and apparent activation energy of CWS obtained is 16.362 kJ/mol, which is a little higher than that of its parent coal, 12.691 kJ/mol. The experimental curves show lean S shape and the heating rate are obtained, which are 617 K/s and 834 K/s, respectively. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Heating rate is one of the important parameters which affects the devolatilization process including the release and composition of volatiles [1]. It has a clear effect on coal pyrolysis and evolution of volatile products, shifting the maximum rate of gas evolution to higher temperatures and enhancing the plastic properties of the coals. Generally, the process is divided into four groups based on the heating rate [2]: low, medium, high and flash. The low heating rate is of the magnitude order of 101 K/s, the medium is of 102– 103 K/s, the high is of 104–105 K/s and the flash is of up to 106 K/ s. Due to mass/energy transfer effects, activation energy varies with heating rate [3], too. Niksa and Lau [4] found that when heating rate increases one magnitude order, the pyrolysis rate will increase five times, and apparent activation energy change little. In a paper on the swelling and porosity of bituminous coals [5], the author found that the heating rate is approximately 5 103 K/s for swelling coals at which the transition from increasing swelling to decreasing swelling occurs. And near this heating rate swelling coals also reach a maximum porosity. Among the apparatus realizing various heating rates, each has its advantages and disadvantages [2,6]. TGA. is a typical low heating rate apparatus which can record the temperature and weight loss history precisely and the temperature measurement is relatively accurate, but its heating rate is far from the intense combustion condition in applications and the extrapolation range of the results is limited. The grid heater and Curie-point pyrolyser repre* Corresponding author. Tel./fax: +86 21 34205681. E-mail address: [email protected] (X. Jiang). 0196-8904/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2009.04.012
sent the medium heating rate case. The grid heater can obtain a relatively high terminal temperature and minimize the secondary reaction, but the temperature history cannot be measured precisely. The Curie-point can accurately control the heating time and avoid secondary reactions and measure sample temperature by contacting directly, but the sample mass requirement is so low that the mass loss error will be significant. Some entrained reactors can achieve a high heating rate, for example drop tube furnace can heat up the samples at 104 s1, but the secondary reaction happens easily and the sample temperature history is difficult to measure, either. Another example is heated tube reactor which can record the sample temperature and residence time precisely at high heating rate, but the coal type is restricted because caking coal will stick to the tube walls. Radiative heating and shock tube represent flash heating rate. The advantages of the radiative heating are very high heating rate and little secondary reaction, and the time and energy can be controlled independently, but the tar and char cannot be separated thoroughly with disadvantages of large thermal gradients in the particles and possible soot production. In spite of the disadvantages of the Curie-point pyrolyser, it is a widely used apparatus in pyrolysis analysis and often used to analyze powder or slurry samples at medium heating rate. Wiktorsson and Wanzl [7] used it in coal pyrolysis and compared the results with that of thermogravimetric analysis and grid heater. And the original kinetic parameters for Curie-point pyrolyser were obtained by applying an isothermal expression of the model. Marriet et al. [8] investigated a high-volatile bituminous (upper carboniferous) coal and its constituting maceral fractions to study the relationships between the chemical structures of coals, coal macerals, and their precursors (plant tissues) using Curie-point
H. Wang et al. / Energy Conversion and Management 50 (2009) 1976–1980
1977
Nomenclature V t V1 K A E R T b
the mass fraction of volatiles released up to time t (%) time (s) value of V as t ? 1 rate constant pre-exponential factor (s1) apparent activation energy (J mol1) ideal gas constant (J K1 mol1) absolute temperature (K) heating rate
pyrolysis–gas chromatography and Curie-point pyrolysis-gas chromatography-mass spectrometry. Gao [9] used Curie-point pyrolysis-gas chromatography to analyze five coal samples and obtained their pyrolysis parameters. Being a new-developed coal-based fuel, CWS is made of about 65–70% coal, 30–35% water and 1–2% additive. It has qualities just like oil and can be burned directly after sprayed. It is a kind of perfect fuel which can take the place of oil in many situations but has different characters from coal for its water content. Zhou et al. [10] studied pyrolysis of coal black liquor slurry using thermogravimetric analysis at low heating rate, which is far from industrial conditions. Curie-point pyrolyser offers an option for its medium heating rate but no one used it to analyze CWS to study its pyrolysis process and characteristics. In this paper, the CWS fuel and its parent coal were processed in a Curie-point pyrolyser to do a comparison analysis to reveal the effect of water in CWS pyrolysis, which will be valuable for studying CWS combustion, too. 2. Experimental 2.1. Apparatus One of the emphases of coal combustion and gasification is its pyrolysis process at medium or high heating rate. Curie-point pyrolyser is an attractive apparatus because nearly a heating rate of industrial units can be obtained. The principle [2,9] is based on the magnetic effect of pyrolysis wire which is fixed in a electromagnetic coil powered by high frequency current. See Figs. 1 and 2, which show the system diagram of Curie-point pyrolyser experiment and the installation detail of the pyrolysis wire in a XP-12 Curie-point pyrolyser used in this experiment which is produced by Shanghai Analysis Instrument Factory. The high frequency magnetic field induced by the electromagnetic coil will make the wire generate an alternating magnetic flux and be heated at medium heating rate. When the wire temperature climbed up to its Curiepoint temperature, its magnetism will change from ferromagne-
Pyrolysis wire stick with sample Quartz glass tube Flow rator Exit
Electromagnetic coil Nitrogen cylinder
High frequency oscillator
Fig. 1. System diagram of Curie-point pyrolysis experiment.
a Vm t0 T0
E/R substitute of V1 in range of [300 K, 1500 K] t + T0/b ambient temperature, 303 K
Subscripts 0 initial 1 terminal
tism to paramagnetism. As a result, the induced flux and hysteresis effect will be lost, so the wire cannot be heated any longer. If wire temperature falls, its magnetism will change to ferromagnetism and the wire gets heated again. This character of pyrolysis wire makes itself a constant temperature holder without any aid of temperature controlling. The other character is the heating rate can also be kept constant before Curie-point temperature is reached. And if any change of terminal temperature is wanted, only the change of component of pyrolysis wire is enough, which makes its Curie-point changed and data analysis very convenient. 2.2. Samples and procedures Samples used in the study are CWS obtained from Shengli CWS Factory and its parent coal (Shanxi Datong bituminous coal). The ultimate and proximate analysis of CWS is shown in Table 1. Size distribution of coal and CWS are shown in Fig. 3, which were same and measured using wet method by a SUCELL CL laser granulometry analyzer produced by SYMPA. The Curie-point temperature of the pyrolysis wire is 770 °C. The procedures of the experiments are as follows: Step 1: See Fig. 2, rotate the six and nine (nut caps) to open seal bolts and drag out the pyrolysis wire carefully. Measure the weight of the pyrolysis wire with eight (silicon rubber) and nine (nut cap). Step 2: Coat the samples on surface of the pyrolysis wire. For CWS, the sample as received is coated and pyrolysed directly. For the parent coal, the coal powder as received should be mixed with a little alcohol and coat the mixture on the surface of the wire, then dry the pyrolysis wire stick with samples and eight and nine in a drying oven at 105–110 °C for about 10 min until the weight is constant. The coal and CWS samples are coated on the pyrolysis wire uniformly and kept a thickness of 1–2 times of coal powder diameter and 10–15 mm long. Mass of samples in each experiment is about 500–600 lg (precision 0.01 mg). Step 3: Measure the weight of the pyrolysis wire stick with samples and eight and nine. The weight of the samples can be calculated. Step 4: Place the pyrolysis wire stick with samples and eight and nine into the quartz glass tube with extra care, then stabilize the pyrolysis wire by pinching the end of nut cap side and rotate the bolts to seal the tube end. Make sure the coated length of the wire to be placed in the middle of electromagnetic coil of Curie-point pyrolyser. Step 5: Set the pyrolysis time by regulating the high frequency oscillator in Fig. 1. After switching on nitrogen as carrier gas, turn on the power of the oscillator immediately. The purity of inlet Nitrogen is 99.999% and its flow flux is 50 ml/min. It goes through quartz glass tube continu-
1978
H. Wang et al. / Energy Conversion and Management 50 (2009) 1976–1980
Carrier gas inlet 10
1
2
3
4
5
6
7
8
9
1 Injector needle 2 Metal shield 3 Induction coil 4 Pyrolysis wire 5 Annular seal ring 6,9Nut cap 7 T-branch pipe 8 Si licon rubber 10 Quartz glass tube Fig. 2. Installation of pyrolysis wire in a XP-12 Curie-point pyrolyser.
Table 1 Ultimate and proximate analysis of CWS sample. Proximate analysis
Ultimate analysis
Mar (%)
Aar (%)
Var (%)
Qar,net (kJ/kg)
Car (%)
Har (%)
Oar (%)
Nar (%)
Sar (%)
32.9
5.64
32.27
18877
50.57
3.27
6.13
0.93
0.56
100 90
Cumulative distribution Density distribution
35
70
Total volatile released /%
Mass percentage /%
80
60 50 40 30 20 10 0
30
coal CWS
25 20 15 10 5 0
1
10
100
Particle diameter /µm Fig. 3. Size distribution of coal and CWS.
ously and quickly so that it can carry out the released volatile gas in time, in order to avoid second reaction between volatile gas with residual carbon. The pyrolysis will complete in the set time and stop by itself. Step 6: After the volatile matters are all swept out, then turn off the power. Keep the pyrolysis wire stable and rotate the two sealed bolts open and avoid rotating of the wire. Then drag out the wire and weigh the pyrolysed wire with eight and nine. Then the weight of the samples after pyrolysis can be calculated. Step 7: Use a fine emery paper to clean the pyrolysis wire, then coat it again and repeat the experiment for three times. The average weight of the samples and samples after pyrolysis will be used in figures and model calculation to minimize the experimental error. The mass loss ratios of the samples are calculated to make curves against reaction time, which are shown in Fig. 4.
0.1
1
10
Pyrolysis time /s Fig. 4. Curve of volatile released vs. pyrolysis time.
Step 8: Regulate the pyrolysis time and repeat the steps 1–7 to complete all the experiments of 0.20, 0.33, 0.5, 0.85, 1, 1.45, 2, 5 and 7.5 s. In each experiment the fresh sample is used and weighed by a TG332A scale with the precision of 0.01 mg produced by Shanghai Scale Instrument Factory. In the experiment, the scale is a key instrument to measure raw mass data of samples. To eliminate the error from scale accuracy and uniform degree of sample layer, the measurement and operation were done carefully by the same operator. In Fig. 2, the needle of an injector (1) is equipped to connect with some other gas analysis instruments. In this experiment, no further gas analysis is done, so the gas generated goes through the needle into the air directly.
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H. Wang et al. / Energy Conversion and Management 50 (2009) 1976–1980
1 1 For T e [300 K, 1500 K], x 2 ½1500 ; 300 : In the coordinates of abscissa axis is x and vertical axis is y = ln x, the equation of line de1 1 ; ln 1500Þ and point ð300 ; ln 300Þ is written as: fined by point ð1500
3. Results and discussion 3.1. Experimental results From Fig. 4, two lean S shape curves are observed. The same shape and tendency of the curve were obtained in other studies of coal pyrolysis in Curie-point pyrolyser [2,12]. For the coal sample, the curve climbs throughout the process and the slope coefficient increases a little in early stages, turns fast in the middle, then slows down in the end. For the CWS, a flatform lasts about 0.5 s is observed in early stages, then a steeper slope than that of coal in middle stages and a flatform again in the end. The evaporation of more water content in CWS delays releasing of volatiles and shows a flatform when the heat provided is not enough to make it start. But for the catalytic effect of water, pyrolysis of volatiles in CWS which completes before 7.5 s proceeds more quickly and thoroughly. For the instrument characteristic of Curie-point pyrolyser, volatile in samples goes through pyrolysis after water content does, and it mainly happens in the middle stages. Each time the temperature will stop at 770 °C exactly and keep constant. Then samples finish their pyrolysis reaction. In short, the early stages of CWS pyrolysis is longer than that of coal, but its slope in the middle stages is steeper, which shows the pyrolysis rate of CWS is faster. And the total volatile yield of CWS is a little higher. In the experiments, no matter how carful the operator is handling, the touch of the samples with the inner surface of the quartz glass tube is sometimes inevitable. And the sample mass is so limited, suggesting a slight mass loss may affect the results relatively significant, which is possibly the reason making a few data in Fig. 4 scattered. The authors improved the data by repeating the experiments several more times. 3.2. Kinetic model Mechanisms of many thermo-analytical processes are unknown or too complicated to be characterized by a simple kinetic model. To describe their kinetics, the methods based on a single-step approximation are often used [2,11]. A pyrolysis model [2,8,13] of first order reaction is selected to analyze pyrolysis kinetic parameters of the coal and CWS samples, and the rate of pyrolysis is expressed as:
ð1Þ
where V is the mass fraction of volatiles released up to time t and V1 is the value of V as t ? 1. Both of them are expressed as mass percentage of initial sample. The rate constant K typically has the Arrhenius form
K ¼ A expðE=RTÞ
ð3Þ
For the reason that the integral of right section of Eq. (3) cannot be solved directly and only the temperature range of 300–1500 K is of interest, exp(E/RT) can be approximated as mTn with enough accuracy. Defining 1/T = x and E/R = a, the function exp(E/ RT) = mTn is now written in the form:
a 1 ln x ¼ x þ ln m n n
y ¼ 603:539x 7:403
ð6Þ
The line between line (5) and line (6) is selected as the approximation line of y = ln x, and it is given as below:
y ¼ 603:539x 7:559
ð7Þ
Compare Eq. (4) with Eq. (7) and it gives:
a ln m ¼ 603:539; ¼ 7:559 n n E ln m ¼ 603:539; ¼ 7:559 i:e: : Rn n
ð8Þ
The integral is solved by using the power function mTn as substitute for exp (E/RT):
V n ln 1 ¼ Amb t 0ðnþ1Þ =ðn þ 1Þ Vm
ð9Þ
Taking logarithms gives
n V Ab ln ln 1 þ ðn þ 1Þ ln t ¼ ln Vm nþ1
ð10Þ
where t0 = t + T0/b is total pyrolysis time deduced from pyrolysis starting temperature to absolute zero, T0 is ambient temperature, 303 K. The experimental data is drawn in logarithmic coordinate paper. From Eq. (10), apparent activation energy E of the two samples can be obtained from intercept of the fit line, and with slope coefficient, the pre-component can be obtained too, see Table 2. Because the heating rate b keeps constant, the variables ln (1V/V1) and (t + T0/b) fit a linear rule in logarithmic coordinates. From Fig. 5, before the critical point, the experiment data Table 2 Kinetic parameters of coal and CWS. Sample
Pre-exponent, A (106 s1)
Apparent activation energy, E (kJ/mol)
Correlation coefficient, R
Coal CWS
62.5 9.4
12.691 16.362
0.943 0.999
Coal
CWS
ð2Þ
where A is the pre-exponential factor, E is apparent activation energy, R is the ideal gas constant and T is the absolute temperature. Under the condition of this experiment, pyrolysis temperature ramps linearly at a constant rate of b with dT = bdt in the investigation range. Then Eqs. (1) and (2) can be rewritten as:
1 A dV ¼ expðE=RTÞdT ðV 1 VÞ b
ð5Þ
and the parallel line tangential to y = ln x is:
1
-ln(1-V/ V )
dV ¼ KðV 1 VÞ dt
y ¼ 603:539x 7:716
0.1
1.25 0.01
0.1
1
1.69 0.1
1
t+303/b/s
ð4Þ
Fig. 5. Determination for heating rate of coal and CWS.
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H. Wang et al. / Energy Conversion and Management 50 (2009) 1976–1980
Table 3 Calculation results of coal and CWS.
4. Conclusions
Sample
Critical time (s)
Heating rate (K/s)
Coal CWS
1.25 1.69
834 617
in each curve agrees to form a line approximately. At the critical point, i.e. the Curie-point temperature of the pyrolysis wire, the sample temperature transforms from climbing linearly to keeping constant. And the heating rate b, can be determined too. See Table 3. 3.3. Kinetic analysis From Table 2, the apparent activation energy of CWS is higher than that of its parent coal because of large water content in CWS. In the experiment, volatiles will not release until the water content in CWS finishes its evaporation, so more heat is needed to complete the pyrolysis of CWS, resulting in apparent activation energy becomes higher. For Curie-point pyrolyser, the heat is directly relative to the time. So CWS absorbs more heat than coal, which can be seen in Fig. 4, the platform in early stages of CWS pyrolysis is longer than that of coal. After that the slope coefficient of CWS gets higher, which shows the pyrolysis rate of CWS after evaporation is higher. As an explanation, when the heating rate becomes higher, the heat transfer in sample speeds up, resulting in gas phase releasing easily and the internal pressure higher, association chance and second reaction decreasing. For CWS, the result of its more water content evaporating heavily under the medium heating rate is to form more porosity than coal, which benefits for heat of pyrolysis wire entering and volatile releasing. In addition, with the improvement of internal heat transfer condition of coal powder in CWS, it will subject to more heat collision on bigger surface and lead to breakage of some side chain and functional group which is not easy to break under low heating rate, then increase the volatile yield of CWS. Comparing with the result from the same CWS sample in the TGA. [14], the apparent activation energies are of the same magnitude and the value from the Curie-point pyrolyser is between the heating rate of 20 K/s and 30 K/ s (10.83 kJ/mol and 21.59 kJ/mol), but the pre-component is about 10 times bigger than that of TGA. (0.005 s1 and 0.089 s1 for 20 K/ s and 30 K/s). The results are consistent to the study of Niksa [4] which indicates the fast pyrolysis of the samples increased the reaction rate significantly. In this experiment, the temperature of pyrolysis wire will keep constant at 770 °C in the end. So the shorter the pyrolysis time, the faster the heating rate. From Table 3, it takes more time for CWS to get to the Critical time, i.e. pyrolysis wire with CWS sample needs more time to get to its Curie-point temperature because a big part of the heat produced by the wire in the high frequency oscillator was used to evaporate water content in CWS, the heat left was used to pyrolyse coal powder in CWS and heat up the wire. But for the coal sample, it takes less time to get to the Curie-point temperature without the heat loss of evaporation, so its heating rate is higher.
The apparent activation energy of CWS pyrolysis in Curie-point pyrolyser is higher than that of its parent coal because a big part of heat is consumed by evaporation of water content in CWS. So in industrial application of CWS, the influence of water content on its ignition in early stages must be carefully considered. For the coal sample, it took 1.25 s to get to the Curie-point temperature, shorter than that of CWS, 1.69 s. Correspondingly, the heating rate are 834 K/s and 617 K/s and both are medium heating rates, see Table 3. Because of the influence of water content, it shows an obvious delay for the pyrolysis of volatile in CWS. The time for CWS gets to 770 °C was prolonged and heating rate was slowed down, too. But the pyrolysis rate of CWS, after the heat reservation stages, gets faster than that of coal, and a more volatile yield is obtained. In the experiment, the apparent activation energy is 16.362 kJ/ mol, which is a little higher than that of Datong coals. When the water evaporates, it will form a steam shelter of char. When the sweeping gas is not fast enough to carry it out, the steam shelter will hinder hot gas entering into its pores and volatiles releasing in time, which makes E value a little higher. In addition, Comparing with the smallest energy condition of deposition, the additive keeps coal powders dispersing increased internal energy of CWS. After the water evaporation ends, the char will change into more porous, which makes it can absorb so much heat to complete its pyrolysis, resulting in the pyrolysis rate of CWS higher than that of coals. References [1] Strezov V, Lucas JA, Evans TJ, Strezov L. Effect of heating rate on the thermal properties and devolatilisation of coal. J Therm Anal Calorim 2004;78(2):385–97. [2] Yu J. Study and Modelling on the Interaction of Volatile Flame, CO Flame and Char Particle Combustion. Dissertation of Shanghai Jiaotong University, Shanghai. 2003. [3] Sima-Ella E, Mays TJ. Analysis of the oxidation reactivity of carbonaceous materials using thermogravimetric analysis. J Therm Anal Calorim 2005;80(1):109–13. [4] Niksa S, Lau CH-W. Global rates of devolatilization for various coal types. Combust Flame 1993;94(3):293–307. [5] Thomas KG, Calvin HB, Thomas HF. Decreases in the swelling and porosity of bituminous coals during devolatilization at high heating rates. Combust Flame 1995;100(1–2):94–100. [6] Solomon PR, Serio MA, Suuberg EM. Coal pyrolysis. Experiments, kinetic rates and mechanisms. Prog Energy Combust Sci 1992;18(2):133–220. [7] Wiktorsson LP, Wanzl W. Kinetic parameters for coal pyrolysis at low and high heating rates – a comparison of data from different laboratory equipment. Fuel 2000;79(6):701–16. [8] Marrie Nip, de Leeuw Jan W, Crelling John C. Chemical structure of bituminous coal and its constituting mackerel fractions as revealed by flash pyrolysis. Energy Fuels 1992;6(2):125–36. [9] Gao KL, Zhang MC. Ignition temperature experiment and forecasting of coal. Therm Power Generat 1988;4:7–14. [10] Zhou JH, Kuang JP, Zhou ZJ, Lai KZ, Cen KF. Thermal analysis on combustibility of coal and coal black liquor slurry. J Fuel Chem Technol 2005;33(1):33–7. [11] Simon P. Considerations on the single-step kinetics approximation. J Therm Anal Calorim 2005;82(3):651–7. [12] Yu J, Zhang M. A simple method for predicting the rate constant of pulverizedcoal pyrolysis at higher heating rate. Energy Fuels 2003;17(4):1085–90. [13] Wu J, Zhang MC, Chen QF. Mathematical process on dynamic description of coal fast pyrolysis. Chin J Power Eng 2002;22(6):2093–5. 2066. [14] Wang Hui, Jiang Xiumin, Liu Jianguo, et al. Analysis of pyrolytic properties of coal–water slurry under various heating rates. J Power Eng 2007;27(2):263–6.
Contents lists available at ScienceDirect
Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman
Fast pyrolysis comparison of coal–water slurry with its parent coal in Curie-point pyrolyser Hui Wang a, Xiumin Jiang a,*, Hui Liu b, Shaohua Wu b a b
Institute of Thermal Energy Eng., School of Mechanical Engineering, Shanghai Jiao Tong Univ., Shanghai 200240, China Institute of Combustion Eng., School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 10 December 2007 Received in revised form 29 August 2008 Accepted 6 April 2009 Available online 12 May 2009 Key words: Coal–water slurry Curie-point pyrolyser Pyrolysis
a b s t r a c t Curie-point pyrolyser is an instrument that can be used to analyze powder or slurry samples at medium heating rates. It can keep a constant heating rate to heat up the samples until the wire temperature reaches Curie-point temperature and remains the same temperature. In this paper, coal–water slurry (CWS) and its parent coal were studied by using a Curie-point pyrolyser. Kinetic parameters of pyrolysis were calculated and apparent activation energy of CWS obtained is 16.362 kJ/mol, which is a little higher than that of its parent coal, 12.691 kJ/mol. The experimental curves show lean S shape and the heating rate are obtained, which are 617 K/s and 834 K/s, respectively. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Heating rate is one of the important parameters which affects the devolatilization process including the release and composition of volatiles [1]. It has a clear effect on coal pyrolysis and evolution of volatile products, shifting the maximum rate of gas evolution to higher temperatures and enhancing the plastic properties of the coals. Generally, the process is divided into four groups based on the heating rate [2]: low, medium, high and flash. The low heating rate is of the magnitude order of 101 K/s, the medium is of 102– 103 K/s, the high is of 104–105 K/s and the flash is of up to 106 K/ s. Due to mass/energy transfer effects, activation energy varies with heating rate [3], too. Niksa and Lau [4] found that when heating rate increases one magnitude order, the pyrolysis rate will increase five times, and apparent activation energy change little. In a paper on the swelling and porosity of bituminous coals [5], the author found that the heating rate is approximately 5 103 K/s for swelling coals at which the transition from increasing swelling to decreasing swelling occurs. And near this heating rate swelling coals also reach a maximum porosity. Among the apparatus realizing various heating rates, each has its advantages and disadvantages [2,6]. TGA. is a typical low heating rate apparatus which can record the temperature and weight loss history precisely and the temperature measurement is relatively accurate, but its heating rate is far from the intense combustion condition in applications and the extrapolation range of the results is limited. The grid heater and Curie-point pyrolyser repre* Corresponding author. Tel./fax: +86 21 34205681. E-mail address: [email protected] (X. Jiang). 0196-8904/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2009.04.012
sent the medium heating rate case. The grid heater can obtain a relatively high terminal temperature and minimize the secondary reaction, but the temperature history cannot be measured precisely. The Curie-point can accurately control the heating time and avoid secondary reactions and measure sample temperature by contacting directly, but the sample mass requirement is so low that the mass loss error will be significant. Some entrained reactors can achieve a high heating rate, for example drop tube furnace can heat up the samples at 104 s1, but the secondary reaction happens easily and the sample temperature history is difficult to measure, either. Another example is heated tube reactor which can record the sample temperature and residence time precisely at high heating rate, but the coal type is restricted because caking coal will stick to the tube walls. Radiative heating and shock tube represent flash heating rate. The advantages of the radiative heating are very high heating rate and little secondary reaction, and the time and energy can be controlled independently, but the tar and char cannot be separated thoroughly with disadvantages of large thermal gradients in the particles and possible soot production. In spite of the disadvantages of the Curie-point pyrolyser, it is a widely used apparatus in pyrolysis analysis and often used to analyze powder or slurry samples at medium heating rate. Wiktorsson and Wanzl [7] used it in coal pyrolysis and compared the results with that of thermogravimetric analysis and grid heater. And the original kinetic parameters for Curie-point pyrolyser were obtained by applying an isothermal expression of the model. Marriet et al. [8] investigated a high-volatile bituminous (upper carboniferous) coal and its constituting maceral fractions to study the relationships between the chemical structures of coals, coal macerals, and their precursors (plant tissues) using Curie-point
H. Wang et al. / Energy Conversion and Management 50 (2009) 1976–1980
1977
Nomenclature V t V1 K A E R T b
the mass fraction of volatiles released up to time t (%) time (s) value of V as t ? 1 rate constant pre-exponential factor (s1) apparent activation energy (J mol1) ideal gas constant (J K1 mol1) absolute temperature (K) heating rate
pyrolysis–gas chromatography and Curie-point pyrolysis-gas chromatography-mass spectrometry. Gao [9] used Curie-point pyrolysis-gas chromatography to analyze five coal samples and obtained their pyrolysis parameters. Being a new-developed coal-based fuel, CWS is made of about 65–70% coal, 30–35% water and 1–2% additive. It has qualities just like oil and can be burned directly after sprayed. It is a kind of perfect fuel which can take the place of oil in many situations but has different characters from coal for its water content. Zhou et al. [10] studied pyrolysis of coal black liquor slurry using thermogravimetric analysis at low heating rate, which is far from industrial conditions. Curie-point pyrolyser offers an option for its medium heating rate but no one used it to analyze CWS to study its pyrolysis process and characteristics. In this paper, the CWS fuel and its parent coal were processed in a Curie-point pyrolyser to do a comparison analysis to reveal the effect of water in CWS pyrolysis, which will be valuable for studying CWS combustion, too. 2. Experimental 2.1. Apparatus One of the emphases of coal combustion and gasification is its pyrolysis process at medium or high heating rate. Curie-point pyrolyser is an attractive apparatus because nearly a heating rate of industrial units can be obtained. The principle [2,9] is based on the magnetic effect of pyrolysis wire which is fixed in a electromagnetic coil powered by high frequency current. See Figs. 1 and 2, which show the system diagram of Curie-point pyrolyser experiment and the installation detail of the pyrolysis wire in a XP-12 Curie-point pyrolyser used in this experiment which is produced by Shanghai Analysis Instrument Factory. The high frequency magnetic field induced by the electromagnetic coil will make the wire generate an alternating magnetic flux and be heated at medium heating rate. When the wire temperature climbed up to its Curiepoint temperature, its magnetism will change from ferromagne-
Pyrolysis wire stick with sample Quartz glass tube Flow rator Exit
Electromagnetic coil Nitrogen cylinder
High frequency oscillator
Fig. 1. System diagram of Curie-point pyrolysis experiment.
a Vm t0 T0
E/R substitute of V1 in range of [300 K, 1500 K] t + T0/b ambient temperature, 303 K
Subscripts 0 initial 1 terminal
tism to paramagnetism. As a result, the induced flux and hysteresis effect will be lost, so the wire cannot be heated any longer. If wire temperature falls, its magnetism will change to ferromagnetism and the wire gets heated again. This character of pyrolysis wire makes itself a constant temperature holder without any aid of temperature controlling. The other character is the heating rate can also be kept constant before Curie-point temperature is reached. And if any change of terminal temperature is wanted, only the change of component of pyrolysis wire is enough, which makes its Curie-point changed and data analysis very convenient. 2.2. Samples and procedures Samples used in the study are CWS obtained from Shengli CWS Factory and its parent coal (Shanxi Datong bituminous coal). The ultimate and proximate analysis of CWS is shown in Table 1. Size distribution of coal and CWS are shown in Fig. 3, which were same and measured using wet method by a SUCELL CL laser granulometry analyzer produced by SYMPA. The Curie-point temperature of the pyrolysis wire is 770 °C. The procedures of the experiments are as follows: Step 1: See Fig. 2, rotate the six and nine (nut caps) to open seal bolts and drag out the pyrolysis wire carefully. Measure the weight of the pyrolysis wire with eight (silicon rubber) and nine (nut cap). Step 2: Coat the samples on surface of the pyrolysis wire. For CWS, the sample as received is coated and pyrolysed directly. For the parent coal, the coal powder as received should be mixed with a little alcohol and coat the mixture on the surface of the wire, then dry the pyrolysis wire stick with samples and eight and nine in a drying oven at 105–110 °C for about 10 min until the weight is constant. The coal and CWS samples are coated on the pyrolysis wire uniformly and kept a thickness of 1–2 times of coal powder diameter and 10–15 mm long. Mass of samples in each experiment is about 500–600 lg (precision 0.01 mg). Step 3: Measure the weight of the pyrolysis wire stick with samples and eight and nine. The weight of the samples can be calculated. Step 4: Place the pyrolysis wire stick with samples and eight and nine into the quartz glass tube with extra care, then stabilize the pyrolysis wire by pinching the end of nut cap side and rotate the bolts to seal the tube end. Make sure the coated length of the wire to be placed in the middle of electromagnetic coil of Curie-point pyrolyser. Step 5: Set the pyrolysis time by regulating the high frequency oscillator in Fig. 1. After switching on nitrogen as carrier gas, turn on the power of the oscillator immediately. The purity of inlet Nitrogen is 99.999% and its flow flux is 50 ml/min. It goes through quartz glass tube continu-
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H. Wang et al. / Energy Conversion and Management 50 (2009) 1976–1980
Carrier gas inlet 10
1
2
3
4
5
6
7
8
9
1 Injector needle 2 Metal shield 3 Induction coil 4 Pyrolysis wire 5 Annular seal ring 6,9Nut cap 7 T-branch pipe 8 Si licon rubber 10 Quartz glass tube Fig. 2. Installation of pyrolysis wire in a XP-12 Curie-point pyrolyser.
Table 1 Ultimate and proximate analysis of CWS sample. Proximate analysis
Ultimate analysis
Mar (%)
Aar (%)
Var (%)
Qar,net (kJ/kg)
Car (%)
Har (%)
Oar (%)
Nar (%)
Sar (%)
32.9
5.64
32.27
18877
50.57
3.27
6.13
0.93
0.56
100 90
Cumulative distribution Density distribution
35
70
Total volatile released /%
Mass percentage /%
80
60 50 40 30 20 10 0
30
coal CWS
25 20 15 10 5 0
1
10
100
Particle diameter /µm Fig. 3. Size distribution of coal and CWS.
ously and quickly so that it can carry out the released volatile gas in time, in order to avoid second reaction between volatile gas with residual carbon. The pyrolysis will complete in the set time and stop by itself. Step 6: After the volatile matters are all swept out, then turn off the power. Keep the pyrolysis wire stable and rotate the two sealed bolts open and avoid rotating of the wire. Then drag out the wire and weigh the pyrolysed wire with eight and nine. Then the weight of the samples after pyrolysis can be calculated. Step 7: Use a fine emery paper to clean the pyrolysis wire, then coat it again and repeat the experiment for three times. The average weight of the samples and samples after pyrolysis will be used in figures and model calculation to minimize the experimental error. The mass loss ratios of the samples are calculated to make curves against reaction time, which are shown in Fig. 4.
0.1
1
10
Pyrolysis time /s Fig. 4. Curve of volatile released vs. pyrolysis time.
Step 8: Regulate the pyrolysis time and repeat the steps 1–7 to complete all the experiments of 0.20, 0.33, 0.5, 0.85, 1, 1.45, 2, 5 and 7.5 s. In each experiment the fresh sample is used and weighed by a TG332A scale with the precision of 0.01 mg produced by Shanghai Scale Instrument Factory. In the experiment, the scale is a key instrument to measure raw mass data of samples. To eliminate the error from scale accuracy and uniform degree of sample layer, the measurement and operation were done carefully by the same operator. In Fig. 2, the needle of an injector (1) is equipped to connect with some other gas analysis instruments. In this experiment, no further gas analysis is done, so the gas generated goes through the needle into the air directly.
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1 1 For T e [300 K, 1500 K], x 2 ½1500 ; 300 : In the coordinates of abscissa axis is x and vertical axis is y = ln x, the equation of line de1 1 ; ln 1500Þ and point ð300 ; ln 300Þ is written as: fined by point ð1500
3. Results and discussion 3.1. Experimental results From Fig. 4, two lean S shape curves are observed. The same shape and tendency of the curve were obtained in other studies of coal pyrolysis in Curie-point pyrolyser [2,12]. For the coal sample, the curve climbs throughout the process and the slope coefficient increases a little in early stages, turns fast in the middle, then slows down in the end. For the CWS, a flatform lasts about 0.5 s is observed in early stages, then a steeper slope than that of coal in middle stages and a flatform again in the end. The evaporation of more water content in CWS delays releasing of volatiles and shows a flatform when the heat provided is not enough to make it start. But for the catalytic effect of water, pyrolysis of volatiles in CWS which completes before 7.5 s proceeds more quickly and thoroughly. For the instrument characteristic of Curie-point pyrolyser, volatile in samples goes through pyrolysis after water content does, and it mainly happens in the middle stages. Each time the temperature will stop at 770 °C exactly and keep constant. Then samples finish their pyrolysis reaction. In short, the early stages of CWS pyrolysis is longer than that of coal, but its slope in the middle stages is steeper, which shows the pyrolysis rate of CWS is faster. And the total volatile yield of CWS is a little higher. In the experiments, no matter how carful the operator is handling, the touch of the samples with the inner surface of the quartz glass tube is sometimes inevitable. And the sample mass is so limited, suggesting a slight mass loss may affect the results relatively significant, which is possibly the reason making a few data in Fig. 4 scattered. The authors improved the data by repeating the experiments several more times. 3.2. Kinetic model Mechanisms of many thermo-analytical processes are unknown or too complicated to be characterized by a simple kinetic model. To describe their kinetics, the methods based on a single-step approximation are often used [2,11]. A pyrolysis model [2,8,13] of first order reaction is selected to analyze pyrolysis kinetic parameters of the coal and CWS samples, and the rate of pyrolysis is expressed as:
ð1Þ
where V is the mass fraction of volatiles released up to time t and V1 is the value of V as t ? 1. Both of them are expressed as mass percentage of initial sample. The rate constant K typically has the Arrhenius form
K ¼ A expðE=RTÞ
ð3Þ
For the reason that the integral of right section of Eq. (3) cannot be solved directly and only the temperature range of 300–1500 K is of interest, exp(E/RT) can be approximated as mTn with enough accuracy. Defining 1/T = x and E/R = a, the function exp(E/ RT) = mTn is now written in the form:
a 1 ln x ¼ x þ ln m n n
y ¼ 603:539x 7:403
ð6Þ
The line between line (5) and line (6) is selected as the approximation line of y = ln x, and it is given as below:
y ¼ 603:539x 7:559
ð7Þ
Compare Eq. (4) with Eq. (7) and it gives:
a ln m ¼ 603:539; ¼ 7:559 n n E ln m ¼ 603:539; ¼ 7:559 i:e: : Rn n
ð8Þ
The integral is solved by using the power function mTn as substitute for exp (E/RT):
V n ln 1 ¼ Amb t 0ðnþ1Þ =ðn þ 1Þ Vm
ð9Þ
Taking logarithms gives
n V Ab ln ln 1 þ ðn þ 1Þ ln t ¼ ln Vm nþ1
ð10Þ
where t0 = t + T0/b is total pyrolysis time deduced from pyrolysis starting temperature to absolute zero, T0 is ambient temperature, 303 K. The experimental data is drawn in logarithmic coordinate paper. From Eq. (10), apparent activation energy E of the two samples can be obtained from intercept of the fit line, and with slope coefficient, the pre-component can be obtained too, see Table 2. Because the heating rate b keeps constant, the variables ln (1V/V1) and (t + T0/b) fit a linear rule in logarithmic coordinates. From Fig. 5, before the critical point, the experiment data Table 2 Kinetic parameters of coal and CWS. Sample
Pre-exponent, A (106 s1)
Apparent activation energy, E (kJ/mol)
Correlation coefficient, R
Coal CWS
62.5 9.4
12.691 16.362
0.943 0.999
Coal
CWS
ð2Þ
where A is the pre-exponential factor, E is apparent activation energy, R is the ideal gas constant and T is the absolute temperature. Under the condition of this experiment, pyrolysis temperature ramps linearly at a constant rate of b with dT = bdt in the investigation range. Then Eqs. (1) and (2) can be rewritten as:
1 A dV ¼ expðE=RTÞdT ðV 1 VÞ b
ð5Þ
and the parallel line tangential to y = ln x is:
1
-ln(1-V/ V )
dV ¼ KðV 1 VÞ dt
y ¼ 603:539x 7:716
0.1
1.25 0.01
0.1
1
1.69 0.1
1
t+303/b/s
ð4Þ
Fig. 5. Determination for heating rate of coal and CWS.
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H. Wang et al. / Energy Conversion and Management 50 (2009) 1976–1980
Table 3 Calculation results of coal and CWS.
4. Conclusions
Sample
Critical time (s)
Heating rate (K/s)
Coal CWS
1.25 1.69
834 617
in each curve agrees to form a line approximately. At the critical point, i.e. the Curie-point temperature of the pyrolysis wire, the sample temperature transforms from climbing linearly to keeping constant. And the heating rate b, can be determined too. See Table 3. 3.3. Kinetic analysis From Table 2, the apparent activation energy of CWS is higher than that of its parent coal because of large water content in CWS. In the experiment, volatiles will not release until the water content in CWS finishes its evaporation, so more heat is needed to complete the pyrolysis of CWS, resulting in apparent activation energy becomes higher. For Curie-point pyrolyser, the heat is directly relative to the time. So CWS absorbs more heat than coal, which can be seen in Fig. 4, the platform in early stages of CWS pyrolysis is longer than that of coal. After that the slope coefficient of CWS gets higher, which shows the pyrolysis rate of CWS after evaporation is higher. As an explanation, when the heating rate becomes higher, the heat transfer in sample speeds up, resulting in gas phase releasing easily and the internal pressure higher, association chance and second reaction decreasing. For CWS, the result of its more water content evaporating heavily under the medium heating rate is to form more porosity than coal, which benefits for heat of pyrolysis wire entering and volatile releasing. In addition, with the improvement of internal heat transfer condition of coal powder in CWS, it will subject to more heat collision on bigger surface and lead to breakage of some side chain and functional group which is not easy to break under low heating rate, then increase the volatile yield of CWS. Comparing with the result from the same CWS sample in the TGA. [14], the apparent activation energies are of the same magnitude and the value from the Curie-point pyrolyser is between the heating rate of 20 K/s and 30 K/ s (10.83 kJ/mol and 21.59 kJ/mol), but the pre-component is about 10 times bigger than that of TGA. (0.005 s1 and 0.089 s1 for 20 K/ s and 30 K/s). The results are consistent to the study of Niksa [4] which indicates the fast pyrolysis of the samples increased the reaction rate significantly. In this experiment, the temperature of pyrolysis wire will keep constant at 770 °C in the end. So the shorter the pyrolysis time, the faster the heating rate. From Table 3, it takes more time for CWS to get to the Critical time, i.e. pyrolysis wire with CWS sample needs more time to get to its Curie-point temperature because a big part of the heat produced by the wire in the high frequency oscillator was used to evaporate water content in CWS, the heat left was used to pyrolyse coal powder in CWS and heat up the wire. But for the coal sample, it takes less time to get to the Curie-point temperature without the heat loss of evaporation, so its heating rate is higher.
The apparent activation energy of CWS pyrolysis in Curie-point pyrolyser is higher than that of its parent coal because a big part of heat is consumed by evaporation of water content in CWS. So in industrial application of CWS, the influence of water content on its ignition in early stages must be carefully considered. For the coal sample, it took 1.25 s to get to the Curie-point temperature, shorter than that of CWS, 1.69 s. Correspondingly, the heating rate are 834 K/s and 617 K/s and both are medium heating rates, see Table 3. Because of the influence of water content, it shows an obvious delay for the pyrolysis of volatile in CWS. The time for CWS gets to 770 °C was prolonged and heating rate was slowed down, too. But the pyrolysis rate of CWS, after the heat reservation stages, gets faster than that of coal, and a more volatile yield is obtained. In the experiment, the apparent activation energy is 16.362 kJ/ mol, which is a little higher than that of Datong coals. When the water evaporates, it will form a steam shelter of char. When the sweeping gas is not fast enough to carry it out, the steam shelter will hinder hot gas entering into its pores and volatiles releasing in time, which makes E value a little higher. In addition, Comparing with the smallest energy condition of deposition, the additive keeps coal powders dispersing increased internal energy of CWS. After the water evaporation ends, the char will change into more porous, which makes it can absorb so much heat to complete its pyrolysis, resulting in the pyrolysis rate of CWS higher than that of coals. References [1] Strezov V, Lucas JA, Evans TJ, Strezov L. Effect of heating rate on the thermal properties and devolatilisation of coal. J Therm Anal Calorim 2004;78(2):385–97. [2] Yu J. Study and Modelling on the Interaction of Volatile Flame, CO Flame and Char Particle Combustion. Dissertation of Shanghai Jiaotong University, Shanghai. 2003. [3] Sima-Ella E, Mays TJ. Analysis of the oxidation reactivity of carbonaceous materials using thermogravimetric analysis. J Therm Anal Calorim 2005;80(1):109–13. [4] Niksa S, Lau CH-W. Global rates of devolatilization for various coal types. Combust Flame 1993;94(3):293–307. [5] Thomas KG, Calvin HB, Thomas HF. Decreases in the swelling and porosity of bituminous coals during devolatilization at high heating rates. Combust Flame 1995;100(1–2):94–100. [6] Solomon PR, Serio MA, Suuberg EM. Coal pyrolysis. Experiments, kinetic rates and mechanisms. Prog Energy Combust Sci 1992;18(2):133–220. [7] Wiktorsson LP, Wanzl W. Kinetic parameters for coal pyrolysis at low and high heating rates – a comparison of data from different laboratory equipment. Fuel 2000;79(6):701–16. [8] Marrie Nip, de Leeuw Jan W, Crelling John C. Chemical structure of bituminous coal and its constituting mackerel fractions as revealed by flash pyrolysis. Energy Fuels 1992;6(2):125–36. [9] Gao KL, Zhang MC. Ignition temperature experiment and forecasting of coal. Therm Power Generat 1988;4:7–14. [10] Zhou JH, Kuang JP, Zhou ZJ, Lai KZ, Cen KF. Thermal analysis on combustibility of coal and coal black liquor slurry. J Fuel Chem Technol 2005;33(1):33–7. [11] Simon P. Considerations on the single-step kinetics approximation. J Therm Anal Calorim 2005;82(3):651–7. [12] Yu J, Zhang M. A simple method for predicting the rate constant of pulverizedcoal pyrolysis at higher heating rate. Energy Fuels 2003;17(4):1085–90. [13] Wu J, Zhang MC, Chen QF. Mathematical process on dynamic description of coal fast pyrolysis. Chin J Power Eng 2002;22(6):2093–5. 2066. [14] Wang Hui, Jiang Xiumin, Liu Jianguo, et al. Analysis of pyrolytic properties of coal–water slurry under various heating rates. J Power Eng 2007;27(2):263–6.