Mass Loss Behavior and Volatile Composition during Pyrolysis of a Bituminous Coal

Fan Junfeng; Tian Bin; An Xiaoxi; Zhang Yaqing; Yin Mengmeng; Tian Yuanyu

(1. College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590; 2. Advanced Chemical Technology for Utilization of Northern Shaanxi Energy of the Engineering Research Center of the Ministry of Education, Shaanxi Research Center of Engineering Technology for Clean Coal Conversion, School of Chemical Engineering, Northwest University, Xi'an 710069)

Abstract: The thermogravimetry analyzer coupled with the pyrolysis gas chromatography/time-of-f light mass spectrometry technology was used in this study to investigate the mass loss behavior and volatile release characteristics of a bituminous coal. The results showed that with an increasing heating rate, the characteristic parameters and TG/DTG curves shifted obviously to the higher temperature region. The pyrolysis of a bituminous coal at different heating rates can be divided into two stages according to the Coats-Redfern (C-R) plots. The activation energy obtained from the C-R method is 50.21—85.14 kJ/mol and 5.14—7.24 kJ/mol at a heating rate range of 8—300 °C/min for the f irst and second pyrolysis stages, respectively. Aromatic hydrocarbons were dominant in the volatile products during fast pyrolysis of the coal, followed by the olef ins, whereas the phenols were the third major components. With the increase of pyrolysis temperature, the heavy components in the volatile species increased; meanwhile the phenol cracking reactions were intensified. The carbon number of olef ins was mainly concentrated in 3—9, and the aromatics were mainly composed of the compounds of C6—C13. This study can provide a basic reference for fast pyrolysis of coal and other thermal chemical conversion processes.

Key words: fast pyrolysis; bituminous coal; kinetics; volatiles

1 Introduction

Pyrolysis technology is one of the most important clean coal technologies, which has the advantages of low energy consumption, low water consumption, low investment, and low CO2emission. Pyrolysis can produce the high quality char, valuable gas, fuel and feedstocks for industry and consumer applications. However, the reactions involved in the coal pyrolysis process are very complex, which is governed by both bond scission caused by heating and complex free radical reactions[1].

During the pyrolysis process, the gas and liquid products are mainly derived from coal volatiles which are closely related to the chemical structure of coals and process parameters[2]. In addition, coal pyrolysis is also the initial stage of other chemical conversion technologies, and has an important impact on the subsequent reactions[3]. In the entrained flow gasification, the coal pyrolysis occurs first under the high temperature environment. Then the gasification and combustion reactions take place simultaneously as the char moving downward to the bottom of the gasifier. Compared with the pyrolysis pressure, atmosphere, and coal particle size, the pyrolysis temperature and heating rate are two main factors affecting the pyrolysis behavior significantly. These two factors have a great influence on the yield and composition of the volatiles as well as the reactivity of the chars. Researchers have made extensive attempts on the kinetics of coal pyrolysis and the compositions of volatile species[4]. Du, et al.[5]applied the amended Arrhenius equation to predict the effect of heating rate on the reaction rate constant of pulverized coal. The results showed that the amended Arrhenius equation not only could predict the pyrolysis kinetics but also had better extrapolation reliability in a wide range. By using different reactors, Wiktorsson, et al.[6]compared the pyrolysis kinetic parameters at different heating rates. The kinetic parameters of CH4and C2H6at different heating rates showed stronger mutually extrapolated reliability. Upon combining thermogravimetry with mass spectrometry (TG-MS), Zhang, et al.[7]investigated the kinetic parameters and characteristics of the released typical volatile species during coal pyrolysis with solid heat carrier. However, the heating rates of TG experiments were relatively far slower than that from the real solid heat carrier pyrolysis. It is known that the basic data and research results of fast coal pyrolysis are more close to the conditions of the current advanced coal utilization technology and the results of the fast pyrolysis can provide a useful guidance to the practical industrial process. Although the study on coal pyrolysis has been making great progress, the researches on the pyrolysis behavior and the characteristics of gas products released under the severe conditions (at high temperature with rapid heating rate) are still insufficient.

In order to better understand the reaction behavior and characteristics of gas products formed during fast coal pyrolysis at high temperature, a fast TG analyzer and a pyrolysis gas chromatography/time-of-flight mass spectrometry (Py-GC/TOF-MS) were combined to conduct the experiments for pyrolysis of a bituminous coal. The pyrolysis kinetics of coal was also investigated. The aim of this study is to provide the basic data and theoretical reference for the efficient coal utilization process.

2 Experimental

2.1 Material

Coal sample was obtained from the Jining coal mine located in Shandong province, which was a kind of bituminous coal (JB). The coal was pulverized to particles with a grain size of less than 74 μm and was dried at 110 °C for 5 h prior to use. The proximate and ultimate analyses of the coal are listed in Table 1.

Table 1 Proximate and ultimate analyses of the coal

2.2 Pyrolysis method

Pyrolysis studies of coal were performed in a thermogravimetric analyzer (STA449F3, NETZSCH–Ger?tebau GmbH, Germany) at different heating rates (8, 30, 100, and 300oC/min) with a N2flow rate of 100 mL/min. In each test, about 10 mg of sample were used and heated from the room temperature up to 1 000oC. The initial devolatilization temperature (Tin), the temperature of maximum decomposition rate (Tmax), the maximum decomposition rate (Rmax), and the terminal devolatilization temperature (Tf) at different heating rates were calculated from the TG and DTG curves for interpreting the pyrolysis profiles of JB. The devolatilization index (Di) of coal was determined via evaluating the performance of volatile release during pyrolysis, as defined in Eq. (1)[8-9]:

whereRmax,Tin, andTmaxcan be obtained from the TG and DTG curves[10]. ΔT1/2is the temperature interval, whenRd/Rmaxis equal to 0.5.Rdis the decomposition rate, which is defined by Eq. (2):

where mtis mass of the raw sample at timet.RmaxandRdcan be obtained from the DTG curves.

Furthermore, the detailed volatile compositions during fast pyrolysis of the coal were determined using a Py-GC/TOF-MS system (Pyroprobe 5200 series pyrolyser, CDS Analytical Inc., GC and TOF-MS, DANI MASTER, Italy) at 1 000 °C, 1 100 °C, and 1 200 °C, respectively. The methods of fast pyrolysis experiments were described in detail elsewhere[11], in which the heating rate of the coal samples was 1 000 °C/s.

2.3 Pyrolysis kinetics

The Coats-Redfern kinetic model was used to describe the pyrolysis process of the coal, the total reaction rate equation can be expressed as follows[12]:

where the mass fractional conversion is presented as:

In the non-isothermal TG experiment with a constant heating rate, the pyrolysis temperatureTis linear at the timet:T=T0+βt(βis the heating rate, K/min). By substituting this relationship into Equation (1), the rearranged new results are given by:

After performing the both sides integration of Eq. (5), the integral expressions can be presented as follows:

After having performed the integration and logarithm of both sides of Eq. (6), the result can be expressed as:

TheEvalues in Eq. (7) and (8) are very large, so the 2RT/Eterm can be approximated to zero. If the reaction order is made right, the left side in Eq. (6) or (7) is linear with 1/T. According to the slope and intercept of the plotted straight line, the activation energyEand the preexponential factor A can be obtained, respectively.

3 Results and Discussion

3.1 Pyrolysis profiles of the coal

As shown in Figure 1, the mass loss (TG) and derivative mass loss (DTG) curves showed different evolution trends with variation of heating rate. In general, the pyrolysis of JB coal showed a mass loss stage except for an ignorable stage at 800 °C for mineral decomposition in coal (such as, carbonates). The characteristic parameters during pyrolysis of JB coal at different heating rates are summarized in Table 2. The initial pyrolysis temperature at 8 °C/min for JB was 405.2 °C, which was higher than that of the low rank coals[13]. This is ascribed to the fact that the bond energies in the surface and bulk structures of the bituminous coal are normally higher than those of low-rank coals and the cleavage of these bonds needs higher temperature. With an increasing heating rate, the values of each characteristic parameter increased gradually and the TG/DTG curves also shifted to the higher temperatures. The poor thermal conductivity of coal and the temperature difference between the heater and the coal were considered to be the main reason for this phenomenon[14]. The characteristic parameter ofDiwas used to describe the devolatilization rate during pyrolysis.Divalue increased from 15.32 to 357.09 upon enhancing the heat rate from 8 °C/min to 300 °C/min. The result indicated that the fast pyrolysis process was beneficial to the release of volatiles, so that the pyrolysis reaction could be completed within a shorter time in the rapid heating process. Furthermore, when the heating rate increased, the released total volatiles before 600 °C decreased significantly. The above results were mainly attributed to reduction of the reaction directions between the volatile components caused by rapid heating. Therefore, the pyrolysis mechanism has changed to a certain extent when the slow pyrolysis is gradually converted into fast pyrolysis.

Figure 1 TG and DTG curves of coal at different heating rates

Table 2 Pyrolysis parameters at different heating rates

3.2 Pyrolysis kinetics

The fitting results of TG/DTG curves by using the Coats-Redfern integral method are shown in Figure 2. The corresponding kinetic parameters are calculated and summarized in Table 3.

Figure 2 Coats-Redfern plots for determination of the active energy and pre-exponential factors (n=1)

It can be seen that the pyrolysis of JB bituminous coal at different heating rates can be divided into two stages. In the f irst stage, the bridged bonds in the coal macromolecules break down to generate the free radicals. Cleavage of the alkyl side chains on the aromatic clusters also takes place in this stage, which can give rise to the formation of gas and tar. Therefore, the activation energy of the first stage is the largest since more energy needs to be provided in this bond scission stage. In the second stage, the polycondensation reactions of aromatic nuclei into char can produce a large amount of H2and CH4. In addition, the mineral decomposition also contributes to some mass loss in the secondary stage. Since these reactions are not very f ierce, the intensity of pyrolysis reaction has been greatly weakened, and lower activation energy is identified.

Table 3 Kinetic parameters of JB coal at different heating rates (n=1)

In addition, the heating rate has a great influence on the apparent activation energy. With the increase of heating rate, the activation energy of the two stages of coal pyrolysis increases gradually. The apparent activation energy mainly ref lects the difficulty degree of the reaction occurrence in the coal pyrolysis process. As mentioned above, the rapid heating process can speed up the breaking of coal chemical bonds, so that the pyrolysis reactions are more concentrated. Meanwhile, more energy is needed when a large number of chemical bonds cracks, so the apparent activation energy becomes larger at the faster heating rate.

3.3 Volatile composition during fast pyrolysis process

The Py-GC/TOF-MS technique can be applied to online identification of the volatile products generated during pyrolysis, which can accurately obtain the detailed molecular structures and the relative contents of the components. Figure 3 shows the total ion flow diagram for different pyrolysis temperatures. According to the NIST library, 203 compounds were identified in the fast pyrolysis products at different pyrolysis temperatures.

Figure 3 Total ion chromatograms of JB coal during fast pyrolysis at different temperatures

The volatile species were divided into five groups according to their structural similarity. As shown in Figure 4, the aromatic hydrocarbons were most abundant, followed by the olefins, and the phenol compounds were the third major components, while the relative content of aliphatic hydrocarbons was the least. As we know, bituminous coal is rich in aromatic clusters and it contains a large number of alkyl side chains on the aromatic clusters, which make this type of coal suitable for producing hydrocarbon species during pyrolysis. Therefore, the volatiles of JB coal contain more aromatics. Owing to the high pyrolysis temperature, cracking and dehydrogenation reactions are dominant, which can result in the formation of more olefins. Phenol compounds are the main form of O-containing species in the pyrolysis volatiles of coal. Furthermore, the content of N-containing compounds in volatiles of JB coal is also high, which is consistent with its elemental composition.

Figure 4 shows the volatile distribution at different pyrolysis temperatures. When the pyrolysis temperature increased from 1 000 °C to 1 200 °C, the alkane yield decreased from 7.61% to 2.29% and the arene yield increased from 36.87% to 51.18%, however the yield of olefins showed a least ratio at the temperature of 1 100 °C. The results indicated that with the increase of pyrolysis temperature, the dehydrogenation of alkyl side chains and the formation of aromatic hydrocarbons through the bridge bonds cleavage were improved. Therefore, the content of aromatic compounds in volatile compounds increased. On the contrary, the yields of phenols and N-containing compounds decreased from 17.89% and 8.98% to 12.09% and 0.30%, respectively. Then the result of higher pyrolysis temperature might contribute to the aromatic generation through the removal of phenolic hydroxyl groups, as well as the ring-opening reaction of the N-containing heterocycles[15].

Figure 4 The volatile component distribution at different pyrolysis temperatures

Figure 5 displays the distribution of carbon numbers for each of the groups in the volatile products formed at different temperatures. It can be seen from Figure 5(a) that with the increase of the reaction temperature, the yield of alkane with the same carbon atom number gradually decreased. The content of C9alkanes formed at the reaction temperatures of 1 000 °C and 1 100 °C was 3.08% and 2.66%, respectively, which was the highest value among the alkane species. However, the yield of C6alkanes was the highest (1.06%) when the reaction temperature reached 1 200 °C. The above phenomenon might be interpreted by supposing that the severe thermal shock could accelerate the secondary cracking reaction of the light tar components. As illustrated in Figure 5(b), the carbon number of olef ins was mainly distributed in the range of 3—9. Propylene was the only one detected at a pyrolysis temperature of 1 200 °C, and there were no apparent trends concerning the production of other olefins with variation in the pyrolysis temperature. In addition, the yields of olefins decreased with the increase in their carbon numbers. As demonstrated in Figure 5(c), the aromatics were mainly composed of C6–C13compounds. As the reaction temperature increased, the aromaticity degree of tar also increased. Because of the large bond energy of C-C in benzene, the benzene ring had high stability[16-17], so the aromatics yield ratio of benzene was the highest. The benzene yield obtained at the reaction temperature of 1 100 °C and 1 200 °C was 8.13% and 8.30%, respectively. In Figure 5(d), there were two main reaction types that constituted the reduced yield of phenolic compounds. One type was the direct benzene formation after the breakage of phenolic hydroxy radicals with the increase of reaction temperature. The other was the gradually intensified generation of cyclopentadiene and CO in the phenol cracking reactions[18].

4 Conclusions

Figure 5 Carbon number distribution of typical products at different reaction temperatures

The decomposition temperature during pyrolysis of JB commenced at 405 °C, indicating that JB could hardly be broken down. With an increasing heating rate, the characteristic parameters and TG/DTG curves shifted obviously to the higher temperature region. The pyrolysis of a bituminous coal at different heating rates can be divided into two stages according to the Coats-Redfern plots. The activation energy obtained from the C-R method is 50.21―85.14 kJ/mol and 5.14―7.24 kJ/mol for the first and second pyrolysis stages, respectively, at the heating rate range of 8―300 °C/min. Aromatic hydrocarbons were most abundant in the volatile products during fast pyrolysis of JB, followed by the olefins, and the phenol compounds constituted the third major components, while the relative content of aliphatic hydrocarbons was the least. With the increase of pyrolysis temperature, the heavy components in the volatile species increased, and meanwhile the phenol cracking reactions were intensified. The high temperature could promote the increase of the aromatic content in volatiles. The carbon number of olefins was mainly concentrated in 3—9, and the aromatics were mainly composed of the C6—C13compounds. This study can provide a basic reference for fast pyrolysis of coal.

Acknowledgement:The authors gratefully acknowledge the f inancial support from the National Natural Science Foundation of China (Grant Nos. 21576293 and 21576294).