The Catalytic Effect from Alkaline Elements on the Tar-Rich Coal Pyrolysis

: Tar-rich coal has been widely concerned because of its high tar yield. Two kinds of tar-rich coals were studied by Thermogravimetric-Mass spectrometer-Fourier transform infrared (TG-MS-FTIR) to obtain the pyrolysis characteristics. TG-MS-FTIR was used to study the mass loss, gaseous compounds evolution, and functional group information of tar-rich coal during pyrolysis. Mass loss is mainly caused by water release and macromolecular decomposition. The results showed that there were two stages of mass loss in the pyrolysis process. In addition, the gas release signal detected by mass spectrometry is consistent with the functional group information detected by FTIR. The main gaseous products include H 2 , H 2 O, CO, CO 2 , and CH 4 . In addition, the effect of ash content on the pyrolysis of oil-rich coal and the catalytic effect of internal minerals on coal pyrolysis are also discussed, and the thermal pyrolysis characteristics of coke-rich oil coal are put forward. The results provide a new idea for the study of pyrolysis characteristics of tar-rich coal.


Introduction
As a coal resource integrating kerosene and gas attributes, tar-rich coal has attracted the attention of scholars all over the world [1][2][3][4]. Coal with a low temperature (500-700 • C) tar yield of more than 7% and less than or equal to 12% is called tar-rich coal [5]. The excellence of oil-rich coal is mainly reflected in its hydrogen-rich structure, which can produce oil and gas by pyrolysis. Moreover, the tar yield after pyrolysis is high, and the semicoke produced by pyrolysis can become a substitute for anthracite and coking coal. Based on the advantages, tar-rich coal is considered to be an important way to increase oil and gas supply and realize clean and efficient utilization of coal resources.
The research on the pyrolysis behavior of coal has been proved to be significant for the efficient production and utilization of coal [6][7][8][9]. Under isolated air conditions, coal can generate oil, gases, and semicoke at low or medium temperatures [10]. Tar-rich coal makes it unique in terms of the yield of tar produced by pyrolysis. As is well-known, the pyrolysis behavior of coal is always of primary concern [11][12][13][14]. A large amount of literature focused on investigating the thermal behavior of tar-rich coal under different conditions to improve the yield of pyrolysis products. Jiang et al. investigated an integrated process of coal pyrolysis with steam reforming of propane (CP-SRP) to improve tar yield using Ni/Al 2 O 3 as a reforming catalyst [15]. The results suggested that the CP-SRP and the integrated coal pyrolysis with steam reforming of methane (CP-SRM) or methane with a few propane (CP-SRMP) showed a higher tar yield. Yao et al. reported detailed research on the effects of ion-exchanged metal cations on the pyrolysis reactivity of Shendong demineralized coal (SD-coal) and the gasification performance of pyrolysis chars [16]. It was suggested that the char yields decreased, and the pyrolysis gas yields increased during the pyrolysis of metal ion-exchanged coals. The catalytic effect of different ion exchanges on the yield of pyrolysis products was different. Li et al. analyzed the characteristics of coal pyrolysis products at various temperatures using a drop tube furnace, and the composition of pyrolysis products The pyrolysis evolution curves of samples can be divided into three stages in regard to temperature (Figure 2). The first stage is the drying and degassing stage, which ranges from room temperature to 350 °C. This stage was mainly caused by the volatilization of coal moisture (surface water and pore water), methane, and carbon dioxide adsorbed on the surface [36,37]. The temperature range of the second stage is 350-550 °C, which is a severe pyrolysis stage. The depolymerization and decomposition of coal mainly occurred at this stage, resulting in a significant increase in the weight loss of coal samples. With the softening of coal particles, semicoke began to form, and pyrolysis water, pyrolysis gas, and pyrolysis oil were separated. In the third stage, tar production began to decrease, hydrogen gas generation increased gradually, and aromatic nuclei of coal increased, which were arranged regularly and had better structure [38].  The pyrolysis evolution curves of samples can be divided into three stages in regard to temperature ( Figure 2). The first stage is the drying and degassing stage, which ranges from room temperature to 350 • C. This stage was mainly caused by the volatilization of coal moisture (surface water and pore water), methane, and carbon dioxide adsorbed on the surface [36,37]. The temperature range of the second stage is 350-550 • C, which is a severe pyrolysis stage. The depolymerization and decomposition of coal mainly occurred at this stage, resulting in a significant increase in the weight loss of coal samples. With the softening of coal particles, semicoke began to form, and pyrolysis water, pyrolysis gas, and pyrolysis oil were separated. In the third stage, tar production began to decrease, hydrogen gas generation increased gradually, and aromatic nuclei of coal increased, which were arranged regularly and had better structure [38]. shown in Figure 1, the DTG curve was asymmetric and could obviously be divided into multiple steps [35]. Therefore, PeakFit software was used to separate the overlapping peaks by adopting the Gaussian multipeak fitting method. The pyrolysis evolution curves of samples can be divided into three stages in regard to temperature ( Figure 2). The first stage is the drying and degassing stage, which ranges from room temperature to 350 °C. This stage was mainly caused by the volatilization of coal moisture (surface water and pore water), methane, and carbon dioxide adsorbed on the surface [36,37]. The temperature range of the second stage is 350-550 °C, which is a severe pyrolysis stage. The depolymerization and decomposition of coal mainly occurred at this stage, resulting in a significant increase in the weight loss of coal samples. With the softening of coal particles, semicoke began to form, and pyrolysis water, pyrolysis gas, and pyrolysis oil were separated. In the third stage, tar production began to decrease, hydrogen gas generation increased gradually, and aromatic nuclei of coal increased, which were arranged regularly and had better structure [38].  It can be seen from Figure 1 that the slopes of the four samples are roughly the same, which means that the decomposition rates of the four samples are the same under the same conditions. The DTG curves display that the samples experienced two weight losses during pyrolysis. The first weight loss was due to the evaporation of water in the sample [39]. The second weight loss was caused by the thermal decomposition of coal samples. The DTG curve fluctuated around 700 • C, mainly due to the decomposition of minerals in coal [40]. DTG curves of SGT-3 and SGT-4 were significantly different from other samples. The main reason was that the yield of volatile matter in SGT-3 and SGT-4 coal samples was higher than that of other coal samples. During this period, the devolatilization reaction mainly occurred in coal samples, including the depolymerization of macromolecules, through weak bridge bond and the removal of fatty side chain-cracking reactions in macromolecular structure, which led to the weight loss rate of coal samples higher than other coal samples [41]. Table 1 shows several special data of samples during pyrolysis. The temperatures of maximum weight loss of four samples were approximately the same. Besides, the maximum rates of weight loss of SGT-3 and SGT-4 were higher than other samples. The final weight loss was maintained at 26~28%. T max is the temperature of maximum weight loss; (dw/dτ) max is the maximum rate of weight loss; W loss is the final weight loss.

The Gas Composition for Pyrolysis
Generally, the breaking of bridge bonds between aromatic rings, the disappearance of heteroatom functional groups, and the ring breaking of macromolecular networks will occur in the process of coal pyrolysis [42]. These reactions result in the production of gases and tar. Gas compounds can be collected by mass spectrometry.
The generation curves of CH 4 are shown in Figure 3. CH 4 is the main gaseous hydrocarbon product in the pyrolysis process [43]. From Figure 3, it can be seen that the main temperature range of methane generated was 400~900 • C. With the increase in temperature, the release rate of CH 4 increased first and was then reduced, reaching a peak at near 550 • C. In the low-temperature stage, CH 4 mainly came from the breaking of the C-C bond in aliphatic chains, whereas the formation of CH 4 at higher temperatures was due to the cleavage of stronger bonds [44]. CO 2 appeared at nearly 100 • C and reached its peak at about 500 • C (See Figure 4). Then, with the increase in temperature, the release rate of CO 2 began to come down. According to previous articles [45][46][47], the sources of CO 2 included carbon dioxide adsorbed in coal, decomposition of carbonate, and fracture of oxygen-containing functional groups. The formation of CO 2 in the early stage was related to the decomposition of the carboxyl group, while the latter was mainly the decomposition of ether structures and oxygenbearing heterocycles.  CO2 appeared at nearly 100 °C and reached its peak at about 500 °C (See Figure 4). Then, with the increase in temperature, the release rate of CO2 began to come down. According to previous articles [45][46][47], the sources of CO2 included carbon dioxide adsorbed in coal, decomposition of carbonate, and fracture of oxygen-containing functional groups. The formation of CO2 in the early stage was related to the decomposition of the carboxyl group, while the latter was mainly the decomposition of ether structures and oxygenbearing heterocycles.   CO2 appeared at nearly 100 °C and reached its peak at about 500 °C (See Figure 4). Then, with the increase in temperature, the release rate of CO2 began to come down. According to previous articles [45][46][47], the sources of CO2 included carbon dioxide adsorbed in coal, decomposition of carbonate, and fracture of oxygen-containing functional groups. The formation of CO2 in the early stage was related to the decomposition of the carboxyl group, while the latter was mainly the decomposition of ether structures and oxygenbearing heterocycles.  The temperature range of the first peak was from 70 • C to 200 • C. At this stage, the H 2 O mainly came from the sample [48]. The other peak temperature range was about 400~800 • C. The release of water in the high-temperature section was mainly due to the decomposition of oxygen-containing functional groups [49]. temperature range of the first peak was from 70 °C to 200 °C. At this stage, the H2O mainly came from the sample [48]. The other peak temperature range was about 400~800 °C. The release of water in the high-temperature section was mainly due to the decomposition of oxygen-containing functional groups [49]. Compared with other gases, the evolution of CO is more complex, and there is no obvious downward trend before 1000 °C (See Figure 6). At about 450 °C, the release rate of CO increased sharply, mainly due to the decomposition of the carbonyl group. At nearly 780 °C, the evolution curve of CO fluctuated again, which might be related to the decomposition of phenolic groups [50]. Actually, there is a process of mutual transformation between CO2 and CO. CO2 can decompose into CO and O2, while CO can also be produced by the Boudouard reaction [51,52].  Compared with other gases, the evolution of CO is more complex, and there is no obvious downward trend before 1000 • C (See Figure 6). At about 450 • C, the release rate of CO increased sharply, mainly due to the decomposition of the carbonyl group. At nearly 780 • C, the evolution curve of CO fluctuated again, which might be related to the decomposition of phenolic groups [50]. Actually, there is a process of mutual transformation between CO 2 and CO. CO 2 can decompose into CO and O 2 , while CO can also be produced by the Boudouard reaction [51,52].
Catalysts 2022, 12, x FOR PEER REVIEW 6 of 15 temperature range of the first peak was from 70 °C to 200 °C. At this stage, the H2O mainly came from the sample [48]. The other peak temperature range was about 400~800 °C. The release of water in the high-temperature section was mainly due to the decomposition of oxygen-containing functional groups [49]. Compared with other gases, the evolution of CO is more complex, and there is no obvious downward trend before 1000 °C (See Figure 6). At about 450 °C, the release rate of CO increased sharply, mainly due to the decomposition of the carbonyl group. At nearly 780 °C, the evolution curve of CO fluctuated again, which might be related to the decomposition of phenolic groups [50]. Actually, there is a process of mutual transformation between CO2 and CO. CO2 can decompose into CO and O2, while CO can also be produced by the Boudouard reaction [51,52].  H 2 was mainly generated from 400 • C to 1000 • C and reached a peak at about 770 • C (See Figure 7). Before 400 • C, the generation of H 2 related to the degradation of the hydrogen-rich matrix. At about 780 • C, the increase in H 2 release rate was due to the condensation of aromatic and hydroaromatic structures [53].  H2 was mainly generated from 400 °C to 1000 °C and reached a peak at about 770 °C (See Figure 7). Before 400 °C, the generation of H2 related to the degradation of the hydrogen-rich matrix. At about 780 °C, the increase in H2 release rate was due to the condensation of aromatic and hydroaromatic structures [53].

The Evolution of Carbon Compounds during Pyrolysis
FTIR technology can be used to capture the volatile products of coal pyrolysis on line. The change of absorption peak of the infrared spectrum can indirectly reflect the generation law of gas species and relative yield [54,55]. The reaction and volatile species in the pyrolysis process mainly depend on the amount of oxygen-containing functional groups, aromatic rings, and aliphatic branched chains [56,57].

The Evolution of Carbon Compounds during Pyrolysis
FTIR technology can be used to capture the volatile products of coal pyrolysis on line. The change of absorption peak of the infrared spectrum can indirectly reflect the generation law of gas species and relative yield [54,55]. The reaction and volatile species in the pyrolysis process mainly depend on the amount of oxygen-containing functional groups, aromatic rings, and aliphatic branched chains [56,57].
The important bands are enumerated in Table 2. From Figure 9, the asymmetric stretching vibrations of carbonyl (C=O) lead to the peak wavenumbers of 2350 cm −1 and 2345 cm −1 . A weak spectrum band at 1600-1450 cm −1 reveals the presence of an aromatic ring (skeleton stretching vibrations of aromatic ring C=C). The bands at 2200-2050 cm −1 (C-O stretch vibration) and 2480-2250 cm −1 (C=O stretch vibration) represent CO and CO 2 , respectively. Moreover, the peak wavenumber of 3740 cm −1 indicates the release of H 2 O. The bands of 3400-3100 cm −1 are owing to N-H stretching vibration, which indicated the presence of NH 3 . The band at 3010 cm −1 corresponds to C-H stretching vibration, which leads to the formation of CH 4 . In addition, the intensity of the absorption peak increases with the increase in temperature. The release of CH 4 is related to the pyrolysis of the benzene ring of coal. The amount of CH 4 produced at 600 • C is much greater than the other two temperatures, which indicates the macromolecular structure of coal was seriously damaged at 600 • C. The release of gas products at higher temperatures may come from the pyrolysis of aromatic carbon.  The important bands are enumerated in Table 2. From Figure 9, the asymmetric stretching vibrations of carbonyl (C=O) lead to the peak wavenumbers of 2350 cm −1 and 2345 cm −1 . A weak spectrum band at 1600-1450 cm −1 reveals the presence of an aromatic ring (skeleton stretching vibrations of aromatic ring C=C). The bands at 2200-2050 cm −1 (C-O stretch vibration) and 2480-2250 cm −1 (C=O stretch vibration) represent CO and CO2, respectively. Moreover, the peak wavenumber of 3740 cm −1 indicates the release of H2O. The bands of 3400-3100 cm −1 are owing to N-H stretching vibration, which indicated the presence of NH3. The band at 3010 cm −1 corresponds to C-H stretching vibration, which leads to the formation of CH4. In addition, the intensity of the absorption peak increases with the increase in temperature. The release of CH4 is related to the pyrolysis of the benzene ring of coal. The amount of CH4 produced at 600 °C is much greater than the other two temperatures, which indicates the macromolecular structure of coal was seriously   The important bands are enumerated in Table 2. From Figure 9, the asymmetric stretching vibrations of carbonyl (C=O) lead to the peak wavenumbers of 2350 cm −1 and 2345 cm −1 . A weak spectrum band at 1600-1450 cm −1 reveals the presence of an aromatic ring (skeleton stretching vibrations of aromatic ring C=C). The bands at 2200-2050 cm −1 (C-O stretch vibration) and 2480-2250 cm −1 (C=O stretch vibration) represent CO and CO2, respectively. Moreover, the peak wavenumber of 3740 cm −1 indicates the release of H2O. The bands of 3400-3100 cm −1 are owing to N-H stretching vibration, which indicated the presence of NH3. The band at 3010 cm −1 corresponds to C-H stretching vibration, which leads to the formation of CH4. In addition, the intensity of the absorption peak increases with the increase in temperature. The release of CH4 is related to the pyrolysis of the benzene ring of coal. The amount of CH4 produced at 600 °C is much greater than the other two temperatures, which indicates the macromolecular structure of coal was seriously   Figure 10 shows the relationship between ash content and maximum weight loss rate, which is negatively correlated. The decrease in ash content deepens the degree of pyrolysis products, which leads to gas products increasing in varying degrees. Ash content has an obvious impact on the generation of CH 4 and CO and a weak impact on the generation of H 2 O and H 2 , which is consistent with previous research results [69]. The influence of ash on coal pyrolysis mainly comes from the minerals in coal. Minerals can catalyze or inhibit coal pyrolysis.

CO2
C=O  Figure 10 shows the relationship between ash content and maximum weight loss rate, which is negatively correlated. The decrease in ash content deepens the degree of pyrolysis products, which leads to gas products increasing in varying degrees. Ash content has an obvious impact on the generation of CH4 and CO and a weak impact on the generation of H2O and H2, which is consistent with previous research results [69]. The influence of ash on coal pyrolysis mainly comes from the minerals in coal. Minerals can catalyze or inhibit coal pyrolysis. The mineral composition of the sample is shown in Table 2. It can be seen from Table  2 that SiO2 and Al2O3 are the main minerals in the sample. At the same time, the component contents of alkali metals, alkaline earth metals, and iron oxides (MgO + Fe2O3 + CaO + K2O + Na2O) with carbon solution catalysis are very high, all above 25%. Therefore, we mainly discuss the effects of SiO2, Al2O3, alkali metals, and alkaline earth metals on the pyrolysis of samples. The mineral composition of the sample is shown in Table 2. It can be seen from Table 2 that SiO 2 and Al 2 O 3 are the main minerals in the sample. At the same time, the component contents of alkali metals, alkaline earth metals, and iron oxides (MgO + Fe 2 O 3 + CaO + K 2 O + Na 2 O) with carbon solution catalysis are very high, all above 25%. Therefore, we mainly discuss the effects of SiO 2 , Al 2 O 3 , alkali metals, and alkaline earth metals on the pyrolysis of samples.

The Catalytic Effect from the Inorganic Compounds
SiO 2 is an inert mineral with a high melting point. At the initial stage of the pyrolysis reaction, the chemical properties of SiO 2 will not change. With the progress of the reaction process, SiO 2 becomes the diluent in contact with the heated, softened particles of the tissue, which reduces the repolymerization reaction of coal tar macromolecules, and then generates more tar small molecules and gas-phase products, especially the formation of CO and H 2 in the later stage of the reaction (>800 • C). However, SiO 2 will not participate in the reaction of product formation and has no catalytic effect on the formation of gas-phase products. As the main material in clay minerals, Al 2 O 3 has been widely recognized for its catalysis [31,70]. The catalytic reaction of Al 2 O 3 on coal pyrolysis is mainly reflected in the high-temperature region, especially in dealkylation and dehydrogenation. However, due to the limitation of the experimental temperature, the catalytic effect of Al 2 O 3 has not been well displayed.
Li et al. found that alkali metals and alkaline earth metals (AAEM) can inhibit the precipitation of large aromatic ring hydrocarbon compounds and then form solid products such as semicoke [71]. AAEM plays an important role in the fracture of old bonds and the formation of new bonds in the process of coal pyrolysis, especially the minerals connected with the organic structure of coal or semicoke. AAEM promotes the thermal decomposition of aliphatic hydrocarbons and small aromatic ring substances to produce gas-phase products, but some studies believe that the internal minerals in coal have little effect on the pyrolysis activity of coal, and the effect on the yield of gas products can be ignored [72]. It can be seen from the above gas-phase product yield that the gas-phase product yield of SGT-3 and SGT-4 is significantly higher than that of SGT-1 and SGT-2. Therefore, AAEM inhibits the formation of tar and other products in the process of this pyrolysis, so that the precursor unit forming tar is repeatedly broken and bonded with the free radical in semicoke, Aliphatic fragments forming tar and some aromatic compounds with low molecular weight are precipitated in the form of gas.

The Mechanism for Thermal Pyrolysis of Tar-Rich Coal
According to the experimental results of TG-MS-FTIR in the argon atmosphere, the thermal pyrolysis characteristics of tar-rich coal under a nonoxidizing gaseous environment can be proposed. Firstly, the unstable bonds in the molecular structure of coal are removed with the increase in temperature, such as C-O-C and phenolic hydroxyl groups. The main gas generated in the low-temperature stage is H 2 O because the water in the coal pores will escape with heating. Meanwhile, many small molecular products are formed at this stage. Secondly, the generation of volatiles increased with the increase in temperature. From the results of MS and FTIR data, volatiles are generated in large quantities after 400 • C, which means the cleavage of the benzene ring and small molecular structure. The cleavage of the aliphatic chain and aromatic chain is the source of CH 4 generation. H 2 is formed by the condensation of hydrogen radical formed by the cleavage of the aliphatic chain. The generation of CO is caused by the decomposition of alkyl and aromatic ether. The generation of CO 2 is related to oxygen-containing hybridization. In the high-temperature stage, the secondary cracking reaction reduces the yield of tar and increases the gaseous products. Due to the high tar yield of tar-rich coal, the yield of the volatile matter still increased after 900 • C.

Coal Samples
The experimental samples used in this work were from Shigetai Coal Mine, Shaanxi, China. This coal sample is typical tar-rich coal; there are high tar yields at low temperatures in samples. Four coal samples, Shigetai-1, Shigetai-2, Shigetai-3 and Shigetai-4 (SGT-1, SGT-2, SGT-3 and SGT-4), were used for this experiment. SGT-1 and SGT-2 were taken from one coal seam, SGT-3 and SGT-4 were taken from another coal seam. The coal samples had been placed in an air oven under 60 • C for 24 h to remove moisture. Then, the dried coal was crushed to a particle size below 80 mesh by a ball mill. Table 3 shows the results of the proximate and ultimate analysis of coal samples. Table 4 shows the ash composition analysis results of the samples. Proximate and ultimate analyses were carried out in the Testing Center of China National Administration Coal Geology (Xuzhou, China). C, H, O, N, S were determined following GB/T476-2008, GB/T19227-2008, and GB/T214-2007. Meanwhile, proximate analysis followed GB/T212-2008 and was used to detect moisture, volatile, ash, and fixed carbon. The analysis of ash composition was tested in the Testing Center of China National Administration Coal Geology (Xuzhou, China). Ash composition was determined following GB/T1574-2007. As shown in Table 2

TG/MS/FTIR Analyses
TG-MS-FTIR analyses were performed at the Institute of Coal Chemistry, Chinese Academy of Sciences (See the Figure S1). The experimental instrument consists of three parts: Thermogravimetry (SETARAM, Lyon, France), Mass spectrometry (PFEIFFER, Asslar, Germany), and Fourier transform infrared spectrometer (BRUKER, Ettlingen, Germany).
The thermal pyrolysis experiments were carried out in SETSYS EVOLUTION TGA 16/18 thermogravimetric analyzer. The dried samples were crushed below 200 mesh. About 20 mg sample was put into an alumina crucible and heated from 30 • C to 1000 • C at a heating rate of 10 • C/min under a high purity argon atmosphere with a gas flow of 50 mL/min. The thermal pyrolysis gas was detected by the mass spectrometer. According to previous studies, the most important gaseous products of coal pyrolysis are H 2 O, CO 2 , CO, H 2 , and hydrocarbons [73,74]. The mass spectrometer has two functions: full scanning mode and selected ion recording (MID) mode. The former mode was used to detect the signals of all the fragment ions (m/z 1-160), and the latter mode tracked MS signals of some specific ions marked accurately as m/z 2, 16, 18, 28, 44. FTIR is a technique to characterize the chemical structure of coal, which is used to determine the functional group in coal [75][76][77][78]. By FTIR, the evolution of functional groups in coal during pyrolysis can be effectively detected. The wavelength ranges of the Fourier transform infrared spectrometer is from 4000 to 350 cm −1 .

Conclusions
In this work, efforts have been made to study the pyrolysis characteristics of two kinds of tar-rich coal through the TG-MS-FTIR. TG was used to detect the quality change of samples during pyrolysis. MS and FTIR were used to detect the evolution of gaseous products and functional groups, respectively. Moreover, the results measured by MS are compared with the data of FTIR analysis. As evidenced by the good consistency between these two approaches, the result is reliable and accurate. Through the experimental results, the following conclusions are obtained: During the pyrolysis process, the sample experienced two mass losses, the first was due to the loss of water in the sample, and the second mass loss was due to the formation of CO 2 . The release of water and the depolymerization of macromolecules are the main reasons for the mass loss of samples.
The results of the MS and FTIR showed good consistency. The evolutions of gaseous compounds (i.e., H 2 , H 2 O, CO, CO 2 , and CH 4 ) during pyrolysis could be measured by TG-MS-FTIR. The formation of gaseous compounds was related to the evolution of different functional groups in coal.
Ash content has a certain influence on coal pyrolysis. Specifically, the less ash, the more sufficient the coal pyrolysis reaction, which is caused by the minerals in coal. AAEM inhibited the formation of tar, promoted the pyrolysis of tar precursor products to form small molecular substances, and improved the yield of gas-phase products.
Based on the analysis of MS and FTIR data, the thermal pyrolysis characteristics of tar-rich coal are proposed.