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Simulation Experiment of TSR Promotes Cracking of Coal Generation H2S

Thermochemical sulfate reduction (TSR) is one of the main contributors to the formation of hydrogen sulfide (H2S) in coal seam strata. Four reaction systems (coal, coal+water, coal+water and MgSO4, and coal+water and MgSO4 and AlCl3) were selected and simulated from 250C to 600C with eight temperature steps using a high-temperature and high-pressure reaction device, and the evolution characteristics of the gaseous products of hydrocarbons (methane, C2-5) and nonhydrocarbon gases (CO2, H2, and H2S) were studied. Thermal simulation experiments showed that the TSR led to the reduction of heavy hydrocarbons, and the presence of salts accelerated the evolution of hydrocarbons; SO4 , Al, and Mg had a certain promoting effect on the TSR, which increased the total amount of alkane gas, H2S, and CO2 production. Improving the salinity of the reaction system can promote the occurrence of TSR, and water plays a key role in hydrocarbon generation evolution and the TSR.


Introduction
Abnormal emission and casualty accidents caused by the enrichment of hydrogen sulfide (H 2 S) in coal mines occur frequently domestically and internationally. High-sulfur natural gas reservoirs (volume percentage of H 2 S greater than 5%) are widely distributed around the world, and all are closely related to the distribution of sulfate reservoirs. Previous studies have shown that thermochemical sulfate reduction (TSR) is one of the main ways to form hydrogen sulfide in oil and gas reservoirs and coal seam strata [1,2]. The simulation research of scholars domestically and internationally about the thermochemical reduction reaction are mostly concentrated in the field of oil and natural gas, and a large number of cracking and thermal simulation experiments have been carried out [3][4][5][6]. TSR research has been carried out for more than 30 years, involving oil and gas geology, mineralogy, geochemistry, experimental geology, and many other fields, and TSR initiation mechanism, production products, influencing factors, isotope fractionation, reaction mechanism, discriminatory markers, limit temperature, and kinetic characteristics have been studied [7][8][9][10].
However, there are still few experimental studies on the simulation of thermochemical reduction of coal and sulfate to produce hydrogen sulfide-containing gas. Based on the previous studies, this study adopts the method of high temperature and high pressure where M ad is the air-dried moisture and A d is the ash dry and thermal simulation experiments under various media conditions in a closed system. The temperature of hydrocarbon formation under geological conditions is about 70~200°C, and the temperature of hydrogen sulfide formed by TSR is 120~200°C. In order to accelerate the reaction, the study chooses a higher temperature and conducts TSR thermal simulation experiments on coal in order to provide theoretical guidance on the evolution and formation mechanism of hydrogen sulfide gas in coal-series strata.

Experimental
Samples. An anthracite fresh coal sample used in this experiment was selected from the no. 15 coal seam in a coal mine of Mountain Fenghuang, southern Qinshui Basin, Shanxi Province, China, where the H 2 S is abnormally enriched, and the parameters of the samples were measured. The industrial analysis, elemental analysis, and sulfur composition are shown in Table 1. After the coal sample was crushed, it was sieved out with the particle size of 80-150 μm in the constant temperature vacuum drying oven at 50°C to make a dry sample of 8000 g as the experimental coal sample, and the sieved sample was divided into 8 portions; each portion of 1000 g was stored in a dry bottle and injected with nitrogen and sealed for use.
M ad is the air-dried moisture, A d is the dry ash, V daf is the dry ash-free volatile, S t,d is the dry basis total sulfur, S o,d is the dry basis organic sulfur, and S p,d is the dry basis pyretic sulfur. The coal of sample no. 15 is mostly black and powdery, and a small part is massive. It is a semibright briquette with mirrored coal ribbon and silk carbon lens. It has a thinmedium strip structure and occasionally white calcite and dispersed, nodular, and film-shaped pyrite.

Experimental
Method. The experimental device mainly consists of a reactor, control device, degassing device, loading device, and gas collection and analysis device. The system diagram is shown in Figure 1 [11]. The maximum temperature control of the system is 650°C, and the accuracy is ±1°C. The maximum pressure control is 25.0 MPa, and the accuracy is ±0.5 MPa. During the experiment, the quartz tube containing 1000 ± 0:1 g of the experimental samples was successively put into the kettle. After connecting all air circuits and temperature sensors of the equipment and detecting the air tightness of each air circuit, the vacuum degasses the entire system. After degassing, different media were injected into the quartz tube in the reactor from the loading device according to the experimental scheme. Into the quartz tube in the reaction kettle, 300 mL of an ion aqueous solution with a molar concentration of 1.0 mol/L of mineral aqueous solution containing additives was injected. The initial pressure in the kettle is set to 5.0 MPa, and the final pressure of the reaction system is between 12.0 and 20.0 MPa. The reaction is heated at a heating rate of 20°C/h, simulating gaseous products at 8 temperature points of 250°C, 300°C, 350°C, 400°C, 450°C, 500°C, 550°C, and 600°C, and these were analyzed. The heating time for each simulation experiment stage was 24 hours. At the end of each experimental stage, the gaseous products in the reactor were collected using a special sampler for hydrogen sulfide and a Tedlar gas sample bag. Using a gas chromatograph, the gaseous products of hydrocarbon gases (methane, heavy hydrocarbons) and nonhydrocarbon gases (CO 2 , H 2 , and H 2 S) were measured.

Experimental Results and Discussion
The coal sample used in this experiment can be regarded as a typical porous medium. Its fine particle size and its connected internal channels allow the gas to flow smoothly through its surface and inside, and it is also a place for heat transfer and physical and chemical reactions. The gaseous product test results of hydrocarbon gas (CH 4 , C 2~5 ) and nonhydrocarbon gas (CO 2 , H 2 , and H 2 S) are shown in Table 2.
Coal is a complex polymer compound with condensed aromatic nuclei derived from lignin as the main body, con-nected by various bridges and carrying various types of fatty and nonfatty functional groups [12]. The coal pyrolysis process begins with the breaking of the fatty carbon-fatty carbon bond (C al -C al ), and at about 500~600°C, the aliphatic carbonhydrogen bond (Cal-H) is broken. The aromatic carbo-n=aromatic carbon bond (C ar =C ar ) on the aromatic ring is very difficult to break during pyrolysis due to its very stable structure. The oxygen-containing structures and other heterocyclic structures in coal break at 400-700°C. Fat carbonsulfur bridge bond (Cal-S) fracture temperature is usually lower than that of C al -C al , and C al -S bond cleavage promotes free radical formation. In the early stage of thermal evolution at low temperature, the gas adsorbed in the pores of the coal and the weak bonds in the coal structure are broken, and the gases such as CH 4 , CO 2 , and N 2 are removed. As the thermal simulation temperature further increases, the weakest bridge bonds in coal, such as ether bonds, thioether bonds, disulfides, aromatics, and fatty side chains, begin to break down and decompose. Oxygen-containing functional groups in coal will also be cracked, showing that fatty methyl groups are shed to form CH 4 and H 2 , carbonyl groups are cracked to form CO, and the fatty bridge bond (-CH 2 -CH 2 -) is broken and sulfide (sulfur radical) is combined to form H 2 S. The reaction can generate CH 4 , C 2 H 6 , C 2 H 4 , and H 2 O, CO, CO 2 , and H 2 S, and other gases, as shown in Figure 2 [13].
Hydrocarbon and nonhydrocarbon gaseous product changes are characterized in Figures 3-8. The generation of CH 4 is related to 5 aspects [14][15][16]: (1) adsorption of CH 4 in coal pores by thermal desorption, (2) fat side chain breakage on the coal surface, (3) pyrolysis of methylene bridge bonds in coal main structure, (4) pyrolysis of oxidized aromatic ring structure, and (5) broken fatty side chain in the aromatic structure. In the first stage, 250°C, the desorption of CH 4 adsorbed in coal pores happened. The second stage is 300~450°C, mainly formed by the breaking of aryl, alkyl, and ether bonds. The third stage 450/500~550°C, from relatively stable chemical bond cleavage, for example, the alkyl on the side chain of aromatic compounds breaks to form low molecular alkanes such as CH 4 . After 550°C in the fourth stage, it was mainly the polycondensation between aromatic rings to form a condensed ring aromatic structure, which releases CH 4 . With the addition of water and additives, the yield of methane and total gas is higher, which is expressed as "coal+water and MgSO 4 and AlCl 3 " > "coal+water and MgSO 4 " > "coal+water" > "coal." Comparison of the gas yield results showed that the presence of water and additives led to a significant increase in the total gas yield and methane gas yield, as shown in Figures 3 and 4.
Heavy hydrocarbons are dominated by C 2 H 6 , and their output increases first and then decreases as the temperature of the simulated experiment increases. Compared with the  groups of "coal" and "coal+water," the addition of MgSO 4 makes the heavy hydrocarbon reach its maximum value at 400°C, which obviously reduces the temperature required to reach the peak of the heavy hydrocarbon yield and has a certain promotion effect on the formation of heavy hydrocarbon, as shown in Figure 5. Heavy hydrocarbons are mainly produced by the reaction of -OH attached to the C atom in the middle part of the aliphatic hydrocarbon chain in coal to form -COOH, and its formation can be described as follows [11]: The heavy hydrocarbon yield decreases rapidly with increasing thermal simulation temperature after 400/450°C. The comparison revealed that the decrease in heavy hydrocarbon content was intensified in the presence of water, indicating that water was involved in the TSR reaction process, while the presence of additives accelerated the reaction of heavy hydrocarbons with sulfur-containing compounds and played a certain role in promoting it. The equation can be described as follows [11]: The sources of H 2 were mainly polycondensation between free radicals and aromatic structures and polycondensation dehydrogenation reaction of hydrogenated aromatic structures [17][18][19]. The generation of H 2 in the first stage mainly includes hydrogen radicals mainly generated by the oxidation of hydrocarbons by minerals (sulfates) before 400°C, the cracking of long-chain aliphatic hydrocarbons to generate short-chain fatty radicals, and the secondary of light paraffins. Hydrogen radicals generated by cracking are combined with each other to generate hydrogen gas. At the second stage of 400~600°C, it is mainly formed by polycondensation between free radicals. They are the hydrogen radicals generated by the C-H bond breaking, cyclization, and aromatization of aliphatic hydrocarbons and the hydrogen radicals generated by the polycondensation reaction between aromatic rings and the reaction of C, CO, and H 2 O. After 350°C, the H 2 yield drops significantly, and the H 2 formed by the C-H bond breakage reacts with sulfur radicals to generate H 2 S. H 2 is generated only when there is a slight surplus, thereby reducing the hydrogen production rate. After the temperature reaches 500°C, it may be due to the insufficient supply of sulfur radicals in the sample and the deep reaction in the coal, such as the reaction of carbon and water and the reaction of CO and water to form H 2 , which leads to an increase in the yield of H 2 . In particular in the experimental conditions of anhydrous and water-free and salt-free addition of hydrogen, the hydrogen production has always been higher under the conditions of water-free and salt-free addition. This indicates that under anhydrous conditions, hydrogen consumption is lower and TSR is more difficult to occur, and water plays a very important role in TSR occurrence, which is consistent with the involvement of water in the TSR reaction mentioned earlier. The variation characteristics of H 2 production are shown in Figure 6, and its formation can be described as follows.

CO + H 2 O ⟶ CO 2 + H 2 ð5Þ
C + 2H 2 O ⟶ CO 2 + 2H 2 ð6Þ CO 2 is the hallmark product of TSR [12,20,21]. The precipitation of CO 2 between 250 and 350°C is mainly due to the  Figure 1: Schematic diagram of the experimental system. 1, 12, 13, 14-carrier gas cylinder; 2-controller; 3-pressure device; 4-liquid loading device; 5-pressure sensor; 6-temperature measuring device; 7-water circulation cooling device; 8-discharge port; 9-reactor body; 10-cooling circulation hole; 11-vacuum pump; 15-host; 16-chromatograph.       5 Geofluids decomposition of oxygen-containing carboxyl groups in coal by heat. After the temperature hits 350°C, most of the fatty bonds, oxygen-containing functional groups, and some aromatic weak bonds in coal are broken. Part of the broken carbonyl group is precipitated in the form of CO, and a part of it is combined with the oxygen atoms in the coal and precipitated in the form of CO 2 . After the temperature reaches 450°C, the CO 2 precipitation gradually increases, indicating that deep TSR has occurred. From the four yield curves, it can be seen that the amount of CO 2 production is significantly increased with the participation of water compared with the "coal" group alone, while MgSO 4 and AlCl 3 have a certain driving effect on the TSR reaction. The variation characteristics of CO 2 production are shown in Figure 7.

Geofluids
Decomposition of carboxyl groups, ether bonds, and other oxygen-containing functional groups occurs [20]. The reaction occurs as follows: Sulfur in coal includes organic sulfur and inorganic sulfur. Most of the organic sulfur in coal is part of the molecular structure of kerogen, mainly including mercaptans, sulfides, disulfides, thiophenes, sulfoxides, and sulfones, and most of them are mainly thiophene sulfur [22][23][24]. Sulfur in coal includes organic and inorganic sulfur, and organic sulfur in coal is mostly a component of the molecular structure of cheese root, mainly including mercaptans, sulfides, disulfides, thiophene, sulfoxide, and sulfone and mainly thiophene sulfur [22][23][24]. Its shape is complex and diverse, and the thermal stability varies greatly. The precipitation, migration, and morphological distribution of sulfur during the TSR and pyrolysis process are affected by the pyrolysis environment and the kerogen structure and mineral species in coal. Inorganic sulfur usually exists in the form of sulfide, sulfate, and trace elemental sulfur, mainly pyrite [22,25].
In the initial low-temperature stage, the TSR was low, and a small amount of H 2 S may form by the bond breakage of unstable organic sulfur (such as thioether, thiol), hydrocarbon oxidation, and sulfate reduction. H 2 S formed by hydrocarbon oxidation and sulfate reduction can in turn act as a catalyst to react with the dissolved form of sulfate to form S 0 , so the production of H 2 S was less. The possible reactions are as follows [8,26,27]: During thermal evolution, the formed hydrocarbons can react with the generated H 2 S to produce organic sulfides.
As the temperature rises, unstable organic sulfides can undergo reduction reactions with sulfates.
The presence and formation of S 0 in turn provide a way for saturated hydrocarbons to be converted into naphthenic acids, aromatic hydrocarbons, and other sulfides, which usually contain two major blocks [28]: The first block is dehydrogenation (oxidation): R-CH 2 CH 3 + S R-CH=CH 2 + H 2 S + 3S 3H 2 S + The second block is sulfide formation.
R is the fatty group. Thiophene only begins to decompose at 450°C. The alkylsubstituted phosgene loses its alkyl group around 500°C, and the thiophene begins to decompose at 550°C. The thiophene 6 Geofluids ring is stable, and decomposition begins only after at 800°C [29,30]. The pyrolysis products of thiophene are usually some small-molecule compounds, such as sulfur, carbon, hydrogen sulfide, and acetylene. Under a hydrogen-rich atmosphere, these compounds react as follows [11, 31, Thiophene compounds can be generated by the reaction of sulfur or H 2 S with organic matter, or pyrite with organic molecules similar to ethylene, and salts such as bauxite and magnesium sulfate may promote the reaction.
The sulfur in this experimental sample is mainly organic sulfur. The peak of H 2 S precipitation reaches its maximum at 450~500°C, which obviously belongs to the form of sulfide. Subsequently, hydrogen sulfide is mainly formed by pyrite and thiophene sulfur, and FeS 2 can be reduced to FeS and H 2 S at about 500°C. Structures such as aliphatic hydrocarbons and hydrogenated aromatic hydrocarbons in coal generate internal hydrogen radicals with hydrogen supply capability during the thermochemical reduction reaction, which promotes the reduction and decomposition of pyrite. The comparison of the hydrogen sulfide gas yield shows that the maximum yield of H 2 S in the "coal+water and MgSO 4 " system is 2.5 times higher than the maximum yield of H 2 S in the "coal+water" system and 8.5 times higher than the maximum yield of H 2 S in the "coal" system. This shows that the addition of MgSO 4 intensified the TSR reaction and promoted the production of H 2 S gas. The variation characteristics of H 2 S production are shown in Figure 8.
Water plays an important role as a solvent, reactant, and reaction carrier in aqueous thermal simulation reactions. The H element in the water is added to the coal sample under the synergistic effect of free radicals and ion effects, or ion exchange occurs with the minerals in the coal; it changes the environment of hydrothermal reaction to promote the cleavage of weak chemical bonds in the coal structure. In the process, it will also replace the oxygen-containing functional groups in the coal, so that the oxygen-containing functional groups decompose to form small-molecule gases and polar substances remaining in the water. It can also exchange with the hydrogen in coal and transfer in the coal structure. At the same time, the hydrogen and oxygen elements in the water may also be replaced by hydrogen atoms on the ben-zene ring under the synergistic effect of free radicals and ionic effects to form phenols.

Conclusion
The results of thermal simulation experiments showed that TSR promoted the cracking of coal, which led to an increase in the production of hydrogen sulfide and methane and a decrease in the production of heavy hydrocarbons. The presence of salts accelerates the evolution of hydrocarbon formation, and SO 4 2-, Al 3+ , and Mg 2+ contributed greatly to the TSR, resulting in an increase in the total amount of alkane gas and the production of H 2 S and CO 2 . Improving the salinity of the reaction system can promote the development of TSR.
In the thermal simulation system of coal, water is actually a hydrogen donor, providing some protonic hydrogen to stabilize the unstable intermediate material and accompanying the generation of H 2 . In terms of gas production, the participation of water provides an additional source of hydrogen for the TSR reaction to produce gas, which accelerates the reaction and promotes gas production. Water media play an important role in the evolution of hydrocarbon generation.

Data Availability
The data used to support the findings of this study are available from the corresponding author upon request.

Conflicts of Interest
The authors declare that they have no conflicts of interest.