# Study on H2S Occurrence in Low Sulfur Coal Seams

Authors: Fei Gao; Zhe Jia; Ji Xia; Dapeng Wang; Yulong Yang; Yafei Shan; Jiaqi Shen
Journal: Adsorption Science & Technology
 (2022)

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## Abstract

Coal samples from the Shanxi Shaping coal mine were selected to investigate the occurrence of H2S in low sulfur coal seams. The adsorption mechanism of coal to H2S was explored, and an occurrence equation for H2S in coal seams was fitted through adsorption experiment results. The results showed that under ambient temperature and pressure conditions, the H2S adsorbed by coal reached equilibrium within 24 h. The increase in H2S concentrations and the moisture content of coal samples resulted in an increase in the adsorption capacity of H2S. Chemical adsorption of H2S by the coal also occurred. The total sulfur content in the coal increased, and water promoted the conversion from H2S to sulfur in coal. After adsorption, most of the H2S remains in the coal structure in the form of inorganic sulfur, such as sulfur hydride, iron sulfide sulfur, and monomeric sulfur, and a small proportion of H2S is bonded in the structure of the coal in the form of organic sulfur such as thiophene, C-S-C, and C-SH. Therefore, the higher the total sulfur content in coal, the greater the occurrence of H2S. The total amount of H2S increased exponentially with the concentration of free H2S and the moisture content of coal at equilibrium. This meant that the total amount of H2S in the coal seam could be estimated by fitting an equation according to the concentration of free H2S and the moisture content of coal seams. The concentration of free H2S decreased linearly with the increase in moisture content of the coal, therefore, the concentration of H2S in space could be reduced by injecting water into coal seams.


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## Body

## 1. Introduction

Hydrogen sulfide (H2S) is the most common toxic gas found in coal mines [1]. Excessive concentrations of H2S can cause vital harm, including fatality and the risk of explosion. H2S reacts readily with metal equipment and causes electrochemical and stress corrosion, hydrogen embrittlement rupture, and other damage, reducing the service life of underground metal equipment. This causes significant safety risks [2, 3]. The Coal Mine Safety Regulations clearly stipulate that the H2S gas concentration in coal mines should not exceed 6.6 ppm. The content of H2S in coal seams is an important indicator that can be monitored to prevent H2S accidents in mines. It is also the basis for selecting treatment methods.Studies have found that the H2S content in coal seams is related to the total sulfur content of the coal [4]. In the process of geological evolution, sulfur element has remained in coal seams to form sulfur-containing coal, which can produce a large amount of H2S gas through biological sulfate reduction (BSR) and sulfate thermal reduction (TSR) [5–7]. Liu et al. [8] found that sulfate in coal seams could be transformed into organic sulfur and iron sulfide, and H2S appeared as an intermediate product of this transformation. However, Deng et al. [9] found that due to change in geological conditions, H2S dissolved in water infiltrated into deeper coal seams and remained in the coal body through a series of physical and chemical reactions. This eventually led to an increase in the total sulfur content in coal. Lin et al. [10] found that the active iron ions in coal react with H2S to form iron-sulfur compounds, resulting in a higher total sulfur content of the coal seams which contained H2S. Asaoka et al. [11] found that H2S would be adsorbed on the coal fly ash of coal seams with a higher oxidation degree, and occurred the redox reaction with that. Most of the H2S would be oxidized to form elemental sulfur and a small part would be oxidized to form sulfate.At present, the methods for measuring H2S in coal seams can be divided into direct and indirect methods [12, 13]. Direct methods involve studying the desorption rule of coal samples and estimating the H2S content of coal seams by underground drilling. Harvey et al. [14] considered that the H2S content of coal seams in southern Australia was approximately 4×10−2m3/t from the H2S content desorbed during mining. Xu [15] analyzed the desorption rule of coal samples by a drilling cutting method, and concluded that the H2S content in the coal seams of the Gaojiapu mine was approximately 2.704×10−3m3/t. Indirect methods estimate the content of H2S in coal seams by measuring the coal seam pressure, adsorption constant, porosity, and industrial analysis parameters. Gao [16] concluded that the H2S content in the coal seam of Wudong Mine was 4.57×10−4m3/t through onsite testing of the coal seam pressure, laboratory determination of coal adsorption parameters, and industrial analysis data.Both direct and indirect methods need to be based on the adsorption data of the coal to H2S. The research on the adsorption of coalbed methane mainly focuses on the adsorption of multiple mixed gases composed of CO2, N2, and CH4 [17–20], but there are few research results on the H2S adsorption by coal. Lu [21] found that the H2S adsorption by coal belonged to physical adsorption, and the adsorption capacity of H2S is greater than that of CH4 and N2. The main factors affecting the adsorption of H2S by coal are pore, maceral, water, pressure, and coalification degree. Ye et al. [22] studied the adsorption of H2S by coal via a partial pressure test method, and considered that the adsorption amount of H2S by coal increased with pressure and decreased with temperature. Cheng et al. [23] concluded that the higher the degree of coal metamorphism, the more favorable the adsorption of H2S by isothermal adsorption under equilibrium water conditions. Yang et al. [4] used H2S aqueous solution to soak coal samples and found that the solution created a significant increase in the pore volume of coal. The volume of open pores and semiclosed pores increased, the total pore volume of the coal increased by 23.09%, so the adsorption capacity of the samples increased on the side. At present, research on the adsorption rule change of coal to H2S only accounts for factors such as pressure, temperature, and the metamorphic degree. The influence of other factors on the adsorption of H2S by coal has not been included, and research on the adsorption mechanism of coal to H2S has not been explored in depth. On the other hand, both the direct and indirect methods estimate the total content of H2S in coal seam which only originates from the content of adsorbed H2S in coal seam. In addition to the adsorbed state, the occurrence of H2S in the coal body is also found in the form of free and water-soluble states. Therefore, the current method of measuring the content of H2S has the disadvantages of difficulty in measurement and low accuracy.In this paper, a self-made experimental device was used to carry out H2S adsorption experiment at the atmosphere temperature and pressure. The quantitative influence of the H2S concentration, and the moisture content of coal, on the adsorption of H2S by coal was analyzed. The total amount of H2S in a coal seam can be calculated by measuring the free H2S concentration and the moisture content of the seam. At the same time, X-ray photoelectron spectroscopy (XPS) were used to analyze changes of sulfur-containing groups in coal before and after H2S adsorption, to explore the transformation relationship between H2S and sulfur-containing groups. It is of significant importance that a method of estimating the amount of H2S in coal seams is outlined, and that a theoretical basis is provided for the prevention and control of H2S in coal mines.


## 2. Experiment

### 2.1. Sample Preparation

#### 2.1.1. Coal Sample Preparation

Experimental coal samples were collected from Shanxi Shaping (SP) Coal Mine, coal seam No.13, which has the H2S concentration exceedance problem. According to the national standard industrial analysis method of coal (GB/T212-2008) and the standard for the determination method of total sulfur in coal (GB/T214-2007), the industrial and total sulfur content analysis of raw coal were carried out. The results are shown in Table 1. According to the national standard coal quality classification (GB/T15224.2-2021), the sulfur content of SP coal is between 0.5% and 1%, which is low sulfur coal.Table 1
Industrial analysis and total sulfur content analysis of SP coal.
Industrial analysis(%)Total sulfur(%)AadVadMadFCadSt, d26.9825.381.945.740.73Note: daf means dry ash-free basis, ad means air-dry basis, Aad-Ash content, Vad-Volatile content, Mad-Moisture, FCad-Carbon content, St, d-Total sulfur.SP raw coal was crushed, sieved, and pulverized until a particle size of 60–80 mesh (0.18–0.25 mm) was obtained for the adsorption test. To prevent coal samples from exposure to moisture and possible oxidation, the sieved coal samples were vacuum dried and stored. Five 1 g samples of dry coal were weighed by electronic balance, and four samples were sprayed with 0.031, 0.053, 0.111, and 0.25 g of distilled water, respectively. This method ensured that samples with a water content of 0, 3, 5, 10, and 20% were prepared successfully.When preparing water-immersed coal samples, 5 g of coal was weighed, and completely immersed in distilled water with a solid-liquid ratio of 1 : 1. The experiment was carried out after mechanical stirring for 24 h.


#### 2.1.2. Adsorption Gas Preparation

The H2S used in the experiment was a self-made adsorption gas. To eliminate the interference of other gases on the adsorption of H2S by coal, the N2 was used as the background gas. The gasification process is shown in Figure 1. In the experiment, N2 was used to purge the entire gas system. A certain amount of dilute sulfuric acid and ferrous sulfide were placed in the Kipp’s apparatus, and the gas collection bag was opened. The H2S produced by the reaction was diluted with N2 and dried. The N2 flow rate was controlled by a pressure regulator to control the collected H2S concentration. H2S gas mixtures with concentrations of 52, 96, 260, 400, 465, 750, 800, and 860 ppm were prepared.Figure 1
Flow diagram of gas production.


### 2.2. H2S Adsorption Experiment

#### 2.2.1. Experimental System

The coal adsorption H2S experimental system is shown in Figure 2, including the gas supply device, vacuum pump, and H2S detector. The gas supply device, the vacuum pump, and the H2S detector were connected with the sealed adsorption chamber through the pipeline. The adsorption chamber was made of an acrylic material that does not react with H2S. The adsorption chamber length (l) was 19 cm, width (w) was 19 cm, height (h) was 20 cm, volume:
(1)V=lwh=19×19×20cm3=7.22×103cm3=7.22L.Figure 2
Experimental system for H2S adsorption in coal at the atmosphere temperature and pressure (Vacuum Pump, Adsorption chamber, H2S detector, and H2S collection bag photograph courtesy of Zhe Jia, Copyright 2022).After degassing, 400 ppm of H2S was introduced into the adsorption chamber to a pressure slightly higher than 0.1 MPa. After 24 h, the pressure in the adsorption chamber remained unchanged. The H2S detector showed that there was no gas leakage outside the adsorption chamber, and the H2S gas concentration in the adsorption chamber remained at 400 ppm, indicating adsorption chamber was good at air tightness.


#### 2.2.2. Experimental Scheme

To study the effect of the gas concentration on the adsorption of H2S by coal, the five dry samples were placed in the adsorption chamber. A vacuum pump was used to degas the adsorption chamber over 2 h. Using a gas delivery pump, H2S was introduced to the adsorption chamber at a pressure 0.1 MPa greater than the existing chamber pressure. This was done for each sample with concentrations of 52, 96, 260, 465, and 850 ppm. The H2S concentration in the adsorption chamber was measured by a H2S detector every 2 h. The pressure of the adsorption chamber and the ambient temperature changes were recorded for 24 h. The adsorption experiments with different H2S concentrations were sequentially labeled as SP-52, SP-96, SP-260, SP-465, and SP-850.To study the effect of water content on the adsorption of H2S by coal, adsorption experiments using 750 ppm H2S gas and coal samples with a water content of 0, 3, 5, 7, 10, and 20% were carried out in turn. The experimental steps were the same as those above. The adsorption of H2S by coal samples with different water content was labeled as SP-0%, SP-3%, SP-5%, SP-7%, SP-10%, and SP-20%.According to the ideal gas equation, the amount of H2S in the gas phase space of the adsorption chamber at any time during the adsorption process can be calculated:
(2)ni=PiVRTi,where Pi is the partial pressure of H2S in adsorption chamber (Pa); V is the volume of the adsorption chamber (0.00722 m3); ni is the amount of substance adsorbing H2S in the chamber (mol); Ti is the ambient temperature during the adsorption process (K); R is the gas constant of ideal gas, approximately 8.314 J/(mol⋅K);
(3)Vi=n0−niVmm×103,where Vi is the adsorption capacity of coal to H2S at a certain time point (m3/t); Vm is the molar volume of gas at the atmosphere temperature and pressure (24.5 L/mol); n0 is the total amount of H2S in the adsorption chamber (mol); ni is the amount of free H2S in the adsorption chamber (mol); m is the mass of coal (g).


### 2.3. Experiment on the Influence of H2S Adsorption on Sulfur Forms in Coal

A 5 g dry coal sample, raw coal, and water-immersed coal, use the above adsorption experiment method to adsorb H2S with a concentration of 400 and 800 ppm, respectively. After adsorption, the adsorbed H2S was removed by vacuum pump for 2 h. According to the Coulomb titration method, the total sulfur content in coal before and after adsorption of H2S was analyzed by LCS-430 automatic sulfur analyzer. The sample mass was 50 mg, the catalyst used was tungsten trioxide, and the combustion temperature was 1050°C.X-ray photoelectron spectroscopy (XPS) technology was used to analyze the morphological changes of sulfur on the coal surface before and after the adsorption of H2S by using an AXISULTRAD LD instrument from the Kratos Analytical Company. The X-ray source used was Al Kα radiation (hv=1486.6eV), and the sample analysis area was 700×300μm. The measurement parameters were as follows: working power 150 W, full scanning transmittance 160 eV, energy analyzer fixed transmission energy 40 eV, vacuum degree 10-8 Pa, and step size 0.05 eV. The binding energy correction was performed at C 1 s (284.6 eV). The Casa XPS software was used to fit the sulfur peaks onto the coal surface.


## 2.1. Sample Preparation

### 2.1.1. Coal Sample Preparation

Experimental coal samples were collected from Shanxi Shaping (SP) Coal Mine, coal seam No.13, which has the H2S concentration exceedance problem. According to the national standard industrial analysis method of coal (GB/T212-2008) and the standard for the determination method of total sulfur in coal (GB/T214-2007), the industrial and total sulfur content analysis of raw coal were carried out. The results are shown in Table 1. According to the national standard coal quality classification (GB/T15224.2-2021), the sulfur content of SP coal is between 0.5% and 1%, which is low sulfur coal.Table 1
Industrial analysis and total sulfur content analysis of SP coal.
Industrial analysis(%)Total sulfur(%)AadVadMadFCadSt, d26.9825.381.945.740.73Note: daf means dry ash-free basis, ad means air-dry basis, Aad-Ash content, Vad-Volatile content, Mad-Moisture, FCad-Carbon content, St, d-Total sulfur.SP raw coal was crushed, sieved, and pulverized until a particle size of 60–80 mesh (0.18–0.25 mm) was obtained for the adsorption test. To prevent coal samples from exposure to moisture and possible oxidation, the sieved coal samples were vacuum dried and stored. Five 1 g samples of dry coal were weighed by electronic balance, and four samples were sprayed with 0.031, 0.053, 0.111, and 0.25 g of distilled water, respectively. This method ensured that samples with a water content of 0, 3, 5, 10, and 20% were prepared successfully.When preparing water-immersed coal samples, 5 g of coal was weighed, and completely immersed in distilled water with a solid-liquid ratio of 1 : 1. The experiment was carried out after mechanical stirring for 24 h.


### 2.1.2. Adsorption Gas Preparation

The H2S used in the experiment was a self-made adsorption gas. To eliminate the interference of other gases on the adsorption of H2S by coal, the N2 was used as the background gas. The gasification process is shown in Figure 1. In the experiment, N2 was used to purge the entire gas system. A certain amount of dilute sulfuric acid and ferrous sulfide were placed in the Kipp’s apparatus, and the gas collection bag was opened. The H2S produced by the reaction was diluted with N2 and dried. The N2 flow rate was controlled by a pressure regulator to control the collected H2S concentration. H2S gas mixtures with concentrations of 52, 96, 260, 400, 465, 750, 800, and 860 ppm were prepared.Figure 1
Flow diagram of gas production.


## 2.1.1. Coal Sample Preparation

Experimental coal samples were collected from Shanxi Shaping (SP) Coal Mine, coal seam No.13, which has the H2S concentration exceedance problem. According to the national standard industrial analysis method of coal (GB/T212-2008) and the standard for the determination method of total sulfur in coal (GB/T214-2007), the industrial and total sulfur content analysis of raw coal were carried out. The results are shown in Table 1. According to the national standard coal quality classification (GB/T15224.2-2021), the sulfur content of SP coal is between 0.5% and 1%, which is low sulfur coal.Table 1
Industrial analysis and total sulfur content analysis of SP coal.
Industrial analysis(%)Total sulfur(%)AadVadMadFCadSt, d26.9825.381.945.740.73Note: daf means dry ash-free basis, ad means air-dry basis, Aad-Ash content, Vad-Volatile content, Mad-Moisture, FCad-Carbon content, St, d-Total sulfur.SP raw coal was crushed, sieved, and pulverized until a particle size of 60–80 mesh (0.18–0.25 mm) was obtained for the adsorption test. To prevent coal samples from exposure to moisture and possible oxidation, the sieved coal samples were vacuum dried and stored. Five 1 g samples of dry coal were weighed by electronic balance, and four samples were sprayed with 0.031, 0.053, 0.111, and 0.25 g of distilled water, respectively. This method ensured that samples with a water content of 0, 3, 5, 10, and 20% were prepared successfully.When preparing water-immersed coal samples, 5 g of coal was weighed, and completely immersed in distilled water with a solid-liquid ratio of 1 : 1. The experiment was carried out after mechanical stirring for 24 h.


## 2.1.2. Adsorption Gas Preparation

The H2S used in the experiment was a self-made adsorption gas. To eliminate the interference of other gases on the adsorption of H2S by coal, the N2 was used as the background gas. The gasification process is shown in Figure 1. In the experiment, N2 was used to purge the entire gas system. A certain amount of dilute sulfuric acid and ferrous sulfide were placed in the Kipp’s apparatus, and the gas collection bag was opened. The H2S produced by the reaction was diluted with N2 and dried. The N2 flow rate was controlled by a pressure regulator to control the collected H2S concentration. H2S gas mixtures with concentrations of 52, 96, 260, 400, 465, 750, 800, and 860 ppm were prepared.Figure 1
Flow diagram of gas production.


## 2.2. H2S Adsorption Experiment

### 2.2.1. Experimental System

The coal adsorption H2S experimental system is shown in Figure 2, including the gas supply device, vacuum pump, and H2S detector. The gas supply device, the vacuum pump, and the H2S detector were connected with the sealed adsorption chamber through the pipeline. The adsorption chamber was made of an acrylic material that does not react with H2S. The adsorption chamber length (l) was 19 cm, width (w) was 19 cm, height (h) was 20 cm, volume:
(1)V=lwh=19×19×20cm3=7.22×103cm3=7.22L.Figure 2
Experimental system for H2S adsorption in coal at the atmosphere temperature and pressure (Vacuum Pump, Adsorption chamber, H2S detector, and H2S collection bag photograph courtesy of Zhe Jia, Copyright 2022).After degassing, 400 ppm of H2S was introduced into the adsorption chamber to a pressure slightly higher than 0.1 MPa. After 24 h, the pressure in the adsorption chamber remained unchanged. The H2S detector showed that there was no gas leakage outside the adsorption chamber, and the H2S gas concentration in the adsorption chamber remained at 400 ppm, indicating adsorption chamber was good at air tightness.


### 2.2.2. Experimental Scheme

To study the effect of the gas concentration on the adsorption of H2S by coal, the five dry samples were placed in the adsorption chamber. A vacuum pump was used to degas the adsorption chamber over 2 h. Using a gas delivery pump, H2S was introduced to the adsorption chamber at a pressure 0.1 MPa greater than the existing chamber pressure. This was done for each sample with concentrations of 52, 96, 260, 465, and 850 ppm. The H2S concentration in the adsorption chamber was measured by a H2S detector every 2 h. The pressure of the adsorption chamber and the ambient temperature changes were recorded for 24 h. The adsorption experiments with different H2S concentrations were sequentially labeled as SP-52, SP-96, SP-260, SP-465, and SP-850.To study the effect of water content on the adsorption of H2S by coal, adsorption experiments using 750 ppm H2S gas and coal samples with a water content of 0, 3, 5, 7, 10, and 20% were carried out in turn. The experimental steps were the same as those above. The adsorption of H2S by coal samples with different water content was labeled as SP-0%, SP-3%, SP-5%, SP-7%, SP-10%, and SP-20%.According to the ideal gas equation, the amount of H2S in the gas phase space of the adsorption chamber at any time during the adsorption process can be calculated:
(2)ni=PiVRTi,where Pi is the partial pressure of H2S in adsorption chamber (Pa); V is the volume of the adsorption chamber (0.00722 m3); ni is the amount of substance adsorbing H2S in the chamber (mol); Ti is the ambient temperature during the adsorption process (K); R is the gas constant of ideal gas, approximately 8.314 J/(mol⋅K);
(3)Vi=n0−niVmm×103,where Vi is the adsorption capacity of coal to H2S at a certain time point (m3/t); Vm is the molar volume of gas at the atmosphere temperature and pressure (24.5 L/mol); n0 is the total amount of H2S in the adsorption chamber (mol); ni is the amount of free H2S in the adsorption chamber (mol); m is the mass of coal (g).


## 2.2.1. Experimental System

The coal adsorption H2S experimental system is shown in Figure 2, including the gas supply device, vacuum pump, and H2S detector. The gas supply device, the vacuum pump, and the H2S detector were connected with the sealed adsorption chamber through the pipeline. The adsorption chamber was made of an acrylic material that does not react with H2S. The adsorption chamber length (l) was 19 cm, width (w) was 19 cm, height (h) was 20 cm, volume:
(1)V=lwh=19×19×20cm3=7.22×103cm3=7.22L.Figure 2
Experimental system for H2S adsorption in coal at the atmosphere temperature and pressure (Vacuum Pump, Adsorption chamber, H2S detector, and H2S collection bag photograph courtesy of Zhe Jia, Copyright 2022).After degassing, 400 ppm of H2S was introduced into the adsorption chamber to a pressure slightly higher than 0.1 MPa. After 24 h, the pressure in the adsorption chamber remained unchanged. The H2S detector showed that there was no gas leakage outside the adsorption chamber, and the H2S gas concentration in the adsorption chamber remained at 400 ppm, indicating adsorption chamber was good at air tightness.


## 2.2.2. Experimental Scheme

To study the effect of the gas concentration on the adsorption of H2S by coal, the five dry samples were placed in the adsorption chamber. A vacuum pump was used to degas the adsorption chamber over 2 h. Using a gas delivery pump, H2S was introduced to the adsorption chamber at a pressure 0.1 MPa greater than the existing chamber pressure. This was done for each sample with concentrations of 52, 96, 260, 465, and 850 ppm. The H2S concentration in the adsorption chamber was measured by a H2S detector every 2 h. The pressure of the adsorption chamber and the ambient temperature changes were recorded for 24 h. The adsorption experiments with different H2S concentrations were sequentially labeled as SP-52, SP-96, SP-260, SP-465, and SP-850.To study the effect of water content on the adsorption of H2S by coal, adsorption experiments using 750 ppm H2S gas and coal samples with a water content of 0, 3, 5, 7, 10, and 20% were carried out in turn. The experimental steps were the same as those above. The adsorption of H2S by coal samples with different water content was labeled as SP-0%, SP-3%, SP-5%, SP-7%, SP-10%, and SP-20%.According to the ideal gas equation, the amount of H2S in the gas phase space of the adsorption chamber at any time during the adsorption process can be calculated:
(2)ni=PiVRTi,where Pi is the partial pressure of H2S in adsorption chamber (Pa); V is the volume of the adsorption chamber (0.00722 m3); ni is the amount of substance adsorbing H2S in the chamber (mol); Ti is the ambient temperature during the adsorption process (K); R is the gas constant of ideal gas, approximately 8.314 J/(mol⋅K);
(3)Vi=n0−niVmm×103,where Vi is the adsorption capacity of coal to H2S at a certain time point (m3/t); Vm is the molar volume of gas at the atmosphere temperature and pressure (24.5 L/mol); n0 is the total amount of H2S in the adsorption chamber (mol); ni is the amount of free H2S in the adsorption chamber (mol); m is the mass of coal (g).


## 2.3. Experiment on the Influence of H2S Adsorption on Sulfur Forms in Coal

A 5 g dry coal sample, raw coal, and water-immersed coal, use the above adsorption experiment method to adsorb H2S with a concentration of 400 and 800 ppm, respectively. After adsorption, the adsorbed H2S was removed by vacuum pump for 2 h. According to the Coulomb titration method, the total sulfur content in coal before and after adsorption of H2S was analyzed by LCS-430 automatic sulfur analyzer. The sample mass was 50 mg, the catalyst used was tungsten trioxide, and the combustion temperature was 1050°C.X-ray photoelectron spectroscopy (XPS) technology was used to analyze the morphological changes of sulfur on the coal surface before and after the adsorption of H2S by using an AXISULTRAD LD instrument from the Kratos Analytical Company. The X-ray source used was Al Kα radiation (hv=1486.6eV), and the sample analysis area was 700×300μm. The measurement parameters were as follows: working power 150 W, full scanning transmittance 160 eV, energy analyzer fixed transmission energy 40 eV, vacuum degree 10-8 Pa, and step size 0.05 eV. The binding energy correction was performed at C 1 s (284.6 eV). The Casa XPS software was used to fit the sulfur peaks onto the coal surface.


## 3. Results and Discussion

### 3.1. Effect of H2S Concentration on H2S Adsorption by Coal

The changes in temperature, pressure, and H2S concentration in the adsorption chamber during the adsorption of coal at different concentrations of H2S are shown in Table S1.1 and Table S1.2 in the Appendix. From the data, the curve for the adsorption amount of H2S on the dry coal sample over time can be obtained at different concentrations of H2S, as shown in Figure 3. The relationship between H2S concentration and the saturation adsorption amount is shown in Figure 4.Figure 3
Variation curve of amount of H2S adsorbed over time for different concentrations of H2S.Figure 4
Relationship between H2S concentration and saturation adsorption capacity.In Figure3, the rate of adsorption of H2S on the coal samples at different concentrations of H2S was the fastest at the beginning of the 2 h; the adsorption rate of H2S on the coal samples gradually slowed down between 2 h and 20 h; the adsorption generally reached equilibrium between 20 h and 24 h. The average value of the adsorption amount during this period was taken as the saturation adsorption amount of H2S on the coal samples. Figure 4 shows that the saturation adsorption capacity of the coal sample is the smallest at 52 ppm H2S concentration, which is 0.40946 m3/t, and the saturation adsorption capacity of the coal sample was the largest at 850 ppm H2S concentration, which is 4.76215 m3/t. At any adsorption time, the magnitude of coal adsorption for different concentrations of H2S was ranked as follows: VSP−850>VSP−465>VSP−260>VSP−96>VSP−52, indicating that the adsorption of H2S gas on the coal samples showed an increasing trend with the increase in H2S concentration. This is because, according to the adsorption equilibrium theory, the greater the spatial H2S concentration, the greater the number of H2S molecules within the pores of the coal, which leads to greater adsorption of H2S on the coal.It was found that H2S has three main occurrence states in coal mines: adsorbed state, water-soluble state, and free state [24], among which the free state H2S concentration is most easily measured by instruments. In order to simulate the relationship between the total amount of H2S endowed in the coal seam and the free H2S concentration at the working face, the free H2S concentration in the adsorption chamber in the above experiment was plotted against the total amount of H2S without considering the H2S content in the water-soluble state. As shown in Figure 5, the linear correlation of the fitted Equation (4) was good, with an R2 of 0.99917.Figure 5
Relation curve between free H2S concentration and total H2S in dry coal.In Figure5, the total amount of H2S is shown to increase exponentially with the increase in free state H2S concentration at equilibrium. According to Equation (4), the total amount of H2S in anhydrous coal seams can be inferred by measuring the free state H2S concentration at the working face.
(4)V=3.07922×eC/287.87395−2.55212,where V is the total amount of H2S in the coal seam, m3/t; C is the concentration of free state H2S in the coal seam when the dry coal sample is at adsorption equilibrium, ppm.


### 3.2. Effect of Water Content on H2S Adsorption by Coal

In the experiment that investigates the effect of water content on the adsorption of H2S by coal, the changes in temperature, pressure, and H2S concentration in the adsorption chamber are shown in Table S2.1 and Table S2.2 in the Appendix. According to the data, the curve for the adsorption of H2S over time for coal samples having different water contents is as shown in Figure 6, and the relationship between the water content of the coal samples and the saturation adsorption amount is shown in Figure 7.Figure 6
Curve of adsorption amount over time for coal samples having different water contents.Figure 7
Relationship between water content of coal samples and saturation adsorption capacity.According to Figure6, the adsorption rate of H2S on coal samples having different moisture contents increased gradually over time, and the adsorption rate was the fastest at the beginning of the 2 h period. After that, the adsorption rate of H2S on the coal samples slowed down gradually and reached an equilibrium at between 20 h and 24 h. In Figure 7, the saturated adsorption of H2S on the dry coal sample was the smallest at 3.35899 m3/t; the saturated adsorption of H2S on the coal sample was the largest at 20% water content, which is 5.78075 m3/t. The magnitude of H2S adsorption on coal having different moisture contents over any adsorption time period was ranked as: VSP−20%>VSP−10%>VSP−7%>VSP−5%>VSP−3%>VSP−0%, i.e., the adsorption of H2S on coal increased as the moisture content of the coal samples increased. This is because when the coal sample is dry, H2S exists in the pores of the coal only in the adsorbed state. As the water content increases, H2S is easily soluble in water, and part of H2S remains in the coal in the water-soluble state, which in turn leads to a decrease in the concentration of H2S in the space-free state and an increase in the saturated adsorption of H2S on the coal.It is known that the occurrence of H2S in coal seams is related to the water content of the coal. Therefore, to explore the total amount of H2S in water-bearing coal seams, the curve for the water content of the coal samples versus free state H2S concentration at equilibrium was fitted. As shown in Figure 8, the linear correlation of the fitted Equation (5) was good with R2 of 0.99552.Figure 8
Fitting curve of water content and free H2S concentration.In Figure8, the concentration of free H2S at equilibrium decreases linearly with the increase in the water content of the coal sample under the conditions of a certain total H2S. Therefore, it is presumed that the concentration of free H2S gas in space can be reduced by injecting water into the coal seam and increasing the water content of the coal seam.
(5)CH2S=−12.74949Wt+C,where CH2S is the free state H2S concentration at adsorption equilibrium for aqueous coal, ppm; Wt is the moisture content of the coal sample, %; C is the free state H2S concentration at adsorption equilibrium for dry coal, ppm.The equation of the relationship between the total amount of H2S in the coal seam and the free H2S concentration and water content of the coal seam can be obtained by associating Equation (4) with Equation (5):
(6)V=3.07922×e12.74949Wt+CH2S/287.87395−2.55212where V is the total amount of H2S in the coal seam, m3/t; Wt is the water content of the coal sample, %; CH2S is the free state H2S concentration in the water-bearing coal seam, ppm.According to Equation (6), the total amount of H2S in coal seams can be deduced by measuring the water content of coal and the concentration of free state H2S in space; the total amount of H2S provides a theoretical basis for H2S management in coal seams.


### 3.3. Changes in Sulfur State in Coal before and after H2S Adsorption

The changes in the total sulfur content of coal before and after the adsorption of H2S on dry coal samples, raw coal, and water-soaked coal samples are shown in Figure 9. The total sulfur content of all three coal samples increased after adsorption in 400 ppm and 800 ppm H2S, and the sulfur content increased more significantly after adsorption in 800 ppm H2S than in 400 ppm, which indicates that the H2S adsorption process can increase the total sulfur content of coal, and the element S stays in the coal structure in some states after H2S adsorption. The increase in total sulfur content of the three coal samples after H2S adsorption was the largest for the water-soaked coal, and the increase in total sulfur content after H2S adsorption was the smallest for dry coal, indicating that water promotes the conversion of H2S to sulfur in coal. There are two possible reasons for the increase in the total sulfur content of coal samples after the adsorption of H2S: one is due to experimental error, where the H2S molecules that are partially adsorbed into the pores of the coal before the sulfur measurement are not completely removed, which results in an increase in the total sulfur content of the coal; the other is due to the chemisorption of H2S during the adsorption on coal, where H2S reacts physically and chemically with active substances in the coal, which results in the elemental sulfur remaining in the structure of the coal.Figure 9
Changes of total sulfur content of coal before and after adsorption of H2S.To investigate the real reason for the increase in the total sulfur content in coal, X-ray photoelectron spectroscopy (XPS) tests were performed on coal samples before and after 800 ppm H2S adsorption. Organic sulfur in coal is mainly in the form of mercaptans, thioethers, thiophenes, sulfoxides, and sulfones, whereas inorganic sulfur is mainly in the form of sulfide sulfur, sulfate sulfur, and trace amounts of monomeric sulfur [25, 26]. During the XPS test, the 2p layer electrons of sulfur under X-ray excitation undergo energy level splitting, which results in a bimodal form of 2p3/2 and 2p1/2 in the XPS spectra. A bimodal spacing of 1.18 eV, a peak area ratio of about 2 : 1, and the same half-peak width [27] were used to fit the XPS spectra of elemental sulfur in coal by this method. The range of binding energies of different sulfur-containing groups are as follows: (160.7–162.3 eV) for FeS, FeS2; (162–163.0 eV) for thiols and thioethers; (163.9–164.9 eV) for thiophenes; (165.0–166.0 eV) for sulfoxides; (167.7–169.3 eV) for sulfones; (169.2–170.8 eV) for sulfates, and monomeric sulfur. The XPS split-peak fitted sulfur spectrum of SP coal is shown in Figure 10, with 6 binding energies and 12 characteristic peaks. The types and content of each sulfur-containing functional group are shown in Table 2.Figure 10
XPS spectrum of elemental S before and after H2S adsorption by SP coal, (a) before adsorption and (b) after adsorption.
(a)(b)Table 2
Distribution of sulfur-containing functional groups before and after H2S adsorption in SP coal.
Coal samplesFunctional groupsPeak position (eV)Content(%)Coal samplesFunctional groupsPeak position (eV)Content(%)Before adsorptionFeS, FeS2161.911.19After adsorptionFeS, FeS2161.404.98Thiol sulfide162.442.79Thiol sulfide162.661.33Thiophene164.0557.45Thiophene164.1050.39Sulfoxide165.9813.10Sulfoxide165.9313.00Sulfone168.6519.11Sulfone168.2318.42Sulfate, S170.776.36Sulfate, S170.1311.79According to the XPS fitting results, the organic sulfur in SP raw coal is mainly thiol, thioether, thiophene, sulfoxide, and sulfone, and the inorganic sulfur is mainly FeS, FeS2, sulfate, and monomeric sulfur. The proportion of thiophene is the highest, at 57.45%, indicating that thiophene is the main form of sulfur in SP coal, whereas the proportion of FeS and FeS2 is the lowest, at 1.19%. The content column shows that the percentage content of organic sulfur, such as mercaptan, thioether, thiophene, sulfoxide, and sulfone in the coal samples exhibits a decreasing trend after the adsorption of H2S gas, whereas the percentage content of inorganic sulfur, such as FeS, FeS2, sulfate, and monomeric sulfur, shows an increasing trend. This may be because the increase in the organic sulfur content of coal is small or unchanged after H2S adsorption, whereas the increase of inorganic sulfur content is significant, which leads to a decrease in the proportion of organic sulfur content. This suggests that the increase in the total sulfur content of coal during H2S adsorption is mainly due to the conversion of H2S to the inorganic sulfur component of coal.


### 3.4. Occurrence Relationship of Sulfur and H2S in Coal

From the above experiments, the adsorption of H2S on coal is a chemical adsorption process in addition to a physical adsorption process, which is mainly reflected in the increase of total sulfur content in the coal, with the most obvious increase of inorganic sulfur content. Because the mechanism of H2S adsorption on coal is still inconclusive, and the properties and structures of activated carbon are similar to coal, the mechanism of H2S adsorption on coal can be inferred from the research results of H2S adsorption on activated carbon. Deng et al. [28] found that there is both physical and chemical adsorption of H2S on activated carbon, and when chemisorption occurs, H2S combines with an aromatic carbon in activated carbon under the action of electric charge to form structures such as thiophene, C-S-C, and C-SH. Based on this, the occurrence relationship between sulfur and H2S in coal is presumed to be as shown in Figure 11.Figure 11
Occurrence relationship between sulfur and H2S in coal.H2S gas endowed in coal pores and fissures has two main sources. A small part is from the decomposition of dead plants and animals, and most comes from the reduction of SO42- [6], where in poorly sealed coal seams, H2S can be oxidized again by O2 to SO42-. Similar to activated carbon, coal contains a large number of aromatic carbon functional groups, and it is speculated that after coal adsorption of H2S, H2S combines with these functional groups and remains in the coal structure in the form of organic sulfur such as thiophene, C-S-C, and C-SH, which in turn leads to an increase in the total sulfur content of the coal.On the other hand, during the coal-forming stage, a large number of Fe ions are carried out of the deposited strata in the form of Fe (OH)3 colloidal solution and remain in the coal seam as stable complexes. Some Fe3+ are reduced to Fe2+ within the coal seam, and Fe2+ can react with H2S to form hydrometallic iron sulfide (FeS·nH2O), which reacts with sulfur monomers and transforms into pyrite after crystallization and dehydration; [29, 30] while the other Fe3+ can react directly with H2S under acidic conditions to form ferrous sulfide and S monomers according to the following reaction equation.
(7)Fe2++H2S+nH2O⟶FeS∙nH2O+2H+,S+FeS∙nH2O⟶FeS2∙nH2O⟶FeS2,2Fe3++3H2S⟶2FeS+S+6H+.In addition, it is known from the composition of coal [31] that coal contains a large amount of carbonate components, and with the participation of water, some soluble carbonates can react with H2S, which results in a portion of H2S remaining in the coal in the form of sulfur hydrides. The reaction equation is as follows:
(8)CO32−+2H2S⟶2HS−+H2O+CO2.Because the reaction process of H2S converting to inorganic sulfur is ionic, water promotes the conversion of H2S to inorganic sulfur in coal, which is consistent with the experimental result that water-soaked coal has the largest increase, of the three coal samples in Figure 9, in total sulfur content after the adsorption of H2S. In Figure 9, the total sulfur content of dry coal also increases after adsorption of H2S. Therefore, it can be inferred that after adsorption of H2S, a small portion of H2S is converted to organic sulfur forms such as thiophene, C-S-C, and C-SH, and most H2S is converted into inorganic sulfur forms such as sulfur hydride, iron sulfide sulfur, and monomeric sulfur with the participation of water, which in turn leads to an increase in the total sulfur content of coal. Therefore, during the process of H2S adsorption by coal, both physical and chemical adsorption processes occur, and there is a positive correlation between the total sulfur content of coal and the occurrence of H2S in coal seams, i.e., the higher the total sulfur content in coal, the greater the occurrence of H2S in coal seams.


## 3.1. Effect of H2S Concentration on H2S Adsorption by Coal

The changes in temperature, pressure, and H2S concentration in the adsorption chamber during the adsorption of coal at different concentrations of H2S are shown in Table S1.1 and Table S1.2 in the Appendix. From the data, the curve for the adsorption amount of H2S on the dry coal sample over time can be obtained at different concentrations of H2S, as shown in Figure 3. The relationship between H2S concentration and the saturation adsorption amount is shown in Figure 4.Figure 3
Variation curve of amount of H2S adsorbed over time for different concentrations of H2S.Figure 4
Relationship between H2S concentration and saturation adsorption capacity.In Figure3, the rate of adsorption of H2S on the coal samples at different concentrations of H2S was the fastest at the beginning of the 2 h; the adsorption rate of H2S on the coal samples gradually slowed down between 2 h and 20 h; the adsorption generally reached equilibrium between 20 h and 24 h. The average value of the adsorption amount during this period was taken as the saturation adsorption amount of H2S on the coal samples. Figure 4 shows that the saturation adsorption capacity of the coal sample is the smallest at 52 ppm H2S concentration, which is 0.40946 m3/t, and the saturation adsorption capacity of the coal sample was the largest at 850 ppm H2S concentration, which is 4.76215 m3/t. At any adsorption time, the magnitude of coal adsorption for different concentrations of H2S was ranked as follows: VSP−850>VSP−465>VSP−260>VSP−96>VSP−52, indicating that the adsorption of H2S gas on the coal samples showed an increasing trend with the increase in H2S concentration. This is because, according to the adsorption equilibrium theory, the greater the spatial H2S concentration, the greater the number of H2S molecules within the pores of the coal, which leads to greater adsorption of H2S on the coal.It was found that H2S has three main occurrence states in coal mines: adsorbed state, water-soluble state, and free state [24], among which the free state H2S concentration is most easily measured by instruments. In order to simulate the relationship between the total amount of H2S endowed in the coal seam and the free H2S concentration at the working face, the free H2S concentration in the adsorption chamber in the above experiment was plotted against the total amount of H2S without considering the H2S content in the water-soluble state. As shown in Figure 5, the linear correlation of the fitted Equation (4) was good, with an R2 of 0.99917.Figure 5
Relation curve between free H2S concentration and total H2S in dry coal.In Figure5, the total amount of H2S is shown to increase exponentially with the increase in free state H2S concentration at equilibrium. According to Equation (4), the total amount of H2S in anhydrous coal seams can be inferred by measuring the free state H2S concentration at the working face.
(4)V=3.07922×eC/287.87395−2.55212,where V is the total amount of H2S in the coal seam, m3/t; C is the concentration of free state H2S in the coal seam when the dry coal sample is at adsorption equilibrium, ppm.


## 3.2. Effect of Water Content on H2S Adsorption by Coal

In the experiment that investigates the effect of water content on the adsorption of H2S by coal, the changes in temperature, pressure, and H2S concentration in the adsorption chamber are shown in Table S2.1 and Table S2.2 in the Appendix. According to the data, the curve for the adsorption of H2S over time for coal samples having different water contents is as shown in Figure 6, and the relationship between the water content of the coal samples and the saturation adsorption amount is shown in Figure 7.Figure 6
Curve of adsorption amount over time for coal samples having different water contents.Figure 7
Relationship between water content of coal samples and saturation adsorption capacity.According to Figure6, the adsorption rate of H2S on coal samples having different moisture contents increased gradually over time, and the adsorption rate was the fastest at the beginning of the 2 h period. After that, the adsorption rate of H2S on the coal samples slowed down gradually and reached an equilibrium at between 20 h and 24 h. In Figure 7, the saturated adsorption of H2S on the dry coal sample was the smallest at 3.35899 m3/t; the saturated adsorption of H2S on the coal sample was the largest at 20% water content, which is 5.78075 m3/t. The magnitude of H2S adsorption on coal having different moisture contents over any adsorption time period was ranked as: VSP−20%>VSP−10%>VSP−7%>VSP−5%>VSP−3%>VSP−0%, i.e., the adsorption of H2S on coal increased as the moisture content of the coal samples increased. This is because when the coal sample is dry, H2S exists in the pores of the coal only in the adsorbed state. As the water content increases, H2S is easily soluble in water, and part of H2S remains in the coal in the water-soluble state, which in turn leads to a decrease in the concentration of H2S in the space-free state and an increase in the saturated adsorption of H2S on the coal.It is known that the occurrence of H2S in coal seams is related to the water content of the coal. Therefore, to explore the total amount of H2S in water-bearing coal seams, the curve for the water content of the coal samples versus free state H2S concentration at equilibrium was fitted. As shown in Figure 8, the linear correlation of the fitted Equation (5) was good with R2 of 0.99552.Figure 8
Fitting curve of water content and free H2S concentration.In Figure8, the concentration of free H2S at equilibrium decreases linearly with the increase in the water content of the coal sample under the conditions of a certain total H2S. Therefore, it is presumed that the concentration of free H2S gas in space can be reduced by injecting water into the coal seam and increasing the water content of the coal seam.
(5)CH2S=−12.74949Wt+C,where CH2S is the free state H2S concentration at adsorption equilibrium for aqueous coal, ppm; Wt is the moisture content of the coal sample, %; C is the free state H2S concentration at adsorption equilibrium for dry coal, ppm.The equation of the relationship between the total amount of H2S in the coal seam and the free H2S concentration and water content of the coal seam can be obtained by associating Equation (4) with Equation (5):
(6)V=3.07922×e12.74949Wt+CH2S/287.87395−2.55212where V is the total amount of H2S in the coal seam, m3/t; Wt is the water content of the coal sample, %; CH2S is the free state H2S concentration in the water-bearing coal seam, ppm.According to Equation (6), the total amount of H2S in coal seams can be deduced by measuring the water content of coal and the concentration of free state H2S in space; the total amount of H2S provides a theoretical basis for H2S management in coal seams.


## 3.3. Changes in Sulfur State in Coal before and after H2S Adsorption

The changes in the total sulfur content of coal before and after the adsorption of H2S on dry coal samples, raw coal, and water-soaked coal samples are shown in Figure 9. The total sulfur content of all three coal samples increased after adsorption in 400 ppm and 800 ppm H2S, and the sulfur content increased more significantly after adsorption in 800 ppm H2S than in 400 ppm, which indicates that the H2S adsorption process can increase the total sulfur content of coal, and the element S stays in the coal structure in some states after H2S adsorption. The increase in total sulfur content of the three coal samples after H2S adsorption was the largest for the water-soaked coal, and the increase in total sulfur content after H2S adsorption was the smallest for dry coal, indicating that water promotes the conversion of H2S to sulfur in coal. There are two possible reasons for the increase in the total sulfur content of coal samples after the adsorption of H2S: one is due to experimental error, where the H2S molecules that are partially adsorbed into the pores of the coal before the sulfur measurement are not completely removed, which results in an increase in the total sulfur content of the coal; the other is due to the chemisorption of H2S during the adsorption on coal, where H2S reacts physically and chemically with active substances in the coal, which results in the elemental sulfur remaining in the structure of the coal.Figure 9
Changes of total sulfur content of coal before and after adsorption of H2S.To investigate the real reason for the increase in the total sulfur content in coal, X-ray photoelectron spectroscopy (XPS) tests were performed on coal samples before and after 800 ppm H2S adsorption. Organic sulfur in coal is mainly in the form of mercaptans, thioethers, thiophenes, sulfoxides, and sulfones, whereas inorganic sulfur is mainly in the form of sulfide sulfur, sulfate sulfur, and trace amounts of monomeric sulfur [25, 26]. During the XPS test, the 2p layer electrons of sulfur under X-ray excitation undergo energy level splitting, which results in a bimodal form of 2p3/2 and 2p1/2 in the XPS spectra. A bimodal spacing of 1.18 eV, a peak area ratio of about 2 : 1, and the same half-peak width [27] were used to fit the XPS spectra of elemental sulfur in coal by this method. The range of binding energies of different sulfur-containing groups are as follows: (160.7–162.3 eV) for FeS, FeS2; (162–163.0 eV) for thiols and thioethers; (163.9–164.9 eV) for thiophenes; (165.0–166.0 eV) for sulfoxides; (167.7–169.3 eV) for sulfones; (169.2–170.8 eV) for sulfates, and monomeric sulfur. The XPS split-peak fitted sulfur spectrum of SP coal is shown in Figure 10, with 6 binding energies and 12 characteristic peaks. The types and content of each sulfur-containing functional group are shown in Table 2.Figure 10
XPS spectrum of elemental S before and after H2S adsorption by SP coal, (a) before adsorption and (b) after adsorption.
(a)(b)Table 2
Distribution of sulfur-containing functional groups before and after H2S adsorption in SP coal.
Coal samplesFunctional groupsPeak position (eV)Content(%)Coal samplesFunctional groupsPeak position (eV)Content(%)Before adsorptionFeS, FeS2161.911.19After adsorptionFeS, FeS2161.404.98Thiol sulfide162.442.79Thiol sulfide162.661.33Thiophene164.0557.45Thiophene164.1050.39Sulfoxide165.9813.10Sulfoxide165.9313.00Sulfone168.6519.11Sulfone168.2318.42Sulfate, S170.776.36Sulfate, S170.1311.79According to the XPS fitting results, the organic sulfur in SP raw coal is mainly thiol, thioether, thiophene, sulfoxide, and sulfone, and the inorganic sulfur is mainly FeS, FeS2, sulfate, and monomeric sulfur. The proportion of thiophene is the highest, at 57.45%, indicating that thiophene is the main form of sulfur in SP coal, whereas the proportion of FeS and FeS2 is the lowest, at 1.19%. The content column shows that the percentage content of organic sulfur, such as mercaptan, thioether, thiophene, sulfoxide, and sulfone in the coal samples exhibits a decreasing trend after the adsorption of H2S gas, whereas the percentage content of inorganic sulfur, such as FeS, FeS2, sulfate, and monomeric sulfur, shows an increasing trend. This may be because the increase in the organic sulfur content of coal is small or unchanged after H2S adsorption, whereas the increase of inorganic sulfur content is significant, which leads to a decrease in the proportion of organic sulfur content. This suggests that the increase in the total sulfur content of coal during H2S adsorption is mainly due to the conversion of H2S to the inorganic sulfur component of coal.


## 3.4. Occurrence Relationship of Sulfur and H2S in Coal

From the above experiments, the adsorption of H2S on coal is a chemical adsorption process in addition to a physical adsorption process, which is mainly reflected in the increase of total sulfur content in the coal, with the most obvious increase of inorganic sulfur content. Because the mechanism of H2S adsorption on coal is still inconclusive, and the properties and structures of activated carbon are similar to coal, the mechanism of H2S adsorption on coal can be inferred from the research results of H2S adsorption on activated carbon. Deng et al. [28] found that there is both physical and chemical adsorption of H2S on activated carbon, and when chemisorption occurs, H2S combines with an aromatic carbon in activated carbon under the action of electric charge to form structures such as thiophene, C-S-C, and C-SH. Based on this, the occurrence relationship between sulfur and H2S in coal is presumed to be as shown in Figure 11.Figure 11
Occurrence relationship between sulfur and H2S in coal.H2S gas endowed in coal pores and fissures has two main sources. A small part is from the decomposition of dead plants and animals, and most comes from the reduction of SO42- [6], where in poorly sealed coal seams, H2S can be oxidized again by O2 to SO42-. Similar to activated carbon, coal contains a large number of aromatic carbon functional groups, and it is speculated that after coal adsorption of H2S, H2S combines with these functional groups and remains in the coal structure in the form of organic sulfur such as thiophene, C-S-C, and C-SH, which in turn leads to an increase in the total sulfur content of the coal.On the other hand, during the coal-forming stage, a large number of Fe ions are carried out of the deposited strata in the form of Fe (OH)3 colloidal solution and remain in the coal seam as stable complexes. Some Fe3+ are reduced to Fe2+ within the coal seam, and Fe2+ can react with H2S to form hydrometallic iron sulfide (FeS·nH2O), which reacts with sulfur monomers and transforms into pyrite after crystallization and dehydration; [29, 30] while the other Fe3+ can react directly with H2S under acidic conditions to form ferrous sulfide and S monomers according to the following reaction equation.
(7)Fe2++H2S+nH2O⟶FeS∙nH2O+2H+,S+FeS∙nH2O⟶FeS2∙nH2O⟶FeS2,2Fe3++3H2S⟶2FeS+S+6H+.In addition, it is known from the composition of coal [31] that coal contains a large amount of carbonate components, and with the participation of water, some soluble carbonates can react with H2S, which results in a portion of H2S remaining in the coal in the form of sulfur hydrides. The reaction equation is as follows:
(8)CO32−+2H2S⟶2HS−+H2O+CO2.Because the reaction process of H2S converting to inorganic sulfur is ionic, water promotes the conversion of H2S to inorganic sulfur in coal, which is consistent with the experimental result that water-soaked coal has the largest increase, of the three coal samples in Figure 9, in total sulfur content after the adsorption of H2S. In Figure 9, the total sulfur content of dry coal also increases after adsorption of H2S. Therefore, it can be inferred that after adsorption of H2S, a small portion of H2S is converted to organic sulfur forms such as thiophene, C-S-C, and C-SH, and most H2S is converted into inorganic sulfur forms such as sulfur hydride, iron sulfide sulfur, and monomeric sulfur with the participation of water, which in turn leads to an increase in the total sulfur content of coal. Therefore, during the process of H2S adsorption by coal, both physical and chemical adsorption processes occur, and there is a positive correlation between the total sulfur content of coal and the occurrence of H2S in coal seams, i.e., the higher the total sulfur content in coal, the greater the occurrence of H2S in coal seams.


## 4. Conclusions

(1)
At the atmosphere temperature and pressure, the adsorption of H2S by coal generally reaches equilibrium at 24 h. The adsorption of H2S increases with the increase in spatial H2S concentration and the water content of the coal samples. In addition to physical adsorption, the chemical adsorption of H2S on coal also occurs. The total sulfur content of coal increases after adsorption of H2S, and water promotes the conversion process of H2S to sulfur in coal(2)
SP raw coal has the highest proportion of thiophene sulfur and the lowest proportion of FeS and FeS2. The inorganic sulfur content increases significantly after adsorption of H2S, and most of the H2S remains in the coal structure in the form of inorganic sulfur, such as sulfur hydride, iron sulfide sulfur, and monomeric sulfur, and a small proportion of H2S is bonded in the structure of the coal in the form of organic sulfur such as thiophene, C-S-C, and C-SH. Therefore, the higher the total sulfur content in coal, the greater the occurrence of H2S(3)
The total amount of H2S increases exponentially with the concentration of free H2S at equilibrium and the water content of the coal. The total amount of H2S in the coal seam can be inferred by measuring the concentration of free H2S in space and the water content of the coal seam according to the fitting equation. Under the condition that the total amount of H2S is certain, the concentration of free H2S decreases linearly with the increase in the water content of the coal sample. It is therefore inferred that the concentration of H2S in space can be reduced by injecting water into the coal seam, which providing a theoretical basis for the management of H2S in underground coal mines



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*Source: 5455101-2022-12-21.xml*