Field conditions for the synergistic increase of biomethane in the goaf of coal mines filled with corn straw

Synergistic fermentation of coal and corn straw is an effective tool to increase biomethane production. However, a large gap exists between the biomethane production conditions of corn straw filling coal mine goafs and laboratory experiments. In order to determine the effect of the field environment on synergistic biomethane production, biomethane production experiments with coal and corn straw were carried out under different conditions to find the key factors restricting the potential of biomethane production. The obtained results showed that various bacterial sources had significant influences on the biomethane production of coal and corn straw, and domesticated bacterial sources provided fermentation systems with more efficient biomethane production capacities than mine water sources. Biomethane production of coal and corn straw was relatively high under mixed conditions, but it was also promoted under unmixed conditions. Different inorganic minerals had different effects on synergistic biomethane production, which varied. For example, calcite, montmorillonite, and kaolin are common minerals in coal‐bearing strata that significantly enhance synergistic biomethane production of coal and corn straw. However, pyrite was found to significantly inhibit the synergistic biomethane production effect of coal and corn straw. Highly metamorphosed anthracite coal also presented biomethane production potential when stimulated by corn straw as a carbon source. The obtained results revealed the influences of different field conditions on the biomethane production of coal and corn straw and provided a reference for the field application of corn straw filling in coal mine goafs.


| INTRODUCTION
As an unconventional natural gas, coalbed methane (CBM) has a higher calorific value and cleaner combustion characteristics when compared to coal (Senthamaraikkannan et al., 2016).The CBM industry contributes to realizing carbon peak and carbon neutrality goals.Currently, annual CBM production in the United States is about 200 × 10 8 m 3 , in Australia it is over 400 × 10 8 m 3 , and in China, CBM production reached 115.5 × 10 8 m 3 in 2022 (Zhang et al., 2018).Biogenic gas is a key supplementary source of CBM, which is generated by the activity of microbial communities on organic matter in coal, resulting in hydrocarbon gases that are predominantly composed of methane.Nature and Science journals have published several reports on complex organic compounds that can be directly converted into biomethane by highly efficient single methanogens (Daisuke et al., 2016;Zhou et al., 2021).In recent years, coalbed gas bioengineering (CGB) has gradually progressed from indoor theoretical research to engineering practice.The United States, Australia, Indonesia, and China have performed several field experiments to inject methanogenic bacteria or nutrient solutions into CBM wells, which has confirmed the feasibility of this technology (Bai et al., 2014;Faiz et al., 2003;Ritter et al., 2015;Sun et al., 2018;Xia et al., 2021).Yoon and Guo et al. reported that co-degradation of coal with rice, corn, wheat, and sweet sorghum resulted in higher biomethane yields compared to degradation of coal or straw alone (Guo et al., 2019;Yoon et al., 2016).Guo et al. showed that corn straw had different promoting effects on the gas production of different coal types, and the methanogenic archaeal community of mixed fermentation of coal and corn straw was remarkably enhanced compared with that of single coal fermentation (Guo et al., 2018(Guo et al., , 2020)).
Coal mining has produced large areas of goaf and large amounts of residual coal, which have not only caused waste of resources but also posed a serious threat to the safe production of coal mines.Currently, the filling materials employed in coal mine goaf filling mainly include gangue, paste, and high-moisture materials.However, they suffer from disadvantages such as insufficient supply of gangue aggregates, low filling rates, and high investment costs (Cai et al., 2019;Li et al., 2023;Ruan et al., 2023;Shi et al., 2019;Zhao et al., 2020).Application of waste straw as filling material in coal mine goafs not only helps mitigate surface subsidence but also promotes residual coal conversion into methane.However, after straw, there is a certain degree of isolation between straw and residual coal, deviating from the ideal condition of complete mixing in indoor experimental setups.There are a large number of inorganic minerals in roof and floor strata as well as residual coal, such as clay minerals (kaolinite, illite, montmorillonite, etc.), calcite, pyrite, etc (He et al., 2022).Research has shown that inorganic minerals have a certain impact on coal biogas production (Zhang et al., 2020).However, when straw is filled into a coal mine goaf, the effects of these inorganic minerals on the synergistic conversion of both materials into biogenic methane are not well understood.Coal rank was found to have the most significant effect on biogas production, with lower coal ranks giving better results (Zhao et al., 2022).However, whether straw can serve as a carbon source for stimulating high-rank coals and improving the efficiency of biogas production is yet to be understood and requires further investigation.
Therefore, this research investigated the factors affecting the conversion of coal and corn straw into biogenic methane under field conditions, including the microbial source, substrate mixing degree, inorganic minerals, and high-rank coal.The aim of this study was to provide guidance for the practical application of filling coal mine goafs with straw by understanding the factors affecting the conversion of the two into methane.

| Sample collection and preparation
Bituminous coal A, B, and C collected from Shoushan No. 1 mine in Pingdingshan, Henan Province, Yukou mine in Datong Coal, Shanxi Province, and Wangjialing mine in Yuncheng, Shanxi Province were considered as experimental coal samples.Jiaozuo corn stalks collected from Henan Province were adopted as straw samples.Coal samples were crushed to 80-100 mesh, and corn straw was crushed through a 30 mesh sieve.Proximate and ultimate analyses were performed on coal and straw samples in accordance with ISO 17246-2010 andISO 17247-2013, respectively.The bacteria originated from the water resources of Jiaozuo Guhanshan Mine and coal seam located in the southern margin of the Junggar Basin in Xinjiang.Guhanshan mine's water flora in Jiaozuo was enriched and cultured, and one generation was grown for the stimulation of floral activities.Optimizing the microbiota structure through multi-generational domestication of the microflora in coal seams on the southern edge of the Junggar Basin in Xinjiang.
The media formulation was: 0.4 g K 2 HPO 4 , 0.1 g MgCl 2 , 0.2 g KH 2 PO 4 , 1.0 g yeast paste, 0.1 g tryptone, 1.0 g NH 4 Cl, 0.5 g cysteine, 0.2 g Na 2 S, 2 g NaHCO 3 , 0.5 g sodium acetate, 0.5 g sodium format, 10.0 mL trace element solution per 1 L of mine water, and distilled water were mixed.Domesticated bacteria liquid was mixed with the medium at a 1:5 ratio, and after configuration, nitrogen was purged for 5 min to replace air in the bottle and quickly sealed with parafilm.Then, the bottles were placed in a constant temperature incubator at 35°C for enrichment culture for 7 days.

| Experimental methods
To increase methanogen activity, it was important to nourish and develop the flora of the two separate sources prior to the biological gas generation simulation experiment.Samples of bituminous coal types A, B, and C, as well as corn stover samples and 200 mL enriched bacterial liquid, were added to an autoclaved 250 mL Erlenmeyer flask.The gas production device is shown in Figure 1.The reaction flask was rinsed with N 2 to provide an anaerobic environment.The conical flask was sealed and placed in a constant temperature incubator (35 ± 1°C) to be cultured.Three parallel samples were tested for each gas production experiment, and the experimental data were averaged.Gas production was recorded every 3 days, and gas components were analyzed qualitatively and quantitatively.Gas chromatography (Agilent 7890GC, Agilent Technologies) was used for the detection of the collected gas.

| Effects of different bacterial source conditions on biogas production
An optimal gas production combination of bituminous coal types A, B, and C and straw mixed fermentation was simulated by different bacterial sources.Gas production results are summarized in Table 2.
From Table 2 and Figure 2a, it was seen that the optimal biogas production combination of bituminous coal types A and B and straw mixed fermentation reached the maximum when mine water was used as bacteria source.The biogas production rates of M-A-2 and M-B-3 reached 367 and 366 mL, respectively.F I G U R E 1 Gas production device.
From Figure 2b, it was seen that the methane production of bituminous coals of type A, B, and C and straw biogas production combination with domesticated coal-origin bacteria was the highest.It was seen from Figure 2b that the methane production of the three biological gas production combinations of bituminous coals of type A, B, and C and straw with domesticated coal-origin bacteria was the highest.The methane productions of M-A-2, M-B-3, and M-C-2 reached 17.28, 14.88, and 12.51 mL/g, which were 52.11, 58.64, and 81.83% higher than those of D-A-2, D-B-3, and D-C-2, respectively.

| Effect of substrate mixing state on biogas production
Straws were wrapped with self-made nylon nets, and then coal samples, wrapped straws, and 200 mL enriched bacterial solution were added to 250 mL conical flasks after autoclaved sterilization to perform biological gas production experiments in non-mixed state.The results are summarized in Table 3.
As illustrated in Figure 3a-c, when bituminous coal type A and straw were mixed with mine water as bacteria source, biological gas production was maximized (367 mL), which was 6.10% higher than that in unmixed state.Using domesticated bacteria as bacteria source, gas production in unmixed state was 58.50% higher than that in mixed state.Biological gas production of bituminous coal type B and straw in mixed state with mine water as bacteria source was the largest (366 mL), which was 36.10% higher than that in non-mixed state.Using domesticated bacteria as bacteria source, gas production in unmixed state was 53.70% higher than that in mixed state.Biological gas production of bituminous coal type C and straw in mixed state with mine water as bacteria source was the lowest (266 mL), which was 15% lower than that in non-mixed state.When domesticated bacteria were employed as bacteria source, gas production in unmixed state was 57.50% higher than that in mixed state.Biomethane production under different coal and straw mixing conditions is shown in Figures 3d,e.When mine water was applied as bacteria source, biomethane production of bituminous coal type A and straw in unmixed state was the highest (11.79 mL), which was only 3.8% higher than that in mixed state, while that of bituminous coal types B and C in mixed and unmixed states showed a small difference.When domesticated bacteria were applied as bacteria source, biomethane production of bituminous coal type A and straw in mixed state was the highest (17.28 mL), which was 31.5% higher than that in unmixed state, while that of bituminous coal types B and C in mixed state were 21.00% and 10.5% higher than those in unmixed state, respectively.

| Effect of inorganic minerals on co-fermentation gas production
In this experiment, bituminous coal type B and straw with the highest biogas production were adopted for biogas production simulation experiments with a single inorganic mineral at a mass ratio of 3:1.Common inorganic minerals, such as pyrite, kaolin, calcite, and montmorillonite, were selected.The bacteria were collected from the coal seam in the southern margin of the Junggar Basin in Xinjiang.The pre-prepared bituminous coal type B, straw, 200 mL enriched bacterial solution, and 1 g inorganic mineral (pyrite, kaolin, calcite, and montmorillonite) were added to a 250 mL autoclaved conical flask, and high-purity nitrogen was purged for 3 min to replace air in the flask to ensure a strict anaerobic environment.Gas production results are summarized in Table 4.
According to Table 4, the biological gas production of bituminous coal type B and straw co-fermentation was 287 mL.From Figure 4a, it was seen that calcite, montmorillonite, and kaolin had significant effects on the biological gas production of bituminous coal type B and straw, which increased gas production by 57.49%, 55.75%, and 20.21%, respectively.At the same time, it was found that pyrite inhibited the biogas production of bituminous coal type B and straw co-fermentation.D-B-3(P) produced only 81 mL gas, which was 71.78% lower than that of the highest gas production combination of bituminous coal type B and straw co-fermentation.
It was seen from Figure 4b that the promotion order of different inorganic minerals on gas production of bituminous coal type B and straw was D-B-3(C) (30.41 mL/g) > D-B-3(M) (29.77 mL/g) > D-B-3(K) (20.41 mL/g) > D-B-3(P) (3.90 mL/g).Calcite, montmorillonite, and kaolin increased methane production of bituminous coal type B and straw by 104.37, 100.00, and 37.16%, respectively, while pyrite reduced methane production of the same mixture by 73.79%, indicating that pyrite had a strong inhibitory effect on biogas production of bituminous coal type B and straw.

| Effects of corn straw on anthracite biogas production
Anthracite (Ro, max = 3.54%) from the Shanxi Yangquan mining area and anthracite (Ro, max = 3.31%) from the Henan Jiaozuo mining area were applied in experiments.A bacteria source was obtained from a coal seam in the southern margin of the Junggar Basin in Xinjiang.The prepared two kinds of anthracite (4, 6, and 8 g), straw (2 g), and 200 mL enriched bacterial solution were added to a 250 mL conical flask after autoclaved sterilization, and air in the flask was replaced with high-purity nitrogen by purging nitrogen gas for 3 min to ensure a strict anaerobic environment.Gas production results are presented in Table 5.   was seen from Table 5 and Figure 5a, it was seen that Yangquan anthracite and straw had the highest biogas production (471 mL) at a mass ratio of 2:1.The mass ratio of Jiaozuo anthracite to straw was 3:1, which was considered as the optimal gas production combination, and gas production of ZY-3 could reach 355 mL.Based on Table 5 and Figure 5b, the best biological methane production mass ratio of Yangquan and Jiaozuo anthracite mixed with straw was 2:1, reaching 33.93 and 21.97 mL/g, respectively.

| Multi-factor coupling analysis of synergistic gas production of coal and corn straw
Influencing factors of synergistic gas production under different conditions of coal and corn straw were divided into four groups: A, B, C, and D. A1, A2, B1, B2, C1, C2, D1, and D2 were different combinations of biological gas and methane production.Multi-factor coupling analysis was performed on the data platform Genes cloud.In Figure 6, an increase in the size of the circle increased gas production, and vice versa.From Figure 6a, it was seen that biological gas production of high-rank coal and corn straw played a controlling role in gas production, and through comparative analysis of methane production (Figure 6b), it was seen that the addition of different minerals also had a great influence on synergistic gas production.
As was seen in Figure 7, clay mineral, coal rank, bacterial flora, and substrate mixing status had significant effects on methane production from corn straw and residual coal filled in mine void during field application.

| DISCUSSION
4.1 | Effects of strain source and substrate mixing degree on biogas production Gas production results with domesticated bacteria showed that bituminous coal type B and straw were the optimal gas production combination, which was different from the gas production combination of bituminous coal types A and B and straw with mine water as bacteria source.The biological gas production of bituminous coal type C and straw under the two bacterial sources was the lowest, while the mixed fermentation of bituminous coals of type A and B with straw presented the best gas production effect.Therefore, although mine water, as a common bacteria source in coal seams, could be provided to residual coal and filled with corn straw in goafs, from the perspective of methane production, injection of domesticated bacteria into coal seam could achieve better biological gas production.
In summary, it was seen that the biomethane production of coal and corn straw was relatively high in mixed but it still showed a good for producing biomethane in unmixed state.Although straw-filled goafs could not be completely mixed with residual coal, it could also promote residual coal conversion to biomethane production.

| Effects of inorganic minerals and coal rank on biogas production
Different inorganic minerals had different effects on the gas production of straw and residual coal.When calcite, montmorillonite, and kaolin existed in residual coal, roof, and floor, synergistic conversion of residual coal and straw was promoted, and biological gas production was increased.The reason is that the existence of kaolin, calcite, and montmorillonite will affect the distribution, activity, diversity, gene expression, and electron transfer of microorganisms.Hwang et al. pointed out that the survival rate of cells was significantly improved in the medium supplemented with kaolin and montmorillonite (Huang & Tate, 1997).The reason for this phenomenon is that minerals change the way bacteria survive and metabolize.
Most previous studies have suggested that anthracite was difficult to achieve biological gas production, only 3.52 mL/g, even with a single straw (3.78 mL/g) of the sum of biological methane production, but also far less than the two synergistic gas production effect (10.38 mL/g) (Guo et al., 2019).Therefore, straw as a carbon source promoted efficient biogas production of anthracite, and this research provided a reference for the popularization and application of this technology in anthracite.In summary, inorganic minerals and coal rank played decisive roles in the synergistic gas production of the two.

| Analysis of key factors affecting the synergistic fermentation of straw filling goaf and residual coal
First of all, surface subsidence will be caused in coal mining areas.Filling corn straw into mining areas and residual coal for biological gas production can not only increase the production of biological coalbed methane but also slow down surface subsidence.In addition to making full use of the original bacteria, it is also a good choice to domesticate and cultivate the flora to increase the production of biogas.At the same time, in the process of injecting corn straw and bacterial liquid into the coal seam, it is inevitable to contact with the shale and mudstone of the roof and floor of the coal seam, so that the clay minerals will affect the biogas production.At the same time, corn straw cannot be fully mixed with coal in the process of filling, and the coal ranks in different regions are different.Identifying actual conditions that might exist in the goafs of straw-filled coal mines and clarifying synergistic fermentation processes of different influencing factors on biogas production by coal and corn straw can provide a reference for the field application of corn straw-filling goafs.This research not only slowed down ground subsidence but also achieved efficient resource consumption and environmental protection.

| CONCLUSION
This study explored the influencing factors of a synergistic increase of biomethane in coal mine goafs filled with corn straw.The obtained results showed that domesticated bacteria had higher methane production than that of native bacteria.Mixtures of coal and corn straw increased the production of biomethane, while different showed prosperous methane production effects, and some differences were not significant.Calcite, montmorillonite, and kaolin significantly increased the methane production of mixed fermentation of residual coal and corn straw, while pyrite inhibited biomethane production of on both.Straw could stimulate anthracite with poor gas production, and synergistic fermentation of the two also had high potential of gas production.
Gas production results of the mixed fermentation of coal and corn straw under different bacterial sources.Gas production results of the unmixed state of coal and corn straw.

F
Biogas production characteristics of coal and corn straw under different mixing conditions (a) bituminous coal A and corn straw; (b) bituminous coal B and corn straw; (c) bituminous coal C and corn straw; (d) mine water as the source of bacteria; (e) domesticated bacteria as the source of bacteria.
Biogas production results of different anthracite and corn straw mixed fermentation.

F
Biological gas production characteristics of co-fermentation of coal with inorganic minerals and corn straw (a) Biological gas production (b) Methane production.T A B L E 4 Effects of different inorganic minerals on gas production by co-fermentation of coal and corn straw.

F
Biological gas production characteristics of different anthracite and corn straw mixed fermentation (a) Biological gas production; (b) Methane production.F I G U R E 6 Circular stacking diagram (a) biological gas production; (b) methane production.A: different bacterial source groups; B: different mixed state groups; C: different mineral addition groups; D: different high-rank coal groups.

F
Schematic diagram of influencing factors of synergistic gas production of corn straw and residual coal filled in the goaf of coal mine.
Table 1 summarizes research findings.
Proximate and ultimate analysis of samples/%.