Article Cite This: Energy Fuels 2019, 33, 287−295
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Potential and Constraints of Biogenic Methane Generation from Coals and Mudstones from Huaibei Coalfield, Eastern China Yuan Bao,*,†,‡ Yiwen Ju,*,‡ Haiping Huang,§ Juanli Yun,∥ and Chen Guo† †
College of Geology and Environment, Xi’an University of Science and Technology, Xi’an 710054, People’s Republic of China Key Laboratory of Computational Geodynamics of Chinese Academy of Sciences, College of Earth Science, University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China § Department of Geoscience, University of Calgary, Calgary, Alberta T2N 1N4, Canada ∥ Institute of Microbiology, Chinese Academy of Sciences, 1 Beichen Road, Beijing 100101, People’s Republic of China Energy Fuels 2019.33:287-295. Downloaded from pubs.acs.org by UNIV OF WINNIPEG on 01/17/19. For personal use only.
‡
ABSTRACT: Coalbed methane (CBM) resources formed biogenically and thermogenically have been discovered in the Permian coalbeds of the Huaibei coalfield. Four coals, three mudstones, and five coalbed-produced water samples collected from the Linhuan, Luling, and Haizi coal mines of the Huaibei coalfield were characterized geochemically and biologically to gain an understanding of the biogenic methane generation potential and the microbial communities involved in situ and in coalbed-produced water-enriched samples. The 16S rRNA gene high-throughput sequencing results showed that the archaeal communities from in situ and enriched cultures were dominated by Methanolobus and Methanobacterium species. The organic material of coals and mudstones could be biodegraded under an anaerobic incubation. The maximum biogenic methane generation potentials of coal and mudstones were 98.5 and 72.5 μmol/g within 123 days, respectively. Volatile matter and total organic carbon (TOC) content were the most important internal factors affecting biogenic methane generation from coals and mudstones. The Na-SO4 water type resulted in a low methane generation potential.
1. INTRODUCTION Coalbed methane (CBM), also called coal-seam gas or coalbed natural gas, is a clean and important energy resource.1 The majority of CBM is absorbed on the inner surface of particles and secondly exists as a free gas in coal pores and cracks.2−4 CBM is formed either biogenically or thermogenically via different processes.5,6 Biogenic (microbial) methane is mainly derived from the low-rank coals during early diagenesis; however, secondary biogenic gas can be generated from any ranks of coals at shallow horizons (burial depth less than 3000 ft7).8 Thermogenic methane occurs in middle- and high-rank coals, with a R0 more than 0.6%, due to high temperature and pressure during coal rank advance.5 Estimates of global inplace CBM resources were different.9−15 Holditch et al.9 suggested 67−241 trillion m3 (tcm) of CBM, while Rogner10 proposed a total of 252 tcm. Early estimations of biogenic gas suggested that ∼20% of total natural gas production (including CBM) in the world was microbial.11,12 Recently, estimates of the overall contribution of biogenic-related gas was updated to a range of 40−50% in CBM.13 Biogenic CBM is the most important component of the biogenic gas inventory. Secondary biogenic CBM accounts for approximately 5−11% of CBM.14 The mixed biogenic and thermogenic CBM accounts for more than 10%.15 Therefore, secondary biogenic CBM has a great resource potential worldwide. Considerable work on microbial methanogenesis in coalbeds and biogenic gas production in coalbeds have been accomplished.16−25 Recent investigations have shown not only biogenic CBM generated in the geologic past and retained in formations in merchant quantities but also an active, ongoing microbial methane generation in some sedimentary basins.26,27 16S rRNA is an ingredient of the © 2018 American Chemical Society
small subunit of a prionuclear ribosome that was found to bind to the Shine-Dalgarno sequence in the 1930s.28 The 16S rRNA gene high-throughput sequencing approach involves DNA extraction and amplification of the 16S rRNA gene followed by a high-throughput sequencing. Through sequence analysis, several thousands of bacteria can be distinguished in each sample. The high-throughput sequencing technology, demonstrating the presence of native coal-to-methane consortia, was widely applied to native CBM communities. For instance, Guo et al.29 described that the Methanolobus was the main genus of methanogens in the Liulin area of the Eastern Ordos Basin of China by 454 pyrosequencing. Methanolobus and Methanosarcina were also identified in the coalbed produced water at the Cook Inlet Basin of Alaska applying 16S rRNA gene tag pyrosequencing.30 A metagenome dominated by Rhodobacteraceae genotypes and Celeribacter spp. was extracted from a volatile-bituminous coal at the Alberta Basin. This methanogen could degrade a wide range of aromatic compounds and produce methanogenic substrates by acidizing fermentation.31 Recently, Methermicoccus shengliensis (M. shengliensis), a methanogen that can transform the coal-derived methoxylated aromatic compounds directly into methane, has been identified in coal, suggesting that another methanogen besides M. shengliensis can metabolize the unknown and complex organic compounds in coal.32,33 Bioconversion of coal to methane is of great significance as an unexploited clean energy and has many economic and environmental advantages over the traditional techniques.22,34 Received: August 10, 2018 Revised: December 3, 2018 Published: December 4, 2018 287
DOI: 10.1021/acs.energyfuels.8b02782 Energy Fuels 2019, 33, 287−295
Article
Energy & Fuels
Figure 1. Locations of coal mines and tectonics (A, modified with permission of Jiang et al.45) and stratigraphic column of coal measure (B, modified with permission of Wang et al.44 and Wei46) of the Huaibei coalfield, eastern China.
processes and factors affecting the microbial methane generation from coal and mudstone.
The methanogenesis stimulated by physical and chemical reservoir treatments has been proved by the presence of live methanogens in situ.23 Coal conversion technologies can convert huge low-rank coal into derivative fuels. The biogenic methane was generated from various coals of different ranks.6,17 The maximum methane yields of 83.7 m3/t and 10.8 m3/t occur from bituminous coal with a fed-batch scheme and from subbituminous coal incubated without hydrogen addition, respectively.8,35,36 A preliminary investigation conducted on shale organic matter provided the evidence of biodegradation, such as losses of light aromatic hydrocarbons, short-chain (C15−C19) n-alkanes, and acyclic isoprenoid alkanes in shale extracts.37,38 However, to the authors’ knowledge, there is not any research on the biodegradation of mudstone (TOC content less than 6%) and carbonaceous mudstone (TOC content ranging from 6% to 40%). Mudstone and carbonaceous mudstone with a high content of organic matter were proven to be an effective biogenic gas source rock in the Sanhu depression of Qaidam basin.39,40 The abundant organic matter can largely afford microbial sulfate reduction and reach the methanogenic process.40 The Huaibei−Huainan coalfield, located at the southeastern margin of the North China Plate, is one of the most important coal mining and CBM exploitation sites in eastern China. Both biogenic and thermogenic CBM occur in the Huaibei− Huainan Permian coalbeds.6,7,41−43 Methane in the Luling and Linhuan coals of the Huaibei coalfield is partially derived from the microbial processes.6,43 The Nos. 8 and 10 coalbeds are the main mining objects.44 Mudstone overlaid coalbeds were well developed. The purpose of this study was to understand the biogenic methane generation potential and microbial communities involved in situ and in the coalbedproduced water-enriched samples and to shed light on the
2. MATERIALS AND METHODS 2.1. Samples. Four coal and three mudstone samples were collected from the underground fresh working face of the Luling, Linhuan, and Haizi coal mines (Figure 1A). Coal samples are labeled as LLM03, LHM03, LHM14, and HZM10. Mudstone samples are coded with LL-1, LH-2, and HZ-2. LLM03 and LL-1 were gathered from the Luling coal mine in July 2013. LHM03, LHM14, and LH-2 were collected from the Linhuan coal mine in August 2013. HZM10 and HZ-2 were gathered from the Haizi coal mine in August 2013. Coal and mudstone samples were ground to a mesh size of 60. Five coalbed-produced water samples were gathered from hydraulically fractured CBM wells at the Luling coal mine in early June of 2014 to understand the hydrogeochemical characteristics of the coal measure. The water-producing wells (LG01, WLG01, WLG02, WLG03, and WLG04) were drilled and fractured in 2008 (LG series well) and 2010 (WLG series wells). Coalbed-produced water and CBM were produced immediately after drilling and fracturing. Two representative water samples (LG01 and WLG04) were chosen for microbe incubation and sequencing. LG01 was a commingled sample from the Nos. 8, 9, and 10 coalbeds (0.80 m3 per day when sampling). WLG04 was derived from the No. 10 coalbed (2.88 m3 per day when sampling). Others were also fracturing from the Nos. 8, 9, and 10 coalbeds. The water samples were gathered and sealed in sterile glass bottles after continuous drainage for more than 1 h. The water samples were taken to the laboratory and stored at 4 °C for the inoculation experiment. The mean vitrinite reflectance (Ro,ave) was performed by reflected light optical microscopy using oil immersion in accordance with the method of GB/T 8899-1998.47 The maceral analysis and the proximate analysis were measured according to methods of GB/T 8899-1998 and GB/T212-2001.47,48 The mineral composition and total organic carbon (TOC) content were measured using X-ray diffraction (XRD) and Eval-rock pyrolysis. XRD was performed on a RINT-TTR3 Diffractometer with Cu Kα radiation operated at 40 kV and 40 mA. The TOC data were measured with the TOC-II at a 288
DOI: 10.1021/acs.energyfuels.8b02782 Energy Fuels 2019, 33, 287−295
Article
Energy & Fuels Table 1. Information about Coal Rank, Maceral, and Proximate Compositions of Coalsa maceral compositions (%) sample ID LLM03 LHM03 LHM14 HZM10
proximate compositions (%)
position
coal mine
Ro,ave (%)
V
I
E
M
Mad
Aad
Vad
FCad
seam seam seam seam
Luling Linhuan Linhuan Haizi
1.07 1.75 1.44 1.93
66.6 94.0 73.6 76.4
14.4 2.5 15.4 21.8
6.5 0.5 8.0 1.3
12.5 3.0 3.0 0.6
1.74 1.07 0.70 2.07
31.66 24.24 13.48 3.45
24.40 17.20 19.98 19.43
42.20 57.49 65.84 75.05
8 8 10 8
Note: V, vitrinite; I, inertinite; E, exinite; M, mineral; Mad, moisture on air-dry basis; Aad, ash yield; Vad, volatile matter; FCad, fixed carbon.
a
Table 2. Maturity, TOC, and Mineral Composition of Mudstonesa mineral compositions (wt %) sample ID
position
coal mine
Ro ave (%)
TOC (%)
Si
Carb
Clay
others (Sd., Py., Ant.)
LL-1 LH-2 HZ-2
roof of seam 8 roof of seam 10 roof of seam 8
Luling Linhuan Haizi
0.63 0.82 1.20
4.17 3.08 5.41
32.0 31.1 32.9
0.9 0.0 16.9
67.1 44.5 45.9
0.0 24.4 4.3
a
Note: TOC, total organic carbon; Si., silica, containing quartz, K-feldspar, and plagioclase; Carb., carbonate, containing calcite and dolomite; Clay, containing Illite, kaolinite, chlorite and mixed I/S; Sd., siderite; Py., pyrite; Ant., anatase. temperature of 90 °C for 2 min, then to 300 °C for 3 min, and finally to 600 °C with a constant step of 25 °C/min.44 The hydrogeochemical analysis was performed according to the People’s Republic of China coal industry standard.49 2.2. Culturing Setup. The culture media, trace metals, and vitamin solutions were prepared according to Strap̨ oć et al.50 The trace mineral solution included CuCl2 (2 mg/L), ZnCl2 (70 mg/L), FeCl2·4H2O (1,500 mg/L), MnCl2·4H2O (100 mg/L), CoCl2·6H2O (190 mg/L), NiCl2·6H2O (24 mg/L), AlK(SO4)2(10 mg/L), Na2MoO4 (6 mg/L), H3BO3 (36 mg/L), and 10 mL/L 25% HCl. The 3 L culture medium was prepared in three Erlenmeyer flasks, which was placed into the high-pressure steam sterilization pot at a temperature of 121 °C for 20 min. 2.3. Habituated and Enriched Culture. Microbial consortia from 100 mL of the coalbed associated water sample LG01, enriched with 10 g of coal and 0.4 g of sodium formate added as a carbon substrate, were inoculated into 100 mL of culture medium. At the end of a 60-day incubation period (designated as a habituated culture), methane was detected by the gas chromatography, indicating the presence of methanogens. To obtain additional enriched and indigenous methanogens, 10 mL of habituated culture solution was inoculated into 100 mL of culture medium with 10 g of coal for 30 days (designated as an enriched culture). The headspace of enriched bottles were filled with H2:CO2 (4:1, v/v) at 1.01 × 105 Pa pressure. 2.4. Grouping Experiment. The grouping experiment of simulating biogenic gas generation was performed in eight groups, including four coal groups, three mudstone groups, and one control group. The inoculation process was the same as the above habituated and enriched cultures. The solution consisting of 10 mL of enriched culture solution, 100 mL of culture medium, and 10 g of powder coal/ mudstone sample with a mesh size of 60 was sealed with butyl rubber stoppers in 250 mL serum bottles. No coal or mudstone samples were put in the control group. The headspace of the inoculation bottles was replaced with N2:CO2 (4:1 v/v) at 1.01 × 105 Pa pressure. Each group experiment was inoculated in triplicate and incubated without shaking at 35 °C for 123 days. 2.5. Gas Analysis. Cumulative headspace gas concentrations of methane, ethane, propane, and carbon dioxide were determined every 3−5 days using an Agilent 7890A gas chromatograph. The gases were sampled with a gastight syringe from the bioreactor headspace. The gas chromatograph was equipped with a Chrompack Carboplot P7 25 m × 0.53 mm fused silica capillary column (HP 19095P-CO2), a thermal conductivity detector (TCD), and a flame ionization detector (FID). Helium was the carrier gas with a flow rate of 5 mL/min and a split ratio of 4.5:1. Calibration standards, consisting of 1% methane, 1% ethane, 1% propane, 10% carbon dioxide, and 87% nitrogen or 20% methane, 15% carbon dioxide, and 65% nitrogen (Beijing AP
BAIF GASES Industry Co., Ltd., China), were injected at atmospheric pressure to generate the calibration plot. 2.6. DNA Extraction, PCR Amplification, and Sequence Analysis. Total deoxyribonucleic acid (DNA) was extracted from 100 mL of coalbed produced waters with 0.5 g of coal addition and 10 mL of slurry samples from the enriched cultures35 using the FastDNA Spin Kit (MPBio Systems, Carlsbad, CA) based on the manufacturer’s instructions. The concentration of DNA was measured by a Qubit 2.0 (Life Technologies, USA), ensuring that a sufficient amount of highquality genomic DNA was extracted. Polymerase chain reaction (PCR) was performed using the extracted DNA as template. Sequence analysis was performed using the Illumina MiSeq system (Illumina MiSeq, USA) according to the manufacturer’s instructions.18 For MiSeq sequencing, the V3 and V4 regions of bacterial 16S rRNA gene were amplified by bacterial specific primer set 27F/ 1492R;51 the second PCR amplification of bacteria was with 27F/ 533R,52 which was widely used for the preparation of 16S rRNA libraries.53−55 As described previously, an archaea-specific primer set (Arch21F/958R) was used for the first amplification step56 followed by Arch519F/915R for the second PCR amplification step.57,58 The reaction condition was set up as Su et al.59 Sequencing data were collected, and chimeras were detected according to Bu et al.60
3. RESULTS AND DISCUSSION 3.1. Geological Parameters of Coal and Mudstone Samples. Coal rank, maceral component, and proximate compositions of coals are shown in Table 1. LLM03 belongs to a highly volatile bituminous A coal (Ro,ave = 1.07%). The mineral, ash, and volatile matter contents are the highest and the vitrinite and fixed carbon contents are the lowest in LLM03. Both LHM03 and LHM14 are bituminous coals. The former contains higher vitrinite content and lower volatile matter, while the latter has higher exinite content and lower moisture. HZM10 is an anthracitic coal. The inertinite, moisture, and fixed carbon contents are the highest, and the mineral and ash contents are the lowest. Maturity, TOC, and mineral compositions of mudstones overlying the roofs of the coalbeds are shown in Table 2. LL-1 has the highest clays and lowest siderite, pyrite, and anatase contents. It is the least mature sample with a Ro,ave of 0.63%. LH-2 has the highest siderite, pyrite, and anatase contents and lowest TOC, silica, carbonate, and clay contents. HZ-2 has the highest TOC and silica and carbonate contents. It is the most mature sample with a Ro,ave of 1.20%. 289
DOI: 10.1021/acs.energyfuels.8b02782 Energy Fuels 2019, 33, 287−295
Article
Energy & Fuels Table 3. Hydrogeochemical Data of Coalbed-Produced Water at the Luling, Linhuan, and Haizi Coal Minesa sample ID
pH
TDS (mg/L)
Na+ (mg/L)
K+ (mg/L)
Mg2+ (mg/L)
Ca2+ (mg/L)
F− (mg/L)
Cl− (mg/L)
NO3− (mg/L)
SO42− (mg/L)
HCO3− (mg/L)
CO32− (mg/L)
LG01 WLG01 WLG02 WLG03 WLG04 LH01b HZ01b HZ02b
8.7 8.3 8.8 8.8 8.4 7.8 8.3 8.3
4666 5571 7122 6462 6021 3016 3323 3089
1299 1564 1986 1810 1822 655.3 877.2 927.5
5.6 34.7 78.7 36.8 38.1 / / /
2.0 2.6 3.7 9.9 4.6 89.9 67.7 27.5
26.2 7.0 8.2 41.9 11.0 202.7 132.8 55.3
3.8 4.6 3.4 4.0 4.3 / / /
157.0 279.0 550.0 208.0 451.0 170.8 263.0 98.0
/
