Coal Bioavailability Research Articles

A laboratory study of coal-to-gas bioconversion through bio-seeding sludge in the primary coal substrate

Coal waste, a by-product of coal extraction, adversely poses environmental hazards as it releases harmful substances into the air, water, and soil, damaging the ecosystem and localized biodiversity. The coal-to-energy sludge-seeded bioconversion shows its potential as an environmentally friendly technology to mitigate the harmfulness of coal-derived hazards. This study uses blended coal and anaerobic digestion sludge in batch reactors at a mesophilic temperature (35 °C) to generate methane-rich biogas for energy production and waste elimination. Nutrient solution and ethanol were added as stimuli to boost bioactivities and enhance gas production. These results showed the potential of lignite coal in biomethane generation over an extended period, even with a lower volume of sludge addition. Adding nutrients and ethanol enhances the ultimate biogas production as an extra feed for microorganisms and as a key parameter in increasing the bioavailability of coal. The ultimate biogas production from the kinetic model indicates a remarkable volume of 111504 mL/g-sludge using lignite with 20 mol ethanol compared to the blank reactor with 701 mL/g-sludge of biogas production. The intricate analysis of results highlights the complex interplay between coal, sludge, nutrients, and additives, where varying factors impact methane production rates. Despite challenges in interpreting data, this study underscores the potential for managing coal waste through wastewater utilization, transforming it into methane-rich biogas—a sustainable green technology for energy production.

Biogenic methane clumped isotope signatures: Insights from microbially enhanced coal bed methane

Biogenic (microbial) methane is an imperative natural gas resource and plays an important role in the global carbon cycle. Determining isotope systematics in microbially enhanced coal bed methane (MECBM) informs methane biogeochemical processes and environmental impacts. Here, we conduct laboratory MECBM experiments using coal samples of differing maturities (from lignite to anthracite) and report analytical results of methane production, bulk and clumped isotopes (δ13C, δD, Δ13CH3D and Δ12CH2D2). We reproduce the differential reversibility in methanogenic reactions, which exhibits a correlation with kinetic clumped isotope fractionation (Δ13CH3D from +0.17 ‰ to +4.04 ‰). The exogenous combinatorial effects play a greater role in determining the negative Δ12CH2D2 (<–30 ‰). In the microcosm of MECBM, coal and amendments are suggested to significantly increase methane production (from 82.4 ± 17.1 to 344.6 ± 22.8 μmol CH4/g coal), with effects controlled by substrate coal bioavailability rather than coal thermal maturity. This suggests the possibility of MECBM attempts to utilize high thermal maturity coal. We perform machine learning models and improve the accuracy of biogenic 12CH2D2 predictions, which help understand the mechanism of methane formation, and further provide new insights into clumped isotope effects among different biogenic methane sources. Our incubation experimental and compiled dataset (n = 275) better constrain biological participation in the methane formation process and potentially be used for methane sources tracing.

Enhancement of biogenic methane production from coal using supercritical CO2

Supercritical CO2 (ScCO2) extraction could improve the bioavailability of coal for the enhancement of biogenic methane production. Here, the influence of prolonged ScCO2 extraction on methane yield was investigated since in-situ ScCO2 injection is time-consuming. The results showed that the enhancement of biomethane production from coal was negatively correlated with extraction time and extraction yield. With the extension of ScCO2 extraction time from 0.5 day to 28 days, the methane yield of extracted coals increased by 20.1∼111.4% than that of raw coal. ScCO2 mainly reacted with the organic structures on the surface of coal (such as the C-O and CH2-CO), rarely the skeleton structure. In the prolonged extraction, aromatics were first extracted, then oxygen-containing matters and aliphatics which reduces the methane production of extracted coal but increases the methane generation potential of extractable organic matter. Then, a conceptual model of ScCO2-MECBM was proposed for enhancing methane recovery in-situ.

Enhancement of biogenic methane production from subbituminous coal by reduced iron-bearing clay mineral

Microorganisms in coal bed can potentially convert coal to natural gas. A number of methods, including nutrient injection and microbial introduction, have been used to enhance coalbed methane (CBM) production. However, the recalcitrant composition and hydrophobic character of coal greatly limit its bioavailability, thus limiting the coal-to-methane conversion efficiency. Ubiquitous iron-bearing clay minerals may be able to improve the bioavailability of coal through oxygenation of structural Fe(II). However, its effect has remained unknown. The objective of this study was to use an Fe(II)-rich clay mineral (nontronite) to oxidize coal and to increase its bioavailability through production of strongly-oxidizing hydroxyl radicals. A coal sample from an underground Dahuangshan coal mine of the southern margin of Junggar Basin of Xingjiang, China was oxidized by hydroxyl radicals generated upon oxygenation of reduced nontornite. The pretreated coal sample was then anaerobically incubated with a methanogenic consortium to assess methane production potential. During the oxidation process, 28–46 mg/g dissolved organic carbon was released from the coal sample as a result of oxidation of coal by hydroxyl radicles. When the condition switched to anoxic for 62 days, the coal sample pre-treated with reduced nontronite produced much more methane than the one without pre-treatment (164 μmol versus 40 μmol methane per gram of coal). During this stage, structural Fe(III) in nontronite was reduced to Fe(II), which can be used in the next redox cycle to sustain production of hydroxyl radicles and oxidation of coal. A potentially low-cost scheme for microbial CBM enhancement is proposed: injection of reduced iron-bearing clay mineral, injection of air/O2, and methane production by indigenous microbial community. Cycling of oxidation/reduction of structural iron in clay mineral can be modulated by injection and consumption of air, and sustainable production of methane may be expected. Overall, this study demonstrated that treatment by iron-bearing clay mineral is a potential method to improve methane production from coal.

Biogenic methane generation from Vietnamese coal after pretreatment with hydrogen peroxide

The application of coal as a methanogenic substrate is limited due to its recalcitrant nature, thus, requiring pretreatment for increasing bioavailability of coal. This paper describes the pretreatment of Vietnamese coal from the Red River Basin. The coal sample was treated with 3% (vol/vol) hydrogen peroxide, which resulted in 45.4% (weight %) solubilization of coal and release of various organics into the liquid phase. This dissolved fraction was analyzed for total dissolved organic carbon, Fourier transform infrared spectroscopy, high-performance size exclusion chromatography, and high-performance liquid chromatography. The detailed analytical investigations revealed the presence of aliphatics and aromatics hydrocarbons in the liquid phase. Short-chain organic acids, including oxalic, citric, malic, malonic, formic, and acetic acids were also observed. The dissolved fraction was subjected to methanogenesis. Methane generation achieved the peak at 218.5 μmol/g coal after 500 days of incubation. The effect of the pretreatment process on inorganic matter of the residual coal samples was also assessed using inductively coupled plasma mass spectrometry. The pretreatment caused a reduction in metal content of the residual coal sample. The findings of the paper can be prospected as a feasibility study of methane production from Vietnamese coal after its pretreatment using hydrogen peroxide.

Deciphering the initial products of coal during methanogenic bioconversion: Based on an untargeted metabolomics approach

AbstractAlthough the process of microbial degradation of coal to produce biomethane has got much attention from many research works, the products profiles of coal at the initial stage of methanogenic bioconversion are not clear yet. In this study, five coal‐degrading bacterial strains (CD1, CD10, CD20, CD24, and CD25) from a methanogenic community were isolated and identified. Among them, CD1 and CD24 belong to Paenibacillus sp., CD10 and CD20 belong to Bacillus sp., and CD25 belongs to Stenotrophomonas sp. After biotreatment of lignite and bituminous coal, the kinds of newly produced compounds were 33 and 45, respectively. Metabolomics analysis showed that a large number of alkane compounds and heterocyclic aromatic compounds were produced after degradation of bituminous coal and lignite by isolated bacteria, and most of the compounds had been produced in a hydroxylated or acylated manner, indicating that the initial microbial treatment enhanced the bioavailability of coal. Some alkaloids and biosurfactants were also detected in the aforementioned products, such as glycerophosphocholine, proveratrol A, proveratrol B, surfactin, etc. These microbial metabolites may play an important role in solubilization during the degradation of coal. This study added to the understanding of the complicated metabolic process of methanogenic coal bioconversion and enabled effective production of biomethane with appropriate metabolic strategies.

Feasibility study of enhanced biogenic coalbed methane production by super-critical CO2 extraction

Super-critical CO2 enhanced coalbed methane (Sc-CO2-ECBM) and microbially enhanced coalbed methane (MECBM) are environment-friendly technologies that can improve CBM recovery and generation. In this study, a new approach of MECBM based on Sc-CO2 extraction is presented. The Sc-CO2 pretreatment experiments of anthracite and bituminous coal were conducted to produce biomethane. The extracted organics and the changes of coal structure caused by Sc-CO2 were also analyzed to discuss the mechanism of methane stimulation. The results indicated that methane yields have been greatly improved after Sc-CO2 extraction by 734.85% and 148.15% for anthracite and bituminous coal, respectively. The extractions observed by GC-MS analysis is also favored to generate methane by microorganisms. The little increment of methane production from coal treated by subcritical CO2 indicated that the special characteristics of Sc-CO2 were critical for the stimulation of methane production. More functional groups and even new functional groups were formed to increase coal bioavailability after Sc-CO2 extraction. The specific surface area and total pore volume of bituminous coal increase after Sc-CO2 extraction that could provide more action sites for microorganisms and enzymes. These results strongly proved the feasibility of enhancing CBM by microbial degradation based on Sc-CO2 extraction.

Importance of specific substrate utilization by microbes in microbially enhanced coal-bed methane production: A modelling study

This study addresses a major gap in the understanding and control of microbially enhanced coal-bed methane (MECBM) production. A mathematical and conceptual model comprises a food-web that includes two types of bacteria and three types of archaea representing substrate-specific members of the community; the microbial community members are potentially interacting by competing for or being inhibited by substrates or products of other microbial community members. The model was calibrated using data sets from two different experimental setups. The calibrated model effectively predicted the methane concentrations within a 7% range of deviation from the experimental results. The results of additional batch experiments using varied conditions are also reproduced in an attempt to validate the model and to test the hypothesis of amendment-induced stimulation of microbial community members capable of converting coal into substrates available to methane producing microbes. This study significantly enhances the understanding of the complex interactions between microbial activity, substrate-specificity and bio-availability of coal for methane production, and provides the basis for including hydraulic flow and transport processes into future mathematical models important for the design and implementation of more sustainable methods of harvesting methane from un-mineable coalbeds.

The effect of NaOH pretreatment on coal structure and biomethane production.

Biogenic CBM is an important component of detected CBM, which is formed by coal biodegradation and can be regenerated by anaerobic microorganisms. One of the rate-limiting factors for microbial degradation is the bioavailability of coal molecules, especially for anthracite which is more condense and has higher aromaticity compared with low-rank coal. In this paper, NaOH solution with different concentrations and treating time was employed to pretreat anthracite from Qinshui Basin to alter the coal structure and facilitate the biodegradation. The results showed that the optimal pretreatment conditions were 1.5 M NaOH treating for 12 h, under which the biomethane production was increased by 17.65% compared with untreated coal. The results of FTIR and XRD showed that NaOH pretreatment mainly reduced the multi-substituted aromatics, increased the C-O in alcohols and aromatic ethers and the branching degree of aliphatic chain, and decreased the aromatic ring structure, resulting in the improvement of coal bioavailability and enhancement of biomethane yield. And some organics with potential to generate methane were released to filtrate as revealed by GC-MS. Our results suggested that NaOH was an effective solution for pretreating coal to enhance biogenic methane production, and anthracite after treating with NaOH could be the better substrate for methanogenesis.

Contribution of Microbial Acclimation to Lignite Biomethanization

Understanding the bioconversion process of lignite to methane is an important contribution to the establishment of effective utilization of the existing coal resources and the acquisition of environmental biofuels. Microorganisms from coal seams were acclimatized using lignite as the sole carbon source to improve the bioavailability of coal. The contribution of acclimation to biomethanation was discussed from the aspects of mathematical models, the changes of key intermediates in the liquid phase, and the dynamic evolution of the community structure. The results showed that the anaerobic digestion (AD) efficiency of lignite was significantly improved after acclimation. The methane yield revealed that the methane-producing capacity of the bacteria and the bioavailability of lignite were enhanced. The data of 16S rDNA also suggested that the diversity of the bacterial and archaeal communities increased significantly. The stability of the AD system was enhanced. The abundance and types of bacterial genera that had a symbiosis with methanogens increased. The gene function abundance and the reading number of the bacteria in terms of membrane transport, cell motility, and xenobiotic biodegradation and metabolism were significantly improved with acclimation. This study provides an experimental basis for microbially enhancing coalbed methane (MECBM) and clean energy production.

Enhanced production of secondary biogenic coalbed natural gas from a subbituminous coal treated by hydrogen peroxide and its geochemical and microbiological analyses

Unmineable coal accounts for over 90% of the world’s fossil fuel resources. Fortunately, many coal seams contain indigenous microorganisms capable of utilizing the coal as a carbon source to produce secondary biogenic coalbed natural gas. However, coal bioavailability has been shown to be a significant factor that limits the extent of bioconversion. In this study, we have analyzed the rate and yield of the biogas production and assessed the gas potential, carbon balance, stable carbon isotopes, microbial communities, and microbial pathways changes resulting from the hydrogen peroxide pretreatment with the Wyoming’s Powder River Basin subbituminous coal. The results showed that coal pretreated with hydrogen peroxide can significantly enhance the bioavailability of coal for enhancing the biogas production, with a peak yield of 552.6 μmol/g coal (437.1 Scf/ton coal) at the day 184. The stable carbon isotopic analysis indicated that the δ13C values of the methane and carbon dioxide were much less negative than the published field data. This suggested that the enrichment or depletion of the precursor 13C could contribute to the shift of the carbon isotopic composition in the subsequence processes. Therefore, the data should be used with cautions for interpreting genesis of the thermogenic/biogenic methane and the methanogenic pathways. The methanogenic pathways were also investigated with re-fed experiment and microbial community analysis. The results indicated that the hydrogenotrophic pathway was not active in the original inoculum became activated after the gas production. The microbial community analysis demonstrated that the obligate hydrogenotrophic methanobacterium was the most dominant methanogens in the microcosms after the gas production. This suggested that the chemical treatment of coal has impacts on the microbial structure during the subsequent methane production phase.

Improving coal bioavailability for biogenic methane production via hydrogen peroxide oxidation

Hydrogen peroxide solution was used to pretreat coal samples for the purpose of improving bioavailability for methanogenesis. Crushed coals of subbituminous (PEN9-003) and medium-volatile-bituminous (C.D.) ranks were oxidized with three different dosages of H2O2 solution over four time periods (1, 4, 7, and 30 days) in the presence of UV light. Bioassays were set up using the oxidation products as the sole carbons substrates for biomethane production. To characterize the change in coal composition after oxidation, concentrations of volatile fatty acids (VFAs) and total organic carbon (TOC) in the filtered liquid sample of the pretreatment reactor were measured. The solid phase was dried and solvent-extracted with a mixture of dichloromethane and methanol. Compounds in the organic extract were analyzed using a GC–MS. H2O2 oxidation has managed to boost the biomethane yield in the subbituminous PEN9-003 coal by a maximum 10 times to 223.7 μmol/g. Production of methane was found to increase exponentially with the concentration of H2O2 relative to coal, but less significantly with pretreatment length. In contrast, the bituminous C.D. coal was barely affected by pretreatment. In addition, oxidation has considerably enhanced solubilization of PEN9-003 coal in water, with contents of VFAs and dissolved TOC surged by up to 28.2 and 22.3 times. The pretreatment has also increased the abundance of solvent-extractable matter in PEN9-003 coal. Oxygen-containing compounds, including aliphatic carboxylic acids, alcohols, ethers, ketones, and oxygenated monocyclic aliphatic and aromatic hydrocarbons were the most affected. The enrichment was due both to formation of new chemical species and an increase in the abundance of existing compounds. In comparison, oxidation of C.D. coal failed to generate any new organic species, nor did it increase the content of dissolved TOC. Biodegradation has resulted in depletion of all dissolved organic carbon, suggesting water-soluble compounds are highly bioavailable. Bioconversion of solvent-extractable matter in PEN9-003 has been improved by a maximum 29.5% upon pretreatment, while that of C.D. coal was lowered. Molecules with heterogeneous moieties, especially those that have been enriched after pretreatment demonstrated the highest bioavailability. Thermal maturation of coal from subbituminous to medium-volatile bituminous rank has been associated with a decrease in extractable matter contents, liptinite proportion, H/C ratio, elemental oxygen, heterogeneous moieties, as well as physical accessibility to microorganisms. They are thought to be responsible for the poor bioavailability and amenability of C.D. coal to H2O2 oxidation. Application of the pretreatment is only likely to be ex-situ, in which coal beneficiation waste may be used as a feed to bioreactors. The study offers a good basis for further process optimization and upscaling.

Characterization of organic substances in lignite before and after hydrogen peroxide treatment: Implications for microbially enhanced coalbed methane

The reaction of coal with hydrogen peroxide (H2O2) has been reported to accelerate the microbial transformation of coal into biogenic methane. To understand the mechanism of the increased bioavailability of coal as a result of H2O2 treatment, we investigated the effects of H2O2 reaction on humic substances of lignite by characterizing their structure and relative abundance. The H2O2-treated lignite, referred to as lignite-H2O2, was also examined in column experiments to confirm the increases in lignite solubilization and organic acids as indicators of enhanced bioavailability. The results showed that the alkali-soluble carbon content of lignite increased by 4.9 times (from 0.4 to 2.1g C) after H2O2 treatment, and the humic acid (HA) content of this fraction increased by 7.7 times (from 0.2 to 1.5g C). Previously, the enhanced solubility of lignite after H2O2 treatment was attributed to increased hydrophilicity of the lignite. However, we propose that this phenomenon is actually caused by chemical fragmentation (i.e., β-fragmentation) involving a radical reaction. In the column experiments, lignite treated with H2O2 showed higher dissolved organic carbon (up to 84.8mg/L) and organic acid (up to 18.9mg/L for acetic acid, and up to 19.9mg/L for formic acid) concentrations than lignite without treatment. These results indicate the potential to optimize CH4 production from microbially enhanced coalbed methane operations using H2O2.

Low carbon renewable natural gas production from coalbeds and implications for carbon capture and storage

Isotopic studies have shown that many of the world’s coalbed natural gas plays are secondary biogenic in origin, suggesting a potential for gas regeneration through enhanced microbial activities. The generation of biogas through biostimulation and bioaugmentation is limited to the bioavailability of coal-derived compounds and is considered carbon positive. Here we show that plant-derived carbohydrates can be used as alternative substrates for gas generation by the indigenous coal seam microorganisms. The results suggest that coalbeds can act as natural geobioreactors to produce low carbon renewable natural gas, which can be considered carbon neutral, or perhaps even carbon negative depending on the amount of carbon sequestered within the coal. In addition, coal bioavailability is no longer a limiting factor. This approach has the potential of bridging the gap between fossil fuels and renewable energy by utilizing existing coalbed natural gas infrastructure to produce low carbon renewable natural gas and reducing global warming.

The effect of coal bed dewatering and partial oxidation on biogenic methane potential

Coal formation dewatering at a site in the Powder River Basin was associated with enhanced potential for secondary biogenic methane determined by using a bioassay. We hypothesized that dewatering can stimulate microbial activity and increase the bioavailability of coal. We analyzed one dewatered and two water-saturated coals to examine possible ways in which dewatering influences coal bed natural gas biogenesis by looking at differences with respect to the native coal microbial community, coal-methane organic intermediates, and residual coal oxidation potential. Microbial biomass did not increase in response to dewatering. Small Subunit rRNA sequences retrieved from all coals sampled represented members from genera known to be aerobic, anaerobic and facultatively anaerobic. A Bray Curtis similarity analysis indicated that the microbial communities in water-saturated coals were more similar to each other than to the dewatered coal, suggesting an effect of dewatering. There was a higher incidence of long chain and volatile fatty acid intermediates in incubations of the dewatered coal compared to the water-saturated coals, and this could either be due to differences in microbial enzymatic activities or to chemical oxidation of the coal associated with O2 exposure. Dilute H2O2 treatment of two fractions of structural coal (kerogen and bitumen+kerogen) was used as a proxy for chemical oxidation by O2. The dewatered coal had a low residual oxidation potential compared to the water-saturated coals. Oxidation with 5% H2O2 did increase the bioavailability of structural coal, and the increase in residual oxidation potential in the water saturated coals was approximately equivalent to the higher methanogenic potential measured in the dewatered coal. Evidence from this study supports the idea that coal bed dewatering could stimulate biogenic methanogenesis through partial oxidation of the structural organics in coal once anaerobic conditions are restored.

Biogenic methane potential for Surat Basin, Queensland coal seams

Produced water samples collected from Surat Basin coal seams in eastern Queensland, Australia were shown to contain viable microbial consortia with the ability to convert native Walloon coal into methane. Methane generation rates of up to 1.0 m3/t coal/day (30 scf/ton/day) were observed, with overall yields up to 6.5 m3/t coal (210 scf/ton); in comparison, total methane reserves for the Surat Basin are typically 4 to 8 m3/t. This is the first direct evidence of real-time biogenic coal-to-methane potential for an Australian coal seam sample. Six of the eight Surat Basin water samples tested positive for biomethane production when H2–CO2 was provided as the sole methanogenic substrate; five produced methane from methanol, but acetoclastic methanogenesis was not observed. An active H2–CO2 pathway could have implications for the conversion of sequestered CO2, if an in-situ source of reduced hydrogen is present in excess. In a simple experiment, biomethane production from a Walloon coal was not enhanced when the bicarbonate concentration in the medium was doubled. It is anticipated that the low surface area/solubility of coals may limit their bioavailability; coal-to-methane rates and final yields increased in proportion to the coal particle surface area. Similarly, when a Zonyl FSN surfactant was added to improve coal bioavailability, the initial methane production rate increased by 240%, and the final methane yield increased by 180% in comparison to a no-surfactant control. Overall, these results suggest an opportunity to enhance Surat Basin coalbed methane reserves via in-situ stimulation of indigenous microbial consortia.

Bioassay for estimating the biogenic methane-generating potential of coal samples

Generation of secondary biogenic methane in coal beds is likely controlled by a combination of factors such as the bioavailability of coal carbon, the presence of a microbial community to convert coal carbon to methane, and an environment supporting microbial growth and methanogenesis. A set of treatments and controls was developed to bioassay the bioavailability of coal for conversion to methane under defined laboratory conditions. Treatments included adding a well-characterized consortium of bacteria and methanogens (enriched from modern wetland sediments) and providing conditions to support endemic microbial activity. The contribution of desorbed methane in the bioassays was determined in treatments with bromoethane sulfonic acid, an inhibitor of microbial methanogenesis. The bioassay compared 16 subbituminous coal samples collected from beds in Texas (TX), Wyoming (WY), and Alaska (AK), and two bituminous coal samples from Pennsylvania (PA). New biogenic methane was observed in several samples of subbituminous coal with the microbial consortium added, but endemic activity was less commonly observed. The highest methane generation [80 µmol methane/g coal (56 scf/ton or 1.75 cm3/g)] was from a south TX coal sample that was collected from a non-gas-producing well. Subbituminous coals from the Powder River Basin, WY and North Slope Borough, AK contained more sorbed (original) methane than the TX coal sample and generated 0–23 µmol/g (up to 16 scf/ton or 0.5 cm3/g) new biogenic methane in the bioassay. Standard indicators of thermal maturity such as burial depth, nitrogen content, and calorific value did not explain differences in biogenic methane among subbituminous coal samples. No original methane was observed in two bituminous samples from PA, nor was any new methane generated in bioassays of these samples. The bioassay offers a new tool for assessing the potential of coal for biogenic methane generation, and provides a platform for studying the mechanisms involved in this economically important activity.