07122-14

Goal
This project seeks to systematically investigate processes involved in methanogenesis from coal to better understand how the process can be enhanced and accelerated. This is the first critical step to ultimately stimulating methanogenesis in situ.

Performers
Colorado School of Mines (CSM), Golden, CO 80401
United States Geological Survey, Denver, CO 80202
University of Wyoming, Laramie, WY 82071
Ciris Energy, Centennial, CO 80112
Pioneer Natural Resources, Denver, CO 80202
Pinnacle Gas Resources, Sheridan, WY 82801
Coleman Oil & Gas, Inc., Denver, CO 80202

Background
The U.S. has an immense coal resource. It is estimated that 13% of the U.S. land mass is underlain by coal deposits. Some of this coal resource, while too deep for mining, contains methane that can be extracted. One option is to drill wells and produce the methane gas adsorbed to the coal by dewatering the coal seam. However, coalbed methane (CBM) wells have a relatively short life, a large volume of organic material (coal) is left in place, and there are environmental and economic challenges related to the massive amount of water production required to dewater the coal seam before gas production can begin. In-situ combustion of the coal can also create environmental problems. Many of these problems could be resolved by finding a means of converting coal to methane in-situ. This naturally occurring process, methanogenesis, is responsible for most of the adsorbed gas in the coal seams of the Powder River Basin (PRB) of Wyoming. The challenge is that the natural process is extremely slow.

During this project, the research team will conduct studies to characterize the methane-generating potential of coals of different rank in response to nutrient additions, chemical pretreatments of the coal and/or environmental manipulations. By identifying the chemical constituents, natural microbial communities and the microbial metabolic products formed en route to methanogenesis, a greater understanding of the processes involved will be gained. Because microorganisms are responsible for catalyzing the transformation of coal to methane, the research team will take advantage of culture-independent phospholipid and molecular biology techniques to describe the salient microbial communities involved in coal conversion.

The scope of this project will be to carefully collect coal and water samples from at least 30 sites in the PRB, San Juan Basin, Sand Wash Basin, and Green River Basin. The samples will be systematically analyzed to better understand how the processes involved in methanogenesis can be enhanced and accelerated. Chemical constituents of the coal will be identified and specific microorganisms will be identified and cultured under controlled conditions to evaluate their role in the methanogenesis process. The chemical pathways of methanogenesis, the rate limiting steps, and the interactions between the microbial communities will be analyzed and captured in a computer model. Chemical pre-treatment of coal with acids, bases, oxidants, solvents, and/or enzymes will be performed to hopefully stimulate the native microorganisms. The goal will be to provide a broader understanding of microbial methane production from coal. This will be a first step leading to development of a process for stimulating methanogenesis from coal in-situ.

Potential Impacts
This project will result in the production of knowledge that could eventually help to produce a large benefit if it should lead to a means of stimulating methanogenesis from coal in-situ. Successful results from the study could result in field testing of the process. This process would result in both increased resource and increased production from existing wells as new methane would be created from existing coal reserves. In addition, if sufficient methane could be produced to exceed the solubility of methane in water, the gas could be produced without dewatering the coals, thus avoiding the costly dewatering step and its associated environmental complications.

Accomplishments
Water and coal samples were collected and characterized. Ongoing experiments include coal solubilization studies, gas chromatography-mass spectrometry (GC-MS) analysis, robust aerobic (fungal) and anaerobic culture growth, microbial characterization and particle size analysis.

Results from the coal solubilization studies have indicated significantly enhanced solubilization for several treatments. Preliminary results from enzyme mediator experiments showed that the mediator did not limit coal solubilization. However, initial results from microbial culture studies indicate that at least some of the solubilization products may have an inhibitory effect on methane production. Similarly, an examination of the effect of coal concentration on methanogenesis in microcosms suggests that increased coal concentration may inhibit methane production. The pH appears to decrease with increasing coal concentration, and coal leaching tests show that carbon dioxide leaches from coal and depresses pH but carbon dioxide does not account for the full pH depression. Further research is underway to identify and assess the potential inhibitory compounds.

Initial solid-liquid extractions from coal were analyzed by GC-MS and resulted in no peaks eluting/detected. The solvent first tested was methylene chloride. In the chromatogram, no peaks eluted except the solvent and the column bleed (very distinct 73, 207, and 281 peaks that show up on the mass spectrum).

To characterize the microbial communities, DNA and lipid samples have been collected from inoculated and uninoculated microcosms and from mud and digester sludge used as inocula.

Current Status
Major tasks to complete this project are summarized below. Work has begun on all of these tasks. In addition, a Project Management Plan consisting of a work breakdown structure and supporting narrative with a summary of the objectives and approach has been submitted. As well, a Technology Status Assessment summary report describing the state-of-the-art of the proposed technology has also been submitted as required.

Sample Collection. Water, fresh drill cuttings, and potentially some coal core samples will be collected with the cooperation of three partner CBM operating companies, Coleman Oil and Gas, Pinnacle Gas Resources, and Pioneer Natural Resources. These contributors will supply samples from the Wyodak and Big George formations of the Powder River Basin (WY and MT), the Raton and Vermejo formations of the Raton Basin (southern CO), the Sand Wash Basin (northwestern CO), and the Green River Basin (WY). Coal ranks range from lignite though sub-bituminous to high volatile C, B, and A. At least 4 of these samples will be collected across a gradient at different distances from the recharge zone of a coal seam, to enable characterization of differences in water and coal chemistry associated with typically higher rates of microbially generated methane near the recharge zone.

Coal Pretreatment. In addition to studying methane production resulting from naturally occurring solubilization processes, various chemical coal pretreatments designed to enhance coal solubilization will be examined. Treatment agents including acids, bases, oxidants, solvents, and enzymes may increase the extent and rate of coal solubilization and depolymerization, resulting in enhanced methane production. Fractional and full-factorial bench-scale studies will be used to evaluate treatment effectiveness and assess the feasibility of reagent coupling. Variables including the types of coal and reagents, reagent-to-coal ratio, residence time, temperature, and pressure will be evaluated using stainless steel micro-reactors. The rate of solubilization is also important and will be examined in separate time-series experiments.

Chemical Characterization of Different Coals, Pre-treated Coals, and Associated Waters. Organic matter (OM) will be extracted from the various coal samples to determine the amount of bioavailable carbon and reveal the identity of putative metabolites that may represent a parent substrate or chemical intermediate in the microbial food chain. The OM extracted by the coal pre-treatments and from untreated coals will be analyzed. Solvents will be selected that, when used separately, will allow extraction of either polar or non-polar OM (e.g. water and chloroform, respectively) from the untreated coals. Additionally, water samples collected from corresponding coal zones will be analyzed to establish the quantity and character of OM present in the coal formations before additional biological or chemical treatment.

A broad-spectrum analysis (BSA) approach will be used to examine the above samples. Solid-phase micro-extraction (SPME) and solvent extraction will be used to simultaneously extract compounds with a wide range of properties that will subsequently be analyzed by Gas Chromatography (GC-MS) and/or Liquid Chromatography-Electrospray Ionization-Mass Spectrometry (LC-ESI-MS). Identified substrates from the water samples and coal extractions will be tested for bioavailability in the following task.

Microbial Enrichment and Characterization. Untreated and pretreated coal and water samples will be incubated under a variety of conditions to stimulate methane generation by associated microbes. Microcosms containing slurries of ground coal samples and liquid media will be prepared for laboratory incubations. The incubation parameters to be tested include coal type, pH, temperature, nutrient and salt addition, carbon amendments, and hydrogen concentrations. Appropriate controls will be run in parallel to ensure that methane production is biogenic and due to coal conversion; these controls will include autoclaved coal, autoclaved coal plus amendments, and amendments with no coal. The rate and extent of methane production will be quantified in these slurries and normalized to the mass and surface area of coal used in the incubation. The microcosms will also be periodically sampled and analyzed for parameters such as hydrogen concentration and production, volatile fatty acid concentrations (especially acetic acid, butyric acid and propionic acid), and dissolved organic carbon. In addition, the dissolved organic constituents will be periodically characterized using both GC-MS and LC-MS analyses on select microcosms. Upon confirmation of a culture’s ability to produce methane from coal, sub-cultures will be made from selected replicates.

The microbes associated with coal will be characterized by physiological and genetic methods such as phospholipid analysis and DNA sequencing. Microbial communities will be characterized in selected native coals and associated waters (at least one sample per site), as well as in those enrichment cultures that produce significant methane from coal. Using GC-MS techniques, CSM will measure the abundance of sulfate-reducing bacteria and methanogenic archaea in groundwaters associated with CBM deposits from structural analysis of microbial membrane phospholipid fatty acids (PLFAs) and phosphoether lipids (PELs), respectively. CSM will conduct similar analyses of selected (high methane production) microcosms. In particular, the methanogenic communities in the microcosms will be characterized from the composition and d13C of the PLFAs and PELs. Similarly, DNA will be extracted from coal, formation water samples, and selected high-methane microcosms. Extracted DNA of eukaryotes, bacteria, and archaea will be amplified, cloned, and sequenced. The DNA of different microbes can be quantified, allowing for a determination of the relative abundance of different microorganisms within the microbial community.

Metabolic intermediates and capabilities of the microbial community will be identified. The rate-limiting metabolic process(es) will be determined by inhibiting specific processes and measuring the production rates of intermediates and also by adding different known intermediates and measuring the removal rates. The first test will be to inhibit methanogenesis from coal conversion using methanogenic-specific inhibitors such as bromoethanesulfonic acid. The accumulation rate of the expected intermediates, hydrogen and acetic acid, will be determined, providing an indication of their production rate by other organisms. The accumulation rate of other constituents, such as phenolic compounds and long-chain fatty acids, will also be determined. To stop acetic acid metabolism by all organisms including sulfate-reducing organisms, fluoroacetic acid, a potent inhibitor of acetate metabolism, may be used. Additional studies using non-methanogenic inhibitors, including antibiotics, will allow examination of the inherent methanogenic activity as well as fermentative processes that may be required to initiate degradation of the parent coal constituent. Selected intermediate compounds, determined from the inhibition studies, will then be added to inhibited and uninhibited cultures and their rates of removal measured. Hydrogen, acetic acid, methane, biomass concentration, and other constituents will be measured using established methods.

Modeling of anaerobic processes. The standard model for anaerobic processes, Anaerobic Digestion Model 1 (ADM1), considers a number of different metabolically active groups but was developed primarily for wastewater and sludge treatment. The approach used in ADM1 can be readily used to model other anaerobic systems, but the microbial populations and the metabolic pathways must be defined appropriately. The model developed for this work will examine hydrolysis and fermentation separately and include hydrogen production, hydrogen consumption, and hydrogen feedback inhibition for intermediate degradation. Parameter values will be estimated using an inverse modeling approach appropriate to upscaling. The data generated in the three previous Tasks will be used to develop a coupled microbial kinetic model that tracks C and N compounds from coal through intermediates to methane. Collection of data required for modeling will include changes in reactant concentrations that include coal mass, bulk coal chemical composition, added nitrogen and phosphorus, and changes in product concentrations including soluble carbon products, methane, and carbon dioxide. Additionally, data on the changes in community structure and active biomass concentration will be collected under the previous task.

Project Start: September 12, 2008
Project End: September 11, 2010

DOE Contribution: $864,333
Performer Contribution: $382,407

Contact Information:
RPSEA – Kent Perry (kent.perry@gastechnology.org or 847-768-0961)
Colorado School of Mines – Dr. Junko Munakata-Marr (junko@mines.edu)

Additional Information

Final Project Report [PDF-4.40MB]