www.acsprf.org

Our research has examined coal as a potential source of biogenic methane.  Stimulation of biogenic coalbed methane (CBM) can be considered a more environmentally benign method of harvesting energy from coal compared to combustion or traditional chemical gasification.  At the current time, it is not clear which organic compound classes in the coal are the primary sources of H₂ or acetate, the likely immediate biogenic CBM precursors.  Based on their extractability using various solvents, we have operationally defined three classes of organic compounds in the coal and studied microbial CH₄ generation using these compound classes as sole sources of carbon.  In addition, we have analyzed the composition of the extracts prior to and following methanogenesis to determine the nature of the organic compounds transformed during CH₄ generation.

 Class 1 compounds were defined as those that are soluble in water, Class 2 compounds as those that are soluble in methanol, and Class 3 compounds as those soluble in dichloromethane (DCM).  The Class 1 compounds are expected to include water-soluble compounds such as low-molecular-weight organic acids, amino acids, and some alcohols.  These compounds are expected to have a relatively short half-life since they are also biodegraded rapidly.  At the other end of the solubility spectrum, the Class 3 compounds are very hydrophobic compounds, whereas the moderately-polar, alcohol-extractable Class 2 compounds include an intermediate-range of compounds with limited water solubility.

  In the first year of our PRF-funding, we focused on developing the necessary extraction protocols and analytical procedures needed to produce and characterize the extracts prepared from two, highly-volatile, bituminous coals from the Lower Block Coal Member (Viking Corning Pit) and Springfield Coal Member (Somerville Mine) in southwestern Indiana.  Springfield (SPR) coal is rich in vitrinite and Lower Block coal (LB) coal has greater liptinite and inertinite contributions.  Extractions were done with the solvents described above to obtain the various compound classes.  In addition to extracting each coal sample individually with each solvent, we also extracted each coal sequentially to first remove Class 1 compounds followed by sequential extraction of the freeze-dried residue with methanol and dichloromethane. 

In the past year, we studied methane production and carbon metabolism in multiple anoxic batch reactors containing a liquid culture medium, the various coal extracts as the sole carbon source, and a microbial inoculum.   The microbial inoculum consisted of a mixed community of microorganisms anoxically removed via filtration from formation water from a CBM-producing well in the Seelyville Coal Member in Sullivan County, western Indiana.    The same inoculum source was used in all experiments.  Gas phase analyses of reactors were monitored over the approximately 6-month experimental duration to measure production of CH₄.  Changes in the composition of the organic extract were analyzed at the end of the experiments after sacrifice of replicate inoculated and control reactors, re-extraction of the added coal extracts, and analysis by FTIR, GC, and HPLC.  GC/MS was used where necessary for further identification.  This allowed us to compare the molecular composition of the initial organic extracts with their final, biodegraded residues.

Periodic methane measurements in bioreactors containing aqueous extracts did not produce CH₄ in amounts significantly greater than in reactors lacking coal extracts.  Inoculated controls containing H₂:CO₂ or acetate instead of coal extracts showed significant methanogenesis indicating that a viable inoculum was used in all reactors. We observed greater CH₄ generation in reactors containing methanol- and DCM-extracts from SPR coal than LB coal, and slightly more CH₄ was produced from methanol-extracts than from DCM-extracts.  The greater methanogenesis in the vitrinite-rich (92% vitrinite) SPR coal compared to the LB coal (66% vitrinite) may be partially a result of nitrogen-, sulfur, and oxygen-rich functional groups in vitrinite which may provide low-activation-energy sites for initiation of biodegradation.  In all cases, however, the amount of carbon in the extracts that was converted to methane was small, i.e., < 1%.  The major portion of the organic carbon extracted using our solvent systems was therefore resistant to complete anaerobic degradation by our inoculum.  Partial degradation patterns revealed by analysis of biodegraded residues extracted from reactors at the conclusion of the experiment were more interesting.

FTIR and GC/MS characterization of organic extracts in uninoculated controls and inoculated bioreactors enabled assessment of the distribution of constituent n-alkanes, acyclic isoprenoid alkanes, hopanes, phenanthrene, anthracene, fluoranthene, and pyrene.  The complete evaluation of extensive analytical data continues but results to date have established certain patterns.  Methanol-extracted organic carbon from both coal types was generally degraded more extensively than the organic carbon in DCM-extracts.  FTIR analyses showed that biodegradation generally resulted in increased aromaticity of residuals, likely as a result of preferential degradation of aliphatics by microbial consortia and an overall chain-shortening of aliphatic compounds.

GC/MS results also showed greater degradation (14 to 91%) of n-alkanes than that observed for aromatics (6 to 58%) when considering both coal types.  Biodegradation of acyclic isoprenoids was not observed and hopane degradation was quite limited.  Although overall degradation of aromatics was less extensive than that for n-alkanes, it should be noted that degradation of low-molecular-weight polycyclic aromatic compounds (PAHs) was quite striking.  In the methanol extracts from LB coal, for example, we observed extensive removal of phenanthrene (89%), anthracene (82%), fluoranthene (35%), and pyrene (27%).  The extent (partial or complete) of PAH degradation is unclear, however, since we did not attempt to identify dead-end metabolites.

Similarly, the overall extent of methanogenesis was not clearly correlated with organic carbon degradation, likely as a result of partial biodegradation.  The apparent simultaneous degradation of both n-alkanes and aromatic compounds is nevertheless particularly interesting and contrasts with the anoxic biodegradation of petroleum during which aromatics are generally degraded only after n-alkanes are severely depleted.

Based on the limited data developed from experiments with only two coals and the greater methane production with the SBR coal, we hypothesize that in situ stimulation of microbial methanogenesis in coal beds may be more effective in vitrinite-rich, high-sulfur coals.  Additional work is clearly needed with a wider variety of coals to test this hypothesis.