AmericanChemicalSociety.com

Our research targets 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.  Effective stimulation of microbial CBM production requires basic knowledge of the microbial pathways by which the complex carbonaceous material in coal is converted to simple methane precursors.  At the current time, it is not at all clear which 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, in the coming year, will identify which compound class can most readily be transformed to methane by microbial communities. 

We have defined Class 1 compounds 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.  The Class 1 compounds include water-soluble compounds such as low-molecular-weight organic acids, amino acids, and some alcohols.  These water-soluble compounds are likely produced as fermentative products of more complex compounds, but we expect that they would have a relatively short half-life since they are also biodegraded rapidly.  They would therefore have a low steady-state concentration in the coals although they may be 'protected' in pores that are not accessible to microorganisms.     At the other end of the solubility spectrum, the Class 3 compounds are very hydrophobic compounds that are likely not microbially transformed at appreciable rates.  The moderately-polar, alcohol-extractable Class 2 compounds include an intermediate-range of compounds with limited water solubility but which are less refractory than the Class 3 compound class.  

In the first year of our PRF-funding, we have concentrated on developing the necessary extraction protocols and analytical procedures (e.g., gas chromatography, high performance liquid chromatography, and Fourier transform infrared spectroscopy) needed to produce and characterize the extracts.  To prepare our extracts, coal samples from the Lower Block Coal Member (Viking, Corning Pit) and Springfield Coal Member (Somerville Mine) in southwestern Indiana were collected in canisters filled with water to reduce exposure to oxygen.  Both coals are from the southeastern part of the Illinois Basin and are characterized as highly volatile bituminous coals.  Coals were pulverized under a nitrogen atmosphere prior to extraction.

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.  Water extracts (Class 1 compounds) were prepared by ultrasonication of powdered coal with deionized water, filtration to remove particulates, and lyophilization after raising the pH to 10 to avoid loss of low-molecular-weight organic acids.  Methanol and dichloromethane extracts were done using Soxhlet extraction of coal powders.

As expected, the amount of water soluble, Class 1 compounds was relatively small.  Water extraction of 700 g of the Springfield and Lower Block coals resulted in 0.94 and 0.55 g of freeze-dried extract, respectively.  The organic carbon content (as measured using dissolved organic carbon analysis) of these extracts, respectively, were only about 5.6% and 3.5% due to the presumed presence of carboxylates and other functional groups, in addition to a minor amount of salts that resulted from pH adjustment.  Preliminary analysis of water extracts from similar coals showed the presence of organic acids such as acetic acid and oxalic acid, in addition to as yet unidentified compounds.  The amount of Class 1 material extracted and the low organic carbon content was somewhat less than expected based on preliminary work with similar coals.  We will therefore need to modify our microbial experiments and utilize smaller reactor volumes, i.e., 10-20 mL instead of 100 mL, than were originally planned.  This should not cause significant difficulties, however, in moving forward.

The amount of material extracted in both the methanol and dichloromethane extracts was, as expected, significantly higher (over one order-of-magnitude greater) than in the water extracts.  Interestingly, the sequential methanol extraction (following water extraction) recovered more organic carbon than was recovered by methanol extraction of the non-water-extracted coal, possibly due to dissolution of water-soluble mineral coatings that impeded methanol extraction.  Preliminary analysis of  methanol and dichloromethane extracts showed complex mixtures that will be further resolved in ongoing analyses.

In the coming year, we will study methane production and carbon metabolism in anaerobic batch reactors containing a liquid culture medium, various coal extracts, and a microbial inoculum.   The microbial inoculum will consist of microorganisms anoxically removed via filtration from formation water from a CBM producing well in the Seelyville Coal Member in Sullivan County, western Indiana.  Collected water will be filtered through sterile 0.22 µm filters in an anaerobic chamber and filters will be used to inoculate the anaerobic batch reactors containing the various coal extracts.  The same inoculum source will be used in all experiments.  The Class 1, 2, and 3 coal extracts on sand will be added to the reactors as described above.  Aqueous and gas phase analyses of reactors will also be monitored over time to measure production of CH₄, H₂, CO₂, dissolved inorganic carbon (DIC), and simple organic compounds, e.g., acetate, formate, low MW alcohols, expected to be produced by fermentation of more complex organic compounds.  Changes in the composition of the added extract will be monitored at several time points by sacrifice of replicate inoculated and control reactors, re-extraction of the added compounds, and analysis by FTIR, GC, and HPLC (high performance liquid chromatography).  GC/MS will be used where necessary for further identification.  Knowledge of which extracts support long-term growth and analysis of changes in extract composition resulting from degradation will provide useful information about possible pathways leading to methane production.