| Description |
Biogenic gas production is a promising alternative or supplement to conventional methane extraction from coalbeds. Microbial consortia native to coalbeds play an important role during biodegradation of carbonaceous sources to produce methane. It is possible to supplement and/or enhance the ability of indigenous microbial communities to produce methane from coal. Presume that adsorbed gas, generated over geologic time, can be supplemented with intentionally-generated biogenic gas during short-term engineering operations. There are two generic procedures for this. The first is to contact the coal with nutrients to support native bacterial development. The second concept is to inject appropriately cultured ex situ consortia. The research presented here involves this latter strategy. Microbial populations were collected from various hydrocarbon-rich environments and locations characterized by biogenic methane production. Different rank coals, complex hydrocarbon sources, hydrocarbon seeps, and natural biogenic environments were incorporated in the sampling. Three levels of screening allowed selection of consortia, favorable nutrient amendments, and quantification of methane produced from various coal types. Incubation periods of up to twenty-four weeks were evaluated at 23 °C. After a two-week incubation period, generated headspace gas concentrations reached 873,400 ppm (154 sft3/ton) for methane and 176,370 ppm (31 sft3/ton) for carbon dioxide. It was demonstrated that microbial communities from coal and lake sediments can be enriched and adapted to effectively generate methane after initial atmospheric exposure. Promising microbial consortia were subsequently incubated using low concentration of nutrient amendments (e.g., 22.4% v/v, 3.36 mg/cm3 TSB) and [NaCl] 6.6 mg/cm3 as a possible scenario and foresee the elevated costs of nutrient utilization at large-scale operations. Incubation periods of up to four months were evaluated at 23 °C. After two months of incubation, generated headspace gas concentrations reached 95,700 ppm (14 sft3 /ton) for methane and 37,560 ppm (5.5 sft3/ton) for carbon dioxide. Finally, environmental conditions that led to increased methane production from subbituminous coal with a methanogenic consortium at low concentration of nutrient amendment were evaluated. A central composite design (CCD) was used to explore a broad range of operational conditions, examine the effects of the important environmental factors, such as temperature, pH, and salt concentration, and query a feasible region of operation to maximize methane production from coal. An anticipated detrimental effect of NaCl concentration on methane production was observed. The feasible region of operational conditions comprised pH values between 4.1 and 6.8, temperatures between 23 °C and 37 °C, and NaCl concentrations between 3.68 mg/cm3 and 9.0 mg/cm3. Coal biogasification was optimal at an initial pH value of 5.5, at 30 °C, and a NaCl concentration 3.68 mg/cm 3 (i.e., 145,165 ppm, which is 25.6 sft3/ton). Results infer that microbial consortia can be used as an attractive low-cost biological complements for coal biogasification. |
