DESIGN OF A SMALL-SCALE HIGH-PRESSURE REACTOR SYSTEM TO STUDY MICROBIAL BIOCONVERSION OF COAL TO METHANE

Microbial generation of coal bed methane (CBM) represents a significant source of natural gas on Earth. While its biostimulation has been demonstrated in batch cultures, environmental parameters such as overburden pressure and formation water flow need to be tested at the laboratory scale before field experimentation. We have designed and fabricated a reactor system that simulates in situ conditions of underground coal seams. It contains an upflow reactor that can be operated at high pressure and low flow using ISCO pumps. The first reactor was filled with coal from the Powder River Basin (PRB, USA), and the second contained a coal/sand mixture to represent the interface of coal seams with sand layers, which are hypothesized to exhibit higher methanogenesis rates in situ. The system was filled with CBM formation water degassed through a retention membrane to allow for anaerobic culture conditions. After colonizing the reactors for ten days, a methanogenic inoculum enriched from a PRB coal bed was stimulated with algal biomass. During the incubation, the reactors were pressurized at 80 psi and a flow of 0.01 mL/min was applied to the system for five hours a day, to allow for a residence time of 20 days. Control batch cultures (no pressure, no flow, ± amendment) were setup in parallel to estimate the time to methane production and observe the effect of pressure on the inoculum. Dissolved methane concentration was analyzed over time by gas chromatography for 75 days. At the end of the experiment, the reactors were depressurized and sampled. The coal/sand reactor exhibited higher methane production than the coal-only reactor: a total of 400 and 150 μg of methane per gram of coal were produced, respectively. This pattern was also observed in the corresponding controls, suggesting an interface effect on methanogenesis. In comparison, the batch cultures generated up to 1000 and 600 μg/g coal, which might reflect the impact of pressure and flow on the methanogenic inoculum. This study indicates that the high-pressure test system we designed is well suited for the study of methanogenesis, and provides a first demonstration of CBM generation from PRB coal under pressure in the laboratory. Further efforts will investigate methanogenic bioconversion inside the fracture system of a coal block using a mesoscale high-pressure reactor.