Mechanical Properties of Abandoned and Closed Roadways in the Kushiro Coal Mine, Japan

The objective of this research is to clarify the mechanical properties and self-healing ability of Excavation Damaged Zone (EDZ) around rock caverns in clastic rock. Observations of nearly 100 closed roadways up to 50 years old, which can be regarded as very severe EDZs with no initial sealability and are up to 300 m deep in clastic rock, were made at Kushiro Coal Mine, Japan, to accomplish the objective. Most old roadways were closed, though a few remain open. Closure of old roadways was mainly due to roof deflection and/or floor heave. Large plastic deformations dominated; however, severe fractures were seldom observed in closed old roadways. Rayleigh wave velocity and hydraulic conductivity in the closed old roadways were in the range of 0.3 - 1.2 km/s and 5 x 10^[-7] - 1 x 10^[-7]m/s, respectively, and those in EDZ and EdZ (Excavation disturbed Zone) around recently excavated roadways were 1.1 - 1.8 km/s and 1 x 10^[-8] - 5 x 10^[-8] m/s, respectively. The extent of EDZ around the present tailgate was in the range of 1 m to 5 m. Mechanical excavation and prevention from water are suggested as the key points for long-term maintenance of rock repositories. Pressurization from inside the cavern to decrease the permeability of EDZ is proposed for maintenance of rock repositories in medium-hard clastic rock masses at similar depths for long periods

Accelerated Bioconversion of Chemically Solubilized Lignite Solution to Methane by Methanogenic Consortium: Experimental Results and Their Application to the Subsurface Cultivation and Gasification Method

Lignite is an obsolete and less commercially circulated natural resource due to its low calorific value worldwide. The effective conversion of lignite into methane is important considering the global energy crunch. This study reported the effective bioconversion of organic matter released from chemically solubilized lignite to methane using two methanogenic consortia types: mixed methanogenic enrichment culture (mMEC) and SAL25-2. We demonstrated in a microcosm study that the start of methane generation was observed within seven days. Furthermore, the methane yield increased as the total organic carbon concentration of the chemically solubilized lignite solution increased. Surprisingly, methane production using mMEC was drastically enhanced by approximately 50–fold when pulverized lignite was added as conductive material (CM) to the microcosms. To the best of our knowledge, this is the highest number of times methane production increased relative to the control. Our results demonstrated that bioaugmentation using a methanogenic consortium and adding pulverized lignite as CM could facilitate the bioconversion of chemically solubilized lignite solution to methane and lead to effective utilization of subterranean lignite, regarded as a neglected natural resource, without any further excavation processes