Oxidation of lignite with H2O2 solution to produce dissolved organic carbon (DOC) for generating biomethane by methanogen cultivation is an important stage of subsurface cultivation and gasification methods. To obtain more insights into this process, changes of lignite mechanical properties were investigated after its oxidation to produce DOC. Core specimens 30 mm in diameter and 60 mm in height were immersed into 1 wt.% H2O2 to achieve a liquid-to-solid ratio of 5:1. Values of pH and Eh were measured at arbitrary time intervals together with concentrations of H2O2 and DOC. P-wave velocity and density were measured before and after immersion. A series of uniaxial compression tests was carried out for both chemically reacted (H2O2-immersed) and non-reacted (H2O-immersed) specimens. The concentration of DOC, which is the substrate of methanogen cultivation, increased due to the oxidation of lignite at decreased pH by H2O2. P-wave velocity showed positive correlations with strength, static tangent modulus, and dynamic Young's modulus. The average P-wave velocity (Vp) decreased by about 1.5% from its initial value due to the average density decrease of 0.6% resulting from the above chemical reaction. This decrease was associated with microcracking caused by swelling and grain boundary change due to leaching. The influence of the above chemical reaction on the mechanical properties of lignite is small, despite the formation of DOC to produce biogenic methane.

1. Introduction

Lignite seams of the Tenpoku coal field in Hokkaido, Japan, are considered to be used for biomethane production by subsurface cultivation and gasification (SCG, Aramaki et al., 2015; Tamamura et al., 2016), with the formation of dissolved organic carbon (DOC) from lignite by induced oxidation using hydrogen peroxide (H2O2) being the first stage of this method. Subsequent stages feature methanogen cultivation to produce biomethane, using the DOC as a substrate, and the last stage corresponds to gas recovery. The formation of DOC for methanogen cultivation via chemical reactions can change the mechanical properties of lignite, which is the subject of this research.

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