Changes in Pore Structure of Coal Caused by CS₂ Treatment and Its Methane Adsorption Response

Abstract

The pore structure and gas adsorption are two key issues that affect the coal bed methane recovery process significantly. To change pore structure and gas adsorption, 5 coals with different ranks were treated by CS 2 for 3 h using a Soxhlet extractor under ultrasonic oscillation conditions; the evolutions of pore structure and methane adsorption were examined using a high-pressure mercury intrusion porosimeter (MIP) with an AutoPore IV 9310 series mercury instrument. The results show that the cumulative pore volume and specific surface area (SSA) were increased after CS 2 treatment, and the incremental micropore volume and SSA were increased and decreased before and after <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M1"><mml:msub><mml:mrow><mml:mi>R</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="normal">o</mml:mi><mml:mo>,</mml:mo><mml:mi mathvariant="normal">max</mml:mi></mml:mrow></mml:msub><mml:mo>=</mml:mo><mml:mn>1.3</mml:mn><mml:mo>%</mml:mo></mml:math>, respectively; the incremental big pore (greater than 10 nm in diameter) volumes were increased and SSA was decreased for all coals, and pore connectivity was improved. Methane adsorption capacity on coal before and after <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M2"><mml:msub><mml:mrow><mml:mi>R</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="normal">o</mml:mi><mml:mo>,</mml:mo><mml:mi mathvariant="normal">max</mml:mi></mml:mrow></mml:msub><mml:mo>=</mml:mo><mml:mn>1.3</mml:mn><mml:mo>%</mml:mo></mml:math> also was increased and decreased, respectively. There is a positive correlation between the changes in the micropore SSA and the Langmuir volume. It confirms that the changes in pore structure and methane adsorption capacity due to CS 2 treatment are controlled by the rank, and the change in methane adsorption is impacted by the change of micropore SSA and suggests that the changes in pore structure are better for gas migration; the alteration in methane adsorption capacity is worse and better for methane recovery before and after <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M3"><mml:msub><mml:mrow><mml:mi>R</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="normal">o</mml:mi><mml:mo>,</mml:mo><mml:mi mathvariant="normal">max</mml:mi></mml:mrow></mml:msub><mml:mo>=</mml:mo><mml:mn>1.3</mml:mn><mml:mo>%</mml:mo></mml:math>. A conceptual mechanism of pore structure is proposed to explain methane adsorption capacity on CS 2 treated coal around the <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" id="M4"><mml:msub><mml:mrow><mml:mi>R</mml:mi></mml:mrow><mml:mrow><mml:mi mathvariant="normal">o</mml:mi><mml:mo>,</mml:mo><mml:mi mathvariant="normal">max</mml:mi></mml:mrow></mml:msub><mml:mo>=</mml:mo><mml:mn>1.3</mml:mn><mml:mo>%</mml:mo></mml:math>.