High-gravity assisted coal mine gas separation based on clathrate hydrates: Implication for methane recovery

Abstract

Hydrate-based gas separation offers a promising approach for coalbed methane recovery, reaching energy conservation and emissions reduction. This study innovatively applied high-gravity technology to enhance hydrate formation in separating 25%CH4/67%N2/8% O2 for achieving rapid and efficient methane recovery. Systematic investigations were conducted at 283.2 K and 3.0 MPa with tetrahydrofuran at a molar concentration of 5.56% and L-tryptophan at a mass concentration of 0.5% additives, first evaluating liquid flow rate effects (0–20 mL/min) on mixed hydrate kinetic performance and separation efficiency, followed by rotating speed optimization (0–1200 r min−1) under the optimal liquid flow rate. The high-gravity system amplified the gas–liquid contact area by ∼1155 times through cascaded liquid supply and secondary shear effects, methane molecules entered the hydrate phase rapidly under the highest driving force with the significantly intensified mass transfer. Optimal conditions (20 mL/min, 600 r min−1) yielded an exceptional initial hydrate growth rate of 58.59 mmol/(mol h) and methane recovery of 50.76%, about 71.33 and 0.58 times higher than the static system, respectively. Gas chromatography and Raman spectrometer analyses revealed superior methane enrichment in hydrate phase at 90% gas uptake completion, with a concurrent 41.17% reduction in process duration. These findings demonstrate the efficacy of high-gravity-enhanced hydrate technology for coalbed methane separation, offering valuable insights for optimizing clean energy utilization.

1. Introduction

Coalbed methane (CBM) is an unconventional natural gas resource that exists abundantly in coal mines. According to the International Energy Agency (IEA), it was estimated that the CBM resources amount to 260 trillion cubic meters in the world [1]. CBM is composed mostly of methane (CH4) and has been considered a high-quality and promising energy for the future energy structure. The energy released in the combustion of 1 m3 of methane is 35.9 million Joules, which is considerable with the combustion of 1.2 kg of standard coal [2–4]. However, most extracted CBM is mixed with air due to the exploitation technology limitations and reservoir-forming conditions, which results in a relatively low concentration of CH4 in CBM (low concentration coalbed methane, LCCBM, cCH4 < 30%) that cannot be utilized directly thus a large portion released into the atmosphere, producing up to 28 billion m3 of CH4 emissions each year [3,5–7]. Being the second-largest greenhouse gas, CH4 has a global warming potential (over 100 years) 34 times higher than CO2 [8], and it is responsible for around 30% of the rise in global temperatures since the Industrial Revolution [9]. Therefore, it is of great significance to separate and recover CH4 from LCCBM for clean energy supplement and greenhouse effect mitigation.

Recently, attention has been paid to the hydrate-based CBM separation for methane recovery. Compared to traditional separation methods such as chemical absorption, pressure swing adsorption, membrane separation and low-temperature liquefaction, the hydrate-based technology has the advantages of safety, cleanliness, and relatively low investment [10,11]. However, the relatively harsh hydrate formation condition and mass transfer limitation result in sluggish formation kinetics and low separation efficiency, posing major challenges for industrial applications [12]. Adding chemical additives