Greenhouse gas (GHG) transport as solid natural gas (SNG) hydrate Pellets: Assessment of Self-Preservation & dissociation controls

Abstract

Safe greenhouse gas (GHG) transport is an integral part of any carbon capture, utilization, and storage (CCUS) operations. This manuscript discusses the assessment of solid natural gas (SNG) hydrate technology to safely transport GHG utilizing the self-preservation capability of hydrates around 248 K. Ice shielding around hydrate grains has been previously implicated as a key mechanism for self-preservation of hydrates. However, there is a lack of thorough understanding of the hydrate grains sintering and compaction effects on self-preservation prior to the ice crust formation around the grains. In a first of its kind study, the promotional effects of hydrate grains compaction and sintering via pellet porosities, aging times, and formation times on self-preservation are presented. Furthermore, real time operational variables such as depressurization rates and system pressure build-up are studied. A unique integrated apparatus was designed to assess in-situ hydrate pellets formation, compaction, and dissociation. This method eliminated the uncertainties of possible hydrate dissociation during material transfer between the operational stages. Around 248 K, hydrate pellet stability occurs in two states: pre-self-preservation (initial fast hydrate dissociation) and extended self-preservation (slow hydrate dissociation over 200 + hours). Decreasing porosity via compaction and increasing formation and pellet-aging times via prolonged sintering increases hydrate pellet stability. The pellet stability can be further promoted by slowly depressurizing the pellet to ambient pressure and allowing pressure build-up from the dissociating hydrates. Additional transport risks are assessed through a sudden loss of temperature from 248 K, which are missing in the literature. Refrigeration failure of the pellet progresses in two stages: (1) a safe insulation stage characterized by slow dissociation and minimal gas evolution, followed by (2) a rapid dissociation stage during which complete dissociation of the pellet took place. Overall, this work elucidates the transportation factors governing self-preservation and dissociation, thus indicating the feasibility for safe GHG transport via SNG technology.