Carbon capture and sequestration is one of the critical approaches to reduce the carbon footprint and achieve net-zero carbon emissions by 2050 as per the Paris Agreement in 2015. The United Nations Climate Change Conference COP26 Glasgow in 2021 also intended to finalize implementation plans for the Paris Agreement. Capturing industrial CO2 emissions and injecting them as compressed liquid CO2 into the deep-ocean sediments to store as gas hydrates provides a potentially sustainable and long-term approach to reduce atmospheric CO2 emissions. However, the application of this method depends upon how fast liquid CO2 becomes converted to hydrates and the understanding of the underlying crystal morphological changes. In this work, the kinetics of CO2 hydrate formation using high-pressure liquid CO2 with/without an environmentally friendly hydrate promoter l-tryptophan has been examined along with the morphological imaging of interfacial interactions of liquid CO2, water, and the hydrate phase simultaneously. In addition to that, a mathematical model for quantifying hydrate formation kinetics employing liquid CO2 has also been presented. The experimental results indicate that the water to hydrate conversion of about 82% can be achieved using liquid CO2 in an experimental hydrate growth time of about 11 h. Three different stages of hydrate formation were identified in the experiments, namely, hydrate nucleation (stage 1), hydrate film formation (stage 2), and hydrate film breakup and bulk hydrate formation (stage 3). The kinetic enhancement effect of a kinetic promoter was elucidated using different dosages of l-tryptophan (300–1000 ppm). The experimental results indicated that the green kinetic promoter helps to enhance the overall water to hydrate conversion from 82 to 98% and reduces the overall hydrate formation process time by 2.5 times. These findings suggest a way forward to an enhanced formation of CO2 hydrates from liquid CO2.

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Carbon capture and storage [CCS] is crucial for mitigating CO2 emissions. One of the potential CCS concepts is to compress and store the captured CO2 into deep oceanic sediments as gas hydrates. However, seawater is highly saline [brine], which may impair the formation/dissociation kinetics and storage of CO2 hydrates. Therefore, it is essential to understand the liquid CO2 [LCO2] hydrate formation and dissociation kinetics in static brine systems. In this experimental study, we have examined the formation/dissociation kinetics and morphology of high-pressure LCO2 hydrates in brine using a static [unstirred] high-pressure crystallizer at deep oceanic [1 km] thermodynamic conditions [10 MPa, 1–2 °C]. The results are compared with [unstirred/stirred] freshwater systems with/without hydrate promoters. Three key stages have been identified in the experiments: nucleation [stage 1], LCO2-hydrate-brine film formation [stage 2], and LCO2-hydrate-brine film breakage [stage 3]. In the absence of stirring, the formation of the LCO2-hydrate-brine film resists the mass transfer of LCO2 into the brine, and most likely, the volume expansion during hydrate formation causes the LCO2-hydrate-brine film to break. New hydrate morphological growth patterns have been identified. It was estimated that the hydrate conversion in the freshwater system was higher [27.5% (±3.04%) in 21.1 (±1.26) h] compared to the brine system [25.0% in 24.2 (±0.58) h]. LCO2 hydrates dissociate faster in brine [1.7 (±0.14) h] compared to the freshwater system [5.7 (±1.77) h]. Finally, the presence of the eco-friendly hydrate promoter 500 ppm l-tryptophan can delay the dissociation process.