Molecular Dynamics Simulation On Decomposition Mechanism And Stability Of CO₂ Hydrate In Porous Media

Carbon dioxide（CO₂）,as an important greenhouse gas,which is the main cause of global warming,greenhouse effect and other environmental problems.The goal of"carbon dioxide emissions peak and carbon neutrality"has become a national strategy,thus controlling and reducing CO₂emissions has become an important challenge faced by all countries in the world.The capture and storage of CO₂gas by hydrate method is an effective way to realize CO₂emission reduction treatment and emission control.Hydrate technology has great development potential and application prospect in the separation and capture of CO₂from industrial flue gas,geological storage of CO₂hydrate method,and replacement exploitation of nature gas hydrate by CO₂.Therefore,the accurate understanding of the decomposition mechanism and stability of CO₂hydrates in porous media systems will help to more clearly understand the generation,decomposition and storage characteristics of CO₂hydrates in complex environments.At present,the research on CO₂hydrate in porous media is mainly based on macroscopic experiments,and the explanation of the decomposition mechanism and stability of CO₂hydrate mainly focuses on pure hydrate phase or additive system.There are still many controversies on the explanation of the influence mechanism of porous media.Therefore,it is necessary to further investigate the decomposition mechanism and stability of CO₂hydrate in porous media.In this paper,three types of porous media model systems based on the cristaleite（Si O₂）as the single crystal material with the same number of CO₂hydrates were constructed by using the molecular dynamics simulation platform,then decomposition process was simulated using the single variable method.Taking the pure hydrate phase as the control group,the decomposition mechanism of CO₂hydrate in different porous media models was studied,and the effects of cage relative occupancy and symmetrical media spacing on the decomposition mechanism were further explored.In addition,the effect law of porous medium model,temperature,relative cage share,hydrophicity of medium surface and thickness of interstitial water on the stability of CO₂hydrate were summarized.The main findings are as follows:（1）The decomposition process of CO₂hydrate in different model systems is mainly divided into two stages:the break stage of initial cage structure and CO₂bubble evolution stage.The whole hydrate phase follows a not very strict contraction-core decomposition mechanism from the outside to the inside,and the decomposition rate of each layer is different between the critical and non-critical,or between the upper half and the lower half.There is obvious decomposition priority in the critical region,specially in the porous medium system（due to Si O₂shows a strong adsorption effect on CO₂molecules）,while the decomposition of hydrate in the non-critical region is closely related to the surrounding cell state.（2）During the decomposition process of CO₂hydrate,the CO₂molecules will experience four stages:the internal stable existence in the initial cage structure;rotation,transfer and gradually escape from the intact water molecular cage by overcoming the van der Waals force;existence in a residual cage;gradual aggregation and evolution into molecular bubbles.The main influence of porous medium on the second stage is to cause the escape of CO₂molecules in the critical hydrate layer without seriously damage of the initial cage structure,as well as the main influence on the fourth stage is the interface aggregation and molecular diffusion gradient along the Z direction caused by the mass transfer obstruction and the interaction with the molecules in the bulk phase.（3）The spacing of the symmetrical two-plate system has a significant effect on both stages of CO₂hydrate decomposition.As the spacing is small,the stability of the non-critical hydrate layer is enhanced,and the first stage of CO₂hydrate decomposition is significantly slowed down.As the spacing is large,the effect of medium layer on the hydrate layer is obviously weakened with the increase of distance,and the decomposition mechanism of non-critical hydrate layer is more inclined to pure hydrate phase system.Additionally,it is found that in this kind of confined space model,the decomposition degree of hydrate in the middle region is greater than that of the critical and even adjacent hydrate layers,which is different from the mechanism that the decomposition degree of the outer layer is always greater than that of the inner layer in the pure hydrate phase system.（4）The structural strength of the empty cage is low,by the driving force,the surrounding molecules will diffuse and collapse to the inside of the empty cage,and the empty cage will be like a“ignition point”,then“the fire”will gradually spread and lead to the acceleration of the decomposition rate of hydrate phase.Comparative analysis of the relative share of four cage types（D:100%,T:100%;D:0%,T:100%;D:0%,T:50%;D:50%,T:50%）,it is found that there is a significant difference in the stability of CO₂hydrate between the large and small cages.The calculation shows that the effect of the vacancy of CO₂molecule in the large cage on the stability of CO₂hydrate is about 1.29 times than that in the small cage.（5）The stability of CO₂hydrate is weakened with the increase of temperature.The results show that,with the temperature intervals of 15 K within 255 to 315 K,the difference of CO₂molecular diffusion coefficient under adjacent temperature systems is as follows:0.664×10^-9m/s,0.411×10^-9m/s,0.45×10^-9m/s and 0.584×10^-9m/s,that is,the influence degree of temperature on its stability is positively correlated but nonlinear.（6）By increasing the hydroxylation ratio on the surface of the medium,it is found that the influence of enhanced hydrophicity on the overall stability of CO₂hydrate is weakened,and the specific influence law is related to the medium model,the hydroxylation ratio,as well as the interaction force between the medium layer and the guest molecules.In the analysis of the influence of interstitial water thickness on the stability of CO₂hydrate,it is found that there is an appropriate thickness（close to the size of one hydrate cell in the paper）that can enhance the stability,while smaller or larger will reduce the stability.