Mechanistic Investigation Of CO₂ Capture In Porous Carbon Materials And Sequestration In Brown Coal With CO₂-ECBM

With the continuous development of economy and society,the environment and energy issues have become increasingly prominent,e.g.,energy crisis because of the reduction of fossil fuels,global warming and sea level rising caused by the emission of greenhouse gases including mainly CO₂.Carbon capture and sequestration?CCS?technology becomes one of the effective ways to relieve excessive greenhouse gases due to its huge potential of emission reduction and economy.Sequestration of CO₂ in geological formations as a way to reduce the greenhouse effect has aroused a great deal of attention.Coal-bed methane?CBM?is considered as a good source which is an alternative of cleaner energy and has been exploited for energy production.Efforts have been concentrated on the process of enhanced coal-bed methane recovery?ECBM?to mitigate the increasing energy demand.Here,coal seam sequestration is defined as the storage of CO₂ from anthropogenic sources into deep,unmineable coal seams,which simultaneously enhance coal bed methane recovery?ECBM?.This process is termed CO₂-ECBM which has both environmental and economic benefits.This technology involves two processes,one is the capture of CO₂,and then the adsorption of CO₂ and CH₄ in coal.Porous carbon materials have been widely utilized in CCS due to its properties of higher surface area,more excellent thermal and chemical stability,and less humidity sensitivity.The unique structural features and surface chemical properties make brown coal the ideal system for the study of gas adsorptions.Graphdiyne family has already attracted the high degree of concern,due to its intriguing and promising properties.A series of graphdiyne-based frameworks?GDY-Rs and Li@GDY-Rs?with the introduction of a variety of functional groups?-NH₂,-OH,-COOH,-F?and doping metal?Li?are constructed from molecular building blocks.The effects of pore topology and morphology,functional groups,and doping metals on CO₂ adsorption properties are systematically investigated by combining density functional theory?DFT?and grand canonical Monte Carlo?GCMC?simulations.The results indicate that surface functionality in conjunction with ultramicropores structure plays a determinant rule in CO₂ adsorption at ultra-low pressure,while the CO₂ adsorption is governed by the total pore volume,surface area and wider PSDs?7.0-20.0??at high pressures.The amino functionalities provoked an exceptionally improvements in CO₂ adsorption,and the positive effects of hydroxyl and carboxyl groups are weaker,while fluorine functionalities bring about a negative influence at low pressures.Excitingly,combining Li-doping and hydroxyl groups strategies offer an unexpected synergistic effect for efficient CO₂ capture with an extremely CO₂ uptake of 4.83 mmol g^-1 at298 K and 1 bar.Combined with its superior selectivity?13 at 298 K and 1 bar?for CO₂ over CH₄,these findings verify that the Li@GDY-OH is one of the most promising material for CO₂capture and separation.GCMC combined with MD and DFT calculations have been performed to investigate the effect of oxygen-,nitrogen-,and sulfur-containing functional groups on competitive adsorption of binary CO₂/CH₄ mixture in brown coal.Our results indicate that the absolute adsorption isotherms of single-component CH₄ and CO₂ exhibit type-I Langmuir adsorption behavior and temperature has a negative influence on the gas adsorptions.For the binary CO₂/CH₄ mixture,the brown coal shows super high selectivity of CO₂ over CH₄ at low pressures,which then decreases quickly and tends to be constant when the pressure increases.Both the physical and chemical characteristics of brown coal account for the favorable adsorption of CO₂ relative to CH₄ in the binary CO₂/CH₄ mixture.First,the electrostatic interactions make a large positive contribution to the CO₂ adsorption,but a negative effect on CH₄ in the competitive CO₂/CH₄adsorption.Second,the brown coal with the pore sizes of 0.38–0.56 nm provides a conducive micropore environment for the adsorption of CO₂ rather than CH₄.Third,the high-density oxygen-containing,pyridine,and thiophene functional groups are energetically favorable for CO₂ adsorption with the sequence of pyridine-N>-C=O>-C₂O>-OH>-COOH>thiophene-S,whereas they has less influence on CH₄ adsorption.Furthermore,the influence of various nitrogen-and sulfur-containing functional groups on the CO₂ adsorption capacity was also investigated.The results indicate that the basicity of the oxygen-and nitrogen-containing groups has large influence on the CO₂ adsorption,while the adsorption of CO₂ at the sulfur functional groups is determined by their polarity.The knowledge obtained in the work may provide useful information in the application of ECBM.