Simulation of the interaction of methane, carbon dioxide and coal

International Journal of Mining Science and Technology 23 (2013)919-923Contents lists available at Science DirectInternational Journal of Mining Science and TechnologyELSEVIERurnalhomepagewww.elsevier.com/locate/ijmstSimulation of the interaction of methane carbon dioxide and coalNie Baisheng Wang Longkang, Li Xiangchun, Wang Chao, LiliSchool of resources Safety Engineering, China University of Mining S Technology, Beijing 100083, ChinaState Key laboratory of Coal Resources and Safe Mining, China University of Mining &r Technology, Beijing 100083, ChinaARTICLE INFOA BSTRACTGas adsorptio12 March 2013coal and gas adsorption is the theoretical basis for studying gas flow in coal. In this paper the interactionin revised form 21 April 2013Accepted 1 May 2013between methane, carbon dioxide and surface molecules of anthracite was simulated using the quantAvailable online 2 December 201chemistry method. Adsorption energy and adsorption configurations of different quantities of gas mole-les absorbed on the coal surface were calculated The results show that adsorption between coal andthe two kinds of gas molecules is a physical adsorption process and there is an optimal configurationGas molecules are more easily adsorbed in the hydroxyl-containing side chain, while it is difficult forCarbon dioxidethem to be adsorbed at the position of the benzene ring. Besides, carbon dioxide molecules are moreCoal surface moleculeseadily adsorbed on the coal surface than methane molecules. the findings have an important signifiInteraction energycance in revealing the nature of gas adsorption in coaluantum chemistryo 2013 Published by Elsevier B V on behalf of China University of Mining technology1 Introductionthe total energy of the system after adsorption At first, the adsorption energy is calculated fromThe characteristics of adsorption and desorption of gas in coalhas become an important research topic in recent years, from theEads=EM Ew +Ew/Mpoint of view of both gas accident prevention and clean energy where eads is the interaction energy when coal molecules and gasexploitation [1-7]. However, these researches remain mainly at molecules reach adsorption equilibrium, kJ/mol; EM the energy ofthe experimental stage on a macroscopic scale. Studies at a micro- coal molecules before adsorption, kJ/mol: Ew the energy of gasscopic scale are few in number and tend to focus on the adsorption molecules before adsorption, kJ/mol: EwM the total energy of theof single molecules in coal. The process of coal and gas adsorption system when coal and gas adsorption reaches a balance, kJ/molwas discussed by Nie et al. [8 using theoretical calculations. Chen The calculatThe calculations in this paper were carried out using the geet al. 9] simulated the interaction between CHa and the coal suln 03 program. Density function theory(dFr)is adopted to studyface with Ab initio calculations. The interaction between CHa and the process of coal and gas absorption based on B3LYP/STO-3Gcoals having varying degrees of metamorphism was studied byJiang et al. [10 using the quantum chemistry method. In this pa- 2.2. Determination of anthracite surface molecule modelper, the quantum chemistry density functional theory and a coalsurface molecular model of anthracite are selected for the studof the interaction between methane, carbon dioxide and coal sur- ture of which is a combination of identical, or similar, molecularface molecules from the perspective of function configuration, fragments [11]. A coal molecular fragment is a partly formed coalfunction energy and electron transfer laws. This approach is signif- organic macro-molecule. Based on the structural model of anthraicant in revealing the nature of coal and gas adsorptioncite having the highest metamorphic grade and with appropriatesimplification, hydrogen saturation is used to deal with cutoff2. Calculation modeland thereby a surface molecule model of coal is obtained and thebenzene ring is numbered [12]. Due to its symmetric structure,2. 1. Construction of the calculation modelonly one side of the symmetric structure of anthracite is numbered, as shown in Fig. 1. Being the representative body of coal,he process of coal and gas adsorption is an exothermic process. anthracite is used to study the adsorption of gas molecules onThe sum of their separate energies before adsorption is larger than the coal surfaceThe anthracite中国煤化工 is depicted in theCorresponding author. Tel. +86 10 82375620Gaussianview softd, and the optimaE-mailaddress:bshnie@163.com(B,Nie).structure of theHCNMHGed, as shown inh t: /d. do, orer o m imes.213. 12 7 hed Dy Elsevier Bv, on ehalt of China University ot Mining technolOgyB Nie et al/International Journal of Mining Science and Technology 23(2013)919-923Fig. 2. The dark gray circles marked with"C "represent carbonatoms, light gray circles marked with"H"represent hydrogenatoms, and red circles marked with" O"represent oxygen atoms.From the optimization results, the energy values can beobtained-the unit is the 'Hartree, where 1 Hartree= 2625.500kJ/mol, and the energy of coal surface molecules is -17412898403900 Hartree°3. Calculation of the interaction of methane and carbon dioxidewith coal surface moleculesDuring the simulation of adsorption between gas molecules andcoal surface molecules, the number of gas molecules adsorbed bycoal is increased gradually, with each of the resulting structuresing optimal. The optimal position of coalcules can thus be obtained. The relationship between adsorptionenergy and the corresponding number of gas molecules adsorbedis linearly related. The difficulty or ease by which the coal surfaceadsorbs methane and carbon dioxide molecules can therefore beFig. 2. Optimal configuration of anthracite.determined by comparing the slope of the fitted straight lines3. 1. Calculation of the interaction between coal surface and methaneTable 1molecules/13-16Adsorption energy between anthracite and various number of CHa(kJ/mol)No of CHa3. 1. Adsorption energyThe Gaussian03 software package is applied first and then the7.6725445961Side chainDFT is used to optimize methane molecules at the b3LYP/STO-3G 279914575170Side chain and one benzene ringlevel. It can be determined that the energy of a single methaneSide chain and two benzene ringmolecule is -40.0389245125 Hartree8.5351854400Side chain and three benzene rings8.7462428210When methane molecules are adsorbed on the coal surfa8.9137694130 Side chain and five benzene ringsthey reach the current optimal adsorption position successively9.163815469Side chain and six benzene ringsthrough competition. The composite structure of different numbers of gas molecules adsorbed on a benzene ring and the sidechain of a coal surface can be formed Optimization calculationson a few kinds of structures formed from coal surface moleculesAs can be seen from Fig 3, when a small number of methaneand different numbers of methane molecules have been carried molecules are adsorbed on a coal surface, the relationship betweenout and the parameters that include the binding energy, bond adsorption energy and the number of adsorbed methane moleculeslengths, bond angles and other micro parameters determinedis linear and can be expressed by Eq (2)Different numbers of methane molecules were placed on a ben- Eads=0.245x+7.48zene rIngneously. The adsorption configuration of methane molecules where x is the number of methane molecules adsorbedadsorbed on the benzene ring or the side chain (hydroxyl) wasthen obtained through optimization. Therefore the adsorption en- 3. 1. 2. Analysis of adsorption configurationergy of different configurations can be obtained, as shown inThe configurations of anthracite adsorbing multiple methaneTable 1molecules can be analyzed by Gaussian 03. Here the case of twoAs can be seen from Table 1, the adsorption energy is different methane molecules is taken as an example. With the Quantumwhen anthracite adsorbs different numbers of methane molecules. Chemistry software package and density functional theory the georstly, methane molecules are adsorbed on the side chain of coal, metric configuration of physical adsorption between anthracitewhere the adsorption energy is the largest. Then they are adsorbed and two methane molecules at the b3LYP/STo-3G level can be aton the benzene ring with an increase in the total number of meth- tained, as shown in Fig 4. Two methane molecules are adsorbed onane molecules adsorbed. In general, the adsorption energy in- hydroxyl and its adjacent benzene ring respectivelycreases with an increase in the number of adsorbed methaneThe bond length and bond angles when two methane molechmolecules. The relationship between adsorption energy, except en- are adsorbed on the surface of anthracite are shown in Tables 2ergy adsorbed on the side chain, and the number of adsorbedmethane molecules is shown in Fig 3The distance of equilibrium adsorption is the distance betweenwo closest atoms of methane molecules and coal surface mole.cules when adsorption is equilibrated. So the distances of equilibrium adsorption between anthracite and methane molecules are1.96848, 4.61023 and 1.96848 A respectively These distances areless than that in genecannot occur duringV凵中国煤化工mtmadsorption processIICNMHGldl LUlllguration betweenanthracite adsorbing methane molecules can be attained. At theFig. 1. Structure of anthracite coaltop of the benzene ring, methane molecules form an inversehnology23(2013)919EaA=0.2451x+7.4804Bond angle between two CHa molecules and coal ().Bond angleAtomic rolesBond angleA(61,73,70)175.77A(6261,73)106.52A(1361,73)13.61702.3110790A(8.68,67.5A(13.6865)A(146865)159.370123456A(136665)72.11Number of CHa adsorbedA(6261,70)Fig 3. Adsorption energy between anthracite and methaneof the former is also lower when two methane molecules areadsorbed by anthracite simultaneously, the equilibrium distanceof methane molecules adsorbed on the side chain is far less thanthat adsorbed on the benzene ring and the absorption energy isreversed3. 2. Calculation of the interaction between coal surface and carbondioxide molecules3. 2. 1. Adsorption energyBy optimizing the carbon dioxide molecule it can be determinedthat the energy of a single carbon dioxide molecule isThe calculation methods are the same as those described in sec-tion 3. 1. 1. The adsorption configuration of multiple carbon dioxidemolecules can be obtained and hence the adsorption energy of different configurations can also be obtained. The calculation resultsare shown in Table 4As can be seen from Table 4, the adsorption energy is differentFig 4. Adsorption configuration of two CHa molecules and anthracitewhen adsorption occurs between the surface molecules of anthracite and different numbers of carbon dioxide molecules, Carbondioxide molecules are adsorbed firstly on the side chain of coalsurface molecules, where the adsorption energy is the highest -al-Bond length of coal adsorbing two CH4 molecules (a)most one order of magnitude larger than that adsorbed on the benBondAtomiczene ring. With increasing numbers of adsorbed carbon dioxidemolecules, adsorption on the benzene ring commencesR(6273)R(13,73)point, the adsorption energy generally increases along with theR(13,70)4increasing number of adsorbed carbon dioxide molecules. The relaR(6271)39R(13,71)4.73tionship between adsorption energy and the number of adsorbed363(6272)424(13,72)R(61,74)3.56carbon dioxide molecules is illustrated in Fig. 5R(688)R(658)577R(66,8)As can be seen from Fig. 5, The relationship between adsorption4.61R(659)568R669)6.23energy and the number of adsorbed carbon dioxide molecules is68,114,6R6511)574R66,11)606linear and can be expressed by Eq. ( 3)R(6813)483R(65,13)5.89R(66,13)6.13R(68,14)4.93R(6514)597R(6614)637Eads=2.129X+22.61R(65,15)591R(66,15)6.5R67,8)6.08R(67,11R6714)6.66where x is the number of carbon dioxide molecules adsorbR67R(67,15R69,11)635R(69,14622R(6913)633R(6915)6.043. 2.2. Analysis of adsorption configurationThe configurations of anthracite adsorbing multiple carbondioxide molecules can be analyzed by gaussian 03. Here, the caseof two carbon dioxide molecules is taken as an example. The calcutriangular shape and on the side chain of anthracite the c-h bond lation methods are the same as those described in Section 3. 1. 2of methane molecules is in a straight line with the oxygen atoms The geometric configuration is shown in Fig. 6 where two carbonon the side. The length of the C-H bond of a methane molecule be- dioxide molecules are adsorbed on hydroxyl and its adjacent benfore adsorption is 1.09684 A and after adsorption the values are zene ring respectively1.10305 and 1.09685 A when a methane molecule is adsorbed onThe bond length and bond angles when two carbon dioxidethe side chain and benzene ring of anthracite respectively. The molecules are adsorbed on the surface of anthracite are shown inchange in C-H bond length is minimal, which does not cause Tables 5 and 6breaking and re-generation of the bond and therefore this adsorp- anthracite and中国煤化工re 168844 andtion process can also be proved to be physical adsorption and not 4. A. theseCNMHGe in generating achemical adsorption. Furthermore, the change in C-H bond length chemical bond, whllIt dusun Puun between coal sur-of methane molecules adsorbed on the benzene ring is smaller face molecules and two carbon dioxide molecules, is phythan that adsorbed on the hydroxyl and so the adsorption energy adsorptionB Nie et al/International Journal of Mining Science and Technology 23(2013)919-923Table 4Table 5dsorption energy of anthracite and various number of COz molecules(kJ/mol).Bond length between two COz molecules and coal (A).No of CO2 Adsorption energy Adsorption sitesAtomicAtomicBondtomBond24.70813154de chainR(60,701234R(57,70)25526.53358655side chain and one benzene ringR(60.66)3.529.30046710Side chain and twoR60,69)37030.88335217Side chain and three benzene ringsR(65,47)4.55R(67,47)4835784.60407960699Side chain and four benzene ringsR(6848)4.95ve benzene ring6750)4.5Side chain and six benzene ringsR(6552)56752)R(6553)5.016753)48R(6554)4.76R(67,54)4.97E21202+2607Table 6Bond angle between two COz molecules and coal (o).Bond anglA(70.66,69)178,7A(52.5960)104.54A(5960.70)173.7A(53.57,70)12719A(57,70,66)174.65A(47,67,52)35.32A(486753A(48,65,53)Number of co, adsorbedA(5067545065,54A(4748,5011987A(676568179.715253,57)11896Fig. 5. Adsorption energy chart of Co, and anthracite coal124.97A(48,6853)1.43A(476852)30.00A(50.6854)3095When two carbon dioxide molecules are adsorbed simultaneously by anthracite the equilibrium distance of carbon dioxidemolecules adsorbed on the side chain is only 1/3 of that adsorbedon the benzene ring and the adsorption energy is reduced4. Conclusions(1) When methane or carbon dioxide interacts with coal surfacemolecules, there is little change in adsorption energy, equilibrium distance and molecular bond this indicates thatno chemical bond forms or in other words there is no substance generation and no chemical reaction takes placeHence only physical adsorption occurs between the twokinds of gas molecules and coal(2)When methane or carbon dioxide interacts with coal surfacemolecules, the energy of hydroxyl side chain adsorption isat least one order of magnitude. This demonstrates thatFig. 6. Adsorption configuration of two CO2 and anthraciteadsorption of gas molecules takes place more readily onthe side chain of coal molecules. it can be inferred that whenlarge numbers of methane molecules are adsorbed on coalAs can be seen from table 6. the carbon dioxide molecules aremolecules, they adsorbed firstly at the side chainsorbed at the top of the benzene ring of anthracite and they(3)When methane adsorbs on the benzene ring of a coal surfaceare substantially parallelplane of the benzene ring, whilemolecule, methane molecules form an inverse triangularthe carbon dioxide molecules adsorbed on the side chain of anthra-shape at the right top of the benzene ring whilst carboncite are substantially co-planar with the plane of the benzene ringdioxide molecules occur at the top right, parallel with theof coal surface moleculesplane on which the benzene ring is located. The configuraIt is known that the C-o bond of carbon dioxide before adsorp-tion is optimal when methane adsorbs on different coal surtion is 1.22743 A and after adsorption the values are 1. 23297 andface molecules1.22733A when carbon dioxide is absorbed on the side chain(4)A coal surface molecule can simultaneously adsorb a largeand its adjacent benzene ring of anthracite respectively. thenumber of gas molecules. When a coal molecule absorbs ahange of C-o bond length is minimal, which further confirms thatsmall amount of gas on the benzene ring, it can be considhe interaction between coal and carbon dioxide is physicalered as mono-adsorption. Furthermore, the change in the c-o bond length of car中国煤化工巾 bed moleculbon dioxide molecules adsorbed on the benzene ring is smallerare positivelyCNMHGthan that where adsorption takes place on the hydroxyl, which(5)The slopes of the incan lai ves Ill une enei gy diagram areindicates that there is a positive correlation between adsorption0.245 and 2. 129, indicating that the energy from carbonenergy and bond length.dioxide molecules adsorbing on coal molecules is greaterB, Nie et al /International Journal of Mining Science and Technology 23(2013)919-923than that from methane molecules adsorbing on coal mole5 Krooss BM, Van Bergen F. Highe methane and carbon dioxidecules. 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