Spontaneous coal combustion producing carbon dioxide and water

leonlineatwww.sciencedirect.comMININGScienceDirectSCIENCE ANDTECHNOLOGYELSEVIERMining Science and Technology 20(2010)0082-0087www.elsevier.com/locate/jcumtSpontaneous coal combustion producingcarbon dioxide and waterDENG Cunbao,, WANG Jiren, WANG Xuefeng, DENG HanzhongCollege of safety Science and Engineering, Liaoning Technical University Fuxin 123000, ChinaCollege of Materials Science and Engineering, Liaoning Technical University, Fuxin 123000, ChinaAbstract: Gas products from the process of coal oxidization and spontaneous combustion have been studied at different temperatures with FTI spectroscopic tests. With temperatures rising to about 30-100 C, water and carbon dioxide gas were formed andfrom about 105-150 C, carbon monoxide was produced Using the DFT B3LYP method with a 6-311G basis set, the reaction sys-tem, where spontaneous combustion between coal and oxygen occurs and produces water and monoxide, has been studied, with thegeometric configuration for all stagnation points on the potential reaction energy surface optimized. with a frequency analysis andan IRC method, transient formations were tested. Our results indicate that in the reaction of coal oxidization and spontaneous com-bustion producing carbon dioxide and water, oxygen molecules attack carbon atoms of the terminal of the propyl alcohol group onthe lateral chain of benzene rings, which causes this propyl alcohol group to produce the acid (-CH2-CH2-COOH)group and waterThis acid group continues its break up into carbon dioxide and the (-CHy-CH3)ethyl group. We have come to the conclusion thathis water-and-carbon dioxide-production reaction is spontaneous, based on the observation of the energy released by the reactionKeywords: spontaneous coal combustion; infrared spectrum; DFT; reaction mechanismIntroductionpanol group based on the side chain of a benzene ringcauses it to produce a(CH2-CH2-COOH) groupFire, as a result of spontaneous coal combustion, without water. In our empirical investigation andthreatens safety and production in coal mines. Acci- given theoretical methods, the process and mechadents, caused by spontaneous fire not only affectnism of chemical reactions of oxidation and sponta-production but also cause gas and dust explosions andneous coal combustion, producing carbon dioxide andresult in casualties!) of theor national coal water has been studied. This should lay a theoreticalmines in China 47.3% have hazardous conditions. foundation for establishing a theory of spontaneousthreatening spontaneous combustion. For small mines, coal combustionthe figure is 85.3%. Because of spontaneous coalcombustion, about 200 million tons of coal resources 2 Experiment and calculationswill be lost every year in China. The main reason fora lack of control of spontaneous combustion is that 2.1 Reagents and instrumentsthere is, fundamentally, no method to solve its me-chanical problems. Already for a long time, efforts 2.5-i3T and a Fourier transformation infrared specApplying a tubular electric resistance furnace, SKhave been made, both qualitatively and from a mac-roscopic point of view413), to solve the essential astroscope, TEN SOR27, 55 coal samples as the re-pects of spontaneous coal combustion. Due to its agents from coal mining districts such as Datong,complexity, these efforts did not quite manageTiefa, Shuangheshan and Hegang, were analyzedsolve all probles. More recently, spontaneous coal The coal samples were ground less than 50 paticlecombustion has been studied from a microscopicsize unit in the laboratory, dried for 24 hours in apoint of view", but given that in chemical reactionsvacuum and preserved in a drying vesseL.oxygen molecules attack C atoms at the end of a pro- 2.2 Experimental procedureReceived 13 May 2009: accepted 20 July 2009Corresponding author. Tel: 86418 3351708tube中国煤化工四1E-mailaddressdengcunbao323@163.comCN MHGgen in a proportiondoi:lo.lo6sl6745264(09)601654of 2ared spectrum mapDENG Cunbao et alcoal combustion producing carbon dioxide and wateof oxidation and spontaneous coal combustion,pro-According to the analyses of infrared spectrogramsdifferent temperatures. The rate of in- the temperatures at which the gases were producedcrease in temperature in the tubular electric resistance from the coal samples varied. Gas products from coalfurnace was5℃/min.combustion have five different forms, i. e H2O, CO2,2.3 Calculation methodCO, CH4 and C2H4. Between temperatures of 30-100C, H20 and CO2 were formed. When the temperaturewith the use of Discrete Fourier Transformation ranged from 105-150 C, co was formed and when(DFT) and computer programme, B3LYP/6-3llG a the temperature exceeded 120 and until 170 Cgeometrical optimization of the reactants and prod- and C2Ha were produced. when the temperature wasucts was obtained and a state of intermediate and between 200-300 C, the curves of H2o, co,, comolecular transition was determined. After calculat- CH and C2H4 appear crested, CH4 and C2H4 changeing the vibration frequency of each stagnation point from a strong to a weak state on the curves and thenand carrying out a vibration analysis, the authenticity became strong again. This phenomenon indicated thatof each transition state was confirmed. As well, the CH4 and C2H4 were produced from side-chains, benzero-point energy(ZPE)was obtained. The internal zene rings and cycloalkanes. CH4 was produced fromreaction coordinates (IRC) were calculated similarly a methyl branched chain and C2H4 produced from aand the structural and functional variation among vinyl side chain, when the temperature was lowinteractional molecules along the minimal energy These two gases were produced from aromatic ringspathway are discussed. From the discussion the right and cycloalkanes, when the temperature was veryconnections between the transition states and reac- hightants, as well as the intermediate products were confirmed, by means of a Gaussian03 programme on our 3.2 Active part of reaction between coal and oxy-ig. 2 shows a simplified model of an organic coal3 Results and discussionmacromolecule after optimization3.1 Infrared spectral product analysis from oxidation and spontaneous coal combustionAfter a Fourier transformation interferometenalysis, the gases produced from the various coalsamples in the process of spontaneous combustion atdifferent temperatures were confirmed. Fig. I showsthe infrared spectrogram of coal samples producingas during spontaneous combustion0020H2OFig. 2 Chemical constitution of coal molecule simplified65°CHighest occupied molecular orbital and lowest un-13coccupied molecular orbital(HOMO-LUMO)and anearby molecular orbit have the greatest effect onreactivity of the starting material. Spontaneous coalcombustion should occur at the site of the maximumelectron cloud density of each group in HOMOLUMO. When the oxygen molecule attacks the coal350030002500200015001000molecule, it should occur at the site of the atom ofone group with the maximum electron cloud densityFig 1 Infrared gas spectrogram produced from coal samples in the highest occupied molecular orbit. Tableof the Datong Sitai Mine #ll, layer 8429 at theshows the electronic density distribution of each atomemperatures of 133 and 165 Con a side chain in a coal moleculeTable 1 Analysis of the frontier orbital(HOMO-LUMO)of a coal moleculeElectron densityOrbital energy中国煤化工-CHr-NH2-CH2- CH=CH2-r-CH:-CHzOH0.259383CNMHG0.18848071848003252770.3397110518700.090654Mining Science and Technolvol 20 No. 1According to the calculation of the basic structu3.3 Geometrie structure of stationary pointsunit of the molecular orbit of a simplified coal molecule, the oxidation reaction and spontaneous combus-In the end, the reaction of oxidation and spontane-s coal combustion produced COz and H2O in aelectron density, so that the atoms in the group posi- complex physicochemical process. Its step by steption, including N22, C25, C29, C46, C49, C54, C33 is the reactant, MI the intermediate, product TS theand C36, easily oxidate and spontaneous combustionoccurs when they are attacked by oxygen molecules. transition state and P the product)The oxidation reaction and spontaneaR+O2Ml→TSl→MI2→TS2→MI3→tion produced CO2 and water. The oxygen moleculesP+H2O→TS3→M→TS4→MI5→Pl+COattacked the C atoms at the end of the propanol groutFig. 3 shows the reaction system of oxidation andbased on the side chain of a benzene ring and pro- spontaneous combustion producing CO2 and H2O, theduced the group(-CH2-CH2-COOH) and water. The micro-mechanism of each reaction after calculation(-CH2-CH2-COOH)group continued to decompose as well as the molecular configuration and parametersand produced COz and(-CHz-CH3)of the reactant, the intermediate products and transition states and the product after optimization3中国煤化工●eoCNMHGFig 3 Mayactants, intermediate products, transient formation and product configuration in the reactionDENG Cunbao et alSpontancous coal combustion producing carbon dioxide and waterTables 2 and 3 show the variable parameters of the bond in the-C(39)(OH)3 group changed greatly. Thechemical structure of groups participating in the change in bond lengths of the intermediate and tran-chemical reaction of spontaneous combustion of oxi- sient states goes through a process in which a stabledized coal molecules producing H2O and CO2.intermediate state becomes an unstable transient stateTable 2 Side chain of the intermediate products containingand then retums to its stable intermediate statetransient formation structural changes of propanol groupother bond lengths changed little. The bond anglesin the reaction of coal oxidization and spontaneousA(39, 40,41)and A(40, 39, 58)changed the most, whilcombustion producingthe remaining angles changed but little. From ourTSIcalculated results, we can see that the c-o bond in151781.518414958the-C(39)(OH)3 group became longer and favored14151142651.85433.3271the development towards producing water. Based onR(3958)1426914558142691.37371.3712the B3LYP/6-31IG set, optimizing the structure ofR(3940)145111.41971,4221.34531.2415TS1, from the optimal result, the-O(40)H(41)groupR(40,41)0.972909614097291.23211.8987turned in the direction of the -O(56)H(57) group.Along this direction there was only one big imaginaryfrequency of 584.016 icmR(5859)09749097450977609781a great change occurred in the transition from theA(36,3940)1133363114853113.3301122226l1254317intermediate Mi2 to the transient state TS2. One bondA(36,3956)1072381075144107246103.4609702596of -OH of the-C(39)(OH)3 group broke from C(39)A(36, 39,58)1115895 112.3762 111 5921 113 3784 113.6329 and o and H in another bond of-OH were broken. H(39.5657)1094132109099310941351200815144.4890moved in the direction of the bond of-oh. From ourA(39,58, 59) 108 8252 106.3451 108.8256 109 5219 110.8315 calculations, we concluded that h turned in the direcA(39,40,41)1102055.6072 110. 1979 86.1768 112.4492 tion of th- CH3 group. Again, there was only one bigA(40.3958)103961999521410396331659411209163 imaginary frequency,ie,l653. lI icmA(40,3956)11009211262691004985473555.1762According to the transition of the transient stateA(56.39.58) 110.6586 109.8276 110.6494 106.6898 175.5624 TS2 to the intermediate MI3 state, the H and-O-Hup combined to form the chemical bond H-O-HTable 3 Side chain of intermediate, products containingand then formed the stable intermediate MI3 statetransient formation of structural change in propanol groupM13 went on producing P and water. Through chemi-in the reaction of coal oxidization and spontaneouscombustion producing carbon dioxidecal reaction, P formed the transient state TS3 by achemical reaction. Bond lengths and bond anglesMI4changed but a little via the process from the transient122231.1973TS3 state to the intermediate ml4 state.19235After optimizing the geometric structure of MllMI2, MI3, MI4, MI5, TS1, TS2, tS3 and TS4, the72845intermediate states MIl. MI2. MI3, Ml4 and mis doA(535556)1155539113484770.7lnot have their own imaginary frequency; tsl, TS2,A(50.5354)125.71221254815864717489TS3 and TS4 have only one imaginary freAs0s3s513289152721042916373ach:ie,58406,16531.5675:and206oicmrespectivelAccording to the theory of chemical rereaction processes of quantum chemistry calculation 3. 4 Analysis of IRC reaction pathduring the process of oxidation and spontaneous coal Using an IRC path analysis to the reaction channelcombustion, oxygen molecules attack the C(39)atom or reaction process given a DFT theory and thein the side chain group-C(33)H2-C(36)H2-C(39) B3LYP/6-3lIG set, with the Saddle Point as the centerHyOH in coal molecules. One atom of an oxygen and 0.05 Borm/s(unit of step length) as step length,molecule absorbs two h atoms in the-C(39)H2OH 50 points were found forward and backward. Thegroup, enlarges the C-H bond in C(39)H OH, breaks result shows that the change of bond lengths, bondand forms an-O-H chemical bond with the oxygen angles and dihedral angles among each atom of theatom. In oor case then, along with a complex process propanol-based group on the side chain of a benzeneof change, such as a reversal process, etc, the oxygen ring and fractures and formation of chemical bondsin the -o-H group absorbed C(39)to produce the agree with the theoretical analysis. According to theC(33)H2-C(36)Hr-C(39)(OH)3 group and a stable energy analysis and parameter oPtimization of theintermediate MIl state was finally reached中国煤化工 ary point, interme-The transition state TSI was formed with the ac- diateverified to be corcumulation of energy from the chemical reaction. rect aCNMHGion and spontaneWith the change of MllTSI-MI2, according to ous coal combustion producing methane, has alsothe calculated results in Table 2, the length of the C-o proved to be a reasonable proposition. Fig. 4 showsMining Science and TechnolVol 20 Nothe potential energy changes along with internal reac- energy of the system was at a maximum. The figuretion coordinates(IRC)in the reaction processes. It is also shows the authenticity of reaction transient stateseasy to see that the transient states TSl, tS2, tS3 and from another sideTS4 were formed during the reaction process and the0.5260.92808-04000408080400040806“020.20608-040.00408(a)Ml→Tsl→MI2(d)MI4-TS4-MISFig 4 Reaction energy change potential along reaction coordinates(IRC) in the processes3.5 Calculation of reaction potential barrierformation TSI: TSl was unstable and released 45.93k/mol energy to form the intermediate state MI2Tables 4 and 5 show the energy, zero point energy MI2 absorbed 137.65 kJ/mol energy to form the tran-nd energy after correcting for zero point energy ofthe reactants transient formation intermediate statessient state TS2; TS2 was also very unstable and re-ind products based on the B3LYP/6-3llG set andleased 175. 47 kJ/mol energy to form the intermediatestate MI3; MI3 absorbed 33.31 k/mol energy againhow the relative energy Erel of the intermediate states, and formed the product P and watertransient formation and products with the energy ofthe C23H29 NO4+O2 and C23H27 NOs groups as referFig 5 is a profile map based on the B3LYP/6-311Gence. From the value of the relative energy Erel, the set after correction of the zero point energy. the en-eaction activation energy of the controlling step of ergy value marked in the figure is total energy of thethe reaction channel was calculatedreactant C23H29 NO4+02, treated as the relative valueof the zero point. From this figure it is seen that theTable 4 Reactants, transient formation, intermediate and total reaction pathway where C23Hz NOs+H2o are prproduct energy in the reaction(energy unit: Hartree)duced, is not an elementary reaction. The reaction hasE(b3lyp)ZPE(b3lyp)E+ZPE Ere(kJ/mol) two steps. The first step goes through the process12495l1518690480887-1249030632from the intermediate mil state to the transient stateO2-150.259126810.003273-150.255854TSI and then to the intermediate state MI2. The en-M1-1399964875480488223-1399476653-49928ergy barrier was 38.01 kJ/mol. The intermediate stateTsl-1399949050420486877-13994621743801M2 was thermodynamically stable, with an energyMI2-1399964875640485209-1399479667-4593507.20 kJ/mol lower than the reactants. At theTs2-1399909956060482718-139942723813765time, the intermediate state MI2 was dynamicallyMi3-1399980945340486876-1399.494070-17547unstable and went on to react to form TS2 and a po-20-764159288800206487639528133.311323.5485184704624161323.08610k/mol of energy. Moreover, the potential energy proTable 5 Reactants transient formation intermediate and totalfile showed that the thermal energy released fromproduct energy in the reaction(energy unit: Hartree)reactant R to Mll and TSI to MIl provided enoughenergy for theof MIl to TSI and MI2 to TS2ZPE(b3lyp) E+ZPEEel (k/mol)The energy released from the chemical reaction of1323.548518470.462416-1323.0861030TS2 to MI3 caused the chemical reaction of Mi3 toTS3-1323.527335620.4603391323.0produce C23H27NO5+H2OFrom Table 5, we can see that the reactant P at first132341396231045339913229absorbed 50. 16 k/mol energy to form the transient13235561093504588781323.097232-35882state TS3; TS3 released 22.86 kJ/mol energy to formP1-1134.993626020.447749-1134545877the intermediate state Ml4: MI4 absorbed 302. 30CO2-188.557232690.010606242k/mol energy to form the unstable transient state TS4ed 358.82 kJ/mol energy to form the in-From Table 4. it is seen that the reaction between中国煤化工bed1242kmolthe reactant and oxygen released 499. 28 k/mol enmYergy to form the intermediate state MIl; MIl abCNMHGsorbed 38.01 kJ/mol energy to form the transientDENG Cunbao et alus coal combustion producing carbon dioxide and watevarious gases produced from different coal samples2)Gas products of coal combustion have five dif-ferent forms, i.e., H2O, CO2, CO, CH4 and C2H. Attemperatures between 30-100 C, H2O and COz areformed. When the temperature reached 200-300 Ccurves of H2O, CO2, CO, CH4 and C2H4 appear500crested70)3)According to the theory of molecular orbits, in01020304050607080the reaction of coal oxidization and spontaneocombustion producing carbon dioxide and water,Fig 5 Sketch map of the corrected potential reactionoxygen molecules attack carbon atoms of the terminalenergy profile of the B3LYP energyof the propyl alcohol group on the lateral chain ofbenzene rings, which causes this propyl alcohol groupFig 6 is a profile map based on the B3LYP/6-311G toproduce the acid (-CH2-CH2-COOH) group andet after correcting for the zero point energy. The en- water The (-CH2-CH2-COOH)group continued torgy value indicated in the figure is total energy of decompose to produce CO2 and(-CH2-CH3) In thethe reactant C23H2 NOs treated as a relative value of end, spontaneous coal combustion produced CO2 andthe zero point. From this figure, we can see that, H20. The reaction processes can be denoted as fol-compared to the reactant, the total energy, -16. lowsk/mol of c2H2sNO3+CO2 is lower and given the R+O2→MIl→TS1→M2→TS2→MI3→P+H2Othermodynamic angle, the reaction occurred easilyTS3→MI4→TS4→M5→PI+CO2Moreover, the potential energy profile shows that the4) According to the potential reaction energy pro-spontaneous coal combustion, produc- file, the reaction of spontaneous coal combustioning water can form the unstable TS3 through the ac- producing CO2 and H20 is spontaneouscumulation of energy to break a small barrier (50.16kJ/mol). TS3 released 22.86 kJ/mol energy to form AcknowledgementsMI4, which needed to overcome a big barrier to formTS4: TS4 released much heat to form MI5, which isFinancial support for this work, provided by thean activated intermediate state, able to dissociate fur- National Natural Science Foundation of Chinather and obtain PI+CO. The entire system is an exo- (No. 50834002)and the National Eleventh Five-yearthermic reactionPlan Science and Technology Key Project (No2006BAK03 B05), is gratefully acknowledgedReference[I] Wang J R, Deng C B, Hong L. Dynamics study onfractal reaction of oxygen in granular coal. Journalof China Coalnese)(-16.80)[2] Lu W, Hu Q T. Theory of coal spontaneous combus-tion stepwise self-actived reaction. Journal of chinese University, 2007, 36(1): 111-115. (In Chinese)es JC. Towards an altermanping safety of activated carbons. Journal of Loss Preven-Fig. 6 Sketch map of the corrected potential reactiontion in the Process industries, 1998(11): 407-411nergy profile of the B3LYP energyHazards. Amsterdam Elsevier Science Publisher, 19844 Conclusions[5] Shinnjh. Study of coal molecular structure. Fuel, 1984,63(11):1187-1196Empirically and with calculations based on DFt [6] Nordon P, Young B C, Bainbridge N W. The rate of oxi-spontaneous combustion of coal producing water and [7] Garcia P. 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