Thermogravimetric study of the effect of a PVA oxygeninsulating barrier on the spontaneous combustio

MININGScience directSCIENCE ANDTECHNOLOGYELSEVIERMining Science and Technology 20(2010)0882-0885Thermogravimetric study of the effect of a Pva oxygeninsulating barrier on the spontaneous combustion of coalMENG Xianliang.2, CHU Ruizhi, wU GuoguangXU Hongfeng, ZHU Jiamei, WANG Zhihua'School of Chemical Engineering and Technology China University of Mining Technology, Xuzhou 221008, chinaKey Laboratory of Coal Processing and Efficient Utilization, China University of Mining& Technology,Ministry of Education, Xuzhou 221008, ChinaAbstract: Coal samples in the air for three months were characterized by Thermogravimetric Analysis(TGA). The effect of a PVAoxygen-insulating barrier on the spontaneous combustion of coal was examined. The moisture loss activation energy, oxidationactivation energy and combustion activation energy were calculated by an integral method using the Coats-Redfen formula. Theresults show that the tendency for spontaneous combustion of three coal samples (judged by the activation energy) falls in the order:CYW>YJL>Sw. The oxidation activation energy and combustion activation energy of coal protected by the Pva oxygenbarrier increased. a significant increase in the combustion activation energy was noted, especially for the CYw coal wherease was 28.53 kJ/mol. Hence, oxidation of the protected coal samples was more difficult. The PVA oxygen-insulating barrierhelps to prevent spontaneous combustion of the coalKeywords: coal spontaneous combustion; oxygen-insulating barrier; activation energy; thermogravimetric analysis1 Introductionconcerning how the inhibitors affect the coal qualitIn this study, the flame retardant effect of a newSelf-heating and spontaneous combustion of coal is type of PVA oxygen-insulating barrier material prea natural phenomenon that creates serious problems pared in our laboratory was investigated by TGA. Thein the coal mining industry and in related coal trans- activation energy of coal samples covered and un-portation and storage activities. It has been reported covered by the PVA oxygen-insulating barrier werethat about 51.3% of the national mines in China were calculated with the coal oxidation kinetic model. Thein danger of spontaneous combustion and that 90% of effect of the material on coal spontaneous combustionthe fires were caused by spontaneous combustion of is evaluated by comparing the activation energy ofcoal, which seriously threaten people's lives and the csa samples/9.i11Furthermore, SO,, Hs, CO and COspontaneous combustion of coal 2 Coal oxidation kineticsseriously pollute the environment. Therefore, preventing and treating coal spontaneous combustionOxidative decomposition of coal is a typicalwith inhibitors plays an important role in the coal gas-solid reaction". The oxidation reaction rate ofcoal can be calculated by Eq (1)Currently used inhibitors are mainly inorganic saltdasolutions(CaCl2, MgClz, NaCl, etc. ) new organicAexp(-E/RT)f(a)materials such as non-ammonia silica acid gel, com-drposite gel, structural polymer swelling gel and other where a is the conversion rate of oxidative de-new inhibitors4-8I. However, the flame retardant ef- composition of coal; t, T and B the reaction time,fect of these inhibitors is limited due to their resis- temperature and heating rate respectively: A thetance to air leakage. There are also considerations pi中国煤化工a) a functionReceived 10 January 2010, accepted 20 AprilmodelCN MH Ghe coal oxidatCorresponding author. Tel: 8651683883682E-mailaddresskevin-meng@163.comreactiondoi:10.016s1674526409)603008Because B=dT/dr Eq (1)becomesMENG Xianliang et alThermogravimetric study of the effect of a PvAexp(E/RT)dT(2) The experimental details have been described in ourf(a) Bprevious repoMass loss of the coals was determined with anIntegrating thismnSTA409C-DTA/DSC-TG simultaneously-integratingthermal analyzer made by the German companyexp(-E/RT)dr (3)f(a)NETZSCH. The temperature was scanned from 25 to600C at a heating rate of 5 K/min. Gas flow rates off(a) on the left of Eq (3)is the integrated con-N2 and O2 were 40 and 10 mL/min to simulate thecomposition of air.version rate function; I exp(E/RT)dt on the 4 Results and discussionright of Bq. (3)is the integrated temperature. Eq- 3) 4.1 Thermocannot be integrated over time analytically, so nu-metric curvesmerical analysis is used to find an approximate soluFig. 1 shows the TG and DTG curves of the Swtion. The Coats-Redfen integral formula is commonly coal after standing covered for three months with theused to calculate an approximate solutionPVA oxygen-insulating barrier. The oxidation processis divided into three stages: moisture loss(To to Ti),AR( 2RTE(4) oxidative weight gain(, sto T2), and combustionweight loss(T2 to TendThe To to T1 zone iswhere moisture is lost as water evaporates from thewhere g(a) is the integral function of the TG sample. In the region from Ti to T2 weight is gainedas oxygen reacts with the coal. During this stagecurves, 8()=-In(-a). Plotting t versus weight gain from oxidation is greater than the physi-In gives a straight line with a slope-E cal desorption so the mass increases From T, to Tendcombustion weight loss occurs as combustion fol-lowed by loss of volatile combustion products causesfrom which E can be obtaineda rapid decline in the coal mass In Fig. I the differenttemperature regimes may be identified as: To. 24.2C,3 ExperimentalT1,133.0℃C;T2,3120°; and Tend,496.0°C.3.1 MethodsTvae:1330℃93.21%309072%Three experimental coal samples were crushed to aparticle size less than 1.25 mm. Each coal sample wasdivided into two coal piles for a total of six piles. Thebottom diameter of a pile was 200 mm and the height 2was about 100 mm. One of each pair of two coal pileswas covered with the PVA oxygen-insulating barrierhe remaining three coal piles were exposed to the airSamples of the coals were analyzed by TGA after100200300months to investigateenergies of oxidationI Moisture loss; 2. Oxidative weight gain;: 3. Combustion weight loss3.2 Properties of the coal samplesFig 1 TG curve, Sw coal, 25 to 600CThree kinds of coal were used. Table 1 shows their 4.2 Coal-oxidation activation energyproximate and ultimate analysis.Table 1 Quality analysis of the coalsThe value of a can be calculated from the TGCoal Proximate analysisUltimate analycurves in Fig. 1. Then In/8(a)Y697.9636763.382264881.030291l51sW9217.3833.1366.8780.134210.780.301456using an Excel spreadsheet. IfT and &(a)725(xcYw 8.99 6.69 38.45 61.55 81.69 5.02 1.10 0.41 11.75 then plotting x versus InF(r) gives the straight lines中国煤化工en separated int3.3 Preparation of the Pva oxygen-insulating the thloss oxidativeCNMH GEht loss. At thisThe PvVA oxygen-insulating barrier was composed point u awwvauOn tugavi the sw coal isknown, see Table 2.of PVA, a modification agent, surfactants and waterMining Science and TechnolVol 20 No 62.0-148Moisture lossOxidativeCombustion130158102y--27218x6952152|y=-27895x10283y=-14357x7.763000270.00290.0031000330001600017000180.00140001500016Fig 2 Graphs of x versus In F(x): Sw coal, three stagesTable 2 Activation energies, E, for the three stages oxygen-insulating barrier covered coals changes fromE(/mol Correlation coefficMoisture loss(25.0-133, 0) 22.6309862tivation energy increases from 20.71 to 22 13 kJ/mol,on average, an average increase of 1. 42 k/mol. The(1330-3120)combustion activation energy increases from 93.50 toCombustion weight loss110.65 kJ/mol, on average, which is significant. note3120-4960)1195109833especially that for CYW the increase is 28.53 k/molSince the PVA oxygen-insulating barrier increases4.3 Effect of the PVA oxygen- insulating barrier the oxidation activation energy it can help preventon activation energyspontaneous combustion of the coal. The moistureloss activation energy changes only slightly, but theAfter three months each of the six coal samples oxidation and combustion activation energy both in-(three coals, protected and un-protected by PVA oxy- crease significantly. The increase is particularly nota-gen-insulating barrier) was studied by the TG tech- ble for the combustion activation energy. Uncoverednique. The activation energy of each coal sample was coal samples suffer chemical adsorption of oxygencalculated as shown above and the results are tabu- after three months the active sites in the coal com-bine with oxygen, oxygen-containing functionalTable 3 Activation energy of three coal samples (kJ/mol) groups increase and heat is released and accumulatedEThis results in spontaneous combustion of the coalCoal△EHowever, samples covered with the PVA oxygen-Moisture lossinsulating barrier material cannot react with oxygenand the activation energy is higher, so the possibilityCombustion weight loss 71.3877.08630of spontaneous combustion decreasesMoisture los2263ative weight gain 23 19-2.005 ConclusionsCombustion weight loss 11951136.731)Moisture loss activation energy, oxidation acti-ration energy and combustion activation energy durCYw Oxidative weight gain 18.070.14ing the oxidation of coal were calculated by an inte-Combustion weight loss 89.62l18.1553 gral method using the Coats-Redfen formula. Thendency for spontaneous combustion of theTable 3 shows that moisture loss activation energy, coal samples can be estimated from the activationtion energy ofa on energy and combustion activa- energies, which fall in the order: CYW>YJLSWoxidation acthree unprotected coal samples in-2)The oxidation activation energy and combustioncrease in that order. This is consistent with the gen- activation energy of samples protected by the Pvaeral rules of reaction. The oxidation activation energy oxygen-insulating barrier both increase. A particularlyfor the three coal samples falls in the order: significant increase in the combustion activation en-CYW terThis helps to pre-YJL>SWvent sCNMH GoalThe moisture loss activation energy of the PyalThermogravimetric study of the effect of a PvaAcknowledgementspreventing fire. Zhongzhou Coal, 1996(1): 32-33. (InChinese)We gratefully acknowledge the financial support[8] Zhao Q, Xu J. 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