Experimental Study On The Effect Of Ash Fusion Characteristics On Secondary Mass Loss Of Coal In High-temperature Pyrolysis And Char Burnout

As a county with coal dominating its energy structure,China has to improve the coal combustion efficiency,reduce pollutant emission and the content of unburned carbon in fly ash to achieve the goal from 13 th Five-Year Plan.At present,most research on coal high-temperature pyrolysis and char combustion are noly considering the effect of temperature,pressure,heating rate and atmosphere on the structure and reactivity of coal,which would then affect the coal high-temperature pyrolysis,char combustion and ash formation.Only few researchers have noticed the influence of ash components evolution on coal high-temperature pyrolysis and char combustion,not to mention considering the ash melt behaviour when temperature is much higher than ash flow temperature(AFT).a thermo-gravimetric isothermal method is employed in this paper to research the effect of ash fusion characteristics on secondary mass loss of coal in high-temperature pyrolysis and char burnout,the detailed description is presented below:First,five Chinese coals with varied AFTs were prepared and pyrolysis in a thermal gravimetric analyzer(TGA)at fixed temperatures below and above their AFTs.All char samples were then analyzed via X-Ray Diffraction(XRD)and associated with chemical thermodynamic calculation.There was a second weightlessness appeared in the TG curves of four coals with higher AFT but absent from lignite coal with lowest AFT.This second weightlessness phenomenon was caused by the carbothermal reduction and carbothermal reduction nitridation reaction between minerals and char,which was leading to the appearance of SiC,AlN and Si3N4.The reason why the second weightlessness was absent from lignite coal possibly was: the high content of Fe and Ca,which would formed lots of low-temperature eutectic crystal with SiO2 and Al2O3;and when the temperature is increasing,the coalification of lignite char low in initial coal would be growed much higher and turned to graphitization.This was leading to the hard fracture of aromatic fused ring in lignite char with high temperature.Meantime,the low-temperature eutectic crystal would fused and lots of fused amorphous polymer appeared and partially or even totally covered the char particles.This was leading to the harder fracture of aromatic fused ring in lignite char and the produce of SiC,AlN and Si3N4 with high temperature.Secondly,the Xinjing coal with secondary weightlessness and lignite coal were employed.CaO and Fe2O3 were then mixed into the Xinjing coal,Al2O3 and SiO2 were mixed into the lignite coal with different proportion.Then,the high tempearture pyrolysis characteristics of all the mixed coal samples were achieved in TGA.The TGA and XRD results showed that there was no effect of Fe2O3 on the second weightlessness of Xinjing coal.While the effect of Ca O on pyrolysis is inhibited and the second weightlessness phenomenon was disappeared when the mass fraction of CaO was up to 75%.The second weightlessness phenomenon with Hulunbeier coal was disappeared when Al2O3 and SiO2 were mixed into the lignite coal separately until the Al2O3 and SiO2 were mixed into the lignite together with specific proportion.Then,six Chinese coals with varied AFTs were prepared,pyrolysis and then burned in a TGA at fixed temperatures below and above their AFTs.All char and ash samples were then analyzed via scanning electron microscope(SEM)and energy dispersive spectroscopy(EDS).The results showed that the mass loss weights of 5 coals in Mode I(combustion temperature higher than AFT)were lower than that in Mode II(combustion temperature lower than AFT).The fused ash formed a shell around the char particle which decreased the char burnout ratio significantly when combustion temperature exceeded the AFT.Besides,the carbothermal reduction and carbothermal reduction nitridation reaction between minerals and char were also contributed to the char burnout.Finally,the combustion temperatures were divided into 25 levels from 500-1450°C,of which the interval was 50°C from 500-1200°C and 25°C from 1200-1450°C.The kinetic parameters of char combustion containing activation energy E and pre-exponential factor A were obtained from TGA data.A model predicted the char combustion rate was then established with the kinetic parameters of char combustion and validated with the expermental data.The results showed that the influence of coal AFT on the char combustion rate calculated by the predicted model was nonnegligible.All cases were devided into two groups with the combustion temperature below or exceed AFT.The relative errors were decreased signifcantly when considering the influence of coal AFT on the char combustion rate,whose max relative errors were less than 6% and mean related errors were less than 2%.It showed that the model was suitable for predicting the char isothermal combusiton in TGA and adaptable to the change of coal-kind.