Experimental Investigation Of Thermal Effect In Coal Pyrolysis Process

Coal pyrolysis is the initial and concomitant reaction of thermochemical conversion processes such as coal combustion,gasification,and liquefaction.Therefore,the research of coal pyrolysis plays an extremely important role in the efficient and clean utilization of coal.The thermal effect of coal pyrolysis is an important basis for exploring the reaction mechanism,model establishment,and energy efficiency evaluation of coal pyrolysis process.In the existing research on the pyrolysis of coal,due to the difference in coal type,experimental methods and conditions,the pyrolysis thermal effect data is quite different,and even the pyrolysis process is exothermic or endothermic.This paper mainly used thermogravimetric-differential scanning calorimetry(TG-DSC)combined with gas phase product analysis(mass spectrometry)to systematically study the pyrolysis process and thermal effects of different coal types under different conditions,and establish the relationship between coal quality parameters and heat absorption.Firstly,the pyrolysis process of five coals based on TG-DSC-MS analysis was studied.According to the weight loss curve,differential weight loss curve,heat flow curve and heat curve of the coal sample,the pyrolysis process of the coal sample is divided into four stages:drying and dewatering stage S1(30-150?),which mainly precipitate free water and adsorbed water in the sample;degassing stage S2(150-350?),CO2,CO,N2 and other gases adsorbed in the coal sample pores are precipitated;pyrolysis stage S3(350-650v),the removal of unstable bridge bonds,functional groups and side chains in coal samples,generating primary gases such as CH4,H2,CO,H2,tar and a small amount of water vapor;polycondensation stage S4(650-1000?),semi-coke further polycondensation,a small amount of secondary gas(mainly H2).Among them,the stage S1,S2,S3 is the endothermic process,and the stage S4 is the exothermic process.Secondly,the influence of coal type,heating rate and particle size on the pyrolysis thermal effect was studied.The experimental results show that for the coal types used in this research(IN lignite,SL lignite,XW bituminous coal,JJ bituminous coal,AR anthracite),whether the endothermic process,the exothermic process,or the entire coal pyrolysis process,the heat absorption of JJ bituminous coal and XW bituminous coal is always the largest,followed by SL lignite and IN lignite,and AR anthracite is the smallest.For the same coal sample,the temperature at the transition point from the endothermic stage(S1+S2+S3)to the exothermic stage(S4)increases with the increase of the heating rate.The heat absorption in the endothermic stage remains basically unchanged,and the heat released in the exothermic stage decreases as the heating rate increases.When the heating rate is less than 10?/min-150C/min,the entire pyrolysis process of the coal sample is exothermic.When the heating rate is higher than 15?/min-20?/min,the pyrolysis process is endothermic.For the same coal type,when the particle size increases from 75 ?m to 175?m,there is no obvious change in the heat flow curve.The temperature deviation of the conversion point from endothermic to exothermic is less than 5?,for the weight loss of the coal sample,the temperature of peak rate increase about 30?.During the process of coal pyrolysis,the temperature corresponding to the peak of the weightlessness curve is different from the temperature corresponding to the peak of the heat flow curve.The coal pyrolysis reaction process characterized by the weightlessness curve and the heat flow curve is inconsistent.Finally,20 coal samples with different coalification degree were measured on TG-DSC to obtain the basic data of pyrolysis thermal effect,the gray correlation degree method was used to analyze the correlation degree of coal composition,cohesiveness index,vitrinite reflectance and pyrolysis activation energy with heat absorption of coal pyrolysis.The results show that the fixed carbon content and the pyrolysis activation energy have the greatest correlation with the heat absorption.The activation energy and the heat absor:ption is linear positive related function,and the fixed carbon content and the heat absorption is a piecewise linear positive correlation function,which can provide a basis for estimating the heat absorption of coal pyrolysis.