Study On Structural Change And Pyrolysis Characteristics Of Coal Pretreated At Different Temperatures

In this paper,Indonesian lignite?IL?,Shenfu coal?SF?,Huainan gas coal?HN?,shenjialin fat coal?SJL?,Yangquanqu coking coal?YQQ?,and Qinglongshan lean coal?QLS?were used as the coal samples.The samples were pre-treated by cryogenic treatment,which were then extracted with tetrahydrofuran?THF?.All samples were characterized by Fouriertransform infrared spectroscopy?FT-IR?,X-ray photoelectron spectroscopy?XPS?,pore size distribution,surface morphology,and TG-DTG.On the other hand,the two low-rank coal?IL and SF?were heat-treated at different temperatures?350?,400?,450?,and 500??,and the samples were analyzed by basic properties,scanning electron microscope?SEM?,BET,X-ray diffraction?XRD?,Raman,XPS,TG-DTG and electron paramagnetic resonance?EPR?.The effects of different temperatures on surface morphology,pore structure,crystal structure,functional group,pyrolysis kinetics,and free radical concentration of coal were studied.The results showed that:?1?The yields of tetrahydrofuran extraction of IL and SF increased after cryogenic treatment.The yields of extraction of IL increased from 2.34%to 2.46%,and the yields of extraction of SF increased from 6.16%to 7.60%.Pore volume,pore diameter,and surface area of IL and SF increased after cryogenic treatment.In detail,the surface areas for IL and SF increased by 24.36%and 6.50%,respectively.?2?According to XPS results,carbon element on the surface of IL and SF mainly existed in the forms of C-C and C-H.In the range 350-500?,with increasing heat treatment temperature,the free radical concentration of IL increased from 1.02×10¹⁹spins/g to 1.51×10¹99 spins/g,and the free radical concentration of SF increased from2.21×10¹99 spins/g to 4.82×10¹99 spins/g.The trend of the disorder degree in the carbon structure of IL and SF was enhanced with the increase in temperature.?3?The TG results showed that the total weight loss rate(?w_max)of coal pyrolysis all presented a decreased trend with increasing coal rank.The temperature of maximum weight loss rate(T_DTGmax)showed an enhanced trend with increasing coal rank.The?w_maxax values for IL and QLS were 72.70%and 39.06%,respectively.The T_DTGmaxTGmax values for IL and QLS were 419?and 492?,respectively.The total package n-stage reaction model was used to investigate the pyrolysis kinetics of coal samples,and the kinetic parameters obtained at different reaction stages were employed to determine the reaction series.In the main pyrolysis stage?300-600??interval,the correlation coefficient of the curve fitting for IL and SF were higher than0.96 when reaction order of 2 was used.When reaction order of 1 was used,the correlation coefficient of the curve fitting for HN,YQQ,SJL,and QLS are higher than 0.95.Relationship between C_dafaf and the activation energy in the main pyrolysis temperature range?300-600??for the six different ranks of coal samples was analyzed.The results showed that activation energy increased first and then decreased with the increase in C_daf.The activation energy reached the maximum at C_dafaf of 78%.?4?TG-DTG curve was used to analyze the change in pyrolysis performance of the coal samples,and pyrolysis kinetic parameters were obtained.The pyrolysis process of the heat-treated IL and SF coal samples was mainly divided into three stages:400-500?,500-600?,and 600-800?.With the increase in treatment temperature,the activation energy of the pyrolysis for IL and SF in the temperature range400-500?gradually reduced.