Structural Characteristics Of Soluble Species And Their Evolution Profile During Pyrolysis Of Baiyinhua Lignite

Lignites is not suitable for direct combustion and gasification because of its high moistre content. However, high volatile matter and oxygen contents of lignite are benefit for its pyrolysis and thermal dissolution under mild conditon. Meanwhile, pyrolysis is the first step of coal gasificaiton, liquefication, combustion and coking processes. Therefore, the study on the evolution profile of organic species in lignite during pyrolysis is important to scientifically regulate these coal utiliaztion processes.In this work, Baiyinhua lignite(BYH) was firstly sequentially extracted using petroleum ether, cyclohexane, carbon disulfide, acetone, methanol and isometric acetone/carbon disulfide mixed solvent affording extracts and extration residue(RE). Then, RE was sequentially thermal dissolved using methanol, toluene and isomeric methanol/toluene mixed solvent(IMT) at 350 oC affording soluble portions(SPs) and thermal dissolution(RTD). Meanwhile, Baiyinhua lignite was pyrolyzed under the temperature range from 300 to 700 oC in a fix-bed reactor, acquiring tars, chars and gaseous products.Subsequently, the chars were thermal dissolved under the same condition with RE. Combined with the characterization of gas chromatograpy/mass spectrometer(GC/MS), direct analyze in real time-mass spectrometer(DART-MS), Fourier transform infrared(FTIR) spectrometer and thermogravimetric analyzer(TGA), the composition and structural characteristics of extracts, SPs, tar, chars, RE and RTD were chareterzied.The extraction yield of BYH in methanol is 7.02%, while the total franctional extraction yield of BYH is only 9.37%, indicating that methanol possesses a wonderful solubility to the free organic species in the macromolecular network structure of BYH. GC/MS anslysis shows that the realtive content(RC) of alkanes in petroleum ether extract is 65.76%, while the dominant compounds in methanol extract are arenes, especially for the derivants of phenanthrene. DART-MS analysis suggest that the average molecular weight of these extracts range from 271 to 464 Da, and their double bond equivalent(DBE) value range from 5 to 20.The SPs yields of RE in methanol, toluene and IMT are 9.29, 6.62 and 5.93%, respectively, and the total SPs yield is 21.84% during sequential thermal dissolution. The GC/MS dectectable compounds in SPs are mainly composed of aliphatic hydrocarbons and arenes, and the total RC of aliphatic hydrocarbons and arenes in the SPs dissolved in methanol, toluene, and ITM are 94.76, 94.58 and 85.20%, respectively. DART-MS analysis shows that the average molecular weight of these three SPs range from 433 to 501 Da。Pyrolysis reactivity and kinetics of BYH was investigatd by a TGA. The result shows that the average reaction order of BYH non-isothermal pyrolysis is 1.52, and the average activation energy values are 402.65 and 413.45 k J/mol calculated by Flynn-Wall-Ozawa and Kissinger-Akahira-Sunose methods, respectively. The results from the pyrolysis experiments in the fixed-bed reactor indicate that pyrolysis conversion and gas yield increase with pyrolysis temperature increasing, while the tar yield increase with pyrolyisis temperature increasing below 600 oC, then decrease with pyrolysis temperature furthur increasing. GC/MS analysis shows that the RC of phenols in tars increase with the pyrolysis temperature increasing, and the phenols in tars are dominated by phenol and methyl pheonls. The RC of alkanas and alkanes in tars firstly increases, then decreases with pyrolysis temperature increasing, and alkanes continuously distribute from C11 to C28.The organic species in BYH release rapidly, and the SPs yileds of chars obviously decrease with pyrolysis temperature increasing at the temperature between 300 and 500 oC. Contrary to the tar, the phenols in the SPs of chars obtained high pyrolysis temperature is very low, indicating that much phenols release from BYH macromolecular network and much oxygen-containing functional groups decompose during pyrolysis between 300 and 500 oC. The compounds in the SPs of chars thermal disoslved in both toluene and ITM are mainly composed of alphatic hydrocarbons and arenes.