Influence Mechanism Of CO₂ Atmosphere On Products Composition And Distribution During Lignite Pyrolysis

As an important factor in coal pyrolysis,atmosphere has an important impact on the composition and distribution of pyrolysis products.Partially replacing steam with CO₂ to reduce phenolic wastewater in pressurized fixed-bed dry-bottom gasification,the CO₂ cycle poly-generation system and CO₂/CH₄ catalytic reforming technology are all involved in the coal pyrolysis characteristics with CO₂ participation.Hence,to achieve resource utilization of CO₂ and provide a theoretical and technical basis for the coal pyrolysis process with CO₂participation,it is necessary to conduct in-depth research on the utilization and conversion of CO₂ in coal pyrolysis process.In this work,a kind of lignite has been chosen for the fixed-bed pyrolysis test,and comprehensive analysis of its pyrolysis products is investigated using Raman spectroscopy,Fourier transform infrared?FT-IR?spectroscopy,X-Ray Diffraction?XRD?,Raman laser gas analysis?RLGA?,gas chromatography and mass spectrometry?GC-MS?,elemental analysis,and NMR spectrometry to reveal how CO₂ atmosphere influences product composition and distribution during lignite pyrolysis.Meanwhile,based on the model compound pyrolysis experiment,the influence mechanism of CO₂ atmosphere on lignite pyrolysis is proposed.The main results of this study are as follows:?1?Compared with N₂ atmosphere,lignite pyrolysis under CO₂ atmosphere consumes more semi-coke and produces more pyrolysis gas and water.At 600oC,the yield of pyrolytic water increases significantly and the pyrolysis yields of CO/CH₄/C₂H₆/C₃H₈ at four pyrolysis temperatures under the CO₂ atmosphere are higher than those under the N₂ atmosphere.The yield of H₂ under the CO₂atmosphere below 600°C is higher than that under the N₂ atmosphere.However,when the temperature is higher than 600°C,the yield of H₂ begins to decrease.The tar yield is lower at 450oC,but higher when the temperature rises to 500oC.?2?Compared with N₂ atmosphere,pyrolysis of lignite under CO₂atmosphere at 450oC is beneficial to the formation of BTEX,PAHs,Phenols with smaller molecular weight and the increase of aromatic C content in tar.At 600oC,pyrolysis is beneficial to increase the content of aliphatic hydrocarbons and ethers and decrease the content of phenols in tar.At the same time,the content of total C and fat C in tar increases,while the content of aromatic C and O decreases.?3?At low temperature?450°C?,CO₂ atmosphere can promote the pyrolysis of tar more obviously.Especially,it can promote the pyrolysis of fat side chains and macrocyclic substances?greater than 3 rings?in volatile matter,resulting in higher aromatic carbon content in tar.?4?CO₂ enhances thermal cracking of volatile matter evolved from the thermal degradation of lignite and also reacts with volatile matter at 600°C.The CO₂ atmosphere promotes the cracking of volatiles and the combination of hydroxyl radicals and hydrogen radicals,thus forming more pyrolysis water.In addition,the CO₂ atmosphere benefits the cracking of the small ring structures in the volatile matter,breakage of the aliphatic side chain in the volatile matter and cracking of the aromatic ring itself in the volatile matter,thereby forming more aliphatic compounds.Part of the CO₂ also participates in reactions with volatiles and provides C to form tar.?5?The CO₂ atmosphere facilitates the formation of small molecular gases and organic compounds by H radicals.H radicals are more likely to be transferred to high molecular weight pyrolysis gases and liquid products at higher temperature.