Selective deoxygenation of lignite with microwave torrefaction and its action mechanisms on pyrolysis process

Thermal torrefaction is an important tool for lignite deoxygenation, but traditional thermal torrefaction either causes an obvious loss of aliphatic structures. This work proposed a novel approach to selectively remove oxygenated functional groups of lignite by microwave torrefaction. The fuel properties, changes of chemical structure, and the pyrolysis processes of Zhaotong lignite (ZTL) during torrefaction were investigated coupled with quantitative analysis and pyrolysis experiments for revealing the role of microwave torrefaction. It was found that the deoxygenation of microwave torrefaction was superior to traditional furnace torrefaction, especially for C=O and COO. Microwave torrefaction was not expected to destroy the alkyl side chains, but had the positive role of activating and relaxing the compact structures of ZTL. The CH₂/CH₃ ratio for ZTL increased from 3.390 to 4.167 during microwave torrefaction so that the torrefied lignite was more prone to decompose. Besides, all oxygen containing gases in the pyrolysis of ZTL before 400 °C were reduced after torrefaction, while hydrocarbon gases increased. The increased activated energy of ZTL during torrefaction confirmed that the strong bonds in ZTL were not influenced by torrefaction. Phenolic compounds and hydrocarbon moieties in the volatile of ZTL have increased significantly after microwave torrefaction, while the groups of ethers/alcohols and ketones/aldehydes were all decreased in this process. These results are expected to give better understand about pyrolysis mechanisms of lignite pretreated by microwave torrefaction.