Drying and depolymerization technologies of Zhaotong lignite: A review

Zhaotong lignite (ZL), a soft lignite from southwest of China, is an abundant coal resource. However, it has not been utilized on large scale due to its very high moisture content and low calorific value. Drying and depolymerization technologies of ZL for upgrading or producing valuable gases, value-added chemicals, and/or liquid fuels have been widely investigated in laboratories. This paper introduces geological setting and chemical structures of ZL. Drying and depolymerization technologies (including thermal dissolution, pyrolysis, and supercritical water gasification) of ZL are reviewed in detail. Different drying methods have different merits and demerits. Thermal dissolution (especially ethanolysis) of ZL under mild conditions can be used to produce oxygen-containing compounds. Pyrolysis of ZL follows the second-order kinetics and optimal temperatures for preparing tar and valuable gases are distinct. High yields of H2 and CH4 are generated from supercritical water gasification of ZL. In addition, future investigations on efficient utilization of ZL are advised. Although the topic of this review mainly focused on a specific lignite, the drying and depolymerization technologies are also applied to other low-rank coals, especially lignites. •Drying and depolymerization technologies of Zhaotong lignite (ZL) are reviewed.•Advantages and disadvantages of different drying approaches are presented.•High yield of ESP rich in phenols and esters can be obtained from thermal dissolution of ZL.•Optimal temperatures for producing tar and gases from ZL pyrolysis are 600 and 700–800 °C, respectively.•SCWG of ZL without extra dry pretreatment can be used to obtain H2 and CH4.