Organic oxygen transformation during pyrolysis of Baiyinhua lignite

• Organic oxygen transformation during pyrolysis of Baiyinhua lignite was investigated.
• Organic oxygen transformation significantly depended on the pyrolysis temperature.
• The tar obtained below 500 °C is rich in organooxygen species, especially phenols.
• Above 400 °C, the organic oxygen species mainly released as pyrolysis water and gas.
• The oxygen yield in water mainly due to thermal cracking of volatile matter.

Fast pyrolysis of a lignite containing a high content of oxygen was investigated to understand the effects of pyrolysis temperature and gas resident time on the yields of pyrolysis products and the distributions of organic oxygen species (OOSs). OOSs in the lignite contain carboxyl, carbonyl, hydroxyl and ether by the results of XPS and FTIR analyses. The tar yield reached the maximum of 15.3% at 500 °C and a gas resident time of 2.5 s. CO2 was found to be the predominant oxygenous gas at 600 °C, and the significant release of CO2 at temperatures lower than 400 °C should be related to the high carboxyl content in the lignite. The CO yield increased significantly with the raising temperature and reached 46.4 mL/g (daf) at 700 °C. The O yield in water was lower than 20% during pyrolysis at 200 °C, and sharply increased to 30.6% at 700 °C because of thermal cracking of volatiles. Above 400 °C, the OOSs are mainly released as pyrolysis water and gas. The OOSs detected in the tar include phenols, ketones, ethers, alcohols, and dibenzofuran and its substitute, among which phenols (phenol, cresol, and xylenol) are the main components with yield up to 27.1% (based on tar) at 500 °C and gas residence time of 10 s. The possible oxygen transformation routes for BYHL pyrolysis are also discussed in this work. Such an approach may lead to the development of efficient lignite upgrading technology.