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• The torrefaction method using CO 2 as a medium was used to pretreat the sludge. • The functional groups of CO 2 torrefaction-pretreated sludge has changed. • The CO 2 torrefaction-pretreated sludge can be applied to coal gasification. Take CO 2 as a torrefaction medium is effective in improving fuel performance. This study discusses the possibility of industrial solid waste sludge (SL) pretreated by torrefaction under CO 2 for gasification with Yangchangwan coal (YC). The study was performed in a thermogravimetric analyzer and a high-frequency furnace. The results showed that the optimal torrefaction temperature of SL was 300°C under CO 2 , and its release intensity of gases higher than that of Ar. –OH and C–H were decreased relative to Ar. At 1100°C, the reactivity index R 0.9 of SL pretreated under CO 2 was 1.78 times higher than that of untreated chars. At 1200–1400°C, the R 0.9 of chars was lower than that of 1100°C. The activation energy of chars was showed an overall increasing trend with increasing carbon conversion levels. The torrefaction-pretreated SL under CO 2 can be better applied to high-temperature gasification of YC. [ABSTRACT FROM AUTHOR]