Combining torrefaction pretreatment and co-pyrolysis to upgrade biochar derived from bio-oil distillation residue and walnut shell

•Torrefaction combined with co-pyrolysis could effectively increase biochar yield.•The positive synergistic effects on solid yield were found both in torrefaction and co-pyrolysis.•There were opposite synergistic effects on H/Ceff of torrefied feedstocks and biochar.•Torrefaction didn't change...

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