Hybrid microwave-ultrasound assisted catalyst-free depolymerization of Kraft lignin to bio-oil

•Highest bio-oil yield of 48.2% was obtained for hybrid MW + US process.•Simultaneous effect of MW and US reduced the repolymerization of intermediates.•HHVBoie of the bio-oils obtained from MW + US and US processes were quite comparable.•Demethoxylation and alkylation played significant roles during the depolymerization.•MW + US bio-oil was superior in both quality and quantity than MW and US bio-oil.