Combination of hydrothermal carbonization and oxy-fuel combustion process for sewage sludge treatment: Combustion characteristics and kinetics analysis

•Sewage sludge was pretreated by hydrothermal carbonization before combustion.•The combustion behaviors under air and oxy-fuel atmosphere were investigated.•Hydrochar had lower activation energy than sewage sludge.•The fouling and slagging issues were mitigated by hydrothermal carbonization.•Hydrochars showed more stable combustion behaviors than sewage sludge.