N migration and transformation during the co-combustion of sewage sludge and coal slime

•N migration during SS-CS co-combustion were investigated by TG-MS combined with XPS.•N migration routes were obtained and related interactions were found.•Interaction increased the selectivity of N species to NH3 and Amino-N.•Interaction reduced NO by 36.9% when CS ratio was 50%.•Possible interaction mechanisms were proposed.