Interactions in NOX chemistry during fluidized bed co-combustion of residual biomass and sewage sludge

•NOx emissions from fluidized bed mono- and co-combustion of biomass studied at 850 °C.•Local and effluent concentration during combustion obtained.•Synergy effect observed during co-combustion with sewage sludge.•Synergy caused by interaction between volatile and ash chemistry.•Impact of ash on the N-chemistry reduced by air staging.