Emissions of NOx and N2O were measured during mono-combustion of dried sewage sludge and co-combustion with coal in a bench-scale bubbling fluidized bed combustor. After starting the sludge feed, emissions of NOx increased with time, but N2O emissions changed only slightly. After a certain amount of sludge was burned, the fuel was switched from sludge to coal. Emissions of NOx from coal combustion after sludge combustion were higher than those before sludge combustion. These results suggest that the accumulation of sludge ash influenced NOx emissions. A simple model of ash accumulation and removal was proposed. The transient change in NOx after starting co-combustion was explained using the model presented here. (c) 2006 Elsevier Ltd. All rights reserved.