Emissions of NOx and N2O were measured during mono-combustion of dried sewage sludge and co-combustion with coal in a bench-scale circulating fluidized bed combustor (CFBC). The results were compared with previous results obtained using a bubbling fluidized bed combustor (BFBC). The increase in NOx with sludge ash accumulation in the combustor was less for the CFBC than the BFBC, partly because of the higher attrition rate of sludge ash in CFBC resulting from the higher gas velocity. The influence of sludge ash on the formation of NOx in CFBC was less than that in BFBC during sludge combustion. The effects of fuel type on NOx and N2O emissions were also evaluated. (C) 2007 Elsevier Ltd. All rights reserved.