One sustainable use of sewage sludge (SS) is to use it as fuel in existing coal-fired plants. Towards this end, this study evaluated thermal characteristics and co-combustion efficiency of dried SS samples from six wastewater treatment plants with different sludge treatment units. Fuel quality of SS based on proximate and ultimate analyses and calorific value, and ash composition by XRF analysis were investigated. Then the SS samples were co-combusted in a laboratory batch reactor in mixtures with coal (3%, 5%, 10%, 20% and 30%). Results showed that samples had good calorific values (between 1931 and 3852 cal/g). Furthermore, the type of sludge stabilization processes had an important effect on thermal characteristics of samples and the point where the sludge addition started to intrude the combustion efficiency. Among all stabilization methods, lime stabilization was observed to affect the thermal characteristics the most. Sludge treated with anaerobic stabilization had lower calorific values than the ones stabilized aerobically. The results from co-combustion experiments showed that as the percentage of SS in the mixture increased from 5% to 30%, the combustion efficiency decreased gradually from 99.5% to 973%. Furthermore, according to XRF analysis result, fouling and slagging indices of samples were higher than the limit values. (C) 2018 Elsevier Ltd. All rights reserved.