Co-incineration of sludge in the existing coal-fired utility boilers is a promising sludge disposal approach, but study on the combustion characteristics and the economic impacts of co-firing sludge in full-scale utility boilers is in absence. In this study, a series of full-scale field experiments and numerical experiments were conducted to study the coal-sludge co-combustion characteristic in a 100 MW coal-fired utility boiler. The effects of sludge blending ratio and moisture content in sludge are concerned. Subsequently, the economic impacts of co-firing sludge is presented. The results demonstrate that the combustion characteristics is acceptable when co-firing 10% sludge. The ignition property and the flame stability are significantly affected when the mass content of co-fired sludge is over 10%. Decreasing the moisture content in sludge is beneficial to the combustion characteristics but the improvements are limited. Co-firing sludge is profitable for power plants as the savings from coal and sludge disposal allowance is quite substantial. Considering both the combustion characteristics and the economic impacts, co-firing 10% sludge with moisture content ranging from 40% to 56% is optimal. The annual profit from co-firing sludge can reach 4.2 million to 5.5 million Chinese yuan (CNY), which is remarkable for the power plant. (C) 2016 Elsevier Ltd. All rights reserved.