Co-combustion characteristics of sludge and coal in fluidized beds

Co-combustion of sludge with coal can effectively improve the ignition and combustion characteristics and promote the harmless and resourceful utilization of sludge. Inorganic or organic chlorine can be introduced into municipal sewage during the disinfection and dehydration. The increased chlorine content in municipal sludge may adversely affect boiler heat exchange surfaces and environmental protection equipment. The influence of operating parameters on ignition, Cl element release, particle size, and element distribution of fly ash was investigated using a thermogravimetric-Fourier Transform Infrared Analyzer and a laboratory-scale bubbling fluidized bed. The results show that the volatile content in the sludge is high, and there are three weight loss peaks in its derivative thermogravimetric (DTG) curves, and the maximum weight loss peak appears in the volatile release and combustion stage. As the sludge blending ratio increases, the ignition and burnout temperatures of the fuel gradually decrease. The emission concentrations of CO, NO and SO₂ increase as the sludge blending ratio increases. The higher volatile content in the sludge promotes the production of CO, while weakening the reducing atmosphere in the dense phase, which is not conducive to the reduction of NO. During the combustion process, both thermo-gravimetric and fluidized bed experiments show that more than 85% of the chlorine elements in the fuel are released in form of HCl, and the self-trapping effect of ash on HCl is weak, and most of the Cl elements in the fuel eventually are emitted in the form of gaseous HCl. The average particle size of fly ash gradually decreases as the sludge blending ratio increases, indicating that sludge ash breaks into finer particles at high temperatures. As the sludge blending ratio increases, the contents of K, P, and Fe elements in fly ash increase significantly, with P showing the most notable increase.