Co-combustion of Sewage Sludge in the Coal-Fired Heating Plant

Abstract

Due to greening of production of electricity and heat some owners of heating plant are changing their fuel mix from fossil fuels to biomass one. The stabilized sewage sludge is in this biomass category as well. New Czech national legislation limits are stricter in parameters of biological activity, so the owners of water treatment plant are looking for new utilization of their stabilized sludge. There are some possibilities in combustion or co-combustion of this sludge in currently heating plant. The article describes the trial test of stabilized sludge combustion on heating plant technology. The heating plant is originally designed for combustion of brown coal. The stabilized sludge was produced at the municipal wastewater treatment plant, from where it was transported to low-temperature (78 °C) drying technology, from where it was transported in granular form to the heating plant in Mladá Boleslav. The fuel mixture of trial test was in energetic ratio of 23% dried sludge and 77% brown coal. The sludge was transported into the boilers by pneumatic conveying, which is used in normal operation for conveying pelletized phytomass. The result shows an increase in SO2 emissions from the original 189.91 mg∙m-3 to 407.98 mg∙m-3 and an increase in NOx emissions from 148.45 mg∙m-3 to 181.46 mg∙m-3. There were also detected increasing NH3 emissions due to reac-tion of control system in SNCR technology od decreasing of NOx emissions. The trial test was done during full op-eration of the heating plant. During the test there was also a lower steam production, from originally 38.17 kg∙s-1 to 34.44 kg∙s-1, which is related to the lower LHV compared to the reference fuel. There were no major operation problems, except higher dustiness during unloading of sludge. Finally, the Czech legislative problems associated with the combustion of stabilized sewage sludge are mentioned. In the current system, the sludge is established as waste, which forbids the combustion of it in conventional heating plants, even though all emission legislative limits are complied with. Nowadays the Ministry of the Environment of the Czech Republic is working on new regulation, which will determine new category of solid alternative fuels and it will be solution of this situation.