Presentation of the project Thermal processes with energy recovery for sludge and special or hazardous waste disposal

Main results obtained in the framework of the project Thermal processes with energy recovery for sludge and special waste (also hazardous) disposal granted for 2.16 M Euro by the Italian Ministry of Education through Structural Funds are reported. This project is subdivided into the following four main sub projects: combustion of hazardous sludges on a demonstrative plant; combustion of sludges in fluidized bed reactors on pilot and laboratory scale; set up of a flue gas sampling device at high temperature; fluid dynamic modelling of combustion. Original and interesting results were produced mainly connected with the following aspects: combined incineration of sewage and hazardous sludges appeared to be reliable considering that gaseous emissions did not show any deterioration following the addition of chlorinated hydrocarbons to sewage sludge. Gaseous emissions were found to be not conform with the standards only in very few cases generally not linked with critical conditions: polynuclear aromatic hydrocarbons (PAHs) were found to be a reliable parameter for emissions monitoring considering that they are produced in abundance following up set conditions due to a temperature decline or an insufficient oxygen supply; the very stringent limit at the emissions of 0.1 ng/m3 for dioxins and furans (TE) can be respected in the most of the cases but this monitoring is very complex, time consuming and expensive. Sampling and extraction errors might considerably impact the measured value, which is much more dependent on the conditions inside the recovery boiler rather than on the combustion efficiency; metals are enriched onto the fly ashes produced in the tests carried out by rotating drum furnace more than in the tests by fluidized bed furnace; after burning chamber mode of operation did not display a direct influence on the emissions; incineration tests carried out on reduced pilot scale have shown a total absence of fragmentation during volatilisation while particulate generation takes place only during combustion. Dried sludge is definitely a very good fuel comparing to coal. Fluid dynamic modelling of the demonstrative fluidized bed furnace has conditions. A proper design of the furnace in relation to the diameter, the position of inlet of primary and secondary air and the sand size can be developed accordingly. For a proper design of a incineration process from the energy point of view, a thermal drying section has to be included. To this purpose the sensible heat of the exhaust gases can be recovered in a boiler downstream the furnace with production of medium pressure steam to be used for drying. The optimal dryness of the sludge before feeding the furnace is the minimum value of solid concentration allowing an autogenous combustion, while the overcome of such value is detrimental both for capital and operating costs