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FLY AND BOTTOM ASH TREATMENT IN 2.45 GHZ

MULTIMODE CAVITY AT VARIABLE POWERS

C. Leonelli*, P.Veronesi, C.Siligardi, F.Carrea+, G.C. Pellacani

Faculty of Engineering, University of Modena and Reggio Emilia, Modena, Italy

* Dept di Ingegneria dei materiali e della produzione, University Federico II, Naples, Italy

+ Faculty of Chemical Engineering, University of Genoa, Italy

Many industrial activities, as well as the conversion of combustible into energy, involve the creation

and subsequent disposal of ashes, which represents a noticeable cost in terms of money and pollution.

Moreover, the ashes resulting from a partially completed process of combustion still posses a high

energetic content which could be in part recovered by post-combustion treatments.

At the moment, one of the main problems involving ashes is their rapid drying prior to the performing

of other treatments, as well as their volume reduction in order to achieve better transportability and

ease of disposal.

It is well know that some materials can be efficiently treated in presence of a microwave field, since it

can lead to rapid, selective and volumetric heating. The drying process, in particular, can be easily

carried out employing hybrid systems, which couple microwave heating with conventional moist

removal by conveying on the material hot air produced by flame burners. Even the volume reduction

or inhertization of hazardous materials found in microwave assisted vitrification an efficient and rapid

technique to obtain results similar to the more expensive and time costing ones subsequent to

traditional heating.

It is clear that the validity in terms of money, time and environement-friendlyness strictly depend on

the kind of material to be treated, in particular, the possibility of heating a material in a microwave

field is mainly connected to its dielectric loss factor (tan ) at a given frequency. Since the aim of this

study was to achieve some results useful for a future large scale transfer of the microwave technology

to the sector of the waste disposal, the frequency had to be chosen among the permitted ones for

industrial use. The 2.45 GHz has been chosen for the experimental tests, which involved ashes of

various provenience, wet and dried.

Ashes, as a matter of fact, could be regarded as a multiphase mixture whose properties vary in such a

wide range to make difficult a theoretical approach of the study of their interaction with microwave

field, without undertaking severe experimental investigations. However, performing laboratory tests

on ashes is not always an easy task, nor a meaningful one, if the available instrumentation is not

explicitly designed for the applications. This is particularly true when working with microwavedevices originally developed for different uses, which lack of adequate power control as well as a

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sufficient number of openings to allow measurement acquiring during the heat treatment. For this

reason the studies to be carried out required the design and development of a small microwave furnace

adequate to the sample investigation during the whole treatment time.

Temperature measurement required the adoption of a steel shielded K-thermocouple, accompanied by

an optical pyrometer to detect, respectively, the inner and surface temperature of the samples. Two

different power controlled furnaces were used to perform drying experiments: the on-off power

controlled CEM MAS 7000 incinerator and a continuously variable power Radatherm furnace. The

heating rate of the ashes was so fast, not to be influenced by the way the power was controlled either

time slicing or continuously varying, as shown in table1.

During the tests it has been possible to measure the power absorbed by the different typologies of

ashes, thus confirming their high absorption of microwave power even at room temperature

Since some of the inhertization or post-combustion treatments were meant to be held on hot ashes

directly coming out from combustion chamber, the microwave furnace was supplied with devices to

create an inhert atmosphere inside the resonant cavity during the heating of the samples, by

introducing nitrogen, which could be substituted by air or oxygen once the ashes reached the proper

temperature. This helped simulating the conditions existing immediately after the combustion process,

without degrading the volatile fraction contained in the samples.

The new furnaces allowed the treatment of ashes coming from a lignite combustion plant for electric

power production and of fly and bottom ashes collected on the filters and grids of an incinerator plant

for urban waste disposal. In the first case the treatment involved the drying and pre-heating of the

ashes to start their post-combustion, in the latter, the treatment was meant to lead to volume reduction

and inhertisation of the ashes by vitrification. The sample treated in the microwave furnace have been

characterised by chemical analysis, leaching tests, SEM-EDS, X-ray diffraction and compared to the

results obtained by conventional heating.

As for lignite ashes, their integrity was preserved during the microwave heating, while the longer

conventional heating modified the samples to an extent which were useless for post combustion tests.

The urban waste ashes have been vitrified using microwaves in a 4 time shorter time, and the

temperature detected by the thermocouple has been more than 200C lower than these of conventionalheating.