View
219
Download
0
Category
Preview:
Citation preview
7/28/2019 Fly Ash and Bottom Ash Treatment in a 2.45GHz microwave applicator
1/2
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
7/28/2019 Fly Ash and Bottom Ash Treatment in a 2.45GHz microwave applicator
2/2
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.
Recommended