International Journal of Engineering Science
Heat treatment of chlorinated waste in a rotating kiln: Problems with intermediate reaction products and solutions applied
Introduction
High-temperature processing of organic solid waste is often used to decrease the waste volume and neutralize its inorganic content by oxidation. Among the various processes available, the CEA selected incineration in a rotating kiln as the most suitable to meet the requirements arising from contamination of the material by radioelements, all of which must be recovered from the ashes with very low-carbon content. The IRIS two-step process [1] fully meets these requirements. It comprises an oxidizing pyrolysis step during which chlorine is released at moderate temperature to minimize corrosion problems, and a calcining step to eliminate virtually all the carbon. However, although satisfactory results have been obtained for processing waste of a given composition, the behavior of the kiln may diverge when the composition changes. The malfunctions occur mainly in the first pyrolysis step where very high temperatures are recorded and plugging results from deposits on the kiln walls.
The work described in this article was undertaken to demonstrate that the deposits are involved in overheating of the pyrolysis kiln, and to account for their presence by the formation of intermediate PVC decomposition compounds. Revising the furnace operating parameters by specifying a waste feed sequence significantly improved the operation of the unit.
Section snippets
The IRIS process
The IRIS process is intended for heat treatment of chlorinated organic waste contaminated by radioelements. It uses two rotating kilns: the first ensures oxidizing pyrolysis at 550 °C to release the chlorine at moderate temperature in order to limit corrosion phenomena, and is followed by calcining at 900°C to produce low-carbon ash (<1 wt%) that can subsequently be processed to recover selected radioelements or vitrified for ultimate disposal. The off-gas arising from both steps is ducted to an
Investigation of pyrolytic decomposition of PVC and cellulose
Basic studies were carried out to identify the decomposition mechanisms of some of the compounds present in the waste feed stream. They involved thermogravimetric analysis, and the results were interpreted using existing degradation models.
Implementation of raw material feed sequencing in the furnace
Fig. 5 shows that the intermediate compound degrades between 600 and 700 °C for a heating rate of 20 °C min−1. This temperature exceeds that of the pyrolysis kiln in the IRIS process, but could be maintained for a residence time of about 5 min. It was therefore considered advantageous to modify the furnace operation by including periodic 6 min pauses to ensure degradation of the intermediate compounds.
The following sequence was initially proposed:
Feed: 20 min.
Pause: 6 min.
Conclusion
Pyrolytic processing of chlorinated organic waste in a rotating kiln revealed that some waste mixtures containing a large proportion of cellulose result in major malfunctions due to deposits in the furnace. A thermogravimetric analysis of PVC degradation supported by modelling with a known degradation mechanism highlighted the appearance of intermediate reaction compounds with an appreciable lifetime. By combining with other compounds such as carbonaceous residues from pyrolytic degradation of
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Disposal of chlorine-containing wastes
2016, Chemistry Beyond Chlorine