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New combustion plant of the gas emission treatment

 

Combustion plant

Within the framework of a programme for the requalification of its gas emission treatment plants run since 2002 in collaboration with DIIAR (Department of Hydraulic Engineering, Environmental, various Infrastructures and Surveys) Environmental Section of the Polytechnic of Milan, LATI had found, at the E 6 emission point of its site at Gornate Olona (VA), the locality of Torba, a possible source of olfactory disturbance for the people living in the houses closest to the production site, as demonstrated by a specific study (see M. Gadda, S. Cernuschi, G. Lonati, 2003 - "Valutazione delle molestie olfattive associate alle emissioni del nuovo impianto di trattamento del flusso gassoso proveniente dalle vasche di decantazione delle acque di degasaggio dello stabilimento di Gornate Olona") at the time handed in to the Mayor and the competent A.R.P.A. agency.

In particular it was found that, together with special production regimes characterised by the working of thermoplastic material containing phosphorous, the emission coming from the stocking tanks and the re-circulation of the degassing waters (E 6), despite the passage through two sequential washing oxidation towers, could contain traces of phosphine, a fairly critical substance for the environment as it is characterised by a very low olfactory perception threshold (0,02 ppm) still lower than the value of the TLV (Threshold Limit Value = 0,3 ppm - TWA; 1 ppm -STEL) or that is to say the value over which sanitary evaluation is necessary.

Following to a somewhat extensive series of tests conducted in 2003 using a thermal conversion plant, the thermo-oxidation technology was found to be the most suitable system of treatment for the gas flow in question. Consequently, during the last year, LATI has adopted this technical solution installing a treatment unit composed of a wet-way dust pre-laving column (scrubber), followed by a regenerative-type thermo-combustion plant.

In particular, the gas to be purified which comes from production enters into the scrubber where they are chilled and saturated and where the water flow necessary to obtain the removal of the dust is re-circulated against the current; a part of the volume of water is discarded in order to maintain a constant level in the scrubber and avoid an excessive decline in the quality of the water which is constantly reintegrated by the column head.

The regenerative-type thermo-combustion plant has three chambers
Each regenerative chamber contains a bed with ceramic filling bodies, which work as heat accumulators, as they are heated or chilled according to the direction of the gas flow which crosses them. The air which must be purified reaches the first chamber and crosses the bed with the ceramic filling bodies, heated during the previous phase, vertically from the bottom to the top.

During this passage the polluted air is heated until it reaches the closest possible temperature to that of oxidation (about 850° C), consequently lowering the temperature of this ceramic bed gradually. If the above mentioned oxidation temperature isn't reached thanks to the auto-ignition of the organic substances present in the exhaust air, an auxiliary burner supplied by fuel (methane) and installed in the combustion chamber is used.

The medium duration of the gas at the combustion temperature is higher than one second. After having left the combustion chamber, the purified gas passes vertically, from the top to the bottom, into the second chamber transferring the heat and they are therefore directed to the emission stack. That way the second bed is heated and is ready for the next sequence, which is to heat the gas on arrival at the plant.

The special type of ceramic filler used allows the optimisation of both fuel consumption, thanks to the elevated specific surface, and energy consumption, thanks to the reduced pressure drop.

The solution with three chambers avoids the possibility of a certain volume of untreated gas being directed to the chimney during the flow inversion, thereby guaranteeing maximum efficiency of the system which can reach values even higher than 99 %.
Thanks to the third chamber it is also possible to degass the towers at every inversion.

Although regenerative-type thermo-combustion plants with two chambers, which are less costly and less technically complicated than the one adopted, and in the event of installation would in any case have respected the restraints imposed by the Region for the emission in question, are commercially available, LATI opted for a technology with three chambers for all of the above mentioned reasons, in line with its environmental policy which has enabled it to be one of the very first completely Italian companies to have obtained, in 1995, the environmental certification according to the UNI EN ISO14001 normative.

The information obtained from the first internal campaign of analysis conducted on the new plant (see the analysis report given to the Mayors in question and the competent A.R.P.A. on 20.12.2004) have confirmed that the objectives set have been completely satisfied, having evidenced characteristic values of the combustor outlet gas flow which are comparable with those of the atmosphere, but no doubt very soon the competent regional body will carry out their own control of the results obtained.

5 July 2005
Contact:
email: marketing@it.lati.com
LATI Industria Termoplastici S.p.A.
Via F. Baracca, 7 21040
Vedano Olona (VA) - Italy
Tel. : +390332409111 - Fax: +390332409307

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