Rifiuti da centrali termoelettriche - ARPAL

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PHARE TWINNING PROJECT RO2004/IB/EN-07 GUIDELINES ON INDUSTRIAL - THERMOELECTRIC PLANTS XX Tabella 9.4: quantità dei residui da trattamento di scarichi di acqua [58, Eurelectric 2001] Waste water from flue-gas cleaning systems (Acque di lavaggio) All wet-type flue-gas cleaning systems produce waste water that, due to the fuel and materials used, contain heavy metals among other components. One of the main sources of waste water in this context is the wet limestone scrubber that is used in a large number of power plants for desulphurisation of the fluegas, although this can be reduced by using fuels with a lower chlorine content. This results in a reduction in the purge to the waste water treatment plant, which in turn reduces emissions to water. A conventional waste water treatment system should be noted that there are many different types of systems, depending on the different national regulations and site-specific factors. The pH value of FGD waste water must be increased in order to precipitate heavy metals. This is generally achieved using either lime milk or caustic soda, causing the formation of metal hydroxides. By adding flocculents (iron(III)chloride), flakes are formed. The addition of coagulation aids (polyelectrolyte), allows the agglomeration of individual flakes, so that a greater flake formation ensues. The sludge is then presedimented, drained and disposed of or co-combusted in case of slag tap combustion. Part of the ‘thin’ sludge is recycled to the flocculation stage where the sludge particles serve as initial crystallisation nuclei promoting more rapid flocculation. The treated waste water from the pre-sedimentation stage can be supplied to a baffle plate thickener for further sedimentation. The suspended micro-particles deposit on the inclined baffle plates. The sludge falling off the plates is gathered at the lower point of the baffle plate thickener and can also be recycled. The cleaned waste water is fed to the drain via the overflow of the baffle plate thickener, provided the regulatory limit values are met. In addition, the pH value is not permitted to exceed the range of 6 to 9.5, otherwise the water must be neutralised. If ammonia is present in the waste water it is first fed to an ammonia stripping plant before it is discharged to the drains. In some processes, e.g. with higher input of Hg because of cocombustion of wastes, it is customary to add also organic sulphide after addition of lime milk, precipitating heavy metals as sulphides, which is more effective than using the hydroxide. The 96
PHARE TWINNING PROJECT RO2004/IB/EN-07 GUIDELINES ON INDUSTRIAL - THERMOELECTRIC PLANTS XX disadvantages, that the heavy metal sulphides (greater quantities) need to be disposed of, because by cocombustion of these residues, the sulphur is released as sulphur dioxide and Hg is released again. Various plants treat FGD waste waters differently. Thus some companies use, e.g. flocculents and flocculation aids, others use only flocculation auxiliaries and organic sulphide. There are, however, also operators who use flocculents, flocculation auxiliaries and organic sulphide. Sanitary waste water This includes special waste waters originating form toilets and canteens. Current emission levels are usually estimated at approximately 75 l/person/day. Emissions are characterised by high organic content. It is noted that waste oils (lubricating or working oils) are considered as solid wastes and they are usually gathered separately and disposed off-site by an authorised contractor for further management (e.g. incineration, regeneration, etc.). Emerging techniques for the co-combustion of waste and recovered fuels It is expected that in the future, due to the extra experience being gained with pretreatment and abatement techniques, that the degree of co-combustion will be increased above the level of 10 % on a thermal basis. 97
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Rifiuti da centrali termoelettriche - ARPAL

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