Rifiuti da centrali termoelettriche - ARPAL

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PHARE TWINNING PROJECT RO2004/IB/EN-07 GUIDELINES ON INDUSTRIAL - THERMOELECTRIC PLANTS XX Combustion of biomass and other plant residues Properties of biomass and peat ashes The mineral content of peat ash is very similar to the constituents of sandy soil. Due to the limestone base on which the bogs were formed, peat ash typically contains 25 – 55 % CaO when used as a fuel. The combustion technique applied has an effect on the character of ash produced, although the quality of peat is the determining factor. Peat ash is formed from mineral matter in peat. The bottom ash from fluidised bed combustion also contains bed sand from the combustion chamber. Peat fly ash is a fine powder consisting mainly of particles of variable silica, alumina and iron oxide (65 – 75 %). Other major species are compounds of the alkali and earth alkali metals (10 – 55 %) and unburned peat particles (0 – 5 %). Due to the limestone base underlying the peat bogs, peat ash in Ireland typically contains 25 – 55 % CaO. The fly ash also contains trace elements, i.e. metals. Peat ash can be used as a raw material and an additive in the construction and building material industry and, e.g. as a road construction material. It may also be used in sewage treatment. The geotechnical properties and mineral constituent of ash have to be investigated on a case-by-case basis before commercial use. The co-combustion of wood and peat results in a slightly different composition of ash, which makes it more difficult to use it as construction material. In the stabilisation of mineral aggregate and in concrete, peat fly ash can be utilised to replace either the finest aggregate material or part of the cement. Ash from a peat-fired power plant can be transported, e.g. to bog areas. Disposal of ash on cutaway bogs is environmentally acceptable and also economically sensible. In some countries, ash must be disposed of to controlled landfills, but even then the quality of the ash must have first been tested (e.g. solubility testing) and later, environmental monitoring of the landfill site must be carried out. Environmental and health aspects are taken into account in site planning. As far as landfilling is concerned, peat ash and coal ash are quite similar. Some temporary nuisance, due to dust or noise caused by traffic and working machines, might occur. Landscaping of the site will a<strong>da</strong>pt cut-away bogs to the surrounding landscape. Peat fly ash may be used as fertiliser on the forest and fields. Peat ash contains some plant nutrients and lime necessary for plants. However, both environmental and health aspects and regulations have to be taken into account before there is any commercial use of peat ash as a fertiliser. The fly ash from straw combustion units is disposed of primarily due to its high content of cadmium. The bottom ash is normally taken back to the fields or used as a road construction material. 88
PHARE TWINNING PROJECT RO2004/IB/EN-07 GUIDELINES ON INDUSTRIAL - THERMOELECTRIC PLANTS XX Ash from combustion with limestone injection The ash from fluidised bed combustion with limestone injection contains the end-product of the desulphurisation reaction, unreacted calcium oxide and limestone (about 15 wt-%). The solubility of trace elements in fly ash is an important factor when assessing the potential environmental impacts. The alkali and earth alkali metals (e.g. sodium, potassium) and other mineral elements like boron. Combustion residues and by-products treatment (from liquid fuel-fired gas turbines) Oil treatment includes centrifugal separation units and modules, filters, combined cleaning units, and complete fuel conditioning systems. An oil recovery and sludge treatment system by flotation and precipitation is integrated in this treatment system. The oil or fuel recovered is burned in an auxiliary oilfired boiler. Final sludges are dewatered, dried, solidified and incinerated, or disposed of by authorised contractors. Water from sludge dewatering, which is contaminated with oil or fluids containing oil, is usually collected in a specific system and discharged separately. Sludge is also collected from the treatment of washing. 89
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Rifiuti da centrali termoelettriche - ARPAL

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