Rifiuti da centrali termoelettriche - ARPAL

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 adapt 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.
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