Utilisation of residues. At Rodenhuize PP the amount of wood pellet co-firing was increased from 25 to 50% (Advanced Green) and later 100% wood pellet was fuelled (Xxx Xxxxx). Xxxx pellet fuels of the type fired at Rodenhuize have a fairly wide range of chemical compositions, normally depending on the level of bark present. The wood pellets have much lower ash contents (0.6-2.3%) than most coals and, as a consequence, the co-firing of wood pellets normally has only a modest impact on the chemistry of the mixed ash produced. In most systems, the control of the UBC levels in the ashes can be controlled by the maintenance of acceptable combustion conditions. The industry has already agreed on a quality scheme for wood pellets which help to maintain adequate control over the ash levels in the wood fuels. These standards will normally permit the ash quality standards for the products of the co-firing of these fuels to be met under normal combustion conditions. At present, the standard for fly ash for concrete EN 450-1 (13) restricts the amount of co- firing to 20% by mass. However, due to the experience with co-firing over the past few years, the level of co-firing will be increased to 40% by mass, and 50% by mass in case of green wood. The results of the ash tests for wood pellets co-firing within DEBCO are consistent with this revision of the standard. 100% wood pellet combustion With 100% wood pellet combustion, the ash residues differ significantly from those originating from coal combustion and from co-firing. The ashes are rich in calcium- and magnesium oxide, alkali metals, phosphate and sulphur. The levels of silica, alumina and iron oxide are lower, as would be expected for wood ash. For utilisation of this material as a constituent of cement, it will be the task of the cement producer to decide about the ratio of this ash in his raw meal mix. The ash may be used as a lime or an alkali agent. and its use as a low grade fertiliser can be considered. These applications may be restricted by trace elements contents, and these need to be considered for a specific country or region and for the respective national and/or regional requirements.
Utilisation of residues. At Kardia PP, co-firing was tested up to 10% cardoon thermal share. The main fuel of the plant is lignite from nearby lignite mines. A number of different lignite types are fired and a relatively wide range for the ash chemical parameters have to be considered. The cardoon biomass used for the tests is characterised by an ash content ranging from 9.5 to 14.5% by mass. The impact of co-firing on the ash quality was modest and, in general terms, the co- firing of the biomass resulted in a more consistent ash chemical composition than for the normal lignite fuel diet. As the investigated ashes from lignite are not covered in the fly ash standard for concrete and as the chemical composition of these ashes is varying widely for a use as construction material, as e.g. filling material, the physical properties have to be considered more intensively as well as the trace element concentration, especially for the leaching limit values. If the co-firing is combined with lignite from a specific seam, also ash qualities for the use in cement and concrete may be obtained.
Utilisation of residues. The fly ashes of Fusina PP were characterised in the long term in order to evaluate the impact of RDF on their composition and quality. The RDF ash composition is dominated by silica, alumina and lime, largely associated with the mineral fillers in paper, and there are significant levels of potash and soda. Compared with pure bituminous coal combustion, the analyses of the ashes sampled during 5% RDF co-firing showed a slight increase in the concentrations of same trace elements (Cu, Fe, Zn), alkalis and chlorides. The trace elements need to be taken into account in light of the future requirements for the product standards as well as in light of the existing national application rules. The main conclusion of the fly ash characterisation is that RDF co-firing up to 5% thermal input has slightly increased the trace element contents but not affected the main oxide composition and, therefore, the use as concrete addition could be possible provided an ETA (European Technical Approval) for co-firing of RDF is granted.
