Koukouzas, and 4 co-authors, reported in 2007 that the chemical and mineralogical composition of fly ash samples collected from different parts of a laboratory and a pilot scale CFB facility has been investigated. The fabric filter and the second cyclone of the two facilities were chosen as sampling points. The fuels used were Greek lignite (from the Florina basin), Polish coal and wood chips. Characterization of the fly ash samples was conducted by means of X-ray fluorescence (XRF), inductive coupled plasma-optical emission spectrometry (ICP-OES), thermogravimetric analysis (TGA), particle size distribution (PSD) and X-ray diffraction (XRD). According to the chemical analyses the produced fly ashes are rich in CaO. Moreover, SiO2 is the dominant oxide in fly ash with Al2O3 and Fe2O3 found in considerable quantities. Results obtained by XRD showed that the major mineral phase of fly ash is quartz, while other mineral phases that are occurred are maghemite, hematite, periclase, rutile, gehlenite and anhydrite. The ICP-OES analysis showed rather low levels of trace elements, especially for As and Cr, in many of the ashes included in this study compared to coal ash from fluidized bed combustion in general.
The authors concluded that compared to fly ash from combustion of wood chips, the co-combustion ashes studied had lower contents of calcium, potassium and magnesium. The blend of coal and wood chips adds aluminium and iron to the ash and the trace element levels were generally in the same ranges as for wood fly ash. The trace elements which may cause some concern are As and Cr in the case of sample F1 (lignite) and F2 (lignite/wood chips), which in addition are of small particle size. As aforementioned, many of the trace elements in fly ash show a definite concentration trend with decreasing particle size. Fly ash deriving from the combustion of pure lignite and pure wood chips has a low LOI level. If the SO3 content is taken additionally into account (SO3 < 5%), then the most attractive fly ash to the cement industry is C1 (lignite), collected at the second cyclone.
Moreover, the particle size distribution of the examined fly ashes is related to the fuels burned. The higher the percentage of biomass in the blends, the smaller the ash particle size. The mineralogical composition of the fly ashes depends on the components of the blends and the combustion temperature. The presence of minerals such as anhydrite and portlandite is more possible when the used fuels contain calcium; therefore fly ash derived from wood chips contains the aforementioned minerals. Also, the occurrence of minerals such as sylvite is noticed when the fly ash derives from the combustion of biomass, due to the higher concentration of alkalis in this fuel.
The chemical characteristics of the blend of biomass coals depend on the system of combustion and the fly ash capturing mechanism. That is mainly because the combustion furnace type and combustion process are closely related to the degree of coal combustion, combustion temperature and combustion efficiency.
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