Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Chemical tests - determination of major and minor elements

For appraising quality of solid biofuels important chemical parameters are the content of major (i.e. (Al, Ca, Fe, K, Mg, Na, P, Si, Ti) and minor (i.e. As, Ba, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Sb, Tl, V, Zn) elements. Major elements are of key relevance referring to ash melting, deposit and slag formation as well as to corrosion. Minor elements are of special importance for particulate emissions as well as the environmental impact of produced ashes and their subsequent utilisation.

In term of sample preparation, particle sizes < 1mm or < 0.25mm should provide satisfactory homogeneity to be used for the analyses of wood and bark mixtures or straw. Regarding this, it is noted to pay attention during size reduction in order to avoid contamination from the inner materials of the used mills. Thus, materials of that mill parts having contact with the sample should be chosen depending on the elements to be determined. If, for instance, minor elements such as Cr and Ni have to be determined, it is not advised to use stainless steel materials, and recommended to use e.g. tungsten carbide or titanium instead. Generally, high-speed mills should not be used due to the higher abrasion rate.

In conclusion of the validation results, for solid biofuel analyses wet digestion with H2O2 / HNO3 / HF / H3BO3 are proved to be the most suitable for the determination of major elements. For minor elements wet digestion using H2O2 /HNO3 /HF are recommended.

Depending on the specific element to be determined, the application ranges revealed the suitability of tested methods for a wide concentration range, including potential concentrations in both natural and contaminated solid biofuels. According to detection limits, the most suitable determination methods are summarised for the different elements in solid biofuels. Thus, (i) atomic absorption spectrometry (AAS) such as FAAS ("flame"), GFAAS ("graphite furnace"), CVAAS ("cold vapour") and (ii) inductively coupled plasma spectrometry such as ICP-OES ("inductively coupled plasma optical emission spectrometry") or ICP-MS (¿inductively coupled plasma mass spectrometry") can be applied.

These determinations showed good agreement between the different results for many elements investigated. Other determination methods applied are hydride generation AAS ("atomic absorption spectrometry"), XRF ("X-ray fluorescence spectrometry") as well as direct Hg determination. Basically, applying XRF detection would be a suitable and fast alternative method for the determination of several major and minor elements. However, due to the requirement of reliable calibration standards, which are not available for solid biofuels so far, XRF systems for element determination presently not be recommended. Thus, the development of such standards is desirable.

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