Rapid Chemical Analysis of Renewable Solid Biofuels by X-ray Fluorescence

Abstract

The chemical composition of solid biofuels is an important quality factor in the operation of biomass-fired heat and power plants (CHP), impacting economic efficiency, maintenance costs, and pollutant emissions (e.g., CO, NOx, and particulate matter). Furthermore, the chemical composition during waste wood processing is also vital due to the legal limits of various elements (regulated in Germany by the Waste Wood Ordinance). While fuel quality parameters are increasingly included in supply contracts, they have traditionally focused on physical aspects due to the high cost and time requirement for chemical analysis. Rapid analysers are expected to facilitate on-site quality assessment, measuring minor and trace elements upon fuel delivery to resolve this. This thesis investigates the suitability of energy-dispersive X-ray fluorescence (ED-XRF) for detecting minor and trace elements in solid biofuels, identifying potential measurement errors, and the further potential of rapid analysis (not only for quality assurance). The results confirm that ED-XRF is usable for rapidly determining elements such as Mg, Al, Si, P, K, Ca, Cr, Mn, Fe, Cu, Zn, and Pb. This was shown by the comparison with the reference approach using inductively coupled plasma optical emission spectroscopy (ICP-OES), which provided comparable analysis results. However, elements like S, Cl, Ti, and Ni require refined calibration, while Na, As, and Cd remain challenging to measure. Furthermore, it has been shown that a certain amount of sample preparation is necessary, whereby grinding to ≤ 250 µm optimises homogeneity, reduces the grain-size-effect, and drying to < 10 % wt. minimises water-based measurement errors. As a result, the measurement procedure is extended because the samples should be slightly prepared and cannot be measured directly. However, the measurement time can be reduced to 60 seconds without significant accuracy loss, resulting in faster on-site analysis than external laboratory tests. The empirical calibration of the ED-XRF analyser improved measurement accuracy for most elements (e.g., Na, P, Ca, Fe, Ni, Cu, Cd, Pb), but slightly reduced accuracy for others (Mg, S, Cl, K, Zn). While some limitations remain, ED-XRF presents a viable, efficient solution for rapid biofuel quality assessment.