Analysis of the fly ash from the processing of wood chips in a pilot-scale downdraft gasifier: Comparison of inorganic constituents determined by PIXE and ICP-AES

Gasification of biomass ultimately generates at least one solid byproduct in which the inorganic constituents of the biomass are concentrated. Given the potential for utilization, or issues with disposal, facile methods are needed for determining the compositions of the fly ashes from recently-available gasifier-based bioenergy systems. Proton induced x-ray emission spectroscopy (PIXE) and inductively coupled plasma atomic emission spectroscopy (ICP-AES) were used to characterize the fly ash recovered from a pilot-scale (25 kW) modular bioenergy system operated with wood chips as the feedstock. The composition of the fly ash from the downdraft gasifier showed some similarities to compositions reported for boiler wood ashes, apart from one half of the material being unburned carbon. Although ICP-AES showed greater sensitivity for the analysis of the fly ash, especially for small amounts of heavy metal contaminants, PIXE proved to be a powerful analytical tool for screening of elements from sodium to uranium. Such broad spectrum screenings could prevent the inadvertent land application of unsuspected pollutant elements. Fly ashes from biomass gasification appear to be suitable for use as ash-based fertilizers for forest lands; however, combustion to remove unburned carbon may be advisable. -- Highlights: ► Fly ash composition data for commercial modular bioenergy systems are now reported. ► PIXE analysis better suited for analysis of fly ashes rich in silicates. ► Fly ashes generated by biomass gasification may be suitable for land application. ► Combustion of biomass-derived fly ashes is recommended before use as soil amendments