Aiming at investigating the influence of minerals in co-firing applications in existing and developing systems, as well as their environmental impact upon recycling to soils, we used a combination of techniques such as X-ray fluorescence spectroscopy, ultraviolet and visible spectroscopy, inductive coupled plasma spectroscopy, X-ray diffractometry, differential thermal analysis and fusibility analysis to characterize various biomass and coal ashes and their blends, with biomass proportions up to 20%. Slagging and fouling propensities were predicted.

The results showed that biomass ashes were richer in calcium, silicon and alkali minerals and micronutrients such as Zn, Cu and Mn, in comparison to coal ashes. Some could be useful for soil amendment or the cement industry. Slagging/fouling problems should be expected in boilers operating above 1000 C, especially those firing cotton residue, vine shoots and bituminous coal without pre-treatment. However, the environmental impact of either biomass or coal ashes upon their disposal is expected to be very low, as leaching tests have shown. For coal/biomass blends, the composition and the fusibility of the ashes varied between those of the individual components. Thus co-firing processes using the alternative fuels studied up to 20% would not entail significant limitations in the system operation or the management strategies of ashes.