The flame emission spectra were measured from the combustion of biomass in a boiler, from which the ratio of the alkali metals’ number densities (sodium and potassium) were precursory derived. Fly and bottom ash samples were collected to determine their composition via scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDX). A comparison of the alkali metals’ number density ratios (from the flame emission spectra) and the ash composition provided information on potential pathways for the alkali metals. The comparison also showed that the number density ratio can be taken as a tentative in-situ indicator of the alkali metals’ ratio. The ash composition also allowed the determination of several slagging and fouling indices.