Chemical forms of ash-forming elements in woody biomass fuels

Abstract

Advanced fuel characterization helps to predict ash fouling and slagging. Chemical fractionation analysis, i.e. sequential leaching in H 2O, NH 4Ac( aq), and HCl( aq), was applied to the biomass of spruce, pine, birch, and aspen. All of the Cl in the samples and most of the K, Na, and P were water-soluble; most of the Mg and Mn, and some of the Ca were leached in NH 4Ac; most of the Ca was leached in HCl; and most of the Si and S remained insoluble in the biomass. Ion Chromatography found the water-soluble Cl, P, and S present as Cl −, PO 4 3 - , and SO 4 2 - , respectively, and equimolar concentrations of C 2 O 4 2 - as leached Ca in the acid fraction. The biomass solids were determined for anionic groups by methylene blue sorption. The contents were lowest in the wood samples (22–118 mmol/kg D.S.) and highest in the bark samples (130–453 mmol/kg D.S.). The closing of the ion charge balance led to a quantitative model for the ash-forming matter: water-soluble salts (KCl, K 2HPO 4, and K 2SO 4), acid-soluble minerals (CaC 2O 4), non-soluble minerals (SiO 2), and organically associated ash-forming elements (ionically bonded Ca 2+, Mg 2+, Mn 2+, and K +, and covalently bonded P and S).