Automatic dynamic chemical fractionation method with detection by plasma spectrometry for advanced characterization of solid biofuels†
A flow-through dynamic leaching test capitalizing on sequential injection (SI) stirred-flow chamber (SFC) extraction is proposed as a front end to inductively coupled plasma optical emission spectrometry (ICP-OES) for automatic and expedient evaluation of potentially mobilisable pools of ash-forming elements (viz., K, Na, Ca, Mg) in troublesome solid biofuels (namely, pine twigs) and ascertainment of the kinetics of metal release. A three-step sequential extraction scheme exploiting increasingly aggressive solvents and encompassing distilled water, 1.0 mol L−1 ammonium acetate and 1.0 mol L−1 hydrochloric acid is herein selected for partitioning of targeted elements in the water-soluble, the exchangeable and the acid-soluble nominal fractions, respectively. The relative amounts of elements in the first two steps are regarded as valuable measures of the release of reactive inorganic species into the flue gases in the course of combustion in boilers and furnaces with the subsequent associated fireside problems. The SI-SFC-based setup coupled to ICP-OES features high tolerance to flow rates (≤2.5 mL min−1) without pressure drop and affords more accurate evaluation of accessible pools of ash-forming matter compared to equilibrium-based methods as a consequence of the exhaustive extraction nature of dynamic fractionation, the solid to liquid equilibria shift, the absence of metal redistribution effects and the release of metals under the nominal pH of the extractant. No significant differences were found at the 0.05 significance level between summation of leached concentrations in each step plus residual fraction and total concentration of ash-forming elements in twigs as determined by microwave digestion, thus revealing the reliability and lack of bias (trueness) of the automatic method. The overall extractable pools of ash-forming elements were quantified in merely 3 h versus 7 days in batchwise leaching tests. The automatic hyphenated technique is thus suitable for expeditious advanced characterization of woody biofuels so as to assist in deciding whether firing biomass fuels leads to potential corrosion risks in combustion devices.