Comparison of trace element contents in low-temperature and high-temperature ash from coals and biomass

Abstract

Four coal samples and three biomass samples have been ashed by low-temperature (LTA) and high-temperature (HTA) methods and a range of trace elements have been measured in the coals, biomass and ash samples. Digestion of solids was by both wet ashing and by microwave digestion with examination of the resulting solutions by inductively coupled plasma mass spectrometry and by inductively coupled plasma atomic emission spectrometry for certain elements. Hg was estimated by a spectrometric method. The trace elements examined were Be, V, Cr, Mn, Co, Ni, Cu, Zn, Ga, As, Se, Mo, Cd, Sb, Ba, Pb and Hg. Changes in concentration in the LTA and HTA compared with the raw materials were observed. For LTAs, Se and Hg were partially lost while Ba became more insoluble in nitric acid, indicating a change to Ba sulfate. The behaviour of trace elements in the coals on conversion to HTAs was as established by earlier work and followed the expected trends, with Se, Hg, As, Cd, Pb, Zn being at least partially volatile. Trace element concentrations were generally greater in the coals than in the biomass samples, except for Mn, Cu and Zn which were more concentrated in the biomass samples.