Trace Element Partitioning and Leaching in Solids Derived from Gasification of Australian Coals

Abstract

Trace element concentrations vary between coals from ppb to ppm levels and can depend on the rank of the coal and its geological origins. During gasification, some of the trace elements are volatilised at high temperatures and may condense and deposit in cooler downstream parts of the system or in quench water streams. Some species may appear in condensed phases such as slag or fly ash. Changes in the trace element concentrations in the slag and flyash from that of the parent coal are expected due to the reactions occurring at high temperatures and the different chemical activity of the trace element phases in the slag, fly ash, and syngas. Four Australian coals were used in an entrained flow gasification test program conducted in the Siemens 5 MWth gasification test facility. Solid samples were collected from different points in the gasification process during each test. Compositions of these samples were analysed and the distribution of trace elements was studied. The elements can be classified as follows, according to their tendency to appear in the slag and fly ash: Partitioned between slag and fly ash: Cu, W, Mo, Cd, Bi, Zn, Sn, Sb Partially volatile and depleted from either slag or fly ash: Be, Th, Sc, Y, Li, Mn, Ni, Sr, Ba Highly volatile (i.e. were not observed in either slag or fly ash): As, Se, B, Hg, F, Pb, V. Comparison of these experimental results with equilibrium calculations of trace element appearance in the condensed phases suggests that the modelling approach is suitable only for certain elements. For several of the trace elements of significance in this study, kinetic factors have to be considered in conjunction with thermodynamic modelling. The leaching behaviour of the trace elements in the slag was also studied. This work shows very low leachability for most of the trace elements except Zn and Sb, which, due to their relatively high volatility, reported to the slag samples in very low concentrations. f 2011 The University of Kentucky Center for Applied Energy Research and the American Coal Ash Association All rights reserved. A R T I C L E I N F O Article history: Received 28 October 2010; Received in revised form 26 January 2011; Accepted 14 March 2011