Trace element behaviour in the Sasol-Lurgi fixed-bed dry-bottom gasifier. Part 3. The non-volatile elements: Ba, Co, Cr, Mn, and V

Abstract

Coal contains most of the naturally occurring chemical elements in (at least) trace amounts, with specific elements and their concentrations dependent on the rank of the coal and its geological origins. The focus of this paper is to discuss more recent environmentally-focused research developments by Sasol, where trace element simulation and validation of model predictions have been undertaken for the gasification process operating on low-rank bituminous Highveld coal. A Sasol-Lurgi fixed-bed dry-bottom (FBDB) gasifier was mined via turn-out sampling in order to determine the trace element changes through the gasifier, results being used for comparison with Fact-Sage modelled data for the non-volatile trace elements Ba, Co, Cr, Mn and V. Considering the experimental error, good agreement between measured results and model predictions in terms of ash phase partitioning behaviour was obtained for Ba, Co, Mn and V. On the contrary, rather poor agreement between model predicted and measured results were obtained for Cr partitioning to the solid ash fraction, which yielded a large overbalance (outside of experimental error) in the case of the measured results. This anomaly was found to not be caused by erosion of the gasifier internals, but rather possibly be ascribed to accumulation and contamination caused by likely condensation and vaporisation of this species during the gasifier sampling campaign, as well as by the particle size reduction processes utilized prior to elemental analyses. When considering the predicted speciation behaviour of the elements studied, the model output in some cases needs to be treated with some caution when validating findings with standard text book data for the elements studied, but was found to correctly model the elemental ash phase partitioning behaviour during fixed-bed gasification. Leaching tests have been conducted on the bottom ash collected from the gasifier and results have shown that the trace elements studied are firmly bound into the ash matrix and therefore would not be released during later disposal. The relative enrichment in trace element content observed for Cr within the gasifier should be further investigated