Trace element behaviour in the Sasol-Lurgi MK IV FBDB gasifier. Part 2-The semi-volatile elements: Cu, Mo, Ni and Zn

Gasification is a coal conversion process that could be considered to be more amenable with regards to environmental impact factors when compared to combustion, as it provides minimum direct emission to the atmosphere due to the opportunity to apply a series of gas cleaning processes. Emissions could be in the form of the well known trace elements labelled as toxic present in feed coal. Due to the minimal literature available on coal gasification when compared to coal combustion, a large amount of inference to coal combustion has been applied in discussing the partitioning behaviour of trace elements during coal utilization. Conducting mass balance calculations of trace elements around gasification processes have proven to be a challenging task. This is due to the limitation of the analytical techniques employed to quantify at the parts per million levels at which trace elements exist. The other challenge is analyzing for trace elements in all the different stream phases that occur after gasification. The availability of thermodynamic equilibrium packages i.e. Fact-Sage to perform high temperature calculations, at the same time handling all phases of material involved has simplified the challenges. Results obtained from such calculations have also proved to be close to reality, but have not been related to the fixed-bed countercurrent gasification reactor operating on lump coal. 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. Fact-Sage thermodynamic equilibrium modelling was used to simulate the semi-volatile trace elements (Cu, Mo, Ni and Zn) gas phase and ash phase partitioning and speciation behaviour occurring in a fixed-bed pressurized gasifier. A Sasol-Lurgi Mark IV FBDB gasifier was mined via turn-out sampling in order to determine the trace element changes through the gasifier, results being used to validate the modelled results. The semi-volatile elements: Cu, Mo, Ni and Zn all showed limited (5% in the case of Zn) de-volatilization behaviour in the drying and pyrolysis zone of the fixed-bed gasifier. Predictions revealed that within the reduction zone of the fixed-bed gasifier that they are all highly volatile, producing gaseous species with an increase in temperature, varying in the order: Zn > Mo > Cu > Ni, which is contrary to what was found from the experimental results. This could imply that thermodynamic equilibrium conditions do not necessarily prevail in a fixed-bed gasifier operating on lump coal, since in reality mass and heat transfer limitations across coarse coal particles apply and the reactions are therefore more kinetically limited. Overbalances of Ni and Mo partitioning to the solid ash fraction, was found for 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 these species during the gasifier sampling campaign, as well as by the particle size reduction processes utilized prior to elemental analyses. Leaching tests conducted on the bottom ash collected from the gasifier 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 Ni and Mo within the gasifier should be further investigated. (C) 2008 Elsevier Ltd. All rights reserved.