The partition behavior and the chemical speciation of selected trace elements in a typical coal sample during pyrolysis
Hlatshwayo, Tivo Bafana
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Sasol is by far the world's leading company in upgrading of low-grade coal into high value chemicals and fuels. Such plants also utilise fine particles or pulverised coal in the combustion process to generate steam and electricity for their processes. Certain trace elements released from coal during utilisation may be of environmental concern. From the literature findings it appears that the elements of interest are mercury, arsenic and selenium due to their potential health hazard and as they are included in the United States of America, Europe and China Clean Air Act for their potential hazardous effects. It has been observed in the literature that these elements show a nature of high volatility compared to the rest of the elements and further mercury provides an important chemistry to investigate as it has the ability to accumulate in the food chain. It is likely that environmental legislation in South Africa could be implemented soon to control environmental pollution of selected trace element from coal plants. The toxicity of heavy metals is dependent on the chemical form in which the metal is released. Available analytical techniques for the measurement of trace metals are in many cases not able to differentiate between different chemical species in which the elements occur and are not very reliable at low concentration levels. This makes it difficult to estimate the real environmental impact of the emission of heavy metals as the result of coal processing. The project centred on a study of the partition behaviour of mercury during coal pyrolysis. Some other environmentally important trace elements, e.g. selenium and arsenic were also investigated. The stated aims of the project were reached by the following objectives: 1. Testing of suitable pyrolysis instrumentation. 2. Testing of absorption apparatus. 3. Testing of analytical methods. 4. Comparison of two coal types with regard to trace metal emission 5. Obtaining mass-balance closure with regard to mercury during pyrolysis. 6. Application of the FACTSAGE modelling package Samples were prepared and analysed for trace elements concentrations using advanced analytical techniques including modelling packages for comparison purposes. The conclusions of the project can be summarised as follows: • The partitioning behaviour of trace elements on a plant scale was successfully determined. • The modes of occurrence of some elements of environmental concern were successfully determined. It was not easy to compare these results with literature findings in this area as the coal used in this work was different from that used by other researchers. • The coal pyrolysis process was well understood. • The FACTSAGE package was applied successfully in this project in that it predicted the species of trace elements thus explaining the observed experimental findings, especially the analytical method used to analyse the concentration of mercury in solution. It can be concluded that research should start with theoretical studies like a model than an actual experiment as the model predicts direction of the reaction.
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