Emissions of organically bound trace elements during brown coal air and oxyfuel combustion

The significant and consistent use of coal as a major source for power generation results in the emissions of various hazardous trace elements into the environment. Coupled with the advancement of oxyfuel combustion, a promising CO₂ abatement technology, there is a pressing need to fully understand trace element behaviour. Therefore, this research project aims at contributing towards fundamental understanding regarding the mechanisms governing the behaviour of trace elements under the conditions that are typically encountered in brown coal air and oxyfuel combustion. To address literature gaps regarding trace element emissions and partitioning, both laboratory-scale and pilot-scale studies have been conducted. The highlight of these studies is the focused use of brown coals (lignites), e.g. Victorian brown coal (VBC), due to the limited knowledge for these coals which possess distinct properties and burns differently from high-rank coals that have been studied intensively in the literature. In terms of mode of occurrence in the original coal, trace elements in brown coals are mostly organically bound rather than being present in discrete minerals typical to that of other high-rank coals. For these coals, a wide variety of trace elements have been examined, including As, Ba, Be, Co, Cr, Cu, Mn, Ni, Pb, Sr, V, and Zn. Of these, As and Cr are highlighted as elements of major environmental concern based on their known adverse health and ecological effects. These two elements are studied in greater detail since their speciation affects their toxicity. To accomplish trace element quantification and characterisation for these purposes, a number of advanced analytical instruments and methods were utilised, including inductively-coupled plasma optical emission spectroscopy (ICP-OES), X-ray fluorescence spectroscopy (XRF), X-ray absorption near-edge structure spectroscopy (XANES), and X-ray diffraction (XRD). The scope of this thesis includes firstly establishing and standardising the trace element quantification analysis method, microwave-assisted digestion performed in conjunction with ICP-OES, for accurate communication of the main body results. The laboratory-scale studies, which utilises a drop-tube furnace, then compare the trace element behaviour derived from a VBC to that from a Chinese lignite. For this, the emission dynamics of their respective trace elements during pyrolysis and char oxidation, different stages of the coal combustion process, were studied in both air and oxyfuel gaseous environments. Further to that, the laboratory-scale studies encompass the additional focus on the emission and speciation of As and Cr. For the study on As, three coals of different type and origin were tested for its As valency using the synchrotron-based XANES during coal combustion in air and oxyfuel combustion mode. On the other hand, the novel study on Cr mechanisms for speciation involved monitoring the evolution of Cr species from reacting reagent-grade compounds using in-situ high-temperature XRD. Finally, as means to validate the laboratory-scale results on a larger-scale, the Chinese lignite was then subjected to air combustion in a pilot-scale 30MWth pulverised coal-fired boiler. Here, a side objective of this work was to investigate the effects of using a silica additive on the emissions and partitioning of trace elements in brown coal. The use of fuel additives is commonly adopted by various facilities as they have been proven to inhibit ash slagging and fouling issues, however, their effects on trace element emissions have not been fully documented. Overall, clarifying trace element emissions and partitioning behaviour is of mainstream interest, and this research ultimately provide a clearer picture for the management of trace elements derived from the use of brown coal for power generation