Sequential and Single Leaching of Victorian Brown Coal to Elucidate the Elution of Inorganic Elements at Room Temperature

摘要

The presence of ash-forming inorganic elements in coal causes slagging and fouling problems in a combustor, as well as the emission of inhalable particulates and toxic metals into air in conjunction to greenhouse gases.Ultra clean coal (UCC) is a coal-derived solid fuel with overall ash content in the order of 0.1 wt %, the utilisation of which instead of raw coal is deemed as one of the promising solutions to promote clean coal combustion with near-zero emissions of both greenhouse gas and environmentally harmful pollutants. As the mineral matter is removed, UCC has the potential to burn directly in gas turbine combined cycle or direct carbon fuel cell for a net power generation efficiency of no less than 48% on the higher heating value basis, given the contents of overall ash and Na/K within UCC are lower than 200 mg/kg and 0.5 mg/kg, respectively. UCC can also be used in a variety of value-added applications such as alternative for heavy fuel oil as well as carbon based materials and chemicals. In order to develop an efficient process for coal demineralization, an understanding on the modes of occurrences of original mineral matter in a raw coal is primarily essential. To date very limited studies have been conducted on the structures of mineral matter in low-rank coal compared with the high-rank coal. This in turn influences the development of a process for low-rank coal demineralization. This paper aims to elucidate the modes of occurrence of inorganic elements in two brown coals collected from Latrobe Valley, Victoria, Australia. Victorian brown coal meets >85% of the local electricity requirement. Apart from the conventional sequential leaching method, a number of advanced analytical instruments were employed to characterize the microstructures of mineral matter within the raw coals, thereby providing a detailed understanding of the distinct inorganic elements in Victorian brown coals. Based on sequential leaching results, several single reagents (acid/chelate) were further tested for brown coal demineralization. The single reagents tested include pyroligneous acid, citric acid and disodium ethylenediaminetetraacetic (Na-EDTA) with strong capability for the mobilization of metals in natural matter such as soil. Pyroligneous acid is a waste solution derived from wood carbonization, the majority of which is disposed of with little being used in a value-added and efficient way.