Mineralogy and trace elements of the Cretaceous Greymouth coals and their combustion products.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Understanding the spatial distribution trends and modes of occurrence (e.g. physical and chemical forms) of trace elements in coal seams is a key factor to any assessment of their potential impacts on the environment and human health. Two Cretaceous coal seams (E and Main) from the Greymouth coalfield, which is one of the most important coal-producing basins in New Zealand, were chosen for this study. The main objectives of this study are: (a) to delineate the concentration and spatial distribution of trace elements in the two coal seams, (b) to determine the modes of occurrence of trace elements in these coals, (c) to develop models of how trace elements from the Greymouth coalfield behave during combustion, and to assess the potential environmental impacts from mining and combustion of these coals. Examination of 184 samples from both the E and Main seams as well as 94 Main seam-composite samples from a drilling coal quality database, showed that both seams are characterized by great thickness (up to 35m) and low ash yield (<0.6% in some cases). The coals are laterally discontinuous as a result of both faulting and rapid sedimentary facies changes. In general a reverse relationship exists between ash content and coal thickness. Modeling of the lateral variations of major elements in the coal seams identified two possible inorganic sources for these coals. Furthermore, morphological features of minerals (quartz and clays) and the secondary mobilisation of liptinitic materials suggest that the Greymouth coals may have undergone extensive leaching in both peat and post-burial stages. Based on the physical and chemical evidence, a model for the leaching mechanisms has been proposed to explain how the leaching processes may have operated in the Greymouth coals. The major points of this model are: 1. Abundant organic acids in peat stage favor plant degradation and produce ~90% of the inorganics as exchangeable ions, which are active and readily leachable. 2. With rising temperature (30 to 70°C) in the lignite stage, minerals partly dissolve and become mobile. The porous structure of lignite allows exchangeable ions to be leached out of the coalbed through incipient cleats and/or micropores. 3. In the subbituminous to bituminous stages (70 to 170°C), organically bound inorganic elements are expelled due to organic matter breakdown and leached away by percolating solutions through cleat networks. Concentrations of all trace elements of interest are relatively low compared with many overseas coals of similar rank and age, although a few environmentally sensitive elements (As, Pb, Cl) are locally enriched. Due to the low concentrations in the coal beds studied, it is impossible to determine the modes of occurrence of trace elements by direct identification and examination of the host minerals. Thus, a group of direct (SEMEDXA, microprobe and INAA analysis of trace elements in minerals) and complimentarily indirect (float-sink test, sequential leaching tests, inter-elemental correlations) techniques and approaches have been employed to determine the modes of occurrence of trace elements in the Greymouth coals. The following results have been obtained: (1) Most trace elements including some hazardous air pollutants (HAPs) (As, Be, Cr, Hg, Sb and U) are predominantly associated with minerals (mainly clays) in the coal. (2) Some trace elements (B, Br, Cd, Cl, Co, Ni and Se) are bound organically or as sub-micron-sized minerals intimately admixed in the organic matrix. (3) REE, Th and Yare mainly associated with phosphates. (4) The mineral crocoite (PbCr04) is identified for the first time in coal and is a major host for Pb and Cr6+. (5) Some elements usually only associated with sulphides (e.g., As, Hg, Sb and Zn) are mainly within clays in the Greymouth coal seams studied. In addition to examining the distribution and mode of occurrence of trace elements in the Greymouth coal seams, their combustion characteristics were also investigated. The distribution and partitioning of trace elements during combustion was as follows: (1) Most trace elements including some HAPs (As, Co, Cr, Ni, Pb) are predominantly partitioned in the bottom ash, especially in three major phases (Fe-oxides, Si-Al-O glass, and Fe-bearing silicate). As and Mn are predominantly partitioned in the Fe-oxides; most other trace elements are partitioned between the other two phases. (2) In contrast, Sand Hg, and to a lesser extent, B, Cd and Cl, are chiefly partitioned in the flue gas fraction. However, a proportion of Hg (<10%) is probably absorbed onto the surface of fly ash particles, and this fraction of Hg may be freely released into the soil or water. (3) Although the low ash Greymouth coals have the advantage of generating small volumes of solid combustion ashes, one of the consequences is that combustion can enrich the trace elements in the ashes very significantly. For example, B, S, and Cl were enriched to 1100 ppm, 4.4% and 2000 ppm, respectively. Due to the low concentrations of toxic metals in the Greymouth coals and their combustion ashes, no major adverse potential environmental impacts would be expected based on the knowledge gained in this study. But it is prudent to further investigate the release of some toxic metals in the surface and subsurface waters in the coal mining areas and the long-term stockpile sites in the future.