Lignite drying technologies for power generation physiochemical properties, pyrolysis and combustion of dried lignite products

Three technologies have been under serious consideration for reducing the moisture content of Latrobe Valley lignites – namely, hydrothermal dewatering (HTD), mechanical thermal expression (MTE) and steam drying (SD). This thesis compares the relative effectiveness of these technologies in terms of water retention in the product, solids recovery from the process and organic carbon in the by-product water. For the same Loy Yang lignite, containing 59% water (on a wet basis), it is shown that the temperature at which 50% of this water is removed is, for MTE: 125°C, for SD: 210°C and for HTD: 350°C. With MTE, the processing temperature required to achieve high moisture reductions was significantly reduced by simultaneous application of mechanical pressure. For Loy Yang lignite the optimum applied pressure was identified as approximately 5.1MPa. Further increases had relatively little effect on the moisture content of the product pellet. The relatively high temperatures required to achieve 50% water removal by HTD also leads to poor solids recovery (86%) due to decarboxylation and devolatilisation processes that concomitantly occur under these conditions. Poor solids recovery is also observed when SD is carried out at high temperatures (>250°C). Much of the sodium removed from drying was in the form of NaCl. In MTE, processing temperature was more effective in removing sodium than applied pressure (within the experimental parameters tested). In contrast, SD processing temperatures above 280°C had no effect on the proportion of sodium leached out from the lignite. The slurrying of the lignite with water in HTD facilitated removal of sodium, magnesium, calcium and chlorine during the process compared to MTE and SD. This considerably higher removal of inorganic material, particularly sodium, is an advantage in that it will reduce fouling and slagging propensities during combustion of the product. Rapid pyrolysis of the HTD product gave significantly lower yields of methane, ethene, carbon monoxide and carbon dioxide when compared to corresponding yields of the raw lignite, MTE and SD products. The lower yields for the HTD product were attributed to volatilisation during hydrothermal dewatering and also to the reduction of catalytic reforming cations. The significant volatile yield loss during hydrothermal dewatering could be disadvantageous in some industrial processes, which convert the carbon matter to lower molecular weight fractions (eg gasification, liquefaction). The combustion reactivity of MTE, HTD and SD products did not demonstrate significant differences from the raw lignite. The combustion reactivity of the thermally dried products decreased with increasing severity of the conditions. However these changes were quite minimal suggesting that the conventional boiler systems currently in operation in the Latrobe Valley would be more than adequate in combusting thermally treated products from any of the three drying processes. The effectiveness of catalytic inorganic species clearly outweighed pore volume effects for affecting the combustion reactivity of the lignites. The chemical form and concentration of the inorganic component in the hydrocarbon macromolecular matrix can influence the rate of combustion at 400°C. The removal of inorganic components from the coal by washing or by HTD or MTE has a much larger impact on reducing the combustion reactivity and reducing the peak temperature of the sample when compared to significant reductions in porosity resulting from thermal drying. Additional combustion experiments on a diverse suite of well-characterised lignites sourced from the Latrobe Valley confirmed that catalytic inorganic species clearly outweighed other physico-chemical effects for affecting the combustion reactivity of the lignites. The combined catalytic effects of iron, magnesium and calcium accounted for more than 95% of the variation in the combustion reactivity of the raw coals. In addition, the combustion reactivity of Latrobe Valley lignite samples was strongly correlated to the sum of the univalent charges of the AAEM species plus the univalent charge of the acid-extractable iron. Finally, the surface area and pore volumes of carbons/chars have negligible effect on the combustion reactivity and the inorganic components in the carbon macromolecular structure are the major influences in the combustion rate of the sample at 400°C.