Hydrothermal-mechanical dewatering of brown coal

There are enormous reserves of brown coal in the world. In Australia, brown coal is used to generate most of electricity in the states of Victoria and South Australia. Brown coal is characterised by very high moisture content (around 60 wt% on a wet basis). Therefore, boilers used in the power station are very large and have low thermal efficiency, leading to high cost and large emissions of green house gas. High moisture content also makes brown coal uneconomical for transportation. Hence, an effective dewatering process for brown coal is imperatively sought for. In an effort to upgrade brown coal and produce a transportable fuel, a new upgrading process, hydrothermal-mechanical dewatering (HMD), has been developed. It is a combination of hydrothermal dewatering and mechanical dewatering and therefore, takes advantages from both processes. In this process, water of brown coal is removed in liquid form under combined action of heat and mechanical pressure, leading to a dry, dense and hydrophobic coal product. The major advantage of the HMD process over conventional evaporative processes is the energy efficiency due to the absence of latent heat for vaporisation. Fundamental studies on the HMD process have been carried out usmg a batch compression-permeability cell for different brown coal slurries including hydrothermally dried coal slurry and raw brown coal slurry, as well as as-mined brown coal. In this thesis, the pore destruction and dewatering characteristics of brown coals in the HMD process have been investigated deeply over a wide range of process-related variables, such as processing temperature, expression pressure and expression rate. Results from batch tests have shown that the HMD process can lead to a greater pore destruction and moisture reduction for brown coals using relatively milder processing conditions. The expression pressure required in the HMD process is much lower than that used in the normal mechanical dewatering, while the processing temperature is lower than that used in the hydrothermal dewatering for achieving a certain moisture reduction. Such conditions make the HMD process easier for scale-up. Therefore, hydrothermal-mechanical dewatering promises to be an effective and practicable process for the upgrading of brown coal. The processing temperature is a dominant parameter in the HMD process. Increasing temperature can significantly improve the dewatering characteristics and increase the extent of pore destruction of brown coals. It was found that higher temperature leads to the destruction of both macroporosity and microporosity. Higher temperature also contributes to the alteration of surface properties of coal particles from hydrophilic to hydrophobic. An optimum temperature can be obtained for a given expression pressure. Expression pressure is also a critical variable in the HMD process. The extent of pore destruction and moisture reduction of brown coal is proportional to the increase in the effective compression pressure. However, the expression pressure required to achieve a given amount of moisture reduction can be minimised by means of increasing the processing temperature. The performances of the HMD process using different expression techniques have been studied. It was proven that the process using a technique of combined constant-rate and constant-pressure expression could achieve a greater pore destruction and moisture reduction. The HMD treatment changes not only the physical properties of brown coals but also improves the chemical properties of the coal. The carbon content and specific energy of the coal are increased while the volatile matter is decreased. More importantly, a significant amount of inorganics can be removed from those coals with high ash content, which reduces the ash formation during coal combustion. The HMD process of brown coal includes a series of chemical and physical separation processes. The complex mechanisms of the HMD process can be analysed by separately considering the contributions of hydrothermal reactions and physical expression. The expression process of the HMD process consists of a filtration process and a consolidation process, which can be described using existing filtration and consolidation theories. A combined Terzaghi-Voigt model has been used successfully to simulate the HMD process of brown coal. The model parameters are dependent on the processing temperature and pressure. The high porosity and hydrophilic nature of raw brown coal are the limiting factors in the preparation of a coal-water slurry with high solids concentration. The HMD upgrading process provides a feasibility of preparing such slurry because it can produce a hydrophobic product with very low porosity. By optimising particle size distribution and adding an appropriate surfactant, a coal-water slurry with a solids concentration up to 60 \/t% has been achieved, which can potentially be sold as an export fuel for the substitution of oil in power stations. The reconstituted coal-water slurry shows a strongly thixotropic behaviour and has a high initial yield stress, which is essential for stable storage. The studies conducted in this thesis have exposed the fundamentals of hydrothermalmechanical dewatering of brown coal, which enables the mechanisms of the process to be understood. The results obtained in this study will lead to the establishment of a continuous dewatering process and consequently, the development of an effective process for the upgrading of low-rank coal.