Fundamental Research Of Upgrading Brown Coal By Hydrothermal Dewatering To Prepare Coal Water Slurry With High Solid Concentration

Coal water slurry (CWS), as an oil substitute, is a clean liquid fuel. Preparing for CWS is necessary process to gasify by texaco technology. Low rank coals, such as brown coal, are very abundant and low price in China. Brown coal with high reaction activity is a good source for gasification. Preparing brown coal for slurry, which can reduce the cost of CWS and increase the field of application, has an important function on high efficient and clean utilization of coal resource.Drying is the first essential step in brown coal utilization process because of its high moisture content and low heat value. Hydrothermal dewatering (HTD) is a non-evaporative method by which water is removed as a liquid. The coal rank can be upgraded and heat value of brown coal increased effectively after HTD process. And the HTD products reabsorb water no more.A series of work are performed under the subjects of CWS preparation from brown coal, the effect of HTD process and the moisture distribution in brown coal.Many types of brown coals are made into CWS. And the slurryability, rheological characteristics and stability of those brown coal water slurries (BCWS) are investigated. Results show that the solid ratio of BCWS is lower than bitumite water slurry, and the most important reason is the high inherent moisture content resulted from the high oxygen functional groups content.Brown coals were hydrothermally treated for dewatering and upgrading in an autoclave. The chemical composition of solid liquid and gas products during HTD process are studied. It can be concluded that the equilibrium moisture and volatile decrease, while the fixed carbon and heating value increase. And the coal rank is upgraded with the mole ratio of oxygen or hydrogen and carbon element goes down. Many organic matters and inorganic salts dissolve into the liquid product. The total organic carbon and total inorganic carbon concentrations in the wastewater increase as the increase of HTD temperature. CO2content Make up a large proportion of gas product, which consist of CO、CH4、O2、H2S、H2. Especially, CO and CH4appear when the temperature is above240℃ Some physical and chemical properties of brown coal have improved after HTD. It can be concluded as follows:phenolic hydroxyl, carbonyl and carboxyl content decrease, leading a decrease of hydrophilicity of coal surface, which is expressed as the increase of contact angle between coal and water, and the point of zero charge. Moreover, the pore volume, surface and diameter of HTD products are smaller than raw coals. As a result, water holding capacity of brown coals weakened.The fixed-viscosity concentration of BCWS is significant increased. When HTD temperature is up to320℃, the increment of fixed-viscosity concentration is about9-13%. At the same time, the fluidity and stability of BCWS are also improved. Preparing the liquid products for BCWS can recover the energy of combustible organic matter from the wastewater. Furthermore, the fixed-viscosity concentration can raise1%more using liquid products.The method of thermogravimetric analysis-differential scanning calorimetry is employed to analyze the moisture distribution in brown coal. According to coal-water binding energy (Eb), the moisture in brown coal can be divided into four parts:free water (Eb=0), physical absorption water (Eb<2000kJ/kg), interim water (2000<Eb<9000kJ/kg) and chemical adsorption water (Eb>9000kJ/kg). The ratios of free water and physical water increase during HTD. The dewatering activation energy of brown, which is about21～27kJ/mol during isothermal drying process, decreased by HTD.The density functional theory of quantum chemistry is used to optimize the structure and energy of brown coal. Result show that the long side chain with high active reaction in brown coal has weaken chemical bonds, which will rupture firstly during HTD process. As a result, the side chains are shortened and concentration increase in brown coal. The adsorption energy of H2O on coal surface is about10kJ/mol. And the effects on hydrophilicity of brown coal:carboxyl> hydroxyl> methyl. Thermodynamic and kinetic analyses of deoxidization reaction of brown coal show that the activation energy of decarboxylation is lower thande-carbonyl reaction. It is more difficulty to remove carbonyl.