Modeling Analysis Of The Zero Emission Coal System And Research On The Mechanism Of Coal Hydrogasification In The System

For the traveling transportation vehicles drived by electricity, most of the power is generated in the coal-fired power station. For gas driveling vehicles, it would be very suitable for the Chinese situation of "rich coal, meager oil and little gas" if part of the gas could be produced by coal gasification. It is, therefore, very meaningful to study the novel, clean and efficient coal utilization technologies. The present work aims to study the integrated operation property of a zero emission coal (ZEC) system and the coal hydro-gasification (CHG) characteristics in the system.First, a detailed ZEC system is setup based on the original ZEC concept and the effects of operating parameters including H2recycling ratio (Rh), calcium to carbon ratio (Rctc) and fuel utilization factor (Uf) on the energy efficiency (Een), exergy efficiency (Eex), total energy efficiency (Een), total exergy efficiency (Eex) and carbon dioxide (CO2) sequestration ratio (Rcs) are analyzed for this system. Rh of0.75, Uf of0.8and Rctc of1.5are found the optimum operation parameters. With these parameters, the ZEC system can achieve Een of36.2%, Eten of46.8%, Eex of35.7%, Etex of46.2%, and Rcs of87.4%. The energy and exergy analyses of the system are then implemented and the maximum energy loss is found occuring in the steam turbine (ST) while the maximum exergy loss occuring in the solid oxide fuel cell (SOFC). Thus, to further improve the system efficiency, the exergy efficiency of SOFC should be improved.After the performance of the ZEC system is evaluated in detail, the CHG component of the system is then focused and studied deeply from the views of chemical equilibrium, chemical kinetics, numerical simulation and experiments. In view of the universality of the chemical equilibrium method and the lack of study on CHG with this method, a chemical equilibrium model (CEM) for CHG is firstly proposed to study the effects of different reaction conditions on the CHG characteristics. The results from the model are then validated against literature available experimental data and the model is proved reliable. When pt is7MPa and T is1,000K, the carbon will be totally converted when Rh/c is about0.25and the maximum methane mole fraction (MMF) will be obtained. If T is not controlled, carbon will be all converted at7MPa when Rh/c is about0.5.Then, in view of the lack of kinetic models reported for CHG, a hydrogasification kinetic model is established and validated against experimental data available in literatures. The model is then used to predict the effects of different reaction conditions on the CHG properties. When pt is13MPa, the reaction time t is10s and other parameters are kept consistent with those of the baseline case, the coal conversion ratio (CCR) is nearly0.9, CH4mole fraction is about0.32and H2mole fraction is about0.6. Increasing T can promote the hydrogasification process when it is not higher than1273K. When T is higher than1273K, however, increasing T will restrain the gasification process.After that, the CHG kinetic model is studied further and a Combined Random Pore and Shrinking Core Model with Pressure Correction (CRPSC-PC) is developed for the heterogeneous reactions. Then, considering the muti-dimensional n umerical simulations about entrained flow coal hydrogasifier are still rarely reported, a three-dimensional mathematical model about the gas-solid turbulent flow with chemical reaction is set up and solved with the assistant of Fluent. The simulation methods are validated and are then used to analyze the hydrogasification properties of an entrained flow bed gasifier. After comprehensive analyses, the synthetically optimal combination of the operating condition is found to he pt=7MPa, Rh/c=0.3, and Ro/h=1.5. With this combination, the char conversion ratio (Rchar) can be96.78%, MMF can be17.42%, and the cold gas efficiency (CGE) can reach76.4%.Finally, in view of the lack of experimental data about the coal hydropyrolysis (CHP) or CHG kinetics, the hydropyrolysis and hydrogasification kinetic characteristics of a lignite coal and a bituminous coal are studied in a pressurized thermo-gravimetric analyzer (P-TGA). The thermo-gravimetric (TG) and derivative thermo-gravimetric (DTG) curves at different reaction pressures are illustrated and compared. The kinetic mechanism function is determined and the kinetic parameters are calculated. In addition, the kinetic compensation effects of the hydropyrolysis and hydrogasification processes of the lignite coal and bituminous coal are analyzed and the corresponding isokinetic points are calculated.