Study On Chemical-looping Combustion Reaction Mechanism And Influence Of Fe-based Oxygen Carrier Structure Evolution Based On Fluidized Bed

Fossil Fuels are both abundant and inexpensive, making it a dominant source of energy. The capacity of fossil-fuel-fired electricity generating is forecast to increase to nearly 450 GW by 2030. However, only about 6 GW of the existing coal-fired electricity generating fleet is projected to retire. Major drawbacks to the use of coal are that coal combustion releases air pollutants. Those pollutants have been strictly regulated by the U. S. EPA, including a sizable amount of CO₂ emitted by the fossil-fuel-fired electric power sector. In 2013, President Obama renewed the previous commitment to address global warming, citing both moral and economic imperatives, for the future regulation to reduce GHG in existing coal-fired power plants soon. To meet mid- to long-term CO₂ reduction targets, CO₂ capture and storage options need to be developed. A novel power generation system, called chemical-looping combustion?CLC?, can exclude both separations of air and flue gas, to capture CO₂ in higher power efficiency, initially uses gas fuel. This was later expanded to the solid fuels, such as coal and petcoke. Chemical looping combustion and gasification have been suggested to combine as the most promising technologies of the inherent separation of CO₂ with a reduced energy penalty.The char-CO₂ gasification reactions in isothermal and pressurized conditions were studied kinetically by high pressure thermo-gravimetric analyzer?HP-TGA?, the primary objective of this paper was to study the effects of pressure, temperature and char rank on the intrinsic reaction kinetics of the char-CO₂ gasification by the Langmuir-Hinshelwood model?nth order?Random pore. With the increasing of the pressure and temperature, both the intrinsic reaction rate at the same carbon conversion and the carbon conversion efficiency at the same reaction time increased. The intrinsic gasification rate experienced an initially slow increase when the carbon conversion below 0.6?Zone I?, then a rapid increase when the carbon conversion between 0.6 and 0.9?Zone II? and finally a sharp decrease when carbon conversion above 0.9?Zone III? corresponding to the carbon conversion efficiency. For more accurate interpretation of the char gasification kinetics, proved kinetic models, based on the random pore model and mixed model were developed to predicate the intrinsic reaction parameters of the Zone I and II, respectively, which were found in a good agreement with the TGA data under different operating conditions. Consequently, the structural parameter of coal char, the reaction order, the pressure order, the activation energies and the intrinsic pre-exponential factor were determined. The combined effects of surface area, pore structure, the degrees of graphitization and graphite crystallites on the char gasification kinetics were also studied under elevated pressures and temperatures conditions using a High-Pressure Thermo-Gravimetric Analyzer?HP-TGA? and characterization methods, such as BET, XRD, Raman spectroscopy, FTIR and SEM. The BET and SEM results indicated that the porous structures of chars experienced a quick developed until the carbon conversion reaching 0.9, and then quickly collapsed to the end of the whole carbon conversion. The changes of char structures were well agreeable to the gasification rate. The XRD, Raman and FTIR analysis provided facts regarding an increased ratio of graphite to amorphous carbons in bone-structure of chars, which was attributed to the consumption of the amorphous carbon and the retention of the graphite carbon during the gasification process. The collected evidence implied that the contributions of the surface area and pore volume of the char on its kinetics were much greater than that of its graphite crystalline during the char CO₂ gasification.Sintered iron ore fines were selected as oxygen carriers in chemical looping combustion. The reactivity of the sintered iron ore was investigated in redox cycles using thermogravimetric methods under isothermal or non-isothermal conditions. The used sintered iron ore samples from the redox cycles in isothermal tests, which were chosen to evaluate their structural changes, were characterized using the N2 adsorption-desorption, SEM-EDS, XRD and Raman spectroscopy. Results revealed that multiple redox cycle was an activation process of the selected sintered iron ore to promote its redox kinetics, as well as formations of the new crystalline phase. This was attributed to variations of the sintered iron ore in both their physical and chemical structural changes during redox cycles. The N2 adsorption-desorption analysis indicated an increase of its surface areas of sintered iron ore during redox cycles. The SEM-EDS results revealed the appearance of tiny cracks on the tested ore sample surfaces. Both XRD and Raman results presented appearance of a new crystalline phase, such as the lepidocrocite??-FeOOH?, which was apparently generated in the reduction reaction of chemical looping cycles. The formation of the new lepidocrocite phase seemed correlated to the decrease of the oxygen transport capacity of the sintered iron ore. The sintered iron ore performed properly in the carbon-laden atmospheres, and there was no carbon deposition on its surface.In this paper, the process of syngas production by means of coal-direct chemical looping gasification process?CDCLG? was modeled under thermochemical equilibrium with the Gibbs free energy approach. The model was developed using a complete and comprehensive Aspen Plus model. Due to the kinetic limits of the char gasification, the carbon conversion efficiency was not suggested to be above 0.9 in the fuel reactor of the CDCLG process. Besides, the residual carbon should be burnt in the air reactor. The effects of various conditions?carbon conversion, temperature, pressure and steam feed ratio? on the both input parameters?air feed ratio? and the energy output parameters?such as efficiencies of the electricity and syngas quality? were studied in this paper., the process to produce power by means of chemical looping combustion process and traditional combustion-turbine system were modeled under thermo chemical equilibrium with the Gibbs free energy approach. The models were developed using a complete and comprehensive Aspen Plus model. The main objective was to study the effect of pressure on the energy output parameters of the CLC power generation system and traditional combustion-turbine system. The results showed that higher operational temperature can help to obtain high electricity efficiency, and the best operational pressure for the CLC system was about 5.0 atm. This work also provided a reasonable operational condition for the CLC system, and to reveal the special feature during the conversion and utilization of chemical energy of methane in CLC cycles.