| The concentration of CO2 in the atmosphere has increased strongly as a result of the dependency on fossil fuels for energy production. Due to the greenhouse gas such as CO2, the global warming has caused huge disasters and economic losses. Chemical looping combustion (CLC) is a clean and efficient combustion method with inherent separation of CO2 without energy penalty. CLC is based on transfer oxygen from air to fuel by oxygen carrier, which is continuously circulating between air reactor and fuel reactor to avoid direct contact reaction between fuel and air. Due to the flue gas from the fuel reactor consists carbon dioxide and steam, the highly pure carbon dioxide can be obtained and storage by condensing the steam. The oxygen carrier is the key factor for the CLC technology. Iron ore which has the advantage of a wide range of raw materials, inexpensive, and environmentally friendly etc., is an ideal oxygen carrier material for the CLC system. The use of iron ore could reduce the cost for capturing CO2 and its reaction characteristic is necessary to be discussed in detail.The objective of this dissertation was to investigate systematically the mechanisms of the reaction of iron ore as oxygen carrier with CO and the fluid dynamics in a continuous CLC unit, and to further study the process of real-time cycle and dynamic characteristics by simulating the gas-solid two phases flow pattern in interconnected fluidized bed reactor of CLC.The reactivity of iron ore with CO was investigated in a batched fluidized bed reactor. The results show that the conversion of iron ore as oxygen carrier to low valence iron oxide increased with the reduction temperature and time increased in the temperature range of 750℃ to 850℃. The effect of temperature on the conversion of iron ore as oxygen carrier to low valence iron oxide from 800 to 850℃ diminished compared with that results at the temperature 800℃ or below. After the reduction reaction, the surface pores of iron ore oxygen carrier particles are almost disappear. Part of iron ore particles surface is covered with sintered material which blocks the diffusion from CO to the active center of iron ore particles and further causes the lost of reduction rate. When The concentration of CO is below 20%, and reaction temperature is between 750 and 950℃, Fe3O4 reacted by Fe2O3 can exist stably and hard to be reduced to FeO and Fe.Under a same temperature, K-decorated iron ore oxygen carrier can increase the concentration of CO2 and the conversion ratio of CO effectively in comparison with pure iron ore oxygen carrier, because the alkali metal ion K+improves the reactivity of iron ore oxygen carrier, increases the reaction rate and promotes the implementation of reduction reaction. In the fuel reactor, due to the catalysis of iron oxygen, the reaction of carbon separation occurs to CO, and the carbon separated attaches on the particle surface of iron ore oxygen carrier and then enters the air reactor to react with O2 to create CO2. KFe11O17 is found on the surface of impregnated and calcined iron ore through test, which shows spinel structure, with good stability. The reduction index of iron ore oxygen carrier and K-decorated iron ore oxygen carrier is 0.74 and 1.45 respectively, and in comparison with iron ore, there are more Fe2O3 reverted to Fe3O4 in K-decorated iron ore oxygen carrier, and the speed of oxygen transfer is higher.The software of Computational Fluid Dynamics FLUENT 14.0 is adopted to carry out the simulations for the chemical-looping interconnected fluidized bed of 1kWth in Southeast University, to study the cyclic process and dynamic characteristics of gas-solid two phases flow model and analyze the concentration of solid particles, the speed contribution, the reactor pressure and the phenomenon of mixing. Based on the results, it can be shown that the transfer of oxygen carrier particles can be realized between the air reactor and the fuel reactor for the chemical-looping interconnected fluidized bed, self-balancing can be realized in the fluidization3 seconds after launching, and then the air reactor and the fuel reactor reaches the state of stable fluidization and maintains material balance. The gas-solid interface is clear in the fuel reactor, the characteristic of bubbles is obvious, the fluidization quality of particles is high and the particles in the bed are mixed evenly, which can meet the requirements for the reduction reaction between oxygen carrier particles and gas fuel. There are three different sections of fluidization areas in the fuel reactor the bottom is bubble section, with obvious fluctuation in concentration; the middle part is fluidization section, with relatively even concentration; and the top is suspension section, with low concentration distribution. Bubbles can move upwards with particles, and when they reach a certain height, bubbles break and particles fall down. The volume share of bubbles in the center of bed is big, which causes the void faction in some parts of this bed increases. The air reactor shows a state of rapid fluidization, the concentration of particles is relatively low, and the volume concentration of particles on the top is lower than 0.15%. Core-annular flow occurs in some areas at the bottom of air reactor, and the concentration distribution of particles is unevenly; on the top, the concentration distribution is relatively even, core-annular flow disappears and particles rise quickly. As the oxygen carrier particles reach the state of steady cycle, the pressure in the interconnected fluidized bed has basically become stable, and pressure difference distribution in various reaction sections is obvious. Given that the oxidization rate of oxygen carrier is higher than its reduction reaction speed obviously, gas fuel needs sufficient stay time in the fuel reactor in comparison with the air oxidation process of oxygen carrier, which meets the requirement that gas fuel is oxidized sufficiently. The pressure drop in the fuel reactor is obviously higher than the pressure drop in the air reactor, which reflects that the oxygen carriers are mainly distributed in the fuel reactor in the process of chemical-looping combustion, accounting for 80%, but there are only 10% of oxygen carriers in isolator and air reactor; and it also shows that the reactor is able to meet the requirement that gas fuel is oxidized sufficiently by oxygen carriers in the fuel reactor. Meanwhile, based on the results of numerical simulation, it can be shown that the newly designed isolator can prevent the mixing between fuel reactor and air reactor effectively. |
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