Mechanism And Reaction Characteristics Of Calcium Looping Cycle For Carbon Capture At High Temperatures

As there is growing concern that the fast increase of carbon dioxide (CO2) concentrations in atmosphere is greatly contributing to global warming, the reduction of greenhouse gases emission, especially CO2, has attracted great attention of governments from all over the world. In China, the major CO2emissions are from the combustion of coal, thus it is important to develop novel clean coal combustion technologies with CO2capture with high efficiency. The carbonation/calcination reactions of calcium-based sorbents for cyclic CO2capture, also called as calcium looping, has been identified as one of the most promising post-combustion CO2capture technologies for coal combustion power plants. However, calcium looping technology is still facing a lot of critical problems. The CO2capture capacities and the carbonation reaction rates of the common calcium-based sorbents experienced rapid decrease after multiple cycles. Besides, the presence of SO2and steam in flue gas greatly influenced the calcium looping.In this thesis, the cyclic performances and morphology changes of common and modified calcium-based sorbents were compared during high temperature cyclic reactions. It was found that the CO2capture capacities of the micro-and nano-sized CaCO3were close after multiple cycles. A sol-gel process that could be contributed to the synthesis of calcium-based sorbent with high reactivity was put forward. The calcium-based sorbent derived from the sol-gel process was composed of nano-sized grains with hollow microstructure. It achieved high CO2capture capacities and low deactivation constant in long-term cyclic reactions. The CO2capture ability of the new sorbent was three times as large as that of common micro-and nano-sized CaCO3after multiple cycles. The kinetics of CaO-CO2reactions of the calcium-based sorbents under chemical reaction controlled period was described by a grain model.Investigating the sulfation behavior of calcium-based sorbents in the presence of SO2and steam during calcium looping, it was found that SO2concentrations, O2concentrations, steam, temperature and reaction duration in carbonation or calcination stage greatly influenced the CO2capture performance of natural limestone sorbent and the calcium-based sorbent derived from sol-gel process. It was proposed that reducing reaction duration in each cycle should be an effective way to limit sulfation of the sorbents during calcium looping.The effect of different atomic skeleton materials on the CO2capture performance of synthetic calcium-based sorbents was investigated. Different atomic skeleton materials displayed different properties in calcium looping. The performances of3kinds of synthetic sorbents with mole ratio of atomic skeleton materials to CaO of1:10was compared, it was found that different properties of the atomic skeleton materials greatly influenced cyclic performance of the synthetic sorbents. If calcium looping proceeded under mild calcination condition of850℃, CaO/MgO sorbent showed highest CO2capture capacity of0.51g CO2/g sorbent after20cycles. If calcium looping proceeded under severe calcination condition of950℃, CaO/Ca12Al14O33sorbent showed highest CO2capture capacity of0.31g CO2/g sorbent after20cycles.The optimal mass ratio of support materials of20-25%was put forward. The synthetic sorbents with MgO and Ca12Al14O33supports obtained excellent CO2capture behaviors, under ideal conditions and in the presence of SO2and steam, respectively. The smallest average grain sizes of MgO should be the reason that CaO/MgO sorbent showed strong affinity to CO2and SO2capture as well as its higher capacities in the presence of steam.The calcium-based pellets were manufactured through two routes. It was found the chemical compositions of the pellets were CaO and Ca12Al14O33, and the pellets derived from re-making of calcium-based powders by sol-gel process showed better performance. The influence of preparation conditions on the friability of the pellets was tested, the results showed that mechanical strength of the pellets was increased with the increase of mass ratio of cement in the pellets.