| SO2and CO2, genetated by the combustion of fossil fuels, are considered as the main contributors towards acid rain and global warming, separately. Electric power plants which use fossil fuels are the largest global source of SO2and CO2emissions. CaO-based sorbents such as limestone are widely used to retain SO2in coal-fired flue gas, due to its ease of availability, low price and large absorption capacity. Further studies have revealed that CaO-based sorbents can also capture CO2cyclically in the flue gas at high temperature. Based on previous work of other researchers, this thesis proposes the idea of using CaO-based sorbent to capture CO2cyclically and then to retain SO2. The flow chart of running with circulating fluidized beds is also ploted. To avoid the negative effect of SO2on carbonation and realize sequential removal of both gases, fresh sorbents are used to capture CO2cyclically first and then the spent ones are used to retain SO2, while flue gas is desulphated first and then decarbonated. It is obvious that using CaO-based sorbent to capture CO2and SO2sequentially is a feasible scheme and will have great economic advantages.With the equipments of thermogravimetric analyzer (TGA) and fixed bed, this paper begins with basic kinetic research, investigating the effect of particle size and calcination atmosphere on CO2and SO2capture, and the effect of CO2looping cycles on SO2capture. As CO2carrying capacity decreases with increasing cycles, steam hydration is used to reactivate CaO-based sorbent. Under simulated flue gas atmosphere, the sequential CO2and SO2carrying capacity of CaO-based sorbent is tested in a bubbling fluidized bed and the fluidized effect and particle attrition is observed. With the idea of "treating waste with waste", coal ash and CaO are hydrated in hot water to synthesize new sorbents with specific surface area and pore volume enhanced dramatically and their CO2and SO2carrying capacity is tested using TGA. During all experimental process, a lot of auxiliary equipment such as electron microscopy scanner, surface area analyzer, particle size analyzer and X-ray diffraction analyzer, are used to identify properties of the sorbent at different stage.The results show that the particle size in the range of180-400mesh has little effect on CO2capture while having a great effect on SO2capture, with SO2carrying capacity increasing with decreasing particle size. Calcination condition such as CO2concentration has a large effect on CO2capture while little on SO2capture. CO2carrying capacity decreases as the number of cycles increases, while the cycle number has little influence on SO2capture. For example, the SO2carrying capacity of sorbent that has experienced40cycles is close to that of fresh sorbent. That means the CaO-based sorbent which is spent in CO2capture is still active in SO2capture and can be reused. It is proved that the CO2carrying capacity of spent CaO-based sorbent can be enhanced to the equivalent of fresh sorbent by steam hydration, and the new carrying capacity decreases in next few cycles at the same rate as fresh sorbent. This process can be repeated. The SO2carrying capacity of steam reactived sorbent is much higher than that of fresh sorbent. The hydration process generates a large amount of cracks on the particle surface, which benefits gas diffusion and increases the outer surface area where products can grow freely, but also intensifies sorbent attrition which is bad for fluidization.This thesis identifies the problem of sorbent agglomeration in fluidized beds. When the natural limestone runs in bench scale bubbling fluidized bed for few cycles, the sorbent particles agglomerate and fluidization deteriorates, even resulting in a "dead area". Apart from the reason of a small reactor size, agglomeration is a built-in problem of CaO-based sorbent at high temperature, as it can be seen that fresh limestone powder cakes after the first calcination in a fixed bed. Although the CaO-based pellets have a lower CO2carrying capacity than natural limestone for the first few cycles, they show much better performace in cycling stability, SO2carrying capacity, anti-attrition, and anti-agglomerationBased on the idea of "treating waste with waste" and inspired by that a pozzolanic reaction can enhance surface area, coal ash and CaO were stirred together in hot water for few hours to synthesize surface area and activity improved CaO-sorbent. The main product of the pozzolanic reaction is CaSiO3, showing network structure, and its development is related to the ratio of CaO/coal ash, hydration time, amount of CaSO4and NaOH. The synthesized sorbent has a better cycling stability than natural sorbent, however, the CaO/coal ash ratio should not be too low, otherwise the free CaO content of the synthesized sorbent decreases and consequently the overall CO2and SO2carrying capacity. The synthesized sorbent which experienced multi-cycles still keep a high SO2carrying capacity. Adding a small amount of NaOH decreases cyclic CO2carrying capacity of synthesized sorbent but enhances SO2carrying capacity dramatically. The reason is that sulphation reaction is controlled not only by gas diffusion but also solid-state iron diffusion. Na+ions generate more crystal lattice defects which can accelerate iron diffusion rate in product layer, and consequentially enhance overall SO2carrying capacity. |
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