| The CO2 capture utilization and storage (CCUS) technology can achieve nearly zero CO2 emission, which is of great significance to realizing reduction target of CO2 emission and developing low carbon economy in China. Oxy-fuel combustion is considered as the most promising CCUS technology. It has the advantages of low cost, can be scaled easily and can be applied to the existed combustion equipment. Oxy-fuel circulating fluidized bed (CFB) combustion has several advantages that makes it a better choice for CO2 capture. CFB combustion has the benefit of uniform heat flux, fuel flexibility, inherently low NOx emissions, and the potential to achieve SO2 emission reduction of 90% or more by addition of limestone to the combustion furnaces, high oxygen concentration combustion etc. During oxy-fuel combustion process, a part of flue gas is recycled to the combustion furnace, and SO2 in the flue gas would be gradually accumulated. The high SO2 concentration would cause corrosion of the tube and device and be harmful for CO2 compression, purification and transportation. Therefore, it is necessary to remove SO2 in oxy-fuel CFB combustion. However, there are few reports on the SO2 emission under the oxy-fuel CFB combustion.The objective of this dissertation is to study the SO2 emission and limestone desulfurization characteristics under the oxy-fuel CFB combustion. The optimum operating parameters for the reduction of SO2 emission were discussed. And the limestone sulfation reaction mechanism of limestone was presented. The results can provide theoretical support and experimental data for the development of lower SO2 emission technology under the oxy-fuel CFB combustion. The major research contents are as follows:1. The evolution of sulfur of coal under the O2/CO2 combustion condition was investigated by thermogravimetry-fourier transform infrared spectrometry (TG-FTIR), and the effect of oxygen concentration, heating rate and biomass blending ratio on the formation of sulfur-containing gases were studied. The results show that the H2S and SO2 yields under the O2/CO2 combustion condition are much lower than that under the air combustion condition, but the COS yield is reversed; Under the O2/CO2 combustion condition, the increasing of the oxygen concentration can accelerate the evolution and increase the yields of H2S, SO2 and COS.2. A 50 kW oxy-fuel CFB combustor was built to investigate the formation and reduction mechanism of SO2. The results show that the SO2 emission increases with the increasing in the combustion temperature and decreases with the increasing in the inlet oxygen concentration and the oxygen excess; With the same Ca/S molar ratio, the optimum combustion temperature to achieve the highest desulfurization efficiency is 900℃; As the inlet oxygen concentration increase, the desulfurization efficiency of limestone increases significantly. The product of limestone sulfurization was analyzed by Barrett-Joyner-Halenda (XRF), Brunauer-Emmett-Teller (BET) and scanning electron microscopy (SEM). During the indirect sulfation process, the CO2 gas release during the calcination causes the porosity sulfation product layer, which is benefited to the sulfation of limestone. However, the high CO2 concentration would inhibit the calcination of limestone. The sinter of the CaO has obviously effect on the calcium conversion during the indirect sulfation, and the higher temperature would enhance the sinter of CaO, resulting in the lower calcium conversion. The different structure of the product layers of the direct sulfation reaction are attributed to the different chemical free energy changes under the O2/CO2 combustion condition; The sulfation product layer consists of larger grains and looks loose and porous under the lower inlet oxygen concentration, whereas the sulfation product layer consists of smaller grains and looks dense and non-porous under the higher inlet oxygen concentration.3. Oxy-fuel combustion experiments with recycled flue gas (RFG) employing high oxygen concentrations were successfully carried out in a 0.1 MWth oxy-fuel CFB combustion unit. The desulfurization efficiency under the O2/RFG combustion condition is apparently lower than that under O2/N2 and O2/CO2 combustion condition; Under the O2/RFG combustion condition, a lower secondary flow ratio and excess oxygen ratio can improve the desulfurization efficiency. But the decreases of oxygen concentration in the primary flow will bring lower desulfurization efficiency. |
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