Study of Radical Reaction Mechanism and Catalysis Reaction Mechanism for Reduction of Sulfur Dioxide by Carbon Monoxide in Flue Gas
|Keywords||CO CO2 SO2 photodissociation reaction desulphurization doped catalyst|
SO2 is one of the most serious pollutants and many researchers devoted to study on the removing technology of SO2 from the flue gas. At present the method of wet flue gas desulfurization (WFGD) has disadvantages such as high investment, high operation cost and causing secondary pollution. Direct reduction of SO2 by CO to elemental sulfur is one green environment technology. Therefore, it was significant to develop novel catalyst, study on catalytic reduction mechanism of SO2 by CO and get proper reduction environment for catalysts in the real composition of flue gas. The main content of this thesis is as follows.The real flue gas contains over 10% (vol.%) CO2, which can be photolyzed by vacuum ultraviolet under 120～180nm wavelength. Firstly, the vacuum ultraviolet(VUV) photochemical reactor was designed to produce spectrum with wavelength less than 180nm. The atomic spectrum intensity of the VUV light source was measured by a pre-calibrated VUV spectrum detecting system using N2 for luminescence gas. It was found that the main atomic lines was 109nm,120nm, 149nm and 174nm, respectively. Secondly, the CO2 photodissociation reaction was carried out in this photochemical reactor. Experimental results showed that CO2 was photolyzed into CO and the yield depended on reactant gas pressure at same electric current. The maximum CO yield was obtained at about 2500 Pa of the reactant gas pressure for CO2 and N2 system. After adding CH4 into this system, CO yield was improved significantly while no maximum value of CO yield was found in the experiment.It was found that oxygen and vapor in the flue gas affect the catalytic activity and stability directly. The reaction path can be changed by the radical produced by photodissociation reaction of SO2 or additional photolyte. SO2 photodissociation was carried out by the low-voltage mercury light of wavelength 254nm. The results showed that the reaction paths are different for the radical of SO2 with SO2, O2 and CO reaction. Without oxygen, the radical of SO2 and CO reacted to produce radical SO and then elemental S, which was the reductive path. With oxygen in the system,the reductive path and oxidative path coexist. The radical of SO2 and O2 reacted to produce SO3 and simultaneously the radical of SO2 and CO reacted to produce elemental S. The conversion of SO2 can be improved by the radical CH3 and CO produced by adding acetone at wavelength 254nm since acetone was favorable for the reductive path.The key point is to prepare catalyst and make the catalytic activity and stability high with oxygen and vapor in the system. For this purpose, the LaCoO3 and 1%,10%,20%,30% doped LaCo1-xBBxO3 (B=Cu, Ti, Fe) were prepared and studied. Two pretreated process were studied to investigate the effect of pretreated process of the catalysts on the reduction of SO2 by CO. One was using the mixture of SO2 and CO for catalysts sulfurized and poisoned in the presence of oxygen and the other was using CO pre-reduced in the absence of oxygen. The evaluations were based on the activity, selectivity of sulfur as well as the COS amount in a gradientless, quartz tubular packed-bed reactor. It was found that the catalysts sulfurized and poisoned by the mixture of SO2 and CO in the presence of oxygen could sustain its activity better than the pre-reduced catalysts. The activity of the catalysts sulfurized and poisoned decreased a little after 4h reaction, but the pre-reduced catalyst deactivated fully after 140min reaction. It was also found that all the doped catalysts presented higher oxygen resistance and lower COS formation than the undoped catalysts. The 10% Cu- LaCoO3 was the most effective catalyst to remove sulfur with little COS formation. The deactivation mechanism was speculated and it was considered that two catalytic circulations occur on the catalyst surface after being sulfurized and poisoned in the presence of oxygen, which may cause the catalytic difference of pre-reduced catalyst and sulfurized and poisoned catalyst.