Studies On Sulfur Transfer Additives For FCC Flue Gas

According to the national requirements of saving energy and reducing emissions, many companys committed to reduce SOx emission. The study on reducing SOx emission for FCC flue gas has already begun in the early 1970s. So far, general methods include washing, hydrotreating and adding sulfur transfer additive. Adding sulfur transfer additive to the reactor and regenerator system of FCCU is the fastest, cheapest, most convenient and effective method for flue gas desulphurization. Comparatively, the study on magnesium aluminum spinel-based sulfur transfer additive is more mature. However, most of these additives contain vanadium oxide, which causes new pollution in the process of production and utilization. Therefore, developing the sulfur transfer additive without vanadium is very important.The role of different active components in the sulfur transfer additive on SO2 removal and regeneration performances were discussed in this paper. Moreover, the effect of preparation methods and raw material properties were investigated. The results showed that rare earth may increase the rate of SO2 converting to SO3, and iron may accelerate the reducing rate of sulfur transfer additive. The fact that the vanadium oxide in sulfur transfer additive system with iron does not play important role was verified by experiments. Comparing alkali method, acid method with impregnation method, it was found that the sulfur transfer additive prepared by gel with acid exhibited better desulfurization performance, and possessed the characterization of simple preparation and suitability to produce in large scale. The sequence of adding raw materials during the production of sulfur transfer additives affected SO2 removal performance, so proper sequence should be chosen. Using metal oxide instead of nitrate may reduce NOx emissions and keep high SO2 removal. Sulfur transfer additives need both abundant active sites and pores. The method, which increasing the BET specific surface area and pore volume by using pore-expanding agents, was devised and adopted in this paper. This method could obviously increase oxidizability and reducibility of the sulfur transfer additives. The affecting factors of SO2 formation in FCC reaction-regeneration systems and the reducibility of sulfur transfer additives were investigated in the riser circulating fluidized bed units. The SO2 concentration in flue gas related to sulfur content and raw materials closely. It increases with sulfur content of raw materials non-linearly. In addition, the concentration of SO2 decreases with the prolonging of residence time; and it increases first and then decreases with the increasing reaction temperature, catalyst/oil ratio and stripping steam content. But reaction temperature and stripping steam do not affect it much. These operating conditions also affect the performance of sulfur transfer additives in various extents. The regeneration performance of sulfur transfer additives increased with increasing reaction temperature and prolonging residence time in the riser. The SO2 removal performance rises with the increase in regeneration temperature, quantity of regeneration air and the extention of residence time in the regenerator.The sulfer transfer additive was evaluated in the riser circulating fluidized bed unit. The results confirmed that the additives without vanadium oxide show excellent SO2 removal and regeneration performances. The rate of SO2 removal is more than 80% during the long running process. The commercial test of the sulfur transfer additive was carried out in a 1.6 million t/a FCCU in Dagang Petrochemical Company, PetroChina Corporation. The results also confirmed that the additive may remove most of SO2 in the FCCU flue gas. Adding 2% additive, more than 90% SO2 was removed. Moreover, adding the additive did not affect the distribution and quality of the products, and the operation of the unit.If sulfur transfer additives can promote the combustion of CO, the cost of using additives may be reduced because it is unnecessary to use CO combustion-promoting additives alone. Through introducing Cu and adjusting the amount of Ce in the sulfur transfer additives, a bifunctional additive combining SO2 removal and CO combustion was prepared. The combustion-promoting performance of the bifunctional additives improves with the content of Cu or Ce. However, the SO2 removal performance drops obviously when excessive Cu was loaded. The additive with 2% CuO has ideal SO2 removal performance and the similar combustion-promoting performance as that of copper-cerium aluminium composite oxides.