Numerical Simulation And Experimental Study On Adsorption Removal Of SO₂ By Powder Activated Carbon In Fluidized Bed

Compared with the traditional desulfurization technology, this technology is characterized by low water consumption, no waste water discharge, no secondary pollution, reclamation of sulfurs, sulfur resource recycling, multi-pollutant removal and so on. So it has a broad application prospect in coal-fired flue gas purification field. At present, the industrial application technologies of adsorption desulfurization by activated carbon have a variety of adsorption process including fixed bed, moving bed and so on. Fixed bed desulphurization process has drawbacks of complex equipment, increasing the desulfurization resistance, reducing the desulfurization efficiency as time goes on. Moving bed desulphurization process has disadvantages of high cost and complicated preparation technology of carbon, low utilization rate of the inner surface, low mass transfer rate, high degree of mechanical loss and so on. Therefore, this paper adopted a new adsorption process named fluidized bed desulfurization process. This process has the advantage of low cost, high performance of gas-solid mixing, excellent heat and mass transfer, low pressure drop, large range of load regulation, fast load regulation, small footprint and so on. Meanwhile, this paper used the powder activated carbon which has reduced the wear and tear of the equipment to a great extent.Based on the predecessors’research, this paper aimed at improving the desulfurization efficiency of fluidized bed by powder activated carbon, and analyzed the effects of different process conditions on the desulfurization performance of fluidized bed by powder activated carbon through numerical simulation and experimental research.First, SO2 dynamic adsorption characteristics by powder activated carbon ware investigated on a low temperature drop tube furnace experimental system. The effect of activated carbon equivalent ratio, adsorption temperature, SO2 volume fraction, O2 volume fraction and H2O volume fraction on SO2 adsorption over activated carbon was discussed. The experimental results showed that SO2 in the gas phase quickly spread to the activated carbon surface in the initial stage of the reaction. The adsorption rate was high. Afterwards, the adsorption rate slowed down because it was controlled by gaseous diffusion and internal diffusion. With the increase of carbon equivalent ratio, the specific surface area and the surface active site went up, which promoted the transfer of SO2 to activated carbon, and led to the increase of the desulfurization efficiency. But due to the competitive adsorption, with the increase of carbon equivalent ratio, a SO2 adsorption per unit quality activated carbon increased before it was reduced. The higher the temperature is, the lower the physical adsorption rate is, the higher the chemical adsorption rate is. But the effects of reaction temperature on the chemical adsorption rate can be negligible relative to the physical adsorption rate. So the higher the temperature was, the lower the adsorption rate was, the lower the desulfurization efficiency was. SO2 and H2O occupy the same kind of active site in the surface of activated carbon. The SO2 in the adsorption state is oxidized to SO3 in the gas phase O2. The SO3 can be combined with adsorption H2O which is in the same active site into H2SO4. The H2SO4 can be away from the active site by the rest of H2O, and store in the micropore of activated carbon. The active site is free, and can continue to adsorption. So the higher the volume fraction of O2, H2O and SO2 was, the higher the desulfurization efficiency was, the higher the SO2 adsorption per unit quality activated carbon was.Based on the experimental research and the Computational Fluid Dynamics (CFD), SO2 adsorption process by activated carbon was calculated, and a contrast experimental research was made on the pilot test system. The conclusion of numerical simulation was in conformity with the conclusion of drop tube furnace. The simulation value was close to the pilot test value. These all showed that numerical simulation could simulate adsorption process of SO2 by powder activated carbon in fluidized bed well, which provided the basis for the test and field application. the simulation results showed that with the increase of flue gas flow rate, the detention time was shortened, the desulfurization efficiency became lower. But, with the increase of flue gas velocity, the gas turbulence increased, the mass transfer resistance was reduced, so the desulfurization efficiency was decreased slower. At the same time, the pilot experimental results showed that when activated carbon equivalent ratio was 10.8, SO2 concentration was 22.88 mg/m3 in the outlet of desulfurization tower, which had achieved the level of super clean emissions. New activated carbon had high SO2 adsorption rate, the desulfurization efficiency of new activated carbon was 69.4%, but the desulfurization efficiency of cyclic activated carbon was only 29.4%. Due to the competitive adsorption of activated carbon, the desulfurization efficiency of new activated carbon and cyclic activated carbon was lower than the sum of them. Micropores are the main area of SO2 adsorption and transformation. After adsorption, the micropore volume was reduced to 46.3%, the adsorption capacity became very low.