Experimental Study On Simultaneous Removal Of Trace Selenium And Arsenic In Flue Gas Desulphurization Within Medium Temperature Range

This study concentrates on simultaneous removal of trace selenium and trace arsenic in the process of medium temperature dry flue gas desulfurization (MTD-FGD). The followings have been studied through bench-scale experiments performed in a thermogravimetric analyzer: reaction mechanism of CaO and SeO₂; reaction mechanism of CaO and As₂O₃; effects of CaSO4 product layer, SO₂ and CO₂ on the capture of trace selenium and trace arsenic; kinetics of the reactions of SO₂-CaO, As₂O₃-CaO and SeO₂-CaO. The removal efficiencies of trace selenium and trace arsenic in the process of MTD-FGD have been predicted through theoretical analysis.The mechanism of reaction between CaO and SeO₂ has been studied via Fourier Transform Infrared (FTIR). The results have shown that the product of the reaction of CaO and SeO₂ is CaSeO₃. The selenite reaction rate increases with increasing temperture in the range of 300-700 oC and decreases with increasing temperture in the range of 700-800 oC. The reaction mechanism of CaO and As₂O₃ has been studied by X-ray Diffusion (XRD). The results have shown that the product of the reaction of CaO and As₂O₃ is Ca3(AsO4)2. The arsenate reation rate increases with increasing temperture over the range of 400-1000 oC.As a single factor, the CaSO4 product layer does not affect the capture of trace selenium and trace arsenic because this reaction step is controlled by the outward transfer of Ca2+ and O₂- through the product layer.In the process of simultaneous removal of SO₂ and trace elements, the presence of SO₂ does not affect the capture of trace selenium and trace arsenic when the sulfate reaction is kinetic contolled. When the sulfate reaction is in the product layer diffusion controlled stage, the diffusion resistance and the competition of SO₂ jointly reduce the selenium capture. However, the capture of trace arsenic is not reduced in this stage, which may due to the fast arsenate reaction rate and the ability of CaSO4 to absorb As₂O₃.As for the effect of CO₂, it has been found that the coexisting CO₂ with the concentration of 15% can reduce the capture of trace selenium by 12-35% in the range of 700-760 oC. However, the presence of CO₂ does not reduce the trace arsenic capture in the range of 600-1000 oC. It has also been found that SO₂, SeO₂ and As₂O₃ can be absorbed at 700 oC through the way of substituting the absorbed CO₂ and then reacting with CaO.Based on the experimental results and kinetics studies, the models to describe the removal efficiencies of sulfur, trace selenium and trace arsenic have been founded. The removal efficiencies of trace selenium and trace arsenic in the process of MTD-FGD have been predicted as follows: In the range of 700-800oC, sulfur, trace selenium and trace arsenic can be simultaneously removaled by CaO in a circulating-fluidized-bed reactor. The removal efficiency of selenium may reduce gradually with increasing temperature in this range, however, the temperature range of 700-850oC is very suitable for highly effective simultaneous removal of sulfur and trace arsenic.