Study of the Mercury Sorption Mechanism on Activated Carbon in Coal Combustion Flue Gas by the Temperature-Programmed Decomposition Desorption Technique

摘要

Effects of coexistent gases (HCl, SO_2, O_2, CO_2, and H_2O) in simulated coal combustion flue gas on mercury removal by a commercial activated carbon (coconut shell AC) were investigated in a laboratory-scale fixed-bed reactor at 80 ℃. To clarify the contribution of the Deacon reaction 2HCl + 1/2O_2 = C1_2 + H_2O (1) on the mercury sorption mechanisms, the experiments were also conducted in the presence of Cl_2 (in the absence of HC1). The characteristics (thermal stability) of the mercury species formed on the AC under the various sorption conditions were investigated by the temperature-programmed decomposition desorption (TPDD) technique. It was found that O_2 promoted mercury removal in the presence of SO_2; however, SO_2 suppressed mercury removal irrespective of the presence of O2. The promotion of mercury removal by the presence of O_2 may result from the Deacon reaction. However, SO_2 seemed to inhibit the Deacon reaction. It is thought that mercury species formed on AC through the Deacon reaction was HgCl_x (including HgCl_2), which decomposed and desorbed at around 300 ℃. This was supported by confirmation of the presence of HgCl_2 vapor in the reactor effluent gas in TPDD experiments. From the comparison of mercury removal in the gas containing 1 ppmv HC1 and gas containing 0.5 ppmv Cl_2 system in the presence of SO_2,O_2, CO_2 and H_2O, it was suggested that Cl_2 enhances mercury removal more efficiently than HC1. Therefore, efficient removal of mercury with AC is possible to achieve via promoting the Deacon reaction. The high-temperature TPDD peaks were observed at around 500 ℃ in TPDD spectra of the spent sorbents used in mercury removal in the presence of Cl_2 (or high concentrations of HC1), SO_2, O_2, CO_2, and H_2O. This TPDD peak temperature range is very close to the decomposition temperature of HgSO_4. We suggest that the high-temperature mercury desorption peaks are related to the decomposition of mercury species similar to mercury sulfate containing chlorine (HgS_xO_yCl_z) on AC.