| In recent years, concerns about mercury emissions from coal combustion have risen greatly. Mercury removal from coal combustion flue gas contains two main methods. For the first method, mercury is adsorbed to adsorbents, and captured by particulate matter control devices (PMCDs). The second method for the removal of Hg0from coal combustion flue gas is catalytic oxidation of elemental mercury (Hg0) to oxidized mercury (Hg2+) over the selective catalytic reduction (SCR) process, which can be subsequently captured by PMCDs and/or wet flue gas desulfurization (WFGD) systems downstream. Using existing air pollution control devices (APCDs) such as SCR, PMCDs and WFGD to control mercury emission form coal combustion is believed to be the most economical efficiency method. Chlorine is the most important species affecting Hg0oxidation and capture in flue gas. Increasing chlorine content in coal or flue gas could promote mercury removal by APCDs. Wastewater generated from the wet FGD process is rich in chlorine. An economical efficient treatment named wastewater evaporation system (WES), that treat the wastewater by injecting wastewater to the flue gas. With the purpose of enhancing mercury removal, wastewater evaporation treatment is combined with mercury removal in the present study by sending WFGD wastewater to the flue gas.the WES process productions was studied. Hydrogen chlorine (HC1) and hydrogen fluoride (HF) was found in the gas when the wastewater was dried by the gas at350℃, while most chlorine crystallized as calcium chloride (CaCl2) in the solid phase productions. The capability of CaCl2to oxidize Hg0was investigated at350℃. Hg0oxidation over CaCl2was promoted with increasing temperature from100℃to400℃. The introduction of oxygen or water vapor to N2promoted the Hg0oxidation efficiency.46.9%oxidizing efficiency was resulted in wet O2contained gas at400℃. the oxidation was due to the hydrolysis process of CaCl2. Mercury oxidation capacity over the catalyst particles drying mixed with powder was studied. In simulated coal combustion flue gas at350℃, the calcium chloride addition dramatically enhanced the Hg0oxidation activity over the catalyst. By adding1.0wt%calcium chloride to the catalyst, the Hg0oxidation efficiency rose from7.7%to78.8%. With a temperature increase from100℃to400℃, the transformation of elemental mercury to oxidized mercury considerably increased, and mercury captured by the catalyst dropped. Both oxygen and sulfur dioxide exhibited promotional effects on Hg0oxidation, whereas water vapor inhibited it. No apparent difference was observed by varying the nitric oxide concentration. The positive function of calcium chloride addition on elemental mercury oxidation is due to two reasons:hydrogen chloride formed through the hydrolysis process of calcium chloride and active chlorine species generated from solid interaction between the catalyst and CaCl2. Experimental study showed that no significant effect of calcium chloride addition on NOx reduction and SO2-SO3conversion has been observed.The effect of wastewater injection on mercury adsorbed to coal combustion fly ash was also studied by adding chloride (KCl、CaCl2、NaCl、MgCl2、AlC13、FeCl3、 CuCl2、NH4Cl) to the coal combustion fly ash. Elemental mercury adsorption over coal combustion fly ash with chlorides addition were investigated on the fixed-bed system. The results indicates that CuCl2has the best positive effect which improves the adsorption capacity of coal combustion fly ash ZQ with2.9μg/g. CaCl2impregnated ZQ increased adsorption capacity with1.1μg/g. The effect of CaCl2was related to the LOI of fly ash, and chlorine active sites was formed on the surface of unburned carbon which promoted elemental mercury adsorption capacity. |
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