利用含硫活性碳捕捉燃煤電廠脫硫廢水中氧化態汞之研究

Abstract

由於世界各國對於火力發電的依賴性仍然相當高，燃煤電廠之汞排放議題一直以來備受大眾關注。在許多空污控制技術中，濕式煙氣脫硫系統(WFGD)是一個可有效去除汞並同時去除酸性氣體的設備。但因為水中含有還原性物質，例如亞硫酸根離子，最初被WFGD系統所捕捉的Hg2+會再還原並以元素汞的型態再釋放進入大氣中，導致WFGD脫汞效率的降低，造成二次汙染。 含硫活性碳具有高比表面積和表面官能基，對於吸附氣相或水相中的汞都有極高的去除效率。有鑑於此，本研究利用批次式實驗，經由實廠WFGD廢水中加入含硫粒狀活性碳進行實驗，測量液相中殘留汞含量，並控制溫度、pH、含硫活性碳劑量、初始汞濃度及SO32-濃度以觀察活性碳之吸附效果並同步探討等溫吸附及吸附動力模式。研究結果顯示，含硫活性碳比表面積為736.7 m2/ g，含硫量為4.6 wt%，其活性碳內微孔隙結構之比表面積較多，有利於活性碳吸附效率。不同pH值下，活性碳之吸附量隨pH上升而下降。而等溫吸附模擬結果顯示在比較低的汞濃度下實驗結果較符合線性等溫吸附模式。在氣態汞逸散連續監測結果發現，當添加SO32- = 5-100 mM時，氣態汞逸散有明顯上升的趨勢，此結果符合文獻所提HSO3-容易導致Hg0再還原。然而當SO32-濃度持續提升至200 mM及300 mM時，氣態汞的產生則有下降的趨勢，可能的原因為Hg(SO3)22-錯合物產生，導致Hg0再還原的情形降低。另外，動力吸附模式結果指出擬二階模式結果較符合實驗所得數據。而熱力學的參數計算後得到△H°= -19.14 kJ/mole、△S°= -0.037 kJ/mole、△G≒-30 kJ/mole，表示利用含硫活性碳吸附廢水中的汞是自發性和放熱性的吸附反應。

Because thermal power generation is still extensively used globally, mercury (Hg) emissions from coal-fired power plants have been of greatest concerns to public. Among the available technologies for avoiding flue gas emissions, Wet Flue Gas Desulfurization (WFGD) has received considerable attention due to its capability to remove SO2 and Hg simultaneously. Under certain circumstances, however, oxidized mercury (Hg2+) captured by the WFGD system might be reduced by the reducing compounds, such as sulfites, and reemitted to the atmosphere in the form of Hg0 that causes secondary pollution and results in the lower efficiency of Hg removal by WFGD. Activated carbon (AC) containing sulfur is highly effective in adsorption of gaseous and aqueous Hg pollutants because of its suitable physical and chemical properties. In this study, a series of designed batch experiments were conducted to obtain the optimal adsorption conditions for removing the aqueous Hg from WFGD wastewater by using a sulfur-containing activated carbon (SAC). The test variables included temperature, pH value, SAC dosage, initial Hg2+ concentration, and the SO32- concentrations of WFGD wastewater. The adsorption isotherms and kinetics were subsequently obtained and better understood by using theoretical and empirical simulation models. The total surface area and sulfur content of SAC was measured to be 736.7 m2/g and 4.6%, respectively. The high microporosity of SAC made the adsorbent suitable for the adsorption of Hg. The experimental results indicated that the Hg adsorption capacity of SAC decreased with increasing pH value. Furthermore, Hg adsorption capacity was better fitted with linear adsorption isotherm model, which is mainly due to the low Hg concentration range tested in this study. By measuring the gaseous Hg concentration, the reemission of gaseous Hg was found to ascend with increasing the SO32- concentration from 5 to 100 mM, which may be resulted from Hg0 formation from Hg2+ reduction due to the presence of HSO3-. However, the reemission of reduced Hg was markedly decreased as increasing SO32- addition from 100 to 300 mM, which may stem from the formation of Hg(SO3)22- stably present in aqueous phase. Kinetic simulation showed that the fitting by pseudo-second order equation possessed a higher R2 compared to that by pseudo-first order equation. Thermodynamic parameter calculation concluded that △H°= -19.14 kJ/mole, △S°= -0.037 kJ/mole, and △G≒-30 kJ/mole. These analytical results indicate that Hg adsorption by SAC is thermodynamically spontaneous and exothermic.