酸-碘改性壳聚糖-膨润土脱除单质汞特性及机理分析

为探明烟气中酸性气体(SO2,NO,HCl)和H2O对酸-碘改性壳聚糖-膨润土吸附剂脱除单质汞(Hg0)的影响和脱除机理,在小型固定床试验台架上开展了Hg0脱除的实验研究.结果表明:由于SO2与Hg0在吸附剂表面活性位上存在竞争吸附,SO2抑制吸附剂对Hg0的脱除.H2O和较高浓度(1000μg/m3)的NO对脱汞具有明显地促进作用.由于低温下HCl与Hg0较高的反应能垒,HCl对脱汞没有影响.与单一烟气成分影响相比,在SO2、NO、HCl和H2O的联合作用下,吸附剂的长时间脱汞性能得到大幅提高.机理分析表明壳聚糖首先被酸中的H+质子化,之后再吸附酸与KI反应生成的I2,进而有效地吸附Hg0.量子化学计算表明壳聚糖在酸性条件下对I2具有良好的吸附特性,改性壳聚糖吸附剂对Hg0具有较好的吸附能力,其吸附能约为127kJ/mol.

Removal of gas-phase elemental mercury by acid-iodine modified chitosan-bentonite adsorbent

To inquire the impacts of acid gases (such as SO2, NO, and HCl), water vapor and mechanism on gas-phase elemental mercury removal by acid-iodine modified chitosan-bentonite adsorbents, the adsorption tests were conducted in a laboratory-scale fixed-bed reactor. The results showed that the presence of SO2 inhibited mercury capture due to the competition adsorption of SO2 and Hg0 on the active site of the adsorbent surface. An obvious promotion was observed when the concentration of NO added to 1000μg/m3and H2O was added. However, the addition of HCl had no favor to improve the efficiency of mercury removal due to higher energy barrier of the HCl and Hg0 reaction. Compared with single flue gas constituent, the long-term mercury capture efficiency was greatly enhanced because of the cooperative effects of SO2, NO, HCl and H2O. The mechanism for mercury removal by modified adsorbent was that in the acidic medium, the amine group of chitosan was protonated with a positive charge. KI reacted with H2SO4 and produced the I2. Then the produced I2 could be strongly adsorbed by protonated chitosan. Thus, Hg0 reacted with the active I2 of the adsorbent. The quantum chemical calculations indicated that chitosan exhibited strong adsorptive ability for elemental iodine under acidic conditions, and the modified adsorbents showed excellent mercury adsorption performance and its adsorption energy for Hg0 was approximately 127kJ/mol.