温度和氧含量对NH4Br改性稻壳焦汞氧化吸附特性的影响

采用10 g·L^-1 NH₄Br溶液对原始稻壳焦进行浸渍改性制备了脱汞吸附剂。利用比表面积及孔隙度分析仪、扫描电子显微镜/X射线能谱分析仪对改性前后稻壳焦吸附剂物理化学性质进行表征。在固定床汞吸附实验台上对吸附剂床层出口元素汞(Hg⁰)和二价汞(Hg²⁺)进行同步检测,研究了不同温度下N₂气氛和N₂+6.4 %(体积)O₂气氛中改性稻壳焦的汞氧化吸附特性。50℃下N₂气氛、N₂+6.4%(体积)O₂气氛以及150℃下N₂气氛中固定床出口均未明显检测到Hg²⁺,50℃和150℃时脱汞率均可达90%。温度为150℃时,在N₂+6.4%(体积)O₂气氛中吸附剂的总汞(Hg^T)脱除效率为98.2%,其中82.2%的Hg⁰被吸附脱除,其余则被O₂非均相氧化为Hg²⁺而氧化脱除。通过对程序升温管式炉出口Hg^T进行检测研究了吸附剂表面吸附态汞的脱附特性。汞脱附峰值温度300℃表明吸附态汞可能以Hg-Br化合物的形式赋存于改性稻壳焦表面,220~350℃的脱附温度说明吸附态汞有较高的热稳定性。100.14%~118.62%的汞平衡率验证了实验结果的准确性。

Effects of temperature and oxygen content on Hg0 oxidation and adsorption by rice husk char impregnated with NH4Br

The mercury sorbent was prepared from raw rice husk char impregnated with 10 g·L^-1 NH₄Br solution. The physical-chemical characteristics of sorbents before and after modification were analyzed by surface area and porosity analyzer, scanning electron microscopy and X-ray energy dispersive spectroscope. The mercury oxidation and adsorption performance of the modified sorbents were investigated by analyzing mercury speciation (Hg⁰ and Hg²⁺) in the outlet of a fixed bed reactor system in N₂ and N₂+6.4%(vol)O₂ atmosphere at different temperatures. No obvious Hg²⁺ was detected in N₂, N₂+6.4%(vol)O₂ atmosphere at 50℃ and N₂ atmosphere at 150℃. Modified rice husk char exhibited a mercury removal efficiency of 90% at 50℃ and 150℃. Total mercury (Hg^T) removal efficiency of 98.2% was obtained in N₂+6.4%(vol)O₂ atmosphere at 150℃, 82.2% Hg⁰ was bound to the sorbent surface, the remaining was removed in the form of Hg²⁺ which was generated from heterogeneous oxidation by O₂ at the surface of sorbent. The heterogeneous reaction between O₂ and gaseous Hg⁰ at relatively elevated adsorption temperature promotes the Hg⁰ oxidation efficiency of modified sorbents. Desorption performance of adsorbed mercury was identified by analyzing Hg^T at the outlet of temperature programmed tube furnace. Peak mercury desorption temperature of 300℃ manifested that mercury was bound to sorbent surface in the form of Hg-Br compound most likely. Mercury desorption temperature between 220℃ and 350℃ suggested high thermal stability of adsorbed mercury. Mercury balance ratio of 100.14%-118.62% verified the accuracy of experimental results in this study.