2. 中国科学院大学, 北京 100049;
3. 天津市生态环境监测中心, 天津 300191
2. University of Chinese Academy of Sciences, Beijing 100049, China;
3. Tianjin Eco-Environmental Monitoring Center, Tianjin 300191, China
焦炉煤气是炼焦过程副产品, 作为燃料燃烧时产生的废气中通常含有较高浓度的氮氧化物(NOx), 同时也会包含多氯代二苯并二
V2O5-WO3/TiO2是使用最多的一种SCR催化剂[14~17], 在欧洲和日本, 商业化的V2O5-WO3/TiO2催化剂已经被应用到部分城市生活垃圾焚烧处理系统, 通常设置在余热锅炉后或布袋除尘器后, 对焚烧废气中的二
自制烟道气等动力采样器(中国科学院大连化学物理研究所), 符合国家标准HJ 77.2-2008[21]; R-205型旋转蒸发仪购于瑞士Buchi公司, Waters AutoSpec Premier高分辨气相色谱-高分辨质谱联用仪(HRGC/HRMS)购于美国Waters公司. XAD-2树脂和碱性氧化铝分别购于美国Supeleo公司和美国MP Biomedicals公司; 农残级正己烷和二氯甲烷购于美国J&T Baker公司; 硅胶(75~150 μm, 德国Merck公司), 经650℃活化4 h; 无水硫酸钠(分析纯, 天津大茂), 正己烷超声清洗后晾干.二
在国内某一大型焦化厂焦炉煤气燃烧系统的SCR装置入口和出口各采集3个废气样品, 同时采集2个SCR装置降尘样品.该燃烧系统的废气量为17 000 m3·h-1. SCR催化剂为V2O5-WO3/TiO2蜂窝状催化剂脱硝模块, 通入氨水作为还原剂, 运行温度为280~350℃.废气采样方法遵照国家标准HJ 77.2-2008[21], 采用等动力模式采样, 采样体积大于1.2 m3·h-1, 采样时间大于1 h.在废气采样前后, 分别采取SCR装置的降尘, 混匀后装在密封袋中, 带回实验室后研磨过60目筛, 待分析.
1.3 样品前处理废气和降尘样品经冷冻干燥后加入13C同位素标记的UP-POPs提取内标, 采用250 mL正己烷和二氯甲烷(1:3)混合溶液进行索氏提取, 提取时间为18 h; 样品提取液经旋转蒸发浓缩后过多层复合硅胶柱, 采用正己烷洗脱, 洗脱液经旋转蒸发浓缩后过氧化铝柱, 采用不同配比的正己烷和二氯甲烷进行洗脱, 分别收集含有二
采用同位素稀释-高分辨气相色谱-高分辨质谱联用技术(HRGC/HRMS)分别分析样品中的二
对所有烟道气和降尘样品进行加标回收率测定, 并进行程序空白分析, 所有测定样品中添加的21种13C标记二
SCR装置对焚烧废气中典型UP-POPs具有很好的脱除效果.如图 1所示, 废气中全部多氯代二苯并二
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图 1 SCR装置进出口废气中不同种类UP-POPs的浓度 Fig. 1 Concentrations of different categories of UP-POPs in the flue gases at the inlet and outlet of the SCR equipment |
废气中全部多氯联苯异构体(ΣPCBs)和全部多氯萘异构体(ΣPCNs)的总浓度在SCR入口处分别为216.7 ng·m-3和552.6 ng·m-3, 在出口处分别为54.7 ng·m-3和119.5 ng·m-3(图 1). SCR装置对ΣPCBs和ΣPCNs的脱除率分别为74.7%和78.4%, 明显低于对ΣPCDD/Fs的脱除率. SCR装置对各类污染物脱除效率的不同显著改变了入口和出口废气中不同类别UP-POPs的相对含量(图 1).在SCR装置入口废气中不同类别UP-POPs相对含量由高到低的顺序为:ΣPCDFs(43.8%)>ΣPCNs(35.3%)>ΣPCBs (13.9%)>ΣPCDDs(7.0%), 而在出口废气中以ΣPCNs(55.0%)>ΣPCBs(25.2%)>ΣPCDFs(17.4%)>ΣPCDDs(2.4%)顺序递减.流经SCR装置后, 废气中ΣPCDD/Fs的相对含量显著降低, 而ΣPCBs和ΣPCNs的相对含量显著增加.
2.2 SCR装置进出口废气中UP-POPs的同系物分布特征为洞悉流经SCR装置前后废气中不同UP-POPs同系物(具有相同分子量的异构体的总和)分布的差别与联系, 对浓度总和归一化处理后的二
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图 2 SCR装置进出口废气中UP-POPs同系物的正交偏最小二乘法判别分析(OPLS-DA)载荷 Fig. 2 OPLS-DA loading plot of the UP-POPs homologue in SCR inlet and outlet flue gases |
从不同UP-POPs同系物浓度分布数据可见, 在SCR装置入口废气中高氯取代UP-POPs同系物的含量较高, 这些高氯取代UP-POPs同系物包括三至八氯代二苯并二
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图 3 SCR装置进出口废气中UP-POPs同系物的分布特征 Fig. 3 Homologue profiles of UP-POPs in SCR inlet and outlet flue gases |
废气流经SCR装置蜂窝状催化剂模块后, 部分废气中的颗粒物会被SCR装置截获, 形成降尘; 部分废气中的UP-POPs会随着降尘的分流得以从废气中脱除.从图 4可见, 沉降灰中ΣPCDDs、ΣPCDFs、ΣPCBs和ΣPCNs的含量分别为1.8、4.2、14.8和4.1 ng·g-1, 以ΣPCBs(59.4%)>ΣPCDFs(16.9%)>ΣPCNs(16.4%)>ΣPCDDs(7.3%)的顺序呈递减趋势.降尘中二
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图 4 SCR装置降尘中UP-POPs的含量和同系物分布特征 Fig. 4 Contents and homologue profiles of UP-POPs in SCR dustfall |
在SCR装置降尘中, 五氯和六氯代二苯并二
以上结果表明, SCR装置对焦炉煤气燃烧废气中的二
如果仅发生催化还原反应, SCR装置出口废气中UP-POPs的同系物浓度分布应该表现出随氯取代数减少而升高的趋势, 或者会有较高的非氯取代母体化合物生成.在本研究中, SCR装置出口废气中一氯取代UP-POPs同系物浓度低于二氯取代UP-POPs, 且二者均低于三氯取代UP-POPs.这意味着, 通过催化还原反应生成的低氯取代UP-POPs也在不断地从废气中脱除.为进一步了解低氯取代UP-POPs的脱除机制, 本研究分析了废气中二
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图 5 SCR装置进出口废气和降尘中DD和DF的浓度和含量 Fig. 5 Concentrations of DD and DF in SCR inlet and outlet flue gas and dustfall |
SCR装置降尘的分流也应该对废气中UP-POPs的脱除有所贡献.根据废气中的颗粒物浓度、SCR装置对颗粒物的截获率及实际观察结果, 笔者估算SCR装置降尘产生速率约为1.5 kg·h-1.根据废气和降尘中UP-POPs的浓度数据, SCR装置降尘分流对废气中二
在氨气还原条件下, SCR装置对焦炉煤气燃烧废气中的典型UP-POPs实现了很好的协同脱除效果, 对二
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