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颗粒物是主要的大气环境污染物[1-2],对全球气候和环境变化有重要影响[3]。国内城市大气颗粒物污染来源解析表明[4-5],城市环境大气颗粒物的主要来源包括燃煤源、工业源、机动车尾气及扬尘。在不同的城市,由于能源供给、产业结构的差异,不同污染源对颗粒物浓度的贡献占比不同。其中,燃煤工业锅炉、燃煤发电锅炉等燃煤源是大气细颗粒物的主要排放源。中国是世界上最大的煤炭消费国[6],应重点关注燃煤锅炉排放颗粒物的排放控制。
燃煤锅炉排放的总颗粒物 (total particulate matter, TPM) 主要分为可过滤颗粒物 (filterable particulate matter, FPM) 、可凝结颗粒物 (condensable particulate matter, CPM) 两大类。FPM是以固态或液态存在,在常温状态下可被滤膜 (或滤筒) 过滤截留的颗粒物[7]。美国环保署 (USEPA) 对可凝结颗粒物定义为:该物质在初始烟道温度状态下为气态,离开烟道后在正常环境状态下降温数秒后凝结成液态或者固态[8]。CPM中组分特征及其在湿法脱硫系统 (wet flue gas desulfurization, WFGD) 等相关净化设备中的转化是目前该领域研究热点。裴冰[9]、CORIO等[10]发现燃煤电厂CPM、FPM二者比例接近1∶1;CPM在燃煤锅炉、燃气锅炉PM10中的占比分别约为76%、50%。学者对国内燃煤锅炉烟气排放水溶性离子特征研究发现,CPM水溶性离子中阳离子主要为Na+、Ca2+,阴离子主要为F−、Cl−、NO3−、SO42−;CPM中有机物占比50.90%~93.17%,其中烃类主要为正构烷烃,脂类主要为邻苯二甲酸酯[11-13];CPM所包含的元素中Na、Ca、K、Mg占所测金属总含量95%以上[12,14]。杨柳等[15]对超低排放下燃煤烟气排放特征研究表明:湿式电除尘装置 (wet electrostatic precipitator,WESP) 对CPM的脱除效果优于WFGD对CPM的脱除效果;WFGD+WESP对CPM的脱除效率达78.90%;WFGD+WESP对CPM有机组分的脱除效率高于对CPM无机组分的脱除效率。
目前,我国针对CPM的监测较为匮乏,故对此类颗粒物的防控研究尚存在诸多不足,无法满足当前大气环境改善的需求。本研究以燃煤烟气中CPM颗粒物为对象,分析其主要金属元素及6种水溶性离子的质量浓度和种类,以及湿法脱硫系统对可过滤颗粒物及可凝结颗粒物的转化作用,以期梳理供热厂可凝结颗粒物的源头排放控制策略,并为有效防控燃煤颗粒污染提供参考。
供热厂燃煤烟气可凝结颗粒物排放特征及其在湿法脱硫系统中的转化
Emission characteristics of condensable particulate matter from coal-fired flue gas of heat supply plant and its transformation in WFGD
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摘要: 燃煤源是城市环境大气颗粒物的重要来源,燃煤产生的颗粒物由可过滤颗粒物 (FPM) 与可凝结颗粒物 (CPM) 两部分组成,且CPM在排放的总颗粒物中占比较高。采用EPA Method 202方法,对燃煤供热厂湿法脱硫系统 (WFGD) 进口及出口烟气进行平行取样,并对燃煤烟气中CPM质量浓度、成分组分、金属元素、水溶性离子及WFGD对CPM的脱除作用进行研究。结果表明,WFGD进口与出口处CPM在TPM中均占有主导地位,且CPM中以CPM无机组分为主。CPM中含量较高的10种金属元素为Na、Ca、Al、K、Fe、Zn、Mn、Ba、Cr、Ti,其中Na、Ca、Al三种金属元素之和占总金属元素的80%以上。燃煤烟气CPM中水溶性离子以SO42−为主,其中WFGD进口处SO42−占CPM总水溶性离子的59.49%,WFGD出口处SO42−占CPM总水溶性离子的81.07%。通过对WFGD进口与出口处颗粒物浓度对比发现WFGD对CPM的脱除效率为69.29%,对CPM有机组分脱除效率为负,对CPM无机组分的脱除效率为77.27%。WFGD对10种金属元素的脱除效率为55.52%,WFGD对总水溶性离子的脱除效率为78.74%。本研究结果可为供热厂的可凝结颗粒物排放源头控制提供参考。Abstract: Coal burning source is an important source of atmospheric particulate matter in urban environment. The particulate matter produced by coal burning is composed of filterable particulate matter (FPM)and condensable particulate matter (CPM),and CPM accounts for a high proportion of the total particulate matter emitted. There is a lack of research on the emission and detection of CPM in China. In this study, EPA method 202 was used to sample the inlet and outlet flue gas of WFGD in coal-fired heating plant in parallel, and the CPM mass concentration, composition, metal elements, water-soluble ions and the removal effect of WFGD on CPM in coal-fired flue gas were studied. The results show that CPM at the inlet and outlet of WFGD plays a dominant role in TPM, and CPM inorganic component is the main component in CPM. The 10 metal elements with high content in CPM were Na, Ca, Al, K, Fe, Zn, Mn, Ba, Cr and Ti, and the sum of Na, Ca and Al accounted for more than 80% of the total metal elements. The water-soluble ions in CPM of coal-fired flue gas are mainly SO42−, of which SO42− at WFGD inlet accounts for 59.49% of the total water-soluble ions in CPM, and SO42− at WFGD outlet accounts for 81.07% of the total water-soluble ions in CPM. By comparing the particle concentration at the inlet and outlet of WFGD, it is found that the removal efficiency of WFGD for CPM is 69.29%, the removal efficiency of organic components of CPM is negative, and the removal efficiency of inorganic components of CPM is 77.27%.The removal efficiency of WFGD for 10 metal elements was 55.52%, and the removal efficiency of WFGD for total water-soluble ions was 78.74%.
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表 1 燃煤供热锅炉水溶性离子的数据
Table 1. Data of water-soluble ions in coal-fired heating boilers
水溶性
离子种类WFGD进口离子
质量浓度/(mg·m−3)WFGD进口离子
所占百分比WFGD出口离子
质量浓度/(mg·m−3)WFGD出口离子
所占百分比Na+ 0.50 2.72% 0.14 3.58% Ca2+ 0.59 3.21% 0.34 8.7% F− 4.16 22.59% 0.11 2.81% Cl− 2.12 11.53% 0.07 1.79% SO42- 10.94 59.49% 3.17 81.07% NO3− 0.08 0.45% 0.08 2.05% 表 2 WFGD对颗粒物的脱除效率
Table 2. WFGD removal efficiency of particulate matter
CPM有机组
分脱除率CPM无机组
分脱除率CPM
脱除率FPM
脱除率−85.79% 77.27% 69.29% −790.28% 表 3 WFGD对金属元素的脱除效率
Table 3. WFGD metal element removal efficiency
Na Ca Al Fe K Zn Mn Ba Cr Ti 总元素 52.26% 39.67% 68.11% 61.13% 54.81% 79.50% −36.72% 67.04% 7.04% 98.89% 55.52% 表 4 WFGD对水溶性离子的脱除效率
Table 4. WFGD to water-soluble ions removal efficiency
Na+ Ca2+ F− Cl− SO42− NO3− 总水溶性离子 72.00% 42.37% 97.36% 96.70% 71.02% 0.00% 78.74% -
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