循环流化床锅炉排放颗粒物中水溶性离子研究*

马 咸 吴建会 张裕芬 孙英明 梁丹妮 冯银厂

(南开大学环境科学与工程学院，国家环境保护城市空气颗粒物污染防治重点实验室，天津 300071)

循环流化床锅炉(CFB)广泛应用于发电、供暖和工业生产中[1]，是大气颗粒物的重要排放源。近年来，大量CFB加装了脱硫装置以控制SO2的排放[2]。然而，脱硫装置在降低颗粒物和SO2排放的同时，会改变颗粒物构成和模态分布，进而影响其消光作用[3-5]。颗粒物中的硫酸盐和硝酸盐等水溶性物质是使可见光散射的主要成分[6]。但国内外对烟气中颗粒物的水溶性离子特征特别是模态分布研究仍相当有限。本研究利用静电低压撞击器(ELPI+)采集CFB中烟气的颗粒物样品，并分析其水溶性离子组成及模态粒径分布。

1研究方法

1.1 样品采样与分析

选择3台CFB(分别为CFB(a)、CFB(b)、CFB(c))进行烟气中的颗粒物样品采集。3台CFB均采用非选择催化还原(SNCR)脱硝技术，未安装除雾器，其他基本情况见表1。CFB(a)和CFB(c)燃煤掺杂20%(质量分数，下同)石灰石，CFB(b)燃煤掺杂20%～30%污泥。

利用颗粒物采样系统在每台CFB的烟道中采集烟气中粒径为0.006～10.000 μm的颗粒物样品，其中粒径为0.006～0.040 μm的颗粒物未检出。图1为颗粒物采样系统示意图。根据3012H型烟气分析仪测得的烟气流速、烟气温度等参数选取等速采样头[7-8]。ELPI+可装载不同粒径段的铝膜或Teflon膜采集颗粒物样品。烟气经过旋风除尘后进入稀释通道，利用经过过滤、加热的洁净空气按烟气与空气体积比为1∶8稀释烟气。稀释通道可在一定程度上反映污染源排放的真实情况[9]。加热器对稀释通道和旋风除尘管路进行加热，以避免烟气温度在管路中迅速下降而使烟气中的水蒸气凝结，从而影响颗粒物样品的采集。单次采样时间为6～8 h，每次采集不同粒径段的膜样品，共计84组样品。颗粒物浓度均使用基准含氧量6%(体积分数)进行转化[10]。测得的标准化质量浓度(c’，mg/m3)通过式(1)可转换得到颗粒物质量浓度。

表1 CFB基本情况

注：1)均为质量分数。

c’=dc/d(lgD)

(1)

式中：c为颗粒物质量浓度，mg/m3；D为颗粒物粒径，μm。

图1 颗粒物采样系统示意图Fig.1 The sampling system for particulate matter

1.2 质量控制

采样前Teflon膜和铝膜均进行60 ℃恒温干燥2 h，以去除表面的有机杂质和水分。采样膜称量前均置于恒温((20.0±2.0) ℃)、恒湿(50%±5%)的室内平衡72 h至恒量。

2 结果与讨论

2.1 烟气中不同粒径颗粒物的浓度分布

3台CFB烟道出口颗粒物标准化质量浓度粒径分布谱如图2所示。PM10、PM2.5和PM1可通过图2积分得到。由于CFB(a)和CFB(c)的除尘方式和脱硫方式相同，因此这两台CFB的颗粒物浓度粒径分布谱基本相似。CFB(a)和CFB(c)的PM10质量浓度分别为29.71、23.40 mg/m3，PM2.5质量浓度分别为4.32、1.78 mg/m3，PM1质量浓度分别为0.05、0.04 mg/m3。CFB(b)的PM10、PM2.5和PM1质量浓度分别为3.38、2.17、0.68 mg/m3。CFB(a)和CFB(c)的PM2.5质量浓度分别占各自PM10的14.54%、7.61%，CFB(b)的PM2.5质量浓度占PM10的64.20%。这可能是由于CFB(b)的袋式除尘器比静电除尘器对大粒径颗粒物有更高的除尘效率[12-13]。

图2 颗粒物标准化质量浓度粒径分布谱Fig.2 Size distribution of particulate matter standard mass concentration

Table 2 Mass percentage of water-soluble ions in particulate matter %

2.2 颗粒物中的水溶性离子分析

颗粒物中水溶性离子的质量分数如表2所示。3台CFB的PM10中水溶性总离子质量分数基本相同。CFB(b)的PM2.5中水溶性总离子质量分数为60.07%，较CFB(a)和CFB(c)中分别高2.86、3.67百分点。CFB(b)的PM1中水溶性总离子质量分数为61.33%，与SIPPULA等[14]2974-2982的研究结果相近，高于CFB(a)和CFB(c)，可能是由于烟气经双碱湿法技术脱硫装置后烟气温度较低、湿度较高，易凝结成超细颗粒物[15-18]。

图3 CFB(a)中水溶性离子的模态粒径分布Fig.3 Mode distribution of water-soluble ions in CFB(a)

图4 CFB(b)中水溶性离子的模态粒径分布Fig.4 Mode distribution of water-soluble ions in CFB(b)

图5 CFB(c)中水溶性离子的模态粒径分布Fig.5 Mode distribution of water-soluble ions in CFB(c)

3 结 论

(1) CFB(a)、CFB(b)、CFB(c)的PM10质量浓度分别为29.71、3.38、23.40 mg/m3，PM2.5质量浓度分别为4.32、2.17、1.78 mg/m3，PM1质量浓度分别为0.05、0.68、0.04 mg/m3。

(2) 3台CFB的PM10中水溶性总离子质量分数基本相同。CFB(b)的PM2.5和PM1中水溶性总离子质量分数高于CFB(a)和CFB(c)。

(4) 3台CFB的颗粒物中水溶性离子大都呈现双模态粒径分布，包括超细粒子模态和粗粒子模态。

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