馬 咸 吳建會 張裕芬 孫英明 梁丹妮 馮銀廠

(南開大學(xué)環(huán)境科學(xué)與工程學(xué)院，國家環(huán)境保護(hù)城市空氣顆粒物污染防治重點(diǎn)實驗室，天津 300071)

循環(huán)流化床鍋爐(CFB)廣泛應(yīng)用于發(fā)電、供暖和工業(yè)生產(chǎn)中[1]，是大氣顆粒物的重要排放源。近年來，大量CFB加裝了脫硫裝置以控制SO2的排放[2]。然而，脫硫裝置在降低顆粒物和SO2排放的同時，會改變顆粒物構(gòu)成和模態(tài)分布，進(jìn)而影響其消光作用[3-5]。顆粒物中的硫酸鹽和硝酸鹽等水溶性物質(zhì)是使可見光散射的主要成分[6]。但國內(nèi)外對煙氣中顆粒物的水溶性離子特征特別是模態(tài)分布研究仍相當(dāng)有限。本研究利用靜電低壓撞擊器(ELPI+)采集CFB中煙氣的顆粒物樣品，并分析其水溶性離子組成及模態(tài)粒徑分布。

1研究方法

1.1 樣品采樣與分析

選擇3臺CFB(分別為CFB(a)、CFB(b)、CFB(c))進(jìn)行煙氣中的顆粒物樣品采集。3臺CFB均采用非選擇催化還原(SNCR)脫硝技術(shù)，未安裝除霧器，其他基本情況見表1。CFB(a)和CFB(c)燃煤摻雜20%(質(zhì)量分?jǐn)?shù)，下同)石灰石，CFB(b)燃煤摻雜20%～30%污泥。

利用顆粒物采樣系統(tǒng)在每臺CFB的煙道中采集煙氣中粒徑為0.006～10.000 μm的顆粒物樣品，其中粒徑為0.006～0.040 μm的顆粒物未檢出。圖1為顆粒物采樣系統(tǒng)示意圖。根據(jù)3012H型煙氣分析儀測得的煙氣流速、煙氣溫度等參數(shù)選取等速采樣頭[7-8]。ELPI+可裝載不同粒徑段的鋁膜或Teflon膜采集顆粒物樣品。煙氣經(jīng)過旋風(fēng)除塵后進(jìn)入稀釋通道，利用經(jīng)過過濾、加熱的潔凈空氣按煙氣與空氣體積比為1∶8稀釋煙氣。稀釋通道可在一定程度上反映污染源排放的真實情況[9]。加熱器對稀釋通道和旋風(fēng)除塵管路進(jìn)行加熱，以避免煙氣溫度在管路中迅速下降而使煙氣中的水蒸氣凝結(jié)，從而影響顆粒物樣品的采集。單次采樣時間為6～8 h，每次采集不同粒徑段的膜樣品，共計84組樣品。顆粒物濃度均使用基準(zhǔn)含氧量6%(體積分?jǐn)?shù))進(jìn)行轉(zhuǎn)化[10]。測得的標(biāo)準(zhǔn)化質(zhì)量濃度(c’，mg/m3)通過式(1)可轉(zhuǎn)換得到顆粒物質(zhì)量濃度。

表1 CFB基本情況

注：1)均為質(zhì)量分?jǐn)?shù)。

c’=dc/d(lgD)

(1)

式中：c為顆粒物質(zhì)量濃度，mg/m3；D為顆粒物粒徑，μm。

圖1 顆粒物采樣系統(tǒng)示意圖Fig.1 The sampling system for particulate matter

1.2 質(zhì)量控制

采樣前Teflon膜和鋁膜均進(jìn)行60 ℃恒溫干燥2 h，以去除表面的有機(jī)雜質(zhì)和水分。采樣膜稱量前均置于恒溫((20.0±2.0) ℃)、恒濕(50%±5%)的室內(nèi)平衡72 h至恒量。

2 結(jié)果與討論

2.1 煙氣中不同粒徑顆粒物的濃度分布

3臺CFB煙道出口顆粒物標(biāo)準(zhǔn)化質(zhì)量濃度粒徑分布譜如圖2所示。PM10、PM2.5和PM1可通過圖2積分得到。由于CFB(a)和CFB(c)的除塵方式和脫硫方式相同，因此這兩臺CFB的顆粒物濃度粒徑分布譜基本相似。CFB(a)和CFB(c)的PM10質(zhì)量濃度分別為29.71、23.40 mg/m3，PM2.5質(zhì)量濃度分別為4.32、1.78 mg/m3，PM1質(zhì)量濃度分別為0.05、0.04 mg/m3。CFB(b)的PM10、PM2.5和PM1質(zhì)量濃度分別為3.38、2.17、0.68 mg/m3。CFB(a)和CFB(c)的PM2.5質(zhì)量濃度分別占各自PM10的14.54%、7.61%，CFB(b)的PM2.5質(zhì)量濃度占PM10的64.20%。這可能是由于CFB(b)的袋式除塵器比靜電除塵器對大粒徑顆粒物有更高的除塵效率[12-13]。

圖2 顆粒物標(biāo)準(zhǔn)化質(zhì)量濃度粒徑分布譜Fig.2 Size distribution of particulate matter standard mass concentration

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

2.2 顆粒物中的水溶性離子分析

顆粒物中水溶性離子的質(zhì)量分?jǐn)?shù)如表2所示。3臺CFB的PM10中水溶性總離子質(zhì)量分?jǐn)?shù)基本相同。CFB(b)的PM2.5中水溶性總離子質(zhì)量分?jǐn)?shù)為60.07%，較CFB(a)和CFB(c)中分別高2.86、3.67百分點(diǎn)。CFB(b)的PM1中水溶性總離子質(zhì)量分?jǐn)?shù)為61.33%，與SIPPULA等[14]2974-2982的研究結(jié)果相近，高于CFB(a)和CFB(c)，可能是由于煙氣經(jīng)雙堿濕法技術(shù)脫硫裝置后煙氣溫度較低、濕度較高，易凝結(jié)成超細(xì)顆粒物[15-18]。

圖3 CFB(a)中水溶性離子的模態(tài)粒徑分布Fig.3 Mode distribution of water-soluble ions in CFB(a)

圖4 CFB(b)中水溶性離子的模態(tài)粒徑分布Fig.4 Mode distribution of water-soluble ions in CFB(b)

圖5 CFB(c)中水溶性離子的模態(tài)粒徑分布Fig.5 Mode distribution of water-soluble ions in CFB(c)

3 結(jié) 論

(1) CFB(a)、CFB(b)、CFB(c)的PM10質(zhì)量濃度分別為29.71、3.38、23.40 mg/m3，PM2.5質(zhì)量濃度分別為4.32、2.17、1.78 mg/m3，PM1質(zhì)量濃度分別為0.05、0.68、0.04 mg/m3。

(2) 3臺CFB的PM10中水溶性總離子質(zhì)量分?jǐn)?shù)基本相同。CFB(b)的PM2.5和PM1中水溶性總離子質(zhì)量分?jǐn)?shù)高于CFB(a)和CFB(c)。

(4) 3臺CFB的顆粒物中水溶性離子大都呈現(xiàn)雙模態(tài)粒徑分布，包括超細(xì)粒子模態(tài)和粗粒子模態(tài)。

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