張 軍, 李存杰, 鄭成航, 翁衛(wèi)國, 朱松強,王丁振, 高 翔, 岑可法

(1.浙江大學(xué) 能源清潔利用國家重點實驗室,浙江 杭州 310027； 2. 浙江能源集團有限公司,浙江 杭州310006)

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篩板塔細(xì)顆粒物協(xié)同脫除特性實驗

張 軍1, 李存杰1, 鄭成航1, 翁衛(wèi)國1, 朱松強2,王丁振1, 高 翔1, 岑可法1

(1.浙江大學(xué) 能源清潔利用國家重點實驗室,浙江 杭州 310027； 2. 浙江能源集團有限公司,浙江 杭州310006)

針對常規(guī)噴淋空塔無法滿足顆粒物協(xié)同控制的難題,提出篩板塔強化傳質(zhì)實現(xiàn)協(xié)同脫除的方法.基于濕法煙氣脫硫中試試驗平臺,考察濕法煙氣脫硫關(guān)鍵工藝參數(shù),包括煙氣流速、漿液噴淋量、飛灰濃度、顆粒粒徑等對細(xì)顆粒物脫除效率的影響規(guī)律,并與噴淋空塔脫除特性進(jìn)行對比.結(jié)果表明,在實驗工況下,細(xì)顆粒物脫除效率大于90%,最高超過95%；脫除效率隨煙氣流速、顆粒物濃度及漿液噴淋量的增大而提高.顆粒物分級脫除效率曲線呈“V”形分布特性,在0.2～1.0 μm粒徑范圍內(nèi)脫除效率最低；在相同條件下,篩板塔細(xì)顆粒物脫除效果顯著優(yōu)于噴淋塔,在0.2～1.0 μm粒徑段的脫除效率與總脫除效率較噴淋塔分別提高11%和5%以上.

篩板塔;燃煤電廠;細(xì)顆粒物;協(xié)同控制;排放特性

近年來,大氣污染排放形勢日趨嚴(yán)峻,煤炭燃燒會產(chǎn)生大量的粉塵、SO2及NOx等污染物,會對人體健康及自然環(huán)境帶來巨大危害[1-2],是我國大氣污染的主要來源之一.目前,我國燃煤電站普遍在除塵系統(tǒng)后加裝濕法煙氣脫硫(wet flue gas desulfurization, WFGD)系統(tǒng),據(jù)統(tǒng)計,90%以上采用了石灰石-石膏濕法脫硫工藝,WFGD系統(tǒng)在高效脫硫的同時,由于脫硫漿液的洗滌作用,還能夠協(xié)同脫除一部分飛灰顆粒,但對PM2.5的捕集效率較低[3]；另外,因為脫硫漿液霧化夾帶、石膏晶體析出,以及形成了可凝結(jié)顆粒物[4-5],脫硫塔出口Ca、S元素含量大幅增加[6],也在一定程度上制約了WFGD對細(xì)顆粒物的控制效果.

國內(nèi)外學(xué)者針對濕法脫硫塔協(xié)同控制細(xì)顆粒物開展了大量研究.鮑靜靜等[7-8]發(fā)現(xiàn)應(yīng)用蒸汽相變技術(shù)后WFGD內(nèi)細(xì)顆粒物數(shù)濃度顯著降低.熊桂龍等[9]采用蒸汽相變和撞擊流耦合的方法,實驗研究了撞擊流相變室中應(yīng)用蒸汽相變促進(jìn)細(xì)顆粒脫除的效果.趙汶等[10]的試驗結(jié)果表明化學(xué)團聚技術(shù)可使細(xì)顆粒物平均粒徑增大約4倍,數(shù)濃度降低40%以上.Kim等[11]電噴射系統(tǒng)引入濕法噴淋洗滌器中,實驗發(fā)現(xiàn)系統(tǒng)產(chǎn)生的帶電液滴能夠有效地捕集氣體中的細(xì)顆粒.除此之外,陳海林等[12]研制了一種中試規(guī)模的螺旋型垂直篩板布?xì)獾膰娚涔呐菝摿虺龎m裝置,除塵效率可達(dá)91%以上.Wang等[13]通過理論分析與實驗研究了浮閥塔板的飛灰脫除特性.通過WFGD實現(xiàn)高效脫硫同時實現(xiàn)低成本的顆粒物控制具有重要意義,篩板塔具有成本低、傳質(zhì)效果強[14-16]等優(yōu)點,在濕法煙氣脫硫領(lǐng)域已有部分工程應(yīng)用,但針對不同工況下篩板塔細(xì)顆粒物脫除效率的變化規(guī)律,篩板對細(xì)顆粒物強化脫除特性,以及篩板塔對顆粒物的協(xié)同控制作用等亟待解決的課題目前尚無研究.

本文基于濕法煙氣脫硫中試試驗平臺,開展了篩板塔強化細(xì)顆粒物脫除特性的實驗研究.研究了濕法煙氣脫硫關(guān)鍵工藝參數(shù),包括煙氣流速、漿液噴淋量、飛灰濃度、顆粒粒徑等,對顆粒物脫除特性的影響,對脫除效率及分級脫除效率進(jìn)行分析,并與噴淋塔脫除特性對比,為進(jìn)一步降低WFGD系統(tǒng)細(xì)顆粒的排放提供技術(shù)支撐.

1 實驗系統(tǒng)及方法

1.1 實驗系統(tǒng)

圖1 篩板塔中試實驗系統(tǒng)示意圖Fig.1 Schematic of sieve tray spray scrubber experimental system

實驗系統(tǒng)流程圖如圖1所示,從圖1可以看出系統(tǒng)主要由污染物發(fā)生系統(tǒng)、煙氣系統(tǒng)、吸收系統(tǒng)和測量系統(tǒng)組成.脫硫塔為噴淋塔結(jié)構(gòu),塔出口設(shè)置有2層折板式除霧器,下方設(shè)有4層噴淋層,在脫硫塔煙氣入口與最下層噴淋之間安裝有一層篩板,篩板塔主要設(shè)計參數(shù)qV-fluegas為煙氣流量；qV-spray為噴嘴流量；qV-slurry為漿液量；ρB-flyash為煙塵濃度；wslurry為洗滌漿液含固量；H為脫硫塔塔高；D為脫硫塔塔徑；d為篩板孔徑；O為篩板開孔率；Δ為篩板孔間距，如表1所示，表中：在實驗過程中,由熱風(fēng)爐燃燒產(chǎn)生熱煙氣,熱風(fēng)爐出口煙道處開有給料口,可變頻給料機以一定頻率向熱煙氣中送入電廠灰,飛灰與煙氣在進(jìn)入脫硫塔前充分混合,經(jīng)增壓風(fēng)機增壓進(jìn)入脫硫塔,煙氣經(jīng)過篩板、噴淋層,與漿液逆向接觸,至塔頂排出.

表1 篩板塔主要設(shè)計參數(shù)Tab.1 Main design parameters of sieve tray spray scrubber

1.2 試驗測試方法

圖2 DGI顆粒物采集系統(tǒng)圖Fig.2 DGI particle matter sampling system

采樣系統(tǒng)如圖2所示,系統(tǒng)連接完畢后,開啟保溫裝置(包括保溫套和采樣管伴熱帶),打開旁路并啟動真空泵,將采樣體積流量設(shè)置為70 L/min,在70 L/min的煙氣流速下,細(xì)顆粒物會被DGI切割成5個粒徑段：>2.5 μm、1.0～2.5 μm、0.5～1.0 μm、0.2～0.5 μm、<0.2 μm,大流量顆粒撞擊式采集器(dekati gravimetric impactor， DGI)的收集盤會采集前4個粒徑段的顆粒物；待保溫裝置溫度升高至150 ℃,關(guān)閉旁路開始采樣.入口與出口采樣時間分別為10和15 min.采樣后,將采樣儀拆解,每級將收集到的顆粒物稱重,計算其質(zhì)量濃度.

分級細(xì)顆粒物脫除效率(簡稱“分級效率”)計算公式如下：

(1)

式中：ηi為第i級細(xì)顆粒脫除效率,ρin,i,ρout,i分別為脫硫塔入口與出口第i級細(xì)顆粒物質(zhì)量濃度(mg/m3).

1.3 試驗飛灰

中試試驗所用的飛灰取自浙江省某電廠電除塵器第4級電場.如圖3所示為使用Mastersizer 2000測得電廠飛灰的累積粒徑分布,w為累積質(zhì)量分?jǐn)?shù).從圖3中可看出,飛灰顆粒粒徑d在0.339 5～389.973 μm之間,以粗顆粒為主.PM2.5所占質(zhì)量比重小于6%,PM10質(zhì)量分?jǐn)?shù)約為20%.飛灰顆粒質(zhì)量分?jǐn)?shù)中位粒徑為38.0 μm.可吸入顆粒物(PM10)占飛灰顆粒總質(zhì)量的21%,PM2.5占PM10總質(zhì)量的30%.

圖3 實驗飛灰累積粒徑分布Fig.3 Cumulative particle size distribution of coal-fired fly-ash particles

2 結(jié)果與討論

2.1 煙氣流速對脫除效率的影響

圖4 煙氣流速對飛灰顆粒脫除效率的影響Fig.4 Effect of gas flow rate on removal efficiency of fly-ash particle

如圖4所示為篩板塔顆粒物總脫除效率η隨塔內(nèi)煙氣流速v的變化曲線,其中ρout為出口飛灰濃度.實驗采用2層噴淋,入口飛灰濃度ρin=25±3 mg/m3.從圖3中可看出,當(dāng)v<2.45 m/s時,隨著煙氣流速的增加,系統(tǒng)對細(xì)顆粒物的脫除效率顯著增加；當(dāng)v=2.45 m/s時,脫除效率可達(dá)90%以上；當(dāng)2.45 m/s

2.2 漿液噴淋量對脫除效率的影響

圖5 漿液噴淋量對飛灰顆粒脫除效率的影響Fig.5 Effect of liquid flow rate on removal efficiency of fly-ash particle

在v=2.45 m/s時,ρin=22±2 mg/m3的工況條件下,考察了漿液噴淋量對顆粒物脫除的影響規(guī)律,如圖5所示為漿液噴淋量對飛灰顆粒脫除效率的影響.從圖5中可看出,顆粒物脫除效率隨著漿液噴淋量的增加逐漸增大.其主要原因是漿液噴淋量增加使得氣液接觸界面增大[20]；此外,在具有足夠大的接觸界面時,細(xì)顆粒分割粒徑主要取決于單個氣泡的液膜捕集效率[21],增大漿液噴淋量雖不能增加氣泡數(shù)量但提高了單個氣泡的脫除效率[22].

2.3 粉塵濃度對脫除效率的影響

如圖6所示為細(xì)顆粒物總脫除效率與入口飛灰濃度關(guān)系的測試結(jié)果.實驗采用三層噴淋,煙氣流速為2.45 m/s.從圖6中可看出,隨著入口飛灰濃度的增加,脫除效率先增大后趨于穩(wěn)定.這是因為增加入口飛灰顆粒物濃度提高了細(xì)顆粒物間碰撞概率[23],強化了顆粒的間團聚,形成了部分大顆粒[19],進(jìn)而顯著提高了細(xì)顆粒的脫除效率；在入口飛灰顆粒濃度較高的情況下,液滴表面會形成不穩(wěn)定的小尺度的團聚體,這些新形成的團聚體容易從液滴表面脫落[24],難以在洗滌塔內(nèi)有效的脫除.

圖6 入口飛灰濃度對顆粒脫除效率的影響Fig.6 Effect of fly-ash loading on removal efficiency of fly-ash particle

圖7 粒徑對顆粒脫除效率的影響Fig.7 Effect of particle size on removal efficiency of fly-ash particle

2.4 顆粒物粒徑對脫除效率的影響

如圖7所示為細(xì)顆粒物分級脫除效率變化曲線.煙氣流速為2.45 m/s,3層噴淋,ρ為飛灰濃度.從圖7中可看出,隨著飛灰顆粒粒徑減小,脫除效率先迅速降低隨后再次回升,最低脫除效率出現(xiàn)在0.2～1.0 μm粒徑區(qū)間,脫除效率呈“V”型分布.這是因為對于0.2 μm 以下的顆粒主要通過擴散作用捕集,隨粒徑的減小,擴散作用加劇,脫除效率提高；對于大于 1.0 μm 以上的顆粒,慣性碰撞機理起主要作用,隨粒徑的增加,捕集效率提高[25].而0.2～1.0 μm粒徑段處于擴散作用與慣性碰撞作用的交叉區(qū)域[26],顆粒不能有效擴散,同時不具備足夠的慣性脫離氣流[27],因此該段顆粒脫除效率相對較低.

2.5 篩板塔與噴淋塔細(xì)顆粒物控制效果對比

如圖8所示為篩板塔與噴淋塔的分級脫除效率與總脫除效率.實驗采用2層噴淋,入口顆粒物濃度為26±2 mg/m3.從圖8中可看出,篩板塔細(xì)顆粒物分級脫除效率與總脫除效率均高于噴淋塔,尤其是對0.2～1.0 μm粒徑段顆粒的脫除效率較噴淋塔有大幅提升.筆者認(rèn)為,這是因為在同一工況下,相對于噴淋空塔,篩板噴淋塔由于形成了持液層而具有更大的氣液接觸面積；另外,篩板持液層近似鼓泡,氣液擾動強烈,促進(jìn)了部分顆粒的團聚長大,強化了細(xì)顆粒的脫除.

圖8 加裝篩板前后顆粒物脫除效果對比Fig.8 Removal efficiency of different scrubbers

3 結(jié) 論

(1)篩板塔煙氣流速、漿液噴淋量、入口飛灰濃度對細(xì)顆粒物的脫除效果存在較大影響,脫除效率隨漿液噴淋量的增大而提高,在一定范圍內(nèi),隨煙氣流速、入口飛灰濃度的增加先增大后趨于穩(wěn)定.

(2)篩板塔細(xì)顆粒脫除效率隨粒徑增加先降低后升高,分級脫除效率曲線呈“V”形分布,0.2～1.0 μm粒徑段的脫除效率最低.

(3)相對噴淋塔,篩板塔具有更強的細(xì)顆粒物脫除能力,對粒徑在0.2～1.0 μm左右細(xì)顆粒的脫除效率比噴淋塔高11%以上,而總脫除效率相對增加了5%左右.

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Experimental of enhancement of simultaneous removing fine particle by sieve tray spray scrubber

ZHANG Jun1, LI Cun-jie1, ZHENG Cheng-hang1, WENG Wei-guo1, ZHU Song-qiang2,WANG Ding-zhen1, GAO Xiang1, CEN Ke-fa1

(1.StateKeyLaboratoryofCleanEnergyUtilization,ZhejiangUniversity,Hangzhou310027,China;2.ZhejiangProvincialEnergyGroupCompanyCo.,Ltd.Hangzhou310006,China)

A new sieve stray spray scrubber, which can promote the mass transfer of gas-liquid-solid system, was proposed in order to improve the fine particle removal efficiency in wet flue gas desulfurization (FGD) system.A pilot-scale wet FGD system was developed to study the removal characteristics of fine particles.The effects of flue gas flow rate, liquid flow rate, fly-ash loading and particle size on the fine particle removal efficiency were investigated.Results show that the fine particle removal efficiency is higher than 90% under typical working condition, of which the maximum exceeds 95%. The removal efficiency increases with the increase of flue gas flow rate, liquid flow rate and the concentration of particles.The fractional removal efficiency is a V-shaped curve with a minimum at 0.2 to 1.0 μm. The sieve tray spray scrubber has a better performance than the ordinary spray scrubber under the same conditions; the total removal efficiency and removal efficiency at 0.2-1.0 μm can be improved more than 5% and 11%, respectively.

sieve tray scrubber; coal-fired power plant; fine particle; synergetic control; emission characteristics

2015-11-05.

浙江省重大科技專項計劃資助項目(2014C03018); 國家杰出青年科學(xué)基金資助項目(51125025).

張軍(1990—), 男, 博士生, 從事燃煤電廠大氣污染物控制技術(shù)等研究. ORCID: 0000-0001-7096-0064. E-mail: stenpher@zju.edu.cn

高翔,男,教授.ORCID: 0000-0002-1732-2132. E-mail: xgao1@zju.edu.cn

10.3785/j.issn.1008-973X.2016.08.013

X 511

A

1008-973X(2016)08-1516-05

浙江大學(xué)學(xué)報(工學(xué)版)網(wǎng)址： www.journals.zju.edu.cn/eng