基于硫循環的煙氣生化脫硫及硫磺回收的過程機理

吳 熙,胡學偉,寧 平,黨雅馨,張 開

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基于硫循環的煙氣生化脫硫及硫磺回收的過程機理

吳 熙,胡學偉*,寧 平,黨雅馨,張 開

(昆明理工大學環境科學與工程學院,云南 昆明 650500)

以元素硫的地球化學循環為指導,系統研究硫酸鹽還原菌(SRB)生化代謝產物還原性硫化物與亞硫酸根的反應過程機理及產物形態.結果表明,在生化煙氣脫硫及硫回收反應體系中,pH=(4.04±0.10)、ORP=(-134±17mV)時硫磺產量最大,硫磺產率大于40%;SO2(aq)—S2-(aq)體系證明pH=(3.99±0.21)、ORP=(-159±40)mV、SO32-/SS2-物質的量比約1:1時為產硫磺最佳條件區間;S2O32-與S2-的產硫磺反應為副反應,有利于硫磺的產生;近中性條件下硫磺的消耗歸因于產多硫化物和連多硫酸鹽反應的進行;SO2(aq)—S2-(aq)體系和生化體系的固體產物表征分析為球狀高純斜方硫,生化體系產生的硫磺被胞外多聚物粘附裹挾.

煙氣脫硫；硫酸鹽還原菌；硫磺回收；硫循環

隨著工業的快速發展,中國不僅是SO2排放大國,同時也是硫磺進口大國(主要用于制備硫酸)[1].傳統煙氣脫硫工藝主要有鈣法脫硫和氨法脫硫,鈣法所產脫硫石膏因含重金屬等而缺乏資源化利用價值,堆置占用大量土地[2-5];氨法脫硫則因液氨價格較高而導致運行成本較高,且存在氨逃逸現象,有助于灰霾的形成[6].

燃煤及硫化礦冶煉等釋放出SO2煙氣,而硫酸鹽還原菌(SRB)可將SO2及硫酸鹽還原為硫化物,從而構成硫元素的地球化學循環[7-13].基于SRB對SO32-還原的特性,目前煙氣生物脫硫主要為荷蘭的Bio-FGD工藝,但其吸收過程堿性物料消耗大,雙生物反應器會導致系統復雜,多菌種間互相抑制等不足[14-21],限制了其推廣應用.本研究提出將含硫煙氣淋洗后,去除煙氣中粉塵、無機鹽組分,以獲得亞硫酸淋洗吸收液;啟動SRB生化反應器,將外加硫源(僅用于反應器啟動階段)經高效生物催化還原產生S2-,后將生化處理出水中S2-與淋洗吸收液中SO32-混合,經氧化還原歸中反應獲得含硫磺產物的處理出水;最終通過固液分離實現煙氣脫硫及硫磺回收(固液分離出水部分回流用于含硫煙氣的淋洗吸收,部分回流至SRB生化反應器用作唯一電子受體來源).本生化煙氣脫硫工藝具有反應條件溫和(常溫常壓)、系統簡單、可回收硫磺等優點,但由于生化代謝過程的復雜性,SRB還原產物中還原性硫化物等與亞硫酸根的反應機理,以及目標產物硫磺產生影響因素等相關研究鮮見文獻報道.

本研究以SRB生化代謝過程中還原性硫化物與亞硫酸根的反應過程為研究對象,對比分析還原性硫化物的理論及生化出水與亞硫酸根的反應過程及產物差異,探索硫磺回收的優化條件,表征分析產物硫磺的形態特征,為SRB煙氣脫硫及硫磺回收工藝提供理論基礎及定向調控策略.

1 材料與方法

1.1 菌種來源及培養

1.2 試驗材料及方法

SO2(aq)—S2-(aq)實驗和SRB生化實驗中分別準確量取S2-溶液和生化出水,逐漸滴入SO2溶液中(滴定量:10mL/min);S2O32-(aq)—S2-(aq)實驗準確量取S2-溶液并逐漸滴入S2O32-溶液中(滴定量: 10mL/min).3組實驗實時監測體系中pH值、ORP的變化,取樣分析體系濁度及SO32-、S2O32-、SO42-、SS2-濃度的變化.反應過程中開啟磁力攪拌,設定溫度為(25±1)℃.

1.3 分析方法

pH計(上海雷磁,PHS-3C)測定體系pH值;自動電位滴定儀(上海雷磁,ZD-2)測定ORP;硫離子濃度計(上海般特,Bante931-S)測定溶液初始S2-,亞甲基藍法[22]測定反應過程中總溶解態硫化物SS2-(H2S(aq)、HS?、S2?);濁度計(上海安亭電子,WZS- 1000)監測體系濁度的變化(稀釋10倍后測定);樣品經孔徑為0.45um的濾膜過濾后,進入離子色譜(北京東西分析,IC-2800)測定SO32-、S2O32-、SO42-的濃度(色譜柱:美國Dionex,IonPac? AS23 4×250mm);真空泵(鄭州科達機械,SHZ-III)將反應產物抽濾到0.45μm濾膜上,收集后利用SEM(捷克TESCAN, VEGA3)和EDS聯用分析反應產物的形態及元素組成;XRD(荷蘭PANalytical,Empyrean)分析反應產物特征.

硫磺產率的測定:進口總硫:[S]進=[S]SO2溶液+[S]滴定(1)

式中:[S]SO2溶液表示SO2溶液中涉硫組分(SO32-、SO42-)濃度換算后的物料量,mg;[S]滴定表示SRB培養液中初始SO42-濃度換算后的物料量,mg.

出口總硫:最大濁度時取樣5mL以轉速10000r/min離心10min,取上清液1mL,滴加30%H2O2溶液(優級純)1mL,充分氧化后稀釋10倍,采用離子色譜測定SO42-濃度并換算為硫元素的物料量.硫磺產率計算式為:

式中:[S0]表示硫磺產率,%;[S]進表示進口總硫,mg; [S]出表示出口總硫,mg.

2 結果與討論

2.1 生化體系反應規律

如圖1所示,隨著SRB生化出水與SO2溶液反應的進行,體系pH值逐漸增加,ORP逐漸減小,溶液濁度呈現先增加后減小的趨勢,并在pH=(4.04±0.10)范圍內達到最大.

本研究中SRB24h硫酸根去除率為80.48%~ 83.03%.pH=(2.00±0.01)~(4.04±0.10)、ORP=(135± 5)~(-134±17)mV區間內,反應體系的pH值、ORP變化率明顯高于其他區間,表明生化體系中該區間內的氧化還原反應最劇烈.依據生化體系中pH值、ORP、濁度的變化,得出:當pH=(4.04±0.10)、ORP=(-134±17)mV時體系中硫磺產量最大,由公式(2)得硫磺產率為42.4%.酸性條件下大量硫化物以HS-和H2S形式存在[23],SO2溶液作為厭氧體系中唯一的氧化劑,與還原性較強的還原態硫發生歸中反應實現硫磺回收[24],由結果與討論2.3可知反應過程中體系監測到S2O32-,于是總反應為:

2S2-+3SO32-+6H+→Sˉ+2S2O32-+3H2O (3)

圖1 生化反應過程中pH值、ORP及濁度的變化Fig.1 Changes of pH, ORP and turbidity in biochemical system

2.2 SO2(aq)—S2-(aq)與S2O32-(aq)—S2-(aq)體系的反應規律

如圖2、3所示,隨著S2-溶液逐漸進入SO2溶液、S2O32-溶液中,體系pH值逐漸增加,伴隨著ORP的逐漸減小.溶液濁度出現先增加后減小的變化,對SO2(aq)—S2-(aq)體系,在pH=(3.99±0.21)、ORP= (-159±40)mV處于最大濁度區間;S2O32-(aq)— S2-(aq)體系,pH=(3.99±0.09)、ORP=(-127±15)mV為最大濁度區間.

圖2 SO2(aq)—S2-(aq)體系中pH值、ORP及濁度的變化Fig.2 Changes of pH, ORP and turbidity in SO2(aq)-S2-(aq) system

2個反應體系中持續性的酸堿反應為復雜氧化還原反應的進行提供適宜的pH值和ORP.pH=(2.00±0.01)~(3.99±0.21)時S2O32-處于不穩定的易分解狀態[25],同時易被S2-還原[26],因此,結合圖5中S2O32-的變化可知S2O32-(aq)—S2-(aq)體系總反應為:

3S2O32-+2S2-+6H+→2SO32-+6S+3H2O(4)

圖3 S2O32-(aq)—S2-(aq)體系中pH值、ORP及濁度的變化Fig.3 Changes of pH, ORP and turbidity in S2O32-(aq)-S2-(aq) system

S2O32-(aq)—S2-(aq)體系的最大濁度區間與SO2(aq)—S2-(aq)體系的最大濁度區間對應,表明S2O32-與S2-的產硫磺反應為SO2(aq)—S2-(aq)反應體系的副反應,對硫磺的產生有促進作用.結合圖5可知SO2(aq)—S2-(aq)體系中SO32-/SS2-物質的量比約1:1時體系處于最大濁度區間,表明pH=(3.99±0.21)、ORP=(-159±40)mV、SO32-/SS2-物質的量比約1:1時為產硫磺最佳條件.

2.3 反應過程中涉硫組分的變化

生化體系(圖4)和SO2(aq)—S2-(aq)體系中(圖5),隨著反應的進行,SO32-濃度不斷降低;隨著S2-溶液滴定量的增加,整個反應過程中只檢測到少量的SS2-,表明SS2-被大量消耗;SO42-濃度在反應初期明顯降低,后趨于穩定;S2O32-濃度則呈現先增加最終趨于穩定的趨勢.

SO2進入水相,發生界面反應電離出H+[27].反應前期SO42-在強酸性條件下與H2S反應形成單質硫[28],導致SO42-濃度降低,隨體系pH值的增加反應受到抑制;該階段SO32-濃度的降低歸因于體系中產硫磺反應(3)的進行[29].反應后期,近中性條件下產硫磺反應(3)處于不利地位,產連多硫酸鹽的反應具有更強的熱力學優勢,成為SO32-濃度緩慢下降及硫磺逐漸消耗的原因.該階段S2O32-呈現逐漸穩定的趨勢,這一現象由于體系中S2O32-的大量存在和SO32-的大量消耗導致更高的連多硫酸鹽產生[24].多硫化物的產生則是反應后期體系中硫磺逐漸消耗的另一重要原因[30].

圖4 生化體系中涉硫組分的變化 Fig.4 Changes of sulfur-related components in biochemical system

圖5 SO2(aq)—S2-(aq)體系中涉硫組分的變化 Fig.5 Changes of sulfur-related components in SO2(aq)-S2-(aq) system

結合圖3和圖6可見,S2O32-(aq)—S2-(aq)體系中,pH=(3.99±0.09)時體系濁度出現最大值,反應前期S2O32-呈現消耗的趨勢,表明該階段反應(4)占據主導,進一步驗證S2O32-(aq)—S2-(aq)體系對產硫磺的貢獻;pH>(3.99±0.09),S2O32-則逐漸積累并最終保持穩定;強酸性條件下S2O32-分解產生的SO32-被迅速還原,僅少量積累,隨著pH值逐漸升高,累積的SO32-消耗并呈現穩定狀態;隨著S2-溶液滴定量的逐漸增加,體系中SS2-僅出現少量累積,表明反應過程中SS2-大量消耗.反應后期,體系濁度逐漸下降, SO42-、SO32-、S2O32-趨于穩定,說明產生的硫磺逐漸消耗并向產多硫化物和連多硫酸鹽方向進行[25,30].

圖6 S2O32-(aq)—S2-(aq)體系中涉硫組分的變化 Fig.6 Changes of sulfur-related components in S2O32-(aq)-S2-(aq) system

2.4 反應產物的表征與對比分析

由圖7可見,SO2(aq)—S2-(aq)體系(A)和生化體系(B)中產生的固體顆粒主要呈規則球狀,不同的是,生化體系(B)產生的部分固體顆粒周圍包裹著胞外多聚物及其他大分子絮狀物質[31].能譜特征C、D表明2種體系產生的固體顆粒元素組成為硫,重量百分比和原子百分比均為100%,表明固體產物的純度高.

SO2(aq)—S2-(aq)體系和生化體系沉淀物的XRD圖譜(圖8)與JCPDS標準圖譜中硫磺標準卡片(PDF#08—0247)高度吻合,且特征衍射峰(1、2)明顯,表明2種體系中產生的沉淀物為硫磺,且全部為斜方硫晶體(α—硫).

圖8 SO2(aq)—S2-(aq)和生化體系產物的XRD圖譜Fig.8 XRD pattern of products from SO2(aq)-S2-(aq) system and biochemical system

3 結論

3.1 在SRB生化出水與SO2溶液反應的生化體系中,當pH=(4.04±0.10)、ORP=(-134±17)mV時硫磺產量最大,最大硫磺產率為42.4%;SO2(aq)—S2-(aq)體系表明pH=(3.99±0.21)、ORP=(-159±40)mV、SO32-/S2-摩爾比約1:1時為產硫磺的最佳條件.

3.2 S2O32-與S2-的產硫磺反應為SO2(aq)—S2-(aq)反應體系的副反應,對硫磺的產生有促進作用;生化體系與SO2(aq)—S2-(aq)體系反應后期,近中性條件下硫磺產量的減少歸因于產多硫化物和連多硫酸鹽反應的進行.

3.3 生化體系與SO2(aq)—S2-(aq)體系的固體產物通過SEM觀察主要呈球狀,EDS和XRD分析固體顆粒為硫磺,其純度高;生化體系產生的硫磺被胞外多聚物粘附.

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致謝：在此感謝我的導師寧平教授、胡學偉教授的培養與指導;同時感謝周成副教授在文章中英文摘要及圖題部分對我的指導.

Biochemical desulfurization of flue gas and process mechanism of sulfur recovery based on sulfur cycle.

WU Xi, HU Xue-wei*, NING Ping, DANG Ya-xin, ZHANG Kai

(Faculty of Environmental Science and Engineering, Kunming University of Science and Technology, Kunming 650500, China)., 2019,39(3)：954~959

Based on the geochemical cycle of elemental sulfur, the reaction mechanism and product morphology of reductive sulfide produced by biochemical metabolism of Sulfate-Reducing Bacteria (SRB) and sulfite were systematically studied. In biochemical desulfurization of flue gas (biochemical system), the largest sulfur yield (more than 40%) achieved at pH=(4.04±0.10) and ORP=(-134±17mV), but the optimal conditions for sulfur production in SO2(aq)-S2-(aq) system were pH=(3.99±0.21), ORP=(-159±40mV) and 1:1 molar ratio of SO32-to S2. In addition, the reaction of sulfur production of S2O32-with S2-was a side reaction, which was beneficial to the production of sulfur. Sulfur consumption under near-neutral conditions was attributed to the production of polysulfide and polysulfate. The solid products of SO2(aq)-S2-(aq) system and biochemical system were characterized as the spherical high-purity orthorhombic sulfur. The sulfur produced in the biochemical system was adhered by the extracellular polymer.

flue gas desulfurization；sulfate-reducing bacteria；sulfur recovery；sulfur cycle

X511

A

1000-6923(2019)03-0954-06

吳 熙(1994-),男,云南宣威人,昆明理工大學環境科學與工程學院在讀碩士研究生,研究方向為大氣污染控制技術.

2018-08-20

國家自然科學基金資助項目(51668026)

* 責任作者, 教授, huxuewei.env@gmail.com