饶毅恒,周 严,汪铁林
(武汉工程大学化工与制药学院,湖北 武汉 430073)
烟气组分对哌嗪捕集CO2过程中形成N-亚硝基哌嗪的影响
饶毅恒,周 严,汪铁林*
(武汉工程大学化工与制药学院,湖北 武汉 430073)
哌嗪(PZ)及其混合胺溶液是极具应用前景的CO2吸收剂,但烟气中的氮氧化物(NOx)可能导致PZ在捕集CO2过程中产生强致癌物N-亚硝基哌嗪(MNPZ)。采用模拟烟气考察了烟气组分对亚硝酸盐累积速率及MNPZ生成速率的影响。结果表明,亚硝酸盐累积速率和MNPZ生成速率与NOx浓度呈线性正相关。即使在无CO2存在的条件下,NOx也能导致PZ发生亚硝基化反应,但CO2的存在会显著加快MNPZ的生成速率。O2的影响较为复杂,低浓度的O2会加快亚硝酸盐累积速率及MNPZ生成速率,O2浓度过高反而会减慢亚硝酸盐累积速率及MNPZ生成速率。
CO2捕集;哌嗪;N-亚硝基哌嗪;NOx
近年来,由于温室气体效应导致的气候变化成为人类所面临的最为严峻的环境问题之一。燃烧传统化石燃料释放的CO2是温室气体的主要来源[1]。要控制全球变暖,必须大幅减少CO2排放。而在未来可预见的一段时间内,人类能源供应仍将主要依靠传统化石燃料。因此,发展作为末端控制的CO2捕集、利用和封存(CO2capture,utilization and storage,CCUS)技术将是作为减缓CO2排放中期方案的一个必然选择。开发CCUS技术已成为环境领域乃至经济与政治领域的重要课题。
CO2捕集技术主要有燃烧前捕集、燃烧后捕集和富氧燃烧等,其中燃烧后捕集对于从已建燃煤电厂烟气等大型排放源中捕集CO2具有明显优势[2-3]。目前最接近工业化的燃烧后捕集技术是基于有机胺溶液的化学吸收。烟气与有机胺溶液在吸收塔中逆流接触,CO2被吸收,富液在解吸塔中升温解吸并释放CO2,再生后的有机胺溶液返回吸收塔,释放出的CO2加压后送至地质埋存或进行其它利用[2-3]。
哌嗪(piperazine,PZ)是一种二元胺,具有吸收速率快、吸收容量大、腐蚀性低等优异的溶剂性能,因此被认为是新一代CO2捕集剂[4-5]。PZ及其混合胺溶液极具应用前景,但PZ在CO2捕集条件下会生成N-亚硝基哌嗪(N-nitrosopiperazine,mononitrosopiperazine,MNPZ)[6]。MNPZ具有强致癌性[7-8],且可能通过形成气溶胶、固废或偶然事故等方式进入周围环境从而影响人类健康[9]。因此,PZ在捕集CO2过程中由于产生MNPZ可能导致二次污染从而阻碍其在该领域的应用。
为了减少MNPZ在基于PZ吸收的CO2捕集系统中的累积从而降低其污染环境的可能性,有必要对PZ捕集CO2过程中形成MNPZ的途径与机理进行研究。尽管已有PZ亚硝基化形成MNPZ的相关报道[10],但都没有考察烟气组分对PZ亚硝基化的影响。作者利用PZ吸收模拟烟气中的CO2,考察了烟气中NOx、O2、CO2等组分对PZ形成MNPZ的影响。
1.1 试剂与仪器
N-亚硝基哌嗪,生物试剂,西格玛奥德里奇;无水哌嗪,分析纯,阿拉丁;乙腈,色谱纯,国药集团化学试剂有限公司。其它试剂均为分析纯。
集热式恒温加热磁力搅拌器,武汉科尔仪器设备有限公司;Agilent 1260 infinity型高效液相色谱仪,美国安捷伦科技有限公司。实验装置如图1所示。
1.温度计 2.医用注射器 3.三口烧瓶 4.磁力搅拌子 5.集热式恒温加热磁力搅拌器 6.CO2钢瓶 7.N2钢瓶 8.O2钢瓶 9.NOx钢瓶 10~13.针型阀 14.缓冲瓶 15.皂沫流量计 16~18.转子流量计
1.2 方法
将200 mL配制好的0.1~0.5 mol·L-1PZ溶液加入三口烧瓶中,启动集热式恒温加热磁力搅拌器,待溶液温度升至50 ℃并稳定后,通入混合气体(模拟烟气)。模拟烟气总流量为100 mL·min-1,通过调节模拟烟气中某一组分流量来改变其浓度,并通过改变N2流量平衡总流量。每隔一定时间用医用注射器取样2 mL,样品经0.45 μm滤膜过滤后置入棕色瓶中待分析。
1.3 分析方法
2.1 NOx的影响
典型烟气中NOx的含量为100~300 ppmv[11],但为加快实验进度及方便产物检测,本实验中NOx的浓度为500~5 000 ppmv,远高于实际烟气浓度。模拟烟气中NOx由NO和NO2组成,NO与NO2物质的量比为9∶1[12]。所有实验均在50 ℃下进行。
图2 NOx对累积量(a)及累积速率(b)的影响Fig.2 Effects of NOx on nitrite accumulation(a) and nitrite accumulation rate(b)
图3 NOx对MNPZ累积量(a)及生成速率(b)的影响Fig.3 Effects of NOx on MNPZ accumulation(a) and MNPZ formation rate(b)
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(2)
(3)
(4)
(5)
2.2 CO2的影响
(6)
图4 CO2对累积量(a)及累积速率(b)的影响Fig.4 Effects of CO2 on nitrite accumulation(a) and nitrite accumulation rate(b)
图5 CO2对MNPZ累积量(a)及MNPZ生成速率(b)的影响Fig.5 Effects of CO2 on MNPZ accumulation(a) and MNPZ formation rate(b)
(7)
(8)
(9)
实验发现,在无CO2存在的条件下仍能检测到MNPZ的生成,表明烟气中的NO和NO2能直接与PZ发生亚硝基化反应[16-17]。PZ与NOx直接发生亚硝基化反应的可能途径如式10、11所示。
(10)
(11)
2.3 O2的影响
图6 O2对累积量(a)及累积速率的影响(b)
图7 O2对MNPZ累积量(a)及MNPZ生成速率的影响(b)
采用PZ吸收模拟烟气中的CO2,考察了烟气中NOx、CO2和O2等组分对PZ溶液中亚硝酸盐累积速率及MNPZ生成速率的影响。结果表明,亚硝酸盐累积速率和MNPZ生成速率与NOx浓度呈线性正相关。即使在无CO2存在的条件下,烟气中的NOx能直接导致PZ发生亚硝基化反应,但CO2的存在会显著加快MNPZ的生成速率。O2并不直接参与PZ亚硝基化反应,但能间接影响PZ溶液中亚硝酸盐累积速率和PZ亚硝基化速率。该研究结果可为评估基于PZ吸收的燃烧后CO2捕集系统中烟气组分对MNPZ累积的影响提供一定的参考依据。
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Effects of Flue Gas Compositions onN-Nitrosopiperazine Formation in Piperazine-Based CO2Capture
RAO Yi-heng,ZHOU Yan,WANG Tie-lin*
(SchoolofChemicalEngineeringandPharmacy,WuhanInstituteofTechnology,Wuhan430073,China)
Piperazine (PZ) and PZ mixed amine solutions have extremely widespread application prospect in post-combustion CO2capture.However,PZ formsN-nitrosopiperazine (MNPZ),which is a carcinogenic compound from the reaction between flue gas NOxand PZ.We evaluated the effect of flue gas composition on the accumulation rate of nitrite and the formation rate of MNPZ using simulated flue gas.The results showed that the accumulation rate of nitrite and the formation rate of MNPZ had positive linear correlation with NOxconcentration.PZ nitrosation could occur by direct reaction with NOxeven without CO2loading,but CO2could significantly increase the formation rate of MNPZ.The impact of O2was complicated,and the accumulation rate of nitrite and the formation rate of MNPZ could increase at low concentration of O2,but could decrease when the concentration of O2was excessively high.
CO2capture;piperazine;N-nitrosopiperazine;NOx
留学回国人员科研启动基金项目
2017-03-16
饶毅恒(1990-),男,湖北武汉人,硕士研究生,研究方向:环境化工,E-mail:290533604@qq.com;通讯作者:汪铁林,博士,教授,E-mail:witwangtl@hotmail.com。
10.3969/j.issn.1672-5425.2017.07.011
X701
A
1672-5425(2017)07-0050-05
饶毅恒,周严,汪铁林.烟气组分对哌嗪捕集CO2过程中形成N-亚硝基哌嗪的影响[J].化学与生物工程,2017,34(7):50-54,60.