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挥发性有机污染物 (volatile organic compounds,VOCs) 是形成细颗粒物 (PM2.5) 及臭氧 (O3) 的关键前体物与参与物[1],为主要大气污染物,其成分复杂、量大面广,对人群健康亦存在危害[2-3]。移动源排放对城市VOCs贡献约为20%[4]。其中,柴油车行驶强度大、里程多,为移动源排放的主要来源。当前柴油机尾气VOCs处理的DOC、NTP与低温等离子体协同催化等[5-8]技术存在运行成本较高、效率不稳定[9]及二次污染等问题。研发一种安全绿色无二次污染的协同处理装置对于柴油机尾气多组分VOCs减排十分必要。而蒸汽相变团聚预处理技术相对简单、易实现,已长期应用于细颗粒物的团聚。其中,电声换能超声雾化技术可将液体破碎生成微米级雾滴[10],后迅速气化并形成过饱和蒸汽环境。相较于传统喷头形式,该技术产生雾滴的比表面积更大、能耗更低[11],更具实用性。而由双膜理论[12]与吸收法[13]基本原理可知,液相雾滴接触气相VOCs时,VOCs分子可通过扩散作用穿过气膜在相界面与含有吸收剂的雾滴发生相似相溶作用,从而进入液膜被雾滴吸收。
超重力收集器是一种具有三级超重力场并联同轴设置的新型污染物分离技术[14],可与雾化系统结合形成雾化超重力技术。目前,雾化超重力技术已成功应用于工业除尘与气态污染物治理领域,其对某电厂转运站不同入口浓度烟尘去除效率均高于99.5%,对拟薄水铝石和高岭土等细颗粒物的去除效率均在98.7%以上。相较于传统Lapple型旋风除尘器提升较大[15-16]。针对气态污染物的去除,超重力技术应用微米级雾滴结合化学试剂,对汽车尾气中的NO与SO2去除效率可达100%[16]。超重力技术主要包括多级联用与结构改进两类。在多级联用方面,根据不同的应用场景,分为串联与并联2种方式,通过改善流场对称性提高效率[17-19]。在结构改进方面,利用多进气道改善气流对称性的多入口超重力技术与增加涡核破碎翼、涡流探测器等部件降低运行压降的内构件超重力技术应用[20-22]较为广泛。
本研究以国Ⅲ标准柴油机为对象,收集其在怠速运行下排放的尾气VOCs,基于雾化超重力技术结合液相吸收法,研究其在不同运行条件与吸收液配比下的最佳吸收效率,以期为深入实施移动源尾气VOCs减排净化与国Ⅵ标准下的达标排放提供参考。
微米级雾滴协同液相吸收柴油机尾气中的VOCs
Absorption of VOCs from diesel exhaust in liquid phase in synergy with micron droplet
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摘要: 针对柴油机尾气中多组分混合的挥发性有机污染物 (VOCs) 开展净化去除研究,整体基于电声换能超声波雾化-超重力收集器联用技术,提出微米级雾滴协同液相吸收脱除柴油机尾气VOCs的应用方案。在对柴油机尾气VOCs排放特征定性与定量分析的基础上,选用不同种类的吸收剂、吸收浓度与分配比助剂,得到吸收剂的最佳使用条件。进而优化运行参数,获得最优工况条件。结果表明:在15%柠檬酸钠作为主吸收剂,0.22%柠檬酸与1%氯化钠作为助剂组成的最佳吸收剂与设定雾滴粒径为5 μm、雾化量为71 mL·min−1,以及雾化后尾气停留1 s的最优运行条件下,本系统对柴油机尾气VOCs的综合吸收效率达到了58.5%。本研究可为移动源尾气VOCs的减排净化提供参考。Abstract: This study was aimed at the purification of multi-component mixed Volatile Organic Compounds (VOCs) from diesel engine exhausts. Based on the electroacoustic transducer ultrasonic atomization-supergravity collector coupling technology, a micron-scale droplet synergistic liquid-phase absorption for the removal of diesel exhaust VOCs was proposed. Firstly, the qualitative and quantitative analysis of the emission characteristics of VOCs from diesel engine exhaust was performed. Moreover, different types of absorbents, absorption concentrations and distribution ratio additives were studied to obtain the optimal conditions for absorbents. In addition, the main operating parameters of the experiment were evaluated to obtain the optimal working conditions. The results showed that the absorption efficiency of this new technology for diesel exhaust VOCs reached 58.5% under the operating conditions of 15% sodium citrate as the main absorber, 0.22% citric acid and 1% sodium chloride as additives, the droplet particle size of 5 μm, atomization volume of 71 mL·min−1 atomization volume and 1 s residence time after atomization, The results of the study can provide technical support for the reduction and purification of mobile source tailpipe VOCs.
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表 1 超重力收集器尺寸
Table 1. Size of Supergravity collector
尺寸参数 符号 尺寸/mm 超重力收集器外径 D 125 入口高度 b 50 入口宽度 a 25 溢流管直径 De 50 溢流管高度 S 25 超重力收集器高度 H 920 灰斗直径 B 40 -
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