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液化天然气 (liquefied natural gas,LNG) 作为船用发动机替代燃料。LNG替代常规燃料,可使得PM和SOx的排放显著降低[1-2]。同时,LNG的碳氢比 (C/H) 低,相比柴油可减少约30%的CO2排放[3],其推广使用对于我国实现双碳目标、改善能源结构具有重要意义。然而,由于LNG发动机甲烷逃逸问题,其排放物中存在CH4。采用低压喷射的LNG发动机,CH4排放量一般为3.0~5.0 g·kWh−1 [4]。CH4是全球第二大温室气体,其全球增温潜势 (global warming potential,GWP) 是二氧化碳的21倍[5]。为控制LNG发动机CH4排放,2016年8月发布的《船舶发动机排气污染物排放限值及测量方法 (中国第一、二阶段) 》规定,根据不同机型及单缸排量,LNG发动机的CH4排放量不应超过1.0~2.0 g·kWh−1 [6]。
船用LNG发动机通常采取缸内稀薄燃烧,导致发动机排气温度较低 (250~400 ℃) ,同时排气为富氧状态,限制了三元催化的使用。目前,尾气中CH4的脱除主要依靠贵金属催化剂,但贵金属催化剂的成本高且低温活性 (<500 ℃) 和抗水性较差[7]。引入低温等离子体 (non-thermal plasma,NTP) 技术可提高催化剂低温活性和抗水性,同时NTP结合催化剂作用系统被证明可有效促进CH4催化还原NOx[8-15]。但由于NTP的加入、排气中N2和O2的存在,会导致额外NOx的生成,从而降低了系统中NOx的脱除效率,且其生成量随着放电功率的升高而显著增大[11]。NOx长期吸入会对人体内脏的功能造成严重伤害,甚至威胁生命安全[16-17]。此外,NOx也是造成光化学烟雾、雾霾和酸雨等有害天气的重要因素之一[18-19],故降低系统中NOx的生成是必要的。优化等离子体反应器结构参数可有效抑制NOx的生成[20-25]。赵如金等[21]用催化剂结合等离子体处理汽油机尾气发现,当放电间隙为2.5~3.5 mm时,3种污染物的去除率变化均小于3%,但放电间隙增大到4 mm时,脱除效率下降明显 (约10%) 。研究者在富氧条件下研究NTP反应器结构参数对NO氧化脱除的影响[23-24],以860
$ \times $ 10−6 NOx (92% NO+8% NO2) / 15% O2 / N2模拟柴油机尾气,发现增大电极直径使·O自由基生成效率增加,减少N2(X, v)和N生成,从而抑制了副反应的生成,提高了NOx氧化效率;而螺纹形电极表面电场强度远高于圆柱形电极,有利于N自由基生成,使得在富氧条件下,更易发生副反应生成NOx。针对低温等离子体脱除CH4的研究仍较少,为明确NTP结构参数对CH4脱除效率及NOx生成的影响,考虑实际LNG发动机尾气,本课题组在625
$ \times $ 10−6 NO/1820$ \times $ 10−6 CH4/10% H2O/6% O2/N2 (余) 气体组分下,针对单独低温等离子体脱除CH4、降低NOx (NO、NO2) 生成系统研究等离子体反应器电极结构参数对系统CH4脱除效率及NOx生成的影响及其机制,以期为提高NTP结合催化剂一体化处理CH4和NOx系统脱除效率提供参考。
DBD结构对船用LNG发动机尾气CH4氧化脱除的影响
Effect of DBD structure on oxidative removal of CH4 from marine LNG engine exhaust
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摘要: 为提高等离子体氧化CH4的脱除效率,同时避免放电过程中副产物NOx的生成,在模拟天然气 (LNG) 发动机排气中,研究了介质阻挡放电 (DBD) 反应器电极结构参数对CH4脱除效率、CO2选择性及NOx生成的影响及其机理。结果表明:增大电极直径使得气隙中折合场强E/N提高,增加了·O和·OH自由基的生成,促进了CH4的氧化,提高了CO2选择性;内电极直径的增大使E/N在气隙中分布更加集中,抑制了N2(X, v)和N自由基的生成,减少了副反应的发生。与圆杆电极相比,螺纹电极的螺牙顶部附近具有过高的电场强度,从而促进N自由基的生成并抑制·O自由基的产生,故圆杆电极比螺纹电极具有更高的CH4脱除效率及更低的NOx生成。电极长度过长降低了折合场强以及电子平均能量,不利于氧化性活性粒子生成,导致CH4脱除效率和CO2选择性降低,同时增加了N2(X,v) ,也使得NOx增加。本研究可为提高低温等离子体协同催化剂促进甲烷还原NOx系统的脱除效率,降低DBD反应器能耗提高能源利用率提供参考。Abstract: In order to improve the removal efficiency of CH4 by plasma and avoid the generation of the by-product—NOx, the influence of electrode structure parameters in the dielectric barrier discharge (DBD) reactor on CH4 removal efficiency and NOx generation, and its mechanism were studied under simulated liquified natural gas (LNG) engine exhaust. The results showed that the reduced field strength (E/N) in the air gap increased with the increase of inner electrode diameter. And the increase of E/N increased the formation of O and OH radicals, promoted the oxidation of CH4, and improved the CO2 selectivity; At the same time, the increase of the inner electrode diameter made the distribution of E/N more concentrated in the air gap, inhibited the formation of N2 (X, v) and N radicals, and reduced the occurrence of side reactions. Compared with the round rod electrode, the screw electrode had a higher electric field strength near the top of the thread, which promoted the generation of N free radicals and inhibited the generation of O free radicals. Therefore, the round rod electrode had a higher CH4 removal efficiency and lower NOx generation than the screw electrode. Excessive electrode length reduced the converted field strength and the average electron energy, which was not conducive to the generation of active particles, leading to the reduction of CH4 removal efficiency, the reduction of CO2 selectivity and the increase of NOx.
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Key words:
- plasma /
- DBD /
- structure parameter /
- CH4 oxidative removal /
- NOx generation
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