Abstract In recent years, liquefied natural gas (LNG) has become a popular alternative to heavy fuel oils for use in marine engines due to its low cost and increasingly stringent emissions regulations from the International Maritime Organization (IMO). While natural gas (NG) combustion produces substantially less soot than other fossil fuels, its soot emissions are not negligible and have not been morphologically characterized. In this study, transmission electron microscopy (TEM) was used to interpret scanning mobility particle sizer (SMPS) distributions of catalytically-denuded particulate matter (soot) from an NG-powered commercial vessel. The vessel studied was powered by a dual-fuel compression ignition engine which uses a diesel pilot only to ignite the NG. Less than 2% of the engine’s power is provided by the diesel pilot for engine loads above 50%. For comparison, tests were also run in diesel-only mode. During NG operation, the SMPS size distributions did not follow a lognormal distribution as expected. Instead, the distribution remained roughly constant from 10 nm to 100 nm. High resolution (HR)-TEM revealed particles as small as 30 nm in diameter were soot. From the TEM images, soot primary particle size and projected-area-equivalent diameter were estimated for individual soot particles. These data were then used to infer the effective density of the particles collected. In general, the primary particle size and correspondingly the estimated effective density of soot from this engine were larger than those from the “universal fit” proposed for soot in previous literature. For both operating conditions the source of the soot was likely the diesel pilot however, there were statistically significant differences between particles produced by the two fuelling modes. With LNG fueling, there were far fewer particles, and mean soot aggregate size decreased, but the primary particle size (for a 100 nm aggregate) increased, relative to diesel. Engine load did not have a statistically significant effect on soot morphology.

中文翻译：

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双燃料船用发动机产生的烟尘尺寸和形态

摘要 近年来，液化天然气 (LNG) 因其低成本和国际海事组织 (IMO) 日益严格的排放法规而成为船用发动机使用的重质燃料油的流行替代品。虽然天然气 (NG) 燃烧产生的烟尘比其他化石燃料少得多，但其烟尘排放量不可忽略且尚未进行形态表征。在这项研究中，透射电子显微镜 (TEM) 用于解释来自 NG 动力商业船舶的催化剥蚀颗粒物质（烟尘）的扫描迁移率粒度仪 (SMPS) 分布。所研究的船舶由双燃料压燃式发动机提供动力，该发动机仅使用柴油引燃器来点燃 NG。当发动机负荷超过 50% 时，柴油机提供的功率不到 2%。为了比较，测试也在纯柴油模式下运行。在 NG 操作期间，SMPS 尺寸分布没有按照预期遵循对数正态分布。相反，分布从 10 纳米到 100 纳米保持大致恒定。高分辨率 (HR)-TEM 显示直径小至 30 nm 的颗粒是烟灰。从 TEM 图像中，估算了单个烟尘颗粒的烟尘初级粒径和投影面积等效直径。然后使用这些数据来推断收集到的颗粒的有效密度。一般而言，该发动机产生的烟尘的初级粒径和相应的估计有效密度大于先前文献中为烟尘提出的“通用拟合”中的那些。对于这两种操作条件，烟灰的来源可能是柴油引燃器，然而，两种燃料模式产生的颗粒之间存在统计学上的显着差异。与柴油相比，使用 LNG 燃料时，颗粒要少得多，平均烟灰聚集体尺寸减小，但初级颗粒尺寸（对于 100 nm 聚集体）增加。发动机负荷对烟尘形态没有统计学上显着的影响。