Investigation of fragmentation during soot oxidation

1133480烟灰的形成和氧化是重要的，因为烟灰对健康的不利影响，在气候强迫中的作用，以及对环境空气质量和能见度的重要性。烟灰对健康的不良影响已经被许多研究人员报道过。煤烟与气候强迫有关；然而，它的实际贡献仍然有些不确定。空气污染控制设备（如柴油微粒过滤器）的制造商也对了解低温下烟尘氧化的速率和机制感兴趣。以前的工作已经确定了各种燃烧条件下煤烟颗粒的破碎，包括高煤烟燃尽（70%碳燃烧）和低燃尽，靠近燃烧器（约10%碳燃烧）在高温，火焰环境中。此外，最近的建模已经确定了碎片在预测颗粒大小分布和其他煤烟特性中的重要性。然而，很少有实验数据可以充分了解碎裂可能发生的条件和机制。另一些人则认为，氧气扩散到微粒中，使得内部燃烧成为可能，从而分解烟灰团块。拟议的工作有以下三个目标：目标1。确定乙烯火焰在低燃尽和高燃尽时发生破碎的条件，即温度、当量比、自由基池等；目标2。确定破碎机理，如氧扩散破桥、煤烟颗粒破碎等；目标3。对替代液体燃料进行测试，以确定目标2中确定的机制对这些燃料的适用性，以及芳香族和脂肪族组分的作用。实验将在两级预混燃烧器系统中进行，该系统可用于气体和液体燃料。在该系统中，第一燃烧器产生烟尘，而第二燃烧器在不同温度和当量比下氧化烟尘。扫描迁移率粒度仪（SMPS），尺寸范围为3-100 nm，将用于确定粒度分布作为燃烧器上方高度的函数。当需要时，长差分迁移率分析仪的测量范围可以扩展到660纳米。关键组分（即O2， H2， CO, CO2）的气相浓度和温度分布将被记录。目标1将使用乙烯来收集数据和开发机制，而目标3将探索液体燃料。利用TEM和HR-TEM以及粒度分布数据，我们将在目标2中探讨通过氧气扩散导致颗粒之间的“桥”材料氧化以及颗粒孔隙度增加导致颗粒破碎的情况。这项研究的智力价值在于了解氧化过程中煤烟破碎的机制以及可能发生这种情况的条件。本研究的新颖之处在于探索高、低烟灰燃尽两种情况下的现象。此外，煤烟氧化还没有深入研究，最近在颗粒测量方面的进展可以增加现有的知识。这些数据可用于修改氧化机制，当用于模拟时，将更准确地预测烟灰氧化时的结构、尺寸分布和平均性质。此外，该项目将开始解决不同燃料成分在破碎中的作用。当工业能够将这些基本原理应用于其系统以减少烟尘排放时，就会实现更广泛的影响，这对保护人类健康和更好地了解/减轻气候强迫很重要。另一方面，对于生产炭黑的行业来说，这些数据将增加现有的烟尘形成/氧化知识库。

1133480LightySoot formation and oxidation are important because of soot's adverse health effects, role in climate forcing, and importance to ambient air quality and visibility. The adverse health effects of soot have been reported by numerous researchers. Soot has been implicated in climate forcing; however, its actual contribution is still somewhat uncertain. Manufacturers of air pollution control equipment, i.e. diesel particulate filters, are also interested in understanding the rates and mechanisms of soot oxidation at lower temperatures.Previous work has identified the fragmentation of soot particles under various combustion conditions, including high soot burnout (70% carbon burned) and low burnout, close to the burner (about 10% carbon burned) in a high temperature, flame environment. In addition, recent modeling has identified the importance of fragmentation in predicting the particle size distribution and other soot properties. However, little experimental data exist for fully understanding the conditions under which fragmentation potentially occurs and the mechanisms. Others have suggested that oxygen diffuses into the particles which allows for internal burning, breaking apart the soot agglomerate. The proposed work has the following three objectives:Objective 1. Identify the conditions, i.e. temperatures, equivalence ratios, radical pool, etc. where fragmentation occurs for ethylene flames at both low burnout and high burnout;Objective 2. Determine the mechanisms of fragmentation, such as oxygen diffusion in breaking of bridges and soot particle break down; Objective 3. Perform tests on surrogate liquid fuels to determine the applicability of the mechanisms identified in Objective 2 to these fuels and the role of aromatic versus aliphatic components.The experiments will be conducted in a two-stage, premixed burner system which can be used for both gaseous and liquid fuels. In this system the first burner generates soot, while the second burner is used to oxidize the soot under various temperatures and equivalence ratios. A Scanning Mobility Particle Sizer (SMPS), with a size range of 3-100 nm, will be used to determine particle size distributions as a function of height above burner. When needed, long-differential mobility analyzer measurements can extend the range to 660 nm. Gas-phase concentrations of key components (i.e. O2, H2, CO, CO2) and temperature profiles will be taken. Objective 1 will use ethylene to gather the data and develop mechanisms, while Objective 3 will explore liquid fuels. Utilizing TEM and HR-TEM and particle size distribution data, we will explore in Objective 2 the oxidation of "bridge" material between particles via oxygen diffusion and also increases in particle porosity which can lead to particle fragmentation. The intellectual merit of the study is in understanding the mechanisms of soot fragmentation during oxidation and the conditions under which it is likely to occur. The novelty of the work is exploring the phenomenon under the two scenarios: high and low soot burnout. Furthermore, soot oxidation has not been studied in depth, and recent advances in particle measurements can add to the existing knowledge. The data can be used to modify oxidation mechanisms which, when used in simulations, will more accurately predict the structure, size distribution and mean properties of soot as it oxidizes. In addition, this project will begin to address the role of different fuel constituents in fragmentation. The broader impacts are realized when industry can apply these fundamentals to their systems to reduce soot emissions, important to protect human health and to better understand/mitigate climate forcing. On the other hand, for industries producing carbon black, these data will add to the existing knowledge base of soot formation/oxidation.