Plasma assisted combustion: Progress, challenges, and opportunities
Plasma assisted combustion: Progress, challenges, and opportunities
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DOI：
10.1016/j.combustflame.2015.01.017
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发表时间：
2015-03
影响因子：
4.4
通讯作者：
Y. Ju;Wenting Sun
中科院分区：
文献类型：
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作者：
Y. Ju;Wenting Sun
提高能源效率和减少排放是可持续能源领域最重要的解决方案之一。在地面运输领域，新的发动机技术，如均质装药压缩点火（HCCI）、部分预混压缩点火（PPCI）和反应性控制压缩点火（RCCI），在更高的压缩比和更低的燃烧温度下工作，因此更多地依赖于体积点火，而不是传统发动机的火焰传播。因此，准确控制点火时间对控制排放和发动机爆震至关重要。然而，现有的双燃料或高燃料分层等点火控制方法大多是被动的，难以在发动机大负荷范围内应用。因此，需要一种主动、快速的点火控制方法。固定式发电采用贫预混高含氢（HHC）合成气燃烧，满足燃气涡轮发动机[2]、[3]2 ppm的nox排放目标。然而，超稀预混HHC燃料燃烧存在严重的火焰不稳定、闪回和吹出问题。此外，替代燃料的能量密度（5-80兆焦耳/公斤）的巨大变化进一步使问题复杂化。因此，需要寻找一种新的方法来延长材料的可燃性极限，提高其点火和燃烧稳定性。等离子体是物质的第四种状态，为燃烧和排放控制提供了新的维度。这些新的维度包括快速热加热（焦耳效应），高电子能量（1-100 eV），电子和振动激发态的高度非平衡，以及库仑和洛伦兹力。如图1所示，等离子体可以通过几种不同的方式改变燃烧过程。第一种增强途径是热增强途径。等离子体可以通过从电子到中性分子的能量转移迅速提高混合物的温度，并加快化学反应速率，这是由阿伦尼乌斯定律控制的。第二种增强途径是动力学途径，其中等离子体产生高能电子和离子以及电子和振动激发的分子(例如，o2 +, n2(∗),o2 （1 Δ g）和n2 (v))，从而导致随后产生活性自由基和活性物质（例如，O， OH， H, HO 2， O 3和NO），以加速和/或创建新的链起始和分支途径。第三种增强途径是通过电子冲击解离直接分解燃料，将燃料大分子破碎/重整为小分子，从而改变燃料的反应性，增加混合物的燃料扩散系数。第四种途径是由于等离子体产生的离子风、流体动力学不稳定性以及库仑和洛伦兹力的流动运动，改变了局部流速，增加了流动的湍流化和混合，从而增强了输运。
Increase of energy efficiency and emission reduction are among the most important solutions of sustainable energy [1]. In ground transportation, new engine technologies, such as the Homogeneous Charge Compression Ignition (HCCI), Partially Premixed Compression Ignition (PPCI), and the Reactivity Controlled Compression Ignition (RCCI), work at higher compression ratio and lower combustion temperature, and thus rely more on volumetric ignition than flame propagation in conventional engines. As such, accurate control of ignition timing is critical to control emissions and engine knocking. Unfortunately, most of the existing ignition control methods such as dual fuel or high fuel stratification are passive and difficult to be applied in a broad range of engine loads. Therefore, an active and rapid ignition control method is needed. In stationary power generation, lean premixed high hydrogen content (HHC) syngas combustion is used to meet the NO x emission target of 2 ppm of gas turbine engines [2],[3]. However, ultra lean premixed HHC fuel combustion suffers from severe flame instability, flashback, and blow-off. Moreover, large variation in the energy density (5–80 MJ/kg) of alternative fuels further complicates the problem. Therefore, a new method to extend the lean flammability limit and improve ignition and flame stability is needed.Plasma, the fourth state of the matter, provides new dimensions for combustion and emission control. These new dimensions include fast thermal heating (the Joule effect), high electron energy (1–100 eV), highly non-equilibrium in electronic and vibrational excited states, and the Coulomb and Lorentz forces. As shown in Fig. 1, plasma can modify combustion processes in several different ways [4]. The first enhancement pathway is the thermal one. Plasma can rapidly raise mixture temperature via energy transfer from electrons to neutral molecules and accelerate chemical reaction rates governed by the Arrhenius law. The second enhancement pathway is the kinetic one in which plasma produces high energy electrons and ions as well as electronically and vibrationally excited molecules (eg, O 2+, N 2 (∗), O 2 (1 Δ g), and N 2 (v)) and thus leads to the subsequent production of active radicals and reactive species (eg, O, OH, H, HO 2, O 3, and NO) to accelerate and/or create new chain-initiation and branching pathways. The third enhancement pathway is the direct fuel decomposition by electron impact dissociation, in which large fuel molecules are broken/reformed into small ones and thus modify the fuel reactivity and increase the fuel diffusivity of the mixture. The fourth pathway is the transport enhancement due to plasma generated ionic wind, hydrodynamic instability, and flow motion via the Coulomb and Lorentz forces, changing local flow velocity and increasing flow turbulization and mixing.