热等离子体裂解甲烷制乙炔过程的数值模拟

Numerical simulation of methane pyrolysis to acetylene via thermal plasma

针对热等离子体甲烷裂解过程,建立了直流电弧反应器的数值模型,使用磁流体力学理论对反应器内的电弧和流场进行数值模拟,考察了电弧运动变化的规律和射流场特点,并分别耦合热力学平衡模型和宏观动力学模型探索了裂解反应的特点及其与电弧的相互影响关系。结果表明反应器内电弧做规律运动和形态变化,惰性无反应气氛下电弧形态变化不显著,运动平稳。放电区发生反应时,一方面气体的组成及热力学性质发生迅速变化,气体放电特性受到影响,等离子体的稳定性下降,化学反应是等离子体不稳定性的重要来源。另一方面,反应和扩散的特征时间小于电弧运动变化的特征时间,各物质在空间的分布较为均匀,受温度场非均匀性的影响较小,模拟的甲烷转化率和乙炔收率与实验结果相近。本工作尤其是等离子体物理模型与甲烷裂解化学反应模型的耦合,为理解热等离子体裂解相关过程提供了直接的帮助和指导。

A numerical model with respect to thermal plasma pyrolysis of methane was established and a numerical study of the plasma and the reaction via the theory of magnetohydrodynamics (MHD) was carried out. The arc motion pattern and the characteristics of the fluid field regarding to temperature and velocity distribution inside the reactor were concluded, and further reaction model respectively considering thermodynamic equilibrium and simplified macro-kinetics was individually coupled into the MHD model to show the influence of pyrolysis reaction on plasma arc and the interactions between them. Generally, with pure argon as discharge media, the arc moved in a smooth and steady pattern, whereas the addition of methane would prolong the arc and accelerate the switch of arc-anode-spot. When reaction kinetics was considered, the gradient of temperature was elevated, yet the distribution of species was much more homogeneous due to the rapid diffusion of gas. The instability of arc under pre-mixing strategy was attributed to the pyrolysis reaction and the differences of thermodynamic properties of individual component.