Abstract:

The surfaces of fine particles that emitted from power generation processes with the direct combustion of biomass and with the co-combustion of biomass and coal usually contain a certain amount of soluble inorganic salt. Based on the classical heterogeneous nucleation theory, an improved model for vapor heterogeneous nucleation on the surfaces of particles containing a spherical insoluble core and soluble inorganic salt was developed, taking into account the embryo growth mechanisms of surface diffusion and direct deposition. Using the numerical simulation method, the nucleation behaviors of four types of particles (insoluble particle and three types of particle containing soluble inorganic salt) were compared and analyzed. The results show that in cases of medium contact angles, the critical free energy of embryo formation and the critical embryo radius of an insoluble particle are the largest, followed by the particles containing KCl, NaCl and CaCl₂, respectively. Under the condition with critical embryo radii, the ratio of water molecule addition rates due to mechanisms of surface diffusion and direct deposition increases slightly at first and then keeps constant with increasing particle radius, and decreases monotonically with increasing contact angle. It is also found that when the contact angle is small, the critical saturation ratio for nucleation of a particle containing soluble inorganic salt is lower than that of the insoluble particle, whereas when the contact angle is larger, the critical saturation for nucleation of particles containing KCl and NaCl exceeds that of insoluble particles successively.

Key words: particle, nucleation, condensation, insoluble core, soluble inorganic salt, numerical simulation

摘要：

鉴于生物质直接燃烧和生物质与煤混合燃烧发电过程排放细颗粒物表面通常含有一定量的可溶无机盐，基于经典异质核化理论，综合考虑晶核生长的表面扩散和直接沉积机制建立了改进的蒸汽在包含球形不可溶核和可溶无机盐的细颗粒物表面的异质核化模型，利用数值模拟方法，对4种组分颗粒(不可溶颗粒以及3种含可溶无机盐的颗粒)的异质核化特性进行对比分析。结果表明，在中等接触角条件下，不可溶颗粒的临界晶核形成自由能和临界晶核半径最大，含KCl颗粒次之，含NaCl颗粒再次之，含CaCl₂颗粒最小；临界晶核条件下，表面扩散机制与直接沉积机制引起的水分子添加速率之比随颗粒半径的增大先略有增加而后保持不变，随接触角的增大而单调下降。研究还发现，当接触角较小时，含可溶无机盐颗粒的成核临界饱和度低于不可溶颗粒；当接触角较大时，含KCl和NaCl颗粒的成核临界饱和度先后超过不可溶颗粒。

关键词: 颗粒, 成核, 凝结, 不可溶核, 可溶无机盐, 数值模拟