炭粒富氧燃烧中传质系数的修正

Correction to Mass Transfer Coefficients for the Combustion of a Char Particle in O_2/CO_2/H_2O Atmosphere

与传统煤粉燃烧技术不同,煤粉在高浓度O2/CO2气氛中燃烧,在炭粒表面产生强烈的斯蒂芬流,对炭粒燃烧有重大的影响.笔者考虑炭的表面氧化(2,C+O2→2,CO)、CO2还原反应(C+CO2→2,CO)和H2O气化反应(C+H2O→CO+H2),给出忽略斯蒂芬流的传质系数修正式.数值计算表明,对单膜模型的修正显著改善了颗粒温度、燃烧速率和燃尽时间的预报.在不同燃烧情况下,采用修正表达式对斯蒂芬流的影响进行了讨论.研究发现,修正因子和反应速率、环境组分浓度、反应产物浓度有关;在典型富氧燃烧工况下,O2的传质系数修正幅度可达18%;如果有水蒸气(摩尔分数为0.1)存在,则修正幅度可达22%;CO2的传质系数修正幅度可达74%,有水蒸气(摩尔分数为0.1)存在时则修正幅度可达78%;仅有水蒸气气化反应时,可以忽略对其传质系数的修正.但如果同时存在炭的表面氧化反应和CO2气化反应,则必须对其进行修正,修正幅度为29%.

In contrast with the conventional coal combustion, the coal is burnt in an O2/CO2 mixture with higher con- tents and strong Stefan flow is formed on the surface of char, which has significant effects on char combustion. In this work, correction factors were presented for the mass transfer coefficients related to oxidation reaction, 2 C ＋ O2→2 CO, reduction reaction C ＋ CO2→2 CO, and steam gasification C ＋ H20→CO ＋ HE, respectively. The numerical simulations showed that the correction to the single-film model neglecting Stefan flow greatly improves the predictions of particle temperature, combustion rates and burnout time. The effects of Stefan flow were also discussed using the improved factors under various combustion conditions. It is found that the correction factors are related to reaction rate, ambient gas content and reaction product. Under typical oxy-fuel combustion conditions, the correc- tion to the mass transfer of 02 is 18% in the absence of steam and 22% with the ambient steam of 0.1 （molar fraction）. The correction for CO2 can be up to 74% in the absence of steam and 78% with the ambient steam of 0.1. The correction for H20 can be neglected when only steam gasification reaction is considered. If surface oxidation and the CO2 gasification are considered at the same time, the correction for H2O will be up to 29%.