Numerical study on heterogeneous reaction characteristics of coal char particle group combustion

Abstract

The reaction of coal char is a unique feature of pulverized coal combustion, which is differed from that of gas or liquid fuels combustion. Since the heterogeneous reaction process accounts for about 90% of the entire time for pulverized coal combustion, it is very important for the efficient utilization of pulverized coal. In general, pulverized coal particles are carried into the furnace in the form of dense phase rather than sparse phase or single particle, and the interaction between particles will affect the heterogeneous reaction of coal char. Therefore, it is necessary to deepen the understandings on the heterogeneous reaction behavior of coal char particles for the efficient utilization of pulverized coal. This study carries out a two-dimensional numerical simulation with the pseudo-steady state approach to study the reaction characteristics of coal char particle group. Firstly, the reaction characteristics of coal char single particle and coal char particle group are analyzed by the comparison, compared with the single particle, the peak temperature of the particle group increases by about 300 K, the surface temperature of the particle decreases by about 100 K, and the carbon consumption rate decreases by 56%−65%, which indicates that the interaction between particles affects the combustion reaction process of coal char particles. Then, the combustion characteristics of the coal char particles dispersed with three typical modes are studied. With the increase of particle spacing, the proportion of coal char oxidation reaction in each dispersion mode increases by 0.22%−2.20%, and the carbon consumption rate increases by 9.8%−26.1%. A larger particle spacing is beneficial to the diffusion of O2 molecules between particles, and the oxidation reaction and carbon consumption are promoted. Finally, the reaction characteristics of coal char particles under different reaction atmospheres are explored. Compared with the air cases, the peak temperature of the particles under high CO2 and H2O concentration atmospheres increases by 4%−30%, and the surface temperature of the particles increases by 0.5%−3.7%, among which the carbon consumption rate under the condition of 30% O2/70% H2O is the largest, which is 80% higher than that under the air atmosphere. By comprehensive comparison, the order of carbon consumption rate of coal particles dispersed with three modes under the conditions of interaction between particles, particle spacing and reaction atmosphere is as follows: Mode C > Mode B > Mode A (Mode A: particle group is rectangular dispersed; Mode B: particle group is diamond dispersed; Mode C: particles are dispersed in a regular triangle). Thus, the disturbing effect between the particles and the air flow on the consumption of coal char cannot be ignored.

Keywords

• heterogeneous reaction
• coal char particle group
• combustion
• particle temperature
• char consumption rate