Combustion mechanism in pulverized coal flames

Abstract

Explosion behaviors of coal dusts were systematically investigated to focus on the roles of moisture in dust explosion. It is found that, when the moisture content is lower, the explosion parameters (Pex, (dp/dt)ex and MEC) basically keep a linear relationship with the moisture content. With the further increase of moisture content, the moisture would gradually agglomerate fine particles by reducing inter-particle distance and forming liquid bridges between particles. This behavior causes the increase of effective particle size and the reduction of dust dispersibility, which would notably heighten the uncertainty of explosion tests. These results indicate that to limit the moisture content is necessary when measuring explosion parameters in practice. Considering that (dp/dt)ex is more sensitive than Pex and MEC to the particle agglomeration, for coal dusts with the particle size <125 μm, the lower moisture content of 5 wt% is proposed as the upper limit to measure (dp/dt)ex, while that of 10 wt% is proposed to measure Pex and MEC. In addition, from the perspective of inert medium, the existence of moisture would narrow the explosible concentration range of coal dusts. When the concentration of dust cloud is near to the worst case, the higher moisture content would be needed to obtain the identical inhibition level. At given conditions, compared with solid inert media, the moisture has higher inhibition level and, thus, is more effective for the inhibition of coal dust explosions. The inhibition mechanisms of moisture mainly involve four categories: heat extraction, oxygen dilution, reaction kinetic inhibition and particle agglomeration. For lower moisture content or larger particle size, the predominant mechanisms are the heat extraction and oxygen dilution. With the increase of moisture content or decrease of particle size, the predominant mechanisms would gradually transform into the reaction kinetic inhibition and particle agglomeration.