Experimental study on microstructure and surface characteristics of fine ash generated in pulverized coal gasification

The ash accumulation in the heat recovery unit of the pulverized coal pressurized gasification technology seriously affects its heat transfer efficiency. The avoidance or reduction of ash accumulation in heat recovery unit plays a key role in the development of this technology. In order to analyze the ash accumulation problem, two kinds of fine ashes which were generated in two typical coal gasification processes (i. e., ash A obtained from the upstream gas quench process and ash B obtained from the downstream water quench process) are studied. Based on the microstructure observation and surface property determination, the adhesion characteristics of these two fine ashes are analyzed, and the main reason for the difference between these two fine ashes is explored. The particle size distribution, microscopic morphology, and surface element content of these fine ashes are measured using laser particle size analyzer, scanning electron microscope and energy spectrum analysis, respectively. The ultra-pure water contact angles on the two fine ashes are determined with the capillary penetration method, and the surface energies of fine ashes are calculated by interpreting contact angle via the Neumann equation of state. The results indicate that the fine ash A particles are regular spherical with no carbon element on the surface, and have the largest volume particle diameter (i. e., dd) of 1.2 μm. Whereas, the fine ash B is mostly large particles (dd=45 μm) with loose structure and high carbon content on the surface. The ultra-pure water contact angles on fine ash A and fine ash B are 39.69° and 79.06°, respectively, and the surface energies are 56.77 and 26.85 mJ/m2, respectively. The surface energy of fine ash B is significantly lower than that of fine ash A. The adhesion work and critical adhesion force analysis suggests that in the area where the Van der Waals force is the main force of adhesion, the fine ash A is easier to adhere on surface compared with the fine ash B, thus, the fine ash A is difficult to remove by gravity or mechanical rapping. The main reason for the difference between these two ashes may be the difference in the separation method. In the upstream gas quench process, the swirling effect in the gasification chamber plays a major role in the separation of fine ash and coarse slag. While, in the downstream water quench process, the effect of water bath in the quench chamber plays a dominant role. In addition, the amount of fine ash also affects the ash accumulation in the heat recovery unit. The surface energy determination method reduces the result deviation caused by the difference in accumulation state of particles, and has wide applications in characterizing the property of coal gasification fine ash. © 2021, Editorial Office of Journal of China Coal Society. All right reserved.