Experimental and kinetics studies on the evolution and effects of ash film during pulverized coal char combustion

Abstract

The char reactivity loss during the burnout stage is closely related to the effects of ash minerals, which show complex transformation behaviors (ash vaporization, ash film, and ash dilution). To assess the impact of ash behaviors, we computed the char consumption characteristics in a 21%O2/79%N2 environment using the Char Burning and Particulate Matter Kinetics (CBPMK) model with a user-design allocation of the ash film and dilution. Regretfully, the ash film fractions have not been experimentally studied due to the irregular ash structure and the high combustion temperatures. On basis of the research on ash formation mechanism, this paper aims to provide appropriately measured ash film fractions of chars as a function of carbon conversion ratio for further development of the kinetic model. The ash film fraction F increases exponentially with carbon conversion ratio N, and their relationship is summarized as F=aN23.66Y (0<a ≤ 1, 0<N<1), where a is the coefficient related to the combustion temperature and mineral components, and Y denotes the initial ash content in coal char. The kinetics model CBPMK is then modified by integrating this self-derived function, and complex ash evolution is predicted during dynamic combustion process. Simulated char combustion characteristics (particle temperature and carbon conversion) are discussed in detail. With higher ash film fraction and a thicker ash film, both the particle temperature and carbon conversion rate decrease. For low to intermediate levels of burnout (N<0.6), the carbon conversion characteristics in the case of full ash dilution (F = 0) are identical to those in the normal combustion (F = f(N)). At the char burnout stage (N>0.8), the relative deviations from the normal carbon conversion ratio are more than +10% when ignoring ash film and -20% when ignoring ash dilution.