The effect of combustion temperature on coal ash fine-fragmentation mode formation mechanisms

Abstract

Ash particles smaller than 2.5μm in pulverized coal combustion ash are difficult to capture in particulate control devices and may pose greater harm to human health than larger particles if emitted into the atmosphere. The fine fragment coal ash mode, centered around 2μm, is less well understood than larger coarse and smaller ultrafine ash. Proposed formation mechanisms include bursting, shedding, or fragmenting of larger particles; particle-to-particle impaction; and adjunct particle formation. Increased heating rate may increase bursting and shedding of large particles and an increased overall temperature may lead to increased mineral vaporization, condensation, and subsequent merging into fine fragment particles. This study aimed at exploring these effects by varying the combustion temperature while maintaining a nearly constant residence time.Pulverized Illinois #6 bituminous coal was combusted at three different peak combustion temperatures in a self-sustaining downflow combustor operated at 15kW to generate ash, resulting in incremental temperature/residence time profiles following the same general trend. The size segregated ash samples were collected from the post-combustion zone in a Dekati low pressure inertial impactor. The mass of each fraction was measured and the ash was analyzed using scanning electron microscopy and X-ray microanalysis. The fine fragment mode ash types were classified into char, coarse, agglomerates, fragments, multi-spheres, and odd/unidentifiable classes. The abundance of these particle types were used to evaluate the significant fine fragment ash formation mechanisms and determine potential impacts of heating rate/temperature on fine-fragment particle formation. The results revealed that the mass fraction of multi-sphere, fragments, char, and agglomerates all increase with decreasing temperature. Further, there appears to be multiple formation mechanisms responsible for the generation of fine fragment sized multi-sphere and fragment type particles.