Effects of moisture on soot generation and ignition of coal particle: Investigation and evaluation with OH-planar laser induced fluorescence

Abstract

Coal with a high moisture content is increasingly being utilized in power generation. However, the influence of moisture on coal ignition and soot generation characteristics remains unclear. Understanding the effect of moisture on coal ignition and soot generation characteristics is important for improving energy efficiency in coal utilization. The characteristics of soot generation and ignition of lignite coal including different contents of moisture (0%, 5%, 10%, 15%) were studied with Planar Laser Induced Fluorescence of OH radicals (OH-PLIF) on a novel concentrating photothermal platform. The results showed that the ignition delay time (tign) of coal and observed soot generation during volatile combustion by the observation of flame luminosity both increased as the moisture content of the coal particles increased from 0% to 15%. The observed increase in tign corresponded to the decreased OH intensity during the ignition. Similarly, the increased generation of soot during volatile combustion corresponded to the increased OH intensity resulting from increased moisture content in the coal. Before ignition, the steam surrounding the coal particle inhibits the release of volatiles. Thus, the formation of fuel molecules (RH) is reduced. Simultaneously, the inhibitory effect of moisture on RH leads to a decrease in H atoms, which further results in a decline in OH radical concentration resulting from the reaction H + O2→OH + O. The combined effect of reduced RH and OH concentrations inhibits ignition for coals containing moisture. During volatile combustion, although OH is commonly acknowledged as the primary oxidant for soot, the increase in OH facilitates gasification reactions, thereby promoting the massive release of volatiles. These volatiles, through processes such as nucleation reactions, contribute to the formation of an increased amount of soot, consequently resulting in an elevated formation of soot in coal with higher moisture content.