High-speed contactless measurements of temperature evolution during ignition and combustion of coal-based fuel pellets

Abstract

The temperature evolution of coal pellets, ignited and burned in an initially motionless high-temperature air medium, was analyzed. The fuel pellet surface temperature was recorded using contactless two-color pyrometry with a time increment of 2 ms by means of a high-speed color video camera and an original video frame processing algorithm developed in Wolfram Mathematica. The obtained results made it possible to explore the patterns and characteristics of coal ignition, considering the detected contribution that the gas-phase combustion of released volatiles makes to the pellet heating. The effect of the heating source temperature on the intensity of the fuel surface temperature evolution during the induction period was shown. When the fuel was heated to temperatures above 1073 K, local extrema were identified on the corresponding temperature trend. These characterize the emergence of hot spots – the sites of volatile ignition, preceding the heterogeneous ignition of the solid carbonaceous residue forming as a result of the thermal decomposition of the near-surface layer of the fuel pellet. The time intervals between consecutive local flashes of volatiles in their series from the first recorded occurrence to the last one, corresponding to the moment of the fuel's heterogeneous ignition, reduce five- to sixfold. The uniformity of temperature distribution over the surface of the fuel pellet at the moment of its heterogeneous ignition allows us to determine the moment when the gas-phase spot ignition of volatiles changes to the heterogeneous combustion of the solid carbonaceous residue at heating source temperatures over 1100 K.