Single fuel particle emissivity evolution of biomass, coal and coal char

Abstract

In solid fuel combustion systems, reacting particles form the hottest surfaces and, therefore, contribute significantly to the radiative heat balance within these environments. Recently, experimental investigations of single-particle emissivity report values for coal and various biomass fuels. Building on this previous research, the experimental setup for single-particle investigations is further refined and applied to determine the reaction time- and wavelength-resolved emissivity of individual fuel particles during burnout. The modifications to the setup, as well as the calibration routines and referencing procedures, are presented. Subsequently, single particle emissivities of raw Colombian bituminous coal, its char and raw walnut shells are presented, representing typical fuels used in pulverized fuel combustion. During combustion, char and walnut shell particles undergo continuous shrinkage, whereas raw coal particles exhibit swelling. Furthermore, the maximum averaged surface temperatures of the fuels differ considerably. The char samples reach the highest temperature (∼1726 K) followed by raw coal (∼1605 K) and walnut shells (∼1575 K). A comparable trend is also observed for emissivity. The maximum surface-averaged emissivity in the mid-infrared (MIR) region (2.4–5.5 µm) is highest for char, followed by raw coal and walnut shells. In the near-infrared (NIR) region (0.85–2.4 µm) the emissivity of all investigated fuels is consistently lower but similarly distributed compared to the MIR. Raw coal and coal char show nearly graybody-like emission behavior, whereas the emissivity of walnut shells is higher at longer wavelengths throughout the reaction. Additionally, radiation emission peaks at 1384 nm are found for raw coal and walnut shells and are attributed to effects related to volatile release.