Abstract
Current work presents an optical setup, its calibration and reference process and the first results from single particle emissivity measurements of pulverized biomass and coal fuel particles. In contrast to earlier attempts, the setup offers the possibility of emissivity measurements during the whole particle burn-off. A laser ignites a single particle, placed in the center of the setup. Two photomultipliers observe the emitted particle radiation in the visible range (550 nm and 700 nm) for temperature calculation, using two-color pyrometry. An InSb-detector records the emitted particle radiation between 2.4 µm and 5.5 µm, which is later used to calculate particle emissivity in this range. The conclusion of multiple particle measurements lead to decreasing particle emissivity with increasing temperature. For coal particles the emissivity decreases from 0.45 at 2300 K to 0.03 at 3400 K. Biomass char shows a similar trend with a decrease from 0.18 (2100 K) to 0.03 (2900 K).
Highlights
The optical properties of fuel particles are of decisive importance for radiative heat transfer in pulverized fuel combustion systems [1,2]
Emissivity especially has a large impact on the calculations of heat transfer in a combustion system [4], burning kinetics [5,6,7] and pyrolysis [8,9]
For all temperatures an uncertainty in the range of ∆ε~ 0.2 seems to be justifiable. This is in agreement with the scatter from most of the other publications dealing with pulverized fuel emissivity [10,11,13]
Summary
The optical properties of fuel particles are of decisive importance for radiative heat transfer in pulverized fuel combustion systems [1,2]. Graeser et al [11] and Schiemann et al [12] achieved significant progress They measured temperature-dependent emissivity of burning coal char in two different spectral ranges. Using the particle diameter and the temperature derived by ratio pyrometry, they calculated emissivity Due to their measurement method, the data of each measurement are just a snapshot of a short phase during the particle burn-off, caused by the fixed optical setup. The current work presents the first results gained from a newly designed setup observing burning particles during their whole burn-off process This involves the setup, the calibration and calculation methods and first.
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