Mechanism on the contribution of coal/char fragmentation to fly ash formation during pulverized coal combustion |
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Authors: | Qi Gao Shuiqing Li Yingqi Zhao Qiang Yao |
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Affiliation: | Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China |
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Abstract: | In this paper, the correlations between coal/char fragmentation and fly ash formation during pulverized coal combustion are investigated. We observed an explosion-like fragmentation of Zhundong coal in the early devolatilization stage by means of high-speed photography in the Hencken flat-flame burner. While high ash-fusion (HAF) bituminous and coal-derived char samples only undergo gentle perimeter fragmentation in the char burning stage. Simultaneously, combustion experiments of two kinds of coals were conducted in a 25?kW down-fired combustor. The particle size distributions (PSDs) of both fine particulates (PM1-10) and bulk fly ash (PM10+) were measured by Electrical Low Pressure Impactor (ELPI) and Malvern Mastersizer 2000, respectively. The results show that the mass PSD of residual fly ash (PM1+) from Zhundong coal exhibits a bi-modal shape with two peaks located at 14?µm and 102?µm, whereas that from HAF coal only possesses a single peak at 74?µm. A hybrid model accounting for multiple-route ash formation processes is developed to predict the PSD of fly ash during coal combustion. By incorporating coal/char fragmentation sub-models, the simulation can quantitatively reproduce the measured PM1+ PSDs for different kinds of coals. The sensitivity analysis further reveals that the bi-modal mass distribution of PM1+ intrinsically results from the coal fragmentation during devolatilization. |
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Keywords: | Multiple-route ash formation Coal/char fragmentation Hard-sphere model |
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