The impact of soot on the combustion characteristics of coal particles of various types

• Published in: Combustion and flame Vol. 95; no. 1; pp. 1 - 21
• Main Authors: Lau, Chun Wai, Niksa, Stephen
• Format: Journal Article
• Language: English
• Published: New York, NY Elsevier Inc 01.10.1993; Elsevier Science
• Subjects: Applied sciences; Combustion of solid fuels; Combustion. Flame; Energy; Energy. Thermal use of fuels; Exact sciences and technology; Theoretical studies. Data and constants. Metering; Pyrolysis; Temperature; Soot; Combustion; Devolatilization; Oxidation; Modeling
• ISSN: 0010-2180; 1556-2921
• DOI: 10.1016/0010-2180(93)90048-8

The products of coal devolatilization are radically transformed by secondary pyrolysis after they are expelled into hot gases until only H 2, C 2H 2, CO, CO 2, H 2O and soot remain. The coal combustion model developed in this article represents the limiting scenario of infinitely fast secondary pyrolysis followed by combustion of gaseous fuels and soot in flame sheets, either on or around individual coal particles. Soot and gases from different coal types are distinguished by different evolution rates, elemental compositions, and transport properties. Thermophoresis and radiation are accounted for in the transport analysis. This study also develops separate limiting behavior for instantaneous soot oxidation in envelope flames and for frozen soot oxidation chemistry. Comparisons among predicted and observed flame lifetimes and maximum flame standoffs select the most realistic modeling scenarios. Radiation from soot into the surroundings dictates the overall impact of secondary pyrolysis on macroscopic combustion characteristics. Soot is almost 1000 times more efficient than its host particle in radiating energy, and dissipates up to 90% of the radiation during the initial stages of combustion. Soot radiation cools flame temperatures by up to 300 K, reducing the differences among flame temperatures for diverse coal types in 8% O 2 to only 100 K around 2200 K. At such temperatures, water/gas shift equilibrium determines the distribution of combustion products, and the energy carried away by intermediates becomes negligible. For envelope flames around 100 μm particles of all coal types, about 60% of the heat of combustion is fedback to the particle, and one-third is conducted or radiated into the surroundings. But for attached flames on smaller particles more than 90% is retained by the particle.