Coal dust flames: A review and development of a model for flame propagation
Coal mine explosions have been a problem for hundreds of years. The projected need for coal as an energy source will demand increased subsurface mining, in turn requiring improved techniques for the prevention of dust explosions. Preventive procedures, however, must rely on fundamental knowledge concerning the behavior of coal dust combustion. To this end, a review of coal dust combustion as reported in the literature is presented, and a mathematical model of flame propagation through coal dust-air mixtures is developed in detail. The model is basically used to predict burning velocities and to study the structure of coal dust-air flames.It is well known that coal particles burn in a very complicated manner. The combustion process includes heterogeneous surface reactions, devolatilization and subsequent reaction of the volatile components, swelling, cracking, and other physiochemical changes to the particles. In addition, it is known that coal particles do not always burn simply as shrinking spheres, but can burn internally and form cenospheres (hollow spheres) as well. The composition of the volatiles released, and the combustion itself, depend upon the rate of heating in a flame, thereby making analytical description of the process extremely difficult.This paper reviews the important studies reported in the literature which describe both the heterogeneous combustion processes for coal particles, including rapid pyrolysis toward volatile matter, and the subsequent gas-phase (homogeneous) reactions of the volatiles with the air. For example, the rate of heating of the coal particles determines the ratio of volatiles to fixed char in dry (ash-free) coal. Therefore the chemical source terms in any model must attempt to include as much as is known of the reported information on this key aspect. In addition, discussion is included of the experimental measurements carried out with coal dust flames in an attempt to categorize the data into two major classes; Class I experiments, having been conducted in open tubes or on small burners, and Class II experiments performed in enclosed furnaces or ducts. The major role of such enclosures is to act as a shield against heat losses, which can have a pronounced effect on the measured flame speed. It is our contention that the Class II tests are described by our model.The model that has been developed predicts steady (laminar) flame structure and flame speeds for premixed coal dust-air mixtures. The model takes into account radiative transport, two-phase flow conservation equations, heterogeneous carbon gasification and rapid devolatilization of the coal particles, and subsequent gas-phase combustion of the volatiles.
آتش گرد و غبار زغالسنگ: بررسی و توسعه مدلی برای انتشار شعله
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