OCR Text |
Show INTRODUCTION The rising cost and reduced availability of fuel oil and natural gas had made coal an attractive blast furnace injectent. Despite fuel handling and flow monitoring problems inherent in a solid injection system the use of coal is becoming more common. Because of the immense difficulties in performing combustion related measurements under actual blast furnace conditions, knowledge about combustion in a blast furnace is'extremely limited. It is believed that it is important for solid fuel injectents to be consumed before leaving the zone of a blast furnace known as the raceway. This belief has arisen from problems with soot collection in the blast furnace gas cleaning plants and raceway blockages encountered during heavy fuel oil injection. The raceway is a cavity formed in the blast furnace charge by forces generated from the hot blast. Several people have measured the size and shape of the chamber in operating blast furnaces. An endoscope was used by Greuel et al. [~l] to investigate the raceway region. This chamber takes the shape of a curved hose expanding in diameter with distance. A typical length for this chamber appears to be between 1.0 and 1.5 m long. Estimating the average blast velocity in the raceway at 200 m/s the residence time of a small particle travelling with the gas would only be about 10 ms. This time is much shorter than in furnaces where pulverized coal is currently used (cement kilns and boilers): in this type of furnace the residence time is on the order of seconds. This short residence time in the raceway region is clearly an important factor in determining the percentage of coal combustion that will occur. Particularly the heterogeneous combustion of coal is a relatively slow process, although burnout times as fast as 20 ms have been reported for optimum conditions in coal engines [2]. However, it is clear that in a blast furnace, conditions such as heating rate, mixing, particle size, and other fuel properties must be optimized to achieve a reasonable level of burnout in the raceway region. The effect of some parameters on burnout times, for example particle sizes, are reasonably well understood. However, intrinsic coal properties such as reactivity and the devolatilization process under high heating rates are not as well understood, and also have an important role in determining combustion times. 10-2 |