Coal Blend Moisture-A Boon or Bane in Cokemaking?

A by-product coke making plant is required to supply sufficient coke of good quality and adequate gas of high calorific value for the integrated steel plant to be a going concern. The one element that influences the handling of coal and impacts the operation and efficiency of the plant is moisture. Compared to other important properties of the coal blend, moisture can be easily manipulated. The coal moisture can be increased simply by adding water through hose pipes. Also, it can be reduced to 5-6 mass percent using Coal Moisture Control (CMC) and 2-4 mass percent using Dry-cleaned & Agglomerated Pre-compaction System (DAPS). Moisture content is one among the many variables affecting the bulk density of coal blend and those controlling the coke qualities and yield. Increase in moisture reduces coal grind-ability, coking pressure and internal gas pressure; helps in dust suppression during charging and hence reduces jamming of ascension pipes and hydraulic main. Batteries charging coals with high moisture content are not troubled with roof carbon deposits. It was observed that when moisture content in coal blend of SAIL-Bokaro Steel Plant increased to more than 8.50%, the calorific value of coke oven gas improved. In the working moisture range of 9-11%, the increase of the yield of coke oven gas per 1% of working moisture is 5.2 m(3). Studies have shown, however, that the increase in moisture content of coal beyond 8% hampers strong coke formation. Pre-carbonization preheating process generally showed an increase in the proportion of 40-80 mm coke, compared with wet charges. For SAIL-Bokaro coke ovens, driving out 1% moisture from coal blend requires 125 Mega-calories of heat/oven. With lesser moisture, the emission of NOx in atmosphere will also be low. On using dry to low moisture coal blend, the swelling of coke mass increases leading to difficulty in oven pushings. Hence, an optimum level of moisture content of charge coal needs to be maintained for improving coke oven productivity, coke quality and operational smoothness. The coke oven managers all around the globe maintain this optimum level according to their requirement, the operating conditions, the quality of product and by products, the oven health & age and the ease of handling.