Coke and Coke making

Coke is a solid carbon fuel and carbon source used to melt and reduce iron ore. Coke production begins with pulverized, bituminous coal. The coke making process involves carbonization of coal to high temperatures (1100°C) in an oxygen deficient atmosphere in order to concentrate the carbon. Coke is produced in batch processes, with multiple coke ovens operating simultaneously. The commercial coke making process can be broken down into two categories: a) By-product Coke making and b) Non-Recovery / Heat Recovery Coke making.

Coal is fed into a coke oven that is sealed and heated to very high temperatures for 14 to 65 hours. The process of heating coal in coke ovens to drive volatile matter from it is called Coking. Ovens are heated by coke-oven gas, which burns in heating flues in an oven's sidewalls. Waste gases from this combustion pass out through a stack or chimney. At either 20- or 30-minute intervals, the flow of gas, air and waste gas are reversed to maintain uniform temperature distribution across the wall. The design of heating systems varies from battery to battery.

Non-Recovery Coke Ovens

Non-recovery Coke Plants are originally referred to as Beehive Ovens, again the coal is carbonized in large oven chambers. The carbonization process takes place from the top by radiant heat transfer and from the bottom by conduction of heat through the sole floor. Primary air for combustion is introduced into the oven chamber through several ports located above the charge level in both pusher and coke side doors of the oven. Partially combusted gases exit the top chamber through "down comer" passages in the oven wall and enter the sole flue, thereby heating the sole of the oven. Combusted gases collect in a common tunnel and exit via a stack, which creates a natural draft in the oven. Since the by-products are not recovered, the process is called Non-Recovery coke making. In non-recovery coke ovens, coke oven slag and other by-products are sent to the battery where they are combusted. As the ovens operate under negative pressure and at a temperature at which all potential pollutants break down into combustible compounds, this technique consumes the by-products, eliminating much of the air and water pollution.

When the waste gas exits into a waste heat recovery boiler, which converts excess heat into steam for power generation, the process is called Heat Recovery Coke making.

The advantages of the Heat recovery process

Emissions – The emissions from this type of ovens are relatively low and can meet the emission norms of most advanced countries

Particulate emission – Particulate emissions out of the stack or into the boiler are normally below 50 mg per Nm#. This is achieved by the trapping of initial dust during charging by the operation of a bag filter on the Pusher car during charging on the one hand and due to system of tertiary air inlet for combustion of the combusting particles in the common flue on the other.

Exit Gas composition – Due to higher coking temperatures (1200-1250 deg C) the hydrocarbons in the coke oven gas are broken into combustible compounds and burnt. Controlled introduction of tertiary air in the common flues ensure that there are no remnant combustible matter in the waste gases going to the stack.