Coke making

In this lecture a very brief discussion on coke making is given. Readers can see the references given at the end of this lecture.

How coke is produced

For material and heat balence a brief account of the coal analysis is given. For the details the readers may refer the lecture 3 of the fuel, furnace and refractory web/video course.

Coal consists of complex organic compounds in which  and S atoms are bonded together. The analysis of coal is reported either by performing proximate or ultimate analysis. In the proximate analysis moisture (m), ash (a) and volatile matter (vm) are determined and expressed in terms of percent of initial mass of coal. Fixed carbon (FC) in percent is obtained by
 

In the ultimate analysis carbon, sulphur , hydrogen and nitrogen are determined by the chemical analysis and expressed on a  moisture-free basis. The ash is determined as it is done for proximate analysis and is expressed on the moisture-free basis. Then percent oxygen (%O) is found by
           

Coke is produced by heating coal to high temperature  out of contact of air until all volatile matters are removed. The process is called “carbonization” or “Destructive distillation of coal”.As a result of heating these bounds are broken and new bounds are formed like  and other complex hydrocarbons like  etc.

The mass of coal during heating fuses and becomes plastic. It swells during coking and then r solidifies. The structure of coke depends much on fusion, swelling and re-solidification. Difference in behavior of different types of coal account for the difference in structure of coke. For example non coking coal may decompose without becoming plastic at any stage. Mixing of two different varieties of coal i.e. non- coking coal with coking coal result in control over coke properties.

By product coke-oven

Metallurgical coke (coke used in blast furnace iron making) is produced in by product coke ovens. By – product coke ovens are flexible in treating different types of coal and to control the coke properties.

Coke is charged batch wise into silica lined or refractory lined retorts and these retorts are heated externally by burning gaseous fuels. Note that retorts are heated indirectly and coal is heated through heat transferred from the walls of the retort. Coal near the wall of the retort is heated faster than coal near the center. As a result, coke near the walls swells much earlier than coal at the center. Therefore, proper distribution of coal in the retort would be desirable.

 The volatile matter from the coal is collected in the byproduct recovery plant where by product are separated from each other. It takes around 18 hours to convert one batch of coal into coke. Coke is discharge from the other end of the retort mechanical hopper into a car, were it is wet quenched.

Note that hot coke so produced cannot be used directly into blast furnace hence coke is cooled to room temperature. During wet quenching of coal considerable amount of sensible heat is lost and pollutants are discharged in atmosphere. The aspect of coke making is dealt separately in next lecture with a quantitative illustration.

The quality of coke depends both on temperature and rate of heating. Metallurgical grade coke is produced at temperature higher than .

Among the by-products, coke oven gas possesses both sensible heat and potential energy. Coke oven gas is used as a fuel to heat the furnace and also in heating the coke oven. The leakage of atmospheric air into coke oven must be avoided as the air causes oxidation of C and results in decrease in yields.