Carbonization of Coal for Metallurgical Coke Production


Carbonization of Coal for Metallurgical Coke Production

Carbonization of coal is also known as coking of coal. The process consists of thermal decomposition of coals either in the absence of air or in controlled atmosphere to produce a carbonaceous residue known as coke.

Carbonization of coal can be carried out at the following three temperature ranges.

  • Low temperature carbonization is normally carried out in the temperature range of 500 deg C to 700 deg C. In this type of carbonization, the yields of liquid products are higher and there is lower gaseous product yield. The coke produced is having higher volatile matter and is free burning.
  • Medium temperature carbonization is done at temperature range of around 800 deg C. This carbonization produces smokeless soft coke. By products produced are similar in characteristics to high temperature carbonization. Medium temperature carbonization is rarely practiced these days.
  • High temperature carbonization is carried out at a temperature which is above 900 deg C. This carbonization gives higher yield of gaseous products and lower yield of liquid products. This carbonization produces hard coke and is normally employed for the production of metallurgical coke from coking coals.

Process of carbonization of coal

The coal to coke transformation takes place as the coal is heated. When the state of fusing is reached, the layer of heated coal softens and fuses. From about 375 deg C to 475 deg C, the coal decomposes to form plastic layer.  Destructive distillation reactions proceed rapidly in the plastic layer with evolution of volatile products. At about 475 deg C to 600 deg C, there is a marked evolution of tar, and aromatic hydrocarbon compounds. The gas and condensable vapour are entrapped in the plastic mass and, as they expand tend to swell it. As the reactions proceed and as the temperature of the fused zone increases, the plasticity of the coal decreases. With continued heating and evolution of the gas the fused layer gradually resolidifies into semi coke having typical, cellular coke structure. The coke at this stage still contains substantial volatile matter. As the temperature increases further beyond 600 deg C, the destructive distillation reaction continues with the evolution of gas and a little tar.  The coke stabilization takes place as the temperature increases from 600 deg C to 1100 deg C.  This is characterized by contraction of coke mass, structural development of coke and final hydrogen evolution. At this stage the final reactions take place. These reactions split off hydrogen from extremely complex, high molecular weight hydro- carbons. With increasing temperature, the coke mass shrinks with the development of shrinkage cracks.

The caking mechanism

When the coking coals are carbonized then first the plastic mass of optical isotropic is formed, and thereafter gradually lamellar nematic liquid crystals are formed. This polymeric phase is called mesophase. This is the intermediate phase between the isotropic fluid coal and the solid anisotropic semi coke ultimately formed from the mesophase, and has properties which are intermediate between solids and liquids. If the fluidity of the intermediate phase is quite high then mesophase coalesce immediately into a single larger unit. Over a range of increasing temperature, mesophase is formed continuously, grows in size and ultimately touches each other. Thus the mesophase can solidify and convert from coking coal into optical anisotropic texture of coke.

History of coking coals

Coke was produced in ancient China as per historical sources dating to the fourth century.  The Chinese people first used coke for heating and cooking no later than the ninth century. In 1709 a coke-fired blast furnace to produce cast iron was established in Great Britain. , During early 18th century the coke was manufactured by burning coal in heaps on the ground in such a way that only the outer layer burned, leaving the interior of the pile in a carbonized state.

The ‘Hearth’ process of coke making, using lump coal, was continued to be used in many areas during the first half of the 19th century. This process was similar to that of charcoal burning but using a heap of coals covered with coke dust instead of a heap of prepared wood, covered with twigs, leaves and earth.

These led subsequently to the development of beehive ovens of different shapes and sizes to meet the increasing demands of hard coke for iron smelting.

Beehive ovens

A beehive oven is a simple firebrick chamber built with an arched roof so that the shape inside is that of an old-fashioned beehive. Its dimensions are typically 4 m in width and 2.5 m in height. Beehive ovens are usually built in rows, one oven beside another with common walls between neighboring ovens. Such a row of ovens is termed a battery. A battery usually consists of many ovens, sometimes hundreds, in a row. Typical cross section of a beehive oven is shown in Fig 1.

Beehive oven

Fig 1 Typical cross section of a beehive oven

Rail tracks for handling the coal to the ovens ran along the tops; and other rail tracks for handling the coke cars ran beside the ovens. The roof has a hole for charging the coal or other kindling from the top. The discharging hole is provided in the circumference of the lower part of the wall.

Coal is charged into an empty oven through the hole at the apex of the dome. It forms a cone-shaped pile which is levelled to a uniform layer by means of a rake passed through the door to produce an even layer of about 600 mm to 900 mm deep.

The carbonization process is started by means of the heat retained in the walls of the oven from the previous charge of coal. Almost immediately after charging gas consisting of volatile matter is produced from the coal. The air for combustion is admitted through an opening at the top of the oven door or through side door. Start of carbonization produces volatile matter which is burnt inside the partially closed side door. Carbonization proceeds from top to bottom. Heat is supplied by the burning volatile matter so no by-products are recovered. The exhaust gases are allowed to escape to the atmosphere.

The time of coking which depends largely on the depth of the layer of coal, ranges from 48 to 72 hours. As coking proceeds, the volume of gas evolved decreases, and the size of the opening in the door is correspondingly decreased or by introducing bricks at the top opening. This regulates the quantity of air and prevents the entrance of an excessive volume of air, which otherwise would burn part of the coke and might be sufficient to cool the oven as well.

The hot coke is quenched with water and discharged, manually through the side door. When coking is complete, the door is opened and the white hot coke is quenched by stream of water directed through the opening. The quenched coke is then raked from the oven manually and loaded into train cars for transport. The walls and roof retain enough heat to initiate carbonization of the next charge.

When coal is burned in a coke oven, the impurities of the coal not already driven off as gases accumulated to form slag, which is effectively a conglomeration of the removed impurities. Since it is not the desired coke product, slag is either discarded or being used as an ingredient in brick-making, mixed cement, and even as a fertilizer.

New ovens are brought up to temperature by heating with coal or wood before charging.

Beehive coking is now an obsolete process because of the small quantity it manufactured and the very large amount of pollution it produced. However it is still being used.

Byproduct coke oven batteries

Maximum amount of global coke production comes from these batteries. Coal in these batteries is carbonized in absence of air and these batteries are operated with positive pressure in the ovens. The coke making process in these coke oven batteries is called byproduct coke making since the off gas is collected and sent to the byproduct plant where various byproducts are recovered. Most of the byproduct coke oven batteries are integrated in an iron and steel plant for the purpose of coke oven gas.

Details of coke making in byproduct coke oven batteries and coke oven byproduct plant are given in separate articles under links http://ispatguru.com/coke-making-in-byproduct-coke-oven-batteries/ and  http://ispatguru.com/coke-oven-by-product-plant/.

Non recovery coke oven batteries

 In non recovery coke oven batteries 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 these batteries usually the waste gas exits into a waste heat recovery boiler which converts the excess heat into steam for power generation hence this the process is also called heat recovery coke making. Details of coke making in non recovery coke oven batteries are given in separate article under link http://ispatguru.com/non-recovery-coke-ovens-battery/.