The Making of Coke

Adapted from (Don Reynolds, 2006)

What is coke used for, and how is it made? 

For many years the main consumer of coke has been the metallurgical industry which has used it as a fuel and reductant for blast furnaces and cupolas.  It was, and still is, used by blacksmiths.  Many years ago it was used as a smokeless fuel for steam trams.  Coke is also used in some manufacturing industries as a filter medium.  Coke is manufactured from coal: the coal is heated in the virtual absence of air to drive off the volatile matter, leaving the relatively strong porous coke as a product.  (Charcoal is made in a similar way, using wood rather than coal.)  In a very few coal mines a natural coke is found in patches where the coal was heated by volcanic action at some time in the past.  

Initially coke was made by heaping coal in a pile and covering it completely with earth and clay.  A small hole through the covering was made in the top of the heap to act as a chimney; another hole, or holes, was left in the covering at the base of the stack.  The coal was then set alight through the hole at the bottom of the heap.  After a period the pile of coal was fully heated and a large plume of smoke issued from the hole in the top.  The bottom holes were then completely plugged with clay and the top hole covered,  the heap being left to slowly burn with the heat driving off the remaining volatiles in the coke.  When the coke burner was satisfied that the process was complete the covering material was removed and the coke oven was opened out to allow it to cool.  Sometimes water was sprayed on the coke to assist the cooling process.  The heap coking of coal was a tedious and inefficient method of manufacturing coke – making a temporary covering over a heap of coal and then having to demolish it after each burn.  

It was an obvious next step to make a permanent vessel or oven into which the coal could be added and the coke removed without disturbing the oven.  The use of fixed ovens to coke coal quickly replaced the heap burning method.  Initially the ovens were circular and domed.  The ovens looked like a beehive and were called “beehive coke ovens“. 

Ovens were clustered together in groups to conserve heat.  After an oven was charged with coal and lit, the bottom door opening was almost closed with bricks and clay.  When the coke burner considered the charge was coked he opened the oven and quenched the burning coke with water while still in the oven.  The coke was manually raked from the oven on to a bench where additional quenching with water could be carried out if necessary.  The ovens could be laid out in such a manner as to allow a ‘larry car’ to run on tracks on top of the ovens to facilitate the charging of coal into the ovens.  Removing the coke from the circular ovens was a tedious operation.  To help overcome these problems the rectangular beehive coke oven was developed.  

These ovens were built side by side to form a battery or bench of ovens.  Initially one end of the oven was completely blocked off while the other end was completely open; the open end was closed with brickwork prior to the oven being charged with coal.  An improvement was made in the design of the rectangular ovens when the closure of both ends was made removable; this allowed a mechanical pusher to push the burnt coke from the oven on to the coke wharf at the other end of the oven.  The removable doors could then be replaced and the oven recharged.  This was the style of oven seen in the larger coke plants in the Illawarra, as seen below.

In later years by-product coke ovens were developed.  In this system the coal is charged into the oven which is completely closed to prevent the entry of air; the coal is heated through the walls of the oven and the volatiles in the coal are driven off and collected. The collected volatiles are treated to recover valuable by-products and the exhaust gas is a high quality fuel which is partially utilised to heat the ovens with the surplus being available for sale or for use in other heating processes. These ovens are tall, thin and quite long and built in multi oven batteries; the charging, pushing and quenching operations are highly automated.  Municipal gas works which are now almost extinct, were designed specifically to produce a heating gas with coke being produced as a byproduct.  This coke was generally inferior to beehive coke or the coke from the above by-product ovens.