Using high temperature carbonization, metallurgical coke (hard coke) can be prepared by two processes.
1. Beehive coke oven process.
2. Otto-hoffmann's by-product coke over process
1.12.1 Beehive coke oven process
It is the earliest and cheapest process. The quality of coke produced is good, but there is a lot of wastage of fuel and the byproducts cannot be recovered. It takes 3-4 days in completion of carbonization.
It is a dome shaped oven of 2.5 m height and 4m diameter having a capacity of 8-10 tonnes of coal. Oven is constructed by refractory bricks. One hole is provided at the top for charging of coal. Another side door is provided for discharging of coke. This door is also used for supply of air. Coal is charged through the top door and leveled to form about 0.6 m thickened layer. Slow carbonization takes place and coke formation will be complete within 3-4 days. After cooling the coke with water spray, it is taken out through side-door. Yield of coke is about 80%.
For economical operation, a number of such ovens should be used in series so that heat od out going gases of one is utilized by the adjacent oven.
Disadvantages of beehive coke oven
The byproducts are not recovered and are allowed to escape.
Lower amount of coke is produced
Process is not flexible and takes more time.
The exhaust gases produce pollution.
Due to non-recovery of valuable by products, this may used as emergency unit" to meet high demands for hard coke (as its capital and running cost is low).
1.12.2 Otto-hoffmann's by-product coke ovens
It is the most developed and modern by product coke oven. The by product like coal gas, ammonia, benzol oil, tar etc. are recovered. Heating is done externally. Heating is done on the basis of regenerative system. I.e. the heat of waste gases is utilized in this process.
Construction and working
Otto-hoffmann's by product coke oven consists of rectangular silica refractory chamber of 10-12 m length, 3-5 m height and 0.3 - 0.45m width. A number of such silica chamber (about 25-1000 ovens) are connected to form a sort of battery. Each chamber is provided with an opening at the top, for chargins of coal, a gas off-take and a door for discharging coke. A battery of such ovens is placed over the regenerators having checker brick work.
The various steps involved in processes are:
Ø Finely crushed coal is introduced through the top charging holes. These holes are then tightly closed to cut off the supply of air.
Ø The ovens are heated to 12000C by burning gaseous fuel like producer gas in vertical flues.
Ø Carbonization takes place towards the central zone.
Ø The waste flue gases leaving the flues are allowed to pass through the checker brick work in the regenerator of the next oven to preheat it. While incoming air and fuel gas pass through other preheated chambers. After some time the direction are reversed.
Ø This cycle goes on. Carbonization of the charge of coal completes in 11 to 18 hrs.
Ø After carbonization, the hot coke is taken to quenching station.
Ø Coke is quenched (rapidly cooled) by wet quenching (spraying water) or by dry quenching (passing inert gas like N2).
Note : dry quenching is advantageous because the coke produced by it is more strong, non reactive, drier and contains less dust.
The coal formed contains 25-30% volatile matter, 1-2% sulphur, 5-6% moisture and about 2% ash.
Yield of coke is about 75%.