Manufacture of Metallurgical Coke

MANUFACTURE OF METALLURGICAL COKE

There are so many types of ovens used for the manufacture of metallurgical coke. But the important one is Otto-Hoffman's by product oven.

1. Otto-Hoffman's by-product oven

Inorder to

1. increase the thermal efficiency of the carbonization process and,

2. recover the valuable by products (like coal gas, ammonia, benzol oil, etc.) Otto-Hoffman developed modern by product coke oven.

The oven consists of a number of silica chambers. Each chamber is about 10 - 12 m long, 3 – 4 m height and 0.4 – 0.45 m wide. Each chamber is provided with a charging hole at the top. It is also provided with a gas off take valve and iron door at each end for discharging coke (Fig 5.1).

Coal is introduced into the silica chamber and the chambers are closed. The chambers are heated to 1200°C by burning the preheated air and the producer gas mixture in the interspaces between the chambers.

The air and gas are preheated by sending them through 2nd and 3rd hot regenerators. Hot flue gases produced during combustion are allowed to pass through 1st and 4th regenerators until the temperature has been raised to 1000°C. While 1st and 4th regenerators are being heated by hot flue gases, the 2nd and 3rd regenerators are used for heating the incoming air and gas mixture.

For economical heating, the direction of inlet gases and flue gases are changed frequently. The above system of recycling the flue gases to produce heat energy is known as regenerative system of heat economy. When the process is complete, the coke is removed and quenched with water.

Time taken for complete carbonisation is about 12-20 hours. The yield of coke is about 70%.

The valuable by products like tar, ammonia, H₂S and benzol, etc. can be recovered from coal gas.

Recovery of by-products

1. Tar

The coal gases are first passed through a tower in which liquor ammonia is sprayed. Tar and dust get dissolved and collected in a tank below, which is heated by steam coils to recover back the ammonia sprayed.

2. Ammonia

The gases are then passed through another tower in which water is sprayed. Here ammonia gets converted to NH₄OH.

3. Naphthalene

The gases are again passed through a tower, in which cooled water is sprayed. Here naphthalene gets condensed.

4. Benzene

The gases are passed through another tower, where petroleum is sprayed. Here benzene gets condensed to liquid.

5. Hydrogen Sulphide

The remaining gases are then passed through a purifier packed with moist Fe₂O₃. Here H₂S is retained.

The final gas left out is pure coal gas, which is used as a gaseous fuel.

Advantages of Otto Hoffman's process

1. Valuable by products like ammonia, coal gas, naphthalene etc., are recovered.

2. The carbonisation time is less.

3. Heating is done externally by producer gas.