Firing machine

There are three types of pellet firing machines that in commercial use which have discuss below :

1. The shaft kiln
2. The grate machine
3. The grate kiln.

The Shaft Kiln :

● It was developed in the 1950's. The green or dried pellets are fed vertically downwards in a central shaft of usually a rectangular cross section. Fuel is burned in two fire chambers, one on each long side of the shaft, and the hot gasses are allowed to enter the main shaft through multiple flues.

● It is in a way counter-current gas solid heater. The fired pellets are cooled in the lower portion and if any chunks are formed these are broken by the chunk breakers and cooled pellets are discharged from the bottom.

● Cooling air, introduced from below, gets pre heated and is either taken out to burn the fuel in the chamber or to make available preheated gas for completing the combustion in the firing zone of the shaft.
The shaft kiln - induration of pellets
The shaft kiln

● Here a typical furnace has a shaft of around 4×2m cross section and is nearly 20m height from the discharge point to the stock level. The feed and the discharge rates are adjusted to maintain a nearly constant stock line level.

● Nearly a third of the total air required for combustion is admitted in the combustion chamber for partial burning of the fuel. The remainder being introduce in the shaft for cooling from below and which is then available as preheated secondary air for completing the fuel combustion inside shaft.

● The temperature in the combustion chamber is around 1300℃. The furnace contains nearly 200t of pellets and production rate of hardened pellets is around 1000 to 1200 ton/day. Now some recent designs have Incorporated external coolers or internal coolers and heat exchanger to improve the thermal efficiency of the process.

The Grate Machine :

This types of machine are modified version of Dwight-Lloyed sintering machine in which the green balls are fed at open end on the continues traveling grate and the hardened end cooled pellets are discharge from the other end. The length of the grate is divided in to four different zones
1. Drying
2. Pre-heating
3. Firing
4. Cooling

● The hot air from the cooling zones is circulated in a complex manner to carry out drying, dehydration, pre-heating etc.
The grate kiln for induration of pellets
The grate kiln for induration of pellets

● Down draught, up-draught or a combination of the two is suitably employed in the design to carry out the preliminary operations before firing. The combustion varies with the mineralogical Constitution of the base feed and as a result the design differ considerably from place to place.

● A peculiar feature is the necessity of the protective layer of previously fired pellets on the bottom as well as on the sides of the bed. The temperature therefore need not be raised to the firing temperature level near bottom or the walls and the danger of overheating the metallic pellet is there by eliminated.
The straight grate kiln for induration of pellets
The straight grate kiln

● If such a precaution is not taken the pellets near the bottom or the walls will remain underscored in trying to protect the metallic grate and the walls.

● The latest modified version of grate machine is the use of a circular grate in place of straight grate. It eliminate the operational difficulties that are encountered in the straight grate. Only one such unit has been recently commissioned in Mexico and it's performance is being watched with interest.

The Grate Kiln :

● It is a combination of a grate and kiln system. The drying and pre-heating is carried out in this machine on the straight traveling grate, as in the grate system and firing is completed in a long rotating kiln, wherein the fuel is burned to generate the necessary temperature.

● Hot gases from the kiln are carried over the grate, where drying and pre-heating are carried out under down draught condition or in combination with up draught movement as well.

● The traveling grate in this case is not subject to very high temperature and hence the life of the grate is very high. It is one single factor that makes the machine a costly operation.

References :

1. Heat treatment principle and techniques  by : T.V. Rajan,  C.P. Sharma,  Ashok Sharma.
2. Physical Metallurgy Principle and Practice by : Raghavan V.,
3. Lectures of IITs & BPUT (ODISHA).

4. Iron making by : Dr. R. H Tupkary & V. R Tupkary

Author :
Subir Kumar Sahu.

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