COAL INJECTION IN ROTARY KILN

COAL INJECTION IN ROTARY KILN

K.Ramu, C.Ramesh, A.Rajasekaran


During the process of manufacture of sponge iron, coal injection plays a very vital role in maintaining the product quality. Due to highly endothermic nature of Boudouard reaction, heat has to be continuously supplied in entire reduction zone of rotary kiln.

Generally the length of reduction zone is 60-70% of total kiln length. If we consider a 100TPD kiln of 42 meter length, coal as to be injected to a distance not less than 25 – 30 meter for maintaining proper reducing atmosphere in reduction zone as well as supply the required heat for Boudouard reaction. Further at it is also essential that the required quantity of coal only is injected at every zone in this reduction area depending upon the rate of reduction in each of the zones. So we need a thorough analysis coal injection pattern for various sizes of coal particles, volume of air, velocity of coal particles, coal injection trajectory.

Based on all these principles, extensive work has been done at all sponge iron plants for the following :

1. Specification of twin lobe Blower
2. Specification of Rotary air lock feeder
3. Injection of Mixture of fine coal and coarse coal
4. Alignment of coal injection system

1. Specification of twin lobe Blower

Twin lobe blowers or compressors designed for free air delivery of 1400cubic meter/hour at 0.8 bar pressure for 100 TPD kilns and 1900cubic meter/hour at 1 bar pressure for 350/500 TPD kilns have been found suitable.

The detailed specification of our twin lobe blowers water cooled model for 100 TPD is as given below:

Type : twin lobe
Application : fine coal inject ion
Capacity : 1500 cubic meter/hour.
Pressure : 0.8 bar.
Motor capacity : 55 kw
RPM : 1450
Model(USHA) : 4RP-1150.





2. Rotary Air lock feeder

Rotary air lock feeder provides positive seal to the injection air as well as medium for feeding coal. In order to ensure proper sealing of blown air, it is essential to maintain clearance between the rotary blade tips and casing within 0.25 to 0.5 mm.

The specification of rotary air lock feeder as given below:

Type : rotary air lock feeder
Material to feed : Fine coal and coarse coal
Bulk density : 0.8 T/cubic meter
Shaft : EN48
Material construction : SS304.
Drive capacity : 2.2kw
Geared motor output rpm : 19/21

3. Mixture of fine and coarse coal

As explained earlier, coal has to be distributed in the entire reduction zone kiln depending upon heat demand in every zone. In order to ensure effective coal injection, we need the mixture of fine coal 0-4mm and coarse coal 4 mm – 10 mm. Generally 0 to 1 mm coal travels a distance of 5 meters and 2 to 4 mm travels a distance 16 meter, 4 to 6mm travels distance of 20 meters and 6 to 8 mm travels at a distance of 22 meters and +8mm travels distance 26 meters. As per this analysis we need the coal all the size fraction for proper distribution in the kiln as per requirement.

Accordingly the mixture 0 – 4 mm fine coal and 4-10mm coarse coal are fed for coal injection. The percentage of 0– 4 mm is generally 60–70%, 4–10 mm is 30– 40%. The details of coal injection trials done during shutdown prior to start up are enclosed the table 1, 2, 3.

4. Alignment of coal injection system

The alignment of coal injection in the kiln is very critical to ensure proper coal particle trajectory. The inclination of the pipe from injector is aligned at 1 to 1.5 degrees. A nozzle is to be provided before rotary air lock feeder to increase the velocity of coal particles and minimize dust losses. One pressure gauge is to be provided before the nozzle to monitor the back pressure of coal injection. In 100 TPD kiln this back pressure varies from 0.15 to 0.2 bar which indicates proper condition of lobe compressor and rotary air lock feeder. If pressure falls below 0.05 bar, either lobe compressor or rotary air lock feeder has to be replaced immediately.



5. Coal injection trials

After completion of alignment of coal injection pipe, the coal injection trial taken prior to start up of kiln as per following parameters :

1. Fine coal 0 – 4 mm : 1.2 ton/hr
2. Coarse or medium 4 – 10 mm : 0.6 ton/hr
3. Blower rpm : 1250 – 1400 rpm
4. Coal injection pressure : 0.2 kg/cm2
5. Time duration : 15 minutes

After the completion of trial, the entire coal is collected for each 2 meter length of the kiln. These samples are analyzed for weight fraction and size analysis. Based on sized analysis, mean particle size can be calculated for every zone.

6. Performance evaluation of coal injection trials

We have done the coal injection trials with old lobe compressor and worn out injector as well as new lobe compressor with new injector. With the poor condition of blower and the rotary air lock feeder, coal could be thrown to a distance of 13 meters only. With a good blower and reconditioned rotary air lock feeder, the distance of throw is 26 meters with good distribution.