1. Suppose a large BF is being fed with Dry Quenched Coke. Suddenly , a breakdown occurs either in the Cove Oven Battery, or in Coke Dry Cooling Plant or in the transport route. The coke feed to the large BF has now to be changed to the wet quenched weathered coke from stock. What precautions need to be taken to ensure safe operation of BF in such scenario ?
2. A large BF is being operated at very high PCI Rate (150-200 kg/THM) and suddenly such a breakdown occurs that total PCI has to be suspended. What immediate actions must be taken for safe operation of BF in this scenario?
You may share some actual experience at your BFs also, if possible.
Dear Salil,
The only difference between the Dry coke and wet coke is the moisture content. For example, Dry coke may have max of 1% moisture, whereas wet coke moisture varies from 4 to 10% depending upon the quenching practice.
While shifting from dry coke to wet coke operation, following points have to consider
1) Burden calculation to match the carbon rate considering the extra fuel required for moisture removal from wet coke.
2) Burden distribution has to be modified slightly to ensure the sufficient coke available at the wall side for direct reduction. otherwise it will result in unreduced FeO moving down.
3) Ratio of Oxide to coke batch has to adjusted with respect to the moisture content from wet coke.
These are my observations. others may comment for better understanding.
Regards
Venkatesan J
Thanks for such prompt response. But need a bit more clarification in P. No. 2 above, for change over from CDQ Coke to wet quenched coke.
Dear Salil
Regarding PCI operation to non PCI operation,
Immediately you may reduce your production drastically and drive the furnace very slowly until you revive the PCI operation or the modified coke batch till it comes to the tuyere level.
Dear Salil Jain ,
Clarification for Point no.2, Ensuring sufficient amount of carbon for direct reduction at wall side.
for example, blast furnace is operating with around 90 kg/thm ( ~22%) of direct reduction. Assuming this direct reduction is uniform throughout the furnace from wall to center.
Assumption : Oxide batch is similar for both the case.
Case 1 : Charging Dry coke ( moisture : 1%)
Assume : 2 tons : Carbon = (2 - (2*1%)) * 88% = 1.7424 tons
Case 2 : Charging Wet coke ( moisture : 7%)
Assume : 2 tons : Carbon = ( 2 - (2*7%))*88% = 1.6368 tons
this difference in carbon feed will affect the reduction behavior at wall side. this may results in insufficient carbon feed for direct reduction ..
To overcome the above issue, charge extra coke at the wall side, in this case approximately around 2.13 tons at wall side.
Regards
Venkatesan J
Final conclusion :
When the coke feed to a large BF changes from Dry Quenched source to Wet Quenched source, following has to be ensured:
1. Frequent cleaning of stock-house screens to avoid clogging and to get better screening
2. Correction in fuel input to the furnace by increasing coke base (reducing Burden ratio) to compensate for extra moisture input and also for the expected deterioration in the coke properties (both hold & cold strength).
3. Some changes in burden distribution may be required to be made (but i m not very clear about what and why: Centre coke to be increased or Periphery coke to be increased ??, what about nut coke charging rate ???)
4. Oxygen enrichment may be reduced a bit to maintain Top gas temperature in desired range (its equivalent to controlling the furnace driving rate within safe limits)
Any comments ??
1. If in your blast furnace top gas temperature is higher than 80oC, than moisture in coke is not affecting your coke rate (of course if your coke weighing system is controlling weight of dry coke by continuous coke moisture measuring and doing relevant corrections).
2. Stop BF, repair PCI plant and start BF again.
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