Restart of BF#2 of Bhushan Steel Limited Using Durfee Method
AK Saxena , KL Sarda, VN.Giridhar, Rajesh Srivastava
Bhushan Steel Limited, Narendrapur, Dist Dhenkanal Odisha
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ABSTRACT
The paper describes the method used for restarting of the 3800 m3 Blast furnace which was stopped unprepared 42 hrs after blow-in. The furnace was restarted after 5 months of stoppage using the durfee pipe technology. The method of durfee involves installation of special pipes for blowing in Hot Blast through the Tap hole of the blast furnace for certain time period. This helps in heating up of the hearth and helps in early connection between Tap hole and Tuyere. Special drilling tools were adopted for opening of the blanked Tuyeres, without any wind reduction/shutdown. Thermocouples were installed in all the 34 Tuyeres to monitor the gas temperature at the Tuyeres. Special cast plugs were used to ensure that Tuyeres open only as per schedule and premature opening was completely avoided. The hot metal and slag separation was started through the pseudo skimmer provided and metal was taken into ladle on the 4th day. Another 4 days the furnace was operated through the pseudo skimmer and finally slag was taken into Inba Slag Granulation system on the 8th day. The early separation of metal and slag helped in fast stabilization of the blast furnace and filling of slag pits was avoided which was the major concern. The blast furnace revival process was completed on the 8th day with all the systems stabilised.
INTRODUCTION
BF#2 of Bhushan Steel is the latest generation of blast furnace designed and engineered based on PWIT technology. The blast furnace is of 3800 m3 internal volume with 34 tuyere’s, 4 tap hole and 2 cast houses. The construction of the blast furnace started in June 2009 and was completed in October 2013. The furnace was blown in during the month of November 2013 however the furnace had to be shut down due to unavoidable reasons unprepared. The furnace was subsequently started in the month of April 2014 using durfee pipe method.
Salient features of the Blast furnace are:
o Inner volume: 3,814 m3
o Working volume: 3,230 m3
o Number of tuyeres: 34
o Charging system: with belt conveyor and Paul Wurth Parallel hopper Bell Less Top®,
o Cooling system
o Under-hearth: water pipes
o Hearth: cast-iron staves
o from Bosh to lower Stack: copper staves
o Tuyere belt: cast iron staves
o Middle Stack to Throat: cast iron staves
PREPARATIONS FOR START UP
The major task of any recovery from a chilled hearth condition is establishing of connection between the Tap hole and the Tuyeres so that the generated liquids can be drained out.
The situation at BF#2 before starting the process of recovery was unique due to:
1. Furnace was stopped unprepared for a very long duration
2. Metal along with slag had solidified in the hearth and Tuyere area
Extensive discussions were held to find out whether to use the oxy-fuel burners in the tap hole or to go for Durfee pipe method which involves blowing of oxygen enriched hot blast through the tap hole. The Durfee method has been used extensively in Europe for revivals after prolonged shutdown of blast furnace, however they have been mostly employed when the shutdowns were prepared. The oxy-fuel method is a good choice when the hearth is severely chilled as it brings in a very large amount of heat and melts away the chilled metal and slag leading to a good connection between the tap hole and tuyere.
Based on lot of deliberations and critical analysis of the situation of burden inside the blast furnace along with experts from PWIT it was concluded to go for the Durfee method for revival of the blast furnace as it was felt that the coke inside may be just sufficient to provide the energy for start of the revival process.
PREPARATION OF THE TUYERES
Any recovery process demands that Tuyeres don’t open up by itself and create drainage issues or Tuyere Burnings leading to unplanned shutdown. Tuyeres were carefully plugged with Ceramic Inserts and gaps were filled up with mortar to ensure no blast can enter the furnace even at higher blast pressures. Also to monitor the situation inside thermocouples were installed in the ceramic plugs. This would give us a fair indication of the status of the BF inside.
To ensure a fast recovery and to avoid any shutdown for opening the tuyeres special drilling tools were used which could drill the ceramic plug even upto blast pressures of 2 bar.
INSTALLATION OF DURFEE PIPE
Tap hole No 2 and 3 were core drilled up to a depth of 4 m horizontally to install the Durfee pipe as away from the hearth refractory to avoid burn out. Stainless steel tube of special construction was pushed inside the tap hole and tightly sealed with castable to avoid leakage of the gases. This was then connected to two modified peephole assembly of Blow pipe with isolation valves.
BUSTLE PIPE HEATING
12 hours before the blast was to be taken through the tap hole, Bustle Main heating was started. 8 to 10 Tuyere peep-hole flanges were opened and hot blast was blown out to heat up the bustle main to upto 650 deg C. After completion of the heating the peephole flanges were closed and preparations were started to start the revival process.
THE START UP
Blast was introduced through both the Durfee’s at a pressure of 2.5 bar at a Temperature of 550 deg C. No flow was observed even upto a pressure of 4.0 bar even after 6 hrs. This indicated a very bad permeability in the hearth and it was decided to remove Durfee from the tap hole and start lancing. It was inferred that slag within the coke interstices has solidified in the lower heart just at the tap hole area and continuous lancing shall help melt the slag and permeability can be achieved. The lancing in TH#2 was continued for around 20 hrs Durfee was re-introduced into the Tap holes.
Blast was introduced into the Durfee pipes along with oxygen. A good delta P was observed between the Durfee which implied a good permeability was present in the hearth area. After around 12 hrs and cumulative flow of around 140000 Nm3 of blast blown through the durfee pipes 4 Tuyeres above TH#2 were opened without reducing blast by using the special drilling methods. Oxygen was introduced into the Tuyeres through the peephole flange by installing branch pipes. 2 hours after opening of the 4 Tuyeres Slag was seen in TH#2 and durfee pipe was removed to divert the hot liquids into the slag pit. Good flow of slag was obtained for around 30 minutes and cast closed.
However after removal of Durfee from TH#3 only gas blow was observed and no liquids were obtained. It was decided to continue opening Tuyeres between TH#2 and TH#3 to have an interconnection between them. TH#1 and TH#4 were not used in the revival process. Further opening of Tuyere was executed based on the furnace working and casting.
TH#2 was cast every two hours with good fluidity of slag and metal and taken into the slag pit. After 71 hours from start up hot metal was tapped into ladle through the upper skimmer and slag was diverted to the slag pit. Slag and Metal separation was good. Gas Cleaning Plant was connected after 75 hrs from start up and gas connected to network.
TH#3 was continued to be lanced and after 135 hours of start up good stream of liquids was obtained and diverted to the slag pit after which Main trough of TH#3 was cleaned of all sand and metal taken through bottom skimmer into ladle. Subsequently TH#2 also was connected through the bottom skimmer.
Conclusion
On the 8th day after start up with 20 of 34 Tuyeres in operation the furnace was successfully commissioned and put into operation. The authors would like to put on record their deep appreciation for the help and support from PWIT team in smoothly commissioning the furnace.
Fig-8 No OF TUYERE & BLAST INCREASE
Fig-9 TUYERE OPENING SEQUENCE
Fig-10 HOT METAL PRODUCTION TONS