Description:FORM 2
THE PATENT ACT 1970
(39 OF 1970)
&
The Patent Rules, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)

1 TITLE OF THE INVENTION :
A METHOD FOR CONTROLLING BLAST FURNACE (BF) HOT BLAST STOVE OPERATION TO MINIMISE BF GAS CONSUMPTION TO MAINTAIN REQUIRED HOT BLAST TEMPERATURE (HBT).



2 APPLICANT (S)

Name : JSW STEEL LIMITED.

Nationality : An Indian Company incorporated under the Companies Act, 1956.

Address : JSW CENTRE,
BANDRA KURLA COMPLEX,
BANDRA(EAST),
MUMBAI-400051,
MAHARASHTRA,INDIA.




3 PREAMBLE TO THE DESCRIPTION

COMPLETE

The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OFTHE INVENTION:

The present invention relates to providing a method of controlled operation of the BF stoves with BF gas alone involving a heating control model based on the fundamental empirical equations and mass & energy balance for stove and to maintain hot blast temperature of 1150 – 1200 °C. The method helps to operate BF stoves without mannual intervention, decreasing the BF gas consumption in each stove by 8,000 Nm3/hr from 57,500 Nm3/hr without any detrimental effect on HBT and consequent decrease of CO2 emission by 30 kg/thm.

BACKGROUND OF THE INVENTION:

Generally, modern blast furnace are operated with four hot blast stoves targeting higher hot blast temperature and lower fuel rate for smooth operation of BF. This operation with four hot stoves is completely interrelated. The operation of BF stoves is critical because of the two alternate main phases during the operation of each stove, namely a ‘on gas’ phase and ‘on blast’ phase. Furthermore, because of the highly interconnected characteristics, both operation parameters and performance of each stove influence the other stoves. For example, if ‘on blast’ duration of one stove cannot be maintained, it hampers the ‘on gas’ duration of other stoves. The fuel gas set point was continuously varied and manually entered by the BF operator to attain the blast temperature, ‘on blast’ duration and ‘on gas’ duration.
Hot blast stove is a tall cylindrical refractoryfilled regenerative heat exchanger, which supplies hot blast air to Blast Furnace (BF). It is used for converting cold blast air at 150-250°C from blower to hot blast 1100-1250°C. Preheating the air increases the efficiency of the ironmaking process by reducing the amount of fuel requirement by enhancing the chemical reactions inside the furnace. It improves BF performance by providing additional heat input to the furnace by maintaining raceway temperature, reducing the fuel rate and improving BF productivity.
Blast furnace hot blast stoves work as regenerative heat exchangers. The hot blast stoves have a cylindrical shape and are lined with refractory bricks. They have two main chambers: a combustion chamber, where the fuel gas is burned, and a regenerative chamber, filled with a chequer-workrefractory brick that absorbs and releases heat. The operation of the hot blast stoves is cyclic, alternating between two phases: on-gas and on-blast.
The first phase is ‘On Gas’, which emphasizes on heating. The hot blast stoves store heat from a fuel source like the gas that comes out of the blast furnace (BF gas) having carbon monoxide. The second phase is ‘On Blast’, where the fuel gas is shut off and the cold air enters through the same path as the gas, but in the opposite direction. The cold blast gains heat from chequer bricks and enter the blast furnace. The chequer bricks cools down at the end of ‘On Blast’ phase as it transfers heat to the cold blast air.
Usually, there are three or four hot blast stoves attached to one blast furnace. While one stove is in the ‘On Blast’ phase, supplying hot blast to the blast furnace, the other stoves are in the ‘On Gas’ phase, being heated by the fuel gas, mainly BF gas. The stoves switch these phases periodically to maintain a consistent flow of blast volume and closer hot blast temperature.

Prior-art:
Indian Patent Application No. 286444 discloses a method of heating of blast furnace stoves to achieve higher hot blast temperature (HBT). Prior art has increased the BF gas input into the stoves by 25-30% to attain higher HBT up to 1000°C in one BF, where stoves were heated by BF gas alone. In another BF, the total gas input into the stoves increased from 85,000 to 1,10,000 Nm3/hr, which resulted in increased HBT from 960 °C to 1100 – 1150 °C. They have increased the total gas input to stoves from 80,000 Nm3/hr to 1,00,000 Nm3/hr to raise HBT from 900 °C to 1000 °C in another blast furnace in another steel plant. However, in present invention, BF stoves were heated by BF gas alone and the BF gas input to each stove was reduced by 8,000 Nm3/hr from 57,500 Nm3/hrwithout any detrimental effect on HBT. HBT was maintained at 1150 – 1200°C for both the cases. This drop in BF gas consumption led to CO2 savings of 30 kg/thm.
United States Patent Application No. US9896735B2 discloses a method for heating of blast furnace stoves with a fuel mixture of BF gas and converter off-gas (higher heating value gas) to get HBT of 1125 °C. Prior art has used slightly oxygen-enriched air for combusting the fuel. They have added an oxidant comprising an oxygen content of at least 85 % into the combustion chamber for recirculating the combustion gases including said oxidant into the combustion zone. However, in present invention, BF stoves were heated by BF gas alone and the BF gas input to each stove was reduced by 8,000 Nm3/hr from 57,500 Nm3/hr without any detrimental effect on HBT. HBT was maintained at 1150 – 1200°C for both the cases without any oxygen enrichment in BF stoves. This drop in BF gas consumption led to CO2 savings of 30 kg/thm.
Chinese Patent Application No. CN202210828463A discloses a method for hot blast stove pressure charging and discharging system and control method thereof. Prior art has developed a utility model that provides a hot blast furnace exhaust pressure system which includes: a first hot blast stove; a second hot blast stove; a third hot blast stove; a fourth hot blast stove; the hot air main pipe can be respectively connected with the first hot air furnace, the second hot air furnace, the third hot air furnace and the fourth hot air furnace in an on-off mode and an outlet of the hot air main pipe is communicated with the blast furnace. However, in present invention, BF stoves were heated by BF gas alone and the BF gas input to each stove was reduced by 8,000 Nm3/hr from 57,500 Nm3/hr without any detrimental effect on HBT. HBT was maintained at 1150 – 1200°C for both the cases without any oxygen enrichment in BF stoves. This drop in BF gas consumption led to CO2 savings of 30 kg/thm.
United States Patent Application No. US7232542B2 discloses a method for preheating cold blast air to blast furnace stoves to get HBT of 2000 °F (1093 °C). Prior art has developed a preheater, which is preferably designed so that the temperature of the heated air may be set to any temperature in the range of about 600 to 1600 °F (315 to 871 °C). They have claimed that preheaters have the ability to generate heated air in a range of temperatures allows the blast furnace system to adapt to the changes in the hot blast temperature of the stoves.However, in present invention, BF stoves were heated by BF gas alone and the BF gas input to each stove was reduced by 8,000 Nm3/hr from 57,500 Nm3/hr without any detrimental effect on HBT. HBT was maintained at 1150 – 1200 °C for both the cases without any oxygen enrichment in BF stoves. This drop in BF gas consumption led to CO2 savings of 30 kg/thm.

OBJECTS OF THE INVENTION:

• The main object of the present invention is to provide a method of controlled operation of the BF stoves with BF gas aloneinvolving a heating control model based on the fundamental empirical equations and mass &energy balance for stoveand to maintain hot blast temperature of 1150 – 1200 °C.
• Another object of the invention is to decrease the BF gas consumption in each stove by 8,000 Nm3/hr from 57,500 Nm3/hr without any detrimental effect on HBT.
• ¬A further object of the invention is to decrease CO2 emission by 30 kg/thm.
• A further object of this invention is to Improve health of stoves due to lower fluctuations in gas flow and On-gas/On-blast cycle times.
• A still further object of the present invention is to eliminate the manual interventions in stove operation.

SUMMARY OF THE INVENTION:

The basic aspect of the present invention is directed to a method for controlling blast furnace (BF) hot blast stove operation comprising:

plurality of hot blast stoves operatively interlinked ;
each said hot blast stove comprising of combustion chamber for bringing fuel gas and a regenerative heat exchanger converting cold blast air at 150-2500C from blower to hot blast 1100 to 12500C for supplying hot blast air to blast furnace, wherein the operation of said hot blast stoves are cyclic alternating between two phases on-gas and on-blast, said on-gas phase comprising involving fuel sources to store heat and said On Blast phase for transferring heat to cold blast air for preheating for sending to the blast furnace; and
wherein the method of controlling blast furnace (BF) hot blast operation comprises operating the said hot blast stoves and heating the same involving as said fuel sources solely from BF gas of low calorific value in the range of 750 to 850kcal/Nm3 alone and reducing the BF gas input into each stove in the range of 7000 to 9000 preferably 8,000 Nm3/hr from 54,000 to 61,000 Nm3/hr preferably 57,500 Nm3/hr with maintaining Hot Blast temperature (HBT) at 1150 – 1200°C.

A further aspect of the present invention is directed to said method wherein said heating of BF gas with low calorific value maintaining said hot blast temperature of 1150-12000C is determined based on mass and energy balance, heat losses at all phases in each stove. In “On Gas” phase, the waste gas volume and waste gas temperature details of each stove was utilized to determine the heat output via waste gas in “On Gas” phase. Energy input and output in “On Gas” phase helped for determining the heat remaining for next “on-Blast” phase.

A still further aspect of the present invention is directed to said method wherein a heating control regime was involved based on predicted energy requirements for BF gas consumption and duration of “On Gas” phase.

Another aspect of the present invention is directed to said method wherein said heat control regime generated heat requirement in “On Gas” phase was followed controlling required valve positioning to minimise blast furnace gas consumption during “On Gas” phase which is based on energy drain rate involving blast furnace parameters, waste gas parameters and heat loss.

A further aspect of the present invention is directed to said method wherein the heat input to stoves was maintained atleast at 0.335 Gcal/thm and HBT of 1153 °C for drop in BF gas consumption and related CO2 savings of 30 kg/thm.

A still further aspect of the present invention is directed to said method which is carried out following stove heating cooling control regime of :
alternating On-Gas of 165-175 min and On Blast of 56-62 min and isolation time between said alternative operations between On-Gas and On-Blast in the range of 6-8 min.

The above and other aspects and advantages of the present invention are described hereunder in greater details with reference to following accompanying non liiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: shows the schematic arrangement of the operatively interconnected four Blast Furnace stoves supplying hot blast to a blast furnace.
Figure 2: shows conventional stove heating cooling cycle timing for BF stoves.
Figure 3: shows the stove heating cooling cycle timing of BF stoves according to the method of present invention.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS:

The present inventionrelatesto a method for controlled operationof BF hot blast stoves with BF gas alone. BF gas is the exit gas from BF; consisting carbon monoxide, carbon dioxide, nitrogen mainly. The heating value of BF gas is lower compared to Coke oven gas, LD off-gas, Corex gas, natural gas etc. The gas wise calorific value is given in Table-1.Generally, all blast furnaces prefer to operate with gas mixture of BF gas and any high caloric value gas in order to maintain higher hot blast temperature.However, a new method has been developed to operate the BF stoves with BF gas alone and to maintain hot blast temperature of 1150 – 1200 °C.

Table 1: analysis of industrial gases
BF gas Corex gas Coke Oven gas LDConverter Gas
CO, % 20 - 25 42 - 46 5 - 8 60 - 66
CO2, % 18 - 24 28 - 32 1 - 3 15 - 20
H2, % 3 - 6 12 - 18 55 - 60 0.5 - 1.5
H2O, % 3 - 6 3 - 6 3 - 6 -
N2, % 43 - 48 - 3 - 6 17 - 20
CH4, % - 1 - 3 23 - 28 -
Calorific value, kcal/Nm3 750 - 850 1800 - 2000 4300 - 4700 1800 - 2000

Based upon mass and energy balance, heat input to system was calculated for ‘On Gas’ phase. The energy going out in waste gas was determined from waste gas volume and waste gas temperature. Finally, total remaining heat was calculated for ‘On Blast’ phase. Energy requirement for cold blast to attain required blast temperature was predicted. Heat losses are also calculated for all the phases. Based upon the final energy requirement for ‘On Blast’ phase, BF gas consumption for ‘On Gas’ phase was predicted and a heating control model was developed. The complete energy balance was considered for the model development.

Accompanying Figure 1 shows the schematic layout of an embodiment of the operatively interconnected four Blast Furnace stoves supplying hot blast to a blast furnace.

BF operation with four hot stoves is completely interrelated. The performance of each stove depends upon other stoves. The operation of BF stoves is critical because of the two alternate main phases during the operation of each stove, namely a ‘on gas’ phase and ‘on blast’ phase. Furthermore, because of the highly interconnected characteristics, both operation parameters and performance of each stove influence the other stoves. For example, if ‘on blast’ duration of one stove cannot be maintained, it hampers the ‘on gas’ duration of other stoves. The fuel gas set point was continuously varied and manually entered by the BF operator to attain the blast temperature, ‘on blast’ duration and ‘on gas’ duration.

This novel method of heating for BF hot blast stoves involves the development of a heating controlmodel based on the fundamental empirical equations and mass &energy balance for stove. Afterwards, the stoves were operated based on energy available in it. As a result, it has been possible to decrease waste gas generation and reduce surface heat loss. Thisheating control model helps in controlling overall heat loss, ‘ongas’ duration and ‘on blast’ duration and to maintain hot blast temperature of 1150 – 1200 °C.

The main purpose of this model is to make the best use of gas. Blast duration of each stove was noted initially. Energy drain rate was calculated based upon blast parameters. Considering all these parameters, waste gas parameters and heat loss; heat requirement in ‘On Gas’ phase was formulated. Accordingly, the amount of BF gas to the stove to generate required amount of heat was predicted. Based on this heat, the position of the valve will be determined so that maximum heat can be utilized. This new controlled approach of valve positioning helped to minimise blast furnace gas consumption during ‘On Gas’ phase.

Table 2: Trial results of reduced BF gas consumption in BF stoves
BF gas consumption per stove, Nm3/hr Energy to Stoves, GCal/thm CO2 emission savings, kg/thm Hot blast temperature, °C
57,500 0.391 0 1158
56,500 0.383 3.48 1157
55,500 0.376 7.22 1158
54,500 0.368 11.03 1156
53,500 0.363 14.76 1157
52,500 0.356 18.48 1156
51,500 0.350 22.25 1157
50,500 0.342 25.96 1155
49,500 0.335 29.79 1153
48,500 0.327 33.45 1149
47,500 0.321 37.22 1145

From the trial results as shown in Table 2, it is observed that with decrease in BF gas consumption per stove, energy input to stoves is also dropped. As heat input to stoves decreased below 0.33 GCal/thm, the hot blast temperature dropped below 1150 °C, which is not beneficial for blast furnace.With 49,500 Nm3/hr BF gas volume, heat input to stoves was maintained at 0.335 Gcal/thm and HBT of 1153 °C. This drop in BF gas consumption led to CO2 savings of 30 kg/thm.

Accompanying Figure 2 shows the conventional stove heating cooling cycle timing for BF stoves operation. In this earlier method of operating a stove, if the on-blast request is activated based on the mixer valve, the mixer valve was previously operated according to hot blast temperature (e.g. HBT=1160 deg C). If the output temperature goes below the set point, an on-blast request will be triggered for another stove.The stove that is on-gas for a longer period will be sent to the on-box and then on-blast.After that, the previous stove will switch to on-box and then to on-gas. During the on-gas phase, the gas flow set point is given by an operator as a fixed value, and the entire on-gas phase will be run with that fixed value. It is not influenced by the gas calorific value or on-gas duration. The stove's heat content or heat carried out by exit gas are not a factor in this. Because of the manual operation by BF operators, the stove isolation time was varied between 8 – 20 minutes.
Accompanying Figure 3 shows the stove heating cooling cycle timing of BF stoves according to the method of present invention for modifiedcontrolled way of operating the stove.
In this new method, the change of stoves from on-blast to on-gas and on-gas to on-blast cycles remains unchanged, but the method has been altered. Now, the on-blast request is activated based on the mixer valve position and output temperature, which is tuned to the heat available in the stove, and selection of the stove to go for on-blast phase is based on the heat content of on-gas phase stoves. The heat is generated in such a way that the output temperature is obtained for a desired period of time (i.e. 60 minutes) and so that heat loss is also reduced through exit gas and surface as well.
Here, the target time is estimated to ensure that sufficient heat is generated and can remain in the on-blast phase for a desired period of time at the selected output (Hot Blast Temperature).
After the modified intervention, the stove isolation time was can be controlled between 6 – 8 minutes. All these modifications helped in lesser gas consumption for BF stoves, which in turn can save Rs. 13 Crores/year.
, Claims:We Claim:

1. A method for controlling blast furnace (BF) hot blast stove operation comprising :
plurality of hot blast stoves operatively interlinked ;
each said hot blast stove comprising of combustion chamber for bringing fuel gas and a regenerative heat exchanger converting cold blast air at 150-2500C from blower to hot blast 1100 to 12500C for supplying hot blast air to blast furnace, wherein the operation of said hot blast stoves are cyclic alternating between two phases on-gas and on-blast, said on-gas phase comprising involving fuel sources to store heat and said On Blast phase for transferring heat to cold blast air for preheating for sending to the blast furnace; and
wherein the method of controlling blast furnace (BF) hot blast operation comprises operating the said hot blast stoves and heating the same involving as said fuel sources solely from BF gas of low calorific value in the range of 750 to 850kcal/Nm3 alone and reducing the BF gas input into each stove in the range of 7000 to 9000 preferably 8,000 Nm3/hr from 54,000 to 61,000 Nm3/hr preferably 57,500 Nm3/hr with maintaining Hot Blast temperature (HBT) at 1150 – 1200°C.

2. The method as claimed in claim 1 wherein said heating of BF gas with low calorific value maintaining said hot blast temperature of 1150-1200 °C is determined based on mass and energy balance, heat losses at all phases in each stove. In “On Gas” phase, the waste gas volume and waste gas temperature details of each stove was utilized to determine the heat output via waste gas in “On Gas” phase. Energy input and output in “On Gas” phase helped for determining the heat remaining for next “on-Blast” phase.
3. The method as claimed in anyone of claims 1 or 2 wherein a heating control regime was involved based on predicted energy requirements for BF gas consumption and duration of “On Gas” phase.
4. The method as claimed in anyone of claims 1 to 3 wherein said heat control regime generated heat requirement in “On Gas” phase was followed controlling required valve positioning to minismise blast furnace gas consumption during “On Gas” phase which is based on energy drain rate involving blast furnace parameters, waste gas parameters and heat loss.
5. The method as claimed in anyone of claims 1 to 4 wherein the heat input to stoves was maintained atleast at 0.335 Gcal/thm and HBT of 1153 °C for drop in BF gas consumption and related CO2 savings of 30 kg/thm.
6. The method as claimed in anyone of claims 1 to 5 which is carried out following stove heating cooling control regime of :
alternating On-Gas of 165-175 min and On Blast of 56-62 min and isolation time between said alternative operations between On Gas and on Blast in the range of 6-8 min.

Dated this the 17th day of February, 2024
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199