In the area of production of hot metal, most promising technology to significantly reduce the CO2 emission is recycling of CO and H2 from the top gas leaving the blast furnace (BF). CO and H2 content of top gas has a potential to act as reducing gas elements, and hence their recirculation to BF is considered as an effective alternative to improve the BF-performance, enhance utilization of C and H2, and reduce emission of CO2. Top gas recycling (TGR) technology is mainly based on lowering the usage of fossil C (coke and coal) with the re-usage of the reducing agents (CO and H2), after the removal of the CO2 from the top gas. This leads to lower the energy requirements. Because of the advantages of high productivity, high PCI (pulverized coal injection) rate, low fuel rate, and low CO2 emission, TGR-BF process is considered to be a promising ironmaking process in future.
Several recycling processes have been suggested, evaluated or practically applied with different objectives. These processes are distinguished by (i) with or without CO2 removal, (ii) with or without preheating, and (iii) the position of injection.
The concept of the TGR-BF (Fig 1) involves many technologies which include (i) injection of reducing top gas components CO and H2 in shaft and hearth tuyeres, (ii) lowering the consumption of fossil C input due to lower coke rate, (iii) usage of pure oxygen instead of hot air at the hearth tuyere (removal of nitrogen from the process), and (iv) recovery of pure CO2 from the top gas for underground storage.
Taking these background investigations into consideration, the concept of ULCOS (ultra-low carbon di-oxide steelmaking) TGR-BF has been developed in 2004. The concept had been experimentally tested at the LKAB’s experimental BF (EBF) in Lulea, Sweden. The EBF was modified and a gas separation plant based on VPSA (vacuum pressure swing adsorption) technology was built near the EBF.
Development of ULCOS TGR-BF
Development work for ULCOS TGR-BF was carried out in two phases. First development phase ran from 2004 to 2009 when three new process concepts have been developed and tested. The second phase started in 2009 and in this phasetwo, additional ULCOS TGR-BF campaigns were conducted.
During the development heat and mass balance models and a 3-D axi-symmetrical model of the BF were used for the calculation of the main data and the inner state of the process for selection of the best operating parameters. Four alternatives were defined and examined for the possible reachable C saving and feasibility of running the BF under these new concepts. The conclusion was that alternatives 1, 3 and 4 should be able to achieve a fossil C saving of 21% or higher with a high PCI level. Alternative 2 was rejected because of the low expected C saving and the necessity of challenging technology to heat the recycle gas in two steps first in a recuperator and then further heating by partial oxidation. All alternatives included CO2 removal and injection of CO-rich product gas into the hearth tuyeres, the usage of pure oxygen and the injection of coal together with the reducing gas.
In alternative 1, the de-carbonated product gas is injected cold with pure oxygen and coal at the hearth tuyeres and hot at the shaft tuyeres. One critical point in this alternative was the small cold gas flow rate at hearth tuyere level leading to smaller raceway sizes and higher flame temperatures compared to the normal BF process. Also, a new tuyere design was necessary because of the small gas flow rates.In alternative 3, the de-carbonated product gas was injected hot at the normal hearth tuyeres together with oxygen and coal. To reach high C saving it was necessary to operate with low RAFT (raceway adiabatic flame temperature) and at the same time with high coal injection rate. In alternative 4, the de-carbonated product gas was injected hot at the hearth tuyeres and hot at the lower shaft. The temperature of the recycled gas varied from room temperature to 1250 deg C.In alternatives 1 and 4, product gas is also injected through shaft tuyeres. The differences are the gas injection temperature, and the position of the injection points. In all the cases, least part of the gas was heated in a regenerative system.
Mathematical modelling of the raceway conditions and gasification tests were then carried out and both laboratory and pilot scale investigations were done for the design and engineering of the tuyeres under the constraints of simultaneous injection of recycled gas, pure oxygen and pulverized coal. The geometry of the tuyere has been improved based on the results of the calculations to avoid hot spots and failure during operation and to keep a sufficient impulse of the gas stream to form a raceway with a sufficient depth.
Results of ULCOS TGR-BFcampaigns
The first conclusion which emerged from the campaigns is that it is possible to operate the ULCOS TGR-BF process. No safety related issue had occurred during the campaigns with the new process. The operation of the VPSA unitwas smooth and without any major failures. EBF coupled with the VPSA unit worked very well during the campaigns. However, the operation of the VPSA unit was influenced by the changes in top gas composition and volume of the gas from the EBF. Hence, both the units were to be operated in a very close relationship.
During the campaigns of the TGR-BF process, stable operation of the BF was experienced with a smooth descent of the burden and it was easy to maintain the thermal stability. Efficiency of the gas in the BF shaft was stable during the different alternatives and there was good gas distribution. No particular process problems were related to the properties of the burden materials. Reduction profile was of a centre working furnace.It could be concluded that the burden properties as used in conventional BF seem to have no problem for the ULCOS TGR-BF process.
The results achieved were very encouraging with regards to saving of C. The trials had shown a substantial decrease in the rate of the reductant which was achieved by injection of the de-carbonated top gas.
Although alternative 1 could not be fully explored because of the early stoppage of the second campaign the maximum reduction in C input via coke was 21% compared to the reference period under conventional BF operation. For this alternative, a new tuyere technology was developed. The tuyere design consisted of co-axial pipes with the inner pipe used for PCI and the outer pipe for the oxygen injection. The tuyeres worked very well and after disassembling neither damages nor wear was noticed.
As regards alternative 3, the C consumption could be reduced upto 15% in the first campaign with a TGR ratio of around 72%. The results of this alternative were lower than expected from the heat and mass balance calculations as this was the first experience with TGR mode and the process was not optimized. In the second campaign the results of this alternative were much better, when the maximum reduction in C input of around 25% was achieved with a TGR ratio upto 90%.
In case of alternative 4, a C saving of 24% was achieved with a TGR ratio of 90%. In terms of coke and coal consumption, there was saving of upto 123 kg/tHM in the new process compared to the reference operation period. The input C via coke and coal could be reduced by 17kg in average per 100 Ncum of gas injected.
The campaigns of EBF have proved that it is possible to run a BF process at a much lower fossil C consumption level. A saving of C upto 25% was proven by the injection of the reducing de-carbonated top gas. The application of BF-TGR technology on modern BF is expected to lead to reduce the C consumption from a present level of around 405kgs C/tHM to a level of around 295kgs C/tHM.
VPSA unit had operated stably. It had been noticed that the VPSA unit could treat 97% of top gas from the BF. The average volume fraction of CO2 in injected gas was around 2.67% and the CO recovery rate was 88%. Combined with VPSA and CCS units, the CO2 emissions reduced by TGR-BF process could reach to 1270kg/tHM.
From process point of view, it can be stated that ULCOS operation is more stable than the conventional BF operation, as far as the temperature and quality of the HM is concerned. This seems essentially due to lower influence of the solution-loss reaction related to the much lower levels of direct reduction rate(DRR). The lowest observed value of this DRR is 5%. There was no indication in the operating results that this was actually the minimum value which could be reached in ULCOS TGR-BF. Quality of HM was largely impacted by the operation of ULCOS TGR-BF. Especially, a substantial decrease of silicon content (> 1% absolute) and correlative increase of the C content were observed. It must however be pointed out that the silicon content in conventional BF operation is much lower (around0.5%against around 2.0% in EBF), and hence no such a big change is expected during the application of the ULCOS BF-TGR process at the industrial scale.
Test campaigns of ULCOS TGR-BF have shown that the new TGR-BF process is feasible and easy to operate. It can be operated with good safety, high efficiency and strong stability. Test results have shown that alternative 4 had the best effect of emission reduction and was selected as the first choice for the trial on the industrial-scale BF during the next stage.