ULCORED is a direct reduction (DR) process, which produces DRI (direct reduced iron) in a shaft furnace, either from natural gas (NG) or from reducing gas obtained by gasification of coal. The process was designed mainly in 2006 by a team led by LKAB, Voest-alpine and MEFOS. Main features of ULCORED process include
(i) use of O2 instead of air resulting into an off-gas of nearly 100% CO2 which is required only to be compressed,
(ii) possibilities to reduce the requirement of NG by 15-20%, and
(iii) coal, biomass, bio waste gasification and H2 can be used as an alternative to NG.
Concept of ULCORED process involves separating CO2 out of the process gas. It is characterized by an effort to adopt gas based DR process to a minimized emission of CO2. The process is based on the utilization of a shifter to convert the CO gas from the shaft furnace toH2 together with a CO2 removal unit.
ULCORED process based on NGMain features of ULCORED process based on NG are
(i) no reformer,
(ii) no heater, and
(iii) high pressure.
Because of high pressure there is less gas velocity in the DR shaft, less fluidization and less fines leaving the DR shaft. Also, due to high pressure, smaller CO2 removal and POx (partial oxidation) units are needed and there is lesser requirement of power needed for recycle compressor.
Originality of the concept is the use of O2 instead of air which means there is either no or low N2 in the gas. Reforming is by a POx unit for conditioning of the reduction gas (Fig 1). The technology replaces the conventional reforming of NG by partial oxidation of the gas for reducing the consumption of NG needed to produce DRI. POx reaction which takes place is CH4 + 0.5O2 = CO + 2H2 with delta H = – 8.6 kcal/mol.
DR shaft has a counter current flow of reducing gas injected at the tuyeres and iron ore is fed cold from the top. The furnace is operated at 6 kg/cm² pressure and around 900?C temperature. The shaft off-gas, containing mainly CO, CO2, H2, and H2O is led through a shifter (one or two stage) converting CO to H2. The use of a water gas shifter results into high H2 content in the reduction shaft. The water gas shifter reaction is CO+H2O <– > CO2+H2. Nearly all CO in the shaft off-gas is shifted to H2 and CO2 is removed in the CO2 removal unit. In the shifter unit, the CO shift reaction is exothermic, and equilibrium favours CO conversion at high steam to gas ratio and low temperatures, while being unaffected by pressure.
Depending on the steam ratio the low temperature shift reactor may be able to operate at a temperature as low as 190?C without concern for condensation. In the concept, the top gas is passing a high and low temperature shifter with heat recovery in between. The high temperature shifter converts 97.2% of the CO and the low temperature shifter completes to 99.5% conversion. The heat exchanger heats the recycled gas and its capacity is calculated based on a set temperature difference on the hot side of 50?C.
CO2 removal unit is used to remove the CO2 from gas producing an H2 rich reduction gas which is returned to the process. The CO2 removal unit is decided by a distribution for the CO2 removal efficiency, H2 and N2 recovery. The two CO2 removal processes which can be used are vacuum pressure swing adsorption (VPSA) process or amine scrubber process. Choice of the CO2 removal unit depends on local requirements. The type of process selected affects the process flow-sheet and the layout.
Part of the cleaned H2 rich gas containing N2 is bled out of the process in order to counteract N2 build-up in reduction gas. Bleed gas is a valuable gas which can be used for production of steam or heating within the system. Flowsheet of ULCORED process based on NG is given in Fig 1.
ULCORED process based on coal gasification
Most CO2 saving option with ULCORED is the use of a coal gasifier producing syngas for the DR-plant and CO2 lean H2 gas for all in-plant users. Coal based concept is based on production of reducing gas using existing coal gasification technology and either cold desulphurization or hot gas desulphurization. Concept uses O2 instead of air and includes CO2 storage. High H2 content in the reduction shaft is achieved through water gas shifter. Excess H2 gas is supplied to the other users of the plant.
A big advantage with coal gasification is the possibility to efficiently clean the syngas (such as sulphur, mercury etc.) before use. Since the shaft furnace for the production of DRI operates at 6 kg/cm² pressure, the gas pressure from the gasifier is decreased through an expansion turbine which also recovers power (from 30 kg/cm² to 6 kg/cm²).
The ULCORED process can easily be integrated with a coal gasification unit, incorporating the advantages of the originality of the process concept. Coal is supplied to the coal gasifying plant. Sulphur is removed from the syngas by either hot or cold desulphurization. The clean syngas is blended with cleaned H2-rich recycle gas, preheated in the DRI cooler or from the heat exchanger between the high temperature and low temperature shifter.
There are three different ways for the gasifier to be integrated with the ULCORED process (Fig 3). The gasifier can be integrated either as a cold syngas or in a way where the thermal energy in the hot gas from the gasifier is utilized.
Fig 3 Integration of coal gasifier with the ULCORED plant
Oxygen is mainly consumed in the coal gasifier, but also before the shaft, in a small POx, ensuring a correct temperature of the reduction gas. Shifted gas is gas which is to be by-passed to the shifter to ensure the right composition of the reduction gas, i.e. increasing the H2 amount. CO2 removed is the amount of CO2 which is separated from the main process stream in the CO2 removal process. Flowsheet of coal based ULCORED process is shown in Fig 4.
The use of a coal gasifier and a shifter in the system makes it possible to by-pass some of the syngas directly to the shifter, generating more gas than necessary for the direct reduction plant. This feature makes it possible to generate a CO2 lean fuel for the steel plant.
Present status of ULCORED process
In depth fundamental model studies for the ULCORED process has been completed. These model studies included pellet scale models, shaft models, and process models by flowsheet simulations. The model studies have helped in the fundamental understanding of the DR process including its dynamics. The flowsheet modelling has helped in optimization of the process layout to fit the ULCORED process in steel plant environment. Material balancing, mass balancing, energy balancing, and CO2 emissions calculations have been carried out for 1 ton of cold DRI output with 92% metallization and 2.76% C.
One of the interesting options is a ULCORED plant is the production of LRI (less reduced iron,65% metallization) and a conventional or N2 free BF to produce the hot metal with an additional savings of CO2 emissions. LRI is an alternative choice instead of DRI considering the successful tests made in the LKAB experimental BF. The LRI test responded very positive in the BF with remarkably stable furnace condition and low consumption of coke which was <200 kg/tHM.
Outcomes of these studies are that ULCORED process can be a ‘quick fix’ for a brown field improvement of CO2 emissions especially where NG is relatively cheap. In case of integrated steel plant with BF route, LRI can be a choice considering the successful tests made in the LKAB experimental BF.
ULCORED process needs to be pilot tested first in the EDRP (experimental direct reduction pilot) furnace, which LKAB is planning to erect in coming yearsin Lulea, Sweden. The specifications of the EDRP are
(i) 1 ton of iron per hour production,
(ii) recirculation of top gas,
(iii) working pressure range of 0-8 kg/sq cm in the shaft furnace,
(iv) gas flow of in the range of 1700-3100 Ncum/hour, and
(v) temperature of the shaft furnace in the range of 900-1050?C.
ULCORED is probably going to be a candidate to retrofitting existing direct reduction plants, once its viability has been demonstrated at pilot and then demonstrator scales, which would take around 10-15 years or more.