ULCORED Process
ULCORED Process
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. Off-gas from the shaft is recycled into the process after carbon di-oxide (CO2) has been captured, which leaves the DR plant in a concentrated stream and goes to storage. The DRI step produces a solid product which is then melted using an electric arc furnace (EAF). The process was designed mainly in 2006 by a team led by LKAB, Voest-alpine and MEFOS.
The objective of the ULCORED process was to reduce the NG consumption needed to produce DRI. It was achieved by replacing traditional reforming technology with partial oxidation (POx) of NG. Combined with CCS device, ULCORED can reduce 70 % CO2 emission compared with the average in the BF route.
The concept of the 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 green- house gases (GHG), using CO2 capture and storage (CCS) technology and at the same time to a minimized use of energy. The process is designed in a way which allows for the extraction and storage of CO2. The process is therefore also dependent on CCS with a similar in-process capture.
The process is based on the utilization of a shifter to convert the carbon monoxide (CO) gas from the shaft to hydrogen (H2) together with a CO2 removal unit. This opens up a new innovative evolution of the process concept.
The main features of the ULCORED DR process include (i) use of oxygen (O2) instead of air resulting into an off gas of nearly 100 % CO2 which is required only to be compressed, (ii) there are possibilities to reduce the requirement of NG by 15 % to 20 %, and (iii) coal, biomass, bio waste gasification and H2 can be used as an alternative to NG.
The concept of ULCORED is to meet the demand of reduced CO2 emission using iron ore and gas based direct reduction for DRI production. The concept includes the use of 100 % oxygen (O2), POx instead of reformers, shifter for production of CO2 free reduction/excess gas and the reducing agent being either natural gas or syngas from coal/biomass gasification. Reducing gas produced by the gasification of coal can also be used in the place of NG.
ULCORED DR process based on NG
The main features of ULCORED DR 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 units are needed and there is lesser requirement of power needed for recycle compressor.
The originality of the concept is the use of O2 instead of air which means there is either no or low nitrogen (N2) in the gas. The 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. The POx reaction which takes place is CH4 + 0.5O2 = CO + 2H2 with delta H = – 8.6 Kcal/mol.
Fig 1 Reducing gas production by partial oxidation
The pilot unit for the partial deoxidation was tested at Linde in two campaigns with new designed burner for H2 rich feed gas. A tube reactor with preheated gas (60 % H2 and 40 % CH4) was used. The conclusions of the pilot unit tests were (i) The burner and reactor could be operated without problems, (ii) a stable flame without significant noise production could be obtained, (iii) the production of soot is expected to be lower than 300 mg/N cum to 460 mg/N cum wet gas volume, and (iv) due to the atmospheric pressure, the CO2 and CH4 content was higher than the pre-calculations on the basis of assumed equilibrium. A higher operating pressure upto 7 kg/sq cm can reduce this content.
The 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/sq cm pressure and around 900 deg C temperature. The shaft off gas is likely to contain mainly CO, CO2, H2, and H2O.
The shaft off-gas, containing mainly CO, CO2, 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 (g) + H2O (g) <– > CO2 +H2. Nearly all CO in the shaft off-gas is shifted to H2 and the CO2 is removed in the CO2 removal unit. Increased production of excess gas increases the power demand for the CO2 removal unit and O2 production. The calculation of the correct level of excess gas is an iterative process. Estimated levels of excess gas will be higher due to this reason. Estimated excess level of 2.2 giga calories per ton of DRI is expected to compensate for the additional power demand. This can be achieved through a by-pass of some gas directly to the shifter. The amount of gas by-passed is determined based on the reduction gas properties into the reduction shaft. Due to increased gas volumes in the system the shifter and CO2 removal units needs additional investment.
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. In an adiabatic system, the achievable CO slip is determined by the exit temperature. Conversion in a single bed of catalyst is equilibrium limited and as the reaction proceeds the high temperature eventually restricts further reaction. This limitation can be overcome with a two stage system consisting of a high temperature shift followed by low temperature shift, with inter-bed cooling. A low operating temperature gives the most favourable thermodynamic equilibrium and hence the minimum slip of CO. Most modern H2 plants have cooling systems upstream of the low temperature shift reactor to allow operation close to the dew point of the process gas. A safety margin above the dew-point is to be used to ensure complete evaporation of water droplets which can form in the cooler.
Depending on the steam ratio the low temperature shift reactor may be able to operate at a temperature as low as 190 deg 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 deg C. An excess of heat in the heat exchanger produces steam to the steam network through a condenser/reboiler.
The 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. The choice of the CO2 removal unit (VPSA or amine) depends on local requirements. If there is steam surplus then the energy demanding CO2 stripping can be made with in plant steam energy instead of electricity needed for the VPSA. Both technologies have pros and cons which need to be further analyzed. Both technologies work with the ULCORED concept. There is also a possibility for using a pressure swing adsorption (PSA) for CO2 removal in place of VPSA unit because of already high pressure available in the process.
The type of process selected affects the process flow-sheet and the layout. The VPSA process requires, including cryogenics and compression to 110 kg/sq cm pressure, 260 kWh of power per ton of CO2 captured. In case of amine scrubber process, with high performance amines (activated methyldiethanolamine, aMDEA), the complete process (amines and compression to 110 kg/sq cm pressure) require around 1.6 tons of low pressure steam and 160 kWh of power per ton of CO2 captured.
The part of the cleaned H2 rich gas containing N2 is bled out of the process in order to counteract N2 build-up in the reduction gas. The bleed gas is a valuable gas which can be used for production of steam or heating within the system.
The flowsheet of the ULCORED process based on natural gas is given in Fig 1.
Fig 2 Flowsheet of ULCORED process based on natural gas
ULCORED DR process based on coal gasification
The 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. This concept can be adapted for both existing systems and for retrofitted systems.
The coal based concept is based on production of reducing gas using existing coal gasification technology and either cold desulphurization (based on existing technology) or hot gas desulphurization. The concept uses O2 instead of air and includes CO2 storage. The 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.
Coal gasification is a well-known technique and is expected to be one of the future energy conversion process techniques after the depletion of the oil and gas reserves. There are several technologies developed for gasification of coal. There are differences between the technologies but they generally work with relatively high C conversion efficiency, usually greater than 80 %. 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/sq cm pressure, the gas pressure from the gasifier is decreased through an expansion turbine which also recovers power (from 30 kg/sq cm to 6 kg/sq cm).
The ULCORED DR 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 DR process. 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. Three different setups have been shown in Fig 3. The possibility to produce excess H2 for the system is enabled through by-pass of cleaned syngas directly to the shifter units.
Fig 3 Integration of coal gasifier with the ULCORED DR 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. The flowsheet of the coal based ULCORED process is shown in Fig 4.
Fig 4 Flowsheet of ULCORED process based on coal gasification
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. It makes it possible to have one CO2 source out of the system, making it possible to basically capture and store all CO2 generated in the steel making system.
Present status of ULCORED DR process
In depth fundamental model studies for the ULCORED DR 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 DR process in steel plant environment. Different approaches adopted during the modelling studies have produced similar results. These studies have created credible basis for evaluation of the concept in different scenarios. Material balancing, mass balancing, energy balancing, and CO2 emissions calculations have been carried out for I ton of cold DRI output with 92 % metallization and 2.76 % C.
One of the interesting options is a ULCORED DR plant is the production of LRI (less reduced iron) and a conventional or N2 free blast furnace (BF) to produce the hot metal (HM) 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 with a DR-product reduced to only 65 % metallization degree responded very positive in the BF with remarkably stable furnace condition and low consumption of coke which was below 200 kg/tHM.
The outcome of these studies are that ULCORED DR process can be a ‘quick fix’ for a brown field improvement of CO2 emissions specially 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 DR process needs to be pilot tested first, a step which might use the opportunity of the EDRP (experimental direct reduction pilot) furnace, which LKAB is planning to erect in coming years as a complement to its Experimental BF in 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 kg/sq cm to 8 kg/sq cm in the shaft furnace, (iv) gas flow of in the range of 1700 N cum to 3100 N cum per hour, and (v) temperature of the shaft furnace in the range of 900 deg C to 1050 deg 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 also take 10 to 15 years or more.
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