ISIJ International, Vol. 55 (2015), No. 2, pp. 340–347
Quantitative Evaluation of CO2 Emission Reduction of Active Carbon Recycling Energy System for Ironmaking by Modeling with Aspen Plus Katsuki SUZUKI,1)* Kentaro HAYASHI,1) Kohei KURIBARA,1) Takao NAKAGAKI1) and Seiji KASAHARA2) 1) Waseda University, 3-4-1 Ookubo, Shinjuku, Tokyo, 169-8555 Japan. 2) Japan Atomic Energy Agency, 4-49 Muramatsu, Tokai-mura, Naka-gun, Ibaraki, 319-1184 Japan. (Received on August 31, 2014; accepted on October 20, 2014)
The use of the Active Carbon Recycling Energy System in ironmaking (iACRES) has been proposed for reducing CO2 emissions. To evaluate the performance of iACRES quantitatively, a process flow diagram of a blast furnace model with iACRES was developed using Aspen Plus, a chemical process simulator. The CO2 emission reduction and exergy analysis was predicted by using the mass and energy balance obtained from the simulation results. iACRES used a solid oxide electrolysis cell (SOEC) with CO2 capture and separation (CCS), an SOEC without CCS, and a reverse water-gas shift reactor as the CO2 reduction reactor powered by a high-temperature gas-cooled reactor. iACRES could provide a CO2 emission reduction of 3–11% by recycling carbon monoxide and hydrogen, whereas the effective exergy ratio decreased in all cases. KEY WORDS: exergy analysis; carbon flow; high-temperature gas-cooled reactor; carbon monoxide.
1. Introduction Steel manufacture accounts for 42%1) of the CO2 emissions from Japan’s industrial sector. Additionally, secure access to coal is a major problem because Japan is the world’s second-largest coal importer and has very limited domestic coal resources.2) To reduce CO2 emissions and carbon usage by the steel industry, the introduction of the Active Carbon Recycling Energy System (ACRES) to the ironmaking process (iACRES) has been proposed. A schematic of iACRES is shown in Fig. 1. The core concept of iACRES is based on the reduction of CO2 to CO by electrochemical or thermochemical processes and recycling the CO to the blast furnace. The exergy used for reducing CO2 must be generated by a low-carbon primary energy source, such as renewable or nuclear energy to decrease CO2 emissions by iACRES. High-temperature gas-cooled reactors (HTGRs), a type of nuclear reactor, are one of the most suitable power sources for iACRES, which can generate electric power, high temperatures (~950°C), hydrogen (H2), and oxygen (O2) without CO2 emissions. In the iACRES process, CO2 is separated from blast furnace gas (BFG) by a conventional amine process and reduced to CO. The CO is recycled to the blast furnace and used for the reduction of iron oxides into pig iron. The trial introduction of ACRES3,4) and unit operations for CO2 reduction, such as those using solid oxide electrolysis cells (SOECs)5) and reverse watergas shift reactors (RWGSs), have already been reported.
Fig. 1.
However, quantitative evaluation and discussion of iACRES has not yet been performed. In this work, a blast furnace using iACRES (Fig. 1) was simulated by Aspen Plus, a commercial chemical process simulator (Aspen Technology, Inc.), to evaluate the saving in the coke input and resulting reduction of CO2 emissions and exergy balance quantitatively. The exergy source is an HTGR and the evaluation boundary is encircled by dashed lines. 2. Blast Furnace Model In a typical blast furnace, solid materials (e.g., coke, iron ore sinter) are introduced from the top of the furnace and
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[email protected] DOI: http://dx.doi.org/10.2355/isijinternational.55.340
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Schematic illustration and evaluation boundary of iACRES.
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ISIJ International, Vol. 55 (2015), No. 2 Table 1.
(a) Stream ID (input). (b) Stream ID (output). (a) Stream ID
Stream Composition
I1-IR-OR
Sintered ore
I2-OT-OR
Other ore
I3-AUX-M
Auxiliary material
I4-COKE
Coke
I5-PC
Powdered coal
I6A-AIR
Hot air
I6B-E-O2
Oxygen
I6C-MOI
Moisture
(b) Stream ID O1-H-MET
Hot metal
O2-SLAG
Slag
O3-BFG
BFG
O4-DUST
Dust
Table 2.
Input material for the calculation.
Material Sintered ore Other ore Auxiliary material
Weight or Volume [kg/t of hot metal (THM) or N m3/THM] 1 254 339 15
Coke
385
Powdered coal
112
Hot air
Fig. 2.
Stream Composition
1 045
Oxygen
25
Moisture
27
based on actual operational data.6) The input conditions for the reference data is listed in Table 2. Tables 3(a) and 3(b) show the simulation results for the output material and heat streams, respectively. Compared with the reference data, the error in the output material and heat streams was within 5%. This strong agreement between the simulation results and reference data confirmed the validity of the model. The scale of blast furnace used in this simulation is shown Table 4; the size of the associated iACRES equipment was scaled to match the blast furnace.
PFD of blast furnace on Aspen Plus.
come into contact with up-flowing gases (e.g., hot air, O2, steam). The process flow diagram (PFD) of the blast furnace simulated by Aspen Plus is shown in Fig. 2. Input streams and output streams are encircled by solid and dashed lines, respectively. The details of the streams are listed in Tables 1(a) and 1(b). Modeling the heat transfer and chemical reactions occurring in a blast furnace as counter-current gas-solid interactions in the PFD accurately reflects actual blast furnace operation. From top to bottom, the blast-furnace shaft is composed of low-, middle-, and high-temperature Gibbs reactors. Additionally, two stoichiometric reactors are placed at the solid outlet of the high-temperature Gibbs reactor to simulate other reactions in the actual blast furnace. One of these reactors directly reduces silicon dioxide (SiO2) and manganese oxide (MnO) with carbon, and the other simulates the partial oxidation of pulverized coal in the raceways. The adiabatic combustion temperature of the raceway was specified at 2 200°C. The validity of this PFD was verified with reference data
3. iACRES Model 3.1. CO2 Separation In iACRES, CO is circulated to the blast furnace as a substitute for additional carbon input. The source of CO is CO2 in BFG. Because BFG has an enormous flow rate and contains large quantities of N2 and CO, CO2 reduction requires a large scale and a large amount of additional energy consumption. However, the CO contained in BFG can be used directly as a gaseous reductant. In this work, we considered that 30 to 40% of BFG is separated at the top of blast fur341
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ISIJ International, Vol. 55 (2015), No. 2 Table 3. (a) Comparison of the simulation result with material balance reference data. (b) Comparison of simulation results with enthalpy balance reference values. (a) Stream
Chemical species
Hot metal
This work [kg/THM]
C
45.04
47.00
Fe
949.29
945.60
Si
5.30
5.30
Mn
2.50
2.50
1 002.14
1 000.40
SiO2
93.31
97.72
Al2O3
37.84
35.88
Sum
Slag
CaO
126.74
116.72
MgO
18.72
14.64
MnO
1.38
1.81
279.99
266.77
Sum
BFG
Reference [kg/THM]
H2
6.17
6.50
N2
1 032.20
1 004.57
CO
433.78
452.85
CO2
708.99
674.33
H2O
32.94
33.05
CH4
1.06
0.00
2 215.14
2 171.30
Sum
Fig. 3.
Fig. 4.
Schematic of BFG separation and CCS model.
Schematic of SOEC and electrochemical reactions.
SOECs can be operated without CCS in the model, whereas N2 and CO from BFG remain in the recirculated gas stream and can disturb the electrolyzation of CO2. However, An RWGS without CCS was not evaluated because circulating CO and H2O strongly disrupt the RWGS reaction.
(b)
Name
Hot metal
Sum [MJ/THM]
Heat and Pressure [MJ/THM]
This work
Reference
This work
Reference
This work
Reference
9 620
9 820
940
1 118
8 680
8 702
Slag
898
890
439
437
459
453
BFG
5 355
5 277
–32
0
5 387
5 277
Electricity
113
112
–
–
–
–
Heat loss
900
744
900
744
–
–
Other loss Sum
307
315
307
315
15 549
15 727
909
1 118
Table 4.
3.2. SOEC Model Figure 4 shows a schematic of the SOEC system. The CO2 introduced into the cathode is reduced to CO and O2– by electrolysis, and the O2– conducted through the electrolyte is oxidized to O2. In this simulation, the operation temperature of the SOEC is assumed to remain constant at 800°C and the conversion rate of CO2 is assumed to be 60% or 70%. The theoretical electrolysis voltage (Et at 800°C) was calculated from the Gibbs free energy change (ΔG = 189 kJ/mol) in the electrolysis reaction of CO2 → CO + 0.5O2. Assuming adiabatic conditions, the electrolysis of 1 mol of CO2 requires enthalpy of 282 kJ (800°C); part of the required enthalpy must be supplied as electric energy, equivalent to ΔG = 189 kJ, and the rest of the enthalpy can be supplied as heat of TΔS = 93 kJ. However, the applied voltage, E, is usually higher than the theoretical electrolysis voltage, Et, due to overvoltage (Fig. 5). Current density, i, for electrolysis is given by the amount of CO2 to be electrolyzed (mol/s) according to Faraday’s law and current leakage. The applied voltage, E, and the overvoltage, η , are determined from the experimental i-V performance curve obtained by Ishihara and co-workers7) as shown in Fig. 5. Figure 6 shows the calculation method for the required electric energy and heat input. Electric energy, E·i, is consumed during the electrolysis of CO2 at the rate of CO2 electrolyzation in an SOEC. Part of the applied electric power, equivalent to η ·i, is converted to heat and makes up part of the required heat input; therefore, the heat input required from external sources decreases. However, the exergy of electric power is destroyed by converting it to heat at a low temperature. Figure 7 shows the breakdown of the total heat input, TΔS, ver-
Chemical [MJ/THM]
–
–
14 527
14 497
Assumed scale of the blast furnace.
Scale indexes
Value
Volume of blast furnace
5 000 m3
Iron trapping ratio Product of hot metal
2.1 t/(dam3) 3 833 kt/y
nace and is recirculated to the blast furnace via ACRES either directly or through separating CO2, as shown in Fig. 3. The conventional mono-ethanol amine process is used for CO2 capture and separation (CCS). A simple component splitter block was used for the CCS model in the ASPEN Plus and an additional heat of 4.0 GJ/t-CO2 at 140°C is included in the total heat input. The CO2 recovery rate was assumed to be 90%. © 2015 ISIJ
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Fig. 8.
Fig. 5.
Experimental i-V performance of SOEC.8)
Equilibrium CO2 conversion vs temperature at various H2/ CO2 ratios.
set by setting number of SOEC modules. 3.3. RWGS Model The catalytic reduction of CO2 by H2 in the RWGS is expressed in Eq. (1), and the unit operation was modeled by an equilibrium reactor in Aspen plus.
CO2 + H 2 → CO + H 2 O, ΔH °298 = +41.2 kJ / mol ... (1)
Fig. 6.
H2 was assumed to be produced in iodine-sulfur (IS) process, a thermochemical water splitting process. Exergy source of the H2 production was HTGRs, hydrogen production thermal efficiency was 37.0% as a result of a process flow simulation by Aspen Plus. The relationship between reaction temperature and conversion of CO2 at molar ratios of H2/CO2 of 1, 2, 2.55, and 3 is shown in Fig. 8. Considering the maximum hydrogen production from the HTGR, the maximum H2/CO2 ratio is 2.55. Higher H2 concentrations and temperatures produce higher yields of CO. However, higher temperatures may induce catalyst degradation. H2O is also produced in Eq. (1) and this can increase the mole fraction of H2O in circulated gas. Excess H2O affects the coal gasification in the blast furnace, as shown in Eq. (2), which may cause a temperature drop due to an endothermic reaction. To prevent the temperature drop, the reduced gas was cooled to 40°C to condense the steam before circulating to the blast furnace. Considering the appropriate temperature for the Ni-based catalyst used in a typical RWGS,9) the operating temperature of the RWGS was set at 640°C, so as to induce 60% CO2 conversion, and a H2/CO2 ratio of 2.55.
Schematic illustration of required enthalpy change, and input of electric power and heat in SOEC process.
C + H 2 O → CO + H 2 , ΔH °298 = +132 kJ / mol ... (2)
Fig. 7.
3.4. Operating Conditions Applying iACRES to the blast furnace model changes the operating conditions from those of the original system in several ways. In particular, the carbon content in hot metal, heat flux ratio, and the temperature of the raceway were important operating conditions. The carbon content in hot metal affects the melting point. The heat flux ratio and raceway temperature affect the temperature distribution in the blast furnace, which may result in incomplete iron ore reduction. The ratio of CO and CO2 in BFG is changed by the recirculated gas and must be calculated by using the non-isothermal rate-based model. In this simulation, the operating temperature of the low-temperature block was
Heat flow calculation of current density.
sus current density. In this calculation, the scale of one SOEC was set by assuming a 10 kW solid oxide fuel cell.8) The total heat input TΔS increases in negative in proportion to current density in accordance with Faraday’s law. Concurrently, the internal heat generation caused by overvoltage, η , also increases, and the remainder has a peak value at i = 0.21 A/cm2. To minimize the exergy loss, the operating current density, i, was set at 0.21 A/cm2 to maximize the direct heat input. In this calculation, the current density was 343
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ISIJ International, Vol. 55 (2015), No. 2 Table 7. Comparison of results of CO2 emission reduction with ACRES. CO2 emission reduction [%]
CCS
SOEC
RWGS
w/
3.43
8.34
w/o
11.4
Table 8. Effect of CO2 conversion and BFG circulation on CO2 emission reduction with SOEC with iACRES. BFG circulation [%]
Fig. 9.
Relationship between CO+CO2 input and the CO/(CO+CO2) ratio of BFG.
Table 5.
Operating conditions and adjusted variables.
Conditions
Value 45.04
Coke input
Heat flux ratio
0.75
Flow rate of enriched O2 and hot air
Temperature of raceway [°C]
2 196
Temperature of circulated gas
Summary of efficiencies and exergy output of HTGR. SOEC
Reactor output Number of reactors
Power generation efficiency ηelec [%]
RWGS
600 MWth 0.5
H2 production efficiency η H2 [%]
2.0 37.0
48.1
H2 exergy [MJ/THM]
47.0 2 905
Electricity exergy [MJ/THM]
1 073
22
Heat exergy [MJ/THM]
72.5
99.4
adjusted by applying the top gas composition of the blast furnace calculated by other one-dimensional simulation code to the gas output stream of the block. Figure 9 shows the CO/(CO+CO2) ratio of the BFG outlet, where z is calculated by one-dimensional simulation code. z is approximated by
z = (0.0056 x − 0.8747) y + 47.4 ................. (3) where x and y are the CO/(CO+CO2) ratio and CO+CO2 molar flow of the circulation gas, respectively. Table 5 summarizes the operating conditions and corresponding adjusted variables. In this simulation, all the exergy inputs, such as the electric power and heat for SOEC or hydrogen and heat for RWGS are supplied by 600 MWth HTGRs. The number of reactors and generation efficiencies are summarized in Table 6. 4. Results and Discussion 4.1. CO2 Emission Reduction Figures 10(a)–10(d) depicts carbon flows for the evaluation boundary shown in Fig. 1, for the original system, © 2015 ISIJ
40
60
3.43
4.99
70
4.11
iACRES using SOEC with CCS, iACRES using SOEC without CCS, and iACRES using RWGS with CCS, respectively The mass flows of carbon in a year (kt/y) for each flow and block calculated based on a 3 833 kt/y hot-metal production are shown in Table 4. Table 7 compares the CO2 emission reduction of iACRES against the original blast furnace system. The base case used the following operating conditions: 30% BFG circulation and 60% CO2 conversion by SOEC/RWGS. The effects of these operating conditions on CO2 emission are discussed in Table 8. When iACRES is used, the circulated CO is used to reduce iron oxides, which decreases the coke input and thus CO2 emissions from the boundary shown in Fig. 1. The original system shown in Fig. 10(a) inputs a 1 288 kt/y carbon flow as coke into the blast furnace in comparison with 1 232, 1 102, and 1 155 kt/y shown in Figs. 10(b)–10(d), respectively. The CO2 emission reduction rate of RWGS was higher than that of SOEC because the H2, which is unused for reducing CO2 in the RWGS, was recirculated into the blast furnace and used to reduce iron oxides. In the carbon flow in Fig. 10(d), the circulated gas and the BFG flow into the stove are less than those in Fig. 10(b). However, the hydrogen flow, which can act both as a reductant in the blast furnace and a heat source in the stove, is hidden in this carbon flow. The CO2 reduction rate of SOEC without CCS was higher than that of SOEC with CCS, because both CO and the circulated H2 were used as a reductant for iron oxides. The circulated gas shown in Fig. 10(c) inputs a 538 kt/y carbon flow, which is mainly contained as carbon monoxide, compared with 244 kt/y shown in Fig. 10(b). However, because the nitrogen in the hot gas is decreased to adjust the heat flux ratio, the heat duty and resulting CO2 emission from the stove is almost comparable in both cases. Table 8 shows the effects of the conversion and BFG circulation on the CO2 emission reduction by using SOEC as the CO2 reduction reactor. The CO2 emission decreased with an increase in both BFG circulation and CO2 conversion because the mole fractions of gaseous reducing agents, such as CO and H2, increased. Meanwhile, an increase in BFG circulation boosted the CO2 emission reduction, whereas the increase in the ratio of CO2 conversion also increased the emission reduction by around 1.2 times.
Adjusted variables
Carbon content [kg/THM]
Table 6.
CO2 conversion [%]
30
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(b)
(c)
Fig. 10.
Continued.
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ISIJ International, Vol. 55 (2015), No. 2 (d)
Fig. 10.
(a) Carbon flow (original system) (unit: kt/y). (b) Carbon flow (iACRES using SOEC with CCS) (unit: kt/y). (c) Carbon flow (iACRES using SOEC without CCS) (unit: kt/y). (d) Carbon flow (iACRES using RWGS with CCS) (unit: kt/y).
Fig. 12. Fig. 11.
Comparison of exergy input and effective exergy output in various iACRES with those of the original system.
For iACRES using an RWGS, exergy inputs increased significantly because of the excess hydrogen. The production efficiency of H2 was lower than the generation efficiency, as shown in Table 6, which means that a greater exergy input is required in an RWGS than that in an SOEC. However, the exergy outputs were almost the same. This resulted in a lower effective exergy ratio in iACRES using an RWGS than that using an SOEC because BFG contained a large amount of CO2 and H2O, which have a low chemical exergy.
4.2. Exergy Figure 11 shows the total exergy input and total effective exergy output for each iACRES setup compared with the original system. Figure 12 shows the effective exergy ratio in each iACRES setup, which is defined as the ratio of the total effective exergy output to the total exergy input. In the iACRES setups using an SOEC and RWGS with CCS, the exergy inputs increased, whereas the exergy outputs were very similar. This indicated that the additional exergy input was more than the reduction in the carbon input arising from CCS. For iACRES using an SOEC and without CCS, the exergy input decreased, in contrast to the system with CCS. A combination of input and output effects resulted in a decrease in effective exergy ratio compared with the original system. In particular, iACRES without CCS had a lower effective exergy ratio than iACRES with CCS because the unused BFG contained a large amount of CO2 and H2O, which have no chemical exergy. © 2015 ISIJ
Comparison of effective exergy ratio in various iACRES with the original system.
4.3. Blast Furnace Temperature Because the temperature drop in the reactor block blast furnace has the potential to make operation impossible, the temperature change in the blast furnace as a result of using iACRES was evaluated. As shown in Fig. 2, the blast furnace model consists of three pairs of adiabatic Gibbs reactors and flash blocks. The high-temperature block is used as an index for temperature change because the highest temperature block is affected significantly by changes in the 346
ISIJ International, Vol. 55 (2015), No. 2 Table 9.
using rate-based blast furnace models is required in future work. To mitigate the negative effect of the temperature drop, additional countermeasures are required. For example, increasing the hot air temperature is a quick way to compensate for the temperature drop, although an additional heat source with an exergy higher than that at 1 160°C is necessary. The combination of an HTGR and coal firing is one possible heat source. CO2 separation from the reduced gas before recirculation is another possible countermeasure.
Calculated temperature of HIGH-TEMP block (°C). Reduction
CCS
Table 10.
SOEC
RWGS
w/
1 097
1 115
w/o
1 082
Calculated temperature of HIGH-TEMP block in SOEC with CCS model (°C). Circulation (%)
Conversion rate (%)
30
40
60
1 097
1 062
70
1 122
5. Conclusions A PFD of iACRES including the blast furnace model was developed and its effect was quantitatively evaluated in terms of CO2 emission reduction resulting from the saving of coke input and exergy analysis. The following results were obtained. (1) Compared with the original blast furnace, iACRES can reduce CO2 emission by 3–11%, and decrease the effective exergy ratio by 1–7%. (2) The CO2 separation (CCS) from BFG before the CO2 reduction reactor contributes to suppressing the decrease in effective exergy ratio. However, because BFG contains H2 and CO, which function as reductants, iACRES without CCS can also reduce the coke input. (3) In iACRES without CCS or with the CO2 reduction reactor operating at a low CO2 conversion, the circulated CO2 causes a temperature drop in the blast furnace of up to 100°C. Detailed analysis using rate-based blast furnace models is necessary to assess whether the system is operationally feasible.
input gas conditions. Table 9 shows the calculated temperature of the high-temperature block in each case. Table 10 shows the sensitivity analysis of the temperature in the SOEC with CCS model. In the original system, the hightemperature block is at 1 160°C and the temperatures in all cases dropped by up to 98°C. Using iACRES changes temperature in the blast furnace because of the carbon solution reaction and the reduction of iron oxides by hydrogen. The unreduced CO2 circulated into the blast furnace can promote the carbon solution reaction in Eq. (4).
C + CO2 → 2CO ΔH °298 = +97.5 kJ / mol ...... (4) The temperature drop in the blast furnace increased with the increase in BFG circulation and thus in CO2 flow rate. For the same reason, the high conversion rate suppresses the temperature drop in Table 10. The hydrogen circulated to the blast furnace reduces the iron oxides, as shown in Eq. (5). However, it also suppresses the direct reduction of iron oxide by carbon as shown in Eq. (6), which has a much larger endothermic enthalpy change than that of the hydrogen reduction. Therefore, the temperature drop in the RWGS is less than that in the SOEC.
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FeO + H 2 → Fe + H 2 O ΔH °298 = +27.2 kJ / mol ... (5) FeO + C → Fe + CO ΔH °298 = +158 kJ / mol .... (6) This simulation model contained only equilibrium reactors and thus cannot give sufficient data to discuss the operational feasibility of the modified blast furnace. Detailed analysis
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