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Energy and exergy analyses of direct ironsmelting processes
Energy 30 (2005) 2772–2783 www.elsevier.com/locate/energy
Energy and exergy analyses of direct ironsmelting processes Oleg Ostrovski, Guangqing Zhang* School of Materials Science and Engineering, The University of New South Wales, E8 Building, UNSW Sydney, NSW 2052, Australia Received 13 January 2004
Abstract The paper discusses the concept of exergy, the energy and exergy balances of blast furnace ironmaking and DIOS-type direct ironsmelting processes, and exergy losses in these processes. The overall fuel efficiency of direct ironsmelting strongly depends on the utilisation of off-gas. It is shown that if off-gas is not utilised efficiently, the fuel efficiency of the direct ironsmelting process is enhanced strongly by increasing heat transfer efficiency and post-combustion ratio. Otherwise, when the off-gas is utilised efficiently, post-combustion ratio and heat transfer efficiency are less significant for the overall fuel efficiency. q 2005 Elsevier Ltd. All rights reserved.
1. Introduction In accordance with the well-known law of energy conservation (the first thermodynamic law), energy can neither be created nor destroyed. However, energy may have different qualities and degradation of energy may occur during its transformation. The energy degradation is unavoidable in any type of our activities and is characterised by energy consumption. The consumption of energy is speeded up along with the progress of industrialisation. This makes a rational usage of energy resources such as coal, petroleum, natural gas and others, which cannot be regenerated, one of the most important tasks of the industrial development. Consumption of energy in iron and steel industry is among the largest in the industrial world [1]. Energy contributes about 30% to the production cost of steel. 40% of total exergy in steel production is consumed in ironmaking processes [2]. Decrease in the exergy loss and energy consumption is a powerful factor in improving the ironmaking efficiency. * Corresponding author. Tel.: C61 2 9385 5440; fax: C61 2 9385 5956. E-mail address: [email protected] (G. Zhang). 0360-5442/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2005.01.007
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Energy management on the basis of the first thermodynamic law (energy conservation or heat balance) allows minimisation of wastes and improvement of energy efficiency by up to 15% [3]. Further, higher level energy savings can be achieved on the basis of the second thermodynamic law, which includes analysis of exergy. The concept of exergy was introduced by Rant in 1956 to characterise the quality of energy and its ability to perform work [2]. Exergy analysis of thermal, chemical and metallurgical processes is used to identify energy saving potentials of these processes. Szargut first employed exergy analysis to metallurgical processes in 1961 [4]. In 1963, Brauer and Jeschar applied exergy to the thermodynamic study of blast furnace process [5]. Akiyama and Yagi used the exergy analysis in application to the conventional blast furnace ironmaking, direct iron reduction-electric furnace smelting, and smelting reduction processes [6,7]. They concluded that smelting reduction is the most efficient process. Akiyama and Yagi considered smelting reduction of iron ores pre-reduced in the shaft furnace and fluidised bed to 70 and 60% extent of reduction, respectively. The post-combustion ratio, heat transfer efficiency, off-gas temperature and other parameters were fixed. However, energy performance of the smelting reduction furnace depends strongly on the post-combustion ratio, heat transfer efficiency and off-gas temperature. Effect of these parameters on exergy loss and coal consumption in the DIOS-type process is a focus of this paper. The exergy balance of a traditional blast furnace ironmaking process is also presented.
2. Process irreversibility, exergy loss and production costs The most exergy efficient process would be a process without exergy loss or a reversible process. The reversible process is a reference against which we can assess a real process. Exergy loss is exclusively due to irreversibility of real processes [8]: 4lost Z Te DSirr
(1)
where Te is the environment temperature and DSirr is the entropy production in the process. All industrial processes are non-equilibrium and practically irreversible. It can be expected that the energy efficiency increases with decreasing reaction rate as a result of decreasing exergy loss (departure from equilibrium decreases). The reactor volume also increases with decreasing reaction rate. This situation is illustrated by Fig. 1 in which the production cost is plotted versus the entropy production. With increasing irreversibility of a process and associated entropy production, the fuel efficiency decreases, and fuel cost increases while the capital cost decreases. If the fuel is relatively cheap, the overall production cost will go down with increasing entropy production. However, if the fuel is expensive, highly irreversible processes with high entropy production above some optimal value will be less cost-efficient. Expecting that the energy cost will increase with time, more attention should be given to fuel efficiency. Certainly this is an illustration only, as industrial technology includes a number of other factors affecting the production cost. Major sources of exergy loss in ironmaking are associated with fuel combustion, heat transfer and reduction reactions. Exergy analysis in combustion process and heat transfer can be found in literature [2,8]. Exergy change in a chemical reaction with enthalpy DH and entropy DS is D4 Z DH K Te DS
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Fig. 1. Production cost as a function of entropy production.
It can be presented as D4 Z DH K Te ðDH K DGÞ=T Z DHð1 K Te =TÞ C DGðTe =TÞ
(2)
where DG is the Gibbs free energy of reaction and T the reaction temperature. The exergy loss due to irreversibility of the reaction, where no work other than work of expansion is involved, is DG(Te/T).
3. Blast furnace ironmaking Major items of energy and exergy balances for the blast furnace are presented in Table 1 [2], where THM represents ton hot metal. A part of blast furnace gas energy is utilised for blast heating (about a quarter of the total blast furnace gas energy) but is not included in the energy and exergy balances in Table 1, because blast stoves can be seen as an internal process of blast furnace processing for energy recovery. Out of all the energy and exergy inputs which are mainly from the metallurgical coke, 37.4% of energy and 32.4% of exergy are outputs with hot metal. Off-gas accounts for 40–50% of the total energy and exergy outputs (Table 1). Its energy and exergy are partly recovered for pre-heating the blast. Including the chemical energy and exergy of the blast furnace gas exported for external use, the energy and exergy efficiencies reach 67.1 and 61%, respectively. It should be said that the efficiencies are relatively high compared to general chemical processes. The rest of energy is rejected with waste materials such as slag and lost blast furnace gas and as various heat losses including the sensible heat of the exported blast furnace gas. Although these parts of energy sum up to 32.7%, corresponding exergy accounts for only 19.3%. Relatively high portion (19.7%) of exergy is destroyed in various irreversible processes of blast furnace processing. In the blast furnace ironmaking, major irreversible processes include the following: (1) coke combustion; (2) heat transfer from the combustion gas (2270–2470 K) to burden (1770– 1820 K); (3) formation of metal and slag solutions; (4) blast furnace gas combustion and heat transfer in
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Table 1 Energy and exergy balances for blast furnace ironmaking [1] Major balance items
Energy (MJ/THM)
%
Exergy (MJ/THM)
%
Input Coke Sinter, ore and flux Open hearth furnace slag Total
25,205 1339 243 26,787
94.1 5 0.9 100
26,415 1086 163 27,664
95.5 3.9 0.6 100
8780 1231
32.8 4.6
8206 754
29.7 2.7
8018 18,029
29.9 67.3
7913 16,873
28.6 61
3261
12.2
2543
9.2
1432 1432 2633 8758
5.3 5.3 9.8 32.7
515 1080 1192 5330
1.9 3.9 4.3 19.3
3623 1838 5461
13.1 6.6 19.7
Output Useful effluents Molten metal – Chemical part – Physical part Blast furnace gas – Chemical part Subtotal Rejected Molten slag Blast furnace gas – Physical part Lost blast furnace gas Other Subtotal Destroyed in irreversible processes In blast furnace In blast stoves Subtotal Total
26,787
100
27,664
100
blast stoves. Mixing of oxygen with air for oxygen enriched blast is also irreversible and is exergy consumable, although almost no heat effect takes place in the process. For blast furnace ironmaking, heat transfer in the thermal reserve zone and solid-state reduction, particularly in the chemically inert zone, can be identified as processes close to reversible.
4. Direct ironsmelting process The conventional blast furnace ironmaking uses coke, agglomerated iron ore and is economical only on large production scale. It is challenged by alternative coke-less processes, which are more environment friendly and more flexible in operation. Fig. 2 shows the schematic of Nippon Steel Corporation Direct Iron Ore Smelting (DIOS) reactor [9]. Iron ore and fluxes are added from the top of the reactor and are dissolved into molten slag. The dissolved iron oxides, along with other minor metal oxides, are reduced from the slag by carbon dissolved in metal and by char in slag. The heat needed for reduction and smelting reactions is generated
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Fig. 2. Scheme of NSC iron ore smelting reduction reactor.
by combustion of coal (char) and post-combustion of CO and H2 in gas phase. To promote the reactions, nitrogen is blown into the reactor from the bottom to produce a stirring effect. Major irreversible processes in direct ironsmelting are similar to those in blast furnace ironmaking, including the following: (1) coal combustion; (2) process gas combustion; (3) heat transfer from the gas phase (2070–2370 K) to the bath (1720–1820 K); (4) iron ore dissolution into slag; (5) iron oxide reduction; (6) formation of metal and slag solutions. Although the water gas shift reaction can be considered close to equilibrium, the combustion of CO and H2 is strongly irreversible. No reactions in the direct ironsmelting process can be identified as reversible. The amount and temperature of off-gas are much higher in the direct ironsmelting processes. The energy and exergy of off-gas depend strongly on the post-combustion ratio and heat transfer efficiency, which are analysed below. A comprehensive modelling of DIOS-type ironsmelting process was done by Panjkovic et al. [10]. The core of this modelling is CFD model coupled with heat and mass balance model for the bath and a droplet tracking model. In this paper, a simpler mass and heat balance model was used to study effects of post-combustion ratio and heat transfer efficiency on the temperature, chemistry and amount of off-gas, and on the energy and exergy balances. A brief description of this model is as follows. Coal was taken of the same composition as examined by Panjkovic et al. [10] using the CFD model (Table 2). It has a gross heat value of 34.2 MJ/kg (dry ash free). According to the composition, 1 kg of coal (dry ash free) contains 69.07 mol C, 26.9 mol H2, 5.8 mol O and 0.61 mol N2. Water-gas shift reaction was assumed to be at equilibrium. The equilibrium constant for this reaction, post-combustion ratio and coal composition define the gas composition through solution of Eqs. (3)–(7). K Z ðPH2 O =PH2 ÞðPCO =PCO2 Þ Z expðKDG8=RTÞ Z expðð25:882T K 26995Þ=RTÞ
(3)
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Table 2 Coal composition Fixed carbon % (dry base)
Volatile matter % (dry base)
55.2 36.5 Element composition, wt% (dry ash free base) C H 82.88 5.38
Ash % (dry base)
Free water %
8.3
1.0
O 9.28
N 1.7
S 0.76
where K is the equilibrium constant for the reaction: H2 C CO2 Z H2 O C CO;
DG8 Z 26995 K 25:882T ðJÞ
Post-combustion ratio was defined in a conventional way: PCR Z ð%CO2 C %H2 OÞ=ð%CO2 C %H2 O C %CO C %H2 Þ
(4)
It was assumed that 40 kg/THM of carbon was consumed for iron carburisation and 8 kg for reduction of manganese and silicon oxides present in iron ore. If C kg/THM of coal (0.8288C kg/THM of carbon) is consumed, then out of total 0.06907C kmol/THM of carbon (0.06907CK40/12) kmol/THM reports to off-gas. This analysis gives additional equations for calculating the off-gas composition: CO C CO2 Z 0:06907C K 40=12 Z 0:06907C K 3:3333
(5)
H2 C H2 O Z ð0:0269 C 0:010=18ÞC Z 0:02746C
(6)
N2 Z 0:000607C C 1:760
(7)
In Eqs. (5)–(7), CO, CO2, H2, H2O and N2 represent kmol/THM of individual species in the gas phase. The heat balance included (per THM) heat of reduction of Fe3O4 and Fe2O3 (6911 MJ), manganese and silicon oxides (319 MJ), heat of carbon dissolution into molten iron (33.5 MJ), heat loss through refractories and lance (418 MJ), heat of water evaporation (1.18 MJ per kg of coal), and heat contents of molten metal (1251 MJ), molten slag (556 MJ), dust (167 MJ) and off-gas (depends on post-combustion ratio and heat transfer efficiency). This heat demand was balanced by heat of oxidation of carbon to CO and CO2 and hydrogen oxidation to H2O in accordance with water-gas shift and post-combustion reactions. Temperature of the off-gas was determined by the heat transfer efficiency, which was described as HTE Z 1 K
Hgas;Tgas K Hgas;Tbath
(8)
KDHpost - combustion where Hgas;Tgas and Hgas;Tbath represent the total enthalpy of off-gas at temperature Tgas, at which gas leaves the reactor, and at the bath temperature, Tbath, respectively; KDHpost-combustion is the heat evolved in the post-combustion reactions. Results of this modelling are compared with data obtained by Panjkovic et al. [10] using CFD model (Table 3). Off-gas temperature and coal consumption calculated in two different models are in reasonable agreement.
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Table 3 Comparison of mass and heat balance model with CFD model (in brackets) HTE
PCR
Off-gas temperature (K)
Coal consumption (kg/THM)
0.714 0.744 0.791
0.387 0.417 0.521
2323 (2369) 2296 (2353) 2276 (2343)
1291 (1366) 1165 (1245) 941 (992)
Coal consumption per THM as a function of PCR, calculated at different HTE, is presented in Fig. 3. Coal consumption decreases with increasing PCR and HTE as expected. Off gas temperature increases with increasing post-combustion ratio and decreasing heat transfer efficiency (Fig. 4). Increase in the off-gas temperature with PCR is more steady at high HTE. At relatively low HTE of 0.75–0.80, the off-gas temperature is very high. At PCR above 0.7, it exceeds 2300 K.
5. Exergy loss in direct ironsmelting Exergy of different materials was calculated using data for the standard exergy from Szargut et al. [2] and heat capacity from Knacke et al. [11]. There is a strong correlation between the coal consumption and off-gas energy (Fig. 5) and off-gas exergy (Fig. 6). This correlation can be explained on the basis of the energy and exergy balances. Heats of reduction of Fe3O4 and Fe2O3, manganese and silicon oxides, carbon dissolution into molten iron, heat losses through refractories and lance, heat contents of molten metal, molten slag, and dust and their exergy are approximately constant. The major variables are enthalpy and exergy of off-gas, which depend on the post-combustion ratio and heat transfer efficiency. The higher the enthalpy or exergy of the off-gas is, the higher the coal consumption is.
Fig. 3. Coal consumption at different PCR and HTE.
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Fig. 4. Off-gas temperature at different PCR and HTE.
Exergy loss was calculated as a difference between the exergy of products and coal. Calculated exergy loss as a function of post-combustion ratio at different heat transfer efficiency is plotted in Fig. 7. This exergy loss is mainly due to irreversibility of reduction reactions, combustion (post-combustion) and heat transfer. The exergy loss due to irreversibility of the reduction reaction was estimated to be 590 MJ/THM. Reduction of iron oxide was considered from molten slag by carbon of char at the bath temperature of 1720 K and PCOZ1 atm. More accurate calculation should also include reduction of iron oxide by carbon of molten iron. However, the exergy loss due to irreversibility of the reduction reaction is relatively small, and does not have a significant effect on the total exergy loss. In the calculation of the exergy loss shown in Fig. 7, exergy of products includes exergy of off-gas. This means that exergy loss in Fig. 7 does not account for inevitable exergy loss in the utilisation of offgas. This gas can be used for pre-heating/pre-reduction of iron ore, pre-heating of blast, or can be exported for electric power generation, etc. However, there is no technology available for utilisation of off-gas having high temperature expected in the direct ironsmelting processes. Before being utilised, offgas is cleaned up and cooled down to temperature of 1070–1370 K, which causes additional exergy loss.
Fig. 5. Coal consumption versus off-gas energy.
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Fig. 6. Coal consumption versus off-gas exergy.
Exergy loss, calculated assuming that off-gas is cooled to 1373 K and fully utilised in a reversible processes afterwards, is shown in Figs. 8–10 as a function of PCR at different HTE. These figures also show exergy losses in the case when off-gas is not utilised at all. Energy and exergy of off-gas at post-combustion ratio of 0.4–0.6 and heat transfer efficiency of 0.7– 0.8 are even higher than energy and exergy requirements for smelting/reduction processes. This means that under given condition, about half of energy and exergy of coal consumed in the direct ironsmelting are in off-gas. Without utilisation of off-gas, fuel efficiency of direct ironsmelting processes would be low. Exergy losses decrease with increasing PCR and HTE. Under any conditions, recovery of energy and exergy of off-gas plays an important role in enhancing the overall fuel efficiency. From the viewpoint of utilisation of off-gas, pressurised vessels offer additional opportunities. Because reduction and combustion reactions are volumetrically expansive, increase in reaction pressure
Fig. 7. Exergy loss under the condition of total utilisation of off-gas exergy.
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Fig. 8. Exergy loss with off-gas (HTEZ0.75).
will decrease the irreversibility of the reactions and hence decrease the exergy loss of these reactions. As a result, more chemical exergy of the reactants is converted into the exergy of products, which is manifested as the increase in the pressure exergy of off-gas. In short, pressurisation decreases exergy loss in the furnace and increases obtainable energy of gas. Additional opportunities in energy saving are created by combination of different technologies. For example, Akiyama et al. [12,13] proposed recovery of heat of high temperature wastes from ironmaking by several chemical reactions including steam reforming and thermal decomposition of limestone, and carried out a feasibility study of methanol synthesis to recover the chemical exergy of blast furnace gas and to decrease greenhouse gas emission. According to Akiyama et al. [13], it is possible to reduce the total exergy consumption by 9%. However, the potential is limited considering the small methanol synthesis industry relative to ironmaking.
Fig. 9. Exergy loss with off-gas (HTEZ0.85).
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Fig. 10. Exergy loss with off-gas (HTEZ1).
6. Conclusions Industrial processes are irreversible and are always accompanied by exergy losses. Exergy destroyed in the blast furnace is about 5460 MJ/THM, while exergy consumed in the direct ironsmelting is in the range of 5000–9200 MJ/THM depending on the PCR and HTE (4140 MJ/THM in the ‘ideal’ reactor with PCRZ1 and HTEZ1). Direct ironsmelting processes are generally farther from equilibrium in comparison with blast furnace ironmaking. For direct smelting reduction processes considered in this paper, exergy of off-gas is 2320 MJ/THM for the ideal reactor. However, this number may increase up to 25,040 MJ/THM when HTEZ0.75 and PCRZ0.4. For HTEZ0.75 and PCRZ0.4–0.5, energy and exergy of off-gas constitute more than half of energy and exergy supplied to the smelting reduction furnace. Off-gas utilisation is critical to the fuel efficiency. If off-gas utilisation is low, the fuel efficiency of the direct ironsmelting process will be enhanced strongly by increasing heat transfer efficiency and post-combustion ratio. When off-gas is utilised efficiently, post-combustion ratio and heat transfer efficiency will be much less significant for the overall fuel efficiency.
References [1] Bisio G, Rubatto G, Martini R. Heat transfer, energy saving and pollution control in UHP electro-arc furnaces. Energy 2000;25:1047–66. [2] Szargut J, Morris DR, Steward FR. Exergy analysis of thermal, chemical and metallurgical processes. New York: Hemisphere; 1988. [3] Rivero R. Application of the exergy concept in the petroleum refining and petrochemical industry. Energy Convers Manage 2002;43(9–12):1199–220. [4] Szargut J. Exergetic balance of metallurgical processes. Arch Hutnictwa 1961;6:23–60. [5] Brauer H, Jeschar R. The significance of exergy for the thermodynamic study of the blast-furnace process. Arch fuer das Eisenhuettenwesen 1963;34:9–16.
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[6] Akiyama T, Yagi J. Exergy analysis of conventional ironmaking, direct reduction-electric furnace and smelting reduction systems. Tetsu-to-Hagane 1988;74(12):2270–7. [7] Akiyama T, Yagi J. Exergy analysis of conventional and new ironmaking systems. In: 1988 Process technology conference proceedings, Toronto, Canada. Warrendale, PA: Iron and Steel Society; 1988. p. 179–93. [8] Gaggioli RA. Second law analysis for process and energy engineering. ACS symposium series; 1983; vol. 235 (efficiency and costing: second law analysis of processes). p. 3–50. [9] Ibaraki T, Yamauchi M, Sakamoto Y, Hirata H, Kanemoto M. Experimental operation of smelting reduction with a 100 mt smelter—I. Operation and the slag in the smelter. Iron Steelmaker 1995;22(3):91–8. [10] Panjkovic V, Truelove J, Ostrovski O. Analysis of performance of an iron-bath reactor using computational fluid dynamics. Appl Math Modell 2002;26(2):203–21. [11] Knacke O, Kubaschewski O, Hesselmann K. Thermochemical properties of inorganic substances. Dusseldorf: Verlag Stahleisen m.b.H.; 1991. [12] Akiyama T, Oikawa K, Shimada T, Kasai E, Yagi J. Thermodynamic analysis of thermochemical recovery of high temperature wastes. ISIJ Int 2000;40(3):286–91. [13] Akiyama T, Sato H, Muramatsu A, Yagi J. Feasibility study on blast furnace ironmaking system integrated with methanol synthesis for reduction of carbon monoxide emission and effective use of exergy. ISIJ Int 1993;33(11):1136–43.
Energy and exergy analyses of direct ironsmelting processes Oleg Ostrovski, Guangqing Zhang* School of Materials Science and Engineering, The University of New South Wales, E8 Building, UNSW Sydney, NSW 2052, Australia Received 13 January 2004
Abstract The paper discusses the concept of exergy, the energy and exergy balances of blast furnace ironmaking and DIOS-type direct ironsmelting processes, and exergy losses in these processes. The overall fuel efficiency of direct ironsmelting strongly depends on the utilisation of off-gas. It is shown that if off-gas is not utilised efficiently, the fuel efficiency of the direct ironsmelting process is enhanced strongly by increasing heat transfer efficiency and post-combustion ratio. Otherwise, when the off-gas is utilised efficiently, post-combustion ratio and heat transfer efficiency are less significant for the overall fuel efficiency. q 2005 Elsevier Ltd. All rights reserved.
1. Introduction In accordance with the well-known law of energy conservation (the first thermodynamic law), energy can neither be created nor destroyed. However, energy may have different qualities and degradation of energy may occur during its transformation. The energy degradation is unavoidable in any type of our activities and is characterised by energy consumption. The consumption of energy is speeded up along with the progress of industrialisation. This makes a rational usage of energy resources such as coal, petroleum, natural gas and others, which cannot be regenerated, one of the most important tasks of the industrial development. Consumption of energy in iron and steel industry is among the largest in the industrial world [1]. Energy contributes about 30% to the production cost of steel. 40% of total exergy in steel production is consumed in ironmaking processes [2]. Decrease in the exergy loss and energy consumption is a powerful factor in improving the ironmaking efficiency. * Corresponding author. Tel.: C61 2 9385 5440; fax: C61 2 9385 5956. E-mail address: [email protected] (G. Zhang). 0360-5442/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2005.01.007
O. Ostrovski, G. Zhang / Energy 30 (2005) 2772–2783
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Energy management on the basis of the first thermodynamic law (energy conservation or heat balance) allows minimisation of wastes and improvement of energy efficiency by up to 15% [3]. Further, higher level energy savings can be achieved on the basis of the second thermodynamic law, which includes analysis of exergy. The concept of exergy was introduced by Rant in 1956 to characterise the quality of energy and its ability to perform work [2]. Exergy analysis of thermal, chemical and metallurgical processes is used to identify energy saving potentials of these processes. Szargut first employed exergy analysis to metallurgical processes in 1961 [4]. In 1963, Brauer and Jeschar applied exergy to the thermodynamic study of blast furnace process [5]. Akiyama and Yagi used the exergy analysis in application to the conventional blast furnace ironmaking, direct iron reduction-electric furnace smelting, and smelting reduction processes [6,7]. They concluded that smelting reduction is the most efficient process. Akiyama and Yagi considered smelting reduction of iron ores pre-reduced in the shaft furnace and fluidised bed to 70 and 60% extent of reduction, respectively. The post-combustion ratio, heat transfer efficiency, off-gas temperature and other parameters were fixed. However, energy performance of the smelting reduction furnace depends strongly on the post-combustion ratio, heat transfer efficiency and off-gas temperature. Effect of these parameters on exergy loss and coal consumption in the DIOS-type process is a focus of this paper. The exergy balance of a traditional blast furnace ironmaking process is also presented.
2. Process irreversibility, exergy loss and production costs The most exergy efficient process would be a process without exergy loss or a reversible process. The reversible process is a reference against which we can assess a real process. Exergy loss is exclusively due to irreversibility of real processes [8]: 4lost Z Te DSirr
(1)
where Te is the environment temperature and DSirr is the entropy production in the process. All industrial processes are non-equilibrium and practically irreversible. It can be expected that the energy efficiency increases with decreasing reaction rate as a result of decreasing exergy loss (departure from equilibrium decreases). The reactor volume also increases with decreasing reaction rate. This situation is illustrated by Fig. 1 in which the production cost is plotted versus the entropy production. With increasing irreversibility of a process and associated entropy production, the fuel efficiency decreases, and fuel cost increases while the capital cost decreases. If the fuel is relatively cheap, the overall production cost will go down with increasing entropy production. However, if the fuel is expensive, highly irreversible processes with high entropy production above some optimal value will be less cost-efficient. Expecting that the energy cost will increase with time, more attention should be given to fuel efficiency. Certainly this is an illustration only, as industrial technology includes a number of other factors affecting the production cost. Major sources of exergy loss in ironmaking are associated with fuel combustion, heat transfer and reduction reactions. Exergy analysis in combustion process and heat transfer can be found in literature [2,8]. Exergy change in a chemical reaction with enthalpy DH and entropy DS is D4 Z DH K Te DS
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Fig. 1. Production cost as a function of entropy production.
It can be presented as D4 Z DH K Te ðDH K DGÞ=T Z DHð1 K Te =TÞ C DGðTe =TÞ
(2)
where DG is the Gibbs free energy of reaction and T the reaction temperature. The exergy loss due to irreversibility of the reaction, where no work other than work of expansion is involved, is DG(Te/T).
3. Blast furnace ironmaking Major items of energy and exergy balances for the blast furnace are presented in Table 1 [2], where THM represents ton hot metal. A part of blast furnace gas energy is utilised for blast heating (about a quarter of the total blast furnace gas energy) but is not included in the energy and exergy balances in Table 1, because blast stoves can be seen as an internal process of blast furnace processing for energy recovery. Out of all the energy and exergy inputs which are mainly from the metallurgical coke, 37.4% of energy and 32.4% of exergy are outputs with hot metal. Off-gas accounts for 40–50% of the total energy and exergy outputs (Table 1). Its energy and exergy are partly recovered for pre-heating the blast. Including the chemical energy and exergy of the blast furnace gas exported for external use, the energy and exergy efficiencies reach 67.1 and 61%, respectively. It should be said that the efficiencies are relatively high compared to general chemical processes. The rest of energy is rejected with waste materials such as slag and lost blast furnace gas and as various heat losses including the sensible heat of the exported blast furnace gas. Although these parts of energy sum up to 32.7%, corresponding exergy accounts for only 19.3%. Relatively high portion (19.7%) of exergy is destroyed in various irreversible processes of blast furnace processing. In the blast furnace ironmaking, major irreversible processes include the following: (1) coke combustion; (2) heat transfer from the combustion gas (2270–2470 K) to burden (1770– 1820 K); (3) formation of metal and slag solutions; (4) blast furnace gas combustion and heat transfer in
O. Ostrovski, G. Zhang / Energy 30 (2005) 2772–2783
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Table 1 Energy and exergy balances for blast furnace ironmaking [1] Major balance items
Energy (MJ/THM)
%
Exergy (MJ/THM)
%
Input Coke Sinter, ore and flux Open hearth furnace slag Total
25,205 1339 243 26,787
94.1 5 0.9 100
26,415 1086 163 27,664
95.5 3.9 0.6 100
8780 1231
32.8 4.6
8206 754
29.7 2.7
8018 18,029
29.9 67.3
7913 16,873
28.6 61
3261
12.2
2543
9.2
1432 1432 2633 8758
5.3 5.3 9.8 32.7
515 1080 1192 5330
1.9 3.9 4.3 19.3
3623 1838 5461
13.1 6.6 19.7
Output Useful effluents Molten metal – Chemical part – Physical part Blast furnace gas – Chemical part Subtotal Rejected Molten slag Blast furnace gas – Physical part Lost blast furnace gas Other Subtotal Destroyed in irreversible processes In blast furnace In blast stoves Subtotal Total
26,787
100
27,664
100
blast stoves. Mixing of oxygen with air for oxygen enriched blast is also irreversible and is exergy consumable, although almost no heat effect takes place in the process. For blast furnace ironmaking, heat transfer in the thermal reserve zone and solid-state reduction, particularly in the chemically inert zone, can be identified as processes close to reversible.
4. Direct ironsmelting process The conventional blast furnace ironmaking uses coke, agglomerated iron ore and is economical only on large production scale. It is challenged by alternative coke-less processes, which are more environment friendly and more flexible in operation. Fig. 2 shows the schematic of Nippon Steel Corporation Direct Iron Ore Smelting (DIOS) reactor [9]. Iron ore and fluxes are added from the top of the reactor and are dissolved into molten slag. The dissolved iron oxides, along with other minor metal oxides, are reduced from the slag by carbon dissolved in metal and by char in slag. The heat needed for reduction and smelting reactions is generated
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Fig. 2. Scheme of NSC iron ore smelting reduction reactor.
by combustion of coal (char) and post-combustion of CO and H2 in gas phase. To promote the reactions, nitrogen is blown into the reactor from the bottom to produce a stirring effect. Major irreversible processes in direct ironsmelting are similar to those in blast furnace ironmaking, including the following: (1) coal combustion; (2) process gas combustion; (3) heat transfer from the gas phase (2070–2370 K) to the bath (1720–1820 K); (4) iron ore dissolution into slag; (5) iron oxide reduction; (6) formation of metal and slag solutions. Although the water gas shift reaction can be considered close to equilibrium, the combustion of CO and H2 is strongly irreversible. No reactions in the direct ironsmelting process can be identified as reversible. The amount and temperature of off-gas are much higher in the direct ironsmelting processes. The energy and exergy of off-gas depend strongly on the post-combustion ratio and heat transfer efficiency, which are analysed below. A comprehensive modelling of DIOS-type ironsmelting process was done by Panjkovic et al. [10]. The core of this modelling is CFD model coupled with heat and mass balance model for the bath and a droplet tracking model. In this paper, a simpler mass and heat balance model was used to study effects of post-combustion ratio and heat transfer efficiency on the temperature, chemistry and amount of off-gas, and on the energy and exergy balances. A brief description of this model is as follows. Coal was taken of the same composition as examined by Panjkovic et al. [10] using the CFD model (Table 2). It has a gross heat value of 34.2 MJ/kg (dry ash free). According to the composition, 1 kg of coal (dry ash free) contains 69.07 mol C, 26.9 mol H2, 5.8 mol O and 0.61 mol N2. Water-gas shift reaction was assumed to be at equilibrium. The equilibrium constant for this reaction, post-combustion ratio and coal composition define the gas composition through solution of Eqs. (3)–(7). K Z ðPH2 O =PH2 ÞðPCO =PCO2 Þ Z expðKDG8=RTÞ Z expðð25:882T K 26995Þ=RTÞ
(3)
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Table 2 Coal composition Fixed carbon % (dry base)
Volatile matter % (dry base)
55.2 36.5 Element composition, wt% (dry ash free base) C H 82.88 5.38
Ash % (dry base)
Free water %
8.3
1.0
O 9.28
N 1.7
S 0.76
where K is the equilibrium constant for the reaction: H2 C CO2 Z H2 O C CO;
DG8 Z 26995 K 25:882T ðJÞ
Post-combustion ratio was defined in a conventional way: PCR Z ð%CO2 C %H2 OÞ=ð%CO2 C %H2 O C %CO C %H2 Þ
(4)
It was assumed that 40 kg/THM of carbon was consumed for iron carburisation and 8 kg for reduction of manganese and silicon oxides present in iron ore. If C kg/THM of coal (0.8288C kg/THM of carbon) is consumed, then out of total 0.06907C kmol/THM of carbon (0.06907CK40/12) kmol/THM reports to off-gas. This analysis gives additional equations for calculating the off-gas composition: CO C CO2 Z 0:06907C K 40=12 Z 0:06907C K 3:3333
(5)
H2 C H2 O Z ð0:0269 C 0:010=18ÞC Z 0:02746C
(6)
N2 Z 0:000607C C 1:760
(7)
In Eqs. (5)–(7), CO, CO2, H2, H2O and N2 represent kmol/THM of individual species in the gas phase. The heat balance included (per THM) heat of reduction of Fe3O4 and Fe2O3 (6911 MJ), manganese and silicon oxides (319 MJ), heat of carbon dissolution into molten iron (33.5 MJ), heat loss through refractories and lance (418 MJ), heat of water evaporation (1.18 MJ per kg of coal), and heat contents of molten metal (1251 MJ), molten slag (556 MJ), dust (167 MJ) and off-gas (depends on post-combustion ratio and heat transfer efficiency). This heat demand was balanced by heat of oxidation of carbon to CO and CO2 and hydrogen oxidation to H2O in accordance with water-gas shift and post-combustion reactions. Temperature of the off-gas was determined by the heat transfer efficiency, which was described as HTE Z 1 K
Hgas;Tgas K Hgas;Tbath
(8)
KDHpost - combustion where Hgas;Tgas and Hgas;Tbath represent the total enthalpy of off-gas at temperature Tgas, at which gas leaves the reactor, and at the bath temperature, Tbath, respectively; KDHpost-combustion is the heat evolved in the post-combustion reactions. Results of this modelling are compared with data obtained by Panjkovic et al. [10] using CFD model (Table 3). Off-gas temperature and coal consumption calculated in two different models are in reasonable agreement.
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Table 3 Comparison of mass and heat balance model with CFD model (in brackets) HTE
PCR
Off-gas temperature (K)
Coal consumption (kg/THM)
0.714 0.744 0.791
0.387 0.417 0.521
2323 (2369) 2296 (2353) 2276 (2343)
1291 (1366) 1165 (1245) 941 (992)
Coal consumption per THM as a function of PCR, calculated at different HTE, is presented in Fig. 3. Coal consumption decreases with increasing PCR and HTE as expected. Off gas temperature increases with increasing post-combustion ratio and decreasing heat transfer efficiency (Fig. 4). Increase in the off-gas temperature with PCR is more steady at high HTE. At relatively low HTE of 0.75–0.80, the off-gas temperature is very high. At PCR above 0.7, it exceeds 2300 K.
5. Exergy loss in direct ironsmelting Exergy of different materials was calculated using data for the standard exergy from Szargut et al. [2] and heat capacity from Knacke et al. [11]. There is a strong correlation between the coal consumption and off-gas energy (Fig. 5) and off-gas exergy (Fig. 6). This correlation can be explained on the basis of the energy and exergy balances. Heats of reduction of Fe3O4 and Fe2O3, manganese and silicon oxides, carbon dissolution into molten iron, heat losses through refractories and lance, heat contents of molten metal, molten slag, and dust and their exergy are approximately constant. The major variables are enthalpy and exergy of off-gas, which depend on the post-combustion ratio and heat transfer efficiency. The higher the enthalpy or exergy of the off-gas is, the higher the coal consumption is.
Fig. 3. Coal consumption at different PCR and HTE.
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Fig. 4. Off-gas temperature at different PCR and HTE.
Exergy loss was calculated as a difference between the exergy of products and coal. Calculated exergy loss as a function of post-combustion ratio at different heat transfer efficiency is plotted in Fig. 7. This exergy loss is mainly due to irreversibility of reduction reactions, combustion (post-combustion) and heat transfer. The exergy loss due to irreversibility of the reduction reaction was estimated to be 590 MJ/THM. Reduction of iron oxide was considered from molten slag by carbon of char at the bath temperature of 1720 K and PCOZ1 atm. More accurate calculation should also include reduction of iron oxide by carbon of molten iron. However, the exergy loss due to irreversibility of the reduction reaction is relatively small, and does not have a significant effect on the total exergy loss. In the calculation of the exergy loss shown in Fig. 7, exergy of products includes exergy of off-gas. This means that exergy loss in Fig. 7 does not account for inevitable exergy loss in the utilisation of offgas. This gas can be used for pre-heating/pre-reduction of iron ore, pre-heating of blast, or can be exported for electric power generation, etc. However, there is no technology available for utilisation of off-gas having high temperature expected in the direct ironsmelting processes. Before being utilised, offgas is cleaned up and cooled down to temperature of 1070–1370 K, which causes additional exergy loss.
Fig. 5. Coal consumption versus off-gas energy.
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Fig. 6. Coal consumption versus off-gas exergy.
Exergy loss, calculated assuming that off-gas is cooled to 1373 K and fully utilised in a reversible processes afterwards, is shown in Figs. 8–10 as a function of PCR at different HTE. These figures also show exergy losses in the case when off-gas is not utilised at all. Energy and exergy of off-gas at post-combustion ratio of 0.4–0.6 and heat transfer efficiency of 0.7– 0.8 are even higher than energy and exergy requirements for smelting/reduction processes. This means that under given condition, about half of energy and exergy of coal consumed in the direct ironsmelting are in off-gas. Without utilisation of off-gas, fuel efficiency of direct ironsmelting processes would be low. Exergy losses decrease with increasing PCR and HTE. Under any conditions, recovery of energy and exergy of off-gas plays an important role in enhancing the overall fuel efficiency. From the viewpoint of utilisation of off-gas, pressurised vessels offer additional opportunities. Because reduction and combustion reactions are volumetrically expansive, increase in reaction pressure
Fig. 7. Exergy loss under the condition of total utilisation of off-gas exergy.
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Fig. 8. Exergy loss with off-gas (HTEZ0.75).
will decrease the irreversibility of the reactions and hence decrease the exergy loss of these reactions. As a result, more chemical exergy of the reactants is converted into the exergy of products, which is manifested as the increase in the pressure exergy of off-gas. In short, pressurisation decreases exergy loss in the furnace and increases obtainable energy of gas. Additional opportunities in energy saving are created by combination of different technologies. For example, Akiyama et al. [12,13] proposed recovery of heat of high temperature wastes from ironmaking by several chemical reactions including steam reforming and thermal decomposition of limestone, and carried out a feasibility study of methanol synthesis to recover the chemical exergy of blast furnace gas and to decrease greenhouse gas emission. According to Akiyama et al. [13], it is possible to reduce the total exergy consumption by 9%. However, the potential is limited considering the small methanol synthesis industry relative to ironmaking.
Fig. 9. Exergy loss with off-gas (HTEZ0.85).
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Fig. 10. Exergy loss with off-gas (HTEZ1).
6. Conclusions Industrial processes are irreversible and are always accompanied by exergy losses. Exergy destroyed in the blast furnace is about 5460 MJ/THM, while exergy consumed in the direct ironsmelting is in the range of 5000–9200 MJ/THM depending on the PCR and HTE (4140 MJ/THM in the ‘ideal’ reactor with PCRZ1 and HTEZ1). Direct ironsmelting processes are generally farther from equilibrium in comparison with blast furnace ironmaking. For direct smelting reduction processes considered in this paper, exergy of off-gas is 2320 MJ/THM for the ideal reactor. However, this number may increase up to 25,040 MJ/THM when HTEZ0.75 and PCRZ0.4. For HTEZ0.75 and PCRZ0.4–0.5, energy and exergy of off-gas constitute more than half of energy and exergy supplied to the smelting reduction furnace. Off-gas utilisation is critical to the fuel efficiency. If off-gas utilisation is low, the fuel efficiency of the direct ironsmelting process will be enhanced strongly by increasing heat transfer efficiency and post-combustion ratio. When off-gas is utilised efficiently, post-combustion ratio and heat transfer efficiency will be much less significant for the overall fuel efficiency.
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