Exergy Analysis of Steel Production Processes

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(1)Materials Transactions, Vol. 43, No. 3 (2002) pp. 379 to 384 Special Issue on Environmentally Benign Manufacturing and Material Processing Toward Dematerialization c 2002 The Japan Institute of Metals. Exergy Analysis of Steel Production Processes Nobuyuki Shigaki1, ∗ , Tomohiro Akiyama2 and Fumitaka Tsukihashi1 1. Department of Advanced Materials Science, Graduate School of Frontier Sciences, The University of Tokyo, Tokyo 113-0033, Japan 2 Department of Chemical Engineering, Graduate School of Engineering, Osaka Prefecture University, Sakai 599-8531, Japan Continuous increase of the annual amount of steel scrap generated presents a problem to the sustainability of human society. The effective utilization of scrap is a serious problem to be solved in the near future, and the optimization of the global system of steel production should be considered from various viewpoints, particularly those of environmental load and material efficiency. For the consideration of a global system that contains many different kinds of flow, a criterion for evaluating the efficiency of the system is necessary for optimizing the utilization of materials in the system. The concept of exergy was adopted in this study and the application of exergy analysis to the evaluation of a complicated system was considered. A simulation model was developed for the steel industry in Japan and the exergy analysis of the Blast Furnace-LD converter (BF-LD) process and the Electric Arc Furnace (EF) process was conducted. The exergy loss was expressed as a function of parameters, such as the mixing rate of pig iron in the EF process and the total exergy loss in the system was calculated. The applicability of exergy as a criterion for the analysis of a production process and for evaluating material efficiency was discussed. (Received October 15, 2001; Accepted January 7, 2002) Keywords: exergy, recycling, material efficiency, steel industry, steel scrap, blast furmace (BF)-LD process, electric arc furmace (EF) process. 1. Introduction Environment and recycling become important keywords in the consideration of a new industry structure for optimizing material efficiency and environmental load for production and utilization of materials. In the present study, a methodology for evaluating the degree of optimization in the system including material recycling was investigated. The Blast FurnaceLD converter (BF-LD) process for the production of highgrade steel and the Electric Arc Furnace (EF) process for the production of low-grade steel are the main steel production processes used in Japan. This division occurs because of contamination by tramp elements as impurities, which is inevitable in processes using low-grade scrap as feed. The percentage of EF steel in the total production of crude steel in Japan is about 33%,1) and the total amount of the accumulative steel stock in Japan is increasing continuously. Generally, processes that consume large amounts of electric energy for heating and smelting are wasteful from the viewpoints of effective energy use and environmental load. However, the optimum steel production ratio in BF-LD and EF processes has not been investigated clearly. For the further improvement of steel productivity, it is necessary to optimize the steel production system from various viewpoints such as the suitable selection of feed materials, the total demand for steel products in society, and the impurities limit in the steel product. The recent state of the steel industry in Japan is that the amount of scrap is increasing annually and its quality is decreasing. Therefore, the effective use of iron resources originating from iron ore for the dilution of scrap has recently become a principal subject in the EF steel industry, together with the improvement of scrap recovery technology. Recent electric furnaces are operated with a high mixing rate of pig iron, from 20 to 50%, as iron feed, and other furnaces have ∗ Graduate. Student, The University of Tokyo.. been combined with a converter to enable the hybrid steelmaking process.2) Since it is very important for the steel industry in Japan to utilize the iron stock effectively as valuable resource, the optimum ratio of EF product to BF product and that of scrap utilized in the EF process to that utilized in the converter were investigated in this study in terms of the efficiency of the use of iron resources originating from iron ore for the dilution. A model of the iron and steel recycling system including material flow was constructed, and the thermodynamic property of exergy was applied to estimate the measure of optimization for the steel production system. 2. The Concept of Exergy Exergy is used to evaluate the degree of qualitative change of energy from available energy to diffused energy that means energy unusable as work. Exergy is defined by eq. (1). E = H − H0 − T0 (S − S0 ). (1). where the subscript 0 indicates values at atmospheric condition. Exergy consists of chemical exergy, E c , and physical exergy, E p , as shown in eq. (2). E = Ec + Ep = εc + εT + εP + εM. (2). where εc is the standard chemical exergy equal to E c . εT , εP and εM are the exergy of temperature, exergy of pressure, and exergy of mixing, respectively. These are expressed by eqs. (3) to (6). · Chemical: εc εc = n i εi◦ (3).

(2) 380. N. Shigaki, T. Akiyama and F. Tsukihashi. · Temperature:εT T εT = n i C p,i T − T0 − T0 ln T0 · Pressure:εP εP =. . n i RT0 ln. · Mixing:εM εM = RT0. . n i ln. Pi P0. ni ni. tion for the estimation of exergy efficiency. (4). − (E out,P1 + E out,P2 + E out,P3 ) (5). (6). εi◦ :. standard chemical exergy of i species C p,i : mean heat capacity of i species n i : molecular number of i species Pi : partial pressure of i species R: gas constant The value of exergy can be calculated for all substances in various states. Many different forms of substances and different kinds of energies are considered in the system. Therefore, the concept of exergy is very useful because it enables us to evaluate the energy efficiency of a system. 3. Model of Recycling System 3.1 Exergy loss in model system To analyze the material recycling system, a model that includes several processes in one system was considered, as shown in Fig. 1. Arrows in Fig. 1 indicate the exergy flows within the system. Each process has inflow and outflow of exergy from and to the outside of the system, and there are also interior flows that circulate through the system. The total exergy loss in this system, E loss,system , can be calculated by the following equa-. E IN ,P1. E OUT , P1. Process 1. E IN , P 2 E OUT , P 2. E PROD , P1. Process 2 E PROD , P 2. E PROD , P 3. Process 3. System E IN , P 3 E OUT , P 3 Fig. 1 Exergy flow in model system.. E loss,system = (E in,P1 + E in,P2 + E in,P3 ). (7). The subscript of each term on the right-hand side gives the direction of exergy flow. The exergy loss contains the dissipation exergy that results from an irreversible reaction and the diffusion exergy that is lost to the outside of the system. The exergy losses in each process and in the system are expressed as eqs. (8) to (11). E loss,P1 = (E in,P1 + E prod,P3 ) − (E out,P1 + E prod,P1 ). (8). E loss,P2 = (E in,P2 + E prod,P1 ) − (E out,P2 + E prod,P2 ). (9). E loss,P3 = (E in,P3 + E prod,P2 ) − (E out,P3 + E prod,P3 ). (10). E loss,P1 + E loss,P2 + E loss,P3 = (E in,P1 + E in,P2 + E in,P3 ) − (E out,P1 + E out,P2 + E out,P3 ) = E loss,system. (11). Equation (12) shows the total exergy loss in the system. E loss,system = E loss,i (12) i. 3.2 Model of the steel recycling system A model of the steel recycling system is shown in Fig. 2, based on the model in Ref. 3). t: Time (31.56 × 106 s (year)) St : Iron stock at time t (t) qt : The amount of iron produced by the reduction of iron ore a: Rate of iron stock scrapped annually b: Rate of scrap recovery I1 : Amount of direct import of iron (t/(31.56 × 106 s)) I2 : Amount of indirect import of iron (t/(31.56 × 106 s)) I3 : Amount of import of scrap (t/(31.56 × 106 s)) I4 : Amount of import of pig iron (t/(31.56 × 106 s)) WM : Amount of process scrap (t/(31.56 × 106 s)) WBF : Amount of home scrap of the BF-LD process used in the EF process (t/(31.56 × 106 s)) P (t/(31.56 × 106 s)) is the amount of pig iron used in the EF process. A is the scrap rate in the BF-LD process. When the system is restricted to BF-LD and EF processes as indicated by the dashed line in Fig. 2, this model can be simplified as in Fig. 3. The total exergy loss in the system shown in Fig. 3 is obtained by summing the exergy losses of all processes. E loss,BF and E loss,EF are the exergy losses in the BF-LD and EF processes per unit steel production, respectively. They are expressed as functions of P and A, and the total exergy loss in the system is obtained from by summing the products of exergy loss of all processes and its production rate (PBF , PEF (ton/year)) by eq. (13). E loss (t) = E loss,BF (P, A) · PBF + E loss,EF (P, A) · PEF (13).

(3) Exergy Analysis of Steel Production Processes. qt. I1. 381. I2. abSt qt WM I1 I 3 I 4. BF-LD. 1 qt 1 A WBF P. St. Manufacturer. abSt WM WBF P A I3 I4 qt 1 A. abSt qt I 2 I3. I1 I4. Stock. WM aSt. P. EF. W BF. Scrap Market abSt WM. A qt 1 A. I3. A qt 1 A. abSt WM. I3 I3. I4 Fig. 2 Scrap A 1. A. 1. A. qt. W (. was assumed to be the loss in the production processes of coke and sinter ore. Therefore, E loss,BF (P, A) is expressed by eq. (16).. P. BF. P BF. Fe (in ores). BF-LD. qt. E loss,BF (P, A) = 14900 − 4147A(MJ/t). Waste materials. Other feeds. Scrap. P. W. BF. Products. abSt WM WBF A I3 qt 1 A. abS I3. WM. t. I4. W BF P A qt ( 1 A. PEF. Pig iron (imported). EF. I4. Waste materials. Other feeds. Fig. 3 Material flow in BF-LD and EF processes.. PBF and PEF are given by eqs. (14) and (15). PBF =. 1 qt − WBF − P 1− A. PEF = abSt + WM + WBF + P + I3 + I4 −. a(1 b)St. Material flow in steel recycling system.. Products 1. qt. abSt. (14) A qt (15) 1− A. 4. Derivation of Eloss,BF ( P, A) and Eloss,EF ( P, A) 4.1 Eloss,BF ( P, A) E loss,BF (P, A) was estimated based on the model described in Refs. 4) to 6). It was assumed that the difference between the heat required for the reduction of iron ore and that required for the melting of scrap is equal to the heat that coke can provide upon its complete combustion. The exergy loss. (16). 4.2 Eloss,EF ( P, A) To obtain E loss,EF (P, A) as a function of P and A, the operation of the EF steelmaking process was simulated using Pyrosim software.7) It was assumed that the EF process can be divided into four zones, that is, the melting zone, foamy slag zone, post-combustion zone, and duct zone, and that chemical equilibrium was attained in each zone, as stated in Ref. 7). The data for feed rates and compositions of feed used for calculation in Ref. 7) were also used in this simulation. Pyrosim simulation was conducted for various mixing rates of pig iron. Figure 4 shows the exergy balance of the EF process in the case where 4% of pig iron was used in scrap. The exergy of electric energy was calculated by multiplying the value of electric energy by 2.5, considering the exergy loss in the power generation process. The results of mass flow obtained by Pyrosim simulation were converted to exergy using eqs. (2) to (6). As the feed materials have relatively high chemical exergy and the operation temperature of the EF process is high, only eqs. (3) and (4) were taken into account in this analysis. As shown in Fig. 4, with regard to the result of input, the exergy of scrap and that of electrical energy are higher than that of carbon. This is quite different from the BF-LD process, which uses high chemical exergy of carbon for the reduction of iron ores, the exergy of which is negligible. The total exergy loss in the EF process was obtained as shown in Fig. 5, including the exergy losses in the casting and rolling processes. The abscissa, X, is the mixing rate of pig iron in feed scrap and is shown in eq. (17). Solid circles show the case in which solid pig iron was used as feed and open.

(4) 382. N. Shigaki, T. Akiyama and F. Tsukihashi. Fig. 4 Exergy analysis of EF process based on Pyrosim simulation in case of using 4% solid pig iron.. Exergy Loss , E /(GJ/t). 12 11 10 9 8 7 6 10 15 20 25 0 5 Rate of Sum of Solid Pig Iron and Molten Pig Iron in. Fig. 5 Relationship between the sum of solid pig iron and molten pig iron at 1800 K in iron feed and exergy loss in EF steel mill including loss of pig iron production.. circles show the case in which 50% of pig iron was replaced with molten pig iron at 1800 K, considering the effective use of sensible heat of molten pig iron. X=. P + I4. (17) A qt abSt + WM + WBF + P + I3 + I4 − 1− A The total exergy losses shown in Fig. 5 include the exergy losses in the processes of pig iron production and are expressed as eqs. (18) and (19). Solid Pig Iron: E loss,EF (P, A) = 10620X + 8910(MJ/t) (18). Fig. 6 Exergy flow in BF-LD and EF processes. (a) BF-LD process, (b) EF process.. Solid Pig Iron + Molten Pig Iron (at 1800 K): E loss,EF (P, A) = 8950X + 8910(MJ/t). (19). 4.3 Comparison of Eloss,BF ( P, A) with Eloss,EF ( P, A) The exergy flows of both BF-LD and EF process are shown schematically in Fig. 6. Each flow in Fig. 6 shows the percentage of the total input. The value of each exergy loss includes the heat loss as diffused exergy. The exergy efficiencies of both processes are almost the same. From the comparison of these two figures, it is evident that the outflow exergy from the BF-LD process is larger than that from the EF process. This means that the feed materials of the BF-LD process provide sufficient energy to support its operation and furthermore they. produce excess energy. Figure 7 shows the exergy loss per unit production in the BF-LD process expressed by eq. (16). Comparing the exergy loss of the BF-LD process with that of the EF process, it is concluded that E loss,BF (P, A) is about 1.5 times larger than E loss,EF (P, A). The reason for this is that the input exergy to the BF-LD process is much larger than that to the EF process, in spite of their having almost the same exergy efficiency. These results show that the EF process is more effective than the BF-LD process in terms of exergy efficiency..

(5) Exergy Analysis of Steel Production Processes. ExergyLoss, E/ (GJ / t). 18. 383. Variable I3 was assumed as expressed in eq. (24) on the basis of the tendency of increase of scrap amount in recent years.9). 16 14. I3 = −0.282(1995 + t) + 561(×106 t/(31.56 × 106 s)) (24). 12 10 8. qt can be derived approximately as a function of t.. 6. qt = 1.64e−0.038t + 0.0282(t + 1995). 4 2. − 51.9(×106 t/(31.56 × 107 s)). 0 0. 5. 10. 15. 20. Substituting eqs. (16), (17), (18), (22), (24) and (25) into eq. (20), E loss (t) is derived as a function of A, P and X.. Fig. 7 Relationship between rate of scrap in iron feed and exergy loss in BF-LD steel production.. 5. Optimization of Material Recycling in Steel Industry 5.1 Derivation of Eloss (t) Substituting eqs. (13) and (14) into eq. (15), E loss (t) is given by eq. (20). 1 qt − WBF − P E loss (t) = E loss,BF (P, A) · 1− A + E loss,BF (P, A) · abSt + WM + WBF (20) A qt + P + I3 + I4 − 1− A. . St is assumed to be expressed by eq. (21).8) St = S − Re−βt. (21). S: convergence value of iron stock (t) R: difference from initial value of St (t) β: attenuation rate of the increase of St per unit time S was assumed to be 1.6 × 109 t, and R and β were 5.3 × 108 t and 0.038, respectively,9) as obtained from the approximate steel stock in Japan. The year 1995 is the origin of time, t, (t = 0). Therefore, St is given as a function of t as eq. (22). St = 160 − 53e−0.038t (×107 t). (22). From the mass balance shown in Fig. 2, qt is derived as eq. (23). qt = β Re−βt + a(1 − b)(S − Re−βt ) − (I1 + I2 + I3 + I4 ) (23) The following data are used as constants.9) a = 0.035 b = 0.8 WM = 7.0 × 106 t/(31.56 × 106 s) WBF = 2.5 × 106 t/(31.56 × 106 s) I1 = −17.0 × 106 t/(31.56 × 106 s) I2 = −15.2 × 106 t/(31.56 × 106 s) I4 = 1.0 × 106 t/(31.56 × 106 s). (25). E loss (t) = f (A, P, X). (26). The system can be optimized by solving eqs. (19) and (26). X should be defined by considering various factors, for example, the content of impurities in low-grade scrap in the future, the required quality of EF products, and the recovery of steel scrap. E loss (t) can be derived as a function of only P or A by solving eqs. (19) and (26), simultaneously, keeping X constant. Three cases were analyzed with regard to X as follows. 5.2 Result of simulation and discussion 5.2.1 Case 1. X = 0.04 is constant In the case that solid pig iron is used in the EF process for the dilution of low-grade scrap, it is known that the cost of steel production generally increases. Therefore, the mount of pig iron mixed kept small, although the quality of EF products is required to be higher. In this case it was assumed that the mixing rate of solid pig iron in scrap, X, would remain constant in the near future. Figures 8 and 9 show that E loss (t) increases with increasing A. As mentioned above, the reason for this is that the exergy loss in the BF-LD process is about 1.5 times larger than that in the EF process. It is generally considered that a value of A more than 0.1 cannot be operated in practice, since the BFLD product is generally used for the high-quality steel. For the reduction of exergy loss, the most desirable solution is to consume all scrap in the EF process (A = 0). However, at the same time, this would mean necessitate the maximum use of pig iron in the EF process for dilution. Therefore, the recovery of sensible heat in the process of producing iron resources for dilution, such as pig iron, DRI, etc. decreases the exergy loss, and therefore this process may be one of the effective solutions for the improvement of overall energy efficiency. In future the exergy loss will increase, according to Figs. 8 and 9. The case with constant X cannot represent the real situation in the future unless there is a radical improvement of scrap recycling technology. 5.2.2 Case 2. X increases constantly The quality of the EF product has improved in recent years, although the quality of scrap will decrease in the future. Therefore, the utilization of resources originating from iron ore is an effective solution. The case that the mixing rate of solid pig iron in scrap will increase with a constant rate annually in the future, together with the decrease of scrap quality, was simulated. It was assumed that the value X for the year 1995 (t = 0) is 0.04 and that X will increase by 0.005 each year. The amount of pig iron used in the EF process, the rate of EF products to total iron products and the total exergy loss.

(6) 1.35. 1 1.3 0.5 1.25. 1.2. 0 0. 0.05. 0.1. 0.15. 0.2. Scrap Rate in BF-LD Steelmaking Pprocess, A. 1.35. 1 1.3 0.5 1.25. 0. 1.2 0.1. 0.15. 0.2. Excerfy Loss,Eloss(t)/(109GJ/(31.56x106s)). 1.4 Pig Iron (P) Rate of EF products E-loss. Rate of EF Steel. Pig Iron, P/ (10-7ton/(31.56x106s)). 1.5. 0.05. Scrap Rate in BF-LD Steelmaking Process, A. 1.4. 1.5. Rate of EF Steel. Pig Iron (P) Rate of EF products E-loss. 1.35. 1 1.3 0.5 1.25. 0. 1.2 0.05. 0.1. 0.15. 0.2. Excergy Loss,Eloss(t)/ (10-9GJ/(31.56x106s)). Fig. 9 Simulation of future prospect of rate of EF steel production and exergy loss at 2030 with constant X = 0.04.. 0. 1.4 Pig Iron (P) Rate of EF products E-loss. 1.35. 1 1.3 0.5 1.25. 0. 1.2 0. 0.05. 0.1. 0.15. 0.2. Scrap Rate in BF-LD Steelmaking Process, A. Fig. 8 Simulation of future prospect of rate of EF steel production and exergy loss at 2010 with constant X = 0.04.. 0. 1.5. Excergy Loss,Eloss(t)/(10-9GJ/(31.56x106s)). 1.4 Pig Iron (P) Rate of EF products E-loss. Pig Iron, P/(10-7ton/(31.56x106s)) Rate of EF Steel. 1.5. Excergy Loss,Eloss(t) / (10-9GJ/(31.56x106s)). N. Shigaki, T. Akiyama and F. Tsukihashi. Pig Iron, P/ (10-7t/(31.56x106s)) Rate of EF Steel. 384. Scrap Rate in BF-LD Steelmaking Process, A. Fig. 10 Simulation of future prospect of rate of EF steel production and exergy loss at 2030 in case that X increases annually with constant rate.. in this system were simulated and are shown as a function of the scrap ratio in the BF-LD process, A, at year 2030 in Fig. 10. P in Fig. 10 is larger than that in Fig. 9. The exergy loss in Fig. 10 is also larger by a few percent than that in Fig. 9; it depends on the exergy loss in the process of solid pig iron production. 5.2.3 Case 3. Molten pig iron at 1800 K is replaced When 50% of the solid pig iron in the case of Fig. 10 is replaced with molten pig iron at 1800 K, the sensible heat of pig iron can be utilized effectively. Figure 11 shows the amount of pig iron used in the EF process, the ratio of EF products to total iron products and the total exergy loss in this system as a function of the scrap ratio in the BF-LD process, A, at year. Fig. 11 Simulation of future prospect of rate of EF steel production and exergy loss at 2030 in case that X increases annually with constant rate and scrap is replaced by molten pig iron at 1800 K.. 2030 in this case. The exergy loss in Fig. 11 is lower than that shown in Fig. 10. As shown in Fig. 5, the use of molten pig iron as feed has an effect on the improvement of exergy efficiency. When the consumption of pig iron becomes large, the potential for the reduction of exergy loss becomes greater. Furthermore, scrap preheating is well known to decrease the unit electric energy consumption. Therefore, a technology for the recovery of sensible heat is necessary for the improvement of overall energy efficiency in the steelmaking process. It is concluded that the exergy may be applied as a measure of the analysis of the steel production process. 6. Conclusions The exergy analysis of steel production and recycling in Japan was conducted. The exergy losses of the BF-LD and EF processes were calculated and compared. The following results were obtained. (1) The total exergy loss in the EF process was simulated and estimated using Pyrosim software. Comparing the EF process with the BF-LD process, the EF process is more desirable from the standpoint of the effective utilization of exergy, although the two processes have almost the same exergy loss. (2) The exergy analysis of the steel recycling system was conducted. The recovery of sensible heat is effective for the improvement of overall energy efficiency in the steelmaking process. REFERENCES 1) Tekko Nenkan, 1999: Tekko Shimbunsha Ltd. 2) Y. Takeuchi: Proc. 165, 166th Nishiyama Memorial Lecture, (Iron Steel Inst. Jpn., 1997) pp. 149–175. 3) A. Toi, J. Sato and H. Katagiri: Energy and Resources 18 (1997) 92–97. 4) T. Akiyama and J. Yagi: ISIJ Int. 38 (1998) 896–903. 5) T. Akiyama and J. Yagi: Tetsu-to-Hagane 74 (1988) 2270–2277. 6) T. Akiyama: private communication. 7) Adrian C. Deneys and Kent D. Peaslee: Proc. Electric Furnace Conf., (Iron & Steel Society, 1997) pp. 417–427. 8) A. Toi and J. Sato: Tetsu-to-Hagane 83 (1997) 407–412. 9) Tetsugen Nenpo, 1998: The Japan Ferrous Raw Materials Association..

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