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and 4). Second, due to the high temperature of flow 1, the combustion takes place at a high temperature (above 1400 ºC), which reduces substantially irreversibility due to heat transfer. I_combustion (kW) 6000 5000 4000 3000 2000 1000 0 I_combustion (kW) phi_combustion (‐) 0 200 400 600 800 1000 1200 Time (s) Fig. 7. Irreversibility of combustion. I_HRSG (kW) 40000 35000 30000 25000 20000 15000 10000 5000 0 I_HRSG (kW) phi_HRSG (‐) 0 200 400 600 800 1000 1200 Time (s) Fig. 8. Irreversibility in the HRSG. 0.1 0.09 0.08 0.07 0.06 0.05 0.04 0.03 0.02 0.01 0 0.5 0.45 0.4 0.35 0.3 0.25 0.2 0.15 0.1 0.05 0 Irreversibility in the heat recovery steam generator is plotted in Fig. 8. These losses can be as high as 30 MW, or more than 15%. They might be reduced by considering another design of the boiler, producing steam of different levels of pressure. It should be noted that the separation of combustion and heat transfer has been made to simplify the model. Accordingly, the actual distribution of losses between these two processes is not exactly the same as the values calculated here. Figure 9 represents irreversibility produced in the first washing venturi, where gases are cooled from more than 700 ºC down to less than 100 ºC. This irreversibility is around 6% of the exergy of flow 1, and might be reduced by modifying the design of the HRSG in order to use a higher part of the thermal exergy of gases. phi_combustion (‐) phi_HRSG (‐) I_venturi1 (kW) 20000 18000 16000 14000 12000 10000 8000 6000 4000 2000 0 I_venturi1 (kW) phi_venturi1 (‐) 0 200 400 600 800 1000 1200 Time(s) Fig. 9. Irreversibility in the first washing venturi. 0.14 0.12 0.1 0.08 0.06 0.04 0.02 Finally, Fig. 10 shows how 2.5 MW of exergy are destroyed without use in the steam condensers. This amount may seem not too high compared to flow 1 (between 1 and 2%), but represents around 20% of the exergy of steam used in steel treatments (flow 104). This value only depends on operation strategy and can be reduced substantially by a proper management of steam storage and steam export to plant net. I_condensers (kW) 3000 2500 2000 1500 1000 500 0 I_condensers (kW) phi_condensers (‐) 0 200 400 600 800 1000 1200 Time (s) Fig. 10. Irreversibility in the condensers. 3.2. Integral cycle indicators. 0 0 0.035 0.03 0.025 0.02 0.015 0.01 0.005 In this section, parameters defined in (7-10) are used to summarize the performance of the whole recovery cycle. The main flows appear in Table 1, and the main components in Table 2. Table 1. Integral cycle indicators for the main flows Flow Bcycle (GJ) η cycle(-) 1, gases from converter 193.31 1.0000 9, flared gas 82.92 0.4290 10, recovered gas 59.70 0.3089 104, steam to treatments 11.55 0.0597 135, steam to net -1.686 -0.0087 Accumulated steam 8.438 0.0437 http://www.ecos2010.ch 6 14-17th june 2010, Lausanne, Switzerland phi_venturi1 (‐) phi_condensers (‐)
Table 2. Integral cycle indicators for the components Component Icycle (GJ) Φcycle (-) HRSG 24.58 0.1271 Venturi 1 12.30 0.0636 Combustion 4.03 0.0209 Condensers 2.44 0.0126 The analysis of these indicators allows one to draw important conclusions regarding the efficiency of the system. First, the most important part of exergy (more than 70%) is available in gas flow which is stored in a gasholder (30.1%) or flared (42.9%). Accordingly, the minimization of gas flared is a key point in increasing efficiency of the system. The first action towards this goal is to avoid flaring when the gas quality is suitable for storing. The second step is to optimize the operation of the system in order to increase the time span when gas accomplishes the requirements for being recovered. 11.55 GJ of steam are consumed for steel treatments, while around 1.7 GJ are imported (this is the meaning of the minus sign) from the general steam net of the steelworks. In this context, the 2.44 GJ of exergy lost in the condenser represent a big potential of steam savings. Accordingly, operation of the steam part of the system should be improved to reduce this amount. Importance of these savings can be even larger, because if steam is properly used, operation can be more easily adapted for maximizing the gas recovered. 12.7% of exergy of entering gas is destroyed in the heat recovery steam generator and 6.4% in the first washing venturi. Accordingly, new developments of gas recovery systems may include more advanced HRSG producing steam at several pressures. Irreversibility in the combustion process is small (2.1%) because it takes place at high temperatures and only affects a part of the heating value of the gas. It should be noted that all these values correspond to a single cycle, and they vary according to the profile of gases leaving the BOF as well as the operation of the recovery system. 4. Conclusions Exergy analysis has been applied to point out the energy savings potential of a system as complex as an installation for BOF gas recovery, which also produces steam and includes storage of both. The approach comprises two parts: a detailed analysis of the evolution of exergy and irreversibility, and the calculation of integral parameters for characterizing the whole recovery cycle. These integral parameters are the key point of the paper, because they allow to rigorously summarize the behavior of batch processes like the one studied here. Results obtained in the analysis of a single cycle point out that there is strong potential for improvement, not only in the reduction of gas flared but also in the operation of the steam section (e.g. condensers). Due to the use of exergy, all these losses can be directly compared. Although these results correspond to an example of converter cycle, the comparison of the integral parameters corresponding to the operation during several months (including different operation strategies) will provide useful information to improve the exergy efficiency of the system and thus the steam and gas recovered. This comparison may include the separation of exergy lost by flared gas into causes (due to gas low quality or to improper operation). Furthermore, the development of a thermoeconomic model based on exergy calculated here may highlight more clearly the potential points for improving. Nomenclature . B exergy, kW Bcycle exergy in a cycle, kJ BOF basic oxygen furnace BREF Best Available Techniques Reference EU Document European Union fm molar rate, kmol/s h specific enthalpy, kJ/kmol HRSG heat recovery steam generator I irreversibility, kW Icycle irreversibility in a cycle, kJ M flow rate, kg/s n number of substances in a flow P pressure, Pa R constant of gases, kJ/(kmol·K) s specific entropy, kJ/(kmol·K) T temperature, K X,x composition Greek symbols η efficiency http://www.ecos2010.ch 7 14-17th june 2010, Lausanne, Switzerland
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