Investigation of energy recovery potential with power generating systems from waste energy of reheat furnace in iron and steel industry

Saving wasted energy has importance as much as using alternative clean energy production systems due to addiction of the developed world to fossil fuels. Therefore, in the scope of the present study, giving an extensive perspective to the manufacturers and researchers about recovering waste heat by power cycle was aimed presented by a case study on a reheat furnace located in iron and steel plant. In the present study, by integrating the Kalina cycle (KC) into the reheat furnace system, parametric optimization of the cycle, energy and exergy analyzes, and economic and environmental analyzes were made. The parametric optimisation, energy and exergy analyses of the KC was performed together with economic and environmental analyses. During the study, the turbine inlet temperature was varied from 130 °C to 430 °C (increased by 20 °C) by taking the turbine working pressure of 80 bar. The ammonia mass fraction was increased from 15% to 95% (increased by 10%). The parametric optimisation of the KC was made for a wide range of parameters to be able to compare the effect of variables on the system. After parametric, energy and exergy analyses of the KC, the maximum net power production, thermal efficiency and exergy efficiency of the KC were calculated as 389.44 kW, 33.86%and 68.96%at 430 °C turbine inlet temperature and 95%ammonia mass fraction. At this working condition, where the best performance obtained from KC, the maximum CO2 emission reduction, investment cost, unit investment price, payback period and unit cost of generated electricity were found as 257.13 kg-CO2/h, 501 115 $, 1243 $/kW, 2.93 years and 0.0265 $/kW. In the second stage, turbine inlet temperature was changed between 130 °C and 430 °C (increasing 20 °C) at the turbine operating pressure of KC between 50 bar and 100 bar. The ammonia concentration was increased from 15% to 95% (increasing 10%). Parametric optimization of KC, evaluations were made for various parameters in order to compare the effects of variables on the system. After the parametric, energy and exergy analysis of KC, the maximum net power generation, thermal efficiency and exergy efficiency of KC were obtained at 430 °C turbine inlet temperature and 95% ammonia mass concentration and 100 bar turbine operating pressure these values were calculated as 410.62 kW, 36.05%and 73.47%. In these operating conditions, the best performance obtained from KC, maximum CO2 emission reduction of the electricity produced, investment cost, unit investment price, payback period and unit cost were found as 274.42 kg-CO2/h, 517 293 $, 1198 $/kW, 2.79 years and 0.0255 $/kW. In the third stage, the steam Rankine Cycle (SRC), Organic Rankine Cycle (ORC) and Kalina Cycle (KC) designed at the same exhaust gas temperatures and operating conditions in order to recover the exhaust gas waste energy of a reheating furnace located in the iron and steel factory, is the most important. The performance optimization results of the systems were compared by deciding on the best performing system. The effect of using the best performing cycle among the systems on CO2 emission reduction was investigated. After deciding on the best cycle configurations and cycle fluid in terms of performance and emissions, economic and environmental analyzes of the system with the best performance were made. In the SRC system, the temperature of the reheat furnace flue exhaust gas was increased to 450 °C, turbine inlet pressure P1 = increased by 10 bar between 10 bar and 100 bar, and the mass flow rate of the exhaust gas was taken as 3.73 kg/s. As a result, the best performance was calculated at 100 bar turbine operating pressure at 440 °C turbine inlet temperature, with net power of 387.94 kW, net thermal efficiency 30.19%, exergy efficiency 64.99%and CO2 emission reduction 256.16 kg-CO2/h. The net power, net thermal efficiency and exergy efficiency changes were investigated for eight working fluids of acetone, benzene, butane, cyclohexane, cyclopentane, ethanol, R11 and R123 selected in the ORC system, depending on different operating pressures and turbine inlet temperatures. In the ORC system, the turbine operating pressure of the ethanol working fluid is between 10 bar and 60 bar and the turbine inlet temperature is evaluated between 125 °C and 400 °C. As a result of the study, at a working pressure of 60 bar and turbine inlet temperature at 375 °C, the net power was calculated as 410.92 kW, the net thermal efficiency was 30.78%, the exergy efficiency was 66.71% and the CO2 emission reduction was 274.61 kg-CO2/h. The total investment cost for ethanol used in the ORC system at 375 °C turbine inlet temperature and 60 bar operating pressure is 440 358 $, the unit cost of electricity produced is 0.0228 $/kW, the investment cost per generated electricity is 1072 $/kW and the minimum payback period is calculated as 2.07 years. As a result, it has been determined that the best performing cycle among SRC, ORC and KC systems designed using the waste heat of exhaust gas in an reheat furnace installed in an iron and steel plant is the Organic Rankine cycle analyzed using Ethanol working fluid. With the ORC system, 410.92 kW net power, 30.78%net thermal efficiency, 66.71%exergy efficiency and 274.61 kg-CO2/h CO2 emission reduction were achieved. At the same time, it was found to be the most appropriate cycle among the systems designed for the current situation, with a total investment cost of 440 358 $, investment cost per 1072 $ / kW of electricity produced, unit cost of electricity produced 0.0228 $ / kW and a minimum payback period of 2.07 years.