Exergy analysis for modified design of the coal-based sponge iron plant

An exergy analysis of a typical coal-based sponge iron plant with a capacity of 500 tpd found in India is carried out to determine its thermodynamic performance and the major sources of irreversibility. A tremendous amount of heat is generated during the process of sponge iron making. About 33% of heat energy, along with waste gases, which are available at useful temperatures, is released into the atmosphere unutilized. For effective utilization of waste heat, two modified configurations, namely, Scenario 1 and Scenario 2, are proposed for process integration within the plant, which reduces the coal consumption, resulting in irreversibility. In Scenario 1, a rotary dryer is incorporated to preheat the feed materials up to 110 °C. Over and above, in Scenario 2, combustion air is additionally preheated up to 385 °C using an air preheater. Exergy analysis is conducted for the major plant equipment. The conventional plant shows an exergy efficiency of 25.09%, which increases to 29.95 and 36.72% in Scenario 1 and Scenario 2, respectively. The rotary kiln is identified as the major source of irreversibility, where the exergy destruction is found to be 72.32 MW in the conventional plant, which decreases by 42.49% to a value of 56.20 in Scenario 2. A sensitivity analysis is conducted by varying the feed moisture content from 10 to 20% and the rotary kiln operating temperature from 800 to 1100 °C. It is found that higher moisture content increases exergy destruction, and a lower kiln operating temperature reduces coal consumption and exergy destruction.