Gour Gopal Roy

Abstract:

A sustainable steelmaking process is characterized by efficient resource use with minimal energy losses, which in turn may be correlated to process exergy efficiency. This parameter depends on fuel type, secondary energy input such as electricity and steel scrap utilization ability. A high process exergy efficiency is likely to lower carbon footprint of the process through utilization of process fuel gases. RHF is based on dual fuel where it uses coal for reduction and natural gas (a low carbon fuel) for heat generation. RHF also generate rich off gas, with subsequent utilization of this gas to generate power for enhancing energy efficiency. Thus a RHF-EAF (Rotary Hearth Furnace-Electric Arc Furnace) process is expected to emit lower CO2 per ton of crude steel for the dual input fuel mix and given scrap input in steel making. Additionally, RHF-EAF process may also run with higher proportion of steel scrap in addition to DRI produced from virgin source, which has the potential to lower the RHF-EAF emissions further. According to WSA, steel scrap can be recycled infinitely without any loss of quality and recycling of 1 ton of steel scrap saves 1.5 ton of CO2.
Exergy analysis and CO2 emission of dual fuel RHF-EAF process is studied using mathematical models and compared with coal based processes like BF-BOF, COREX-BOF. Two variant of RHF producing iron nugget (ITmk3 process) or DRI (FASTMET process), are considered separately. Although, at lower scrap level BF-BOF always yields lower total exergy loss values as compared to RHF-EAF process, at higher scrap level (50% scrap) RHF-EAF yields lesser total exergy loss which is further lowered on hot charging of the DRI produced. The total exergy loss with hot charging of DRI is lower than BF-Tenova (BOF with Arc facility) process at higher scrap levels (50%). At identical scrap level, RHF-EAF processes are always found to produce higher gas based exergy efficiency. Considering the conversion of product gas exergy to generate electricity, metal based exergy indices of RHF-EAF processes are found to be superior or comparable to coal based BF-BOF and COREX-BOF processes. Net CO2 emission through RHF-EAF processes are (around 1.92-2.01 ton/tcs) found to be comparable to BF-BOF process. Again on consideration of product gas conversion to electricity and carbon credit, CO2 emission from RHF-EAF system is always found to be lower than BF-BOF process.