**5. Conclusions**

CFD modeling techniques were applied to ascertain the predicted impacts of varied NG injection rates and pre-heating levels on operation at two different industrial BFs. Simulations indicated that there is a clear possibility to utilize NG pre-heating in conjunction with increased NG injection rates to provide an avenue for improved BF operating efficiency (reduced coke rates while maintaining stable raceway flame and top gas temperatures). Impacts of NG pre-heating in isolation appear linear, though the slope changes significantly with NG injection rate, and the benefits disappear quickly when pushing towards injection rates above 140 kg/mthm. The application of NG pre-heating also appears to have a similar negative impact on top gas temperatures as that of other parameters that increase furnace RAFT, such as hot blast temperature or oxygen enrichment. However, this impact is small enough that it can be offset by the increased NG injection rates made possible under these operating conditions.

As noted in the discussion section, achieving NG pre-heating could be approached from a multitude of different avenues, including pre-combustion or an upstream recuperative heater. Some initial simulation testing of a conceptual system for heating NG near the BF blowpipe indicated that sufficient heating could be provided to the incoming NG to achieve the predicted benefits explored in this research. However, any industrial implementation would doubtless need to account for the control mechanisms, CAPEX considerations, and other limitations that would figure into such a system in a real-world environment. The difficulties and costs would need to be weighed against the potential benefits, with an eye towards furnace life-span and the potential operational flexibility and return on investment that an NG pre-heating system would enable.

**Author Contributions:** Conceptualization, T.O., J.D., and S.S.; Data curation, S.N.; Formal analysis, T.O. and S.N.; Investigation, T.O., S.N., J.D., S.R., and S.S.; Methodology, T.O. and C.Z.; Project administration, C.Z.; Resources, T.O. and C.Z.; Software, T.O. and S.N.; Supervision, T.O. and C.Z.; Validation, T.O., S.N., J.D., S.R., and S.S.; Visualization, T.O. and S.N.; Writing—original draft, T.O. and S.S.; Writing—review and editing, T.O. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** The authors would like to thank the agencies and companies involved in supporting previous and current development of blast furnace research at PNW, including AK Steel, AISI, AIST, ArcelorMittal, the U.S. Dept. of Energy, the Indiana 21st Century Technology and Development Fund, Praxair, U.S. Steel, Stelco, Union Gas, and all members of the Steel Manufacturing Simulation and Visualization Consortium (SMSVC). In particular, the authors would like to acknowledge Megha Jampani of Linde plc for her contributions to conceptual development. The support of many individual industry collaborators, along with CIVS staff and students, are also appreciated.

**Conflicts of Interest:** The authors declare no conflict of interest.
