High-efficient combustion of gas-fired furnace with 21∼36% oxygen-enriched air Liying Qi, JunXiang Guo, Qing Wang, Wenbin Dai, Hao Bai and Daqiang Cang Metall. Res. Technol., 115 6 (2018) 613 Published online: 14 December 2018 DOI: 10.1051/metal/2018098