Systematic investigation of natural gas and pulverized coal co-injection combustion in the blast furnace

A three-dimensional model of the lances-tuyere-raceway-bosh region is developed to comprehensively investigate the co-injection combustion of natural gas (NG) and pulverized coal (PC) under oxygen enriched conditions (21–29% O 2 ) in the lower blast furnace. The model incorporates a reduced mechanism along with the heterogeneous reactions of fixed carbon with O 2 (HR1), CO 2 (HR2), and H 2 O (HR3) to represent the key combustion characteristics of NG–PC co-injection. The oxygen-to-fuel ratio (OFR), defined as the molar ratio of supplied oxygen to the input gaseous fuels, including NG and volatile matter released from PC, is introduced as an operational index for evaluating the influence of NG injection on raceway temperature, bosh-outlet temperature, PC burnout rate, and gas composition. Based on the PC burnout rate, the combustion state in the raceway is classified into three regimes: Condition I (oxygen-sufficient), Condition II (oxygen-moderate), and Condition III (oxygen-deficient). In Condition I, NG addition enhances gaseous-fuel combustion, thereby increasing the overall PC burnout rate through greater contributions from HR2 and HR3. In Condition II, HR2 gradually decreases, whereas HR1 and HR3 remain relatively unchanged. The maximum raceway temperature occurs near the boundary between Conditions I and II. In Condition III, insufficient oxygen supply leads to incomplete NG combustion, causing HR1, HR2, and HR3 to decrease markedly. Consequently, the overall PC burnout rate becomes lower than that in the PC-only case. Moreover, oxygen enrichment increases both raceway and bosh-outlet temperatures, enhances HR1 and HR2, and shifts the onset of oxygen-deficient conditions toward higher NG injection rates. Under Conditions II and III, the relative contribution of HR3 becomes increasingly important with oxygen enrichment, indicating that HR3 plays a growing role in sustaining PC burnout at high NG injection rates.