Effect of hot reducing gas (HRG) injection on blast furnace operational parameters: theoretical investigation

The injection rate of fossil fuels in the blast furnace is limited because of a drop in the flame temperature in the raceway as well as problems in deadman region and the cohesive zone owing to the unburnt char. An alternative option for coke saving, a clean deadman as well as increase in blast furnace productivity is injection of hot reducing gases (HRG) which are produced by low grade coal gasification or top gas recycling after CO2 and N-2 removal. Calculations using a mathematical model show that HRG injection at higher temperature is desirable. Hot reducing gas injection is possible up to 300 Nm(3) thm(-1), above which the top gas temperature shoots up beyond practical limits. Furthermore, it also shows that if the flame temperature is maintained constant by varying steam and oxygen injection, the productivity is increased by 16% and coke rate is reduced by 84 kg thm(-1) with the replacement ratio of 1.4 kg coke/kg gasified coal at 300 Nm(3)/thm of HRG injection. It was also observed that the complete replacement of pulverised coal (PC) injection with HRG injection is more effective over the coinjection of PC and HRG in terms of coke rate saving. However, oxygen enrichment is possible up to 75% with the coinjection of HRG and PC, with a resultant of rise in productivity. Injection of HRG in the form of top gas (blast furnace gas) is more effective over the injection of HRG generated from coal gasification. The productivity is increased by 25% and coke rate is reduced by 83 kg thm(-1) with the replacement ratio of 1.7 kg coke/kg HRG at 250 Nm(3) thm(-1) of HRG injected from top gas.