Technical Market Support » Metallurgical Coal
The blast furnace is a process dominated by carbon. Carbon is required to produce CO to complete the reduction of iron ore. This is provided by the coke, which is also needed to create the packed bed of the furnace, and coal injected through the furnace tuyere. The steel industry's efforts to reduce CO2 emissions have followed two paths; finding alternate ways to produce iron or finding alternative low carbon fuels to feed the blast furnace. Both have focused on the utilisation of hydrogen and hydrogen-rich fuels.
Due to the need for the packed bed in the blast furnace, replacing coke is difficult, but replacing pulverised coal injection (PCI) is relatively simple. However, changing to a hydrogen rich injectant has implications throughout the furnace, and a sudden change could be catastrophic for operations. Therefore, co-injection of coal and hydrogen rich fuels has and will occur. This project considers if any interaction occurs when two fuels co-combust within the tuyere and raceway of a blast furnace.
Different types of fuels, rich in hydrogen, were selected for this study:
- Three plastics were considered: high density polyethylene (HDPE), polypropylene and polystyrene.
- Co-combustion of coal char with hydrogen, synthetic coke ovens gas and ammonia were considered.
The combustion behaviour of the solid fuels was measured in a drop tube furnace and compared to the performance of other coals measured in previous studies.
Thermogravimetric analysis (TGA) was used to determine if interactions occurred during co-combustion of coal and the selected hydrogen rich fuels and to determine the thermodynamic constants of each fuel to predict the time for combustion at PCI conditions.
Changes in the combustion conversion curves were observed when coal char was co-combusted with bamboo and HDPE. Catalytic assistance to the combustion reaction of the coal char is believed to occur associated with bamboo ash. And the presence of coal char appears to have delayed the decomposition of HDPE slightly.
When coal char is combusted in the presence of gaseous hydrogen rich fuels, the gaseous fuels consumption of oxygen from the gas lowers the effective oxygen concentration in the gas, lowering the reaction rate of the char. In the blast furnace, co-combustion of these fuels will lower the flame temperature, requiring adjustment by increasing the blast temperature or increasing the blasts oxygen content. If the flame temperature is allowed to decrease by 250°C, the time for complete conversion of the injected coal will increase by 48.5% but this increase in burnout time will not cause unreacted char to enter the raceway boundary.
Full burnout of the solid fuels at PCI conditions was predicted to be 0.00676 s for coal char, 0.00468 s for Jarrah char and 0.00594 s for bamboo char. The plastics will decompose before heating to temperature is complete.