Char formation, transportation and consumption in the blast furnace and its impact on coke rate (Charfoco)
In the past, blast furnace coal injection research mainly focused on combustion of injection coal in the raceway. However, it could be assumed that coal is anyway not able to fully combust, particularly at high injection rates, and unburnt residues leave the raceway as char. The char evolution and subsequent consumption depends on the raceway characteristics, which depend on coal conversion. Both groups of factors were investigated with conversion tests, as well as raceway measurements. Generally, the char evolution is seen to manifest itself as a negative impact, but this may depend on the... blast furnace conditions and injection coal characteristics. Char effects might deteriorate the blast furnace operation and stability (i.e. lower burden permeability or higher carbon losses in the flue dust). Nonetheless, char could also be advantageous by contributing toward the increase in coke reactivity thereby lowering the thermal reserve zone temperature. It was shown that the coal combustion highly depends on coal grain size and only marginally on oxygen enrichment. Effects of oxygen enrichment on maximum coal injection rate might be more related to char consumption, rather than coal conversion. Despite low conversion of granular coal under blast furnace raceway simulation conditions, granular coal injection is possible at high rates, obviously due to its consumption outside the raceway. Blast furnace operation experience with coals, like operational benefits or transport behaviour could be confirmed. Char impact on ferrous burden behaviour highly depends on temperature and gas composition and is difficult to predict from the performed tests. Several raceway measurement techniques were successfully developed and applied at pilot scale and at a real blast furnace. Raceway size, temperature and changes in the gas composition could be shown. The raceway size depends on the location of the tuyère in relation to blast inlet and tap hole and changes significantly over time, indicating raceway collapses. Gas and temperature measurements of the BF interior were performed: New insights regarding the impact of high coal injection rates on the blast furnace inner state were gained by gas and temperature measurements of the blast furnace inside and confirmed with mathematical modelling.