Blast furnace (BF) coal injection became a routine practice among European BFs; roughly, 40% of total energy required for the process is covered by auxiliary reducing agents. Pulverized coal (PC) remains the most commonly used auxiliary reductant. The key trend is increasing PC injection rates; over 200 kg tHM À1 PC on an annual basis is no rarity any more. Despite numerous measures for intensifying the coal conversion in the raceway, [1] it is hardly possible to combust such a high amount of coal within a very short residence time of few tens of milliseconds. Recent computational fluid dynamics calculations showed that at PC injection rate of 240 kg tHM À1 , about 50% of the coal amount entering the raceway may leave it as so-called char. [2] Another theoretical study from 2011 calculated a maximum PC injection rate of 190-210 kg tHM À1 for some BFs, considering that no ash deposition nor change in the gas flow distribution due to unburnt coal fines trapped in the coke bed takes place. [3] The following types of coal residues appear depending on different conditions and stages of its formation (Figure 1): 1) devolatilized coal particles (after light gases and tar have been released); 2) pyrolyzed (partly or completely) particles (caused by the thermal decomposition of the organic matter); and 3) not completely gasified particles (residues). Char formation, transportation, and behavior outside the raceway may significantly affect the BF process both negatively with respect to process stability and positively by increasing the combustion efficiency by possible consumption of char. The knowledge on these phenomena was limited because the main efforts over the last few decades were focused on the complete conversion of PC within the raceway. A char morphology system was introduced for the characterization of char types. [4] However, few studies are devoted to the
Co-injection of two or more reducing agents via tuyères may ensure technological, economic and environmental benefits and enhance the flexibility of use of various reductants. The blast furnace technology with top gas recycling, recently tested in the scope of the ULCOS program aiming at significant mitigation of CO2 emissions, is also based on co-injection of reducing gas, pulverised coal and pure oxygen. This paper presents results of a study on conversion behaviour of pulverised coal while co-injecting further substances with focus on tuyère assembly design. Factors affecting the coal particle conversion in the raceway derived from the combustion mechanism and constructional features of tuyère stock arrangement are firstly discussed. An experimental work was performed using the Multifunctional Injection Rig for Ironmaking (MIRI) at the Department of Ferrous Metallurgy, RWTH Aachen University. Tests simulating different scenarios for injection of coal, carbon monoxide and oxygen were conducted. Conversion degree determined by three methods and microstructural analysis were used to examine the effect of pre-mixing of coal with above mentioned components on its combustion behaviour. Results obtained give hints for optimising the lance arrangement.
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