Chemical Structure of Chars Prepared under Conditions Prevailing in the Blast Furnace PCI Operation.

Abstract

Using a drop tube furnace, char samples were prepared from coals of different ranks, under conditions similar to those prevailing during pulverized coal injection into the blast furnace. The chemical structure of resultant chars was determined by quantitative X-ray diffraction analysis (QXRDA) and high-resolution transmission electron microscopy (HRTEM), and investigated as a function of pyrolysis temperature, heating rate and coal type. Among the parameters examined, pyrolysis temperature was the key factor influencing char chemical structure. Char obtained at higher temperature is generally more ordered, with the distinctive peaks becoming sharper and the background intensity becoming lower. Heating rate is another important factor affecting char chemical structure. Char is more ordered at lower heating rate due to the longer residence time. Although considerable differences were still observed in the chemical structure of chars prepared from coals of different ranks, it is clear that such differences are reduced after coal pyrolysis. Char structural evolution during post-pyrolysis and combustion was also investigated. The importance and potential applications of this work to the blast furnace PCI operation have been outlined. KEY WORDS: pulverized coal injection; coal pyrolysis; char combustion; char reactivity; char chemical structure. different ranks. However little systematic investigation has been reported on the influence of pyrolysis conditions on char chemical structure. As part of an overall study on char structure and char reactivity, this paper is focused on the effect of pyrolysis conditions on char chemical structure. A subsequent paper will discuss char physical structure. Char structure formed as a result of pyrolysis serves as the starting point of combustion during PCI operation. These wellcharacterized chars have been subsequently used to establish an understanding of the effect of char structure on char combustion reactivity. Experimental Char PreparationA drop tube furnace (DTF) was used in this investigation to prepare chars under conditions similar to those prevailing in the raceway region of a blast furnace. Details regarding the drop tube furnace can be found elsewhere, 8) and only a brief account is given here. It consists of a coal feeding system, a sampling probe, a gas distribution system and an electrically heated furnace. The composition of gaseous reactants, position of the sampling probe as well as feeding rate of coal particles are adjustable depending on experimental conditions. To prepare char, a metered stream of coal particles was entrained into the iso-thermal zone of the furnace by a preheated gas flow (99 vol% N 2 ϩ1 vol% O 2 ), and a char sample was collected using a water-cooled probe after a residence time of about 1 second. A slightly oxidizing atmosphere was used to burn off the volatiles released during coal pyrolysis. This was considered necessary to avoid contamination of char samples with soot and condensed tar. Three size-graded Australian black coals of differ...

Structural Transformation of Agricultural Waste/Coke Blends and Their Implications during High Temperature Processes

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Reactivity to CO2 of chars prepared in O2/N2 and O2/CO2 mixtures for pulverized coal injection (PCI) in blast furnace in relation to char petrographic characteristics

“…The conditions prevailing in the tuyeres of blast furnace are very difficult to reproduce at laboratory scale and different devices have been used to achieve high temperatures, high heating rate, some pressure and short residence time such as horizontal ovens (Vamvuka et al, 1996), a shock wave reactor (Gudenau et al, 2002), a wire mesh reactor (Pipatmanomai et al, 2003) and drop tube furnaces (Borrego et al, 2008;Lu et al, 2002;Wall et al, 2002). None of these devices satisfactorily reproduce the conditions prevailing at industrial scale but reasonably approach to some of the variables to which particles are submitted.…”

Evaluation of petrology and reactivity of coal blends for use in pulverized coal injection (PCI)

“…The combustion behaviour of coal char has a significant impact in the amount of unburned material, although char combustion has been improved in the last decade by increasing hot blast furnace and oxygen enrichment and improving injection lance design (Lu et al, 2002). The reactivity of char strongly depends on the parent coal and the operating conditions (Essenhigh, 1981).…”