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Abstract
Rapid growth of the steel industry in coming years will be strongly dependent upon coal. Understanding of coal behavior in current or emerging ironmaking processes will require improved understanding of coal transformation during direct injection or after carbonization. Recent studies have highlighted the significance of coke minerals on coke reactivity such that the reactivity was shown to increase with increasing contents of magnetite, pyrrhotite, and metallic iron. Therefore, it is imperative to understand how coal minerals transform during carbonization particularly at higher temperatures and how mineral transformation is influenced by carbonization and heat treatment conditions and, in turn, how the transformed minerals affect reactivity. However, there is limited information available about transformation of coal minerals as a function of coal properties, carbonization and heat treatment conditions.
In this study, the effect of coal mineral transformation was investigated, particularly iron-bearing minerals on coke reactivity. Effect of high heating rate on coal mineral transformations was studied in a TGA reactor. Carbonization tests were conducted in a 100 g and a 9 kg coke oven. Coal devolatilization and coke annealing were examined under controlled conditions in a horizontal tube furnace. The mineralogy of coal and coke was analyzed using XRD, QEMSCAN and FESEM/EDS. Implications of minerals transformation on coke reactivity with CO2 was studied in a fixed bed reactor at 1173K.
The study demonstrated that total volatile yield of lump coals was not influenced by either the tested range of temperature or particle size. The devolatilization rates of coal decreased with increasing size of coal, however for coal lumps above 10 mm there appeared to be little effect of particle size. The swelling tendency of lump coals increased with increasing volatile content, particle size and heating rate. Swelling ratio based on measurements of physical dimensions of individual large coal particle or lump coal did not have any relation to the free swelling index commonly reported by the existing coking tests. The study showed that low swelling coals may be more vulnerable to cracking.
The study showed that both the apparent reaction rates of cokes and chars made from the same coals were modified greatly under different carbonization conditions and the differences were much greater than those seen with different coal types. The micropore surface areas and the carbon structure of cokes were also significantly influenced by heating rate, soaking time and temperatures during carbonization and thermal treatment, however the initial apparent reaction rates of cokes and chars was not strongly related to either carbon crystallite height, micropore surface area of cokes and chars or rank and maceral composition of the parent coals as seen in the past for cokes.
The study also demonstrated the strong influence of carbonization conditions on the transformation of coal minerals. In particular the magnitude and type of potential catalytic iron minerals mainly iron sulfide, metallic iron and iron oxide were affected by the carbonization conditions. At higher temperatures they transformed to non-catalytic iron minerals mainly silicide.
The initial reactivities of cokes and chars with CO2 increased with increasing amounts of the catalytic iron minerals present and this relationship was found to be independent of carbonization conditions as well as heat treatment temperature at total catalyst levels below 1.0 wt%. However, at greater catalyst levels CO2 reaction rate was less sensitive to the total amount of catalytic iron phases. The initial apparent reaction rate of cokes annealed at higher temperature (1773 K) significantly decreased due to decreased levels of catalytic iron minerals as well as increased ordering of the carbon structure. The catalytic impact of minerals on reactivity was also found to decline with progressive gasification.
This study suggests that transformation of coal iron minerals to potential catalytic iron-bearing minerals in coke can be optimized by suitable selection of carbonization conditions and hence to control coke quality particularly gasification reactivity. The study demonstrates that understanding of high temperature performance of coal matter, particularly coal mineral matter is important to optimize coke quality for both current and emerging blast furnace operations.
The quantitative mineralogical analysis suggests that transformation of iron-bearing minerals in coal to catalytic iron phases in coke may be optimized by controlling the heating rate and temperature during carbonization.
Further understanding of the influence of coal mineral grain size and morphology on the distribution as well as the catalytic intensity of iron and calcium species in coke is required on establishing a quantitative relationship with coke quality parameters particularly reactivity.