The combustion efficiency of pulverised coal injection PCI is an important factorinfluencing stability and productivity of the blast furnace. It is affected by coal propertiesand the process conditions employed for combustion. Economic considerations have drivenblast furnace operators to commission a wide range of coals, which differ in type and rank.The main objective of the current project is to study the influence of different operatingconditions on combustion performance of coal and to examine the role of coal minerals andtheir transformations on low and high temperature reactivity.The combustion performance of three PCI coals was investigated under a range of combustionconditions including three combustion temperatures of 900°C, 1200°C and 1500°C, and arange of oxygen concentrations in the gas phase at 1200°C in a drop tube furnace (DTF). Thelow temperature oxygen reactivity of pyrolysed chars was also measured by observing weightloss in a thermogravimetric (TGA) furnace at 600°C. Physical and chemical properties ofpyrolysed and partially combusted chars were characterised using a range of analytical toolsincluding X-ray diffraction, scanning electron microscopy, BET N2 surface area and Hgporosity. The correlation between char properties and char reactivity at low and hightemperatures was also investigated.All three coal samples experienced deactivation during progressive combustion at 1200°C and23%v O2 i.e. there was a decrease in the reaction rate with proceeding combustion. The carbonstructure of the chars became increasingly ordered as quantified by an increase in crystalliteheight and a decrease in the amorphous carbon proportion in char. Partially combusted char hadmuch higher surface area than a pyrolysed one, which can be attributed to the opening ofenormous number of closed pores as combustion proceeds. However, this increase in surfacearea did not show a direct correlation with char reactivity. Average particle size of ash increasedwith increasing degree of combustion due to fusion and agglomeration of coal minerals. Underthese conditions, carbon structural ordering of char was found to be one of the key factorsprimarily responsible for loss of char reactivity during combustion.Increasing oxygen content in the gas stream from 23% to 35% at 1200°C resulted in asignificant improvement in the combustion performance of three coals, with burnout increasingfrom ~65% to up more than ~95%. However, increasing oxygen level beyond 35% did not leadto any further significant improvements. Coal burnout was also enhanced by increasingtemperature in the range 900°C to 1500°C, such that the improvement was much more rapid inthe higher temperature range of 1200°C to 1500°C. This could be related to increased reactionrates at higher temperatures.Pyrolysed char reactivity was measured at low temperature 600°C and 10%v O2 using TGA.The results indicated that the presence of iron and calcium minerals could result in enhancedchar reactivity.Coal minerals underwent increased fusion and melting as the combustion temperature wasincreased. At 1500°C, most ash particles were molten. The level of basic oxides in ash as wellas the extent of association between aluminosilicates and basic oxides enhanced the proportionof molten phases. Fusing and melting behaviour of ash particles was found to influence charcombustion reactivity at high temperatures. Ash melting on the char surface may hinder gasaccessibility to the reactive surface of char, thereby decreasing char burnout. The molten ashparticles may coalesce and cover char surface or these molten particles may partially/completelyblock char pores. The amount of slag phases in ash and the distribution of minerals in char areexpected to have a significant influence on ash-char interactions at high temperatures.In summary, the study shows that inorganic matter present in coal can affect coal combustion ina number of ways. Inorganic minerals, such as iron and calcium catalyse char oxidation at lowtemperatures. On the other hand, these minerals may act as fluxing agents at high temperatures,which could lower the melting point of aluminosilicates minerals in char. The molten phases ofash may restrict the accessibility of oxygen to carbon in char through physical obstruction,thereby, retard char oxidation.
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