The combustion efficiency of pulverised coal injection PCI is an important factor influencing stability and productivity of the blast furnace. It is affected by coal properties and the process conditions employed for combustion. Economic considerations have driven blast 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 operating conditions on combustion performance of coal and to examine the role of coal minerals and their transformations on low and high temperature reactivity. The combustion performance of three PCI coals was investigated under a range of combustion conditions including three combustion temperatures of 900??C, 1200??C and 1500??C, and a range of oxygen concentrations in the gas phase at 1200??C in a drop tube furnace (DTF). The low temperature oxygen reactivity of pyrolysed chars was also measured by observing weight loss in a thermogravimetric (TGA) furnace at 600??C. Physical and chemical properties of pyrolysed and partially combusted chars were characterised using a range of analytical tools including X-ray diffraction, scanning electron microscopy, BET N2 surface area and Hg porosity. The correlation between char properties and char reactivity at low and high temperatures was also investigated. All three coal samples experienced deactivation during progressive combustion at 1200??C and 23%v O2 i.e. there was a decrease in the reaction rate with proceeding combustion. The carbon structure of the chars became increasingly ordered as quantified by an increase in crystallite height and a decrease in the amorphous carbon proportion in char. Partially combusted char had much higher surface area than a pyrolysed one, which can be attributed to the opening of enormous number of closed pores as combustion proceeds. However, this increase in surface area did not show a direct correlation with char reactivity. Average particle size of ash increased with increasing degree of combustion due to fusion and agglomeration of coal minerals. Under these conditions, carbon structural ordering of char was found to be one of the key factors primarily responsible for loss of char reactivity during combustion. Increasing oxygen content in the gas stream from 23% to 35% at 1200??C resulted in a significant improvement in the combustion performance of three coals, with burnout increasing from ~65% to up more than ~95%. However, increasing oxygen level beyond 35% did not lead to any further significant improvements. Coal burnout was also enhanced by increasing temperature in the range 900??C to 1500??C, such that the improvement was much more rapid in the higher temperature range of 1200??C to 1500??C. This could be related to increased reaction rates 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 enhanced char reactivity. Coal minerals underwent increased fusion and melting as the combustion temperature was increased. At 1500??C, most ash particles were molten. The level of basic oxides in ash as well as the extent of association between aluminosilicates and basic oxides enhanced the proportion of molten phases. Fusing and melting behaviour of ash particles was found to influence char combustion reactivity at high temperatures. Ash melting on the char surface may hinder gas accessibility to the reactive surface of char, thereby decreasing char burnout. The molten ash particles may coalesce and cover char surface or these molten particles may partially/completely block char pores. The amount of slag phases in ash and the distribution of minerals in char are expected 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 in a number of ways. Inorganic minerals, such as iron and calcium catalyse char oxidation at low temperatures. 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 of ash may restrict the accessibility of oxygen to carbon in char through physical obstruction, thereby, retard char oxidation.
| Identifer | oai:union.ndltd.org:ADTP/212620 |
| Date | January 2004 |
| Creators | Al-Omari, Yaser, Materials Science & Engineering, Faculty of Science, UNSW |
| Publisher | Awarded by:University of New South Wales. School of Materials Science and Engineering |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Yaser Al-Omari, http://unsworks.unsw.edu.au/copyright |
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