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Technical Market Support

Mineral Re-Distribution from Raw Coal to Pulverised Fuel (pf) Coal to Ash in Commercial Power Stations

Technical Market Support » Thermal Coal

Published: August 21Project Number: C28072

Get ReportAuthor: Rohan Stanger, Quang Anh Tran, Terry Wall, Liza Elliott, Andrew Stanger, Rajenda Gupta, Chris Spero | University of Newcastle, University of Alberta, CS Energy

This project was based on understanding how minerals in thermal coal are redistributed during combustion in commercial power stations. The project involved samples taken from two Australian power stations burning the same coal. Samples of pulverised coal and fly ash were obtained under a range of operating conditions and compared using an automated SEM-EDS technique and associated software - Tescan Integrated Mineral Analysis (TIMA). The TIMA software provided a means of mineral identification from elemental content using Energy Dispersive Spectroscopy (EDS) profiles. In this respect, the project's additional aim was to determine the value of such analysis towards thermal coal characterisation.

The TIMA software provided a range of valuable analytical tools to characterise thermal coals. Specifically, its ability to recognise a wide range of minerals based on elemental composition could be combined with particulate information to provide reports of (i) Mineral analysis; (ii) Grain Size distribution for selected minerals; (iii) association between selected minerals.

Combined, these tools provided a means of characterising coal particles to better reflect the distribution of ash chemistries occurring during combustion and provide fundamental enhancements towards predicting deposition. Current testing of ash chemistry and fusibility is based on bulk properties and lack the precision to account for the wide variability in mineral combinations (and hence chemistry) that occurs inside milled coal particles. These advanced analytical tools offer a significant step forward in capability for thermal coal evaluation by distinguishing minerals at a granular level.

The dominant minerals identified in the pulverised coal samples were kaolinite (56-79% of minerals), quartz (13-31.6% minerals), siderite (4-15% minerals) and hematite/magnetite (0.8-4% minerals). These minerals comprised over 99% of the identified mineral compounds and had 2-9% as unclassified.

It was found that the pulverised coal samples contained significant variation in mineral grain size. The size of quartz grains was recommended as a potential indicator for erosion, measuring between 14µm to 155µm for median grain size.

For the fly ash samples, TIMA was able to identify between 45-62% of the fly ash material as kaolinite, quartz and hematite. Kaolinite in the fly ash was observed to vary from 22-32%, quartz from 11-22% and hematite from 6-11%. Overall, the use of the existing library was found to be inadequate due to the large amount of unclassified fused alumina-silicate material.

This research has shown that mineralogical information of coal is highly useful for determining the ash characteristics inside a coal-fired power station. At a particulate level, the mineral distribution of kaolinite/silica/siderite allowed slagging potential to be assessed as a population rather than as a bulk value with current methods. The reactive clay library developed in this work from fly ash provides an alternative metric for assessing this slagging potential.

Apart from evaluating the use of TIMA for minerals-related applications, an outcome of this project has also been to identify a range of TIMA uses potentially relevant to coal utilisation and marketing. Most are based on the TIMA capability which extends the previous CCSEM and QemSCAN methods.

The results of this project have indicated that the TIMA software could also be useful for coal preparation in terms of providing particle density distributions, indicative distributions of organically bound elements (e.g. sulfur) and for higher value minerals such as Rare Earth minerals (e.g. Monazite) and Zircon. Combined maceral and mineral grain data offer new insights for evaluation of coking coal in terms of resultant coke character from high temperature carbon-mineral interactions (e.g. SiC formation, iron reduction). Combining maceral and mineral characterisation with TIMA's particle porosity measurement could also provide opportunities for assessing coal to char formation and burn-out in combustion applications (power generation and PCI).

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