Abundance of Fe-bearing species in coal extract and its implications to the iron speciation in raw coal

Lian Zhang, Eleanor Binner, Sankar Bhattacharya, Toshimasa Takanohashi

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Abstract

Three Argonne Premium Coal Samples (APCS) and two Australian coals were extracted in a Japanese coal extraction process to generate ultra-clean extract (HyperCoal) that is potentially used in an advanced combustion system such as gas turbine to improve coal power generation efficiency. For a gas turbine fuel, the contents of inorganic impurities within it must be lower than 1000 mg/kg in total to avoid the turbine blade corrosion and/or erosion. How to exactly quantify the inorganic metals in a coal extract, especially their chemical species, is however beyond the capabilities of most the conventional analytical instruments. In this study, five raw coal samples were extracted by 1-methynaphthaelene (1- MN) or its mixture with indole (IN) at 360°C and under 1 MPa nitrogen (cold) protection. Concentrations of inorganic elements and their chemical compositions in the resulting extracts were investigated. Inductively coupled plasma-optical emission spectroscopy (ICPOES) was adopted for elemental quantification, whereas a variety of advanced instruments/methods was employed for chemical speciation, including electron spin resonance spectroscopy (ESR), transmission electron microscope (TEM) and sequential leaching. The results indicate that, irrespective of raw coal type (e.g. rank, location), Fe is the most prevalent metal in coal extract, accounting for more than half of the total inorganic metals. Black experiments without using coal precluded the contaminants from reaction and/or analytical systems employed in this study. Fe is also insoluble in any acids such as hydrofluoric acid (48%). In contrast, characterization using the advanced instruments clearly indicates that Fe in coal extract is mainly composed of two forms: octahedral Fe3+ complex associated with coal functional groups and nanometric Fe-bearing particles embedded deeply in the closed voids in coal matrix. These two species are also prevalent in the acid-washed coal samples, showing independence on coal rank. It is highly likely that they are biologically originated from ferritin ion in proteins and iron porphyrin complexes that have undergone complex transformation during coalification. The Fe-bearing species could be biomarkers in coal. To reduce their concentrations in coal extract is essential for a clean and safe combustion in gas turbine, which can be realized by washing either raw coal or coal extract with a chelating agent such as ethylenediaminetetraacetic acid (EDTA).

Original languageEnglish
Title of host publication26th Annual International Pittsburgh Coal Conference 2009, PCC 2009
Pages506-515
Number of pages10
Volume1
Publication statusPublished - 2009
Event26th Annual International Pittsburgh Coal Conference 2009 - Pittsburgh, United States of America
Duration: 20 Sep 200923 Sep 2009
Conference number: 26

Conference

Conference26th Annual International Pittsburgh Coal Conference 2009
CountryUnited States of America
CityPittsburgh
Period20/09/0923/09/09

Keywords

  • Acid-washing
  • Biomarkers
  • Coal extract
  • Nanometric Fe-bearing particles
  • Octahedral Fe complex

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