Partitioning behaviour of trace elements in a stoker-fired combustion unit: An example using bituminous coals from the Greymouth coalfield (Cretaceous), New Zealand

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Abstract

In order to understand trace element behaviour during combustion of coals from the Greymouth coalfield, combustion tests were performed on three seam composite samples. The major and trace elements from sub-samples of feed coal, bottom ash, fly ash, and flue gas were analysed by different techniques including inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma-atomic emission spectrometry (ICP-AES), wavelength dispersive X-ray fluorescence (WD-XRF), and scanning electron microscopy with energy-dispersive X-ray analyser (SEM-EDXA). To help better understand trace element partitioning in combustion ash, float–sink and sequential leaching experiments were also employed to determine the association of trace elements with mineral matter or organic matter. Instrumental Neutron Activation Analysis (INAA) was also employed to determine trace element content in float and sink fractions of fly ash as well as in three major phases in the bottom ash.

The partitioning behaviour of trace elements, including some that are environmentally sensitive, was also investigated through the use of float–sink tests and direct determination of trace elements in different combustion ash types and phases. Mass balance and partitioning of major and trace elements have been studied to determine the fate of trace elements after combustion. The partitioning of trace elements, especially hazardous air pollutants (HAPs), in different combustion ashes can be summarised as follows:

  • 1.

    Most trace elements, especially As, Ba, Co, Cr, Mn, Ni, are partitioned in the glassy and refractory bottom ash fractions.

  • 2.

    A significant proportion of trace elements of As, Se, and Pb are partitioned into fly ash fractions.

  • 3.

    Some volatile elements (e.g. > 90% of S and Hg and up to 64% of Cl) and, to a lesser extent, B (up to 44%) and Cd (up to 50%) are partitioned in the flue gas fraction.

  • 4.

    Although the low ash yield of Greymouth coal seams have the advantage of generating less solid combustion ash, one of the accompanying consequences is that resultant trace elements tend to be enriched in the ash to a greater magnitude than other more clastic sediment influenced coals.

Introduction

With increasing awareness of the environmental impact of coal combustion [U.S. Environmental Protection Agency, 1998a, U.S. Environmental Protection Agency, 1998b, U.S. Environmental Protection Agency, 2000] and stringent regulations of hazardous air pollutants (HAPs) in developed countries, the behaviour of trace elements during mining and subsequent utilisation has attracted a great deal of interest. The US Clean Air Act Amendments of 1990 (U.S. Statutes at Large, 1990) specifically identified As, Be, Cd, Cr, Co, Hg, Mn, Ni, Pb, Sb, Se, and U as potential HAPs. Some coals with a high abundance of potentially toxic trace elements, especially HAPs, may become less useable or unusable as stringent compliance on fly ash disposal and flue gas discharge from utilities comes into effect.

Various workers have studied trace element occurrence in coal (Finkelman, 1981, Finkelman, 1995, Swaine, 1990), trace element partitioning in combustion ashes (Querol et al., 1995, Vassilev and Vassileva, 1997, Hower et al., 2000a, Clemens et al., 2000, Vassilev et al., 2001, Karayigit et al., 2001; among others), and the mobility of those trace elements in coal and combustion ashes (Hower et al., 1993, Fernandez-Turiel et al., 1994, Fleming et al., 1996, Querol et al., 1996, Querol et al., 2001, Karuppiah and Gupta, 1997, Gentzis and Goodarzi, 1999). However, the behaviour of trace elements can vary significantly between different coal beds either simply because they have different modes of occurrence or because they are used under different combustion conditions. There are few geochemical rules that apply universally to all coals because of the complexity of trace element occurrence and behaviour upon combustion.

There are numerous studies of trace element partitioning in combustion ashes, but most have been conducted under pulverised combustion regimes characterised by high temperature (1300–1500 °C), fine feed particle size and longer residence time in the boiler, and a ratio of fly ash to bottom ash being roughly 80:20 (Smith et al., 1980, Meij, 1994, Querol et al., 1995, Bool and Helble, 1995, Robl et al., 1995, Hower et al., 1997, Hower et al., 1999a, Hower et al., 1999b, Hower et al., 1999c, Hower et al., in press, Martinez-Tarazona and Spears, 1996, Vassilev and Vassileva, 1997, Senior et al., 2000a, Senior et al., 2000b, Senior et al., 2000c, Yan et al., 1999, Sakulpitakphon et al., 2000, Sakulpitakphon et al., 2004, Ward, 2002, Sloss, 2002, Narukawa et al., 2003, Ren et al., 2004, Pires and Querol, 2004, Mardon and Hower, 2004, Mastalerz et al., 2004, Vassilev et al., 2005a, Vassilev et al., 2005b; among others). Only a few studies (e.g. Clemens et al., 2000) have been conducted on stoker-fired combustion regimes characterised by lower temperature (1000 to 1200 °C), larger feed particle size, and longer boiler residence time, and a ratio of fly ash to bottom ash being roughly 20:80. Because of the different combustion conditions between the pulverised and stoker-fired combustion regimes, trace element partitioning may not be the same as what has been established in the literature. Many industrial boilers in New Zealand are of the stoker-fired type. Therefore, this study is designed specifically to investigate the behaviour of selected trace elements during stoker-fire combustion regimes. The coal selected is from the Greymouth coalfield and is Cretaceous in age. The study also aims to identify the mineralogical transformations occurring within the combustion chamber in order to gain a better understanding of the factors influencing the partitioning behaviour of trace elements. Understanding of trace element partitioning behaviour allows an assessment of their environmental impact to be made as well as identify any undesirable impacts which then allows potential problems to be avoided or mitigated.

Section snippets

Location and sampling methods

Three seam composite samples were taken as channel samples in accordance with American Society for Testing and Materials (ASTM) standard D4596-86 (ASTM, 1995) from two different underground locations (P532A, 2A–2B) in the same seam (“E” seam) from Strongman No. 2 Mine, Greymouth coalfield, New Zealand. The sampling location is given in Fig. 1. The E seam coal occurs in the Rewanui Member of the Paparoa Coal Measures (Fig. 2). Sample P532A was taken from the northwest margin of Strongman No. 2

Characterisation of the feed coal

Three combustion runs were performed on three underground coal samples taken from Strongman E seam. The coal quality data of these three combustion samples are shown in Table 2.

The major minerals found in the feed coal were clays (mainly kaolinite, illite, and minor smectite), quartz, and carbonates (mainly siderite, ankerite and calcite). The minor minerals include sulphides (pyrite, marcasite) and phosphates (apatite, crandallite, monazite); with trace amounts of sulphates (gypsum), oxides

Environmental significance of trace elements in combustion products

In the present study, most trace elements (including As, Co, Cr, Ni, Pb, and Zn, etc.) are found to be at relatively low concentrations and mostly partitioned in the glassy and refractory bottom ash fraction, particularly with silicates or Al-silicate glass phases. The elements partitioned in bottom ash are (1) fixed or encapsulated into the glassy phases; and (2) at relatively low concentrations. Therefore, the trace elements partitioned in the glassy and refractory bottom ash are difficult to

Conclusions

The behaviour of some environmentally sensitive trace elements in float–sink tests and their partitioning in different combustion ashes have been quantitatively studied based on the respective proportion in different ash types generated from the laboratory-scale combustion runs. The mineralogical transformation during coal combustion has been investigated in relation to the trace element partitioning in bottom and fly ash. The partitioning mechanisms of those trace elements during stoker-fired

Acknowledgements

This study was funded by grants from Department of Geological Sciences, University of Canterbury, with additional funds from a FRST NZ(Foundation for Research, Science and Technology of New Zealand) grant to CRL Energy Ltd. This study would not have been possible without the kind support from the former Greymouth Coal Operating Ltd. and Solid Energy Ltd., especially Jonny McNee, Rob Boyd, Ted Nunn, and Frank Taylor and many others from the Strongman #2 Mine. Thanks are also due to Bob Finkelman

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