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

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.