| In this work, a novel in situ/direct method has been developed to simulate the partitioning and estimate the chemical speciation of three toxic semi-volatile trace elements (TEs), arsenic, antimony and selenium, during coal combustion. The method involves the use of a modified graphite furnace atomic absorption spectrometer (GFAAS) as a simultaneous high temperature (up to 2800 °C) in situ pyrolysis simulator, sample collector and analytical platform. Several methods for calculating activation parameters were compared and a simple, yet accurate method was selected for calculations. It was shown that the activation parameters determined using this method are independent from instrument operational conditions.;Three forms of occurrence of TEs in coal were modeled. The partitioning of organically associated TEs was simulated by placing TEs within the GFAAS graphite furnace, which upon heating generates an environment that simulates the initial (anaerobic) stage of pyrolysis for the organic fraction of coal. For excluded minerals, the effect of the graphite tube matrix was reduced by using either Ta-lined or Zr/W-coated furnace tubes. Mineral inclusions were mimicked by using un-lined graphite furnace tubes and representative water-soluble matrices homogeneously mixed with the TEs.;In the case of organically associated TEs, the following activation energies were obtained for As, Sb and Se, respectively: 60 +/- 4, 36 +/- 3 and 57 +/- 9 kcal•mol-1. These activation energies were then compared to the enthalpies of reactions that could have occurred during vaporization/atomization. It was found that As and Se atoms as well as diatomic species might be present. For Sb, the low activation energy observed may reflect a less energy-demanding rate-limiting physical process, such as evaporation of antimony oxide or chloride.;Excluded minerals were modeled by using either of two types of coated furnace tubes (i.e. Zr-coated and W-coated tubes) or a Ta lining insert. For As, the activation energy obtained in an uncoated graphite tube was the same as the activation energy obtained in coated tubes. Thus, the partitioning of organically associated As and As present in excluded minerals does not appear to involve carbon. For Se, an increase in activation energy was observed in the presence of both coatings and the Ta lining. This indicates either a stronger retention of Se by excluded minerals compared to an organic coal matrix or a significant role of elemental carbon in Se atomization.;In the case of mineral inclusion modeling, two cationic matrices, Ca(CH 3CO2)2 and Fe(NO3)3, significantly increased atomization/vaporization activation energies, indicating an increased TE retention in the solid phase. These observations were attributed to specific interactions of anionic TE species with cationic matrices. By contrast, an anionic matrix, NaAlO2, did not alter the activation energies for these TEs. The other anionic matrix, K2SiO3, decreased the atomization/vaporization activation energy for Se and reduced its atomic absorption signal, apparently due to non-specific blocking of the TE access to the surrounding carbon. |
