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Coal combustion is one of the main anthropogenic sources of toxic trace element emissions to the environment. Various species and oxidation states of the trace elements released from power stations may determine their ultimate environmental fate and health impacts. This study focuses on speciation of arsenic and selenium present in the coal combustion products. Speciation analysis in representative coal, bottom ash, and fly ash obtained from four different Australian power stations was carried out in this work. Laboratory ash and char were also produced by carrying out combustion and pyrolysis experiments in a laboratory based drop tube furnace. The synchrotron based nondestructive speciation analysis method X-ray absorption fine structure spectrometry (XAFS) was applied for arsenic and selenium speciation analysis of the selected coal, ash, and char samples. The semiquantitative analysis of arsenic revealed variations in arsenic species in the coal samples indicating the presence of As/pyrite, arsenite (As 3+ ), and arsenate (As 5+ ) with the latter as a dominant form. Arsenic in power station fly ash samples was found to be mainly in an arsenate form with little presence of arsenite (As 3+ ). Selenium speciation in coal samples indicated organic/reduced or elemental forms as dominant selenium species along with presence of selenite (Se 4+ )/selenate (Se 6+ ). Selenium in fly ash was mainly found to be selenite with a minor presence of selenate. Char produced by pyrolysis indicated different speciation behavior of arsenic and selenium compared to coal and ash samples, which might be due to their further reactions with other volatilized species produced during pyrolysis and/or retained mineral matter.

A dimensionless correlation for the liquid overflow rate from a rising column of foam has been developed that expresses the ratio of liquid to gas superficial velocities as a function of Reynolds, Grashof, and Morton numbers. This correlation is developed using new and previously reported data. An empirical approach is made after noting that models that estimate the overflow rate by adding a gross upward velocity to conventional solutions of stationary foam drainage that take into account viscous losses only at the Plateau border walls are unsatisfactory. Relaxation of the assumption of rigid Plateau borders by allowing for finite surface shear viscosity does not improve such a mechanistic approach. However, an existing semiempirical drainage expression that takes into account losses at the vertices within the foam enables a much better prediction of the overflow rate, although the newly developed correlation gives superior predictions and requires no a priori knowledge of the drainage behavior of the foam. Experiments performed on a shear-thinning solution are also discussed.

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