Donald Herbert Maylotte

Significant parameters in the catalysed CO2 gasification of coal chars

Abstract Alkali and alkaline earth carbonates have been used to catalyse the C02 gasification of coal chars prepared by pyrolysis of Illinois No.6 coal. This study found that alkaline earth carbonates are fair gasification catalysts, though throughputs are insensitive to loadings in the range of 5–20 wt%. The order of efficacy is Ba > Sr > Ca. Alkali carbonates are excellent catalysts, with throughputs showing a dependence on loadings and atomic number. In particular, at high loadings (20 wt%) the order is Cs > K > Na > Li. As kinetic parameters for the alkali carbonate catalysed Boudouard reaction with coal chars differ significantly from those for graphites, an alternative redox cycle mechanism has been proposed involving an alkali hydride intermediate.

Forms of potassium in coal and its combustion products

Abstract High resolution X-ray absorption spectroscopy utilizing synchrotron radiation as a light source was used to probe the bonding and structure of potassium in coal and coal-derived products. The potassium sites in coals of various ranks and in the products of thermochemical processing were identified by comparison of unknown spectra with those of a wide range of selected potassium model compounds and minerals. For most coals examined, potassium was found to occur in a site in illite, a layered clay related to muscovite. Such coals included subbutuminous, high and low volatile bituminous, cannel and anthracite types. The lower rank subbituminous and lignite coals show other forms of alkali, where potassium appears to be in a noncrystalline environment. Examination of the near-edge spectra fails to reveal similarities between potassium in low-rank coal and in any organically bonded potassium model compounds such as carboxylic, phenolic, benzoic, phthalimide or intercalated groups. It is possible that the exchangeable potassium is still inorganically associated on disordered clay surfaces, though this remains equivocal. Thermal treatment of potassium-bearing phases inherent to coals generally leads to formation of a potassium aluminosilicate glass. Samples include those obtained from flash pyrolysis, gasification, laboratory combustion and a large scale pressurized fluidized-bed combustor including bed material, scrubbing hot cyclones, and quench-cooled pins for vapour phase deposition. No potassium sulphates were observed in the deposits.

Experimental and Theoretical XANES Studies of Selected Vanadium Compounds

As the region near an absorption edge is scanned in energy the ejected photoelectron seauentially probes the empty electronic levels of the material. The resulting spectra have long been known to be rich in chemical and structural information [1]. An attempt to correlate a large number of spectra, including vanadium compounds, was made by WHITE and McKINSTRY [2]. Reconciliation of molecular orbital theory with x-ray near-edge spectra as made by BEST [3] for tetra-oxo-bonded transition metals, by SEKA and HANSON [4] for octahedral complexes and by FISCHER and BAUM [5] for Ti compounds. All of these earlier efforts suffered from the (varying) poor resolution of their experiments. With the availability of intense synchrotron radiation sources, these spectra can now be gathered more quickly, simply and with greater resolution than ever before. A number of interesting papers have recently been published on K-edge and L-edge spectra [6] measured with synchrotron sources. For transition metals there are generally weak but distinct features in the spectra just before the onset of absorption. The so-called near-edge features have been attributed by SHULMAN et al. [7] to electronic transitions from ls to some d,s (non-dipole), and p-like (dipole-allowed) empty states. In this paper we report high resolution XANES spectra of vanadium in a series of selected compounds of well-defined coordination geometry and known bond distances. The intensity and position of these near-edge spectral features are analyzed within a quantum molecular orbital framework and a simple coordination charge concept.