Vincenzo Calemma

FT-i.r. study of coal oxidation at low temperature

Abstract Chemical changes occurring during dry phase oxidation with air of a subbituminous coal have been investigated by FT-i.r. spectroscopy. Tests were carried out at 200, 250 and 275 °C, withdrawing coal samples from a fluidized bed reactor at fixed intervals up to 12h. At each temperature there was an increase in oxygenated functional groups (carboxyls, esters and anhydrides), whose concentration was strictly dependent on the oxidation temperature. The results have shown that the aliphatic structure is more reactive than the aromatic one. There is evidence of differing reactivities of some aliphatic structures.

Changes in coal structure accompanying the formation of regenerated humic acids during air oxidation

Abstract Dry-phase oxidation of two subbituminous coals and a lignite was carried out in a fluidized bed reactor at 200 °C, at different oxygen partial pressures and reaction times up to 4 h. The formation and evolution of various oxygenated functional groups (ester, carboxyl, ketone and hydroxyl) was investigated by FT-i.r. spectroscopy using a curve-resolving procedure and acetylation of coal samples. The development of alkali solubility of the coal as a function of reaction time was studied and the average molecular weight of regenerated humic acids (RHA) extracted from the oxidized coal was determined. The lower molecular weight of RHA extracted from highly oxidized coal samples suggests that besides the build-up of various oxygenated functional groups, the development of alkali solubility also results from concomitant ‘depolymerization’ of the coal network. The latter may occur through oxidation of aliphatic cross-links and formation of ester groups hydrolysable in basic solution. The FT-i.r. results were correlated with the development of alkali solubility. Some oxidation pathways are proposed.

Study of low temperature coal oxidation by 13C CP/MAS n.m.r.☆

Abstract Dry phase oxidation of a subbituminous coal with air was carried out in a fluidized bed reactor at 175, 200, 250 and 275 °C. Coal samples were withdrawn during the reaction at fixed intervals up to 12 h. Reaction pathways and chemical changes occurring in coal structure during oxidation were studied by 13 C n.m.r. spectroscopy, elemental analysis and weights variation. By coupling 13 C n.m.r. and carbon mass balance data it was possible to detect semiquantitatively the fate of aliphatic and aromatic carbon. The results show that, at each temperature, aliphatic carbon is more reactive than aromatic carbon, and that aromatic carbon does not seem to be affected by oxidation ⩽ 200 °C. A decrease in the aromatic carbon of 15% at 250 °C, and 20% at 275 °C was detected after 12 h of oxidation. The mass balance data and elemental analyses were used to examine the effect of reaction temperature on the relative importance of reaction pathways which lead to oxygen fixation or coal combustion. The weight losses at 250 and 275 °C are mainly due to combustion of the aliphatic structure.

On the gasification reactivity of Italian sulcis coal

Abstract Some aspects of the gasification behaviour of raw and demineralized chars prepared from Italian Sulcis coal have been studied. The results obtained with demineralized chars showed that when the pyrolysis temperature is raised from 1123 to 1423 K a maximum decrease in reactivity (in CO2) of a factor of four is observed. Regardless of the pyrolysis conditions and the presence or absence of mineral matter, no significant differences in the global energy of activation were observed. In spite of the presence of significant amounts of ion-exchanged calcium in the Sulcis coal, small reactivity differences were observed between raw and demineralized chars. This behaviour was ascribed to the high sulphur content of Sulcis coal, which is responsible for the formation of catalytically inactive CaS during the pyrolysis step.

Changes in optical properties of coals during air oxidation at moderate temperature

Abstract Montana Rosebud coal and North Dakota lignite have been oxidized in a fluidized bed reactor at 200°C, with air at 0.2, 0.6 and 1 MPa for reaction times up to 4 h. Oxidized samples were analysed by reflected light microscopy to investigate the variation of reflectance, formation of rims and changes in maceral morphology. The most remarkable changes concerned the vitrinite. Exinite exhibited an increase of reflectance, but this was not measurable owing to the small size of the fragments. The results indicate that the increase of reflectance at 200°C is only partially dependent on chemical changes caused by oxidation. Data show the existence of a threshold oxidation degree, beyond which the reflectance is not affected by further chemical changes occurring in the coal structure. Furthermore, the changes in elemental composition associated with the degree of threshold oxidation are relatively small. During oxidation both Montana Rosebud coal and North Dakota lignite exhibit the formation of rims, but with large differences as regards the size of the phenomenon. The decreased percentage of vitrinite fragments with rims at higher air pressures, and the relationships between the width of rims and reaction time at the three pressures used, support the view that formation of rims occurs when the reaction rate is controlled by the diffusion of oxygen. During reaction, the telinitic structure of vitrinite tends to disappear to assume a collinitic appearance.